3.1 Introduction

Ever since it started to look as if the dinosaurs were done in by a nagging case of asteroids, the hypothesis has been pursued that every mass extinction has had an extraterrestrial cause, while some have expressed a strong preference for an earthly cause.Footnote ¹ Here I frame the pursuit of the Nemesis hypothesis of an extraterrestrially caused periodicity in mass-extinction events as a process of ‘reading’ the fossil and geologic records in pursuit of narrative closure.Footnote ² In the case of mass extinction, I am particularly keen on understanding how periodicity guides the search for evidence in pursuit of a causal narrative. In contrast to narratives of periodic extinction stand narratives of particular mass extinctions, where the plot is driven by the specific setting, characters, and one-off events. Of course, narratives of periodicity and one-time events do not exhaust the space of possible narrative explanations, and in the end I will describe somewhat of a middle path that seems to be gaining traction.

3.2 Periodicity of Mass Extinctions

In 1979, in Gubbio, Italy, a team of researchers led by Walter Alvarez discovered an iridium anomaly in sedimentary strata dated to be of end-Cretaceous age. This worldwide temporal horizon happens to coincide with the last known fossil occurrence of a number of biological taxa, including non-avian dinosaurs, ammonites, rudist bivalves, pterosaurs, mosasaurs and large numbers of plant and bird species. In terms of severity, the Cretaceous-Tertiary (or K-T) extinction (now known as the Cretaceous-Paleogene, or K-Pg extinction) ranks among the ‘big five’ mass extinctions in the fossil record: the end-Ordovician, Devonian, Permian, Triassic and K-Pg.

Like many discoveries in the earth sciences, the discovery of the iridium anomaly was serendipitous (Reference Glen and HookGlen 2002). The Alvarez team, assuming a statistically constant rain of meteoritic iridium throughout geologic time, thought that they could use that iridium flux to estimate elapsed time represented by sedimentary deposits. But the concentration they found was far off-scale relative to the known rate, and further lab analysis of samples confirmed that there was a ‘spike’ in iridium in a red boundary clay layer at the top of Cretaceous strata. Iridium concentrations in strata immediately above and below that layer fell off exponentially to zero (Reference Alvarez, Alvarez, Asaro and MichelAlvarez et al. 1980). Because Iridium is quite rare in the earth’s crust, the Alvarez team hypothesized an asteroid or comet impact.Footnote ³

Meanwhile, as the Alvarez group pursued evidence for an asteroid or other bolide impact at the Cretaceous–Tertiary boundary, David Raup and Jack Sepkoski were independently at work analysing broad extinction patterns in a synoptic database compiled by Sepkoski, A Compendium of Fossil Marine Families (Reference Sepkoski1982). Sepkoski had been compiling this database for years by combing the published literature for new reports of fossil occurrences, and continually updated this record of the first known and last known fossil appearances of marine families.Footnote ⁴ By tabulating the record of first and last appearances, a diversity curve for the entire Phanerozoic eon could be generated, and the number of families becoming extinct could be chronicled for each subdivision of geologic time. By Reference Raup and Sepkoski1982, Raup and Sepkoski’s statistical analyses of Sepkoski’s data resulted in a clear pattern of five large mass-extinction events – the so-called ‘big five’ – standing as outliers against a backdrop of smaller events (see Figure 3.1).

Figure 3.1 The ‘big five’ mass extinctions

The Ashgillian event at the close of the Ordovician, the Frasnian-Famennian event of the late Devonian, the Guadalupian-Dzhulfian event at the end of the Permian, the Norian event of the late Triassic and the Maestrichtian event at the Cretaceous–Tertiary boundary.

Source: Reference SepkoskiRaup and Sepkoski (1982).

As Jack Sepkoski continued to compile a pen-and-ink database of first and last fossil appearance of marine families, his colleague at Chicago, David Raup, became interested in computerizing, tabulating, plotting and analysing them statistically. Whereas Sepkoski had plotted the data at the level of the stratigraphic series (e.g., upper Cretaceous), Raup decided to plot the data at a finer resolution, that of the stratigraphic stage (e.g., the Maestrichtian stage, a subdivision of the upper Cretaceous; Reference Sepkoski and GlenSepkoski Jr 1994). The gestalt they perceived was one of mass extinctions evenly spaced (Figure 3.2). Could this be a periodic array?

Figure 3.2 Graph of percentage extinction of fossil marine families for each geologic stage of the past 250 million years

With best-fit 26 million-year periodicity.

Source: Raup and Sepkoski Reference Raup and Sepkoski(1984). Reproduced with thanks to the controllers of Raup and Sepkoski’s respective estates.

The stratigraphic record of the twelve largest mass-extinction events of the past 250 million years appeared to be periodic. However, two methodological constraints on the system had the potential to make the fossil record of mass extinction look periodic, regardless of whether it was or not. First, the stratigraphic record is divided into 40 stratigraphic stages (bins) of varying duration, and the dates of mass extinctions are resolved only to the level of the stratigraphic stage. Second, extinction peaks can only be recognized if they occur in non-consecutive stages, imposing some minimum separation between events. Spurious periodicity needed to be distinguished from the real thing. The questions raised were, within these methodological constraints: (1) what periodicity best fit the data? and (2) what is the probability of obtaining such a well-fitting periodicity simply due to chance?

To answer the first question, they needed to determine the best-fit periodicity, which required a measure of goodness of fit. Reference Raup and SepkoskiRaup and Sepkoski (1984) tried a range of periods from 12 million to 60 million years. For each period length they took a perfectly periodic time series and lined it up as closely as possible to the time series of mass extinctions and computed the standard deviation as a goodness-of-fit statistic. The best-fitting period came out to be 26 million years, with some standard deviation (call it sd*) from perfect periodicity. To answer the second question, they asked how frequently such a close fit to periodicity would occur if the timescale were randomized and extinction peaks were assigned to non-adjacent stages. As it turns out, the probability of obtaining a fit of sd* or better by chance was vanishingly small, and on this basis Reference Raup and SepkoskiRaup and Sepkoski (1984) were able to argue that the periodicity of 26 million years is very unlikely to have arisen by chance and thus should be provisionally accepted.Footnote ⁵

3.3 The Nemesis Affair and Narrative Closure

Raup and Sepkoski’s finding of periodicity, coupled with the Alvarez group’s discovery of an iridium anomaly coinciding with the mass extinction of the dinosaurs at the end of the Cretaceous period, led to the formulation of the Nemesis hypothesis (Reference Davis, Hut and MullerDavis, Hut and Muller 1984; Reference Whitmire and JacksonWhitmire and Jackson 1984). According to the Nemesis hypothesis, the sun has a companion star, Nemesis, which every 26 million years perturbs the orbits of comets in the Oort cloud, sending some of them on an earth-crossing orbit, with the resulting impact causing a mass extinction.Footnote ⁶ Linking periodicity with a possible extraterrestrial cause for mass extinction altered the temporality governing palaeontological research to one based on periodicity. In addition, it set in motion a search for a cause capable of producing the extinction periodicity: an astronomical search for a companion star (Reference MullerMuller 1988), a statistical search for periodicity in the ages of impact craters on earth (Reference Rampino and StothersRampino and Stothers 1984b) and a search for indicators of impact at stratigraphic horizons corresponding with mass extinctions around the world (e.g., Reference Claeys, Casier and MargolisClaeys, Casier and Margolis 1992). In short, this new ‘narrative of nature’ was compelling enough to galvanize a coalition of researchers from different disciplines, and changed the nature of extinction research, setting in motion a search for narrative closure.Footnote ⁷ Yet alongside the search, critiques were mounted, falling into one of five categories: general scepticism about the warrant for extraterrestrial causation (e.g., Reference HoffmanHoffman 1989), uncertainties in the ages of the dated events (e.g., Reference Grieve, Sharpton, Goodacre and GarvinGrieve et al. 1985), mismatch between timing of cause and effect, the possibility that periodicity may be spurious (e.g., Reference Stigler and WagnerStigler and Wagner 1987) and alternative explanations for the presence of the indicator in question (e.g., Reference Wang, Attrep and OrthWang, Attrep and Orth 1993).

3.4 Mass Extinction as a Recurring Narrative

While it was already accepted prior to Raup and Sepkoski’s finding of periodicity that there have been major mass extinctions in the history of life, there had been no reason to suspect that each of these mass extinctions had the same cause.Footnote ⁸ There was every reason to believe that if each mass extinction were to yield to any analysis at all, if the cause or causes were to be found, an idiographic approach was called for. Geologists and palaeontologists are highly trained in the identification of traces, in extracting information from remains, in inferring causal sequence, in arriving at consiliences of inductions and in pursuing multiple working hypotheses. In short, they are trained in reconstructing events from their available traces.Footnote ⁹ This may explain why, among many palaeontologists, the Nemesis hypothesis was met with suspicion. One eminent palaeontologist, Steven Stanley of Johns Hopkins, who in his 1987 book Extinction mounted a compelling argument that mass extinction is largely explicable in terms of well-documented changes in climate, summed up the prevailing view well:

This is a paradigmatic idiographic narrative explanation. Stanley is pointing out that the elements of a narrative explanation were beginning to coalesce – approaching narrative closure – when out of nowhere, like an asteroid, comes a new narrative. Note that he is not contesting the plausibility or empirical support for the extraterrestrial narrative (although he would do so elsewhere), but rather whether, given the existence of a climatological narrative, the extraterrestrial narrative was necessary.Footnote ¹¹

3.5 On Rereading the Book of Nature

Historian David Sepkoski has written an account of the rise of analytical palaeobiology entitled Rereading the Fossil Record, focusing on the period from around 1970 to the mid-eighties. Darwin and Lyell are understood to have brought us the metaphor of the fossil record as a book from which are missing several chapters, and from the remaining chapters many pages, and from the remaining pages many words, written in a slowly changing language.Footnote ¹² Sepkoski’s account describes three historical phases of rereading that fossil record: literal, idealized and generalized. The literal rereading of the fossil record is exemplified by Reference Eldredge, Gould and SchopfEldredge and Gould’s (1972) model of punctuated equilibria in which the absence of morphological intermediates from the fossil record is not absence of evidence so much as evidence of absence (of morphologic change in species)! The idealized rereading is exemplified by the nomothetic palaeobiology of the Marine Biological Laboratory (MBL) group, which abstracted away from species as individuals and modelled them as particles in space and time, nomothetism denoting the search for lawlike generalities among historical events.Footnote ¹³ The generalized rereading combines empirical and statistical analysis made possible by the painstaking compilation and digitization of taxonomic data by Sepkoski’s father, Jack, with mathematical modelling undertaken for the most part with David Raup (Reference SepkoskiSepkoski 2012). During the generalized rereading phase of the rise of analytical palaeobiology emerged David Raup and Jack Sepkoski’s work on mass extinctions, first as a statistical phenomenon quantitatively distinct from background extinctions and then as a recurring phenomenon registering a 26 million-year periodicity.

In coming to a better understanding of how scientists reread the fossil record, it may be helpful or at least instructive to appeal explicitly to narrative theory as it has been developed in the study of literature. Clearly this is a vast field encompassing a large body of scholarship. I would like to start with the key distinction in narrative theory, as formulated by the Russian formalists, Vladimir Propp (1895–1970) and Viktor Shklovsky (1893–1984).Footnote ¹⁴

This is the distinction between the supposed chronological sequence of events, referred to as the fabula, and the way they are presented in the narrative discourse, the syuzhet. Notably, fabula and the syuzhet register different orderings.Footnote ¹⁵ The relationship between these two orderings of events contributes to the literary characteristics of a narrative, allowing for it to exert its effects on a reader, and to elicit a certain aesthetic response. For example, in Dostoevsky’s Crime and Punishment, Raskolnikov’s murder of the pawnbroker is presented early in the narrative. It is only after reading for a good number of pages that we learn from Porfiry Petrovich’s cross-examination that several months prior to the murder Raskolnikov had written an essay arguing that the extraordinary man is not bound by common morality. This ordering of the presentation of events between fabula and syuzhet elicits an affective response from the reader, for example a feeling of suspense over whether Raskolnikov will crack under questioning.

Crime and Punishment is rather noteworthy for its subversion of the narrative of a typical murder mystery, so, although it illustrates the difference between fabula and syuzhet, we might be better served using the more conventional genre of the ‘whodunnit’. In this genre, the murder is revealed early on in the syuzhet, and suspense builds until the identity of the murderer is eventually revealed. I will return to this idea later.

If we take the idea of reading (or rereading) the fossil record seriously, we might regard the traces in the fossil record as forming the syuzhet, from which the palaeobiologist infers the fabula. The palaeobiologist ‘reads on’, and keeps rereading in a search for narrative closure. If this is so, then the narrative structure of the mass-extinction account may help explain the search for evidence as the search for closure.

It is important to acknowledge disanalogies between narrative closure in reading a work of fiction and in reading the fossil record. From the reader’s point of view, in a work of fiction, the fabula is something inferred, and, depending on the work in question, there may not be sufficient textual evidence to adjudicate among rival fabulae. At first it might be tempting to think that something analogous is at work in reading the fossil record. Due to underdetermination, scientists may differ in their readings of the fossil evidence, with each reading consistent with the available evidence. In both cases, one might bring in background knowledge, theories of interpretation and the like to provide support for one reading over another. In both cases, we may have no choice but to sit pat with the situation unresolved. Yet there are at least two important disanalogies between reading a work of fiction and reading the fossil record. The first stems from the nature of fiction. It is entirely possible that an author is, to put it glibly, ‘all syuzhet and no fabula’. That is to say, there need not even exist an underlying fabula to which the syuzhet refers.Footnote ¹⁶ The author may present, in whatever order, a set of events in the narrative discourse over which there could be great disagreement as to what their true chronological ordering was, and it is possible that there does not even exist any true chronological ordering: what we have are the words on the page and an argument in favour of one reading or another. Indeed, Reference WalshWalsh (2001) has argued that even in conventional cases of narrative fiction, fabula is not ontologically prior to syuzhet. Rather, from the syuzhet, the reader is constructing – not reconstructing – a fabula (not the fabula) in an ongoing process of interpretation. Fabula is the reader’s working version of what happened in the world of the characters – a fictional world. Yet reading the fossil record differs from this: the history of life is not a fiction. First, the palaeontologist presumes that, whether it is empirically ascertainable, there does exist an ordering of events, wie es eigentlich gewesen, to which the syuzhet (the fossil record as it is read) must in some way be connected. The fabula of the history of life is ontologically (and temporally) prior to the syuzhet (order of presentation in the fossil record that the palaeontologist is reading). It is being reconstructed from the record it has left behind.Footnote ¹⁷ Second, the form of reading on which the palaeontologist is embarked allows her to expand the text, to look to other stratigraphic horizons, to seek out new evidence, to read on in search of narrative closure an ever-expanding text, in which one narrative is better supported than others, at which point narrative closure will have been achieved, at least temporarily. This is not to say that the situation is completely unlike that of rereading a work of literature, in which other information external to the text (e.g., early drafts, memoirs by the author, inter- and extratextual references, theories of interpretation) may help to support both the existence of a fabula and give some notion of what it is. Indeed, in the historical sciences in general, it has been argued that at any given time, even in the face of a fixed set of fossils and geological evidence (analogous to the closed form of the written text), the totality of the rest of science (theory, method, observations), which is constantly changing, enables an assessment of which of many possible fabulae are best supported (Reference JeffaresJeffares 2010).

Under periodicity, which presented a narrative of recurrent, extraterrestrial perturbation of the biosphere, the search for evidence looked completely different. Planetary geologists and astronomers began to reread the record of impact structures (craters, astroblemes) for evidence of periodicity (Reference Grieve, Sharpton, Goodacre and GarvinGrieve et al. 1985). While this record is even more fragmentary and less well-dated then the fossil record, it eventually did yield periodicity (Reference Rampino and StothersRampino and Stothers 1984a; Reference Rampino and Stothers1984b), and the hypothesis of impact periodicity continues to be pursued (Reference Rampino, Caldeira, Prokoph, Koeberl and BiceRampino, Caldeira and Prokoph 2019; Reference Rampino, Caldeira and ZhuRampino, Caldeira and Zhu 2020). At stratigraphic boundaries marking extinction events, iridium anomalies were sought and sometimes detected (although for certain events, such as the end-Permian extinction, iridium anomalies have so far turned out to be spurious; Reference ErwinErwin 2015 and personal communication). Where iridium anomalies proved wanting, other markers of impact were sought: shocked quartz (with a distinctive crystalline lattice), microtektites (bits of molten rock associated with the high heat of impact), buckminsterfullerenes, osmium isotopes and soot (Reference RaupRaup 1986: 75–87). Markers of one type or another proved adequate to justify continued pursuit of the hypothesis. Meanwhile, astrophysicists, chiefly Berkeley astrophysicist Richard A. Muller, continued to scan the heavens searching for Nemesis, which as of 2007 was still an ongoing search. The pursuit of narrative closure does not always end in achieving it.

To summarize, emplotting all mass extinctions of the past 250 million years in the narrative of a cause that recurs with clocklike regularity enabled Raup and Sepkoski to resurrect the nomothetism of the 1970s in which they had been integrally involved by fitting a periodic model to the record of mass extinctions, yet at the same time to create a narrative, a narrative of recurrence which drove scientists from a number of different fields – astronomy, planetary geology, isotope geochemistry, mineralogy and palaeontology – to embark on a quest for narrative closure on the basis of a periodic pattern or cause.

In so doing, Raup and Sepkoski’s research on extinction resolved an ongoing tension in the history of the earth sciences between uniformitarianism and catastrophism by putting forward an exemplar of a catastrophe (asteroid impact) that behaved according to a uniform periodicity rooted in the regularity of astronomical orbits. The Nemesis hypothesis was thus idiographic and nomothetic, catastrophist and uniformitarian, and it was a narrative explanation.

3.6 Rereading the Book of Nature through Diagrams

One step along the way to constructing a narrative of extinction is to ‘read’ and reread the stratigraphic record. In order to test whether patterns in the fossil record are consistent with a given causal narrative, such as sudden, catastrophic extinction, it is helpful to be able to investigate historical counterfactuals, which are narratives of events that could have happened, but did not. In the study of mass extinction, one of the templates for the formulation and articulation of counterfactual narratives has been the stratigraphic diagram. Stratigraphic diagrams do not operate alone to produce these counterfactual narratives, but in the context of tacit knowledge and ‘ways of seeing’ that are an extension of the practices of palaeontological and geological fieldwork. A common visual language and sets of practices makes possible the diagrammatic narratives that have been central to studies of mass extinction.

The stratigraphic diagram thus becomes a template for framing narratives of extinction and even for experimenting with alternative, counterfactual narratives; from reading through different configurations of syuzhet, scientists gain a sense of which fabulae are consistent with it, answering questions thrown up by the Nemesis hypothesis.Footnote ¹⁸

For example, in his 1989 paper, ‘The Case for Extraterrestrial Causes of Extinction,’ David Raup presents a diagram, plotting the distribution of fossil occurrences of different ammonite species in a stratigraphic section of late Cretaceous age in Zumaya, Spain, based on the fieldwork of Peter Ward (Figure 3.3). Ammonites, cephalopods with a coiled morphology, are one of the taxa that became extinct at the K-Pg boundary. The question Raup sets out to answer is whether this extinction was gradual, stepwise or sudden. Here one must distinguish between apparent and actual patterns: the apparent pattern of last known fossils and the actual pattern of last surviving members of the species. If the actual pattern of ammonite extinction (and, by extension, the end-Cretaceous extinction of other species) was gradual leading up to the K-Pg boundary, then a sudden cause such as a bolide impact is not tenable. If the actual pattern of extinction was stepwise, then a multi-phase event such as a comet shower is not ruled out. And if the actual pattern of extinction was sudden, then an impact-caused extinction becomes viable.

Figure 3.3 Stratigraphic ranges of 21 lineages (i.e., species genus Linnaeus) of ammonites found at Zumaya, Spain

Vertical scale marks distance in metres below the Cretaceous-Tertiary (today called the Cretaceous-Paleogene) boundary. Numbered vertical lines refer to ammonite lineages. Each horizontal tick mark designates a horizon at which a specimen of the lineage was found and identified. Note the ‘gappiness’ of the fossil records of the various lineages. For example, specimens of lineage 4 (Pachydictus epiplectus) were found and identified at 3 horizons: 200 m, 180 m, and 135 m below the Cretaceous–Tertiary boundary). The histogram on the right plots the number of lineages (inferred from first and last occurrences of specimens) in each 5 m interval (e.g., the 15 lineages who range through the 130 m to 125 m interval). Based on field data of Peter Ward.

Source: Reference RaupRaup (1989).

The methodological problem palaeontologists face is that of stratigraphic range truncation: due to gaps in preservation or failure to find or identify species, there is often elapsed time between the last appearance datum (LAD) for any given species in the fossil record and the time that the species actually went extinct, a mismatch between apparent and actual patterns of extinction. This is a missing data problem. The consequence is that the fossil record of sudden, simultaneous extinction of many species can look as if the event were smeared out over geologic time: a sudden extinction event in the fabula will appear in the syuzhet as gradual, a phenomenon known as the Signor-Lipps effect (Reference RaupRaup 1986). Conversely, if there is a large hiatus in preservation or sampling, then a gradual extinction in which species became extinct one after another over an extended period of time will leave a record that looks as if species all became extinct simultaneously: a gradual extinction on the level of fabula will be read as sudden in the syuzhet. Alternatively, smaller hiatuses in preservation or sampling can mean an extinction is read as if it happened in a series of bursts – stepwise extinction – even if the extinction was gradual or sudden.

Reference RaupRaup (1989) points out a paradox in how palaeontologists have tended to read the fossil record. On one hand, palaeontologists know that the fossil record is gappy: absence of evidence does not (generally) constitute evidence of absence; the syuzhet requires interpretation in order to reconstruct the fabula. On the other hand, there is a tendency to read the last appearance datum as the time of extinction for a species. Raup believes this to be fundamentally a methodological problem, ultimately to yield to a quantitative treatment, but chooses to illustrate the point using an experiment – a thought experiment – which happens to take the form of a visual, counterfactual narrative. Suppose, he asks, that all fossil occurrences of ammonites were eliminated beginning at a stratigraphic horizon 100 m below the K-T boundary: what would the fossil record of this sudden mass extinction look like? As can be seen from Figure 3.3, he argues, it would look gradual (with a spurious step introduced at the 125 m mark).

As has been pointed out elsewhere, palaeontology has a distinctive visual culture that places a premium on being able to show visually that which might also be demonstrated analytically or mathematically (Reference Huss, Sepkoski and RuseHuss 2009). For example, when a palaeontologist looks at a stratigraphic diagram, he or she can visualize it as an idealized, synoptic representation of a rock outcrop embedded with specimens of fossil species, as well as the fruit of a great deal of integrative inference. It will be second nature for any geologist or palaeontologist to read this diagram from bottom (oldest) to top (youngest). Field skills and geologic training allow the interpreter to give the diagram a spatiotemporal reality that may not be perspicuous to others (Reference Huss, Bouton and HunemanHuss 2017). Embedded in such a diagram as that depicted in Figure 3.3 is a ‘research narrative’, as well as one of nature. Palaeontological field workers sought, found and identified fossils at certain horizons in the stratigraphic record. Tectonic forces may have distorted, tilted or completely inverted the sequence as found in the field. All is righted in the diagram. Laterally dispersed localities needed to be correlated using principles of stratigraphic inference to determine whether specimens of different species were found at the ‘same’ horizon. There are many such sketches of the reconstructive aspect of palaeontology that are encoded in a scientific diagram. While they need not be fleshed out each time, and the identities of those making the scientific contribution would itself need to be reconstructed from other sources, when palaeontologists look at a stratigraphic diagram they see encoded in it a community’s research narrative.Footnote ¹⁹

Yet Figure 3.3 also encodes a ‘narrative of nature’. Beds of sediment were laid down, organisms lived and died and left fossilizable hard parts. Periods of erosion or depositional hiatus, along with dissolution of shells, create gaps in the rock and fossil records. Narratives of morphological change and differentiation – microevolution and macroevolution – leave their traces in the patterns of fossil occurrences. Broad temporal trends in species gain and species loss, ultimately culminating in extinction, can be inferred from the patterns of diversity that are depicted in the running histogram jutting out from the right-hand side of the diagram. Tacit knowledge would enable most palaeontologists to provide a narrative sketch of what they see in Figure 3.3. Experts on ammonites may be able to venture a richer narrative, but some elements of the causal story remain outstanding. While scientists broadly understand some of the processes that gave rise to the patterns of spatiotemporal distribution of fossils in this diagram, ultimately, the causal analysis of evolution and extinction will need to be found elsewhere. The patterns in Figure 3.3 are the explanandum. Specific causal hypotheses are the explanans.

Figure 3.4 enables a visual reading of a counterfactual narrative: given the same evolutionary history and gappy stratigraphic distribution of fossils, what would the pattern of last appearances look like if extinction occurred suddenly at the 100 metre datum?Footnote ²⁰ Because the temporal sequence of geological and evolutionary events leaves a spatial record – a vertical array of fossil occurrences organized into geologic strata consisting of depositional, erosional and quiescent horizons – the resulting visual chronology lends itself to a narrative treatment, including the formulation of alternative narratives to help assess the plausibility of the proposed narrative explanation under consideration. In the same fashion as Figure 3.3, the thought experiment depicted in Figure 3.4 draws upon the knowledge and interpretive habits of palaeontologists, who are now in a position to see that even cases of sudden, simultaneous extinction can leave a misleadingly gradual trace in the fossil record.

Figure 3.4 Thought experiment on causes of extinction

Here a thought experiment is posed: what if all lineages had suddenly become extinct at a datum 100 m below the Cretaceous-Tertiary boundary? Would the pattern of last appearances look sudden or gradual? Note that despite the instantaneousness of this hypothetical extinction event, the apparent pattern of die-off is gradual, with a spurious ‘step’ appearing at around the 125 m mark. The conclusion may be drawn that an extinction event that was in fact sudden and simultaneous may look gradual when filtered through the ‘gappiness’ of the fossil record. From data plotted in Figure 3.3.

Source: Reference RaupRaup (1989).

In the historical sciences, one often wishes to reconstruct what happened – to produce a historical narrative – based on physical traces, background theory and other assumptions.Footnote ²¹ One way to assess a pattern of physical traces as evidence for or against a proposed narrative is to ask whether a similar pattern would have been expected under an alternative narrative scenario. In these diagrams, the focal question is not what caused the extinction, but how to read the fossil record – what combination of species extinction and spotty preservation does it reflect? Understanding their relative contributions can give rise to a corrected pattern of species extinction, which is what, qua historians of life, palaeontologists seek to explain.

Ultimately, however, the narrative that explains the fossil record as we find it, that gives an account of the patterns therein, is relevant to the grander, causal narratives of mass extinction: extraterrestrial, climatological, ecological, volcanogenic, etc. At a minimum, the fossil patterns must be consistent with the proposed mechanism of extinction, but the search for additional evidence – of impact, climate change, trophic shift or volcanism – has taken scientists beyond the fossil patterns themselves to competing narratives of extinction and the evidence relevant to adjudicating among them.

3.7 Narrative Closure in Philosophical Context

Philosophers have disagreed about the epistemic underpinnings of narrative closure in the historical sciences. For starters, there remains the very real possibility that, depending on the question at issue, narrative reconstructions of the past are always one data point or a few data points away from being reopened such that scientists should always be open to the temporariness of narrative closure (Reference TurnerTurner 2007). To this, I should add that narrative explanation is remarkably flexible and resilient in the way that components can be retained as well-established (e.g., suddenness of extinction, periodicity), even as evidence for other components of the narrative is found lacking, inconclusive or is even overturned (e.g., evidence for the existence of Nemesis). Second, there is an ongoing debate about what epistemically grounds narrative closure (Reference ClelandCleland 2002; Reference TurnerTurner 2007; Reference Forber and GriffithForber and Griffith 2011). Cleland has argued that narrative closure is achieved when a ‘smoking gun’ is found: a piece of evidence that is consistent with one narrative but inconsistent with its rivals. In this view, the Chicxulub crater that has been dated to the end of the Cretaceous period played this role in establishing an asteroid impact as the cause of the K-Pg extinction. Yet Forber and Griffith point out that any given datum only has evidentiary value against a background of auxiliary assumptions, which in the historical sciences can be difficult to test. Hence, data that appear to rule in one hypothesis and rule out its rivals may prove to be indecisive, because their doing so is too sensitive to weak auxiliary assumptions: there is no one-to-one mapping between fabula and syuzhet. As we saw earlier, in the discussion of Reference RaupRaup’s (1989) rereading of the stratigraphic record at Zumaya, evidence that the K-Pg extinction was gradual, based on a petering out of certain species as the K-Pg boundary is approached from below, can easily be shown to be consistent with sudden mass extinction if different assumptions are made about how preservation is expected to result in the observed fossil record. It is easy to ‘explain away’ inconsistencies in this way: one can ‘save the narrative’ by deflecting inconsistencies to auxiliary assumptions. Thus, Reference Forber and GriffithForber and Griffith (2011) have argued that a more promising and robust way to achieve closure that is likely to be less ephemeral is to ground historical inferences by a consilience of inductions (Reference WhewellWhewell 1858), namely by finding lines of evidence that each depend on independent sets of auxiliary assumptions. They give the example of several different sets of evidence that were used to predict the size of the asteroid impact at the end of the Cretaceous and the degree to which they did or did not share auxiliary assumptions as crucial factors in assessing the strength of evidence, both in probabilistic terms and in reception by scientists (Reference Forber and GriffithForber and Griffith 2011). For my purposes here, I merely wish to note that historical science as the pursuit of narrative closure is consistent with both of these models.

3.8 Conclusions

A recurrent narrative such as the Nemesis hypothesis challenges some distinctions that have been used to set up oppositions between approaches in palaeontology. Simply put, a narrative of lawlike recurrence has both nomothetic and idiographic components consisting of the mathematical laws governing the periodic forcing agent as well as the overall causal narrative explaining which taxa became extinct, which survived and why. It also challenges the distinction between uniformitarianism and catastrophism, in a sense rendering bolide impact uniformitarian – a periodic catastrophe, as it were (Reference Sepkoski and GlenSepkoski Jr 1994).

The narrative of recurrent extinction known as the Nemesis hypothesis set in motion a search for narrative closure, and for communities of scientists, a quest for evidence that each mass extinction had been caused by an extraterrestrial impact. In the case of the K-Pg extinction, in which the dinosaurs, ammonites and a number of other groups perished, narrative closure was achieved with the discovery of an impact crater of approximately the size predicted on the basis of the iridium anomalies found around the world (Reference Forber and GriffithForber and Griffith 2011). This effectively closed off debate about alternative narrative explanations for that particular extinction.

The legacy of the Nemesis affair is far more complicated. For starters, the periodic pattern in mass extinction appears to be too stable to be compatible with the instability of the calculated orbit of the supposed companion star Nemesis (Reference Melott and BambachMelott and Bambach 2010)! Still, the pursuit of closure in the impact narrative is ongoing, especially on the part of Michael Rampino and colleagues (Reference Rampino, Caldeira, Prokoph, Koeberl and BiceRampino, Caldeira and Prokoph 2019; Reference Rampino, Caldeira and ZhuRampino, Caldeira and Zhu 2020), but in general there is greater pluralism. Volcanism and deep ocean anoxia are among the proposed causal agents at horizons where evidence of impact is lacking (Reference Rampino, Caldeira, Prokoph, Koeberl and BiceRampino, Caldeira and Prokoph 2019), and three episodes of large-scale igneous province (LIP) eruptions are dated at times that coincide with the inferred ages of the three largest known impact craters, all of them falling at or near extinction peaks now computed as having a 27.5 million year periodicity (Reference Rampino, Caldeira and ZhuRampino, Caldeira and Zhu 2020). This periodicity has been found to be statistically significant over the past 500 million years, extending it twice as far back in time as had been found in Raup and Sepkoski’s original analyses (Reference BambachBambach 2017; see also Reference Erlykin, Harper, Sloan and WolfendaleErlykin et al. 2017). It is close to the half-period of passes of the solar system through the plane of the Milky Way galaxy – conjuring the image of a ‘Galactic Carousel’ (Reference Rampino, Haggerty, Rickman and ValtonenRampino and Haggerty 1996). In a bit of brand differentiation, the hypothesis that the concomitant mass-extinction periodicity is due to the resultant galactically governed influx of asteroids, comets or even dark matter has been dubbed the ‘Shiva hypothesis’ (Reference GouldGould 1984; Reference Rampino, Haggerty, Rickman and ValtonenRampino and Haggerty 1996). Statistical searches for periodicity in the timing of mass extinctions, asteroid crater ages and oscillations through the galactic plane have been ongoing (Reference Rampino and StothersRampino and Stothers 1984a; Reference Melott and BambachMelott and Bambach 2014; Reference Rampino, Caldeira, Prokoph, Koeberl and BiceRampino, Caldeira and Prokoph 2019; Reference Rampino, Caldeira and ZhuRampino, Caldeira and Zhu 2020). These analyses have also turned up an approximately 60-million-year periodicity in an isotopic signature in marine sediments that has given rise to a variety of alternative narratives involving internal drivers of plate tectonic activity, galactically driven increases in the influx of cosmic rays with effects on upper atmospheric ionization and climate and possible coupling between astronomical cycles and internal geodynamical cycles (Reference Melott, Bambach, Petersen and McArthurMelott et al. 2012).

In other words, despite the pursuit of narrative closure, the science does not seem to be approaching it. Rather, narrative seems to be a rather flexible tool for adjusting to what scientists find as they ‘read on’. So what does the Nemesis affair teach us about the pursuit of narrative closure in the case of periodicity of mass extinction? First, periodicity in the temporal pattern of the mass extinctions themselves has stood up to improved resolution of the data, revisions to the geological timescale (Reference Melott and BambachMelott and Bambach 2014) and the use of a range of different statistical methods (Reference Rampino, Caldeira and ZhuRampino, Caldeira and Zhu 2020). Closure seems to have been achieved in the pattern in the timing of the mass extinctions themselves. Second, as might be expected when vastly different narratives compete, such as ‘earth-bound’ narratives of particular mass extinctions and astronomically driven recurrent causes of the periodic pattern, attempts to achieve narrative closure in one camp are met with attempts to keep the narrative open in another, sometimes by folding the objections in to produce a unifying narrative (Reference Rampino, Caldeira and ZhuRampino, Caldeira and Zhu 2020). Third, the Nemesis narrative itself, while today finding few adherents, reoriented attitudes such that astronomical processes are deemed worthy candidates for driving biotic and geologic phenomena on planet Earth. Finally, in the case of periodic mass extinction, the search for narrative closure has been empirically and methodologically fruitful. Scientists really are pursuing narratives, seeking to assemble a causal story that can account for the apparent periodicity – we see this particularly in the attempt to connect galactic processes with oceanic, atmospheric and geological processes – drawing on that narrative to guide an empirical quest, and reading on in pursuit of evidence that can provide narrative closure, however elusive it may be.Footnote ²²

4.1 Introduction

In an article entitled ‘The Geologist as Historian’, one of the twentieth century’s foremost British geologists, H. H. Read (1889–1970), characterized geology as ‘earth-history’ (Reference Read1952: 409). Read’s point was that geology has a lot in common with human history, dealing as it does with the reconstruction of particular events that occurred in the past, albeit on a very different timescale (and minus a role for human agency). Read noted that in geology, as with its counterpart in the humanities, ‘no event has ever been exactly repeated’ (Reference Read1952: 411). A possible point of contention emerges, however, when this analogy is extended to the mode of explanation in geology. It is uncontroversial to state that explanations in history have a narrative form, but suggesting that ultimately all explanations of events in time are narrative in structure (as asserted by Reference Richards, Nitecki and NiteckiRichards 1992: 22–23) may well cause unease among many geologists who do not wish their science to be associated with a term which may seem vague and unscientific, or which is suggestive of ‘mere’ storytelling.Footnote ¹

In this chapter, I will seek to emphasize and uphold the narrative nature of geology, a property which may not be widely understood or appreciated, even by its own practitioners, but which has a logic and rigour of its own (Reference FrodemanFrodeman 1995: 966). Unlike those in human history, narratives in geology are always strictly bounded by what is possible according to the general laws of physics and chemistry; they are also tightly constrained by what is geologically plausible, although there is always scope for daring or ‘outrageous’ hypotheses (Reference DavisDavis 1926), which are open to review and testing by the geological community.

Without being too prescriptive, the essence of a narrative statement can be usefully thought of as specifying two time-separated events, so that the prior event is understood to have given rise to, and thereby to explain, the later event;Footnote ² it is typically expressed in the past tense (Reference DantoDanto 1962: 146). More complex narratives, involving multiple events and processes which have interconnected causal relationships, are built on this simple formulation. A narrative explanation therefore specifies the causal connections in a temporal sequence of events or processes. It is important to note that a narrative in a historical science such as geology is neither a chronicle (a chronological listing of disconnected events),Footnote ³ nor is it a merely descriptive exercise. It has been argued that a defining characteristic of historical sciences such as geology is that they rely on narrative sentences for understanding (Reference GriesemerGriesemer 1996: 66). As will become apparent in the course of this chapter, however, narratives in the geological literature are not always explicitly narrative.

4.2 The Case of the Stac Fada Member

The cliffs along the Assynt coastline of Sutherland, north-west Scotland, are formed of some of the oldest sedimentary rocks in the British Isles. The reddish-brown outcrop has been known informally as the Torridonian since the late nineteenth century when it was recognized to be of Pre-Cambrian age.Footnote ¹⁰ Despite their great antiquity, the rocks are acknowledged to be remarkably well preserved. They were originally assumed to be unfossiliferous, although eukaryotic microfossils are now known to be present (Reference Brasier, Culwick, Battison, Callow, Brasier, Brasier, McIlroy and McLoughlinBrasier et al. 2017).

Writing in 1897, J. G. Goodchild of the Geological Survey of Great Britain drew a direct analogy between the ancient sediments that formed the Torridonian rocks and the modern sands currently being deposited in ephemeral rivers and lakes in the Sinai Desert, on the basis of their remarkably similar form and composition:

The Geological Survey studied and mapped the rocks of north-west Scotland in a major campaign that ran from 1883 to 1897. The results were published in a substantial Memoir several years later (Reference Peach, Horne, Gunn and CloughPeach et al. 1907), in which descriptions of the Torridonian rocks occupied one of five subsections.

Following something of a hiatus during the early twentieth century, research on the Torridonian resumed in the 1960s. A research group was set up in the Geology Department of Reading University focused exclusively on furthering knowledge and understanding of the Torridonian (Reference StewartStewart 2002: 3). Initial reconnaissance work revealed the rocks to be ‘unexpectedly complex’ (Reference Gracie and StewartGracie and Stewart 1967: 182), and it took several years of careful fieldwork to unravel the stratigraphic relationships. Among the unexpected complexities encountered by the Reading group was a particular layer which was noted to be generally between 10 m and 30 m thick. In the redefinition of Torridonian stratigraphy undertaken by the Reading group, this layer was named the Stac Fada Member (Reference StewartStewart 2002: 5).Footnote ¹¹ This rock unit, the outcrop of which stretches across more than 50 km of coastline (Figure 4.1), was regarded as unremarkable by the nineteenth-century Survey geologists,Footnote ¹² but was noted by the Reading researchers to differ in a number of significant respects from the layers immediately above and below (Reference StewartStewart 2002: 9–11).

Figure 4.1 Location map of the Stac Fada outcrop

D. E. Lawson’s (1972) study of the Stac Fada Member described some of its constituents as angular shards of pumice, green particles of devitrified glassFootnote ¹³ and accretionary lapilliFootnote ¹⁴ (as shown in Figure 4.2). These were all interpreted as products of a nearby volcano. As with Goodchild’s Sinai Desert comparison, this interpretation was made by analogy with present-day processes. Accordingly, the Stac Fada Member was interpreted either as a pyroclastic flow – an airborne surge of fluidized ash and other fragments derived from a violent eruptive event (Lawson 1972) – or as more of a surface-bound volcanic mudflow (Reference StewartStewart 2002).

Figure 4.2 Ball-shaped accretionary lapilli on the surface of a Stac Fada Member outcrop The largest examples shown here are about 15 mm in diameter.

Source: Image courtesy of Renegade Pictures/Channel 4.

The volcanic interpretation of the Stac Fada Member initially seemed to fit the field observations well and it held sway for several decades. The absence of a volcanic vent in the surrounding landscape was not seen as problematic, given the long-term effects of erosion and burial. Based on the distance that present-day accretionary lapilli are known to travel through the air in an eruption, the volcanic vent was suggested to have lain a short distance offshore (Reference YoungYoung 2002: 7–8; point Y in Figure 4.1). However, it was apparent that there were some aspects that did not add up. For example, the lack of evidence for additional contemporaneous flows was regarded as ‘curious’ by Lawson: he explained that one ‘would not really expect volcanic activity to cease after a single eruption’ (Lawson 1972: 346, 360). Furthermore, several ‘thorny problems’ (Reference StewartStewart 2002: 10–11) were identified in the geological evidence. For instance, indicators of transport directions in the sediments showed that there had been an ‘abrupt change’ in the slope of the land from east to west ‘immediately prior to deposition of the Stac Fada Member’ (Reference StewartStewart 2002: 10–11). This was difficult to explain in terms of the volcanic hypothesis given that the outcrops along the coast were estimated to have been located too far from the putative volcano to have been affected by any associated land movements. Although several modes of volcanic emplacement had been proposed, it was concluded that none of them satisfactorily explained all of the field observations (Reference StewartStewart 2002: 10–11), and Stewart noted that, in 2002, the volcanic hypothesis remained ‘controversial’ (Reference StewartStewart 2002: 65). Despite these unresolved issues, the phenomenon of Stac Fada volcanism was incorporated, albeit with a degree of incongruity, into the body of literature on Scottish geology. For example, a major synthesis of the tectonic and magmatic evolution of Scotland included a short section on Stac Fada volcanism, in which the authors referred to it as ‘enigmatic’ (Reference Macdonald and FettesMacdonald and Fettes 2006: 232–233).

4.2.1 Old Evidence, New Discovery

Since 2004, the Torridonian outcrop has formed part of the North West Highlands Geopark and has become a popular destination for geology undergraduate field trips. On an Oxford University field course in 2006, postgraduate geologist Ken Amor was serving as an assistant to the teaching staff. Amor had recently returned from Ries in Bavaria where he had been studying the rocks around one of Europe’s few recognized meteorite craters. His attention was drawn to the distinctive green fragments of devitrified glass in the Stac Fada outcrop which had been highlighted by the Reading geologists. Although they were consistent with the prevailing volcanic interpretation, he had seen remarkably similar crystals close to the Ries crater, where they were interpreted to have been formed by the melting of the surface rocks in the impact event. There were no known instances of a major meteorite strike in the British Isles, but the green particles aroused Amor’s curiosity. A microscopic examination of thin sections of the rock in question would provide a test of Amor’s hunch, and on returning to Oxford he discovered that his department already held some thin sections that had been made from rocks collected on previous field trips.Footnote ¹⁵ However, he knew that the chances of finding anything new were not promising. ‘How many countless eyes of undergraduates had looked at these very same thin sections over several decades and not spotted anything unusual’?Footnote ¹⁶

What Amor hoped to see down his microscope were crystals of shocked quartz – a form of silica which bears the marks of the instantaneous application of stresses that are far higher than can occur in terrestrial processes, and which is regarded as an unequivocal indicator of a so-called ‘hypervelocity impact’. Against his expectations, Amor did find grains of shocked quartz in the Stac Fada thin sections (Figure 4.3), and the implications of his discovery soon dawned on him. He later reflected: ‘I remember thinking at the time that at that moment I was the only person […] to realise that the UK had been struck by an asteroid […] I didn’t tell my supervisor for two days because I wanted to hold on to that discovery moment for a little longer’.Footnote ¹⁷

Figure 4.3 Photomicrograph of a shocked quartz grain from the Stac Fada Member

Showing two sets of intersecting lines (see inset sketch). These are planar deformation features (PDFs), which represent primary evidence for shock metamorphism. Image is approximately 0.35 mm across.

Source: Reference Amor, Hesselbo, Porcelli, Thackrey and ParnellAmor et al. (2008).

Amor’s moment of insight has led to the Stac Fada Member being reinterpreted as an ejecta blanket – the material violently thrown out of the crater formed by the impact of a massive extraterrestrial body, overturning the previous interpretation of the layer as the product of a volcanic eruption and resolving many of the inconsistencies surrounding that explanation. For example, the sudden change in the slope of the land now made sense as a consequence of the impact. Amor published his findings in a co-authored paper two years after the breakthrough discovery (Reference Amor, Hesselbo, Porcelli, Thackrey and ParnellAmor et al. 2008). In addition to shocked quartz, the paper presented further evidence of an impact origin including the shocked form of another mineral (biotite) and key geochemical indicators such as anomalous chromium isotope values and elevated abundances of platinum group metals such as iridium. Subsequently, grains of shocked zircon were discovered in the Stac Fada deposit (Reference Reddy, Johnson, Fischer, Rickard and TaylorReddy et al. 2015), further confirming the impact ejecta interpretation. The melange of angular fragments and partially melted material in the Stac Fada Member is now routinely referred to as a suevite, the term for such a deposit created by an extraterrestrial impact.Footnote ¹⁸ The particles of devitrified glass and accretionary lapilli which had been assumed by the Reading geologists to be uniquely diagnostic of volcanic eruptions were evidently also capable of being formed in major meteorite impacts. This had been recognized from evidence at the Ries impact site for some time (Reference Kölbl-EbertKölbl-Ebert 2015: 275), a fact of which Amor would have been aware. According to radiometric dating of the Stac Fada Member, the impact occurred approximately 1.2 billion years ago, placing it in the Mesoproterozoic Era (Reference Parnell, Mark, Fallick, Boyce and ThackreyParnell et al. 2011).Footnote ¹⁹

There is no sign of the impact crater from which the Stac Fada Member was ejected. As with the now discarded volcanic interpretation and the absence of a volcano, this is not surprising given the burial or removal by erosion of much of the Torridonian outcrop in the last billion or more years. Different lines of reasoning by different geologists, but based essentially on the same evidence, have resulted in two different locations being posited for the crater (A and S in Figure 4.1). Amor et al. (Reference Amor, Hesselbo, Porcelli, Thackrey and Parnell2008; Reference Amor, Hesselbo, Porcelli and Price2019) suggest that a point in the Minch Basin 15–20 km to the north north-west of Enard Bay, an area that would have been dry land at the time of impact, is the most likely location, while an alternative hypothesis by Simms has the crater deeply buried onshore about 50 km to the east of the outcrops along the coast (Reference SimmsSimms 2015). Further data-gathering work in the form of expensive geophysical surveys or borehole drilling would be required to confirm or deny both proposals. However, Simms’s location has been criticized as being inconsistent with the overarching narrative of the plate tectonic history of northern Scotland (Reference Butler and AlsopButler and Alsop 2019: 443), and, in an example of counterfactual reasoning, Amor et al. Reference Amor, Hesselbo, Porcelli and Price(2019) argue against Simms’s location by pointing out that there would have been a topographic obstruction blocking the path of the ejecta blanket if it came from the east (Reference Amor, Hesselbo, Porcelli and PriceAmor et al. 2019: 842).

Three years after the publication of the discovery paper (Reference Amor, Hesselbo, Porcelli, Thackrey and ParnellAmor et al. 2008), volcanologists Michael Branney and Richard Brown addressed the question of exactly how the Stac Fada Member might have been emplaced. Nothing remotely approaching the scale of the meteorite impact from which the deposit is believed to have originated has ever been observed in recorded history,Footnote ²⁰ and terrestrial impact ejecta blankets are commonly not well preserved, limiting the availability of possible analogues. The example at Ries is a notable exception, however, and Branney and Brown (Reference Branney and Brown2011) highlight parallels between the Stac Fada deposit and the Ries suevite. The absence of a crater, however, means that there is no way of investigating the relationship between it and the ejecta.

The authors note that while there are ‘important differences’ between the ejecta from impacts and those from volcanoes – for example, the presence of shocked minerals and distinctive geochemistry – there are also ‘striking similarities’ (Reference Branney and BrownBranney and Brown 2011: 287–288). The distinctive components of the Stac Fada deposit, including the presence of devitrified glass and accretionary lapilli, as well as the order in which they were deposited, resemble those ejected from large explosive volcanic eruptions. These similarities have led them to deduce that emplacement mechanisms comparable to pyroclastic processes associated with volcanoes were at work and they have coined the analogous term, impactoclastic (Reference Branney and BrownBranney and Brown 2011: 276) to describe their model, which details how the Stac Fada impact ejecta blanket could have been deposited (as we can see in Figure 4.4).

Figure 4.4 The impactoclastic emplacement of the Stac Fada ejecta blanket

For the benefit of the non-geologist reader, three pairs of images show the situation at successive points in time immediately following the meteorite impact. Each pair consists of a panel showing a cross-section through the dust plume thrown up by the impact (on the left) and a column representing the vertical accumulation of different types of debris deposited by the plume by that time (on the right). The time sequence, t₁ to t₃, runs from top to bottom. In the plume cross-sections, the crater lies out of frame to the right and the plume moves from right to left through the time sequence. Along the base of each of these cross-sections is the layer of debris deposited from the plume. This increases in thickness with time as marked by the ticks labelled t₁, t₂ and t₃ at the bottom right of each cross-section. The location of each column of debris is marked by a rectangular outline in the bottom right of each corresponding plume cross-section. An understanding of how the overall diagram is put together, along with some technical (geological) knowledge, enables it to be read as a self-contained narrative.

Source: Reference Branney and BrownBranney and Brown (2011).

4.3 Tracing the Narratives

4.4 Conclusions

Robert Richards (Reference Richards, Nitecki and Nitecki1992: 23) is surely correct to claim that ‘all explanations of events in time are ultimately narrative in character’ (and it is evident that H. H. Read did not extend his analogy of geology with human history far enough explicitly to acknowledge this fact). Accordingly, the case of the interpretation and re-interpretation of the stratigraphic unit known as the Stac Fada Member demonstrates that geology is an inescapably narrative science that follows rigorous standards of internal and external narrative logic and is constrained by physical and chemical laws and by the norms of geology.

Narrative statements in the geological literature on the Stac Fada Member, and on the Torridonian of which it is a part, are commonly presented in the form of what appear at first to be descriptions of observations. However, these contain cues which the geologist automatically picks up, and by virtue of her training and experience she makes a range of default assumptions which are translated into causally connected temporal sequences. This phenomenon is particularly applicable to geology because of the relationship between space and time in the ways in which rocks accumulate, as well as in the clues to past environments which certain types of rock embody. More ‘traditional’ narrative passages that explicitly express the causal relationships between time-separated events also occur but are less common. Textual narratives may be accompanied by images that aim to add clarity but which may often be read as narratives themselves. Beyond the field of communication, a lot of narrative activity in geology is unseen, as it takes place in the minds and conversations of geologists as they try to make sense of observations that may initially be perplexing.

Geology is also an interpretive science, and narratives that attempt to answer the question, ‘What caused these traces?’, are inevitably accompanied by some degree of uncertainty. This is illustrated by the fact that key pieces of evidence, such as the presence of shocked quartz, may be overlooked for a variety of reasons, and by the observation that different conclusions may be drawn from the same field evidence – as illustrated by the disagreement over the most likely location of the missing crater. Geological narratives are therefore prone to be rewritten when new evidence or new ideas emerge. Whether the Stac Fada narrative will be rewritten again remains to be seen. In its latest version, the Stac Fada narrative provides a contribution to the body of knowledge relating to the susceptibility of the Earth to periodic meteorite impacts, a phenomenon which is now recognized as posing an existential threat to humanity.Footnote ³⁰

5.1 Introduction

The ground shaking that an earthquake produces is the result of a complex sequence of events that occur at a fault. This sequence of events is often given a narrative account by seismologists. Here is an example of such an account of the 2011 Tohoku-Oki earthquake. This is the massive earthquake that gave rise to the tsunami that devastated the north-east coast of Japan and caused the nuclear disaster at Fukushima.

The events that are recounted here (e.g., the rupture ‘expanded relatively slowly in the up-dip direction, with fault-slip of ~30 m’, and later ‘expanded more rapidly and erratically down-dip to below the Honshu coast with slip of 1–5 m’) took place along a fault, deep within the earth. I will refer to the sequence of events at the fault, which played out over several minutes in the case of the Tohoku earthquake, as the rupture process. For each earthquake that occurs, there is a particular way in which these events play out – each earthquake has a unique rupture process. Knowing these rupture processes in detail would yield precious information about the faults on which they occur and their history, which can be used to make better determinations of seismic hazard.

The rupture process of an earthquake cannot be observed directly, since it takes place deep within the earth, but its effects can be observed at the earth’s surface. The rupturing of a fault generates seismic waves that travel outwards in all directions from the fault. These seismic waves can be recorded on seismographs at the earth’s surface. An earthquake can also result in permanent ground motion at the earth’s surface, which can be recorded using GPS technology. Data on other effects of an earthquake, such as tsunamis, can also be recorded.

Reconstructing the rupture process of an earthquake from this recorded data is a particularly difficult problem, for several reasons. First, rupture processes are very complex, and highly contingent.Footnote ¹ The way a rupture process unfolds is highly dependent on contingent features of the fault. Second, as I have already mentioned, seismologists generally do not have direct access to faults. This means that the contingent features of the fault are typically not known prior to the earthquake. Third, the data recorded from a major earthquake such as the Tohoku earthquake can come from observations of a number of different phenomena, such as seismic waves, permanent ground motion and tsunamis. This diverse data must be integrated in some manageable and principled way. In short, seismic reconstruction involves inferring from a wide variety of downstream effects a complex, highly contingent process that occurs on a fault that is not directly accessible .

An important tool for seismic reconstruction, slip inversion, produces models (called source models) that capture the rupture process . As we will see, a source model provides a narrative about a possible way the rupture process may have occurred. This narrative cannot, however, be straightforwardly regarded as an accurate account of the events at the fault as they actually occurred. When a large number of different source models of the same earthquake are generated, they will generally conflict with each other, due to differences in the sets of data they utilize, the specific mathematical techniques used and the assumptions that go into these models. A problem that seismologists have faced when attempting to reconstruct the Tohoku and other earthquakes, then, is how to take such conflicting models and reconstruct the actual rupture process.

This chapter examines how seismologists have obtained increasingly detailed knowledge about the rupture process of the Tohoku earthquake in the face of this problem. I will give an account of the growth of this knowledge that is slightly unorthodox, but it exemplifies how thinking about narrative might help us to understand the growth of scientific knowledge.Footnote ² I will focus in particular on three stages on the path from recorded data to increasingly detailed knowledge about the rupture process, and the role of narrative in each of those steps.

Here is an initial sketch of these three stages.Footnote ³ In the first stage, source models are used to produce, from recorded data, narratives that recount the rupture process in detail, which I call rupture narratives. As I have mentioned, these narratives generally conflict with each other due to differences in the data, techniques and assumptions that go into the source models. In the second stage, a set of details that is taken accurately to represent features of the actual rupture process is distilled out of these conflicting rupture narratives. This set of details is arrived at through the use of a research narrative that examines the evolution of source models. In the third stage, these distilled details are strung together into a model-independent rupture narrative, which I call an integrating narrative. This integrating narrative is used as a research tool for formulating questions, the pursuit of which has led to the production of further evidence about the rupture process.Footnote ⁴

This chapter will proceed as follows. In section 5.2, I will lay down some basics about how earthquakes occur, the rupture process of an earthquake and the Tohoku fault. In section 5.3, I examine the construction of source models from data and present an example of a rupture narrative. In section 5.4, I show how details are distilled from source models through the use of a research narrative. In section 5.5, I present an example of an integrating narrative, and show how the pursuit of questions about this narrative results in further evidence about the rupture process. In the concluding section 5.6, I briefly consider the functions of the three types of narratives just mentioned in the growth of knowledge about the rupture process of the Tohoku earthquake.

5.2 Earthquakes and the Tohoku Fault

Most earthquakes are generated at a fault, which may be thought of as a roughly planar surface within the earth where the ground on the two sides of the surface are slowly being pulled in opposite directions. A well-known example is the San Andreas fault, the two sides of which are moving a few centimetres a year relative to each other. If a fault were completely smooth and frictionless, the two sides would simply move very slowly past each other, and we would have no earthquakes. But faults are not frictionless. The two sides are rough, and there are portions, called asperities, where the two sides are locked together.

What happens when the forces on each side of the fault continue to act in opposite directions, while the two sides are locked together? Because rock is elastic, the rock around the fault will slowly bend due to the imposed forces, and it will store up elastic strain energy, much like a wooden ruler would store up elastic energy if you slowly flexed it. Points far away from the fault will tend to move slowly relative to each other, while the fault remains locked together. This will result in strain slowly accumulating in the material surrounding the fault as it gets pushed further and further out of equilibrium. The strain will continue to build until it is sufficient to overcome the friction that keeps the sides locked together. The two sides of the fault will then rupture, suddenly snapping back towards a position of equilibrium. The pent-up elastic energy is released, generating seismic waves.

The largest earthquakes occur on faults that can be hundreds of kilometres long. Several features of large earthquakes are particularly important for understanding this chapter. First, large faults do not rupture along their entire length all at once. The rupture initiates at a particular point on the fault. This rupture will then propagate to other parts of the fault. If the fault is hundreds of kilometres long, the rupture can take several minutes to propagate the entire length of the fault. This series of events at the fault is called the rupture process. Second, the state of friction on a large fault is generally heterogeneous. That is, there can be patches of the fault that are strongly stuck together (the asperities), while there can be other patches that are only weakly coupled. The patches that are weakly coupled rupture easily, while the asperities are resistant to rupture. When the asperities do rupture, however, they typically have built up a lot of elastic energy, so they tend to rupture much more forcefully than the weak patches. Thus, the particular way the rupture propagates will depend on contingent features such as the state of friction at various points of the fault. These features are typically not directly accessible to seismologists, since the fault is buried deep within the earth.

I will now move to specific details about the Tohoku earthquake and the fault on which it occurred. The Tohoku earthquake occurred on a subduction zone off the north-east coast of Japan. There, tectonic forces are driving the Pacific plate underneath Japan and into the mantle, at a rate of roughly 8 centimetres per year. Figure 5.1 is a cutaway diagram showing the subduction zone, as viewed facing roughly northward. Northern Japan sits on top of the Okhotsk plate, towards the left side of the diagram. The fault on which the earthquake occurred is on the border between the Pacific and the Okhotsk plate. In the cutaway view, the fault is represented as a line at 12 degrees to the horizontal, with arrows indicating the relative motion of the two sides of the fault. The direction along the fault, at 12 degrees to the horizontal, is called the dip direction. In actuality, the dip angle of the fault is not known so accurately, and it may vary by a few degrees. Because the fault slopes downwards to the west, the western part of the fault that eventually goes underneath Japan is referred to as the down-dip part of the fault, while the eastern, up-dip part eventually reaches the ocean bottom at the Japan Trench, an extremely deep area of the Pacific Ocean off the coast of Japan.

Figure 5.1 Cutaway view of Tohoku fault

Source: Figure kindly provided by Dr Jeroen Ritsema.

Now it is fairly easy to visualize how large earthquakes occur on this fault. The Pacific plate is slowly getting pushed under the Okhotsk plate, but there are places where the two sides are locked together. The strain accumulates until it is enough to overcome the friction, and the two surfaces at the fault suddenly unlock. The upper surface jolts eastward and upward, releasing elastic energy in the form of seismic waves. The Tohoku earthquake ruptured an area of around 200 kilometres by 500 kilometres, and the entire rupture process took around 150 seconds. This motion also gave rise to a powerful tsunami that inundated the north-east coast of Japan. A detailed understanding of the rupture process, its connection to past and possible future earthquakes in the area, and the way in which it generated the tsunami, is of obvious importance for seismology, as well as the determination of seismic hazard along the coast of Japan.

The Tohoku earthquake was recorded on an unprecedented variety of instruments. Broadly, the data that were recorded for this earthquake can be categorized by the kind of phenomenon that was recorded. Seismic data are recordings of seismic waves. Strong motion seismic data is recorded at stations nearby an earthquake. These kind of data are recorded on several networks of different types of seismographs throughout Japan, including KiK-net, a network of over 600 strong-motion seismographs that are situated in boreholes; and K-NET, a network of over 1,000 strong-motion seismographs at the surface. Geodetic data are recordings of the deformation of the earth’s surface. Such data are typically recorded using GPS technology. Most of the geodetic data for the Tohoku earthquake was recorded on a network of over 1,200 GPS stations distributed throughout Japan, called GEONET. Tsunami data are recordings of the tsunami caused by the earthquake. This type of data was typically recorded by offshore wave and tide gauges. These three categories do not exhaust all the types of data that were recorded for this earthquake. In addition, there were important data recorded of the motion of the ocean bottom at seafloor geodetic sites, data from deep drilling into the fault zone after the earthquake and even gravimetric data recorded by satellites.

5.3 Rupture Narratives: From Data to Details

The data collected from the Tohoku earthquake are rich and diverse, but they consist of recordings of the downstream effects of the earthquake, such as ground motions that occurred far away from the fault. Such data do not immediately reveal any details about the rupture process. An initial step towards a reconstruction of the rupture process is the use of source models,Footnote ⁵ which, as we will see, take this downstream data and provide a detailed account – albeit an unreliable one – of the rupture process.

Most source models for the Tohoku earthquake have been constructed using a method called slip inversion.Footnote ⁶ A good example can be seen in Figure 5.2, taken from Reference Suzuki, Aoi, Sekiguchi and KunugiSuzuki et al. (2011). This is an early source model that was produced entirely from seismic data recorded at 36 stations located throughout northern Japan.Footnote ⁷ Let us first examine the large figure on the right. An outline of the northern part of the Japanese island of Honshu can be seen towards the left. Just off the coast is a rectangle, which has a length of 510 km and a width of 210 km, oriented at a small angle in the north–south direction. This rectangle is a representation of the fault. We are here viewing the fault from directly above (in contrast to Figure 5.1, which is a cutaway view). The dip of the fault cannot be seen in this view, but in this model the dip angle was set at 13 degrees to the horizontal.

Figure 5.2 Representation of the time progression of the rupture for the 2011 Tohoku earthquake

On the left is a representation of the time progression of the rupture given in intervals of 10 seconds. On the right is a representation of the total slip distribution of the Tohoku earthquake.

From Reference Suzuki, Aoi, Sekiguchi and KunugiSuzuki et al. (2011: 3–4).

Slip is a measure of how much the two sides of a fault moved relative to each other during an earthquake. The contours and shading on the figure to the right are an indication of how much various parts of the fault slipped over the course of the Tohoku earthquake. The darker the shading, the more slip occurred. According to this source model, there was an area of very large slip of around 48 m near the Japan Trench (towards the right edge of the fault). The series of 16 small figures on the left are miniature versions of the figure to the right. Each of these small figures represents the amount of slip on the fault in each ten-second slice of time from the beginning of the earthquake to the end (reading from top left to right, and then bottom left to right). As I described earlier, when an earthquake occurs, various parts of the fault rupture in succession. We can think of these as a series of snapshots of this rupture process as it propagates. If we allow for a wide definition of ‘narrative’ that includes visual objects such as diagrams,Footnote ⁸ we can view this series as a visual narrative of the rupture process, indicating spatial changes of the fault over time during the Tohoku earthquake.

Source model studies also provide more straightforward textual narratives of the rupture process, along with such diagrams. For example, Reference Suzuki, Aoi, Sekiguchi and KunugiSuzuki et al. (2011) provides the following:

We can view slip inversion as taking seismic or other data as input, and outputting what I call rupture narratives, which are visual and textual narratives of a rupture process that includes quantitative details. Such details can include temporal details such as the timing of various sub-events within the rupture process. They can also include details about the rupture process as a whole, such as the total amount of slip that occurred at a particular part of the fault. Borrowing a term from Robert Meunier (Chapter 12), the rupture narratives produced by source models present themselves as ‘narratives of nature’ – narratives that recount a process as occurring in nature, independently of any human observers. As we will see, however, they are highly model-dependent – that is, many of the details within these narratives are artefacts of the data, techniques and assumptions that go into the source models.

An indication of this model-dependence is a wide variability among rupture narratives produced by source models of the Tohoku earthquake. Figure 5.3 is a comparison of 45 different source models of the Tohoku earthquake. Each of the lines represents the amount of total slip indicated by each source model. For ease of comparison, only the amount of slip along the corridor off the north-east coast of Japan indicated in the inset map is shown, extending from just underneath the coast to the Japan Trench. There is particularly wide variability in the up-dip regions, near the trench. Some models show slip of 50 m or more here, while other models indicate slip of 10 m or less.

Figure 5.3 Comparison of slip according to 45 different source models of the Tohoku earthquake

Source: Reference LayLay (2018: 26), modified from Reference Sun, Wang, Fujiwara, Kodaira and HeSun et al. (2017).

How could there be such discordance between rupture narratives produced by various source models of the same earthquake? Broadly, there are two reasons. The first has to do with differences in the type of input data. I have mentioned that the data that were recorded for the Tohoku earthquake can be categorized into seismic data, geodetic data and tsunami data. Source models have been constructed using all of these types of data. Different types of data are sensitive to different features of the rupture, and thus models that rely on different types of data tend to emphasize different features. The second reason has to do with differences in the methods used to construct source models. This can include differences in the parameterizations used, differences in the idealizations and assumptions that go into the models, and differences in the mathematical and computational techniques that are used.

5.4 Research Narratives: Distilling Details from Source Models

The first source models of the Tohoku earthquake were produced and published in 2011, within months after the earthquake. A large number of source models of the earthquake have been produced since then – by 2017 there were at least 45 of them (Reference Sun, Wang, Fujiwara, Kodaira and HeSun et al. 2017). From the start, there have been pronounced discordances between various source models of the earthquake. An important question for the reconstruction of the rupture process of the Tohoku earthquake has thus been: exactly what is one to conclude about the actual rupture process given the discordance between the source models? Is there a way of distilling out from these conflicting source models some set of rupture details that can be regarded as accurately representing the actual rupture process? One reasonable thought is that later source models are generally more accurate than earlier ones, since they presumably have more knowledge about the earthquake to draw upon. A more rigorous approach would examine in detail the evolution of source models since 2011 to determine whether indeed later models improve on earlier ones. This is the approach taken in Reference LayLay (2018), a review article of the Tohoku earthquake. Reference LayLay (2018) contains a very long and complex narrative that traces out the evolution of source models, with the aim of distilling out rupture details to which some degree of confidence can be attached. Borrowing again from Robert Meunier (Chapter 12), this is a research narrative – a narrative that provides an account of the activities of researchers.

Let us now take a closer look at the research narrative in Reference LayLay (2018). The general thrust of the narrative is to show how source models of the Tohoku earthquake have gone through an evolution, the result of which is that details in certain later source models have claim to being relatively accurate representations of details of the actual rupture process. For example, in a section of the narrative, Lay examines early source models based purely on geodetic observations made at onshore GPS sites. He notes that such source models ‘can provide good resolution of the spatial distribution of slip if the observation configuration is favorable’ (Reference LayLay 2018: 11). Unfortunately, it turns out that the observation configuration for the Tohoku earthquake is unfavourable – all of the GPS sites are on the Japanese mainland, which is on the down-dip side of the fault. This means that source models based purely on onshore geodetic data have poor sensitivity to slip that happens on the up-dip side of the fault, near the Japan Trench. This is significant, for, as Lay points out, although source models based purely on onshore geodetic data are largely consistent with each other, they are inconsistent with source models based purely on seismic data. Source models based purely on seismic data tend to show the largest slip happening up-dip, near the Japan Trench, as with the source model depicted in Figure 5.2, while source models based purely on onshore geodetic data tend to put the largest slip near the hypocentre, more towards the centre of the fault. One might surmise that the reason for this inconsistency is the unfavourable observation configuration for source models based on onshore geodetic data.

Recognizing this as a limitation, seismologists have attempted to address this problem in later source models utilizing onshore geodetic data by incorporating other types of data that are complementary to onshore geodetic data. Particularly important is a set of geodetic data taken by GPS/Acoustic stations located offshore, on the ocean bottom, which, according to Lay, has ‘proved transformative for geodetic models of the 2011 Tohoku earthquake slip distribution’ (Reference LayLay 2018: 12). Another important kind of additional data is time series data taken at GPS stations, called hr-GPS. Regarding the evolution of source models based on geodetic data, Lay states:

Significantly, these later source models are much more consistent with source models based on seismic data (note, for example, that the source model depicted in Figure 5.2 has a peak slip of 48 m in the up-dip, shallow part of the fault). In other words, the rupture narratives of these later source models look much more like the rupture narratives of source models based on seismic data.

Lay does similar analyses of the evolution of models based on other types of data, showing how later models have improved upon earlier models. Not only are the rupture narratives of later models more consistent with each other, but they are getting more detailed. He takes later source models that incorporate multiple types of data – called joint inversions – to be the most reliable. One reason is because data of different types can be complementary – they are sensitive to different aspects of the rupture process. Another reason is because the later source models generally address the shortcomings of earlier models. Lay summarizes the evolution of source models as follows:

We can think of the research narrative provided by Lay about the evolution of source models as providing a justification that the details that appear in the rupture narratives of the later models are relatively accurate. Greater confidence is placed on the later joint inversion models, but no one model is taken to be best, and the amount of confidence one can place in a particular detail is ultimately based on a judgement that takes into account the commonalities between particular source models, limitations due to the datasets and methods of construction and the overall evolution of source models.

5.5 Integrating Narratives: Pursuing Further Evidence

Typically, in review articles, the details that are distilled from source models are strung together into a new rupture narrative that is independent of any particular model. This is a ‘narrative of nature’ – one that is taken to represent the best current estimate of the actual rupture process. There is such a narrative in Reference LayLay (2018), which he calls a ‘strawman reference model that distills the features that appear most stable and/or plausible’ (Reference LayLay 2018: 29). Although Lay calls it a model, it is not a model in the sense of the source models discussed earlier – it comes in the form of a textual narrative. As source models have evolved from the early source models based on single data sets to more detailed source models based on joint inversions, the details that were taken to be established about the Tohoku earthquake have also evolved. Thus, the model-independent rupture narratives that would be constructed by seismologists at any given time after the Tohoku earthquake would also evolve.Footnote ⁹ I will refer to rupture narratives of this type as integrating narratives because they are used as tools for integrating details of the rupture process with other seismological results.

In this section, I will show that integrating narratives play an important role in the production of new evidence about the rupture process of the Tohoku earthquake. First, let me provide, as an example, the ‘strawman reference model’Footnote ¹⁰ of Reference LayLay (2018):

This is just the beginning of the first paragraph of the narrative. The thing to note about this narrative is that it does not just string together well-established details from rupture narratives. It also makes reference to ‘domains’ of the rupture process, the ‘JFAST drill hole’, and past earthquakes.Footnote ¹¹ The later parts of this narrative, not shown here, continue on to discuss studies of afterslip (ground motions that occurred after the earthquake), seafloor deformation observations and specific earthquakes of the past. Integrating narratives can also highlight loose ends and open questions. Let me note again that this integrating narrative is from a comparatively late stage of analysis of the Tohoku earthquake. Earlier integrating narratives tend to be less detailed, they draw fewer connections to other studies and their references to past earthquakes are framed in a more speculative mode.Footnote ¹²

Integrating narratives have provided a sort of frameworkFootnote ¹³ upon which certain questions about the Tohoku earthquake could be pursued. Some questions have to do with the connection between the rupture process of the Tohoku earthquake and its spatial and temporal context. More specifically, these questions can be about connections to past earthquakes, to seismic events immediately preceding the Tohoku earthquake, seismic events that came after it, and various features of the subduction zone on which it occurred, such as the locations of asperities, the distribution of accumulated strain, the composition of the rock in the subduction zone, and so on. Other questions have to do with anomalies or inconsistencies in the rupture process as laid out in the narrative. The pursuit of such questions has been a driving force for uncovering further evidence about the earthquake.

For example, a major open question has to do with the frequency characteristics of the rupture process. Just a few years before the Tohoku earthquake, a new technique for producing source models from seismic data, called back-projection, had been developed (Reference Kiser and IshiiKiser and Ishii 2017). Back-projection is sensitive to high-frequency seismic waves, and the source model it produces is a kinematic image, not of slip, but of seismic radiation energy release over time. Since most of the energy being radiated at any given time during an earthquake originates from the rupture front, the back-projection image can be taken to show a kinematic image of the rupture front during the earthquake. Early back-projection studies of the Tohoku earthquake were systematically discordant with early slip inversion studies. The back-projection studies indicated that most of the seismic radiation was concentrated in the down-dip part of the fault. On the other hand, slip inversion studies indicated that the maximum slip was in the up-dip part of the fault, and the area of very large slip possibly extended all the way to the trench (Reference Lay, Ammon, Kanamori, Xue and KimLay et al. 2011: 687). An early question about the rupture process was thus: why does back-projection appear to show that rupture occurred mainly down-dip, while slip inversion appears to show that the area of maximum slip was up-dip and close to the trench?

A possible answer is that different parts of the fault produced seismic radiation at different frequencies – the down-dip part producing more high-frequency radiation than the up-dip part. If this answer is right, then it gives rise to another question: is the difference in the frequency characteristics in different parts of the fault just a special feature of this particular earthquake, or is it a feature of the fault – in which case it ought to hold for other earthquakes as well? Reference Koper, Hutko, Lay, Ammon and KanamoriKoper et al. (2011: 602) suggested that the latter is the case – that the difference is due to depth-varying frictional properties of the fault.

The idea fits with the known history of the fault. The down-dip region corresponds to an area where large earthquakes of up to Mw 7.9 had repeatedly occurred over the past century, and these earthquakes would have had similar frequency characteristics as the down-dip part of the Tohoku earthquake. The up-dip region corresponds to an area that had not ruptured since 869 ce, but it also partially overlapped an area that is taken to have ruptured in 1896 during what is known as a ‘tsunami earthquake’. Tsunami earthquakes have characteristics like those exhibited by this part of the fault during the Tohoku earthquake – with large slip but slow rupture velocities, leading to relatively more seismic energy being radiated at lower frequencies.

In this view, then, each part of the fault has its own rupture characteristics that are constant across earthquakes (these are roughly the ‘domains’ that Lay refers to in the extract above). The unusual feature of the Tohoku earthquake was that it ruptured both regions at the same time, so it combined the characteristics of both types of earthquakes. Given this view, the next question to ask would then be why there are regions with different rupture characteristics within the fault – does it have to do, for example, with the composition of materials in different areas of the fault? This has been probed by the use of seismic wave tomography (Reference Tajima, Mori and KennettTajima, Mori and Kennett 2013: 27) and studies (such as JFAST) where holes are drilled directly into the sea floor in the fault area (Reference LayLay 2018: 28).

The pursuit of questions such as these has improved the picture of how the Tohoku earthquake fits into its spatial and temporal context – whether it is, in some sense, a repeat of particular earthquakes in the past, for example. It has also opened up new lines of research that have contributed new evidence about the Tohoku earthquake. In some cases, this new information has been utilized to improve source models – thus contributing to the evolution of source models, and indirectly to the evolution of the integrating narratives themselves. Thus, there is a sort of mutual evolution of source models and integrating narratives, resulting in a more highly resolved, and more ramified, picture of the rupture process of the Tohoku earthquake. That Lay refers to the most recent version of an integrating narrative as a ‘strawman reference model’ is an indication that this is very much an ongoing process.

5.6 Conclusion

In this chapter, I have examined the growth of knowledge about the rupture process of the Tohoku earthquake, with a focus on the role of narrative. I have described three kinds of narratives in this chapter: rupture narratives, research narratives and integrating narratives. I would like to end with some considerations about the functions of these narratives in contributing to the growth of knowledge about the Tohoku earthquake.

6.1 Introduction

If you were thousands of years old, and in your youth had passed by the ancient city of Ur, Babylon,Footnote ¹ at around 2000 bce, you might have been lucky enough to visit Simat-Enlil, King Shulgi’s daughter. Over a cold drink, she may have shown you a gift from her father: a bowl already over a hundred years old that he had inscribed as a gift to her (Reference ThomasonThomason 2005: 74). This is the first documented case of people reusing and reappropriating already old materials in historical archaeology. One and a half thousand years later,Footnote ² Nebuchadnezzar and Nabonidus, successive kings of Babylon, excavated and restored earlier structures at Ur. While doing so, they re-incorporated into their architecture inscriptions made by different kings, thousands of years earlier. Due to written records, we also know that Nabonidus’s daughter, the princess En-nigaldi-Nanna, dug at the temple of Agade and had a room in her palace dedicated to items of antiquity, making her possibly the first known antiquarian (Reference DanielDaniel 1981: 14; Reference OatesOates 1979: 162). These instances of the curation of already old artefacts are usually understood by archaeologists to be efforts to reinforce authority through emphasizing a connection to past rulers or ancestors.

In prehistory (i.e., without the benefit of written information), our understandings are built through narrative explanations based on the suite of physical evidence we encounter. The amount and nature of this evidence vary wildly – sometimes preservation is excellent and at other times very poor – but in general terms there is more evidence from time periods closer to the present day than from the more distant past. Already old materials incorporated into later deposits have been noted by archaeologists at prehistoric sites (Teather 2018; Reference Knight, Boughton and WilkinsonKnight, Boughton and Wilkinson 2019), and recent applications of absolute dating methods in prehistory have led to more instances of out-of-time artefacts being uncovered. I will argue in this chapter that this greater focus on attempting to ascribe more precision to an archaeological event has forced archaeologists to confront their own assumptions of what kinds of time they are trying to measure, and use, to frame accounts of past lives.

Archaeological information can be an important source of identity for human societies, providing an alternative narrative of life in the past and its connection to the present that can sit alongside origin myths, religion and history. As a uniquely integrated discipline of history and science, archaeology has made increasingly sophisticated use of narrative tools in the last 30 years. While it has always used narration to report and discuss evidence retrieved from past people’s lives, this has been problematic. Narrative approaches have sometimes been seen as unduly historical and not scientific or objective enough; but scientific results are equally understood to be subjective and selective (or, at least, ‘theory laden’) and require explanation. In undertaking archaeological analysis, we are inextricably tied to both disciplinary approaches. This chapter traces the structure of knowledge creation in archaeology, how this applies to measuring time and how these are brought into coherent narrative form by archaeologists. In conclusion, archaeological information both contains stories and suggests stories: some that are ours and some that belong to the past.

6.2 Archaeological Knowledge and Narrative

6.2.2 Construction of Archaeological Narratives

Narratives in archaeology weave between these chronicles and genealogies, as if with ribbon, creating individual cat’s cradles by encompassing different facts from different chronicles and genealogies. For example, I conducted a synthesis of prehistoric human burials with strike-a-light kitsFootnote ¹⁰ that determined that the overwhelming majority occurred with male burials between 2200 and 2000 bce (Reference Teather and ChamberlainTeather and Chamberlain 2016). While already considered to be a gendered practice, this research showed it was more common, very strongly male-related, often seen in higher status burials and, as a product of new radiocarbon dating, the duration and peak occurrence of the practice could be ascertained. In terms of scaffolding, this paper relied on many different chronicles of knowledge: experimental work; chemical work on the degradation of iron pyrites in soils over time; a genealogy of situating the practice within European prehistoric analyses of similar types of burials; and finally, the metaphorical work of Reference Lakoff and TurnerLakoff and Turner (1989) (a genealogy based on the use of textual analysis and material culture in archaeology) to suggest that death may have been seen as a type of journey for the dead men during this time period and requiring a portable source of light and/or heat. Each of these elements as brought to that paper have their own histories and scaffolds of knowledge in archaeology and cognate disciplines of anthropology and ethnology, but it was the authors’ preference and choice to bring them together in this particular narrative. Other authors could use the same starting point of evidence and produce a different cat’s cradle of narrative.Footnote ¹¹ The success of this particular approach was that it has stimulated more attention during excavation to record and identify these otherwise quite functional and unremarkable objects; the thorough synthesis accompanied with a compelling narrative proposing a rich metaphorical significance has affected field practices. In Berry’s terms (Chapter 16), the authors are present in the archaeological narrative through this process. Yet, the motive of the original research question or puzzle that stimulated that work is not actually mentioned in that paper.Footnote ¹² As a specialist in artefacts made from chalk in the Neolithic, I have proposed that most chalk artefacts mimic artefacts made from different substances, such as stone or wood (Reference Teather and ShaffreyTeather 2017; Reference Teather, Chamberlain and Parker PearsonTeather, Chamberlain and Parker Pearson 2019). I was puzzledFootnote ¹³ by chalk ‘charms’Footnote ¹⁴ found in a small number of burials of predominantly women and children, and their visual resemblance to iron pyrite strike-stones that were in a few adult male burials (Figure 6.1).

Figure 6.1 Iron pyrites (left) and chalk charms (right) from the burial of a female, dated to 3600 BCE

Cissbury, West Sussex, Shaft 27.

No recent research had been conducted on strike-a-light burials so I had to complete it myself and having done so can argue (and will do so further in a monograph in preparation) that strike-a-light burials may have been male-dominated in this period, but that there was a metaphorical past connection in a different material within the burials of women and children. Therefore, the strike-a-light burials are male-gendered, but a similar practice included women and children in a different, and potentially socially subversive, way. In order to argue that position and create a convincing narrative, the research had to be completed in this sequence.

These examples show that archaeological narratives appear to map well onto a narrative science framework. I will now turn to focus on archaeological dating methods and how these fit into this proposal.

6.3 Archaeological Dating

Chronologies in archaeology are manifold and can refer to temporal changes in certain types of artefact or in modes of an economy or social system. In effect, they are types of chronicle that seek to order selected events by the inclusion and exclusion of information. Relative and absolute chronologies (Figure 6.2) sit side by side in archaeology and can include many different facts of the record, but are constructed in different ways.

Figure 6.2 Schematic representation of narrative reasoning in archaeological chronologies for British prehistory

This shows the end phases of the Stone Age and the Beginning of the Bronze Age, relative chronologies (left) and absolute chronologies (right).

6.4 From Dates to Narratives: Impacts of Recent Studies

As detailed above, relative and absolute dating provide different ways to build narratives. For relative chronology, narratives are built piecemeal from the contributions of seriation, typologies and closed contexts – often by individual scholars working on sets of material with continual reference to the existing genealogies and chronicles. When syntheses are produced, they are agreed in chronicles by the process of accepting individual facts within the scaffolding of each chronicle. Therefore, these have been subjected to continuous revision and refinement over time (Reference Chapman and WylieChapman and Wylie 2016), as new discoveries have been made or inferences or assumptions have been successfully challenged. This process produces rich and subtle narratives that often incorporate previously overlooked evidence and challenge earlier interpretations or narratives, such as the Glastonbury Iron Age village where the excavators’ results were revisited and reworked five times between the 1960s and 1990s (Reference Chapman and WylieChapman and Wylie 2016: 108–136).

On a wider scale and using data from the British and European Neolithic period, the span of data derived from relative dating originates from the accretion and consensus of primary analyses of archaeological finds. These initially consist of collections of multi-authored specialist reports of pottery, flint and other materials, that when taken together lead to a consensus of date or date range that subsequently fits into the broader relational chronological framework. At this stage, it is usual for a few ‘range-finder’ radiocarbon dates to be obtained to confirm the proposed artefact-based chronology. Therefore, at an initial and primary assessment stage, three to five radiocarbon dates may be taken per site to confirm and establish, or conversely challenge, the range of activity and sequence proposed through the relative dating methods. When information from many sites is aggregated, this results in a fairly even spread of absolute dates throughout the chronological sequence, as seen with the numbered dates on the right-hand side of Figure 6.2.

In the last decade and a half, advanced computational approaches have permitted new analyses of absolute dating evidence undertaken on archives of previously excavated materials that have challenged this traditional means of narrative building in archaeology. Intensive radiometric dating studies on archival material that have often focused on a particular category of site or research question have increasingly been undertaken. These dates supplement the range-finder dates typically gained through excavation and have the effect of creating blocks of data in the temporal framework and consequently an uneven distribution of information (please refer to the tightly spaced lines on the right-hand side of Figure 6.2). Two approaches have been particularly prominent: new absolute dating on sequential stratigraphic layers within archaeological deposits to produce Bayesian statistical analyses of site chronologies; and large-scale geo-spatial analysis of aggregated radiocarbon dates. These projects produce narratives and chronologies that are based on either inclusive or reductive reasoning. Inclusive narratives use absolute dating to establish how events are chronologically and spatially related. They are inclusive, as they seek to address all the findings uncovered, and they are narrative in that they provide an explanation for those findings through attention to the type of artefactual material under study and the sequence revealed. Reductive chronologies use absolute dating to provide a more precise date for an event. They are chronologies in that precise time measurement is their goal; and they are reductive in that they exclude dating evidence that does not easily confirm their chronological goal. This is because they assess any type of material culture as solely contributing in terms of chronological (and not cultural) evidence.

6.5 Conclusion

This chapter began with a discussion of the use of already old material culture in later deposits in ancient Babylon. Between the third and first millenniums bce, the connection between the curation, deposition in graves and/or powerful display of material culture initially had the purpose of legitimizing the power of individuals in their leading societal role, both locally and regionally. In the British Neolithic (4000–2000 bce), it is possible that old bones were already incorporated into pits or burial chambers, which may have been for the purpose of integrating past material with those of that present (Teather 2018), and, later in prehistory, human bodies appear to have been deliberately mummified, curated and buried at a later date (Reference Booth, Chamberlain and Parker PearsonBooth, Chamberlain and Parker Pearson 2015). For archaeologists, time and temporality are different faces of the same coin: time is simply a clock; temporality encompasses the human experience of time and is not easily measured.

I have sought to discuss how narratives in archaeology are created through chronicles and genealogies. Archaeologists are familiar with only achieving a temporary success with our narratives; research in our archives and in the field is a continual process, and new discoveries can quickly destabilize existing narratives. By separating our narratives into the use of facts, chronicles and genealogies, it allows us to comprehend the complex structure of archaeological knowledge and how we construct it. New information is readily incorporated into our existing genealogies and chronicles.

I have chosen to discuss chronology in archaeology, and how absolute dating methods have moved our primarily genealogical reasoning into establishing chronologies which, in the end, produce only chronicles. Further, the different methodologies within absolute dating projects have resulted in a diversity of composed narratives. In particular, the creation (or not) of filtered chronicles and use of detached narratives (narratives detached from context) have been seen to be pivotal to this process, and I have termed these inclusive narratives and reductive chronologies.

While more absolute dating allows us to create more chronicles of absolute chronologies, these are not equal. Some will provide us with more adjacent temporal moments that do not necessarily produce better understandings of the archaeology but answer questions of time. Normative approaches will produce a normative view of human behaviour. But the peripheral, inconvenient and subversive facts construct entirely different chronicles, genealogies and narratives. These are where the human stories lie.Footnote ³⁴