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Contents
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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- Climate Modes of the Phanerozoic
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- 19 November 1992, pp v-viii
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9 - The Cenozoic Cool Mode: early Eocene to late Miocene
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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- Climate Modes of the Phanerozoic
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- 19 November 1992, pp 99-114
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Summary
The early stages of the Cenozoic Cool Mode started with the cooling in the early Eocene (55 Ma). From that time onwards the climate of the Earth gradually cooled from the Warm Mode of the late Cretaceous to early Tertiary to the cool glacial climates of today. Important changes which occurred during this phase include the enhancement of climatic zonation and the development of a thermally stratified ocean. Unlike the Palaeozoic record, the early part of the Cenozoic cooling is not recorded simply by the presence of ancient glacial deposits. In fact, any direct evidence of extensive glacial ice at the poles during the Tertiary is scarce, principally because the rocks in these regions are now covered with ice. However, the presence of ice-rafted debris in deep ocean cores provides positive evidence for the presence of at least seasonal ice.
The principal evidence for Tertiary cooling is documented in the oxygen isotope record of calcareous foraminifera from the oceans. This illustrates the decline in ocean temperatures and the build-up of ice at the poles (mainly the South Pole) from about 55 Ma onwards (Fig. 9.1). Climate evidence from fossil plant assemblages reflects the same cooling trend. Documentation of the Tertiary cooling is important because it illustrates the crucial transformation phase from a non-glacial to glacial state, as recorded by several geological parameters. This trend or transformation can be used as a model to determine the history of glacial build-up during former Cool Modes, such as those in the Palaeozoic, for which data are less reliable.
10 - The late Cenozoic Cool Mode: late Miocene to Holocene
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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- Climate Modes of the Phanerozoic
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- 19 November 1992, pp 115-188
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Summary
The late Cenozoic was a time of major palaeoclimatic and palaeoceanographic events (Fig. 10.1). The latest Miocene was the most critical time: there was a global cooling of ocean surface waters at middle and high latitudes; a northward migration of cold Antarctic surface waters; an expansion of the Antarctic ice sheet; a major sea-level fall; isolation and desiccation of the Mediterranean Basin; and a lightening in carbon isotopes. In the early Pliocene, there was a warming and a major sea-level rise, followed by late Pliocene climate deterioration and gradual intensification of northern hemisphere glaciation leading to late Pleistocene climate changes visible in the prominent 100 k.y. cycle. The late Cenozoic thus has the most complete record available for defining global change during development of a Cool Mode, especially as related to orbitally driven glacial cyclicity.
Summary of marine climates from oxygen isotopes
Oxygen isotopic ratios in foraminifera from late Cenozoic deep-sea sediments are primarily a function of the δ18O composition and temperature of the ambient sea water. The abyssal waters of the ocean experience relatively small changes in temperature, thus the δ18O in benthic foraminifera is mainly a function of the isotopic composition of the bottom water, which in turn is controlled largely by global ice volume. However, the effect of diagenesis was emphasized by Killingley (1983) who noted that the longterm decrease in ocean temperatures from the Palaeocene to the Miocene, derived from oxygen isotopes, might reflect progressive recrystallization of calcite and therefore all palaeotemperature estimations for the early Tertiary remain speculative.
1 - Introduction
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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- 19 November 1992, pp 1-6
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Summary
There are many reasons for investigating the climatic history of the Earth. First and foremost is the need to know how the climate of our planet has evolved over the last 600 million years (m.y.). Only by understanding past climate states of the Earth can we discern the driving mechanisms for global climate change, set boundary conditions for numerical modelling and learn how to predict future climates. Improved predictability of climate in the short-term future can be included among these advantages to be gained in the applied sense, as can using palaeoclimate information to predict the distribution of economically significant commodities such as petroleum, phosphorite, bauxite and the accumulation of other sedimentary minerals related to climatically controlled redox changes.
This book first describes, in a concise way, the salient features of Earth climates over the last 600 m.y. or so. With this background information in hand, our purpose is to recognize and then compare similar climatic states in history (e.g. the warm intervals in the early Palaeozoic and in the late Mesozoic), so as to determine whether major episodes display any significant similarities. In cases where strict parallelism seems to exist, whether for relatively warm or cool, wet or dry, or seasonally comparable global conditions, the description of climate for any one interval perhaps can be broadened by inferences from conditions in another interval, for which circumstances may be better known. Thus, we adopt an approach of comparative palaeoclimatology, in the hope that pooled information will provide new insights into the evolution of climate on Earth.
Palaeoclimatologists appear to be inveterate seekers after causes of change and the authors of this book are no exception.
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Climate Modes of the Phanerozoic
- Lawrence A. Frakes, Jane E. Francis, Jozef I. Syktus
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The changes in the Earth's climate over the past 600 million years, from the Cambrian to the Quaternary, come under scrutiny in this book, first published in 1992. The geological evidence for ancient climates is examined, such as the distribution of climate-sensitive sediments. The Earth's climate has changed many times throughout the Phanerozoic. Thus in this book the climate history has been divided into Warm and Cool modes, intervals when either the Earth was in a former 'greenhouse' state with higher levels of atmospheric CO2 and polar regions free of ice, or the global climate was cooler and ice was present in high latitudes. The studies presented here highlight the complex interactions between the carbon cycle, continental distribution, tectonics, sea level variation, ocean circulation and temperature change as well as other parameters. In particular, the potential of the carbon isotope records as an important signal of the past climates of the Earth is explored. This book will be useful to all students and researchers with an interest in palaeoclimates and palaeoenvironments.
7 - The Cool Mode: middle Jurassic to early Cretaceous
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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- 19 November 1992, pp 65-82
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Summary
The Jurassic and Cretaceous have classically been considered very warm intervals, with low temperature gradients, temperate conditions at high latitudes and extensive evaporite deposition. However, there is a great deal of data that indicate that this is an over-simplification and, in fact, parts of the Earth were quite cool during this period. Reports of ice-rafted deposits in high-latitude regions at intervals during the latter part of the Jurassic and into the early Cretaceous (Bajocian to Albian) suggest that freezing conditions occurred near the poles and that temperate glaciers may possibly have existed (Kemper, 1987; Frakes and Francis, 1988,1990). The equatorto-pole temperature gradient therefore appears to have been greater than previously considered. Marked seasonality also seems to be a prominent feature of climate during this interval. The designation of this late middle Jurassic to early Cretaceous interval as a Cool Mode (183–105 Ma, Bajocian to mid-Albian) is therefore based on the presence of at least seasonal ice in high latitudes.
In contrast, there are no reports of ice-rafted deposits for the latter half of the Cretaceous. The climate appears to have changed during the mid-Cretaceous to much globally warmer conditions. We have therefore split the Cretaceous Period into two different climatic modes.
Palaeoclimate information for the latter half of the Jurassic (from the Bajocian onwards) is rather sparser than for later times. The rarity of Jurassic ocean floor sediments and the consequent scarcity of ocean oxygen isotope data prohibit detailed temperature analysis for short-term intervals, as is possible for the late Cretaceous and Tertiary. Most Jurassic climate information is for continental areas.
Frontmatter
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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Preface
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- By Lawrence Frakes, Jane Francis, Jozef Syktus, Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK.
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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- Climate Modes of the Phanerozoic
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Summary
This book covers the history of climate for the last 600 million years. Part of the book is a summary of palaeoclimate information for different slices of geologic time. We believe it is necessary to update and attempt to synthesize the great body of research on palaeoclimates generated over the last ten years or so. But there was a further purpose in the collection of these basic data and that was to establish a framework which we could use to compare and contrast similar climate states of the past, and from there to recognize some causes of climate change. We have divided climate history into Warm Modes and Cool Modes, in a way not unlike Fischer's (1982) ‘Greenhouse’ and Icehouse' states, but our Modes are of shorter duration and contain some controversial elements – we have questioned the theory that the Mesozoic climates were uniformally warm and ice free and instead propose a Cool Mode in the middle Mesozoic. We have also included a chapter on the climates throughout the Quaternary, something which is often missing or abbreviated in texts on geologic climates, not surprisingly considering the huge volume and the increased scale of detailed information available for such a relatively short time.
6 - The Warm Mode: late Permian to middle Jurassic
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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- 19 November 1992, pp 56-64
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Summary
Although the Mesozoic as a whole has been interpreted as a warm and arid interval, it now appears that such a generalization does not take account of significant variations within the Era. Much of the early Mesozoic appears to have been a warm time but extensive aridity developed only after the middle Triassic, and significant cooling occurred in the mid-Jurassic. A Warm Mode is here taken to extend from the end of Gondwanan and Asian ice-rafting in the late Permian (Kazanian) through part of the middle Jurassic (to the end of the Aalenian), after which ice-rafting was again common in high-latitude sites (Frakes and Francis, 1988). Following a cool, though arid, time from middle Jurassic to roughly the middle of the Cretaceous, Mesozoic warming resumed.
Oxygen isotopes
The general thermal state of the earth is deduced by the oxygen isotope method and by interpretations of climatically sensitive indicators. Evidence for the middle Jurassic part of the Warm Mode includes an abundance of isotopic information, most of it gained, however, at a time when techniques were imperfect. Most of such data are summarized in Bowen (1966) and have been reinterpreted by Stevens and Clayton (1971) and Hallam (1975). As stated in Frakes (1979), the problems of changing oceanic composition, vital effects, diagenesis and the practice of data averaging before presentation, all mitigate against acceptance of these data as reliable indications of early Mesozoic palaeotemperatures. Unaveraged data for this part of the geologic record suggest a cool mid-Jurassic in Australia (∼ 18°C to 15°C) and the Soviet Union (∼ 15°C to 11°C).
2 - The Warm Mode: early Cambrian to late Ordovician
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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Summary
The first Warm Mode began at the termination of the late Precambrian glaciation, probably in the earliest Cambrian; definite Cambrian glacial deposits are not known. Over the approximate 100 m.y. of the early Palaeozoic Warm Mode (∼560 to ∼458 Ma) continents were mostly dispersed over the low-latitude zones and glacial deposits are lacking. This Warm Mode eventually terminated with the initiation of glaciers in North Africa in the late Ordovician (Caradocian). Major problems exist in trying to explain, first, the occurrence of Plate Cambrian to Precambrian glaciation at low latitudes, and second, the termination of the previous glaciation and the beginning of this early Palaeozoic Warm Mode.
Age and distribution of the latest Precambrian glaciation
Past research has established only a poor framework for dating and distribution of late Precambrian glacial deposits, and as a result the synchroneity versus diachronism of these deposits is not resolved. When the geochronological dates are assembled (Fig. 2.1) it is apparent that glaciation extended over at least 230 m.y. (∼800 to ∼570 Ma). These data, taken from Hambrey and Harland (1981), are of variable quality and several, included on the diagram, are based on stratigraphic estimates. Moreover, age data are limited to Africa, Asia, Australia and Europe.
The shape of the histogram in Fig. 2.1 is not obviously polymodal but it is not suitable to consider the results as a normal distribution of data about a mean value because, when considered in detail, continents have individual dating patterns. Africa for example, contributes all pre-800 Ma dates and, together with Asia, all dates younger than about 610 Ma.
Bibliography
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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3 - The Cool Mode: late Ordovician to early Silurian
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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Summary
Ranked after the late Palaeozoic and the late Cenozoic, the Ordovician- Silurian is the most extensive and intensive Cool Mode of Phanerozoic time. Glacial effects were felt predominantly in Africa, and in displaced terranes subsequently derived from there, and were also felt in South America. A major ice sheet developed in North and central Africa. The age of glacial deposits covers probably 35 m.y., as opposed to a minimum of 65 m.y. for the late Palaeozoic. Despite this widespread evidence of glaciation, it appears that the Ordovician-Silurian glaciation was limited to high-latitude land masses in the southern hemisphere; cooling effects are discerned with difficulty elsewhere.
Distribution and age of the glacials
Ordovician–Silurian glacial deposits, unlike those of the late Palaeozoic, are found in Gondwana and in regions commonly considered as parts of the Laurentia and Baltica blocks. The latter areas probably were attached to Gondwana in the early Palaeozoic and have since been rifted away to new sites in North America and Europe. The evidence of glaciation in these displaced continental fragments provided a strong incentive for revising continental reconstructions for the early Palaeozoic.
Glaciation at this time was centred on North Africa, the best documented deposits being those of the central Sahara region (Beuf et al., 1971). Tillites and associated glacial features occur over a wide area from Algeria to Libya and Mali. A total of four separate tillites and striated pavements showing generally northward flow have been recognized. Deposits of about the same age occur to the west and south-west in Morocco, Mauretania and Sierra Leone; these include both terrestrial tillites and glacial-marine strata.
Index
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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4 - The Warm Mode: late Silurian to early Carboniferous
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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Summary
The period from the end of the early Silurian until the beginning of the Namurian (early Carboniferous) was characterized by globally warm climates. It might seem that the late Devonian and Visean glacial intervals were exceptions to this but glaciation at these times was apparently limited only to high-latitude regions in South America. A general warming trend took place throughout the Warm Mode and can best be seen in a progressive expansion of the evaporite and carbonate sedimentation belts. Carbonates and evaporites persisted in mid-latitudes until late in the Namurian, after the succeeding late Palaeozoic Cool Mode had begun.
Trends from oxygen isotopes
The tendency for oxygen isotope ratios to be progressively lighter back in time through the Palaeozoic has been confirmed by Popp et al, (1986) and by Hudson and Anderson (1989). This work, on brachiopod shells that show evidence of being chemically unaltered, also showed similarities to δ18O determined on diagenetic cements and other components of carbonate sediments. Karhu and Epstein (1986) estimated the isotopic composition of Palaeozoic oceans at – l%o PDB and calculated temperatures for subequatorial North America over the interval of the Warm Mode to be in the range 36–64°C. Previous measurements on unaltered Warm Mode carbonate fossils gave values between ∼21°C and ∼45°C. Unaltered brachiopods studied by Popp et al included only one Warm Mode sample, but other samples gave a range from about – 12%o to – 2%o. Assuming no polar ice and an oceanic composition of – l%o, this range yields temperatures estimated at between about 65°C and 20°C.
5 - The Cool Mode: early Carboniferous to late Permian
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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Summary
A great deal is known about the late Palaeozoic glacial phase owing to intensive study since its recognition in the early part of this century. We now know that glaciations of this time were centred in polar to subpolar latitudes, that they were developed in parts of Laurasia as well as Gondwana, that their inception was regionally related to both collisional and extensional orogenic activity, that ice sheets developed in the centres of continents, and that glaciers underwent waxing and waning in response to changes in their palaeolatitudinal position. There are many other aspects of the late Palaeozoic Cool Mode about which we are relatively ignorant. What was the full duration of glacier development? What were the climates of the nonglaciated parts of the globe? Why did the glaciation cease?
Distribution and age of the glacials
Late Palaeozoic glacial deposits occur widely in Gondwana and also in displaced terranes which are now part of Asia (Fig. 5.1). These have been summarized most recently by publications in Hambrey and Harland (1981), Frakes (1979) and Crowell (1983). Africa is the continent with the most widespread deposits from this glaciation; glacial deposits are known from South Africa to the Arabian peninsula and from the east (Madagascar) to the west (Namibia). The southern exposures consist of the Dwyka Tillite, which occurs in the Karroo basin across the full length of the Cape Fold Belt and eastward into coastal Natal. An ice sheet occupied the northern part of South Africa and adjacent countries to the north (Zimbabwe, Botswana, Zambia) and radiated lobes to the south-east, and south-west (Kaokoveld Lobe).
8 - The Warm Mode: late Cretaceous to early Tertiary
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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Summary
The period from the mid-Cretaceous (mid-Albian) to the mid-early Eocene (approximately 105 Ma to 55 Ma) was one of the warmest times in the late Phanerozoic. The average global temperature was probably about 6°C higher than that of today (Barron, 1983), allowing polar regions to be free of permanent ice. Temperatures were high enough to allow forests and vertebrates to live near both poles, particularly during the peak of warmth in the early Eocene. In contrast to the mid-Jurassic to early Cretaceous there is no evidence for seasonal ice (as ice-rafted deposits) in high latitudes from the Cenomanian to Maastrichtian, although there may have been seasonal ice present during the Palaeocene in northern high latitudes.
The mid-Cretaceous was a time of globally high sea levels and extensive areas of shallow shelf seas, favouring moderate climates and increased evaporation and precipitation (Arthur, Dean and Schlanger,1985).The oceans are believed to have been more stratified than at present, with little vertical mixing and the deposition of large quantities of organic-rich sediments in anoxic bottom waters. In particular, restricted circulation occurred in the developing Atlantic oceans, although by the latter part of the Cretaceous more open circulation was established and the deposition of black shales diminished. Sea levels reached their Mesozoic-Cenozoic peak in the late Cretaceous and then gradually dropped (Haq et al., 1987). During the latest Cretaceous the seas withdrew, exposing larger continental areas and leading to the establishment of more seasonal, continental climates (Hays and Pittman, 1973).
However, refined palaeotemperature data and reinterpretation of the thermal tolerances of fossil plants indicates that there were distinct warmer and cooler phases within the warm trend, particularly highlighted at high palaeolatitudes.
11 - Causes and chronology of climate change
- Lawrence A. Frakes, University of Adelaide, Jane E. Francis, University of Leeds, Jozef I. Syktus, Division Atmospheric Research CSIRO, Australia
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- Climate Modes of the Phanerozoic
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- 19 November 1992, pp 189-219
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Summary
Seeking the cause of climate change is both an exciting activity and a difficult task, as borne out by the long history of enquiry and the scarcity of firm conclusions. While numerous hypotheses have been advanced, either to explain local or temporally short changes or to provide a global framework for change throughout geologic time, it is safe to say that, although elements of truth exist in many of these hypotheses, no single one takes account of all the variables as they are known at present. To help us to understand the climate system, recent studies have utilized an integrated approach that considers the atmosphere, the hydrosphere, the biosphere and the solid Earth. This approach has tended, justifiably, to be historical in scope in order to make use of geological data bearing on the temporal and spatial variability of processes central to the system. The flood of new information means that there is a constant need to re-evaluate concepts and, given the growing awareness of the complexities of the climate system, it is likely that many more attempts will need to be made before we have a thorough understanding of how the system has worked in geological history.
The search for cyclical climate change
The palaeoclimate information collated here illustrates that the history of climate over the past 600 m.y. was not a simple trend of cooling or warming but was characterized by alternating periods of warm and cool climates. These periods we have defined as Cool or Warm Modes, based on the presence of cool or warm climate indicators as presented in the previous chapters.
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