The standard practice of eliminating apparent “anomalous” results preclude[s] the possibility of discovering the unexpected, identifying the unusual, and uncovering individual short-lived visitation events [Seymour Reference Seymour2010:158].
In the quotation above, Seymour (Reference Seymour2010) argues that chronometric dating outliers can provide critical evidence of periodic short-term occupations, particularly among highly mobile populations. In this article, we argue that Seymour's statement is even more relevant for stable isotope analyses. In radiocarbon dating, tiny amounts of contaminant carbon can have extremely large effects on measurement results and hence on inferred dates (e.g., Wood et al. Reference Wood, Bronk Ramsey and Higham2016; Zazula et al. Reference Zazula, MacPhee, Metcalfe, Reyes, Brock, Druckenmiller, Groves, Richard Harington, Hodgins, Kunz, Longstaffe, Mann, Gregory McDonald, Nalawade-Chavan and Southon2014). In contrast, stable carbon isotope (δ13C) values measured from purified materials such as bone collagen and hair keratin are less susceptible to contamination. Only large-scale biochemical alteration and/or contamination significantly alters δ13C values, and such changes can usually be detected using quality-control indicators (Ambrose Reference Ambrose1990; van Klinken Reference van Klinken1999). Consequently, a single carbon stable isotope outlier can provide robust insight into aspects of human or animal behavior, including migration and mobility. Outlying δ18O and 87Sr/86Sr values measured from tooth enamel are well established as indicators of human and animal migrations where local isotopic baselines are well constrained (e.g., Bentley and Knipper Reference Bentley and Knipper2005; Hodell et al. Reference Hodell, Quinn, Brenner and Kamenov2004). In some contexts, outlying δ13C values obtained from animal tissues may provide similarly valuable insights (Hobson and Wassenaar Reference Hobson and Wassenaar2019). Obtaining a robust baseline of local isotope values is crucial for this type of research.
While engaged in a comprehensive AMS radiocarbon dating study of early Promontory Phase materials from the Promontory caves of northern Utah (Steward's [Reference Steward1937] Promontory Culture), we noticed that one leather sample from the ankle wrap of a fragmentary moccasin recovered in our 2011 excavations had a δ13C value (−12.5‰) that was much higher than any of the other 51 values obtained from materials in the same archaeological context (Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014). In contrast to the moccasin's unusual δ13C value, its AMS and calibrated calendar dates (725 ± 24 years BP, OxA-25234; 95% probability range AD 1252–1290) fell firmly within both the measured and Bayesian-modeled ranges of the other Promontory samples (Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014).
In this study, we examine the significance of this isotopically unusual moccasin leather. Is it feasible that its high δ13C value came from a bison consuming plants near the Promontory caves? Or did it come from somewhere else? If so, where? To address these questions, we present a local carbon isotope baseline of 157 samples of Promontory caves bison dung, hair, hide, and bone collagen; 14 samples of modern Antelope Island bison hair and dung; and 8 samples of Promontory-era Franktown Cave bison hair and hide (Figure 1). We use these isotopic data along with a grassland carbon isoscape, ancient DNA (species and sex identification), tissue turnover rates, and analogies with modern bison behavior to evaluate competing explanations for the outlying Promontory moccasin δ13C value. These alternative hypotheses include the following: (1) the leather was from a local Promontory-area animal that consumed unusually large amounts of C4 and/or CAM plants, (2) the leather was from an animal that migrated into the Promontory region from a faraway location, (3) the leather was obtained from a faraway location through trade, or (4) the leather was obtained during long-distance travel by people.
The Promontory Caves
The Promontory caves were excavated in 1930–1931 by Julian Steward (Reference Steward1937) and again between 2011 and 2014 by Ives, Janetski, and colleagues (Billinger and Ives Reference Billinger and Ives2015; Ives Reference Ives, Parezo and Janetski2014, Reference Ives and Brunswig2020; Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014). Both research episodes recovered an abundance of exquisitely preserved organic remains, including moccasins, basketry, and cordage; many varied gaming pieces; a complete range of stone and bone tools, from hunting weaponry to hide processing tools; ceramics; items of adornment; indications of ceremonial life such as rock art and incised tablets; and animal bone, hide, hair, and dung (Figure 2; Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014; Thomas Reference Thomas2018; Yanicki and Ives Reference Yanicki, Ives and Voorhies2017). Two aspects of the Promontory occupation are abundantly clear: bison and other large game animals were intensively hunted near the caves, and Cave 1 (42BO1) was a residential location at which women, men, subadults, and children were present at various seasons for extended periods of time over one or two human generations.
Cave 1 contains faint, discontinuous traces of Archaic use, but the vast majority of deposits are Promontory Culture. Cave 2 (42BO2) has some preceding Archaic occupations and a substantial Promontory Culture layer, although fewer deposits than Cave 1. Steward (Reference Steward1937) found that the Promontory Culture layer in Cave 1 covered much of the roughly 240 m2 of habitable space and averaged about 65 cm in depth. Although we made similar stratigraphic findings in Steward's Trench B area, in his Trench A area toward the front of Cave 1, we discovered intact midden deposits that were 2 m deep. These Cave 1 deposits produced a large assemblage of radiocarbon dates with a tight range of ages Bayesian modeled between AD 1240 and 1295 (95% probability estimate; Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014).
Although our excavations were minor (roughly 3 m3 of deposit) we recovered more than 30,000 whole and fragmentary faunal remains, supporting Steward's (Reference Steward1937) contention that the Promontory cave occupants focused almost exclusively on large-game hunting, particularly of bison and pronghorn (see Johansson Reference Johansson2013). Our excavations yielded few small mammals or aquatic resources and scant evidence of seed processing (Ives, Johansson, et al. Reference Ives, Johansson, Graham and Lakevold2014). The Promontory Culture occupation postdates the cessation of maize horticulture among the local Fremont, although relatively sedentary populations were still present in the area, including the nearby Chournos Springs site (e.g., Yanicki Reference Yanicki2019).
For Promontory Cave 1, discard and accumulation formulae along with space syntax analyses suggest a group size of roughly 35 persons, an estimate of 1.8 million whole and fragmentary faunal remains, and an extrapolated total of 2,400 moccasins within the deposits (Hallson Reference Hallson2017; Lakevold Reference Lakevold2017). Seasonality indicators are still being refined, but a range of evidence—from deer skulls with cast antlers to abundant bulrush seeds (by-products of summer mat making)—indicates that Cave 1 was occupied during various seasons of the year. Obsidian sourcing and ceramic evidence further suggest that Promontory cave occupants ranged into southern Idaho (Arkush Reference Arkush2016; Ives Reference Ives and Brunswig2020). Altogether, the Cave 1 Promontory Culture deposits reflect a sudden, substantial, but seasonally intermittent residential occupation of about 55 years’ duration, supported by intensive large-game hunting. The high volume of material culture resulted from the domestic presence of many children, subadults, and adult women and men.
Steward (Reference Steward1937) concluded that the Promontory Culture deposits had been created by a population with northern Plains and Subarctic affinities. He voiced the suspicion that they were Dene ancestors who had lingered long enough to absorb material culture characteristics from neighboring terminal Fremont communities. The potential attribution of a proto-Apachean identity to the Promontory cave occupants is an underlying interest in our research group. Triggered by the massive White River Ash east lobe eruption at AD 846–848, a small founding population of Apachean ancestors may have gradually made their way south in ensuing centuries (e.g., Achilli et al. Reference Achilli, Perego, Lancioni, Olivieri, Gandini, Kashani, Battaglia, Grugni, Angerhofer, Rogers, Herrera, Woodward, Labuda, Smith, Cybulski, Semino, Malhi and Torroni2013; Kristensen, Andrews, et al. Reference Kristensen, Andrews, MacKay, Gotthardt, Lynch, John, Duke, Locock and Ives2019; Kristensen, Hare, et al. Reference Kristensen, Hare, Gotthardt, Easton, Ives, Speakman and Rasic2019; Kristensen et al. Reference Kristensen, Beaudoin and Ives2020). Promontory caves material culture linkages to the Subarctic include moccasins constructed in a characteristically Subarctic style (Bata Shoe Museum classifications BSM 2[Bb] and BSM 2[Ab]), fine leather from a sophisticated hide-processing tradition, and distinctive artifacts such as chi-thos (tabular bifaces used for hide softening), which are common in the Subarctic but unknown in Fremont assemblages (Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014; Webber Reference Webber1989). The Promontory moccasins stand in visible contrast to many surrounding forms of footwear, from hide and plant fiber sandals to hock moccasins (e.g., Clark Reference Clark1885:257; Hatt Reference Hatt1916; Lycett Reference Lycett2015). The Promontory moccasins are completely unlike Fremont footwear or most other footwear in a broader arc in interior western North America (Ives Reference Ives and Brunswig2020:103–104; Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014:626–627). Like many other forms of clothing, moccasins signal cultural identity (in the caliber of leather, the knowledge of requisite patterns, execution of fine motor skills, and decorative techniques). As for absorbing regional traits, the rare basketry in the Promontory caves is characteristic of Great Basin forms, and Promontory pottery was influenced by Fremont forms (Ives, Froes, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014; Yanicki Reference Yanicki2019). We are nevertheless mindful of other explanations for the Promontory Culture expressions. These would include the possibility of Kiowa involvement (although this would have its own Dene dimension, given the long-term Kiowa relationship with Ndee [Plains Apache] peoples) or the prospect that Promontory expressions might be yet more broadly polyethnic in character (Ives Reference Ives, Parezo and Janetski2014, Reference Ives and Brunswig2020; Mooney Reference Mooney1898; Ortman and McNeil Reference Ortman and McNeil2017).
Whatever the cultural identities involved, the Promontory record raises an important and broader set of issues. The AD thirteenth century in western North America was turbulent, featuring profound social and environmental shifts in which hunter-gatherer mobility imparted significant advantages for some groups as more sedentary societies vanished (as with Fremont) or retracted (as with Puebloan communities; e.g., Benson et al. Reference Benson, Berry, Jolie, Spangler, Stahle and Hattori2007; Kohler et al. Reference Kohler, Ortman, Grundtisch, Fitzpatrick and Cole2014; Robinson et al. Reference Robinson, Bocinsky, Bird, Freeman and Kelly2021). By the time of Spanish contact, there existed a variety of Plains-Puebloan interactions founded on the trade of Plains bison and other products for Puebloan goods, and a spectrum of relationships ranging from enclavement among Puebloan communities (as with the Jicarilla Apache) to raiding (Eiselt Reference Eiselt2012). Irrespective of the precise cultural identity of the people who produced it, the Promontory record suggests that the foundation for later relationships between Plains bison-oriented economies and more sedentary villagers was being laid in the thirteenth century.
Franktown Cave (5DA272), Colorado (south of Denver on the Palmer Divide; Figure 1), has material cultural affiliations with the late thirteenth-century Promontory caves. Like Promontory, Franktown Cave's assemblage includes Promontory-style ceramic sherds, a netted gaming hoop, incised wood (probably gaming pieces), sinew-backed bows, and two moccasins of the BSM-2(Bb) type (Gilmore Reference Gilmore, Frink and Weedman2005). According to Bayesian modeling (95% probability estimates), these Franktown artifacts date to AD 1180–1280, which overlaps and slightly predates the occupation of the Promontory caves themselves (Gilmore et al. Reference Gilmore, Ives, Hamilton, Ives and Janetski2021; Gilmore, Hamilton, and Ives Reference Gilmore, Hamilton and Ives2017). Both Franktown Cave and Promontory Cave 1 occupy easily defensible positions overlooking the surrounding area, which may reflect only partial integration with local groups in a dynamically changing thirteenth-century sociopolitical landscape (Lakevold Reference Lakevold2017). The Late Prehistoric occupants of Franktown Cave and the Promontory caves shared a distinctive material culture heritage (Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014).
Carbon Isotopes in Bison Diets: Vegetation
Bison are herbivores and therefore derive their carbon isotope compositions from plants. Although they are primarily grazers, bison also occasionally consume sedges, forbs, and, less frequently, woody plants (Gogan et al. Reference Gogan, Larter, Shaw, Gross, Cormack Gates, Truett, Gates, Freese, Gogan and Kotzman2010). Bison prefer nutritious new growth, particularly C3 grasses during cooler seasons and C4 grasses during summer (Gogan et al. Reference Gogan, Larter, Shaw, Gross, Cormack Gates, Truett, Gates, Freese, Gogan and Kotzman2010). Across the Great Plains, modern free-ranging bison tissue δ13C values directly correlate with local abundances of C3 and C4 grasses (Hoppe et al. Reference Hoppe, Adina Paytan and Chamberlain2006).
Carbon isotope analysis can be used to distinguish between plants using different photosynthetic pathways: C3 (cool-season grasses, forbs, and most woody vegetation), C4 (warm-season grasses and some shrubs), and CAM (cacti and succulents). Today, northern Utah, southern Idaho, and western Wyoming vegetation is dominated by shrubs and C3 grasses. Although substantial C3 grasslands exist in this region, C4 grasses are relatively rare (Cotton et al. Reference Cotton, Cerling, Hoppe, Mosier and Still2016). Microhabitats that support higher abundances of C4 grasses (e.g., Distichlis spicata, Sporobolus airoides), however, do exist (Morden et al. Reference Morden, Brotherson and Smith1986). C4 shrubs (e.g., Atriplex spp.) and CAM cacti and succulents (e.g., Opuntia spp.) are relatively abundant in northern Utah (Coltrain and Leavitt Reference Coltrain and Leavitt2002; Sandquist and Ehleringer Reference Sandquist and Ehleringer1995; Ugan and Coltrain Reference Ugan and Coltrain2012), but are not known to be components of bison diets.
Modern C3 plants have a mean δ13C around −27‰, with a range from about −37‰ to −20‰ (Kohn Reference Kohn2010). Modern plant values are lower than those of plants that grew before the onset of large-scale human industrial activities, which are continually lowering the δ13C of atmospheric CO2 (e.g., Graven et al. Reference Graven, Allison, Etheridge, Hammer, Keeling, Levin, Meijer, Rubino, Tans, Trudinger, Vaughn and White2017). The Holocene preindustrial δ13C of CO2 was about −6.4‰ (Schmitt et al. Reference Schmitt, Schneider, Elsig, Leuenberger, Lourantou, Chappellaz, Koehler, Joos, Stocker, Leuenberger and Fischer2012), which is 1.6‰ higher than the postindustrial atmospheric CO2 δ13C of −8.0‰ used by Kohn (Reference Kohn2010) to standardize his modern plant values. Thus, preindustrial C3 plants should have had a global mean δ13C of around −25.4‰ and a range between about −35.4 and −18.4‰. In a global survey of modern C3 plants, only plants from hyper-arid locations (mean annual precipitation <10 mm/year) and genus Pinus had δ13C values >−23‰ (i.e., preindustrial plant δ13C >−21.4‰; Kohn Reference Kohn2010). Because bison do not consume pine, preindustrial bison with inferred dietary δ13C significantly higher than −21.4‰ can reasonably be inferred to indicate consumption of C4 or CAM plants. Given bison dietary preferences, such values would most likely indicate consumption of C4 grasses.
There is a very clear separation between the δ13C values of C3 and C4/CAM plants: their distributions are nonoverlapping and their means differ by about 14‰ (Cerling et al. Reference Cerling, Harris and Leakey1999; O'Leary Reference O'Leary1988). Modern C4 plant δ13C values typically fall within the range of −16‰ to −10‰, and modern CAM plants that are actively utilizing the CAM photosynthetic pathway also tend to have δ13C values within this range (O'Leary Reference O'Leary1988; Sternberg et al. Reference Sternberg, DeNiro and Johnson1984). Modern C4 plants from Utah fall within this range as well (Coltrain and Leavitt Reference Coltrain and Leavitt2002). Accounting for a Suess Effect shift of 1.6‰, the preindustrial C4/CAM plant range would have been about −14.4‰ to −8.4‰. A recent study of well-preserved Late Pleistocene and Early Holocene plants from Utah's Colorado Plateau resulted in CAM plant (Opuntia sp.) δ13C values between −10.8‰ and −9.8‰ (mean = −10.3‰), in agreement with these global generalizations (Metcalfe and Mead Reference Metcalfe and Mead2019).
Carbon Isotopes in Bison Tissues: Diet Estimates
To directly compare animal tissues to one another or to plant δ13C values, one must account for isotopic fractionations that occur when plant carbon is incorporated into each type of biological tissue. Large herbivore bone collagen δ13C values are about 5‰ higher than the δ13C of plants in their diet (Drucker et al. Reference Drucker, Bridault, Hobson, Szuma and Bocherens2008; Tieszen Reference Tieszen, Owsley and Jantz1994). Hair δ13C values are on average about 3‰ higher than δ13C of plants in the diet (Bocherens et al. Reference Bocherens, Grandal-d'Anglade and Hobson2014; Crowley et al. Reference Crowley, Carter, Karpanty, Zihlman, Koch and Dominy2010; Drucker et al. Reference Drucker, Bridault, Hobson, Szuma and Bocherens2008). Dung δ13C values are typically 1‰ lower than those of diet (Sponheimer et al. Reference Sponheimer, Robinson, Ayliffe, Passey, Roeder, Shipley, Lopez, Cerling, Dearing and Ehleringer2003). The isotopic compositions of skin (hide) have seldom been studied, but Tieszen (Reference Tieszen, Owsley and Jantz1994) showed that for one bison, skin/hide δ13C was about 0.5‰ lower than bone collagen and about 2.0‰ higher than hair, which is broadly consistent with experimental studies of manatee and seal skin (Alves-Stanley and Worthy Reference Alves-Stanley and Worthy2009; Ames et al. Reference Ames, Van Vleet and Sackett1996; Hobson et al. Reference Hobson, Schell, Renouf and Noseworthy1996).
Tissues form over different time scales and can therefore provide information about animal diets over different durations of time. Bison dung represents the diet consumed 1–3 days immediately prior to deposition (Rutley and Hudson Reference Rutley and Hudson2000; Schaefer et al. Reference Schaefer, Young and Chimwano1978), and therefore likely includes vegetation consumed in the geographical region of deposition. In contrast, bone collagen δ13C values are derived from the diet consumed over a period of years (Hedges et al. Reference Hedges, Clement, David, Thomas and O'Connell2007; Hobson and Clark Reference Hobson and Clark1992). A recent study found that the cortical bone of bear, deer, serow, and macaque formed primarily during adolescence (i.e., the active bone-growth phase) rather than throughout the animal's life (Matsubayashi and Tayasu Reference Matsubayashi and Tayasu2019). Thus, in some instances, bison cortical bone collagen may represent the diet several years prior to death. If an old animal migrated into a new area during its adolescence, its bone collagen could represent foods consumed in locations quite distant from where the bones were recovered. If animals did not migrate, bone collagen would provide a record of local diet. Regardless, bone collagen integrates diet over a long period of time. Hair contains a short- or medium-term record of diet, depending on the sampling strategy and the hair's growth rate. When hair is sampled in segments, it is possible to recover a time series of changes over a period of days, weeks, or months. Skin is constantly re-forming and therefore represents diet relatively close to the time of death. Skin turnover rates have not been studied in terrestrial mammals. Manatee and dolphin carbon isotope “half-lives” (time to reach half-equilibrium with new values after a controlled-diet switch) ranged from about two weeks to four months, so time to reach full equilibrium would be on the order of several months or more (Alves-Stanley and Worthy Reference Alves-Stanley and Worthy2009; Browning et al. Reference Browning, Dold, I-Fan and Worthy2014). Based on these turnover rates, bone and skin δ13C values would be less likely to record brief and/or occasional consumption of isotopically distinct plants, whereas dung and hair δ13C would hold greater potential for recording such brief deviations from a typical diet.
Based on the research described above, we convert bison dung, hair, hide, or bone collagen δ13C to short-, medium-, and long-term diet δ13C according to the following equations:
Short-term bison diet 1–3 days prior to deposition:
Medium-/short-term bison diet (weeks/months):
Medium-term bison diet (months):
Long-term diet (years):
A further conversion is needed to make diet δ13C values obtained from modern samples directly comparable to those from preindustrial archaeological samples. In this study, the δ13C values of modern Antelope Island samples collected in 2017 are converted to preindustrial equivalents by adding +2.1‰, which accounts for the higher atmospheric CO2 in 2017 (when the samples were collected; ca. −8.5‰) relative to the preindustrial global atmospheric CO2 δ13C value of −6.4‰ (Graven et al. Reference Graven, Allison, Etheridge, Hammer, Keeling, Levin, Meijer, Rubino, Tans, Trudinger, Vaughn and White2017; Schmitt et al. Reference Schmitt, Schneider, Elsig, Leuenberger, Lourantou, Chappellaz, Koehler, Joos, Stocker, Leuenberger and Fischer2012).
Materials and Methods
Bison samples (dung, hair, hide, and bone collagen) used for isotopic analysis are listed in Table 1. Detailed preparation and analytical methods are provided in the Supplemental Methods and explained here in brief. Hair and hide were cleaned and bone collagen was extracted following established procedures. Carbon isotope compositions (δ13C) and carbon and nitrogen contents (%C, %N) were measured using an Elementar Vario Micro Cube elemental analyzer, coupled with an Isoprime stable isotope ratio mass spectrometer in continuous-flow mode. Elemental compositions were calibrated using USGS-40. δ13C values were calibrated to the VPDB scale using USGS-40 and USGS-41 or 41a (accepted values −26.39‰, +37.63‰, and +36.55‰, respectively). Internal check standards (gelatin, methionine, red lentil, amaranth; accepted values −15.30‰, −28.60‰, −26.12‰, −13.59‰, respectively) and sample replicates were included in each run to monitor measurement uncertainty. For bone collagen, hair, hide, and dung (respectively), total analytical uncertainty was estimated to be ±0.22‰, ±0.18‰, ±0.24‰, and ±0.43‰ (for error calculation method, see Szpak et al. Reference Szpak, Metcalfe and Macdonald2017). As shown in Table 1, consistent %C, %N, and C/N ratios indicate excellent preservation of collagen, hair, and hide.
Note: Diet δ13C values were calculated from measured tissue values using the equations in the text. An Excel version of this table is provided as Supplemental Table 1.
a Values for these samples are means are from serial sampling (see Supplemental Table 2 for values of sections).
b Carbon isotope values for these samples were obtained in the process of radiocarbon dating and previously published in Ives and colleagues (Reference Ives, Parezo and Janetski2014). Their diet values are not calculated because the calibration does not provide the necessary precision for paleodiet reconstruction.
c Antelope Island diet calculations account for a +2.1‰ adjustment to make them equivalent to preindustrial values, as described in the text.
Genetic analyses of the FS-305 ankle-wrap hide sample were conducted at the UCSC Paleogenomics Laboratory (PGL) to (1) obtain a taxonomic identification and (2) determine the sex of the animal. Further details of both isotopic and genetic methods can be found in Supplemental Text 1.
Results and Discussion
Moccasin Leather Outlier: Context and Isotopic Composition
The FS-305 sample, which produced the outlying δ13C value that inspired this study, came from an ankle wrap remnant on a fragmentary moccasin recovered from Promontory Cave 1 (Figure 3a). The ankle wrap is one of three separate pieces of leather, along with the body and vamp, used in the construction of Promontory-style moccasins (Figure 4). The FS-305 moccasin was missing the toe area, but its inferred size extrapolated from complete moccasins in the Promontory caves is about 22.5–24 cm (Erika Sutherland, personal communication 2019). Based on correlations between foot size, height, and age, this moccasin was probably worn by a youth approximately 11–13 years old (Billinger and Ives Reference Billinger and Ives2015).
The ankle wrap of FS-305 was originally selected as a convenient (minimally destructive) sampling location for an earlier AMS radiocarbon dating study. For that study, the sample underwent the Oxford Radiocarbon Accelerator Unit (ORAU)'s routine leather pretreatment process (lab code ACJ; Brock et al. Reference Brock, Higham, Ditchfield and Ramsey2010) and yielded an AMS date of 725 ± 24 BP (OxA-25234), calibrated to AD 1252–1290 using OxCal v4 (Bronk Ramsey Reference Bronk Ramsey2009) and the IntCal13 dataset (Reimer et al. Reference Reimer, Bard, Bayliss, Warren Beck, Blackwell, Ramsey, Buck, Cheng, Lawrence Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatte, Heaton, Hoffmann, Hogg, Hughen, Felix Kaiser, Kromer, Manning, Niu, Reimer, Richards, Marian Scott, Southon, Staff, Turney and van der Plicht2013). Its associated δ13C was −12.5‰ and its %C, %N, and C/N were 37%, 14%, and 3.2, respectively. In the present study, a new aliquot of the same sample was independently cleaned as described in the Supplemental Text 1, and its isotopic composition was obtained following a 2-point calibration that produced the accuracy and precision necessary for paleodietary analysis (Szpak et al. Reference Szpak, Metcalfe and Macdonald2017). Duplicate analyses of FS-305 resulted in δ13C values of −12.93‰ and −12.87‰, carbon contents of 35.0% and 34.7%, nitrogen contents of 12.3% and 12.5%, and C/N ratios of 3.32 and 3.24 (means listed in Table 1). The quality-control indicators and reproducibility both within and between labs indicate excellent preservation and support the validity of the isotopic measurements. Converting the mean of the measured δ13C value obtained from the leather (−12.9‰) to a diet δ13C value results in an estimate of −17.4‰ for the plants consumed by the animal while the skin was being formed.
After confirming the ankle wrap's unusual δ13C, we tested the body portion of the same moccasin. The FS-305 moccasin body leather produced a δ13C value of −20.0‰, which results in a dietary δ13C estimate of −24.5‰ (Table 1). The moccasin body δ13C is distinctly different from that of the ankle wrap but is consistent with all 171 other northern Utah bison samples (Table 1).
Local Bison Eating C4 and/or CAM Plants?
The thickness and general appearance of the FS-305 moccasin leather were consistent with bison. Our genetic analysis supports this species attribution (see Supplemental Text 1).
The FS-305 ankle wrap was the only northern Utah bison tissue sample with clear evidence for C4/CAM plant consumption. All of the other carbon isotope measurements for Promontory bison tissues are consistent with purely C3 diets (Table 1; Figure 5). Even bison dung, which would be most likely to record short-term dietary deviations from the norm, clearly reflects a C3 diet. (We note that a single dung sample has a δ13C value 0.3‰ higher than our C3 cutoff, but because analytical uncertainty for dung was ±0.43‰, this sample does not provide solid evidence of C4 plant consumption.) Serially sampled Promontory bison hair, including several long hairs that likely record at least a year of growth, show no evidence of temporary short-term consumption of C4 or CAM plants. Instead, each individual hair shows variations of ≤1.8‰, all of which fall firmly within the C3 range (Figure 6; Supplemental Table 2). Medium- and long-term bison diet records, including the bone collagen and hide/leather values, also show no evidence of C4 or CAM plant consumption (Table 1; Figure 5). Fourteen modern Antelope Island bison dung and hair samples further show no evidence of C4 or CAM plant consumption along the shores of Great Salt Lake (Table 1; Figure 5).
The FS-305 leather sample clearly has a much higher δ13C value than any of the other Promontory samples (Figure 5). In contrast to the abundant evidence that local Promontory bison diets were entirely C3, the FS-305 ankle wrap has a δ13C value that indicates the animal consumed a large amount of C4 (and/or CAM) plants. A linear %C4 estimate can provide a general approximation of the relative amount of C4 versus C3 plants that would have had to have been consumed to produce a particular δ13C value. If we assume a pure C3 dietary endpoint of −24.3‰ (the mean derived from typical Promontory bison values obtained in this study) and a pure C4/CAM endpoint of −10.3‰ (the mean of ancient Colorado Plateau CAM plants, which overlap with C4 plants; Metcalfe and Mead Reference Metcalfe and Mead2019), the FS-305 diet value of −17.4‰ suggests a C4/CAM contribution of approximately 50%. For roughly 50% C4/CAM to be recorded in a relatively slow-forming tissue such as skin, the C4 and/or CAM plants must have comprised a significant proportion of the diet for several weeks to months. Yet, there is no unequivocal evidence for C4 or CAM plant consumption in any other Promontory bison tissue, even those that formed over short timescales and would therefore record brief temporary changes in diet. We conclude that the FS-305 sample came from a bison that was not native to the Promontory region.
Could the outlying δ13C of the FS-305 ankle wrap be the result of a bison migrating into the Promontory area after spending an earlier part of its life in a high-C4 grass area? As demonstrated by the C3 signals obtained from 171 samples of Promontory and Antelope Island bison dung, hair, hide, and bone collagen, this would have been a highly unusual occurrence. But if it did occur, what locations would have had sufficiently high grass δ13C values to produce the FS-305 value?
Cotton and colleagues (Reference Cotton, Cerling, Hoppe, Mosier and Still2016) predictively modeled the distributions of C3/C4 grasses and mean grass δ13C values across modern (preindustrial) North America. Given that this model was based in part on measured bison δ13C values that were not included in the present study, it provides an excellent baseline for determining possible regions of bison origin (Figure 7). Local Promontory bison diet δ13C estimates from dung, hair, hide, and bone collagen are in good agreement with one another and with the predicted values for the Promontory region (Table 2; Figure 7). The nearest areas on the grass isoscape with mean δ13C values similar to that of the FS-305 bison are about 500 km to the south, in southwestern Utah / northwestern Arizona, and about 700 km east, around Franktown Cave in Colorado. We cannot exclude the possibility that closer locations might include microhabitats with high C4, particularly during the summer when C4 grasses flourish. It is clear from our baseline isotopic data, however, that if such areas existed near the caves, local Promontory bison did not utilize them.
a Carbon isotope offset used to convert tissue to diet values, as described in the text.
b Modern (Antelope Island) diet values were converted to preindustrial equivalents by adding +2.1‰, as described in the text.
c Serially sampled hair results were averaged to produce a single value for each individual hair before being included here.
d Includes only data obtained in the present study, using a 2-point calibration suitable for paleodiet reconstruction, as described in the text.
To determine whether a bison could feasibly have traveled under its own power from one of these faraway locations to Promontory while retaining a strong C4 signal in its hide, we consider the distance traveled (estimated using a least-cost path approach), the isotopic turnover rate of skin (because continuous replacement would gradually “dilute” the C4 signal as an animal switched its diet during the journey), and the feasibility of traveling the required distance in the required time (based on modern bison observational studies). Supplemental Text 2 provides details of these inferences. We conclude that it may have been physically possible for a bison to travel those distances in the required amount of time, but it would have been highly unlikely—particularly because genetic analyses indicates that the leather was from a female bison, which would typically travel with calves (Supplemental Texts 1 and 2). Altogether, it is unlikely that the FS-305 ankle wrap came from a wandering bison.
Procurement of Hide via Trade?
If the FS-305 ankle wrap leather was not from a local bison or a bison wanderer, then it must have been procured in a faraway location and transported to Promontory Cave 1. It is possible that the FS-305 ankle wrap leather was obtained through trade. In this scenario, the Promontory people would have obtained the leather from other individuals or groups who had been in a location far to the south or east (or who had traded for bison leather from these areas). However, based on the ready availability of local bison to Promontory people, we believe that trade in bison products was very unlikely. In a less than 3 m2 area of Cave 1, more than 30,000 whole and fragmentary bones were recovered. These are completely dominated by large game animals, with bison the most common genus for identifiable specimens (Ives, Johansson, et al. Reference Ives, Johansson, Graham and Lakevold2014; Johansson Reference Johansson2013). The presence of large, difficult-to-transport skulls and low-utility lower limbs and hooves suggest that a kill site was nearby. Indeed, the landscape above the caves funnels prey into a narrow area that is ideal for ambush strategies and remains an active game trail today. Furthermore, Promontory Cave 1 occupants were not heavily comminuting bones to extract greases as they would if food were scarce. Strata consisting of charred and calcined bone burned at variable temperatures suggest that greases and fats were present and served as accelerants for a fire rather than being extracted for food (Ives, Johansson, et al. Reference Ives, Johansson, Graham and Lakevold2014). Based on the number of discarded moccasins in the caves, Reilly (Reference Reilly2015) estimated that about 250 bison hides would have been required to replace them. In combination with the skeletal evidence, this suggests that the minimum number of bison individuals (MNI) for the entire cave contents would be huge. Thus, multiple lines of evidence indicate that bison were abundant around the Promontory caves and that the Promontory people were highly adept at hunting them. It would have been highly unnecessary for Promontory people to trade for bison leather. Although we cannot completely exclude this possibility, we consider the explanation in the next section more likely.
Procurement of Hide via Long-Distance Travel
The final and likeliest possibility to explain how the FS-305 ankle wrap leather ended up incorporated into a Promontory Cave 1 moccasin is long-distance travel to/from an area far to the south or east. Below, we will first explore potential areas of origin and then discuss possible mechanisms of transportation, including a long-distance round trip, a meeting with individuals from distant regions at an intermediate location, or a visit from outsiders who left behind the hide or leather.
It is not possible to pinpoint a precise location of origin for the FS-305 ankle wrap leather because its δ13C value of −17.4‰ is characteristic of a vast region from South Dakota through Nebraska, eastern Colorado, southeastern New Mexico, and southern Arizona (Figure 7). We also cannot exclude the possibility that the bison inhabited a microhabitat with high C4 some distance from the caves but closer than the areas identified in our regional isoscape. Based on our current knowledge, however, northwestern Arizona and central Colorado are the closest locations to have δ13C values consistent with that of the FS-305 ankle wrap (Figure 7).
The northwestern Arizona region with modeled δ13C values consistent with the FS-305 ankle wrap sample is known to have supported bison during the Late Prehistoric period (Martin et al. Reference Martin, Martin and Mead2017). Open-air sites with Promontory material culture associations dating approximately 100–200 years later than the Promontory cave deposits occur on the eastern shore of Great Salt Lake and Utah Lake (Janetski and Smith Reference Janetski and Smith2007), along the least-cost path route to this part of northwestern Arizona (Supplemental Text 2). With the cessation of Fremont maize agriculture around AD 1150 (Coltrain and Leavitt Reference Coltrain and Leavitt2002), travel along the Wasatch Front and into northwestern Arizona during the late thirteenth century may have been relatively easy. Given that later Promontory Phase assemblages are situated in geographically intermediate areas, it would not be surprising if late thirteenth-century Promontory cave occupants had connections with regions as far south as northern Arizona. We note that Promontory-style moccasins have recently been identified near Mesa Verde (Figure 1), supporting the idea of a connection with the Southwest (Fewkes Reference Fewkes1909; Osborne Reference Osborne2004; Yanicki and Ives Reference Yanicki, Ives and Voorhies2017).
Alternatively, the isoscape model and Franktown Cave bison δ13C values are consistent with a potential origin for the FS-305 ankle wrap leather in central Colorado. Franktown bison hide and hair samples had highly variable δ13C values, ranging from purely C3 to purely C4 (Figure 5). The much larger variability in bison δ13C at Franktown compared to Promontory reflects the site's location in an area that rapidly transitions from C3- to C4-dominated grassland (Figures 5 and 7)—that is, all of the Franktown bison could have been procured relatively close to Franktown Cave, with higher values among bison who had grazed in areas further east and/or were killed after the summer months. The Promontory Cave FS-305 moccasin leather may therefore be evidence of contact between northern Utah and central Colorado. The assemblage similarities between the Promontory and Franktown caves support the possibility of contact in various forms: travel between the two locations, a rendezvous at a geographically intermediate location, or a rendezvous with yet another population that shared this material culture heritage. We note that Promontory-style moccasins are also present in Daugherty Cave (48WA302) and Ross Rockshelter (48NA331) in Wyoming, which are intermediate in longitude (although at higher latitudes) relative to Promontory and Franktown (Figures 1 and 7; Frison Reference Frison1968; Garling Reference Garling1964).Footnote 1
It is significant that the only extralocal leather identified to date in the Promontory caves was used to construct a moccasin ankle wrap. Unlike moccasin bodies—which quickly become worn, patched, further worn, and discarded—ankle wraps can be reused and recycled into new footwear. Within the Promontory caves assemblage, the great majority of moccasins are heavily worn, patched, or both. Cuts that indicate the severing of ankle wraps from the moccasin body are common (Figure 3b–d). Repurposing of leather also occurred at Franktown Cave, where three fringed-seam artifacts dating to the Promontory occupation had been cut from larger leather objects (Gilmore et al. Reference Gilmore, Ives, Hamilton, Ives and Janetski2021). Although some leather scraps occur in both the Promontory and Franktown caves, their occupants were frugal, generally discarding only worn-out moccasin bodies and bison robe edges. Given its longer use life and potential for reuse, an ankle wrap is the one portion of a moccasin most likely to survive a long-distance journey, precisely because it could be usefully reincorporated into new footwear.
Historic sources show that it was essential to carry and repair additional moccasins during a lengthy journey. The distances to origin documented here would require wearing and replacing several pairs of moccasins, which were common discards in abandoned camps (e.g., Ewers Reference Ewers1955:182, 184; Moulton Reference Moulton1987:216, 219; Wilson Reference Wilson1924:212, 232, 283–284). We consider it plausible that the leather was obtained at a distant source and recycled in a series of usages as an ankle wrap over the course of the journey to the Promontory caves. It is also possible that the FS-305 ankle wrap leather was brought to the caves as a piece of clothing, bison robe, or lodge covering that was repurposed (although the larger the original object, the greater the possibility of detecting other anomalous values, which we have not). As discussed above, however, obtaining such items from other groups or individuals would have been like carrying coals to Newcastle.Footnote 2 The Promontory cave occupants had access to a genuine abundance of bison products, including fine leather, and they would have had no need to acquire it from others while residing in the caves. It therefore seems unlikely that a visitor to the caves would leave behind leather that would later become incorporated into a Promontory moccasin. Instead, it seems more likely that the leather was obtained at some distance from the caves and reincorporated into the moccasin during travel. This implies that Promontory people had either traveled to one of these faraway locations themselves or met with others at an intermediate location where the leather changed hands. Either way, the ankle wrap came from far away and was not obtained in the vicinity of the caves, which implies long-distance connections and knowledge of faraway locations.
Independent Evidence for Long-Distance Connections of Promontory Caves Inhabitants
Although the ankle wrap δ13C value is a unique piece of evidence for human movement, it is far from the only evidence for long-distance connections of Promontory people. In addition to the Promontory material culture evidence at Franktown Cave and Promontory's northern artifact associations, gaming and obsidian source studies also provide support for long-distance interactions. The Promontory caves inhabitants were familiar with a diversity of games whose distributions span much of western North America, suggesting a wide range of social interactions and possibly long-distance movements (Yanicki and Ives Reference Yanicki, Ives and Voorhies2017). X-ray fluorescence indicates that the majority of obsidian artifacts recovered from Promontory Caves 1 and 2 were constructed from Malad obsidian sourced in southern Idaho (Hughes Reference Hughes2011, Reference Hughes2012). Around the time of the Promontory and Franktown cave occupations, material from Malad and other northern obsidian sources increased by an order of magnitude on the central and southern Plains (Gilmore, Hedlund, and Clark Reference Gilmore, Hedlund and Clark2017; Hoard et al. Reference Hoard, Bevitt and McLean2008; Hughes et al. Reference Hughes, Geib and Ziska2019). Promontory groups may have been involved in the distribution of Malad obsidian through direct migration, down-the-line trade, or trade involving more distant redistributive centers. Whichever alternatives were involved, the late-period obsidian distribution and gaming evidence support the hypothesis that Promontory Phase communities participated in long-distance interaction spheres from north of Utah far to the south and east.
Implications for Promontory Lifeways
For hunter-gatherer groups, Seymour (Reference Seymour and Seymour2012:378) recommends reserving the term “migration” to describe movement to a new region that was not previously utilized, and “mobility” to describe movements within regularly used territories. Given the fact that only one Promontory caves specimen has been discovered with a C4 isotopic signal, it is unlikely that regular Promontory mobility encompassed regions with abundant C4 grasses. Several lines of evidence support geographically widespread interactions, however, and the isotopic evidence suggests that at least one long-distance voyage was likely made. Anthony (Reference Anthony1990) has identified categories of long-distance activities associated with migration that may have detectable material correlates: scouting (to facilitate long-distance “leapfrog” migrations), return visits to a previous occupation, long-distance movements for seasonal or task-specific purposes, stream migration (movement of successive groups along a well-defined route), and counter-streaming (returning to a former occupation after migrating away). If Promontory people had traveled to a faraway location, long-distance scouting seems the most plausible purpose for such a voyage. Establishing familiarity with distant regions would be an essential strategy for highly mobile groups infiltrating the changing thirteenth-century social landscape of the American Southwest and central/southern Plains. Scouting for new economic, trading, or raiding opportunities could have served as a precursor to a shift in regularly used territories, equipping the Promontory people with broad landscape knowledge that could be mobilized to facilitate migration (Seymour Reference Seymour and Seymour2012). The latter seems to have occurred at the end of the thirteenth century, when Promontory people ceased intensive occupation of the Promontory caves (Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014).
Exceptional organic preservation allowed us to establish a uniquely strong isotopic baseline for bison in the area around the Promontory caves during the late thirteenth century. Analysis of 171 bison samples (dung, hair, hide, bone) that formed over short, medium, and long timescales suggests that Promontory bison consumed purely C3 diets. In contrast, the ankle wrap of one Promontory moccasin was from a bison that consumed a large amount of C4 plants over a period of several weeks or months. Based on the local Promontory isotopic baseline along with our current knowledge of skin turnover rates and C4 grass distributions, we argued that it was unlikely the leather came from either a bison with an unusual local diet or a bison that migrated to Promontory from a distant location. Instead, this animal was killed far to the east or south of the Promontory caves. We argued that trade in bison leather was unlikely given that bison were abundant in areas around the caves. Therefore, we consider long-distance travel to be the likeliest explanation, with a scrap of leather procured in the south/east recycled into an ankle wrap over the course of the journey, as would have been common during long-distance voyages. Possible source regions for the ankle wrap leather include northwestern Arizona, central Colorado, and areas farther to the south and east. Regardless of the precise location, the evidence suggests a connection with an area far from the Promontory caves. Knowledge of distant locations, whether obtained through long-distance scouting or contact with other groups, would have ensured that Promontory people were well prepared for relocation to a new “home territory” when the need arose. It appears that this occurred at the end of the thirteenth century, when intensive occupation of the caves ceased (Ives, Froese, et al. Reference Ives, Froese, Janetski, Brock and Ramsey2014).
This study has demonstrated an approach for investigating long-distance connections that is viable for organic remains in regions with minimal local isotopic variability (i.e., C3-dominated grasslands), where the local isotopic baseline is well constrained and considerably different from potential extralocal source regions. This approach is not useful for areas with highly variable local plant δ13C values (i.e., abundant C3 and C4 plants), such as Franktown Cave. Crucial to our approach was the availability of different “time records” from animal tissues preserved in dry cave contexts (dung, hair, hide, bone), and the integration of multidisciplinary lines of evidence (isotopic, genetic, ecological, archaeological). We echo Seymour's (Reference Seymour2010) argument that “anomalous” data can be sources of valuable information rather than errors to be averaged away, especially for highly mobile populations.
We thank Paul Stavast, Vandy Bowyer, Harvey Friebe, Evangeline Bell, and Amy Sky for sampling and laboratory assistance; Jennifer Cotton for access to isoscape data; Kathy Lemberg, Gabriel Bowen, and Rhy McMillan for map assistance; Joel Janetski for manuscript comments; Steven Bates for permission to sample in Antelope Island State Park; and Wes Olson, Johan du Toit, and Tom Seaton for insights into bison ecology. This research was funded by a Social Sciences and Humanities Research Council of Canada (SSHRC) Banting Postdoctoral Fellowship (awarded to JZM), and by SSHRC Standard Research Grant 410-2010-0480 and SSHRC Insight Grant 435-2012-0140 (awarded to JWI). We are grateful to landowners George and Kumeroa Chournos for the opportunity to conduct research in the Promontory caves.
Data Availability Statement
Isotopic data presented in this article are available in the Supplemental Materials associated with this manuscript. Genetic data are available on the online data storage platform Dryad (DOI:10.7291/D18Q23).
For supplemental materials accompanying this article, visit https://doi.org/10.1017/aaq.2020.116.
Supplemental Text 1. Supplemental materials and methods.
Supplemental Text 2. Supplemental discussion (wandering bison).
Supplemental Table 1. Excel version of Table 1.
Supplemental Table 2. Results from serially sampled Promontory Caves bison hair.