In situ oxygen isotope micro-analysis of faunal material and human teeth using a SHRIMP II: a new tool for palaeo-ecology and archaeology

A Sensitive High Resolution Ion MicroProbe (SHRIMP II) has been used to make high spatial resolution in situ micro-analyses of oxygen isotopes in fish otoliths, and teeth from fossil herbivores and a Neanderthal. Large intra-tooth variations in the oxygen isotopic composition (up to 9‰) were observed in the enamel of herbivores from the Neanderthal fossil site of Payre, consistent with preservation of seasonal cyclicity. The range of isotopic compositions observed in Neanderthal tooth enamel was much smaller (∼3‰), possibly the result of a longer enamel maturation time averaging out variability. An archaeological otolith from a Preceramic site in Northern Peru exhibited marked changes in δ¹⁸O over life, due either to the fish occasionally migrating from the sea to a lower salinity habitat, or to short-lived rises in sea water temperature. A fish otolith from Australia's Willandra Lakes World Heritage Area showed clear seasonal variations, but also a general trend towards isotopically heavier and more saline water, as indicated by higher δ¹⁸O and Sr/Ca values resulting from increased evaporation. The results of these case studies are compared to results of oxygen isotope analysis using more conventional methods and demonstrate the ability of the SHRIMP II to provide precise high spatial resolution in-situ oxygen isotope analyses of a variety of biogenic materials. This approach has major advantages over conventional methods. It can provide rapid, micro-scale isotopic analyses of sub-permil precision without the need for chemical preparation of the sample.     

Investigating foetal and perinatal mortality in prehistoric skeletal samples: a case study from a 3000-year-old Pacific Island cemetery site

The Teouma skeletal sample from Vanuatu represents one of the few truly colonising populations in the Pacific Islands. Therefore, investigating the factors that may have affected foetal/infant mortality in this population is potentially important for understanding the success of settlement in this region of the world. We investigate whether stable isotope analyses of carbon and nitrogen in conjunction with skeletal ageing techniques, can aid in identifying whether the subadults from Teouma died before or after birth in an attempt to understand the potential threats to foetal and infant survival. Multiple skeletal ageing methods using diaphyseal lengths were used to age the young subadults (n = 7). Using regression-based skeletal ageing methods, four of the individuals were aged at around full-term gestation (37–42 weeks gestation), while the remaining three individuals died preterm. The isotope analyses did not assist in identifying the individuals that survived post-birth because none of the subadults displayed the 2–3‰ trophic increase in δ¹⁵N values expected for a breastfed infant, probably as a result of their young age. However, all of the foetal/perinatal individuals exhibited higher δ¹⁵N values in their bone collagen compared with the adult females of the sample, with two of the individuals demonstrating unusually high δ¹⁵N values. The δ¹³C values of the foetuses/perinates did not exhibit the same variation. We explore a number of possible explanations for this elevation of perinatal/foetal δ¹⁵N values and tentatively suggest that this is a result of in utero stress as a consequence of chronic maternal ill-health. The osteological and palaeodemographic evidence supports the assertion that females, foetuses and perinates were susceptible to environmental stress within this colonising population, resulting in early death of the perinatal individuals in addition to early terminations of pregnancy or premature birth possibly caused by infectious and/or metabolic diseases.     

Reconstructing faunal migrations using intra-tooth sampling and strontium and oxygen isotope analyses: a case study of modern caribou (Rangifer tarandus granti)

Isotopic analyses of incrementally developed dental tissues can be used to reveal patterns of movement and diet in animals. However, the suitability of these methods for the reconstruction of herd movements has not yet been demonstrated. Inter-individual behavioural and isotopic variability at the herd scale, and the implications for archaeological and palaeocological applications, can only be demonstrated through the testing of modern animals. In this pilot study, dual-element isotopic profiles were created from incrementally developed dental tissues of five individuals selected from a modern herd of migratory Alaskan caribou (Rangifer tarandus granti). Enamel from second and third molars from the individuals was sequentially sampled in order to reconstruct time-series isotopic profiles. Variation in the strontium (⁸⁷Sr/⁸⁶Sr) and oxygen (δ¹⁸O_CARB) isotope ratios of sequentially sampled enamel were compared to documented herd movement patterns and local geological and environmental conditions. Four individuals displayed the same general trends, although absolute isotopic values varied. One individual displays a very different trend and may represent a behavioural outlier or an immigrant from a semi-domesticated reindeer herd. The implications of this study to herd movement reconstruction in the past are discussed.     

Modern and early-middle Holocene shells of the freshwater mollusc Unio, from Çatalhöyük in the Konya Basin, Turkey: preliminary palaeoclimatic implications from molluscan isotope data

Carbon and oxygen isotope ratios in the shells of the freshwater Unio mollusc yield information on the isotopic composition of the water in which the shell was formed, which in turn relates to climatic conditions prevailing during the bivalves’ life span. Here we analysed shells from one modern Unio, from a modern lake shore in Anatolia, and 4 subfossil Unio shells from Çatalhöyük (dated between 7200 BC and 5000 BC, Neolithic and Chalcolithic periods). Sequential carbon and oxygen isotope analysis along the surface of the shells provides information on seasonal or shorter-term variability of lake waters during the lifetime of the organisms. δ¹⁸O values of the modern shell are consistent with it being collected from a dammed portion of the Seyhan River that does not undergo intensive summer evaporation. This is in contrast to many of the surface water bodies in Anatolia which suffer extensive evaporation in the arid summers. δ¹⁸O values of the subfossil shells from the Çatalhöyük middens indicate that the bivalves came from lakes which evaporated extensively during the summer months but were replenished by high winter rainfall, suggesting that the subfossil shells lived in relatively small lakes or ponds. Stable isotope analysis along the growth of freshwater bivalves is one of the few methods for investigating seasonal water fluctuations in the past. These results may alter current interpretations on the environment of Çatalhöyük during the Neolithic, although more work is needed to confirm these initial findings.     

Endangered species,archaeology, and stable isotopes: huemul (Hippocamelus bisulcus) isotopic ecology in central-western Patagonia (South America)

The management and conservation of animal species should be based upon a long-term analysis that considers its geographical distribution and feeding patterns. The study of carbon and nitrogen stable isotopes on skeletal remains provides a quantitative approach to the paleodietary reconstructions and constitutes a potent tool for comparing behavioral aspects of the fauna. In this paper we present the first set of isotopic values for modern and archaeological samples of Huemul (Hippocamelus bisulcus) from the forests of Central-western Patagonia, Chile. Contrary to initial expectations, our analysis indicates that there is no evidence of an important incidence of the canopy effect on the δ¹³Ccollagen values, which we suggest is due to an ecological selection that drives the huemul to focus its predation on open sectors within the forest. On the other hand, a wide range of δ¹⁵Ncollagen values for huemul was verified. This could reflect the N impoverishment of the forest soils and provide an interesting ecological indicator. The long-term information offered by the archaeological record provides the necessary context for decision-making conducive towards the preservation of Hippocamelus bisulcus in Patagonia.     

Variation in collagen stable nitrogen values in black-tailed jackrabbits (Lepus californicus) in relation to small-scale differences in climate,soil, and topography

Longstanding observations about the relationship between increasing aridity and ¹⁵N enrichment in mammalian collagen values have led to an interest in their use as a paleoclimatic marker. Here we report on variability in collagen nitrogen values from five modern and two archaeological samples of black-tailed jackrabbits (Lepus californicus) from the eastern Great Basin of the United States (N = 178 individuals). Nitrogen δ¹⁵N values were highly variable within all samples. Modern samples showed significant differences despite similarities in average annual precipitation and temperature. Archaeological samples were indistinguishable from each other or from modern samples taken from the same area despite independent evidence for differences in precipitation for the two prehistoric periods considered. Differences between modern samples were most strongly associated with soil characteristics. We discuss these results in light of their relationship with topography and vegetation and highlight their implications for archaeological applications of stable nitrogen analyses in several contexts.     

New evidence for the establishment and management of the European fallow deer (Dama dama dama) in Roman Britain

The semi-domestic status of the European fallow deer (Dama dama dama) renders its ancient biogeography a reflection of human activity with the potential to provide important insights into the movement, trade patterns and ideology of past societies. Given this potential, fallow deer have received surprisingly little attention from scientists within the fields of archaeology, biology and zoology. Here we present new AMS radiocarbon dates, stable carbon and nitrogen isotope data and genetic evidence (the first ancient DNA sequences for the species) resulting from the analysis of a set of remains recovered from the Roman settlement at Monkton on the Isle of Thanet, Kent, UK. By viewing our results against the very limited available comparative data, this paper provides new information for the establishment and management of fallow deer in Britain. We argue that much more could be achieved with even a slight increase in sample sizes and a plea is made for greater research into this culturally significant species.     

Stable carbon- and nitrogen-isotope ratios and electron spin resonance (ESR) g-values of charred bones: changes with heating and a critical evaluation of the utility of g-values for reconstructing thermal history and original isotope ratios

Changes in the stable carbon and nitrogen isotope ratios of modern bone samples heated to a variety of times and temperatures were used to determine the effect of heating on isotope ratios and the retention of organic matter in charred bones. For organic extracts produced by slow demineralization in weak acid, δ¹³C values were unchanged, while δ¹⁵N values increased by up to 5‰ and were primarily determined by heating temperature. Changes in the electron spin resonance (ESR) g-value of whole bone and organic extracts were also measured. For organic extracts from charred bones, the g-value was well-correlated with δ¹⁵N and temperature, suggesting that g-values could be used to estimate the charring temperature and original δ¹⁵N values of charred bones. Thus, g-values from demineralized extracts could be very useful in forensic investigations where it would important to reconstruct the thermal history of burned bones. Isotope ratios and g-values of demineralized extracts from four prehistoric components at three sites that produced cremated human bone were used to test whether the same approach can be applied to archaeological materials. While carbon isotope ratios of the prehistoric samples were similar to those of uncharred specimens, nitrogen isotope ratios were increased and the g-value corrections for nitrogen isotope ratios were not effective.     

A stable isotope method for identifying transatlantic origin of pig (Sus scrofa) remains at French and English fishing stations in Newfoundland

From the 16th century onward, various European nations shared fishing spaces off the coast of Newfoundland in an effort to provide salt-fish products to supplement increasing European demand. Faunal remains excavated at seasonal and permanent Newfoundland fishing stations indicate that pigs were the primary mammal species consumed by cod fishermen. It is not clear whether these pig remains derive from salt pork and/or live pigs imported from Europe or, rather, from pigs bred and raised in Newfoundland. Based on the notion that Newfoundland-raised pigs would have had greater access to marine-derived foods from nearby fisheries compared to their European-raised counterparts, we analyzed stable carbon and nitrogen isotope values from 28 pigs and 117 other faunal specimens from Dos de Cheval (EfAx-09) and Ferryland (CgAf-02) in order to identify individuals with marine or terrestrial isotope dietary signatures. Results indicating two distinct groups of pigs with mean δ¹³C and δ¹⁵N values differing by ∼6‰ and ∼9‰, respectively, suggest differing pig-product origins at each site. This method for identifying the transatlantic origin of pig remains has potential to shed light on patterns in the provisioning of the early European transatlantic fishing industry and inter-community relations. It may also allow for the development of more sophisticated body part representation models for zooarchaeological reconstruction of barreled salt pork use.     

Historical ecology of late Holocene sea otters (Enhydra lutris) from northern British Columbia: isotopic and zooarchaeological perspectives

We examined the isotopic composition (δ¹³C and δ¹⁵N) of sea otter (Enhydra lutris) bone collagen from ten late Holocene (ca. 5200 years BP–AD 1900) archaeological sites in northern British Columbia (BC), Canada. Because sea otters are now extinct from much of this region and have not fully recolonized their former range (e.g. Haida Gwaii and most of northern BC) these data represent an important first step towards better understanding sea otter foraging ecology in BC. The isotopic data suggest a diet composed primarily of benthic invertebrates, with a very low reliance on epibenthic fish. There is very low isotopic and thus inter-individual dietary variability in Holocene BC sea otters during the late Holocene. Furthermore, zooarchaeological abundance data suggest that otters represented a widespread and significant focus of aboriginal hunting practices on the northern BC coast during the mid- to late-Holocene. The consistent reliance on a small number of low-trophic level prey and limited dietary variability in Holocene BC sea otters may reflect top-down impacts on otter populations by aboriginal peoples. As part of our assessment of sea otter diet, we review trophic discrimination factors (Δ¹³C and Δ¹⁵N) for bone collagen from published literature and find marked variability, with mean values of +3.7 ± 1.6 for Δ¹³C (n = 21) and +3.6 ± 1.3 for Δ¹⁵N (n = 15).     

Late Stone Age subsistence in the Tilemsi Valley, Mali: Stable isotope analysis of human and animal remains from the site of Karkarichinkat Nord (KN05) and Karkarichinkat Sud (KS05)

The pathways leading to the adoption of cereal cultivation and pastoralism in West Africa are poorly understood. In order to elucidate the transition to food production during the Late Stone Age in Mali’s Tilemsi Valley samples of ancient and modern human and animal remains were selected for carbon and oxygen isotope analysis. Our results indicate the inhabitants of Karkarichinkat Nord (KN05) consumed considerable quantities (85%) of carbon derived from C₄ plants, either directly in the form of C₄ grasses such as wild Panicum sp. and possibly domestic Pennisetum sp. or indirectly through the consumption of C₄ grazers such as Bos sp. and Ovis sp.     

Stable isotope analysis of dog,fox, and human diets at a Late Holocene Chumash village (CA-SRI-2) on Santa Rosa Island,California

Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analyses of dog (Canis familiaris), island fox (Urocyon littoralis), and human bone collagen from CA-SRI-2 (AD 130–1830) on Santa Rosa Island, California provide a proxy of diet and the relationships between humans and these animals. Carbon isotopic signatures indicate that Native Americans and their dogs at CA-SRI-2 subsisted almost exclusively on marine resources, while the island fox ate primarily terrestrial foods. Nitrogen isotopes and archaeofaunal remains indicate that humans and dogs also ate higher trophic level foods, including finfishes, marine mammals, and seabirds with smaller amounts of shellfish. The CA-SRI-2 island foxes appear to have eaten higher amounts of terrestrial foods, similar to the diets observed in modern fox populations. These data generally confirm the commensal relationship assumed to exist between domesticated dogs and people, but the carbon isotopic composition of dogs is enriched ∼2‰ compared to humans. We hypothesize that the difference in carbon isotopes between dogs and humans may have resulted from a higher consumption of C3 plants with lower δ¹³C values by humans, or less likely from the ingestion by dogs of significant amounts of bone collagen, which is enriched by ∼4‰ over associated muscle.     

Stable isotope investigations of charred barley (Hordeum vulgare) and wheat (Triticum spelta) grains from Danebury Hillfort: implications for palaeodietary reconstructions

Palaeodietary studies typically focus on the analysis of bone collagen due to the limited availability of plant remains. Isotopic analysis of plant remains, however, allow for a more extensive consideration of the contribution of plants to the human diet and can potentially provide information about the environment in which the crops were grown. This paper reports the results of carbon and nitrogen isotope analyses performed on charred barley and wheat grains recovered from pits within Danebury Iron Age hillfort. To the best of our knowledge, this is the first Iron Age site in Britain from which charred grains have been isotopically analysed. Our results suggest that cereals found at the hillfort were grown in several different environmental contexts. The isotope data demonstrate that the herbivores were not consuming a diet primarily based on grains as the δ¹⁵N values of the grains are very similar to those of the herbivores. Palaeodietary investigations typically assume that humans eating plant protein only would have the same δ¹⁵N value as the local herbivores. This assumption is clearly invalid at Danebury, where the humans and animals appear to have consumed either different parts of the same plants or different plants. Researchers typically interpret high differences between human and animal δ¹⁵N values as indicative of diets high in animal protein, however where major plant resources have δ¹⁵N values similar to those of the herbivores our ability to distinguish between plant and animal sources of protein in the diet is limited. Our research has demonstrated that whenever possible it is desirable to measure the isotopic signatures of potential major plant resources in order to understand past subsistence strategies.     

Seasonality and season of birth in early Eneolithic sheep from Cheia (Romania): methodological advances and implications for animal economy

Cheia (early 5th millennium cal BC) is a prehistoric village in the Dobrodgea province in Romania. Its occupation is attributed to the Early Eneolithic period or Hamangia III phase. The exploitation of animal resources is heavily dominated by husbandry. Although cattle are dominant, they were complemented by caprines, mainly sheep, exploited for tender meat (as highlighted by a 6–12 months age class peak in the mortality profile). Sheep reproduction patterns were investigated through stable isotope analyses in order to characterize the annual rhythms of slaughtering for tender meat acquisition while informing a more general picture on sheep demographical management and animal husbandry at the settlement. Results from δ¹⁸O analysis on second and third molar enamel were modeled and compared with modern reference populations. Sheep births took place over less than four months, from late winter to early summer. From this it could be concluded that tender meat could be provided most of the year, excepted over short period in early summer. Finally, comparison of datasets obtained on the M2 and M3 suggests that the M2 presents a more accurate representation of birth seasonality due to lower inter-individual variability in the chronology of tooth growth.     

Dietary patterns during the late prehistoric/historic period in Cikobia island (Fiji): insights from stable isotopes and dental pathologies

The dietary patterns of eight adults and one child interred in the Korotuku burial mound (Cikobia, northern Fiji), dated from the late prehistoric/historic period (around AD 1850), have been investigated using carbon and nitrogen stable isotope signatures and dental and oral pathologies. Bone collagen isotopic compositions showed that (1) males and females had similar diets in terms of terrestrial C₃ resources and marine fish components and proportions; (2) the proportion of marine fish comprised around 25% of the protein fraction of the diet, and the proportion of vegetal food was high; and (3) one canid that was also studied had a different diet, probably rich in shellfish. Oral and dental examinations of the humans evidenced (1) a low level of dental macrowear, (2) a high rate of carious lesions—interproximal and cervical—of about 15%, and (3) a low amount of abscessing, as well as the presence of alveolar bone recession and calculus in most of the adults. This evidence suggests a diet relying mainly on vegetal food with very limited shellfish consumption. When viewed in the light of ethnohistoric information, this pattern suggests a particular sociocultural behaviour, including food selection, since the analyses of the canid remains indicate that more diversified food resources were available than those consumed by the humans. Overall observations suggest that the deceased in the Korotuku burial mound might be members of an elite living on the island, thus pointing to possible social stratification in the late prehistoric/historic Cikobia community.     

Paleodietary analysis of a San Francisco Bay Area shellmound: stable carbon and nitrogen isotope analysis of late Holocene humans from the Ellis Landing site (CA-CCO-295)

The late Holocene archaeofaunal record of the San Francisco Bay Area demonstrates temporal declines in the abundance of low-cost, high-ranked marine and terrestrial resources. During later periods of occupation, faunal assemblages are often dominated by lower-ranked, higher-cost resources, suggesting an increase in diet breadth through time. Archaeological resource intensification models argue that this marks a late Holocene decline in foraging efficiency in the Bay Area, driven by human-induced harvest pressure. This study examines dietary change in the region using stable carbon and nitrogen isotope data from 65 human burials, spanning two temporal components (cal AD 55–890 and cal AD 762–1550) at the Ellis Landing site (CA-CCO-295). The strong linear relationship between collagen carbon and nitrogen isotope values reflects both marine and terrestrial food consumption, with individuals showing a high level of dietary variability at the site. No temporal trend or meaningful sex differences were found in isotope values. The widening of diet breadth predicted by the archaeofaunal record occurred prior to the occupation of the Ellis Landing shellmound site. The trend in exploiting a greater amount of terrestrial resources during the Middle and Late Period in central California was supported by isotopic evidence.     

Fishing ancient Lake Baikal,Siberia: inferences from the reconstruction of harvested perch (Perca fluviatilis) size

Fishing was the foundation for many of the world's foraging peoples and was undertaken using a variety of technologies. Reconstructing fishing technologies can be difficult because these tools were often made of perishable materials. Here we explore fishing technologies employed at the Ityrkhei site on Lake Baikal, Siberia. Specifically, we employ regression analyses to reconstruct the sizes of perch (Perca fluviatilis) captured through time at the site. Our analyses demonstrate that almost no juvenile perch were taken, suggesting some selectivity in harvest. We suggest this selectivity is most consistent with the use of relatively large gauge nets or traps. Such mass harvesting technologies may have been important elements of the subsistence economies of Lake Baikal's foraging peoples throughout much of the Holocene.     

Inferring intermediate‐scale fluid flow in a heterogeneous metasedimentary multilayer sequence during progressive deformation: evidence from the Monts d’Arrée slate belt (Brittany,France)

Quartz veins in the early Variscan Monts d’Arrée slate belt (Central Armorican Terrane, Western France), have been used to determine fluid‐flow characteristics. A combination of a detailed structural analysis, fluid inclusion microthermometry and stable isotope analyses provides insights in the scale of fluid flow and the water–rock interactions. This research suggests that fluids were expelled during progressive deformation and underwent an evolution in fluid chemistry because of changing redox conditions. Seven quartz‐vein generations were identified in the metasedimentary multilayer sequence of the Upper Silurian to Lower Devonian Plougastel Formation, and placed within the time frame of the deformation history. Fluid inclusion data of primary inclusions in syn‐ to post‐tectonic vein generations indicate a gradual increase in methane content of the aqueous–gaseous H₂O–CO₂–NaCl–CH₄–N₂ fluid during similar P–T conditions (350–400°C and 2–3.5 kbar). The heterogeneous centimetre‐ to metre‐scale multilayer sequence of quartzites and phyllites has a range of oxygen‐isotope values (8.0–14.1‰ Vienna Standard Mean Ocean Water), which is comparable with the range in the crosscutting quartz veins (10.5–14.7‰ V‐SMOW). Significant differences between oxygen‐isotope values of veins and adjacent host rock (Δ = ?2.8‰ to +4.9‰ V‐SMOW) suggest an absence of host‐rock buffering on a centimetre scale, but based on the similar range of isotope values in the Plougastel Formation, an intraformational buffering and an intermediate‐scale fluid‐flow system could be inferred. The abundance of veins, their well‐distributed and isolated occurrence, and their direct relationship with the progressive deformation suggests that the intermediate‐scale fluid‐flow system primarily occurred in a dynamically generated network of temporarily open fractures.     

Guanaco (Lama guanicoe) isotopic ecology in southern South America: spatial and temporal tendencies, and archaeological implications

We synthesize and discuss available isotopic data on Holocene guanaco samples from southern South America, extending from Tierra del Fuego Island to northern Patagonia and the Pampean region. We evaluate temporal and spatial tendencies on the basis of 91 samples (mainly based on δ¹³C_collagen values), presenting implications for paleodietary research in archaeology. We conclude that there are no strong correlations with latitude in the macro-spatial scale, while there is a set of interesting patterns at smaller regional scales. These patterns lead us to evaluate the role of ecologic and topographic variables (e.g. canopy effect, variations in altitude, ecotones) in structuring isotopic variability. Finally, on the basis of the present analysis we suggest a number of hypotheses and perspectives for the use of stable isotopes as geographic tracers of guanaco distribution in the past, and specific implications for the paleodietary study of human samples.     

Shell-gathering from mangroves and the seasonality of the Southeast Asian Monsoon using high-resolution stable isotopic analysis of the tropical estuarine bivalve (Geloina erosa) from the Great Cave of Niah, Sarawak: methods and reconnaissance of molluscs of early Holocene and modern times

This paper describes a reconnaissance, high-resolution, study of stable isotopes (δ¹⁸O and δ¹³C) from modern shells of the estuarine bivalve Geloina erosa, and those dated to the early Holocene that were harvested by people from mangroves near the Great Cave of Niah on the north coast of Borneo. This reconnaissance study provides high-resolution palaeoclimatic-palaeohydrological information concerning early human activity in the region and the past character of the Southeast Asian Monsoon. Laser ablation continuous flow isotope ratio mass spectrometry (LA-CF-IRMS) on modern shells of Geloina erosa revealed ‘saw-tooth’ stable isotopic profiles that bear a close resemblance to peaks and troughs of trends in recent local rainfall, including the 1998 El Niño drought, highlighting the potential of Geloina erosa for reconstructing seasonality of the Southeast Asian Palaeomonsoon. LA-CF-IRMS analysis of prehistoric shells of Geloina erosa held in the Harrisson Archives of the Sarawak Museum revealed cyclical shifts in δ¹⁸O with similar amplitudes of variation as found in modern shells of Geloina erosa. As at the present day, this probably reflects the changing seasonality of the monsoon rains with shifts to negative δ¹⁸O values during periods of high runoff. Lighter mean δ¹⁸O values of the prehistoric shells, however, may indicate a greater annual surplus of rainfall and possibly consistent with the early Holocene strengthening of the summer monsoon at that time. The similarity of the last growth increment δ¹⁸O of the prehistoric shells to their mean δ¹⁸O profile values suggests that gathering took place during times of moderate runoff.