Oxygen isotope seasonality in a temperate estuarine shell midden: a case study from CA-ALA-17 on the San Francisco Bay,California

Seasonality determination using stable oxygen isotope (δ¹⁸O) analyses in archaeological mollusk shell has been largely limited to aquatic settings where one of the two factors that control shell δ¹⁸O – water δ¹⁸O (or salinity) and temperature – is assumed to be constant. Open coastal marine environments reflect the former situation, and tropical estuaries constitute the latter. In an effort to expand stable isotope seasonality to an ecological setting where neither variable remains constant, we present a model of annual shell δ¹⁸O cycle of aragonite deposition derived from instrumental data on salinity and temperature from San Francisco Bay, California. The predicted range of modeled shell δ¹⁸O is consistent with observed δ¹⁸O values in prehistoric and modern shells when local conditions are considered. Measurements of δ¹⁸O taken at 0 mm and 2 mm from the terminal growth margin were made on 36 archaeological specimens of Macoma nasuta from a late Holocene hunter-gatherer site CA-ALA-17, and season of collection was inferred using the shell δ¹⁸O model. We conclude that shellfish exploitation occurred through the year with the exception of fall, which may indicate scheduling conflicts with acorn harvesting or other seasonally abundant resources elsewhere. The model supports the feasibility of stable isotope seasonality studies in temperate estuaries, provided that instrumental records are available to quantify the relevant water conditions at appropriate spatial and temporal scales.     

Oxygen isotope analysis of California mussel shells: seasonality and human sedentism at an 8,200-year-old shell midden on Santa Rosa Island,California

To evaluate coastal settlement and land use strategies among maritime hunter-gatherers, we analyzed oxygen isotope (δ¹⁸O) data from 131 marine carbonate samples from 21 California mussel (Mytilus californianus) shells obtained from a large ～8,200-year-old shell midden (CA-SRI-666) on California’s Santa Rosa Island. Seasonal distributions of the isotopic data were assigned using a paleo-sea surface temperature model created by comparing modern sea surface temperatures (SST) to a fully profiled ～8,200-year-old shell. For 20 additional shells, we used two sampling strategies to compare season-of-harvest inferences and explore whether the Early Holocene site occupants were sedentary. Estimated season-of-harvest differed by 35 % between the two sampling methods, corroborating recent isotope analysis of an 8,800-year-old shell midden on San Miguel Island. Shellfish appear to have been collected year-round at CA-SRI-666 from intertidal or subtidal water temperatures similar to modern SST in the vicinity of eastern Santa Rosa Island. The isotope results are consistent with other evidence from CA-SRI-666 that suggest that the site served as a residential base for relatively sedentary maritime people.     

Oxygen isotopic composition of limpet shells from the Beagle Channel: implications for seasonal studies in shell middens of Tierra del Fuego

Shell middens distributed along the coastal areas of the Beagle Channel (Tierra del Fuego) testify the primary role of marine resources in the successful social dynamic of hunter–gatherer–fisher societies (HGF) in extreme high latitude environmental conditions. Intertidal and subtidal limpet species are frequently identified taxa of these archaeological deposits. We investigated whether shell δ¹⁸O of the limpet species Nacella deaurata is a valuable record of local sea surface temperature (SST) and ambient water δ¹⁸O (δ¹⁸Ow) fluctuations, and in turn can be used to estimate the season of its exploitation by Holocene HGF. Sequential shell δ¹⁸O of modern species track seasonal changes of SST and δ¹⁸Ow. The comparison of shell δ¹⁸O profiles of N. deaurata and Nacella magellanica (a coexisting species) suggests the occurrence of distinct growth rate and physiological adaptations between species. Shell δ¹⁸O of zooarchaeological specimens of N. deaurata from Lanashuaia II suggests that animals were exploited predominantly in winter and that experienced similar environmental conditions of present day at ∼1320 years BP. Results also indicate that N. deaurata is a valuable candidate for studying past seasonal changes in SST and δ¹⁸Ow in this southernmost part of South America.     

MARINE RESOURCES IN THE MESOLITHIC AND NEOLITHIC AT THE GROTTA DELL’UZZO (SICILY): EVIDENCE FROM ISOTOPE ANALYSES OF MARINE SHELLS*

Oxygen isotopes in shell carbonate samples from the marine rocky‐shore intertidal gastropod Monodonta turbinata (Born) are investigated in both modern analogue specimens and in archaeological specimens from the Grotta dell’Uzzo (Sicily). Variations in shell edge values of δ¹⁸O in living specimens collected monthly over two years are closely correlated with monthly seawater temperatures measured at the time of collection, showing that the species can be used for palaeoseasonality studies. Analyses of shell edge δ¹⁸O values in archaeological specimens, from Mesolithic through to early Neolithic phases at the Grotta dell’Uzzo, enabled the inference of various seasons of collection of shellfish and how such seasonality varies between the different phases of occupation. Interesting similarities and differences exist between the seasons of marine shellfish exploitation and the seasons inferred from the vertebrate zooarchaeological assemblages. A major inference drawn from the analyses and discussion is that the exploitation of all marine resources (fish and shellfish) increased in the later Mesolithic and early Neolithic periods.     

δ18O Analyses of the Humpbacked Conch (Gibberulus gibberulus): Evaluating a Proxy for Reconstructing Sea-Surface Temperature at Chelechol ra Orrak,Palau

ABSTRACT

In Palau, Micronesia, marine resources, particularly shellfish, played a vital role in human subsistence for millennia. Despite the vast array of molluscan species in archaeological assemblages, there is a dearth of data on nearshore palaeoecology or prehistoric shellfish foraging practices. In this study, we analysed stable oxygen isotopes (δ¹⁸O) values present in the calcium carbonate shell of Gibberulus gibberulus from the Chelechol ra Orrak archaeological site to reconstruct average nearshore sea-surface temperatures (SST) from approximately 1500–1100 cal BP. Modern shellfish samples and environmental data were collected from intertidal zones near the site and x-ray diffraction (XRD) was employed to identify the biomineralogical composition of G. gibberulus. These steps provide necessary information for the selection of the proper oxygen isotope-to-SST conversion formula. The selected formula was applied to δ¹⁸O samples from archaeological shells to reconstruct prehistoric SST averages. The results of this proxy validation study verify that G. gibberulus accurately records ambient SST and can be used to reconstruct ancient nearshore conditions. These findings also contribute to the establishment of an environmental baseline, which can be used to examine how environmental changes may have influenced the availability of molluscan taxa that in turn influenced human subsistence practices through time.     

Analysis of the Size,Shape, and Modeled Age of Common Limpets (Patella vulgata) from Late Norse Middens at Sandwick,Unst, Shetland Islands,UK: Evidence for Anthropogenic and Climatic Impacts

Zooarchaeological faunal remains are commonly examined to investigate harvesting behavior. We determined limpet (Patella vulgata) shell size and shape, and estimated shell age from several middens at the Late Norse Sandwick South Site, Unst, Shetland, UK, whose strata represent distinct occupational phases (Phase 1: AD 1100–1200, Phase 2: AD 1200–1250, Phase 3: AD 1250–1350). Our goal was to determine if the many limpets found there could provide insight into Norse harvesting behavior. Shell length, conicity, and modeled age all declined between Phases 1 and 2, suggesting intensive, size-selective harvesting of limpets and a shift to harvesting lower in the intertidal zone between phases. Length and conicity varied in Phases 2 and 3 and no major changes seem to have occurred over these periods, indicating that harvesting maintained the limpet population at an impacted level throughout the later phases. The conicity decline between Phases 1 and 2 may also have been caused by increased storminess that accompanied the onset of the Little Ice Age. The mean length of modern limpet populations near the Norse site did not differ from the archaeological phases, but did vary among collection years. Limpets were 26% larger in 2015 than in 2012 and 2013, indicating that large interannual variations in population structure can occur over short time periods. Potentially the result of extreme storms removing small limpets, this result raises the possibility that size and conicity changes during the Sandwick South Site occupation, as well as in other early populations, could also be the result of environmental factors rather than human harvesting alone. We feel, however, that the most parsimonious explanation for the patterns we document is human harvesting.     

REFINING ESTIMATES FOR THE SEASON OF SHELLFISH COLLECTION ON THE PACIFIC NORTHWEST COAST: APPLYING HIGH‐RESOLUTION STABLE OXYGEN ISOTOPE ANALYSIS AND SCLEROCHRONOLOGY

Stable oxygen isotopes from estuarine bivalve carbonate from Saxidomus gigantea were analysed combined with high‐resolution sclerochronology from modern and archaeological shells from British Columbia, Canada, to determine the seasonality of shellfish collection from the archaeological site of Namu. The combination of high‐resolution sclerochronology and a micro‐milled sampling strategy for δ¹⁸O analysis permits a precise estimate of archaeological seasonality, because seasonal freshwater influxes and changes in temperature have dual effects on the δ¹⁸O value of the shell. Sclerochronological analysis identifies the timing and duration of growth that is temporally aligned to stable oxygen isotope results, since δ¹⁸O_shell appears to be strongly influenced by seasonal inputs of very low δ¹⁸O snowmelt‐water from adjacent coastal mountain ranges. The results show that shellfish were collected year‐round at this site over a 4000‐year period, and these data combined with other zooarchaeological lines of evidence support the interpretation of year‐round occupation.     

10,000 years of human predation and size changes in the owl limpet (Lottia gigantea) on San Miguel Island,California

To investigate potential human impacts on California Channel Island shellfish stocks over the past 10,000 years, we measured 1718 owl limpet shells from 19 archaeological components on San Miguel Island. Inhabiting the middle intertidal zone of Pacific Coast rocky shores, owl limpets are slow-growing mollusks that can live for 30 years and reach lengths of over 100 mm. Recent ecological studies indicate that owl limpets are highly susceptible to heavy human predation, which results in reduced mean shell sizes among Lottia populations along the coasts of Alta and Baja California. On San Miguel Island, we identified a significant trans-Holocene reduction in mean owl limpet size, beginning more than 6500 years ago and accelerating at least 4400 years ago. Fluctuations in limpet size show no clear correlation with changes in sea surface temperature and marine productivity, suggesting that increased predation by growing human populations was primarily responsible for the reduction in Lottia size over time. Our results provide further evidence that prehistoric peoples significantly influenced nearshore fisheries long before the development of the commercial and industrialized fisheries of historic times.     

Refit and Oxygen Isotope Analysis of Freshwater Mussel Shells from the Tillar Farms Site (3DR30), Southeast Arkansas

ABSTRACT

We investigate the link between the consumption of foodstuffs, excavation of a large pit, and disposal of waste at the Tillar Farms site (3DR30), southeast Arkansas, using refit and oxygen isotope analyses of well-preserved freshwater mussel shells from Feature 1. Only 0.13% of 7,408 valves analyzed were unidentifiable to species.The refit analysis produced 460 refits across 23 species and strongly indicates that the shell midden represents a single episode of shellfish gathering, consumption, and discard. Oxygen isotope analysis of five randomly selected shells are used as a test of the refit results. δ¹⁸O values from the five archaeological shells are compared to modern control samples of live-collected specimens from Bayou Bartholomew in winter of 2011. Refit analysis suggests the accumulation of mussel shells occurred quickly, likely as a result of one collection, consumption, and discard event. δ¹⁸O values suggest this activity took place during a single winter season.     

Exploring Accumulation Rates of Shell Deposits Through Seasonality Data

Shell middens are often analysed as the result of short- or long-term depositional activities. In order to confidently interpret such deposits, it is necessary to have accurate estimations of shell accumulation rates, most commonly produced by radiocarbon dates. This paper introduces the application of seasonality data as a temporal measurement of short-term shell deposition. This gives access to an additional estimate of shell accumulation rates, which work on a shorter timescale than can be analysed through radiocarbon dating. We focus on shell deposits on the Farasan Islands, Saudi Arabia, which comprise over 3000 shell midden sites dating to the mid-Holocene (6500–4500 calBP). One site (JW1727) was chosen to (1) explore the potential of seasonality data to reconstruct accumulation rates, (2) analyse the intensity of exploitation and (3) assess the visibility of short-term shellfish deposits. Stable oxygen isotope values (δ¹⁸O) were obtained from the marine gastropod Conomurex fasciatus (Born 1778), representing 72 % of the shell weight of JW1727, to reconstruct season of capture. Seasonality data was grouped by their spatial distribution, which allowed successive episodes of deposition within a stratigraphic sequence to be connected. This allowed us to make an estimation of exploited shell meat of ～200 kg over a 7-month period (～400 shells/day). We argue that excavation methods and low resolution stratigraphic data cause imprecision in the seasonality data and the low visibility of rapidly accumulated shell deposits. Also, an increase of analysed shells per layer is key to understanding the seasonal brickwork of more middens in the future.     

Ground-Penetrating Radar as a Cultural Resource Management Tool for Assessment of Eroding Shell Middens

ABSTRACT

With an increasing rate of sea level rise inherently linked to climate change, numerous coastal archaeological resources are under threat. In Maine, virtually all 2000+ coastal aboriginal shell middens are eroding. Given limited time and resources, an efficient method for making informed cultural resource management decisions is critical. Herein, we describe a six-step protocol for a minimally invasive and cost-effective ground-penetrating radar delineation of shell midden size and morphology. Benefits of the survey methodology outlined in this study include the efficiency of data collection, the non-destructive aspect of the technique, continuous records of site stratigraphy, and the lower cost as compared to large-scale excavation. Two case studies are presented to illustrate the advantages and challenges associated with this method.     

Oxygen isotopes in cellulose identify source water for archaeological maize in the American Southwest

Maize (Zea mays) was a primary food crop for aboriginal societies of the arid American Southwest. Water used for maize production in these arid zones could have come from precipitation and runoff during the summer monsoon, from perennial streams and springs, or from stored soil water fed by snowmelt. The oxygen stable isotope ratio (δ¹⁸O) of summer and winter precipitation on the Colorado Plateau naturally differ by more than 10‰ providing a powerful tool for distinguishing winter- from summer-derived water sources used in cultivation of maize. We investigated whether variation in δ¹⁸O of potential source waters is preserved in the δ¹⁸O of cellulose (δ¹⁸O_cellulose) of maize cobs by growing four aboriginal and one modern maize variety in pots irrigated with water of known δ¹⁸O composition. The δ¹⁸O_cellulose values of cobs ranged from 26.8 to 36.4‰ (averaged within varieties) and were highly correlated with δ¹⁸O values of the source irrigation water (−15.8 to −8.2‰). Cob δ¹⁸O_cellulose from five archaeological sites on the Colorado Plateau in southeastern Utah ranged from 27.3 to 34.6‰, closely matching the range of values observed in experimental plants. A δ¹⁸O_cellulose model developed originally for tree rings was parameterized and applied to the archaeological maize cobs. The model indicated that monsoonal precipitation accounted for 0–20% of the moisture for archaeological cob samples from a site adjacent to a perennial stream and 43–98% for samples from an upland site, more distant from a perennial water source. These results reveal the potential for using δ¹⁸O_cellulose to investigate prehistoric irrigation practices and source water used for maize production in the American Southwest.     

Stability in the South Pacific surface marine C reservoir over the last 750 years. Evidence from American Samoa, the southern Cook Islands and the Marquesas

Although minor climatic and sea-level changes have been documented for the South Pacific during the late Holocene, our understanding of the consequent impact of these changes on the marine ¹⁴C reservoir, and therefore the ¹⁴C content of shellfish, is limited. Ultimately, this has implications for documenting the chronology of human movement and adaptation in this region. In this paper we compare marine reservoir (ΔR) data obtained from tightly controlled archaeological proveniences with known-age, pre-AD 1950 shells from the southern Cook Islands, American Samoa, and Marquesas Islands. Results indicate that there has been no significant change in the near-shore marine reservoir in these three locations over the last ca. 750 years. Furthermore, known-age, pre-AD 1950 shell samples provide more precise ΔR values for use in sample calibration than archaeological paired shell/charcoal samples. This is attributed in part to the limitations of assigning provenance and age to material from archaeological sites. On the basis of these results we conclude that the known-age, pre-AD 1950 shell derived ΔR values can be used to calibrate shell ¹⁴C results from deposits of late Holocene age.     

Deposition of annual growth lines in the apex of the common limpet (Patella vulgata) from Shetland Islands,UK and Norway: Evidence from field marking and shell mineral content of annual line deposition

The abundance, accessibility and value of limpets as a source of food and bait for coastal peoples have resulted in their high frequency in shell middens worldwide. The limpet Patella vulgata is found in middens from the Mediterranean to Norway, and morphometric and sclerochronological analyses of its shell can provide insight into harvesting patterns and paleoenvironmental variables valuable in reconstructing climate. Previous work with P. vulgata has relied on lines on the exterior of the shell, or on lines exiting on the shell surface in shell cross-section, as annual or sub-annual markers. Shell damage may compromise these lines and limit the use of some shells, but growth lines are also found in the better-preserved shell apex. We investigated whether the growth lines in the apex of P. vulgata from two locations in Northern Europe are annual using calcein-marking and recapture. Investigations were performed at one site in the Shetland Islands (UK) and at one site at the northern limit of P. vulgata's range in Northern Norway. We also used laser ablation to measure the concentration of minerals in the shells of two individuals from Shetland to determine if patterns of minerals suggested as bioproxies for temperature and productivity varied annually. All individuals deposited one growth line in the apex during their year in the field, and the lowest ratios of Sr/Ca and Ba/Ca and to a lesser extent Mg/Ca were coincident with annual lines on the shells from Shetland. Growth at both sites was modelled using the von Bertalanffy growth function, and apex growth was nearly five times faster in Shetland than in Norway, probably a result of differences in temperature between the two locations.     

From the Piedmont to the Coast: LA-ICP-MS 87Sr/86Sr evidence for short-term,long-distance mobility in the American Southeast

Mounting evidence suggests that the Archaic Southeast shell mound builders had large-scale trade networks and engaged in social aggregations. Here, incremental ⁸⁷Sr/⁸⁶Sr values were measured by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in third molar enamel samples of 11 individuals interred in the Middle Archaic Harris Creek shell burial mound in St. John's River Valley (SJRV), Florida. Results reveal that SJRV residents engaged in short-term, long-distance mobility up to the Piedmont margin and excursions into coastal areas, consistent with direct trading and social gatherings. Two individuals are interpreted as migrants from central Tennessee, suggesting a link to the Ohio River Valley shell mound builders.     

Inter-site variability in the season of shellfish collection on the central coast of British Columbia

High-resolution stable oxygen isotope analysis of the bivalve Saxidomus gigantea from shell midden sites was applied to identify seasonal patterns of resource procurement on the central coast of British Columbia, Canada. A total of 90 archaeological shells were examined from eight distinct sites spanning a 4500-year period. Combining micro-growth pattern analysis with high-resolution stable oxygen isotope sampling allows for a precise season of collection to be determined in estuarine bivalves recovered from archaeological sites. The results of the stable oxygen isotope analysis provide insights into seasonally structured harvest of S. gigantea (butter clam), which is associated with different types of sites. The results show a variety of patterns, including multi-seasonal collection, intensive seasonal harvesting and casual, supplemental use of butter clams at different locations.     

High-resolution sclerochronological analysis of the bivalve mollusk Saxidomus gigantea from Alaska and British Columbia: techniques for revealing environmental archives and archaeological seasonality

The butter clam, Saxidomus gigantea, is one of the most commonly recovered bivalves from archaeological shell middens on the Pacific Coast of North America. This study presents the results of the sclerochronology of modern specimens of S. gigantea, collected monthly from Pender Island (British Columbia), and additional modern specimens from the Dundas Islands (BC) and Mink and Little Takli Islands (Alaska). The methods presented can be used as a template to interpret local environmental conditions and increase the precision of seasonality estimates in shellfish using sclerochronology and oxygen isotope analysis. This method can also identify, with a high degree of accuracy, the date of shell collection to the nearest fortnightly cycle, the time of day the shell was collected and the approximate tidal elevation (i.e., approx. water depth and distance from the shoreline) from which the shell was collected.Life-history traits of S. gigantea were analyzed to understand the timing of growth line formation, the duration of the growing season, the growth rate, and the reliability of annual increments. We also examine the influence of the tidal regime and freshwater mixing in estuarine locations and how these variables can affect both incremental structures and oxygen isotope values. The results of the sclerochronological analysis show that there is a latitudinal trend in shell growth that needs to be considered when using shells for seasonality studies.Oxygen isotope analysis reveals clear annual cycles with the most positive values corresponding to the annual winter growth lines, and the most negative values corresponding to high temperatures during the summer. Intra-annual increment widths demonstrate clear seasonal oscillations with broadest increments in summer and very narrow increments or no growth during the winter months. This study provides new insights into the biology, geochemistry and seasonal growth of S. gigantea, which are crucial for paleoclimate reconstructions and interpreting seasonality patterns of past human collection.     

Late-Holocene Dungeness crab (Cancer magister) harvest at an Oregon coast estuary

Coastal peoples worldwide harvested and consumed a wide variety of shellfish. Most archaeological analyses of shellfish remains tend to focus on bivalves such as clams and mussels while other shellfish such as gastropods, barnacles, and crabs have received much less attention. Here we examine the use of Dungeness crabs (Cancer magister) at a late-Holocene village on Netarts Bay, northern Oregon Coast. Ethnographic and ethnohistoric records suggest that crabs were both individually hunted as well as gathered, often en masse. We employ allometric scaling of Dungeness claws (propal fingers) recovered from several household middens to estimate crab body size and age. These data indicate that while a wide age range of crabs were collected, most harvesting efforts focused on juveniles and young adults. This suggests that most Dungeness crabs at the site were gathered (not individually hunted) in Netarts Bay, most likely in shallow subtidal areas where cockles (Clinocardium nuttalli) were also being regularly taken. As such, Dungeness crabs were part of a foraging strategy that involved the efficient mass harvest of small prey using minimal technology.     

Prehistoric Shellfish Exploitation in Coastal New York

Abstract

Sectioning studies of marine bivalves are broadening our understanding of prehistoric shellfish exploitation along the Atlantic coast of North America. We review the evidence for the timing of shellfish exploitation in coastal New York and present the results of a recent sectioning study of Mercenaria mercenaria from the Sungic Midden site on Shelter Island, New York. The results compare favorably with the harvesting patterns of other north Atlantic Sites, indicating that shellfish exploitation took place during more than one season of the year. The implications of these findings are examined in relation to current subsistence-settlement models of coastal New York.     

Comparison of mangrove oyster shells lengths harvested in Cuba during the pre-Columbian era and the present day

Abstract

The Mangrove oyster [Crassostrea rhizophorae (Guilding, 1828)] was intensely harvested in the pre-Columbian era by an aboriginal group of hunters and gatherers known as the Ciboney who lived exclusively in Western Cuba. During a comprehensive study of mangrove oyster fishing areas it was of interest to compare the population characteristics under contemporary exploitation with those in pre-Columbian times and for that purpose two samples were used: one from a Ciboney midden in an archeological site 3 km inland from the Jaruco River mouth and another from a commercially exploited population in the ecologically similar Tacajó River. The samples were studied with a ‘shell classifier’ of original design resulting in a mean length of 51.8 ± 2.5 mm for the midden shells and 51.0 ± 3.1 mm for the contemporary sample. Both values were not statistically different for any P > 0.10 meaning that the two populations they came from had similar characteristics. It was also concluded that the Ciboney's manual harvesting exerted less negative anthropogenic pressure on the mangrove oysters’ populations than the contemporary commercial practices based on the use of metal instruments to gather the oysters along with the mangrove aerial roots they are attached to. This latter conclusion is based on the fact that oyster shells 41–80 mm long from the midden are 12% more abundant than in the modern sample, in spite of 500 years of weathering.