Oxygen isotopes from limpet shells: Implications for palaeothermometry and seasonal shellfish foraging studies in the Mediterranean (original) (raw)
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Geochemistry, Geophysics, Geosystems, 2007
1] Climate archives contained in shells of the European limpet, Patella vulgata, accumulated in archaeological deposits can potentially provide much needed information about Holocene environmental change in midlatitude coastal areas. Before reconstructing climate information preserved in these zooarchaeological records, we studied the controls on oxygen and carbon isotope ratios (d 18 O and d 13 C, respectively) in modern specimens. We tested the hypothesis that P. vulgata precipitates its shell in isotopic equilibrium with the ambient water by comparing d 18 O SHELL with predicted values. Predicted d 18 O SHELL was constructed using observed sea surface temperature (SST) records and the equilibrium fractionation equation for calcite and water. We assumed a constant d 18 O WATER value of +0.10% (VSMOW) based on published regional measurements. Comparison of d 18 O SHELL with predicted values revealed that d 18 O SHELL values were higher than expected by +1.01 ± 0.21%. Consequently, estimated SST calculated from d 18 O SHELL was 4.2 ± 2.3°C lower than observed SST. However, because of the relatively uniform offset between observed and expected d 18 O, an adjustment can be made to account for this predictable vital effect. Thus past climate can be reliably reconstructed using this temperature proxy once the offset is taken into account. d 13 C values have a similar cyclicity to the d 18 O variation and therefore vary seasonally. However, d 13 C is slightly out of phase relative to d 18 O. An overall negative shift in d 13 C SHELL over the lifetime of the individual indicates a vital effect associated with ontogeny. Further study of environmental and ecological factors that influence shell d 13 C is required to evaluate fully the potential of carbon isotope ratios as a useful environmental proxy.
PALAIOS, 2013
In the western North Atlantic and Gulf of Mexico, isotope sclerochronology and field studies of the hard clams, Mercenaria mercenaria and M. campechiensis, collected across a latitudinal gradient show patterns in the seasonal timing of slow versus fast shell incremental growth. It is unknown, however, if similar patterns exist in the eastern North Atlantic. The European limpet, Patella vulgata, is abundant in rocky shore communities and archaeological deposits along the eastern North Atlantic. As such, it is a potentially valuable archive for paleoclimate and archaeological research. We used isotope sclerochronology to identify the seasonal timing of annual growth line formation in shells from the cold-and warm-temperate zones and at the boundary between these zones. Four shells from the cold-temperate zone (United Kingdom and Norway), five shells from the warm-temperate zone (Spain), and six shells from the boundary (near the English Channel) were analyzed. The isotopic records represent between two and eight years of growth. Cold-temperate shells formed annual lines in winter, and warm-temperate shells produced annual lines in summer. A mixed pattern was found at the boundary. This pattern in the seasonal timing of slowed growth across a latitudinal gradient is similar to that shown in studies of Mercenaria in the western North Atlantic. Thermal tolerance is the most likely mechanism for the observed changes in the timing of annual growth line formation with latitude.
Journal of …, 2012
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.► Limpet species are frequent components of shell midden sites in the Beagle Channel. ► Shell δ18O of N. deaurata and N. magellanica track changes in SST and seawater δ18O. ► Distinct growth rate and physiology seems to be involved in shell δ18O profiles. ► Limpet exploitation at Lanashuaia II (∼1320 years ago) occurred mainly in winter. ► New evidence of limpet shell δ18O as proxy for seasonal SST oscillations.
Earth and Planetary …, 2011
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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 hunteregathererefisher 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 d 18 O of the limpet species Nacella deaurata is a valuable record of local sea surface temperature (SST) and ambient water d 18 O (d 18 Ow) fluctuations, and in turn can be used to estimate the season of its exploitation by Holocene HGF. Sequential shell d 18 O of modern species track seasonal changes of SST and d 18 Ow. The comparison of shell d 18 O profiles of N. deaurata and Nacella magellanica (a coexisting species) suggests the occurrence of distinct growth rate and physiological adaptations between species. Shell d 18 O 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 w1320 years BP. Results also indicate that N. deaurata is a valuable candidate for studying past seasonal changes in SST and d 18 Ow in this southernmost part of South America.
Chemical Geology, 345: 77-86.
The marine topshell, Phorcus (Osilinus) turbinatus, is a common component of many archaeological sites in the Mediterranean. This species has been successfully used as a palaeoclimate proxy in Italy. To test whether δ18O from P. turbinatus shells can serve as a reliable palaeoclimate archive for other regions of the Mediterranean, we collected live P. turbinatus from the northeast coast of Malta each month for a year. The δ18OSHELL values of the outermost growth increments of these live-collected shells ranged between -0.4 and +2.4 ‰. These values correspond to growing temperatures calculated from shell edge δ18O of between 15°C to 27°C. Calculated shell edge sea surface temperatures are highly correlated with instrumental records of sea surface temperature recorded over the period of collection. The individuals analysed for this study are smaller than P. turbinatus from populations studied elsewhere in the Mediterranean. Nonetheless, δ18OSHELL provides a robust record of sea surface temperatures, suggesting that smaller/younger shells in archaeological deposits can still provide reliable palaeothermometry records. This study extends the upper growth limit P. turbinatus by 2°C compared with previous studies of P. turbinatus in the Mediterranean and suggests that, contrary to previous studies, growth shutdown does not occur in all P. turbinatus when sea surface temperatures exceed 25°C. This may reflect the higher sample resolution that can be obtained from smaller/faster growing shells, or it may reflect actual higher growth tolerances of P. turbinatus populations in Malta. By showing that P. turbinatus precipitate their shells in δ18O equilibrium with surrounding sea water, this study reinforces the potential for the stable isotope chemistry of P. turbinatus shells preserved in Mediterranean archaeological sites to provide a window into the climate and seasonality regimes of the past.
The seasonal pattern of shellfish foraging at the archaeological site of Haua Fteah in the Gebel Akhdar, Libya was investigated from the Epipaleolithic to the Neolithic via oxygen isotope (d18O) analyses of the topshell Phorcus (Osilinus) turbinatus. To validate this species as faithful year-round palaeoenvironmental recorder, the intra-annual variability of d18O in modern shells and sea water was analysed and compared with measured sea surface temperature (SST). The shells were found to be good candidates for seasonal shellfish forging studies as they preserve nearly the complete annual SST cycle in their shell d18O with minimal slowing or stoppage of growth. During the terminal Pleistocene Early Epipaleolithic (locally known as the Oranian, with modeled dates of 17.2e12.5 ka at 2s probability, Douka et al., 2014), analysis of archaeological specimens indicates that shellfish were foraged year-round. This complements other evidence from the archaeological record that shows that the cave was more intensively occupied in this period than before or afterwards. This finding is significant as the period of the Oranian was the coldest and driest phase of the last glacial cycle in the Gebel Akhdar, adding weight to the theory that the Gebel Akhdar may have served as a refugium for humans in North Africa during times of global climatic ex- tremes. Mollusc exploitation in the Latest Pleistocene and Early Holocene, during the Late Epipaleolithic (locally known as the Capsian, c. 12.7 to 9 ka) and the Neolithic (c. 8.5 to 5.4 ka), occurred predominantly during winter. Other evidence from these archaeological phases shows that hunting activities occurred during the warmer months. Therefore, the timing of Holocene shellfish exploitation in the Gebel Akhdar may have been influenced by the seasonal availability of other resources at these times and possibly shellfish were used as a dietary supplement when other foods were less abundant.
Scientific Reports, 2019
elemental analysis of biogeochemical archives is an established technique used to study climate in a range of applications, including ocean circulation, glacial/interglacial climates, and anthropogenic climate change. Data from mollusc archives are especially important because of their global abundance and sub-annual resolution. Despite this potential, they are underrepresented among palaeoclimate studies, due to enigmatic physiological influences skewing the elemental record. Understanding the patterns behind these influences will improve data interpretation and lead to the development of new climate proxies. Here, we show for the first time that extensive spatial mapping of multiple mollusc specimens using Laser Induced Breakdown Spectroscopy (LIBS) across a wider region can resolve enigmatic patterns within the elemental record caused by physiological influences. 2D elemental (Mg/Ca) maps of whole limpet shells (Patella caerulea) from across the Mediterranean revealed patterns of variability within individual mollusc records as well as within isochronous parts of specimens. By registering and quantifying these patterns, we established previously uninterpretable correlations with temperature (R 2 > 0.8, p < 0.01). This outcome redefines the possibilities of accessing sub-annual climate proxies and presents the means to assess annual temperature ranges using oxygen isotope analysis requiring only 2 samples per shell. Seasonally resolved climate proxies are a valuable tool for overcoming the biases of records reflecting only mean annual conditions 1 as they trace the full extent of variability that is experienced throughout the year rather than only within a specific growth-window 2. This information is essential for evaluating short-term ecological responses 3 , as well as long-term changes of seasonal variability across millenia 4. Most mollusc shells provide this information as biogeochemical archives of their growth-increments and are commonly analysed using oxygen isotope ratios (δ 18 Ο) or elemental ratios (e.g. Mg/Ca, Sr/Ca, Ba/Ca). Their availability along modern and ancient shorelines as well as their good preservation facilitate climatic analyses at a high temporal and spatial resolution 5,6 , in addition to ecological responses across a large population 7-9. Obviously sufficiently resolved geochemical studies require large sample sizes per shell specimen and with elemental analysis being more cost-effective 10 , this approach is becoming increasingly popular within the last decade 11-14. Yet, elemental ratios in mollusc shells are more difficult to establish as climate proxies relative to δ 18 Ο values 15-17. Studies have had mixed success in developing general equations that link elemental ratios of shell car-bonate to specific sea surface temperatures (SST) even within a single species 11. The amount of unpublished negative results is unknown but likely substantial 18. Specifically, for Patella sp. shells a variety of correlations between elemental ratios and SST were found 19-22 , with no consistent explanation for any of the variabilities or anomalies that were encountered. Generally, it is argued that variations in study location as well as vital or physiological effects working between and within individual mollusc specimens prevent a comprehensive and unambiguous interpretation of their elemental records 11,21-28 .
Shellfishing, Seasonality, and Stable Isotopes: A View from the Devil's Head Site, Calais, Maine
Archaeology of Eastern North America, 2021
Season of occupation is a crucial component of understanding hunter-gatherer settlement and mobility strategies archaeologically. In Maine's Quoddy region, for instance, there remains substantial variability about diachronic changes in settlement and subsistence that more precise reckoning of season of occupation could help to address. In this paper we report on a method using isotope values (δ 18 O) derived from powdered archaeological Mya arenaria shell sampled at the terminal growth band. We use this technique on shell from the Devil's Head site, which is a Late Maritime Woodland (1,350-500 BP) to Protohistoric period (ca. 500-350 BP) site. We compare these results with data from modern Mya arenaria. These data provide useful estimates of seasonality, sea surface temperature, and the paleoecological setting of the site during these periods of occupation.