Oyster shells as history books

[FIRST PARAGRAPH] A collaborative project was established in 2002 that has brought together geochemistry and archaeology in order to investigate environmental change and the harvesting strategies of ancient peoples. The objectives of this study are to decipher the life history and environmental information contained in shells of the European oyster, Ostrea edulis, by analyzing geochemical variations along shell growth. This approach provides an independent measure of age and season of death, as well as a record of environmental change in temperature and salinity through the life of the oyster. By understanding the life history and environmental records contained in modern oyster shells, we can analyze shells from archaeological sites to gain an historical perspective of harvesting practices and environmental change in ancient shellfisheries

Evaluating Freshwater Mussel Shell δ13C Values as a Proxy for Dissolved Inorganic Carbon δ13C Values in a Temperate River

The stable isotope ratio of dissolved inorganic carbon (δ13CDIC) in rivers reflects the dominant vegetation type in the surrounding watershed, rates of chemical weathering, atmospheric CO2 fluxes, and the relative rates of photosynthesis and respiration. Reconstructing past δ13CDIC values may reveal changes in these characteristics before watersheds experience land-use change and/or climate change. This study uses freshwater mussels, Elliptio complanata, and coeval environmental data to assess how high-resolution changes in the oxygen and carbon isotope ratios of shell carbonate (δ18Oshell and δ13Cshell, respectively) can be used as proxies of paleotemperature and paleo-DIC in rivers. To test our hypotheses, we analyzed time-series δ18Oshell and δ13Cshell values, δ13C values in bivalve tissues, and environmental data collected fortnightly from the Neuse River, North Carolina. Shell microsamples milled every 150 μm along the maximum growth axis represent an average of 12 ± 5 days (n = 524; excluding periods of growth cessation of >30 days), which is similar to the environmental data resolution (∼14 days). Serially sampled δ18Oshell and δ13Cshell values did not capture the full range of environmental conditions due to growth cessation during winter shutdown and extreme weather events. Low water temperature and elevated turbidity appear to be significant drivers of growth cessation. Spatial and temporal differences in the amount of metabolic carbon incorporated in the shell (∼0%–44%) likely occur due to variable mussel metabolic rates within and among study sites. Though high-resolution δ13Cshell values did not reflect variations in δ13CDIC values, average δ13Cshell values were indicative of average δ13CDIC values in the Neuse River

Lifespan and growth of Astarte borealis (Bivalvia) from Kandalaksha Gulf, White Sea, Russia

Marine bivalves are well known for their impressive lifespans. Like trees, bivalves grow by accretion and record age and size throughout ontogeny in their shell. Bivalves, however, can form growth increments at several different periodicities depending on their local environment. Thus, establishing lifespans and growth rates of marine bivalves requires a proper identification of annual growth increments. Here, we use isotope sclerochronology to decipher the accretionary growth record of modern Astarte borealis from the White Sea, Russia (N 67°05.70′; E 32°40.85′). Unlike winter growth increments observed in many other cold-temperate and boreal bivalve and limpet species, prominent growth increments in A. borealis corresponded to the most negative values in the oxygen isotope (δ18O) time series indicating that they formed during summer. Furthermore, summer growth increments do not coincide with the external concentric ridges on the shell making the latter feature an unreliable indicator of age. Similar to many other polar bivalves, A. borealis shows slow growth and long life. The von Bertalanffy growth equation for our sample is Ht = 29.39*(1 − e(− 0.11(t−(− 1.86))). Lifespans of individuals examined here (n = 18) range from 16 to 48 years. Given its impressive longevity and widespread polar distribution, A. borealis may be a potentially valuable skeletal archive for monitoring environmental conditions in the Arctic Ocean and boreal seas in the face of changing climate

Carbon and nitrogen tracers of land use effects on net ecosystem metabolism in mangrove estuaries, southwest Florida

Four estuaries in southwest Florida with different land-use characteristics in their watersheds were chosen to investigate the effects of anthropogenic land use on estuarine biogeochemical cycling. We compared C:N ratios, concentrations of dissolved inorganic carbon (DIC), chlorophyll-a (chl-a) and particulate organic carbon (POC), stable isotope ratios of DIC (δ13CDIC) and POC (δ13CPOC), and nitrogen isotope ratios of particulate organic nitrogen (δ15NPON) among these estuaries. Values of δ13CDIC ranged from −14.1 to +0.9‰. The more negative values occurred upstream, resulting from DIC inputs derived from both the degradation of organic carbon and dissolution of carbonates. Upstream DIC concentrations were as high as 8066 μmol L−1, suggesting high respiration rates. Further, a comparison of DIC values to a conservative mixing model indicates net heterotrophic metabolic state in all four estuaries. Supporting this interpretation, low δ13CPOC values suggest that terrestrial plants were the main source of POC in the upstream sampling points. However, C:N ratios ranged from 7.2 to 13.4, and were consistent with the decomposition of both terrestrial and aquatic sources. Chl-a concentrations were variable and typically below 20 μg L−1, indicating moderate to low levels of autotrophy in all estuaries. Elevated chl-a concentrations indicative of increased primary productivity occurred at intermediate salinities, and were possibly caused by the mixing front at mid-estuary locations. There were no apparent differences in δ15NPON among estuaries, suggesting that the N sources to these estuaries are comparable. The combined results show no differences between near-natural and anthropogenically influenced estuaries, indicating a minimal effect of anthropogenic activities on the parameters measured, possibly as a result of the filtering capacity of the extensive surrounding mangrove vegetation

Life history patterns of modern and fossil Mercenaria spp. from warm vs. cold climates

Recent work projects significant increases in sea surface temperature by the end of the 21st century. The biological consequences of such temperature increases are poorly understood. Study designs using a conservation paleobiology approach combined with sclerochronology methods can provide a powerful framework in which to assess these consequences. This study focuses on the ecological and economically important hard clam, Mercenaria, from modern and fossil settings that grew during climates that were warmer than or comparable to today. We compared lifespans and growth rates (von Bertalanffy k) of modern Mercenaria spp. populations to those from the Mid Pliocene Warm Period (MPWP) and early Pleistocene to better understand the influences of temperature on life history. We found that growth rates tend to increase with increasing temperature both through space and time. However, the relationship between lifespan and climate state is not as clear. Further, we observe that mid-to high-latitude individuals seem to be more impacted by changes in climate state than low latitude individuals. We suggest in response to increased seawater temperatures, mid- and high-latitude individuals might experience significant shifts towards faster growth rates whereas low latitude individuals might not see as much change. These findings provide insight to how growth rates and lifespans of Mercenaria might shift in response to future increases in seawater temperature. Understanding such impacts are critical for the development of management strategies and policies for future environmental change

Ecological information and water mass properties in the Mediterranean recorded by stable isotope ratios in Pinna nobilis shells

Sclerochronologic and stable isotope records in Pinna nobilis shells potentially record ecological and oceanographic information. P. nobilis is a subtidal bivalve adapted to live in a variety of environments in the Mediterranean. We hypothesized that stable isotope ratios (δ18O and δ13C) and growth increment patterns from individuals living in different environments serve as ecological indicators. Using a new methodology for calcite sampling, we (1) identified annual growth features (nacre tongues) and (2) compared monthly resolved variations in δ18O and δ13C values and calcification temperatures recorded in animals located above and below the thermocline (16 and 30 m depth). The specimens from 16 m showed more negative δ18O values than the specimen from 30 m, likely reflecting differences in salinity. The specimens from 30 m recorded δ13C values less positive than the specimens from 16 m, which we interpreted as an ontogenetic effect observed in previous studies. Estimated calcification temperatures were offset relative to measured water temperature by ∼6.1°C (∼1.4‰). This finding is evident in earlier proxy studies of P. nobilis, although it was not discussed in those studies. Using the seasonal pattern of δ18O and δ13C values, we demonstrated that nacre tongues are deposited annually and that their formation is independent of temperature. Food availability rather than temperature may control nacre tongue formation. An alternative explanation for nacre tongue formation is gonad maturation during spring. Our findings support the idea that sclerochronology in P. nobilis can be used to reconstruct environmental, ecological, and climate archives of the Mediterranean

Oceanic cooling recorded in shells spanning the Medieval Climate Anomaly in the subtropical eastern North Atlantic Ocean

The Medieval Climate Anomaly (MCA; 900–1300 AD) was the most recent period of pre-industrial climatic warming in the northern hemisphere, and thus estimations of MCA signals can illuminate possible impacts of anthropogenic climate change. Current high-resolution MCA climate signals are restricted to mid- and high-latitude regions, which confounds inferences of how the MCA impacted some global/hemispheric climate mechanisms (e.g. North Atlantic Oscillation; NAO). To address this knowledge gap, we estimate seasonally-resolved sea surface temperatures (SSTs) from the oxygen isotope composition (δ18O) of serially sampled Phorcus atratus shells from archaeological sites spanning the MCA in the Canary Islands. Twelve archaeological and six modern P. atratus shells were analyzed, and archaeological shells were dated using carbonate-target radiocarbon dating. SSTs were estimated using the published aragonite-water equilibrium fractionation equation. Modern shells showed a mean SST of 20.0 ± 1.5 °C, with a seasonal amplitude of 5.3 ± 0.9 °C. Archaeological shells exhibited a mean SST of 18.2 ± 0.7 °C, with a mean seasonal amplitude of 5.5 ± 1.0 °C. Thus, shells that span the MCA in the Canary Islands recorded SSTs that were significantly cooler than the modern (P <.05), contrasting with warming estimates and model predictions elsewhere in the Northern Hemisphere. We propose that the observed cooling resulted from increased upwelling in NW Africa due to a strengthening of the prevailing westerlies and coastal winds along the African shoreline. The intensified upwelling scenario during the MCA is partially supported by in-situ carbon isotope data (δ13C) retrieved from the archaeological shells, which was compared to the δ13C values of modern shells and dissolved inorganic carbon in the ambient seawater. These results are consistent with other low-latitude temperature/precipitation anomalies associated with a positive NAO mode, suggesting a transition to a positive NAO index during the middle and late MCA that possibly extended later into the 13th century AD

Calibration of the oxygen isotope ratios of the gastropods Patella candei crenata and Phorcus atratus as high-resolution paleothermometers from the subtropical eastern Atlantic Ocean

The oxygen isotope composition (δ18O) recorded in shells of the gastropods Patella candei crenata and Phorcus atratus from the Canary Islands (27–29°N) potentially provide invaluable high-resolution paleoclimatic data. However, because these two species have never been studied isotopically, it is necessary to calibrate and validate this approach by using live-collected specimens that can be compared to present-day climate data. For P. candei crenata, live organisms were collected at 15-day intervals for nearly one year (between 2011 and 2012) from the rocky-intertidal coast of SE Tenerife along with sea surface temperatures (SST) and seawater δ18O values. The δ18O values of the last growth episode along the shell growth margin, representing the conditions when organisms were collected, illustrate that P. candei crenata δ18O values were 1.3 ± 0.2‰ higher than expected values from isotopic equilibrium. This finding resulted in estimated SST 5.7 ± 0.6 °C lower than observed values. This offset or “vital effect” is uniform and predictable, and therefore can be corrected by subtracting 1.3‰ from the measured shell δ18O value. Adjusted temperatures from the shell coincided with observed SST in the study area. Therefore, Patella candei crenata shells are reliable repositories of SST data, in agreement with other species of Patella from higher latitudes. For Phorcus atratus, a live specimen was analyzed isotopically along shell-growth direction. The intrashell δ18O values showed the entire range of measured SST in the region without an apparent “vital effect”, in correspondence with other Phorcus species from the Mediterranean. Both Patella candei crenata and Phorcus atratus are common components of archaeological and Pleistocene-Holocene paleontological sites across the Canary Archipelago. This study suggests that well-preserved shells of these two taxa retrieved from ancient settings have the capacity to serve as excellent high-resolution paleotemperature archives for the tropical/subtropical eastern Atlantic region

Assessing δ15N values in the carbonate-bound organic matrix and periostracum of bivalve shells as environmental archives

Though previous studies demonstrate the utility of nitrogen and carbon stable isotope ratios (δ15N and δ13C, respectively) in bivalve soft tissues as biogeochemical proxies, it is necessary to develop alternative proxies for environmental reconstructions when soft tissues are unavailable, such as with fossils or in museum-archived specimens. This study assesses the reliability of the δ15N values of carbonate-bound organic matrix (δ15NCBOM) and periostracum (δ15Nperiostracum) in bivalve shells as recorders of the δ15N values of particulate nitrogen (δ15NPN) by comparing the δ15NCBOM and δ15Nperiostracum values of live-collected freshwater mussels (Elliptio complanata) and estuarine clams (Rangia cuneata) to the δ15N values of particulate nitrogen (δ15NPN) in the water column. The δ15NCBOM and δ15Nperiostracum values in both species were within the range of the δ15NPN values that have been corrected for trophic-level enrichment. Thus, our findings illustrate that δ15NCBOM and δ15Nperiostracum values reliably record δ15NPN values in rivers and estuaries. The significant positive correlation between δ15NCBOM and δ15Nperiostracum values in both species indicates that they may be used in a similar manner to record δ15NPN values. The δ15N values in E. complanata muscle, mantle, and gill tissues were enriched by about +3.4‰ compared to δ15NPN from the water column, which suggests that they are primary consumers that reflect baseline trophic levels. On the other hand, δ15N values in the soft tissues of R. cuneata have trophic-level enrichment consistent with both primary and secondary consumption. Therefore, variations in the δ15N values of tissues in R. cuneata may be related to trophic-level shifts and/or changes in N sources. Differences between the δ15N values of soft tissue, CBOM, and periostracum in E. complanata and R. cuneata can be attributed to asynchronous growth, metabolic rate, and organic molecule composition. The δ15NCBOM values vary along a freshwater-estuarine gradient because of land-use change and differences in the trophic level of the compared species. The δ15NCBOM values between neighboring sites reflect influences from biosolid application and treated wastewater discharge. While δ15NCBOM values did not differentiate between sites dominated by urban and forested land-cover, δ15NCBOM values were highest at the site with the highest agricultural land-use. These results demonstrate the potential of δ15NCBOM values in bivalve shells to record long-term changes in watershed land use

Seasonal SIMS δ18O record in Astarte borealis from the Baltic Sea tracks a modern regime shift in the NAO

Introduction: Astarte borealis holds great potential as an archive of seasonal paleoclimate, especially due to its long lifespan (several decades to more than a century) and ubiquitous distribution across high northern latitudes. Furthermore, recent work demonstrates that the isotope geochemistry of the aragonite shell is a faithful proxy of environmental conditions. However, the exceedingly slow growth rates of A. borealis in some locations (<0.2mm/year) make it difficult to achieve seasonal resolution using standard micromilling techniques for conventional stable isotope analysis. Moreover, oxygen isotope (δ18O) records from species inhabiting brackish environments are notoriously difficult to use as paleoclimate archives because of the simultaneous variation in temperature and δ18Owater values. Methods: Here we use secondary ion mass spectrometry (SIMS) to microsample an A. borealis specimen from the southern Baltic Sea, yielding 451 SIMS δ18Oshell values at sub-monthly resolution. Results: SIMS δ18Oshell values exhibit a quasi-sinusoidal pattern with 24 local maxima and minima coinciding with 24 annual growth increments between March 1977 and the month before specimen collection in May 2001. Discussion: Age-modeled SIMS δ18Oshell values correlate significantly with both in situ temperature measured from shipborne CTD casts (r2 = 0.52, p<0.001) and sea surface temperature from the ORAS5-SST global reanalysis product for the Baltic Sea region (r2 = 0.42, p<0.001). We observe the strongest correlation between SIMS δ18Oshell values and salinity when both datasets are run through a 36-month LOWESS function (r2 = 0.71, p < 0.001). Similarly, we find that LOWESS-smoothed SIMS δ18Oshell values exhibit a moderate correlation with the LOWESS-smoothed North Atlantic Oscillation (NAO) Index (r2 = 0.46, p<0.001). Change point analysis supports that SIMS δ18Oshell values capture a well-documented regime shift in the NAO circa 1989. We hypothesize that the correlation between the SIMS δ18Oshell time series and the NAO is enhanced by the latter’s influence on the regional covariance of water temperature and δ18Owater values on interannual and longer timescales in the Baltic Sea. These results showcase the potential for SIMS δ18Oshell values in A. borealis shells to provide robust paleoclimate information regarding hydroclimate variability from seasonal to decadal timescales