Comparisons of multiple isotope systems in the aragonitic shells of cultured Arctica islandica clams

Abstract:

Previous work using oxygen and stable carbon isotopes from Arctica islandica shells has shown that this archive can provide information on past seawater temperatures, carbon cycling and ocean circulation. However, relatively less attention has been devoted to other “non-traditional” isotope systems within this proxy archive. In this study, we report the boron (δ¹¹B) and strontium isotopic values (⁸⁷Sr/⁸⁶Sr and δ^88/86Sr) from A. islandicashells collected and cultured from the Gulf of Maine.

The long-lived ocean quahog, A. islandica was collected and cultured in the Gulf of Maine for 8 months. Our high-resolution δ¹¹B records from the experiment show 5-7‰ of increase through the culture, with low values from January to May and higher values after May. The ⁸⁷Sr/⁸⁶Sr ratios from both tank water and shell samples suggest that the shell material reflects ambient ocean chemistry without interferences from terrestrial sources. Although It has been suggested that stable Sr isotopic ratios (δ^88/86Sr) in biogenic carbonates are influenced by the temperature of the precipitating fluid, our nearly identical δ^88/86Sr data do not support this hypothesis despite a 15 °C temperature change during the experiment.

Based on the in-situ measurements of culture seawater temperature, salinity and pH, and two commonly used fractionation factors (α^3-4) for corals and forams, we predicted the range in shell δ¹¹B values for the experiment. Our boron results are at the extreme ends of the two prediction lines suggesting the potential usage of the bivalve shells as seawater pH indicator. However, the wider range in δ¹¹B in this experiment than the predictions based on other carbonate organisms (only 2 to 3‰) suggests that a species-specific fractionation factor may be required.

Recent work from an additional constant temperature experiment (10 and 15 °C) in the Gulf of Maine will allow us to further evaluate temperature influences and potential vital effects on the shell boron isotope values.