BORON AND STRONTIUM ISOTOPIC COMPOSITION IN THE ARAGONITIC SHELL MATERIAL OF CULTURED ARCTICA ISLANDICA

Previous studies have demonstrated that the traditional oxygen and stable carbon isotope analysis in the shell material of the long-lived ocean quahog Arctica islandica have been useful in addressing environmental and climate change problems. However, relative less attention has been devoted to other “non-traditional” isotope systems within this proxy archive. Here we report the boron (δ¹¹B) and strontium isotopic values (⁸⁷Sr/⁸⁶Sr and δ^88/86Sr) from A. islandica shells collected and cultured from the Gulf of Maine. A. islandicahas a wide habitat range around much of the coastal ocean in the northern North Atlantic and is therefore a potential archive for addressing temporal and spatial changes in ocean chemistry during recent centuries.

High-resolution boron and strontium records from an 8-month culture experiment under ambient conditions from the Gulf of Maine are compared with in-situ measurements of temperature, salinity, and pH to find the relationships between the isotope systems and the environmental factors. The δ¹¹B records from the experiment show at least a 5‰ of increase through the culture season (January, 2010 – August, 2010), with low values for weeks 5-15 and higher values beginning at week 20. The ⁸⁷Sr/⁸⁶Sr ratios from both tank water and shell samples show ratios nearly identical to the open ocean, which suggests that the shell material reflects ambient ocean chemistry without interferences from land or other sources. It has been suggested that stable Sr isotopic ratios (δ^88/86Sr) in biogenic carbonates are influenced by the temperature of the precipitating fluid, however, our δ^88/86Sr data show identical values throughout the experiment despite a temperature of more than 15 °C.

Based on the in-situ measurements of culture conditions (seawater temperature, salinity and pH), and two commonly used fractionation factors (α^3-4) from biogenic carbonates (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 that the bivalve shells generally reflect the boron isotopic composition in the seawater. 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.