BORON AND STRONTIUM ISOTOPIC COMPOSITION IN THE ARAGONITIC SHELL MATERIAL OF CULTURED ARCTICA ISLANDICA
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 (δ88/86Sr) in biogenic carbonates are influenced by the temperature of the precipitating fluid, however, our δ88/86Sr 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 (α3-4) 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.