Ocean Acidification

Ocean acidification, the decrease in ocean pH associated with increasing atmospheric CO₂, is likely to impact marine organisms, particularly those that produce carbonate skeletons or shells. Therefore it is important to investigate how environmental factors (seawater pH, temperature and salinity) influence the chemical compositions in biogenic carbonates. In this study we report the first high-resolution strontium (⁸⁷Sr / ⁸⁶Sr and δ^{88 / 86}Sr) and boron (δ¹¹B) isotopic values in the aragonite shell of cultured Arctica islandica (A. islandica). 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 terrestrial influence. The ⁸⁴Sr–⁸⁷Sr double spike resolved shell δ ^{88 / 86}Sr and Sr concentration data show no resolvable change throughout the culture period and reflect no theoretical kinetic mass fractionation throughout the experiment despite a temperature change of more than 15 °C. The δ¹¹B records from the experiment show at least a 5‰ increase through the culture season (January 2010–August 2010), with low values from beginning to week 19 and higher values hereafter. The larger range in δ¹¹B in this experiment compared to predictions based on other carbonate organisms (2–3‰) suggests that a species-specific fractionation factor may be required. A relatively strong correlation between the Δ pH (pH_shell-pH_sw) and seawater pH (pH_sw) was observed (R² = 0.34), which suggests that A. islandica partly regulates the pH of the extrapallial fluid. However, this proposed mechanism only explains approximately 34% of the variance in the δ¹¹B data. Instead, a rapid rise in δ¹¹B after week 19 suggests that the boron uptake of the shell changes when a temperature threshold of 13 °C is reached.

Liu Y.-W., Aciego S. M. & Wanamaker Jr., A. D., 2015. Environmental controls on the boron and strontium isotopic composition of aragonite shell material of cultured Arctica islandica, Biogeosciences Discussions 12: 2979-3023. Article.