Magnesium (Mg∕Ca, <i>δ</i> ²⁶ Mg), boron (B∕Ca, <i>δ</i> ¹¹ B), and calcium (Ca ²⁺ ) geochemistry of <i>Arctica islandica</i> and <i>Crassostrea virginica</i> extrapallial fluid and shell under ocean acidification

Abstract. The geochemistry of biogenic carbonates has long been used as proxies to record changing seawater parameters. However, the effect of ocean acidification (OA) on seawater chemistry and organism physiology could impact isotopic signatures and how elements are incorporated into the shell. In this study, we investigated the geochemistry of three reservoirs important for biomineralization – seawater, the extrapallial fluid (EPF), and the shell – in two bivalve species: Crassostrea virginica and Arctica islandica. Additionally, we examined the effects of three ocean acidification conditions (ambient: 500 ppm CO2, moderate: 900 ppm CO2, and high: 2800 ppm CO2) on the geochemistry of the same three reservoirs for C. virginica. We present data on calcification rates, EPF pH, measured elemental ratios (Mg/Ca, B/Ca), and isotopic signatures (δ26Mg, δ11B). In both species, comparisons of seawater and EPF Mg/Ca and B/Ca, Ca2+, and δ26Mg indicate that the EPF has a distinct composition that differs from seawater. Shell δ11B did not faithfully record seawater pH, and δ11B-calculated pH values were consistently higher than pH measurements of the EPF with microelectrodes, indicating that the shell δ11B may reflect a localized environment within the entire EPF reservoir. In C. virginica, EPF Mg/Ca and B/Ca, as well as absolute concentrations of Mg2+, B, and Ca2+, were all significantly affected by ocean acidification, indicating that OA affects the physiological pathways regulating or storing these ions, an observation that complicates their use as proxies. Reduction in EPF Ca2+ may represent an additional mechanism underlying reduction in calcification in C. virginica in response to seawater acidification. The complexity of dynamics of EPF chemistry suggests boron proxies in these two mollusk species are not straightforwardly related to seawater pH, but ocean acidification does lead to both a decrease in microelectrode pH and boron-isotope-based pH, potentially showing applicability of boron isotopes in recording physiological changes. Collectively, our findings show that bivalves have high physiological control over the internal calcifying fluid, which presents a challenge in using boron isotopes for reconstructing seawater pH.