Mg/Ca from mussel shells rather than δ18O as a promising temperature proxy for hydrothermal vent ecosystems

Minor and trace metal fluctuations in biogenic carbonates have been demonstrated to be important potential environmental tracers in coastal areas, but remained poorly studied in deep-sea environments. For the first time, this study assesses the use of Mg/Ca signal as a thermometer proxy in hydrothermal vent ecosystems together with the analysis of oxygen stable isotope composition. Bathymodiolus azoricus and B. thermophilus mussels were collected at three hydrothermal vent fields from the mid-Atlantic ridge (Rainbow, Menez Gwen) and the East Pacific Rise (EPR 9°50N), presenting contrasted temperature and chemical habitat conditions. The variation of Mg/Ca was analysed via LA-ICP-MS and is revealed suitable to reconstruct temperature variations in these ecosystems, presumably due to stable Mg compositions of the seawater surrounding hydrothermal systems. Temperature anomalies inferred from Mg/Ca ratios can be good tracers of fluid pulses. Important fluid emissions appear however to generate major growth cessation in shell mineralization. Temperatures inferred from Isotope Ratio Mass Spectrometry (IRMS) δ¹⁸O analyses systematically underestimated the measured values in the environment. The isotopic disequilibrium is likely due to pH fluctuations in the mussel habitat, in the vicinity of vent fluid discharges, and/or interactions with the symbiotic chemosynthetic bacteria. Those results will first benefit to the ecological study of deep-sea mussels, but also provide a promising contribution for the study of the environmental dynamic in hydrothermal systems at short (daily) to long-term (pluri-annual) scales, recorded in the calcite material of bivalves.