Matrix corrected SIMS in-situ oxygen isotope analyses of marine shell aragonite for high resolution seawater temperature measurements

Marine mollusk shells continuously incorporate oxygen isotope signatures during growth that are representative of their surrounding environment and thus produce valuable records of seawater temperature and oxygen isotopic composition. These records of past environmental conditions can be measured in situ at length-scales down to sub-daily growth increments using high resolution ion microprobes (SIMS). However, the determination of oxygen isotope ratios in aragonite, the most common shell mineral, is hampered by a lack of ideal reference materials, limiting the accuracy of isotope calibrations and temperature reconstructions. Here, we cultured marine Anadara trapezia bivalves under controlled environmental conditions at four seawater temperatures ranging from 13 to 28 °C. The start of the growth period was marked in the crossed-lamellar shell ultrastructure using strontium labelling, enabling precise targeting of the SIMS analyses. A novel calibration method, specifically tailored to analyses of biogenic aragonites with organic-inorganic architectures, has been developed to correct matrix biases that affect the accuracy of all such SIMS oxygen isotope analyses. The matrix fractionation bias calibration was achieved by combining two aragonite reference materials in a linear relationship, pinning the SIMS biases to the true composition as a function of calcium abundances. The oxygen isotope calibration provided a novel seawater temperature versus seawater-corrected oxygen isotope fractionation relationship of T (°C) = 23.31 ± 0.34 - 4.31 · (δ18Oaragonite [‰ VPDB] - δ18Oseawater [‰ VSMOW] ± 0.22) that improves the applicability of in-situ oxygen isotope-based paleo-environmental reconstructions of marine bio-aragonite proxy archives.