Multi-proxy assessment of brachiopod shell calcite as a potential archive ofseawater temperature and oxygen isotope composition

Most of our knowledge of past seawater temperature history is based on delta O-18 values of calcium carbonate fossil shells. However, the determination of past temperatures using this proxy requires the knowledge of past seawater delta O-18 values, which is generally poorly constrained. Other paleothermometers using carbonate archives, such as Mg/Ca ratios and clumped isotopes (Delta(47)), have been developed to allow for paleotemperatures to be estimated independently and to allow past ocean delta O-18 values to be calculated using various groups of calcifying organisms. Articulated brachiopod shells are some of the most commonly used archives in studies of past oceanic geochemistry and temperature. They are abundant in the fossil record since the Cambrian, and for decades, their low Mg-calcite mineralogy has been considered relatively resistant to diagenetic alteration. Here, we investigate the potential of brachiopod shells as recorders of seawater temperatures and seawater delta O-18 values using new brachiopod shell geochemical data by testing multiple well-established or suggested paleothermometers applied to carbonate archives. Modern articulated brachiopod shells covering a wide range of temperatures (-1.9 to 25.5 degrees C), depths (5 to 3431 m) and salinities (33.4 to 37.0 PSU) were analysed for their stable isotope compositions (delta O-18, delta O-18 and 147) and their elemental ratios (Mg =Ca, Sr =Ca, Na =Ca and Li =Ca). Our data allowed us to propose a revised oxygen isotope fractionation equation between modern-brachiopod shell calcite and seawater: T = -5.0(+/- 0.2)(delta O-18(c) - delta O-18(sw)) + 19.4(+/- 0.4) (1) where delta O-18(c) is in parts per thousand VPDB, delta O-18(sw) is in parts per thousand VSMOW, and T is in degrees C. Our results strongly support the use of clumped isotopes as an alternative temperature proxy but confirm significant offsets relative to the canonical relationship established for other biogenic and abiogenic calcium carbonate minerals. Brachiopod shell Mg =Ca ratios show no relationship with seawater temperatures, indicating that this ratio is a poor recorder of past changes in temperatures, an observation at variance with several previous studies. Despite significant correlations with brachiopod living temperature, brachiopod shell Sr =Ca, Na =Ca and Li =Ca values indicate the influence of environmental and biological factors unrelated to temperature, which undermines their potential as alternative temperature proxies. Kinetic effects (growth rates) could explain most of the deviation of brachiopod shell calcite from expected isotopic equilibrium with seawater and part of the distribution of Sr =Ca, Na =Ca and Li =Ca ratios.