Comment on egusphere-2022-1144

Abstract

Most of our knowledge of past ocean temperature history is based on the δ18O measurements of calcium carbonate fossil shells. However, the determination of past temperature using this proxy requires the knowledge of past ocean δ18O, which is generally poorly constrained. Other carbonate-based paleothermometers, such as Mg / Ca ratios, and clumped isotopes (Δ47), have been developed to estimate independently paleotemperatures, and allow past ocean δ18O to be calculated using various groups of calcifying organisms. Articulated brachiopods are some of the most commonly used in studies of past oceanic geochemistry and temperature. They are abundant in the fossil record since the Cambrian, and their low Mg-calcite mineralogy has been considered relatively resistant to diagenetic alteration for decades. Here, we investigate the potential of brachiopod shells as recorders of growing temperature and seawater δ18O using new brachiopod shell geochemical data, by testing multiple established or supposed carbonate-based paleothermometers. Modern articulated brachiopod shells covering a wide range of temperatures (-1.9 to 25.5 °C), depths (5 to 3431 m) and salinities (33.4 to 37.0 PSU), were analysed for their stable isotope compositions (δ13C, δ18O and Δ47), and 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=-5.2 (±0.3) (δ18Oc-δ18Osw) + 19.9 (±0.8) R2=0.95 (n=53) Where δ18Oc is in ‰VPDB, δ18Osw is in ‰VSMOW and T is in °C. The measured Δ47 values show a strong correlation with growing temperatures but are significantly offset relative to the canonical relationship established for other biogenic and abiogenic calcium carbonate minerals. Our results strongly support the use of clumped isotopes as an alternative temperature proxy and indicate that brachiopod Δ47 values can be used with δ18O to estimate past δ18Osw with a precision of about ±1 ‰ VSMOW. The obtained Mg / Ca ratios show no relationship with temperatures, indicating that this ratio is a poor recorder of past changes in temperatures, an observation at variance with several previous studies. Brachiopod shell Sr / Ca, Na / Ca and Li / Ca display relatively good and significant correlations with brachiopod living temperature, but data indicate the influence of environmental and biological factors unrelated to temperature. Our proposed revision of marine temperature and water δ18O proxies based on brachiopod shell geochemistry is promising to refine the record of these oceanic parameters in the Phanerozoic.