Isotope Record of Phanerozoic Seawater

The development of a paleothermometer for ancient oceans has been a prime goal of stable isotope geochemistry since its inception. For the pre-Tertiary times, the earlier limitation of suitable carrier phases for the temperature signal is being slowly overcome by utilising low-Mg calcite shells of oysters, belemnites and particularly brachiopods. Their modern counterparts display up to 4‰ spread in oxygen isotope values and a similar bandwidth characterises also the ancient assemblages, reflecting a combination of physical variables (depth, temperature, salinity) and vital factors, the latter genera/species specific. Most modern articulate brachiopods secrete their secondary and tertiary shell layers in apparent oxygen isotopic equilibrium with ambient seawater, but carbon isotopes often display 13-C depletion due to metabolic overprint. The band of baseline data for the Phanerozoic is reasonably well defined and likely represents a primary feature. Oxygen accounts for 60% of all atoms in the calcite (aragonite) lattice and replacement, via dissolution/reprecipitation, by extraneous oxygen would undoubtedly result in disruption of related attributes, such as texture, mineralogy, chemistry and isotopes. Yet the uncontested sulfur, calcium, carbon and strontium Phanerozoic isotope records all emerge from the same collection of shells. Future effort directed at unravelling the details of the internal structure within the Phanerozoic trend, and testing the utility of various genera/species for such a task, is a more constructive approach than the sterile dismissal of the entire body of data as nothing more than an alteration artefact. A higher order structure of greenhouse and icehouse episodes appears to be superimposed on the general Phanerozoic baseline and Permian data suggest an existence of the latitudinal temperature gradient of about 14ºC. Overall, this Phanerozoic pattern shows much better correlation with potential celestial climate drivers than with model concentrations of atmospheric carbon dioxide, with empirical records on shorter time scales providing additional support.