Fossil corals as archives of secular variations in seawater chemistry

Abstract

Records of the elemental and isotopic composition of the oceans can help elucidate the geologic controls on seawater chemistry and climate over million-year timescales. This thesis describes the development of a new fossil coral archive that can be used to reconstruct properties of seawater chemistry for the past ~200 My. It also details the application of this archive to investigate changes in seawater Mg/Ca, Sr/Ca, U/Ca, δ26Mg, and δ44Ca over the Mesozoic and Cenozoic.

Results of diagenetic tests used to validate ~60 fossil coral samples for studies of seawater paleochemistry are presented in Chapters 2 and 4. The validated samples range in age from Triassic through Recent. X-ray diffractometry, scanning electron microscopy, petrographic microscopy, cathodoluminescence microscopy, and micro-raman spectroscopy studies indicate that sample mineralogy is preserved (as aragonite). Studies of 87Sr/86Sr, carbonate clumped isotopes, trace elements sensitive to diagenesis, He/U dating, and U isotopes are used to screen for geochemical signs of alteration.

Records of seawater chemistry inferred from validated fossil coral samples are presented in Chapters 2-4. Mg/Caseawater inferred from fossil corals (Chapter 2) is low during the Mesozoic, and increases by a factor of ~5 between 80 Ma and today – compatible with existing reconstructions. The record helps improve our understanding of the timing of Mg/Caseawater changes since the Triassic. Inferred Sr/Caseawater (Chapter 2) varies between 8 and 13 mmol/mol since ~200 Ma, with a maximum in the Late Cretaceous. This result is consistent with reconstructions from benthic foraminifera and fossil fish teeth. A record of δ26Mgseawater from fossil corals (Chapter 3) helps distinguish between two existing records that give conflicting results, and indicates that the fraction of Mg removed from seawater as dolomite has not changed significantly over the Cenozoic. A reconstruction of fossil coral U/Ca (Chapter 4) suggests that [U]seawater has increased by a factor of ~2 since the Eocene, with implications for our understanding of past seawater [CO32-] and the importance of U removal in reducing sediments. In Chapter 5, a record of δ44Ca from fossil corals is presented. This record may reflect changes in coral Ca isotope discrimination through time, rather than changes in δ44Caseawater.