Geocheminar Winter-2018

The 18O/16O ratios of calcite fossils increase by ~8‰ between the Cambrian and present. It has long been controversial whether this change reflects evolution in the δ18O of seawater, or a decrease in ocean temperatures, or greater extents of diagenesis of older strata. I will present measurements of the oxygen and ‘clumped’ isotope compositions of Phanerozoic dolomites, and compare these data with published oxygen isotope studies of carbonate rocks. The δ18O records of dolomites and calcite fossils overlap one another, suggesting they are controlled by similar processes. Clumped isotope measurements of Cambrian to Pleistocene dolomites imply formation temperatures of 15 to 158˚C and parent waters having δ18OVSMOW values from -2 to +12‰. These data are consistent with dolomitization through a km-scale circulation of seawater and diagenetically modified seawater, over timescales of 100 Myr, and suggest that like dolomite, temporal variations of the calcite fossils δ18O record are largely driven by diagenetic alteration. We find no evidence that Phanerozoic seawater was significantly lower in δ18O than pre-glacial Cenozoic seawater. Thus, the fluxes of oxygen-isotope exchange associated with weathering and hydrothermal alteration reactions have remained stable throughout the Phanerozoic, despite major tectonic, climatic and biologic perturbations. This stability implies that a long-term feedback exists between the global rates of seafloor spreading and weathering. Massive dolomites have formed in pre-Cenozoic units at temperatures >40°C. Since Cenozoic platform strata generally have not reached such conditions, the paucity of dolomite in Cenozoic strata (also known as the ‘dolomite problem’) could simply reflect the thermal immaturity of recent platform sections.