Clara L. Blättler

Explaining the Phanerozoic Ca isotope history of seawater

A new geochemical budget for the modern marine carbonate sink helps to explain the major features of the Phanerozoic Ca isotope record. A large compilation of Ca isotope ratios for modern carbonates, incorporating more than 50 new measurements, represents the quantitatively important components of the system. With this data set, distinct Ca isotope ratios are identified for different types of marine carbonate, the balance of which has changed over time with shifts between calcite and aragonite seas and with the development of pelagic calcification during the Mesozoic. It is suggested that large-scale changes in the Ca isotope ratio of seawater, as exemplified by that in the Carboniferous, were no longer possible after Jurassic time because of the generation of a deep-sea calcite sink expressed by deposition of foraminiferal–coccolith ooze across the world ocean. This work demonstrates the close connection between isotopic cycling, carbonate sedimentation, seawater chemistry, and evolutionary trends.

Calcium isotopes in evaporites record variations in Phanerozoic seawater SO4 and Ca

Reconstructing variations in the major element chemistry of seawater provides constraints on how the global carbon and sulfur cycles have changed over geological time, but archives for this information are rare. This work generates a new independent record of ancient seawater composition, in particular the relative abundance of Ca and SO 4 , through Ca isotope measurements in marine evaporites. Evaporite sequences that reach halite saturation record large Ca isotope variability if SO 4 > Ca, whereas a small range in Ca isotope ratios is observed if Ca > SO 4 . Analyses of geological evaporites indicate SO 4 -rich seawater in the Neogene and Permian and Ca-rich seawater in the Cretaceous and Silurian. These results agree with previous reconstructions using fluid inclusions in halite, and demonstrate a new approach for extending our understanding of oceanic and geochemical evolution.