Possible triggers of the seawater sulfate S-isotope increase between 55 and 40 million years ago

Abstract

The Earth system experienced a climatic transition from a greenhouse state to an icehouse state over the Cenozoic, which may have been induced by tectonic and orbital processes. This transition has triggered many environmental changes including fluctuations in sea levels, seawater chemistry, and biogeochemical cycles of carbon, sulfur, and other elements. Owing to a better-resolved biostratigraphic framework and analytical techniques, high temporal resolution seawater sulfate S-isotope data (δ34Ssw) have become available through the analysis of marine barite and carbonate associated sulfate (CAS). These data provide new insight into changes in global biogeochemical cycles over this time interval and their potential causes. This study presents a review of recent work on the δ34Ssw for the past 65 million years, with a focus on the most salient perturbation of the Cenozoic sulfur cycle, a 5‰ rise over a 15-million-year period in the early-middle Eocene. We discuss the possible causes suggested to date for the δ34Ssw excursion, which include changes in the magnitude, location, or fractionation associated with pyrite formation and burial, changes to the input of sulfate to the ocean, and changes in ocean circulation. We suggest how some new geochemical data and models can improve our understanding of the various Earth's surface processes that affect the global sulfur cycle and outline a framework for interpreting other geologic intervals where δ34Ssw has fluctuated.