Keywords:Sulfur isotope, Carbonate associated sulfate, Barite, Eocene
The marine sulfur cycle over geologic timescales is closely linked to the redox state of the Earth's surface environments due to the burial of redox-sensitive sulfur species. Fluctuations in the δ34Ssw record have been related to a wide range of global environmental changes, such as oxygenation of the biosphere, bacterial evolution, and mass extinctions. In order to investigate the utility of bulk carbonate as a recorder of seawater sulfate sulfur isotope ratios (δ34Ssw), co-existing pelagic barite and bulk carbonate in Eocene sediments from the Equatorial Pacific Ocean (IODP Exp. 320/321 Sites U1331 to U1333) were analyzed for their sulfur isotope ratios (δ34S). The δ34S from both minerals showed parallel fluctuation throughout the Eocene with carbonate associated sulfate (CAS) values about 0.8‰ heavier than those of barite. A similar offset was observed in CAS δ34S obtained using species-specific cleaned planktonic foraminifers (Rennie et al., 2018). The consistent results from two distinct minerals suggest that the original δ34Ssw can be derived from bulk CAS analysis, if post deposition carbonate recrystallization is minimal.
Our δ34Sbarite data added an important data set in middle Eocene (Lutetian, 47.8 to 41.2 Ma). The data set combined with previous studies showed a ~5‰ increase of δ34Ssw during the Eocene (from 53 Ma to 36 Ma) with the majority of the shift within 7 myr between 53 and 46 Ma. This change is more gradual than previously reported. The timing of this δ34Ssw shift coincides with extensive pyrite burial in the Arctic Ocean, supporting the hypothesis that the 5‰ increase in Eocene δ34Ssw has been caused by 34S-enriched water outflow from the Arctic Ocean as suggested in the previous study.