Using the Sulfur Cycle to Constrain Changes in Seawater Chemistry During the Paleogene

The sulfur isotopic composition of seawater sulfate (δ34S) has varied significantly throughout the Phanerozoic and is related to variability in Earth’s sulfur and carbon cycles. During the Early Eocene (~55 Ma to 45 Ma) the δ34S composition of seawater sulfate increased from +18‰ to +22‰ and has remained largely invariant over the last 34 Ma. The two principal hypotheses invoked to explain this positive excursion are: (1) a rapid increase in the flux of weathering-derived sulfate and an increase in sulfate concentrations, coupled with a compensatory increase in pyrite burial, or (2) an increase in the pyrite burial flux due to an expansion in the volume of anoxic waters. A clear understanding of this significant change in seawater sulfate chemistry is hampered by the relatively low temporal sampling resolution of data that reflect seawater sulfate δ34S during this period, and potential problems with the preservation of primary signals. Here, we use δ34S analysis of carbonate associated sulfate (CAS) from IODP Expedition 342, Newfoundland Drifts to test the pattern observed in marine barite δ34S records (Paytan et al., 1998) and to increase the temporal resolution of the δ34S isotope curve through the Eocene excursion. Despite the broad variability in the CAS record, these new data largely confirm the magnitude and timing of change observed in the Paytan et al. (1998) marine barite δ34S records, but the magnitude of the excursion is higher in the CAS record. Additionally, we observe a series of anomalous δ34S values that appear to the result from oxidation of 34S-depleted pyrite during CAS extraction or incorporation of 34S-enriched sulfate during authigenesis. These depleted or enriched values in the CAS record correlate with a lower concentration of CAS, suggesting that such samples are more susceptible to contamination of non-CA¬S sulfur during either extraction or original precipitation. We also performed δ34S analyses on pore-water sulfate to better constrain possible influences from pore-water sulfate on CAS δ 34S. These pore-water sulfate data indicate that microbial sulfate reduction is ongoing at sub-seafloor depths, yielding enriched pore-water sulfate values with depth. But, we observe no strong correlation between pore-water and CAS δ34S, suggesting that pore water sulfate contamination may not be the mechanism causing enriched CAS δ34S values. Our data do not provide a clear consensus between the two proposed mechanisms for this excursion, weathering-derived sulfate or an increase volume of anoxic waters; a surge in volcanically derived sulfur associated with sea-floor spreading offers an alternative mechanism for generating this positive excursion