Effects of a transient marine sulfur reservoir on seawater δ¹⁸O_SO4 during the Paleocene-Eocene Thermal Maximum

Recent work suggests that during the Paleocene-Eocene Thermal Maximum (PETM) the volume of the oceanic oxygen minimum zone (OMZ) has expanded by one order of magnitude and acted as a transient reservoir for reduced sulfur. Fluctuations in the seawater S-isotope ratios (δ³⁴S_SO4) can be used to estimate the size of this reservoir, but do not capture the gross fluxes associated with hydrogen sulfide reoxidation at the upper and lower boundaries of the OMZ. Isotope exchange processes during microbially mediated sulfate reduction as well as constant reoxidation of hydrogen sulfide to sulfate, however, affect the oxygen isotope ratio of seawater sulfate (δ¹⁸O_SO4). Here we present a high-resolution (10-kyr) barite δ¹⁸O_SO4 record. Our data shows that the δ¹⁸O_SO4 value increased by 1.5‰ within 48 kyrs from the onset of the PETM and then returned to the pre-excursion value over the next 200 kyrs. Quantitative modeling suggests that the gross microbial sulfate reduction (MSR) flux was on the order of 4 × 10¹⁴ mol/yr, which is about one order of magnitude higher than the gross sulfate reduction flux in the modern ocean.