IMIS

Mediterranean sapropels represent some of the largest scale deoxygenation events in recent Earth history. Here, we use high resolution Mo isotope data for seven such events (sapropels S3 to S9) to semi-quantitatively constrain past H₂S concentrations using a new interpretive framework. Bottom water H₂S was present for all studied sapropels, but the extent of redox changes varied considerably between them, the ultimate driver likely being variations in monsoon strength. Near-quantitative removal of Mo (δ⁹⁸Mo > 2 ‰) during deposition of sapropels S5 and S7 suggests predominantly highly sulfidic conditions with long deep water residence times, comparable to the modern Black Sea, whereas considerably lower δ⁹⁸Mo values for sapropels S3, S4, S8, and S9 (−0.4 to +0.9 ‰) imply mildly euxinic conditions only (0 < H₂S < 11 μmol/L). The high resolution data reveal consistent temporal patterns that track the development of basin restriction and euxinia over several kyr. These observations illustrate how Mo isotopes can provide quantitative constraints on basin wide redox changes on relatively short time scales.