Molybdenum isotope constraints on the temporal development of sulfidic conditions during Mediterranean sapropel intervals

Abstract

Mediterranean sapropels represent some of the largest scale deoxygenation events in recent Earth history. Here, we use high resolutionMo 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. Nearquantitative 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.