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 resolution Mo isotope data for seven such events (sapropels S3 to S9) to semi-quantitatively constrain past H 2 S concentrations using a new interpretive framework. Bottom water H 2 S 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 (δ 98 Mo > 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 δ 98 Mo values for sapropels S3, S4, S8, and S9 (−0.4 to þ0.9 ‰) imply mildly euxinic conditions only (0 < H 2 S < 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.

Environmental controls on very high δ238U values in reducing sediments: Implications for Neoproterozoic seawater records

“…6). At the peak of these two events, watercolumn H 2 S total concentrations were greater than the thiomolybdate switch point of 100 μmol/mol (Neubert et al, 2008) resulting in largely quantitative Mo sulfidation and δ 98 Mo auth (Sweere et al, 2021;Chiu et al, 2022) similar to modern seawater (~2.3‰; Archer and Vance, 2008). By contrast, S3, 4, 6, 8 and 9 were less well developed and moderately euxinic with lower Mo and U enrichments (Fig.…”

“…New and published sapropel data (see Table 1, S3 & S4) come from two localities in the Eastern Mediterranean; namely ODP Site 967 (Andersen et al, 2020) and Site 46PE406-E1 (Clarkson et al, 2021a;Sweere et al, 2021;Chiu et al, 2022). Calculated δ 238 U auth , δ 98 Mo auth and enrichment factor data follow previously described methods (Chiu et al, 2022).…”

“…For Site 46PE406-E1, linear sedimentation rates are calculated using previously published age models (Grant et al, 2016;Sweere et al, 2021) and combined with dry bulk densities to calculate bulk accumulation rates (BAR). Accumulation rates for Mo, U (MoAR and UAR) and organic carbon (OCAR) were then calculated by multiplying concentrations by BAR and expressed as ng/cm 2 /yr and g/m 2 /yr, respectively, to facilitate comparison to modern environments.…”

“…Sampling depths for Unit II samples from Montoyo-Pino et al (2010) are not reported so these depths were set to the same depth as Brüske et al (2020). and cover several sapropel events (S1 to S9 deposited during the last 250 kyr) that systematically capture different degrees of deoxygenation, H 2 S concentrations and bottom-water renewal times (Rohling et al, 2002;Rohling et al, 2015;Andersen et al, 2018;Andersen et al, 2020;Hennekam et al, 2020;Clarkson et al, 2021a;Sweere et al, 2021;Chiu et al, 2022). By using multiple sapropels from two nearby cores, the effect of secondary environmental variabilities on δ 238 U auth should be minimised, allowing for a more direct comparison of U reduction mechanisms under different redox regimes.…”

“…6) and δ 98 Mo auth data (Fig. 7) are used to qualitatively define the degree of euxinia for these samples (This study; Andersen et al, 2018;Andersen et al, 2020;Sweere et al, 2021;Chiu et al, 2022). Sapropels from Site 967 tend to show larger Mo and U enrichments compared to Site 64PE406-E1 (Fig.…”