Rapid onset of ocean anoxia shown by high U and low Mo isotope compositions of sapropel S1

Abstract

Authigenic uranium isotope compositions of Holocene sapropel S1 (δ 238 U auth = þ 0.10 to þ0.52 ‰; ODP core 967, 2550 mbsl) are significantly higher than the proposed upper boundary (þ0.2 ‰) associated with the transport-porewater diffusion model for sediment uranium uptake. It is shown that these high δ 238 U auth values are compatible with rapid initial slowdown of thermohaline overturning and the development of an anoxic water column. These conditions would favour U uptake in an organic-rich floccule layer overlying the sediment-water interface. The high δ 238 U auth values correlate with low δ 98 Mo auth values (þ0.02 to −0.88 ‰), interpreted to reflect weakly euxinic conditions controlled by thiomolybdate-molybdate solution equilibria. The S1 data contrast markedly with published data from last interglacial sapropel S5 from the same core, which show δ 238 U auth and δ 98 Mo auth characteristics compatible with a restricted euxinic basin due to progressive slowdown in the thermohaline circulation. The U-Mo isotope data for S1 are similar to a range of published palaeosettings. Sapropels are therefore shown to be useful templates for the unravelling of the interplay between productivity and deep water renewal times in ancient settings.

Eastern Mediterranean climate change deduced from the Soreq Cave fluid inclusion stable isotopes and carbonate clumped isotopes record of the last 160 ka

“…The formation of these organic carbon rich sediments is related to slowdown in the Mediterranean thermohaline intermediate and deep water formation (Rohling et al, 2015). Recent U-Mo isotope studies have validated the slowdown in EMS deep water circulation rate during two major sapropel events covered in this study, S5 and S1 (Andersen et al, 2018(Andersen et al, , 2020. This occurs through the creation of a low density surface water layer due to enhanced influx of low   Ow freshwater from the Nile River and North African rivers (that are dry at present) during periods of African monsoon maxima, together with increased rainfall from Atlantic westerlies in the Mediterranean region, and post-glacial sea level rise (Rossignol-Strick, 1985;Rohling, 1994;Thomson et al, 1999;Kallel et al, 2000;Emeis et al, 1998Emeis et al, , 2000aEmeis et al, ,b, 2003Scrivner et al, 2004;Bar-Matthews et al, 2000;de Lange et al, 2008;Osborne et al, 2008;Bar-Matthews, 2014;Grant et al, 2016).…”

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

“…1; Supplementary Information) are similar to those previously published values for sapropel S1, suggesting that these sediments were deposited under mildly euxinic conditions ([H 2 S] < 11 μM) ( Fig. 2; Azrieli-Tall et al, 2014;Matthews et al, 2017;Andersen et al, 2020). Alternative models for low δ 98 Mo values for these sediments, such as deposition associated with Fe or Mn (oxyhydr)oxides (e.g., Reitz et al, 2007) are not supported by the Mn, Mo, and U concentration data as these data do not show evidence for a strong Fe-Mn particulate shuttle (Supplementary Information; Figs.…”

“…High δ 98 Mo (up to ∼2.3 ‰) values have been found for S5, reflecting strongly euxinic conditions with long deep water renewal times (Andersen et al, 2018). By contrast, low δ 98 Mo values (mostly <1 ‰) have been found for sapropel S1, indicating on average lower bottom water H 2 S concentrations (Reitz et al, 2007;Azrieli-Tal et al, 2014;Matthews et al, 2017;Andersen et al, 2020).…”

“…The Mo isotope composition of organic-rich sediments is generally lighter than seawater (2.34 ‰; Nakagawa et al, 2012), with reported δ 98 Mo values as low as ∼ −0.7 ‰ (e.g., Andersen et al, 2020;Brüske et al, 2020). Low δ 98 Mo values in mildly euxinic environments can be attributed to isotope fractionation associated with the incomplete conversion of molybdate to thiomolybdate species, with the more sulfidised species becoming progressively isotopically lighter (Tossell, 2005;Kerl et al, 2017).…”

Enhanced phosphorus recycling during past oceanic anoxia amplified by low rates of apatite authigenesis

“…Intense rainfall during stronger monsoon intervals resulted in stratification and deoxygenation, which, in turn, increased the burial of C ORG and the recycling of P (38,64). Bottom water conditions during the formation of S1 (10 ka ago) were generally less reducing than for S5 (125 ka ago) and i-282c (2.943 Ma ago) (36,38).…”