Rapid onset of ocean anoxia shown by high U and low Mo isotope compositions of sapropel S1
Morten B. Andersen, Alan Matthews, Miryam Bar‐Matthews, Derek Vance
Abstract
Authigenic uranium isotope compositions of Holocene sapropel S1 ( 238 U auth = 0.10 to 0.52 ; ODP core 967, 2550 mbsl) are significantly higher than the proposed upper boundary (0.2 ) associated with the transport-porewater diffusion model for sediment uranium uptake. It is shown that these high 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 238 U auth values correlate with low 98 Mo auth values (0.02 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 238 U auth and 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.