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

Authigenic uranium isotope compositions of Holocene sapropel S1 (delta U-238(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 delta U-238(aut) 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 delta U-238(aut) values correlate with low delta Mo-98(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 delta U-238(aut) and delta Mo-98(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 palaeo-settings. Sapropels are therefore shown to be useful templates for the unravelling of the interplay between productivity and deep water renewal times in ancient settings.