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doi: 10.7185/geochemlet.2027 Authigenic uranium isotope compositions of Holocene sapropel S1 (δUauth=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 δUauth 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 δUauth values correlate with low δMoauth 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 δUauth and δMoauth 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. Received 20 December 2019 | Accepted 14 July 2020 | Published 2 September 2020

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