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Abstract: Patterns of uranium–molybdenum covariation in marine sediments have the potential to provide insights regarding depositional conditions and processes in paleoceanographic systems. Specifically, such patterns can be used to assess bottom water redox conditions, the operation of metal-oxyhydroxide particulate shuttles in the water column, and the degree of water mass restriction. The utility of this paleoenvironmental proxy is due to the differential geochemical behavior of U and Mo: (1) uptake of authigenic U by marine sediments begins at the Fe(II)–Fe(III) redox boundary (i.e., suboxic conditions), whereas authigenic Mo enrichment requires the presence of H2S (i.e., euxinic conditions), and (2) transfer of aqueous Mo to the sediment may be enhanced through particulate shuttles, whereas aqueous U is unaffected by this process. In the present study, we examine U–Mo covariation in organic-rich sediments deposited mostly in the western Tethyan region during oceanic anoxic events (OAEs) of Early Jurassic to Late Cretaceous age. Our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regarding water mass restriction and the operation of particulate shuttles in depositional systems. These insights will help to address contentious issues pertaining to the character and origin of Mesozoic OAEs, such as the degree to which regional paleoceanographic factors controlled the development of the OAEs. [Copyright &y& Elsevier]

Abstract: Modern low-oxygen marine systems exhibit patterns of molybdenum–uranium covariation that can be linked to specific attributes and processes of the depositional system, including (1) variation in benthic redox conditions, (2) the operation of particulate shuttles within the water column, and (3) the evolution of watermass chemistry. The importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum (Moauth) and uranium (Uauth) and from the (Mo/U)auth ratio of the sediment relative to the seawater Mo/U molar ratio of ∼7.5–7.9. In open-ocean systems with suboxic bottomwaters, Uauth enrichment tends to exceed that of Moauth owing to onset of Uauth accumulation at the Fe(II)–Fe(III) redox boundary, resulting in sediment (Mo/U)auth ratios less than that of seawater. As bottomwaters become increasingly reducing and at least occasionally sulfidic, the rate of accumulation of Moauth increases relative to that of Uauth, and sediment (Mo/U)auth ratios equal or exceed that of seawater. In restricted marine systems with permanently sulfidic deepwaters, the relative enrichment of Moauth and Uauth depends on additional factors. In the Cariaco Basin, which has an aqueous Mo/U ratio similar to that of seawater, the operation of a particulate Mn–Fe-oxyhydroxide shuttle serves to accelerate the transfer of Mo to the seafloor, leading to strong enrichments in Moauth relative to Uauth. In the Black Sea, the chemistry of the deep watermass has evolved to the point where its aqueous Mo/U ratio is only ∼0.04 that of seawater, as a consequence of which sediments deposited under deepwater influence are depleted in Moauth relative to Uauth. These Mo–U covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low-oxygen marine systems. Analysis of anoxic facies from two North American paleomarine systems, the Late Pennsylvanian Midcontinent Sea (LPMS) and the Late Devonian Seaway (LDS), reveals authigenic Mo–U relationships similar to those of the modern marine environments above, implying similar redox and hydrographic controls. The observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the LPMS, and with markedly restricted deepwater circulation in silled basins of the LDS. Patterns of authigenic Mo–U covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics. [Copyright &y& Elsevier]