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 (Mo auth) and uranium (U auth) 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, U auth enrichment tends to exceed that of Mo auth owing to onset of U auth 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 Mo auth increases relative to that of U auth, and sediment (Mo/U) auth ratios equal or exceed that of seawater. In restricted marine systems with permanently sulfidic deepwaters, the relative enrichment of Mo auth and U auth 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 Mo auth relative to U auth. 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 Mo auth relative to U auth. 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.