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Mercury (Hg) is a globally distributed trace metal with natural and anthropogenic sources to the environment. Marine reservoirs provide the primary vector of human exposure to Hg and are integral to global Hg cycling. To elucidate environmental influences on marine Hg cycling and discern prominent sources of Hg to modern estuaries, Hg isotopic compositions and concentrations were analyzed in pre-Industrial and modern marine sediments. Mid-Pleistocene Mediterranean Sea sapropels had Hg concentrations (HgT) 6-fold higher than non-sapropels, reaching 400 ng/g. Comparison between Hg and other trace metals indicated that elevated primary productivity promoted sediment Hg accumulation. In the Baltic Sea, surface sediments had HgT up to 150 ng/g and 3-times higher than co-located pre-Industrial sediments, and Hg isotope analysis demonstrated that an additional source of Hg also contributed to increased sediment HgT. Pre-Industrial and modern Baltic Sea sediment Hg can be explained by mixing between marine Hg (delta-202Hg = -1.1 permil) and anthropogenic Hg (delta-202Hg = -0.5 permil). In San Francisco Bay (SF Bay), surface sediment HgT ranged from 160 to 850 ng/g, with highest HgT in Central and Lower South Bay. The Hg isotopic compositions of sediments displayed a geographic gradient, with delta-202Hg values ranging from -0.3 permil in the south to -1.0 permil in the north. Fluvial sediments in southern and northern tributaries had delta-202Hg values of -0.3 and -0.9 permil, and demonstrated that two large regional sources dominated Hg input into SF Bay. Small fish co-located with sediments demonstrated a similar spatial gradient in Hg isotope compositions, with delta-202Hg values ranging from +0.6 to -0.3 permil. A consistent offset between sediment and fish delta-202Hg values demonstrated that sediments were the primary source of Hg to SF Bay small fish. Additionally, this work used Hg isotopes to identify perturbations to the global Hg cycle at the Paleocene-Eocene Boundary (PEB). A large increase in HgT and delta-202Hg values was observed at the PEB and provides the basis for future Hg isotope research in paleoceanography.