Changes in sedimentary diagenetic processes resulting from the transition from a terrestrial to a marine environment were studied through a series of riverbed cores collected from the White Oak River Estuary. Analyses of interstitial water and associated solid phases reveal systematic geochemical and mineralogical trends over an 18-km stretch of estuary and river channel related to the transition from a dominantly marine to a dominantly terrestrial setting. The most striking of these trends involves the nature of the metabolic pathways by which bacterial decomposition of organic matter occurs and probable differences in the composition of organic matter. In the riverbed stretches under periodic marine influence, sulfate reduction is a major process, owing to the availability of seawater sulfate. Upstream, however, methane generation becomes more important. Consistent with this trend is a decrease in sedimentary pyrite in methane-rich sediments. Total inorganic carbon decreases by nearly an order of magnitude upstream, and pH is significantly lower in the terrestrial environment. Chemically reactive products of organic decomposition, such as ammonia and phosphate, are considerably less concentrated in the cores taken farthest from marine influence.
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