This thesis investigates the influence of early diagenesis on trace metal and molybdenum isotope behavior in marine and lacustrine environments. Chapter one is a synthesis of previous research in all the marine environments investigated, providing an essential geochemical context for interpreting the observed behavior of Mo in these settings. Chapter two discusses Mo behavior in three sites from the anoxic Mexican continental margin. The data from these sites suggest that a unique Mo isotopic signature exists for authigenic Mo enrichments in anoxic sediments. Chapter three discusses Mo geochemical and isotopic behavior from a variety of marine environments to further constrain Mo behavior during early diagenesis. At sites representing end-member cases for oxic and anoxic conditions, the observed sediment Mo isotope compositions agree with those predicted from previously reported natural and laboratory fractionations. However, data from surface sediments of several study sites suggest that Mo associated with organic matter has an isotopic composition that is less fractionated (relative to modern seawater) than either oxic or anoxic authigenic Mo phases, and that this biogenic Mo may dominate the bulk sediment Mo pool in certain environments. In addition, redox cycling of Mn within the sediment column appears to strongly influence Mo geochemical and isotopic behavior. Chapter four investigates sediment geochemistry along a depth transect in Lake Tanganyika, East Africa. Permanent stratification of waters in this lake has produced a strong chemocline, with oxic conditions in the surface and sulfidic waters at depth. The sediment distributions of trace metals (specifically Mo and U) along this transect are investigated herein to evaluate changes in sediment geochemistry across this transition. Despite sulfur limitation in this freshwater system, conditions are sufficiently reducing (particularly at depths below the chemocline) to generate substantial authigenic metal enrichments.