Geochemical processes that occur in marine sediments have significant impacts on the seafloor-seawater exchange of soluble elements and the subsequent composition of sediments. Postdepositional oxidative degradation of sedimentary organic matter is a central element of many of these processes. The oxidation of organic matter follows a general sequence of terminal electron acceptors of, from first to last, interstitial oxygen, nitrate, Mn(IV) oxides, Fe(III) oxides, and sulfate (Froelich et al., 1979; Schulz et al., 1994) in the transition from oxic to suboxic to anoxic sediment conditions. Various mineral dissolution and precipitation processes occur during this sequence (Berner, 1980; von Breymann et al, 1991; Canfield, 1993), and the interplay of oxidizing and reducing conditions influences the mobilities of many metals (Wilson et al., 1986; Pruysers et al., 1993; Thomson et al., 1993). Moreover, evolution of interstitial dissolved CO2 from oxidation of organic matter increases in situ dissolution of CaCO3 (Emerson and Bender, 1981; Berelson et al., 1990).
Most models of organic matter degradation and related geochemical processes assume that steady-state reaction and diffusion are dominant, yet non-steady-state conditions prevail in many areas of the floor of the deep-sea, particularly in those regions where turbidites accumulate (Buckley and Cranston, 1988). An opportunity to evaluate the consequences of turbiditic sediment accumulation on sedimentary geochemical processes that subsequently occurred over a time span of several million years is provided by the transect of sites drilled by Ocean Drilling Program (ODP) Leg 149 across the landward edge of the Iberia Abyssal Plain. In this synthesis, we use interstitial sulfate and methane concentrations as indicators of oxidation-reduction processes that have happened within and are influenced by a turbidite unit emplaced during Pliocene-Pleistocene times.