Three Ocean Drilling Program sites in the Iberia Abyssal Plain were compared to study the impact of turbidite emplacement on redox conditions in abyssal sediments. Pore-water sulfate and methane concentrations were used as indicators of redox conditions. Two sites showed massive turbidite emplacements in the Pliocene–Pleistocene sequence, coincident with the onset of climate-driven sea-level change. These sites showed sediment redox conditions more characteristic of shallow margin sediments than abyssal sediments. Calculations suggest that oxygen diffusion into thick turbidite sequences is limited, thus most of the degradation of emplaced organic carbon occurs under anoxic conditions. Sulfate reduction is the primary mode of carbon degradation in recent turbidites. However, profiles indicate that a significant fraction of the older buried carbon is refractory with respect to sulfate reduction, but not to methanogenesis. Thus, in the deeper sections of the cores, methane is the primary carbon source for sulfate reduction.

Date of initial receipt: 5 December 1994
Date of acceptance: 22 May 1995

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