Early organic diagenesis: The significance of progressive subsurface oxidation fronts in pelagic sediments

Abstract

Porewater and solid phase geochemical data at two contrasting NE Atlantic stations are reported. Station 10552, on the Cape Verde abyssal plain, is a site of slow pelagic accumulation ( ca. 0.4 cm kyr −1). Molecular oxygen is present in the sediment column to at least 2 m, and probably much deeper, labile organic-carbon is almost totally consumed in the upper few centimetres of the sediment. By contrast, at station 10554 on the Madeira abyssal plain, the pelagic sequence has been interrupted by the occasional deposition of organic-rich turbidites. Porewater oxygen and nitrate profiles show that subsurface organic metabolism of the organic-carbon associated with the uppermost turbidite layer is a significant fraction of the overall metabolism in the sediment column. This metabolism occurs at a relatively thin reaction front which progresses deeper into the turbidite with time. This phenomenon exerts a controlling influence on the present nutrient profile and redox succession. In a less extreme form, substrate distributions of this latter type are not uncommon in Atlantic sediments. A model has been developed which is controlled by both oxygen and nitrate data. This model permits a vertical profile of metabolic activity to be derived, and also gives estimates of the reaction rate constants and solid phase mixing rates at these two contrasting stations. About 30% of the total activity at station 10554 is located within the turbidite at the deepening reaction front; this is a non-steady-state condition. In fact, it is found that the integrated metabolic activity at the two stations is not dissimilar ( ca. 1–2 × 10 −13 moles cm −2 sec −1 ). The striking differences in redox profile are therefore primarily attributable to differences in the distribution of metabolic activity within the column.