Extents and implications of organic matter alteration at oxidation fronts in turbidites from the Madeira Abyssal Plain

Organic geochemical, stable N isotope, and mineral surface area analyses were made on samples spanning oxidation fronts at the tops of organic matter-rich turbidites from Sites 951 and 952 on the Madeira Abyssal Plain (MAP), off northwest Africa. These oxidation fronts permit the isolation of the effects of oxygen exposure on sedimentary organic matter preservation. Organic matter contents of the unoxidized turbidites, which originate on the productive northwest African margin, vary, but contents within a given turbidite are uniform. Across the oxidation fronts, most of the organic material previously preserved for extended periods in the absence of oxygen is lost, resulting in contents typical of pelagic sediments. Organic carbon-to-total nitrogen ratios drop, partly or wholly the result of preservation or immobilization of inorganic N. Lignin yields in the unoxidized turbidites are low, consistent with a minor terrestrial organic component, as is also indicated by C:N ratios and previous biomarker studies. Lignin losses across the fronts parallel those of organic C, indicating nonselective degradation of terrestrial relative to marine organic material. Stable N isotope compositions shift to heavier values across the fronts, from compositions typical of many marine sediments. The shifts resemble early diagenetic patterns in oxygenated marine settings, but the degree to which they result from preservation or immobilization of inorganic N remains unclear. Mineral surface areas show a roughly inverse relationship with relative contents of CaCO₃, both within the unoxidized turbidites and across the oxidation fronts. In the latter case, surface areas rise with a decrease in CaCO₃, presumably because of dissolution of the (coarse) carbonate fraction. Organic carbon contents drop from loadings near the 'monolayer equivalent' relative to the available surface area, which is typical of continental margin sediments deposited under suboxic conditions, to 'submonolayer' loadings typical of oxygenated pelagic sediments. Thus, in effect, oxidation occurring at the MAP oxidation fronts converts organic matter-rich continental margin sediments into organic matter-lean pelagic deposits. This result clearly indicates that, in the pelagic realm, where exposure is normally long, oxygen availability is a primary control on sedimentary organic matter content.