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. 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.
N remains unclear.
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. 2 Marine Geosciences Unit, Geology and Geophysics Department, University of Edinburgh, West Mains Road, EH9 3JW, Scotland.