Gert J. De Lange2


Upon deposition, downward diffusing oxygen results in the "burn-down" of the upper part of organic-rich turbidites in the Madeira Abyssal Plain. This oxidation front is terminated after a few thousand years, on average, at the moment of deposition of the next turbidite. These initially homogeneous sediments form an excellent setting for studying long-term organic matter remineralization processes to establish the extent and possible selectivity of oxic vs. anoxic degradation of organic matter in a natural environment. Samples for this study have been taken from the oxidized upper part and the unoxidized lower part (from 42 to 280 meters below seafloor) of 13 organic-rich turbidites 1.2–14 m.y. old. The organic fraction in the lower part of organic-rich turbidites has remained virtually unchanged for periods as long as 14 m.y. under suboxic to anoxic conditions. In contrast, dramatic changes have occurred during the post-depositional oxidation of sediments in the tops of the same turbidites by oxygen. This has resulted in a large decrease in the organic carbon (-80%) and total nitrogen content (-50%) of the upper, relative to the lower, parts of these turbidites. In addition, this oxidation process has resulted in a 13 C org that is, on average, 1.5‰ more negative and a 15 N tot that is 1.3‰ more positive in the upper oxidized samples than it is in the lower unoxidized ones from within the same turbidite. The seemingly selective preservation of organic matter that is low in C/N and has a relatively negative 13C org is usually interpreted as being indicative of better preservation of terrestrial organic matter relative to marine organic matter. However, it could equally be explained by the different preservation of certain marine compounds with, on average, distinctly different
C org and C/N ratios. Furthermore, a decrease is observed in the total carbonate content, which is related in part to the oxidation of organic matter and S-compounds. For some samples, however, it may also be related to a more shallow carbonate compensation depth and/or some bioturbation during the downward oxidation process. Major and minor element compositions of sediments do not seem to be affected by the oxidation process if they are normalized to Al. Sulfur is the main exception. Upon oxidation, the reduced (mostly pyrite) S-compounds produce acid and sulfate, which dissolves carbonate and diffuses away in the pore water, respectively.

1 Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998. Proc. ODP, Sci. Results, 157: College Station, TX (Ocean Drilling Program).
2 Utrecht University, Institute of Earth Sciences, Department of Geochemistry, Budapestlaan 4, 3584 CD Utrecht, The Netherlands. gdlange@earth.ruu.nl