Gert J. De Lange, Utrecht University (Netherlands)
Caroline Slomp, Utrecht University (Netherlands)
John Thomson, Ex-SOC (United Kingdom)
Daniela Crudeli, Milano Bicocca University (Italy)
Fred Prahl, Oeregon University (United States)
Ana Ferreira, Institue for Fisheries (Portugal)
Jaap Sinninghe-Damste, NIOZ (Netherlands)
Jack Middelburg, Netherlands institute for Ecology (Netherlands)
Karin Zonneveld, Bremen University (Germany)
Gerard Versteegh, Bremen University (Germany)
Carme Huguet, NIOZ (Netherlands)
Stefan Schouten, NIOZ (Netherlands)
After deposition, the downward diffusion of oxygen has resulted in the "burn-down" of the upper part of organic-rich turbidites in the Madeira Abyssal Plain. These initially homogeneous sediments form an excellent setting for studying long-term organic matter remineralisation processes and thus provide excellent material to establish the extent and possible selectivity of oxic vs. anoxic degradation of organic matter in a natural environment.
Comparing the oxidized upper part and the unoxidized lower part for a set of 13 organic-rich turbidites with depositional ages of 0.2?14 Ma old, it appears that the organic fraction in the lower part has remained virtually unchanged, whereas dramatic changes have occurred in the upper parts of the same turbidites. Post-depositional oxidation alone has resulted in a large decrease in the organic carbon (-80%) and total nitrogen content (-50%), and in a change of d13Corg (-1.5), and d15Ntot (+1.3). The seemingly selective preservation of organic matter that is low in C/N and has a relatively negative d13Corg is usually interpreted as being indicative of better preservation of terrestrial organic matter relative to marine organic matter. Additional dino cysts and biomarker studies indicate that selective preservation may also influence their interpretation as paleo proxies. Similar results appear to occur in eastern Mediterranean sediments with normal pelagic deposition (∼3 cm/ka), but with rather contrasting, paleoceanographically controlled, organic matter contents versus depth. Again such postdepositional processess severely influence the potential interpretation of data if not recognized properly. Pertinent examples from both areas will be illustrated.