Why some Mediterranean sapropels survived burn-down (and others did not)

We have evaluated, using a numerical model, the burn-down depth (depth of post-depositional oxygenation) of organic matter-rich sapropels under aerobic conditions at the seafloor. The model suggests that sapropels may be erased by burn-down to depths of at least 5 cm after 10 kyr of bottom-water reoxygenation. A test of the sensitivity shows that the parameters exerting a major control over the burn-down process are the flux of freshly sedimenting organic carbon, the oxygen Concentration of the bottom water, the amount of organic matter in the sapropel, and the sedimentation rate. In comparison, bioturbation is of minor importance. The result implies that burial of elevated concentrations of non-refractory organic carbon is generally improbable when the sedimentation rate is lower than 1-2 cm kyr(-1). The record of sapropel deposition in the Mediterranean Sea and elsewhere may thus be biased towards depositional environments where the sedimentation rate is large enough to shield the organic-rich layer from oxygenation. (C) 1997 Elsevier Science B.V.