Sedimentary organic matter preservation: A test for selective degradation under oxic conditions

We report here a test of the hypothesis that the extent of organic matter p reservation in continental margin sediments is controlled by the average pe riod accumulating particles reside in oxic porewater immediately beneath th e water/sediment interface. Oxygen penetration depths, organic element comp ositions, and mineral surface areas were determined for 16 sediment cores c ollected along an offshore transect across the Washington continental shelf , slope, and adjacent Cascadia Basin. Individual amino acid, sugar, and pol len distributions were analyzed for a 11 to 12 cm horizon from each core, a nd C-14-based sediment accumulation rates and stable carbon isotope composi tions were determined from depth profiles within a subset of six cores from representative sites. Sediment accumulation rates decreased, and dissolved O-2 penetration depths increased offshore along the sampling transect. As a result, oxygen exposure times (OET) increased seaward from decades (mid-s helf and upper slope) to more than a thousand years (outer Cascadia Basin). Organic contents and compositions were essentially constant within individ ual sediment cores but varied consistently with location. In particular, or ganic carbon/surface area ratios decreased progressively offshore and with increasing OET. Three independent compositional parameters demonstrated tha t the remnant organic matter in farther offshore sediments is more degraded . Both concentration and compositional patterns indicated that sedimentary organic matter exhibits a distinct and reproducible "oxic effect" OET helps integrate and explain organic matter preservation in accumulating continen tal margin sediments and hence provides a useful tool for assessing transfe r of organic matter from the biosphere to the geosphere.