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 preservation in continental margin sediments is controlled by the average period accumulating particles reside in oxic porewater immediately beneath the water/sediment interface. Oxygen penetration depths, organic element compositions, and mineral surface areas were determined for 16 sediment cores collected along an offshore transect across the Washington continental shelf, slope, and adjacent Cascadia Basin. Individual amino acid, sugar, and pollen distributions were analyzed for a 11 to 12 cm horizon from each core, and 14C-based sediment accumulation rates and stable carbon isotope compositions were determined from depth profiles within a subset of six cores from representative sites. Sediment accumulation rates decreased, and dissolved O2 penetration depths increased offshore along the sampling transect. As a result, oxygen exposure times (OET) increased seaward from decades (mid-shelf and upper slope) to more than a thousand years (outer Cascadia Basin). Organic contents and compositions were essentially constant within individual sediment cores but varied consistently with location. In particular, organic carbon/surface area ratios decreased progressively offshore and with increasing OET. Three independent compositional parameters demonstrated that 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 continental margin sediments and hence provides a useful tool for assessing transfer of organic matter from the biosphere to the geosphere.