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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 14 C-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 O 2 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.

We determined the concentrations and compositions of coarse particulate (Ͼ63 m), fine particulate (0.1-63 m), and dissolved (0.001-0.1 m) organic matter collected along a river reach extending from a first-order stream in the Bolivian Andes, through the Beni River system, to the lower Madeira and Amazon Rivers. Dissolved organic carbon (DOC) concentrations increased down the total reach from ϳ80 to 350 M. The percentage of total DOC with a molecular weight greater than ϳ1,000 atomic mass units that could be isolated by ultrafiltration also increased downstream from 40 to 80%. Weight percentages of organic carbon in the ultrafiltered isolates also grew downstream from 5% at the uppermost station to 37% in the Amazon mainstem. Organic carbon composed only 0.4-1.2 weight percentage of the total mass of the fine particulate fraction, which accounted for 70-80% of the total organic carbon (TOC) in transport through the highly turbid (ϳ600-2000 mg L Ϫ1 ) Beni sequence. Observed compositional differences were related primarily to the size fractions in which the organic matter occurred. On average, coarse particulate organic material exhibited an atomic C : N of 24, whereas ultrafiltered DOM was nitrogen poor, (C : N)a ϭ 34, and fine particulate material was nitrogen rich, (C : N)a ϭ 15. The lignin and stable-carbon isotopic compositions of these fractions indicate tree leaves and other nonwoody tissues from C3 land plants as predominant sources. Three molecular parameters demonstrate that the coarse, fine, and dissolved fractions of individual water samples are increasingly degraded downstream. Elemental nitrogen, amino acids, and basic amino acids are all preferentially associated with fine minerals. Observed geographical patterns included more positive ␦ 13 C values in particulate organic matter from high altitude sites and an increase in the abundance and degradation of ultrafiltered dissolved organic matter down the drainage system. Many of these compositional patterns are imprinted within materials carried by low-order, high-altitude tributaries and appear to reflect processes occurring on the landscape.

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