TERRESTRIAL ORGANIC-CARBON CONTRIBUTIONS TO SEDIMENTS ON THE WASHINGTON MARGIN

Elemental and stable carbon isotopic compositions and biomarker concen trations were determined in sediments from the Columbia River basin an d the Washington margin in order to evaluate geochemical approaches fo r quantifying terrestrial organic matter in marine sediments. The biom arkers include: an homologous series of long-chain n-alkanes derived f rom the surface waxes of higher plants; phenolic and hydroxyalkanoic c ompounds produced by CuO oxidation of two major vascular plant biopoly mers, lignin and cutin. All marine sediments, including samples collec ted from the most remote sites in Cascadia Basin, showed organic geoch emical evidence for the presence of terrestrial organic carbon. Using endmember values for the various biomarkers determined empirically by two independent means, we estimate that the terrestrial contribution t o the Washington margin is approximately 60% for shelf sediments, appr oximately 30% for slope sediments, and decreases further to less-than- or-equal-to 15% in basin sediments. Results from the same geochemical measurements made with depth in gravity core 6705-7 from Cascadia Seac hannel suggest that our approach to assess terrestrial organic carbon contributions to contemporary deposits on the Washington margin can be applied to the study of sediments depositing in this region since the last glacial period.