Multi-biomarker characterization of sedimentary organic carbon in small rivers draining the Northwestern United States

The sources of sedimentary organic carbon in various river systems draining the Pacific Northwest (USA) were assessed using a biomarker approach. Sediment samples were collected in creeks and small rivers from Oregon and Washington states (Cascades and Coastal Range ecoregions) and analyzed as total and silylated total extracts using gas chromatography–mass spectrometry (GC–MS). In general, the individual organic compounds were similar in the river samples from both the Cascades and the Coast Range, with apparent higher averaged abundances (normalized to organic carbon content) observed for the Cascades sediments. Major contributions in the river samples were from sterols and triterpenoids derived mainly from the vegetation bordering the rivers, followed by n-alkanols and n-alkanes from epicuticular plant wax. Fatty acids (16:0, 16:1, 18:0, 18:1, 18:2), cholesterol, monoacylglycerols (e.g., 1-O-hexadecanoyl glycerol) and glucose were important organic tracers detected especially in higher altitude systems, presumably due to autochthonous production, namely from algal blooms during late spring. Resin acids (e.g., dehydroabietic acid, a biomarker for conifers) were expected to be the major tracer input to these sediments. However, diterpenoids were detected in significant amounts only at sites directly bordering conifer forests, suggesting that those compounds are not transported downstream during normal river flow.