Sediment and Organic Carbon Burial in Englebright Lake, CA over the Last Century

• Main Author: Pondell, Christina Rose
• Format: Others
• Language: English
• Published: W&M ScholarWorks 2014
• Subjects: Geochemistry
• Online Access: https://scholarworks.wm.edu/etd/1539616814; https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=2381&context=etd

The proliferation of dams has impacted over 67% of the world's largest rivers, degrading ecosystems, severing river-watershed connectivity, and reducing water, sediment, and carbon supply from rivers to the coastal ocean by 20 to 100%. Many studies have focused on the downstream impacts of dams, but few have quantified the amounts and quality of organic matter stored within impoundments. This study examined organic carbon (OC) sources and accumulation in a reservoir in northern California to characterize the organic matter retained in terrestrial systems by these man-made structures and relate the amounts and quality of OC to mining, damming and flood events in the watershed. Englebright Lake was chosen for the study site because high sedimentation rates, resulting from 19th century hydraulic gold mining, offered high-resolution records of historic (1940-2002) change, detailed records of land-use and flood events provided a history of the watershed, and the construction of an upstream dam in 1970 offered the opportunity to analyze its hydrologic impacts. Sediments from Englebright Lake as well as soil, vegetation and aquatic samples from the lake and its watershed were analyzed as part of this study. Measured plutonium radioisotope profiles were used to calculate sediment and OC accumulation rates, and organic matter was characterized using total organic carbon (TOC) and nitrogen (TN) content, stable carbon and nitrogen isotopes (delta 13C and delta15N), and fatty acid, sterol, and lignin biomarkers. OC signatures from end-member samples were used to describe aquatic (plankton and algae) and terrigenous (soil and plant) sources and interpret sediment OC profiles in Englebright Lake. Mass accumulation rates of OC ranged from 0.8 to 216 kgoc m-2 yr-1, with the highest mass accumulation rates corresponding to flood events. During the 60-year record measured in this study, 036 Tg OC were buried in Englebright Lake, 64% of which was from terrigenous sources (based on data from a two end-member delta13C mixing model). Flood events in 1964 and 1997 had a significant impact on OC accumulation, and terrigenous biomarkers, including long chain fatty acids (LCFA), lignin phenols (Sigma8), and plant sterols (stigmasterol, campesterol, sitosterol), increased 2 to 5 times in delta front cores during these events. Aquatic biomarkers in bottomset deposits responded to the upstream dam construction, and concentrations of aquatic sterols (27-nor-24-cholesta-5,22-dien-3beta-ol and cholesta5,22-dien-3beta-ol) increased after 1970, while diacid concentrations (a terrigenous biomarker) decreased following the construction of the dam. Records of hydraulic mining impacts were observed as a thick layer of OC-poor (TOC < 0.1%) sediments accumulating between 1950 and 1980 in topset deposits. Overall, the OC sequestered in Englebright Lake suggests that the magnitude of the carbon sink provided by dams and impoundments is substantial, and should be considered in global carbon cycle budgets as well as in plans for future dam construction or removal projects.