Isotope Based Inference of Organic Carbon Storage and Turnover Rates in Buried Sediments.

Lakes and reservoirs play significant role in the global carbon cycle as sites of rapid accumulation and preservation for carbon (C). Lakes and reservoirs have been estimated to remove up to 5.5 percent from the 5.5 x 1015 g C yr-1 that is anthropologically released to the atmosphere. As a result of increased erosion and sedimentation many of the 68,000 reservoirs in the United States are approaching the end of their useful lifetime, less than 100 years after they were built. We studied sediments from Searsville Reservoir, California (with almost 100% sediment trapping efficiency) to elucidate the fate of eroded soil organic carbon (SOC) after sedimentation and its potential preservation after burial in lacustrine environments. Assuming fluvially regulated sediment-bound-nutrient flux in the riverine system was in dynamic equilibrium, the stocks of soil organic and inorganic C, the carbon to nitrogen (N) ratio (C/N), and the stable isotope signatures of C and N (13C and 15N) of the reservoir sediments were correlated to the reservoir's sedimentation history (derived from 137Cs, 210Pb). We found that sediments in the deep-water portions of the reservoir resist mineralization whereas episodically drained sediments in the delta portions of the reservoir initially exhibit significant mineralization until the C gets protected with subsequent sedimentation. From dated C sedimentation profiles, we infer that turnover rate of C decreases with depth (i.e. time since deposition). This empirical analysis of the fate of eroded SOC after terrestrial sedimentation provides a mechanistic explanation of the spatial and temporal variability of buried-C storage and role of reservoirs as C sinks.