The Impacts of Flood, Drought, and Turbidites on Organic Carbon Burial Over the Past 2,000 years in the Santa Barbara Basin, California

Climate conditions and instantaneous depositional events can influence the relative contribution of sediments from terrestrial and marine environments and ultimately the quantity and composition of carbon buried in the sediment record. Here, we analyze the elemental, isotopic, and organic geochemical composition of marine sediments to identify terrestrial and marine sources in sediment horizons associated with droughts, turbidites, and floods in the Santa Barbara Basin (SBB), California, during the last 2,000 years. Stable isotopes (delta C-13 and delta N-15) indicate that more terrestrial organic carbon (OC) was deposited during floods relative to background sediment, while bulk C to nitrogen (C/N) ratios remained relatively constant (similar to 10). Long-chainn-alkanes (C-27, C-29, C-31, and C-33), characteristic of terrestrial OC, dominated all types of sediment deposition but were 4 times more abundant in flood layers. Marine algae (C-15, C-17, and C-19) and macrophytes (C(21)and C-23) were also 2 times higher in flood versus background sediments. Turbidites contained twice the terrestrialn-alkanes relative to background sediment. Conversely, drought intervals were only distinguishable from background sediment by their higher proportion of marine algaln-alkanes. Combined, our data indicate that 15% of the total OC buried in SBB over the past 2,000 years was deposited during 11 flood events where the sediment was mostly terrestrially derived, and another 12% of deep sediment OC burial was derived from shelf remobilization during six turbidite events. Relative to twentieth century river runoff, our data suggest that floods result in considerable terrestrial OC burial on the continental margins of California.