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

Abstract

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 (δ 13 C and δ 15 N) 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 (~10). Long-chain n-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 terrestrial n-alkanes relative to background sediment. Conversely, drought intervals were only distinguishable from background sediment by their higher proportion of marine algal n-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.

Interannual Variations of D14CTOC and Elemental Contents in the Laminated Sediments of the Santa Barbara Basin During the Past 200 Years

“…With anoxic conditions on the bottom of Santa Barbara Basin (SBB) offshore of California, laminated sediments have regularly been used to reconstruct high-resolution paleoclimate records (Kennett and Brassell, 1992;Schimmelmann and Lange, 1992;Kennett and Ingram, 1995;Heusser, 1998;Schimmelmann et al, 1998;Schimmelmann et al, 2003;Robert, 2004;Schimmelmann et al, 2006;Barron and Bukry, 2010;Barron et al, 2015;Sarno et al, 2020), and natural disaster signals such as earthquake and flooding events (Schimmelmann et al, 2001;Schimmelmann et al, 2013;Hendy et al, 2015;Du et al, 2018). Numerous investigations of SBB water column, sediment trap and sediment core analyses have been conducted on carbon and nitrogen cycles (e.g., Andrew and Peter, 1977;Pisias, 1978;Lynn and Simpson, 1990;Schimmelmann and Tegner, 1991;Thunell, 1998;Emmer and Thunell, 2000;Thunell, 2003;Tems et al, 2015;Davis et al, 2019;White et al, 2019), biological productivity and chemical conditions in the basin and their linkage to climate and water mixing (e.g., Browne, 1994;Gorsline, 1996;Weinheimer and Cayan, 1997;Warrick et al, 2005;Lund, 2011;Berelson et al, 2019;Napier et al, 2019).…”

“…Terrestrial organic carbon input by episodic discharge events from the mountain river systems can survive in coastal processing near Santa Barbara Channel (Kolpack and Drake, 1984;Thunell et al, 1995;Schimmelmann and Lange, 1996). The organic carbon (OC) can be rapidly transported to low-oxygen seafloor sediments in SBB where remineralization is less efficient (Sarno et al, 2020). Due to restricted circulation below the basin sills, oxygen supply to the bottom water of SBB is limited, so a suboxic environment exists at a depth below 480 m (Li et al, 2009).…”

Local and remote forcing on the interannual variations of the sedimentary δ15N in Santa Barbara Basin during the past 80 years

“…It is a shallow nearshore basin with a maximum depth of ~600 m. The basin is bordered with narrow sills to the west (~475-m depth) and east (~230-m depth), which further reduces ventilation combined with the re-mineralization of organic matter, creating an anoxic water environment (dissolved oxygen<0.1 ml/L) below the sills (Emmer and Thunell, 2000;Bograd et al, 2002;Goericke et al, 2015). The high sedimentation rate and anoxic depositional environment minimize bioturbation, creating high-resolution and low sedimentary diagenesis records in SBB (Hendy et al, 2015;Sarno et al, 2020). A minimal (<1‰) offset between the sediment trap d 15 N time series and down core d 15 N sed records in SBB provides further support that diagenetic isotopic alteration on d 15 N sed is negligible (Altabet et al, 1999;Emmer and Thunell, 2000;Davis et al, 2019;Wang et al, 2019).…”