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

Caitlyn T Sarno,Catherine V Davis,Robert C Thunell,Eric J Tappa,Claudia R Benitez‐Nelson,Ingrid L Hendy,Lori A Ziolkowski

doi:10.1029/2020pa003849

Caitlyn T Sarno, Catherine V Davis + Show 5 more

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https://doi.org/10.1029/2020pa003849

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Abstract

AbstractClimate 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 (δ13C and δ15N) 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 (C27, C29, C31, and C33), characteristic of terrestrial OC, dominated all types of sediment deposition but were 4 times more abundant in flood layers. Marine algae (C15, C17, and C19) and macrophytes (C21 and C23) 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.

Highlights

Burial of organic carbon (OC) in marine sediments and coastal environments in particular, transfers carbon from the short term atmosphere-biosphere carbon cycle and into semi-permanent geological repositories
Turbidite sediment is visually characterized by a homogenous olive coloration and larger grain size, and are derived from turbidity currents stimulated by mass failure on the basin slope [Rack and Merill, 1995; Schimmelmann et al, 1998; Hendy et al, 2015; Du et al, 2018]
Turbidites contain a mixture of flood and background sediment characteristics, with Total Organic Carbon (TOC) and Total Nitrogen (TN) concentrations significantly different from flood and background sedimentation (p < 0.001)

Summary

Introduction

Burial of organic carbon (OC) in marine sediments and coastal environments in particular, transfers carbon from the short term atmosphere-biosphere carbon cycle and into semi-permanent geological repositories This carbon sequestration in the rock record influences global climate via the regulation of the greenhouse gas, carbon dioxide [Berner, 1982; Martin et al, 1987; Sarmiento and Sundquist, 1992; Hedges and Keil, 1995]. Non-event sedimentation is responsible for 75% of the TOC buried over the past 2,000 years (Figure 7). Flood events account for 11 of the past 2,000 years (less than 1% of time and assuming instantaneous burial) and are responsible for 8% of the TOC buried.

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