Controls on the stratigraphic distribution of organic carbon in ancient deep-marine levees

Abstract

Modern deep-marine levees have been shown to be an important reservoir of the world's total buried organic carbon; however, few studies have attempted to assess this in ancient systems. Deep-marine levees are extensive features that experience high rates of sedimentation, making them ideal sites for significant carbon burial, and therefore examining the distribution of organic material in ancient levee deposits could provide insight into paleoenvironmental conditions and the evolution of ancient ocean and climate systems. In this study, a 350-m-thick succession of mudstone-dominated levee deposits of the Windermere Supergroup in British Columbia, Canada, was stratigraphically logged in centimeter-scale detail and samples taken with 4–10 m spacing. X-ray fluorescence (XRF) and total organic carbon (TOC), were conducted to evaluate the elemental composition and distribution of organic carbon to evaluate trends in paleoenvironmental conditions such as primary productivity, ocean redox, weathering intensity, and detrital flux. TOC ranges from <0.2% to 15.55% (corrected for the effects of metamorphism). Organic-rich strata, taken to be ≥ 1% TOC, are principally confined to a single 60 m-thick stratigraphic interval, and typically occur as anomalously thick, mud-rich sandstone turbidites, with organic matter occurring mostly as micro-scale carbon sorbed onto the surface of clay grains. In this same interval, trends in XRF data indicate an increase in primary productivity, weathering intensity, and detrital influx, and a decrease in ocean oxygenation levels. These data suggest that intense continental weathering, high terrigenous input, elevated sea level, and relatively low oxygenation conditions all act to enhance organic matter production, accumulation, and preservation. However, although all these components contributed to increased organic production, accumulation, and preservation on their own, the results of this study suggest that it is the temporal coincidence of all of them in a “perfect storm” that is required for significant organic carbon enrichment.