Sea Level as a Controlling Factor in Deep-Marine Neoproterozoic Organic Carbon Sequestration

The late Neoproterozoic was a time of significant change in Earth systems and climate, including widespread oxygenation of the atmosphere and oceans, the evolution of Ediacaran metazoans, and large shifts in ocean chemistry and the carbon cycle following three major glaciations in the Cryogenian and Ediacaran. Previous studies have found that total organic carbon contents (TOC) are generally low throughout the Neoproterozoic, suggesting low levels of primary productivity and/or low rates of marine carbon burial, although uncommon TOC-rich intervals have been observed.

In this study, a 350-meter-thick succession of mudstone-dominated levee deposits of the Neoproterozoic Windermere Supergroup in B.C., Canada, was stratigraphically logged and samples were taken with 4–10 m spacing. TOC and X-ray fluorescence analyses were conducted to evaluate the distribution of organic carbon and model paleoenvironmental proxies. TOC ranged from <0.2% to 4% (uncorrected for the effects of metamorphism). Organic-rich strata are principally confined to a single 60-m-thick stratigraphic interval corresponding to sea-level highstand, with organic matter occurring mostly as micro-scale carbon sorbed onto the surface of clay grains. Sea level was found to be a controlling factor in organic carbon enrichment in this interval and a necessary condition for high rates of primary productivity, but was required in conjunction with other paleoenvironmental factors, including high rates of continental weathering and terrigenous input. These trends can be used to understand how modern deep marine deposits can sequester organic matter and impact the carbon cycle during sea-level transgression and highstand.