Paleoenvironmental Factors Controlling Organic Carbon Sequestration in Neoproterozoic Deep-Marine Levees

Abstract

Levees in modern deep-marine systems have been shown to sequester significant amounts of organic carbon, due largely to their wide expanse and high rates of sedimentation. However, relatively few studies have examined organic carbon sequestration in ancient deep-marine leveed slope channel systems. 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.Deep marine levee deposits of the Neoproterozoic Windermere Supergroup are exceptionally well-exposed in the Southern Canadian Cordillera of western Canada, where detailed physical description and stratigraphic logging were combined with total organic carbon (TOC) and X-ray fluorescence (XRF) to evaluate trends in the distribution of organic carbon and elemental composition within a 300 m-thick succession. These geochemical analyses were then used to reconstruct paleoenvironmental conditions such as primary productivity, ocean redox, weathering intensity, and detrital flux. In this succession, TOC ranges from < 0.1% to 4.04% (uncorrected for the effects of greenschist metamorphism). Organic-rich strata, taken to be ≥ 1% TOC, are principally confined to a single 60 m-thick stratigraphic interval, where they typically occur as anomalously thick, mud-rich sandstone turbidites. Organic matter in these beds occurs mostly as micro-scale carbon sorbed onto the surface of clay grains but can also occur as uncommon sand-sized organomineralic aggregates or discrete sand-sized amorphous grains.In this same interval, trends in elemental 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 in shallow marine environments and organic matter accumulation and preservation in the deep marine. 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. Additionally, because these strata are Neoproterozoic in age the preserved organic matter is exclusively marine in origin, which then raises the possibility that the conditions described here are unique to deep-sea turbidite systems before the evolution of metazoans or terrestrial plants. By studying the geochemical trends of both organic-rich and organic-poor rocks in this ancient outcrop, this study helps to elucidate the role of various paleoenvironmental factors in deep-marine organic matter enrichment throughout geologic time. This will ultimately improve our understanding of the complex interplay of physical, chemical, and biological processes that govern marine sedimentation and their relationship with the carbon cycle and past global climate, particularly in systems that pre-date terrestrial vegetation.