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Organic Carbon in Deep-Marine Levees as a Possible Driver of Neoproterozoic Atmospheric and Oceanic Conditions, Windermere Supergroup, British Columbia, Canada


Deep-marine levees, which commonly extend up to a few tens of km beyond the margins of the coeval channel, in addition to being areas of exceptionally high sedimentation rate, have received little research attention compared to the adjacent channels - an artefact of generally poor exposure in the ancient rock record and far-distant control points in the modern. Because of this bias many important features of levees remain poorly documented, including their potential as sites for significant carbon burial. Levee deposits in the Neoproterozoic Windermere Supergroup at the Castle Creek study area (BC, Canada) were sampled for total organic carbon (TOC) in order to detect potential changes in environmental conditions like organic production on the shelf and sea level. Previous work in the area has shown that TOC in levee deposits is highly variable and ranges from <1% to ~3.5%, although this is the first work to systematically study TOC in these strata. High TOC was recorded in both silty and sandy facies, but is less common in sandy strata. Sand beds with elevated TOC are usually Tbcde or Tcde turbidites with discrete dark grey-black graphite laminae. This graphite is interpreted to have formed through in-situ metamorphism of detrital organic matter that was sourced from disaggregated bacterial mats on the shelf before being remobilized and transported downslope by turbidity currents. Organic particles, which were hydraulically equivalent to medium sand grains, were then preferentially sorted during bedload and avalanche transport. The organic detritus in the silty and muddy facies is interpreted to have been concentrated in the upper silt- and clay-rich portion of turbidity currents that overspilled the channel margins. Although modern levees have been shown to account for a large proportion of the world’s total buried organic carbon, there have been few studies to evaluate this in their ancient counterparts. The Neoproterozoic was characterized by several large, potentially global glaciations that likely had a significant effect on sediment and possibly also nutrient supply to the oceans. The build-up of numerous post-glacial continent-margin turbidite systems with well developed levees and significant carbon burial may have had a major effect on atmospheric CO2 and Neoproterozoic climate. Work here aims to link large-scale climatic and oceanic conditions with organic carbon deposition in ancient, and by extension, modern turbidite systems.