REDOX CONTROLS ON TERRESTRIAL CARBON BURIAL IN LATE MESOPROTEROZOIC RIFT BASIN LAKES

While life is ubiquitous in terrestrial environments today, harsh environmental conditions limited the extent of the terrestrial biosphere for much of Earth’s history. Terrestrial life was restricted to certain niches, and it is important to understand the environmental changes that allowed for the expansion of terrestrial life. The Mesoproterozoic is an era of relative environmental stability yet it was characterized by a dynamic biosphere. The 1.1 Ga Midcontinent Rift in North America offers an opportunity to characterize terrestrial life and paleoenvironments at this time, as it contains a thick series of terrestrial sedimentary deposits that were never deeply buried nor subjected to substantial metasomatism. Riverine stromatolites and microbially induced sedimentary structures have been identified in the alluvial sequences of the Midcontinent Rift. Within these thick alluvial sequences lies the relatively thin and organic-rich Nonesuch Formation (~200 m).

The Nonesuch Formation represents a series of rift basin lakes that may have been intermittently connected. It preserves both organic-poor marginal lacustrine and organic-rich (~2%) deep lacustrine facies. However, the nature and amount of organic matter preserved in the Nonesuch Formation is highly variable. The mechanism behind this uneven distribution remains enigmatic, although it has been suggested that the spatial variability may be the result of differential preservation throughout the basin. For this study, samples were analyzed from six different cores taken across northern Wisconsin and Michigan. Organic carbon isotope results will be compared with trace element geochemistry (Mo, U, V) and iron speciation data in order to test the hypothesis that the abundance and composition of organic matter changes as a function of spatially variable paleo-redox conditions. Bulk carbon isotope values range between -26 and -35‰, which is suggestive of a metabolically diverse lacustrine community that is ¹³C depleted as compared to penecontemporaneous floodplain deposits (-24 to -27‰). Enrichments in redox sensitive metals display spatial patterns that appear to be related to variable enrichments in organic carbon, indicating that separate lake basins may have become isolated explaining the differential preservation.