BIOGEOCHEMICAL PATHWAYS OF NUTRIENT AND METAL BURIAL IN PROTEROZOIC LAKES AND IMPLICATIONS FOR ATMOSPHERIC OXYGEN LEVELS

While atmospheric oxygenation history and microbial evolution are broadly understood from records of Proterozoic oceans, relatively little is known about Proterozoic lake chemistry and about how well it records atmospheric chemistry. The preservation of metals and nutrients in a modern analogue and late-Mesoproterozoic lake sediments can be used to understand ancient freshwater biogeochemistry and possibly atmospheric oxygenation. Previous work from the Stoer Group (~1.18 Ga; Scotland) used enhanced Mo burial and Δ³⁴S to indicate that, in contrast to marine environments, there was no nutrient limitation in Proterozoic lakes due to sufficient atmospheric pO₂ to weather nutrients and support biota adapted for an oxygenated atmosphere. However, more geochemical data and sedimentary features in the Stoer Group have been interpreted as recording a marine influence. As a result, because of the uncertain origin of the Stoer Group, the implications of terrestrial Mesoproterozoic habitats as recorders of atmospheric oxygenation and potential biological evolution remains an open question. We can test this idea by comparing nutrient and metal (e.g. Fe and Mo) burial in the lacustrine Nonesuch Formation (~1.1 Ga; USA), and a modern analogue: the Middle Island Sinkhole (MIS). MIS is a shallow karst feature at the bottom of Lake Huron fed by low oxygen, high-sulfate groundwater that limits eukaryotic activity and harbors metabolically flexible cyanobacteria similar to those proposed to inhabit Proterozoic oceans. Due to its redox chemistry, salinity, and biology freshwater MIS can be considered a modern analogue for Proterozoic waters. The Nonesuch Formation and MIS both lack enhanced Mo burial and indicate ferruginous conditions, which is consistent in the paleorecord with both low atmospheric pO₂ and low biotic productivity. This contrasts with the initial Stoer Group conclusions, indicating that a non-limited nutrient state cannot be generalized to all Proterozoic lakes. Given as well that MIS matches many of our assumptions about what sediment burial in a low oxygen atmosphere should look like even in a fully oxygenated atmosphere, apparent oxygenation of Proterozoic lacustrine systems may indicate only local water column and sediment burial conditions rather than the composition of the atmosphere.