Distinguished Speaker Sheldon Looks at "Life on Land One Billion Years Ago"
EES welcomes Dr. Nathan Sheldon, University of Michigan
"What was it like for life on land one billion years ago?"
September 27, 2019, 12:30 PM, Room 204, Natural Science Building
The 1.1 billion year old Midcontinent Rift spans from Ontario to Kansas, with some of the best surface exposures on Michigan's Upper Peninsula and in northern Minnesota along Lake Superior. The North Shore Volcanic Group (NSVG; Minnesota) and the Copper Harbor Conglomerate (Michigan) both preserve microbially-induced sedimentary structure, microbialites, and unaltered organic carbon in floodplain facies and the lacustrine Nonesuch Formation (Michigan, Wisconsin) preserves organic carbon, degraded biomarkers, and acritarchs. Data from the microbialites and paleosols allow us to reconstruct the atmospheric atmospheric carbon dioxide and oxygen levels through the use of mass-balance models, organic and carbonate geochemistry, and stable Cr isotopes. When used in concert with a coupled atmosphere-ocean general circulation model, it becomes clear the atmospheric carbon dioxide levels were too low overcome the "faint young Sun" paradox and that another greenhouse gas such as methane or nitrous oxide was needed to maintain equable environments for the life that flourished in both the subaqueous Nonesuch Formation and the subaerial NSVG. To understand how the low atmospheric oxygen levels impacted the biosphere, my group also studies a modern analogue site in Lake Huron at the Middle Island Sinkhole (MIS). MIS is nearly anoxic, but has significant groundwater-sourced sulfide, making it analogous both the shallow marine and lacustrine Proterozoic environments. Metagenomic and metatranscriptomic analyses suggest a relatively low diversity community, but one with significant metabolic flexibility as has been inferred for Proterozoic ecoysystems. Comparison of metal biogeochemistry between MIS and the Nonesuch Formation indicates that the water column in the Nonesuch Formation lake was intermittently oxic all the way down the the bottom but that the relative oxygen level had little impact on acritarch diversity or abundance, suggesting that they were well-adapted to fluctuating environmental conditions. Similarly, organic carbon in NSVG paleosols demonstrates Rayleigh fraction to depths meters below the surface, suggesting the low atmospheric oxygen levels did not limit terrestrial microbial productivity. Taken together, the Midcontinental Rift recovers a widespread terrestrial biosphere that inhabited a wide variety of ecological niches in a moderate carbon dioxide, low oxygen world.