Though there is some record of Precambrian life in the oceans from stromatolites, carbonates, and organic matter, relatively little is known about Precambrian life in non-marine settings. The non-marine record is also temporally incomplete with only limited evidence in the Archean, no convincing reports in the Paleoproterozoic, and weak sedimentological evidence for life in the Mesoproterozoic. Recent identification of unaltered ~1100 Ma old paleosols in the North Shore Volcanic Group (Minnesota) of the western arm of the Midcontinent Rift System (MCR) and of microbial mat structures both at the top of some of the paleosols and also in the associated sediments suggests that this gap in the non-marine Mesoproterozoic record can be filled. In particular, the presence of organic carbon in the microbial mats, paleosols, and as detritus in associated sediments suggests microbes fixing carbon via photosynthesis. The PI will test the hypothesis that there was an extensive non-marine biosphere by ~1100 Ma ago, and evidence for it is preserved throughout the MCR. These questions will be answered using a combination of whole rock and stable isotope geochemistry, sedimentology, and mass-balance modelling. Based upon preliminary results from in situ (i.e., within paleosols, microbial mat structures, and stromatolites) and detrital organic carbon in the MCR, C fixation by photosynthesis has been inferred as the primary metabolic pathway for the microbial community there. The abundance of organic carbon in the MCR sediments also makes it possible to use n-alkane biomarkers to make a preliminary analysis of in situ wholly terrestrial Proterozoic biomass for the first time. Finally, if metasomatism can be discounted, whole rock geochemical data from paleosols on both sides of the MCR can be used with a previous published quantitative mass-balance model to reconstruct atmospheric CO2 levels at the time.
Broader Impacts: The proposed research will form the basis of a post-doctoral research for a promising young scientist, while providing continued broad interdisciplinary training in a variety of analytical geochemistry techniques, research supervision, and science outreach. It will aslo provide research experience for two undergraduates a year, including both field and laboratory work, and long-term mentoring. Lastly, the proposed research will inform the education and outreach mission of Univ. of Michigan both by adding local data and a fieldtrip to an earth science class, but more importantly, by providing the basis for an Exhibit Museum of Natural History display that will integrate both the history of copper mining on Michigan?s Upper Peninsula and also the evidence of Mesoproterozoic life on land. The Exhibit Museum serves 70?80,000 visitors per year (including >20,000 school children from area schools), including many groups that are traditionally under-represented in science from the neighboring Detroit metropolitan area.
