EVIDENCE FOR EXTENSIVE LIFE ON LAND 1100 MA AGO

Recent results from the Mesoproterozoic of Scotland indicate the presence of complex organisms in lacustrine systems that were broadly comparable to marine organisms at the same time. However, less is known about the sub-aerial biosphere at that time. The Keweenawan Midcontinental Rift event ~1100 Ma ago resulted in the accumulation of thick (km-scale) packages of continental clastic sediments that have never been deeply buried and that are exposed today at the margins of Lake Superior as part of the North Shore Volcanic Group (NSVG; Minnesota) and the Powder Mill Group (PMG; Wisconsin, Michigan). Within those sediments, there are a variety of depositional environments including lacustrine, paludal, and fluvial facies, along with paleosols and dm- to m-scale riverine stromatolites. The paleosols and stromatolites both contain evidence of microbial mat structures that satisfy criteria for marine microbially induced sedimentary structures in clastic rocks. In addition, organic matter and authigenic carbonate are preserved, allowing for stable isotopic analysis. δ¹³C_org values range from the NSVG range from -29.6 to -25.5‰ (detrital and paleosol C) and from the PMG range from -27.4 to -24.0‰ (detrital and stromatolitic occluded organic C), suggesting both carbon fixation by photosynthesis and that similar biochemical pathways were used by microbial organisms on both sides of the rift. In addition, "clumped isotope" analyses (Δ₄₇) of the PMG stromatolites indicate that the organic matter has not been affected by significant post-burial heating (<60°C). Taken together, the sedimentary and isotopic evidence from both sides of the Midcontinent Rift indicates the presence of a significant terrestrial biosphere by 1100 Ma ago. Given that there are relatively few reports of life on land in the Archaean, none in the Paleoproterozoic, and scarce definitive evidence for life on land in the Mesoproterozoic outside of lacustrine systems, this new evidence for extensive microbial life from the Keweenawan Rift represents a significant expansion of the Precambrian record of microbial structures in clastic sediments and of the history of life on land.