EVALUATING THE ENVIRONMENTAL AND ECOLOGICAL SETTING OF A MESOPROTEROZOIC ICE AGE THROUGH THE ANALYSIS OF LIPID BIOMARKERS

We present a basin-wide analysis of lipid biomarkers and time-series elemental and isotopic trends from the Upper Vazante Group, Brazil in order to evaluate basin-scale paleoenvironmental and paleoecological conditions during a newly identified Mesoproterozoic ice age. The Upper Vazante Group contains two intervals of sub-greenschist, organic-rich shale with intermittent ice rafted debris, which accumulated during post-glacial transgression of the Morro do Calcário and Lapa ice ages. Percent total organic Carbon (% TOC) of the Morro do Calcário and Lapa shales range from 0.32 - 9.80 % and 0.82 - 3.46 % respectively. Re-Os and U-Pb detrital zircon ages of the two glacial intervals are consistent with a late Mesoproterozoic depositional age (1.1 and 1.0 Ga respectively). The Upper Vazante Group therefore represents the only radiometrically constrained ice age sediments of the Mesoproterozoic. This, along with the low metamorphic grade and high organic content of the shale units, offers a unique opportunity for the extraction and analysis of biomarkers that has not previously been afforded to other sedimentary units of similar age. Biomarkers are fossilized organic compounds from once-living organisms that are preserved through burial and digenesis and can provide direct information about the metabolisms, physiologies, and identities of organisms present at the time of deposition. Based on an initial data set from a basin-wide distribution of exploratory drill cores through the Morro do Calcário shale we identified a suite of biomarkers and compounds, which included n-alkanes, pristane, phytane, hopanes, and steranes. The ratio of pristane to phytane (0.43-0.66) and the C₃₅ hopane index (4.45-6.74 %) indicate reducing conditions at the time of deposition. Additionally, the ratio of steranes to hopanes (0.20-0.23) indicates that although eukaryotes were present, bacteria dominated the microbial community. The biomarker analysis of the Morro do Calcário shale as well as the Lapa and other shale units from the Upper Vazante Group provide insights into the paleontological and environmental conditions present during a previously unknown Mesoproterozoic ice age.