BIOMARKER GEOCHEMISTRY OF A POST-GLACIAL NEOPROTEROZOIC SUCCESSION IN BRAZIL

Studies of organic biomarkers in post-glacial shale may help to characterize biomass contributions from various micro-organisms thriving in the aftermath of Neoproterozoic ice ages. Understanding the suite of possible metabolisms should also provide clues to redox conditions and sources of alkalinity during post-glacial transgression. Sequence stratigraphic evidence for significant sea level drawdown, and deposition of a dropstone-laden and iron-formation bearing diamictite at the top of the Neoproterozoic Vazante Group (Pamplona Formation) in east-central Brazil, is consistent with the interpretation of the overlying Lapa Formation as a post-glacial succession. The Lapa Fm. contains rhythmic carbonate and siltstone that is moderately depleted in ¹³C overlain by bituminous shale deposited during maximum flooding. Shale samples taken from continuous, non-mineralized cores contain abundant finely disseminated pyrite (0.8 to 4.0 mg sulfide/g sample) remarkably enriched in ³⁴S (up to +30‰). Bitumen extracted by Soxhlet extraction with dichloromethane:methanol (10:1) comprises ~0.1 to 0.5% of the whole rock; residual kerogen contents range between 2.8 and 4.2 wt.% C, and carbon isotopic compositions hover around -22.7‰. Bitumen fractions (saturated, aromatic, and polar hydrocarbons) were concentrated to 100 µl and 1 µl was injected into a HP 6890 gas chromatographer interfaced with a 5972 MSD mass spectrometer to identify molecules based on elution times and comparisons to published mass spectra. Low resolution runs identified the presence of a wide suite of saturated and aromatic hydrocarbons, including hopanes of bacterial origin and steranes derived from eukaryotic membrane lipids; selected-ion mass spectrometry and compound-specific carbon isotope analyses will be carried out on the same fractions to further characterize the sources of these molecules, and to search for relationships among the biomarkers and other geochemical records of the post-glacial environment. The key challenge is to determine which molecular constituents are indigenous to the shale — those that truly reflect the ancient biosphere.