An Integrated Isotopic and Biomarker Analysis of the Glaciogenic Vazante Group, Brazil

Abstract

The Vazante Group, a meta-sedimentary succession located in south-central Brazil, contains several intervals of diamictite, interpreted as glacial in origin, bracketed by well-preserved carbonate and shale. This glacial succession was previously associated with the global occurrence of Neoproterozoic low latitude glacial deposits (aka Snowball Earth), and biomarkers (molecular fossils) identified from an organic-rich interval within this succession were used to infer active photosynthesis during the ice age (Olcott et al., 2005). However, new Re-Os and detrital zircon U-Pb ages suggest that the upper Vazante Group is ca. 1.3 to 1.0 billion-years old (Geboy, 2006; Azmy et al., 2008; Rodrigues et al., 2008) and thus may preserve evidence for hitherto unknown Mesoproterozoic ice ages. Within this context, I present biomarker and time-series stable isotope data from a basin-wide distribution of pre- and post-glacial sedimentary units in order to i) evaluate the Mesoproterozoic interpretation of this succession, ii) assess the biologic and environmental conditions present when these sediments were deposited, and iii) to understand the co-evolution of life and ocean chemistry in response to rapidly changing environmental conditions. Biomarker distributions and abundances from the Serra do Garrote Formation, a pre-glacial shale, and the Serra do Poço Verde and Lapa formations, both post-glacial shales, show evidence of a diverse microbial community that would have only existed in a redox stratified water column. Additionally, the presence of aryl isoprenoids, biomarkers indicative of green sulfur bacteria, in the Serra do Garrote and Serra do Poço Verde formations suggest that reducing, sulfide-rich water was present in the photic zone. These biomarkers however, are absent from the Lapa Formation suggesting that sulfidic conditions either receded to deeper water or collapsed entirely. Carbon and sulfur isotopic signatures support the conclusions drawn from this biomarker study. Carbon and sulfur trends from the Serra do Garrote and Serra do Poço Verde formations show evidence of a large, anoxic, isotopically stable, dissolved organic carbon pool (relative to inorganic carbon) and extensive bacterial sulfate reduction of a small, oceanic sulfate reservoir. The Lapa Formation, on the other hand, displays evidence for a smaller, isotopically responsive, dissolved organic carbon pool. The carbon isotopic compositions of carbonates from these three units are consistent with other Mesoproterozic successions supporting the geochronological age constraints. Taken together, biomarker and time-series stable isotope data from the upper Vazante Group map a transition from a sulfide-rich, stratified, water column to one that, while still stratified, was no longer sulfidic. This environmental transition occurred in response to consecutive Mesoproterozoic ice ages.