BIOMARKERS OF A LOW-LATITUDE NEOPROTEROZOIC GLACIATION

Molecular, isotopic, and petrographic analyses suggest that laterally extensive, organic rich strata (up to 3.0% TOC) deposited in southeastern Brazil during a Neoproterozoic low-latitude glaciation likely represent the preserved remains of synglacial, marine primary production. Extractable biomarkers, including 2-α-methyl hopanes, 2,3,6-trimethylarylisoprenoids, C₂₉-C₃₁ hopanes, and C₂₇-C₂₉ steranes, reflect the presence of a complex and productive microbial ecosystem comprised of both aerobic and anaerobic phototrophs, heterotrophs, and eukaryotes. The occurrence of photosynthetic microbes at this time indicates that sea-ice cover was thin to nonexistent, and is incompatible with models for snowball Earth that envision kilometers of ice thickness. Our data also indicate that euxinia extended into the photic zone and provide evidence that the oceans were strongly stratified. Although we cannot rule out the possibility of mass extinctions accompanying the low-latitude glaciation, the recovered biomarkers indicate a record of high microbial diversity and strong primary production. We hypothesize that this Neoproterozoic glaciation was thus not a significant evolutionary bottleneck.