BIOMARKERS OF A SNOWBALL EARTH

Laterally extensive strata deposited in southeastern Brazil during the Neoproterozoic low-latitude glaciation contain up to 0.7% organic carbon. Molecular, chemical, and petrographic analyses suggest that this is not recycled organic matter carried in with glacial debris, but rather the preserved remains of marine primary production. Extractable biomarkers, including 2-alpha-methyl hopanes, 2,3,6-trimethylarylisoprenoids, C₂₉-C₃₁ hopanes, and C₂₇-C₂₉ steranes, reflect the presence of a complex and productive microbial ecosystem, which included both aerobic and anaerobic phototrophs, heterotrophs, and eukaryotes.  These results indicate that photosynthesis was prolific, either in open waters or in relatively thin sea ice, and is incompatible with models for snowball Earth that envision kilometers of ice thickness. Additionally, the results indicate that euxinia extended into the photic zone.  Although we cannot rule out the possibility that extinctions in diversity accompanied the low-latitude glaciation, our data suggest a reasonable amount of primary productivity, both oxic and anoxic, was present during Neoproterozoic low latitude glaciations.