2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 8
Presentation Time: 9:55 AM


OLCOTT, Alison N.1, CORSETTI, Frank A.1, SESSIONS, Alex L.2 and KAUFMAN, Alan J.3, (1)Department of Earth Sciences, Univ. of Southern California, Los Angeles, CA 90089, (2)Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, (3)Geology, Univ of Maryland, College Park, MD 20742, olcott@earth.usc.edu

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.