HR: 16:35h
AN: B14A-03    [Abstracts]
TI: Biomarkers of a Low-Latitude Neoproterozoic Glaciation
AU: * Olcott, A N
EM: olcott@usc.edu
AF: University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089 United States
AU: Sessions, A L
EM: Sessions@its.Caltech.edu
AF: California Institute of Technology, Geological and Planetary Sciences, Pasadena, CA 91125 United States
AU: Corsetti, F A
EM: fcorsett@usc.edu
AF: University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089 United States
AU: Kaufman, A J
EM: kaufman@geol.umd.edu
AF: University of Maryland, Geology Department, College Park, MD 20742 United States
AB: Neoproterozoic low-latitude glaciations are often considered times of great biologic limitation because of the hypothesized presence of thick, global sea ice. Alternatively, climate models have suggested that tropical oceans could have remained ice-free, or covered by only thin sea ice, allowing life to continue unimpeded throughout the glaciations. The analysis of organic remains from synglacial sediments provides an approach to address the debate. Here we describe molecular, isotopic, and petrographic analyses of organic rich strata (up to 3.0 percent TOC) deposited in southeastern Brazil during Neoproterozoic low-latitude glaciation ca. 700 Ma. These strata contain extractable biomarkers, including 2-α-methyl hopanes, 2,3,6-trimethylarylisoprenoids, C29-C31 hopanes, and C27-C29 steranes. The preserved biomarkers reflect the presence of a complex and productive ecosystem comprised of both aerobic and anaerobic phototrophs, heterotrophs, and eukaryotes. The biomarker data indicate euxinia extending into the photic zone, providing evidence that the oceans were strongly stratified. Significantly, the occurrence of photosynthetic cyanobacteria and green sulfur bacteria at this time indicates that sea-ice cover at this location was thin to nonexistent, and is incompatible with models for snowball Earth that envision kilometers of ice thickness.
DE: 0420 Biomolecular and chemical tracers
DE: 0424 Biosignatures and proxies
SC: Biogeosciences [B]
MN: Fall Meeting 2005