| Summary: | Laterally extensive black shales were deposited on the São Francisco craton in southeastern Brazil during low-latitude Neoproterozoic glaciation ∼740 to 700 million years ago. These rocks contain up to 3.0 weight % organic carbon, which we interpret as representing the preserved record of abundant marine primary productivity from glacial times. Extractable biomarkers reflect a complex and productive microbial ecosystem, including both phototrophic bacteria and eukaryotes, living in a stratified ocean with thin or absent sea ice, oxic surface waters, and euxinic conditions within the photic zone. Such an environment provides important constraints for parts of the "Snowball Earth" hypothesis. © 2005 American Association for the Advancement of Science. Received for publication 6 June 2005. Accepted for publication 21 September 2005. We thank Companhia Mineira de Metais for samples, R. Summons and E. Grosjean for helpful discussion and metastable-reaction monitoring gas chromatography–mass spectroscopy (GC-MS) analyses; G. Love, A. Bradley, J. Brocks, C. Li, M. Eek, K. Nealson, and W. Berelson for helpful discussion; J. Pinho and K. B. Brody for help in Brazil; N. Geboy and C. France for TOC abundance data; G. McDonald for lab access; as well as two anonymous reviewers for comments. Supported by NSF grants EAR0418083 (F.C., A.S., A.O.) and EAR0126378 (A.J.K.) and NASA Exobiology grant 42000-62153 (F.C. and A.O.). Supplemental Material - Olcott.SOM.pdf
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