T249 The Co-Evolution of Life and Its Environment during the Precambrian: The Rise of Oxygenic Photosynthesis and the Great Oxidation Event

Tuesday, 24 October 2017
1:30 PM - 5:30 PM
Skagit 5

STABLE ISOTOPE GEOCHEMISTRY OF A LATE ARCHEAN MICROBIAL ECOSYSTEM: DIVERSITY IN THE PRE-GOE OCEANS

Paper number: T249-11
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

  • Jeffrey Osterhout Presenting Author
    University of California-Los Angeles Dept. of Earth Planetary and Space Sci, Los Angeles, CA
  • Andrew D. Czaja
    University of Cincinnati, Cincinnati, OH

Abstract

The evolution of microbial communities preceding the Great Oxidation Event (GOE) has become a subject of considerable interest and exploration, particularly regarding the early diversity of microbial metabolisms and the onset of oxygenic photosynthesis. The paleobiological record of Archean deposits continues to grow, and the morphological range of ancient microfossils has been greatly expanded in recent years. However, most studies of early microbial ecosystems have been focused on shallow-marine deposits due to their enhanced preservation, and offshore deposits are often overlooked, providing a partially restricted view of biological diversity in the Archean oceans. To address these issues, we studied the stable isotope geochemistry of fossiliferous cherts from the deep-marine unit of the upper Gamohaan Formation, part of the Neoarchean (~2.52 Ga) Transvaal Supergroup in South Africa. High-resolution isotopic measurements were obtained using secondary ion mass spectrometry (SIMS) to analyze carbon isotopes (δ13C) of several large, organic-walled microfossils, described as possible sulfur-oxidizing bacteria, and sulfur isotopes (δ34S) of associated pyrite grains. δ13C values of the fossils range from -41 to -32‰ (VPDB), whereas δ34S values of pyrite grains range from -0.7 to +6.3‰ (VCDT), and the isotopic variability appears to be correlated with differences in fossil morphologies and pyrite textures, respectively. Such fine-scale isotopic heterogeneity within this unit is consistent with the presence of an active and diverse microbial ecosystem capable of cycling carbon and sulfur in an offshore environment of the late Archean ocean. Comparing these data with similar studies of other Archean deposits will further improve our understanding of biological evolution and diversity in Earth’s oceans prior to the GOE.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-307829

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.