HR: 1340h
AN: B33B-0260    [Abstracts]
TI: Biogeochemical Processes in Late Archean Marine Biosphere Revealed by Isotopic and Molecular Records
AU: Eigenbrode, J L
EM: j.eigenbrode@gl.ciw.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Eigenbrode, J L
EM: j.eigenbrode@gl.ciw.edu
AF: The Carnegie Institution of Washington, Geophysical Laboratory, Washington, DC 20015 United States
AU: * Freeman, K H
EM: kate@essc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Summons, R E
EM: rsummons@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139 United States
AB: The presence of shallow-marine oxygen oases and associated aerobic ecosystems in an otherwise anoxic and anaerobic world has been proposed by researchers to explain the anomalous 40 permil spread in organic-carbon isotope values during the late Archean. To test this hypothesis, we studied isotopic, molecular, and lithologic records of 2.7-2.5 Ga rocks of different depositional facies from the Hamersley Province, Western Australia. Kerogen carbon-isotopic compositions indicate that extreme 13C-depletion (more than -45 permil) was associated with shallow-marine-carbonate environments at 2.72 Ga and with deepwater environments thereafter. Moreover, kerogen-carbon-isotope values associated with carbonate environments became enriched by more than 10 permil over 100-150 Ma. These observations suggest that microbial processes responsible for extreme 13C-depletion became less significant in shallow carbonate environments, but remained important in deeper settings. Molecular biomarker ratios determined for associated bitumens: 1) strongly correlate to kerogen carbon-isotope values and other biomarker ratios, and, 2) show relationships with depositional facies and dolomite abundance giving credence to a syngenetic relationship with host rocks. The biomarker data confirm aerobic methanotrophs in the Late Archean biosphere, but not in strong association with extreme 13C-depletion. Biomarker patterns reflect a greater association of aerobic respiration and oxygenic photosynthesis in shallow carbonate environments compared to deeper settings. Collectively, the data record dramatic changes in carbon cycling associated with environmental partitioning of microbial processes and ecosystems over 100-150 Ma. Most likely, this represents increased bioavailability of strong electron acceptors with the expansion of oxidant-rich oases prior to rise in atmospheric oxygen.
DE: 4267 Paleoceanography
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting