HR: 1340h
AN: B53B-0996    [Abstracts]
TI: Microbial Ecosystem In The Oldest Freshwater Lake Revealed From A Drill Core Of The 2.76 Ga Hardey Formation, Pilbara District, Western Australia
AU: * Watanabe, Y
EM: yumiko@geosc.psu.edu
AF: Astrobiology Research Centere, Penn State University, 435 Deike Bldg., University Park, PA 16802 United States
AU: Bevacqua, D C
EM: dbevacqu@geosc.psu.edu
AF: Astrobiology Research Centere, Penn State University, 435 Deike Bldg., University Park, PA 16802 United States
AU: Ohmoto, H
EM: ohmoto@geosc.psu.edu
AF: Astrobiology Research Centere, Penn State University, 435 Deike Bldg., University Park, PA 16802 United States
AB: The Hardey Formation in the Pilbara district of Western Australia, is the oldest known (2.76 Ga) lacustrine deposit, was one of eight targets of the Archean Biosphere Drilling Project (ABDP). Rocks in the drilling area have been subjected to very low-grade metamorphism (zeolites facies). The recovered drill core (~145 m in depth) is divisible into two depth zones: (1) the upper zone (<~80 m), comprising mostly coare-grained, gray, arkosic sandstones, and (2) the lower zone, comprising alternating layers of finely laminated, black shales and calcareous sandstones. We selected 26 samples from the lower zone at 2.5-m intervals, and conducted petrographical, mineralogical, and geochemical investigations (e.g., chemical mapping of major and trace element (including REEs); organic and carbonate C, and S contents; C isotope). Bedding planes are well preserved, comprising alternating layers of organic-C rich clay and organic-C poor carbonate (10-20 æm). Presence of cross-lamination suggests a low energy depositional environment, probably a shallow lake. Major and trace element contents of all samples are quite homogeneous. Organic C- and pyrite S contents fall in narrow ranges, from 0.2 to 0.7wt% (a mean value = 0.5wt% and s.d. = 0.1wt%) and from 0.01 to 0.1wt% (a mean value = 0.04 wt%; s.d. = 0.02 wt%), respectively. Compared to average Archean marine shales (organic C = 0.45 wt%; S = 0.1 wt%), the Hardy shales are comparable in organic C but are much depleted in pyrite S contents. The low S/C ratios of the Hardy shales concur with a freshwater depositional interpretation. The C isotope values for the organic C vary from -36.6 to -27.6% (a mean value = -32.7%; s.d. = 1.8%). There is no stratigraphic trend, but theycorrelate well with the lithology: -31.9 to -27.6% (a mean value = -30.4%) for disseminated-grains and seams of organic C in calcareous sandstone; and -36.6 to 32.6% (a mean value = -33.4%) for finely-laminated black shales. The heavier values may represent microbial communities in shallow water, while the lighter values represent microbial communities in deeper water. The carbon isotope systematics in the Hardy Formation are essentially identical to those of the 2.75 Ga marine shales at Kidd Creek, Ontario, Canada. These data suggest that the fresh-water ecosystems were already as diversed as marine ecosystems at 2.76 Ga ago.
DE: 1055 Organic geochemistry
DE: 0325 Evolution of the atmosphere
DE: 1010 Chemical evolution
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting