Evidence for a diverse microbial community in a 3.46 Ga ocean from ABDP#1 core
Abstract
The 3.46 Ga Marble Bar Chert/Jasper Formation (MBC) in the Pilbara Craton, Western Australia is known as one of the oldest jasper units which contains abundant hematite. In order to understand a formation process(es) of the jasper, particularly an origin of hematite, the MBC is the first target of Archean Biosphere Drilling Project (ABDP), which aims at obtaining modern-oxidation-free samples to increase our understandings of the environmental and biological conditions on the early Earth. Our petrolographical and mineralogical study of ABDP#1 revealed that: (I) The dominant Fe-bearing mineral in the MBC changed with the stratigraphic sequence, from siderite in the lower section, to hematite, and then to hematite/magnetite in the uppermost section; (II) These Fe-bearing minerals, including hematite, formed by mixing of submarine hydrothermal fluids (T <~200°C) and local seawater at depth >200m in a large (~30 km diameter) submarine caldera (Hoashi et al., 2009); and (III) The redox state of bottom water in the submarine caldera changed from anoxic (during siderite precipitation) to oxic (during hematite precipitation). We estimated that dissolved O2 content of the deep water during hematite precipitation >10 µM. The early emergence of oxygenic photoautotroph has been implied by our conclusion on the origin of hematite. This study was conducted to understand the nature of microbial communities in the caldera during the deposition of the MBC. We have investigated changes in contents of organic- and carbonate C, and δ13C values of extracted kerogens and carbonates. We also studied the nature of the extracted kerogens using an elemental analyzer and a Raman spectrometer. The results show the following general trends accompanying the change in environment from anoxic to oxic (and increased hydrothermal): (1) decreasing organic C contents, from 0.01 to 0.7wt%; (2) increasing δ13Corg values from -27±5‰ to -20±5‰; (3) decreasing carbonate (siderite) C contents from 3.7 to 0.01 wt%; (4) increasing δ13Ccarb values from -2.4 to +5.8‰; and (5) increasing Δ13Ccarb-org values from ~30 to 15‰. These data suggest that: (A) Organic matter accumulated on the seafloor during the anoxic stage with siderite precipitation was mostly remnant of cyanobacteria (δ13Corg = -27±5‰); and (B) During oxic (and intensive hydrothermal) stage of hematite precipitation, remnants of cyanobacteria were decomposed within unconsolidated sediments to CO2 and CH4 by chemolithoheterotrophic methanogens; and this CO2 together with H2 from hydrothermal fluids were used by chemolithoautotrophic methanogens to produce isotopically heavy organic matter. A variety of organisms that thrived in different chemical and thermal conditions (aerobes and anaerobes; photoautotrophs and chemolithoautotrophs; mesophylic and thermophylic) already existed in an ocean 3.46 Ga ago.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.B41B0305W
- Keywords:
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- 0444 BIOGEOSCIENCES / Evolutionary geobiology;
- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry;
- 9623 INFORMATION RELATED TO GEOLOGIC TIME / Archean