Late Archean Microbial Ecology: An Integration of Molecular, Isotopic, and Lithologic Studies

Open Access
Author:
Eigenbrode, Jennifer L
Graduate Program:
Earth Sciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 26, 2004
Committee Members:
  • Katherine Haines Freeman, Committee Chair
  • Christopher Howard House, Committee Member
  • Robert David Minard, Committee Member
  • Mark E Patzkowsky, Committee Member
  • Roger E Summons, Committee Member
Keywords:
  • Precambrian
  • biomarkers
  • oxygen
  • bacteria
  • respiration
  • methylotrophy
  • carbon
  • Hamersley Province
  • hopanes
  • steranes
  • isoprenoids
  • sulfur
  • paleoenvironments
  • marine
  • anoxia
Abstract:
This thesis explores the microbial ecology of depositional environments in order to shed light on the 40‰ range of organic-carbon isotope compositions recognized in the global record of the late Archean. Kerogen-carbon isotopic compositions (delta-[super]13C[sub]ker) for a 150 Ma record from the Hamersley Province, Western Australia show a -58 to -27‰ spread in kerogen-carbon isotopic values. Lithofacies association with kerogen-carbon isotope patterns for the entire period indicate that shallow-water carbonate facies received relatively greater inputs of photosynthate biomass relative to biomass from recycled organic carbon (i.e. methane recycling and chemoautotrophic recycling of [super]13C-depleted carbon dioxide). In addition, results indicate that a “methane signature” (extreme [super]13C-depletion; <-45‰) is strongly associated with restricted and deep-water environments and that in other environments, particularly open, shallow-water carbonates, the relative proportion of recycled organic carbon was lower and declined further over time. Molecular analysis of both extracted free biomarkers and thermally cleaved bound-biomarkers provides strong supporting evidence for the syngenicity of molecular fossils in these late Archean rocks. Biomarkers indicate microbial diversity including evidence for oxygenic photosynthesizing cyanobacteria, eukaryotic algae, aerobic methylotrophs, and, possibly, green sulfur bacteria. The discovery of >C20 acyclic isoprenoids, including squalane, in these bitumens provides the first molecular evidence for the Archaea Domain at 2.72 Ga. Moreover, the presence of quaternary-branched alkanes suggests input from chemoautotrophic sulfide-oxidizing bacteria in slope environments. Both isotopic and molecular results support the onset and expansion of marine oxygenation during the late Archean prior to the rise of atmospheric oxygen at ~2.3 Ga. The extreme [super]13C-depletion is attributed to recycling of organic carbon, potentially by anaerobic methane oxidation, despite molecular evidence for the activities of aerobic methylotrophs in late Archean environments.