Micro-Scale Sulfur and Carbon Isotope and Trace Metal Analysis of a Neoarchean Stromatolite: Evidence for a Redox Transition in Epeiric Platform Carbonates Prior to the Great Oxidation Event

Open Access
Author:
Ilhardt, Peter Donald
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 21, 2016
Committee Members:
  • Christopher Howard House, Thesis Advisor
  • Peter J Heaney, Committee Member
  • Michael Allan Arthur, Committee Member
Keywords:
  • Neoarchean
  • Stromatolite
  • Geochemistry
  • Sulfur Isotopes
  • Carbon Isotopes
  • Trace Metals
  • Great Oxidation Event
  • Pyrite
  • Microanalysis
  • Mass-Independent Fractionation
Abstract:
Neoarchean (2.8-2.5 Ga) platform carbonate environments such as the Campbellrand-Malmani platform are believed to have been sites of substantial O2 accumulation and nutrient cycling prior to the Great Oxidation Event (GOE). Stromatolites in particular serve as biogeochemical “hotspots” where evidence of various metabolic pathways and bacterial lineages can be traced through geochemical fingerprints. We identified morphologically-distinct, organic-rimmed pyrite grains (~2-10 μm) embedded in the dolomitic lamina of a Campbellrand Subgroup stromatolite (2.52-2.55 Ga) from the Lime Acres Member of the Kogelbeen Formation at Lime Acres limestone mine, South Africa. Carbon and sulfur isotopes measured in situ on different layers of the finely-laminated, LL-C columnar stromatolite revealed a multi-layered microbial community employing photoautotrophic carbon fixation, organic matter respiration, sulfate reduction, and potentially methane assimilation. In particular, unusually high kerogen δ13Corg and pyrite δ34S compositions are consistent with an aerobic ecosystem recycling photosynthetic biomass and sulfate reduction in sulfate-limited porewaters, respectively. An array of positive Δ33S values suggests incorporation of atmospherically-processed sulfur formed from volcanic SO2 photodissociation, isolated in particulate form in these stromatolites. We argue the Δ33S-δ34S trend is best explained by mixing between a δ34S-enriched (~18-24‰) coastal marine sulfate reservoir and stratospheric Δ33S-positive sulfate precipitating as oceanic barite particles. The hypothesized predominance of sulfate rather than elemental sulfur agrees with prior arguments for increased subaerial volcanism and intense plume activity coinciding with oxidation of the upper mantle. We suggest explosive subaerial eruptions sustained a stratospheric SO2 reservoir that was photo-oxidized by long-wavelength (250-330 nm) UV radiation to produce positive MIF-carrying sulfate in the Neoarchean. This contrasts with Paleoarchean sulfur chemistry dominated by SO2 photolysis in the 190-220 nm excitation band and points to an evolving Archean atmosphere, culminating in a coupled biogeochemical-tectonic redox transformation that fundamentally changed the atmospheric sulfur cycle and ultimately prompted the GOE.