WHAT IMPACTS THE RECORD OF THE BIOLOGICAL SULFUR CYCLE AT 2.63 GA?

2.63 Ga shales and carbonates from South Africa and Western Australia contain a distal spherule layer from a large bolide impact as well as potential evidence of euxinia and pre-Great Oxidation Event oxidative weathering. Understanding this biologically-influenced environment and the changes caused by the impact requires untangling the timing of events recorded in pre-, syn-, and post-impact outcrop and cores.

Stratigraphic and lateral correlations between pyrite and microbial features were characterized in a section through impact strata at Monteville Farm, South Africa, with particular attention on the stratigraphic distribution of microbial features and oxidized pyrite in the form of iron-rich nodules, sheets, tubes, and laminae. Rolled laminae were cohesive but softly deformable when deposited, suggesting these textures are microbial in origin. Brittle deformation of laminae and sheets within calcite-filled voids suggest that many pyritized features lithified pre-compaction. Thus, at least some synsedimentary or early diagenetic pyrite likely reflects microbially influenced pore water chemistry. Additionally, the distribution of sulfide-rich facies changes above the impact stratum.

Oxidized sulfides in outcrop are inappropriate for geochemical analyses, so unoxidized, time-correlative cores from Western Australia (AIDP-2 and AIDP-3) were observed petrologically and in polished thin sections. Reflected light petrography revealed that sulfide minerals preserve softly deformed crinkled laminae of probable microbial origin. SEM-EDS revealed disseminated pyrite closely associated with kerogen, suggesting a link between pyrite and kerogen that may be biological. Late-stage carbonate veins also contain sulfides, revealing burial alteration.

Preliminary data suggests that less sulfide accumulated in shallow water environments in the aftermath of the 2.63 Ga giant impact, a change in sediment composition and texture that may reflect changing biological activity.