EARLY OXIDATION EVENTS IN THE NEOARCHEAN AS DETERMINED BY WHOLE ROCK GEOCHEMISTRY, LA-ICPMS AND S ISOTOPES

Early examples of both atmospheric oxygenation and euxinia are found within the Hamersley and Fortescue Basins. Chemical changes in the Neoarchean are examined using a combination of whole rock geochemistry, LA-ICPMS analyses and S isotope measurements (SHRIMP-SI) of sedimentary pyrite and pyrrhotite from shales from drill core (ABDP9 and WRL1). Three different periods of oxygen enrichment have been identified in these basins. The earliest event occurred during the deposition of the Jeerinah Formation and is indicated by an increase of O₂ sensitive trace elements in whole rock and pyrite near the middle of the formation. This event does not exhibit a corresponding S isotope signature indicative of sulfide oxidation and thus, is considered to be minor and did not significantly alter the Neoarchean S cycle. The second enrichment phase occurs from the Paraburdoo Member through to the midpoint of the Bee Gorge member of the Hamersley Basin. This is indicated by an increase in O₂ sensitive trace elements in pyrite (Mo and Se) and is supported by the expected S isotope fractionation for the onset of major sulfide oxidation. Increased biologic productivity during the same interval, is indicated by whole rock enrichment factors symptomatic of organic matter burial (Ni, Zn, Co and Cu). Cyanobacteria are thought to be the likely organism which would explain the increase in oxygen at the same time. Increased atmospheric oxygen would be available to oxidize surface sulfides releasing productivity limiting trace elements (Mo and Se). A positive feedback loop may be envisaged whereby flourishing cyanobacteria promote increased surface sulfide oxidation releasing limiting trace elements for further cyanobacteria growth. These periods of increased oxygen end with the onset of BIF deposition, possibly due to oxygen levels reaching a threshold necessary to convert Fe²⁺ to Fe³⁺ and precipitate the BIFs. This BIF deposition severely reduced the quantity of phosphate in the ocean system, resulting in an extreme decrease in the population of cyanobacteria, which in turn limited O₂ production. The final phase of O₂ enrichment occurs at the top of the McRae Shale and has the highest increase in whole rock enrichment factors (Mo, Co, Ni, Cu and Zn). It is interpreted to be the greatest level of oxygenation of the three events identified in this study.