This proposal will characterize element abundances and isotopes in two Neoarchean drill cores to investigate the oxidation state of the environment ~ 2.5 billion years ago. The goal is to determine if transient or low-level quantities of O 2 were present in the environment before the Great Oxidation Event (G.O.E.). The presence or absence of O 2 at this time has bearing on the relationship between the timing of the evolution of oxygenic photosynthesis and the rise of atmospheric O 2 – a relationship that will inform the eventual interpretation of spectra from extrasolar Earth-like planets.

The proposed work builds directly on prior research on one of the two cores conducted by the PI, Co-I and others with support from the NASA Astrobiology program. In that study, summarized here, high-resolution chemostratigraphy revealed an episode of enrichment of the redox-sensitive transition metals Mo and Re in pyritic shales from the late Archean Mt. McRae Shale, Western Australia. Correlations with organic carbon and Re-Os data indicated these metals were derived from contemporaneous seawater. We proposed that Mo and Re were supplied to Archean oceans by oxidative weathering of crustal sulfide minerals and, hence, that small but significant amounts of O2 existed in the environment > 50 Ma before the start of the G.O.E.

We propose to conduct similar analyses on a drill core that samples pyritic shales from the late Archean Klein Naute Formation, S. Africa, believed to be a time-correlative deposit with the Mt. McRae Shale. We also propose to conduct Fe, Mo, U and S isotope analyses and to characterize sedimentary Fe speciation in both cores. The proposed research will test key aspects of the O 2 interpretation as well as alternative hypotheses, and will help determine if the event recorded in the Mt. McRae Shale was of local or global extent.