New Fe isotope insights into the oxidation history of a Neoarchean Earth

Abstract:

Our understanding of early Earth’s oxidation history has been dramatically sharpened over the past two decades by non-traditional stable isotope studies of Archean sedimentary rocks and minerals (4.0 to 2.5 billion years ago, or Ga). One of the isotope systems most widely applied to this end has been Fe (reported as δ⁵⁶Fe). Iron isotopes are fractionated substantially during Fe redox reactions, and these effects are sometimes preserved in the geologic record. We generated 91 bulk-shale δ⁵⁶Fe values across three well-characterized shale units deposited during the Neoarchean (2.8 Ga to 2.5 Ga). These units are the ~2.65 Ga Jeerinah Formation (n = 43) and ~2.50 Ga Mt. McRae Shale (n = 22) from Western Australia and the ~2.50 Ga Klein Naute Formation (n = 26) from South Africa. In the case of the Jeerinah and Klein Naute formations, we collected samples from each in two separate drill cores, with one core deposited in a more proximal setting than the other. In the case of the Mt. McRae Shale, we complemented our bulk-shale δ⁵⁶Fe measurements with 18 δ⁵⁶Fe measurements of the pyrite fraction leached from shales during sequential extraction. The first strong trend that emerges from our data is increasingly negative δ⁵⁶Fe found in bulk-shales with higher pyrite Fe contents from all units (δ⁵⁶Fe as low as –2.06 ± 0.15‰; 2SD in the upper Mt. McRae Shale). We also find that δ⁵⁶Fe measured in the pyrite fractions from the Mt. McRae Shale are in all but one instance lighter than the corresponding bulk-shale value (δ⁵⁶Fe as low as –2.31 ± 0.15‰; 2SD). These first two trends strongly implicate pyrite in the generation of our most negative δ⁵⁶Fe values. Lastly, and more interestingly, the magnitude of negative d⁵⁶Fe signal seems to increase with increasing distance from the paleo-shoreline. Plausible drivers of this trend include: (1) varying relative Fe contributions from detrital versus hydrothermal sources, (2) changes in Fe and S availability in the overlying water column, (3) controls linked to local sedimentation rates, and (4) proximity to a particulate oxide “shuttle.” The pros and cons of each will be discussed, as well as the likelihood that our data and the most plausible mechanism(s) may capture Earth’s early steps toward oxygenation on the eve of the Great Oxidation Event.