New geochemical data from the South African Agouron drill cores paint a clearer picture of oxygen distribution in the Late Archean marine environment! Rhenium and molybdenum abundances and sedimentary iron geochemistry in 2.6-2.5 Gyr old black shales indicate that photosynthetic production of oxygen by cyanobacteria was vigorous in the Late Archean surface ocean near the continents. Mildly oxygenated waters extended below the photic zone before giving way to anoxic deep and open oceans. Mass independent fractionation of S isotopes indicates that atmospheric oxygen abundances remained below 0.001% of the present atmospheric level. Our results show that substantial oxygen accumulation began along productive ocean margins more than 100 million years before the first significant increase in atmospheric oxygen concentrations. This research was a collaborative effort with Chris Reinhard and Timothy Lyons from the University of California (Riverside), Alan Jay Kaufman from the University of Maryland, and Simon Poulton from Newcastle University. The paper is now available in the September 2010 issue of Nature Geoscience.
FIGURE CAPTION: Simplified representation of the redox conditions along Late Archean ocean margins beneath a low-oxygen atmosphere (credit: Susan Selkirk). The orange cells in the inset figure are Synechococcus, a unicellular cyanobacterium only about 1 um in size (credit: Susanne Neuer/Amy Hansen). Organisms like Synechococcus were responsible for pumping oxygen into the environment 2.6-2.5 Gyr ago.