ASU researchers Brian Kendall & Ariel Anbar have a new paper published online at Nature Geoscience that concludes "the productive regions along ocean margins during the late Archaean eon [2.5-2.6 bya] were sites of substantial O2 accumulation, at least 100million years before the first significant increase in atmospheric O2 concentration."
[right, "the orange cells in this microscope image are Synechococcus, a unicellular cyanobacterium only about 1 um in size. Organisms like Synechococcus were responsible for pumping oxygen into the environment 2.5 billion years ago." Credit, Susanne Neuer/Amy Hansen]
Ref: Pervasive oxygenation along late Archaean ocean margins - pp647 - 652 Brian Kendall, Christopher T. Reinhard, Timothy W. Lyons, Alan J. Kaufman, Simon W. Poulton & Ariel D. Anbar, Published online: Nature Geoscience, 22 August 2010 | doi:10.1038/ngeo942