Identification of an Archean marine oxygen oasis

•2800 Ma Steep Rock carbonate platform is an early example of a marine oxygen oasis.•REE analyses show that Steep Rock limestones precipitated from oxygenated water.•We infer that oxygenation promoted Ca-carbonate over Fe-carbonate precipitation.•Oxygenation could be linked to the locally common stromatolites in the limestone.•The Archean inception of marine carbonate platforms reflects oxygenation.

The early Earth was essentially anoxic. A number of indicators suggest the presence of oxygenic photosynthesis ∼2700–3000 million years (Ma) ago, but direct evidence for molecular oxygen (O2) in seawater has remained elusive. Here we report rare earth element (REE) analyses of ∼2800 million year old shallow-marine limestones and deep-water iron-rich sediments at Steep Rock Lake, Canada. These show that the seawater from which extensive shallow-water limestones precipitated was oxygenated, whereas the adjacent deeper waters where iron-rich sediments formed were not. We propose that oxygen promoted limestone precipitation by oxidative removal of dissolved ferrous iron species, Fe(II), to insoluble Fe(III) oxyhydroxide, and estimate that at least 10.25 μM oxygen concentration in seawater was required to accomplish this at Steep Rock. This agrees with the hypothesis that an ample supply of dissolved Fe(II) in Archean oceans would have hindered limestone formation. There is no direct evidence for the oxygen source at Steep Rock, but organic carbon isotope values and diverse stromatolites in the limestones suggest the presence of cyanobacteria. Our findings support the view that during the Archean significant oxygen levels first developed in protected nutrient-rich shallow marine habitats. They indicate that these environments were spatially restricted, transient, and promoted limestone precipitation. If Archean marine limestones in general reflect localized oxygenic removal of dissolved iron at the margins of otherwise anoxic iron-rich seas, then early oxygen oases are less elusive than has been assumed.