Abstract
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 shallowmarine
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 M 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.