Abstract
We report the (13)C content of preserved organic carbon for a 150 million-year section of late Archean shallow and deepwater sediments of the Hamersley Province in Western Australia. We find a (13)C enrichment of approximately 10 per thousand in organic carbon of post-2.7-billion-year-old shallow-water carbonate rocks relative to deepwater sediments. The shallow-water organic-carbon (13)C content has a 29 per thousand range in values (-57 to -28 per thousand), and it contrasts with the less variable but strongly (13)C-depleted (-40 to -45 per thousand) organic carbon in deepwater sediments. The (13)C enrichment likely represents microbial habitats not as strongly influenced by assimilation of methane or other (13)C-depleted substrates. We propose that continued oxidation of shallow settings favored the expansion of aerobic ecosystems and respiring organisms, and, as a result, isotopic signatures of preserved organic carbon in shallow settings approached that of photosynthetic biomass. Facies analysis of published carbon-isotopic records indicates that the Hamersley shallow-water signal may be representative of a late Archean global signature and that it preceded a similar, but delayed, (13)C enrichment of deepwater deposits. The data suggest that a global-scale expansion of oxygenated habitats accompanied the progression away from anaerobic ecosystems toward respiring microbial communities fueled by oxygenic photosynthesis before the oxygenation of the atmosphere after 2.45 billion years ago.