Paul Mason: Biological controls on the Earth's earliest sulfur cycle

Please note: this "Wednesday seminar" will be on Tuesday (usual time and place)

Paul Mason from Utrecht University is visiting CGB. He is studying traces of early life, working in the Barberton Greenstone Belt, SA. widely recognized as an experts in LA-ICP-MS and will speak about his experiences during the Wednesday Seminar.

Biological controls on the Earth's earliest sulfur cycle

Stable isotope variations in early Archean sedimentary rocks represent once of the best lines of evidence for the evolution of microbial life very early in Earth history. Multiple sulfur isotope analysis of sulfides contained in the North Pole barite deposit from the Pilbara Block, NW Australia support the appearance of microbial sulfate reduction and elemental sulfur disproportionation by 3470 million years ago in a sulfate-rich niche environment [1-3]. This is an important result as until relatively recently the Archean sulfur cycle was considered to be controlled only by abiotic processes. It is widely considered to be one of the strongest pieces of evidence for an Archean biosphere. However, it remains unclear how representative the North Pole evidence is for the wider Archean environment.

Our research group has been active in the Barberton Greenstone Belt in South Africa where we have studied barite-rich and barite-free parts of the stratigraphy spanning the whole of the Palaeoarchean. Results from different sites are contrasting but it has been possible to make some general conclusions:
- the most oxidized sulfur species, sulfate, was exclusively derived from the photolysis of volcanic sulfur dioxide in the atmosphere. No sulfate was derived directly from the mantle. Oceanic sulfate evolved in composition through the Palaeoarchean
- the oceans were very low in sulfate (approximately 10-3 lower than today). Our observations from Archean pyrites fit experimental predictions
- hydrothermal reworking of marine sulfate was unlikely to have been important. Evidence for widespread abiotic sulfate reduction is lacking
- it was not easy to produce pyrite from iron monosulfide precipitates and the incorporation of elemental sulfur produced by atmospheric photolysis may have been critical during diagenesis
- the isotopic signature of sulfur disproportionation is localized whilst that of sulfate reduction is more widespread
- sulfur was largely immobile in its reduced form in the Archean ocean basins
- sulfur isotopes remain one of our strongest biosignatures for the early Archean and biological processes were critical in controlling the distribution of sulfur across the early oceans

References

[1] Phillippot et al. (2007) Science 317, 1534-1537. [2] Ueno et al.
(2008) GCA 72, 5675-5691. [3] Shen et al. (2009) EPSL 279, 383-391.