Australia: The Land Where Time Began

A biography of the Australian continent 

A sulphidic Sea in the Late Archaean Stimulated by Early Oxidative Continental Weathering

In the Mount McRae Shale, from the Late Archaean, iron speciation data provides evidence for a water column that was euxinic (anoxic and sulphidic) 2.5 Ga. Sulphur isotope data that was obtained from samples of the same stratigraphic section suggest it was increased oceanic sulphate levels that resulted  from  the weathering of sulphide minerals on the land that were exposed to the trace free oxygen levels in the atmospheric that had been produced by photosynthesis. Euxinic conditions are suggested to have been confined to the mid-levels of the water column on the margin of the basin by variability of the local flux of organic matter. The authors1 suggest that it is indicated by these findings that on a variety of spatial and temporal scales, prior to as well as immediately following rise in atmospheric oxygen in the Palaeoproterozoic, which hints that in the deep ocean chemistry during the early history of the Earth there may have been a hidden texture that was previously unexplored.

The atmosphere of the early Earth, for the first 2 Gy, was characterised by little if any free oxygen (1,2). A sharp rise in the concentration of atmospheric O2 between 2.45-2.32 Ga in the Palaeoproterozoic (1-3), as indicated by a large body of evidence, though what is less well-known is the history of oxygenation in the deep ocean. From about 3.8-1.8 Ga, in the Archaean and Early Proterozoic, the deposition of banded iron formation has been interpreted as implying that in the deep ocean water masses, which were believed to be anoxic, there were high levels of dissolved ferrous iron (Fe2+) derived by high-temperature weathering of the basalt of the sea floor under low oceanic concentrations of sulphate (SO42-) (4,5). It is believed that for most of the early history of the Earth conditions in the deep ocean were persistently reducing and rich in ferrous iron (ferruginous), though between 2.4-2.0 Ga there was a relatively paucity of BIF (6), rendering the chemistry of the deep ocean of the time obscure. Whatever the case was, when BIF deposition ended at about 1.8 Ga, this is linked to atmospheric oxygen accumulation as a result of the eventual removal from the ocean of Fe2+, either as ferric (hydr)oxides (7), or in euxinic basins as pyrite (8). A corollary of the latter model is that delivery to the ocean of oxidative sulphate became sufficient to remove reactive iron from the ocean through the bacterial production of sulphide until later than about 1.8 Ga. It is suggested by recent studies of the Mount McRae Shale, of Late Archaean age, oxidative sulphur cycling may have occurred prior to the rise of atmospheric oxygen in the Palaeoproterozoic (9) and at about 2.5 Ga the conditions existed in marine sediments sufficient to authigenically (in situ) enrich molybdenum (Mo). Substantial enrichment of Mo into sediments occurs at present, following the conversion of soluble molybdate (MoO42-) to thiomolybdates (MoO4x2-Sx2-), that is particle-reactive, in stable sulphidic environments (11), indicating that the enrichment of Mo in the Mount McCrae Shale may have been the result of the development of a euxinic water column associated with increased oxidative transport of crustal sulphur as SO42-.


Sources & Further reading

  1. Reinhard, Christopher T., Rob Raiswell, Clint Scott, Ariel D. Anbar, and Timothy W. Lyons. "A Late Archean Sulfidic Sea Stimulated by Early Oxidative Weathering of the Continents." Science 326, no. 5953 (October 30, 2009 2009): 713-16. 


Author: M. H. Monroe
Last updated  10/06/2013
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