Sulphides in the ocean

About 2.3 billion years ago, ancient soils begin to reveal that Earth, or more precisely life upon it had developed an atmosphere that contained oxygen, albeit at quite low levels.  One of the most interesting events during the Proterozoic Aeon was the world-wide disappearance of vast deposits of iron oxides known as banded iron formations or BIFs, at about 1.8 billion years.  Many authorities view that as the time when sufficient oxygen was dissolved in seawater to have removed soluble Fe-2 at its source, on the ocean floor near hydrothermal vents – BIFs formed in shallow water, and that requires Fe-2 to have permeated the entire oceans.  There is another possibility.  The presence of atmospheric oxygen would have ensured the oxidation of iron sulphide exposed at the land surface, thereby adding sulphate ions to river water, and eventually seawater.  Another line of evidence for atmospheric oxygen is the disappearance of detrital sulphide grains from sedimentary rocks younger than 2.3 billion years, so a build-up of sulphate ions in later seawater is quite plausible.  Should deep-ocean chemistry have been reducing, it is possible that sulphide ions would form there.  The insolubility of iron sulphides would then remove Fe-2 from seawater equally as efficiently as would oxygen.  Danish and Canadian geochemists have investigated this possibility using data from sediments in Canada that mark the last phase of major BIF deposition around 1.8 billion years (Poulton, S.W. et al. 2004.  The transition to a sulphidic ocean ~1.84 billion years ago.  Nature, v. 431, p. 173-177).  They found that conditions changed from one in which seawater contained dissolved Fe-2 at the time of the last BIF deposition to one dominated by sulphide ions, similar to that found in modern anoxic waters such as those in the Black Sea.  That would have sequestered any available Fe-2 to pyrite in sediments, a feature typical of many later Proterozoic sediments.  Since seawater during the Phanerozoic was dominated by sulphate ions, except in periods of ocean anoxia, it looks likely that late Precambrian sulphidic oceans gave way to more modern sulphur chemistry following a rapid rise in atmospheric oxygen at the end of the Proterozoic.  One consequence of highly-reducing deep ocean water would have been very efficient burial of dead organic matter while it lasted, because anaerobic bacteria do not fully convert organic molecules back to water and carbon dioxide.  During the Neoproterozoic d13C in seawater underwent rapid swings from highly negative to highly positive, on which all kinds of connotations have been placed.  Another explanation for the carbon hiccups might be that periodically there were short-lived increases in oxygenation of deep ocean water.


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