OPEN OCEAN VS. CONTINENTALLY-DERIVED IRON CYCLES ALONG THE NEOARCHEAN CAMPBELLRAND-MALMANI CARBONATE PLATFORM, SOUTH AFRICA

摘要

The deposition of large amounts of mixed-valence Fe minerals in iron formations during the Archean and Paleoproterozoic indicates that the Fe(II)(aq) (aqueous) content of coeval anoxic seawater was likely several hundred mu M, compared to ca. 1 to 20 nM of the modern oxygenated ocean. It has been suggested that oxygen production along shallow marine continental shelves, which probably started several hundred million years before the rise of atmospheric oxygen, effectively oxidized Fe(II)(aq) from deeper seawater and removed it as Fe(III)(ppt) (poorly soluble precipitates). However, the reconstruction of the marine Fe cycle during the Archean is still incomplete, partly because of diagenetic redox processes that challenge the interpretation of Fe concentration and isotope signatures of sedimentary archives.