Iron cycling in the Archean as recorded by the Fe isotopes in shales from the Barberton Greenstone Belt
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The iron cycle on the early earth was significantly different from today due to anoxic oceans and atmosphere. Today, the iron-rich deposits found show proof of this distinct cycle. For a long time, iron formations have been of particular interest because of their economic value and Fe isotopes were used to trace sources and depositional processes. Other sediments from the Precambrian are also distinctively iron-rich and might reflect iron cycling. Here, we present bulk iron isotope measurements of rocks from the Middle Mapepe formation of the Fig-Tree group from the Barb 5 drill core, Barberton Greenstone Belt, South Africa. 42 selected reference samples represent the basin development of the Middle Mapepe from deep anoxic to shallow water settings, including alluvial fans. Iron in the samples was chemically separated from its matrix and analysed for δ56Fe (relative to IRMM-014) with a MC-ICPMS at the Vrije Universiteit (VU) Amsterdam. Values ranged from -0.46‰ to 0.42‰ and showed a trend with negative values at the bottom of the core and more positive values at its top. This trend is interpreted to reflect variations in sedimentation and geochemical cycling in the Mapepe basin induced by changes in basin topography. Negative δ56Fe at the bottom of the core represent incorporation of isotopically light iron in diagenetic minerals, including pyrite, with only minor mixing with sediment input. In contrast, more positive values towards the top of the core suggest a stronger influence of detrital iron derived from weathering of mainly igneous rocks. This is mixed with iron that is withdrawn from the water column by an increasingly active anoxygenic photosynthesis in shallow conditions. A negative excursion of δ56Fe in a unit dominated by volcanoclastic debris coincided with a negative excursion in δ34S and an increase in inorganic carbon. This indicates active sulphur cycling and sulphide production by bacterial sulphur reduction and related Fe withdrawal and pyrite formation.