Iron-rich Archean Oceans and Implications for the Carbon Cycle

Dawn Y. Sumner

1998, AGU Annual Meeting, Abstracts

Prior to 2.0-2.2 Ga, the atmosphere and oceans contained very little O2. Because Fe2+ is soluble in anoxic water, one of the consequences of low [O2] was the maintenance of substantial [Fe2+] dissolved in seawater. The abundance of marine Archean and Paleoproterozoic iron-formations demonstrates that large quantities of Fe2+ were transported through the deep oceans to shelves, probably at concentrations >100's m/l. [Fe2+] in the mixed zone of the oceans was probably substantially lower due to oxidation by UV radiation and photosynthetic O2. However, concentrations of ~1 m/l may have been dynamically maintained through upwelling and re-reduction of iron at depth. Evidence for a gradient from lower to high [Fe2+] with depth is preserved in carbonate platform to basinal iron-formation transitions in the Transvaal (South Africa) and Hamersley (Australia) basins.

Experimental results demonstrate that Fe2+ dramatically slows calcite precipitation rates. However, long term removal of C and Ca2+ from the ocean-atmosphere system must balance influxes from volcanism, metamorphism, and weathering. If carbonate precipitation rates were slowed by the presence of Fe2+, saturation states would have increased to the point where long term accumulation rates balanced the Ca2+ influx. Carbon removal was balanced by both carbonate accumulation and Corg burial. Late Archean carbonate d13C0, implying that the balance of carbon removed as carbonate versus Corg was approximately 3:1 if d13Corg-25. However, average d13Corg may have been closer to -45 (Des Marais et al, 1992, Nature, v 359, p 605-609) requiring a carbonate to Corg burial ratio of 7:1. If CO2 outgassing was more rapid during Archean time, large volumes of carbonate must have accumulated even with the presence of inhibiting Fe2+. Thus, carbonate saturation states were probably substantially higher prior to the rise in O2 than afterward. High saturation states can be achieved with high [Ca2+] or high CO2. High pH alone could not be the cause because high pCO2 is required for a substantial greenhouse effect.

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616