WARC DISCOVERIES

Can the iron contents of ancient soils really be used to estimate atmospheric oxygen contents in the early Earth?

Ancient soils, called paleosols, have been one of the key components to estimations of ancient atmospheric oxygen contents. Because Fe(III) forms insoluble compounds at neutral pH, paleosols that have “normal” iron abundances and consist largely of Fe(III) minerals have been taken as evidence for an oxygenated atmosphere. Moreover, by assuming that iron has been immobile during soil formation, the amount of atmospheric oxygen required to oxidize the measured quantities of iron have been calculated. Alternatively, other workers have argued that the complex processes involved in soil formation, which may involve mobilization of iron by organic acids, precludes their use as an indicator of ancient atmospheric oxygen contents. In collaboration with WARC researchers Johnson and Beard, NAI members Kosei Yamaguchi and Hiroshi Ohmoto conducted an iron isotope study of the Hekpoort paleosol (Africa), which has been proposed to be one of the youngest (~2 b.y. old) iron-depleted paleosols that reflects a time of relatively low atmospheric oxygen contents.  Their findings, published in Earth and Planetary Science Letters, indicate that iron behaved as an open system, probably mobilized by groundwater flow, which indicates that the inferred low atmospheric oxygen contents for the Hekpoort paleosol are not correct. Because iron isotopes place important mass-balance constraints on iron mobilization, this study suggests that this type of analysis must be done on all paleosol studies that attempt to look at relative iron mobility.

Citation:
Yamaguchi, KE, Johnson, CM, Beard, BL, Beukes, NJ, Grutzmer, J, and Ohmoto (2007) Isotopic evidence for iron mobilization during Paleoproterozoic lateritization of the Hekpoort paleosol profile from Gaborone, Botswana.  Earth and Planetary Science Letters 256:577-587. A PDF copy of the paper can be found here.

Posted January 11, 2008