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Pedogenic Translocation of Fe in Modern and Ancient Vertisols and Implications for Interpretations of the Hekpoort Paleosol (2.25 Ga)

Steven G. Driese
The Journal of Geology
Vol. 112, No. 5 (September 2004), pp. 543-560
DOI: 10.1086/422665
Stable URL: http://www.jstor.org/stable/10.1086/422665
Page Count: 18
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Pedogenic Translocation of Fe in Modern and Ancient Vertisols and Implications for Interpretations of the Hekpoort Paleosol (2.25 Ga)
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Abstract

Abstract Many interpretations of oxygen levels in the Precambrian atmosphere use interpretations of Fe distributions measured in paleosols. This article addresses the current lack of knowledge concerning “baseline” Fe translocations in modern soil analogs for the 2.25‐Ga Hekpoort paleosol at Waterval Onder, South Africa, as well as the lack of understanding of diagnostic petrographic features related to Fe translocation preserved in the Precambrian paleosols. Petrographic features related to Fe translocations include redox‐related Fe depletions and enrichments, and possibly Fe‐Mn nodules, which compare favorably with similar pedogenic features in Paleozoic paleo‐Vertisols and modern analog Vertisols. Additionally, preservation of sepic‐plasmic microfabrics in the Hekpoort paleosol, which are characteristic of clay soils experiencing shrink‐swell, supports previous paleo‐Vertisol interpretations. Total Fe losses in Vertisols are 10%–50%; losses increase with increasing soil age and with increasing mean annual precipitation but appear to be independent of parent material differences (unconsolidated sediments, sedimentary rocks, mafic rocks). Total Fe losses in Paleozoic paleo‐Vertisols are comparable to those of Vertisols. Nearly complete (75%–99%) loss of total Fe characterizes the upper 220 cm of the Hekpoort paleosol, if the underlying basalt is assumed to be the parent material. The apparent greater mobility of Fe in the Hekpoort paleosol most likely reflects the lower (but not anoxic) Po2 conditions postulated for this time by some researchers but could also indicate oxic conditions and the presence of a significant terrestrial biomass as a source of organic ligands that enhanced Fe mobility, which has also been recently proposed.

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