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Limited iron isotope variations in recent lateritic soils from Nsimi, Cameroon: Implications for the global Fe geochemical cycle

Poitrasson, Franck and Viers, Jerome and Martin, Francois and Braun, Jean-Jacques (2008) Limited iron isotope variations in recent lateritic soils from Nsimi, Cameroon: Implications for the global Fe geochemical cycle. In: Chemical Geology, 253 (1-2). pp. 54-63.

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Abstract

Laterites are oxidized Fe-rich soils covering one third of the continents and are drained by half of the continental waters. They therefore represent a key component of the iron geochemical cycle at the Earth's surface, yet no iron isotope study has been conducted on recent laterites so far. Building on previous integrated morpho-pedological studies of soils located in an equatorial rainforest, Southern Cameroon, we have undertaken a mineralogical, elemental and isotopic study of iron in two lateritic profiles. One is a borehole 3620 cm deep going to the parent granodioritic rock, located at the top of a hill, whereas the other is 675 cm deep and is located downhill. Mossbauer spectroscopy reveals that typically $\geq 96%$ of the soil's iron is held in nanocrystalline hematite and goethite. Iron isotope measurements performed by plasma source mass spectrometry show that Fe isotopic equilibrium was rarely reached between these iron oxide and hydroxide despite their small size. Overall, it is found that most samples display iron isotope signatures very close to the mean crustal value, with a maximum range of 0.2% in $\delta^{57}Fe$. Given that lateritic soils evolve over millions of years, show large variations in Fe concentrations and mineralogical abundances, this range is surprisingly small when compared to other Fe isotope studies of soils from different climatological contexts, that can easily show $\delta^{57}F$e ranges in excess of 1% at the bulk sample scale. The most likely explanation of this finding is that despite notable vertical and lateral Fe mobility in the studied laterites from Cameroon, as computed using the open-system mass fraction transport function,$^{\tau}Fe,W$ iron remained mostly in the oxidized form as shown by Mossbauer pectroscopy. This probably results from the strong bioturbation and pedoturbation of these lateritic soils that lead them to remain porous over a large thickness and facilitated the flow of oxygenated waters from the surface down to the saprolite horizon. This study therefore reveals that soils that remained as an open system for iron do not necessarily show large Fe isotopic variations. It is suggested that the Fe drained from such lateritic soils should have $\delta^{57} Fe$ values within $\sim 0.1$ of that of the continental crust. This is in contrast with previous soil studies, notably from sites located at higher latitudes, that imply that these soils should release isotopically more variable Fe to surface waters. Such possible contrasted isotopic signatures from different surface waters will likely lead to an isotopically heterogeneous ocean given the short residence time of Fe in seawater.

Item Type: Journal Article Copyright for this article belongs to Elsevier. Laterite;soil;goethite;hematite;iron isotopes;continental surfaces. Others 23 Sep 2008 06:34 19 Sep 2010 04:50 http://eprints.iisc.ernet.in/id/eprint/15933