Rayleigh fractionation of iron isotopes during pedogene along a climate sequence of Hawaiian basalt Aaron Thompson a , Joaquin Ruiz b , Oliver A.Chadwick c , Monica Titus a , Jon Chorover a, ⁎ a Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85721, United States b Department of Geosciences, University of Arizona, Tucson, AZ 85721, United States c Department of Geography, University of California-Santa Barbara, CA 93106, United States Received 13 April 2006; received in revised form 31 October 2006; accepted 2 November 2006 Editor: C.Gopel Abstract We measured iron isotopic composition of surface (10–20 cm) and subsurface (50–70 cm) basaltic soil horizo of Mauialong a climate gradient (MCG) ranging from 2.2 to 4.2 m mean annual precipitation (MAP). All soilforming factors except climate were conserved. The MCG has a documented decrease in Fe with increasing rainfall that is highl decreasing mean annual Eh values. We found that increasing MAP from 2.8 to 4.2 m resulted in a surface plus s increase of 0.56‰ ± 0.09‰ δ 56 Fe with the subsurface consistently 0.33 ± 0.06‰ δ 56 Fe greater than the surface horizons. Based o loss of Fe relative to Nb, Rayleigh fractionation was observed with 10 3 lnα lost–retained values of − 0.37 ± 0.03 and − 0.34 ± 0.04 fo surface and subsurface, respectively. Equivalent 10 3 lnα lost–retained values for the surface and subsurface soils suggests Fe los driven by similar mechanisms throughout the soilprofile.Ourcalculated fractionation factor is about1/3 the magnitude of laboratory determined fractionation factors for Fe reduction, suggesting other processes (organic complexation modulate the net Fe loss along the MCG. These results offer field-scale confirmation of laboratory experiments that show anoxic weathering reactions produce materials enriched in heavy Fe isotopes. © 2006 Elsevier B.V. All rights reserved. Keywords: Fe isotopes; Soil; Redox; Pedogenesis 1. Introduction The weathering reactions of iron (Fe)intersect with many aspects of the biogeochemical functioning of soils (Crewset al., 1995;Torn et al., 1997;Darkeand Walbridge, 2000; Miller et al., 2001; Küsel et al., 2002 ). Thermodynamic transformation of initially ferrous-bearing primary basalt minerals in oxic environments is thou involverapid Feoxidation to secondary short-range- ordered minerals like ferrihydrite or nano-goethite fol- lowed by slow transformation to crystalline Fe-oxide as goethite [α-FeO(OH)] and hematite [α-Fe 2 O 3 ] (Shoji et al.,1993).Conservation of Fe relative to Si and non- hydrolyzing cations (i.e., Na, Ca, etc.) is predicted b on the low aqueous solubility of Fe III -solids,although some mobilization of Fe does occur via proton-prom dissolution at pH < pznpc (point of zero net proton c Chemical Geology 238 (2007) 72 – 83 www.elsevier.com/locate/chemgeo ⁎ Corresponding author. E-mail address: Chorover@cals.arizona.edu (J. Chorover). 0009-2541/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2006.11.005