Rayleigh fractionation of iron isotopes during pedogenesis along a climate sequence of Hawaiian basalt

We measured iron isotopic composition of surface (10-20 cm) and subsurface (50-70 cm) basaltic soil horizons from the Island of Maui along a climate gradient (MCG) ranging from 2.2 to 4.2 m mean annual precipitation (MAP). All soil forming factors except climate were conserved. The MCG has a documented decrease in Fe with increasing rainfall that is highly correlated with decreasing mean annual Eh values. We found that increasing MAP from 2.8 to 4.2 m resulted in a surface plus subsurface average increase of 0.56‰ ± 0.09‰ δ⁵⁶Fe with the subsurface consistently 0.33 ± 0.06‰ δ⁵⁶Fe greater than the surface horizons. Based on loss of Fe relative to Nb, Rayleigh fractionation was observed with 10³lnα_{lost-retained} values of - 0.37 ± 0.03 and - 0.34 ± 0.04 for the surface and subsurface, respectively. Equivalent 10³lnα_{lost-retained} values for the surface and subsurface soils suggests Fe loss is driven by similar mechanisms throughout the soil profile. Our calculated fractionation factor is about 1/3 the magnitude of laboratory determined fractionation factors for Fe reduction, suggesting other processes (organic complexation, Fe re-precipitation) modulate the net Fe loss along the MCG. These results offer field-scale confirmation of laboratory experiments on model systems that show anoxic weathering reactions produce materials enriched in heavy Fe isotopes.