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 103lnαlost–retained values of − 0.37 ± 0.03 and − 0.34 ± 0.04 for the surface and subsurface, respectively. Equivalent 103lnαlost–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.