Fe cycling in the Shale Hills Critical Zone Observatory, Pennsylvania: An analysis of microbiology, chemical weathering, and Fe isotope fractionation

Background/Question/Methods

During weathering, iron in primary minerals is solubilized with ligands or reduced by bacteria and released into soil porewaters. Such Fe is then removed or re-precipitated in soils. To understand these processes, we analyzed Fe chemistry and isotopic composition in regolith in the Shale Hills watershed, a Critical Zone Observatory in central Pennsylvania overlying iron-rich shale of the Rose Hill Formation. Elemental concentrations were measured in soil from a well-drained catena on a planar hillslope on the south (north-facing) side of the catchment.

Results/Conclusions

Based upon X-ray diffraction data, loss of Fe commences as illite begins to weather 15 cm below the depth of auger-refusal. More Fe was oxidized than reduced in all soil samples from the ridge top to the valley floor. Both total (<80%) and ferrous (≤75%) iron are depleted from the land surface of catena soils relative to the bedrock. In terms of microbiology, aerobic Fe-related bacteria were cultured from soils across the catena, while Fe reducers of the genus Shewanella were observed only in the upper valley floor soils. The isotopic compositions (d⁵⁶Fe, relative to IRMM-014) of bulk Fe and 0.5 N HCl-extracted Fe (operationally designed to remove amorphous Fe (oxyhydr)oxides) range between -0.3 and +0.3‰, with D⁵⁶_{bulk-extractable} values between ~0.2 and 0.4‰. Throughout the watershed, d⁵⁶Fe signatures of both bulk Fe and HCl-extracted Fe become isotopically lighter as the extent of weathering proceeds. The isotopic trends are attributed to incomplete precipitation and retention of isotopically light Fe (oxyhydr)oxides, and release of isotopically heavy micron-sized Fe particulates into pore fluids and streams. The preferential release of ⁵⁶Fe between retained and released particulate Fe may be important in other watersheds as well.