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

During weathering, Fe in primary minerals is solubilized by ligands and/or reduced by bacteria and released into soil porewaters. Such Fe is then removed or reprecipitated in soils. To understand these processes, we analyzed Fe chemistry and isotopic composition in regolith of 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 side of the catchment. Based upon X-ray diffraction and bulk elemental data, loss of Fe commences as clay begins to weather ~15cm below the depth of auger-refusal. More Fe(III) was present than Fe(II) in all soil samples from the ridge top to the valley floor. Both total and ferrous iron are depleted from the land surface of catena soils relative to the bedrock. Loss of ferrous Fe is attributed mostly to abiotic or biotic oxidation. Loss of Fe is most likely due to transport of micron-sized particles that are not sampled by porous-cup lysimeters, but which are sampled in stream and ground waters. The isotopic compositions (δ⁵⁶Fe, relative to IRMM-014) of bulk Fe and 0.5N HCl-extracted Fe (operationally designed to remove amorphous Fe (oxyhydr)oxides) range between -0.3‰ and +0.3‰, with Δ⁵⁶Fe_{bulk-extractable} values between ~0.2‰ and 0.4‰. Throughout the soils along the catena, δ⁵⁶Fe signatures of both bulk Fe and HCl-extracted Fe become isotopically lighter as the extent of weathering proceeds. The isotopic trends are attributed to one of two proposed mechanisms. One mechanism involves Fe fractionation during mobilization of Fe from the parent material due to either Fe reduction or ligand-promoted dissolution. The other mechanism involves fractionation during immobilization of Fe (oxyhydr)oxides. If the latter mechanism is true, then shale - which comprises one quarter of continental rocks - could be an important source of isotopically heavy Fe for rivers.