『Abstract
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 Fills 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 〜15 cm 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 IEMM-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 Δ56Febulk-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.』
1. Introduction
1.1. Fe isotope, soils, and rivers
2. Methods
2.1. Site description
2.2. Sample collection
2.3. Dissolved oxygen concentrations
2.4. Sample preparation and analysis
2.5. Microbiological methods
2.6. Stable iron isotopes
3. Results
3.1. Field observations
3.2. Soil oxygen and temperature
3.3. Chemical analysis
3.4. Chemical composition of water samples
3.5. Microbiological, C, and N observations
3.6. Iron stable isotope results
4. Discussion
4.1. Abiotic and biotic iron transformations
4.2. Hillslope mass balance
4.3. Iron isotope systematics in the Shale Hills watershed
4.3.1. Significance of iron isotope systematics during shale
weathering
4.3.2. Mechanism I
4.3.3. Mechanism II
5. Conclusions
Acknowledgements
Appendix A. Supplementary data
References