Chemical and physical weathering in regolith: An investigation of three different Fe-rich sites of varying climate and lithology.

摘要

The goal of this thesis is to understand chemical weathering in regolith at three very different sites that vary with respect to climate and lithology. The first study involves an analysis of chemical weathering and Fe isotope fractionation in soils from a shale-derived catchment in central Pennsylvania. Our investigation of this site is unique in the sense that we sampled regolith not only as a function of depth, but also from along a transect from the ridge to the valley floor of the watershed. The oxidation of Fe(II) occurs in a gradual manner in the augerable soil at this site, with no abrupt oxidation fronts with depth noted across the soil profile. Ultimately, we observed that both bulk and amorphous Fe become isotopically lighter in moving from the younger, shallow ridge top regolith into the older, deeper valley floor. One explanation for this trend involves dissolution processes (i.e., ligand-promoted dissolution and dissimilatory Fe reduction) that preferentially release isotopically light Fe during weathering. As weathering proceeds, we suggest that this isotopically light Fe travels along preferential zones of flow and is then partially precipitated, accumulating in the older valley floor. A second explanation for these trends involves the inference that the accumulation preferentially retains isotopically light Fe oxides. Although dissolved Fe in most streams is thought to be isotopically light, this second mechanism requires the loss of heavy Fe from the Shale Hills watershed.;In addition to the shale-derived watershed, we also investigated regolith weathering in two very different profiles developed on basaltic material. The first of these profiles is located at the Sverrefjell volcano, a Mars analog site in the Arctic Circle characterized by a cold, dry climate (described in Chapter 3). Despite the limited amount of time for weathering, very high concentrations of amorphous Al, Fe and Si were observed. Analysis of the clay-sized fraction from this site documents allophane as the predominant secondary phase, while abundant silica coatings of up to 100 microm on rock surfaces may help to explain anomalously low extracted amorphous Al/Si ratios. Based on batch experiments designed to study colloidal dispersion, elements such as Na, Mg and Ca are lost from the regolith in the dissolved form, while Al, Fe and Ti are mobilized as colloids in the 260-415 nm size range. In contrast, Si is mobilized simultaneously both in the dissolved and colloidal fractions. Findings from these colloidal dispersion experiments help to explain the relative loss of specific elements from regolith (Na, K >Ca, Mg >Si >Fe, Al, Ti) as well as the presence of silica coatings on weathered rocks.;In the final regolith profile studied (Chapter 4), analysis of pore-water samples in weathering diabase regolith allows for a further understanding of how various constituents (pH, DOC, anion and cation concentrations) change as a function of depth and season. In contrast to the other two sites, Fe(II) has been oxidized in close proximity to the bedrock/regolith interface of the Pennsylvania diabase profile. Elevated amorphous Fe concentrations near the bedrock/regolith interface are attributed to the release of Fe(II) and precipitation of amorphous Fe in a zone inferred to have originally been a set of rindlets created during spheroidal weathering. While bulk Fe isotope signatures within the diabase regolith did not vary significantly with depth, amorphous Fe values are isotopically lighter near the bedrock/regolith interface. Isotopically light Fe at depth is explained as resulting from kinetic effects during the rapid precipitation of Fe oxides from Fe(III) in solution. Similar observations at sites in Virginia and Puerto Rico suggest that spheroidal weathering results in unique chemical and Fe isotopic trends.