Investigation of a diabase-derived regolith profile from Pennsylvania: Mineralogy, chemistry and Fe isotope fractionation

Abstract

At Earth's surface, bedrock transforms to regolith in a process that has many implications for water storage, soil formation, and nutrient availability to ecosystems. To understand deep regolith formation, we investigate three zones demarcating changes in mineralogy, chemistry and Fe isotopic composition in a 4-m thick regolith profile developed at a ridge top in Pennsylvania (U.S.A.) that is underlain entirely by diabase. Zone 1, at the bottom, is characterized by major element depletion, abrupt oxidation of Fe(II), a rapid decrease in sulfur concentrations and the presence of short-range ordered (SRO) Fe with depleted δ 56 Fe values. We attribute many of the observations at this depth (which we refer to as the lithogenic zone) to the effects of spheroidal weathering, which may be releasing a flux of Fe(II) with low δ 56 Fe values as the mineral pyroxene is weathered. This Fe(II) is oxidized and likely precipitated as isotopically light SRO Fe phases, which then recrystallize with time. Crystallization in Zone 2 is inferred to result in loss of isotopically light SRO Fe. Finally, SRO Fe isotopic compositions become lighter again near the land surface in Zone 3, the zone of most intense biogenic weathering related to exudation of protons and organic acids by plant roots. In this zone, these acids solubilize and remove isotopically light SRO Fe. This pattern of SRO Fe isotopic variability, which has been observed in other deep regolith profiles on spheroidally weathering bedrock, may be explained best by a combination of lithogenic and biogenic processes. The effects of biota reach deeper than the dominantly biogenic zone due to high concentrations of carbon dioxide, trapped in the soil by the low-permeability Bt/BC interface. Dissolution based on modern pore fluids in the upper two meters of regolith is occurring at a rate that is within an order of magnitude of the time-integrated rate of dissolution constrained mineralogically in the upper two meters. This correspondence is consistent with the conclusion that the rate of regolith formation remains roughly constant over time. Constant rates of weathering are expected in zones where erosion rates and rainfall are also constant. In that case, the thickness of the diabase may be considered to be at steady state (i.e., the rate of weathering advance due to deep lithogenic processes is roughly equivalent to the rate of advance at shallow depths due to biogenic processes).

Iron and silicon isotope behaviour accompanying weathering in Icelandic soils, and the implications for iron export from peatlands

“…EA-3a). The poorly drained conditions of this profile are likely to favour the release of isotopically light Fe(II) under anoxic conditions, followed by the quantitative precipitation of Fe-oxides during fluctuating oxic conditions (e.g., Fekiacova et al, 2013;Yesavage et al, 2016) resulting in the enrichment of light Fe isotopes in pedogenic oxides in these soils (Wiederhold et al, 2007a;Guelke et al, 2010;Kiczka et al, 2011). This hypothesis is supported by the lighter d 57 Fe DCB composition of the Fe d pool relative to the bulk soils (Fig.…”

Pervasively anoxic surface conditions at the onset of the Great Oxidation Event: New multi-proxy constraints from the Cooper Lake paleosol

“…The Fe isotopic fractionation preserved in modern soils is related to incomplete reductive dissolution of Fe(III)-oxides or ligand-promoted dissolution of primary Fe-bearing minerals (e.g., Brantley, 2001;Yesavage et al, 2012), with rapid Fe(II) oxidation and removal of colloidal Fe(III) imparting only a minimal isotopic effect (Li et al, 2017;Poitrasson et al, 2008;Yesavage et al, 2016). The Fe speciation chemistry of the Cooper Lake saprolith eliminates Fe redox as a major control on Fe isotopic fractionation, leaving the selective removal of isotopically light, dissolved aqueous Fe(II) during incongruent ferromagnesian mineral weathering as the most parsimonious explanation for increasing δ 56/54 Fe (Figure 7).…”

The Fe-Zn Isotopic Characteristics and Fractionation Models: Implications for the Genesis of the Zhaxikang Sb-Pb-Zn-Ag Deposit in Southern Tibet

“…The Fe isotopes were measured on the Neptune MC-ICP-MS at Pennsylvania State University. The instrument setup, sample introduction, and running conditions are discussed in greater detail in Yesavage et al [93]. Samples were diluted to a 3 ppm Fe solution which produced approximately a 10 V signal on the shoulder to the argon interference peak ( 56 Fe and 40 Ar 16 O).…”