Sources and cycling of Mn: Located in the heart of a historic iron producing region, the Susquehanna Shale Hills CZO provides a model system to investigate anthropogenic metal inputs to soils. Detailed geochemical studies have shown that excess manganese (Mn) contaminates the soils at SSHO due to past atmospheric deposition (Herndon et al., 2011).

  This Mn, emitted as a byproduct of iron and steel production, coal combustion, and other industries, remains in the soil as the legacy of Huntingdon County's industrial past.

We are currently evaluating the role of vegetation in retaining Mn within the watershed (Figure 1). Low concentrations of Mn are found in soil pore fluids, indicating that losses to chemical weathering are small. In contrast, high concentrations of Mn are found in tree leaves, suggesting that Mn is being rapidly taken up from soil by the trees. In this way, vegetation acts as a “capacitor” that stores Mn and releases it slowly to the environment over time. We are using X-ray absorption spectroscopy (XAS) to characterize Mn in soils and vegetation. Preliminary results show that Mn in foliar tissue is reduced and complexed with organic compounds (Figure 2), while Mn in soils is present as a tri-/tetravalent oxide, indicating that Mn is oxidized and immobilized as tree leaves fall to the soil and are decomposed.

Additionally, we are coupling our study of Mn at SSHO with analysis of global Mn contamination. Data gathered from multiple geochemical databases show that Mn addition is common in soils, but patchy in distribution. Air and water data indicate that air concentrations of Mn peaked in the 1960s before declining by an order of magnitude, while riverine outputs of Mn peaked ~1980 before also declining.

This research is spearheaded by Elizabeth Herndon. [Herndon, E.M., Jin, L., and Brantley, S.L. (2011) Soils reveal widespread manganese enrichment from industrial sources, Environmental Science & Technology 45(1), 241-247.]

Iron Isotopes: This research involves understanding how Fe is transformed and cycled as a result of weathering and soil formation in the Shale Hills watershed. In order to investigate this question, a series of soil samples were collected from along a transect and analyzed for bulk chemistry and Fe(II) concentrations. Chemical weathering results in a moderate but incomplete loss of both total Fe and Fe(II) from soil profiles. Furthermore, mass balance calculations are used to evaluate fluxes of these two constituents from different topographic positions along the transect.

  In order to better elucidate microbiological processes that may be cycling Fe within the catchment, both Fe-oxidizing and Fe-reducing bacteria were cultured. The abundance of Fe-reducing bacteria is well predicted by soil moisture level, while Fe-oxidizing bacteria are often prevalent near the soil/bedrock interface.

A final goal of this research involves understanding how different processes associated with Fe weathering and cycling may fractionate the stable isotope values of Fe in soils. Previous experimental work suggests that many common processes (including bacterial Fe reduction and the ligand-promoted dissolution of minerals) tend to produce Fe isotope signatures in bulk soils that are isotopically heavy relative to starting material. In contrast, although the overall extent of Fe fractionation in SSHO soils is low, we observe a very different type of trend that involves isotopic depletion with increased weathering extent toward the valley floor (Figure 3). At the moment, we are not fully sure of how to explain this trend. It may result from the preferential uptake of isotopically light Fe by vegetation over time or from the loss of isotopically enriched dissolved Fe from the watershed. This research is spearheaded by graduate student Tiffany Yesavage.

Uranium-series disequilibrium isotopes: U-series disequilibrium isotopes (238U, 234U, and 230Th) of shale rocks and soils are being analyzed at the Shale Hills catchment to derive the soil production function and estimate the residence time of regolith particles using mass balance models (Figure 4).

  The activity ratios observed in the soils are explained by loss of U-series isotopes during water-rock interactions and re-precipitation of 234U and 238U downslope. Regolith production rates calculated with these isotopes decrease exponentially from 45 to 15 m/Myr (regolith residence times increase from 6.7 to 45 kyr) on the south planar transect, with increasing soil thickness from the ridge top to the valley floor. By contrast, regolith production rates from the north planar transect are ~40-52 m/Myr (regolith residence times are 8-10 kyr). The regolith profiles on the northern, sun-facing slope are thus characterized by faster regolith production rates and shorter duration of chemical weathering compared to the southern, more shaded slope. These observations are consistent with the conclusion that aspect exerts an important control on the regolith formation at the Shale Hills catchment. The aspect asymmetry induces different microclimatic conditions that affect slope stability and erosion and set the residence time for the materials available for chemical weathering. Given that the SSHO experienced a peri-glacial climate ~15 ky ago, we conclude that the hillslope retains regolith formed before that glacial period and that the hillslope is not at geomorphological steady state. This research is spearheaded by postdoctoral scholar Lin Ma (present address: Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968). [Ma, L., Chabaux, F., Pelt, E., Blaes, E., Jin, L., and Brantley, S.L., Regolith production rates calculated with Uranium-series isotopes at Shale Hills Critical Zone Observatory, Earth and Planetary Science Letters vol 297, 211-225; Ma, L., Chabaux, F., Pelt, E., Jin, L., and Brantley, S., North versus south: Implications from U-series isotopes about regolith formation at Shale Hills, to be submitted]

Water Chemistry: An intense field study was conducted at the Susquehanna/Shale Hills Critical Zone Observatory to use water chemistry to probe the subsurface hydrogeochemical conditions and water flow dynamics. Soil pore water sampled through tension lysimeters suggest that contemporary reaction rates depend on preferential flowpaths and solute residence times, and are limited by dissolution kinetics.

  Thus weathering processes are strongly coupled with hydrological features of shale rocks and soils. The chemistry of ground- and streamwater is being analyzed in relation to that of the soil porewaters to elucidate the mineral reactions occurring below the soils and to derive elemental fluxes over the whole catchment.

Soil water solutes are predominantly contributed by shale weathering, which is limited by clay dissolution kinetics. As such, solute concentrations are primarily controlled by the residence time of water in soil. Translocation and deposition of secondary clays created B-horizons that are more conductive to lateral flow right above or below it than vertical flow. Consistent with soil moisture monitoring data, water flows out of the steep-sloping hillslope predominantly laterally through preferred pathways that include soil horizon and soil-bedrock interfaces, and vertically via macropores. As a consequence, much higher concentrations of major cations are observed in porefluids within the B-horizon because water flows more slowly and mineral-water contact time is much longer. The amplitude of seasonal variations in O and H isotopes decreases in the order: precipitation>> soil water > stream > groundwater, suggesting water becomes progressively older as rainfall infiltrates the soil and eventually recharges to ground water. Interestingly, Mg concentrations increase with the residence time of the water due to increasing contact time with minerals (clay and ankerite). Stream water is a mixture of groundwater and shallow soil water where the relative proportions change seasonally. The discharge of the first-order stream responds to precipitation closely, documenting the rapid release of both old groundwater and relatively young soil water during storms.

[Jin, L., Andrews, D.M., Holmes, G.H., Duffy, C.J., Lin, H and Brantley, S.L. (2011) Opening the "Black Box":  Water chemistry reveals hydrological controls on weathering in Susquehanna/Shale Hills Critical Zone Observatory (Central Pennsylvania, USA), Vadose Zone Journal 10:928-942.]

Neutron Scattering: We use neutron scattering techniques to characterize the evolution of nanoscale features (e.g., nano-porosity, surface area, surface roughness) in shales and to reveal that changes of these physical properties are coupled with chemical weathering reactions. 

  At ~ 20 m depth, dissolution is inferred to have depleted the bedrock of ankerite and all the chips investigated with neutron scattering are from above the ankerite dissolution zone. Scattering data documents that 5-6% of the total ankerite-free rock volume is comprised of isolated, intraparticle pores. At 5 m depth, an abrupt increase in porosity and surface area corresponds with onset of feldspar dissolution in the saprock and is attributed mainly to peri-glacial processes from 15 ka. At tens of centimeters below the saprock-regolith interface, the porosity and surface area increase markedly as chlorite and illite begin to dissolve. These clay reactions contribute to the transformation of saprock to regolith. Throughout the regolith, intraparticle pores in chips connect to form larger interparticle pores and scattering changes from a mass fractal at depth to a surface fractal near the land surface. Pore geometry also changes from anisotropic at depth, perhaps related to pencil cleavage created in the rock by previous tectonic activity, to isotropic at the uppermost surface as clays weather. In the most weathered regolith, kaolinite and Fe oxyhydroxides precipitate, blocking some connected pores. These precipitates, coupled with exposure of more quartz by clay weathering, contribute to the decreased mineral-pore interfacial area in the uppermost samples.

These observations are consistent with conversion of bedrock to saprock to regolith at SSHO due to i) transport of reactants (e.g., water, O2) into primary pores and fractures created by tectonic events and peri-glacial effects, ii) mineral-water reactions and particle loss that increase porosity and the access of water into the rock. From deep to shallow, mineral-water reactions may change from largely transport-limited where porosity was set largely by ancient tectonic activity to kinetic-limited where porosity is changing due to climate-driven processes.

[Jin, L., Rother, G., Cole, D.R., Mildner, D.F.R., Duffy, C.J. and Brantley, S.L. (2011) Characterization of deep weathering and nanoporosity development in shale – a neutron study. American Mineralogist 96:498–512.]

Swale Chemistry: The soil chemistry data (major and REEs) from a swale transect are characterized for comparison to similar measurements on a planar transect for the Susquehanna/Shale Hills critical zone observatory. Similar reaction fronts are observed: plagioclase dissolution is indicated by Na and Ca depletion and a negative Eu anomaly; clay dissolution followed by particle loss is accompanied by loss of Mg, K, Fe, Al and Si.

  However, different from the planar transect, soils along the swale transect, especially in the topographically depressional site, do not show smooth elemental profiles. This documents both residuum soils and accumulation of colluvium sediments. The soils at the swale transect are thicker and on average wetter than those along the planar transect; however, the Ce anomaly observed in the swale soils is consistent with a generally oxic environment. Thus, preferential flowpaths are an important mechanism for water transport, preventing swale soils from water saturation.

[Jin, L. and Brantley S.L. (2011) Soil chemistry and shale weathering on a hillslope influenced by convergent hydrologic flow regime at the Susquehanna/Shale Hills Critical Zone Observatory. Applied Geochemistry 26:S51–S56].

Deep shale weathering: Molly Holleran was a undergrad in the Geosciences Department, who worked as a field assistant at Shale Hills for the three years. She assists with lysimeter sample collection during the spring, summer and fall seasons. Molly is now working on her senior thesis based at Shale Hills, on the quantification of deep shale weathering. Research goals include evaluating the change of mineralogy with depth, and determining if changes seen can be attributed to chemical weathering or heterogeneity in the lithology. Carbonate, pyrite, feldspar, and illite/chlorite reaction fonts are being investigated, from the ridge top to the valley floor. Overall, a goal of this study is to be able to relate the process of shale weathering and associated reaction fronts with the overall water-table in the catchment.

Contacts: Susan Brantley (PI), Carmen Enid Martinez