Soil chemistry and shale weathering on a hillslope influenced by convergent hydrologic flow regime at the Susquehanna/Shale Hills Critical Zone Observatory
Highlights
► Soils in a swale are controlled by weathering and colluvium sediment transport. ► Plagioclase and clay dissolution dictates elemental profiles at Shale Hills soils. ► Swales at Shale Hills are wetter but oxic, evidenced by the positive Ce∗ anomaly.
Introduction
Over the long term, rates of weathering and erosion combine to control soil chemistry and evolution of landscapes (White et al., 1996, Anderson et al., 2007, Brantley and White, 2009). One approach to investigate watersheds that are developed on a single lithology is to work on 1D, 2D and 3D sites (representing different patterns of sediment erosion and water flow) from pedon to catena to catchment scales. The Susquehanna/Shale Hills Critical Zone Observatory (SSHO) is a small watershed located in central Pennsylvania developed on gray shale (Silurian Rose Hill Formation), and soil weathering reactions and elemental transport have previously been studied in 1D (ridge top) and 2D (planar transect) sites on the southern slope (Jin et al., 2010). Here, three weathering reactions transform bedrock into saprock and regolith. The deepest inferred reactive interface (or “reaction front”) involves ankeritic carbonate dissolution at about 20 m below ground surface at the ridge. The next and shallower reaction front begins ∼5 m below ground surface and is defined by plagioclase feldspar dissolution, accelerated by a fracture associated with periglacial activities (Jin et al., 2011). The clay weathering front initiates just below the regolith, where chlorite and illite dissolve to form kaolinite and Fe-oxyhydroxide.
Here the focus is on major and trace element observations for a 3D swale transect (Fig. 1a) for discussion of weathering and sediment transport as a function of landscape position. These physical and chemical processes are recorded by elemental profiles in regolith. Soil samples were collected by augering to the refusal depth at three sites: south swale ridge top (SSRT), south swale middle slope (SSMS) and south swale valley floor (SSVF). Elemental abundances were measured following lithium metaborate fusion at 950 °C and resultant solutions were analyzed by inductively coupled plasma emission spectrometer (ICP-AES) for major elements and inductively coupled plasma mass spectrometer (ICP-MS) for Zr and rare earth elements (REEs).
Section snippets
Results
The regolith thickness varies significantly along the swale transect: 29 cm at ridge top (SSRT), 163 cm at the mid-slope (SSMS) and 95 cm at the valley floor (SSVF). The swale shows a sharp drop in elevation near SSMS, and the slope is also steeper to the side (Fig. 1b). Concentrations of major elements and REEs in the swale soils are reported in Table 1, Table 2, respectively, as well as the corresponding τj,Zr values, calculated as the mass transfer parameter τj,Zr = (Cj,wCi,p)/(Cj,pCi,w) − 1 (e.g.,
Discussion
Our previous study has shown that planar hillslope soils exhibit smooth depletion profiles under the influence of coupled chemical dissolution and physical sediment erosion processes (Jin et al., 2010). The swale transect develops analogously to the planar hillslope soils and shows element concentrations that vary consistently with depth (SSRT and SSVF in Fig. 2). Even though this is on a swale transect, the SSRT site is located at the ridge line, and its elemental profiles are as steep as
Conclusions
This study reports soil chemistry along a swale transect, where elemental profiles are primarily controlled by clay dissolution, similar to those in the planar transect. However, this convergent slope is also influenced by colluvial sediments, leading to inconsistent variation of element concentrations with depth.
Depletion of Ca and Na, as well as Eu negative anomaly reveals that plagioclase dissolution is an important weathering reaction at Shale Hills, and it occurs at an even faster rate
Acknowledgements
We thank those who helped in the field, especially Danielle Andrews, Jennifer Williams, Elizabeth Herndon and Molly Holleran for sampling soil cores. Financial support is provided by National Science Foundation under Grant No. CHE-0431328 for the Center for Environmental Kinetics Analysis and under Grant No. EAR-0725019 for Susquehanna/Shale Hills Critical Zone Observatory. Logistical support and/or data were provided by the NSF-supported Shale Hills Susquehanna Critical Zone Observatory.
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