How mineralogy and slope aspect affect REE release and fractionation during shale weathering in the Susquehanna/Shale Hills Critical Zone Observatory
Highlights
► Significant REE depletions during formation of acidic and organic-rich soils. ► Slope aspect controls REE depletion by generating different microclimate conditions. ► Dissolution of rhabdophane controls the MREE fractionation during regolith formation. ► Strong Ce anomalies point to prevalent oxidizing conditions. ► Positive Eu anomalies attributed to weathering of plagioclase.
Introduction
Chemical weathering of silicate rocks plays an important role in providing nutrients to ecosystems, controlling river and ocean chemistry, and regulating atmospheric CO2 over geological time (e.g., Kump et al., 2000, Drever, 2004). The weathering products, soils, are vital for the sustainability of ecosystems and human society (Montgomery, 2007, Porder et al., 2007, Brantley, 2008). It is thus of great importance to understand the factors that control chemical weathering and soil formation at Earth's surface.
Because of their similar chemical properties as one group, rare earth elements (REE: La to Lu) have the great potential to serve as powerful tracers in many geochemical processes, e.g., to trace the sedimentary source provenance (McLennan, 1989, Nesbitt et al., 1990, Schatzel and Stewart, 2003, Lev and Filer, 2004, Song and Choi, 2009). Systematic fractionation of REE commonly occurs during weathering, transport, and deposition and thus provides natural tracers to decipher these different processes. For example, cases of enhanced mobility for light REE (LREE: La to Nd), middle REE (MREE: Sm to Dy), or heavy REE (HREE: Ho to Lu) have been commonly reported in studies of weathered bedrock, soil and weathering profiles, sediments, and natural waters (Nesbitt, 1979, Elderfield et al., 1990, Sholkovitz, 1993, Sholkovitz, 1995, Nesbitt and Markovics, 1997, Shiller, 1997, Shiller, 2010, Johannesson and Zhou, 1999, Land et al., 1999, Ingri et al., 2000, Viers et al., 2000, Aubert et al., 2001, Brantley et al., 2001, Hannigan and Sholkovitz, 2001, Compton et al., 2003, Bau et al., 2004, Johannesson et al., 2004, Andersson et al., 2006, Stille et al., 2006). In addition, Ce and Eu are redox sensitive — Ce oxidizes from III to IV and Eu reduces from III to II under oxidizing and reducing conditions, respectively. This behavior gives rise to redox-induced Ce and Eu anomalies and thus records changes of redox conditions in geological processes (e.g., Brookins, 1989, McLennan, 1989, Prudencio et al., 1995, Bau, 1999, Patino et al., 2003, Guo et al., 2010). Finally, REE are also commonly used as analogs to some toxic heavy metals such as actinides to predict their mobility, potential toxicity, and contamination effects because of their similar chemical properties in the environment (e.g., Brookins, 1989, Wood, 1990, Koppi et al., 1996).
The release of REE into streams and rivers by chemical weathering is initiated in the thin porous layer at Earth's surface (the “Critical Zone” where fresh rocks, water, atmospheric gases and vegetation first interact; Brantley et al., 2007). Studies of REE in small first-order catchments hold great promise to ascertain the mobility of REE during regolith formation and to reveal REE fractionation signatures inherited directly from mineral weathering. Many REE studies have been focused on watersheds underlain by igneous and metamorphic bedrock (Nesbitt, 1979, Mongelli et al., 1998, Tricca et al., 1999, Aubert et al., 2001, Leybourne and Johannesson, 2008, Steinmann and Stille, 2008, Shiller, 2010) and a few studies have been focused on sedimentary rocks such as sandstone and shale (e.g., Duddy, 1980, Condie, 1991, Yan et al., 2000, Lev and Filer, 2004). It is well known that sedimentary rocks cover about 70% of the Earth's land surface (Amiotte-Suchet et al., 2003). Because weathering of shale controls global geochemical fluxes of C, P, and Pt-group elements (Petsch et al., 2001, Kolowith and Berner, 2002, Amiotte-Suchet et al., 2003, Tuttle and Breit, 2009, Tuttle et al., 2009), it is of great importance to fully understand the weathering of this common lithology.
Here, we study REE behaviors in weathering profiles, pore waters, stream waters and stream sediments from a first-order catchment developed entirely on gray shales in central Pennsylvania, USA. These REE data allow for an improved understanding of the mobility, redistribution, and fractionation of REE during chemical weathering in a well-constrained experimental natural catchment under temperate climate. Furthermore, the roles of climate, landscape positions, and hydrological processes on REE fractionation during chemical weathering are assessed by comparing weathering profiles from two different slope aspects within this catchment.
Section snippets
Geological setting
To understand inter-relationships among chemical, physical, hydrological and biological processes impacting the near-surface environment, the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) was established in central Pennsylvania (Fig. 1a; http://www.czo.psu.edu/). This 8-ha watershed is located on the northern part of the Appalachian Mountains and managed as a Pennsylvania State University experimental forest. Extensive data sets are available at SSHO from forestry experiments,
Samples, experimental protocol and analytical methods
To characterize shale weathering and regolith production at sites representing different patterns of sediment erosion and water flow, selected pedons and catenas have been chosen for elemental and isotopic analyses (Jin et al., 2010a, Jin et al., 2010b, Ma et al., 2010, West et al., 2010). We focus here on the REE composition of weathering profiles along a southern and a northern planar/hillslope from the catchment (Fig. 1). The two catenas were chosen from hillsides experiencing generally
Results
REE, organic carbon, phosphorus and Zr contents and loss on ignition values in solid samples (regolith, sediments and bedrock) are reported in Tables 1. REE concentrations, pH, and DOC values (from Andrews et al., 2011) in pore waters and stream waters are reported in Table 2. REE, Al, Fe, and Mn concentrations in sequential extracts (normalized to bulk soils) are reported in Table 3. Major elements and Zr contents, mineralogy, and cation exchange capacities of the three southern weathering
Mobility of REE during shale weathering and regolith formation
REE mobilization, redistribution, and fractionation during chemical weathering have been documented in many soils and weathering profiles over the past few decades (e.g., Nesbitt, 1979, Duddy, 1980, Braun et al., 1993, Condie et al., 1995, Land et al., 1999, Compton et al., 2003, Tyler, 2004). Most values of τ for REEs in SSHO southern slope profiles become more negative towards the surface (Fig. 9). Such depletion profiles (Brantley and White, 2009) are generally interpreted to reveal
Summary and conclusions
REE concentrations in regolith, stream sediments, stream waters, pore waters, and regolith extractions at SSHO reveal how mineralogy and slope aspect impact the release and fractionation of REE during shale weathering and regolith formation processes.
Significant depletions in concentrations of REEs are observed in the weathering profiles in this catchment. Depletion is attributed to chemical leaching in soils with acidic and organic-rich porefluids. In addition, newly formed fine particles
Acknowledgments
We thank Z. Ruge and B. Ketchum for help with regolith sampling and W. Castro and E. Herndon for help with sample analysis and N. West and E. Kirby for discussion. Logistical and financial support and/or data were provided by the NSF-supported Susquehanna/Shale Hills Critical Zone Observatory (Grant EAR-0725019 to C. Duffy, Penn State). Financial support for L.M. was partially provided by National Science Foundation Grant CHE-0431328 to SLB for Center for Environmental Kinetics Analysis. SLB
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