How mineralogy and slope aspect affect REE release and fractionation during shale weathering in the Susquehanna/Shale Hills Critical Zone Observatory

Abstract

To understand the factors that control rare earth elements (REE) release and fractionation during shale weathering, we investigate the REE contents in solid (bedrock, regolith, stream sediments), and natural waters (stream, and pore waters) from a first-order catchment developed entirely on gray shales in central Pennsylvania, USA. Up to 65% of the REE (relative to parent bedrock) is depleted from the weathering profiles in the acidic and organic-rich soils due to chemical leaching. In addition, newly formed fine particles were also lost along with the down-slope movement of soil waters. Weathering profiles on the south-facing slope show less depletion of REE than those on the north-facing slope (33% vs. 45% on average). We hypothesize that the different degrees of REE depletion on the two transects reflect a history of different chemical weathering rates and possible different surface erosion rates, controlled by contrasting slope aspect-induced microclimate conditions. In addition, weathering profiles, natural waters and sequential extractions all show a preferential removal of Middle REE (up to 22% more) relative to Light REE and Heavy REE during shale weathering, due to preferential release of MREE from rhabdophane. Furthermore, the long-term phosphate mineral dissolution rates (e.g., rhabdophane) were estimated at 10^− 15 to 10^− 14 mol m^− 2 s^− 1 under field conditions, based on REE depletion profiles.

Strong positive Ce anomalies (average [Ce/Ce*]_N value: 1.79) observed in the regolith, stream sediments, and regolith extractions point to the fractionation and preferential precipitation of Ce as compared to other REE, due to the generally oxidizing conditions during release, transport, and redistribution of REE in the surface and subsurface environments. Positive Eu anomalies (average [Eu/Eu*]_N value: 1.30) observed in the natural waters of the catchment are attributed to weathering of plagioclases in the shale bedrock. This study highlights the use of REE as natural tracers for low-temperature geochemical processes.

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.