Applied Geochemistry

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1 Applied Geochemistry 26 (2011) S89 S93 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: Geochemical behaviors of different element groups during shale weathering at the Susquehanna/Shale Hills Critical Zone Observatory Lin Ma a,b,, Lixin Jin a,b, Susan L. Brantley a a Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA b Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA article info abstract Article history: Available online 22 March 2011 Major and trace element concentrations were measured in bedrock, regolith and stream sediments from a first-order catchment developed entirely on grey shale in central Pennsylvania, USA. These elements can be classified into five major groups based on statistical data analysis. The presence of different elemental groups is due to the mineralogical origin, cycling processes, and geochemical properties of these elements during soil formation. A better understanding of the behaviors of these elements during chemical weathering would allow for their possible use as natural tracers in Critical Zone processes. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Chemical weathering of bedrock at the Earth s surface contributes to important Critical Zone processes such as nutrient cycling, C sequestration, erosion, and sediment transport (e.g., Drever, 2004; Brantley et al., 2007). The weathering products, soils, are vital for the sustainability of ecosystems and human society. It is thus of great importance to understand the factors that control chemical weathering and soil formation. The weathering reactions that occur during bedrock soil transformation represent one of the most difficult systems to study with respect to chemical, physical, hydrologic and biological coupling over time. Despite the complexity of these interactions, certain elemental groups are observed to have similar patterns during soil formation due to the same mineralogical origins, common cycling processes, and/or similar geochemical properties. Such patterns have been classified as immobile, depletion, addition, depletion addition and biogenic profiles (Brantley et al., 2007; Brantley and White, 2009). To understand the geochemical behaviors of different element groups during shale weathering, major and trace element concentrations were determined in bedrock, regolith and stream sediments from a first-order catchment developed entirely on grey shale in central Pennsylvania, USA (Fig. 1). Statistical data analysis indicates that these elements can be classified into five major groups depending on their geochemical behaviors during soil formation. As such, different elemental depth profiles are observed, indicating different cycling processes of these elements during Corresponding author at: Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA. Tel.: ; fax: address: lma@utep.edu (L. Ma). shale weathering. This study provides a better understanding of the behaviors of elements during chemical weathering. 2. Geological setting The study area is the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) in central Pennsylvania, USA (Fig. 1). The climate at SSHO is temperate and humid with mean annual temperature of 10 C and mean annual precipitation of 107 cm. A first-order stream flows within the catchment from east to west (Fig. 1). The V-shaped catchment is characterized by an average local relief of 30 m. Soil thickness averages 1.4 m over the catchment, ranging from 0.3 m at the ridge top to greater than 3 m in the valley floor and topographic depressions (Fig. 1). The bedrock is a Silurian-age shale with a few interbedded limestones (Rose Hill Formation). The shale is composed predominantly of illite (58 wt.%), quartz (30 wt.%), vermiculited chlorite (referred here as chlorite, 11 wt.%), and trace amounts of feldspar (plagioclase and K-feldspar), anatase (TiO 2 ), Fe-oxides (magnetite and hematite) and zircon (Jin et al., 2010). The carbonate mineral, ankerite, was observed at the northern ridge at tens of meters below surface. Chemical weathering reactions at SSHO are dominated by clay transformations wherein illite and chlorite weather to vermiculite, hydroxyl-interlayered vermiculite, and kaolinite in soil zones, and dissolution of ankerite and plagioclase feldspar occurs at depth (Jin et al., 2010). U-series isotope activity ratios in regolith profiles provide the duration of chemical weathering and regolith formation rates at SSHO (Ma et al., 2010). The focus here is on the major and trace element compositions of three weathering profiles along a southern planar transect (SPRT, SPMS and SPVF) from the catchment (Fig. 1). Stream /$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi: /j.apgeochem

2 S90 L. Ma et al. / Applied Geochemistry 26 (2011) S89 S93 Fig. 1. Study area and sample locations for the Shale Hills catchment in central Pennsylvania. sediments (SS) trapped behind a sediment fence built in 1970s at the center of the valley floor (Fig. 1) were also collected. In addition, a 25 m deep drill core (DC1) on the northern ridge of the catchment provided bedrock samples which have been identified as relatively unaltered parent material (Jin et al., 2010). Major and trace element concentrations were measured by Minerals Services Laboratory (SGS Canada Inc., Toronto, Ontario) by inductively coupled plasma optical emission spectrometry (method ICP95A) and inductively coupled plasma mass spectrometry (method IMS95A), respectively, after LiBO 2 fusion, with precision estimated to be better than ±5%. 3. Results Major and trace element concentrations of the bedrock, regolith, and stream sediment samples are listed in Table A1. Major elements of the bedrock and regolith samples have been previously discussed by Jin et al. (2010) Matrix of correlation coefficients A statistical analysis of the correlation coefficients (r 2 ) for any given element pairs of these samples is shown in Fig. 2, where significant positive correlations (r 2 > 0.5 and p-values < 0.05) are shown in red and significant negative correlations are shown in blue. The p-value is the probability of getting a correlation as large as the observed value by random chance when the true correlation is zero; if p is less than 0.05, then the correlation is significant. These elements can be classified into five groups based on the correlation matrix (Fig. 2): an element shows strong positive correlation with the other elements in the same group. Group 1 includes Si, Zr, Hf, Ti, Nb and Ta; group 2: Al, Fe, K, Mg, Ga, Rb, V, Cs, Th and Ni; group 3: Ba, Sr, Na, Cr and Zn; group 4: rare earth elements, Y, Co and U; group 5: Mn, P, and loss on ignition (LOI). In addition, elements in group 1 show strong negative correlations with elements in most of the other groups Element profiles To evaluate the loss or gain of elements in a weathering profile (especially to correct for the effects of element additions/depletions), the concentration (C) of an immobile (i.e. conservative) element i is commonly compared with the relative loss or gain of a more mobile element (j) by calculating the mass transfer coefficient s i,j (e.g., Brimhall and Dietrich, 1987): s i;j ¼ C j;w C j;p C i;p C i;w 1 Positive s i,j values indicate the extent of enrichment of element j and negative values define the fractional depletion. A value of zero means that element j is as immobile in the weathered regolith (w) as the assumed immobile element i (with respect to parent material p). To calculate s values for the SSHO samples, the average concentrations of core material (DC1) were used as the parent material (Jin et al., 2010). Zirconium was found to be immobile at SSHO,

3 L. Ma et al. / Applied Geochemistry 26 (2011) S89 S93 S91 Fig. 2. Matrix of correlation coefficients for measured elements. Gray boxes represent five major groups of elements that can be classified (see text for details). Fig. 3. values for elements in group 2: (a) ridge top (SPRT); (b) middle slope (SPMS); (c) valley floor (SPVF); and (d) stream sediments (SS).

4 S92 L. Ma et al. / Applied Geochemistry 26 (2011) S89 S93 Fig. 4. values for elements in group 5: (a) ridge top (SPRT); (b) middle slope (SPMS); (c) valley floor (SPVF); and (d) stream sediments (SS). (only s values for group 2 are shown; Fig. 3a c). These data are characteristic depletion profiles (Brantley and White, 2009). By contrast, the elements in group 5 show positive s values that increase towards the surface (Fig. 4a and b), consistent with addition profiles or biogenic profiles (Brantley and White, 2009). For stream sediments, elements from group 5 show addition profiles (Fig. 4d). However, elements from other groups show more complicated behaviors: e.g., group 2 elements in stream sediments all show depletion profiles at depth but addition profiles for shallow depths (Fig. 3d). 4. Discussion 4.1. Regolith profiles Fig. 5. Loss on ignition (%) vs. depth for ridge top (SPRT), middle slope (SPMS), valley floor (SPVF) and stream sediments (SS). presumably because it was observed to occur in the relatively insoluble and stable mineral zircon (Jin et al., 2010). For regolith profiles, the elements in group 1 show s values close to 0 throughout the profiles, termed as immobile profiles (Brantley and White, 2009). Most elements in groups 2 4 show negative s values that decrease continuously towards the surface Elements from group 1 show significant negative correlation with the elements that show depletion profiles (e.g., group 2), suggesting a conservative behavior of group 1 elements during chemical weathering. This is consistent with the fact that these elements Zr, Hf, Ti, Ta and Nb are commonly immobile in soils (e.g., Brimhall and Dietrich, 1987; Chadwick et al., 1990). In addition, quartz, the main carrier for Si, is generally very stable during weathering. Group 2 includes Fe, Al, K and Mg, elements that are commonly associated with primary clay minerals such as illite and chlorite in the bedrock (Jin et al., 2010). Hence, the observed depletion profiles of K and Mg in the regolith samples are related to clay mineral dissolution during shale weathering at SSHO (Jin et al., 2010).

5 L. Ma et al. / Applied Geochemistry 26 (2011) S89 S93 S93 Aluminium and Fe also show depletion profiles at SSHO, indicating that chemical weathering processes involve loss of particles at SSHO (Jin et al., 2010). Gallium, V and Rb (Cs) are also included in this group because their well-known geochemical behaviors are similar to Al, Fe and K, respectively (Kabata-Pendias, 2001). In group 3, Sr, Na and Ba also show depletion profiles. In addition to clay mineral dissolution, feldspar dissolution also occurs as documented from previous major and rare earth element studies at SSHO (Jin et al., 2010; Ma et al., in preparation). It is most likely that the association of group 3 elements reflects the occurrence of feldspar dissolution. Because of their similar geochemical behaviors, rare earth elements in group 4 commonly vary as one unique group during chemical weathering, related to dissolution of clay minerals, P minerals, and feldspars (Ma et al., in preparation). The pairing of U to group 4 elements probably reflects its distribution in the clay minerals. Because P is a bio-essential element but a limiting nutrient, it is often pumped upward and recycled by vegetation after being released by chemical weathering from bedrock and thus shows a biogenic profile in group 5 (Fig. 4a c). Similarly, LOI values, which approximate organic matter contents, of these three regolith profiles increase towards the surface, suggesting significant addition of leaves and roots at the surface in these profiles (Fig. 5). Manganese in group 5 shows an addition profile, as a result of anthropogenic atmospheric Mn loading to surface soils and also biological recycling (Herndon et al., 2011). This observation reveals that human activity has significantly perturbed Earth s natural biogeochemical cycles Stream sediments Stream sediments at SSHO show similar addition profiles to planar transect soils for group 5 elements (Figs. 4d and 5), suggesting that the previously discussed biochemical processes also lead to the addition of such elements to the stream sediments. By contrast, group 2 elements in stream sediments all show addition profiles at shallow depths, complementary to the regolith depletion profiles (Fig. 3). This observation confirms that part of the elements that are released and mobilized from shale weathering and particle transport are re-deposited in the valley with stream sediments (Jin et al., 2010; Ma et al., in preparation). 5. Conclusions A statistical analysis on the measured major and trace element concentrations of bedrock, regolith, and stream sediment samples at SSHO reveals that elements can be classified into five major groups with different geochemical behaviors during soil formation. The presence of different elemental groups is related to the mineralogical origin, cycling processes, and geochemical properties of these elements during chemical weathering. This study provides a better understanding of the behaviors of elements during chemical weathering that would allow for possible uses as natural tracers for low-temperature geochemical processes. Acknowledgements We thank W. Castro and E. Herndon for help with sample analysis and discussion. Logistical support and/or data were provided by the NSF-supported Susquehanna/Shale Hills Critical Zone Observatory. Financial support was provided by NSF Grant CHE to SLB for Center for Environmental Kinetics Analysis and NSF Grant EAR to C. Duffy (Penn State) for Susquehanna Shale Hills Critical Zone Observatory, and by Department of Energy Grant DE-FG02-05ER15675 to SLB. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi: /j.apgeochem References Brantley, S.L., White, A.F., Approaches to modeling weathered regolith. Rev. Mineral. Geochem. 70, Brantley, S.L., Godhaber, M.B., Ragnarsdottir, K.V., Crossing disciplines and scales to understand the Critical Zone. Elements 3, Brimhall, G.H., Dietrich, W.E., Constitutive mass balance relations between chemical composition, volume, density, porosity, and strain in metosomatic hydrochemical systems: results on weathering and pedogenesis. Geochim. Cosmochim. Acta 51, Chadwick, O.A., Brimhall, G.H., Hendricks, D.M., From a black to a grey box a mass balance interpretation of pedogenesis. Geomorphology 3, Drever, J.I. (Ed.), Surface and Ground Water, Weathering, and Soils. Holland, H.D., Turekian, K.K. (Exec. Eds.), Treatise on Geochemistry, vol. 5. Elsevier. Herndon, E., Jin, L., Brantley, S.L., Soils reveal widespread manganese enrichment from industrial sources. Environ. Sci. Technol. 45, Jin, L., Ravella, R., Ketchum, B., Bieman, P.R., Heaney, P., White, T., Brantley, S.L., Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory. Geochim. Cosmochim. Acta 74, Kabata-Pendias, A., Trace Elements in Soils and Plants, third ed. CRC Press, NY. Ma, L., Chabaux, F., Pelt, E., Blaes, E., Jin, L., Brantley, S., Regolith production rates calculated with uranium-series isotopes at Susquehanna/Shale Hills Critical Zone Observatory. Earth Planet. Sci. Lett. 297, Ma, L., Jin, L., Brantley, S.L., in preparation. Controls of mineralogy and slope aspect on REE release and fractionation during shale weathering in the Susquehanna/ Shale Hills Critical Zone Observatory. Chem. Geol.