Controls on granitic weathering fronts in contrasting climates
Introduction
The chemical weathering of silicate minerals is generally accepted to be one of the primary controls on atmospheric CO2 over geologic timescales and thus the long-term climate (>106 yr) of the Earth (Berner et al., 1983). Although numerous factors operate on a range of scales to determine long-term silicate weathering rates, it is believed that the primary controls on these rates are climate and lithology (e.g., White and Blum, 1995; Riebe et al., 2004; Maher, 2010). The relative influence of these two factors remains disputed, as well as the relative influence of different aspects of climate (i.e., temperature and precipitation), but typically, mafic lithologies weather more rapidly than felsic ones, and warmer, wetter climates promote more rapid weathering, with broad latitudinal trends identified in weathering profile morphology (Strakhov, 1967). Granitic lithologies, in particular, display large variations in weathering intensity compared to their mafic counterparts (Bazilevskaya et al., 2013). Although granitic weathering rates and mechanisms have been well studied (e.g., Melfi et al., 1983; White and Blum, 1995; Oliva et al., 2003; Riebe et al., 2004; Buss et al., 2005; Fletcher et al., 2006; Pierson-Wickmann et al., 2009; Frey et al., 2010), the controls that produce the variations in weathering front morphology, depth, and rates are not well understood. Greater constraint on these controls would enhance our ability to predict weathering responses through geologic time, as well as responses to future land use and climate change.
Most chemical weathering occurs in the Critical Zone, commonly defined as the region of Earth spanning the upper extent of vegetation to the lower extent of bedrock weathering. Significant differences exist in the intensity and depth of granitic weathering and the thickness of the weathering fronts in the critical zone. Some critical zone profiles exhibit only a few meters of weathering, whereas others show weathering to tens of meters deep (Bazilevskaya et al., 2013). The intensity of granitic weathering also varies significantly; some profiles weather from fresh bedrock to completely depleted regolith over weathering fronts < 1 m thick, whereas other profiles show gradual alteration over tens of meters (e.g., Schaffhauser et al., 2014; Buss et al., 2017). It is unclear to what extent these variations are a product of the weathering environment or subtle differences in the mineralogy or intrinsic rock structure such as porosity and permeability. Regardless, these variations in weathering are likely to have implications for water flow rates and residence times within granitic profiles, which in turn could affect the coupling (or decoupling) of surface processes to those at depth and the flux of weathering products from the watersheds.
The depth of the bedrock-weathering boundary may also impact the initial weathering reactions. Weathering and biological reactions above weathering fronts can alter saturation indices and consume or produce reactants, such as CO2 or O2 in reactive fluids (Buss et al., 2005; Brantley et al., 2014). The manner and rate at which secondary porosity develops also affects weathering mechanisms and fluid transport and can lead to distinct weathering morphologies (Navarre-Sitchler et al., 2011; Goodfellow et al., 2016).
Initiation of weathering in granitic rocks generally begins with either dissolution of highly reactive accessory minerals such as calcite (White et al., 2005) or oxidation of Fe(II) (Buss et al., 2008; Goodfellow et al., 2016). Similar reaction-driven fracturing mechanisms involving expansion during hydration of biotite (e.g., Isherwood and Street, 1976) or during hydrothermal alteration of mafic rocks (Røyne et al., 2008; Jamtveit et al., 2009) have been identified, but these mechanisms require connected pore space or exposed surfaces. Fracturing via oxidation of Fe(II), however, typically occurs within rocks where water cannot penetrate (Buss et al., 2008; Behrens et al., 2015) However, the majority of porosity formed during granitic weathering stems from the dissolution of aluminosilicate minerals, dominantly plagioclase (e.g., Buss et al., 2008). Typically, Ca and Na rich feldspars dissolve more rapidly than K rich feldspars (e.g., Bandstra et al., 2008). Relatively small variations in the composition and abundance of feldspars amongst granitic rocks may affect weathering rates within granitic lithologies (e.g., White et al., 2001). Furthermore, as the most abundant Ca-containing silicate mineral within most granitic rocks, the weathering of plagioclase is the dominant granitic weathering reaction contributing to CO2 drawdown (e.g., Brantley et al., 2014).
As such, the chemical weathering of plagioclase minerals within the critical zone represents a major control on long-term climate. The initiation of weathering can begin 10s of meters below the rock-regolith interface (e.g., Riebe et al., 2017), but the practical difficulties in accessing and sampling weathering bedrock at depth means there are few available datasets appropriate for weathering studies that extend below the augerable, or outcropping, regolith (here defined as all disaggregated material overlying intact bedrock). The relatively limited number of such investigations means that our understanding of the fundamental processes that govern weathering profile development remain poorly understood (e.g., Bazilevskaya et al., 2013). In this study, we present new elemental and mineralogical data from a core drilled 30.3 m into a temperate granitic weathering profile, representing one of the first deep critical zone granitic weathering studies from such settings (e.g., White et al., 2001; Schaffhauser et al., 2014). Bulk rock and mineral-specific chemical weathering rates were calculated, and the intensity, depth, and morphology of the weathering fronts were analysed. We compare these findings to granitic weathering profiles from the literature, spanning a range of precipitation and temperature regimes. These comparisons were used to assess the relative impact of weathering factors such as climate, lithology, and subsurface architecture (i.e., variations in primary and secondary porosity and permeability with depth) on the resulting weathering front morphologies and chemical weathering rates.
Section snippets
Primary field site
The small granitic catchment (0.273 km2) of Lysina is part of the Slavkov Forest Critical Zone Observatory in the NW Czech Republic (Krám et al., 2012; Fig. 1; Krám et al., 2017). The region is located within the so-called ‘Black Triangle’, an area of Central Europe heavily affected by pollution from coal-fired power plants (Krám et al., 1999; Kopacek et al., 2016). The region has been formally monitored by the Czech Geological Survey since 1989, with a focus on the recovery from anthropogenic
Lysina mineralogy
Phase analysis of the Lysina core samples indicate the primary minerals are quartz, K-feldspar, and albite (defined here as An0–10 Ab90–110), with some variation in their relative abundances through the core (Table 3). Albite abundance decreases significantly from 2.77 to 1.85 m depth, across the rock-regolith interface (Table 3). No Ca-rich plagioclase phases were identified, in contrast to previous studies on fresher samples which found small quantities (<1% normalised volume) of Ca
Discussion
The depth and thickness of weathering fronts affect numerous critical zone characteristics such as water flow paths (e.g., Schaffhauser et al., 2014; Orlando et al., 2016), subsurface microbial communities (e.g., Buss et al., 2005), stream solute sources (e.g., Calmels et al., 2011) and isotope fractionation mechanisms (e.g., Schaffhauser et al., 2014; Chapela Lara et al., 2017). Granitic weathering front depth and thickness, as well as weathering intensity and rates, vary significantly around
Conclusions
We presented new geochemical analyses of a granitic weathering profile in a temperate forest, Lysina, from which we calculated mineral specific weathering rates and identified key weathering processes. We then compared the weathering intensities (CIA), mineral-specific weathering rates and weathering front morphology with additional granitic profiles from the literature from different climatic regimes to identify dominant weathering controls.
Lysina exhibits thin regolith overlying a weathered
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by European Research Council under the European Union Seventh Framework Programme (FP/2007–2013)/ERC Project “The Greenhouse Earth System” (T-GRES), Grant Agreement no. 340923. Drilling in the Slavkov Forest CZO was funded by the European Commission FP7 Collaborative Project “Soil Transformations in European Catchments” (SoilTrEC), Grant Agreement no. 244118. P. Krám was supported by the internal project 310010 of the Czech Geological Survey. We thank S. Kearns and B.
References (93)
Soil functions in Earth’s critical zone: key results and conclusions
Adv. Agron.
(2017)- et al.
Mineralogical transformations set slow weathering rates in low-porosity metamorphic bedrock on mountain slopes in a tropical climate
Chem. Geol.
(2015) - et al.
Relating weathering fronts for acid neutralization and oxidation to pCO2 and pO2
- et al.
Toward a conceptual model relating chemical reaction fronts to water flow paths in hills
Geomorphology
(2017) - et al.
Elemental weathering fluxes and saprolite production rate in a Central African lateritic terrain (Nsimi, South Cameroon)
GCA
(2012) - et al.
Constitutive mass balance relations between chemical composition, volume, density, porosity, and strain in metasomatic hydrochemical systems: results on weathering and pedogenisis
GCA
(1987) - et al.
Effects of a tectonically-triggered wave of incision on riverine exports and soil mineralogy in the Luquillo Mountains of Puerto Rico
Appl. Geochem.
(2015) - et al.
Denudation rates determined from the accumulation of in situ-produced 10Be in the Luquillo Experimental Forest, Puerto Rico
Earth Planet. Sci. Lett.
(1995) - et al.
Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: coupling oxidation, dissolution, and fracturing
GCA
(2008) - et al.
Phosphorus and iron cycling in deep saprolite, Luquillo Mountains, Puerto Rico
Chem. Geol.
(2010)
Lithological influences on contemporary and long-term regolith weathering at the Luquillo Critical Zone Observatory
GCA
Contribution of deep groundwater to the weathering budget in a rapidly eroding mountain belt, Taiwan
Earth Planet. Sci. Lett.
Regolith formation rate from U-series nuclides: implications from the study of a spheroidal weathering profile in the Rio Icacos watershed (Puerto Rico)
GCA
From black box to a grey box: a mass balance interpretation of pedogenesis
Geomorphology
The influence of critical zone processes on the Mg isotope budget in a tropical, highly weathered andesitic catchment
Geochim. Cosmochim. Acta
Catchment-wide weathering and erosion rates of mafic, ultramafic, and granitic rock from cosmogenic meteoric 10Be/9Be ratios
GCA
Soils as pacemakers and limiters of global silicate weathering
Compt. Rendus Geosci.
Responses of chemical erosion rates to transient perturbations in physical erosion rates, and implications for relationships between chemical and physical erosion rates in regolith-mantled hillslopes
Earth Planet. Sci. Lett.
A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation
Earth Planet. Sci. Lett.
Introduction to the critical zone
The applicability of the Chemical Index of Alteration as a paleoclimatic indicator: an example from the Permian of the Paraná Basin, Brazil
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Slow advance of the weathering front during deep, supply-limited saprolite formation in the tropical Highlands of Sri Lanka
GCA
Experimental evidence for the mobility of Zr and other trace elements in soils
GCA
Recovery from acidification in central Europe - observed and predicted changes of soil and streamwater chemistry in the Lysina Catchment, Czech Republic
Environ. Pollut.
The effect of fractures on weathering of igneous and volcaniclastic sedimentary rocks in the Puerto Rican tropical rain forest
Procedia Earth Planet. Sci.
Differential weathering of basaltic and granitic catchments from concentration–discharge relationships
GCA
Effect of industrial dust on precipitation chemistry in the Czech Republic (Central Europe) from 1850 to 2013
Water Res.
Application of the forest-soil-water model (PnET-BGC/CHESS) to the Lysina catchment, Czech Republic
Ecol. Model.
Streamwater chemistry in three contrasting monolithologic Czech catchments
Appl. Geochem.
Bedrock weathering and stream water chemistry in felsic and ultramafic forest catchments
Procedia Earth Planet. Sci.
Hydrochemical fluxes and bedrock chemistry in three contrasting catchments underlain by felsic, mafic and ultramafic rocks
Procedia Earth Planet. Sci.
The frequency and distribution of recent landslides in three montane tropical regions of Puerto Rico
Geomorphology
The dependence of chemical weathering rates on fluid residence time
Earth Planet. Sci. Lett.
Relationships between the transit time of water and the fluxes of weathered elements through the critical zone
Procedia Earth Planet. Sci.
Incipient chemical weathering at bedrock fracture interfaces in a tropical critical zone, Puerto Rico
GCA
A reactive-transport model for weathering rind formation on basalt
GCA
Chemical weathering in granitic environments
Chem. Geol.
Regolith residence time and the concept of surface age of the Piedmont peneplain
Geomorphology
The convenient fiction of steady-state soil thickness
Geoderma
High chemical weathering rates in first-order granitic catchments induced by agricultural stress
Chem. Geol.
Hidden erosion on volcanic islands
Earth Planet. Sci. Lett.
Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes
Earth Planet. Sci. Lett.
Controls on rock weathering rates by reaction-induced hierarchical fracturing
Earth Planet. Sci. Lett.
Geochemical and isotopic (U, Sr) tracing of water pathways in the granitic Ringelbach catchment (Vosges Mountains, France)
Chem. Geol.
Large-scale erosion rates from in situ-produced cosmogenic nuclides in European river sediments
Earth Planet. Sci. Lett.
Regolith production and chemical weathering of granitic rocks in central Chile
Chem. Geol.
Cited by (34)
Pyroclastic component influence on the weathering indices assessment in marine sediments – Lessons from Upper Ordovician of the Baltic Basin
2024, Palaeogeography, Palaeoclimatology, PalaeoecologyShear-wave velocity imaging of weathered granite in La Campana (Chile) from Bayesian inversion of micro-tremor H/V spectral ratios
2023, Journal of Applied GeophysicsIs there still something to eat for trees in the soils of the Strengbach catchment?
2023, Forest Ecology and ManagementCitation Excerpt :The initial water content of each soil layer was set at its maximum (θsat), corresponding to solids fully immersed in aqueous solution. Last, the specific surface area of minerals was fixed to 10 m2/g for secondary phases and to 0.1 m2/g for the primary phases (Hayes et al., 2020). At the end of the simulation, the WITCH code allows to estimate the evolution of the concentration of each cation in the solution of the considered box, the fluxes resulting from the exchange between the solution and the clay-humic complex, the solution saturation index with respect to each mineral and the fluxes resulting from mineral dissolution/precipitation.