A reactive-transport model for weathering rind formation on basalt
Introduction
As bedrock weathers in place without disturbance, regolith can form that retains evidence of bedrock structure. This regolith, generally known as saprolite, is more erodible than bedrock. Understanding the rates at which saprolite forms on silicate rocks is necessary to develop basic understanding of processes such as nutrient cycling (Hyman et al., 1998, Chadwick et al., 1999, Derry et al., 2005, Starr and Lindroos, 2006), landform evolution (Dethier, 1986, Pavich, 1986, Pope et al., 1995), and regulation of CO2 concentration in Earth’s atmosphere over geologic timescales (Walker et al., 1981, Berner et al., 1983).
Basalt is of particular interest with respect to saprolite formation because it is easily weathered and accounts for approximately 30–35% of the atmospheric CO2 drawdown attributed to silicate weathering (Dessert et al., 2003). A number of researchers have put forth considerable effort to measure basalt weathering rates across watersheds of varying extent (e.g. Gislason et al., 1996, Louvat and Allegre, 1997, Louvat and Allegre, 1998, Dessert et al., 2001, Gaillardet et al., 2003, Das et al., 2005) as well as to quantify weathering rates in laboratory dissolution experiments for basalt glass (e.g. Gislason and Eugster, 1987, Eick et al., 1996, Gislason and Oelkers, 2003, Wolff-Boenisch et al., 2004) and crystalline basalt (Gudbrandson et al., 2008, Hausrath et al., 2009).
To compare rates measured in different systems, weathering rates are typically normalized to mineral surface area. For example, dissolution rates measured in the laboratory are generally normalized to the BET surface area of dissolving minerals. This is determined by adsorbing an inert gas to mineral powder and then assessing the surface area through the use of the Brunauer–Edwards–Teller, i.e. BET, adsorption isotherm (Brunauer et al., 1938). Laboratory dissolution rates normalized by such BET surface areas are often 2–5 orders of magnitude faster than field-derived weathering rates that have been normalized to estimates of BET surface area for the field samples (White and Brantley, 2003). This so-called field-laboratory discrepancy has been observed for many lithologies or mineralogies (Velbel, 1993, Anbeek et al., 1994, Blum et al., 1994, Gislason et al., 1996, Drever and Stillings, 1997, White and Brantley, 2003, Beig and Luttge, 2006, Maher et al., 2006). Part of the problem is that BET surface area is difficult to estimate for field systems. Given this difficulty, most field scale weathering rates are generally reported after normalization by geographic instead of mineral surface area. Navarre-Sitchler and Brantley (2007) argued that much of the discrepancy observed when comparing field- and laboratory-derived weathering rates is related to scaling of surface area, but that weathering rates for basalt can also vary by up to two orders of magnitude even after the surface area scaling problem is corrected (Navarre-Sitchler and Brantley, 2007).
This remaining discrepancy has been attributed to biological effects such as the presence or absence of organic acids (Drever and Stillings, 1997), the exposure age or residence time of weathered material (White and Brantley, 2003, Maher et al., 2004), differences in hydrologic regime (Velbel, 1993, Pacheco and Alencoao, 2006), armoring of natural surfaces (Nugent, 1998), or affinity effects that can sometimes be related to secondary precipitates (White and Brantley, 2003, Zhu, 2005, Maher et al., 2006, Maher et al., 2009, Steefel and Maher, 2009). Additionally, large-scale field systems are often impacted by physical erosion that removes weathered material and may influence the interpretation of chemical weathering rates (Bluth and Kump, 1994, Millot et al., 2002). In order to understand weathering of natural systems, it is necessary to examine end members where primary processes that control chemical and physical weathering can be considered separately.
One example of weathering that is relatively uncomplicated by erosion and perhaps biological effects is an alteration rind that has formed on a weathered clast at depth within a soil (Cernohouz and Solc, 1966, Chinn, 1981, Colman and Pierce, 1981, Gellatly, 1984, Oguchi and Matsukura, 1999). Weathering rinds, defined as permeable crusts enriched in immobile oxides that envelop cores of unweathered rock material (Colman, 1982), are often used as indicators of relative ages of sediment deposits in glacial (Porter, 1975, Colman and Pierce, 1981) and alluvial (Ricker et al., 1993, Fisher et al., 1998, Sak et al., 2004) systems. Rinds have even been analyzed on Martian rocks (Hausrath et al., 2008). In effect, weathering rinds can be conceptualized as small-scale “saprolites” that allow detailed observations of the chemical and physical transformations that occur during weathering (Sak et al., 2004, Pelt et al., 2008, Sak et al., 2010).
Here we investigate rinds developed on weathered clasts in terraces of different ages but within the same climate and weathering environment. According to IUGS classification using total alkalis (Na2O + K2O weight%) and silica (SiO2 weight%) reported in Sak et al. (2004) the clasts range in composition between basalt and basaltic andesite (Raymond, 1995). Nonetheless, all of the clasts were referred to as basalts in previous studies (Sak et al., 2004, Hausrath et al., 2008, Navarre-Sitchler et al., 2009) and we will follow the same convention here. The weathered basalt clasts from the Costa Rica chronosequence provide a time series of observations of the geometry of the reaction front and its position over time, i.e. the weathering advance rate. A reaction front is defined as the transition zone across which a mineral reaction occurs. The basalt clasts were deposited in three fill-type alluvial terraces (Qt1, Qt2, and Qt3) along the central Pacific coast of Costa Rica (Fisher et al., 1998, Sak et al., 2004, Fisher and Ryan, 2006) and they weathered in an environment where they were surrounded by terrace material (Sak et al., 2004). Therefore, chemical weathering processes can be studied in these clasts without the additional complications associated with the removal of weathered material by physical erosion.
The terraces are preserved in the lower reaches of the Rios Parrita and Barranca on the Pacific side of Costa Rica (Fig. 1). The basalt clasts are characterized by a central core of relatively unweathered material surrounded by a rind dominated by secondary minerals with a few residual grains of ilmenite and apatite. Rinds increase in thickness as weathering advances into the clasts with terrace age. Rinds on basalt clasts were measured by Sak et al. (2004) to be 12 ± 1, 29 ± 1, and 69 ± 6 mm thick for Qt3, Qt2, and Qt1, respectively. The mean annual temperature and precipitation are 27.3 °C and 3085 mm/y, respectively, for the time period from 1941–1982 (Instituto Meterologico Nacional de Costa Rica, 1992, see Sak et al., 2004). While temperature is relatively constant seasonally, precipitation varies from a low of 28.5 mm in March to a high of 570 mm in October (Sak et al., 2004). Additional detailed descriptions of the terraces are available (Sak, 1999, Marshall, 2000, Sak et al., 2004).
In this study, we collected detailed chemical data across the core/rind interface of five clasts to delineate the weathering advance rate through quantification of the rind thickness and reaction front thickness through time. We present full results of numerical simulation efforts using the multicomponent software CrunchFlow (Giambalvo et al., 2002, Maher et al., 2009) which we used to develop a fully-coupled reactive transport model of weathering rind formation. Model predictions were compared to observations from the basalt clasts. The numerical model successfully predicts weathering advance rates and reaction front thicknesses. Such a model is a first step in providing insight into how to numerically model weathering advance or saprolite formation at larger scales.
Section snippets
Weathering age
An estimate of the time period of weathering, termed here the weathering age or duration, is needed in order to quantify rates of weathering in basalt clasts. The basalt clasts considered here come from a series of three poorly consolidated alluvial fill terraces near the Pacific coast of Costa Rica. For these terraces, the weathering age is considered to be equivalent to the time since deposition. The terraces are distinguished on the basis of pedogenic maturity of the terrace deposit and the
Methods
Weathered basalt clasts were collected from the same series of terraces as Sak et al. (2004) along the Pacific Coast of Costa Rica for detailed characterization of the mineralogical and physical changes that occur with weathering (Fig. 3).The sampling location for terrace Qt1 (sample QE1-1) was located near the town of Barranca (Fig. 1). Samples from Qt2 (samples CR03-02.1 and CR03-03.1) and Qt3 (samples DP7-2 and DP7-3) were collected near Esterillos (Fig. 1), approximately 60 km southeast of
Weathering rind thickness in Qt3
The measurements of total weathering rind thickness – including both rind that remained on the clast during sampling and that which remained in the soil after removal – in terrace Qt3 range from 5 to 32 mm with an average thickness of 13 ± 1 mm (Table 3). The reported uncertainty is equal to the standard error of the mean of 67 measurements with a standard deviation of 5 mm. In comparison, an average rind thickness of 12 ± 1 mm was measured and reported by Sak et al. (2004) for terrace Qt3, within the
Features of the weathering rinds
The clasts weather when water infiltrating the matrix material in the terrace brings reactive constituents (primarily hydrogen ions) into contact with the parent clasts. For two minerals with different dissolution rate constants – such as pyroxene and plagioclase – to dissolve at the same rate and location (that is, for them to have coincident reaction fronts), transport must play at least some part in controlling their reaction kinetics. Since these clasts have very low porosity (1–3%) and the
Scenario 1 – uncoupled model with no update of porosity
We first built a basic model for predicting weathering rind formation based strictly on chemical and physical observations and literature values for the reactive transport parameters (i.e., rate constants, specific surface areas, diffusivities). For this model, like all the models discussed herein, diffusion is assumed to be the only mode of transport. For this first scenario, it was assumed that as parent minerals weathered to secondary products, the change in porosity could be ignored (i.e.,
Reaction front thickness
In a single component diffusion-dominated porous water + mineral system, the reaction front thickness (h) is proportional to the square root of the product of porosity () and diffusivity (D) divided by the product of reaction rate constant (k) and mineral-water interfacial area (A) (Lichtner, 1988):
A similar equation, derived by Lichtner (1988) for advective-dominated systems, was shown by Brantley et al. (2008) to be useful to interpret the thickness of multicomponent multiphase
Implications for weathering studies across scales
It is well established that the position and geometry of a reaction front can provide information about long-term mineral dissolution kinetics (e.g. Murphy et al., 1998, White, 2002, Maher et al., 2004, Hausrath et al., 2008). Our modeling efforts show that in the Costa Rica basalt system both the linear advance rate and reaction front thickness are in part surface-reaction controlled. Since the transport rates in this system are well constrained as discussed in Navarre-Sitchler et al. (2009),
Conclusions
Four observations were used to constrain reactive-transport models for a suite of basaltic clasts weathered in a chronosequence of terraces in Costa Rica: (i) the main weathering products in the alteration rinds were gibbsite and goethite, (ii) the weathering rind thicknesses increased from 12 ± 1 to 69 ± 6 mm with increasing time, (iii) coincident reaction fronts developed for dissolution of plagiocase and augite, and (iv) the thickness of these reaction fronts remained constant through time. In
Acknowledgments
We acknowledge Don Fisher (Penn State) and Thomas Gardner (Trinity University) for introducing us to the Costa Rica terraces and for use of the unpublished OSL date for terrace Qt3. We thank P. Lichtner, R. Fletcher, A.F. White, and M. Lebedeva for many conversations. Material presented in this paper is based upon work supported by the National Science Foundation under grants DGE-9972759 (Biogeochemical Research Initiative for Education, BRIE) and CHE-041328 (Center for Environmental Kinetics
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Formerly at Department of Geosciences, Penn State, University Park, PA, United States.