Elsevier

Geochimica et Cosmochimica Acta

Volume 195, 15 December 2016, Pages 29-67
Geochimica et Cosmochimica Acta

Quantifying chemical weathering rates along a precipitation gradient on Basse-Terre Island, French Guadeloupe: New insight from U-series isotopes in weathering rinds

https://doi.org/10.1016/j.gca.2016.08.040Get rights and content

Abstract

Inside soil and saprolite, rock fragments can form weathering clasts (alteration rinds surrounding an unweathered core) and these weathering rinds provide an excellent field system for investigating the initiation of weathering and long term weathering rates. Recently, uranium-series (U-series) disequilibria have shown great potential for determining rind formation rates and quantifying factors controlling weathering advance rates in weathering rinds. To further investigate whether the U-series isotope technique can document differences in long term weathering rates as a function of precipitation, we conducted a new weathering rind study on tropical volcanic Basse-Terre Island in the Lesser Antilles Archipelago. In this study, for the first time we characterized weathering reactions and quantified weathering advance rates in multiple weathering rinds across a steep precipitation gradient. Electron microprobe (EMP) point measurements, bulk major element contents, and U-series isotope compositions were determined in two weathering clasts from the Deshaies watershed with mean annual precipitation (MAP) = 1800 mm and temperature (MAT) = 23 °C. On these clasts, five core-rind transects were measured for locations with different curvature (high, medium, and low) of the rind-core boundary. Results reveal that during rind formation the fraction of elemental loss decreases in the order: Ca  Na > K  Mg > Si  Al > Zr  Ti  Fe. Such observations are consistent with the sequence of reactions after the initiation of weathering: specifically, glass matrix and primary minerals (plagioclase, pyroxene) weather to produce Fe oxyhydroxides, gibbsite and minor kaolinite.

Uranium shows addition profiles in the rind due to the infiltration of U-containing soil pore water into the rind as dissolved U phases. U is then incorporated into the rind as Fe-Al oxides precipitate. Such processes lead to significant U-series isotope disequilibria in the rinds. This is the first time that multiple weathering clasts from the same watershed were analyzed for U-series isotope disequlibrian and show consistent results. The U-series disequilibria allowed for the determination of rind formation ages and weathering advance rates with a U-series mass balance model. The weathering advance rates generally decreased with decreasing curvature: ∼0.17 ± 0.10 mm/kyr for high curvature, ∼0.12 ± 0.05 mm/kyr for medium curvature, and ∼0.11 ± 0.04, 0.08 ± 0.03, 0.06 ± 0.03 mm/kyr for low curvature locations. The observed positive correlation between the curvature and the weathering rates is well supported by predictions of weathering models, i.e., that the curvature of the rind-core boundary controls the porosity creation and weathering advance rates at the clast scale.

At the watershed scale, the new weathering advance rates derived on the low curvature transects for the relatively dry Deshaies watershed (average rate of 0.08 mm/kyr; MAP = 1800 mm and MAT = 23 °C) are ∼60% slower than the rind formation rates previously determined in the much wetter Bras David watershed (∼0.18 mm/kyr, low curvature transect; MAP = 3400 mm and MAT = 23 °C) also on Basse-Terre Island. Thus, a doubling of MAP roughly correlates with a doubling of weathering advance rate. The new rind study highlights the effect of precipitation on weathering rates over a time scale of ∼100 kyr. Weathering rinds are thus a suitable system for investigating long-term chemical weathering across environmental gradients, complementing short-term riverine solute fluxes.

Introduction

Quantifying weathering rates for small tropical volcanic islands is of particular interest in understanding weathering processes at global scales. For example, tropical volcanic islands weather at rapid rates due to the reactive nature of the volcanic rocks and hot and humid climate (Louvat and Allègre, 1997, Gaillardet et al., 1999, Gaillardet et al., 2011, Chadwick et al., 1999, Dessert et al., 2003, Derry et al., 2005, Goldsmith et al., 2010, Lloret et al., 2011). Weathering on small volcanic islands, despite covering only a small portion of Earth’s surface, accounts for up to ∼25% of CO2 consumed by global silicate weathering (Dessert et al., 2003). The rapid removal of weathering products by rivers generates extremely high chemical denudation fluxes (Goldsmith et al., 2010, Gaillardet et al., 2011) and high sediment yields (Carpentier et al., 2008) on small volcanic islands. Many researchers have investigated how chemical weathering rates respond to changes in climate, landscape and tectonic regimes by measuring riverine solute fluxes. For example, previous studies have documented a strong positive correlation between riverine solute fluxes and river runoff or mean annual precipitation (MAP) on volcanic islands, highlighting particularly the important control of water cycle and hydrology on chemical weathering and CO2 consumption (e.g., Goldsmith et al., 2010, Gaillardet et al., 2011, Eiriksdottir et al., 2013).

However, the use of riverine solute fluxes to infer weathering rates has several limitations. First, the measured solute fluxes correspond to weathering rates in catchments if the residence time of water in catchment systems is known. These rates are snapshots in time that may only document the current climate. Their potential use to investigate the influence of climate patterns over long timescales is limited. Second, the calculation of riverine solute fluxes involves the use of river runoff values. Specifically, the riverine solute fluxes are solute concentrations multiplied by river runoff values, and the fact that solute concentrations can be chemostatic and vary much less than runoff values (Gaillardet et al., 2011) may introduce an autocorrelation between solute fluxes and runoff. Therefore the actual relationship between weathering fluxes and river runoff (or MAP) may be complicated. Third, the measured river chemical fluxes are generally normalized to the geographic area of watersheds, not the actual reactive mineral surface area where chemical reactions occur. Hence, a comparison of weathering rates measured at different watersheds should be reconciled by approaches such as introducing the surface roughness or flow path factors (e.g. Navarre-Sitchler and Brantley, 2007, Maher, 2010). One way to circumvent some of these complications is to measure directly long-term chemical weathering rates in solid-state weathering products.

Critical Zone processes such as chemical weathering and physical disaggregation transform bedrock into saprolite and soil. During the transformation, some rock fragments within regolith may alter and form porous alteration rinds surrounding unweathered core materials (collectively known as a weathering clast). Rind formation initiates at the core-rind boundary as unweathered material is exposed to infiltrating pore water. The water carries weathering reagents such as H+ and dissolved O2 that initiate release of soluble elements (e.g., Mg, Ca, Na, and K). The resulting rind is generally composed of oxides and secondary minerals enriched in immobile elements (e.g. Ti, Fe, Th, and Al). Overtime, the rind thickness increases as the core-rind boundary advances into the core. In some instances, the outermost layer of rind may disintegrate into surrounding soil. Weathering rinds provide a unique and excellent small-scale system for investigating the initiation of weathering reactions, because most large-scale field systems such as soil profiles or watersheds may be impacted by additional processes such as physical erosion, mixing, variations in fracture density, parent material compositional heterogeneity, and dust inputs.

Previous alteration rind research efforts have focused on using geometry and thickness of weathering rinds as a relative age indicator of terraces (e.g., Cernohouz and Solc, 1966, Porter, 1975, Colman and Pierce, 1981, Kirkbride and Bell, 2010), and have studied changes in the chemistry and mineralogy across the core-rind boundary to characterize chemical reactions, porosity development, and environmental conditions (Colman, 1982a, Colman, 1982b, Sak et al., 2004, Graham et al., 2010, Yoshida et al., 2011). Some researchers have developed mathematical models to simulate porosity development, phase transformation, and advance rates of core-rind boundary during rind formation (Oguchi and Matsukura, 2000, Gordon and Dorn, 2005, Hausrath et al., 2008, Navarre-Sitchler et al., 2009, Navarre-Sitchler et al., 2011, Lebedeva et al., 2010, Lebedeva et al., 2015, Rossi and Graham, 2010, Sak et al., 2010, Reeves and Rothman, 2014).

Recently, a uranium-series (U-series) chronometer in weathering rinds has been successfully applied to determine rates of weathering rind formation and to quantitatively understand factors controlling weathering advance rates at the clast scale (Pelt et al., 2008, Ma et al., 2012). The radioactive U-series isotopes (e.g., 238U, 234U, and 230Th) are characterized by various half-lives and distinct geochemical properties in geological environments (e.g., Ivanovich and Harmon, 1992, Bourdon et al., 2003). Understanding the mobility of U-series isotopes at Earth’s surface has led to recent advancements in U-series as a chronometer to constrain the rates and duration of chemical weathering in soils, rinds, and riverine sediments with time scales ranging from several kyr to 1 Myr (Sarin et al., 1990, Chabaux et al., 2003a, Chabaux et al., 2008 and references therein; Dosseto et al., 2006, Dosseto et al., 2008a, Dosseto et al., 2008b, Dosseto et al., 2014, Granet et al., 2007, Granet et al., 2010, Pelt et al., 2008, Ma et al., 2010, Ma et al., 2012). The two recent weathering rind studies (Pelt et al., 2008, Ma et al., 2012) documented that during the formation of basaltic/andesitic weathering rinds in Costa Rica and Guadeloupe, soil pore waters transported dissolved U (238U and 234U) into the rind, in addition to leaching U from the rind as previously expected. The mechanism of U deposition in rinds is likely related to sorption along with the formation of secondary minerals such as Fe-oxides after creation of porosity in these low porosity rocks. The accumulation of U and its subsequent modification of U-series decay chain with time has provided a tool to determine radiometric rind formation ages and weathering rates, highlighting the use of U-series isotopes as a highly desired geochronometer for chemical weathering studies, especially for tropical volcanic regions (Pelt et al., 2008, Ma et al., 2012).

To investigate whether the U-series isotope technique can further document differences in rind formation rates as a function of precipitation over long time scales, we have conducted a new rind study on Basse-Terre Island in the Guadeloupe Archipelago (Fig. 1). Basse-Terre is a tropical volcanic island characterized by a steep precipitation gradient. In this study we focus on multiple weathering rinds collected from the northern relatively dry part of the island (Deshaies watershed: MAP = 1800 mm; mean annual temperature, or MAT = 23 °C), an ideal site to compare to the previously studied site in the wet central island (Bras David watershed: MAP = 3400 mm; MAT = 23 °C) (Sak et al., 2010, Ma et al., 2012). Weathering clasts and bedrock were systematically characterized by petrographic, bulk chemical, electron microprobe (EMP), and U-series isotope analyses. This study provides new direct evidence that U-series isotopic systematics in weathering rinds chosen along a gradient in MAP elucidate the control of precipitation on long term weathering rates. Results from this study also reveal new details about how U was immobilized during rind formation as well as important controlling factors on rind formation rates at the clast scale, i.e. curvature of rind-core boundary, reactive phases, and porosity development. Weathering advance rates determined at the clast scale also reveal important information to assess the role of surface roughness and flow path factors when compared to weathering rates determined at watershed scales.

Section snippets

Geological setting and climate

Basse-Terre Island is a part of the French Guadeloupe archipelago in the Lesser Antilles arc (Fig. 1a), which was generated during subduction of the South American plate beneath the Caribbean plate (e.g. Jordan, 1975, Hawkesworth and Powell, 1980). Despite its small exposed surface (850 km2), Basse-Terre is a suitable location for exploring the effect of precipitation on rates of chemical weathering and denudation because of the steep gradients in the bedrock ages, relief, and precipitation (

Site description and sample collection

The study site is located in a quarry operated by Societe Antillaise de Granulats (SADG) within the Deshaies watershed in the northwestern portions of Basse-Terre (N 16°18.635′ W 61°46.601′, Elevation 248 m; Fig. 2a). The andesitic bedrock at this site formed ∼1.6 Ma (Samper et al., 2007). The climate within the watershed is tropical and humid (MAP = 1800 mm and MAT = 23 °C). Vegetation cover in this area is classified as a semi-deciduous and seasonal evergreen forest (Rousteau, 1996). Thick soil and

Results

In this study, two weathering clasts (AN-14-7.6 and AN-14-7.5) were characterized: both were oblate in shape, i.e., about 10 cm long, 7 cm wide, and 6 cm high and 15 cm long, 10 cm wide and 7 cm high, respectively (Fig. 3). The cores of both clasts are dark gray in color and are surrounded by ∼<1 –3 cm thick brownish-yellow weathering rinds. The core-rind boundary of both clasts is characterized with low to high values of curvature. Previous study has suggested that rind thickness increase with the

Sequences of weathering reactions during rind formation

During rind formation, the extent of elemental loss, based on the τTi,j values in the rinds from both clasts in this study decreases in the order Ca  Na > K  Mg > Si  Al > Zr  Ti  Fe (Fig. 6, Fig. 7). The order of element depletion is consistent with the sequence of weathering reactions starting at the rind-core boundary including weathering of plagioclase, pyroxene in both groundmass matrix and phenocrysts, and glass matrix. Similar sequences of weathering reactions have been observed in the weathering

Conclusions

To further investigate whether U-series disequilibria in weathering rinds can be used to determine changes of long-term weathering rates as a function of precipitation, we systematically studied two weathering clasts collected from the Deshaies watershed (MAP = 1800 mm and MAT = 23 °C) on the Basse-Terre Island. Five rind transects with distinct curvatures on the rind-core boundary for the clasts were characterized by petrographic, bulk chemical, electron microprobe, and U-series isotope analyses.

Acknowledgements

This research was funded by the National Science Foundation grants EAR1251952 to L.M., EAR1251969 to P.B.S., and EAR1251875 to S.L.B., and. We thank Celine Dessert (IPGP) and L’Observatoire Volcanologique et Sismologique de Guadeloupe (IPGP) for providing logistical support. L.M. also acknowledged analytical assistant of Dr. Adam Ianno from Center of Earth and Environmental Isotope Research at UTEP. J.G. has benefited from a grant from Institut Universitaire de France. We thank Julien Bouchez

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