Age and rate of weathering determined using uranium-series isotopes: Testing various approaches

doi:10.1016/j.gca.2018.11.038

Geochimica et Cosmochimica Acta

Volume 246, 1 February 2019, Pages 213-233

https://doi.org/10.1016/j.gca.2018.11.038 Get rights and content

Abstract

The development of weathering profiles shapes Earth’s surface and regulates its climate via chemical weathering. Hence, it is essential to be able to determine the age of weathering profiles and quantify how fast they form. Uranium-series isotopes allow for such quantification. However, isotope compositions are generally measured in bulk regolith, which represents a complex mixture of mineral and organic phases of different origins that can impact the reliability of the information derived from U-series isotopes. Thus, in this study, we assess whether sequential extraction and mineral separation could provide more reliable estimates of weathering ages and rates. We focus on a granitic profile developed under temperate climate in southeastern Australia, a tectonically quiescent environment. Regolith production rates have been independently estimated in the region using cosmogenic isotopes.

As expected, the mineralogy and geochemistry of the bulk regolith show that biotite and feldspar are the main phases lost during weathering, progressively replaced by clay minerals. There is no evidence for significant input of element from external sources, such as via aerosol deposition. While sequential extraction does not seem to affect major mineral phases and element concentrations, it is suspected of producing artificial radioactive disequilibrium. Biotite separates show very large accumulation of U and Th, which increases with decreasing depth.

Regolith production rates and mineral dissolution rates calculated with weathering rates estimated using the bulk saprolite and quartz separate compositions yield values comparable to independent estimates. Conversely, weathering ages derived from the compositions of saprolite leached experimentally or biotite separates underestimate regolith production rates and mineral dissolution rates. Thus, sequential extraction or biotite separation are not recommended methods to derive reliable rates of regolith production and mineral dissolution. Despite the potential complexity of the composition of bulk regolith, the use of regolith without any pre-treatment seems to yield satisfying estimates of regolith production and mineral dissolution rates. The composition of quartz separates yields rates similar to those derived from bulk compositions. This provides an alternative method, potentially allowing reliable results to be obtained from a single mineral phase rather than a complex mixture of weathering products.

Introduction

Quantifying the rate of conversion of bedrock into regolith is crucial for understanding landscape evolution over millennia and Earth’s climate regulation by chemical weathering. Isotopic techniques have allowed us to address this aim. For instance, Heimsath et al. (1997) have used in-situ cosmogenic isotopes to show that soil production rates are inversely correlated to soil depth. In order to be able to discuss the evolution of soil resources, one needs to quantify soil erosion and production rates independently. However, this is not possible using cosmogenic isotopes since they require assuming that these two rates are equal. Uranium-series isotopes in regolith can be used to determine the time elapsed since inception of bedrock weathering, termed weathering age, and thus independently quantify regolith production and mineral dissolution rates (Rosholt et al., 1966, Boulad et al., 1977, Moreira-Nordemann, 1980, Mathieu et al., 1995, Dequincey et al., 1999, Dequincey et al., 2002, Vigier et al., 2001, Vigier et al., 2005, Vigier et al., 2006, Chabaux et al., 2006, Chabaux et al., 2008, Chabaux et al., 2003b, Chabaux et al., 2012, Chabaux et al., 2013, Dosseto et al., 2006a, Dosseto et al., 2006b, Dosseto et al., 2006c, Dosseto et al., 2008b, Dosseto et al., 2008a, Dosseto et al., 2010, Dosseto et al., 2011, Dosseto et al., 2014, Dosseto et al., 2012b, Granet et al., 2007, Granet et al., 2010, Pelt et al., 2008b, Pelt et al., 2013, Ma et al., 2010, Ma et al., 2012, Ma et al., 2013, Keech et al., 2013, Suresh et al., 2013, Suresh et al., 2014b, Dosseto, 2015, Gontier et al., 2015, Ackerer et al., 2016, Bosia et al., 2016, Bosia et al., 2018, Schoonejans et al., 2016). The weathering age is calculated using a gain/loss nuclide model which describes the evolution of nuclide abundance in the regolith (Dequincey et al., 2002, Chabaux et al., 2003b, Dosseto et al., 2008b, Dosseto, 2015). This model seeks to estimate the amount of time required for U-series isotope ratios to evolve from an initial composition (generally the composition of the parent material) to that measured in the regolith (exhibiting radioactive disequilibrium). However, the robustness of this model can be complicated by the complex mixture of phases that make up the regolith: residual phases from the parent material (primary minerals), neo-formed phases from incongruent dissolution of primary minerals or from precipitation from soil pore water, and organic compounds. In particular, the presence of mineral phases precipitated from pore water as well as organic matter can introduce uncertainties in the model that may result in inaccurate estimation of regolith production rates, if these phases contain a significant amount of U or Th. To address this potential issue it is necessary to isolate bedrock-derived phases, i.e. residual primary minerals and secondary minerals produced by incongruent dissolution of primary minerals (Suresh et al., 2014a). Sequential extraction has been proposed as a possible methodology (e.g. Tessier et al., 1979, Schultz et al., 1998, Blanco et al., 2004, Lee, 2009, Suresh et al., 2010, Martin et al., 2015, Menozzi et al., 2016; Francke et al., 2018). This treatment aims at selectively removing the exchangeable fraction, carbonates, manganese and iron oxides, as well as organic matter. However, its effect on the U-series isotopic composition of regolith is not clear and needs thorough testing (Schultz et al., 1998, Blanco et al., 2004, Suresh et al., 2014a, Menozzi et al., 2016). In a basaltic weathering profile, Menozzi et al. (2016) have shown that sequential extraction can mobilise labile U and Th, resulting in artificial radioactive disequilibria. An alternate approach to isolate bedrock-derived phases is to physically separate mineral phases of interest. While time consuming, this method does not require extensive chemical treatment and thus avoid potentially modifying the composition of the sample.

In this study, sequential extraction and mineral separation are applied to a granitic weathering profile. Their effect on the geochemical composition of the regolith and their suitability to derive robust weathering ages are discussed. The study site is located in southeastern Australia, where independent estimates of regolith production rates have been previously determined using cosmogenic isotopes (Heimsath et al., 2000, Heimsath et al., 2001, Heimsath et al., 2006, Heimsath et al., 2010, Burke et al., 2009).

Section snippets

Study site & sample preparation

A weathering profile was sampled from a road cut located on a hilltop near Crookwell, NSW, Australia (34°26′42.67″S 149°12′1.28″E, WGS 86, elevation: 734 m above sea level; Fig. 1). The soil is a red kandosol, developed on a Silurian granite of the Gunning Formation (Bodorkos et al., 2010). A sample of the bedrock amounting to several kg, was collected from boulders that were extracted during road construction; the boulders did not display any sign of weathering. Two soil and four saprolite

Bulk samples

The mineralogy of the bedrock is dominated by quartz, plagioclase and biotite (Table 2). Magnesium, Al, K, Ca, Fe, U and Th concentrations (Table 3, Table 7) are similar to values previously reported for granitoid rocks (Harmon and Rosholt, 1982, Rosholt, 1983, Cowart and Burnett, 1994, Shimizu et al., 2000). The (²³⁴U/²³⁸U) activity ratio is within error of secular equilibrium, while the (²³⁰Th/²³⁸U) is lower than unity (Table 7).

In the bulk regolith, quartz concentrations vary between 39 and

Mineralogical and geochemical evolution of the bulk regolith

The mineralogy of the bulk regolith indicates that biotite and plagioclase are the main phases dissolved during regolith development (Fig. 2). While during the conversion of bedrock into saprolite, most biotite is lost, plagioclase persists and is lost mainly in the soil. Clay abundance increases with decreasing depth, illustrating the conversion of primary minerals into clay minerals. This increase is dominated by illite in the saprolite, and chlorite and kaolinite in the soil (Fig. 2).

Conclusions

In the granitic weathering profile studied, mineralogical and geochemical data show that as expected, biotite and feldspar are the main phases dissolving, replaced by clays. Loss of alkali and alkali earth elements is less abrupt than in a nearby basaltic profile, illustrating that basalts weather more readily than granitic rocks. Geochemical data show no evidence of elemental gain to the profile, thus suggesting that aeolian deposition does not play a major role on the geochemical budget of

Acknowledgments

We would like to thank Lili Yu for help with sample preparation. This work was funded by an Australian Research Council Future Fellowship FT0990447 to AD.

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Age and rate of weathering determined using uranium-series isotopes: Testing various approaches

Abstract

Introduction

Section snippets

Study site & sample preparation

Bulk samples

Mineralogical and geochemical evolution of the bulk regolith

Conclusions

Acknowledgments

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