Vignettes > Time evolution of soils

Time evolution of soils

Anthony Dosseto
University of Wollongong

Simon Turner (Macquarie University) and John Chappell (The Australian National University)

Author Profile

Shortcut URL: http://serc.carleton.edu/36030

Location

Continent: Australia
Country: Australia
State/Province:NSW
City/Town: Bemboka
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Chronology


(a) Location map for the Bega basin (Heimsath et al., 2000). (b) Inset from (a) showing the studied area (open square). Topography is shown with 20m contour intervals (modified from (Heimsath et al., 2000)). Details


Soil production rates (in mm/ka) calculated with U-series isotopes for different scenarios versus the distance from the ridge (in m) Details



Description

Soils are vital for the sustainability of ecosystems and human societies (e.g. (Montgomery, 2007)), and therefore it is important to understand how their rates of production and erosion are balanced. Furthermore, the rate of soil erosion is strongly linked to water salinity and can dramatically affect drainage systems (e.g. (Chhabra, 1996)). Previously, cosmogenic isotopes, such as in-situ beryllium-10 (10Be), have been used to determine rates of soil production (Heimsath et al., 1997). However, this approach requires the assumption that soil erosion and production are balanced (that the soil thickness is in steady-state) and it has only been possible to test this hypothesis in a few instances (Heimsath et al., 2000). Moreover, because soil production and erosion rates are assumed to be equal, it is impossible to use this approach to evaluate whether the soil is aggrading or degrading. Recently, uranium-series (U-series) isotopes have been proposed as an independent means to determine the rates of both soil and saprolite production. By comparing soil production rates inferred from U-series isotopes to erosion rates derived from 10Be, albeit is possible to quantitatively evaluate soil evolution.

For a system that has remained closed for more than 1 Ma (such as bedrock) all of the uranium-series nuclides will be in secular equilibrium (i.e. a daughter-parent activity ratio equal to unity) (Bourdon et al., 2003). Because the relative mobility of radionuclides during weathering is believed to be uranium-234 (234U) > uranium-238 (238U) >> thorium-230 (230Th), the residues of recent weathering (e.g. soils) deviate from secular equilibrium and are anticipated to have (234U/238U) < 1 and (230Th/238U) > 1 (Chabaux et al., 2003; Dosseto et al., 2008a) (parentheses denote activity ratios). The amount of disequilibrium depends on both the extent and age of weathering processes. For example, 238U-234U and 234U-230Th can record weathering events up to 1 million years (Ma) and 300 thousand years (ka) old, respectively, because 234U and 230Th, the daughter nuclides of each system, have half-lives of 245 and 75 ka, respectively.

In a study of a lateritic profile in the Brazilian Amazon basin, Mathieu et al. (1995) have shown that the time required to produce a 15m thick weathering profile is about 300,000 years, implying a migration rate of the weathering front of ~ 50 mm/ka. In a lateritic profile from Burkina-Faso, Dequincey et al. (2002) measured 238U-234U-230Th compositions which were difficult to reconcile with the common assumption that the relative radionuclide mobility during weathering is 234U>238U>>230Th. Nevertheless, they developed a model to estimate that the profile was developed over a timescale > 300 ka.

In a recent study, Dosseto et al. (2008b) have investigated soils of moderate thickness (< 1m), developed in a temperate climate. They selected a site in southeastern Australia where soil erosion rates have been previously determined using cosmogenic isotopes (Heimsath et al., 2000). At this site, Heimsath et al. (2000) also showed that soil thickness is likely to be in steady-state and soil erosion rates could be used to infer rates of soil production. Soils are relatively thin, but they overlay a thick saprolite horizon (20-30 m; (Green et al., 2006)) developed over a granodiorite. Dosseto et al. (2008b) measured significant radioactive disequilibrium between 238U-234U and 234U-230Th in saprolite and soil material. They used these results to model that it takes 0.55 to 6.2 Ma to develop the thick saprolite unit, which is equivalent to saprolite production rates of 4 to 46 mm/ka. The average residence time of material in the soil was also constrained and ranges from 8 to 38 ka. Dosseto et al. investigated several models possible to calculate soil production rates and derived values ranging from 13 to 59 mm/ka. These values are comparable to the soil erosion rates obtained using cosmogenic isotopes. Thus, the combination of U-series and cosmogenic isotope techniques provide an independent mean to assess any potential imbalance between soil loss (erosion) and gain (production from the saprolite or bedrock). At the site studied in southeastern Australia, devoid of any human activity, it was shown that there is no major imbalance and that the soil thickness is in steady-state. Future studies should look at sites that have undergone severe soil loss as a result of intensive land use.

Associated References


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