Soil formation rates determined from Uranium-series isotope disequilibria in soil profiles from the southeastern Australian highlands

Abstract

The sustainability of soil resources is determined by the balance between the rates of production and removal of soils. Samples from four weathering profiles at Frogs Hollow in the upper catchment area of the Murrumbidgee River (southeastern Australia) were analyzed for their uranium-series (U-series) isotopic composition to estimate soil production rates. Sequential leaching was conducted on sample aliquots to assess how U-series nuclides are distributed between primary and secondary minerals. Soil is increasingly weathered from bottom to top which is evident from the decrease in (234U/238U) ratios and increase in relative quartz content with decreasing soil depth. One soil profile shows little variation in mineralogy and U-series geochemistry with depth, explained by the occurrence of already extensively weathered saprolite, so that further weathering has minimal effect on mineralogy and geochemistry. Al2O3 is mobilized from these soils, and hence a silicon-based weathering index treating Al2O3 as mobile is introduced, which increases with decreasing soil depth, in all profiles. Leached and unleached aliquots show similar mineralogy with slight variation in relative concentrations, whereas the elemental and isotopic composition of uranium and thorium show notable differences between leached and unleached samples. Unleached samples show systematic variations in uranium-series isotopic compositions with depth compared to leached samples. This is most likely explained by the mobilization of U and Th from the samples during leaching. Soil residence times are calculated by modeling U-series activity ratios for each profile separately. Inferred timescales vary up to 30 kyr for unleached aliquots from profile F1 to up to 12 kyr for both leached and unleached aliquots from profile F2. Muscovite content shows a linear relationship with U-series derived soil residence times. This relationship provides an alternative method to estimate residence timescales for profiles with significant U-series data scatter. Using this alternative approach, inferred soil residence times up to 33 kyr for leached samples of profile F1 and up to 34 kyr for leached samples of profile F3 were determined. A linear relationship between soil residence times and WIS (Si-based Weathering Index) exists and is used to estimate soil residence times for profile F3 (up to 28 kyr) and F4 (up to 37 kyr). The linear relationship between soil depth and calculated residence time allows determination of soil production rates, which range from 10 to 24 mm/kyr and are comparable to the rates determined previously using cosmogenic isotopes at the same site (Heimsath et al., 2001b). This implies that at this site, on the highland plateau of southeastern Australia, soil thickness has reached steady-state, possibly as a result of stable tectonic conditions but despite variable climatic conditions over the timescale of soil development. Soil-mantled landscapes are the geomorphic expression of this balance between soil production and denudation, and our res...

A late Holocene onset of Aboriginal burning in southeastern Australia

“…Be inventories. We use previous studies (e.g., Heimsath et al, 2001;Suresh et al, 2013) to verify the assumption that 10 Be concentrations in soils and bedrock had reached secular equilibrium with regard to erosion and production prior to the onset of Aboriginal burning. We then compare predicted 10 Be concentrations under a range of background erosion rates with measured 10 Be concentrations to assess the timing of the onset and continued use of Aboriginal burning.…”

“…1). We compared 10 Be-derived erosion rates to those estimated by radiometric and soil production studies (Bishop, 1985;Bishop and Goldrick, 2000;Heimsath et al, 2001;Suresh et al, 2013) to assess the consistency of erosion rates averaged over different time scales. The production rate of 10 Be through time and as a function of depth in rock or soil is well understood (Lal, 1991), enabling us to model 10 Be inventories in fluvial sediment under different Aboriginal fire regimes, given background erosion rates of 5-10 mm k.y.…”

“…-1 geological erosion rate range sets limits on what we consider to be plausible interpretations of land-use chronologies, and because the Tablelands is currently a soil-mantled landscape (Suresh et al, 2013), we only consider model results that show 10 Be in fluvial sediment being derived from soil-mantled rather than bedrock landscapes (Fig. 2).…”

Estimates of late Holocene soil production and erosion in the Snowy Mountains, Australia

“…In contrast, production rates inferred from U-series isotopes are independent of this assumption and essentially measure the rate of downward migration of the soil/saprolite or saprolite/bedrock boundary. Soil production rates estimated using the U-series method at Frogs Hollow, NSW (930 AHD, 500 MAP), approximately 100 km northeast of the Snowy Mountains, were 10-24 mm/ky (for soils developed on Devonian granites) (Suresh et al, 2013). Assuming a bulk density of 1.6 g/cm 3…”

The fate of atmospheric metal pollutants in the landscape, Snowy Mountains, south-eastern Australia

“…since Previously estimated rates for soil formation, undertaken in Australia using cosmogenic 10 Be nuclides, range from 10 ± 4 mm/ka on Devonian, Carboniferous and Permian aged sedimentary and metamorphic substrates in warm temperate New South Wales to 75 ± 24 mm/ka on sandstone in the tropical Northern Territory (Stockmann et al, 2014). Closer to the Snowy Mountains at Frogs Hollow, (930 AHD, rainfall 500 mm year), production rates inferred from U-series isotopes for soils developed on Devonian granites were 10 -24 mm/ky (Suresh et al, 2013 (Egli et al, 2014)). Some of the highest rates of soil development have been measured in alpine areas experiencing both rapid uplift and very high annual precipitation.…”