'Humped' soil production function: eroding Arnhem Land, Australia.

We report erosion rates and processes, determined from in situ-produced beryllium-10 (Be-10) and aluminum-26 (Al-26), across a soil-mantled landscape of Arnhem Land, northern Australia. Soil production rates peak under a soil thickness of about 35 cm and we observe no soil thicknesses between exposed bedrock and this thickness. These results thus quantify a well-defined 'humped' soil-production function, in contrast to functions reported for other landscapes. We compare this function to a previously reported exponential decline of soil production rates with increasing soil thickness across the passive margin exposed in the Bega Valley, south-eastern Australia, and found remarkable similarities in rates. The critical difference in this work was that the Arnhem Land landscapes were either bedrock or mantled with soils greater than about 35 cm deep, with peak soil production rates of about 20 m/Ma under 35-40 cm of soil, thus supporting previous theory and modeling results for a humped soil production function. We also show how coupling point-specific with catchment-averaged erosion rate measurements lead to a better understanding of landscape denudation. Specifically, we report a nested sampling scheme where we quantify average erosion rates from the first-order, upland catchments to the main, sixth-order channel of Tin Camp Creek. The low (similar to 5 m/Ma) rates from the main channel sediments reflect contributions from the slowly eroding stony highlands, while the channels draining our study area reflect local soil production rates (similar to 10 m/Ma off the rocky ridge; similar to 20 m/Ma from the soil mantled regions). Quantifying such rates and processes help determine spatial variations of soil thickness as well as helping to predict the sustainability of the Earth's soil resource under different erosional regimes. © 2009, Wiley-Blackwell. The definitive version is available at www3.interscience.wiley.co

Improving the utility of detrital zircon studies through chemical abrasion [abstract only]

U-Pb isotopic dating of detrital zircon has the potential to yield important insight into a variety of geologic processes, including, but not limited to: understanding sedimentary provenance and deciphering the exhumation history of mountain belts. Most studies of detrital zircon use either SIMS or ICP-MS methods to obtain U-Pb isotopic ages on large numbers of single crystals. However, an almost ubiqutious problem with this approach is the presence of large numbers (up to 50% for random detrital zircons) of crystals that yield discordant ages. Discordance generally results from radiogenic lead loss, either through radation-damage and/or diffusion along imperfections within the crystal lattice. The presence of a significant number of discordant analyses from an individual sample complicates data interpretation and severely limits the geologic utility of this approach. Following the ‘chemical abrasion’ method developed by Mattinson [1], we investigate the potential for this preparation technique to significantly improve the overall concordance of a detrital zircon data set where Pb-loss is the cause of discordance. Previous applications of this approach to magmatic samples suggest >90% of analyses from any given sample could be moved to within 5% of concordance. Initial data indicate that annealed and non-annealed aliquots of the same sample have comparable age distributions, suggesting that this method does not introduce any additional bias into the age-population spectra. As a consqeuence of this approach, high-U and/or radiation-damaged zircon are often reduced to small irregular fragments or skeletal morphologies. In order to compensate for this, such that these types of grains are not underrepresented in any analysis, we have developed novel sampling strategies in order to obtain isotopic information without degradation of data quality. Through chemical abrasion, the proportion of detrital zircon analyses that can be included in an age spectrum could be increased dramatically without significant sampling bias. Although further work is required, the potential to improve the number of useful data points and confidence in interpreting difficult detrital zircon spectra, is significant

Topographic control of asynchronous glacial advances: A case study from Annapurna, Nepal

Differences in the timing of glacial advances, which are commonly attributed to climatic changes, can be due to variations in valley topography. Cosmogenic 10Be dates from 24 glacial moraine boulders in 5 valleys define two age populations, late-glacial and early Holocene. Moraine ages correlate with paleoglacier valley hypsometries. Moraines in valleys with lower maximum altitudes date to the late-glacial, whereas those in valleys with higher maximum altitudes are early Holocene. Two valleys with similar equilibrium-line altitudes (ELAs), but contrasting ages, are <5 km apart and share the same aspect, such that spatial differences in climate can be excluded. A glacial mass-balance cellular automata model of these two neighboring valleys predicts that change from a cooler-drier to warmer-wetter climate (as at the Holocene onset) would lead to the glacier in the higher altitude catchment advancing, while the lower one retreats or disappears, even though the ELA only shifted by ∼120 m. Copyright 2011 by the American Geophysical Union