Cosmogenic nuclide inventories of river sediments reveal the geomorphic character of their source areas
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The realization that surface processes can play a key role in moderating tectonics, and possibly climate (Molnar & England, 1990; Raymo & Ruddiman, 1992; see also Bishop 2007) has meant that there is now a heightened need for improving our understanding of how surface processes operate and how landscapes respond to these surface processes. This need in turn requires the development of techniques and methodologies that enable unravelling the 'history' of landscapes by quantifying erosion rates and sediment fluxes over the relevant spatial and temporal scales.
Cosmogenic nuclide analysis is such a technique and terrestrial cosmogenic nuclides can detect landscape changes over the spatial and temporal scales that are relevant to studying the links between surface processes, tectonics, and climate. Yet, their use as sediment tracers is currently limited.
Terrestrial cosmogenic nuclides are trace amounts of nuclides produced by the interaction of high-energy cosmic rays (mainly neutrons) with minerals in the Earth's crust. Several nuclides, in particular He-3, Be-10, Ne-21, Al-26, and Cl-36, are now routinely measured and have been used in geomorphological studies for the last two decades (Bierman and Nichols, 2004). The production of cosmogenic nuclides is confined to the upper few metres of the Earth's surface, and production rates are highly sensitive to elevation. The latter means that the total cosmogenic nuclide concentration acquired by a sediment grain, before being detached from bedrock, is sensitive to variations in bedrock erosion rate (i.e., how long the grain spends within the upper few metres of the Earth's surface) and to changes in surface elevation (or where the grain is exposed).
Cosmogenic nuclide concentrations in alluvial sediment are routinely used to estimate time- and space-averaged basin-wide erosion rates but have the potential to offer considerably more. Each individual sediment grain has a unique history as it is eroded from the parent material, and then transported via hillslope processes into the fluvial network and through this network to the point of sampling. Grains accumulate cosmogenic nuclides prior to their detachment and throughout all stages of their transport and storage, so long as they are not deeply buried or shielded. Just as the cosmogenic nuclide concentration of a grain reflects its history of erosion and transportation, so the frequency distribution of cosmogenic nuclide concentrations in a large number of grains leaving a drainage basin should reflect the geomorphological character and history of the basin. Thus, the frequency distribution of nuclide concentrations in exported sediment has the potential to provide not only a mean erosion rate but also a signature of the range and spatial distribution of erosion rates across a basin.
Recent measurements of He-3 in alluvial olivine grains from the Waimea River basin on the island of Kauai, Hawaii (Gayer et al., 2008) and of Ne-21 in individual alluvial quartz pebbles from the upper Gaub River basin in central-western Namibia (Codilean et al., 2008) have confirmed that the spatial non-uniformity of erosion rates in a drainage basin is reflected in the frequency distribution of cosmogenic nuclide concentrations in sediment leaving the basin. Further, using a simple numerical model, Codilean et al. (2010) have shown that the form of the frequency distribution of cosmogenic Ne-21 concentrations in exported sediment is sensitive to the range and spatial distribution of geomorphic processes operating in the sediment's source areas and that this distribution can be used to infer aspects of source area geomorphology. Notably, Codilean et al.'s (2010) modelling shows that the source area characteristics that can be inferred from cosmogenic nuclide data in detrital grains include the range of erosion rates that characterise the drainage basin, with, in principle, a probability attached to that inference, even for relatively small sample sizes (~30 grains).
Cosmogenic nuclide analyses of larger numbers of detrital grains potentially permit the determination of the probable range of erosion rates in the source area basin with higher confidence. Thus, if sediment source drainage basin area is known, it should in principle be possible to use cosmogenic nuclide concentrations in detrital grains to determine the range of erosion rates responsible for the production of that sediment, complementing the more 'traditional' sedimentological tools for analysis of source area and sediment transport.
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