Cosmogenic nuclide inventories of river sediments reveal the geomorphic character of their source areas part of Vignettes:Vignette Collection
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.
Quantifying rates of erosion in Namibia using cosmogenic nuclides part of Vignettes:Vignette Collection
Quantifying rates of erosion on millennial timescales is fundamental to understanding the processes that shape the Earth's surface. Cosmogenic nuclide analysis is probably the most suitable technique for this purpose as it is capable of quantifying rates of erosion over a variety of spatial scales and over timescales of the order of thousands to millions of years. Cosmogenic nuclide analysis is a relatively new technique with the first studies being published in the early 1990's. Cosmogenic nuclides are minute amounts of nuclides such as He-3, Be-10, C-14, Ne-21, Al-26, and Cl-36, and are produced by the interaction of high-energy cosmic particles (mainly neutrons) with minerals in the Earth's upper crust. The technique is based on the principle that the concentration of cosmogenic nuclides is proportional to the amount of time the mineral (or host rock) is exposed to cosmic radiation. The production of cosmogenic nuclides is confined to the upper few metres of the crust and the production rate is strongly dependent on elevation. The production rate also depends on other factors, such as geomagnetic latitude and the configuration of surrounding topography, but these are somewhat less important. The strong dependence of the production rate on altitude means that the total cosmogenic nuclide concentration acquired by a 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 also to changes in surface elevation (or where the grain is exposed). Cosmogenic nuclides have been used extensively to estimate rates of erosion at both the outcrop- and the landscape-scales. The Namibian sector of the Great Escarpment, especially the area in the vicinity of the Gamsberg Mountain, has been extensively studied, and given a comprehensive published cosmogenic nuclide dataset, this area is ideal for illustrating the power of the technique. Namibia's landscape is dominated by three elements: the Namib Desert, ~2000 km long and ~200 km wide; the Great Escarpment, a major escarpment zone parallel to the coast; and an upland plateau inland of the escarpment, characterised by low relief and gentle slopes. Namibia has an arid climate as subtropical easterly winds loose moisture while crossing the African continent before descending the escarpment and drying further. The south Atlantic anticyclone, the cold Benguela current, and coastal upwelling offshore of Namibia mean that little precipitation reaches the coast. Rivers and streams originating in Namibia are ephemeral, being dry most of the year except during floods that may last up to two weeks. Cosmogenic nuclide analyses in samples collected from bedrock outcrops from the coastal plain and upland plateau by Bierman and Caffee (2001) and van der Wateren and Dunai (2001) have yielded very low bedrock erosion rates, averaging around 2.5 meters per million years. Bedrock samples collected by Cockburn et al. (2000) along a profile perpendicular to the Great Escarpment in the Gamsberg area have yielded slightly higher (although still relatively very low) bedrock erosion rates averaging around 7.9 meters per million years. Basin-wide erosion rates obtained by Bierman and Caffee (2001) and Codilean et al. (2008) based on cosmogenic nuclide analyses of sediment are higher than their bedrock counterparts but exhibit a similar regional pattern: 6.4 and 5.8 meters per million years on the coastal plain and upland plateau respectively, and 12.9 meters per million years on the escarpment. The results of the cosmogenic nuclide analyses in both bedrock and river sediment samples suggest that the landscape of central-western Namibia has virtually remained unchanged in the last couple of million years. Comparing the bedrock- and sediment-based erosion rates indicates that bedrock outcrops are more resistant to erosion than the landscape as a whole, suggesting that relief may fluctuate over time as inselbergs grow and then decay due to lateral erosion (Twidale and Bourne, 1975). Despite this local variability, illustrated by the difference between the rates recorded by bedrock and those recorded by sediment, both bedrock and sediment tell a similar regional story, that is, the steeper escarpment area is eroding more rapidly than either the more gently sloping coastal plain or the upland plateau. Further, Bierman and Caffee's (2001) and Codilean et al.'s (2008) sediment-based cosmogenic nuclide data show a strong linear correlation with the average slope of the sediment's source drainage basins. This finding is important, confirming other studies that have identified mean basin slope as a dominant control on rates of erosion and the associated development of topography.