Vignettes > Cosmogenic nuclide inventories of river sediments reveal the geomorphic character of their source areas

Cosmogenic nuclide inventories of river sediments reveal the geomorphic character of their source areas

Alexandru T Codilean
University of Glasgow
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Shortcut URL: http://serc.carleton.edu/31892

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Type: Process, Computation

Plot illustrating the different methods available for quantifying erosion rates. It also shows the range of temporal and spatial scales over which these methods are applicable. Any of these methods can in principle be used to quantify erosion over geomorphological timescales but cosmogenic nuclides are the most suitable because: (1) are produced at shallow depths and so unlike low-temperature thermochronometers are highly sensitive to changes in surface morphology, and (2) accumulate over longer timescales and so unlike historical methods (such as using sediment volume data) are not prone to anthropogenic disturbance. Original image courtesy of Roderick Brown (University of Glasgow). Details



Schematic representation of the pathways of two different sediment grains from source to the location of sampling. The two grains originate from different locations possibly characterised by very different elevations and erosion rates, and can spend different amounts of time at different elevations (and depths within the transport layer) while transported through the drainage systems. Thus, the final cosmogenic nuclide concentration of the two grains reflects their potentially unique (and different) histories of erosion, transport, and deposition. Similarly the frequency distribution of cosmogenic nuclide concentrations in a large number of grains leaving the drainage basin should reflect the geomorphological character and history of the basin. Details


Predicted Ne-21 concentration distributions for the upper Gaub River basin, Namibia, (blue lines) obtained assuming spatially uniform bedrock erosion throughout the drainage basin. Each blue line represents one predicted Ne-21 distribution and was obtained by sampling 100 sediment grains from a Ne-21 concentration ‘map’ produced for a given bedrock erosion rate. There are 1000 predicted Ne-21 concentration distributions. Note how the shape of the predicted envelope of Ne-21 concentration distributions mirrors the shape of hypsometric curve (distribution of elevation values in the drainage basin) illustrating the strong dependence of cosmogenic nuclide production rates on elevation. Details


The hypsometric curve of the study basin and two predicted Ne-21 distributions obtained using different ranges of bedrock erosion rates that vary linearly with slope. Note how the two Ne-21 distributions are markedly different from the hypsometric curve (and also different from each other), illustrating the sensitivity of the cosmogenic nuclide concentration distribution to the spatial distribution and range of erosion rates in the drainage basin. Details


Cumulative frequency distribution plot comparing Codilean et al.'s (2008) Ne-21 concentrations measured in pebbles (black circles) with a predicted Ne-21 concentration distribution envelope obtained for the upper Gaub River catchment assuming bedrock erosion rates that vary linearly with slope. The envelopes are based on 1000 predicted Ne-21 concentration distributions, each obtained by sampling 32 grains from a Ne-21 concentration ‘map’. In each plot, 95% of the predicted Ne-21 concentration distributions lie within the envelope labelled 95% and 50% lie within the envelope labelled 50%. (Source: Codilean et al., 2010, Earth Surface Processes and Landforms, v. 35.) Details


Description

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.

Associated References

Bierman, P.R., and Nichols, K.K., (2004). Rock to sediment - slope to sea with 10Be - rates of landscape change. Annual Review of Earth and Planetary Sciences. v. 32, doi: 10.1146/annurev.earth.32.101802.120539, p. 215-255.

Bishop, P., (2007). Long-term landscape evolution: linking tectonics and surface processes. Earth Surface Processes and Landforms. v. 32, doi: 10.1002/esp.1493, p. 329-365.

Codilean, A.T., Bishop, P., Hoey, T.B., Stuart, F.M., and Fabel, D., (2010). Cosmogenic 21Ne analysis of individual detrital grains: Opportunities and limitations. Earth Surface Processes and Landforms. v. 35, (in press).

Codilean, A.T., Bishop, P., Stuart, F.M., Hoey, T.B., Fabel, D., and Freeman, S.P.H.T., (2008). Single-grain cosmogenic 21Ne concentrations in fluvial sediments reveal spatially variable erosion rates. Geology. v. 36, doi: 10.1130/g24360a.1, p. 159-162.

Gayer, E, Mukhopadhyay, S, Meade, B.J., (2008). Spatial variability of erosion rates inferred from the frequency distribution of cosmogenic 3He in olivines from Hawaiian river sediments. Earth and Planetary Science Letters. v. 266, doi: 10.1016/j.epsl.2007.11.019, p. 303-315.

Molnar, P., and England, P., (1990). Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg? Nature. v. 346, doi: 10.1038/346029a0, p. 29-34.

Raymo, M.E., and Ruddiman, W.F., (1992). Tectonic forcing of late Cenozoic climate. Nature. v. 359, doi: 10.1038/359117a0, p. 117-122.

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