Characteristics of aeolian grain transport over a fluvio-glacial lacustrine braid delta, Lake Tekapo, New Zealand

Abstract

This paper presents results from one of the few scientific studies to examine the physical characteristics of aeolian sediment transport in an alpine area, where topographically reinforced foehn winds initiate dust storm events. The major objective of this study is to improve knowledge of aeolian processes in mid-latitude alpine regions experiencing extreme wind speeds. Of particular interest is the role of surface characteristics in contributing to the unusually deep saltation layer which is seen to form over fluvio-glacial deposits in the Southern Alps of New Zealand. Sediment was collected at several heights (0·5, 1, 2 and 4 m) and locations over a large alpine braided river delta, and standard laboratory techniques used to examine grain size characteristics. An image processing technique was also used to evaluate grain roundness.Grains filtered from the airstream at 0·5 m and 1m above such surfaces were found to display a mean grain size of approximately 300 to 435 µm, resembling grain size characteristics of saltation clouds previously observed in high latitude, cold climate locations, in contrast to desert and prairie environments. Samples collected at 2 and 4 m above the surface were found to consist of 60 to 65 per cent sand-sized material, with some grains exceeding 1-1·5 mm in diameter. Grain shape analysis conducted on silt-and clay-sized grains filtered from the airstream above mixed sand and gravel surfaces showed such grains to display an increase in grain roundness with height. This characteristic is thought to reflect the airstream's shape-sorting ability and has important implications with respect to the often observed increase in grain roundness in aeolian deposits with increasing distance from source areas. Namely, if more rounded grains are preferentially carried higher into the airstream and therefore into regions of higher wind speed, they should theoretically be transported further from the entrainment zone before being deposited.The high wind speeds observed, often exceeding 30 m s −1 , are seen to transport significantly larger sediment than reported in the literature for desert and prairie environments. In addition, the mixture of grain sizes, and especially the pebble-and cobble-sized clasts that dominate the fluvio-glacial deposits associated with the braided rivers in this mountain region, also appear to increase significantly the trajectory height of saltating sand grains. As a result of these two factors, the depth of the saltation cloud often exceeds 1m.Observations made in this study therefore highlight the need for field and laboratory aeolian process studies to be extended to examine grain transport over coarse-grained beds during much higher wind velocities than typically reported in the literature. Such studies would provide a valuable insight into aeolian processes in high latitude/altitude environments, such as loess genesis.

Frequency, magnitude, and characteristics of aeolian sediment transport: McMurdo Dry Valleys, Antarctica

“…The different patterns of mean grain size as a function of height above the surface, and the notable change in the patterns for the Victoria Valley east and Wright Valley sites, suggest that there may be two well‐defined species of grain in transport, which correspond to what Andreotti [] called saltons (high‐energy grains) and reptons (grains ejected from the sand bed by the impact of saltons); this may, in part, be affecting the grain size distribution below 0.24 m. We suggest that in the natural setting of an unconstrained boundary and saltation layer in the MDV, the length dimensions of the particles in the phases of transport may be much greater than has been observed in wind tunnel studies. This is supported by other researchers examining saltation processes and aerodynamic effects in wind tunnels [e.g., Sherman and Farrell , ] and by field studies such as that by McGowan and Sturman []. They measured aeolian transport up to 4 m above ground level over a coarse‐grained bed in an alpine environment with high wind speeds (>30 m s −1 ) and also observed that the trajectory height of saltating particles exceeded 2 m, similar to what we observed in the MDV.…”

Contemporary glacigenic inputs to the dust cycle

“…These occur when moist air rises over a topographic barrier such as a mountain range or high plateau, loses its moisture as precipitation and then descends on the lee side of the barrier to produce very dry, often warm, strong downslope winds. In New Zealand, wind speeds during föhn wind storms at Lake Tekapo have been recorded in excess of 30 m s -1 lifting particles over 1500 mm in diameter to heights over 4 m above the glaciofluvial deposits (McGowan, 1997;McGowan and Sturman, 1997). Fristrup (1953) attributed much of the wind erosion in the high Arctic to föhn winds, highlighting their desiccating effect on surface moisture and the consequent increase in sediment erodibility.…”

“…To date, the vast majority of aeolian research has focused on these prominent global dust sources in the tropics and mid-latitudes. Notably, modern dust emissions in high latitudes are not confined to arid regions and can occur in relatively humid areas of Alaska ), New Zealand (McGowan et al, 1996McGowan and Sturman, 1997), Patagonia (Gassó et al, 2010;Johnson et al, 2011), Iceland (Arnalds, 2010;Prospero et al, 2012) and Greenland (Bullard and Austin, 2011). Contemporary glacigenic dust sources are typically associated with cold climates and are primarily, but not exclusively, located at high latitudes.…”

Testing an approach to OSL dating of Late Devensian glaciofluvial sediments of the British Isles

“…Those deposited during waning flow in shallow, slow-flowing or stagnant water columns have the highest chance of bleaching (in comparison to ice-proximal sub-environments) such as bar-top and back-bar deposits (rippled sands, Sr; horizontally laminated sands, Sh; and finely laminated sands, Fl). Subaerial exposure of these deposits under low-flow conditions presents an opportunity for these bar-top and back-bar deposits to be subject to aeolian transport (McGowan et al, 1996;McGowan and Sturman, 1997). If this occurs during daylight hours then the potential for complete bleaching of the optical signal is much greater.…”

“…Subaerial exposure of the bar-top environment is also possible as part of both the annual and diurnal flow cycle. Once subaerially exposed, transport via aeolian processes is possible, enhancing the opportunity for sufficient bleaching (McGowan et al, 1996;McGowan and Sturman, 1997).…”