Paper No. 4
Presentation Time: 9:00 AM
Sand Dunes Are a Major Proximal Coarse Dust Source for Late Pleistocene Desert Margin Loess, the Negev Desert, Israel
Sources and transport mode of coarse silt grains depoited as loess are still in debate. Although recent laboratory experiments indicate the potential of abrading coarse silt grains from moving eolian sand, the concept was widly rejected before as field-based evidence is rare. In the Negev, Israel the contrast between the regional carbonate lithology and the coarse silt, quartz-rich loess provides an opportunity in addressing this problem applicable to all world deserts. Analyses of three hilltop, primary eolian loess sequences from the Negev show bimodal grain size distribution at 50-60µm and 3-8µm. Using mineralogy and OSL ages we demonstrate that the coarse mode is composed of ~70% quartz grains with amplitude and grainsize mode increasing regionally during the late Pleistocene. As the bedrock is carbonate, this quartz grain increase indicates a time-transgressive enhancement of a proximal, relatively coarse dust source; the distal Saharan source probably provides the finer grain size mode of dust of the loess. The only potential proximal dust source for the large amount of coarse silt quartz grains were the quartz-rich sands advancing into Sinai and the Negev. They are concurrent with the loess accretion during the late Pleistocene and available as a result of the exposure of the Mediterranean shelf or processes associated with the eastern Nile delta. We therefore propose that the coarse silt quartz grains were formed through eolian abrasion within the margins of an advancing sand sea in accordance with experiments. This relationship between desert sand seas as a source for proximal coarse dust and desert margin loess deposition is applicable also in other deserts such as in North Africa, China and Australia. Our results emphasize the crucial role of analysing primary hilltop eolian and not valley fill, partially fluvial reworked loess sequences in interpreting sources, transport, and rates of dust-forming loess.