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Johannes Gutenberg-Universität Mainz (2010)

Aeolian geomorphodynamics in endorheic basins of the Mongolian Gobi Desert

Hempelmann, Nils Hannes

Titre : Aeolian geomorphodynamics in endorheic basins of the Mongolian Gobi Desert

Auteur : Hempelmann, Nils Hannes

Université de soutenance : Johannes Gutenberg-Universität Mainz

Grade : Doktor der Naturwissenschaften im Promotionsfach Geographie 2010

The following thesis about actual aeolian geomorphodynamics in endorheic basins of the Mongolian Gobi desert was facilitated by the DGF Project "Late Holocene, Pleistocene and actual geomorphodynamics in endorheic basins of the Mongolian Gobi desert". The working area is situated in the south of Mongolia in the northern Gobi desert. Along with some Saharan sites (Heintzenberg, 2009), especially the Bodélé region in northern Chad (e.g. Washington et al., 2006a ; Todd et al., 2006 ; Warren et al., 2007) Central Asia is assumed to be a very important source area for particle emission into the global atmospheric system (Goudie, 2009). Explicitly the endorheic basins with their temporary lakes are source areas for particle emission and particle dispersion of their sediments. The vulnerable bare soil of the dried out lake sediments in the endorheic basins is the main contributor of particle emission and their dispersion. In a geomorphological context of landscape development and the connectivity of the basin sediments to the hill slope depositions, as published in Grunert and Lehmkuhl (2004) based on ideas of Pye (1995), are numerical modelled. The following study attempts to model the dust dispersion on a regional scale (simulation site 200x150 km) based on punctual sampling sites. The sampling sites represent the distinct geomorphological system parts assumed to be contributing to aeolian geomorphodynamic. The surface covered by pavement, typical for the Gobi region, and their surface sediment, focusing on the grain size distribution, were investigated. Furthermore a 10 year time series (Jan 1998 to Dec 2007) of data of 32 Mongolian governmental weather stations was analyzed, with regards to the conditions of aeolian geomorphodynamics and parts of the data set used to perform the simulation. Additional atmospheric investigations focusing on the changing of atmospheric stability during night and day time with kite born measurements were done. The values of the field investigations and the laboratory analyses as well as the time series of the Mongolian governmental weather stations and the kite borne atmospheric investigations were used as input parameter for the simulations. Particle emission for the sampling sites and their dispersion in a 3D diagnostic wind field afterwards were performed to model connectivity of basin sediments to the hill slope deposits. In case of high mechanical turbulence in the atmospheric surface layer (i.e. high wind friction velocity (U*)) the atmospheric stability was determined as neutral in this period and particle emission and dispersion was calculated for this condition. The calculation of the dust production was performed in a simplified particle emission model following suggested models (Laurent et al., 2006 ; Darmenova et al., 2009 ; Shao and Dong, 2006 ; Alfaro, 2008). The 3D wind field calculation as well as the performed Lagrange simulation of aerosol transport was realized with the commercial programem LASATr. This program is based on the Lagrange algorithm and calculates the dispersion and deposition of single particles in a stochastic possibility. The study is showing different particle concentrations in the near ground air masses and their deposition rates depending on the grain size in a spatial distribution. A second part of this study is the investigation of the geochemical property of the distributed sediments of different sites in the area of interest. The geochemical properties should be used to track the simulated dispersion transects of particles in their spatial distribution. In case of the sediments the investigations show a relative homogeneity irrespective to different source areas or grain size fractions. Trace element investigations on single coarse sand grains with a laser application revealed only small variations depending on the different source regions. The mineral and elemental distribution pointing towards granite rock formations as being the sediment contributor. It was concluded that wide spread maphic and ultramaphic alkaline granitoides (Jahn et al., 2009) are basically responsible for the sediment production. Beside the element investigations the quartz characteristic of the light mineral fraction was investigated focusing on the amount of quartz, crystallization of the quartz crystal grid and the electron spin resonance signal of the E’1 -center in defect oxygen vacancies in the SiO2 grid. The investigation follows studies of Sun et al. (2007) to determine fingerprints of sediments based on these values. The relative homogeneity was also detectable in these proxies

Mots Clés : aeolian geomorphodynamic, dust dispersion, Mongolia, Gobi desert, ESR, REE, particle emission


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Page publiée le 17 mars 2011, mise à jour le 12 janvier 2019