Geochemistry of aerosols of northwestern part of India adjoining the Thar desert

Abstract

The northwestern part of India, which includes the Thar desert on its western side, is a hot and arid region with intense aeolian activity and transport of aerosols by the prevailing SW-W summer winds. Different size fractions of aerosols (free fall=FF, suspended particulate matter=SPM, PM₁₀=<10 μm, and >10 μm) from air were sampled simultaneously at four locations along the dominant wind trajectory for not, vert, ∼600 km. These aerosols and deposited surface sediments were characterized for their texture, mineralogy and geochemistry including REE and Sr isotopes. Within each size fraction, a bimodal distribution of grain size was observed. Quartz is the dominant mineral followed by K-feldspar, mica, calcite, chlorite and plagioclase. Garnet, amphibole, titanite and zircon are some of the identified heavy minerals. All samples, particularly those collected during summer, are compositionally homogeneous, including in their REE geochemistry, and are similar to average upper continental crust (UCC). However, in the winter aerosol samples, large deviations from the UCC composition are observed. This is attributed to meteorological parameters such as low wind velocity and temperature inversions in the winter season. During winter, secondary non-silicate and anthropogenic materials become important sources to Ca, Na, Mg, K, Ba and Ni budget; also reduction in the uptake and transport of heavy minerals lowered the concentration of Ti, Zr, Y, Cr, and REE in the aerosols. Geochemical coherence among aerosols, deposited surface sediments and the Thar sands, and the limited Sr isotopic data indicate that the Thar sediments and certain lithotectonic units of the Himalayan orogen are the proximal and distal crustal sources, respectively, for the aerosols in this region. Prevailing aridity and strong summer winds, and the presence of river alluvium in the Thar act together to transport silt rich dust, the removal of which could be a possible mechanism of ongoing desertification.