Hf-Nd-Sr isotopic fingerprinting of mineral dust from Asian and North African deserts

Abstract:

Mineral dust accounts for more than 50% of the atmospheric dust loading and plays an important role in the marine and terrestrial geochemical cycles. The deserts in North Africa, Northern China and Southern Mongolia are the major sources of mineral dust and have been studied intensively over past decades, especially with Sr, Nd and recently Hf isotopes which are seen as powerful tools to identify source areas. However, the isotopic compositions of dust are highly dependent on particle size hindering the ability to accurately identify dust provenance.

The clay fraction (<2 μm) comprises about half of all mineral dust and has unique minerals phases dominanted by clay minerals. Once the clay-sized particles are deflated to the upper troposphere, they are transported over long distances and are removed from the atmosphere mainly by wet deposition. Thus, the clay-sized isotopic fingerprints from deserts may be ideal targets not only for tracking the provenance tracing of long-distance transported mineral dust, but also to provide an unparalleled window for understanding the global dust cycle, especially eolian dust preserved in deep-sea sediments and ice cores.

In this work we investigate multivariate joint radiogenic Sr, Nd, and Hf isotopic compositions obtained from complete dissolution of clay-sized fractions of surface sediments from Asian and North African deserts. Asian dust source samples included the ten Northern China deserts and sandy lands - the Taklimakan, Gurbantunggut, Qaidam, Badaim Jaran, Tengger and Mu Us deserts, and the Hobq, Hulun Buirm, Onqin Daga and Horqin sandy land - and Mongolian Gobi desert. North African dust samples were from four transects in the Sahara and Sahel from Mali, Togo, Egypt and Morocco .

Our results on the clay-sized isotopic measurements of these samples describe (1) the general characteristics of dusts from the Asian with ε_Nd from -17.3 to 0.98, ε_Hf from -5.95 to 3.68 and ⁸⁷Sr/⁸⁶Sr from 0.710113 to 0.73306, and North African with ε_Nd -18.55 to -4.75, ε_Hf from -14.89 to 3.47 and ⁸⁷Sr/⁸⁶Sr from 0.708007 to 0.739651; (2) the tectonic and weathering controls on clay-sized Sr-Nd-Hf isotopic composition of deserts and (3) the implications for the source and transport pathway of eolian dusts found in the North Pacific Ocean, Atlantic Ocean and Greenland ice cores.