Uranium isotopes of aeolian dust deposited in northern Tibetan Plateau glaciers: Implications for tracing aeolian dust provenance

Abstract

Asian dust comprises a large portion of the northern hemisphere atmospheric dust load, thereby exerting substantial influence on the Earth's climate, global biogeochemistry and hydrological cycle through accelerated snow and ice melt. Dust deposited on alpine glaciers encodes information on broad scale atmospheric-environmental processes. The (234U/238U) values of dust fine particulates can reflect the comminuting time and intermediate processes; thus, it provides a new method for the provenance of aeolian dust in the glacial snowpack/cryoconite. Here we present results from a comprehensive survey of uranium isotopic concentrations in dust collected from cryoconites on the glaciers of the northern Tibetan Plateau (TP). These results indicate significant spatial heterogeneity in the (234U/238U) values associated with snowpack/cryoconite dust over a broad range of glaciers in the northern TP. The values of the (234U/238U) ratio in the glaciers of western Qilian Mountains (Qiyi Glacier, Shiyi Glacier, Laohugou Glacier No.12) were the highest, followed by the Tanggula (Dunkemadi Glacier) and Kunlun Mountains (Yuzhufeng Glacier), whereas these values were the lowest in the eastern Qilian Mountains (Jingyangling Snowpack, Dabanshan Snowpack, Lenglongling Glacier). By including the analysis of Sr-Nd isotopic compositions, we find the spatial isotopic distribution reflects a combination of local dust, which is associated with short comminuting times, and dust transported over long ranges. Meteorological data indicate that the dust production in the west and north TP, Alxa arid lands and Gobi Deserts, may have a significant impact on the TP glaciers. Moreover, U-Sr-Nd isotopic composition and end-member mixing models (EMMA) were used in our study to find out the relative contribution of distinct Asian dust sources to the dust budget in the TP glaciers. The results reveal that snowpack/cryoconite dust is derived from both local sources (low comminution signatures) as well as other dust sources in the Asian region. Our study demonstrates the potential of U isotope composition as a dust tracing method. In particular, by investigating this composition on dust collected from glacier snowpack and cryoconite holes, we arrive at a map of the distribution characteristics of (234U/238U) values in different regions of the TP. Our study is the first to deploy uranium comminution age for Tibetan dust source tracing, and the results are important to elucidate the multiple origins and dynamics of dust in the Tibetan Plateau.