Major element, rare earth element, and sulfur isotopic composition of a high‐elevation firn core: Sources and transport of mineral dust in central Asia

Abstract

The arid region extending from the Caspian Sea into China contains some of the most extensive dust aerosol production areas in the world. Major element (Al, Fe, Ca, and S), rare earth element (REE), and isotopic (δ34S and δ18O) data from a 14.4‐m‐long firn core recovered on the Inilchek Glacier (Tien Shan Mountains, Kyrgyzstan; 5100‐m elevation) are presented and used to investigate the composition, sources, and transport of mineral dust to this central Asian site. The combination of geochemical compositions points to three distinct types of mineral deposits (loess, calcium carbonate, and gypsum), which do not covary in a linear fashion during the period recorded in the firn core, (1992–1998). Loess, identified by its REE composition, is characterized by distinct large‐scale deposition events and by ubiquitous background deposition at other times. Significant interannual‐scale variability is evident, with the two largest loess events occurring within a single accumulation year, (1997) and background conditions prevailing during 1994–1996. REE compositions, specifically Nd/Yb ratios, point to multiple sources and transport directions of loess to Inilchek Glacier. Nonloess sources provide a significant fraction of the total Ca and S deposited at the Inilchek Glacier. Hence the downcore profiles of Ca and S reflect variability in the deposition of calcium carbonate and gypsum. Differences in the sulfur isotopic composition (δ34S) of high dust period (+15.0‰) and low (background) dust period (+5.4‰) samples provide strong evidence for two sources of sulfate. The former isotopic composition reflects a marine evaporite source of which there are many to the west of the Tien Shan Mountains. The latter composition indicates that a major portion of the atmospheric SO42− was derived from anthropogenic emissions. While the deposition of non‐loess Ca (calcite) and non‐loess S (gypsum) is also high during the periods of high loess deposition, there is pronounced Ca and S variability during background loess periods, (1994–1996). We suggest that the observed geochemical variability in the firn core can be explained by changes in local‐ to regional‐scale atmospheric circulation and associated transport from multiple dust sources (western Kyrgyzstan/Kazakhstan versus the Taklimakan Desert, China).