Regional Sr–Nd isotopic ratios of soil minerals in northern China as Asian dust fingerprints

Abstract

We report that arid soils in various areas of northern China can be distinguished by using Sr–Nd isotopic ratios of acid-resistant minerals and Sr isotopic ratios of water- and weak-acid-soluble minerals. Our results show that contemporary dust falling on Beijing is transported mainly from the adjacent northwestern to western areas and is more likely to be related to desertification than dust from the remote Takla Makan desert, the southwestern Gobi desert, or the Loess Plateau. Mineral isotope fingerprinting of arid soils is a powerful tool for source identification and impact assessment of mineral dust, and can serve as a desertification index.

Introduction

Mineral dust from the desert and loess areas of northern China and southern Mongolia (Fig. 1) affects climate, human health, and biological activity over the Northern Hemisphere on an event to geological timescale (Duce, 1995; Chadwick et al., 1999; Wilkening et al., 2000). These arid areas have become desertified by both natural processes or human activity during the course of Chinese history. At present, desertification is accelerating and extending even farther north as a result of socio-economic pressures induced by population growth (Sheehy, 1992; Zhu et al., 1992; Zhang et al., 2001). Several studies have reported that Asian dust events, which occur throughout the year but predominantly in spring, have become more frequent in the late 20th century, particularly since 1999–2000 (Chun et al., 2001; Sun et al., 2002; Gao et al., 2003; Kurosaki and Mikami, 2003), but the relationship between desertification and recent dust events remains unresolved.

Recent increases in the amount of dust have caused serious problems, particularly in areas which are near arid regions as Beijing (Sun et al., 2002; Guo and Jiang, 2002). However, despite modeling and climatological studies (Sun et al., 2000; Husar et al., 2001; Xuan and Sokolik, 2002; Chung et al., 2003), the identification of major Asian dust sources at downwind depositional sites across the northern Pacific region by event-scale research only is difficult because the geochemical characterization of source areas is insufficient. Such identification is important not only for paleoclimate reconstruction over geological time (Biscaye et al., 1997; Asahara, 1999; Bory et al., 2002) but also for accurate environmental assessment and the planning of possible countermeasures to dust events.

Stable isotopes of Sr and Nd have potential as source-area fingerprints because minerals and rocks have distinct ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, depending on their geological derivation. Also, these isotope ratios are less altered than elemental composition during transport in the atmosphere or after deposition as sediments (Biscaye et al., 1997; Asahara, 1999; Chadwick et al., 1999; Bory et al., 2002; Grousset et al., 2003). Desert sand and loess in China are composed of a mixture of salinization minerals (formed by evaporation of soil water) and other minerals derived mainly from provenance rocks and their weathered materials; the former consist of water-soluble minerals (i.e., halite, gypsum, and anhydrite) and acetic acid (HOAc)-soluble minerals (mainly calcite), whereas the latter consist of acid-resistant minerals, especially silicates (Zhang et al., 1990). The salinization minerals of several arid soils in northern China have ⁸⁷Sr/⁸⁶Sr ratios distinct from those of acid-resistant minerals (Yokoo and Nakano, 2001; Yokoo et al., 2004), and are found dissolved in spring rain in Japan (Nakano and Tanaka, 1997). In order to provide key constraints on the source-areas of Asian dust and to identify the major dust source on Beijing, we collected surface soil samples at 35 locations from the desert and loess areas of northern China and five of wind-blown origin in Beijing (Fig. 1), and determined their Sr–Nd isotopic ratios of salinization and acid-resistant minerals.