Environmental dynamics since the last glacial in arid Central Asia: evidence from grain size distribution and magnetic properties of loess from the Ili Valley, western China
Yue Li1,2, Yougui Song1, Kathryn E. Fitzsimmons3, Hong Chang1, Rustam Orozbaev4,5, and Xinxin Li1,21State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, 710061, China 2College of Earth Science, University of Chinese Academy of Sciences, Beijing, 100049, China 3Research Group for Terrestrial Palaeoclimates, Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany 4Research Center for Ecology and Environment of Central Asia, Chinese Academy of Sciences, Urumqi, 830011, China 5Institute of Geology, National Academy of Sciences, Bishkek, 720040, Kyrgyzstan
Received: 19 Mar 2017 – Accepted for review: 25 Mar 2017 – Discussion started: 27 Mar 2017
Abstract. The extensive loess deposits of the Eurasian mid-latitudes provide important terrestrial records of Quaternary climatic change. As yet, however, loess records in Central Asia are poorly understood. Here we investigate the grain size and magnetic characteristics of loess from the Nilka (NLK) section in the Ili Basin of eastern Central Asia. Magnetic parameters indicate very weak pedogenesis compared with loess from other regions in Eurasia. The higher χlf values occur in primary loess, rather than in weak paleosols, and the variations in magnetic susceptibility (MS) value correlate closely with the proportions of the sand fraction. We attribute this result to high wind strength at the time of loess deposition. To explore the dust transport patterns further, we identified three grain size end members (EM1, mode size 47.5 µm; EM2, 33.6 µm; EM3, 18.9 µm) which represent distinct aerodynamic environments. EM1 and EM2 represent the grain-size fractions transported from proximal sources in short-term, near-surface suspension during dust outbreaks. EM3 appears to represent the continuous background dust fraction under non-dust storm processes. Of the three end members, EM1 is most likely the most sensitive recorder of wind strength. A lack of correlation between EM1 proportions and GISP δ18O values at the millennial scale, combined with modern weather data, suggests that Arctic polar front predominates in the Ili Basin and the Kyrgyz Tian Shan piedmont during cold phases, which leads to the dust transport and accumulation of loess deposits, while the shift of mid-latitude westerlies towards the south and north controls the patterns of precipitation/moisture variations in this region. Comparison of EM1 proportions with Northern Hemisphere summer insolation clearly illustrate local insolation-based control on wind dynamics in the region, and humdity can also influence grain size of loess over MIS3 in particular. Although, the polar front dominated wind dynamics for loess deposition in the region, the Central Asian high mountains obstructed its migration further south. Our results may also support the significance of the mid-latitude westerlies in transmitting North Atlantic climate signals to East Asia.
Li, Y., Song, Y., Fitzsimmons, K. E., Chang, H., Orozbaev, R., and Li, X.: Environmental dynamics since the last glacial in arid Central Asia: evidence from grain size distribution and magnetic properties of loess from the Ili Valley, western China, Clim. Past Discuss., doi:10.5194/cp-2017-50, in review, 2017.