Holocene climate changes in westerly-dominated areas of central Asia: evidence from optical dating of two loess sections in Tianshan Mountain, China
Optical dating was applied to two loess-paleosol sections (Lujiaowan and Shuixigou) from the northern piedmont of Tianshan Mountain, Xinjiang province, China. The two sections are over 200 km apart and have a similar depositional sequence, which consists of two paleosol layers embedded by one loess layer. Two difficulties were met in optical dating. First, because the sections are located on the slope of the mountain, it was found that some cliff debris, with coarse grains (>200 μm), were mixed with the eolian sediments by rainfall, especially in the paleosol layers. Second, the optically stimulated luminescence (OSL) signals of quartz grains from the deposits were too dim to obtain a reliable equivalent dose (De). The 63-90 μm K-feldspar grains were separated to decrease the debris portion, and they yielded bright infrared stimulated luminescence (IRSL) signals. A multiple-elevated-temperature post-IR IRSL (MET-pIRIR) procedure was applied to determine De. Comparing the optical dating ages of the two sections, the Lujiaowan (LJW) and Shuixigou (SXG) sections recorded almost the same depositional process during the Holocene. The ages of the two loess layers (2.44-3.38 ka at LJW; 2.47-4.36 ka at SXG) suggested that one drought event happened widely in this westerly dominated area. The same drought event 2.5-3.5 ka ago also happened in the Chinese Loess Plateau (CLP), where the summer monsoon dominated. However, the paleosol development period (6.6-4 ka) in the study area was distinguished from the monsoon dominated area (8-4 ka), which suggests an arid early Holocene in the westerly area.