Clay mineralogy and its palaeoclimatic significance in the Luochuan loess-palaeosols over similar to 1.3 Ma, Shaanxi, northwestern China

To understand climate changes recorded in the Luochuan loess-palaeosols, Shaanxi province, northwestern China, clay mineralogy was studied using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM) methods. XRD results show that clay mineral compositions in the Luochuan loess-palaeosols are dominantly illite, with minor chlorite, kaolinite, smectite, and illite-smectite mixed-layer clays (I/S). Illite is the most abundant species in the sediments, with a content of 61%-83%. The content of chlorite ranges from 5%-22%, and the content of kaolinite ranges from 5%-19%. Smectite (or I/S) occurs discontinuously along the loess profile, with a content of 0-8%. The Kubler index of illite (IC) ranges from 0.255A degrees-0.491A degrees, and the illite chemical index (ICI) ranges from 0.294-0.394. The CIA values of the loesspalaeosols are 61.9-69.02, and the R3+/(R3+ + R2+ + M+) values are 0.508-0.589. HRTEM observations show that transformation of illite to illite-smectite has occurred in both the loess and palaeosol, suggesting that the Luochuan loess-palaeosols have experienced a certain degree of chemical weathering. The Luochuan loess-palaeosols have the same clay mineral assemblage along the profile. However, the relative contents of clay mineral species, CIA, ICI, and IC values fluctuate frequently along the profile, and all these parameters display a similar trend. Moreover, climate changes suggested by the clay index are consistent with variations in the deep-sea delta O-18 records and the magnetic susceptibility value, and thus, climate changes in the Luochuan region have been controlled by global climate change.