Mixed-layer illite-vermiculite as a paleoclimatic indicator in the Pleistocene red soil sediments in Jiujiang, southern China

Abstract

The mineralogy and chemical composition of the Jiujiang red soil sediments were investigated using X-ray diffraction (XRD), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). Mixed-layer illite–vermiculite (I/V) was confirmed based on the conditions that a broad peak of 1.0 to 1.4nm appeared in XRD after Mg-saturation, and its d-spacing did not change after glycol treatment, but collapsed to 1.0nm after heating, owing to the loss of water and hydroxy-cations from its interlayer. The mixed-layer I/V contents fluctuate throughout the section but show a decreasing trend upwards, suggesting decreased weathering towards the surface of the section. The chemical index of alteration (CIA) values and Rb/Sr ratios also exhibit upward decreasing trends, jointly revealing the climate shift from warm/humid to cool/dry from the Middle to the Late Pleistocene. Although the general climate evolution can be evaluated by CIA values and Rb/Sr ratios, sub-climatic events within the Jiujiang red soil sediments cannot be identified. The SiO2/Al2O3, Al2O3/Fe2O3, and SiO2/Fe2O3 ratios clearly fluctuate along the section, especially within the net-like red soil sediments, but they do not reveal multiple climate cycles. This is because of an inhomogeneous distribution of SiO2, Al2O3, and Fe2O3 in the white veins and the red matrix and also non-uniform sampling. The variation in the content of mixed-layer I/V show a strong positive correlation with changes in magnetic susceptibility of loess-paleosol sediments and in marine oxygen isotope ratios, revealing multiple sub-climate cycles on a global scale since the Middle Pleistocene. Therefore, the mixed-layer I/V contents not only reveal the whole climate trend documented in the red soil sediments since the Middle Pleistocene, but also disclose the sub-climate cycles related to the deposit-pedogenesis process.