Geochemical evidence of windblown origin of the Late Cenozoic lacustrine sediments in Beijing and implications for weathering and climate change

摘要

The origin of lacustrine sediments in the middle latitudes of Asia remains controversial. In this study, we conducted major and trace element analyses on 86 lacustrine samples from the X5 core in Beijing to determine their origin and provenance. The results indicate that their abundances all varied in relatively narrow ranges, similar to those of the samples in the Chinese Loess Plateau (CLP). A high correlation of major and trace element abundances exists between our lacustrine samples and the CLP samples. UCC-normalized major and trace element abundances also have a similar pattern between the lacustrine and the CLP samples, with the exception of a few elements with high solubility, such as Mg. In the ternary diagrams of A-CN-K, La-Th-Sc and Zr/10-Th-Sc, and the correlation maps of major/trace elements and their ratios, four groups of the X5 lacustrine samples with different sedimentation stages (3.16-2.7 Ma, 2.7-1.8 Ma, 1.8-0.9 Ma and 0.9-0 Ma) overlap well with each other and with the CLP samples, indicating that the X5 lacustrine samples are likely of windblown origin and possibly shared common, unchanged dust source regions with the CLP since the Late Pliocene. This interpretation is supported by the overlap of the lacustrine samples from Beijing with the CLP samples in the correlation maps of Ms vs. sorting, Ms vs. skewness, Ms vs. kurtosis, and kurtosis vs. skewness. Analysis of the frequency and cumulative frequency curves indicates that the X5 lacustrine sediments were dominantly transported by the winter monsoon and the westerly circulation. Given more scattered distributions of different elemental ratios and grain size parameters of the X5 core lacustrine sediments compared to those of the CLP samples, local dust particles must have contributed occasionally, which deserves further investigation in the future. Several chemical ratios, such as Rb/Sr, Na2O/Al2O3, CaO/Al2O3, and chemical index of alteration (CIA), were calculated for the lacustrine samples in Beijing. Comparison of these ratios' variations with those of the sea level of the South China Sea and the global benthic delta O-18 record indicates that Na2O/Al2O3 and CIA can be regarded as sensitive indicators of chemical weathering of lacustrine dust sediments in the mid-latitudes of Northern Hemisphere. (C) 2016 Elsevier B.V. All rights reserved.