Geochemistry and Provenance of Loess on the Miaodao Islands, China

Abstract

Loess deposits are widely distributed across the globe and provide detailed records of climatic changes since the Quaternary period. Their geochemical element characteristics are important indicators of paleoenvironmental evolution and provenance. Therefore, four typical loess sections from four different islands of the Miaodao Islands were selected for systematically geochemical analysis of major and trace elements. The geochemical data of major and trace elements are very similar, indicating that the loess of all islands on the Miaodao have a common provenance. The geochemical test results show that t SiO2, Al2O3, Fe2O3 and CaO are the major chemical components of loess, with an average total content of 85–90%. The average Eu/Eu*, ΣLREE/ΣHREE, LaN/YbN, GdN/YbN values of the Miaodao Islands loess range from 0.65 to 0.69, 7.84 to 8.31, 8.02 to 9.99, 1.40 to 1.76. These data are similar to and different from those of the Chinese Loess Plateau, indicating the diversity of Miaodao Islands Loess provenance. The CIA (Chemical Index of Alteration) (50–65) and Na/K results suggest that the loess here had experienced incipient chemical weathering. The A-CN-K (Al2O3-CaO* + Na2O-K2O) diagram indicates that the weathering trend of the loess sections is consistent with that of continental weathering. The local loess data points are close and parallel to the A-CN line, suggesting that the loess dust material on the Miaodao Islands originated from the extensive upper continental crust, and was highly mixed in the process of wind transport and deposition. The relationships of Log[(CaO + Na2O)/K2O] versus Log(SiO2/Al2O3), Na2O/Al2O3 versus K2O/Al2O3, LaN/YbN versus Eu/Eu*, Sc-Th-La and Zr-Sc-Th plots of major and trace elements reveal that the loess sources for the Miaodao Islands are similar to those of the Loess Plateau, which were derived from alluvial fan deposits flanking the Qilian Shan in China, the Gobi Altay and Hangayn Mountains in Mongolia. However, the loess of the Miaodao Islands is coarser in average grain size and contains abundant marine fossils, with gravel layers, indicating it is allochthous and near-source, which suggests it mainly originated from the adjacent exposed sea floor sediments of the Bohai Sea during glacial periods. Finally, we conclude that the loess of the Miaodao Islands is the result of a gradual accumulation process, in which the relative amount of distant-source material decreased and the near-source material increased in response to changes in sea level and paleoclimate. Our findings support that the loess of the Miaodao Islands was formed by mixing material from distant and proximal sources.