Tectonic Subsidence on the East China Coast Recorded by Magnetic Properties of Pliocene Red Clay in the Yangtze Delta

Thick red clay in northern China contains rich information about the uplifting of the Qinghai–Tibet Plateau, the drying process of Asian inland, the East Asian monsoon changes, and global cooling since 22 Ma. In comparison, the red clay widely distributed in southern China is generally much younger (<1 million years), thus limiting the paleoclimate and paleoenvironment reconstruction over a longer geological time. We conducted a comprehensive magnetic investigation on the Pliocene red clay of the core LQ11, located in the Yangtze Delta, to reveal its paleoclimate and paleoenvironment implications for the eastern China coast. Our results revealed that the Pliocene red clay in the Yangtze Delta has higher S-ratio and lower HIRM (Hard isothermal remanent magnetizations) values than Quaternary vermiculate red clay of hot–humid climate origin in southern China. This indicates a weaker transformation from maghemite to hematite during the process of pedogenesis. The lack of net-like white veins in the Pliocene red clay also indicates a relatively low intensity of pedogenesis. We believe that the Pliocene red clay, which is presently 250 m below the mean sea level, was formed in high-altitude topography before the Quaternary period, where paedogenic intensity was remarkably low. This finding shows rapid tectonic subsidence occurring on the eastern China coast since the late Pliocene and enriches the theoretical research on paleoenvironment reconstruction based on red clay.