Sedimentary trace-element records of natural and human-induced environmental changes in the East China Sea

Abstract

Total and chemical fractions of nine potential harmful elements (V, Cr, Co, Ni, Cu, Zn, Cd, Pb and Bi) and five lithogenic elements (Li, Sc, Rb, Cs and Th) in two Pb-210-dated cores from the East China Sea were analyzed to investigate their applications in paleoenvironment studies, and to reconstruct the histories of environmental changes. The residual fraction was the largest pool of trace elements in sediments. Potential harmful elements exhibited distributions that were similar to residual fractions and the lithogenic elements, indicating their terrestrial origin mainly derived from the Changjiang River and the old Huanghe delta. In the coastal core, the distributions of total and residual trace elements recorded the dry/wet variations of the Changjiang runoff since the 1850s. Total potential harmful elements and lithogenic elements in the offshore core reflected fluctuations in the strength of the Jiangsu coastal current and the East Asia Winter Monsoon over the last century. The response mechanisms of sedimentary trace elements to the runoff and monsoon variations involved direct terrestrial input of elements and the impacts of TOC and sediment grain size on trace elements in sediments. Enrichment factors (EFs), chemical fractions and principal component analysis (PCA) were used to evaluate the anthropogenic disturbance on potential harmful elements. PCA identified the lithogenic fraction of trace elements in both cores and the anthropogenic/authigenic input in the coastal core. Increases of the EFs and labile fractions of Zn, Pb and Bi in upper sediments of the coastal core indicated increased anthropogenic input of Zn and Pb since the 1980s, and increased Bi input since the 1940s. Increases in oxidizable Co, Ni, Cu, Zn, Pb and Bi above 16 cm were related to eutrophication and elevated marine organic matter in inshore East China Sea after the 1990s. Sediment records in offshore did not show any evidence of anthropogenic influence on the potential harmful elements. This study revealed that trace elements in sediments were good proxies for natural and human-induced environmental changes in waters.