Elemental geochemistry of northern slope sediments from the South China Sea: Implications for provenance and source area weathering since Early Miocene

Abstract

The South China Sea (SCS) is the largest marginal sea in the Western Pacific. A total of 106 sediment core samples was taken from Ocean Drilling Program (ODP) Site 1146 in the SCS and analyzed for major and trace elements. Three stratigraphic sedimentary units (I, II and III) were recognized at this site. Except for the Ca and Na contents, the contents of all the major elements in the sediments taken from Site 1146 were significantly positively correlated with the Al contents, indicating that they were associated with micaceous/clay minerals in the sediments. The sediments were strongly enriched for Ca and Sr in Unit IIA compared to the average upper continental crust (UCC), relative to the other units, while all major elements (except for Na and Ca) were significantly depleted in the unit. High correlations among the contents of Ca, Sr and calcium carbonate suggested that the down-core distribution of most major elements might be affected by the dilution effect of biogenic carbonate. The samples were strongly enriched in transition elements (TTEs) and slightly depleted in high field strength elements (HFSEs) (e.g., Nb and Y), and light rare earth elements (LREEs), such as La and Ce, relative to the UCC.The chemical index of alteration (CIA) in the sediment core ranged between 54 and 72 with an average of 64, reflecting low to intermediate source area weathering intensity. The chemical weathering intensity, in combination with the geochemical proxies of Ti/Na, Al/Na and K/Na, and the mineralogical features of the sediment core, indicated that the climate in the source area had cooled down gradually since the Early Miocene.By comparing the geochemical characteristics of suspended sediments from the Pearl River, the Yangtze River, the Red River, the Chinese Loess and sedimentary rock from Taiwan, we inferred that the sedimentary rock of Taiwan, as well as the suspended sediments from the Yangtze River and the Chinese Loess, might be similar in provenance to the sediments from Site 1146, whereas the Pearl River and the Red River might not play a significant role in contributing detrital material to the site. The detrital material of the northern part of the SCS could be derived from a siliceous igneous source with no major contribution from basic igneous rocks (e.g., the volcanic Taiwan–Luzon Arc). The triangular diagrams of A–CN–K and La–Th–Sc suggested that the samples were not affected by K-metasomatism and came from the same source area; granodiorite was the most likely source rock for these samples. Therefore, the South China craton is believed to be the dominant source of the Site 1146 sediments.