Sediment pathway of the East China Sea inferred from an R-mode factor analysis of surface sediments in the Okinawa Trough

Abstract

Major and trace elements, loss on ignition (LOI), and grain-size distributions were determined on 190 surface sediment samples collected from the continental shelf, slope and the north Okinawa Trough in the East China Sea, at water depths of 100–200, 200–600 and more than 600 m respectively. The sediments are composed mainly of sand and silt, generally low clay contents (average 10.5%), increasing over the range of 0.12–53.6% from the shelf and slope to the trough. Sediments were sandier on the continental shelf, particularly in the northern shelf sector. Spatial distributions in chemical compositions show that the contents of SiO 2, CaO, Sr, and LOI values were relatively high on the sandy continental shelf. By contrast, the contents of Al 2O 3, MgO, Na 2O, MnO, FeO, TiO 2, Cu, Zn, Co, Ni and Cr were relatively high in the trough and along the central western shelf margin, where the sediments were composed mainly of silt. Fe 2O 3, P 2O 5 and Zr enrichment was highest in the slope sediments. Three factors were extracted by means of an R-mode factor analysis of chemical composition. Factor 1 has two inversely correlated elemental assemblages: (1) SiO 2, K 2O, TiO 2, Al 2O 3 and Zr, which can be associated with terrigenous debris input and show high positive factor scores in the southern sector of the shelf; and (2) CaO, Sr, and LOI, which can be related with biogenic debris enrichment and show high negative factor scores in the northern shelf sector as well as in a more restricted area along the slope. CaO, SiO 2, K 2O and Sr represent the element assemblage of the shelf. Factor 2 comprises Na 2O, FeO, Al 2O 3, TiO 2, Cu, Zn and Ni, and shows high scores in the trough sediments. Na 2O, Cu, Zn and Ni represent the elemental assemblage of the Okinawa Trough, and can be associated with volcanic matter. Factor 3 comprises Fe 2O 3, MgO, Cr and Co, with high factor scores on the slope and along the central western shelf margin of the study area, due to upwelling-induced enrichments in nutrients and biogenic shells. Fe 2O 3 and MgO are the slope elements. Based on the results of the distribution of the chemical compositions and the elemental assemblage of the surface sediment in the source-to-sink system, the influence of the Yellow River on the continental shelf is stronger than that of the Yangtze River. From the continental shelf and slope to the trough, the influence of the Yellow River decreased. From north to south in the Okinawa Trough, the sediment from the Yellow River decreases and from the Yangtze River increases, and the chemical compositions changed.