Characteristic Of Element Geochemistry And Sediment Provenance In Sediment Cores From Shikoku Basin

Marine sediment is very huge database, which has abundant records about the evolution of the environment and the earth. The sediment from the continent shelf and outspread area is characteristic of rapid sedimentary rate and high resolution. Besides, massive terrigenous clastic material is included, which is propitious to the study of land-ocean interaction. Shikoku basin, located at the margin of northwest Pacific, is a typical back-arc basin. The smooth terrain there has stored a mass of sediment which contains a lot of information for the environment evolution. Thus, the study of the sediment in Shikoku basin and the adjacent area is of great importance to understanding the paleo-environment of northwest Pacific.In this paper, two sediment cores F120909 (54cm) and F130601 (88cm) from south and middle Shikoku basin are chosen for grain size analysis and element geochemical analysis, including major elements and rare elements. Further more the sedimentary environment and their provenance are also discussed.Grain size analysis shows that, the two sediment cores are mainly made of clayey silt, generally uniform but small changes in different depths. Subsection comparision indicates that the grain size coarsing upwards gradually, especially in the surface where sand is unignored. It probably reveals the enhancement of terrigenous clastic material.Major elements analysis shows that the main components in the sediment cores are SiO2, Al2O3 and Fe2O3, which account for as much as 75%. There is no obverse change in both cores, which are quite similar with ocean mean value. Through comparision of SiO2/Al2O3, Fe2O3/Al2O3 and TiO2/Al2O3 with sediment from other typical areas, the sediment in Shikoku Basin is most close to deep-sea clay. For core F130601, it has an obvious MnO enrichment at 30cm to 20cm depth, probably by seafloor hydrothermal activity. Rare elements analysis reveals both cores are riched in Ba and Th. The other elements are more or less the same with NASC. According to the NASC-normalized REE distribution, both curves are nearly aclinic and there is no Ce and Eu abnormity. According to the geochemistry result above, it is concluded that Shikoku basin is characterized by hemipelagic and pelagic deposit.Considering the samples location and current mode, as well as the result from provenance discriminable diagram, it is inferred that: sediment in F130601 is mainly from Nankai Trough due to the contour current, including terrigenous clastic and volcaniclastic material; F120909, which is far from the continent, is less affected by Nankai Trough but appears more volcanic action affect, probably from the central Shikoku basin spreading center or Kyushu Ridge.