Sediment provenance and paleoenvironmental change in the middle Okinawa Trough during the last 18.5 ky: Clay mineral and geochemical evidence

摘要

We investigated grain size, clay mineralogy, elemental geochemistry, organic composition, and AMS C-14 dating ages in Core KX12-3 from the middle Okinawa Trough in order to better understand the sediment provenances and transport processes and also the forcing mechanisms behind their variations over the last 18.5 ky. The geochemical-mineralogical indices reveal three notable phases of change in the sediment provenance of the core, which are related to the development of the Kuroshio Current coupled with sea-level fluctuations, the position shift of the mouths of large rivers, the magnitude of tidal bottom stress, and the East Asian monsoon intensity. The sea-level lowstand deposits of Interval 1 (18.5-9.3 ka) formed from mixed sediments originating mostly from Chinese rivers (the paleo-Huanghe and the paleo-Changjiang) as well as from materials eroded from seafloor by the strong tidal stress. Thereafter during Interval 2 (9.3-8.7 ka), a significant sea-level rise, the moderate tidal bottom stress, and a gradual landward retreat of river mouths led to obviously decreased sediment supplies from both seafloor erosion and mainland China rivers (especially the paleo-Huanghe) into to the study area. After this transitional period, the dominant sediment provenance during the mid-to-late Holocene (Interval 3) changed to the Changjiang and seafloor erosion, resulting in lower detrital linear sedimentation rate and finer grain size. At the same time, some fine-grained materials from Taiwan may have been transported northward to the study area by the Kuroshio Current, as evidenced by higher values of chlorite/kaolinite ratio and chemical index of illite, and lower TOC/TN values. In addition, a prominent decline in chlorite/kaolinite values that occurred at 5.0-3.5 ka may have been linked to a suppression of the Kuroshio Current, probably related to the Pulleniatina minimum event and/or the late Holocene Neoglacial cold event commonly found in the northwestern Pacific. (C) 2016 Elsevier Ltd and INQUA. All rights reserved.