Terrestrial, Atmospheric and Oceanic Sciences (Jan 2005)
Calcareous Nannofossils and Variation of the Kuroshio Current in the Okinawa Tro ugh During the Last 14000 Years
Abstract
A quantitative census study on calcareous nannofossils from sediments of Site 1202 recovered by ODP Leg 195 and surface sediments from the East China Sea was carried out to obtain a high-resolution nannofossil record of the change of the Kuroshio Current during the late Quaternary. Two nannofossil ratio indices were designed and employed in this study: (1) ratio of Florisphaera profunda against F. profunda, Emiliania huxleyi and Gephyrocapsa oceanica (F-EG ratio) as a nannofossil proxy of the Kuroshio Current, (2) ratio of G. oceanica against F. profunda and E. huxleyi and G. oceanica (G-FE ratio) as a proxy of near-coast environment. Results from the 14 surface nannofossil samples demonstrate that the F-EG ratio is > 15% in the assemblage lying directly under the main route of the Kuroshio Current, whereas it is very low ( 30%) were seen from all samples on the East China Sea continental shelf or from near-coast cores. Down hole nannofossil record from the top 60 mbsf sediment interval at ODP Hole 1202B reflects the change of the Kuroshio Current in the last glacial and postglacial period since 14 ka. Extreme low F-EG ratio together with very high G-FE ratio at Hole 1202B during the time of the latest Pleistocene and the earliest Holocene suggest the absence of the Kuroshio Current in the area studied. The event of intrusion of the Kuroshio Current was clearly recorded by a dramatically increase of F-EG ratio and notably a reduction in the G-FE ratio around 9 ka. Furthermore, based on the variation of the F-EG ratio and _ of planktonic foraminifera Neogloboquadrina dutertrei, variation of the Kuroshio Current in the Holocene shows three long-term cycles (with a periodicity of ~3000 yr).
