| Fifty-nine surfacial sediment samples and four gravity piston cores(E017: 126?01.38?E, 26?34.45?N, water depth: 1826 m, core length: 297 cm; DOC-42: 128?20.9?E, 30?18.64?N, water depth: 389 m, core length: 190 cm; DOC-24: 128?59.61?E, 30?49.25?N, water depth: 740m; core length: 350cm; DGKS96-03: 28?08.869?N, 127?16.238?E, water depth: 1100m; core length: 585cm)collected from the Okinawa Trough were examined to reveal the characteristics of surface water and deep-water evolution from late Pleistocene to the present, calcareous microfossils-including calcareous nannonfossils and benthonic foraminifera were counted quantitatively and analyzed on the basis of planktonic foraminiferal oxygen curves and AMS14C dating.Latitude, ocean currents, water masses and correlative level of nutrients, salinity, temperature and available light are all thought to control the distribution of coccolithophores in the modern ocean, in order to extract effective palaeoceanographical information from their fossil parts, analysis has been carried out for some surfacial sediment samples collected from the Okinawa Trough to investigate distribution patterns of nannofossils and related environmental characteristics. All species found here have been reported from the marginal seas around China, the relative abundance of dominant species G. oceanica, E. huxleyi and other subordinate species along with the presentation of U. tenuis show the nannofossil assemblage of this area is similar to that of the outer-shelf of the East China Sea, so this area belongs to the central zone of the North Pacific in biogeographic zonations of nannoplankton as the East China Sea(ECS) and the South China Sea(SCS). A relatively higher abundance of warm-water species U. irregularis, U. tenuis and less abundance of G. caribbeanica and C. leptoporus favoring cold water again reflect the area under the control of warm Kuroshio and its extension. Variation of fossil assemblage diversity indexes and distribution pattern also show a rather shallow lysocline lying in the Okinawa Trough.The newly development in Oceanography largely stimulate the study of surface water structure in the past. Records from three gravity piston cores collected in the northern and central southern Okinawa Trough show that, downcore variation of F. profunda-one of the two nannonplankton species living in the lower euphotic zone, can be used as a perfect proxy to reveal the variance of the nutricline depth, transparency etc. of the upper water column. In Core E017, the percentage of F. profunda varied in accordance to the planktonic foraminifera P. obliquiloculata-a believed indicatorof the Kuroshio Current, reached a high level since the coming of the Holocene, inferred a significant change of surface water structure have occurred with the re-entering of the Kuroshio Current to the Okinawa Trough. Around 10.9 ka.BP, relatively cold water species Calcidiscus leptoporus reached its maximum while warm water species Umbellosphaera spp. and Helicosphaera spp. dropping prominently, paleotemperature estimates from planktonic foraminifera also record a winter drop, by comparison it can be found this cooling event coincides with the Younger Dryas events found in the ECS and SCS. Two high levels of G. oceanica were recorded around 5.7 ka.BP and 3.4 ka.BP by reducing of F. profunda, referring increasing lithogenic particles, organic particles input and decreasing transparency, suggested two fresh -water events have occurred. Cold water species also increased considerably around 2 ka.BP with the minimum percentage of P. obliquiloculata and a winter paleotemperature drop of about 3℃, indicated a regional cooling event.Among the nannonfossil species found in the northern Okinawa Trough, the most regular variance were found to be of G. oceanica and E. huxleyi especially in Core DOC-24(they varied in eminent negative correlation), while a rather imperceptible change of F. profunda was observed. Around 2-3ka.BP and 5-6 ka.BP, G. oceanica and E. huxleyi alter reversely, accordingly cold water spec...
|
PDF Full Text Request