| 其他摘要 | Two gravity cores MD05-2908 and PC-1, which are under the main axis of Kuroshio Current (KC), are adopted in this thesis. Based on detailed AMS14C dates, by using high-resolution organic geochemistry records (such as the content and isotopic composition of carbon and nitrogen, the content of long-chain n-alkane, n-alkanol and unsaturated alkenone) we reconstructed the history of sea surface environmental evolution of the Kuroshio Current main flow for the past 25,000a B.P.. In this study, sea surface temperature (SST) and salnity (SSS) of the past 25,000a B.P. were calculated by using the Uk’37-derived paleo-thermometer index and the foraminifer oxygen isotopic composition. The evolution of history of terrigenous material input during the last 7,000a B.P. was reconstructed via the content and composition of carbon and nitrogen and biomarkers. And the history of sea-surface paleoproductivity for the last 7,000a B.P. was reconstructed by using long-chain unsaturated alkenone and organic carbon isotopic composition. By correlation with records from north hemisphere ice-raft events, GRIP2 ice core and records from Okinawa trough, we analyzed the response mechanisms of sea surface circulation on the past global climate changes. By correlation with those global high frequency climate events we discussed the responses of Kuroshio Current evolution to the global high-frequency climate events. In assistant with spectrum and wavetest methods, we discussed the controlling mechanisms of high-frequency climatic events recorded in the Okinawa Trough. By all these methods and approaches we got the following viewpoints and conclusions.
There are good correlations between our organic geochemical based paleoenvironmental records and those reported records in the Kuroshio Current flow. Those former reported Kuroshio variation events (especially during the middle to late Holocene) can be recognized from our high-frequency geochemistry records. There are good correspondings between the Kuroshio variations and global climate changes for the past 25,000 a B.P.. The variation trend of Kuroshio strength is similar to that of global climate changes, and the records of high-frequency climate events identified from our organic geochemistry records are synchronous with those records reported elsewhere in the world. This synchronization reveals the fact that the climate changes in the Kuroshio Current is influenced by global climate changes and there is no obvious time lag or leading.
Global high-frequency events show prominent controls on both the main flow of the Kuroshio Current and the atmospheric and oceanic circulation in the nearby areas. This controlling pattern is carried on by the three climatic controlling factors in this area, namely the western Pacific subtropic high (WPSH), the intertropical convergence zone (ITCZ) and the East Asian monsoon (EAM). These three factors correlate and restrict each other, and they are dominated by the solar radiant intensity . Concretely, the decrease of the solar radiant intensity induces a reduced heat-difference between the tropical western pacific ocean and the high latitude land area, which weakens the convection activities in the tropical western pacific, as a result, the WPSH is restricted to the south and east part of China, heavy and concentrated rainfall maintains at the east and south part of China. The intensified rainfall enhances the weathering velocity and the river discharges, which together results a increased mass supply into the East China Sea. Furthermore, the East Asian winter monsoon (EAWM) is influenced by the solar radiant intensity. An intensified EAWM always corresponds to a reduced solar radiant intensity, which results in an increasing of “nepheloid layer” transportation. In the South Okinawa Trough (SOT), this kind of “nepheloid layer” transportation is the primary contribution to the mass accumulated in SOT. Thus, intensified EAWM leads the increasing of material supply in the past 7000a B.P..
The organic geochemistry-induced sea-surface paleo-productivity records show similar variations to that of land-material input for the past 7000a B.P.. The increase in land material greatly enriches the nutrient supplement in the sea surface, which stimulates the primary productivity. For the past 7000 a B.P., the increase of primary productivity always correlates to increased land materials input and decreased Kuroshio current. Indicates the increased influences of land nutrient supplement together with diluted land water.
The high-sedimentation rate events correspond to the decreased Kuroshio intensity and the increased ENSO frequency. There are closely relationships with ENSO and the WPSH, the EAM and the migrations of ITCZ.
The Kuroshio current shows a decreased SST difference between the middle and the north part of Okinawa Trough from the past 25000a B.P. to present, the maximum difference occurred in the LGM, indicates a decreased Kuroshio Current influences over that time. However, the SST differences between the middle and north part of the Okinawa trough show very little variations between LGM and the Holocene, suggests the Kuroshio Current had always been in the Okinawa Trough both in the Holocene and in the LGM.
Periodicity analysis shows the Kuroshio Current give a good records of global high-frequency cycles including the millennial scale D/O cycle and the atmosphere—ocean inter-oscillation induced multi-decadal cycles such as the PDO and NAO, reveals quite sensitive corresponds to the global and regional high-frequency climate events. Periods records picked-up from different proxies show similar periods, which reveals these paleo-environmental proxies have the same controlling factors. The periodicities show that the solar activity has the basic influences on the Kuroshio current and the adjacent areas while those regional climate factors superimposed their influences on the backgroud climate variations and enlarged those high-frequency climate changes. |
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