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Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/15217

Title: 沖繩海槽最近十萬年之古海洋記錄研究
The Last 100,000 Years Paleoceanographic Records in the Okinawa Trough
Authors: Yuan-Pin Chang
張詠斌
Contributors: NTOU:Institute of Applied Geosciences
國立臺灣海洋大學:應用地球科學研究所
Keywords: 東海;沖繩海槽;黑潮;古海洋學
East China Sea;Okinawa Trough;Kuroshio;Paleoceanography
Date: 2008
Issue Date: 2011-06-30T08:08:57Z
Abstract: 利用取自沖繩海槽中部的深海沈積物岩心MD012404,本論文進行了東海晚第四紀古海洋研究。主要研究成果包含有四個方面:1.建立高解析度與高精準度的年代模式;2.有孔蟲及矽藻化石群集鑑定與豐度分析;3.東海軌道力時間尺度之古海洋學探討;4.東亞邊緣海千年至百年時間尺度之海洋與氣候變化記錄研究。 由本研究利用碳十四定年與穩定氧同位素地層所建立之年代模式顯示,沈積物岩心MD012404可以記錄過去十萬年來東海地區的氣候與環境變化訊息,並可涵蓋最近一次間冰期-冰期的氣候循環。岩心MD012404之平均沈積速率約為每千年50公分,經與本區其他已發表的岩心記錄比較,本研究所建立的古海洋記錄在涵蓋的時間範圍與分辨率方面皆為東海地區目前已知的最佳記錄,不僅可幫助我們了解東亞氣候在軌道力時間尺度上面的變化,甚至可以達到千年以至百年時間尺度的解析度,因而可以與南、北兩極的冰心記錄做良好對比,增加我們對主導全球氣候循環改變營力的了解。 影響浮游生物化石群集的環境因子包括海水溫度、鹽度、營養鹽、日照及洋流等,因此重建過去化石群集的豐度變化資料,可以提供我們關於東海地區環境因子改變的資訊。本研究主要建立了包含浮游有孔蟲及矽藻的化石群集豐度變化資料,重建了過去十萬年本研究區域包括海水表面溫度、鹽度、營養鹽及洋流變化的記錄。由海表溫度記錄來看,顯示東海在幾個北半球快速變冷事件中,都有相對應的1至3℃變冷趨勢,然而與北極冰心與東亞陸地石筍古氣候記錄相較之下,東海的海表溫度變冷事件持續的時間較短,且其發生的時間均呈現約為500到1000年的領先趨勢,與南極的冰心所記錄的氣候轉冷事件較為吻合,顯示東海的氣候變化可能同時受到南、北半球高緯的氣候改變影響,而這高低緯氣候連繫的徒徑推測可能是藉由東亞季風與太平洋表面洋流–黑潮的熱量與水氣傳輸有關。 由本論文鑑定出的浮游有孔蟲與矽藻化石群豐度變化記錄顯示,沖繩海槽在氧同位素第五階之前應為一半封閉的海盆,南邊因當時存在著琉球島弧的海欄而與菲律賓海隔開,因此底部水團呈現較不流通的缺氧狀態,此缺氧狀態的訊息便反映在底棲有孔蟲殼體的碳同位素上。氧同位素第五階之氣候與海水面高度與全新世相似,但沖繩海槽氧同位素第五階的底棲有孔蟲殼體碳同位素比值比全新世較輕,可達1‰左右。此時黑潮應是受限於海欄地形而未完全進入沖繩海槽,導致矽藻化石中的近岸種與淺海種比例高於大洋種,且黑潮指標種完全消失。這種半封閉情況持續到距今約六萬到五萬年前,由於海槽南邊與那國海盆的沉降,促使了底層水的流通與表層黑潮的進入沖繩海槽。而於末次冰盛期時,由於全球海水面下降了約120公尺,因此再次限制了黑潮進入沖繩海槽,於此時,東海的浮游生物主要以較適應低溫的矽藻為主,浮游有孔蟲的數量相對減少,直到氣候回暖,海水面開始上升,黑潮於距今1萬6千年前重新開始影響沖繩海槽的水文環境直到今日。 經由分析沈積物中的碳氮比值與有機物的碳穩定同位素比值顯示,岩心MD012404沈積物中的有機碳成份主要為海洋性來源,因此可以當作東海表面生物生產力變化的代用指標。經與陸地上的洞穴石筍記錄相對比顯示,當東亞氣候相對較潮溼溫暖,夏季季風較盛行時,東海的生物生產力較高,而當東亞氣候變得乾冷,冬季季風較盛行時,則東海生物生產力也相對偏低,並呈現明顯的歲差週期變化,且可以與北半球高緯地區冰心記錄中的千年時間尺度氣候變化事件做良好對比,顯示當太陽入射量增強或北半球氣候快速回暖時,逐漸增強的夏季季風會帶來較多的降雨在東亞大陸上,大量的淡水輸入東海促使了較多的營養鹽輸入或藉由洋流湧昇到東海陸棚上,因而促使了生物生產力的增加,反之,當夏季季風減弱時,乾燥的氣候減少了營養鹽的輸出,並伴隨陸棚湧昇流的減弱,因而降低了東海的生物生產力。
Marine sediment core MD012404 retrieved from the central Okinawa Trough (OT) was used in this thesis research for studying late Quaternary paleoceanography in the East China Sea (ECS). This research has four objectives: 1. establishing high-resolution and high-precision age model that covers the last interglacial to glacial cycle; 2. reconstructing hydrographic records by planktic foraminifer and diatom species abundance analyses; 3. understanding forcing mechanisms that drive orbital-scale paleoceanographic variations in the ECS; 4. investigating millennial- to centennial-scale variability of the East Asian monsoon and ECS productivity by comparing the marine records with stalagmites on land and ice cores from polar regions. The age model of core MD012404 is constructed by 19 AMS 14C dating and 7 Marine Isotope Stage (MIS) events correlated to the SPECMAP time scale. It indicates that the core is composed of sediments accumulated over the last 100,000 years, with a fairly constant sedimentation rate of ~50 cm/kyr. The resolution is necessary to the paleoclimatic analyses of orbital, and millennial to centennial time-scales on the basis of correlating to stalagmite and ice core records, that has not been achieved in any previous paleoceanographic study in the ECS. Sea surface temperature (SST), salinity (SSS), nutrient content, current pattern, and the variation in incoming solar insolation control the productivity and distribution of marine planktons. Planktic foraminifer fauna and diatom flora assemblages preserved in core MD012404 thus provide information about how these environmental factors have varied in the ECS. Planktic foraminifer fauna SST estimation indicates that the surface ECS has experienced several abrupt cooling events by 1-3 oC in the past 100,000 years. These cooling events are analogous to the Heinrich events that have been reconstructed based on North Atlantic marine records, and are coincident to abrupt dry episodes indicated by stalagmite oxygen isotope records (Hulu and Sanbo) in East Asia. These cold and dry events expressed respectively in marine and terrestrial settings suggest that strengthened East Asian winter monsoon is a probable mechanism that drives the climate change. Moreover, the abrupt cold events in the ECS appear to be much shorter-lived in comparison with similar events observed in ice core records, but show ~500-1000 years lead, the same phasing as seen in Antarctic temperature records. This phasing implies effective influences from the South Hemisphere to the ECS by propagating the climatic signatures from the south through heat and moisture transport via East Asian summer monsoon and the Kuroshio. The more frequent occurrence of tychopelagic species and the lack of marine species of diatoms before 50-60 ka in core MD012404 suggest a more semi-enclosed environment in the OT. The microfossil evidence is consistent with that indicated by carbon stable isotope in benthic foraminifers from the same core and is supported by tectonic evidence from the southern part of the OT. The semi-enclosed environment in the OT has not been changed until the Yonaguni Depression was formed at the southern part of the OT in ~50-60 ka. The microfossil data generated in this core suggest that the Kuroshio might have been blocked out of the OT before 50-60 ka. The microfossil evidence also indicates that during the LGM, a lowering of ~120 m of global sea level might have also limited the intrusion of the Kuroshio water into the ECS. The warm and more saline Kuroshio water appeared to re-enter into the OT at ~16 ka when the global sea level raised gradually in response to the melted ice sheets in high latitudes of the North Hemisphere. Analyses of carbon stable isotopes of organic matter and C/N ratios of biogenic sediments indicate that the organic matter and biogenic sediment contents in core MD012404 are mainly marine sources. The precession and millennial to centennial-scale variations in total organic carbon (TOC) in core MD012404 correlate well with the summer monsoon precipitation changes recorded in Hulu and Sanbao stalagmites from Eastern China. The land-sea linkage of strengthened summer monsoon on land and increased marine productivity in the ECS suggests a scenario in that the increased precipitation on land has been responsible for transporting more nutrients via Changjiang river runoff input to the ECS. The increased fresh water runoff from the Changjiang may induce more upwelling on the continental shelf in the ECS, bringing more nutrients to support high productivity in the surface layer of the ECS. Thus, this study leads to a conclusion that the East Asian monsoon system plays a key role in not only controlling climate and ocean variability in the ECS, but also biological productivity that is tightly linked to global carbon cycles and atmospheric pCO2 variations.
URI: http://ethesys.lib.ntou.edu.tw/cdrfb3/record/#G0D91860003
http://ntour.ntou.edu.tw/ir/handle/987654321/15217
Appears in Collections:[應用地球科學研究所] 博碩士論文

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