沖繩海槽岩心近四萬年以來之高解析度古海洋�古氣候變遷記錄

High-resolution Paleoceanography / Paleoclimate fluctuation records from the Okinawa trough cores for the past 40kyrs

黃振祐(Chen-yu Huang)

指導教授 ： 米泓生博士

國立臺灣師範大學/理學院/地球科學系/碩士(2001年)

摘要

本研究分析在沖繩海槽北段深海岩心MD982195 (北緯31度38.33分，東經128度56.63分，水深746公尺)及其鄰近CSH-2(北緯29度25.0分，東經126度34.0分，水深703公尺)岩心中高解析度(前者65-90年；後者30-140年)之烯酮類不飽和脂肪酸( )指標、碳酸鹽及有機碳含量，以研究過去四萬年以來之沖繩海槽附近海域古海洋�古氣候變化。
MD982195岩心長度33.6公尺，為良好而連續的沈積，含有兩層火山灰沈積物；CSH-2岩心長度7.3公尺。兩根岩心經碳十四定年及浮游有孔蟲氧同位素分析的結果顯示岩心底部分別為末次冰期的氧同位素地層第三階及第二階。
兩根岩心的碳酸鹽含量變化均表現出冰期低、間冰期高的大西洋型碳酸鈣地層 型式，同時陸源有機物質在冰期時大幅增加，並帶來豐富的營養鹽，顯見本區域之沈積特性深受陸源物質的輸入及稀釋作用影響；與其他西太平洋邊緣海的五根岩心之碳酸鈣含量記錄相同。MD982195及CSH-2岩心的有機碳含量比南海及沖繩海槽北段其他的岩心高，除了因為陸源有機碳的輸入量較大之外，岩心位於海水剖面中的氧氣含量最低帶，使得有機碳的保存特別良好應也有關係。基本上有機碳也呈現冰期低間冰期高的現象，可見陸源物質同時對於有機碳含量產生稀釋效應。
古溫度結果呈現明顯的冰期—間冰期的氣候旋回特徵，冰期比間冰期較冷約5℃，本研究結果顯示沖繩海槽北段在冰期—間冰期的溫差較沖繩海槽中段及南海的古溫度溫差為高，顯示冰期—間冰期的溫度差異隨緯度增加而加大。沖繩海槽北段的氣候、溫度關係和北半球有一致的變化，約於17-18 ka達到最低溫；千年尺度的溫度變化量隨著時間而遞減，古溫度在Interstadial 8以前可達2℃，在間冰期僅達0.5℃。由表層海水古溫度記錄及浮游有孔蟲氧同位素數值的比較，可以發現在四萬年以來有數次千年至百年尺度的鹽度變化事件，可能與中國大陸河川的淡水注入量變化有關。記錄並顯示約於800-1500 BP有一低溫事件，溫度可下降達2℃，與新仙女木事件的變化等量；而在約9000 BP有一高溫事件，推論可能有地下熱源的區域性影響，也有可能與黑潮的加強有關，尚待進一步研究。

關鍵字

沖繩海槽 ； 古氣候 ； 古海洋 ； 黑潮 ； 烯酮類不飽和脂肪酸 ； 西太平洋 ； 海水表層溫度 ； 碳酸鈣

並列摘要

Two deep-sea cores(MD982195, 31°38.33’N, 128°56.63’E, water depth 746m; CSH-2, 29°25.0’ N, 126°34’ E, water depth 703m) retrieved from the Okinawa Trough, Western Pacific were studied to unravel the high resolution (65-90 years; 30-140 years, respectively) paleoceanography and paleoclimate records of the Okinawa Trough for the last 40 kyrs. Proxies adopted in this study include alkenone index, carbonate fraction, and organic carbon content.
Core MD982195 (33.6 m long) shows continuous and homogenous sedimentation with two vocanic ash layers. The length of core CSH-2 is 7.3 m. Several 14C datings together with planktonic foraminiferal δ18O stratigraphy show the bottom of the cores are respectively down to stage 3(MD982195) and stage 2(CSH-2) in the last glacial period.
Carbonate stratigraphy in both cores shows the typical Atlantic pattern, which the carbonate content is high in the interglacial period and low in glacial time. Terrestrial organic matter concentration increase in the glacial period giving this area planty of nutrient. Obviously, the sedimentation character of the Okinawa Trough is heavily influenced by terrigeous input and its dilution effect. This trend is the same as those records recorded in other five cores along the western Pacific marginal seas.
The organic carbon content in both cores are quite high compare with other records in the South China Sea and Okinawa Trough. This is probably because of great terrestrial organic matter input and the water depth of the cores falls in the oxygen minimum zone in the water column where always have better preservation in organic matter.
The paleo-sea surface temperature (SST) record which is almost parallel with oxygen isotope curve shows that the SST of glacial time was about 5℃ cooler than that of interglacial time in the Okinawa Trough. Similar to other records of northern hemisphere, the lowest paleo-SST occurred at 17-18 ka in the last glacial maximum. The millennial paleo-SST fluctuation magnitude decrease gradually since the glacial period of time. The magnitude reaches 2℃ before IS. 8 but only 0.5℃ in the interglacial period of time. When we compare the record with oxygen isotope record, we can find there are several salinity change events in the last 40kyrs, which might be related to the input of the fresh water from China continent. The record also shows that there are two special paleo-SST change events. One is a 2℃ cooling event occure at 800-1500BP which is the same magnitude as the Younger Dryas event. The other is a warming event around 9000 BP, which might be the result of the strengthen of the Kuroshio current or regional hydrothermal effect. Both of them needs to be further studied.

並列關鍵字

Okinawa trough ； paleoclimate ； paleoceanography ； kuroshio ； Uk-37 ； Western Pacific ； SST ； carbonate

參考文獻

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