臺灣東南外海過去七千五百年來的黑潮水文變化

Abstract

本研究分析臺灣東部外海綠島北方的ORI715-21岩芯過去7.5 ka的浮游有孔蟲氧碳同位素紀錄 (δ18Oc) 與鎂鈣元素比 (Mg/Ca ratio)，進而重建古海表溫 (Sea surface Temperature, SSTs) 及古海水鹽度 (Sea Surface Salinity, SSS) 。ORI715-21岩芯平均沈積速率約為25 cm/kyr，δ18Oc之數值在7.5 ka以來呈現變輕且振盪加劇的趨勢，而重建之浮游有孔蟲鎂鈣比紀錄顯示在7.5 ka以來的趨勢大致呈穩定狀態。 將ORI715-21岩芯鎂鈣元素比所轉換的SST與西太平洋暖池、沖繩海槽 (MD01-2403) 岩芯的SSTs相對比，發現三者的SSTs長期趨勢相似，皆呈現穩定狀態。另與臺灣南投頭社盆地之全新世花粉紀錄所呈現之氣候序列相比，與花粉紀錄所得之森林相轉換暗示之冷暖變化無同時性，但與GISP2冰芯的10Be含量變化曲線有同時性的現象，另將台灣東岸外海及南沖繩海槽兩根岩芯與本州島中南部海域的緯向溫度差值作對比，發現黑潮主流在該海域的擺盪現象與黑潮上游區的溫度變化有連動關係，當黑潮上游區的溫度較高，在日本南部海域的黑潮易於向北擺盪。 在黑潮上游區ORI715-21岩芯經由δ18Oc與鎂鈣元素比所重建的水體氧同位素值δ18OW-I變化與南沖繩海槽區的MD01-2403岩芯的δ18OW-I有相似的趨勢，兩者的振幅自8.0 ka至1.3 ka大體上相近，皆呈現自8.0 ka以來逐漸變輕，但卻與貴州董哥洞的石筍氧同位素值成反向關係，表示東亞地區東亞夏季季風的強度減弱，但是台灣東岸及南沖繩海槽的海水反而變淡，顯示東亞夏季季風的變化並無影響到熱帶太平洋開放海區。但從 1.3 ka至今的水體氧同位素變化則與董哥洞的石筍氧同位素值呈同向變化，顯示東亞夏季季風小幅增強，表層海水氧同素值也相應變淡。 ORI715-21岩芯的水體氧同位素之振幅與MD01-2403岩芯的振幅各約0.8‰ 與0.6‰ 左右，顯示兩者分別皆有明顯的降雨量增加的現象，其值約377±217 mm與277±217 mm，推測應是於東亞夏季季風期間在黑潮上游區與南沖繩海槽區域降水增加所致。兩者之差值亦可視為兩地之間同一黑潮水團受到不同淡水量混合之影響，數值相差越小暗示流黑潮流速越快，期間所受外來水的混合效果越低，故可利用ORI715-21岩芯與MD01-2403岩芯之δ18OW-I之間的差值絕對值當作黑潮流速變化的代用指標，結果顯示黑潮流速變動有百年尺度的週期，而在ITCZ北移、ENSO不顯著的全新世大暖期時，黑潮流速有加快的趨勢，此外，在過去4.0 ka以來，ITCZ南移，ENSO顯著時，黑潮流速有減弱的趨勢。在6.1∼5.9 ka頭社盆地所記錄之暖期事件中，δ18OW-I絕對差值有明顯減小的情形發生，可能暗示黑潮流速加快時使得台灣中部更為暖濕。

We analyzed oxygen, carbon isotopes and Mg/Ca element ratios of planktonic foraminifera, Globigerinoides ruber, in the ORI715-21 marine core off southeast Taiwan during the past 7,500 years and reconstructed the sea surface temperatures (SSTs) and sea surface salinity (SSS). The average sedimentation rate of the core was about 25 cm/kyr. The values of δ18Oc of G. ruber displayed a long-term gradual depletion trend with centennial-scale fluctuations and the reconstructed records of Mg/Ca ratios of G. ruber exhibited the approximately stable trend during the past 7.5 ka. The Mg/Ca-derived SSTs of the core were compared with that of MD98-2181 in the west Pacific warm pool (WPWP) , and MD01-2403 in the southern Okinawa Trough. We found that the long-term SSTs trends are similar in showing stable condition. However, in comparison with the Holocene climate series in Toushe Peat Bog, in central Taiwan, the Mg/Ca-derived SSTs are not closely related to the forest biome shifts reflected from the pollen spectra, yet, the SST records are synchronous with the changes in 10Be flux in the GISP2 ice core. The SST (℃) in the ORI715-21 marine core compared with the latitudinal ΔT (℃) off south Honshu, Japan. The two records are coupled, indicating that the higher temperature in the upper reach area of Kuroshio Current off east Taiwan, the stronger the northern migration of the Kuroshio Extension off the south Japan. The δ18OW-I (calculated from the δ18Oc and Mg/Ca ratio) of the ORI715-21 shows a similar trend with that of MD01-2403, but in opposite direction to the stalagmite δ18O of the Dongge Cave during 8 - 1.3 ka, suggesting that the salinity of the sea-surface waters off east Taiwan had become fresher despite the East Asian monsoon had become weaker. However, during the last 1,300 years, they all exhibited the same direction, namely, the δ18OW-I has become more depleted while the East Asian monsoon intensified. Magnitudes of the δ18OW-I fluctuations shown by the ORI715-21 core and MD01-2403 core are ~0.8‰ and ~0.6‰, respectively, during the past 1,300 years, signifying a salinity difference caused by local fresh-water input in the season of the East Asian summer monsoon equivalent to a 377±217 mm precipitation off southeastern Taiwan, and 277±217 mm precipitation in the south Okinawa Trough. Assuming that the δ18OW-I between these two sites were affected by the amount of precipitation over the east Taiwan region, the slower the Kuroshio, the larger the gradient is expected. Therefore, the δ18OW-I gradient can be used as a measure of the speed of the Kuroshio Current off eastern Taiwan. The result shows that the Kuroshio flew faster during the mid-Holocene when the ITCZ shifted northward and the ENSO was less frequent. The trend has revered during the last 4,000 years when the ENSO became more frequent and the Kuroshio’s speed also increased. Finally, the warm event recorded by the pollen in Toushe Peat Bog at 6.1-5.9 ka, is coincided with a small value of the δ18OW-I gradient, implying that the faster the Kuroshio Current flew, the warmer and wetter the central Taiwan was.