Hydrographic changes of the Kuroshio Current off southeast Taiwan during the past 7500 years
Date Issued
2009
Date
2009
Author(s)
Lai, Yung-Hsiang
Abstract
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.
Subjects
Kuroshio Current
oxygen isotope
oxygen isotope of sea water
Mg/Ca element ratios
East Asia summer monsoon
ITCZ
ENSO
Type
thesis
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