臺灣南部珊瑚骨骼中之氧同位素和鋇鈣比紀錄及其環境解釋

Oxygen Isotope and Ba/Ca Ratio Records in Coral Skeletons and their Environmental Explanations from Southern Taiwan

姜宏偉(Hong-Wei Chiang)

指導教授 ： 陳于高

共同指導教授 ： 沈川洲(Chuan-Chou Shen)

國立臺灣大學/理學院/地質科學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.02606

摘要

珊瑚是理想的研究環境變遷的海洋材料之一，原因即在於其骨骼中的諸多微量元素和氧同位素組成，會與碳酸鈣形成時之海水物理與化學環境相關。本研究乃採取了台灣南部南灣地區的兩株微孔珊瑚，用以測試氧同位素值於傳統海表溫度與鹽度之研究外，是否有其他的可能性，以及鋇鈣比值在不同時間尺度下的環境意義。
首先，在氧同位素的研究上，從1992到2003年，我們發現當聖嬰現象發生時，氧同位素與海表溫迴歸線的斜率會較正常時平緩(-0.12 ± 0.04‰ °C-1 vs. -0.21 ± 0.04‰ °C-1)，而根據台灣南部至菲律賓附近之實際洋流觀測，我們推測其可能原因在於，聖嬰年夏季時，黑潮往北流的強度會較正常年之夏季時弱，部分的黑潮水因此異常地在呂宋海峽往西侵入南海北部，並使得當地之夏季海水氧同位素值變重，進而影響了氧同位素與海表溫迴歸線的斜率，此外，依據前人的研究，我們認為本紀錄在1998年發生的變化，應是與聖嬰現象於東亞夏季季風上之影響，受到太平洋擺盪(Pacific decadal oscillation, PDO)的調控有關。
第二部份關於珊瑚骨骼中鋇鈣比值的研究，我們發現其在月的時間尺度上，會呈現出明顯的雙峰特徵，第一個高鋇鈣比值在時間上早於主要的雨季，因此與一般認為的由降雨帶來陸源鋇無關，在仔細討論各種成因後，我們認為其應是與早春時之藍綠藻繁盛(Cyanophyta bloom)有關。第二個高鋇鈣比值則與降雨的訊號同時，兩者之間的關係可以線性函數描述之，但此一關係在1999時發生改變(1987-1998, Ba/Ca=3.02+0.0024*P；1999-2003, Ba/Ca=2.26+0.0011*P)。同時，在年之時間尺度上，珊瑚骨骼中的鋇鈣比背景值亦從3.0-3.6*10^-6 mol/mol，降至2*10^-6 mol/mol，我們認為上述之改變與南灣地區兩座汙水處理廠，分別從1999與2000年開始運轉有關，其明顯有效地降低了南灣內的鋇鈣比值。綜上所述，台灣南部地區之珊瑚骨骼中的鋇鈣比值的確可直接用作古降雨量的代用指標。

關鍵字

微孔珊瑚 ； 氧同位素 ； 鋇鈣比 ； 降雨量 ； 南灣

並列摘要

Coral has been one of the best marine archives to study climate change from weekly to decadal timescales. The abundant trace elements and stable oxygen isotopes in massive coral skeletons are affected by the environment condition prevailing at the time of skeleton deposition. In this study two coral heads were collected from the Nanwan Bay, southern Taiwan to test the capabilities of traditional d18O values and recent-developed Ba/Ca ratios, with seasonal to sub-decadal timescales. Firstly, during El Niño episodes the skeletal d18O–SST correlation showed a gentler slope of -0.12 ± 0.04‰ °C-1 rather than the value of -0.21 ± 0.04‰ °C-1 during normal year. The difference was likely caused by enhanced penetration of Kuroshio Current waters into the South China Sea during El Niño summers. Secondly, an annually distinct double-peak feature of Ba/Ca ratios in coral was identified. The earlier peak is not concordant with the time of major precipitation but regularly occurred before the Sr/Ca value reached the minimum. It was resulted from a biotic source and possibly related to Cyanophyta bloom in the early-spring. The later peak happened at the time of raining seasons. Two different linear correlations are presented between skeletal Ba/Ca and accumulated precipitation in July-October. Before 1999, the relationship was given by Ba/Ca=3.02+0.0024*P; while after 1999, the relationship is given by Ba/Ca=2.26+0.0011*P. The decreased Ba/Ca baseline from the end of 1999 to 2001 is believed to relate to the operating of two sewage treatment plants since 1999 and 2000. It also can explain the gentler slope value of Ba/Ca-precipitation regression and the decreased amplitude of Ba/Ca maxima after 1999. Thus, the skeletal Ba/Ca ratios can be applied as a quantitative proxy of the paleo-precipitation in southern Taiwan.

並列關鍵字

Porites corals ； stable Oxygen isotopes ； Ba/Ca ratios ； precipitation ； Nanwan Bay

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