臺灣博碩士論文加值系統

海水表層溫度(SST)為各種氣候模式中最重要的邊界條件，因為它會直接或間接影響其它環境因子的變化。熱帶至亞熱帶大洋因能提供大量的水氣，常扮演著氣候變遷的發動機，故其海水表層溫度及水文狀況變化記錄之建立，對於了解氣候變化機制及確認氣候預測模式之正確性有很大的幫助。廣泛分布於熱帶及亞熱帶的珊瑚於形成骨骼時，能記錄當時周圍海水的化學成份，並能反映環境因子的變化，且其具有生長時間長、高解析度及良好的年代控制等優點，故為從事古氣候變遷研究的一項利器。過去數十年來，由花粉、冰芯等陸上研究，顯示全新世大暖期(Holocene Maximum)的存在，故本研究乃採集恆春南灣地區之全新世化石微孔珊瑚(Porites sp., 6.2kyr BP)，希望藉此對當時海表水溫及雨量做定量的分析，以期對此一時期之南台灣氣候有更進一步的了解，作為未來全球人造溫室效應預測之參考。
選擇南灣地區作為採樣點的原因有三：因前人研究已建立"鍶鈣比值-海表水溫"溫度計及雨量回推模式，並具長期之海水氧同位素記錄。所採集到之標本具十七年之連續生長，經沿最大生長軸進行精密採樣後(年平均生長率約16mm，一年取十二樣本)，分別利用熱離子源質譜儀(TIMS)及氣體比值型質譜儀(IRMS)分析珊瑚骨骼之鍶鈣元素比值及氧同位素值。由過去的研究顯示，珊瑚骨骼之鍶鈣比值(Sr/Ca)主要受控於海表水溫(SST)，氧同位素值則由海表水溫及海水本身氧同位素值之變化控制。故假設海水的鍶鈣比值不變，藉由分析化石珊瑚骨骼鍶鈣比值，再利用由現生珊瑚所建立之"鍶鈣比值-海表水溫"溫度計，即可重建過去之海表水溫，再者，測量珊瑚骨骼之氧同位素(d18Ocoral)，一同代入(1)式，即可得到海水的氧同位素值(d18Osea)，藉此可推之當時的水文狀況。
d 18Ocoral ─d 18Osea = -0.409 ─ 0.1886 × SST (1)
研究的結果顯示，南灣地區於全新世大暖期時，其表層海水溫度較現今低，與一般陸上資料所得較現今溫暖的狀態相反，可能是因為季風增強的緣故。此外，因日照量增加及夏季季風增強，導致蒸發旺盛，且增強的季風帶給陸地豐沛的雨量，除此之外，此一時期的四季溫差大，季節性明顯，年雨量約為現今的兩倍，然而，年雨量之變化極大，且乾溼季之分布與現今不同。

In most climate models, sea surface temperature(SST) is a crucial parameter because of its linkage with other climate variables. It is now becoming apparent that West Pacific plays a key role on globe climate, because it contains the largest mass of warm ocean water. Thus it''s useful for validating climate models and for understanding the causes of past climate fluctuation to construct the records of sea surface temperature and hydrology change of the West Pacific. Corals are widely distributed in the tropical and subtropical shallow sea, secrete skeletons, which incorporates some chemical components of surrounding area, and these can reflect the changes of environmental factors. Besides, coral also has some advantages of living for a long duration, high temporal resolution, and well age control, so that it becomes an excellent proxy for paleoclimate reconstruction. In the past decades, according to many terrestrial studies like pollen, ice core, and lake level, it appears that there exits a period ''''Holocene Maximum'''' (6~8kyr BP) warmer and moister than of today in most areas of the world. We collected Holocene Porites coral exposed in the uplifted coral terrace from Nanwan, Southern Taiwan. By quantitative analysis of the SST and precipitation, we expect to understand the paleoclimate of that period.
There are three reasons for us to choice Nanwan as our sampling site: previous study had already established the Sr/Ca-SST thermometer and precipitation reconstruction model, and we have long time sea water d18O record there. The sample we collected provides seventeen years high resolution(9~14 subsamples per year, average growth rate is 16 mm/y). After sampling along the maximum growth direction, high precision determination of Sr/Ca elemental ratios were carried out with a multi-collector mass spectrometer, VG354, using mixed 42Ca-44Ca-84Sr triple spike(a long term reproducibility of ±0.440/00, 2s), and d18O ratios were measured with a isotopic ratio mass spectrometer, Finnigan Delta Plus, and automatic pretreatment system, Kiel Device(a long term reproducibility of ±0.250/00, 2s).
Previous studies show that the ratio of incorporation of Sr to Ca is controlled by two factors: the Sr/Ca activity ratio of the ocean water , and the Sr/Ca distribution coefficient between aragonite and seawater, which depends on the temperature of the seawater in which the coral grew. Because of the long residence times of Sr and Ca in the oceans, it is probable that the seawater Sr/Ca ratio has remained essentially constant over time scale of about 105 years. Thus, the Sr/Ca ratio of corals is a potential monitor of ocean temperature on the time scale. On the other hand, d18O is also controlled by two factors: the temperature of the surrounding seawater in which corals secreted their skeletons, and the fluctuation in the seawater d18O, which varies as a function of the volume of the planetary ice caps, or, in the case of sea surface water, can be modified by rainfall or evaporation effects. For our data explanation, the effect due to the change of the volume of the planetary ice caps can be ignored, because the volume of the ice caps during Holocene Maximum(HM) is supposed as the same as today.
Using the Sr/Ca-SST thermometer(eq. 1) established from living coral, we can reconstruct paleo-SST by analyzing the Sr/Ca ratios of coral skeletons. Besides, precise measurements of Sr/Ca ratios and d18O values in coralline aragonite should make it possible to determine uniquely the past oxygen isotopic composition of seawater, by removal of the temperature component of the coral d18O signal(eq. 2).
Sr/CaN(mmol/mol) ＝ 10.286 － 0.0514 × SST(oC) (1)
d18Ocoral(0/00) －d18Osea ＝ -0.409 － 0.1886 × SST (2)
The results suggested that SST was lower than that of today at Nanwan during Holocene Maximum; it could be due to enhanced Monsoon, and was contrast to the results obtained from the land, which suggested it was warmer than today during that time.
The enhanced solar radiation and summer monsoon were respond for enhanced evaporation near shore and increased precipitation on land. Besides, the annual temperature difference was larger and the precipitation could be two times than those of today, and the variation of annual precipitation is somewhat high.
According to the coral records of West Pacific, it is suggested that there was an enrichment in seawater 18O relative to modern values, which implied the enhanced evaporation in tropical. These could cause strengthen atmospheric circulation, then increase the seawater d18O difference between tropical and polar region.

第一章 緒論 1
1.1 研究動機與目的 1
1.2 前人研究 4
1.2.1全新世大暖期 4
1.2.2古氣候指標 9
1.3 珊瑚骨骼中鍶鈣元素比值法的應用 10
1.4 珊瑚骨骼中氧同位素法的應用 12
第二章 研究方法 14
2.1原理簡介 14
2.1.1氧同位素 14
2.1.1.1同位素基本原理 14
2.1.1.2同位素分異 14
2.1.1.3 d值 15
2.1.1.4氧同位素在珊瑚上之應用原理 16
2.1.2鍶鈣元素比值 17
2.1.2.1同位素稀釋法的原理 17
2.1.2.2珊瑚鍶鈣比值溫度計 20
2.2樣本採集 22
2.3實驗方法 22
2.3.1前處理 22
2.3.2 14C-Dating 24
2.3.3鍶鈣比值分析 24
2.3.4氧同位素分析 25
第三章 結果 28
3.1珊瑚生長紋變化 28
3.2定年結果 30
3.3鍶鈣比值結果 30
3.4珊瑚骨骼氧同位素變化 35
第四章 討論 47
4.1 南灣地區6250 yr BP之表層海水溫度 47
4.2南灣地區6250 yr BP之海水氧同位素值 50
4.3全新世大暖期之氣候變化 50
4.4全新世大暖期以來之表層海水溫度變化 52
第五章 結論 55
附錄
A. 參考書目
B. 分析數據

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