碩士 / 國立臺灣大學 / 地質學研究所 / 87 / 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.
| Identifer | oai:union.ndltd.org:TW/087NTU00138020 |
| Date | January 1999 |
| Creators | HsiaoTien Sun, 孫虓天 |
| Contributors | YuKao Chen, Typhoon Lee, 陳于高, 李太楓 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 55 |
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