| Summary: | 碩士 === 國立臺灣師範大學 === 地球科學系 === 93 === We have analyzed the chemical compositions of brachiopod fossils collected from Toukou and Szekou profiles, Hengchun, Taiwan to construct the Late Pleistocene environment of southern Taiwan. We have collected 124 specimens. Fifty out of these specimens were determined well preserved based on the shell microstructure, luminescent characteristics, minor and trace element compositions, and isotope compositions of samples analyzed.
The δ18O and δ13C values of well preserved samples are between -0.61 and 1.07 (0.15 ± 0.31；1σ；N = 175) and between -0.15 and 2.62 (1.44 ± 0.56), respectively. Because the seasonal fluctuation of δ18O values were observed within the single shell, there was no significant difference in δ18O values between brachial and pedicle valves of the same specimens, and there is no significant difference in δ18O values among different genera of the same stratigraphic interval, we concluded that the samples studied were in oxygen isotope equilibrium with the seawater they lived in.
Sedimentolgic evidence indicate that there was a transgression event from the Hengchun limestone to the overlain Szekou silty mudstone. However, there was no difference in δ18O values crossing this boundary. Therefore, We suggest that this transgression event shown by the facies change, was mainly caused by the tectonic subsidences rather than ice volume effect. We observed the mean δ18O values changed from 0.3 to -0.4 from the bottom of the Szekou Formation to the upper Szekou Formation. The Mg/Ca ratio only showed 0.3 mmol/mol difference (4.3 to 4.0 mmol/mol) between the same interval. Using the Mg/Ca temperature equation established from foraminiferal shells, the 0.3 mmol/mol difference would equivalent to 0.8℃(0.2 in δ18O) change. Thus, the remaining 0.5 difference in δ18O values might indicate decreasing in continental ice volume.
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