台灣南部晚更新世恆春西臺地腕足動物化石殼體化學成份反映之環境變遷

Abstract

本研究採集恆春地區頭溝剖面及貓鼻頭剖面一系列的腕足動物化石，分析其殼體穩定碳氧同位素及微量元素成份，以提供本地區的海洋性同位素地層記錄，並進一步的解析台灣南部恆春西臺地恆春石灰岩及四溝層的古環境變遷。 我們根據殼體之發光性及不發光殼體中Mn、Fe、Al、Si等微量元素超過最低偵測值的特性來找出受到成岩作用影響的標本點，排除了受到成岩作用影響的數據後，保存良好的殼體穩定碳氧同位素數值均明顯大於保存度不佳之殼體及圍岩之數值，其δ18O值落在-0.61~1.07之間(平均值為0.15 ± 0.31；1σ)，δ13C值落在-0.15~2.62之間(平均值為1.44 ± 0.56)，保存良好殼體的Mg/Ca比值介於2.2~6.4mmol/mol之間(平均值為3.9 ± 0.9mmol/mol)。保存良好單一殼體氧同位素數值呈現週期性振盪變化，反映出腕足化石殼體記錄了季節性之變化，這更加證明腕足動物碳酸鈣殼體並無受到成岩作用的影響。同一腕足動物化石腕殼及莖殼的氧同位素數值差異不大，因此腕殼及莖殼應該均能與生長當時周圍海水達到同位素平衡，同層位Pictothyris picta、Lequeus orbicularis、Kikaithyris hanzawai平均δ18O值亦相近，故這三個種屬氧同位素數值均能反映出當時的海洋環境。 恆春石灰岩平均氧同位素數值比四溝層重約0.4，若該地區的溫度變化不大，則恆春石灰岩較偏向形成於冰期時期之環境狀態。當由恆春石灰岩進入四溝層粉砂質泥岩沉積環境時，顯示著一個海進記錄，但氧同位素數值卻不變，故此時的海進主要應是受到構造運動所造成，冰川效應所導致之全球海水面變化可能沒有或只佔極小部份的影響。四溝層下部至上部的腕足動物殼體δ18O值由0.3逐漸變輕至-0.4，但Mg/Ca比值卻只由4.3變輕至4.0 mmol/mol，若腕足動物Mg/Ca比值可類似有孔蟲應用於Mg/Ca比值溫度方程式，則此0.3 mmol/mol之變化僅相當約0.8℃(δ18O約為0.2)之變化，故四溝層沉積時期應處於一融冰的時期。此外，四溝層中期氧同位素數值有一變大0.3 的變化，若溫度及鹽度效應可略，則數值可能反映出全球海水面下降約30公尺，由於此時期恆春半島還是處於一構造活動較劇烈的區域，因此構造活動抬升加上全球海水面下降的雙重影響，造成此時期於四溝層出現一富含生活於半淡水環境下貝類殼體所組成的生物碎屑石灰岩。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.