澎湖的全新世海洋碳儲庫年齡紀錄

Abstract

碳-14定年法廣泛應用於地質、考古、海洋與古氣候等自然科學領域中。當利用此方法測定如珊瑚、貝殼或有孔蟲等海洋生物樣本時，由於大氣與海洋之間存在碳-14含量差異，因此需經過修正才可以取得較準確的年代，此修正值即為碳儲庫年齡。碳儲庫年齡源自於碳在大氣與海洋之間的緩慢交換，經海水混合、天水注入及硬水效應等多種海洋因素影響，使得碳儲庫年齡會隨著時空而發生變化。對於碳-14年代之修正，目前需仰賴Marine20所提供之全球碳儲庫年齡，並利用區域碳儲庫年齡之修正值∆R作為輔助，因而凸顯了區域碳儲庫年齡紀錄的重要性。過去在臺灣東岸及南方的呂宋島已有建立近代區域碳儲庫年齡數值，然而在臺灣西側之臺灣海峽中至今仍缺少碳儲庫年齡之數據，對於此區域碳儲庫年齡隨時間之變化亦缺乏較完整紀錄。因此，本研究採集來自於臺灣海峽中的澎湖之全新世珊瑚樣本，比對其碳-14與釷-230定年結果，計算出臺灣海峽中之碳儲庫年齡。 本研究年代範圍橫跨過去6700年，其中依據碳儲庫年齡數值特徵可分為兩時期，分別為自6.7至5.7 cal kyr BP之高變動期，平均值為407 ± 98 14C yr，與5.1至0 cal kyr BP之較穩定時期，平均值為352 ± 61 14C yr。本研究亦觀察到1.4與3.5 cal kyr BP為兩期低碳儲庫年齡時期，其數值最低可至220-230 14C yr。利用此紀錄中的變化特徵，本研究比對臺灣周遭和範圍較廣之南海、南太平洋等海域中之碳儲庫年齡紀錄，發現澎湖群島周圍海水於全新世之碳儲庫年齡變化趨勢與高低變化，皆與南海和南太平洋類似，但在4.0至2.5 cal kyr BP之間與南海略有差異。此趨勢指示臺灣海峽與南海之碳儲庫年齡主要受到太平洋的海洋環境演變之影響，且構成南海於4.0至2.5 cal kyr BP之間的高碳儲庫年齡的機制影響範圍侷限。最後本研究計算出臺灣海峽6700年以來之平均∆R值為 -146 ± 61 14C yr，可供臺灣周遭之海洋樣本碳-14定年校正時使用，以提升此區碳-14定年結果之準確度。

Marine radiocarbon reservoir age is a necessary correction for the 14C dating of marine samples such as corals, mollusks, and foraminifera. Reservoir age is caused by the long residence time of carbon in the marine reservoir, which causes a difference between marine 14C ages and contemporary terrestrial 14C ages that varies both temporally and spatially. Currently, global reservoir ages can be corrected using the Marine20 calibration curve. Local reservoir ages are accounted for through additional regional corrections known as ΔR. Although reservoir age variations in the Western Pacific and its marginal seas have been discussed in several previous studies, there is currently a lack of reservoir age data in the Taiwan Strait, an important waterway connecting the South China Sea and the East China Sea. In this study, we presented a series of reservoir ages in the Taiwan Strait since 6.7 cal kyr BP. Reservoir ages were calculated using the paired 230Th and 14C ages of Holocene coral samples collected from the Penghu Islands. The record showed a period of higher and more variable reservoir ages from 6.7 to 5.7 cal kyr BP, with a mean value of 407 ± 98 14C yr, and a period of lower value and variability from 5.1 to 0 cal kyr BP, with a mean of 352 ± 61 14C yr. We also identified two periods of low reservoir ages with minimum values reaching 220-230 14C yr around 3.5 and 1.4 cal kyr BP. The evaluation of the local distribution of reservoir ages around Penghu and comparisons to reservoir ages reported from previous studies in the South China Sea and the South Equatorial Pacific showed good correlation of reservoir age trends. This indicated a strong influence of Pacific-sourced waters around the Penghu Islands and in the South China Sea. From differences between Penghu and the South China Sea records, we proposed that the high reservoir age variation observed in the South China Sea during 4.0 to 2.5 cal kyr BP is likely spatially restricted. Lastly, based on the results from the Penghu Islands, we determined that a mean ∆R value of -146 ± 61 14C yr is suitable for the calibration of marine 14C dates near Taiwan during the Holocene.