南中國海現代硨磲貝14C定年及地球化學紀錄

Abstract

本研究對來自南中國海西南部靠近菲律賓海域採集得的硨磲貝(Tridacna gigas)樣本進行14C定年和地球化學研究，旨在構建幾十年來的海洋核爆碳曲線以及海溫變化趨勢。所使用的硨磲貝殼體長約72 cm、高約37 cm、厚約12 cm。殼體可明顯區分為透光的珍珠層(內層)與不透光的棱柱層(外層)，內層經XRD分析結果顯示皆為霰石組成。沿著內層紋層生長方向共計99個的AMS 14C 定年結果顯示皆含核爆碳，現代碳比例介於109.11% 到115.74%之間，D14C範圍介於91.1‰ 到157.4‰之間，利用該樣本的核爆碳曲線對比南中國海珊瑚Δ14C數據可獲得硨磲生長年代大約從西元1970到1985年，這是首次利用14C核爆曲線估算硨磲生長年齡的研究。另外，該核爆碳曲線於反聖嬰年間(約1976年前後) D14C值呈現較低的結果，推測是因為在反聖嬰時期，由呂宋海峽進入南中國海表層的黑潮支流減少，當黑潮流帶來的暖水減少，會使得南海表層垂直溫差減小、南中國海逆時針環流減弱和上浮水動力減弱，使得南中國海西南部海域的湧升流增強，並帶來D14C值較低的深層海水。210Pb定年結果則顯示硨磲貝中的210Pb活度太低，無法用於定年。碳、氧同位素分析結果顯示其值分別介於1.10‰ 至 3.29‰ 與 -3.15‰ 至 -1.34‰ 之間，雖然相較於前人研究其季節性變化較弱，但氧同位素仍可顯示年際變化，從而更為精確的指示硨磲生長年代。δ18O值於1982-1983的強聖嬰年間呈現明顯偏負結果，推測為高水溫與強降雨所致。Sr/Ca比範圍則介於2.09 至 0.947mmol/mol間，無明顯季節性變化。綜合以上研究，可以為我們建立利用硨磲貝紀錄研究近現代和過去氣候變化及海水同位素組成改變的方法，未來或許可用於重建熱帶海洋中水團的運動和水氣交換作用。

A Tridacna gigas shell collected from southwestern area of South China Sea (SCS) was used for studying “nuclear bomb 14C activity” in surface ocean water and geochemical proxies of paleoceanography. The shell is about 30 kg in weight with 72 cm in length, 37 cm in width and maximum thickness of 12 cm. Along a sampling track in the relatively transparent area (Inner layer), a total of 99 samples have been taken for AMS 14C dating. Up to date, all the AMS dates show that all samples contain nuclear bomb signal (percentage of modern carbon (pMC) ranging from 109.11% to 115.74%). The D14C range is between 91.1 ‰ and 157.4 ‰. Using the nuclear bomb carbon curve to compare with the Δ14C data of corals in the South China Sea, the growth period of the Tridacna gigas is ca. 1970-1985 C.E.. This is the first study of using 14C nuclear bomb curve to estimate the growth age of a giant clam. The nuclear bomb carbon curve showed a lower D14C value during the La Niña Year (1976 C.E.). It hypotheses that during La Niña period the surface water from NW equatorial Pacific entering South China Sea via Luzon Strait was reduced, so that upwelling in the SW area of SCS was increased which in term provided lower D14C water. The 210Pb dating on the shell shows very low 210Pb activity, so that no chronology information from the 210Pb dating. The carbon and oxygen isotope analytical results range from 1.104 ‰ to 3.292 ‰ and from -3.154 ‰ to -1.336 ‰, respectively. Although the seasonal variation of stable isotope result is weak, but the δ18O record can be identified by annual variation, which helps us in chronology improvement of the giant clam. The δ18O value showed more negatively during the strong El Niño year from 1982 to 1983. The Sr/Ca ratio of Tridacna gigas ranges from 2.09 to 0.947 mmol/mol, with no obvious seasonal changes. Based on the above research results, we can establish a method by using the giant clam record to study modern and past climate changes and the seawater isotopic composition, which can be used in the future to reconstruct the movement of water masses and the exchange of water and air in tropical oceans