由中國重慶羊口洞石筍氧同位素記錄探討過去12萬至21萬年間亞洲季風強度的變化

Asian monsoon variability during 124-206 ka inferred from oxygen isotope records of stalagmites from Yangkou Cave, Chongqing, China

黃莉容(Li-Jung Huang)

指導教授 ： 沈川洲

國立臺灣大學/理學院/地質科學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.03083

摘要

透過分析中國洞穴石筍氧同位素記錄，前人研究已將過去38萬年來的亞洲季風強弱變化重建，並顯示在天文軌道週期的時間尺度上，亞洲季風強度主要跟隨北半球夏季日照量而改變。然而，在十萬年以前的中國洞穴石筍氧同位素記錄幾乎僅來自於湖北省的三寶洞，此單一的數據可能對於亞洲季風演化過程的解釋造成些許不確定。透過分析來自重慶市的羊口洞的五支鈾濃度介於2-16 ppm的高鈾濃度石筍，我們利用其氧同位素和定年紀錄，重建了12.4萬至20.6萬年前的亞洲季風變化。此研究結果支持三寶洞在此時間段的亞洲季風研究成果，包含深海氧同位素階層(MIS) 6.3、6.5與7.1的強季風時期，以及MIS 6.2、6.4與7.0的弱季風時期。中國洞穴石筍氧同位素變化的一致性說明過去亞洲季風興衰支配整個中國大陸地區，在天文軌道週期的時間尺度上主要跟隨北半球夏季日照量而改變，並且與全球大氣中氧氣的氧同位素變化有相關性存在。透過中國洞穴石筍氧同位素記錄與太平洋海表溫資料的對比，我們的研究提出亞洲季風的發展可能部分受控於沃克環流的強弱。前人研究顯示在MIS 6時期，中國洞穴石筍氧同位素記錄與黃土磁感率在季風興衰的分析上有所衝突，我們則認為此一現象是中國夏季季風系統與西北界線的遷移所造成。利用高精度的鈾釷定年技術，在95%信賴區間（2-sigma）內，我們所分析的羊口洞的石筍定年誤差可達到僅±450年，可讓我們更加確信在MIS 6-7時期亞洲季風事件的持續時間與確切的發生時間，進一步理解此時期亞洲季風的強弱變化與氣候上的影響因素。

關鍵字

亞洲季風 ； 石筍 ； 氧同位素 ； 鈾釷定年

並列摘要

Asian monsoon (AM) variation over past 380 kyrs has been reconstructed using stalagmite oxygen isotope records from caves in China, showing that AM intensity primarily follows Northern Hemisphere summer insolation on orbital timescales. However, those 100s-kyr records were built with stalagmites mainly from Sanbao Cave only, which could bring uncertainty in interpreting long-term AM evolution. Oxygen isotope records of stalagmites with high uranium levels of 2-16 ppm, collected from Yangkou Cave were used to reconstruct AM record from 124 to 206 kyr BP. Our results show that the Yangkou stalagmite-inferred AM variation superimposes on Sanbao record, supporting the strong AM intervals at marine isotope stage (MIS) 6.3, 6.5, and 7.1 and weak AM intervals at MIS 6.2, 6.4, and 7.0. This consistency confirms that the AM events are dominant in the entire mainland and primarily follow Northern Hemisphere summer insolation and relate to the atmospheric "δ" (_^"18" )"O" variation on orbital timescales. Combined with Pacific thermal conditions, our study suggests that AM evolution could be partially controlled by the Walker Circulation. The conflict between stalagmite-inferred and loess-inferred Asian monsoon records during MIS 6 might be caused by the migration of Asian summer monsoon boundary. Advantages of high precision absolute U/Th dates with 2-sigma error as low as only ±450 yrs allow us to precisely determine the event durations and timings and to understand the AM variability and climatic forcings during MIS 6-7.

並列關鍵字

Asian monsoon ； Stalagmite ； Oxygen isotope ； U-Th Dating

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