地中海地區在47萬到36萬年前的水文氣候變遷

Abstract

深海氧同位素 (Marine isotope stage, MIS) 第11階是過去五十萬年以來地球繞日軌道參數與全新世最相似的間冰期。因此，研究區域性MIS 11 的氣候變遷模式有助於了解位於副熱帶及西風帶氣候邊界的地中海地區未來的氣候變化趨勢。然而，目前鮮少有陸上紀錄研究地中海在 MIS 11 的水文氣候變化。本研究材料為義大利西北部的巴蘇拉洞穴 (Basura Cave) 碳酸鹽質沉積岩—BA7-1流石，利用精準的230Th 定年技術建立年代模式，並以碳氧同位素、微量元素分析，重建 MIS 11 的相對降水變化。流石的碳氧同位素在 MIS 11 的變化趨勢與地中海的浮游有孔蟲氧同位素相似，可以反映區域的氣候變遷。同步變動的碳同位素值、鎂鈣及鍶鈣比值則受控於前方解石沉澱 (prior calcite precipitation)，可作為乾燥氣候的指標。本研究重建的降水結果顯示，本研究區域的降水變化與軌道的歲差週期 (precession) 有關。當北半球夏至位於近日點時，此地區越濕潤。歲差週期在未來的數千年內將呈現減少的趨勢，區域的日照量相對增加，在只考慮此因素的情況下，未來數千年，地中海北區的冬季降水可能將持續增加。

Marine Isotope Stage (MIS) 11 is an interglacial period with similar orbital configurations to Holocene in the past 500 thousand years. The regional hydroclimate in MIS 11 can help us understand the current and future trends in the Mediterranean region, located between two essential climate boundaries, subtropical belt and westerlies belt. Yet, few independent-dated terrestrial proxy records in the Mediterranean during MIS 11 was reported. Here, I provide the 230Th dated speleothem-inferred millennial-scale hydroclimate variability during MIS 11 by using C/O isotopes and trace elemental compositions of a flowstone core, BA7-1, from Basura Cave in northwestern Italy. Speleothem BA7-1 δ18O and δ13C series are similar to the Mediterranean surface planktic foraminiferal δ18O records. The covariation of δ13C, Mg/Ca, and Sr/Ca indicates dry/wet conditions controlled by prior calcite precipitation. The inferred precipitation record is anti-phase with precessional cycle. The less the precession increases the northern hemisphere insolation and would lead to a wet condition. Precession will have a decreasing trend in the next thousands of years. If considering only precessional forcing, the precipitation over the Northern Mediterranean will most likely keep increasing in the coming millennia.