南西伯利亞一萬四千年氣候變遷：石筍記錄

Abstract

高緯度地區在地球氣候系統中扮演重要的角色，其中南西伯利亞地區更位處北半球最重要的三個氣候系統：西風、極鋒（polar front）及西伯利亞高壓的交會帶，對於古氣候重建有重大意義。然此地過往的研究往往受限於材料特性與解析度，使得此區域的古氣候記錄仍不完備。本研究利用AMS 14C定年，搭配210Pb定年與平均沉積速率法，為Nadezhda洞（52º38.872’N, 88º39.194’E, 550m a.s.l.）的H1與Lunnaya洞（52º40.729 N, 88º43.854 E, 481 m a.s.l.）的L2，兩支釷放射性比值過低（230Th/232Th < 10）的石筍建立年代模式。在分別扣除400年與500年的死碳效應（dead carbon influence, DCI）後，本研究建立了第一個南西伯利亞14,000年以來的高解析度（平均解析度約10年）δ18O與δ13C石筍記錄。本研究的δ18O記錄與總太陽輻照（total solar irradiance, TSI）呈現反相關，顯示阿爾泰地區石筍碳氧同位素並非主要反映氣溫變化，而是反映降水變化。記錄顯示南西伯利亞自14,000年前的冷乾環境逐漸轉暖濕，在約10,000 ~ 7,500年前進入暖濕的全新世最宜期（optimum），最宜期結束後進入長期的冷乾情況，僅在約6,000年前與1,800 ~ 1,400年前較暖濕。記錄中多次出現總太陽輻照減弱同時δ18O與δ13C變重的冷乾事件（7.5 ~ 6.0 ka, 5.0 ~ 4.2 ka, 2.3 ~ 1.7 ka, 1.4 ka及小冰期），顯示阿爾泰地區的氣候主控因子為總太陽輻照。這些由總太陽輻照驅動的冷乾事件，可能是經由總太陽輻照減弱，促使西伯利亞高壓增強所導致。

The climate system of the high latitude region plays an important role in the Earth’s climate system. Paleoclimate study of South Siberia is very important since South Siberia is affected by the three most important climate systems in the northern hemisphere: westerly, polar front, and Siberia High. However, human impact and natural variability of the climatic changes in the Siberian region are not clear due to limited high-resolution records. This study reported two stalagmite records, H1 and L2, from Nadezhda cave (52º38.872’N, 88º39.194’E, 550m a.s.l.) and Lunnaya cave (52º40.729 N, 88º43.854 E, 481 m a.s.l.), respectively. These two stalagmites had low thorium radio-isotopic ratio (230Th/232Th < 10), which led to unsuccessful 230Th/U dating. Instead, this study used AMS 14C dates to construct the age model. To estimate the dead carbon influence (DCI), we used 210Pb dating and the average growth rate method then found the initial 14C age caused by DCI to be 400 (for H1) and 500 (for L2) years. After removing the DCI, we reconstructed the first high-resolution stalagmite δ18O and δ13C record in South Siberia during the past 14 ka. The δ18O record of the study shows a negative correlation with total solar irradiance (TSI), indicating that δ18O and δ13C of stalagmites in the Altai mountain region mainly reflect changes in precipitation (amount effect). The record shows that South Siberia gradually became warm and humid since 14 ka from the cold and dry environment during the last deglacial, and entered the Holocene Optimum during the period of 10 ~ 7.5 ka. After the end of the optimum, the region entered a long term cold and dry condition. Relatively warmer and wetter conditions occurred at about 6 ka and 1.8 ~ 1.4 ka. Cold and dry episodes were 7.5 ~ 6.0 ka, 5.0 ~ 4.2 ka, 2.3 ~ 1.7 ka, 1.4 ka, and the Little Ice Age, during which TSI decreased, and both δ18O and δ13C became heavier repeatedly. Our records indicate that the major climate control factor in the Altai mountain region is TSI. These cold and dry events may be caused by the enhancement of Siberian high driven by decreasing of TSI.