一個在海洋氧同位素第7階的兩千年南歐驟乾事件

Abstract

突發性的氣候變遷往往會引起劇烈的人文與生態變化，為了探討未來歐洲與地中海地區是否有可能出現突發性的氣候變遷，我們選用了古氣候紀錄上類似的時間區段進行研究討論。海洋氧同位素階段第7階 (Marine Isotope Stage 7) 發生於186至245千年以前 (ka)，而其中的substage MIS 7d 則發生於219至233 ka，為冰期終止期-Termination III後一個明顯變冷的階段，而這個冷期的結束使MIS 7內部出現另一明顯的回暖現象並被命名為Termination IIIA。我分析了位於地中海地區義大利西北部Basura 洞穴中的流石岩心標本 (flowstone) BA2 的多項地質紀錄，包含δ18O、δ13C 和微量元素中的Sr/Ca和U/Ca數值紀錄。利用U-Th技術對64個層位樣本定年，並取用了此流石樣本的220-250 ka的年代範圍。δ18O數值範圍為-7.49至-3.97‰，在232.5 ka時δ18O數值為-6.34‰，並在短短1-2千年後的233.6 ka時變為-3.97‰；δ13C數值範圍為-12.69至-4.11‰，在227.8 ka為-10.34‰，在232.8 ka時驟變為-4.11‰，而在234.2 ka後變回-11.10 ‰。碳同位素與Sr/Ca和U/Ca比值在這兩千年同時變化，根據二氧化碳脫氣作用與Prior calcite precipitation (PCP)等機制，可顯示出此為一個驟旱的時期。比較各種機制後，我認為這個事件可能歸因於全球氣候在進入間冰期初期時，仍有部分冰蓋殘留物覆蓋在北歐地區，而暖期導致了這些殘留物的融解，淡水注入北大西洋改變海洋環流迫使西風帶北移，使南歐地區減少來自西風帶的暖濕水氣來源，在234至232 ka進入了兩千年的乾旱時期。根據在地中海地區的MIS 7d 時期的這項研究，在冰期終止期後一萬年內出現的突發乾燥事件，我們不排除未來的全球氣候變化中也可能會出現類似的千年尺度突發乾燥事件。

The unpredictable climate changes always caused dramatic socio-ecological changes. Abrupt climate changes during past interglacial can provide valuable insights to predict future climate. The Marine Isotope Stage (MIS) 7 occurred from 186 to 245 thousand years ago (ka), and the substage MIS 7d occurred from 219 to 233 ka, which was an obvious cooling after the termination of the ice age so-called Termination III. The end of MIS 7d, the cold period in interglacial period, caused another obvious warming phenomenon inside MIS 7 and was named Termination IIIA. In order to evaluate the possible occurrence of a future abrupt drought event in Europe and the Mediterranean realm, this time window was selected in this thesis. I analyzed/developed multi-proxy records of δ18O, δ13C, Sr/Ca, and U/Ca of a flowstone core BA2, covering 220-250 ka from Basura Cave, northern Italy. The chronology was built with U-Th ages on 64 layers. δ18O value range from -7.49 to -3.97‰ and shift from -6.34‰ at 232.5 ka to -3.97‰ at 233.6 ka. δ13C values range from -12.69 to -4.11‰, dramatically increase from -10.34‰ at 227.8 ka to -4.11‰ at 232.8 ka, and then abruptly decreases to -11.10‰ after 234.2 ka in just 2 thousand years. Covariation of Sr/Ca and U/Ca and δ13C data show an abrupt drought event at 234-232 ka through the mechanism of CO2 degassing and Prior calcite precipitation (PCP). Comparing previous published paleoclimate records and model simulations, I speculate this event could be attributable to the melting of the ice-sheet remnants in Northern Europe. The injection of melting water into the North Atlantic changed the ocean circulation and forced the westerly belt to move northward. Based on the observation of this abrupt dry event during MIS 7d, the possibility of drought event(s) in the future could not be excluded.