重新定年拉赫湖的火山爆發

 原文網址：https://www.uni-mainz.de/presse/aktuell/13879_ENG_HTML.php

拉赫湖的爆發時間經過重新審視後為距今13077年前，這對歷史上發生在末次冰期結束時的氣候波動來說提供了重要資訊。

在德國邁森海姆附近的拉赫湖火山爆發堆積物中挖出13000年前的樹木。圖片來源：Olaf Jöris

拉赫湖火山爆發為歐洲中部過去十萬年來最大的火山爆發，地點為德國西部低矮的艾菲爾山脈。這場爆發噴出了大約20立方公里的火山灰，噴發柱的高度據信至少有20公里，相當於西元1991年菲律賓的皮納土玻火山爆發。一組國際團隊透過在噴發過程中被掩埋的樹木殘骸以及更加進步的技術，成功精準地定年這場事件。根據他們的結果，拉赫湖火山爆發的時間為距今13077年前，比過去推測的還早了126年――這讓研究人員對整個北大西洋與歐洲地區的氣候歷史有了新的資訊，也使歐洲氣候的文獻需要進行改寫。「我們現在可以對末次冰期結束時溫度突然下降的事件進行精確的定年，結果是這項資訊跟格陵蘭冰芯中觀察到的現象是同時發生的，」位在德國美茵茲的約翰尼斯˙谷騰堡大學的樹輪年代學家Frederick Reinig博士表示。這組國際研究團隊包括了考古學、氣候學、生態學、放射碳定年以及火山學的專家。研究結果發表在知名的科學期刊《自然》(Nature)。

白楊與樺樹變成木碳的殘骸仍保存至今

拉赫湖火山爆發堆積物中的炭化樹幹，每道年輪仍清晰可見，這讓研究人員可以精確定年噴發的時間。圖片來源：Olaf Jöris

拉赫湖火山爆發是一場影響歐洲大部分地區的天災。雨點般落下的火山灰範圍南邊最遠可以到達義大利北部，東北方則遠至聖彼得堡。緊鄰火山的地區以及附近的萊因河河谷堆積了非常厚的火山灰與浮石，活埋了所有生命。「噴發期間火山碎屑流掩埋了拉赫湖火山周圍的植被。這些火山灰堆積物裡的樹木有一些變成了木炭而保存至今，」Reinig解釋。他說噴發總共歷經了數個禮拜，時間為晚春到初夏，這讓科學家得以精確地定年事件的發生時間。「當年目擊這場爆發的樹木非常稀少，因此也難以找到它們，」研究第一作者Reinig表示。

「火山爆發造成的區域影響已經研究得十分透徹，但一直以來我們都無法肯定確切的發生時間，」共同作者，劍橋大學自然出版集團的Ulf Büntgen教授強調。現在經由被埋藏的白楊與樺樹樣品，總算能將此確定下來。

分析樹輪得出火山爆發的確切年代

在瑞士蘇黎世發現的松樹準化石的切片，瑞士有篇研究測量了這類松樹的放射性碳。研究人員從拉赫湖爆發堆積物中找到樹木之後，測量它們的放射性碳，再參考該文獻進行校正，而精確地定年爆發時間。圖片來源：Daniel Nievergelt

火山堆積物不僅把木頭保存了超過13000年，同時也讓研究人員可以清楚地辨識出每一道樹輪。「樹輪讓我們可以確切得出樣本的年代，」美茵茲大學的教授Jan Esper表示。他們和瑞士比門斯多夫的瑞士聯邦森林、雪與景觀研究所(WSL)，以及諾伊維德的人類行為演化博物館與考古研究中心共同發起的計畫中，企圖分析新發現的樣品以及之前找到的。為了進行分析，蘇黎世聯邦理工學院的離子束物理實驗室從他們研究的樹木中，取出了157道不同的年輪來測量放射性碳含量。接著他們再參考一篇瑞士的年代學文獻來進行校正，而得到相當準確的定年結果。「放射性碳測量技術的持續進步、我們用的校正方法以及十分仔細地處理這些脆弱的樣品，都是讓我們定年結果的誤差可以不到十年的關鍵因素，」蘇黎世聯邦理工學院的Lukas Wacker表示。

修正後的火山爆發定年結果使歐洲的氣候紀錄可以同步，並讓研究人員對大尺度的氣候動力學有更深的理解

樹輪不只可以透露出樹木的年紀。舉例來說，科學家可以從中得出每道樹輪生長時對應的環境，因此可以間接地瞭解當時的氣候。圖片來源：Jan Esper

之前拉赫湖火山爆發公認的發生時間，是根據德國艾菲爾地區的梅爾費爾德低平火山口的沉積物定年結果。而這篇《自然》的新文章則表示這場爆發的時間是在西元1950年以前的13006年，誤差為9年，比之前的結果還早了126年。

這項時間差異對於歐洲氣候紀錄的同步，以及研究人員對於北大西洋和歐洲氣候史的認知來說都有很深遠的影響。拉赫湖火山爆發產生的火山灰覆蓋了歐洲中部與北部的大範圍地區，因此在古環境紀錄中是相當重要的時間標記。由於定年結果有所更新，因此歐洲的氣候紀錄也得調整它們的年表。於此同時，之前這些紀錄和格陵蘭冰芯數據之間的時間差也縮小了。

在目前所屬普遍暖和的時代，即全新世開始前的1300年前左右，末次冰期發生了一段持續大約130年的急遽降溫事件――這段插曲稱作新仙女木期(Younger Dryas)。新的定年結果代表該事件在歐洲中部的開始時間提早了130年，大約是距今12870年前，與北大西洋地區透過格陵蘭的冰芯所得出開始降溫的時間一致。歐洲中部在新仙女木期溫度下降了最多5度，跟之前想的不一樣，這場劇烈降溫事件的發生時間並非各地不同，而是同時發生在整個北大西洋和歐洲中部，Frederick Reinig表示。這組跨領域研究團隊得出的結果不僅精準定出了艾菲爾山脈的拉赫湖火山爆發的時間，火山灰堆積物修正後的年代以及歐洲氣候紀錄連帶得做出的改變，也讓科學家對於整個北大西洋地區的氣候史有了全新理解。

 

Eruption of the Laacher See volcano redated

Revised date of the Laacher See eruption 13,077 years ago provides crucial information about historical climate fluctuations at the end of the last Ice Age

The eruption of the Laacher See volcano in the Eifel, a low mountain range in western Germany, is one of Central Europe's largest eruptions over the past 100,000 years. The eruption ejected around 20 cubic kilometers of tephra and the eruption column is believed to have reached at least 20 kilometers in height, comparable to the Pinatubo eruption in the Philippines in 1991. Technical advances in combination with tree remains buried in the course of the eruption now enabled an international research team to accurately date the event. Accordingly, the eruption of the Laacher See volcano occurred 13,077 years ago and thus 126 years earlier than previously assumed. This sheds new light on the climate history of the entire North Atlantic and European region and requires an adaptation of the European climate archives. "We can now precisely date a drop in temperature at the end of the last glacial period, so that the information coincides with that observed from the Greenland Ice Sheet cores," said Dr. Frederick Reinig, a dendrochronologist at Johannes Gutenberg University in Mainz (JGU). An international research team with experts in archeology, climatology, ecology, radiocarbon dating, and volcanology was involved in this study. The research results were published in the renowned scientific journal Nature.

Charred remains of birch and poplar wood have been preserved to this day

The eruption of the Laacher See volcano was a natural disaster that affected large parts of Europe. The ash rain reached as far as northern Italy in the south and Saint Petersburg in the Northeast. In the immediate vicinity and the neighboring Rhine Valley, mighty deposits of ash and pumice formed, which buried all life beneath them. "During the eruption, pyroclastic flows buried the local vegetation around the Laacher See volcano. The trees were partially charred within the ash deposits and have been preserved to this day," explained Reinig, describing the eruption process that took place over several weeks in late spring to early summer and which now enables scientists to precisely date the event. "These wooden contemporary witnesses are very rare, and they are difficult to recover," said Reinig, first author of the study.

"The regional effects of the volcanic eruption have been well studied. What we have been missing so far is the certainty of when exactly this happened," emphasized Professor Ulf Büntgen, co-author of the Nature publication from the University of Cambridge. This was now determined based on samples from buried birch and poplar trees.

The analysis of tree rings reveals the precise date of the eruption

The volcanic sediments not only preserved the wood for over 13,000 years but also allowed to identify the individual tree rings. "The tree rings enable us to exactly determine the age of the samples," said Professor Jan Esper from Mainz University. In a joint initiative of the Federal Research Institute for Forests, Snow and Landscape WSL in Birmensdorf, Switzerland, together with the Archaeological Research Center and Museum for Human Behavioral Evolution MONREPOS in Neuwied, both newly discovered samples and older finds were analyzed. For this purpose, the Laboratory for Ion Beam Physics at ETH Zurich carried out radiocarbon measurements on 157 individual tree rings of the examined trees. Calibration of these results against a Swiss reference chronology then resulted in the precise dating. "The constant advances in radiocarbon measurement technology and the calibration methods used, as well as the careful handling of the sensitive samples, were the key to establish this dating with an uncertainty of less than ten years," said Lukas Wacker from ETH Zurich.

Revised dating of the volcanic eruption has consequences for the synchronization of European climate archives and the understanding of large-scale climate dynamics

According to the description in Nature, the eruption of the Laacher See volcano took place 13,006 years before 1950, with an uncertainty of nine years. That is 126 years earlier than the generally accepted dating based on sediments in the Meerfelder Maar from the Eifel region in Germany.

This difference has far-reaching consequences for the synchronization of European climate archives and the understanding of North Atlantic and European climate history. Laacher See eruption ashes were widespread over large areas of Central and Northern Europe as a result of the volcanic eruption and represent an important time marker for paleoenvironmental archives. Due to the new dating, the European archives now have to be temporally adapted. At the same time, a previously existing temporal difference to the data from the Greenland ice cores was closed.

This means that the massive cooling at the beginning of the Younger Dryas – i.e., the last Ice Age intermezzo lasting around 1,300 years before the currently prevailing warm phase, the Holocene – also occurred in Central Europe 130 years earlier, around 12,870 years ago respectively. This is in line with the onset of the cooling in the North Atlantic region identified in ice cores from Greenland. During the Younger Dryas period, temperatures in Central Europe fell by up to 5 degrees Celsius. This strong cooling did not take place time transgressively, as previously thought, but rather synchronously over the entire North Atlantic and Central European region," said Frederick Reinig. The results of the interdisciplinary research team not only set a precise date for the eruption of the Laacher See in the Eifel. The revised age of the ash deposits and the associated shift in the European climate archives now sheds new light on the climate history of the entire North Atlantic region.

原始論文：Frederick Reinig, Lukas Wacker, Olaf Jöris, Clive Oppenheimer, Giulia Guidobaldi, Daniel Nievergelt, Florian Adolphi, Paolo Cherubini, Stefan Engels, Jan Esper, Alexander Land, Christine Lane, Hardy Pfanz, Sabine Remmele, Michael Sigl, Adam Sookdeo, Ulf Büntgen. Precise date for the Laacher See eruption synchronizes the Younger Dryas. Nature, 2021; 595 (7865): 66 DOI: 10.1038/s41586-021-03608-x

引用自：Johannes Gutenberg Universitaet Mainz. "Eruption of the Laacher See volcano redated."