地球冷卻是由火山爆發造成，而非隕石撞擊

原文網址：https://today.tamu.edu/2020/07/31/texas-am-study-cooling-of-earth-caused-by-eruptions-not-meteors/

地球冷卻是由火山爆發造成，而非隕石撞擊

分析霍爾洞的沉積物顯示大約13000年前地球發生的降溫事件是由火山爆發造成

By Keith Randall

由德州農工大學的教授參與的研究指出，在德州中部的一個洞穴之內發現的古代沉積物似乎能解決地球大約13000年前氣溫突然下降的謎團。

工作人員正在挖掘德州中部的霍爾洞。圖片來源：Michael Waters

Michael Waters是德州農工大學美洲原住民研究中心的主任與特聘教授，他和貝勒大學以及休士頓大學的同僚合作的這項成果發表在《科學前緣》(Science Advances)。

這起事件使得地球下降了3℃，可以說是非同小可。有些研究人員相信起因是地外天體撞上地球，像是流星造成的碰撞。

但是Waters和團隊發現殘留在霍爾洞的沉積層裡的證據，卻幾乎可以肯定是火山爆發造成的結果。

Waters表示位在德州丘陵地區的霍爾洞裡擁有延續了二萬年以上的沉積物紀錄。他在2017年首度對霍爾洞展開研究。

他說：「霍爾洞品質良好的紀錄讓我們有這項獨一無二的機會進行跨領域合作，藉此探討一些重要的科學問題。」

「其中一個重大問題是：在末次冰河期的尾聲，大約13000年前當覆蓋加拿大的冰原正在融化的時候，是否發生了地外天體撞擊事件造成氣溫突然下降，將北半球再次推入另一場持續1200年的冰河期？」

Thomas Stafford (任職於科羅拉多州的Stafford實驗研究院)最先對洞穴裡的沉積層進行定年。Waters和團隊發現年代為理論中撞擊時間的沉積層能回答上述問題，甚至可以從中找出過去這段突然變冷的時期最初是由什麼原因引起。

曾經漫步於北美大陸上的大型哺乳類，像是猛瑪象、馬和駱駝等動物的滅亡過程中，這起事件可能也具有推波助瀾的角色。

帶領研究團隊的是休士頓大學的地質科學教授Alan Brandon。他說：「結果顯示九千到一萬五千年前，跟冷化事件有關的地球化學訊號出現了不只一次，而是四次。」

「因此引發冷化事件的原因並非來自外太空。過往認為是大型流星在大氣中爆炸的地球化學證據，其實反映的是一段火山爆發相當活躍的時期。」

「雖然我感到懷疑，」Brandon表示，「但我們已經採用了任何我們能用的方法來得到別的解釋，甚至是避免得出這項結論。雖然科學家過往就認為火山爆發是可行的解釋，但因為沒有相關的地球化學證據，所以通常不被採用。」

團隊表示火山爆發之後散布到全球的氣膠會把太陽輻射反射回去，可能造成噴發過後一到五年的全球氣溫低於以往，持續多久則依據噴發的規模及長度而定。

貝勒大學的地質科學教授Steven Forman是研究共同作者，他說：「末次冰河期結束時全球氣溫明顯地往上攀升，但發生在13000年前的新仙女木期卻中斷了這股趨勢。」

Forman表示新仙女木期的地球氣候可能跨越了某個臨界點，或許是因為冰層的融水流入北大西洋、雪地的覆蓋面積增加、強烈的火山噴發……等。這些因素結合起來可能造成北半球的氣溫劇烈降低。

Waters說：「氣溫快速下降的同時也有一些生物滅絕了，像是駱駝和馬。此外，克洛維斯文化的考古場址也在此時出現。」

Brandon和休士頓大學的科學家Nan Sun合作，對採集自霍爾洞的沉積物進行了同位素分析。他們發現銥、釕、鉑、鈀、錸等元素的比例並不正確，代表這起事件的成因不是流星或小行星。

論文主要作者Sun表示：「這些元素的同位素分析結果以及相對比例符合之前火山氣體的分析結果。」

團隊表示火山爆發造成的降溫現象在噴發地點附近最為劇烈，而且通常是在噴發過後的一年之內最為明顯，幾年之後的降溫幅度便會減少許多。

但是新仙女木期的持續時間為1200年，「因此火山爆發單獨來說是相當重要的觸發條件，但也需要地球系統的其他變化，像是海洋溫度降低以及雪地覆蓋面積增加，才能讓這段時期的氣候都處在低溫當中，」Forman表示。

Waters補充說重點為「在定年結果為新仙女木期開始時形成的沉積物中，發現的化學異常值並非由地外物體撞擊所形成，而是來自於火山爆發。」

 

Cooling of Earth Caused by Eruptions, Not Meteors

Analysis of sediment found in Hall’s Cave shows volcanic eruptions responsible for cooling of Earth around 13,000 years ago.

Ancient sediment found in a central Texas cave appears to solve the mystery of why the Earth cooled suddenly about 13,000 years ago, according to a research study co-authored by a Texas A&M University professor.

Michael Waters, director of The Center for The Study of the First Americans and Distinguished Professor at Texas A&M University, and colleagues from Baylor University and the University of Houston have had their work published in Science Advances.

Some researchers believed the event – which cooled the Earth by about 3 degrees Centigrade, a huge amount – was caused by an extraterrestrial impact with the Earth, such as a meteor collision.

But Waters and the team found that the evidence left in layers of sediment in Hall’s Cave were almost certainly the result of volcanic eruptions.

Waters said that Hall’s Cave, located in the Texas hill country, has a sediment record extending over 20,000 years and he first began researching the cave in 2017.

“It is an exceptional record that offers a unique opportunity for interdisciplinary cooperation to investigate a number of important research questions,” he said.

“One big question was, did an extraterrestrial impact occur near the end of the last ice age, about 13,000 years ago as the ice sheets covering Canada were melting, and cause an abrupt cooling that thrust the northern hemisphere back into the ice age for an extra 1,200 years?”

Waters and the team found that within the cave are layers of sediment, first identified by Thomas Stafford (Stafford Research Laboratories, Colorado), that dated to the time of the proposed impact that could answer the question and perhaps even identify the trigger that started the ancient cold snap.

The event also likely helped cause the extinction of large mammals such as mammoth, horse and camel that once roamed North America.

“This work shows that the geochemical signature associated with the cooling event is not unique but occurred four times between 9,000 and 15,000 years ago,” said Alan Brandon, professor of geosciences at University of Houston and head of the research team.

“Thus, the trigger for this cooling event didn’t come from space. Prior geochemical evidence for a large meteor exploding in the atmosphere instead reflects a period of major volcanic eruptions.

“I was skeptical,” Brandon said. “We took every avenue we could to come up with an alternative explanation, or even avoid, this conclusion. A volcanic eruption had been considered one possible explanation but was generally dismissed because there was no associated geochemical fingerprint.”

After a volcano erupts, the global spread of aerosols reflects incoming solar radiation away from Earth and may lead to global cooling post eruption for one to five years, depending on the size and timescales of the eruption, the team said.

“The Younger Dryas, which occurred about 13,000 years ago, disrupted distinct warming at the end of the last ice age,” said co-author Steven Forman, professor of geosciences at Baylor.

The Earth’s climate may have been at a tipping point at the end of Younger Dryas, possibly from the ice sheet discharge into the North Atlantic Ocean, enhanced snow cover and powerful volcanic eruptions that may have in combination led to intense Northern Hemisphere cooling, Forman said.

“This period of rapid cooling coincides with the extinction of a number of species, including camels and horses, and the appearance of the Clovis archaeological tradition,” said Waters.

Brandon and fellow University of Houston scientist Nan Sun completed the isotopic analysis of sediments collected from Hall’s Cave. They found that elements such as iridium, ruthenium, platinum, palladium and rhenium were not present in the correct proportions, meaning that a meteor or asteroid could not have caused the event.

“The isotope analysis and the relative proportion of the elements matched those that were found in previous volcanic gases,” said Sun, lead author of the report.

Volcanic eruptions cause their most severe cooling near the source, usually in the year of the eruption, with substantially less cooling in the years after the eruption, the team said.

The Younger Dryas cooling lasted about 1,200 years, “so a sole volcanic eruptive cause is an important initiating factor, but other Earth system changes, such as cooling of the oceans and more snow cover were needed to sustain this colder period, “Forman said.

Waters added that the bottom line is that “the chemical anomalies found in sediments dating to the beginning of the Younger Dryas are the result of volcanism and not an extraterrestrial impact.”

原始論文：N. Sun, A. D. Brandon, S. L. Forman, M. R. Waters and K. S. Befus. Volcanic origin for Younger Dryas geochemical anomalies ca. 12,900 cal B.P.. Science Advances, 2020 DOI: 10.1126/sciadv.aax8587

引用自：Texas A&M University. "Cooling of Earth caused by eruptions, not meteors."