古代多峇火山的爆發並未在東非造成火山冬天

原文網址：https://uanews.arizona.edu/story/no-volcanic-winter-east-africa-ancient-toba-eruption

古代多峇火山的爆發並未在東非造成火山冬天

亞利桑那州立大學的地質學家表示，74000年前的超級火山爆發並未在東非引發造成人類數目減少的嚴重環境破壞。

根據亞利桑那州立大學的地質學家所述，大約74000年前發生在蘇門答臘的多峇超級火山爆發並未在東非造成長達6年的「火山冬天」，故該地區的人口也沒有因此而急速下降。

這項新發現駁斥了多峇巨災理論(Toba catastrophe hypothesis)。該理論認為火山噴發和後續效應使東非陷入了數年的酷寒，造成嚴重的生態浩劫。

「這是首度有研究提供直接證據顯示噴發不久之前和之後的植被狀況，進而指出多峇火山爆發對植被的影響。」主要作者，亞利桑那州立大學地質科學系的博士候選人Chad L. Yost表示。「對於東非的植被生長狀況來說，多峇火山爆發並未造成顯著的負面衝擊。」

利用沖刷至湖泊然後堆積在湖底的植物部件，研究人員可以重建某個地區過往的生態系樣貌。Yost和同事研究的微小植物碎屑保存於馬拉威湖的兩根沉積物岩芯當中，這座湖泊長約570公里，是東非大裂谷最南端的湖泊。

之前的研究曾在馬拉威湖的岩芯當中找到來自於多峇火山爆發的物質，它們可以指出噴發時間並讓Yost和同事得以追溯多峇火山爆發之前100年和之後200年間發生的事物。研究團隊在這300年區間中，平均每隔8.5年便分析一個代表性樣品。

「結果相當令人驚訝。」Yost表示。「基於多峇火山爆發的規模，你可能會預料有嚴重的冷化現象發生――但我們卻沒有看到。」

Yost和他的同事並未看到低海拔的植被在噴發過後有顯著變化，但團隊確實發現噴發剛結束時，某些山區植物有消失的跡象。他說噴發造成的冷化可能對不耐寒的植物造成了損害。

Yost表示如果此區域在多峇火山噴發之後歷經了長達數年的嚴重冷化，則岩芯呈現的證據應該會指出該區域各海拔的植被都經歷了大規模死亡。

多峇巨災理論中的一部份認為該次噴發造成人類數目大幅減少。

「我們知道當時馬拉威湖周遭的50公里以內有解剖學上的現代人生活著，」Yost表示。「人類或許可以搬遷到其他棲地或者低海拔地區，多峇火山爆發造成的冷化效應對這些地方只有些微影響或者根本沒有。」

他說該地區已知的多數考古場址都是位在低海拔地區而非山區。

共同作者，亞利桑那州立大學地質科學系的特聘教授 Andrew S. Cohen表示：「發生在地球歷史75000年前的單一事件，造成人類搖籃的人口數大幅縮減的說法並不成立。」

研究團隊的論文「Subdecadal phytolith and charcoal records from Lake Malawi, East Africa imply minimal effects on human evolution from the ~74 ka Toba supereruption」本周發表在《人類演化期刊》(Journal of Human Evolution)。

和Yost與Cohen's共同進行研究的作者包括德州大學奧斯汀分校的Lily J. Jackson和印第安納州立大學的Jeffery R. Stone。資金來源則是美國國家科學基金會和國際陸地科學鑽探計畫。

Cohen是馬拉威湖鑽探計畫的主要研究人員之一，他說這項合作計畫於2005年從湖底成功取出了岩芯。馬拉威湖是地球上最深的湖泊之一，其岩芯所保存的物質紀錄可以追溯至超過100萬年以前。

沖刷至湖裡的動植物組織會一年一年地層層堆積在湖底，因此湖泊本身和周遭陸地的過往環境變化會被記錄在沉積物岩芯當中。

Yost的研究取自於湖泊的兩根岩芯：一根來自靠近山區的湖泊北端，另一根則是在湖泊的中心地帶。Cohen表示過往其他研究人員已經精確指出岩芯中的哪個層位含有來自多峇火山爆發的玻璃質和晶體。

Yost從岩芯中取出橫跨火山爆發前後區段的樣品，並分析其中的木炭以及含有矽質的植物組織――植矽體(phytolith)。

Yost的專長為辨識特定植矽體是來自於何種植物，他說這項研究需要盯著顯微鏡數百個小時才得以完成。

如果多峇巨災理論為真，那麼大規模死亡的植被應該會使野火的發生次數增加，造成更多木炭被沖刷到湖裡。然而，他在噴發之後堆積的沉積物中，並沒有觀察到木炭數量增加至超過正常變動範圍之外。

「我們判定多峇火山爆發對生長在東非的植被並沒有造成重大的負面影響，」Yost表示。「我們希望這項結果可以成為宣告多峇巨災理論為錯誤的關鍵證據。」

No Volcanic Winter in East Africa From Ancient Toba Eruption

The supereruption 74,000 years ago did not trigger major environmental disruption that caused human populations in East Africa to decline, UA geoscientists say.

The massive Toba volcanic eruption on the island of Sumatra about 74,000 years ago did not cause a six-year-long "volcanic winter" in East Africa and thereby cause the human population in the region to plummet, according to new research from University of Arizona geoscientists.

The new findings disagree with the Toba catastrophe hypothesis, which says the eruption and its aftermath caused drastic, multiyear cooling and severe ecological disruption in East Africa.

"This is the first research that provides direct evidence for the effects of the Toba eruption on vegetation just before and just after the eruption," said lead author Chad L. Yost, a doctoral candidate in the UA Department of Geosciences. "The Toba eruption had no significant negative impact on vegetation growing in East Africa."

Researchers can use ancient plant parts that wash into and accumulate on the bottoms of lakes to reconstruct a region's past ecosystem. Yost and his colleagues studied microscopic bits of plants preserved in two sediment cores from Lake Malawi, which is approximately 570 kilometers (354 miles) long and is the southernmost of the East African Rift lakes.

Previous investigators found material from the Toba eruption in the Lake Malawi cores. That material pinpoints the time of the eruption and allowed Yost and colleagues to peer back in time 100 years before to 200 years after the Toba eruption. The team analyzed samples that represented, on average, every 8.5 years within that 300-year interval.

"It is surprising," Yost said. "You would have expected severe cooling based on the size of the Toba eruption — yet that's not what we see."

Yost and his colleagues did not find marked changes in lower-elevation vegetation post-eruption. The team did find some die-off of mountain plants just after the eruption. Cooling from the eruption might have injured frost-intolerant plants, he said.

Had the region experienced the drastic, multiyear cooling post-Toba, the cores would have evidence of a massive die-off of the region's vegetation at all elevations, Yost said.

Part of the Toba catastrophe hypothesis suggests the eruption caused human populations to shrink.

"We know anatomically modern humans were living within 50 kilometers of Lake Malawi," Yost said. "People would have been able to travel to habitats and lower elevations that had little to no cooling effect from the Toba eruption."

Most of the region's known archaeological sites are from low elevations, not the mountains, he said.

Co-author Andrew S. Cohen, UA Distinguished Professor of Geosciences, said, "That a singular event in Earth history 75,000 years ago caused human populations in the cradle of humankind to drop is not a tenable idea."

The team's paper, "Subdecadal phytolith and charcoal records from Lake Malawi, East Africa imply minimal effects on human evolution from the ~74 ka Toba supereruption," is published online this week in the Journal of Human Evolution.

Yost's and Cohen's co-authors are Lily J. Jackson of the University of Texas, Austin, and Jeffery R. Stone of Indiana State University. The National Science Foundation and the International Continental Scientific Drilling Program funded the research.

The Lake Malawi Drilling Project took the cores from the lake bottom in 2005, said Cohen, one of the principal investigators on the collaborative project. The lake is one of the deepest in the world. The material archived in the cores goes back more than 1 million years.

Plant and animal material washes into lakes and is deposited on the bottom in annual layers, so a sediment core contains a record of the past environments of a lake and of the surrounding land.

Yost studied two cores taken from the lake: one from the north end of the lake, which is closer to the mountains, and the other from the central part of the lake. Other researchers had pinpointed what layer in those cores had glass and crystals from the Toba eruption, Cohen said.

Yost took samples from the cores that straddled the eruption and analyzed the samples for charcoal and for silica-containing plant parts called phytoliths.

The work required hundreds of hours of peering through a microscope, said Yost, an expert in identifying the type of plant a particular phytolith came from.

If the Toba catastrophe hypothesis is true, the massive die-off of vegetation would have resulted in more wildfires and therefore more charcoal washing into the lake. However, he did not find an increase in charcoal outside the range of normal variability in the sediments deposited after the eruption.

"We determined that the Toba eruption had no significant negative impact on vegetation growing in East Africa," Yost said. "We hope this will put the final nail in the coffin of the Toba catastrophe hypothesis."

原始論文：Chad L. Yost, Lily J. Jackson, Jeffery R. Stone, Andrew S. Cohen. Subdecadal phytolith and charcoal records from Lake Malawi, East Africa imply minimal effects on human evolution from the ∼74 ka Toba supereruption. Journal of Human Evolution, 2018; 116: 75 DOI: 10.1016/j.jhevol.2017.11.005

引用自：University of Arizona. "No volcanic winter in East Africa from ancient Toba eruption."