於74000年前的古代超級火山爆發之後仍然繁盛的一群現代人

原文網址：ScienceDaily, 12 March 2018. www.sciencedaily.com/releases/2018/03/180312132956.htm

於74000年前的古代超級火山爆發之後仍然繁盛的一群現代人

想像非洲某年夏天從未到來：天空在白晝是一片灰濛，夜晚則閃耀著紅光；春花未開，冬日樹木枯萎；羚羊之類的大型哺乳類因挨餓而日漸消瘦，倚賴牠們維生的掠食者(肉食動物和獵人)也無法從牠們身上得到什麼養分。接著想像如此怵目驚心的景象年復一年地不斷重演，便是在74000年前左右的印尼多峇(Toba)超級火山爆發之後，地球上的生靈所處的景象。但在本周發表於期刊《自然》(Nature)的一篇論文中，科學家於南非海岸發現一群早期現代人在此事件下仍繁榮發展。

西元1815年同樣發生在印尼的坦博拉(Tambora)火山爆發，被認為是西元1816年夏季從未到來的元兇――而此次爆發比多峇火山爆發規模還小了數百倍。坦博拉火山爆發造成的影響十分嚴重：歐亞大陸和北美的農作物歉收，導致飢荒發生並引起大規模的人口遷徙。多峇超級火山爆發連黃石公園在遠古時代的劇烈噴發都相形見絀，它對全球各地人類造成的影響勢必更加劇烈且持續更久。

單從火山灰落的範圍便能證實這場環境災害的規模有多大。從火山噴發至高層大氣的巨量氣膠可以大幅遮蔽陽光，據估計當時陽光減弱了25%至90%不等。在此情況下，可以預料植物會大量死亡。證據指出多峇火山爆發不久之後，東非發生了嚴重乾旱且野火叢生，植物群集的組成也有所變化。

如果坦博拉火山爆發可以在一年之內造成如此嚴重的破壞――而它跟多峇火山相比不過是一縷輕煙――那我們不難想像多峇火山爆發在全世界造成的浩劫可以持續好幾年，並將生物推向瀕臨絕種的地步。

這場劫難的根源在當時位處印尼的目擊者眼中想必相當駭人――烙印在他們死前的那一刻。然而，若你是74000年前生活於非洲的一個狩獵採集家族，你大概不會有任何線索得知為何忽然之間天氣變得如此惡劣。接著飢荒到來，家中老小接連死去。你在社會上所屬的團體被漸漸摧毀，而整個社會則處於崩潰邊緣。

多峇火山爆發對某些生態系的影響勢必較其他生態系來的嚴重，或許造成生活於某些區域的人類族群可以比其他族群更加成功地度過整起事件――這些區域即稱為「避難所」(refugia)。你的族群所居住的地方是否為此種避難所，有很大一部份取決於你們能取得的資源類型。從海岸可以取得的資源，像是貝類，不但營養豐富而且相較於內陸地區的動植物，也比較不容易受到火山噴發的影響。

當火焰、煙霧和塵埃組成的柱狀雲從多峇火山頂端爆發出來的時候，也噴濺出許多岩石、氣體和只能在顯微鏡底下看見的微小玻璃質碎片(顯微火山灰，cryptotephra)，這種擁有特殊鐮刀狀外形的碎片是由玻璃質氣泡整個炸碎而形成。當它們被噴入大氣層之後，這些肉眼不可見的碎片便能散布到世界各地。

Malcolm H. Wiener考古學實驗室位於希臘的美國古典研究學院，其主持人Panagiotis (Takis) Karkanas在顯微鏡底下觀察一片以樹脂密封起來的考古遺址沉積物時，看到了一塊由該次爆發形成的碎片。

Karkanas表示：「在我當時觀察的數百萬個礦物顆粒中就只有這麼一塊碎片。但它確實在那，不可能是別的東西。」

出土這塊碎片的考古學場址位於一處稱為尖峰角(Pinnacle Point)5-6的岩架，地點為南非南部莫塞爾灣小鎮附近的海岸。沉積物的定年結果大約為距今74000年以前。

尖峰角挖掘計畫的主持人Curtis W. Marean說明：「Takis和我之前就在猜想也許我們能在這處考古場址的沉積物中找到多峇火山爆發的碎片，而他也真的憑藉著他的鷹眼找到了一個。」Marean是美國亞利桑那州立大學人類起源研究所的副主任，同時也是南非納爾遜．曼德拉大學古海岸科學研究中心的榮譽教授。

Marean讓Eugene Smith看看這塊碎片的影像，而這位美國內華達大學拉斯維加斯分校的火山學家證實它確實是由火山產生的碎片。

Smith表示：「尖峰角的研究讓我回想起40年前我在寫博士論文時對火山玻璃碎片進行的研究。」

研究早期階段團隊曾邀請專長為顯微火山灰的科學家Christine Lane來訓練研究生Amber Ciravolo這方面所需的技術。之後Racheal Johnsen把Ciravalo加入團隊成為實驗室管理員並發展新的技術。

在美國國家科學基金的資助下，他們從零開始建立起考古學與地質學研究用的顯微火山灰實驗室。現在他們參與計畫的研究地點不僅只有在非洲，還包含了義大利、美國內華達州和猶他州。

封存在火山碎片中的特有化學訊號可以讓科學家依循這條線索來找出火山爆發的元凶是誰。在這篇發表於《自然》的論文中，團隊描述了他們在這兩座南非海岸的考古場址發現到的火山碎片，透過化學指紋他們查出這些碎片來自多峇火山，而他們也證明在火山爆發事件前後一直都有人類定居於此。

「之前有許多研究都在試著證明多峇火山爆發消滅人類族群的假說，」Marean強調，「但他們都以失敗收場。因為他們無法呈現出決定性證據顯示人類的居住模式恰好能和這起事件的發生時間畫上關聯。」

之前多數研究都著眼於多峇火山爆發是否造成環境變遷。雖然答案是肯定的，但這類研究缺乏必要的考古資料來指出多峇火山爆發對人類有何影響。

尖峰角的研究團隊一直以來都身處第一線發展並運用極為先進的考古技術。他們利用「全站儀」(total station)而能以毫米級的精準度測量考古現場的一切事物，這種雷射測量儀器配合行動裝置可以進行十分精準且毫無誤差的錄像。

美國德州大學阿靈頓分校的Naomi Cleghorn在研究人員取出尖峰角的樣品時同步進行紀錄。

Cleghorn對此說明：「我們採集了一連串樣品。這是從之前開挖的岩壁上所輕挖下來的少量沉積物，每採集一個樣品，我們就用全站儀拍下它的所在位置。之後我們就能精確比對樣品所在位置和我們爾後挖掘出的文化遺產―也就是古代人在該處遺留的垃圾―之間的位置關係。同時我們也可以把顯微火山灰樣品的所在位置，跟用來定年與進行環境分析樣品的位置進行比對。」

通過這項研究除了可以瞭解多峇火山爆發對該區域人類的影響，此研究對考古定年技術來說也具有其他重要意義。在此年代區間使用的考古定年方法並不太精確，誤差達到10%(也就是數千年)是常有的事。然而多峇火山爆發造成的火山灰落是已經準確定年過的短暫事件。碎片沉積下來所花費的時間大概只有兩星期這麼長――就地質學角度來看相當於一瞬間。

Marean進一步解釋：「我們在兩個地點發現多峇火山的碎片。其一是尖峰角的岩架，人們在此居住、用餐、工作並睡覺；另外一處則是十公里之外稱為Vleesbaai的開闊空間。於後者發現一群人曾在此圍坐成一小圈打造石器，成員跟生活在尖峰角的可能是同一團體。在兩個地點都找到多峇火山的碎片使我們可以歸納出兩地的文明紀錄幾乎是在同一時刻形成。」

碎片出土的位置不只可以告訴我們上述訊息，還讓科學家能以別種方法對考古場址的年代進行獨立檢定。人們生活在尖峰角5-6場址的年代為距今9萬至5萬年前。澳洲臥龍崗大學的Zenobia Jacobs利用光螢光(ptically stimulated luminescence, OSL)定年90具樣品後推導出可以定年所有層位的年代模式。光螢光定年法得出的是沙石顆粒最後一次暴露在陽光之下的時間。

Marean表示：「光螢光定年的準確度一直有所爭議，但以Jacobs的年代模式來定年發現多峇火山爆發碎片的層位，結果為距今74000年前左右――分毫不差。」這對Jacobs最新的光螢光定年方法來說是個十分有力的支持，她已經運用此方法來定年南非和世界各地的許多考古場址。

Jacobs表示：「在重建我們歷史發展的時間線時，有很大一部份是仰賴光螢光定年做為一種主力工具。因此，測試這副時鐘是否以正確的步調運行是很重要的。可以得到如此有力的支持相當令人開心。」

多峇火山爆發是過去200萬年以來最劇烈的火山爆發。從1990年代開始，科學家就開始爭論多峇火山爆發是否造成一段持續許久的火山冬天，這也許會摧毀世界各地的生態系並造成各地人口銳減，甚至使我們的祖先幾近滅絕而形成所謂的「瓶頸效應」(bottleneck)。

這項研究顯示住在非洲南部海岸線，食物來源豐富的人們成功度過了此次超級火山噴發事件，或許是因為這段海岸為食物特別豐饒的區域。現在其他研究團隊可以運用這篇研究新發展出來的方法和改良過後的方法，來研究他們在非洲其他地方的考古場址以瞭解度過這段黑暗時刻的人類族群是否只有本研究中的這群人而已。

Modern humans flourished through ancient supervolcano eruption 74,000 years ago

Imagine a year in Africa that summer never arrives. The sky takes on a gray hue during the day and glows red at night. Flowers do not bloom. Trees die in the winter. Large mammals like antelope become thin, starve and provide little fat to the predators (carnivores and human hunters) that depend on them. Then, this same disheartening cycle repeats itself, year after year. This is a picture of life on earth after the eruption of the super-volcano, Mount Toba in Indonesia, about 74,000 years ago. In a paper published this week in Nature, scientists show that early modern humans on the coast of South Africa thrived through this event.

An eruption a hundred times smaller than Mount Toba -- that of Mount Tambora, also in Indonesia, in 1815 -- is thought to have been responsible for a year without summer in 1816. The impact on the human population was dire -- crop failures in Eurasia and North America, famine and mass migrations. The effect of Mount Toba, a super-volcano that dwarfs even the massive Yellowstone eruptions of the deeper past, would have had a much larger, and longer-felt, impact on people around the globe.

The scale of the ash-fall alone attests to the magnitude of the environmental disaster. Huge quantities of aerosols injected high into the atmosphere would have severely diminished sunlight -- with estimates ranging from a 25 to 90 percent reduction in light. Under these conditions, plant die-off is predictable, and there is evidence of significant drying, wildfires and plant community change in East Africa just after the Toba eruption.

If Mount Tambora created such devastation over a full year -- and Tambora was a hiccup compared to Toba -- we can imagine a worldwide catastrophe with the Toba eruption, an event lasting several years and pushing life to the brink of extinctions.

In Indonesia, the source of the destruction would have been evident to terrified witnesses -- just before they died. However, as a family of hunter-gatherers in Africa 74,000 years ago, you would have had no clue as to the reason for the sudden and devastating change in the weather. Famine sets in and the very young and old die. Your social groups are devastated, and your society is on the brink of collapse.

The effect of the Toba eruption would have certainly impacted some ecosystems more than others, possibly creating areas -- called refugia -- in which some human groups did better than others throughout the event. Whether or not your group lived in such a refuge would have largely depended on the type of resources available. Coastal resources, like shellfish, are highly nutritious and less susceptible to the eruption than the plants and animals of inland areas.

When the column of fire, smoke and debris blasted out the top of Mount Toba, it spewed rock, gas and tiny microscopic pieces (cryptotephra) of glass that, under a microscope, have a characteristic hook shape produced when the glass fractures across a bubble. Pumped into the atmosphere, these invisible fragments spread across the world.

Panagiotis (Takis) Karkanas, director of the Malcolm H. Wiener Laboratory for Archaeological Science, American School of Classical Studies, Greece, saw a single shard of this explosion under a microscope in a slice of archaeological sediment encased in resin.

"It was one shard particle out of millions of other mineral particles that I was investigating. But it was there, and it couldn't be anything else," says Karkanas.

The shard came from an archaeological site in a rockshelter called Pinnacle Point 5-6, on the south coast of South Africa near the town of Mossel Bay. The sediments dated to about 74,000 years ago.

"Takis and I had discussed the potential of finding the Toba shards in the sediments of our archaeological site, and with his eagle eye, he found one," explains Curtis W. Marean, project director of the Pinnacle Point excavations. Marean is the associate director of the Institute of Human Origins at Arizona State University and honorary professor at the Centre for Coastal Palaeoscience at Nelson Mandela University, South Africa.

Marean showed the shard image to Eugene Smith, a volcanologist with the University of Nevada at Las Vegas, and Smith confirmed it was a volcanic shard.

"The Pinnacle Point study brought me back to the study of glass shards from my master's thesis 40 years earlier," says Smith.

Early in the study, the team brought in expert cryptotephra scientist Christine Lane who trained graduate student Amber Ciravolo in the needed techniques. Racheal Johnsen later joined Ciravalo as lab manager and developed new techniques.

From scratch, with National Science Foundation support, they developed the Cryptotephra Laboratory for Archaeological and Geological Research, which is now involved in projects not only in Africa, but in Italy, Nevada and Utah.

Encased in that shard of volcanic glass is a distinct chemical signature, a fingerprint that scientists can use to trace to the killer eruption. In their paper in Nature, the team describes finding these shards in two archaeological sites in coastal South Africa, tracing those shards to Toba through chemical fingerprinting and documenting a continuous human occupation across the volcanic event.

"Many previous studies have tried to test the hypothesis that Toba devastated human populations," Marean notes. "But they have failed because they have been unable to present definitive evidence linking a human occupation to the exact moment of the event."

Most studies have looked at whether or not Toba caused environmental change. It did, but such studies lack the archaeological data needed to show how Toba affected humans.

The Pinnacle Point team has been at the forefront of development and application of highly advanced archaeological techniques. They measure everything on site to millimetric accuracy with a "total station," a laser-measurement device integrated to handheld computers for precise and error-free recording.

Naomi Cleghorn with the University of Texas at Arlington, recorded the Pinnacle Point samples as they were removed.

Cleghorn explains, "We collected a long column of samples -- digging out a small amount of sediment from the wall of our previous excavation. Each time we collected a sample, we shot its position with the total station. We could then precisely compare the position of the sample to our excavated cultural remains -- the trash ancient humans left at the site. We could also compare our cryptotephra sample position with that of samples taken for dating and environmental analyses."

In addition to understanding how Toba affected humans in this region, the study has other important implications for archaeological dating techniques. Archaeological dates at these age ranges are imprecise -- 10 percent (or 1000s of years) error is typical. Toba ash-fall, however, was a very quick event that has been precisely dated. The time of shard deposition was likely about two weeks in duration -- instantaneous in geological terms.

"We found the shards at two sites," explains Marean. "The Pinnacle Point rockshelter (where people lived, ate, worked and slept) and an open air site about 10 kilometers away called Vleesbaai. This latter site is where a group of people, possibly members of the same group as those at Pinnacle Point, sat in a small circle and made stone tools. Finding the shards at both sites allows us to link these two records at almost the same moment in time."

Not only that, but the shard location allows the scientists to provide an independent test of the age of the site estimated by other techniques. People lived at the Pinnacle Point 5-6 site from 90,000 to 50,000 years ago. Zenobia Jacobs with the University of Wollongong, Australia, used optically stimulated luminescence (OSL) to date 90 samples and develop a model of the age of all the layers. OSL dates the last time individual sand grains were exposed to light.

"There has been some debate over the accuracy of OSL dating, but Jacobs' age model dated the layers where we found the Toba shards to about 74,000 years ago -- right on the money," says Marean. This lends very strong support to Jacobs' cutting-edge approach to OSL dating, which she has applied to sites across southern Africa and the world.

"OSL dating is the workhorse method for construction of timelines for a large part of our own history. Testing whether the clock ticks at the correct rate is important. So getting this degree of confirmation is pleasing," says Jacobs.

In the 1990s, scientists began arguing that this eruption of Mount Toba, the most powerful in the last two million years, caused a long-lived volcanic winter that may have devastated the ecosystems of the world and caused widespread population crashes, perhaps even a near-extinction event in our own lineage, a so-called bottleneck.

This study shows that along the food-rich coastline of southern Africa, people thrived through this mega-eruption, perhaps because of the uniquely rich food regime on this coastline. Now other research teams can take the new and advanced methods developed in this study and apply them to their sites elsewhere in Africa so researchers can see if this was the only population that made it through these devastating times.

原始論文：Eugene I. Smith, Zenobia Jacobs, Racheal Johnsen, Minghua Ren, Erich C. Fisher, Simen Oestmo, Jayne Wilkins, Jacob A. Harris, Panagiotis Karkanas, Shelby Fitch, Amber Ciravolo, Deborah Keenan, Naomi Cleghorn, Christine S. Lane, Thalassa Matthews, Curtis W. Marean. Humans thrived in South Africa through the Toba eruption about 74,000 years ago. Nature, 2018; DOI: 10.1038/nature25967

引用自：University of Cape Town. "Modern humans flourished through ancient supervolcano eruption 74,000 years ago: Modern humans flourished through ancient supervolcano eruption." ScienceDaily.