遠古證物指出地球歷史在四萬二千年前有個重大的轉捩點

 原文網址：https://newsroom.unsw.edu.au/news/science-tech/ancient-relic-points-turning-point-earths-history-42000-years-ago

就像《銀河便車指南》中所說的，終極答案是42。

By Sherry Landow

由雪梨新南威爾斯大學和南澳博物館共同主持的國際研究顯示，在四萬二千年前地球磁場暫時消失引發了劇烈的氣候變遷，進而造成全球環境變化與大滅絕事件。

這場以大範圍出現的極光為特徵的劇烈氣候變遷，或許能解釋其他演化上的謎團，像是尼安德塔人為何滅絕。圖片來源：Unsplash

伴隨著電磁風暴、大範圍的極光以及宇宙輻射，這個地球歷史上重大的轉捩點是由地球磁極反轉以及太陽風的變化所造成。

研究人員把這段災害發生的時期稱作「亞當斯地磁轉換事件」(Adams Transitional Geomagnetic Event)，或者簡稱為「亞當斯事件」(Adams Even)。這是為了致敬科幻作家道格拉斯．亞當斯(Douglas Adams)，他在《銀河便車指南》(The Hitchhiker's Guide to the Galaxy)中寫道所有生命、全宇宙、乃至於萬物的解答為「42」。

此發現今日發表在《科學》(Science)。

「有史以來第一次，我們能夠精確的定出上次磁極反轉的時間以及對環境造成的衝擊，」研究共同主持人，新南威爾斯大學的教授Chris Turney表示。

「利用在沉積物裡保存了超過四萬年的古代紐西蘭貝殼杉(kauri tree)，我們才能達成這項發現。」

「地球磁場消失會造成大氣中的放射性碳含量出現高峰，利用這些古時候的樹木，我們可以測出這些高峰並且定出它們發生的時間。。」

這株在紐西蘭Ngāwhā發現的古代貝殼杉生存在亞當斯事件期間。圖片來源：www.nelsonskaihukauri.co.nz

雖然科學家早就知道地球的磁極大約在四萬一千年到四萬二千年前曾經暫時翻轉(稱作「拉尚漂移」(Laschamps Excursion))，但他們尚未完全知曉如果這次事件有對地球生物造成衝擊的話，程度又有多大。

但是研究人員可以經由分析古代的貝殼杉，而把地球大氣在這段時間的詳細變化歷程給建立起來。

「貝殼杉就像羅塞塔石碑一樣，幫助我們結合世界各地留存在洞穴、冰芯和泥沼裡的環境變遷紀錄，」共同主持這項研究的南澳博物館榮譽研究員Alan Cooper教授表示。

研究人員比較這份新建立起的時間表和太平洋其他地點的紀錄，同時利用涵蓋全球的氣候模型。結果發現北美冰層與冰河的增長，以及主要風帶與熱帶風暴系統的大範圍移動，都可以追溯到亞當斯事件發生的時候。

他們的頭幾條線索之一是澳洲大陸各處和塔斯馬尼亞的巨型動物群都在四萬二千年前同時邁向滅亡。

「這個時間點看起來總是不太對勁，因為澳洲的原住民已經到達很久了。但差不多在同樣的時間，澳洲的環境也始轉變成現今的乾燥氣候。」Cooper教授表示。

論文提出亞當斯事件還可以解釋演化上的其他諸多謎團，像是尼安德塔人滅絕，以及世界各地的許多洞窟突然出現具象藝術。

「這是我參與過的研究中最令人驚訝也最重要的發現，」Cooper教授說。

完美(宇宙)風暴

磁北極是羅盤指針所指的方向，但它並非一直保持不動。由於地核內部的活躍運動，磁北極通常會隨著時間而在地理北極(地球自轉軸的最北端)附近游移，磁南極也是一樣。

由於某些還不清楚的原因，有時候磁極的運動會變得較為激烈。大約在四萬一千到四萬二千年前，兩者甚至完全調換了位置。

「磁極最近一次翻轉便是拉尚漂移，」Turney教授表示。「它們在交換位置八百年後又改變了主意，換回原本的位置。」

在此之前的科學家進行研究時著重於磁極反轉期間發生的變化。那時磁場強度比現在弱了28%左右。

但是團隊的發現指出變化最劇烈的部份發生在反轉的前導階段，也就是磁極跨越整個地球移動的時候。

「地球磁場在亞當斯事件中，強度跌到了只有現今的0.6%，」Turney教授表示。

「基本上這代表地球完全失去了磁場――為我們抵擋宇宙射線的屏障徹底消失了。」

在地球磁場停擺的期間，太陽經歷了數次「太陽活動極小期」(Grand Solar Minima, GSM)，也就是太陽長時間處於活動低落的狀態。

雖然太陽活動極小期代表太陽表面的活動較為安靜，但是地球磁場弱化則意味著更多的太空天氣，像是太陽閃焰和銀河宇宙射線可以直接撲向地球。

「從太空進來地球未經過濾的輻射會破壞大氣層的空氣粒子，造成電子脫離出來並放出光線，此過程稱為游離作用。」

「這些游離的空氣會加熱臭氧層，引發一股襲捲全球的氣候變遷。」

亞當斯事件期間的天空可能會上演一連串從極光到閃電風暴等精彩的表演。圖片來源：Unsplash

逃進洞穴

亞當斯事件期間的天空可能常常上演炫目的燈光秀。

當太陽風跟地球大氣撞擊，便會產生北極光與南極光。

這些絢爛的光線通常只能在地球北極或南極附近看見，但是地球磁場停擺期間它們的出現範圍可能會擴大許多。

「全世界的早期人類應該能看到整片天空都是這些迷人的極光所形成的光幕與光帶，」Cooper教授表示。

游離的空氣具有相當好的導電度，因此會增加電磁風暴的頻率。

「當時的情景看起來勢必就像世界末日一樣，」Cooper教授說。

研究人員推論強烈的環境變遷可能會促使早期人類尋找更多可以避難的地方。這可以解釋為什麼全世界的洞穴藝術都突然出現在大約在四萬二千年前。

「我們認為紫外線強度激增，特別是在太陽閃焰期間，會讓洞穴搖身一變成為相當珍貴的庇護所，」Cooper教授解釋。「洞穴藝術中常常可以看到用紅色赭土蓋下的手印為主題。我們認為這可能代表當時人類會塗抹赭土來防曬，時至今日某些民族仍會運用這項技巧。」

「那段時期在洞穴中創造的迷人圖像仍留存至今，而其他在開闊地區創造的可能已經被侵蝕殆盡，造成藝術似乎是在四萬二千年前突然出現的樣子。」

圖片中央為西班牙卡斯蒂略洞穴的洞穴藝術，科學家認為它們的年代為四萬二年前――跟亞當斯事件同個時候。圖片來源：Paul Pettitt, courtesy Gobierno de Cantabria

挖掘古代線索

這項發現的兩年之前，在紐西蘭北島的Ngāwhā找到了一棵特別重要的古代貝殼杉。

這棵樹幹寬度超過兩公尺半的大樹生活在拉尚漂移期間。

「就像其他被埋藏起來的貝殼杉的樹幹，Ngāwhā的木頭也保存得相當完整，就連樹皮都還連在上面，」新南威爾斯大學的樹輪定年專家Jonathan Palmer表示。他在新南威爾斯大學理學院的柯羅諾斯碳十四循環設施中研究這些樹木的橫剖面。

放射性碳定年可以得出古物或事件的年代，團隊運用這項技術來追查磁極反轉期間放射性碳含量如何變化。接著再把這項數據和樹木的年度生長輪畫在同一張圖表上，後者可以做為精準的天然時間標記。

新的年代表有助於具體描繪出這段地球歷史上變化劇烈的時期。團隊因而可以利用氣候模型來重建出接連發生的環境變遷以及滅絕事件。

「我們看的數據越多，就越能看出一切都指向42，」Turney教授表示。「真的很不可思議。」

「最終來看，道格拉斯．亞當斯毫無疑問地意會到了某些重要的事。」

無與倫比的催化劑

雖然磁極經常有所漂移，不過現在磁北極迅速地在北半球移動還是令某些科學家感到擔憂。

「這種移動速度加上地球磁場的強度在過去170年已經減弱了9%左右，或許是下一次磁極反轉的徵兆，」Cooper教授說。

「如果今天發生了類似的事件，將會對現代社會產生非常嚴重的後果。侵入地球的宇宙輻射會破壞我們的輸電網路以及衛星網路。」

Turney教授表示就算沒有劇烈的太陽活動變化以及磁極反轉同時襲來，人為造成的氣候危機所造成的災難也已經夠強大了。

「我們讓大氣裡充滿了碳，含量之高是人類前所未見，」他說。「這時再有一次磁極反轉或是太陽活動的極端變化，將會是無於倫比的氣候變遷催化劑。」

「我們急需降低我們的碳排放量――在這類隨機事件再次發生之前。」

 

Ancient relic points to a turning point in Earth's history 42,000 years ago

Just like in The Hitchhiker’s Guide to the Galaxy, the answer was 42.

The temporary breakdown of Earth’s magnetic field 42,000 years ago sparked major climate shifts that led to global environmental change and mass extinctions, a new international study co-led by UNSW Sydney and the South Australian Museum shows.

This dramatic turning point in Earth’s history – laced with electrical storms, widespread auroras, and cosmic radiation – was triggered by the reversal of Earth’s magnetic poles and changing solar winds.

The researchers dubbed this danger period the ‘Adams Transitional Geomagnetic Event’, or ‘Adams Event’ for short – a tribute to science fiction writer Douglas Adams, who wrote in The Hitchhiker's Guide to the Galaxy that ‘42’ was the answer to life, the universe, and everything.

The findings are published today in Science.

“For the first time ever, we have been able to precisely date the timing and environmental impacts of the last magnetic pole switch,” says Chris Turney, a professor at UNSW Science and co-lead author of the study.

“The findings were made possible with ancient New Zealand kauri trees, which have been preserved in sediments for over 40,000 years.

“Using the ancient trees we could measure, and date, the spike in atmospheric radiocarbon levels caused by the collapse of Earth’s magnetic field.”

While scientists already knew the magnetic poles temporarily flipped around 41-42,000 years ago (known as the ‘Laschamps Excursion’), they didn’t know exactly how it impacted life on Earth – if at all.

But the researchers were able to create a detailed timescale of how Earth’s atmosphere changed over this time by analysing rings on the ancient kauri trees.

“The kauri trees are like the Rosetta Stone, helping us tie together records of environmental change in caves, ice cores and peat bogs around the world,” says co-lead Professor Alan Cooper, Honorary Researcher at the South Australian Museum.

The researchers compared the newly-created timescale with records from sites across the Pacific and used it in global climate modelling, finding that the growth of ice sheets and glaciers over North America and large shifts in major wind belts and tropical storm systems could be traced back to the Adams Event.

One of their first clues was that megafauna across mainland Australia and Tasmania went through simultaneous extinctions 42,000 years ago.

“This had never seemed right, because it was long after Aboriginal people arrived, but around the same time that the Australian environment shifted to the current arid state,” says Prof. Cooper.

The paper suggests that the Adams Event could explain a lot of other evolutionary mysteries, like the extinction of Neandertals and the sudden widespread appearance of figurative art in caves around the world.

“It’s the most surprising and important discovery I’ve ever been involved in,” says Prof. Cooper.

The perfect (cosmic) storm

The magnetic north pole – that is, the direction a compass needle points to – doesn’t have a fixed location. It usually wobbles close to the North Pole (the northern-most point of Earth’s axis) over time due to dynamic movements within the Earth’s core, just like the magnetic south pole.

Sometimes, for reasons that aren’t clear, the magnetic poles’ movements can be more drastic. Around 41,000-42,000 years ago they swapped places entirely.

“The Laschamps Excursion was the last time the magnetic poles flipped,” says Prof. Turney. “They swapped places for about 800 years before changing their minds and swapping back again.”

Until now, scientific research has focused on changes that happened while the magnetic poles were reversed, when the magnetic field was weakened to about 28 per cent of its present-day strength.

But according to the team’s findings, the most dramatic part was the lead-up to the reversal, when the poles were migrating across the Earth.

“Earth’s magnetic field dropped to only 0-6 per cent strength during the Adams Event,” says Prof. Turney.

“We essentially had no magnetic field at all – our cosmic radiation shield was totally gone.”

During the magnetic field breakdown, the Sun experienced several ‘Grand Solar Minima’ (GSM), long-term periods of quiet solar activity.

Even though a GSM means less activity on the Sun’s surface, the weakening of its magnetic field can mean more space weather – like solar flares and galactic cosmic rays – could head Earth’s way.

“Unfiltered radiation from space ripped apart air particles in Earth’s atmosphere, separating electrons and emitting light – a process called ionisation,” says Prof. Turney.

“The ionised air ‘fried’ the Ozone layer, triggering a ripple of climate change across the globe.”

Into the caves

Dazzling light shows would have been frequent in the sky during the Adams Event.

Aurora borealis and aurora australis, also known as the northern and southern lights, are caused by solar winds hitting the Earth’s atmosphere.

Usually confined to the polar northern and southern parts of the globe, the colourful sights would have been widespread during the breakdown of Earth’s magnetic field.

“Early humans around the world would have seen amazing auroras, shimmering veils and sheets across the sky,” says Prof. Cooper.

Ionised air – which is a great conductor for electricity – would have also increased the frequency of electrical storms.

“It must have seemed like the end of days,” says Prof. Cooper.

The researchers theorise that the dramatic environmental changes may have caused early humans to seek more shelter. This could explain the sudden appearance of cave art around the world roughly 42,000 years ago.

“We think that the sharp increases in UV levels, particularly during solar flares, would suddenly make caves very valuable shelters,” says Prof. Cooper. “The common cave art motif of red ochre handprints may signal it was being used as sunscreen, a technique still used today by some groups.

“The amazing images created in the caves during this time have been preserved, while other art out in open areas has since eroded, making it appear that art suddenly starts 42,000 years ago.”

Uncovering ancient clues

These findings come two years after a particularly important ancient kauri tree was uncovered at Ngāwhā, Northland.

The massive tree – with a trunk spanning over two and a half metres – was alive during the Laschamps.

“Like other entombed kauri logs, the wood of the Ngāwhā tree is so well preserved that the bark is still attached,” says UNSW’s Dr Jonathan Palmer, a specialist in dating tree-rings (dendrochronology). Dr Palmer studied cross sections of the trees at UNSW Science’s Chronos 14Carbon-Cycle Facility.

Using radiocarbon dating – a technique to date ancient relics or events – the team tracked the changes in radiocarbon levels during the magnetic pole reversal. This data was charted alongside the trees’ annual growth rings, which acts as an accurate, natural timestamp.

The new timescale helped reveal the picture of this dramatic period in Earth’s history. The team were able to reconstruct the chain of environmental and extinction events using climate modelling.

“The more we looked at the data, the more everything pointed to 42,” says Prof. Turney. “It was uncanny.

“Douglas Adams was clearly on to something, after all.”

An accelerant like no other

While the magnetic poles often wander, some scientists are concerned about the current rapid movement of the north magnetic pole across the Northern Hemisphere.

“This speed – alongside the weakening of Earth’s magnetic field by around nine per cent in the past 170 years – could indicate an upcoming reversal,” says Prof. Cooper.

“If a similar event happened today, the consequences would be huge for modern society. Incoming cosmic radiation would destroy our electric power grids and satellite networks.”

Prof. Turney says the human-induced climate crisis is catastrophic enough without throwing major solar changes or a pole reversal in the mix. 

“Our atmosphere is already filled with carbon at levels never seen by humanity before,” he says. “A magnetic pole reversal or extreme change in Sun activity would be unprecedented climate change accelerants.

“We urgently need to get carbon emissions down before such a random event happens again.”

原始論文：Alan Cooper, Chris S. M. Turney, Jonathan Palmer, Alan Hogg, Matt Mcglone, Janet Wilmshurst, Andrew M. Lorrey, Timothy J. Heaton, James M. Russell, Ken Mccracken, Julien G. Anet, Eugene Rozanov, Marina Friedel, Ivo Suter, Thomas Peter, Raimund Muscheler, Florian Adolphi, Anthony Dosseto, J. Tyler Faith, Pavla Fenwick, Christopher J. Fogwill, Konrad Hughen, Mathew Lipson, Jiabo Liu, Norbert Nowaczyk, Eleanor Rainsley, Christopher Bronk Ramsey, Paolo Sebastianelli, Yassine Souilmi, Janelle Stevenson, Zoë Thomas, Raymond Tobler, Roland Zech. A global environmental crisis 42,000 years ago. Science, 2021 DOI: 10.1126/science.abb8677

引用自：University of New South Wales. "Ancient relic points to a turning point in Earth's history 42,000 years ago.”