原文網址：https://www.ucl.ac.uk/news/2024/dec/massive-asteroid-impacts-did-not-change-earths-climate-long-term
根據倫敦大學學院的新研究，大約在3565萬年前有兩顆大型小行星撞上地球，但它們並未對地球氣候造成任何持續性的影響。
藝術家描繪一顆小行星正在接近地球的場景。圖片來源：iStock / dottedhippo
這兩顆寬達數公里的岩石以相隔25000年的時間一前一後撞上地球，分別形成了俄羅斯西伯利亞100公里寬(60英里)的波皮蓋(Popigai)隕石坑，以及美國乞沙比克灣40-85公里(25-55英里)寬的隕石坑——也是世上已知第四與第五大的隕石坑。
這項新研究發表於期刊《通訊-地球與環境》(Communications Earth & Environment)，結果發現在撞擊過後的15萬年之間，沒有證據顯示氣候有出現持續性的改變。
當時生活在海水或是海底的微小帶殼生物會形成化石，研究人員透過觀察其中的同位素
(原子類型)
組成，就可以推論出過去的氣候，因為同位素的模式能反映出這些生物存活當下的海水溫度。
倫敦大學學院地球科學系的Bridget
Wade教授是共同作者，他說：「我們的結果令人相當詫異，因為它指出撞擊之後氣候並未發生實質的變化。我們預期同位素值會朝其中一個方向偏移，代表海水的溫度有所升高或是下降，但結果卻不然。雖然發生了這些大型小行星的撞擊事件，但從長遠來看，地球運作的步調似乎還是一如往常。」
「然而，因為我們研究的取樣間隔為11000年，因此無法捕捉到十年或百年之間發生的短期變化。以人類的時間尺度來說，這些小行星撞擊事件肯定會是災難一場。它們可以產生強烈的衝擊波與海嘯、面積廣大的野火，並揚起大量的塵埃到空氣當中而遮蔽陽光。」
「模擬規模更大、把恐龍消滅的西克蘇魯伯撞擊事件的研究也指出，將時間尺度大幅縮短至不到25年的情況下，氣候會因撞擊而有所改變。」
「因此我們還是得去了解撞擊的後果，並且提供經費給防止未來有小行星撞上地球的任務。」
Wade教授與地球科學系的碩士生Natalie
Cheng都是研究團隊的成員。他們分析了超過1500個稱作有孔蟲的單細胞生物化石中的同位素，包括了生活在海洋表面附近(浮游性)以及海床上方(底棲性)兩種類型的有孔蟲。
這些化石的年代為距今3550萬至3590萬年前。它們保存在一根三公尺長，由深海鑽探科學計畫從墨西哥灣海底取出來的岩芯當中。
前人推估這段期間撞上地球兩顆最大的小行星，寬度分別為5-8公里(3-5英里)與3-5公里(2-3英里)
。較大的那顆寬度跟埃佛勒斯峰的高度差不多，撞擊後形成了波皮蓋隕石坑。
屬於始新世晚期的該段期間除了這兩次撞擊事件之外，現存的證據指出還有三顆較小的小行星也撞上了地球，代表此時太陽系的小行星帶受到了擾動。
研究人員提出過往對於這段時期氣候的研究並沒有得出定論，有些研究認為小行星撞擊跟氣候加速冷化有所關聯，而另一些則認為其跟幾個較暖的短期有關。
然而，這些研究的解析度都比較低，它們所用的樣品時間間隔都大於11000年；而且它們的分析方式也較為侷限，像是只研究生活在海床表面的底棲性有孔蟲。
透過生活在不同海洋深度的化石，新的研究結果得出了更為全面的圖像，讓我們了解海洋對於撞擊事件的反應。
研究人員探討了幾種浮游性與底棲性有孔蟲的碳氧同位素組成。
他們發現在兩次小行星撞擊事件發生的100,000年前左右，同位素的組成出現了變化，意味著表層海水的溫度大約升高了2℃，而深層海水則降低了1℃。但是在撞擊事件附近與之後卻沒有發現任何變化。
研究人員在岩石中也發現了這兩次主要撞擊事件留下的證據：數千顆微小的玻璃珠(二氧化矽)。當含有二氧化矽的岩石被小行星撞擊給蒸發就會被噴到空中，冷卻固化之後便形成了這些玻璃珠而保存下來。
岩石中發現的二氧化矽珠(微型球體)在顯微鏡下的影像(每條白線為100微米)。圖片來源：Natalie
Cheng / Bridget Wade.
共同作者，地球科學系的碩士生Natalie
Cheng表示：「由於希克蘇魯伯撞擊事件很可能造成了大滅絕，因此我們也好奇發生在始新世一連串規模不算小的小行星撞擊事件，是否也造成了持續許久的氣候變遷，而進行了這項研究。結果令我們感到訝異，氣候並沒有因為這些撞擊事件而有顯著的變化。」
「從微體化石保存的化學成分來解讀地球氣候的歷史是件非常神奇的事。而且這項研究特別有趣的地方是，在我們挑選有孔蟲種類的過程當中，還發現到某些相當美麗的微型球體樣本。」
研究經費來自英國國家自然環境研究委員會。
Massive asteroid
impacts did not change Earth’s climate in the long term
Two massive asteroids hit Earth around
35.65 million years ago, but did not lead to any lasting changes in the Earth’s
climate, according to a new study by UCL researchers.
The rocks, both several miles wide, hit Earth about
25,000 years apart, leaving the 60-mile (100km) Popigai crater in Siberia,
Russia, and the 25-55 mile (40-85km) crater in the Chesapeake Bay, in the
United States - the fourth and fifth largest known asteroid craters on Earth.
The new study, published in the journal Communications Earth & Environment,
found no evidence of a lasting shift in climate in the 150,000 years that
followed the impacts.
The researchers inferred the past climate by looking
at isotopes (atom types) in the fossils of tiny, shelled organisms that lived
in the sea or on the seafloor at the time. The pattern of isotopes reflects how
warm the waters were when the organisms were alive.
Co-author Professor Bridget Wade (UCL Earth Sciences)
said: “What is remarkable about our results is that there was no real change
following the impacts. We expected the isotopes to shift in one direction or
another, indicating warmer or cooler waters, but this did not happen. These
large asteroid impacts occurred and, over the long term, our planet seemed to
carry on as usual.
“However, our study would not have picked up
shorter-term changes over tens or hundreds of years, as the samples were every
11,000 years. Over a human time scale, these asteroid impacts would be a disaster.
They would create a massive shockwave and tsunami, there would be widespread
fires, and large amounts of dust would be sent into the air, blocking out
sunlight.
“Modelling studies of the larger Chicxulub impact,
which killed off the dinosaurs, also suggest a shift in climate on a much
smaller time scale of less than 25 years.
“So we still need to know what is coming and fund
missions to prevent future collisions.”
The research team, including Professor Wade and MSc
Geosciences student Natalie Cheng, analysed isotopes in over 1,500 fossils of
single-celled organisms called foraminifera, both those that lived close to the
surface of the ocean (planktonic foraminifera) and on the seafloor (benthic
foraminifera).
These fossils ranged from 35.5 to 35.9 million years
old and were found embedded within three metres of a rock core taken from
underneath the Gulf of Mexico by the scientific Deep Sea Drilling Project.
The two major asteroids that hit during that time
have been estimated to be 3-5 miles (5-8km) and 2-3 miles (3-5km) wide. The
larger of the two, which created the Popigai crater, was about as wide as
Everest is tall.
In addition to these two impacts, existing evidence
suggests three smaller asteroids also hit Earth during this time – the late
Eocene epoch – pointing to a disturbance in our solar system’s asteroid belt.
Previous investigations into the climate of the time
had been inconclusive, the researchers noted, with some linking the asteroid
impacts with accelerated cooling and others with episodes of warmer
temperatures.
However, these studies were conducted at lower
resolution, looking at samples at greater intervals than 11,000 years, and
their analysis was more limited – for instance, only looking at species of
benthic foraminifera that lived on the seafloor.
By using fossils that lived at different ocean
depths, the new study provides a more complete picture of how the oceans
responded to the impact events.
The researchers looked at carbon and oxygen isotopes
in multiple species of planktonic and benthic foraminifera.
They found shifts in isotopes about 100,000 years
prior to the two asteroid impacts, suggesting a warming of about 2 degrees C in
the surface ocean and a 1 degree C cooling in deep water. But no shifts were
found around the time of the impacts or afterwards.
Within the rock, the researchers also found evidence
of the two major impacts in the form of thousands of tiny droplets of glass, or
silica. These form after silica-containing rocks get vaporised by an asteroid.
The silica end up in the atmosphere, but solidify into droplets as they cool.
Co-author and MSc Geosciences graduate Natalie Cheng
said: “Given that the Chicxulub impact likely led to a major extinction event,
we were curious to investigate whether what appeared as a series of sizeable
asteroid impacts during the Eocene also caused long-lasting climate changes. We
were surprised to discover that there were no significant climate responses to
these impacts.
“It was fascinating to read Earth's climate history
from the chemistry preserved in microfossils. It was especially interesting to
work with our selection of foraminifera species and discover beautiful
specimens of microspherules along the way.”
The study received funding from the UK’s Natural
Environment Research Council (NERC).
原始論文：Bridget S.
Wade, Natalie K. Y. Cheng. No paleoclimatic anomalies are associated
with the late Eocene extraterrestrial impact events. Communications
Earth & Environment, 2024; 5 (1) DOI: 10.1038/s43247-024-01874-x
引用自：University College London. "Massive
asteroid impacts did not change Earth's climate in the long term."
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