一組研究團隊最近發表在《自然―地球科學》(Nature Geoscience)的新研究，探討了奧陶紀晚期大滅絕的成因
By Dan Bernardi
我們都知道恐龍在一次大滅絕當中就此滅亡，但是你知道其實還有別的大滅絕事件嗎？過去總共發生了五次最為嚴重的大滅絕事件，合稱為「big
five」，每次都是在一段特定的地質時期之中，當時地球上活著的所有物種至少有四分之三走入滅絕。在面臨現今全球暖化與氣候變遷的趨勢之下，許多研究人員相信我們可能正處於第六次大滅絕。
研究人員正在檢視一座海岸邊的奧陶紀岩石露頭，該地位於加拿大魁北克的安蒂科斯蒂島。圖片來源：André Desrochers/渥太華大學
對於科學家來說，找出地球發生大滅絕事件的根本原因，一直以來都是熱門的題目。因為了解什麼樣的環境條件在過去可以造成絕大部分的物種消失，或許可以避免未來再度發生類似的事件。
雪城大學地球與環境科學系、加州大學柏克萊分校、加州大學河濱分校、勃艮第弗朗什―孔泰大學、新墨西哥大學、渥太華大學、中國科學技術大學以及史丹佛大學組成的科學團隊最近合力撰寫了一篇論文，探討了「big
five」中第一個，也是最古老的奧陶紀晚期大滅絕(LOME，發生在約4億4500萬年前)。當時85%左右的海洋生物都消失了，牠們大部分生活在靠近陸地的淺海區域。
論文主要作者是加州大學河濱分校的Alexandre
Pohl (現為法國第戎的勃艮第弗朗什―孔泰大學的博士後研究員)。他跟共同作者為了判斷釀成該次滅絕事件的原因以及引發過程，他們調查了事件之前、當下、之後的海洋環境。研究結果發表在期刊《自然―地球科學》。
加州大學柏克萊分校的副教授Seth
Finnegan是研究大滅絕事件的專家，對於奧陶紀時的海洋生態系他如此描述：當時海裡的生物多樣性相當豐富，其中含有動物製造出來的第一批礁體，不過脊椎動物的數量並不多。
「如果你潛進奧陶紀的海中，會看到一些熟悉的動物，像是蚌殼、海螺與海綿，不過還有許多在今日多樣性已經大幅減少、甚至是完全消失的動物，例如三葉蟲、腕足類和海百合，」Finnegan說。
奧陶紀的腕足類化石，採自加拿大魁北克的安蒂科斯蒂島上的岩石露頭。圖片來源：André Desrochers/渥太華大學
不同於快速發生的大滅絕，例如6550萬年前左右的白堊紀―第三紀大滅絕事件，當時恐龍與其他物種在瞬間就消失了，Finnegan說LOME的過程花了相當長的一段時間，估計從不到五百萬年到將近二億年都有可能。
關於LOME最大的爭議之一是，海水缺氧是否為這段期間發生大滅絕事件的原因？為了探討這道問題，團隊將地球化學的試驗結果整合到數值模擬與電腦模型當中。
雪城大學地球與環境科學的教授Zunli
Lu和他的學生，在海洋不同深度的氧含量這方面做出了重要發現。他們測量了這段時期形成的碳酸鹽中碘的濃度有多高，該數值可以作為地球歷史上海洋氧氣含量變化的指標。
他們結合了實驗數據與電腦模型的模擬結果，得出在滅絕事件的發生期間，並沒有證據顯示大部分動物居住的淺海棲地有缺氧加重的現象。意謂在奧陶紀晚期，可能是氣候冷化以及其他因素共同作用之下造成了LOME。
另一方面，證據卻指出同一期間深海的缺氧情形有擴增的現象。氣候模擬的專家Alexandre
Pohl表示對於傳統的海洋氧氣模型來說，這是無法解釋的一道難題。
「隨著氣溫降低，海洋上層的氧氣增加在預料之中，因為大氣裡的氧氣比較容易溶解在冷水，」Pohl表示。「然而，我們卻看到海洋下層的缺氧情形擴大了，這令我們感到驚訝，因為地球歷史上海洋缺氧事件通常跟火山引起的全球暖化有關。」
他們認為深海缺氧的原因在於流經全球海洋的環流。Pohl表示海洋環流是氣候系統當中非常重要的一個環節，這是需要牢記的關鍵。
Pohl是加州大學河濱分校的教授Andy
Ridgwell團隊的成員之一，他在氣候模擬這方面相當資深。Ridgwell的模擬結果顯示氣候冷化可能會改變海洋環流的模式，使得富含氧氣的海水停止從淺海流入深海。
Lu表示認證氣候冷化也可以造成海洋某些地方的氧含量下降，是他們這項研究最大的重點。
「數十年來，我們領域當中的主流學派認為全球暖化會造成海洋的氧氣減少，使得海洋適合居住的程度受到衝擊，還可能讓整個生態系陷入不穩定，」Lu表示。「最近幾年有越來越多證據指出地球歷史上某些時期，氣候變冷的時候氧含量也跟著降低。」
雖然科學家們對於奧陶紀晚期大滅絕的成因還是沒有取得完全的共識，而且在短時間內大概也不會，不過團隊的研究顯示只用氧氣變化並不足以解釋滅絕事件為何發生，他們的新數據更傾向於溫度變化才是LOME消滅動物的機制。
Pohl希望隨著品質更好的氣候數據以及更精密的數值模型問世，未來他們可以更有根據地描述引發奧陶紀晚期大滅絕的因素。
Uncovering the secrets behind Earth’s
first major mass extinction
A team of researchers have published a
new study in Nature Geoscience
exploring the cause of the Late Ordovician mass extinction.
We all know that the dinosaurs died in a
mass extinction. But did you know that there were other mass extinctions? There
are five most significant mass extinctions, known as the “big five,” where at
least three quarters of all species in existence across the entire Earth faced
extinction during a particular geological period of time. With current trends
of global warming and climate change, many researchers now believe we may be in
a sixth.
Discovering the root cause of Earth’s mass
extinctions has long been a hot topic for scientists, as understanding the
environmental conditions that led to the elimination of the majority of species
in the past could potentially help prevent a similar event from occurring in
the future.
A team of scientists from Syracuse University’s
Department of Earth and Environmental Sciences, the University of California,
Berkeley and the University of California, Riverside, Université Bourgogne
Franche-Comté, the University of New Mexico, the University of Ottawa, the
University of Science and Technology of China and Stanford University recently
co-authored a paper exploring the Late Ordovician mass extinction (LOME), which
is the first, or oldest of the “big five (~445 million years ago).” Around 85%
of marine species, most of which lived in shallow oceans near continents,
disappeared during that time.
Lead author Alexandre Pohl, from UC Riverside (now a
postdoctoral research fellow at Université Bourgogne Franche-Comté in Dijon,
France) and his co-authors investigated the ocean environment before, during,
and after the extinction in order to determine how the event was brewed and
triggered. The results from their study have been published in the journal Nature Geoscience.
To paint a picture of the oceanic ecosystem during
the Ordovician Period, mass extinction expert Seth Finnegan, associate
professor at UC Berkeley, says that seas were full of biodiversity. Oceans
contained some of the first reefs made by animals, but lacked an abundance of
vertebrates.
“If you had gone snorkeling in an Ordovician sea you would
have seen some familiar groups like clams and snails and sponges, but also many
other groups that are now very reduced in diversity or entirely extinct like
trilobites, brachiopods and crinoids” says Finnegan.
Unlike with rapid mass extinctions, like the
Cretaceous-Tertiary extinction event where dinosaurs and other species died off
suddenly some 65.5 million years ago, Finnegan says LOME played out over a
substantial period of time, with estimates between less than half a million to
almost two million years.
One of the major debates surrounding LOME is whether lack
of oxygen in seawater caused that period’s mass extinction. To investigate this
question, the team integrated geochemical testing with numerical simulations
and computer modeling.
Zunli Lu, professor of Earth and environmental
sciences at Syracuse University, and his students took measurements of iodine
concentration in carbonate rocks from that period, contributing important
findings about oxygen levels at various ocean depths. The concentration of the
element iodine in carbonate rocks serves as an indicator for changes in oceanic
oxygen level in Earth's history.
Their data, combined with computer modeling
simulations, suggested that there was no evidence of anoxia – or lack of
oxygen – strengthening during the extinction event in the shallow ocean animal
habitat where most organisms lived, meaning that climate cooling that occurred
during the Late Ordovician period combined with additional factors likely was
responsible for LOME.
On the other hand, there is evidence that anoxia in
deep oceans expanded during that same time, a mystery that cannot be explained
by the classic model of ocean oxygen, climate modeling expert Alexandre Pohl
says.
“Upper-ocean oxygenation in response to cooling was
anticipated, because atmospheric oxygen preferentially dissolves in cold
waters,” Pohl says. “However, we were surprised to see expanded anoxia in the
lower ocean since anoxia in Earth’s history is generally associated with
volcanism-induced global warming.”
They attribute the deep-sea anoxia to the circulation
of seawater through global oceans. Pohl says that a key point to keep in mind
is that ocean circulation is a very important component of the climatic system.
He was part of a team led by senior modeler Andy
Ridgwell, professor at UC Riverside, whose computer modeling results show that
climate cooling likely altered ocean circulation pattern, halting the flow of
oxygen-rich water in shallow seas to the deeper ocean.
According to Lu, recognizing that climate cooling can
also lead to lower oxygen levels in some parts of the ocean is a key takeaway
from their study.
“For decades, the prevailing school of thoughts in
our field is that global warming causes the oceans to lose oxygen and thus
impact marine habitability, potentially destabilizing the entire ecosystem,” Lu
says. “In recent years, mounting evidence point to several episodes in Earth’s history
when oxygen levels also dropped in cooling climates.”
While the causes of Late Ordovician extinction have
not been fully agreed upon, nor will they for some time, the team’s study rules
out changes in oxygenation as a single explanation for this extinction and adds
new data favoring temperature change being the killing mechanism for LOME.
Pohl is hopeful that as better climate data and more
sophisticated numerical models become available, they will be able to offer a
more robust representation of the factors that may have led to the Late Ordovician
mass extinction.
原始論文：Alexandre
Pohl, Zunli Lu, Wanyi Lu, Richard G. Stockey, Maya Elrick, Menghan Li, André
Desrochers, Yanan Shen, Ruliang He, Seth Finnegan, Andy Ridgwell. Vertical
decoupling in Late Ordovician anoxia due to reorganization of ocean circulation. Nature
Geoscience, 2021; DOI: 10.1038/s41561-021-00843-9
引用自：Syracuse University. "Uncovering the
secrets behind Earth’s first major mass extinction.”
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