發現「改變世界的碰撞」產生的另一道餘波

原文網址：https://www.princeton.edu/news/2019/04/25/princeton-geoscientists-find-new-fallout-collision-changed-world

發現「改變世界的碰撞」產生的另一道餘波

Liz Fuller-Wright

五千萬年前，現為印度次大陸的陸塊撞上了亞洲，改變了陸地的分布模式，也對地形、全球氣候等方面造成了諸多影響。美國普林斯頓大學的科學家團隊最近發現了這起事件的另一道效應：全球海洋氧氣增加，進而改變了生物的生存環境。

陸地與海洋的樣貌都會隨時間改變。現在的印度次大陸曾是一個陸塊，之後它往北用力撞上亞洲，造成特提斯洋關閉並擠出喜馬拉雅山，史稱「改變世界的碰撞」。這兩張「古地圖」顯示了發生前(上)與發生期間(下)的海陸分佈。當時全球海平面比現今還高，使得大部分的非洲北部以及各大陸的部分地區被高鹽度的淺海(淺藍色區塊)覆蓋。最近普林斯頓大學的研究團隊利用從三個地點(星號標記)採來的樣品，首度建立出7000萬年前至3000萬年前海洋氮和氧的濃度變化，結果顯示印度和亞洲碰撞之後海洋化學發生了重大變化。另一次變化則發生在3500萬年前，南極洲的冰層開始變厚，造成全球海平面下降的時候。圖片由普林斯頓大學的Emma Kast製作，古地理的重建結果經Deep Time Maps授權。

「研究結果跟人們過去的認知截然不同。」普林斯頓大學地球科學系的研究生Emma Kast表示。她是這篇4月26日發表於《科學》(Science)的論文主要作者。「重建出來的變化幅度讓我們大為驚訝。」

Kast利用海洋生物的微小殼體來建立距今7000萬年前――也就是恐龍滅亡不久之前――到3000萬年前海洋氮含量的變化。論文共同作者，普林斯頓大學地球科學系的副教授 John Higgins說，這份紀錄對於地球氣候領域而言是相當重要的貢獻。

「在我們這個領域有些紀錄就像金科玉律，任何假說想描述生物和地球化學之間關聯的同時，也必須能解釋這些紀錄。」Higgins說。「這類紀錄相當稀少，一部分的原因是可以追溯到許久之前的紀錄相當難建立――五千萬年前的岩石可不會輕易透露出它的秘密。因此，Emma的這份紀錄絕對足以成為那些金科玉律之一。從現在起，人們想要探索最近七千萬年來地球如何演變的時候，勢必要把Emma的數據也納入其中。」

氮是大氣含量最多的氣體，除此之外，也是地球所有生命生存的關鍵。論文資深作者，普林斯頓大學地質和地球物理系的教授Daniel Sigman表示：「我研究氮的原因是為了探討全球氣候。」Sigman、Higgins 和Daniel Stolper一同發起了這項計畫。Stolper當時為普林斯頓大學的博士後研究員，現在為加州大學柏克萊分校地球和行星科學系的助理教授。

地球上的所有生物都需要「固定氮」，有時也稱作「生物可利用的氮」。雖然地球大氣的78%都是氮氣，但可以把氮氣轉換成生物可利用的形式，也就是把氮「固定」下來的生物卻相當稀少。海洋中生活在表層的藍綠菌可以把氮固定下來，供海中其他所有生物使用。這些藍綠菌和其他生物死亡之後，便會往下沉並遭到分解。

氮有兩種穩定同位素： ¹⁵N和¹⁴N。在氧氣含量低的水中，分解作用會耗盡所有的固定氮，過程中會稍微傾向使用較輕的氮同位素，因此 海水¹⁵N和¹⁴N的比例便可以反映海水的氧含量。

稱為有孔蟲的微小海洋生物在活著的時候會吸收海水裡的氮，死掉的時候便會將當時海水的氮同位素比保存在殼體之中。大洋鑽探計畫從北大西洋、北太平洋和南大西洋採集了許多有孔蟲化石，Kast和她的同事分析這些有孔蟲之後，便能重建古代海水¹⁵N和¹⁴N的比例，從而建立海水氧含量的變化歷程。

氧含量決定了海洋生物如何分布，因為大部份的海洋生物都難以生存在缺氧的海水當中。過去許多氣候暖化事件都造成了海水氧含量下降，因而縮減了海洋生物的棲地――從微小的浮游生物，到以牠們維生的魚類和鯨魚都受到了影響。科學家試著預測當前以及未來的氣候暖化會造成什麼後果，他們警告海洋氧含量降低將重創海洋生態系，包括許多十分重要的魚類族群。

研究人員在整合這份絕無僅有的海洋氮同位素紀錄時，發現恐龍滅亡後的1000萬年間，海水的¹⁵N對¹⁴N的比例較高，顯示這段期間海洋的氧含量較低。他們最初認為此現象的成因是當時溫暖的氣候，因為溫度較高的水溶氧也較少。然而，隨著時間經過，故事卻出現了轉折：5500萬年前海洋氧濃度開始升高，但是此時的氣候依舊溫暖。

「與我們最初預期的相反，全球氣候並非是海洋氧濃度和氮循環發生變化的主要原因。」Kast說。那麼最有可能的嫌犯是誰？答案是板塊運動。當代氣候研究的創始者之一，偉大的地質學家Wally Broecker曾把印度與亞州的碰撞喻為「改變世界的碰撞」――這起事件關閉了古海洋「特提斯洋」，因而劇烈改變了大陸棚的分布，以及大陸棚跟大洋之間的連結。

Sigman說：「在數百萬年之間，板塊運動可能會對海洋環流產生重大影響。」但他接著表示，這不代表可以輕忽氣候變遷的影響，「在千年到萬年的時間尺度下，氣候則占有主導地位。」

New fallout from ‘the collision that changed the world’

When the landmass that is now the Indian subcontinent slammed into Asia about 50 million years ago, the collision changed the configuration of the continents, the landscape, global climate and more. Now a team of Princeton University scientists has identified one more effect: the oxygen in the world’s oceans increased, altering the conditions for life.

“These results are different from anything people have previously seen,” said Emma Kast, a graduate student in geosciences and the lead author on a paper coming out in Science on April 26. “The magnitude of the reconstructed change took us by surprise.”

Kast used microscopic seashells to create a record of ocean nitrogen over a period from 70 million years ago — shortly before the extinction of the dinosaurs — until 30 million years ago. This record is an enormous contribution to the field of global climate studies, said John Higgins, an associate professor of geosciences at Princeton and a co-author on the paper.

“In our field, there are records that you look at as fundamental, that need to be explained by any sort of hypothesis that wants to make biogeochemical connections,” Higgins said. “Those are few and far between, in part because it’s very hard to create records that go far back in time. Fifty-million-year-old rocks don’t willingly give up their secrets. I would certainly consider Emma’s record to be one of those fundamental records. From now on, people who want to engage with how the Earth has changed over the last 70 million years will have to engage with Emma’s data.”

In addition to being the most abundant gas in the atmosphere, nitrogen is key to all life on Earth. “I study nitrogen so that I can study the global environment,” said Daniel Sigman, Princeton’s Dusenbury Professor of Geological and Geophysical Sciences and the senior author on the paper. Sigman initiated this project with Higgins and then-Princeton postdoctoral researcher Daniel Stolper, who is now an assistant professor of Earth and planetary science at the University of California-Berkeley.

Every organism on Earth requires “fixed” nitrogen — sometimes called “biologically available nitrogen.” Nitrogen makes up 78% of our planet’s atmosphere, but few organisms can “fix” it by converting the gas into a biologically useful form. In the oceans, cyanobacteria in surface waters fix nitrogen for all other ocean life. As the cyanobacteria and other creatures die and sink downward, they decompose.

Nitrogen has two stable isotopes, ¹⁵N and ¹⁴N. In oxygen-poor waters, decomposition uses up “fixed” nitrogen. This occurs with a slight preference for the lighter nitrogen isotope, ¹⁴N, so the ocean’s ¹⁵N-to-¹⁴N ratio reflects its oxygen levels.

That ratio is incorporated into tiny sea creatures called foraminifera during their lives, and then preserved in their shells when they die. By analyzing their fossils — collected by the Ocean Drilling Program from the North Atlantic, North Pacific, and South Atlantic — Kast and her colleagues were able to reconstruct the ¹⁵N-to-¹⁴N ratio of the ancient ocean, and therefore identify past changes in oxygen levels.

Oxygen controls the distribution of marine organisms, with oxygen-poor waters being bad for most ocean life. Many past climate warming events caused decreases in ocean oxygen that limited the habitats of sea creatures, from microscopic plankton to the fish and whales that feed on them. Scientists trying to predict the impact of current and future global warming have warned that low levels of ocean oxygen could decimate marine ecosystems, including important fish populations.

When the researchers assembled their unprecedented geologic record of ocean nitrogen, they found that in the 10 million years after dinosaurs went extinct, the ¹⁵N-to-¹⁴N ratio was high, suggesting that ocean oxygen levels were low. They first thought that the warm climate of the time was responsible, as oxygen is less soluble in warmer water. But the timing told another story: the change to higher ocean oxygen occurred around 55 million years ago, during a time of continuously warm climate.

“Contrary to our first expectations, global climate was not the primary cause of this change in ocean oxygen and nitrogen cycling,” Kast said. The more likely culprit? Plate tectonics. The collision of India with Asia — dubbed “the collision that changed the world” by legendary geoscientist Wally Broecker, a founder of modern climate research — closed off an ancient sea called the Tethys, disturbing the continental shelves and their connections with the open ocean.

“Over millions of years, tectonic changes have the potential to have massive effects on ocean circulation,” said Sigman. But that doesn’t mean climate change can be discounted, he added. “On timescales of years to millenia, climate has the upper hand.”

原始論文：Emma R. Kast, Daniel A. Stolper, Alexandra Auderset, John A. Higgins, Haojia Ren, Xingchen T. Wang, Alfredo Martínez-García, Gerald H. Haug, Daniel M. Sigman. Nitrogen isotope evidence for expanded ocean suboxia in the early Cenozoic. Science, 2019; 364 (6438): 386 DOI: 10.1126/science.aau5784

引用自：Princeton University. "New fallout from 'the collision that changed the world'."