日本的遠古岩石揭露了關於海洋缺氧事件的線索

 原文網址：https://news.northwestern.edu/stories/2024/12/prehistoric-rock-in-japan-reveals-clues-to-major-ocean-anoxic-event/

By Amanda Morris

在研究日本盧別岳一帶出露的遠古岩石與化石之後，研究人員對於海洋缺氧事件1a(OAE 1a)的發生年代與持續時間重新得到了更加精準的數值。這場極為嚴重的環境惡化事件使得地球海洋無法吸到氧氣，造成大量生物滅絕，又以浮游生物受到的打擊特別嚴重。

在新研究當中，研究人員探討的對象並非深海，而是日本北海道盧別岳的西北側露出的古代岩層。這些稱為凝灰岩的岩石是由經年累月堆積下來的火山灰固結而成。圖片來源：Luca Podrecca

研究人員長久以來推測中生代的二氧化碳增加、全球暖化以及海洋缺少氧氣，是由巨大的海底火山爆發所造成。最近西北大學的地球科學家參與在內的國際團隊得出這場火山爆發的精確時間，並將OAE 1a的開始時間標定在距今1億1950萬年前。這項成果連同越來越多的證據指出造成缺氧事件的直接原因，便是火山排放的二氧化碳。

新研究也定出OAE 1a的持續時間為110幾萬年。這項新資訊有助於科學家更加了解地球的氣候與海洋系統，在面對壓力時會如何運作以及出現什麼樣的反應——尤其考量到該事件和目前的氣候暖化有可比之處。

新研究上個月底發表於期刊《科學前緣》(Science Advances)。這是文獻中迄今對於單次海洋缺氧事件的定年研究中，最詳細且解析度也非常高的結果。

「在一個溫室世界當中，氣候暖化造成的結果之一便是海洋缺氧事件，」此研究的資深作者，西北大學的Brad Sageman表示。「如果我們想要精準預測在人類造成的暖化之下，未來數十年我們會看見什麼樣的景象，這類資訊便十分重要。觀察來自過去的數據是洞悉未來的最好方式。」

Sageman教授是研究過去氣候的專家，目前任職於西北大學文理學院的地球、環境與行星科學系，也是永續發展與能源研究所的聯合主任。

與西北大學的淵源

白堊紀經歷了兩次大型的海洋缺氧事件與數次小型的，而OAE 1a便是最大的兩次之一。OAE 1a最有可能的成因為火山爆發迅速把大量二氧化碳注入到海洋與大氣當中。這些火山並不是一般的火山，而是持續噴發數百萬年、流出的玄武岩多達一百萬立方公里的大型火成岩區域。當二氧化碳和海水產生反應會形成碳酸這種弱酸，可以溶解海洋生物的殼體。對於海洋生物來說，碳酸和低落的氧濃度聯手造成了嚴重的後果。

研究人員開始仔細考慮海洋發生缺氧事件的可能性是在1970年代中期，起因為西北大學的地質學家Seymour Schlanger與牛津大學的教授Hugh Jenkyns的發現。他們檢視了從太平洋海床取得的沉積物樣品，發現其中有黑色、富含有機碳的頁岩，而且它們的成分和年代竟然跟大西洋的沉積物以及義大利的岩層樣品都互相吻合。

最能解釋這些沉積物如何形成的說法便是範圍廣大的缺氧事件。缺氧可以防止植物與動物殘骸裡的有機質被分解，造成全世界的環境變成富含有機質。這些沉到海底的浮游生物與其他化石並未被分解，而是持續堆積成富含有機碳的岩層並分布在世界各處。

「深海與陸上要怎麼樣才能同時形成黑色頁岩？」Sageman問道。「Schlanger和Jenkyns理解到當時必定發生了規模浩大的全球性事件，使得海洋從表面到底層的氧氣含量都大幅降低。」

固結在石頭中的歷史

在新研究當中，研究人員不只探討了海洋深處，還有日本北海道盧別岳的西北側出露的古老地層。這些稱為凝灰岩的岩石是由經年累月堆積的火山灰固結而成。在日本群島形成的過程中，板塊構造運動把這些凝灰岩抬到海平面之上，接著河川切穿這座北海道的溫帶雨林，使得它們露出地表而被人發現。透過採集並分析這些凝灰岩，Sageman和他的博士生Luca Podrecca與同僚得以窺見地質歷史當中的一段時光。

「熔岩以液體的形式從火山出來之後便開始冷卻，」Sageman表示。「在過程中會逐漸形成礦物晶體。當火山灰固結之後，這些晶體成為了微小的封閉系統。被它們封鎖在內的原子有一些會開始衰變，也就是從一種同位素轉變成另一種。這可以作為一種工具來讓我們定年該次噴發的時間，進而定年一套沉積岩當中的特定層位。團隊當中來自日本北海道大學、英國杜倫大學和西北大學的成員專長主要是判別地層的特性並對比世界各地的地層；而威斯康辛麥迪遜分校與博伊西州立大學的同僚則是地質年代學分析的專家。」

研究人員也運用了其他種類的同位素，像是透過碳來追蹤碳循環中的同步變化，並用鋨來追蹤火山活動以及海洋化學的變化。

「這些同位素系統讓我們可以對比北海道、法國南部以及全球各處的其他研究地點中，發生OAE 1a的區間，」Sageman表示。「它們讓我們可以標定出地質時間裡特定的某一瞬間。」

畫出精確的時間線

這份證據顯示碳同位素的比例在白堊紀初期，OAE 1a開始之際發生了突然的變化，最初的原因是碳循環裡出現大量來自火山的二氧化碳(之後則是因為過多有機物質遭到埋藏)。於此同時鋨同位素比也出現了變化，反映出有許多來自火山的物質進到海水當中。這些事件的發生時間可以對應到翁通爪哇—努伊複合體(Ontong Java Nui complex)的形成年代，其為一座位在西南太平洋、面積約為阿拉斯加的龐大火成岩區域。

在知道海洋花了110萬年才從二氧化碳突然增加所造成的傷害復原之後，對於二氧化碳造成的暖化事件所帶來的影響可以持續多久，以及它會連帶造成什麼樣的效應(像是海洋缺氧)，研究人員現在有了更加深入的理解。

「我們現在已經能在墨西哥灣看到低氧含量的區域，」Sageman表示。「然而最主要的區別在於，過去的事件是在數十萬年到數百萬年之間逐漸形成，但我們卻只用了不到200年的時間，就讓氣候暖化達到了相似(甚至更高)的程度。」

此研究標題為「Radioisotopic chronology of Ocean Anoxic Event 1a: Framework for analysis of driving mechanisms(海洋缺氧事件1a的放射性同位素年代學：做為分析驅動機制的框架)」。經費來源為美國國家科學基金會、英國國家自然環境研究委員會、日本科學技術振興財團。

 

Prehistoric rock in Japan reveals clues to ocean extinction event

By studying prehistoric rocks and fossils emerging from the side of Mount Ashibetsu in Japan, researchers have precisely refined the timing and duration of Ocean Anoxic Event 1a (OAE 1a), an extreme environmental disruption that choked oxygen from Earth’s oceans to cause significant extinction, especially among plankton.

Researchers have long suspected that massive volcanic eruptions undersea caused carbon dioxide (CO₂) increases, global warming and depleted oxygen (called anoxia) in the ocean during the Mesozoic Period. Now, an international team of researchers, including Northwestern University Earth scientists, determined the precise timing of the volcanic eruption and OAE 1a, which started 119.5 million years ago. The work adds to a growing volume of evidence that volcanic CO₂ emissions directly triggered the anoxic event.

The new study also determined that OAE 1a lasted for just over 1.1 million years. This new information helps scientists better understand how the Earth’s climate and ocean system operates and responds to stress — especially as it relates to current warming.

The study was published late last month in the journal Science Advances. It marks the most detailed and highly resolved dating of an ocean anoxic event ever achieved.

“Ocean anoxic events occur in part as a consequence of climatic warming in a greenhouse world,” said Northwestern’s Brad Sageman, a senior author of the study. “If we want to make accurate predictions about what we will see in the decades ahead with human-caused warming, this information is invaluable. The best way to understand the future is to look at data from the past.”

An expert on ancient climates, Sageman is a professor of Earth, Environmental and Planetary Sciences at Northwestern’s Weinberg College of Arts and Sciences and a co-director of the Paula M. Trienens Institute for Sustainability and Energy.

A Northwestern connection

The Cretaceous Period experienced two major and several minor ocean anoxic events, with OAE 1a as one of the two largest. The most likely cause: volcanic eruptions rapidly injected massive amounts of CO₂ into the ocean and atmosphere. These aren’t ordinary volcanoes but large igneous provinces that erupt up to a million cubic kilometers of basalt over several millions of years. When CO₂ reacts with seawater, it forms a weak carbonic acid, which literally dissolves sea creatures’ shells. The acid, combined with low oxygen levels, has significant consequences for sea life.

Researchers first began pondering ocean anoxic events in the mid-1970s, after a discovery by Northwestern geologist Seymour Schlanger and Oxford professor Hugh Jenkyns. When examining sediment samples from the Pacific Ocean floor, Schlanger and Jenkyns discovered black, organic carbon-rich shales that matched samples — in composition and age — from both the Atlantic Ocean and rock formations in Italy.

Widespread lack of oxygen was the most likely explanation for these deposits. Anoxia prevents the breakdown of organic matter from dead plants and animals, leading to a global pattern of organic enrichment. Instead of decomposing, the settling plankton and other fossils accumulated to form organic carbon-rich strata scattered around the globe.

“How were black shales forming at the same time in the deep oceans and up on land?” Sageman asked. “Schlanger and Jenkyns realized there must have been a massive global event that caused oxygen to decrease from the ocean surface all the way down to the seafloor.”

History solidified in stone

In the new study, researchers looked not to the depths of the oceans but to ancient strata along the northwest edge of a mountain on Japan’s Hokkaido Island. The rocks, or tuffs, formed from volcanic ash that settled and solidified over time. Tectonic activity lifted these layers above sea level during formation of the Japanese islands, leaving them exposed and accessible where streams carve through the temperate rainforest of Hokkaido. By collecting and analyzing the tuffs, Sageman, his Ph.D. student, Luca Podrecca, and their collaborators gained a glimpse into geologic history.

“Magma comes out of a volcano in liquid form and then begins to cool,” Sageman said. “During this process, crystals start to form. By the time the tuff solidifies, the crystals become a tiny closed system. They lock in atoms, and some of those atoms, like uranium, start to decay, meaning they convert from one isotope to another. That provides a tool to date the eruption, and, thus, date a specific layer within a stack of sedimentary rock. While the expertise of team members from Tohuku University in Japan, Durham University in the U.K. and Northwestern focuses on the characterization and global correlation of the strata, our collaborators at the University of Wisconsin-Madison and Boise State University are experts in the geochronological analyses.”

The researchers also used other types of isotopes, such as carbon, which tracks synchronous changes in the carbon cycle, and osmium, which tracks volcanic activity and changes in ocean chemistry.

“These isotope systems provide tools for correlating the OAE1a interval between sites in Hokkaido, southern France and other sites all around the globe,” Sageman said. “They give us markers for instants in geologic time.”

Pinpointing the exact timeline

According to this evidence, an abrupt shift in carbon isotope ratios — caused first by the spike in volcanic CO₂ added to the carbon cycle (and later by the excess burial of organic matter) — occurred in the early Cretaceous at the beginning of OAE 1a. A concurrent shift in the isotopic ratios of osmium reflects a massive input of volcanic material into ocean waters. The timing of these events corresponds to eruption of the Ontong Java Nui complex, an enormous igneous province about the size of Alaska located in the southwestern Pacific Ocean.

Now that researchers know it took the oceans 1.1 million years to recover from the sharp increase in CO₂, they have more insight into how long the effects of CO₂-driven warming events might last and what the associated effects, such as ocean anoxia, may be.

“We’re already seeing zones with low oxygen levels in the Gulf of Mexico,” Sageman said. “The main difference is that past events unfolded over tens of thousands to millions of years. We’re driving roughly similar levels of warming (or more) but doing so in less than 200 years.”

The study, “Radioisotopic chronology of Ocean Anoxic Event 1a: Framework for analysis of driving mechanisms,” was supported by the National Science Foundation, the U.K. Natural Environment Research Council and the Japan Science Foundation.

原始論文：Youjuan Li, Brad S. Singer, Reishi Takashima, Mark D. Schmitz, Luca G. Podrecca, Bradley B. Sageman, David Selby, Toshiro Yamanaka, Michael T. Mohr, Keiichi Hayashi, Taiga Tomaru, Katarina Savatic. Radioisotopic chronology of Ocean Anoxic Event 1a: Framework for analysis of driving mechanisms. Science Advances, 2024; 10 (47) DOI: 10.1126/sciadv.adn8365

引用自：Northwestern University. "Prehistoric rock in Japan reveals clues to major ocean anoxic event."