在古代「冰室」世界中，碳、氣候變遷與海洋缺氧間的關聯

 原文網址：https://www.ucdavis.edu/climate/news/carbon-climate-change-and-ocean-anoxia-ancient-icehouse-world

by Andy Fell

一篇新研究描述了過去一段迅速發生的全球氣候變遷，當時的地球跟現在非常相似，某些地方有冰層覆蓋——只不過是在3億40萬年前。大約30萬年之間大氣的二氧化碳含量就增加了一倍，海洋變得缺乏氧氣，陸地及海洋的生物多樣性也跟著驟降。

「這是地球歷史上暖化速度最快的事件之一，」Isabel Montañez表示。她是加州大學戴維斯分校地球與行星科學系的特聘教授。

雖然已知地球歷史上還有其他幾次「超高溫」(迅速暖化)事件，但這是首次確認發生在冰室地球的事件。類似於現在的環境，冰室地球是指地球擁有冰層及冰河的時期。研究結果發表於5/2這週的《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)，顯示和二氧化碳濃度本來就比較高的暖期相比，冰室氣候對於大氣二氧化碳濃度的變化可能更加敏感。

Montañez的實驗室之前便在研究3億到2億6千萬年前的這段時期，當時的地球氣候正從寒冷的冰室轉變成炎熱無冰的溫室。2007年，他們指出該時期的氣候曾經來回動盪好幾次。

最近Montañez的團隊以及其他人成功聚焦在3億40萬年前的一次過渡期，稱為Kasimovian–Gzhelian(卡西莫夫—基什爾)邊界，簡稱KGB。他們運用了許多種代用指標，像是岩石與植物化石中的碳同位素以及稀有元素，同時也對這段時期的大氣二氧化碳進行了模擬。

研究人員估計在KGB不久前大約有9兆噸的碳排放到大氣當中。

「雖然我們沒辦法算出速率，但它肯定是地球歷史上暖化最快的事件之一，」Montañez表示。當時的大氣二氧化碳含量從350 ppm左右(跟工業革命前差不多)倍增到將近700 ppm。

深海死區

全球暖化的後果之一是海洋缺氧，也就是溶在海水裡的氧氣大幅減少。冰層融化所釋出的淡水流到海洋表面之後，會形成阻礙深海環流的屏障，因而切斷氧氣的來源。沒有了氧氣，海洋生物便接連死亡。

缺氧會在鈾同位素留下記號，並在海底形成岩石的時候成為其中一部份。研究人員從現今的中國取得碳酸岩並測量其中的鈾同位素，就能得到代用指標來知道岩石沉積的時候海洋裡含有多少氧氣——或是根本沒有氧氣。

他們估計當時全世界的海床大約有23%變成了缺氧的死區。這和其他研究得出來的結果吻合：同一時期海洋和陸地的生物多樣性也下降了許多。

跟其他研究「溫室」條件下發生的迅速暖化的結果相比，這段時期釋放到大氣的碳對海洋缺氧造成的效應要明顯許多。原因可能是溫室地球的大氣二氧化碳基準值本來就高出不少。

「如果溫室地球的二氧化碳上升同樣的量並不會造成太大影響，但是冰室地球似乎對於這樣的變化要更加敏感而發生海洋缺氧，」Montañez表示。

Montañez說穿透石炭紀煤層的火山爆發可能是如此大量的二氧化碳釋放出來的原因。此外，火山爆發可能會引起大火，而暖化也會使永凍土融化，這些都能讓更多有機碳釋放出來。

Montañez是這篇論文的共同通訊作者，另外兩位為Jitao Chen(之前為加州大學戴維斯分校的博士後研究員，現在任職於中國南京地質古生物研究所)以及Xiang-dong Wang(中國南京大學)。其他共同作者包括德州農工大學的Shuang Zhang；紐西蘭懷卡托大學的Terry Isson、Sofia Rauzi、Kierstin Daviau；中國南京地質古生物研究所的Le Yao、Yu-ping Qi、Yue Wang；密西根大學安娜堡分校的Sophia Macarewich、Christopher Poulsen；耶魯大學的Noah Planavsky；南京大學的Feifei Zhang、Jun-xuan Fan、Shu-zhong Shen；亞利桑那州立大學的Ariel Anbar。

研究經費來自中國國家自然科學基金會、中國科學院、美國國家科學基金會。

 

Carbon, climate change and ocean anoxia in an ancient icehouse world

A new study describes a period of rapid global climate change in an ice-capped world much like the present — but 304 million years ago. Within about 300,000 years, atmospheric carbon dioxide levels doubled, oceans became anoxic, and biodiversity dropped on land and at sea.

“It was one of the fastest warming events in Earth’s history,” said Isabel Montañez, distinguished professor in the Department of Earth and Planetary Sciences at the University of California, Davis.

Although several other “hyperthermal,” or rapid warming events, are known in Earth’s history, this is the first identified in an icehouse Earth, when the planet had ice caps and glaciers, comparable to the present day. It shows that an icehouse climate may be more sensitive to changes in atmospheric carbon dioxide than warmer conditions, when CO2 levels are already higher. The work is published this week (May 2) in Proceedings of the National Academy of Sciences.

Montañez’s lab has studied the period from 300 million to 260 million years ago, when Earth’s climate went from a glacial icehouse to a hot, ice-free greenhouse. In 2007, they showed that the climate swung back and forth several times during this period.

More recently, Montañez’s team and others have been able to home in on a transition 304 million years ago, the Kasimovian–Gzhelian boundary, or KGB. They used multiple proxies, including carbon isotopes and trace elements from rocks and plant fossils, and modeling to estimate atmospheric CO2 at the time.

The researchers estimate that about 9,000 gigatons of carbon were released into the atmosphere just before the KGB.

“We don’t have a rate, but it was one of the fastest in Earth’s history,” Montañez said. That doubled atmospheric CO2 from approximately 350 parts per million, comparable to modern pre-industrial levels, to about 700 ppm.

Deep ocean dead zones

One of the consequences of global warming is marine anoxia, or a drop in dissolved oxygen in the ocean. Melting ice caps release fresh water onto the ocean surface, creating a barrier to deep water circulation and cutting off the supply of oxygen. Without oxygen, marine life dies.

Lack of oxygen leaves its mark in uranium isotopes incorporated into rocks forming at the bottom of the ocean. By measuring uranium isotopes in carbonate rocks in present-day China, the researchers could get a proxy for the amount of oxygen — or lack of it — in the ocean when those rocks were laid down.

About 23% of the seafloor worldwide became anoxic dead zones, they estimate. That lines up with other studies showing big losses in biodiversity on land and at sea at the same time.

The effect of carbon release on ocean anoxia was significantly greater than that seen in other studies of rapid warming during “greenhouse” conditions. That may be because the baseline level of atmospheric CO2 was already much higher.

“If you raised CO2 by the same amount in a greenhouse world, there isn’t much effect, but icehouses seem to be much more sensitive to change and marine anoxia,” Montañez said.

The massive carbon release may have been triggered by volcanic eruptions that tore through carboniferous coal beds, Montañez said. The eruptions would also have started fires, and warming may have melted permafrost, leading to the release of more organic carbon.

Montañez is co-corresponding author on the paper with Jitao Chen, formerly a postdoctoral scholar at UC Davis and now at the Nanjing Institute of Geology and Palaeontology, China, and Xiang-dong Wang, Nanjing University, China. Additional co-authors are: Shuang Zhang, Texas A&M University; Terry Isson, Sofia Rauzi and Kierstin Daviau, University of Waikato, New Zealand; Le Yao, Yu-ping Qi and Yue Wang, Nanjing Institute of Geology and Palaeontology; Sophia Macarewich and Christopher Poulsen, University of Michigan, Ann Arbor; Noah Planavsky, Yale University; Feifei Zhang, Jun-xuan Fan and Shu-zhong Shen, Nanjing University; and Ariel Anbar, Arizona State University.

The work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences and the U.S. National Science Foundation.

原始論文：Jitao Chen, Isabel P. Montañez, Shuang Zhang, Terry T. Isson, Sophia I. Macarewich, Noah J. Planavsky, Feifei Zhang, Sofia Rauzi, Kierstin Daviau, Le Yao, Yu-ping Qi, Yue Wang, Jun-xuan Fan, Christopher J. Poulsen, Ariel D. Anbar, Shu-zhong Shen, Xiang-dong Wang. Marine anoxia linked to abrupt global warming during Earth’s penultimate icehouse. Proceedings of the National Academy of Sciences, 2022; 119 (19) DOI: 10.1073/pnas.2115231119

引用自：University of California - Davis. "Carbon, climate change and ocean anoxia in an ancient icehouse world."