大型火山爆發或許造成了首次生物大滅絕

原文網址：www.sciencedaily.com/releases/2017/05/170517090638.htm

大型火山爆發或許造成了首次生物大滅絕

日本和美國的科學家表示他們也許找出了世上首次生物大滅絕的成因

自原始動物於6億至4.5億年前演化成各種形式的動物之後，總共發生了5次大滅絕事件。第三次與第四次大滅絕肇因於火山活動，而小行星撞擊則造成了第五次大滅絕。但是引發第一次和第二次大滅絕的元凶，直到現在仍然不清楚。

第一次大滅絕發生於奧陶紀末期。這個年代位於奧陶紀的生物分化，和維管束植物與動物入侵陸地的時間之間。奧陶紀―志留紀的海洋動物包括了珊瑚、三葉蟲、海蠍、直角石、腕足類、筆石、海百合和水母等。大約有80%的物種在奧陶紀結束時就此滅絕。

由美國安默斯特學院的David S. Jones博士和日本東北大學的海保邦夫教授領導的團隊，探討了首次生物大滅絕的可能成因。他們從兩個地方――北美和中國南方――採集岩石樣品並分析其中的汞含量。結果發現在兩個地區，汞富集的時間點都跟大滅絕重合。

他們認為這是大型火山爆發的結果，因為在其他由大火成岩省火山活動造成的事件中，也能觀察到汞異常現象。

巨型火山爆發事件可以在平流層產生許多硫酸氣膠。硫酸氣膠是種可以大量反射陽光的氣膠分子，因此會造成全球冷化。如此快速的氣候變遷被認為是海洋生物死亡的幕後推手。

海保教授的團隊目前正在研究第二次大滅絕事件，希望可以更加瞭解該事件背後的原因及過程。

Large volcanic eruption may have caused the first mass extinction

Researchers in the USA and Japan say they may have found the cause of the first mass extinction of life.

There have been five mass extinctions since the divergent evolution of early animals 600 -450 million years ago. The cause of the third and fourth was volcanic activity, while an asteroid impact led to the fifth. But triggers of the first and second mass extinctions had, until now, been unknown.

The first mass extinction occurred at the end of the Ordovician. This age is between the divergence of the Ordovician and land invasion of vascular land plant and animals. Animals in the Ordovician-Silurian comprised marine animals like corals, trilobites, sea scorpion, orthoceras, brachiopods, graptolite, crinoid and jawless fish. Approximately 80% of species disappeared at the end of the Ordovician.

A team led by Dr. David S. Jones of Amherst College and Professor Kunio Kaiho of Tohoku University, looked into possible triggers of the first mass extinction. They took sedimentary rock samples from two places -- North America and southern China -- and analyzed the mercury (Hg) in them. They found Hg enrichments coinciding with the mass extinction in both areas.

This, they believe, is the product of large volcanic eruptions because Hg anomaly was also observed in other large igneous province volcanisms.

Huge volcanic eruptions can produce sulfate aerosols in the stratosphere. Sulfate aerosols are strong, light-reflecting aerosols, and cause global cooling. This rapid climate change is believed to be behind the loss of marine creatures.

Kaiho's team is now studying the second mass extinction in the hopes of further understanding the cause and processes behind it.

原始論文：David S. Jones, Anna M. Martini, David A. Fike, Kunio Kaiho. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, 2017; G38940.1 DOI: 10.1130/G38940.1

引用自：Tohoku University. "Large volcanic eruption may have caused the first mass extinction." ScienceDaily. ScienceDaily, 17 May 2017.