侏儸紀的海洋氧含量下降事件持續了100萬年之久

原文網址：www.sciencedaily.com/releases/2017/05/170512081327.htm

侏儸紀的海洋氧含量下降事件持續了100萬年之久

海洋氧氣的劇烈下降造成了海洋生物的大滅絕，雖然最終自然落幕了，但卻耗費了將近百萬年的時間。

海洋缺乏氧氣的狀況被稱作「缺氧」(anoxia)。艾希特大學的科學家之前一直在研究缺氧時期是如何結束的。

他們發現氧含量降低會造成更多有機碳被埋藏在海床上的沉積物裡面，最終造成大氣氧含量上升，進一步讓海洋重新充滿氧氣。

科學家認為現在海洋正處於「缺氧邊緣」，艾希特大學的研究人員表示限制碳排放以避免這種情況的發生是「相當重要的」。

主要研究人員，艾希特大學的地理學家Sarah Baker表示：「像海洋缺氧這種大型事件一旦開始發生，就得花上很長一段時間才能讓地球系統重新回到平衡狀態。」

「這代表至關重要的是要限制人類對碳循環的擾動，如此才能調節地球系統使其維持在適居範圍之中。」

包括康伯恩工礦學院的Stephen Hesselbo教授在內的研究人員，探討了發生在1億8300萬年前的托阿爾階海洋缺氧事件(Toarcian Oceanic Anoxic Event)，其特徵為全球碳循環受到重大擾動，使得海洋缺氧並造成大量海洋生物滅亡。

數值模型預測有機碳的埋藏量會增加，這是因為分解作用降低，加上在溫度較高、含碳量較多的環境中，植物以及海洋生產力的提高所導致。有機碳的埋藏量增加會讓大氣氧含量上升，而在缺氧事件開始之後的100萬年讓整起事件落幕。

為了證實該理論，科學家檢視了木炭化石的樣品來尋找發生野火的證據――因為在氧氣含量高的時期會更頻發的發生這類火災。

他們發現在缺氧事件發生的100萬年之後，開始進入一段野火活動增加的時期，持續了將近80萬年。

「我們主張野火活動的頻率大為增加主要是由大氣氧含量上升所導致。」

「我們的研究呈現出第一筆基於化石的證據，顯示像這樣的大氣氧含量變化事件，發生時間會長達數百萬年。」

野火活動增加造成陸地上的植物因為燃燒而減少，或許也有助於終結海洋缺氧的情況。

這是因為陸地上的岩石含有海洋生物所需的營養鹽，而植物可以促進陸上岩石的侵蝕作用。因此，當植物數目變少，可以被帶到海洋供海洋生物使用的營養鹽就會跟著變少。

會呼吸氧氣的海洋生物數量下降，意味著海中被用掉的氧氣就比較少，因此有助於氧氣重新在海中累積到較高濃度，而終結缺氧事件。

研究人員表示，維持野火活動在自然界的功用，或許有助於長期調節地球系統。

木炭沉積物的實驗是在英國威爾斯的Mochras和葡萄牙的佩尼謝進行。

研究由英國自然環境研究委員會補助。

Jurassic drop in ocean oxygen lasted a million years

Dramatic drops in oceanic oxygen, which cause mass extinctions of sea life, come to a natural end -- but it takes about a million years.

The depletion of oxygen in the oceans is known as "anoxia," and scientists from the University of Exeter have been studying how periods of anoxia end.

They found that the drop in oxygen causes more organic carbon to be buried in sediment on the ocean floor, eventually leading to rising oxygen in the atmosphere which ultimately re-oxygenates the ocean.

Scientists believe the modern ocean is "on the edge of anoxia" -- and the Exeter researchers say it is "critical" to limit carbon emissions to prevent this.

"Once you get into a major event like anoxia, it takes a long time for the Earth's system to rebalance," said lead researcher Sarah Baker, a geographer at the University of Exeter.

"This shows the vital importance of limiting disruption to the carbon cycle to regulate the Earth system and keep it within habitable bounds."

The researchers, who also include Professor Stephen Hesselbo from the Camborne School of Mines, studied the Toarcian Oceanic Anoxic Event, which took place 183 million years ago and was characterized by a major disturbance to the global carbon cycle, depleted oxygen in Earth's oceans and mass extinction of marine life.

Numerical models predicted that increased burial of organic carbon -- due to less decomposition and more plant and marine productivity in the warmer, carbon-rich environment -- should drive a rise in atmospheric oxygen, causing the end of an anoxic event after one million years.

To test the theory, the scientists examined fossil charcoal samples to see evidence of wildfires -- as such fires would be more common in oxygen-rich times.

They found a period of increased wildfire activity started one million years after the onset of the anoxic event, and lasted for about 800,000 years.

"We argue that this major increase in fire activity was primarily driven by increased atmospheric oxygen," said Baker.

"Our study provides the first fossil-based evidence that such a change in atmospheric oxygen levels could occur in a period of one million years."

The increase in fire activity may have also helped end ocean anoxia by burning and reducing the amount of plants on land.

This is because plants can help to erode rocks on the land that contain nutrients needed for marine life -- therefore with fewer plants, fewer nutrients are available to be carried to the sea and used to support marine life in the oceans.

Less marine life -- that would use oxygen to breathe -- would mean less oxygen being used in the oceans, and could therefore help the oceans to build up a higher oxygen content, ending anoxia.

It may therefore be essential to maintain the natural functioning of wildfire activity to help regulate the Earth system in the long-term, the researchers say.

The charcoal sediment tests were carried out at Mochras in Wales and Peniche, Portugal.

The research was funded by the Natural Environment Research Council (NERC).

原始論文：Sarah J. Baker, Stephen P. Hesselbo, Timothy M. Lenton, Luís V. Duarte, Claire M. Belcher. Charcoal evidence that rising atmospheric oxygen terminated Early Jurassic ocean anoxia. Nature Communications, 2017; 8: 15018 DOI: 10.1038/ncomms15018

引用自：University of Exeter. "Jurassic drop in ocean oxygen lasted a million years." ScienceDaily. ScienceDaily, 12 May 2017.