By Bill Wellock
二億五千萬年前，地球上的大多數生物都滅亡了。
經過純化的岩石樣品會在國家強磁場實驗室中測量它們的鉈同位素。圖片來源：Stephen Bilenky/ National High Magnetic Field Laboratory
在標記二疊紀結束的事件中，地球海洋裡超過96%的物種以及陸地上超過70%的物種都在一夕之間走向滅絕，這也是地球歷史上規模最大的滅絕事件。
佛羅里達州立大學的研究人員最近發現這起滅絕事件發生的同時，海洋裡的氧含量突然出現了一道高峰然後下降。他們的發現發表在《自然―地球科學》(Nature
Geoscience)。
「之前進行的研究顯示當時環境裡的氧氣下降而造成了滅絕事件，不過一直以來我們都假設這股變化是逐漸發生的，」主要作者，佛羅里達州立大學的研究生研究助理Sean
Newby表示。「我們看到在滅絕事件開始的時候，出現了這道氧化速度飛快的事件，接著環境又回到還原的狀態，這讓我們相當驚訝。」
科學家早就知道滅絕事件期間的氧濃度有逐漸下降的現象，但是在滅絕事件開始的時候氧含量迅速升高則是全新的發現。研究人員認為這次氧化事件只歷經了數萬年，對於地球以百萬年為尺度的地質歷史來說，這是相當短的一道時間。
「從地質紀錄來說，幾乎是一瞬間的事情，」Newby表示。「理所當然的，你可以把它和現今由人類造成的氣候變遷相比，跟那次大滅絕事件比較起來，我們在短時間內就造成了重大且迅速的變化。」
研究人員還不清楚造成海洋氧含量出現這道高峰的確切原因，但他們推測有個大型火山區域持續噴發了至少數十萬年，造成氣溫暫時降低，同時海洋的氧氣則突然升高接著暴跌。
雖然在這道高峰之前古代的海洋氧氣含量就已經呈現下降的趨勢，而且之後一直處於低迷狀態，但是以地質角度來說氧氣快速上升下降以及長期缺氧，對於生物而言似乎比逐漸下降還要更加致命。火山爆發釋放出來的二氧化碳加熱了地球大氣，造成海裡的氧氣降低，使得海洋有數百萬年的時間相對來說不適合生命居住。
要直接測量古代大氣或海洋裡的氧含量是不可能的，因此研究人員測量的對象是鉈同位素，它提供的間接資訊可以讓他們瞭解過去海洋裡的氧含量。
研究人員接下來的計畫是研究其他過往的滅絕事件，試著找出那些大滅絕發生的時候，氧氣是否也有類似的劇烈震盪。對於因為氣候變遷以及更多營養鹽流入使得氧氣逐漸減少的當今海洋來說，這些結果必定能帶來啟發。
「海洋目前的情況不僅是氧氣減少而已，」論文共同作者，地球、海洋與大氣科學系的副教授Jeremy
Owens表示。「氧氣減少是很重要，因為現今存活的生物已經適應了氧含量高的環境，不過在低氧狀況下或許還是有很多生物可以適應。然而劇烈的變動――不論是往哪個方向――勢必都會造成衝擊。」
來自佛羅里達州立大學、西卡羅萊納大學以及辛辛那提大學的研究人員參與了這項工作。
Oxygen spike coincided with ancient
global extinction
Two hundred fifty-two million years ago,
much of life on planet Earth was dying.
In an event that marked the end of the Permian
period, more than 96 percent of the planet’s marine species and 70 percent of
its terrestrial life suddenly went extinct. It was the largest extinction in
Earth’s history.
Now Florida State University researchers have found
that the extinction coincided with a sudden spike and subsequent drop in the
ocean’s oxygen content. Their findings were published in Nature Geoscience.
“There’s previous work that’s been done that shows
the environment becoming less oxygenated leading into the extinction event, but
it has been hypothesized as a gradual change,” said lead author and FSU
graduate research assistant Sean Newby. “We were surprised to see this really
rapid oxygenation event coinciding with the start of the extinction and then a
return to reducing conditions.”
Scientists have previously seen a gradual decrease in
oxygen during this extinction, but the rapid oxygen increase at the beginning
of the extinction was a new finding. The researchers think the oxygenation
occurred over a few tens of thousands of years, a very brief period on the
scale of the millions of years of the Earth’s geological history.
“For the geological record, that’s practically
instantaneous,” Newby said. “And then you can of course compare that to modern,
human-induced climate change, where we’re having huge, rapid changes on
fractions of the time compared to this mass extinction.”
The exact cause of this spike in ocean oxygenation is
unknown, but the researchers hypothesized that the continual eruption of at
least several hundred thousand years of a massive volcanic region led to a
brief cooling and the sudden marine oxygenation spike and subsequent crash.
Although ancient marine oxygen levels were on a
downward trend ahead of the spike and remained low afterward, it’s the
geologically rapid shift back and forth and long-term oxygen deficiency that
seemed to be more detrimental to life than the gradual decrease. The carbon
dioxide released during that volcanic eruption caused the Earth’s atmosphere to
warm, which lowered oxygen in the oceans and caused the oceans to become
relatively inhospitable for millions of years.
It is impossible to directly measure ancient marine
or atmospheric oxygen levels, so the research team instead measured thallium
isotopes, which indirectly provided information to understand the marine oxygen
levels of the past.
The researchers plan to study other ancient
extinctions to see if similar dramatic swings in oxygen coincided with any of
those mass extinctions, which could have modern-day implications as climate
change and increased nutrient discharge decrease the amount of oxygen in our
present-day ocean.
“It’s not just the loss of oxygen in the modern
ocean,” said Jeremy Owens, an associate professor in the Department of Earth,
Ocean and Atmospheric Science and paper-co-author. “The loss of oxygen is
important because the organisms living now are adapted for high oxygen, but if
you have low oxygen there’s also many organisms that may be able to adapt. Any
rapid fluctuation in either direction will have an impact.”
Researchers from Florida State University, Western
Carolina University and the University of Cincinnati contributed to this work.
原始論文：Sean M. Newby, Jeremy D. Owens, Shane D. Schoepfer, Thomas J.
Algeo. Transient
ocean oxygenation at end-Permian mass extinction onset shown by thallium
isotopes. Nature
Geoscience, 2021; DOI: 10.1038/s41561-021-00802-4
引用自：Florida
State University. “FSU researchers find oxygen spike coincided with ancient
global extinction.”
沒有留言:
張貼留言