原文網址：https://news.ucr.edu/articles/2022/08/17/sleeping-giant-could-end-deep-ocean-life
陸地移動可以剝奪海洋裡的氧氣
By Jules
Bernstein
在促使地球海洋充滿生命所需的氧氣這方面，有一項因素之前並未受到重視：陸地的位置。但陸地移動也能導致相反的效果，殺死海洋裡絕大部分的生物。
在阿拉斯加發現的深海水母。圖片來源：Hidden Ocean 2005
「大陸漂移看似相當緩慢，好像不會造成任何突如其來的影響。然而，當海裡已經湊齊了所需條件，就算是看起來相當微小的事件也能引起海洋生物大量死亡，」加州大學河濱分校的地質學家Andy
Ridgwell表示。他是這篇影響海洋氧氣的作用力的研究共同作者。
海洋表層的水會從大氣中帶走氧氣，當海水接近南北極的時候會變冷且密度變高，接著便往下沉，而把這些氧氣運往海床。
最後海水回流到表層時，會把下沉的有機物釋放出的養分帶回來，促使表層的浮游植物生長。在今日的海洋，順暢無阻運送到深處的氧氣，以及在表層製造出來的有機物聯合之下，使得魚類和其他動物得以展現出驚人的多樣性。
由加州大學河濱分校的研究人員主持的新研究發現，這道氧氣與養分的循環可以在短時間內突然停止下來。研究人員運用繁複的電腦模型，探討大陸板塊的位置是否能夠影響周圍的海水運送氧氣的路徑。令他們驚訝的是，結果顯示確實如此。
今日發表於期刊《自然》(Nature)的文章中詳述了這項發現。
「在好幾億年前，海洋中的動物出現不久之後，全球的海洋循環似乎有週期性的中斷現象，」Ridgwell表示。「我們發現陸地移動可能造成表層海水和氧氣停止往下沉，進而對地球生命的演化歷程造成重大影響。這是我們沒有預料到的。」
在此之前，用來研究過去5億4000萬年海洋氧氣如何演變的模型，相對來說較為簡單而且沒有考量到海洋循環。在那些模型中，海洋氧氣消失——即缺氧的期間也意味著大氣氧濃度降低。
「之前科學家推測海洋氧氣含量的變化，絕大部分反映了大氣裡的氧氣出現了類似的波動，」研究第一作者Alexandre
Pohl表示。他之前在加州大學河濱分校從事古氣候的模擬，現在任職於法國勃根地的法蘭西康提大學。
這道差異代表除了靠近岸邊的淺海地帶，有數千萬年的時間整個海床的氧氣都消失了。這種情況到了大約4億4000萬年前志留紀開始的時候才解除。
「雖然大氣中仍有生命所需的氧氣，但是對於無法游到靠近水面獲取氧氣的生物來說，循環中斷無疑是判了牠們死刑，」Ridgwell表示。這些深海生物包括了外表特異的魚類、巨大的蠕蟲、甲殼類、烏賊、海綿……等等。
這篇論文並無法解決未來類似的事件會在什麼時候或者是否發生的問題，而且也很難從中得知循環中斷可能發生的時間以及原因。然而，現存的氣候模型證實了愈加嚴重的全球暖化會弱化海洋環流，有些模型則預測環流的分支最終會從北大西洋開始停擺。
「我們需要解析度更高的模型來預測大滅絕事件是否會發生，」Ridgwell表示。「話雖如此，目前北大西洋的海水環流已經讓我們感到憂心，某些證據指出流到深處的海水正在減少當中。」
Ridgwell表示理論上，一場超乎尋常的炎熱夏季或者一面海崖崩落就有可能引發一連串的反應，大幅擾亂今日所見的生命。
「你可能會認為我們衝浪或是航行其上的海洋表層，這一小部分就是海中所有的作用發生的地方。但是下方的海洋無時無刻都在運作著，將重要的氧氣供給住在漆黑深海的動物，」Ridgwell表示。
「海洋讓生命得以滋長，但也能奪走生命。隨著大陸板塊不斷地移動，沒有任何生物可以一直統治海洋。」
Sleeping giant could end deep ocean
life
Continental movement capable of
throttling marine oxygen
A previously overlooked factor — the
position of continents — helps fill Earth’s oceans with life-supporting oxygen.
Continental movement could ultimately have the opposite effect, killing most
deep ocean creatures.
“Continental drift seems so slow, like nothing
drastic could come from it, but when the ocean is primed, even a seemingly tiny
event could trigger the widespread death of marine life,” said Andy Ridgwell,
UC Riverside geologist and co-author of a new study on forces affecting oceanic
oxygen.
The water at the ocean’s surface becomes colder and
denser as it approaches the north or south pole, then sinks. As the water
sinks, it transports oxygen pulled from Earth’s atmosphere down to the ocean
floor.
Eventually, a return flow brings nutrients released
from sunken organic matter back to the ocean’s surface, where it fuels the
growth of plankton. Both the uninterrupted supply of oxygen to lower depths and
organic matter produced at the surface support an incredible diversity of fish
and other animals in today’s ocean.
New findings led by researchers based at UC Riverside
have found this circulation of oxygen and nutrients can end quite suddenly.
Using complex computer models, the researchers investigated whether the
locations of continental plates affect how the ocean moves oxygen around. To
their surprise, it does.
This finding, published today, is detailed in the
journal Nature.
“Many millions of years ago, not so long after animal
life in the ocean got started, the entire global ocean circulation seemed to
periodically shut down,” said Ridgwell. “We were not expecting to find that the
movement of continents could cause surface waters and oxygen to stop sinking,
and possibly dramatically affecting the way life evolved on Earth.”
Until now, models used to study the evolution of
marine oxygen over the last 540 million years were relatively simple and did
not account for ocean circulation. In these models, ocean anoxia — times when
oceanic oxygen disappeared — implied a drop in atmospheric oxygen
concentrations.
“Scientists previously assumed that changing oxygen
levels in the ocean mostly reflected similar fluctuations in the atmosphere,”
said Alexandre Pohl, first author of the study and former UCR paleoclimate
modeler, now at Université Bourgogne Franche-Comté in France.
This study used, for the first time, a model in which
the ocean was represented in three dimensions, and in which ocean currents were
accounted for. Results show that collapse in global water circulation lead to a
stark separation between oxygen levels in the upper and lower depths.
這項研究運用的模型首次把海洋用三維來表示，並且將洋流給考慮進去。結果顯示全球海洋環流停擺會導致海洋上層與下層的氧含量出現驚人的差異。
That separation meant the entire seafloor, except for
shallow places close to the coast, entirely lost oxygen for many tens of
millions of years, until about 440 million years ago at the start of the
Silurian period.
“Circulation
collapse would have been a death sentence for anything that could not swim
closer to the surface and the life-giving oxygen still present in the
atmosphere,” Ridgwell said. Creatures of the deep include bizarre-looking fish,
giant worms and crustaceans, squid, sponges and more.
The paper does not address if or when Earth might
expect a similar event in the future, and it is difficult to identify when a
collapse might occur, or what triggers it. However, existing climate models
confirm that increasing global warming will weaken ocean circulation, and some
models predict an eventual collapse of the branch of circulation that starts in
the North Atlantic.
“We’d need a higher resolution climate model to
predict a mass extinction event,” Ridgwell said. “That said, we do already have
concerns about water circulation in the North Atlantic today, and there is
evidence that the flow of water to depth is declining.”
In theory, Ridgwell said an unusually warm summer or
the erosion of a cliff could trigger a cascade of processes that upends life as
it appears today.
“You’d think the surface of the ocean, the bit you
might surf or sail on, is where all the action is. But underneath, the ocean is
tirelessly working away, providing vital oxygen to animals in the dark depths,”
Ridgwell said.
“The ocean allows life to flourish, but it can take
that life away again. Nothing rules that out as continental plates continue to
move.”
原始論文：Alexandre
Pohl, Andy Ridgwell, Richard G. Stockey, Christophe Thomazo, Andrew Keane,
Emmanuelle Vennin, Christopher R. Scotese. Continental configuration
controls ocean oxygenation during the Phanerozoic. Nature,
2022; 608 (7923): 523 DOI: 10.1038/s41586-022-05018-z
引用自：University of California - Riverside.
"Sleeping giant could end deep ocean life.”
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