原文網址：https://www.rutgers.edu/news/there-may-be-good-news-about-oceans-globally-warmed-world-0
研究提出海洋因為氣候變遷而持續失去氧氣的趨勢，未來有可能逆轉回去
By Kitta MacPherson
一篇關於地球海洋氧氣含量的分析結果，對於未來全球暖化世界中的海洋健康來說，可能是個難得的好消息。
由羅格斯大學的研究人員主持，發表在《自然》的研究分析了海洋沉積物之後，顯示一處重要的海域在中新世暖期有更高的氧氣含量。中新世暖期大概是在1600萬年前，當時地球的溫度比現在還要高。
氧氣使生命得以存活，但在最近數十年，海洋的氧氣含量持續下降，讓人擔憂在重要的海域缺氧區域會繼續擴張，進而對海洋生物造成危害。
科學家將這股趨勢歸咎到氣候變遷引發的溫度上升，原因是溫度影響了海洋可以從大氣吸收多少氧氣。
羅格斯大學的博士生Anya
Hess是研究主要作者，她說：「目前海洋最大的缺氧區域位在赤道太平洋東部，我們的研究顯示該地點在中新世暖期有充足的氧氣，儘管當時的全球氣溫比現在還高。」Hess跟專長為海洋與地球科學的Yair
Rosenthal共同進行了這篇研究。Rosenthal是羅傑斯大學文理學院與環境和生物科學院的特聘教授。
Hess接著表示：「這意味著目前的氧氣減少趨勢最終有可能反轉回去。」
最近數十年氧氣減少速度最快的地方發生在缺氧區域，科學家預計它們的面積會持續擴張並往淺處擴散，使得魚群的棲地縮減而威脅到漁業。然而，不同氣候模型在預測2100之後這些區域的反應時卻得出了分歧的結果，促使研究團隊進行更加深入的探討。
科學家預測在目前的氣侯變遷時代，幾個世紀之後的氣候條件會類似於中新世中期，因此研究人員便選擇這個時間點來驗證氣候模型。他們檢視了赤道太平洋東部在中新世中期堆積下來的沉積物。這些沉積物是聯合果敢號(JOIDES
Resolution)上的科學家從海床取出，該研究船的經費來自美國國家科學基金會，為國際海洋發現計畫(International
Ocean Discovery Program, IODP)的研究船之一。
研究人員從沉積物分離出只有砂粒大小、活在水層當中的微生物——有孔蟲的化石遺骸。它們的化學成分反映了古代海洋化學的深度剖面，研究人員分析這些成分之後再透過某些方法把氧含量區分出來，比方說以氮同位素(相同元素但原子量不同的形式)來偵測氧氣有無。原理是氮同位素對於脫氮作用相當敏感，而該作用只會發生在氧含量相當低的環境。此外，他們還運用了一種比較碘和鈣濃度的分析方法，其讀數的細微變化可以區分出充分氧化和中度氧化的環境。
這些方法顯示此區域在中新世暖期的高峰期間有充足的氧氣，甚至接近現代南太平洋這類開放海洋中所看到的數值。
「這些出乎意料的結果意謂氣候變遷對於海洋氧氣造成的影響來說，最近數十年因為溶解度而導致的氧氣流失還不是整個故事的結局，」Rosenthal表示。
研究其他作者包括了羅格斯大學文理學院地球與行星科學系的特聘教授Ken
Miller、德國馬克思•普朗克化學研究所的Alexandra
Auderset與Alfredo
Martinez-Garcia、普林斯頓大學的Daniel
Sigman、中國同濟大學的Xiaoli
Zhou。
There may be good news about the
oceans in a globally warmed world
Study suggests ongoing oxygen loss
from the seas due to climate change may reverse in the future
An analysis of oxygen levels in Earth’s
oceans may provide some rare, good news about the health of the seas in a
future, globally warmed world.
A Rutgers-led study published in Nature analyzing ocean sediment shows that ocean oxygen levels in a
key area were higher during the Miocene warm period, some 16 million years ago
when the Earth’s temperature was hotter than it is today.
In recent decades, levels of life-sustaining oxygen
in the ocean have been decreasing, raising concerns that oxygen-deficient zones
in key parts of the world oceans will expand, further harming marine life.
Scientists have attributed the trend to climate
change-induced rising temperatures, which affect the amount of oxygen that can
be absorbed from the atmosphere.
“Our study shows that the eastern equatorial Pacific,
which today is home to the largest oxygen-deficient zone in the oceans, was
well oxygenated during the Miocene warm period, despite the fact that global
temperatures at that time were higher than at present,” said Anya Hess, the
lead author of the study and a Rutgers doctoral student working with Yair
Rosenthal, a Distinguished Professor focused on marine and Earth sciences with
the Rutgers School of Art and Sciences and the School of Environmental and
Biological Sciences.
Hess added: “This suggests that current oxygen loss
may ultimately reverse.”
The fastest rates of oxygen loss in recent decades
have been in oxygen-deficient zones, and they are expected to continue to
expand and become shallower, threatening fisheries by shrinking fish habitat.
However, climate models diverge in their predictions of how these zones will
respond beyond the year 2100, inspiring the team to investigate more.
To test current climate models, researchers chose the
mid-Miocene, when climate conditions were similar to those predicted for the
next few centuries in the current era of climate change. Researchers examined
ocean sediments deposited during the mid-Miocene in the eastern equatorial
Pacific. The sediments were recovered from the seafloor by scientists aboard
the National Science Foundation-funded research vessel JOIDES Resolution as
part of what is now known as the International Ocean Discovery Program (IODP).
The researchers isolated the fossilized remains of
microorganisms the size of individual grains of sand that live in the water
column called foraminifera. The scientists analyzed the chemical composition of
the foraminifera, which reflects the chemical profile of the ancient ocean.
They discerned oxygen levels of ancient oceans in a few ways, including using
isotopes of nitrogen – forms of the element that have a different relative
atomic mass – as detectors. The isotopes are sensitive to a process called denitrification
that only occurs at very low oxygen levels. They also employed a method of
analysis that compares levels of iodine and calcium and gives subtle readings
that can differentiate between well-oxygenated conditions and moderately
well-oxygenated conditions.
The methods showed the area was well oxygenated
during the height of Miocene warmth, even approaching modern day levels seen in
the open-ocean South Pacific.
“These results were unexpected and suggest that the
solubility-driven loss of oxygen that has occurred in recent decades is not the
end of the story for oxygen’s response to climate change,” Rosenthal said.
Other authors on the study include Ken Miller, a
Distinguished Professor in the Department of Earth and Planetary Sciences in
the Rutgers School of Arts and Sciences, Alexandra Auderset and Alfredo
Martinez-Garcia of the Max Planck Institute for Chemistry in Germany, Daniel
Sigman of Princeton University and Xiaoli Zhou of Tongji University in China.
原始論文：Anya V. Hess,
Alexandra Auderset, Yair Rosenthal, Kenneth G. Miller, Xiaoli Zhou, Daniel M.
Sigman, Alfredo Martínez-García. A well-oxygenated eastern tropical
Pacific during the warm Miocene. Nature, 2023; DOI: 10.1038/s41586-023-06104-6
引用自：Rutgers University. "There may be good
news about the oceans in a globally warmed world: Study suggests ongoing oxygen
loss from the seas due to climate change may reverse in the future."
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