原文網址：https://www.brown.edu/news/2021-01-04/seaice
布朗大學的研究人員在新研究中證明之前認為會造成麻煩的一種分子，實際上是用來重建海冰的良好代用指標。
海冰對於地球氣候的變化來說是相當重要的指標。布朗大學的研究人員得到的最新發現，提供了科學家新的方法來重建許久以前的海冰多寡以及分布範圍，進而幫助我們更加瞭解目前人類引發的氣候變遷。
研究人員在發表於《自然通訊》(Nature Communications)的研究當中，證明一種經常在高緯度的海洋沉積物裡面發現的有機分子「四元不飽和烯酮」(tetra-unsaturated alkenone ，C37:4))，是由一或多種居住在冰裡的未知藻類所製造。當海冰密度有所增減的時候，這些居住於其中的藻類以及它們留下的分子也會跟著變多變少。
「我們證明這種分子是海冰密度的良好代用指標，」研究主要作者，布朗大學的博士生Karen
Wang表示。「觀察不同年代的沉積物中這種分子的濃度，可以讓我們重建海冰密度隨時間的變化。」
其他種類的烯酮分子許多年來被科學家用作海洋表面溫度的代用指標。由於生活在海洋表面的藻類在不同溫度下製造出來的烯酮C37:2和C37:3的量也不同，因此科學家可以利用海洋沉積物中這兩種分子的比例來估計過往的海溫。而這項新研究的探討對象C37:4長久以來則被視為測量溫度時會遇到的一些麻煩。它們會出現在北極附近採集的沉積物，造成C37:2和C37:3的比例出現誤差。
「對於C37:4烯酮最常見的概念便是它們會『混淆溫度比例』。」布朗大學地球、環境與行星科學系的教授Yongsong
Huang表示。他是這項經費來自美國國家科學基金會的計劃主持人。「沒有人知道它們來自哪裡，是否有任何利用價值。雖然之前有人提出一些理論，但都沒法確定下來。」
研究人員為了解答這些問題，他們從北極附近寒冷的海水與沉積物中採集含有C37:4的樣本並加以研究。利用最新的DNA定序技術來辨認樣品中含有的生物，他們找到了幾種隸屬於等鞭金藻目(Isochrysidales)的未知藻類。接著在實驗室培養這些新發現的藻類，證實了它們就是會製造出特別多C37:4的生物。
下一步是探討這些居住在冰裡的藻類遺留下來的分子，是否能當作海冰的可靠代用指標。研究人員從較近的歷史得知，目前北極海海冰的邊界對於區域的溫度變化十分敏感，因此他們從這些地方取得沉積物岩芯，然後觀察其中的C37:4濃度。結果發現C37:4濃度最高的時候氣候也最為寒冷，海冰的面積也達到高峰。他們定出濃度最高的時期為12000年前的新仙女木期，這是一段氣候狀況非常嚴寒的時期。研究人員也發現當氣候最溫暖、海冰消退的時候，C37:4的量也變得相當少。
「我們利用這項新的代用指標得出的相關性比其他人用的標記還要強烈許多，」布朗大學環境與社會研究所的研究員Huang表示。「由於海冰模擬起來是種相當複雜的過程，因此不可能得到完美的相關性，但這項指標或許是你可以得到結果中最佳的。」
研究人員表示這項新的代用指標跟其他的比起來還有一項優勢。另外一種重建海冰的方法需要尋找稱為矽藻的藻類化石殘骸。但此方法用的分子化石會遭到分解，因此時間越往前推就變得越不可靠。然而C37:4這類分子卻可以保存得較為完好，因此這項方法在重建許久之前的海冰時，跟其他方法比起來可以得到更好的結果。
研究人員計畫進一步探討這些新的藻類，深入瞭解它們是怎麼被包覆在海冰裡面，又是如何製造出這些烯酮化合物。這些藻類似乎是生長在海冰裡面含有海水的氣泡與通道，但也有可能會在海冰融化之後立即大量繁殖。瞭解它們的活動有助於研究人員把C37:4校正成更好的海冰代用指標。
最後，研究人員希望新的代用指標可以讓科學家更加瞭解從古至今的海冰活動。這項訊息可以讓科學家改良重建過去氣候的模型，進而對未來的氣候變遷作出更為準確的預測。
Researchers discover a new tool for
reconstructing ancient sea ice to study climate change
A previously problematic molecule
turns out to be a reliable proxy for reconstructing sea ice, a new study by
Brown University researchers shows.
Sea ice is a critical indicator of
changes in the Earth’s climate. A new discovery by Brown University researchers
could provide scientists a new way to reconstruct sea ice abundance and
distribution information from the ancient past, which could aid in
understanding human-induced climate change happening now.
In a study published in Nature Communications, the researchers show that an organic
molecule often found in high-latitude ocean sediments, known as
tetra-unsaturated alkenone (C37:4), is produced by one or more previously unknown
species of ice-dwelling algae. As sea ice concentration ebbs and flows, so do
the algae associated with it, as well as the molecules they leave behind.
“We’ve shown that this molecule is a strong proxy for
sea ice concentration,” said Karen Wang, a Ph.D. student at Brown and lead
author of the research. “Looking at the concentration of this molecule in
sediments of different ages could allow us to reconstruct sea ice concentration
through time.”
Other types of alkenone molecules have been used for
years as proxies for sea surface temperature. At different temperatures, algae
that live on the sea surface make differing amounts of alkenones known as C37:2
and C37:3. Scientists can use the ratios between those two molecules found in
sea sediments to estimate past temperature. C37:4 — the focus of this new study
— had been long considered a bit of problem for temperature measurements. It
turns up in sediments taken from closer to the Arctic, throwing off the
C37:2/C37:3 ratios.
“That was mostly what the C37:4 alkenone was known
for — throwing off the temperature ratios,” said Yongsong Huang, principal
investigator of the National Science Foundation-funded project and a professor
in Brown’s Department of Earth, Environmental and Planetary Science. “Nobody
knew where it came from, or whether it was useful for anything. People had some
theories, but no one knew for sure.”
To figure it out, the researchers studied sediment
and sea water samples containing C37:4 taken from icy spots around the Arctic.
They used advanced DNA sequencing techniques to identify the organisms present
in the samples. That work yielded previously unknown species of algae from the
order Isochrysidales. The researchers then cultured those new species in the
lab and showed that they were indeed the ones that produced an exceptionally
high abundance of C37:4.
The next step was to see whether the molecules left
behind by these ice-dwelling algae could be used as a reliable sea ice proxy.
To do that, the researchers looked at concentrations of C37:4 in sediment cores
from several spots in the Arctic Ocean near the present-day sea ice margins. In
the recent past, sea ice in these spots is known to have been highly sensitive
to regional temperature variation. That work found that the highest concentrations
of C37:4 occurred when climate was coldest and ice was at its peak. The highest
concentrations dated back to the Younger-Dryas, a period of very cold and icy
conditions that occurred around 12,000 years ago. When climate was at its warmest and ice ebbed,
C37:4 was sparse, the research found.
“The correlations we found with this new proxy were
far stronger than other markers people use,” said Huang, a research fellow at
the Institute at Brown for Environment and Society. “No correlation will be
perfect because modeling sea ice is a messy process, but this is probably about
as strong as you’re going to get.”
And this new proxy has some additional advantages
over others, the researchers say. One other method for reconstructing sea ice
involves looking for fossil remains of another kind of algae called diatoms.
But that method becomes less reliable further back in time because fossil
molecules can degrade. Molecules like C37:4 tend to be more robustly preserved,
making them potentially better for reconstructions over deep time than other
methods.
The researchers plan to further research these new
algae species to better understand how they become embedded in sea ice, and how
they produce this alkenone compound. The algae appear to live in brine bubbles
and channels inside sea ice, but it may also bloom just after the ice melts.
Understanding those dynamics will help the researchers to better calibrate
C37:4 as a sea ice proxy.
Ultimately, the researchers hope that the new proxy
will enable better understanding of sea ice dynamics through time. That
information would improve models of past climate, which would make for better
predictions of future climate change.
原始論文：Karen Jiaxi
Wang, Yongsong Huang, Markus Majaneva, Simon T. Belt, Sian Liao, Joseph Novak,
Tyler R. Kartzinel, Timothy D. Herbert, Nora Richter, Patricia Cabedo-Sanz. Group
2i Isochrysidales produce characteristic alkenones reflecting sea ice
distribution. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-020-20187-z
引用自：Brown
University. "New tool for reconstructing ancient sea ice to study climate
change."
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