北極海在過去十五萬年至少有兩度曾被厚達900公尺的冰棚覆蓋，而且裡面裝的都是淡水。這項發表在《自然》(Nature)期刊的驚人發現是阿爾弗雷德˙韋格納研究所和MARUM的科學家長期研究之下的成果。他們詳細分析海洋沉積物的成分之後，證實了至少在兩次冰河期的期間北極海和北海並不含海鹽；反之，當時它們被一層厚厚的冰蓋住，裏頭裝滿了大量的淡水。這些水接著可能會在很短的時間內釋放到北大西洋。之前對於這段期間的快速氣候動盪未能找到滿意的解釋，像這種突如其來的淡水輸入事件或許就是原因。
冰河期北極海與北極冰層的示意圖。冰河期海面降低的時候，北極海海盆與太平洋會完全停止交流，和北大西洋的交流則會劇烈減少，但本身仍會接收大量淡水。與外界的交換只能透過格陵蘭―蘇格蘭洋脊中狹窄的海道來進行。這三張照時間順序的圖分別代表了(1)北極海淡化的時期，接著是(2)當鹽水進入北極海會讓大量的淡水釋放到北大西洋，(3)鹽度以及溫度都比較高的大西洋海水接觸到北極冰層時，就會造成冰層突然融化。圖片來源：Martin Künsting
在六萬到七萬年前，上次冰河期特別寒冷的時候，歐洲北部和北美洲大部分都覆蓋在冰層之下。在歐洲，冰層綿延了超過五千公里，從冰島和蘇格蘭開始，一路經過斯堪地那維亞半島，直到喀拉海(北極海)的東緣。在北美洲，現今的加拿大許多地方都覆蓋在兩座大型冰層之下。此外，格陵蘭和白令海一部分的海岸線也變成了冰河。但在更北的北極海，冰層是怎麼分布的？是覆蓋於厚厚的海冰之下；還是這些大型冰層的前緣會深入北極地區，漂浮在北極海之上？
目前科學家對於這些問題的答案大概都只是推測而已。不同於冰河在陸地上會留下漂礫、冰磧石等沉積物與冰河谷這些顯著的標記，目前在北極海只有找到少數由大型冰棚留下的痕跡。不過，阿爾弗雷德˙魏格納極地與海洋研究所以及不萊梅大學海洋環境科學中心(MARUM)的地球科學家最近彙整了北極海和北海現有的證據，並且結合新的數據得出了驚人的結論。
根據他們的研究成果，這些北方的冰層漂在海上的部分在過去十五萬年曾經把北極海許多地方給覆蓋住。其中一次發生在大約七萬到六萬年前，另外一次則發生在大約十五萬到十三萬年前。在這兩段時期冰塊之下積滿了淡水，使得北極海有數千年的時間徹底變成了一座淡水湖。
「這項結果確實改變了我們認知中冰期氣候時的北極海。就我們所知，這是首度有人提出北極海和北海曾經完全淡化，而且不只發生了一次，是兩次。」研究第一作者阿爾弗雷德˙韋格納研究所的地球化學家Walter
Geibert博士表示。
沉積物沒有釷，水必定沒有鹽
這群地球化學家從北極海、弗拉姆海峽和北海的不同地區採集了十根沉積物岩芯，進行地質分析之後得到了此發現。這些層層堆疊的沉積物反映了過去冰河期的氣候歷史。他們探討並比較沉積物的紀錄之後，發現一種重要的指標總是在相同的兩個時期消失。「在含鹽的海水當中，自然界原有的鈾衰變必定會產生同位素釷230。這種物質會累積在海床，由於它們的半衰期長達七萬五千年，所以在許久之後仍然可以偵測出來。」Walter
Geibert解釋。
也因此地質學家常常利用釷230來當作自然界的時鐘。「此處的釷230一而再地於許多地方消失，這項訊息讓我們得知當時發生了什麼事情。據我們所知，要形成這種模式唯一可能的解釋便是在近期的歷史當中，北極海曾經兩度裝滿了淡水，無論它們是處於凍結或液體的狀態。」共同作者，同樣來自AWI的微體古生物學家Jutta
Wollenburg博士解釋。
北極海的新樣貌
這麼大的一個海盆，而且跟北大西洋和太平洋還有好幾條海峽相連的狀況下，要如何變成只有淡水？「如果我們可以理解冰河期的全球海平面最多比現在低130公尺，而且北極的冰層也許還能進一步的限制海水循環，那麼這樣的情景確實是可以想見的。」共同作者，阿爾弗雷德˙韋格納研究所暨MARUM的地質學家Ruediger
Stein教授表示。
白令海峽這類較淺的通道以及加拿大群島的水道在當時會露出海平面，使得北極海跟太平洋的連結完全斷開。北海則有大型冰山或者延伸到海床上的冰層，使得水體之間的交換受到限制。在北極海，夏季冰河融化流出的雪水，以及流進來的河川會持續供給大量的淡水，每年至少有1200立方公里。其中有一部份會經由格陵蘭―蘇格蘭洋脊當中狹窄的淺海水道，從北海流入北大西洋，這股力量能防止鹹水滲入北方，結果便是北極海持續淡化。
「這些冰塊形成的屏障機制一旦失效，較重的鹹水便能再次填滿北極海。」Walter
Geibert表示。「我們相信鹹水可以快速把較輕的淡水替換掉，造成本來累積於北極海的淡水從北海南部較淺的邊界，也就是從格陵蘭―蘇格蘭海脊一口氣大量流入北大西洋。」
推測北極海曾經囤積大量淡水而且能夠快速釋放出來的這道想法，有助於解釋過往氣候一連串的波動之間的關係。此外，這也可以解釋為什麼在重建過去的海平面時，用不同方法結果會有明顯的差異。「在重建某些冷期的海平面時，比起運用南極冰芯以及微體海洋生物的鈣質殼體，從殘留下來的珊瑚礁得出的海平面會高出一些。」Walter
Geibert解釋。「如果我們可以認同淡水不只能以固體形式儲存在陸地，有些還能以液體形式待在海洋，那麼這些不同的海平面重建方式就能得出更加一致的結果，而且也能透過計算淡水儲量來得出一致的珊瑚礁位置。」
北極海釋放出來的淡水或許也能解釋上次冰河期期間一些突然發生的氣候變遷事件。在這些事件當中，格陵蘭的溫度會在數年之間飆升攝氏8到10度，但在接下來的數百年或者數千年又會逐漸回到原本的低溫。「過往的北極氣候可以看成是地球系統具有臨界點的一則範例。我們現在需要更加仔細地探討這些過程有何關聯，並且評估關於北極海的新觀念可以讓我們的知識提升多少，特別是考量到我們現在正面臨人為氣候變遷所帶來的風險。」
The Arctic Ocean was covered by a
shelf ice and filled with freshwater
The Arctic Ocean was covered by up to 900
m thick shelf ice and was filled entirely with freshwater at least twice in the
last 150,000 years. This surprising finding, reported in the latest issue of
the journal Nature, is the result of
long-term research by scientists from the Alfred Wegener Institute and the
MARUM. With a detailed analysis of the composition of marine deposits, the
scientists could demonstrate that the Arctic Ocean as well as the Nordic Seas
did not contain sea-salt in at least two glacial periods. Instead, these oceans
were filled with large amounts of freshwater under a thick ice shield. This
water could then be released into the North Atlantic in very short periods of
time. Such sudden freshwater inputs could explain rapid climate oscillations
for which no satisfying explanation had been previously found.
About 60.000 to 70.000 years ago, in a particularly
cold part of the last glacial period, large parts of Northern Europe and North
America were covered by ice sheets. The European ice sheet spanned a distance
of more than 5000 kilometres, from Ireland and Scotland via Scandinavia to the
Eastern rim of the Kara Sea (Arctic Ocean). In North America, large parts of
what is now known as Canada were buried under two large ice sheets. Greenland
and parts of the Bering Sea coastline were glaciated too. What was the ice
situation like even further North, in the Arctic Ocean? Was it covered by thick
sea-ice, or maybe with the tongues of these vast ice sheets were floating on
it, far beyond the North Pole?
Scientific answers to these questions were more or
less hypothetical so far. In contrast to deposits on land, where erratic
boulders, moraines and glacial valleys are the obvious landmarks of glaciers,
only few traces of vast ice shelves had been found so far in the Arctic Ocean.
Geoscientists from the Alfred Wegener Institute Helmholtz Centre for Polar and
Marine Research (AWI) and MARUM Center for Marine Environmental Sciences at the
University of Bremen have now compiled existing evidence from the Arctic Ocean
and the Nordic Seas, and combined it with new data to arrive at a surprising
conclusion.
According to their study, the floating parts of the
northern ice sheets covered large parts of the Arctic Ocean in the past 150,000
years. Once about 70,000-60,000 years ago and also about 150,000-130,000 years
ago. In both periods, freshwater accumulated under the ice, creating a
completely fresh Arctic Ocean for thousands of years.
“These results mean a real change to our
understanding of the Arctic Ocean in glacial climates. To our knowledge, this
is the first time that a complete freshening of the Arctic Ocean and the Nordic
Seas has been considered - happening not just once, but twice,” says the first
author, Dr Walter Geibert, geochemist at the Alfred Wegener Institute.
Thorium is absent
in the sediments, so saline water must have been absent
Their finding is based on geological analyses of ten
sediment cores from different parts of the Arctic Ocean, Fram Strait and the
Nordic Seas. The stacked deposits mirror the climate history of the past
glacials. When investigating and comparing the sediment records, the
geoscientists found that an important indicator was missing, always in the same
two intervals. “In saline sea water, the decay of naturally occurring uranium
always results in the production of the isotope thorium-230. This substance
accumulates at the sea floor, where it remains detectable for a very long time
due to its half-life of 75,000 years,” Walter Geibert explains.
Therefore, geologists often use this thorium-isotope
as a natural clock. “Here, its repeated and wide-spread absence is the giveaway
that reveals to us what happened. According to our knowledge, the only
reasonable explanation for this pattern is that the Arctic Ocean was filled
with freshwater twice in its younger history- in frozen and liquid form,”
co-author and micropalaeontologist Dr Jutta Wollenburg, also from the AWI,
explains.
A new picture of
the Arctic Ocean
How can a large ocean basin, connected by several
straits with the North Atlantic and the Pacific Ocean, turn entirely fresh?
“Such a scenario is perceivable if we realize that in glacial periods, global
sea levels were up to 130 m lower than today, and ice masses in the Arctic may
have restricted ocean circulation even further,” states co-author Professor
Ruediger Stein, geologist at the AWI and the MARUM.
Shallow connections like Bering Strait or the sounds
of the Canadian Archipelago were above sea level at the time, cutting off the
connection with the Pacific Ocean entirely. In the Nordic Seas, large icebergs
or ice sheets extending onto the sea floor restricted the exchange of water
masses. The flow of glaciers, ice melt in summer, and rivers draining into the
Arctic Ocean kept delivering large amounts of fresh water to the system, at
least 1200 cubic kilometres per year. A part of this amount would have been
forced via the Nordic Seas through the sparse narrow deeper connections in the
Greenland-Scotland Ridge into the North Atlantic, hindering saline water from
penetrating further north. This resulted in the freshening of the Arctic Ocean.
“Once the mechanism of ice barriers failed, heavier
saline water could fill the Arctic Ocean again,” Walter Geibert says. “We
believe that it could then quickly displace the lighter freshwater, resulting
in a sudden discharge of the accumulated amount of freshwater over the shallow
southern boundary of the Nordic Seas, the Greenland-Scotland-Ridge, into the
North Atlantic.”
A concept that assumes that enormous amounts of
freshwater were stored in the Arctic Ocean and available for rapid release
would help understanding the connection between a range of past climate
fluctuations. It would also offer an explanation for some apparent
discrepancies between different ways of reconstructing past sea levels. “The
remains of coral reefs have pointed to a somewhat higher sea level in certain
cold periods than reconstructions from Antarctic ice cores, or reconstructions
from the calcareous shells of small marine organisms, would suggest,” explains Walter
Geibert. “If we now accept that freshwater may not only have been stored in
solid form on land, but some of it also in liquid form in the ocean, the
different sea level reconstructions agree better and we can reconcile the
location of the coral reefs with calculations of the freshwater budget.”
Freshwater release from the Arctic Ocean might also
serve as an explanation for some abrupt climate change events during the last
glacial period. During such events, temperatures in Greenland could rise by
8-10 degree centigrade within a few years, only returning to the original cold
glacial temperatures over the course of hundreds or thousands of years. “We see
an example here of a past Arctic climate tipping point of the Earth system. Now
we need to investigate in more detail how these processes were interconnected,
and evaluate how this new concept of the Arctic Ocean helps in closing further
gaps in our knowledge, in particular in view of the risks of manmade climate
change,” says Walter Geibert.
原始論文：Walter
Geibert, Jens Matthiessen, Ingrid Stimac, Jutta Wollenburg, Ruediger
Stein. Glacial episodes of a freshwater Arctic Ocean covered by a thick
ice shelf. Nature, 2021; 590 (7844): 97 DOI: 10.1038/s41586-021-03186-y
引用自：Alfred Wegener Institute, Helmholtz Centre for
Polar and Marine Research. "The Arctic Ocean was covered by a shelf ice
and filled with freshwater."
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