近年來全球暖化在南極冰層留下了明顯的痕跡。這些「亙古不變」的南極冰雪融化速度比過往預估的還快，而南極西部又比南極東部超出更多。這種現象的根本原因或許存在於冰層的形成過程——如同最近阿佛列•韋格納研究所主持的國際團隊得到的發現：他們透過岩芯裡的沉積物樣本並結合複雜的氣候與冰層模擬，發現大約距今3400萬年前開始，南極便一直處在冰河期；但是跟之前推測的不一樣，當時冰層還沒有覆蓋到整個南極大陸，而只侷限在東部。至少要再經過700萬年，冰層才有辦法擴展到南極西部的海岸。研究人員在著名期刊《科學》(Science)中敘述了這項新的發現，顯示南極東部和西部對外在作用力的反應有重大差異。
位於派恩島海灣的極星號研究船。圖片來源：英國南極調查局 / R. La
大概距今3400萬年前，地球氣候經歷了一次十分重大的變化，時至今日仍影響著全球的氣候狀態：當時地球從溫室轉變成冰室(icehouse)，也就是從陸地上幾乎沒有或是很少地方有冰層堆積，轉變成有些區域一直都被廣大的冰河所覆蓋的世界，而南極冰層也是在這段期間逐漸成長起來。然而，我們還是沒有完全明白南極冰層形成的時間，以及最重要的——是從什麼地方開始。這是因為我們還沒從南極的關鍵區域取得有將過去的變化紀錄下來的可靠數據或樣品，尤其是南極西部。不過，德國亥姆霍茲極地與海洋研究中心的阿佛列•韋格納研究所的科學家，與英國南極調查局、海德堡大學、諾桑比亞大學(英國)、布萊梅大學海洋環境科學中心MARUM的同僚合作之下，最近成功填補了這道知識缺口。其他合作人員則來自德國的阿亨大學、萊比錫大學、布萊梅大學和基爾大學；以及塔斯馬尼亞大學(澳洲)、倫敦皇家學院(英國)、弗里堡大學(瑞士)、格拉納達大學(西班牙)、萊斯特大學(英國)、德州農工大學(美國)，以及德國森肯堡海洋研究中心與位在漢諾威的聯邦地球科學與國家資源研究所。
研究人員運用海床鑽機MARUM-MeBo70在南極西部，鄰近史威茲冰河與派恩島冰河的阿蒙森海海床上鑽取了一根岩芯，藉此他們首次重建了南極大陸陷入冰封之初的歷史。令他們感到驚訝的是，在南極冰河期的第一個主要階段，並沒有看到此區域有冰雪出現的痕跡。「這意味第一個持續至今的大規模冰河作用必定是從南極東部的某處開始，」研究團隊的主持人，阿佛列•韋格納研究所的地質學家Johann
Klages博士表示。原因是在第一次冰河最大期的時候，南極西部還沒有被冰雪覆蓋。當時此區域大部分還是生長著濃密的闊葉林，溫涼的氣候使得冰層無法在南極西部形成。
MARUM-MeBo70降落在阿蒙森海海床上的示意圖。圖片來源：布萊梅大學海洋環境科學中心/
Martin Künsting
南極東部與西部對外在作用力的反應相當不同
為了更加了解南極第一座持續存在的冰層在哪形成，阿佛列•韋格納研究所的古氣候模擬專家結合了最新取得與現存的數據中，關於氣溫、海溫和冰量的資料。「模擬支持了地質學家從這根特別的岩芯中得出的結果，」阿佛列•韋格納研究所的古氣候模擬專家Gerrit
Lohmann教授表示。「這徹底改變了我們對於南極冰河期的認知。」根據這項研究，能讓冰層永久存在的基本氣候條件只有在南極東部維多利亞地的北端較為普遍。潮濕的氣團在此會遇到快速隆起的橫貫南極山脈，造成讓冰雪持續堆積，接著形成冰帽的理想條件。冰層以此為起點，快速擴張到南極東部的內陸地區，但是往南極西部的路上卻花了點時間。「還要再過七百萬年左右，環境條件才允許冰層擴張到南極西部的海岸，」阿佛列•韋格納研究所的古氣候模擬專家Hanna
Knahl解釋。「我們的模擬結果清楚顯示溫度需要下降到什麼程度，冰層才能擴張並覆蓋南極西部，當時這裡許多地方已經比海平面還低。」另一個讓人印象深刻的研究結果是，這兩個區域的南極冰層對於外在作用力以及重大氣候變遷的反應有多麼不同。「即使是輕微的暖化就足以讓南極西部的冰層再次融化——這正是我們現在所處的情境，」Johann
Klages表示。
這項國際團隊的發現對於了解氣候的極端轉變——從溫室氣候到我們目前所處的冰室氣候——來說相當關鍵。研究的另一個重要結果是提供了新的觀點，讓氣候模擬專家可以更加精準地模擬持續被冰河覆蓋的地區對於全球氣候的動力學(也就是冰層、海洋、大氣之間的互動)有什麼樣的影響。這項知識至關重要，就如Johann
Klages所說的：「特別是因為在不久的將來，我們可能就要再次面對像當時那樣的重大氣候變遷。」
利用新科技得出獨到的觀點
研究人員之所以能彌補這道知識缺口，有賴於一根獨特的岩芯，其取自於2017年極星號(Polarstern)研究船在南極西部的PS104航次，這也是布萊梅大學海洋環境科學中心研發的鑽機MARUM-MeBo70首次運用在南極。由於南極西部史威茲冰河與派恩島冰河之外的海床相當堅硬，因此之前無法用一般的鑽探方法取得深處的沉積物。而MARUM-MeBo70具備可以旋轉的鑽頭，使其可以鑽進海床10公尺深的地方並取回樣品。
此研究計畫的經費，特別是極星號的PS104航次，來自阿佛列•韋格納研究所、布萊梅大學海洋環境科學中心、英國南極調查局、以及NERC
UK-IODP計畫。
The dawn of the Antarctic ice sheets
In recent years global warming has left
its mark on the Antarctic ice sheets. The "eternal" ice in Antarctica
is melting faster than previously assumed, particularly in West Antarctica more
than East Antarctica. The root for this could lie in its formation, as an
international research team led by the Alfred Wegener Institute has now
discovered: sediment samples from drill cores combined with complex climate and
ice-sheet modelling show that permanent glaciation of Antarctica began around
34 million years ago – but did not encompass the entire continent as previously
assumed, but rather was confined to the eastern region of the continent (East
Antarctica). It was not until at least 7 million years later that ice was able
to advance towards West Antarctic coasts. The results of the new study show how
substantially differently East and West Antarctica react to external forcing,
as the researchers describe in the prestigious journal Science.
Around 34 million years ago, our planet underwent one
of the most fundamental climate shifts that still influences global climate
conditions today: the transition from a greenhouse world, with no or very
little accumulation of continental ice, to an icehouse world, with large
permanently glaciated areas. During this time, the Antarctic ice sheet built
up. How, when and, above all, where, was not yet known due to a lack of
reliable data and samples from key regions, especially from West Antarctica,
that document the changes in the past. Researchers from the Alfred Wegener
Institute, Helmholtz Centre for Polar and Marine Research (AWI) have now been
able to close this knowledge gap, together with colleagues from the British
Antarctic Survey, Heidelberg University, Northumbria University (UK), and the
MARUM - Center for Marine Environmental Sciences at the University of Bremen,
in addition to collaborators from the Universities in Aachen, Leipzig, Hamburg,
Bremen, and Kiel, as well as the University of Tasmania (Australia), Imperial
College London (UK), Université de Fribourg (Switzerland), Universidad de
Granada (Spain), Leicester University (UK), Texas A&M University (USA),
Senckenberg am Meer, and the Federal Institute for Geosciences and Natural
Resources in Hanover, Germany.
Based on a drill core that the researchers retrieved
using the MARUM-MeBo70 seafloor drill rig in a location offshore the Pine
Island and Thwaites glaciers on the Amundsen Sea coast of West Antarctica, they
were able to establish the history of the dawn of the icy Antarctic continent
for the first time. Surprisingly, no signs of the presence of ice can be found
in this region during the first major phase of Antarctic glaciation. “This
means that a large-scale, permanent first glaciation must have begun somewhere
in East Antarctica,” says Dr Johann Klages, geologist at the AWI who led the
research team. This is because West Antarctica remained ice-free during this
first glacial maximum. At this time, it was still largely covered by dense broadleaf
forests and a cool-temperate climate that prevented ice from forming in West
Antarctica.
East and West
Antarctica react very different to external conditions
In order to better understand where the first
permanent ice formed in Antarctica, the AWI paleoclimate modelers combined the
newly available data together with existing data on air and water temperatures
and the occurrence of ice. “The simulation has supported the results of the
geologists' unique core,” says Prof Dr Gerrit Lohmann, paleoclimate modeler at
the AWI. “This completely changes what we know about the first Antarctic
glaciation.” According to the study, the basic climatic conditions for the
formation of permanent ice only prevailed in the coastal regions of the East
Antarctic Northern Victoria Land. Here, moist air masses reached the strongly
rising Transantarctic Mountains – ideal conditions for permanent snow and
subsequent formation of ice caps. From there, the ice sheet spread rapidly into
the East Antarctic hinterland. However, it took some time before it reached
West Antarctica: “It wasn't until about seven million years later that
conditions allowed for advance of an ice sheet to the West Antarctic coast,”
explains Hanna Knahl, a paleoclimate modeler at the AWI. “Our results clearly
show how cold it had to get before the ice could advance to cover West
Antarctica that, at that time, was already below sea level in many parts.” What
the investigations also impressively show is how different the two regions of
the Antarctic ice sheet react to external influences and fundamental climatic
changes. “Even a slight warming is enough to cause the ice in West Antarctica
to melt again - and that's exactly where we are right now,” adds Johann Klages.
The findings of the international research team are
critical for understanding the extreme climate transition from the greenhouse
climate to our current icehouse climate. Importantly, the study also provides
new insight that allows climate models to simulate more accurately how
permanently glaciated areas affect global climate dynamics, that is the
interactions between ice, ocean and atmosphere. This is of crucial importance,
as Johann Klages says: “Especially in light of the fact that we could be facing
such a fundamental climate change again in the near future.”
Using new
technology to gain unique insights
The researchers were able to close this knowledge gap
with the help of a unique drill core that they retrieved during the expedition
PS104 on the research vessel Polarstern in West Antarctica in 2017. The
MARUM-MeBo70 drill rig developed at MARUM in Bremen was used for the first time
in Antarctica. The seabed off the West Antarctic Pine Island and Thwaites
glaciers is so hard that it was previously impossible to reach deep sediments
using conventional drilling methods. The MARUM-MeBo70 has a rotating
cutterhead, which made it possible to drill about 10 meters into the seabed and
retrieve the samples.
The research project, and the Polarstern expedition
PS104 in particular, was funded by the AWI, MARUM, the British Antarctic
Survey, and the NERC UK-IODP Programme.
原始論文：J. P. Klages,
C.-D. Hillenbrand, S. M. Bohaty, U. Salzmann, T. Bickert, G. Lohmann, H. S.
Knahl, P. Gierz, L. Niu, J. Titschack, G. Kuhn, T. Frederichs, J. Müller, T.
Bauersachs, R. D. Larter, K. Hochmuth, W. Ehrmann, G. Nehrke, F. J.
Rodríguez-Tovar, G. Schmiedl, S. Spezzaferri, A. Läufer, F. Lisker, T. van de
Flierdt, A. Eisenhauer, G. Uenzelmann-Neben, O. Esper, J. A. Smith, H. Pälike,
C. Spiegel, R. Dziadek, T. A. Ronge, T. Freudenthal, K. Gohl. Ice
sheet–free West Antarctica during peak early Oligocene glaciation. Science,
2024; DOI: 10.1126/science.adj3931
引用自：Alfred Wegener Institute, Helmholtz Centre for
Polar and Marine Research. "The dawn of the Antarctic ice
sheets."
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