研究南極羅斯冰棚之下的融化模式得到了驚人發現
By Marie Denoia
Aronsohn
由許多研究機構合作進行的「羅塞塔冰層計畫」(ROSETTA-Ice
project)，三年來調查了南極的冰層並蒐集各類資料，統整起來對羅斯冰棚得出了全新見解，包括它的結構與變化歷程。今日(5/27)發表於《自然―地球科學》(Nature Geoscience)的研究中，羅塞塔冰層計畫的團隊成員詳述他們如何發現過往形成的地質構造限制了海水流動的路徑。這項發現顯示羅斯冰棚未來消融的時候，當地的海流可能會有相當重要的影響。
羅塞塔冰層計畫的一部分是運用IcePod飛過南極羅斯冰棚上方。這個吊艙掛在LC130貨機的下方，其中含有的儀器在羅塞塔冰層計畫中相當重要。照片來源：Winnie
Chu
羅塞塔冰層計畫從巨大的羅斯冰棚蒐集了許多資料。冰棚是漂浮在海上、面積十分遼闊的巨大冰層，它們可以減緩後方南極冰層進到海洋的速度。而羅斯冰棚阻擋了南極陸地冰層的20%進到海洋，如果這部分的冰層進到海中會讓全球海平面上升將近12公尺。目前南極的冰層正在加速融化，若要預測全球持續暖化會讓冰棚出現什麼樣的變化，就需要瞭解冰層、海洋、大氣、地質彼此之間複雜的交互作用。
為了更加了解這些作用，羅塞塔冰層的跨領域團隊前往羅斯冰棚進行研究，猶如初次探索一顆新的星球。羅斯冰棚的面積跟西班牙差不多大，時常有數百公尺高的冰山出沒，使得傳統的研究船無法近來調查海底。要在這種地方蒐集數據是團隊面臨的主要挑戰。他們的解決方法是設計出一項全新的系統，用來在極區各處蒐集高解析度的數據，稱為「IcePod」。由哥倫比亞大學拉蒙特―多爾蒂地球觀測所開發的IcePod可以裝設在貨機上，其中的儀器可以量測冰棚的高度、厚度和內部構造，還有下方岩石的磁力重力性質。
每次IcePod飛過冰棚，磁強計(用來測量地球磁場強度的儀器)一開始的訊號都很平穩，幾乎沒有變化。但到了半途讀數便開始活躍起來，呈現出明顯的變化，就像心電圖上的心跳一樣。團隊把結果畫在地圖上面之後，清楚顯示出「心跳」總是出現在冰棚的中央，代表南極東部和西部中間有一道未曾發現的地質界線。
團隊接著利用IcePod的重力場測量結果來模擬冰棚底下的海床型態。研究主要作者，拉蒙特―多爾蒂地球觀測所的研究員Kirsty
Tinto解釋：「我們可以看到這道地質邊界造成南極東部的海床比西部還深，進而影響冰棚底下的海水如何流動。」這三次實地考察都是由Kirsty
Tinto主持。
利用冰棚底下新的海洋地形圖，團隊模擬了此地的海洋環流以及它對冰棚融化的影響。結果顯示只有少量的溫暖海水可以到達羅斯冰棚下方。相較之下在東邊的阿蒙森海，溫暖的海水可以越過大陸棚，造成該地冰棚快速融化。羅斯冰棚內部有一處稱為冰中湖(Ross
Shelf Polynya)的開放水域，深層海水在到達冰棚下方之前，含有的熱量會被冰中湖上方寒冷的冬季空氣帶走。模型顯示雖然冰冷的海水還是可以融化南極東部冰河的深處，但過往形成的地質構造界線使兩側的深度出現差異，造成這些海水無法流到南極西部。
然而團隊也驚訝地發現，羅斯冰棚前緣的某個區域反倒是因為冰中湖而在夏季快速融化。冰棚內部結構的雷達影像也證實該處確實如此。共同作者，地球和太空研究所(Earth
and Space Research)的資深科學家Laurie
Padman說：「我們發現冰棚前緣發生的作用出現的變化，像是海冰或雲層減少，造成夏季溫度升高的時候，羅斯冰棚的減小狀況以及鄰近陸上冰層的流動速度，很容易就會受到影響。」
總體來說，結果指出要用模型預測南極冰層因為未來的氣候變遷而流失的狀況時，不能只納入深層溫暖海水環流的大尺度變化，也要考慮冰棚前緣的局部環境如何變化。Tinto表示：「我們發現如果想作出確實的預測，必須瞭解發生在局部地區的作用。」
Study uncovers surprising melting
patterns beneath Antarctica's Ross Ice Shelf
The ROSETTA-Ice project, a three-year,
multi-institutional data collection survey of Antarctic ice, has assembled an
unprecedented view of the Ross Ice Shelf, its structure and how it has been
changing over time. In a study published today in Nature Geoscience, the ROSETTA-Ice team members detail how they
discovered an ancient geologic structure that restricts where ocean water
flows. The discovery suggests that local ocean currents may play a critical
role in the ice shelf’s future retreat.
Ice shelves are massive expanses of floating ice that
slow down the flow of Antarctic ice into the ocean. ROSETTA-Ice collected data
from the massive Ross Ice Shelf, which helps slow the flow of about 20 percent
of Antarctica’s grounded ice into the ocean — equivalent to 38 feet of global
sea level rise. Antarctica’s ice is already melting at an accelerating rate.
Predicting how the ice shelf will change as the planet continues to warm
requires understanding the complex ways in which the ice, ocean, atmosphere and
geology interact with each other.
To gain a better understanding of these processes,
the multidisciplinary ROSETTA-Ice team approached the Ross Ice Shelf much like
explorers visiting a new planet for the first time. The team faced the key
challenge of how to gather data from a region the size of Spain, and where ice
that is frequently more than a thousand feet thick prevents more traditional
ship-based surveys of the seabed. The solution was IcePod, a first-of-its kind
system designed to collect high-resolution data across the polar regions.
IcePod was developed at Columbia University’s Lamont-Doherty Earth Observatory
and mounted on a cargo plane. Its instruments measure ice shelf height,
thickness and internal structure, and the magnetic and gravity signal of the
underlying rock.
Each time the team flew across the ice shelf, the
IcePod’s magnetometer (which measures Earth’s magnetic field) showed a flat and
almost unchanging signal. That is, until halfway across the ice shelf, when the
instrument came alive, displaying large variations, much like the heartbeat on
a cardiogram. When the team mapped their results, it became clear that this
“heartbeat” always appeared in the middle of the ice shelf, identifying a
previously unmapped segment of the geologic boundary between East and West
Antarctica.
The team then used IcePod’s measurements of Earth’s
gravity field to model the shape of the sea floor beneath the ice shelf. “We
could see that the geological boundary was making the seafloor on the East
Antarctic side much deeper than the West, and that affects the way the ocean
water circulates under the ice shelf,” explained Kirsty Tinto, the Lamont
research scientist who led all three field expeditions and is lead author of
the study.
Using the new map of the seabed under the ice shelf,
the team ran a model of ocean circulation and its effect on ice shelf melting.
Compared with the Amundsen Sea to the east, where warm water crosses the
continental shelf to cause rapid melting of the ice shelves, little warm water
reaches the Ross Ice Shelf. In the Ross Sea heat from the deep ocean is removed
by the cold winter atmosphere in a region of open water, called the Ross Shelf
Polynya, before flowing under the ice shelf. The model showed that this cold
water melts deeper portions of east Antarctic glaciers, but it is steered away
from the west Antarctic side by the depth change at the ancient tectonic
boundary.
In a surprise twist, however, the team found that the
polynya also contributes to a region of intense summertime melting along the
ice shelf’s leading edge. This melting was confirmed in the radar images of the
ice shelf’s internal structure. “We found that the ice loss from the Ross Ice
Shelf and flow of the adjoining grounded ice are sensitive to changes in
processes along the ice front, such as increased summer warming if sea ice or
clouds decrease,” said Laurie Padman, a co-author and senior scientist at Earth
and Space Research.
Overall, the results indicate that models used to
predict Antarctic ice loss in future climates must consider changing local
conditions near the ice front, not just the large-scale changes in the
circulation of warm deep water. “We found out that it’s these local processes
we need to understand to make sound predictions,” said Tinto.
原始論文：K. J. Tinto,
L. Padman, C. S. Siddoway, S. R. Springer, H. A. Fricker, I. Das, F. Caratori
Tontini, D. F. Porter, N. P. Frearson, S. L. Howard, M. R. Siegfried, C.
Mosbeux, M. K. Becker, C. Bertinato, A. Boghosian, N. Brady, B. L. Burton, W.
Chu, S. I. Cordero, T. Dhakal, L. Dong, C. D. Gustafson, S. Keeshin, C. Locke,
A. Lockett, G. O’Brien, J. J. Spergel, S. E. Starke, M. Tankersley, M. G.
Wearing, R. E. Bell. Ross Ice Shelf response to climate driven by the
tectonic imprint on seafloor bathymetry. Nature Geoscience,
2019; DOI: 10.1038/s41561-019-0370-2
引用自：Earth Institute at Columbia University.
"Study uncovers surprising melting patterns beneath Antarctica's Ross Ice
Shelf.”
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