流到北太平洋的冰塊促成末次冰期發生的一連串氣候事件
By Michelle Klampe
最近發表在《科學》(Science)的新研究表示末次冰期期間，北美西部屢次有大量冰塊流入北太平洋，促成了規模涵蓋半個地球的氣候變化，甚至有可能是直接原因。
奧勒岡州立大學地球、海洋與大氣科學院的古氣候學家Maureen
Walczak是研究主要作者。他說：「冰層快速融化，流入北太平洋之後可能會對整個地球的氣候造成什麼樣的影響？這項發現讓我們得到了全新的見解。」
Walczak說：「了解過去海洋和冰河的交互作用，可以幫助我們預測未來會發生什麼事。」
柯迪勒拉冰蓋(Cordilleran
ice sheet)曾經覆蓋了北美西部的大片地區，從阿拉斯加、華盛頓和蒙大拿西部都隸屬於它的範圍。分析海洋沉積物紀錄並運用放射性碳定年法，結果顯示過去42000年中這處冰蓋屢次會有大量冰塊流入海洋，接著開啟一連串的連鎖反應使得全球氣候受到擾動，引發深海環流變化並讓北大西洋的冰蓋後退。
這項發現挑戰了上述影響全球的大規模氣候擾動，是起源自北大西洋勞倫泰德冰蓋(Laurentide
ice sheet)快速流失，又稱作海因裏希事件(Heinrich
Event)而導致的理論。勞倫泰德冰蓋是另一座巨大的冰蓋，覆蓋了大部分的加拿大以及美國北部，包含美國中西部的北側和東北各州。
「這項研究的結果讓人相當意外。數據明明白白地指出先是太平洋的冰層出現變化，接著才發生海因裏希事件和其他一連串的變化。因此決定這一切如何進行的其實是太平洋才對。」Walczak 表示。「研究結果改變了我們思考這些事件彼此之間關聯的常用方式。」
這項研究是國際海洋發現計畫的一部分。為了更加瞭解北太平洋的氣候歷史，一組國際研究團隊在阿拉斯加灣的北部進行鑽探，取出沉積物岩芯並加以分析。
Alan Mix是計畫主持人與這篇論文的共同作者，他說：「這些全新的見解是我們研究多年才得到的成果。我們一開始是在2004年繪製這片海床的地形並取回較短的沉積物岩芯，接著在2013年取得了更長的岩芯。16年來我們在實驗室對它們進行了極為詳盡的分析，過程中也培養了幾位博士生。」
Mix也是奧勒岡州立大學地球、海洋和大氣科學院的特聘教授。他說：「我們開始研究時這片區域對人類來說幾乎是未知的領域，但現在它卻給出了地球在冰河時期有關海洋的長期變化，最詳細也是定年最好的紀錄。」
研究人員利用兩台粒子加速器來測量放射性碳同位素，以便建立事件的發生年代；此外，他們也詳細計算了從冰山掉出來的小石頭(ice-rafted
debris，漂冰碎屑)有多少。
研究團隊追查這些漂冰碎屑的來源，認為它們是從柯迪勒拉冰蓋的大型冰流傾瀉而出。從70000年至17000年前，華盛頓北方、英屬哥倫比亞和阿拉斯加南部的大部分地區都覆蓋在柯迪勒拉冰蓋之下。
從快速流動的冰流中斷裂出來的冰山帶有許多髒汙。它們會隨著洋流漂向北方，最後拋下它們攜帶的沙子、卵石和礫石。就像沉入海底的寶船一樣，這份紀錄會留在深海當中，顯示出曾有一段冰河快速後退的時期。
研究作者把這些阿拉斯加的冰山拋下許多物質的事件稱做「Siku」，來源為因紐特語當中的冰。結合冰川碎屑的紀錄和放射性碳定年的結果，他們十分驚訝地發現Siku
事件發生的時間點剛好就在海因裏希事件不久之前，其為類似的冰塊大量流出事件，但是發生在大西洋。
科學家三十幾年前就已經發覺了海因裏希事件，證據為北大西洋也有發現類似的漂冰碎屑。不過研究人員表示海因裏希事件的成因至今仍然沒有一個令人信服的解釋。
Mix說太平洋會牽涉地球的重大變化是合理的想法。透過大尺度的大氣環流，或者直接經由環繞南極的海水，太平洋可以和地球其他地方產生連結；此外在海平面較高的時期，也可以藉著白令海峽和北極海連到北大西洋。
「在地球上，熱量、水、二氧化碳可以互相交換的儲庫中太平洋是最大的，原因很簡單，太平洋實在是太大了，」他說。「如果這些氣候因子是動物園中的猛獸，那太平洋就是重達800磅的大猩猩。」
由於氣候暖化，目前還在阿拉斯加沿岸的冰層大部分都正在後退，而且在本世紀可能就會完全消失。融化的冰會流入太平洋和大西洋，造成海平面上升，並且影響海洋裡密度較輕的淡水和較重的鹹水之間的平衡，這和過去的情形十分類似。
如果此處目前冰川融化的模式和過去一樣且步調更加快速，那麼遠處北大西洋和北極的冰川系統可能便會因此後退。
Mix 說：「這不過是另外一個理由，告誡我們減少使用化石燃料，降低暖化速度才是明智的做法。」
Walczak說：「這項新發現或許能讓科學家燃起對於北太平洋的興趣。相較於地球其他地方，這裡一直以來受到的研究都比較少。」
柯迪勒拉冰蓋的冰塊為什麼會大量流出是研究人員還沒解開的問題之一；此外，他們也希望可以更加瞭解從柯迪勒拉流出的冰塊和其他氣候事件之間有什麼樣的關聯。
Walczak問道：「為什麼柯迪勒拉冰蓋後退會影響到其他的冰蓋？這些事件之間的骨牌效應又有多快發生？」這是研究團隊正在繼續調查的幾項問題之一。
其他共同作者包括了奧勒岡州立大學(OSU)地球、海洋與大氣科學院的Andreas
Schmittner、Joseph
Stoner、Brian
Haley、June
Padman，以及最近在此獲得博士學位的Jianghui
Du；同時任職於OSU和美國地質調查所的Jay
Alder；之前任職於OSU，現為美國地質調查所的Summer
Praetorius；阿帕拉契州立大學的Ellen
Cowan；澳洲國立大學的Stewart
Fallon和L.
Keith Fifield；最後還有中央密蘇里大學的Sarah
Zellers。
研究經費來自美國國家科學基金會、澳大利亞研究委員會、澳大利亞―紐西蘭國際海洋鑽探計畫委員會、美國澳洲協會。
Ice discharge in the North Pacific set
off series of climate events during the last ice age
Repeated catastrophic ice discharges from
western North America into the North Pacific contributed to, and perhaps
triggered, hemispheric-scale changes in the Earth’s climate during the last ice
age, new research published online today in Science
reveals.
The discovery provides new insight into the impact
rapidly melting ice flowing into the North Pacific may have on the climate
across the planet, said Maureen Walczak, a paleoclimatologist in Oregon State
University’s College of Earth, Ocean, and Atmospheric Sciences and the study’s
lead author.
“Understanding how the ocean has interacted with
glacial ice in the past helps us predict what could happen next,” Walczak said.
The Cordilleran ice sheet once covered large portions
of western North America from Alaska to Washington state and western Montana.
Radiocarbon dating and analyses of the marine sediment record revealed that
recurrent episodes of discharge from this ice sheet over the past 42,000 years
were early events in a chain reaction of disturbances to the global climate.
These disturbances triggered changes in deep ocean circulation and retreat of
ice sheets in the North Atlantic.
The findings challenge theories that those massive,
globally-reaching disturbances originated in the North Atlantic as rapid ice
loss from the Laurentide ice sheet, another massive ice sheet that covered much
of Canada and the northern United States, including the upper Midwest and
Northeast. The Laurentide ice loss events are known as Heinrich Events.
“The outcome of this research was unexpected. The
data irrevocably says that the Pacific ice goes first, with Heinrich Events and
other changes following in a rhythm. The Pacific Ocean sets the drum beat,”
Walczak said. “This is a paradigm shift in our thinking about how these events
are connected.”
To gain insight into the climate history of the North
Pacific, an international team of researchers collected and analyzed sediment
cores from the northern Gulf of Alaska that were recovered by drilling as part
of the International Ocean Discovery Program.
“Getting these new insights took years of work. We
first mapped the seafloor and recovered short sediment cores in 2004, drilled
longer cores in 2013 and had 16 years of painstaking laboratory work involving
several Ph.D. students,” said Alan Mix, the project’s principal investigator and
co-author of the paper.
“This was a
virtually unknown area when we started, and now it offers among the most detailed
and best-dated long records of ocean change on the planet during the ice age,”
said Mix, a distinguished professor in OSU’s College of Earth, Ocean, and
Atmospheric Sciences.
Researchers measured radioactive isotopes of carbon
using two particle accelerators to establish the chronology of events and also
added meticulous counts of small rocks dropped by icebergs known as ice-rafted
debris.
The research team traced the source of the ice-rafted
debris back to purges of massive ice streams emanating from the Cordilleran ice
sheet, which covered northern Washington, most of British Columbia and southern
Alaska from about 70,000 to 17,000 years ago.
Dirty icebergs broke off from surging ice streams and
drifted northward in ocean currents, carrying and eventually dropping their
load of sand, pebbles and gravel, leaving a record of rapid ice retreat buried
in the deep sea like sunken treasure.
The authors of the study named these Alaskan iceberg
dumps “Siku Events” after the Inuit word for ice. The big surprise, discovered
by combining the record of glacial debris with the radiocarbon chronology, was
that Siku Events immediately preceded Heinrich events, which are a similar type
of ice purge in the North Atlantic.
Scientists have been aware of Heinrich Events, from
similar evidence of ice-rafted debris in the North Atlantic, for more than 30
years but the trigger for those events has never been convincingly explained,
the researchers said.
It makes sense for the Pacific Ocean to be involved
in major planetary changes, Mix said. The Pacific Ocean is connected to the
rest of the world by large-scale atmospheric circulation and physically around
Antarctica, and during times of high sea level, through the Bering Strait and
the Arctic Ocean to the North Atlantic.
“The Pacific
Ocean is the largest exchangeable reservoir of heat and water and carbon
dioxide on Earth, simply because of its massive size,” he said. “It really is
the 800-pound gorilla in the zoo of climate beasts.”
Today the ice that remains along the coast of Alaska
is mostly retreating and may be gone within this century as the climate warms.
The melting ice will drain to the Pacific and the Arctic, contributing to sea
level rise and impacting the balance of buoyant fresh and dense salty water in
the ocean, much as it did in the past.
If the current ice melt follows patterns of the past,
and happens quickly, it could contribute to the retreat of distant glacial
systems in the North Atlantic and the Arctic.
“This is yet another reason that it is prudent to
slow down warming by reducing our fossil-fuel use,” Mix said.
“The new findings are likely to fuel increased
interest in the North Pacific, an area that has not been as well-studied as
other parts of the planet,” Walczak said.
One thing that remains unclear is why the discharges
from the Cordilleran ice sheet occurred. Researchers also would like to better
understand the relationship between the discharges of the Cordilleran and the
other climate events.
“Why did the other ice sheets respond to the retreat
of the Cordilleran? How fast do the dominoes fall in this sequence of events?”
Walczak asked. Those are among the questions the research team is continuing to
investigate.
Additional coauthors of the study include Andreas
Schmittner, Joseph Stoner, Brian Haley; and June Padman, all of OSU’s College
of Earth, Ocean, and Atmospheric Sciences; Jianghui Du, who recently earned his
doctorate at Oregon State; Jay Alder of OSU and the U.S. Geological Survey;
Summer Praetorius, previously of OSU and now at the U.S. Geological Survey;
Ellen Cowan of Appalachian State University; Stewart Fallon and L. Keith
Fifield of Australian National University; and Sarah Zellers of University of Central
Missouri.
The research was supported by the National Science
Foundation, the Australian Research Council, the Australian-New Zealand IODP
Commission and the American Australian Association.
原始論文：Maureen H.
Walczak, Alan C. Mix, Ellen A. Cowan, Stewart Fallon, L. Keith Fifield, Jay R.
Alder, Jianghui Du, Brian Haley, Tim Hobern, June Padman, Summer K. Praetorius,
Andreas Schmittner, Joseph S. Stoner, Sarah D. Zellers. Phasing of
millennial-scale climate variability in the Pacific and Atlantic Oceans. Science,
2020; eaba7096 DOI: 10.1126/science.aba7096
引用自：Oregon State University. "Ice discharge in
the North Pacific set off series of climate events during last ice age."
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