來自陸地的碳在上次冰河期消退時具有重要地位

原文網址：www.sciencedaily.com/releases/2016/03/160314161245.htm

Carbon from land played a role during last deglaciation

來自陸地的碳在上次冰河期消退時具有重要地位

As the Earth emerged from its last ice age several thousand years ago, atmospheric carbon dioxide increased and further warmed the planet. Scientists have long speculated that the primary source of this CO2 was from the deep ocean around Antarctica, though it has been difficult to prove.

自地球於數萬年前進入末次冰河期起，大氣中的二氧化碳便不斷累積而進一步暖化地球。雖然相當難以證實，但科學家長期以來猜測這些二氧化碳的主要來源是南極周圍的深海。

A new study published this week in Proceedings of the National Academy of Sciences confirmed that the ocean played a significant role in the rise of atmospheric carbon dioxide, but also documents the signature of land-based carbon sources in Antarctic ice cores that contributed to abrupt increases in CO2.

刊登於本周《美國國家科學院學報》(Proceedings of the National Academy of Sciences, PNAS)的新研究證實海洋的確在大氣二氧化碳濃度的上升中具有重要地位，但同時也論述南極冰芯中的碳有來自陸地的跡象，顯示陸地在二氧化碳的突然上升中佔有一席之地。

"There wasn't a steady rate of rising carbon dioxide during the last deglaciation," said Edward Brook, an Oregon State University paleoclimatologist and co-author on the PNAS study. "It happened in fits and starts. With the new precise techniques we developed to fingerprint the sources, it is apparent that the early carbon largely came from the ocean, but we think the system got a jolt from an influx of land-based carbon a few times as the climate warmed."

「在末次冰消期( last deglaciation)時二氧化碳的上升速率並非固定。」此篇刊於PNAS的研究的共同作者，奧勒岡州立大學的古氣候學家 Edward Brook說。「這起事件發生地時快時慢。利用我們研發可以追查二氧化碳來源的精準新技術，結果顯示碳起初大都來自海洋，但我們認為隨著氣候逐漸變暖，這個系統會屢次受到來自陸地的碳影響而擺盪。」

The study was funded by the National Science Foundation with support from the Marsden Fund Council in New Zealand.

這起研究的資金來自國家科學基金會，並由紐西蘭的Marsden基金委員會提供協助。

The breakthrough came from the comparison of carbon isotope ratios in pristine samples of ice mined from the Taylor Glacier in Antarctica. Although such isotopic fingerprinting strategies have been attempted before, the key was detailed work both in the field and in the laboratory that improved the precision to read the record in fine detail.

這起突破性的研究來自於完整冰芯樣本中的碳同位素比對結果，其採自於南極的Taylor冰河。雖然早已有人試過這種同位素測蹤技術，但這起研究的關鍵是他們的野外和實驗室作業皆相當詳盡，使得他們的精確度更加提高而能夠徹頭徹尾地解讀這些紀錄。

The study found that during the initial rise in atmospheric CO2 -- from 17,600 years ago to 15,500 years ago -- the light isotope 12-C increased faster than the heavier isotopes, pointing to a release of carbon from the deep ocean. However, at about 16,300 years ago and 12,900 years ago, there were abrupt, century-scale perturbations in the carbon ratio that suggested rapid release of carbon from land sources such as plants and soils.

研究發現從17,600至15,500年前大氣二氧化碳濃度剛開始上升的階段，較輕的同位素碳-12上升得比較重的同位素還要快，顯示碳從深海中釋放了出來。然而在16,300和12,900年前，碳同位素卻突然有些長達數百年的擾動，代表來自陸地(像是植物和土壤)的碳在這段期間迅速排放至大氣。

Although the region of the CO2 source is not clear, the scientists say, at least one of the two events may come from the tropics because methane from tropical swamps rose at the same time.

科學家說雖然這些二氧化碳的來源地區尚未明朗，但這兩起事件的其中之一可能出自熱帶，因為同一時間來自熱帶沼澤的甲烷量也提升了。

"One theory," Brooks said, "is that an influx of icebergs in the Northern Hemisphere at about 16,300 years ago -- from retreating ice sheets -- cooled the North Atlantic Ocean and pushed the tropical rain belt southward over Brazil, expanding the wetlands. Swamps in the Southern Hemisphere, in places like Brazil, may have become wetter and produced methane, while plants and soils in the Northern Hemisphere, in places like China, may have been hit by drought and produced CO2."

「有項理論認為在16,300年前因為冰棚退縮使得北半球冰山流入海洋的速率增加，造成北大西洋冷卻，並將熱帶雨帶推至巴西以南，濕地因而擴展。位處南半球，比如巴西的沼澤地或許會因此變得更加潮濕而製造更多甲烷；而在北半球，像是中國的植物和土壤卻可能因遭受乾旱而產生二氧化碳。」

During the next 4,000 years, the continued rise of atmospheric CO2 -- by about 40 parts per million -- was marked by small changes in the carbon-13 to carbon-12 ratio indicating additional sources of carbon from rising ocean temperatures. This CO2 source, analogous to the bubbles released from warming soda pop, may have added to the biological carbon sources.

在接下來4000年大氣二氧化碳濃度持續攀升約10億分之4的過程中，碳-13與碳-12的比例有幾處些微的變化，顯示有額外來源的碳因為海洋溫度升高而出現。這些碳的來源可以比喻成碳酸飲料加熱後冒出的氣泡，它們也許會加入那些來自生物的碳。

The application of this carbon isotope technique became possible because of a unique site along the margin of the Antarctic ice sheet where old ice that flowed from the interior is exposed at the surface of a large glacier -- Taylor Glacier -- named for a geologist on an early expedition to the frozen continent. Ice that normally would be a mile or more below the surface is available to easily sample in large quantities.

這種碳同位素技術得以運用的原因在於樣品來自得天獨厚的環境。這些樣品的採集地點為一處南極冰棚的邊緣地帶，這是一條大型冰川-Taylor冰川(以一位早期遠征此冰封大陸的地質學家命名)中的古老冰塊從內部流出至表面處，因此正常來說要在冰河表面下一英哩甚至更深處才能取得的冰可以在此輕易地大量採樣。

These large samples, laboriously cut from the exposed ice layers, allowed the precise measurements, the Oregon State researchers report.

據奧勒岡州立大學的研究人員描述，他們可以精確分析這些從出露的冰層上辛勤切割下的大量樣品。

"The isotope ratio technique gives us a sort of 'return address' for carbon dioxide," noted Thomas Bauska, a former Ph.D. student and post-doctoral researcher in OSU's College of Earth, Ocean, and Atmospheric Sciences, who was lead author on the PNAS study. "The technique is new, extremely precise and gives us one of the best windows into the Earth's past climate."

「同位素技術提供給我們的資訊有點像是二氧化碳的『寄件地址』。」此篇刊登於PNAS的研究的第一作者 Thomas Bauska如此解釋。他之前是奧勒岡州立大學地球、海洋暨大氣科學系的博士生及博士後研究員。「這項技術不但新穎且極度精確，還給了我們窺向地球過去氣候的絕佳管道。」

Bauska is now a post-doctoral researcher at the University of Cambridge in England.

Bauska現在是英國劍橋大學的博士後研究員。

That window into the past may provide hints at what may happen in the future under a new global warming regime, noted Alan Mix, an Oregon State oceanographer and co-author on the study. However, he cautioned, it isn't always simple to predict the future based on past events.

此篇研究的共同作者，奧勒岡州立大學的海洋學家 Alan Mix點出這個管道也許可以提供我們在新的全球暖化局勢下，未來可能會發生何種事物的蛛絲馬跡。然而，他也提醒以過去的事件來預測未來並非總是那麼單純。

"The rise of CO2 is a complicated beast, with different behaviors triggered at different times," Mix said. "Although the natural changes at the end of the ice age are not a direct analogy for the future, the rapid changes do provide a cautionary tale. Humanmade warming from CO2 pollution may trigger further release from 'natural sources,' and this could exacerbate greenhouse gases and warming."

「二氧化碳的上升是個相當錯綜複雜的過程，不同的時間點可能會有截然不同的反應發生。」 Mix說。「雖然上次冰河期末的自然變化並不能直接類推至將來，但這些迅速變化確實可成為借鏡。排放二氧化碳造成的人為暖化可能會進一步造成其他『自然來源』隨之釋放溫室氣體，而增加其濃度並加劇暖化。」

Other authors on the PNAS study include Daniel Baggenstos and Jeffrey Severinghaus, Scripps Institution of Oceanography; Shaun Marcott, University of Wisconsin-Madison; Vasillii Petrenko, University of Rochester; Hinrich Schaefer, National Institute of Water and Atmospheric Research in New Zealand; and James Lee, Oregon State University.

此篇PNAS的研究的其他作者還有斯克里普斯海洋研究所的 Daniel Baggenstos和Jeffrey Severinghaus；威斯康辛大學麥迪遜分校的 Shaun Marcott；羅徹斯特大學的 Vasillii Petrenko；紐西蘭國家水資源與大氣研究院的 Hinrich Schaefer及奧勒岡州立大學的 James Lee。

引用自：Oregon State University. "Carbon from land played a role during last deglaciation." ScienceDaily. ScienceDaily, 14 March 2016.