北極海怎麼變鹹的？

原文網址：www.sciencedaily.com/releases/2017/06/170606112753.htm

北極海怎麼變鹹的？

研究人員模擬了格陵蘭―蘇格蘭海脊沉沒造成的氣候變遷

北極海曾是一座巨大的淡水湖。在格陵蘭和蘇格蘭之間的陸橋沉沒到一定程度之後，來自大西洋的鹹水才開始大量灌入。阿爾弗雷德·韋格納研究所的研究人員利用氣候模型來描繪出整個過程是如何發生，讓我們首度能更精確地了解今日所見地大西洋環流是如何誕生。研究結果現刊登於期刊《自然通訊》(Nature Communications)。

每年大約有3300立方公里的淡水流到北極海中，等同於世界所有河川每年流入海洋水量的十分之一。在氣候較溫暖潮濕的始新世(大約5600萬到3400萬年前)，淡水流入的量恐怕更多。但跟今日不同的是，在那段地質時期北極海的水並不會跟其他海洋交換。現今流入北極海的鹹水一部份是太平洋的海水經由白令海峽流入，另一部份則是大西洋的海水經由格陵蘭―蘇格蘭海脊流入，但在當時卻無法發生。因為這兩處通道現在雖然完全沉於水下，但過去卻佇立於海面之上。

只有在格陵蘭和蘇格蘭之間的陸橋消失，第一條海道產生之際，北極海跟北大西洋才能互相連通，使得海水的交換得以發生。德國亥姆霍茲極地和海洋研究中心的阿爾弗雷德˙韋格納研究所的研究人員，近日成功模擬出這件發生於地質上的變化對氣候的影響。在他們的模擬中，他們逐漸將陸橋沉至200公尺的深度。研究第一作者，氣候科學家Michael Stärz表示：「在現實世界中，這樣的構造沉沒作用會歷經數百萬年的時間。有趣的是，只有在陸橋沉沒至水下超過50公尺的深度後，北極海的環流模式和特性才會出現重大改變。」

此門檻深度可以對應至海洋表面混和層(surface mixed layer)的深度，並且標示了密度較輕的北極海表層水的末端，以及下層的北大西洋海水開始流入的位置。Stärz解釋：「只有當海脊位在表面混和層的深度以下，密度較高的北大西洋鹹水才能不受阻礙地流入北極海中。一旦格陵蘭和蘇格蘭之間的海道到達此關鍵深度，今日我們所知由鹹水組成的北極海便開始形成了。」由於海道的形成會造成中緯度和極區海洋的熱量交換，使其在全球氣候歷史中扮演相當重要的腳色。

2004年在北極附近的國際鑽探計畫期間，於始新世的深海沉積物中發現了淡水藻類化石，進而支持了北極海盆地曾一度孤立的理論。這道幾乎由火山玄武岩組成的海床曾經一度為陸橋，如今卻位在海表下方大約500公尺深，僅剩冰島還佇立於海面之上。

How the Arctic Ocean became saline

Researchers model climate changes caused by the submersion of the Greenland-Scotland Ridge

The Arctic Ocean was once a gigantic freshwater lake. Only after the land bridge between Greenland and Scotland had submerged far enough did vast quantities of salt water pour in from the Atlantic. With the help of a climate model, researchers from the Alfred Wegener Institute have demonstrated how this process took place, allowing us for the first time to understand more accurately how Atlantic circulation, as we know it today came about. The results of the study have now been published in the journal Nature Communications.

Every year, ca. 3,300 cubic kilometres of fresh water flows into the Arctic Ocean. This is equivalent to ten percent of the total volume of water that all the world's rivers transport to the oceans per year. In the warm and humid climate of the Eocene (ca. 56 to 34 million years ago), the inflow of freshwater was probably even greater. However, in contrast to today, during that geological period there was no exchange of water with other oceans. The influx of saline Atlantic and Pacific water, which today finds its way into the Arctic Ocean from the Pacific via the Bering Strait and from the North Atlantic via the Greenland-Scotland Ridge, wasn't possible -- the region that is today completely submerged was above the sea at that time.

Only once the land bridge between Greenland and Scotland disappeared did the first ocean passages emerge, connecting the Arctic with the North Atlantic and making water exchange possible. Using a climate model, researchers from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) have now successfully simulated the effect of this geological transformation on the climate. In their simulations, they gradually submerged the land bridge to a depth of 200 metres. "In reality, this tectonic submersion process lasted several million years," says Climate Scientist Michael Stärz, first author of the study. "Interestingly, the greatest changes in the circulation patterns and characteristics of the of the Arctic Ocean only occurred when the land bridge had reached a depth of over 50 metres below the surface."

This threshold depth corresponds to the depth of the surface mixed layer, and determines where the relatively light Arctic surface water ends and the underlying layer of inflowing North Atlantic water begins. "Only when the oceanic ridge lies below the surface mixed layer can the heavier saline water of the North Atlantic flow into the Arctic with relatively little hindrance," explains Stärz. "Once the ocean passage between Greenland and Scotland had reached this critical depth, the saline Arctic Ocean as we know it today was created." The formation of ocean passages plays a vital role in global climate history, as it leads to changes in heat transport in the ocean between the middle and polar latitudes.

The theory that the Arctic Basin was once isolated is supported by the discovery of freshwater algae fossils in Eocene deep-sea sediments that have been obtained during international drilling near the North Pole in 2004. What was once a land bridge now lies ca. 500 metres under the ocean and consists almost entirely of volcanic basalt. Iceland is the only section remaining above the surface.

原始論文：Michael Stärz, Wilfried Jokat, Gregor Knorr, Gerrit Lohmann. Threshold in North Atlantic-Arctic Ocean circulation controlled by the subsidence of the Greenland-Scotland Ridge. Nature Communications, 2017; 8: 15681 DOI: 10.1038/ncomms15681

引用自；Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research. "How the Arctic Ocean became saline: Researchers model climate changes caused by the submersion of the Greenland-Scotland Ridge." ScienceDaily. ScienceDaily, 6 June 2017.