深海「陰影帶」如何困住世上最古老的海水

原文網址：http://www.su.se/english/research/research-news/how-a-shadow-zone-traps-the-world-s-oldest-ocean-water-1.356556

深海「陰影帶」如何困住世上最古老的海水

由一組國際團隊進行的新研究闡明世上最古老的海水，為何會滯留在北太平洋深度2公里處的陰影帶超過一千多年。

具體來說，這團海水最後一次跟空氣接觸時哥德人才剛入侵西羅馬帝國(大約為西元五世紀末，可對應至東晉末年)。研究提出海床的形狀對垂直流動造成的影響決定了古代海水會停留在海底多久。

「我們早就利用碳14定年得知世上最古老的海水位在北太平洋深處。但我們到現在仍難以理解為何這些相當古老的海水會聚集在深度大約2公里的位置。」主要作者，南威爾斯大學的Casimir de Lavergne博士表示。「我們發現印度洋和太平洋表面下方2公里處有個『陰影帶』，由於此處海水幾乎沒有任何垂直流動使得它們停留在此長達數個世紀。」

在海洋深度2.5公里以下的地方，崎嶇的海底地形和地熱供給之處使得水流湧升；而接近表面的海洋淺處則有受風力驅動的洋流；在此之間即為幾乎停滯的海水所處的陰影帶。本篇研究發表之前，模擬深海海流循環的模型並未將海床對底層水流動的限制考慮得相當周全。研究人員把它納入參數詳加模擬之後，他們發現底層水無法湧升至超過海洋表面下方2.5公里深的位置，使得深度2.5公里以上的地區直接跟其他地方孤立開來。雖然研究解開了這道謎題中的一部份，但他們的結果或許還具有更多意義。

「當此孤立的陰影帶困住海水數千年之久時，也困住了營養鹽和碳。這些物質對海洋的生產力有直接影響，進而可以從數百年的時間尺度上來調節氣候。」共同作者，斯德哥爾摩大學氣象學系的研究員Fabien Roquet博士表示。

這篇文章刊登於科學期刊《自然》(Nature)，題名為「Abyssal ocean overturning shaped by seafloor distribution」。

How a “shadow zone” traps the world’s oldest ocean water

New research from an international team has revealed why the oldest water in the ocean in the North Pacific has remained trapped in a shadow zone around 2km below the sea surface for over 1000 years.

To put it in context, the last time this water encountered the atmosphere the Goths had just invaded the Western Roman Empire. The research suggests the time the ancient water spent below the surface is a consequence of the shape of the ocean floor and its impact on vertical circulation.

 “Carbon-14 dating had already told us the most ancient water lied in the deep North Pacific. But until now we had struggled to understand why the very oldest waters huddle around the depth of 2km,” said lead author from the University of New South Wales, Dr Casimir de Lavergne. “What we have found is that at around 2km below the surface of the Indian and Pacific Oceans there is a ‘shadow zone’ with barely any vertical movement that suspends ocean water in an area for centuries.

The shadow zone is an area of almost stagnant water sitting between the rising currents caused by the rough topography and geothermal heat sources below 2.5km and the shallower wind driven currents closer to the surface. Before this research, models of deep ocean circulation did not accurately account for the constraint of the ocean floor on bottom waters. Once the researchers precisely factored it in they found the bottom water can not rise above 2.5km below the surface, leaving the region directly above isolated. While the researchers have unlocked one part of the puzzle their results also have the potential to tell us much more.

 “When this isolated shadow zone traps millennia old ocean water it also traps nutrients and carbon which have a direct impact on the capacity of the ocean to modify climate over centennial time scales,” said fellow author from Stockholm University, Dr Fabien Roquet, researcher at the Department of Meteorology.

The article "Abyssal ocean overturning shaped by seafloor distribution" is published in the scientific journal Nature.

原始論文：C. de Lavergne, G. Madec, F. Roquet, R. M. Holmes, T. J. McDougall. Abyssal ocean overturning shaped by seafloor distribution. Nature, 2017; 551 (7679): 181 DOI: 10.1038/nature24472

引用自：Stockholm University. "How a 'shadow zone' traps the world's oldest ocean water."