海底熱泉為海洋中鐵的重要來源

原文網址：www.sciencedaily.com/releases/2015/07/150708133904.htm

Seafloor hot springs a significant source of iron in the oceans

海底熱泉為海洋中鐵的重要來源

At the bottom of the sea, volcanic and magmatic forces create hot springs that spew super-heated water into the deep sea. The hot, acidic water scours metals from Earth's crust, and the warm chemical-rich water from these remote geysers supports exotic deep-sea ecosystems.

在海底由火山與岩漿活動形成的熱泉會往深海噴出超高熱的泉水。在地底這些滾燙的酸性泉水會將地殼中的金屬沖刷出來，之後從那些人類難以抵達的噴泉噴出。這種富含礦物質的溫熱泉水支撐了噴泉附近的奇異深海生態系。

It had been widely thought the story stopped there. Metals such as iron and manganese were thought to quickly react and form particles that would either clump together or stick to other things, causing them to sink to the seafloor close to the source. But new research proves that the metals remain dissolved and follow deep-sea currents to provide a major source of iron to the world's oceans. The findings are published on the cover of Nature.

過去大都認為故事在此畫下句點。一般認為鐵與鎂這類的金屬會很快地發生化學反應而形成固體顆粒，之後聚集成團塊或是依附在別的東西上面，造成它們僅能沉降在來源附近的海床。然而新的研究顯示這些金屬可以維持在溶解態並且乘著深海洋流到達遠方，而成為全世界海洋中重要的鐵來源。此發現被刊登在《自然》( Nature)的封面。

"This proves that hydrothermal activity at the mid-ocean ridges impacts global ocean chemistry of important trace metals," said lead author Joseph Resing, a senior research scientist at the University of Washington's Joint Institute for the Study of the Atmosphere and Ocean, a partnership with the National Oceanic and Atmospheric Administration. "On longer timescales, it also impacts the productivity of the oceans."

「這證實了中洋脊的熱泉活動會影響全世界海洋的稀有金屬化學性質。」第一作者Joseph Resing說，他是華盛頓大學大氣海洋聯合研究中心(University of Washington's Joint Institute for the Study of the Atmosphere and Ocean)的高級研究員。此機構與美國國家海洋與大氣管理局(National Oceanic and Atmospheric Administration)有合作關係。「在長時間尺度下，這也會影響海洋的生產力。」

Metals, especially iron, are crucial to the growth of phytoplankton in the oceans. In many parts of the ocean iron controls the growth of marine life even though it is only present at concentrations of parts per trillion.

金屬，尤其是鐵，對於海洋中浮游植物的成長來說相當重要。在海洋的許多區域裡雖然鐵的濃度可能僅有一兆分之幾，但卻控制了當地海洋生物的繁盛與否。

Most of the iron in the ocean comes from dust blown off deserts, or from rivers that discharge into the sea. But recent research, some conducted by co-author Christopher German at Woods Hole Oceanographic Institution, hinted that iron might also be escaping from the volcanic ridge crest by exploiting some type of chemical trick to make the long-distance voyage.

海洋中大部分的鐵來自從沙漠吹至海上的沙塵，或是流入海洋的河流。然而最近的研究顯示(其中幾篇來自共同作者，伍茲霍爾海洋研究所(Woods Hole Oceanographic Institution)的Christopher German)，鐵或許能以某些化學戲法從火山洋脊頂端脫離出來，長途航行至遠方。

The new study, part of the U.S. National Science Foundation's GEOTRACES program, locates the "smoking gun" -- a plume of hydrothermal metals drawn westward by a slow-moving, deep-ocean current that carries these metals for decades to distant parts of the ocean.

此篇研究為美國國家科學基金會GEOTRACES計畫中的一部分，研究對象為「菸槍(smoking gun)」—其為一富含溶解金屬的熱液柱。往西方緩緩流動的深海洋流會拖曳此熱液柱，數十年來不斷地將其內的金屬帶至遠方的海洋。

A 57-day cruise in fall 2013 aboard the UW's research vessel, the Thomas G. Thompson, tracked water venting from the East Pacific Rise, a chain of underwater volcanoes west of Ecuador that is one of the most volcanically active places on Earth. The oceanographers followed the trail for more than 4,000 kilometers (2,500 miles) west across the South Pacific to Tahiti, using extremely sensitive tools to make measurements of the metals from the ocean''s surface to the seafloor.

在2013年秋天這趟57的航行中，華盛頓大學的研究船Thomas G. Thompson號追循著東太平洋隆起(East Pacific Rise)上的成串海底噴泉進行研究。東太平洋隆起為厄瓜多西方的海底火山鍊，是地球上火山活動最活躍的地方之一。這群海洋學家沿著南太平洋至大溪地這段長達4,000公里(2,500哩)的航線，利用極為靈敏的儀器來測量海洋表面至底層的金屬濃度。

While the aluminum eventually petered out, every station west of the ridge crest revealed evidence of hydrothermal manganese and, surprisingly, of iron, at about 2.5 kilometers (1.5 miles) depth.

測量結果顯示在洋脊頂峰以西每個測站的水深2.5公里(1.5哩)處，雖然鋁會逐漸消失，但都能測到來自熱泉的鎂，而且令人驚訝的，還有鐵。

"Every single day we were out there, we were surprised to see that the plume of dissolved iron was still present," Resing said. "We have never before documented dissolved iron carried so far in the ocean currents."

「在那裏的每一天我們都驚訝地看到含有溶解鐵的液柱依然存在，」Resing說。「我們先前從未記錄到溶解鐵能被洋流攜帶那麼長的一段距離。」

The finding is especially important for the Southern Ocean, circling Antarctica, where massive phytoplankton blooms are known to be limited by iron supplies, and where winds are less likely to carry iron-rich dust.

此發現對於環繞南極洲的南冰洋(Southern Ocean)來說特別重要，因為目前所知當地大規模浮游植物藻華的發生受限於鐵的供應，而那裏並沒有什麼機會能接收風帶來的沙塵。

Co-author Alessandro Tagliabue at the University of Liverpool, England, placed the results within an ocean model and found that phytoplankton growth in the Southern Ocean is supported by iron from deep-sea vents. Iron from vent systems thus helps sustain a major ecosystem that consumes carbon dioxide from the atmosphere. Much of this carbon is exported from the ocean surface to the deep sea, and in the Southern Ocean 15 to 30 percent of this carbon export is supported by hydrothermal iron.

共同作者英國利物浦大學的Alessandro Tagliabue將研究結果帶入海洋模型當中，發現南冰洋中浮游植物的生長的確有賴來自深海熱泉的鐵。從這些熱泉系統中釋放的鐵因此有助於支撐這個能夠吸收大氣二氧化碳的重要生態系。海洋吸收的二氧化碳大多數會從海洋表層被傳輸至深海，在南冰洋中這種傳輸方式有百分之15至30依靠熱泉系統產生的鐵來進行。

"To properly model the uptake of carbon dioxide by the Southern Ocean and to understand how this uptake impacts climate, you must account for this iron," Resing said.

「為了更正確地模擬南冰洋如何從大氣中汲取二氧化碳，以及瞭解這種吸收過程對氣候的影響，你必須將鐵納入考量。」Resing說。

Ongoing research by other collaborators will analyze additional water samples collected during the same cruise to figure out what allows the iron to be transported so far. Two leading theories are that it attaches to large organic molecules, similar to how iron clings to hemoglobin in our bloodstream, or that it separates into tiny nanoparticles that can remain suspended in the water for decades.

由其他協同人員進行的後續研究將會分析同樣從這趟航程中採集的水樣，以了解鐵為何可以被運送那麼長的距離。目前有兩個主要理論，其一認為鐵依附在大型有機分子上，就像鐵在我們的血液中攀附於血紅素而能到處流動；另一則認為它裂解成微小的奈米顆粒，此型態的鐵能夠在水中維持懸浮狀態數十年之久。

 引用自：University of Washington. "Seafloor hot springs a significant source of iron in the oceans." ScienceDaily. ScienceDaily, 8 July 2015.