地球表層的水可以滲到地下深處，使地核外層發生轉變

 原文網址：https://news.asu.edu/20231113-earths-surface-water-dives-deep-transforming-cores-outer-layer

數十年前，拍攝地球深處影像的地震學家辨識出一層只有幾百公里厚的構造。這圈被稱為E’層的起源長久以來都是道謎題——直到現在。

這幅示意圖顯示了由水引發的化學反應，使得二氧化矽晶體從地球外核的液態金屬中冒出來。圖片來源：Dan Shim/ASU

亞利桑那州立大學地球與太空探勘學院的科學家Dan Shim、 Taehyun Kim與Joseph O’Rourke參與在內的國際研究團隊，發現地球表面的水可以滲到地球深處，使得液態金屬地核最外層區域的成分產生改變，形成一圈獨立出來的薄層。

他們的研究最近發表在《自然—地球科學》。

研究指出數十億年來，地球表面的水分持續經由下沉的隱沒板塊運送到地球深處。這些水分到達大約1800英里深(約2900公里)的地核地函邊界，就會觸發化學反應，對地核的結構造成深遠的影響。

Shim與他的團隊和南韓延世大學的Yong Jae Lee合作之下，透過高壓實驗證明了隱沒的水分會和地核物質發生化學反應，產生一層富含氫且缺乏矽的構造，使得外核最表層的區域變得像是一道薄膜。此外，還會產生往上浮的二氧化矽晶體並跟地函結合。他們預測這層受到改變的液態金屬密度會比較低，使得地震波波速下降，與地震學家探測到的異常性質相符。

「許多年來科學家認為地核與地函之間不太有物質的交流，但是從我們最近的高壓實驗得出的故事情節卻大不相同。我們發現當水分到達地函與地核的交界，它會和地核中的矽反應而形成二氧化矽，」Shim表示。「我們之前也觀察到在極端高壓的環境，液態鐵裡的碳會跟水反應而形成鑽石。連同最近這項發現，它們都指出了地核與地函的互動其實比過往認為的活躍許多，也代表兩者之間有大量的物質交流。」

此發現拓展了我們對於地球內部運作過程的瞭解，顯示全球水循環的範圍比過往認定的還要更廣。這層遭到置換的地核外膜連結了地球表面的水循環以及深處的金屬地核，對於地球化學循環來說具有相當重大的意義。

由地球科學家組成的國際團隊，利用了阿貢國家實驗室的先進光子源提供的尖端實驗技術而完成這項研究，並且透過德國同步加速器研究所的PETRA III來複製地核地函邊界的極端環境條件。

研究團隊隸屬亞利桑那州立大學(ASU)的成員及他們的職掌如下：Kim，他在就讀博士訪問ASU的期間開始這項研究，現為地球與太空探勘學院的博士後研究員；Shim是地球與太空探勘學院的教授，主導高壓實驗的工作；O’Rourke是地球與太空探勘學院的助理教授，透過電腦模擬來剖析地核表面遭到置換的薄層是如何形成並維持。其他重要的研究人員還有領導延世大學研究團隊的Lee；阿貢國家實驗室先進光子源的Vitali Prakapenka與Stella Chariton；德國同步加速器研究所的Nico Giordano與 Hanns-Peter Liermann。

 

Earth's surface water dives deep, transforming core's outer layer

A few decades ago, seismologists imaging the deep planet identified a thin layer, just over a few hundred kilometers thick. The origin of this layer, known as the E prime layer, has been a mystery — until now.

An international team of researchers, including Arizona State University scientists Dan Shim, Taehyun Kim and Joseph O’Rourke of the School of Earth and Space Exploration, has revealed that water from the Earth's surface can penetrate deep into the planet, altering the composition of the outermost region of the metallic liquid core and creating a distinct, thin layer.

Their research was recently published in Nature Geoscience.

Research indicates that over billions of years, surface water has been transported deep into the Earth by descending, or subducted, tectonic plates. Upon reaching the core-mantle boundary, about 1,800 miles below the surface, this water triggers a profound chemical interaction, altering the core’s structure.

Along with Yong Jae Lee of Yonsei University in South Korea, Shim and his team have demonstrated through high-pressure experiments that subducted water chemically reacts with core materials. This reaction forms a hydrogen-rich, silicon-depleted layer, altering the topmost outer core region into a film-like structure. Additionally, the reaction generates silica crystals that rise and integrate into the mantle. This modified liquid metallic layer is predicted to be less dense, with reduced seismic velocities, in alignment with anomalous characteristics mapped by seismologists.

“For years, it has been believed that material exchange between Earth's core and mantle is small. Yet, our recent high-pressure experiments reveal a different story. We found that when water reaches the core-mantle boundary, it reacts with silicon in the core, forming silica," said Shim. "This discovery, along with our previous observation of diamonds forming from water reacting with carbon in iron liquid under extreme pressure, points to a far more dynamic core-mantle interaction, suggesting substantial material exchange.”

This finding advances our understanding of Earth's internal processes, suggesting a more extensive global water cycle than previously recognized. The altered "film" of the core has profound implications for the geochemical cycles that connect the surface-water cycle with the deep metallic core.

This study was conducted by an international team of geoscientists using advanced experimental techniques at the Advanced Photon Source of Argonne National Lab and PETRA III of Deutsches Elektronen-Synchrotron in Germany to replicate the extreme conditions at the core-mantle boundary.

Members of the team and their key roles from ASU are Kim, who began this project as a visiting PhD student and is now a postdoctoral researcher at the School of Earth and Space Exploration; Shim, a professor at the School of Earth and Space Exploration, who spearheaded the high-pressure experimental work; and O’Rourke, an assistant professor at the School of Earth and Space Exploration, who performed computational simulations to comprehend the formation and persistence of the core's altered thin layer. Lee led the research team from Yonsei University, along with key research scientists Vitali Prakapenka and Stella Chariton at the Advanced Photon Source and Rachel Husband, Nico Giordano and Hanns-Peter Liermann at the Deutsches Elektronen-Synchrotron.

原始論文：Taehyun Kim, Joseph G. O’Rourke, Jeongmin Lee, Stella Chariton, Vitali Prakapenka, Rachel J. Husband, Nico Giordano, Hanns-Peter Liermann, Sang-Heon Shim, Yongjae Lee. A hydrogen-enriched layer in the topmost outer core sourced from deeply subducted water. Nature Geoscience, 2023; DOI: 10.1038/s41561-023-01324-x

引用至：Arizona State University. "Earth's surface water dives deep, transforming core's outer layer."