科學家在中國西部發現世上年代最輕的帶狀鐵礦

原文網址：https://www.ualberta.ca/science/science-news/2018/july/earths-youngest-banded-iron-formation-china-discovered

科學家在中國西部發現世上年代最輕的帶狀鐵礦

此發現證明富含鐵的海水的消失時間比過往認為的還要晚了許多

Katie Willis

根據亞伯達大學的地質學家進行的研究，這座地球史上年代最輕的帶狀鐵礦，將會改變科學家對複雜生物演化歷程的理解。

位於中國西部，地球年代最輕的帶狀鐵礦。圖片來源：Zhiquan Li

精確的定年結果顯示這座位於中國西部的帶狀鐵礦(banded iron formation)屬於寒武紀。確切來說此地層的年紀大約為5.27億年，跟迄今發現的多數帶狀鐵礦相比年輕許多。距今38億年前左右開始有帶狀鐵礦沉積，科學家長久以來認為在寒武紀開始的5.4億年前，帶狀鐵礦早就已經停止形成。

共同作者，亞伯達大學地球和大氣科學系的教授Kurt Konhauser表示：「這項發現相當關鍵，因為這是首次在寒武紀早期觀察到類似前寒武紀才會出現的帶狀鐵礦。最近從其他地球化學指標得到的結論認為當時的環境普遍來說含有大量的鐵，而此帶狀鐵礦提供了最具決定性的證據顯示的確如此。」Konhauser負責指導研究主持人Zhiquan Li，這位北京大學的博士候選人在亞伯達大學進行交換的期間進行了此研究。

眾所皆知動物是在寒武紀早期開始崛起，因此當時海水中的氧濃度應該已經跟現在差不多。論文共同作者，Konhauser實驗室的博士候選人Leslie Robbins表示：「這項推論相當重要。長久以來科學家認為氧氣不足限制了複雜生命的演化過程，因此這股壓力應該是在寒武紀早期有所減輕，動物才能出現。」

研究人員將此帶狀鐵礦的地質和地球化學性質跟古代和現代的樣品對比，以找出和它們沉積時類似的環境。研究人員依據稀土元素的分布模式，提出這座帶狀鐵礦是在一個富含鐵的分層盆地中(stratified iron-rich basin)的化學躍變層之內或附近形成。

Kurt Konhauser表示：「我們未來的研究目標是定量這些寒武紀的帶狀鐵礦在中國的分佈有多廣，以及類似的沉積物是否能在世上其他地方找到。」

Scientists discover Earth’s youngest banded iron formation in western China

Discovery provides evidence of iron-rich seawater much later than previously thought

The discovery of Earth’s youngest-ever banded iron formation is changing how scientists understand the evolution of complex life, according to a study by University of Alberta geologists.

The banded iron formation, located in western China, has been conclusively dated as Cambrian in age. Approximately 527 million years old, this formation is young by comparison to the majority of discoveries to date. The deposition of banded iron formations, which began approximately 3.8 billion years ago, had long been thought to terminate before the beginning of the Cambrian Period at 540 million years ago.

“This is critical, as it is the first observation of a Precambrian-like banded iron formation that is Early Cambrian in age. This offers the most conclusive evidence for the presence of widespread iron-rich conditions at a time, confirming what has recently been suggested from geochemical proxies,” said Kurt Konhauser, professor in the Department of Earth and Atmospheric Sciences and co-author. Konhauser supervised the research that was led by Zhiquan Li, a PhD candidate from Beijing while on exchange at UAlberta.

The Early Cambrian is known for the rise of animals, so the level of oxygen in seawater should have been closer to near modern levels. “This is important as the availability of oxygen has long been thought to be a handbrake on the evolution of complex life, and one that should have been alleviated by the Early Cambrian,” says Leslie Robbins, a PhD candidate in Konhauser’s lab and a co-author on the paper.

The researchers compared the geological characteristics and geochemistry to ancient and modern samples to find an analogue for their deposition. The team relied on the use of rare earth element patterns to demonstrate that the deposit formed in, or near, a chemocline in a stratified iron-rich basin.

“Future studies will aim to quantify the full extent of these Cambrian banded iron formations in China and whether similar deposits can be found elsewhere,” says Kurt Konhauser.

原始論文：Zhiquan Li et al. Earth's youngest banded iron formation implies ferruginous conditions in the Early Cambrian ocean. Scientific Reports, 2018 DOI: 10.103841598-018-28187-2

引用自：University of Alberta. "Scientists discover Earth's youngest banded iron formation in western China”