科學家發現「永恆之河」尼羅河比過去認為的更加古老
古埃及人視尼羅河為所有生命的源頭。尼羅河千百萬年來穩定地向北流動，滋養出非洲東北肥沃的河谷，也因此決定了人類文明的軌跡。
流過埃及開羅的尼羅河。德州大學的科學家最近發現尼羅河的年齡是過去認為的六倍左右。圖片來源：Nina
R.
然而在地質學中，尼羅河的路徑如此固定卻是一道未解之謎，因為年代久遠的河流隨著時間經過通常會改道。德州大學奧斯汀分校的研究人員最近指出河流流向跟地函深處的岩石流動有所關連，進而破解了這道謎題。在調查過程中他們也發現這條「永恆之河」的年紀超出所有人的理解――他們估計尼羅河已經存在了3000萬年，是過去認為的六倍。
這項研究11月11日發表於期刊《自然―地球科學》。作者發現如果沒有地函流動讓尼羅河的路徑保持固定，那麼在許久之前尼羅河就會往西偏轉，可能會讓人類歷史的進程跟著改變。
德州大學奧斯汀分校傑克遜地球科學院的教授Claudio
Faccenna是主要作者，他說：「關於尼羅河最大的幾個謎題是它起源自什麼時候，以及為什麼長久以來都保持不變。而我們給出的解答確實會讓人大吃一驚。」
研究結果或許能平息長久以來關於尼羅河年齡的爭論，同時也給出證據顯示深層地函的緩慢運動，是塑造地表地景與地質作用的重要力量。地函是由固體岩石組成，但在長時間下會像液體般流動。猶如海裡的洋流一樣，地函中不同區塊的岩石流動模式也不同。
研究團隊包括同為傑克遜地球科學院的Petar
Glisovic(現於魁北克大學進行合作研究)與Thorsten
Becker教授(和Faccenna同為德州大學地球物理研究所的研究員)。其他共同作者包括佛羅里達大學、米蘭比可卡大學、以色列地質調查局和耶路撒冷希伯來大學的研究人員。
研究人員在論文中表示尼羅河傾斜的地形特徵和一道地函岩石的輸送帶有關。這道地函岩石流動時把南方的伊索比亞高原往上推，同時把北方的地表向下拉。這讓尼羅河從源頭到河口的坡度相當平緩，而能夠持續往北流動。
研究內容包括追查尼羅河的地質歷史，方法是研究伊索比亞高原的古代火山岩，並且和埋藏在尼羅河三角洲下的大量河流沉積物進行對比。研究人員從中得知伊索比亞高原在一開始劇烈地抬升之後，接下來數千萬年都保持在差不多的高度。他們認為原因是底下有地函岩石在支撐。
「我們知道伊索比亞的高凸地形大概是在3000萬年前形成。」
但科學家之前不清楚為什麼它可以長久以來保持在同樣的高度。
團隊運用電腦模擬重現出過去4000萬年地球的板塊活動，由此證明他們的發現。
伊索比亞高原的成因可能是因為有道高溫地函柱在下方使岩漿湧出地表，模型顯示它到達的時候也在地函啟動了一條至今仍在運作的輸送帶。模擬重現出來的地貌變化幾乎就和科學家預期中的如出一轍，甚至連那些地形變化中的微小細節，像是尼羅河沿途可見的洶湧急流都重現了出來。
Glisovic表示模型可以改良到顯現出如此微小的細節，不但令人大為訝異，也是相當重要的研究成果。
他說：「我認為這項技術可以帶給我們過去無法瞭解的事物。」
奧勒岡州立大學的教授Eric
Kirby是研究地表抬升的專家，他說結合多項地質數據與尖端的地球物理模型是這項研究的成功關鍵。
並未參與這項研究的Kirby如此評論：「少了兩者其中之一，就無法得到這麼有說服力的結果。」
團隊冀望這項技術接下來可以用在研究其他河流，像是長江和剛果河。研究經費來自NASA，一部分的補助來自於義大利的教育、大學與研究部。
Scientists find
Eternal Nile to be more ancient than previously thought
Ancient Egyptians considered the Nile
river to be the source of all life. The steady northward path of the river has
nourished the fertile valleys of northeast Africa for millions of years and in
doing so, shaped the course of human civilization.
The Nile’s unchanging path, however, has been a
geologic mystery because long-lived rivers usually move over time. Researchers
at The University of Texas at Austin have cracked the case by linking the
river’s flow to the movement of rock in the Earth’s deep mantle. In the course
of their investigation, they found the eternal river to be much older than
anyone realized, with the scientists estimating the age of the Nile to be 30
million years – about six times as long as previously thought.
The research, published on Nov. 11 in the journal Nature Geoscience, found that if it
weren’t for the mantle movement keeping the river on course, the Nile would
have turned west long ago, probably changing the course of history along with
it.
“One of the big questions about the Nile is when it
originated and why it has persisted for so long,” said lead author Claudio
Faccenna, a professor at the UT Jackson School of Geosciences. “Our solution is
actually quite exciting.”
The results should settle a long-running debate about
the age of the river and provide evidence that the slow movement of the deep
mantle is one of the key forces shaping our Earth’s landscape and geological
processes. The Earth’s mantle is composed of solid rock that flows like a fluid
over long periods. Like currents in an ocean, different areas of the mantle
have different circulation patterns.
The research team included Jackson School scientists
Petar Glisovic, who is now a research collaborator at the University of Quebec;
and Thorsten Becker, a professor at the Jackson School and research scientist
at its University of Texas Institute for Geophysics, where Faccenna is also a
research scientist. Other collaborators include researchers from the University
of Florida, the University of Milano-Bicocca, the Geological Survey of Israel
and the Hebrew University of Jerusalem.
In the paper, the researchers connected the tilted
nature of the Nile’s topography to a conveyor belt of mantle rock pushing up
against the Ethiopian Highlands in the south and pulling the surface down in
the north. From beginning to end, the gentle gradient keeps the Nile on a
consistent northward course.
The research involved tracing the geological history
of the Nile by studying ancient volcanic rock in the Ethiopian Highlands and
correlating it with enormous deposits of river sediment buried under the Nile
Delta. This told the researchers that after rising dramatically, the Ethiopian
Highlands have remained at a similar height for millions of years, which the
researchers attribute to the support of the mantle rock from below.
“We know that the high topography of the Ethiopian
plateau was formed about 30 million years ago,” Becker said.
Until now, however, it was unclear how this
topography has been maintained for so long.
The team verified their findings using computer
simulations that re-created 40 million years of Earth’s plate tectonic
activity.
The model showed the arrival of a hot mantle plume
that probably led to the outpouring of lava that formed the Ethiopian Highlands
while activating a conveyor belt in the mantle that persists to this day. The
simulation reproduced changes in the landscape almost exactly as the scientists
had expected – including small details in the landscape such as whitewater
rapids found along the length of the Nile.
The ability of the model to refine such small details
was a big surprise and a significant research finding, said Glisovic.
“I think this technique gives us something we didn’t
have in the past,” he said.
Eric Kirby, a professor at Oregon State University
and an expert in uplifting landscapes, said that combining diverse geological
data with state-of-the-art, geophysical modeling was key for the research.
“Without either piece, you wouldn’t get such a
compelling result,” said Kirby, who was not involved in the study.
The team now hopes to apply the technique to other
rivers such as the Yangtze and Congo. The research was supported by NASA and a
grant from the Italian government’s Ministry of Education, University and
Research.
原始論文：Claudio
Faccenna, Petar Glišović, Alessandro Forte, Thorsten W. Becker, Eduardo
Garzanti, Andrea Sembroni, Zohar Gvirtzman. Role of dynamic topography
in sustaining the Nile River over 30 million years. Nature
Geoscience, 2019; DOI: 10.1038/s41561-019-0472-x
引用自：University of Texas at Austin. "Scientists
find eternal Nile to be more ancient than previously thought."
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