By Felix
Würsten
動物在5億4000萬年前的迅速發展永久改變了地球，甚至達到下部地函。由蘇黎世聯邦理工學院的研究員Andrea
Giuliani主持的團隊，發現地函深處的岩石具有那次生物的迅速發展留下的痕跡。
金柏利岩是一種成分複雜的岩石，它們是從非常深的地方來到地球表面。圖中是一片富含碳酸鹽的金柏利岩薄片。圖片來源：David Swart / Messengers of the Mantle Exhibition
我們很容易就能看到地球內部的作用對地表事物造成的影響。舉例來說，火山會噴出變成岩漿的岩石並且釋放氣體到大氣當中，因而影響地球表面的生物地球化學循環。
不過比較不明顯的是，反過來看也沒錯：地表發生的事物可以影響地球內部，甚至達到非常深的地方。蘇黎世聯邦理工學院地球科學系的Andrea
Giuliani是瑞士國家科學基金會Ambizione計畫資助的研究員，她主持的國際團隊在發表於期刊《科學前緣》(Science
Advances)的新研究中得出了上述結論。根據該研究，地球表面生物的發展過程對下部地函的一部分造成了影響。
碳帶來的訊息
在此研究中，研究人員檢視了地球歷史不同時期形成的金柏利岩，這是一種含有鑽石的稀有岩石，它們攜帶了地函最底部的訊息。科學家測量了大約150個這種特殊的岩石樣品中的碳同位素組成。他們發現年代較近(小於2億5000萬年)的金柏利岩的組成跟年代較老的有很大的差異。在許多年代較近的樣品當中，碳同位素的組成竟然落在地函岩石預期要有的範圍之外。
研究人員認為年代較近的金柏利岩的組成會有這種變化，關鍵原因在於寒武紀大爆發。這起事件發生在大約5億4000萬年前寒武紀剛開始的時候，長度大概只有數千萬年，以地質角度而言是相當短的一段時間。幾乎世上所有現存的動物類型都是在這場劇烈的變革當中第一次出現在地球上。「海洋裡的生命形式巨幅增加，對地表的事物造成了決定性的改變，」Giuliani解釋。「這又會影響海底沉積物的組成。」
從海洋進入地函之後再回來
這項轉變也會牽連到地球的下部地函，原因是生物死亡之後遺骸會堆積在海床上的沉積物，其中一部份可以經由板塊運動進入地函。這些沉積物在隱沒帶會隨著下方的海洋地殼一同被運往非常深的地方。透過此方式，沉積物裡的有機質含有的碳便能到達地函。在此沉積物會跟地函其他的岩石物質混和，經過一段時間之後(據估計至少要2到3億年)，這些物質會在其他地方回到地球表面，像是變成金柏利岩的岩漿。
令人驚訝的是海洋沉積物的變化竟然能留下如此深刻的痕跡，因為整體來說，在隱沒帶只有一丁點的沉積物可以運送到地函深處。「這證實了隱沒到地函的岩石材料並非均勻分布，而是會沿著特定的軌跡移動，」Giuliani解釋。
地球是一個整體的系統
除了碳以外，研究人員也檢視了其他化學元素的同位素組成。舉例來說，鍶和鉿這兩種元素的組成也跟碳有相似的模式。「這代表碳的訊號無法用其他作用，像是脫氣作用來解釋，因為那些作用不會讓鍶和鉿的同位素組成與碳的組成之間有相關性，」Giuliani強調。
這些新發現也啟發了更加深入的研究。比方說磷和鋅這類因為生命出現而深受影響的元素，或許也能提供線索顯示出地表發生的作用如何影響地球內部。「地球整體是一個極為複雜的系統，」Giuliani表示。「而我們現在想做的是更加仔細地了解這個系統。」
Traces of life in the Earth's deep
mantle
The rapid development of fauna 540
million years ago has permanently changed the Earth - deep into its lower mantle.
A team led by ETH researcher Andrea Giuliani found traces of this development
in rocks from this zone.
It is easy to see that the processes in the Earth's interior
influence what happens on the surface. For example, volcanoes unearth magmatic
rocks and emit gases into the atmosphere, and thus influence the biogeochemical
cycles on our planet.
What is less obvious, however, is that the reverse is
also true: what happens on the Earth's surface effect the Earth's interior -
even down to great depths. This is the conclusion reached by an international
group of researchers led by Andrea Giuliani, SNSF Ambizione Fellow in the
Department of Earth Sciences at ETH Zurich, in a new study published in the
journal Science Advances. According
to this study, the development of life on our planet affects parts of Earth’s
lower mantle.
Carbon as a
messenger
In their study, the researchers examined rare
diamond-bearing volcanic rocks called kimberlites from different epochs of the
Earth's history. These special rocks are messengers from the lowest regions of
the Earth's mantle. Scientists measured the isotopic composition of carbon in
about 150 samples of these special rocks. They found that the composition of
younger kimberlites, which are less than 250 million years old, varies
considerably from that of older rocks. In many of the younger samples, the
composition of the carbon isotopes is outside the range that would be expected
for rocks from the mantle.
The researchers see a decisive trigger for this
change in composition of younger kimberlites in the Cambrian Explosion. This
relatively short phase - geologically speaking - took place over a period of
few tens of million years at the beginning of the Cambrian Epoch, about 540
million years ago. During this drastic transition, almost all of today's
existing animal tribes appeared on Earth for the first time. "The enormous
increase in life forms in the oceans decisively changed what was happening on
the Earth's surface," Giuliani explains. "And this in turn affected the
composition of sediments at the bottom of the ocean."
From the oceans
to the mantle and back
For the Earth's lower mantle, this changeover is
relevant because some of the sediments on the seafloor, in which material from
dead living creatures is deposited, enter the mantle through plate tectonics. Along
the subduction zones, these sediments - along with the underlying oceanic crust
- are transported to great depths. In this way, the carbon that was stored as
organic material in the sediments also reaches the Earth's mantle. There the
sediments mix with other rock material from the Earth's mantle and after a
certain time, estimated to at least 200-300 million years, rise to the Earth's
surface again in other places - for example in the form of kimberlite magmas.
It is remarkable that changes in marine sediments
leave such profound traces, because overall, only small amounts of sediment are
transported into the depths of the mantle along a subduction zone. "This
confirms that the subducted rock material in the Earth's mantle is not
distributed homogeneously, but moves along specific trajectories,"
Giuliani explains.
The Earth as a
total system
In addition to carbon, the researchers also examined
the isotopic composition of other chemical elements. For example, the two
elements strontium and hafnium showed a similar pattern to carbon. "This
means that the signature for carbon cannot be explained by other processes such
as degassing, because otherwise the isotopes of strontium and hafnium would not
be correlated with those of carbon," Giuliani notes.
The new findings open the door for further studies.
For example, elements such as phosphorus or zinc, which were significantly
affected by the emergence of life, could also provide clues as to how processes
at the Earth's surface influence the Earth's interior. "The Earth is
really a complex overall system," Giuliani says. "And we now want to
understand this system in more detail."
原始論文：Andrea
Giuliani, Russell N. Drysdale, Jon D. Woodhead, Noah J. Planavsky, David
Phillips, Janet Hergt, William L. Griffin, Senan Oesch, Hayden Dalton, Gareth
R. Davies. Perturbation of the deep-Earth carbon cycle in response to
the Cambrian Explosion. Science Advances, 2022; 8 (9)
DOI: 10.1126/sciadv.abj1325
引用自：ETH
Zurich. "Traces of life in the Earth's deep mantle."
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