板塊運動可能促成了「寒武紀大爆發」
追查地球生命史上最重要的演化事件之一是由什麼引起的過程中，出現了引人注目的全新轉折。
五億多年前發生的寒武紀大爆發期間，動物以極快的速度開發出各式各樣的型態。一組科學家最近對此事件的發生原因提出了嶄新觀點。
迄今科學家提出了許多理論來解釋這段黃金年代的成因，其中呼聲最高的理論認為原因是氧濃度大幅提升，使得許多類型的動物都能茁壯成長。
這項新研究則提出全球板塊運動出現了重大變化，是氧濃度飆升的原因
大陸板塊和海洋板塊碰撞的時候往往會形成數千公里長的火山鍊，稱作「陸弧」。岡瓦那超大陸形成的時候，陸弧的火山活動也更加活躍，造成隱沒至地函的古老沉積岩釋放出許多二氧化碳。
研究團隊的計算結果顯示這會造成大氣裡的二氧化碳增加，地球隨之暖化，繼而讓陸地上的岩石受到更加強烈的風化作用，持續提供海洋含磷的營養鹽，使得光合作用提升，產生更多氧氣。
領導這項研究的為愛丁堡大學的博士生Josh
Williams。他在艾希特大學讀碩士時開始進行這項研究，當時他利用精密的生物地球化學模型，首度量化寒武紀大爆發不久之前大氣氧濃度的變化過程。
艾希特大學全球系統研究所的Tim
Lenton是研究共同作者與指導教授，他說：「由達爾文最先指出的一項重大難題是：在我們所稱的寒武紀大爆發期間，為什麼型態複雜的生命會突然出現，留下多采多姿的動物化石？」
「許多研究指出這和氧濃度提升有關，但卻無法說明氧氣增加的原因，也沒有試著算出增加的量有多少。」
他們的模型預測結果不僅指出板塊運動變得更加活躍是氧濃度顯著提高的原因，也得出氧氣大概提升至現今大氣氧濃度的四分之一，比估計中要讓寒武紀大爆發的動物出現所需的臨界值還高。
Williams接著表示：「寒武紀敏捷的大型掠食動物需要足夠的氧氣才能生存，而模型預測的氧濃度比預估所需的值還高；此外，模型其他預測結果也和現存的地球化學證據非常一致，使我們這項研究極具說服力。」
「想到某些最為古老的動物祖先――因而包括全人類在內――能夠存在的原因都得歸功於五千多萬年前板塊運動有一段特別活躍的時期，這實在是非常驚人。」
Plate tectonics may have driven “Cambrian Explosion”
The quest to discover what drove one of
the most important evolutionary events in the history of life on Earth has
taken a new, fascinating twist.
A team of scientists have given a fresh insight into
what may have driven the “Cambrian Explosion” – a period of rapid expansion of
different forms of animal life that occurred over 500 million years ago.
While a number of theories have been put forward to
explain this landmark period, the most credible is that it was fuelled by a
significant rise in oxygen levels which allowed a wide variety of animals to
thrive.
The new study suggests that such a rise in oxygen
levels was the result of extraordinary changes in global plate tectonics.
During the formation of the supercontinent
‘Gondwana’, there was a major increase in continental arc volcanism – chains of
volcanoes often thousands of miles long formed where continental and oceanic
tectonic plates collided. This in turn led to increased ‘degassing’ of CO2
from ancient, subducted sedimentary rocks.
This, the team calculated, led to an increase in
atmospheric CO2 and warming of the planet, which in turn amplified
the weathering of continental rocks, which supplied the nutrient phosphorus to
the ocean to drive photosynthesis and oxygen production.
The study was led by Josh Williams, who began the
research as an MSc student at the University of Exeter and is now studying for
a PhD at the University of Edinburgh. During his MSc project he used a
sophisticated biogeochemical model to make the first quantification of changes
in atmospheric oxygen levels just prior to this explosion of life.
Co-author and project supervisor Professor Tim
Lenton, from the University of Exeter’s Global Systems Institute said: “One of
the great dilemmas originally recognised by Darwin is why complex life, in the
form of fossil animals, appeared so abruptly in what is now known as the
Cambrian explosion.
“Many studies have suggested this was linked to a
rise in oxygen levels – but without a clear cause for such a rise, or any
attempt to quantify it.”
Not only did the model predict a marked rise in
oxygen levels due to changes in plate tectonic activity, but that rise in
oxygen – to about a quarter of the level in today’s atmosphere – crossed the
critical levels estimated to be needed by the animals seen in the Cambrian
explosion.
Williams added: “What is particularly compelling
about this research is that not only does the model predict a rise in oxygen to
levels estimated to be necessary to support the large, mobile, predatory animal
life of the Cambrian, but the model predictions also show strong agreement with
existing geochemical evidence."
“It is remarkable to think that our oldest animal
ancestors - and therefore all of us - may owe our existence, in part, to an
unusual episode of plate tectonics over half a billion years ago” said
Professor Lenton.
原始論文：Joshua J.
Williams, Benjamin J. W. Mills, Timothy M. Lenton. A tectonically
driven Ediacaran oxygenation event. Nature Communications,
2019; 10 (1) DOI: 10.1038/s41467-019-10286-x
引用自：University of Exeter. "Plate tectonics may
have driven 'Cambrian Explosion'."
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