原文網址：https://home.dartmouth.edu/news/2022/01/study-links-glaciers-earths-great-unconformity
發表在《美國國家科學院院刊》(Proceedings of the National Academy of
Sciences)的新研究提出了更多證據，指出象徵地質歷史十億年之久的岩石被刮除掉，是「雪球地球」期間的古代冰河所造成。
稱為「大不整合面」的大量岩石缺失，最初的命名地點為大峽谷。圖片來源：Saad Chaudhry on Unsplash
「大不整合面」(Great Unconformity)是指在地球各處都有厚達3英里的岩石被侵蝕掉，造成地質紀錄當中出現一大段空白的現象。對於它的成因仍有爭議，而這項研究給出了最新發現。
「岩石紀錄長久以來最令人困惑的地方之一，便是這段時期為什麼許多地方的沉積岩都消失了，」此研究的資深作者，地質科學的助理教授C. Brenhin Keller表示。「隨著更多研究結果出爐，我們對這種模式的理解也增加許多。」
在1800年代晚期，科學家首次在大峽谷把這些大量消失的岩石命名為「大不整合面」。這道宏偉的地質景觀在外表上像是三明治般堆疊起來的遠古岩層，找出含有化石的岩層直接堆疊在不具化石的岩層上方是辨識它的常用方法。
「那是地質歷史上一段迷人的時光，」這篇論文的主要作者，達特茅斯學院的博士後研究員Kalin McDannell表示。「大不整合面鋪陳了之後的寒武紀生命大爆發。由於地質和演化過程通常來說是漸變的，因此寒武紀大爆發就這樣突然出現在化石紀錄，一直以來都很令人費解。」
研究人員從一個多世紀之前便開始尋找可以解釋這段地質時間消失的原因。
過去五年，有兩個對立的理論開始受到矚目：其中一個解釋認為7億到6億3500萬年前的雪球地球期間，遠古冰河將這些岩石給刮除殆盡；另一個則著眼在歷時較長的一連串板塊運動，認為10億到5億5000萬年前羅迪尼亞超大陸的分分合合才是原因。
2019年Keller主持的研究首次提出成冰紀(Cryogenian)期間，大陸冰層在許多地方造成的侵蝕作用使得岩石消失。他們根據的地球化學代用指標指出在雪球地球的同一期間，也發生了許多大規模侵蝕作用。
「新的研究證實了之前的發現並且使其更加完備，」Keller表示。「我們在此提出了岩石降溫的獨立證據，顯示成冰紀期間北美大部分地方有數英里厚的岩石遭到剝蝕。」
這項研究詳細解讀熱定年(thermochronology)結果之後得出了上述評估。
熱定年法可以讓研究人員預估隨著時間經過礦物晶體的溫度如何變化，以及在給定的溫度結構下礦物晶體在大陸地殼中的位置如何移動。這些歷史變化可以提供證據讓研究人員知道消失的岩石是在什麼時候被移走，以及現今暴露在地表的岩石是在什麼時候露出。
研究人員依據先前發表的熱定年資料進行多次測量，這些資料跨越了北美洲四個不同的地點。它們位在穩定地塊，是陸地中化學與物理性質皆相當穩定的部分，因此板塊構造運動當時在這些地方應該不常發生。
他們透過模擬找出這些岩石過往時間對溫度的變化路徑。結果顯示在雪球地球的冰河作用期間，北美洲內陸許多地方的岩石溫度都迅速地大幅降低，相當於上方有2到3英里厚的岩石被侵蝕掉。
「雖然其他研究曾經利用熱定年法來質疑大不整合面的起源是否為冰河，但是像這種全球可見的現象也需要從全球的角度進行評估，」McDannell表示。「一般認為雪球地球期間冰層曾覆蓋北美大部分地方，而且它們挖掘岩石的效率非常高，因此當時要發生大面積的侵蝕作用最簡單的解釋便是冰河。」
研究團隊表示另一個團隊在2020年提出了對立的理論，他們認為板塊構造運動才是把這些消失的岩石搬除的主要原因，他們也質疑古代的冰河是否有這個能力侵蝕如此巨量的岩石。雖然該研究也利用了熱定年法，但他們使用的技術並不同，而且只用在一處板塊運動活躍的地方，得出的結果認為侵蝕發生在雪球地球之前。
「基本概念相當簡單：有某種事物移除了一大堆岩石，造成時間出現了一大片空白，」Keller表示。「我們的研究證實唯有冰河造成的侵蝕作用可以達到如此大的規模。」
研究團隊表示新發現也有助於解釋岩石的侵蝕作用，以及大約5億3000萬年前複雜生物在寒武紀大爆發中崛起，兩者之間有什麼關聯。科學家相信雪球地球期間侵蝕作用造成富含養分的沉積物堆積在海洋，這讓環境變得相當肥沃而成為複雜生命出現的基石。
研究強調岩石如何受到侵蝕的這兩種假說並非互不相容。在大不整合面形成的時候地球系統在全球各地都有可能因為構造作用和冰河作用而受到擾動。不過在大陸中心遠離構造邊界的地方，應該只有冰河可以解釋此處的侵蝕作用。
研究論文寫道：「關於大不整合面，或許我們最後證明出來的結果是當時的地質條件出現了整部地球歷史上從沒看過的巧合：普遍接受的陸塊重建結果指出羅迪尼亞大陸當時集中在赤道，於此同時新元古代也出現了獨一無二的環境條件。」
團隊表示這是他們第一篇運用熱定年模型，探討時間長度遠遠超過十億年的研究。未來團隊希望對其他大陸重複進行這類研究，希望可以進一步驗證這些有關大不整合面如何形成並保存下來的理論。
團隊指出解開不同研究的差異，對於了解地球早期歷史，以及氣候、構造運動和生物地質化學作用之間的關聯來說相當重要。
「構造運動可能會侵蝕穩定地塊的邊緣，但這並不能否決冰河作用曾經發生，」McDannell表示。「不整合面是一種複合式的構造。我們的成果指出成冰紀的侵蝕作用是關鍵因素，但是在不同地方，稍早或稍晚發生的侵蝕作用也有可能參與了不整合面的形成過程。探討世界各地的不整合面可以告訴我們更多訊息。」
論文共同作者還有伊利諾大學厄巴納－香檳分校的William Guenthner、理海大學的Peter Zeitler、加州大學柏克萊分校暨柏克萊地質年代學中心的David Shuster。
New research strengthens link between
glaciers and Earth's 'Great Unconformity'
New research provides further evidence that rocks
representing up to a billion years of geological time were carved away by
ancient glaciers during the planet's "Snowball Earth" period,
according to a study published in Proceedings
of the National Academy of Sciences.
The research presents the latest findings in a debate over what caused the
Earth's "Great Unconformity"—a time gap in the geological record
associated with the erosion of rock up to 3 miles thick in areas across the
globe.
"The fact that so many places are missing the sedimentary rocks from
this time period has been one of the most puzzling features of the rock
record," said C. Brenhin Keller, an assistant professor of earth sciences
and senior researcher on the study. "With these results, the pattern is
starting to make a lot more sense."
The massive amount of missing rock that has come to be known as the Great
Unconformity was first named in the Grand Canyon in the late 1800s. The
conspicuous geological feature is visible where rock layers from distant time
periods are sandwiched together, and it is often identified where rocks with
fossils sit directly above those that do not contain fossils.
"This was a fascinating time in Earth's history," said Kalin
McDannell, a postdoctoral researcher at Dartmouth and the lead author of the
paper. "The Great Unconformity sets the stage for the Cambrian explosion
of life, which has always been puzzling since it is so abrupt in the fossil
record—geological and evolutionary processes are usually gradual."
For over a century, researchers have sought to explain the cause of the
missing geological time.
In the last five years, two opposing theories have come into focus: One
explains that the rock was carved away by ancient glaciers during the Snowball
Earth period about 700 to 635 million years ago. The other focuses on a series
of plate tectonic events over a much longer period during the assembly and
breakup of the supercontinent Rodinia from about 1 billion to 550 million years
ago.
Research led by Keller in 2019 first proposed that widespread erosion by
continental ice sheets during the Cryogenian glacial interval caused the loss
of rock. This was based on geochemical proxies that suggested that large
amounts of mass erosion matched with the Snowball Earth period.
"The new research verifies and advances the findings in the earlier
study," said Keller. "Here we are providing independent evidence of
rock cooling and miles of exhumation in the Cryogenian period across a large
area of North America."
The study relies on a detailed interpretation of thermochronology to make
the assessment.
Thermochronology allows researchers to estimate the temperature that
mineral crystals experience over time as well as their position in the
continental crust given a particular thermal structure. Those histories can
provide evidence of when missing rock was removed and when rocks currently
exposed at the surface may have been exhumed.
The researchers used multiple measurements from previously published
thermochronometric data taken across four North American locations. The areas,
known as cratons, are parts of the continent that are chemically and physically
stable, and where plate tectonic activity would not have been common during
that time.
By running simulations that searched for the time-temperature path the
rocks experienced, the research recorded a widespread signal of rapid, high
magnitude cooling that is consistent with about 2-3 miles of erosion during
Snowball Earth glaciations across the interior of North America.
"While other studies have used thermochronology to question the
glacial origin, a global phenomenon like the Great Unconformity requires a
global assessment," said McDannell. "Glaciation is the simplest
explanation for erosion across a vast area during the Snowball Earth period
since ice sheets were believed to cover most of North America at that time and
can be efficient excavators of rock."
According to the research team, the competing theory that tectonic
activity carved out the missing rock was put forth in 2020 when a separate
research group questioned whether ancient glaciers were erosive enough to cause
the massive loss of rock. While that research also used thermochronology, it
applied an alternate technique at only a single tectonically active location
and suggested that the erosion occurred prior to Snowball Earth.
"The underlying concept is pretty simple: Something removed a whole
lot of rock, resulting in a whole lot of missing time," said Keller.
"Our research demonstrates that only glacial erosion could be responsible
at this scale."
According to the researchers, the new findings also help explain links
between the erosion of rock and the emergence of complex organisms about 530
million years ago during the Cambrian explosion. It is believed that erosion
during the Snowball Earth period deposited nutrient-rich sediment in the ocean
that could have provided a fertile environment for the building blocks of
complex life.
The study notes that the two hypotheses of how the rock eroded are not
mutually exclusive—it is possible that both tectonics and glaciation
contributed to global Earth system disruption during the formation of the Great
Unconformity. It appears, however, that only glaciation can explain erosion in
the center of the continent, far from the tectonic margins.
"Ultimately with respect to the Great Unconformity, it may be that
the generally accepted reconstruction(s) of more concentrated equatorial
packing of the Rodinian continents along with the unique environmental
conditions of the Neoproterozoic, proved to be a time of geologic serendipity
unlike most any other in Earth history," the research paper says.
According to the team, this is the first research that uses their
thermochronology modeling approach to study a period that extends well beyond a
billion years. In the future, the team will repeat their work on other
continents, where they hope to further test these hypotheses about how the
Great Unconformity was created and preserved.
According to the team, resolving differences in the research is critical
to understanding early Earth history and the interconnection of climatic,
tectonic and biogeochemical processes.
"The fact that there may have been tectonic erosion along the craton
margins does not rule out glaciation," said McDannell.
"Unconformities are composite features, and our work suggests Cryogenian
erosion was a key contributor, but it is possible that both earlier and later
erosion were involved in forming the unconformity surface in different places.
A global examination will tell us more."
William Guenthner, from the University of Illinois at Urbana-Champaign;
Peter Zeitler from Lehigh University; and David Shuster from the University of
California, Berkeley and the Berkeley Geochronology Center served as co-authors
of the paper.
原始論文：Kalin T. McDannell, C. Brenhin Keller, William R. Guenthner,
Peter K. Zeitler, David L. Shuster. Thermochronologic
constraints on the origin of the Great Unconformity. Proceedings of the National
Academy of Sciences, 2022; 119 (5): e2118682119 DOI: 10.1073/pnas.2118682119
引用自：Dartmouth College. "New research strengthens
link between glaciers and Earth’s 'Great Unconformity’”
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