原文網址：https://www.titech.ac.jp/english/news/2021/062153
扶好你的帽子――因為科學家找到了更多證據指出地球時不時會倒向一旁。我們知道陸地正因為板塊運動而緩慢地移動，但是陸地漂移只會讓板塊之間互相推擠。過去數十年來，科學家持續爭論地球外側的固體殼層是否會相對自轉軸而晃動，甚至整個傾倒。地球的這種運動稱作「真極漂移」(true
polar wander)，但是關於此作用的證據一直都有爭議。東京工業大學地球生命科學系的實驗室主持人Joe
Kirschvink(同時也是加州理工學院的教授)和北京地質與地球物理研究所的Ross
Mitchell教授主持了一項發表在《自然通訊》(Nature Communications)的新研究，他們提出了迄今最為有力的部分證據，顯示地球確實發生過這種整顆行星傾斜的現象。
義大利亞平寧山北部Furlo附近出露的Scaglia Rossa石灰岩。此處的石灰岩是在古地中海一條水道的淺部堆積而成，時間大約為8500萬年前的白堊紀晚期。圖片來源：Ross Mitchell
這裡先解釋一下「真極漂移」。地球是一顆可以分成不同層的球體：從內往外依序是固體金屬組成的內核、液態金屬組成的外核、接著是固體組成的地函以及更上方的地殼――也就是我們居住的地球表面。這些殼層就像陀螺一樣，每天都會旋轉一圈。由於地球的外核是液體，因此屬於固體的地核和地殼可以在上面到處滑動。密度較高的構造，比方說隱沒的海洋板塊和夏威夷之類的大型火山會往赤道靠近，就像你坐在辦公椅上旋轉的時候手會往旁邊張開一樣。
雖然地殼會到處移動，不過外核的鎳-鐵金屬液體對流產生的電流，才是地球有磁場的原因。從長久的時間尺度來看，上方地殼與地函的移動並不會影響地核，因為對於地球磁場來說，這些上覆的岩層就像是空無一物。反之，外核的對流模式其實是受制於地球的自轉軸而舞動，意味著地球磁場的模式整體來說是可以預測的，就像是將鐵粉灑在棒狀磁鐵上方所展開的線條一樣。因此這些數據可以提供絕佳的資訊，顯示地理南極與北極的方向；另一方面，傾角則能顯示與磁極的距離(當磁場的方向為垂直代表在極點，而水平則是赤道)。實際上許多岩石在形成當下都會記錄該地的磁場方向，如同磁帶可以錄下音樂。某些細菌產生的微小磁鐵礦晶體就是很好的粒子，它們會像微型的羅盤指針一樣整整齊齊地排列，當沉積物固化成岩石而把它們封存在裡面的時候，這些「磁場化石」就能用來追蹤自轉軸和地殼的相對移動過程。
「想像從太空看向地球，」Kirschvink解釋。「真極漂移發生時地球就像是往一邊傾斜，不過實情是地球的岩石外殼，也就是固體地函和地殼整個繞著液態外核旋轉。」雖然科學家可以利用衛星精準測量現在發生的真極漂移，不過地殼和地函在地球歷史上是否曾經大幅旋轉，仍是地質學家討論的議題。
其中一個受到激烈討論的事件大概發生在8400萬年前的白堊紀晚期。在過去三十年，地球物理學家反覆地在期刊《科學》以及許多會議上，公開辯論在白堊紀晚期是否曾發生一次大型真極漂移事件。
Mitchell和Kirschvink想出了可以一勞永逸平息這項爭議的計畫。Mitchell借助學生時期在義大利中部亞平寧山研究地質的經驗，知道哪些岩石樣品正好是他們想要的。這組國際研究團隊接著把籌碼押在這些白堊紀(大約為1億4500萬年前至6550萬年前)的義大利石灰岩，希望其中的古地磁紀錄可以提供決定性的證據。50年前科學家研究了同一地區較年輕的岩石的磁性，結果間接讓科學家發現消滅恐龍的小行星撞擊事件。共同作者，達特茅斯學院的地質生物學家Sarah
Slotznick解釋：「研究證實這些義大利的沉積岩非常特別而且可以信賴，原因是這些磁性礦物其實為細菌的化石，它們可以形成鏈狀的磁鐵礦。」
義大利亞平寧山的Scalgia
Rossa石灰岩裡的緯度漂移紀錄。數據顯示義大利在8600萬年前到8000萬年前曾經短暫地往赤道偏移，從太平洋海底岩石的地磁數據也可以觀察到同樣的旋轉現象。圖片來源：Ross
Mitchell與Christopher
Thissen
為了證實他們的真極漂移假說，他們需要非常多重複的數據來追蹤地球自轉軸過去的位置如何移動。團隊指出過去的研究探討的數據量不夠充足，特別是某些主張真極漂移並未發生的研究。休士頓萊斯大學的地球物理學家Richard
Gordon沒有參與此研究，他說：「這項研究會讓人耳目一新的原因，其中一個便是他們呈現了豐富且漂亮的古地磁數據。」
Kirschvink和同僚發現義大利的數據正如真極漂移假說預測的，顯示出8400萬年地球傾斜了12度左右。團隊也發現地球可以自己修正回來：在傾斜過後地球立刻掉頭反轉，在大約五百萬之間總共轉動了將近25度――就像個巨大的溜溜球一樣。
Did the Earth tip on its side 84
million years ago?
Hold on to your hats, because scientists
have found more evidence that Earth tips over from time to time. We know that
the continents are moving slowly due to plate tectonics, but continental drift
only pushes the tectonic plates past each other. It has been debated for the
past few decades whether the outer, solid shell of the Earth can wobble about,
or even tip over relative to the spin axis. Such a shift of Earth is called
"true polar wander", but the evidence for this process has been
contentious. New research published in Nature
Communications, led by the Earth-Life Science Institute (ELSI) at Tokyo
Institute of Technology's Principle Investigator Joe Kirschvink (also a
Professor at Caltech) and Prof. Ross Mitchell at the Institute of Geology and
Geophysics in Beijing, provides some of the most convincing evidence to date
that such planetary tipping has indeed occurred in Earth's past.
True polar wander bears some dissecting. The Earth is
a stratified ball, with a solid metal inner core, a liquid metal outer core,
and a solid mantle and overriding crust at the surface which we live on. All of
this is spinning like a top, once per day. Because the Earth's outer core is
liquid, the solid mantle and crust are able to slide around on top of it.
Relatively dense structures, such as subducting oceanic plates and massive
volcanoes like Hawaii, prefer to be near the Equator, in the same way that your
arms like to be out to your side when you are spinning around in an office
chair.
Despite this wandering of the crust, Earth' magnetic
field is generated by electrical currents in the convecting liquid Ni-Fe metal
of the outer core. On long time scales, the overlying wander of the mantle and
crust does not affect the core, because those overlying rock layers are
transparent to Earth's magnetic field. In contrast, the convection patterns in
this outer core are actually forced to dance around Earth's rotation axis,
which means that the overall pattern of Earth's magnetic field is predictable,
spreading out in the same fashion as iron filings lining up over a small bar
magnet. Hence, these data provide excellent information about the direction of
the North and South geographic poles, and the tilt gives the distance from the
poles (a vertical field means you are at the pole, horizontal tells us it was
on the Equator). Many rocks actually record the direction of the local magnetic
field as they form, in much the same way that a magnetic tape records your
music. For example, tiny crystals of the mineral magnetite produced by some
bacteria actually line up like tiny compass needles, and get trapped in the
sediments when the rock solidifies. This, "fossil" magnetism can be
used to track where the spin axis is wandering relative to the crust.
"Imagine looking at Earth from space,"
explains Kirschvink "True polar wander would look like the Earth tipping
on its side, and what's actually happening is that the whole rocky shell of the
planet—the solid mantle and crust—is rotating around the liquid outer
core." Although scientists can measure true polar wander occurring today
very precisely with satellites, geologists still debate whether large rotations
of the mantle and crust have occurred in Earth's past.
One particularly heated debate has been over events
during the Late Cretaceous, about 84 million years ago. Over the last three
decades, geophysicists have been going back and forth through public arguments
in the journal Science, and at
numerous meetings, about whether a large true polar wander event occurred in
the Cretaceous.
Mitchell and Kirschvink came up with a plan for
settling the debate once and for all. Leveraging Mitchell's experience as a
student studying the geology of the Apennine Mountains of central Italy, he
knew just the right rocks to sample. The international team of researchers then
placed their bet that paleomagnetic data from limestones created in the
Cretaceous (between ~145.5 and 65.5 million years ago) located in Italy would
provide a definitive test. The magnetism of the younger rocks in the same area
was studied nearly 50 years ago, and indirectly led to the discovery of the
asteroid impact that killed the dinosaurs. Sarah Slotznick, co-author and
geobiologist at Dartmouth College explains, "these Italian sedimentary
rocks turn out to be special and very reliable because the magnetic minerals
are actually fossils of bacteria that formed chains of the mineral
magnetite."
To test their hypothesis about true polar wander,
paleomagnetic data with lots of redundancy are required to track the wander of
the ancient location of Earth's spin axis. Prior studies, especially some
claiming that true polar wander does not occur, have failed to explore enough
data points according to the team. Says Richard Gordon, a geophysicist at Rice
University in Houston who wasn't involved in the study, "that is one
reason why it is so refreshing to see this study with its abundant and
beautiful paleomagnetic data."
Kirschvink and colleagues found, as the true polar
wander hypothesis predicted, the Italian data indicate an ~12˚ tilt of the
planet 84 million years ago. The team also found that Earth appears to have
corrected itself—after tipping on its side, Earth reversed course and rotated
right back, for a total excursion of nearly 25˚ of arc in about five million
years. Certainly, this was a cosmic "yo-yo".
Ross N. Mitchell, Christopher J. Thissen, David A. D.
Evans, Sarah P. Slotznick, Rodolfo Coccioni, Toshitsugu Yamazaki & Joseph
L. Kirschvink. A Late Cretaceous true
polar wander oscillation. Nature
Communication, 2021; Doi:10.1038/s41467-021-23803-8.
引用自：Tokyo
Institute of Technology. “Did the Earth tip on its side 84 million years ago?”
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