摘要：真极移（行星旋转轴的重新定向）在其他行星和卫星中都有很好的记录，在地球上通过人造卫星也能够得到观测。但由于板块构造引起的同时的运动，检测真极移在地球过去的普遍性变得复杂。学术界对晚白垩世约84 Ma的真极移是否存在尚有争议。来自意大利 Scaglia Rossa灰岩的经典古地磁数据是反对存在84 Ma真极移的主要依据。近日，岩石圈演化国家重点实验的Ross Mitchell研究员与国际合作者发表在Nature Communications上的研究展示了来自意大利两个平行地层剖面的新的高分辨率古地磁记录，为86 Ma至78 Ma 的～12°的真极移振荡观点提供了证据。 这一观察结果代表了最近记录的大规模真极移事件，并挑战了地球自旋轴在过去 100 Ma中基本稳定的观点。
抓紧坐稳了！科学家刚刚发现地球会侧倾。
现如今我们清楚地知道因为板块运动，大陆在缓慢地漂移，但是大陆漂移只会让板块相对彼此运动。
在过去的几十年里，关于地球能否发生侧倾一直存在争议。这个侧倾的过程——地球相对于自转轴的漂移——被称作“真极移”。关于真极移是否存在的证据有很大争议，但是一篇刚发表在Nature Communications上的文章为真极移提供了坚实的证据。
“想象从外太空观察地球“，加州理工学院和东京地球生命研究所的地球物理学家, 新文章的合作者之一Joe Kirschvink解释道，”真极移看起来会像是地球侧翻了。实际上是地球的整个地幔和地壳相对于液态地球外核发生了旋转。”
尽管科学家能用卫星精确测量现今正在发生的真极移，但是地质学家们依旧在激烈争论着类似的大尺度旋转是否曾在地球历史上出现过。
其中，大概8400万年前的晚白垩纪是一个争议尤其激烈的时期。在过去的30年里，关于在8400万年前地球是否发生过真极移，地球物理学家们各执一词，并在Science上爆发了公开的辩论。
这篇新文章的第一作者，中科院地质与地球物理研究所的Ross Mitchell研究员制定了一个能一劳永逸地解决争议的计划。
因为学生时期的经历，Mitchell知道在意大利中部的亚平宁山脉有再适合不过的目标岩石。由Mitchell带领的多国学者组成的团队认为，一套详尽的的来自于意大利的著名的白垩纪石灰岩的古地磁数据能为前面提到的争议提供一个决定性的检验。
保存于岩石中磁性矿物里的古地磁能用来测量大陆相对于地球磁极的相对运动。文章的另一个合作者，达特茅斯学院的地球生物学家Sarah Slotznick解释道：“因为这些意大利的沉积岩里的链状磁性矿物实际上是细菌化石，所以它们非常特殊和可靠”。
为了检验真极移假说，能够追踪地球自转轴漂移历史的高质量的、并且高度冗余（多个沉积剖面的数据）的古地磁数据必不可少。之前的多项研究里的数据，尤其是那些声称真极移没有发生过的研究，并没有达到这个标准。莱斯大学的地球物理学家Richard Gordon（并非本文作者）说：“这项研究让人耳目一新的其中一个原因正是丰富漂亮的古地磁数据。”
正如真极移假说所预测的，来自意大利的新数据指示了地球在正好8400万年前发生了大约12度的侧倾。
除了检验原本的假说，Mitchell的团队还有新的发现。地球在侧倾以后，似乎又调转了方向，移动回了原来的位置。
“就像是地球买了一张往返票”，Michell解释道。
“Michell的团队做出了关于晚白垩纪真极移震荡的崭新发现，”Gordon说，“这是开拓性的工作。”
当然，尽管这是一个基于坚实数据的结论，肯定还是会有持不同意见的人。Michell说：“见证我们修正过的理论能否经历时间的检验会很有趣。”
研究成果发表于国际顶级专业期刊Nature Communications（Mitchell R N, Thissen C J, Evans D A D, et al. A Late Cretaceous true polar wander oscillation[J]. Nature Communications, 2021, 12: 3629.)（原文链接）
Cretaceous Earth Tipped Over
Hold on to your hats, because scientists have just discovered that the earth can tip over.
We now know very well that the continents are moving slowly due to plate tectonics. But continental drift only pushes the tectonic plates past each other.
It has been also debated for the past few decades whether the whole planet can tip over. Such a shift of Earth on its axis is called “true polar wander”, however the evidence for this process has been very contentious.
But a new paper just published in Nature Communications provides some of the most solid evidence to date that such a planetary tipping point has indeed occurred in Earth’s past.
“Imagine looking at Earth from space,” explains Joe Kirschvink, co-author of the new paper and a geophysicist at the California Institute of Technology in Pasadena and the Earth Life Science Institute in Tokyo. “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 measure true polar wander occurring today very precisely with satellites, geologists still debate to great length whether large rotations have occurred in Earth’s past.
One particularly heated debate has been over Late Cretaceous time about 84 million years ago. Over the last 3 decades, geophysicists have been going back and forth, including very public arguments in the journal Science, about whether a large true polar wander event occurred at that age.
Ross Mitchell, lead author and geophysicist at the Institute of Geology and Geophysics Chinese Academy of Sciences in Beijing, came up with a plan for settling the debate once and for all.
With experience as a student in the Apennines Mountains of central Italy, Mitchell knew just the right rocks to sample. The international team of researchers placed their bet that a wealth of new paleomagnetic data from famous limestones of Cretaceous age in Italy would provide a definitive test.
Paleomagnetism measures the motion of continents relative to the North Pole and is preserved in the orientations of the magnetic minerals within rocks. 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 hypotheses about true polar wander, really good paleomagnetic data with lots of redundancy are required to track the wandering of the ancient location of Earth’s spin axis. Many prior studies, especially some claiming that true polar wander does not occur, fall below such a standard. 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.”
Indeed, as the true polar wander hypothesis had predicted, the new Italian data indicate a ~12？ tilt of the planet at precisely 84 million years ago.
But Mitchell’s team did more than just test an old idea. Notably, Earth appears to have corrected itself—after tipping on its side, Earth reversed course and rotated right back.
“It’s like a roundtrip ticket for planet Earth,” explains Mitchell.
“Mitchell’s team has a totally novel discovery of an oscillation in true polar wander in Late Cretaceous time,” says Gordon. “It is path breaking work.”
Of course, while it is an exciting conclusion from a robust dataset, there will surely be others that are less convinced. Says Mitchell, “it will be interesting to see how our revised theory stands the test of time.”
The article can be found online: https://www.nature.com/articles/s41467-021-23803-8
Ross Mitchell: Professor, Institute of Geology& Geophysics, Chinese Academy of Sciences (IGGCAS) (since 2021). Research interests: How Earth works, from core to crust. My areas of expertise are palaeomagnetism, cyclostratigraphy, and Earth history. The Earth system is completely integrated, but plate tectonic theory is not. Explaining the dynamics of past supercontinents, and even predicting future supercontinents, requires a systems theory approach.
