利物浦大學的新研究提供了更進一步的證據，顯示地球磁場強度有二億年左右的週期。
研究人員從蘇格蘭東部取得由古代熔岩流形成的岩石樣品，接著進行了高溫與微波古地磁分析(後者為利物浦大學的獨特技術)，以測量地球歷史上一段關鍵時期的磁場強度，在此之前幾乎沒有關於那段時期的可靠數據。此外，研究也分析了將近80年來，對於2億至5億年前的岩石樣本進行的所有測量結果可信度有多高。
他們發現年代為3億3200萬年至4億1600萬年的岩石，保存在裡面的地球磁場強度不到現在的四分之一。這和科學家先前辨識出來，開始於1億2000萬年前左右的低磁場強度時期有類似之處。研究人員將新發現的時期稱為「古生代中期弱偶極」(Mid-Palaeozoic
Dipole low ，MPDL)。
這篇發表在《美國國家科學院院刊》(Proceedings of the National Academy of
Sciences)的研究支持了地球磁場強度具有週期性，每過2億年就會變弱的理論。2012年，利物浦大學主持的研究提出此想法，當時的限制之一是磁場強度缺乏比3億年前更早的可信數據，因此新研究填補了這段相當重要的空白時間。
地球的磁場可以保護地球不受太陽輻射足以致命的強力爆發襲擊。不過從時間與空間上來說，磁場的強度與方向都並非完全穩定，它可以徹底翻轉或倒轉過來，這具有相當重大的涵義。
破解地磁強度的變化歷程相當重要，因為它指示了數億年來地球深部進行的作用如何變化，同時也提供線索讓我們推測它在未來可能會有什麼樣的波動，是否會翻轉或倒轉過來。
磁場變弱也會影響地球上的生命。最近有篇研究認為泥盆紀與石炭紀交界的大滅絕，跟中波紫外線強度升高有關，此時大約也是MPDL磁場強度測量出來最弱的時候。
利物浦大學的古地磁學家Louise
Hawkins博士是文章主要作者。他說：「我們對Strathmore與Kinghorn熔岩流地磁強度的詳細分析結果，是把Kiman超期(Superchron)還早的數據給填補起來的關鍵。Kiman超期是持續5000萬年左右，地磁磁極相當穩定並未發生翻轉的時期。」
「這組資料致意了我們最近幾年完成的研究，以及我們在莫斯科和亞伯達的同僚，他們在這兩個地點得到的年代也和我們相符。」
「同時參照我們的發現與現存的資料庫，便能支持地球磁場強度具有大約為2億年的周期，而此現象和地球深部的作用有關。由於板塊運動不停地抹除我們對於地球內部作用的大部分證據，因此作為我們可以用來界定地球深部作用的少數條件，這道訊息可以保存下來是相當珍貴的。」
「我們的發現也更加支持磁極倒轉可以和磁場變弱畫上關聯，而磁極超期的磁場強度一般來說都比較高。這項結果相當重要，因為之前已經證明要把磁極倒轉紀錄推進到3億年以前是幾乎不可能的。」
Further evidence of 200 million-year
cycle for Earth’s magnetic field
The findings of a new study by the
University of Liverpool provides further evidence of an approximately 200
million-year long cycle in the strength of the Earth’s magnetic field.
Researchers performed thermal and microwave (a
technique which is unique to the University of Liverpool) paleomagnetic
analysis on rock samples from ancient lava flows in Eastern Scotland to measure
the strength of the geomagnetic field during key time periods with almost no
pre-existing, reliable data. The study also analysed the reliability of all of
the measurements from samples from 200 to 500 million years ago, collected over
the last ~80 years.
They found that between 332 and 416 million years
ago, the strength of the geomagnetic field preserved in these rocks was less
than quarter of what it is today, and similar to a previously identified period
of low magnetic field strength that started around 120 million years ago. The
researchers have coined this period “the Mid-Palaeozoic Dipole low (MPDL).”
Published in Proceedings
of the National Academy of Sciences, the study supports the theory that the
strength of the earth’s magnetic field is cyclical, and weakens every 200
million years, an idea proposed by a previous study lead by Liverpool in 2012.
One of the limitations at the time was the lack of reliable field strength data
available prior to 300 million years ago, so this new study fills in an
important time gap.
The Earth’s magnetic field shields the planet from
huge blasts of deadly solar radiation. It is not completely stable in strength
and direction, both over time and space, and has the ability to completely flip
or reverse itself with substantial implications.
Deciphering variations in past geomagnetic field
strength is important as it indicates changes in deep Earth processes over
hundreds of millions of years and could provide clues as to how it might
fluctuate, flip or reverse in the future.
A weak field also has implications for life on our
planet. A recent study has suggested that the Devonian-Carboniferous mass
extinction is linked to elevated UV-B levels, around the same as the weakest
field measurements from the MPDL.
Liverpool palaeomagnetist and lead author of the
paper, Dr Louise Hawkins, said: “This comprehensive magnetic analysis of the
Strathmore and Kinghorn lava flows was key for filling in the period leading up
the Kiman Superchron, a period where the geomagnetic poles are stable and do
not flip for about 50 million years.
“This dataset compliments other studies we have
worked on over the last few years, alongside our colleagues in Moscow and
Alberta, that fit between the ages of these two locations.
“Our findings, when considered alongside the existing
datasets, supports the existence of an approximately 200-million-year long
cycle in the strength of the Earth’s magnetic field related to deep Earth
processes. As almost all of our evidence for processes within the Earth’s
interior is being constantly destroyed by plate tectonics, the preservation of
this signal for deep inside the Earth is exceedingly valuable as one of the few
constraints we have.
“Our findings also provide further support that a
weak magnetic field is associated with pole reversals, while the field is
generally strong during a Superchron, which is important as it has proved
nearly impossible to improve the reversal record prior to ~300 million years
ago.”
原始論文：Louise
M. A. Hawkins, J. Michael Grappone, Courtney J. Sprain, Patipan Saengduean,
Edward J. Sage, Sheikerra Thomas-Cunningham, Banusha Kugabalan, Andrew J. Biggin. Intensity of the Earth's magnetic
field: Evidence for a Mid-Paleozoic dipole low. Proceedings of the National Academy of Sciences, 2021. DOI: 10.1073/pnas.2017342118
引用自：University
of Livepool. “Further evidence of 200 million-year cycle for Earth’s magnetic
field.”
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