利用放射線定年更準確地得到地球最近一次磁極轉換的年代

原文網址：www.sciencedaily.com/releases/2015/07/150707101952.htm

An improved age for Earth's latest magnetic field reversal using radiometric dating

利用放射線定年更準確地得到地球最近一次磁極轉換的年代

Earth's magnetic field periodically reverses such that the north magnetic pole becomes the south magnetic pole. The latest reversal is called by geologists the Matuyama-Brunhes boundary (MBB), and occurred approximately 780,000 years ago. The MBB is extremely important for calibrating the ages of rocks and the timing of events that occurred in the geological past; however, the exact age of this event has been imprecise because of uncertainties in the dating methods that have been used.

地球的磁場會週期性的倒轉使得磁北極變成磁南極。地質學家稱呼最近一次的倒轉事件為松山-布容反轉界面(Matuyama-Brunhes boundary，MBB)，其發生在約78萬年前。MBB對於比對過去地質時間中的岩石與事件發生的年代都相當重要，然而受限於過去的定年方法的誤差，此事件的確切年代仍不太準確。

A team of researchers based in Japan and Canada have obtained an improved age for the MBB. The team studied volcanic ash that was deposited immediately before the MBB. This volcanic ash contains small crystals called zircons. Some of these crystals formed at the same time as the ash; thus, radiometric dating of these zircons using the uranium-lead method provided the exact age of the ash. To verify their findings, the researchers also used a different method to date sedimentary rock from the same place that was formed at the time of the MBB. The combined results demonstrate that the age of the MBB is 770.2 ± 7.3 thousand years ago. The research has been published in the journal Geology.

由加拿大與日本的科學家組成的研究團隊得到了更準確的MBB發生年代。此團隊的研究對象為剛好在MBB發生前沉積的火山灰。這層火山灰中含有稱為鋯石(zircon)的微小礦物結晶，其中有一些的形成時間與火山灰正好相同，因此火山灰的形成年代即為這些鋯石用鈾鉛定年(uranium-lead method)得出的年代。為了證實他們的發現，研究人員也利用其他的方法來定年沉積於同一個地方，時間為MBB發生時的沉積岩。這些結果組合起來顯示MBB的發生年代為770.2 ± 7.3千年前。此研究成果刊登於期刊《地質》(Geology)。

Dr. Yusuke Suganuma of the National Institute of Polar Research, Tokyo, who is the lead author on the paper, commented: "This study is the first direct comparison of radiometric dating, dating of sediments, and the geomagnetic reversal for the Matuyama-Brunhes boundary. Our work contributes calibrating the geological time scale, and will be extremely important in future studies of the events that occurred at this time."

本篇論文的第一作者，東京國立極地研究所(the National Institute of Polar Research, Tokyo)的菅沼悠介博士評論：「本研究為首篇對松山-布容反轉界面的放射性定年、沉積物定年與磁極反轉性質直接綜合比較的研究。我們的成果對地質年代表的校正助益良多，也對目標為發生在這段時期的地質事件的未來研究至關重要。」

引用自：Research Organization of Information and Systems. "An improved age for Earth's latest magnetic field reversal using radiometric dating." ScienceDaily. ScienceDaily, 7 July 2015.