Fast Directional Changes during Geomagnetic Transitions: Global Reversals or Local Fluctuations?
Fast Directional Changes during Geomagnetic Transitions: Global Reversals or Local Fluctuations?
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DOI：
10.3390/geosciences11080318
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发表时间：
2021-07
期刊：
影响因子：
2.7
通讯作者：
S. Maffei;P. Livermore;J. Mound;Sam Greenwood;C. Davies
中科院分区：
文献类型：
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作者：
S. Maffei;P. Livermore;J. Mound;Sam Greenwood;C. Davies
意大利中部和南部沉积物的古地磁调查发现，在最后一次地磁场反转(大约发生在780,000年前)期间，方向变化每年约10∘。这些数值比估计的千禧年地磁场逆转时间尺度要大几个数量级。目前尚不清楚这些极端变化是否定义了全球偶极变化的时间尺度，或者它们是否表明了一种快速的、但空间上局部化的特征，而不是全球变化的指示。在这里，我们通过计算在核心顶部流动的最小动能来解决这个问题，这是瞬时再现这两个场景所需的。我们发现，与全球尺度的方向变化解释相兼容的优化流动结构需要的能量比复制局部变化的结构多一个数量级。特别是，我们发现，最近报道的意大利中部苏莫纳盆地的方向变化可以通过核表面流再现，其均方根值与目前估计的约8至22公里/年相当，或显著低于目前的估计值。相反，将观测结果解释为全球变化需要均方根值超过77公里/年，点状最大速度为127公里/年，我们认为这是不可能的。因此，我们得出结论，所报道的苏尔莫纳盆地的极端变化可能是由较长过渡期间的局部瞬时特征造成的。
Paleomagnetic investigations from sediments in Central and Southern Italy found directional changes of the order of 10∘ per year during the last geomagnetic field reversal (which took place about 780,000 years ago). These values are orders of magnitudes larger than what is expected from the estimated millennial timescales for geomagnetic field reversals. It is yet unclear whether these extreme changes define the timescale of global dipolar change or whether they indicate a rapid, but spatially localised feature that is not indicative of global variations. Here, we address this issue by calculating the minimum amount of kinetic energy that flows at the top of the core required to instantaneously reproduce these two scenarios. We found that optimised flow structures compatible with the global-scale interpretation of directional change require about one order of magnitude more energy than those that reproduce local change. In particular, we found that the most recently reported directional variations from the Sulmona Basin, in Central Italy, can be reproduced by a core-surface flow with rms values comparable to, or significantly lower than, present-day estimates of about 8 to 22 km/y. Conversely, interpreting the observations as global changes requires rms flow values in excess of 77 km/y, with pointwise maximal velocities of 127 km/y, which we deem improbable. We therefore concluded that the extreme variations reported for the Sulmona Basin were likely caused by a local, transient feature during a longer transition.