Thermochemical flows couple the Earth's inner core growth to mantle heterogeneity
Thermochemical flows couple the Earth's inner core growth to mantle heterogeneity
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DOI：
10.1038/nature07109
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发表时间：
2008-08-07
期刊：
影响因子：
64.8
通讯作者：
Olson, Peter
中科院分区：
文献类型：
--
作者：
Aubert, Julien;Amit, Hagay;Olson, Peter
在地球内核顶部100公里处采集的地震波显示，东半球(东经40度-东经180度)比西半球地震更快(1，2)，更各向同性(2，3)，更衰减(4)。这种半球二分法的起源对于我们理解地球是一个由动态耦合层组成的系统是一个具有挑战性的问题。此前，实验室实验已经证实，来自下地幔的热控制可以极大地影响外核(5)中的流体流动，这反过来又可以在内核凝固前沿(6)引起结构不均匀。由此产生的结构应该与地球发电机上热地幔控制的其他预期表现相一致，特别是过去5Myr的时间平均古地磁场(9，10)中的磁通量密度(7，8)，以及通过对历史地磁长期变化的分析，在核幔边界下成像的流动中的优选涡流位置(11)(12)。在这里，我们表明，热化学对流和发电机作用的单一模型可以通过产生大规模、长期的外核流来解释所有这些影响，这种外核流将内核的非均质性与下地幔的非均质性耦合在一起。这种热化学“风”的主要特征是亚洲以下的气旋式环流，它将磁场集中在观测位置的核-地幔边界上，并在局部与岩芯流动图像相一致。这股风还在内核边界的东半球造成了异常高的轻元素释放率，这表明内核顶部的横向地震异常是地幔引起的冻结速率变化的结果。
Seismic waves sampling the top 100 km of the Earth's inner core reveal that the eastern hemisphere (40 degrees E-180 degrees E) is seismically faster(1,2), more isotropic(2,3) and more attenuating(4) than the western hemisphere. The origin of this hemispherical dichotomy is a challenging problem for our understanding of the Earth as a system of dynamically coupled layers. Previously, laboratory experiments have established that thermal control from the lower mantle can drastically affect fluid flow in the outer core(5), which in turn can induce textural heterogeneity on the inner core solidification front(6). The resulting texture should be consistent with other expected manifestations of thermal mantle control on the geodynamo, specifically magnetic flux concentrations(7,8) in the timeaverage palaeomagnetic field(9,10) over the past 5 Myr, and preferred eddy locations(11) in flows imaged below the core-mantle boundary by the analysis of historical geomagnetic secular variation(12). Here we show that a single model of thermochemical convection and dynamo action can account for all these effects by producing a large-scale, long-term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle. The main feature of this thermochemical 'wind' is a cyclonic circulation below Asia, which concentrates magnetic field on the core-mantle boundary at the observed location and locally agrees with core flow images. This wind also causes anomalously high rates of light element release in the eastern hemisphere of the inner core boundary, suggesting that lateral seismic anomalies at the top of the inner core result from mantle-induced variations in its freezing rate.