Core-Mantle Interaction and Evolution of the Geodynamo

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。非技术解释：这项研究的目的是了解地球深处两个主要区域——固体地幔和大部分为液态的金属地核之间的相互作用。根据最可靠的估计，有大量的热量正在穿过地核-地幔边界流动——总量接近10太瓦，与当今全球工业能源生产相当。这种巨大的热量传递在地质时期的累积效应对人类来说意义重大，因为它们影响到地球系统的所有部分。在地核-地幔边界之上，热量为地幔的缓慢翻转提供了浮力，从而推动了地表附近的板块构造运动，这是地震和火山爆发的最终原因。在地核-地幔边界以下，地核热量的逐渐散失使固态内核得以结晶，并在仍处于熔融状态的外核中产生对流流动。反过来，外核的对流就像一个自我维持的电磁流体发电机，产生了地磁场的主要部分，这是地球抵御太阳风高能粒子的第一道防线。我们计划利用三条主要的证据来理解这个地球深处的引擎是如何一起工作的。首先，下地幔的地震成像为我们提供了线索，让我们了解当今地核-地幔边界上方缓慢翻转流动的性质。其次，最近在固体内核中发现的地震结构提供了内部结晶的证据。第三，保存在海洋地壳中的地磁极性反转记录为我们提供了地球动力学历史的磁记录，包括没有反转的长期超年表以及地磁场频繁反转的时期。我们的首要目标是通过建立一个地核-地幔相互作用的全地球数值模型，统一这些看似不同的观测结果。我们将模拟地幔侧的缓慢翻转流，并将其与地震图像相结合，以更好地约束来自核心的热流。在边界的核心一侧，我们将计算内核如何随时间结晶，并将其生长历史与观测到的地震结构联系起来。最后，我们将模拟地球发电机在地质时期内受地幔和内核过程影响的演化，以阐明在其反转记录中保存的地球发电机历史与地球深部系统的其他两个部分之间的联系。技术描述：我们将使用两种不同的方法，以新颖的方式结合各种数值工具，在地质时间重要部分的核心-地幔相互作用的广泛背景下研究地球动力学。首先，我们计划对地核、地球动力和地磁场在过去100 Ma的连续演化进行建模，并对它们自固体内核成核以来的不连续演化阶段进行建模。在这段时间里，行星的自转速度、外核的组成和温度、内核的大小和结构，以及从地核到下地幔的热流的大小和分布，都发生了显著的动态变化。地球发电机对这些变化的响应在上一次磁超时以来极性反转频率的增加中是明显的，并提供了一个重要的进化约束。我们将通过在长时间尺度上持续改变发电机控制参数和边界条件来模拟地球发电机的演化，包括地核的冷却、地核-地幔边界热条件的变化、内核的生长和地球的潮汐减速，并受到地核体积平均热历史模型、潮汐减速和地幔历史的板块重建的约束。我们将从演化的发电机模型与非演化的发电机模型以及古地磁记录中比较极性反转行为和其他磁场统计的趋势。我们将进行相当于100迈历的发电机演化模拟，从白垩纪正常超时开始到现在。其次，我们将采用三维球面地幔对流代码来计算内核在其生长的连续阶段的亚固体流，这是由内核边界的表面载荷驱动的，这是由于内核本身的非均匀凝固速率和热化学对流。内核流动的边界条件将来自发电机模型的输出。这些计算将为确定内核结构的可能来源及其对观测到的内核地震非均质性的贡献提供基础。我们还将研究具有内部产热和可压缩性的时间相关地幔对流，以约束核心-地幔边界热流与下地幔非均质性之间的关系（作为地幔参数的函数），从而更好地约束核心边界和地表热流时间变化之间的相位关系。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Non-technical explanation:The purpose of this research is to understand the interaction between the two major regions of Earth's deep interior, the solid mantle and the mostly liquid metallic core. According to best estimates, a very large amount of heat is flowing across the core-mantle boundary -- a total of nearly 10 terawatts, comparable to the present-day global industrial energy production. The cumulative effects of this enormous heat transfer over geologic time are significant for mankind, as they affect all parts of the Earth System. Above the core-mantle boundary, the heat provides buoyancy for the slow overturn of the mantle that drives plate tectonic motions near the surface, the ultimate cause of earthquakes and volcanoes. Below the core-mantle boundary, the progressive loss of heat from the core allows crystallization of the solid inner core and also produces convective flow in the still-molten outer core. In turn, the convection in the outer core acts as a self-sustaining electromagnetic fluid dynamo, generating the main part of the geomagnetic field, our planet's first line of defense from high-energy particles in the solar wind.We plan to exploit three major lines of evidence to understand how the pieces of this deep Earth engine work together. First, seismic imaging of the lower mantle offers clues to the present-day nature of the slow overturning flow above the core-mantle boundary. Second, recently-discovered seismic structure in the solid inner core provides evidence of how the inner crystalized. And third, the record of geomagnetic polarity reversals preserved in the ocean crust gives us a magnetic recording of the geodynamo history, including long-lasting superchrons devoid of reversals as well as periods when the geomagnetic field reversed frequently.Our overarching goal is to unify these seemingly diverse observations, by building a whole-Earth numerical model of core-mantle interaction. We will model the slow overturning flow on the mantle side and combine these with the seismic images to better constrain the heat flow from the core. On the core side of the boundary, we will calculate how the inner core has crystallized over time, and connect its growth history to its observed seismic structure. Finally, we will model the evolution of the geodynamo over geologic time as influenced by the mantle and inner core processes just described, to clarify the links between the history of the geodynamo preserved in its reversal record and the two other parts of the deep Earth system.Technical description:We will investigate the geodynamo in the broad context of core-mantle interaction over significant portions of geologic time, using two different approaches that combine a variety of numerical tools in novel ways.First, we plan to model the evolution of the core, the geodynamo, and the geomagnetic field continuously over the past 100 Ma, and model their evolution in discontinuous stages since the nucleation of the solid inner core. During these periods of time there have been dynamically-significant changes in the rate of planetary rotation, outer core composition and temperature, inner core size and structure, as well as the magnitude and distribution of heat flow from the core into the lower mantle. The response of the geodynamo to these changes is evident in the increased frequency of polarity reversals since the last magnetic superchron, and provides an important evolutionary constraint. We will model geodynamo evolution, including the cooling of the core, the change in thermal conditions at the core-mantle boundary, the growth of the inner core and the tidal deceleration of the Earth by continuously changing the dynamo control parameters and boundary conditions over long time-scales, as constrained by volume-averaged thermal history models of the core, tidal deceleration, and plate reconstructions of mantle history. We will compare the trends in polarity reversal behavior and other magnetic field statistics from the evolving dynamo models with their non-evolving counterparts and with the paleomagnetic record. We will conduct dynamo evolution simulations lasting the equivalent of 100 Myr, starting from the Cretaceous Normal Superchron to the present-day.Second, we will adapt a three-dimensional spherical mantle convection code to calculate the subsolidus flow in the inner core at successive stages of its growth, driven by surface loads at the inner core boundary due to heterogeneous rates of solidification and thermo-chemical convection within the inner core itself. Boundary conditions for the inner core flow will come from the dynamo model output. These calculations will provide a basis for determining possible sources of inner core texture and their contribution to the observed inner core seismic heterogeneity. We will also examine time-dependent mantle convection with internal heat generation and compressibility to constrain the relationship between heat flow at the core-mantle boundary and lower mantle heterogeneity as a function of mantle parameters, to better constrain the phase relationship between time variations in heat flow at the core boundary and the surface.