Core-Mantle Interaction and Evolution of the Geodynamo

核心-地幔相互作用和地球发电机的演化

• 批准号: 0909622
• 负责人: Peter Olson
• 金额: $ 41.34万
• 依托单位: Johns Hopkins University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-08-01 至 2013-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0909622&HistoricalAwards=false
• 关键词: Core; Mantle; Interaction; Evolution; Geodynamo

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.

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。非技术性解释：这项研究的目的是了解地球内部深处的两个主要区域之间的相互作用，固体地幔和大多数液体金属核心。根据最佳估计，有非常大量的热量流过地核-地幔边界-总共将近10太瓦，相当于当今全球工业能源生产。这种巨大的热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。在核幔边界之上，热量为地幔的缓慢翻转提供浮力，推动地表附近的板块构造运动，这是地震和火山的最终原因。在核幔边界之下，地核热量的逐渐流失使得固态的内核结晶，同时也在仍处于熔融状态的外核产生对流。 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，这是我们星球抵御太阳风中高能粒子的第一道防线。我们计划利用三条主要证据来了解这个地球深部发动机的各个部分是如何协同工作的。首先，下地幔的地震成像提供了线索，在核幔边界以上的缓慢翻转流的今天的性质。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发电机历史的磁性记录，包括长期没有反转的超年代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全球核幔相互作用的数值模型来统一这些看似不同的观测结果。我们将模拟地幔一侧的缓慢翻转流，并将这些与地震图像联合收割机结合起来，以更好地限制来自地核的热流。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的演化，并将其视为受到地幔和内核过程的影响，以阐明保留在其反转记录中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，以新颖的方式将各种数值工具联合收割机结合起来，在地质时期的重要部分，在核幔相互作用的广泛背景下研究地球发电机。我们计划模拟过去100 Ma以来地核、地球发电机和地磁场的连续演化，并模拟它们自固体内核成核以来的不连续阶段的演化。在这段时间里，行星自转的速度、外核的成分和温度、内核的大小和结构，以及从内核进入下地幔的热流的大小和分布都发生了动态的重大变化。地球发电机对这些变化的反应是明显的，自最后一个磁超年代以来，极性反转的频率增加，并提供了一个重要的进化约束。 我们将模拟地球发电机的演变，包括冷却的核心，在核-幔边界的热条件的变化，增长的内核和潮汐减速的地球通过不断改变发电机控制参数和边界条件在长时间尺度上，作为约束的体积平均热历史模型的核心，潮汐减速，和板块重建地幔历史。 我们将比较极性反转行为的趋势和其他磁场统计数据，从不断发展的发电机模型与他们的非不断发展的同行和古地磁记录。我们将进行相当于1亿年的发电机演化模拟，从白垩纪正常超时开始到现在。第二，我们将采用三维球形地幔对流代码来计算内核中连续生长阶段的亚固体流，由内芯边界处的表面载荷驱动，该表面载荷是由于内芯本身内的不均匀的固化速率和热化学对流。内核流的边界条件将来自发电机模型输出。这些计算将为确定内核结构的可能来源及其对所观察到的内核地震不均匀性的贡献提供基础。我们还将研究与时间相关的地幔对流与内部生热和压缩性，以约束热流在核幔边界和下地幔不均匀性作为地幔参数的函数之间的关系，以更好地约束热流在核边界和表面的时间变化之间的相位关系。