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.
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.
