Interactions Between Long-Term Mantle Dynamics and the Evolution of Earth’s Surface Volatiles and Climate

Abstract

Elucidating the interactions between Earth's surface and interior is crucial for understanding Earth's long-term climate cycles and surface geochemical processes. In this thesis, I investigate the feedbacks between Earth's mantle dynamics and volatile and climate cycles on timescales ranging from millions to billions of years. In Chapters 1 and 2, I focus on the 4 Ga plate tectonic evolution of Earth and the potential feedbacks involving volatile cycles. The goal of Chapter 1 is to examine how a change in plate tectonic style may influence the outgassing flux of CO2 at ocean ridges. I combine an analytic model of mantle convection with a petrologic model of mantle melting in the presence of CO2 to compute mantle temperature, plate speed, melt production, and CO2 outgassing flux at ocean ridges since the Hadean. A large suite of realistic mantle and lithospheric parameters are explored to map out a full range of possible thermal histories. The results show that in order to satisfy thermal constraints, the Earth must have started in a sluggish-lid mode of plate tectonics (where the mantle speed is much greater than the plate speed) and then transitioned to an active-lid mode (where the plates are strongly coupled to the mantle). Furthermore, I show that plate speed and CO2 outgassing flux do not necessarily scale with mantle temperature, and that it is possible to reach present-day mantle temperatures and plate speeds with a simple force balance without invoking any feedbacks (e.g. grain size evolution, dehydration stiffening) or a fully stagnant-lid mode of convection in the Precambrian. The solutions show a range of evolutionary behaviors depending on the parameters chosen. The transition from sluggish- to active-lid mode can be smooth, abrupt, or be characterized by an intermediate time scale. A smooth transition is difficult to distinguish from a purely active-lid evolutionary path based on temperature, plate speed, melt production, and CO2 outgassing flux. In contrast, an abrupt transition leads to a large increase in the average plate speed with a corresponding increase in melt production and CO2 outgassing flux. Since an abrupt transition would have a much larger effect on CO2 outgassing and melt production than a change in ridge length, I investigate the impact of rapidly changing plate speeds and do not consider changes in ridge length. Finally, I show that carbon recycling is required for a large part of Earth history in order to explain present-day CO2 outgassing. The model highlights the importance of understanding the style of mantle convection when calculating melt production and volatile fluxes through Earth’s history. In Chapter 2, I expand on the thermal evolution model of Chapter 1 by incorporating a deep water cycle where water is brought into the mantle at subduction zones and released from the mantle at ocean ridges. Water is an important element in mantle convection as it has the ability to significantly reduce mantle viscosity and hence influence mantle dynamics. The results show that, even with a water cycle, a long phase of sluggish-lid tectonics is required to reach present-day constraints and that the transition may still be smooth, abrupt, or of intermediate time scale. The overall evolution of the thermal state of the planet and plate tectonic regimes is primarily explained by the forces acting on the plate boundary. However, the water cycle contributes through its effect on mantle viscosity. Specifically, all of the models that go through a phase of sluggish-lid tectonics before transitioning to an active-lid mode have significantly higher initial water contents than the runs that do not transition. This suggests that the Archean Earth may have had higher water content than present-day. Finally, I explore the sensitivity of these results to the depth of hydrous alteration adopted in the regassing parameterization. Again, the general thermal and tectonic evolution remain similar, but the history of the regassing-to-degassing ratio is sensitive to the assumed depth of the hydration. This demonstrates the potential importance of subduction zone metamorphism in models of early Earth climate and volatile cycles. In Chapter 3, I investigate the feedback between mantle dynamics and paleoclimate cycles on timescales of 1-10s of millions of years. The purpose is to revisit the enigma proposed by Morrow et al. (2012) who showed that the two observational estimates of Earth's oblateness (i.e. dynamic ellipticity) based on deep sea sediment cores --- one with a 3 Myr time scale and the other with a 25 Myr time scale --- are contradictory. I calculate Earth's dynamic ellipticity using models of glaciostatic adjustment (GIA) and novel mantle convection simulations based on the adjoint method. While GIA models show decreasing oblateness over the past 3 Myr, mantle convection models predict increasing oblateness going back 25 Myr. Further, estimates from both the GIA models and mantle convection models are inconsistent with the two observational constraints. However, these results suggest systematic inaccuracies in the observational estimates. To illustrate this issue, I reassess the estimate of dynamic ellipticity over the past 3 Myr from Lourens et al. (2001) incorporating the updated orbital modeling software of Laskar et al. (2010) and find that the dynamic ellipticity changes are higher, and of opposite sign, than expected from physical modeling of GIA and mantle convection. This indicates that either the analysis method is not robust or the sediment core used is not representative of actual climate cyclicity. Finally, I discuss the importance of establishing observational constraints at multiple time periods in order to distinguish between different models of the geophysical processes that perturb the oblate form of the Earth over geological time.