Imprints of Geodynamic Processes on the Paleoclimate Record

Abstract

In this thesis I investigate how solid Earth deformation associated with glacial isostatic adjustment and mantle convection impacted ice age climate. In particular, I discard approximations that treat the Earth's internal properties as radially symmetric and demonstrate that lateral variations in viscosity and density within the Earth's mantle play an important role in understanding and interpreting surface observations.

At the beginning of this thesis, I turn my attention to the Last Glacial Maximum, ~ 21 kyr ago. Estimates of the globally averaged sea level low stand, or equivalently maximum (excess) ice volume, have been a source of contention, ranging from -120 m to -135 m. These bounding values were obtained by correcting local sea level records from Barbados and northern Australia, respectively, for deformation due to glacial isostatic adjustment using 1-D viscoelastic Earth models. I demonstrate that including laterally varying mantle structure, and particularly the presence of a high viscosity slab consistent with seismic imaging and the tectonic history of the Caribbean region, leads to a significant reinterpretation of the Barbados sea level record. The revised analysis places the sea level low stand at close to -130 m, bringing it into accord with the inferred value from northern Australia within their relative uncertainties.

In the following three chapters I explore the effects of dynamic topography on sea level records during past warm periods. Dynamic topography is supported by viscous flow and buoyancy variations in the Earth's mantle and lithosphere. I begin by developing a theoretical framework for computing gravitationally self-consistent sea level changes driven by dynamic topography and then combine this framework with models of mantle convective flow to investigate two important time periods in the geologic past. First, I examine the Last Interglacial (LIG) period, approximately 125 kyrs ago, which is considered to be a recent analogue for our warming world. I show that changes in dynamic topography since the LIG are on the order of a few meters, making them a non negligible source of uncertainty in estimates of excess melting during this time period. Second, I turn to the mid-Pliocene warm period (MPWP), ca. 3 Ma ago, which is a more ancient analogue for climate of the near future since temperatures were elevated, on average by ~ 2ºC. Dynamic topography has been shown to significantly deform the elevation of shoreline markers of mid-Pliocene age, particularly along the U.S. Atlantic coastal plain. It has also profoundly altered bedrock topography within the Antarctic over the last 3 Myr. I couple my dynamic topography calculations to an Antarctic Ice Sheet model to explore this previously unrecognized connection and find that changes in topography associated with mantle flow have a significant effect on ice sheet retreat in the marine-based Wilkes basin, suggesting levels of ancient instability that are consistent with offshore geological records from the region. This finding indicates that the degree to which the mid-Pliocene can be regarded as an analogue for future climate is complicated by large-scale dynamic changes in the solid Earth.

In the final section of this thesis, I move to the surface record of large igneous provinces (LIPs) - which are often cited as mantle flow induced drivers of critical events in Earth's ancient climate - and examine whether the location of LIPs carries information about the stability of large-scale structures in the deep mantle that have been imaged by seismic tomography. In particular, I investigate the spatial correlation between LIPs, which are the surface expression of deep sourced mantle plumes, and large low shear wave velocity provinces (LLSVPs) at the core mantle boundary. A correlation between LIPs and margins of LLSVPs has been used to argue that LLSVPs are thermochemical piles that have been stationary over time scales exceeding many hundreds of millions of years. My statistical analysis indicates that there is a statistically significant correlation between LIPs and the overall geographic extent of LLSVPs, and this admits the possibility that LLSVPs may be more transient, thermally dominated structures. I conclude that given the limited record of LIPs, one cannot distinguish between the two hypotheses that they are correlated with the edges or the areal extent of the LLSVPs.