Geodynamic Insights on Critical Climate Events in Earth History

• Main Author: Coulson, Sophie Leanne
• Other Authors: Mitrovica, Jerry X., Fu, Roger, Fischer, Rebecca, Huybers, Peter
• Format: Thesis
• Language: English
• Published: 2021
• Subjects: Mantle Dynamics; Paleoclimate; Sea Level Change; Geophysics; Paleoclimate science; Arctic; Antarctic; Baffin Island; Greenland; Antarc*; Baffin; glacier; Ice Sheet
• Online Access: https://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37368220

This interdisciplinary thesis aims to elucidate processes responsible for sea-level changes during climate events spanning the Miocene to present day, using the tools of numerical geodynamics and theoretical geophysics. Understanding critical events in Earth’s climate history provides valuable insight into the natural variability of ice sheets and sea level; a pressing issue for recognizing anomalous behavior and projecting future changes. In the following chapters, I evaluate the impacts of mantle flow and crustal deformation, across a wide range of timescales, on sea-level changes, topography and ice sheet evolution of the ice-age Earth system. On timescales of millions of years, I investigate the mantle convection process using state-of-the-art models to track perturbations in dynamic topography and their potential link to ancient ice sheet stability. By extending models of glacial isostatic adjustment (GIA), I capture viscoelastic processes acting on the crust and sea surface, on timescales from years to tens of thousands of years, that have been overlooked in previous modeling. In Chapter 1, I evaluate the role of solid-Earth processes in priming the Canadian Arctic for glacial inception in the Northern Hemisphere ~2.7 Ma. I present a range of model-generated dynamic topography predictions and review the geological constraints on dynamically supported topography across Baffin Island. These two independent lines of evidence suggest large scale uplift across northeast Baffin Island, with a significant gradient in elevation change northeast-southwest. Model-generated dynamic topography predictions show uplift of 57-88 m since 5 Ma in most simulations. This change in elevation likely had a substantial impact on long-term trends in climate through the Plio-Pleistocene, driving regional cooling and altering snowfall patterns. Chapters 2-4 focus on applying and extending state-of-the-art glacial isostatic adjustment calculations to determine the gravitational and deformational signal associated with changes in sea level in water bodies that are isolated from the global ocean. In Chapter 2, I use an approximate treatment to model sea-level changes due to oscillations in water level in the Mediterranean Sea during the first phase of the Messinian Salinity Crisis (5.96 Ma). I show that sea-level changes at the Gibraltar Strait depart dramatically from global average sea level, affecting the connectivity between the Mediterranean and Atlantic Ocean. I also consider a model of the cyclical flooding and emptying of the Mediterranean driven by a competition between tectonic uplift and erosion at the Gibraltar sill. Incorporating realistic sea-level physics significantly alters the uplift and erosion parameters that are required to generate the periodicity evident in the stratigraphy. In particular, I find that a tectonic uplift rate at the Gibraltar sill three times smaller than previously published, yields periodic sea-level curve when sea-level physics is included. This revised uplift rate is in line with rates estimated by geodynamic models appropriate for the Gibraltar arc. In Chapter 3, I present an extended ice-age sea-level theory governing gravitationally self-consistent, spatio-temporal sea-level changes within an ocean-plus-lake system intermittently connected by water flux across a sill. This new framework advances previous treatments by allowing water to redistribute within and across the two water bodies in a gravitationally-self consistent manner without pre-defining flood volumes or geometry. I illustrate the new theory using case studies of Black Sea flooding during the last deglacial phase and sea-level fall in the Mediterranean Sea during the Messinian Salinity Crisis. These examples demonstrate the importance of including the geophysical feedbacks associated with sea-level change in an isolated basin in the dynamics of both flooding and desiccation. In Chapter 4, I apply this extended sea-level theory to an idealized Earth system to explore sea-level feedbacks on megaflooding events. I vary both the ratio of ocean-to-lake size and the effective height of water breaching the sill to constrain which scenarios experience strong positive feedbacks that result in complete flooding of the lake within a short period. My preliminary findings suggest that the timescale of flooding for events caused by sill breach from the global ocean - for example the reflooding of the Mediterranean during the Messinian Salinity Crisis, and the periodic flooding of the Black Sea, Red Sea and Persian Gulf within the Pleistocene ice-age cycles - will be limited only by the efficiency of water flux across the sill. In a final chapter, I focus on solid-Earth deformation associated with modern ice-mass loss from ice sheets and glaciers. I show that the fingerprint of melting ice from the Greenland and Antarctic ice sheets and mountain glaciers is not limited to glaciated areas, but is characterized by a global pattern of 3-D crustal deformation. I compute "far-field" vertical and horizontal deformation rates that occurred in response to 21st century mass flux between these areas and the ocean. Both the Greenland Ice Sheet and high latitude glacier systems each generated average crustal motion of 0.1-0.4 mm/yr across much of the Northern Hemisphere, with significant year-to-year variability in magnitude and direction. Horizontal motions associated with melting exceed vertical rates in many far-field areas, and thus both should be considered in future analysis of GNSS measurements. Taken together, this thesis aims to deepen our understanding of the role of solid-Earth processes on sea-level change and ice sheet stability, and advance numerical modeling techniques to account for these interactions.