UC Santa Cruz

Isotopes of uranium and their radioactive decay products are important timekeepers for geologic and planetary processes, capable of recording precise dates across an enormous range of timescales and environments. This dissertationencompasses four distinct studies that employ the U decay systems to explore both time and physical processes in various geologic and planetary systems. While the focus of the chapters vary in both geologic and temporal setting, the studies fall under a unifying theme of pairing geochronologic methods with models that relate chronologies to Earth system and asteroidal processes.

The first two chapters use U-Pb thermochronology to measure high-temperature cooling processes in deep time (million-to-billion-year timescales). The first chapter interprets the cooling dates of U-Pb thermochronometers from middle-to-lower crustal xenoliths with thermal models to reconstruct the thermal history of the Superior craton, constraining the timing, temperature, and lithospheric response of the region to mantle plume heating 1.1 billion years ago. The second chapter interprets Pb-Pb cooling dates of phosphate minerals in LL ordinary chondrites with thermal models to reconstruct the size and timeframes of accretion of the LL parent planetesimal, revealing a large (>150 km diameter) asteroidal body that accreted rapidly after the formation of its constituent chondrule particles.

The latter chapters use intermediate decay products of the ²³⁸U-series to explore subglacial conditions over the Pleistocene. The third chapter explores physical and chemical weathering processes in the hyperarid polar environment of Taylor Valley, Antarctica and confirms that Taylor Glacier actively comminutes sediment at its base, challenging canonical assumptions that glaciers in the McMurdo Dry Valleys are non-erosive. The fourth chapter dates subglacial melting events beneath the northern Laurentide Ice Sheet and reveals their coincidence with Heinrich events, recurring episodes of voluminous iceberg discharge into the North Atlantic during the last glacial period. This terrestrial record of basal ice sheet processes corroborates subsurface ocean warming as the primary stimulus for Heinrich events and identifies subglacial aquifers and permafrost as important reservoirs involved in deglacial perturbations of the Atlantic Ocean uranium budget.

Collectively, the studies herein span timeframes that exceed the age of the Earth and peer into deep planetary interiors as well as the geosphere-cryosphere interface, each applying U decay systems in distinct ways to explore both geologic time and process.