title: The role of fully coupled ice sheet basal processes in quaternary glacial cycles
Creators: name: Drew, Matthew
Publisher: name: Memorial University of Newfoundland
Year: 2023

description: Bed conditions such as meltwater pressurization and unconsolidated sediment cover (soft versus hard bedded) strongly impact ice sheet sliding velocities. How the dynamical processes governing these conditions affect glacial cycle scale ice sheet evolution has been little studied. The influence of subglacial hydrology and glacial sediment production and transport is therefore largely unknown. Here I present a glaciological model Glacial Systems Model (GSM) with the to-date most complete representations of fully coupled subglacial hydrology and sediment production and transport for the glacial cycle continental scale context. I compare the influence of of several types of subglacial hydrology drainage systems on millennial scale variability and examine the role dynamical sediment processes potentially played in the mid-Pleistocene Transition (MPT) from 41 to 100 kyr glacial cycles. Subglacial hydrology has long been inferred to play a role in glacial dynamics at decadal and shorter scales. However, it remains unclear whether subglacial hydrology has a critical role in ice sheet evolution on greater than centennial time-scales. It is also unclear which drainage system is most appropriate for the continental/glacial cycle scale. Here I compare the dynamical role of three subglacial hydrology systems most dominant in the literature in the context of surge behaviour for an idealized Hudson Strait scale ice stream. I find that subglacial hydrology is an important system inductance for realistic ice stream surging and that the three formulations all exhibit similar surge behaviour. Even a detail as fundamental as mass conserving transport of subglacial water is not necessary for simulating a full range of surge frequency and amplitude. However, one difference is apparent: the combined positive and negative feedbacks of the linked-cavity system yields longer duration surges and a broader range of effective pressures than its poro-elastic and leaky-bucket counterparts. The MPT from 41 kyr to 100 kyr glacial cycles was one of the largest changes in the Earth system over the past million years. A change from a low to high friction base under the North American Ice Complex through the removal of pre-glacial regolith has been hypothesized to play a critical role in the transition to longer and stronger glaciations. However, this hypothesis requires constraint on pre-glacial regolith cover as well as mechanistic constraints on whether the appropriate amount of regolith can be removed from the required regions to enable MPT occurrence at the right time. This is the first study to test the regolith hypothesis for a realistic 3D North American ice sheet that treats regolith removal as a system internal process instead of a forced soft to hard transition. The fully coupled climate, ice, subglacial hydrology, and sediment physics capture the progression of Pleistocene glacial cycles within parametric and observational uncertainty. Incorporating the constraint from estimates for the present day sediment distribution, Quaternary erosion, and Atlantic Quaternary sediment volume suggests the mean Pliocene regolith thickness was 40 m or less. Given this constraint, I compare the simulated soft to hard bed transitions with the timing inferred for the MPT. The combined constraint, bedrock erosion, and sediment transport poses a challenge to the Regolith Hypothesis: denudation occurs well in advance of the MPT and the hard bedded area stays largely constant by 1.5 Ma. Furthermore, I examine the sensitivity of glacial cycle evolution to the initial thickness of the regolith in the absence of erosion. Surprisingly, thicker regolith does not delay the transition but produces large glacial cycles in the early Pleistocene even extending the length of some. This is due to the effect from higher topography on ice sheet mass balance. Therefore, I suggest that the regolith removal mechanism is not singularly responsible for the MPT, but that the MPT results from changes in many aspects of the systems. One of these aspects which remains under-studied in the literature is the long term evolution of glacierized beds over the Pleistocene.