Ocean cavity regime shift reversed West Antarctic grounding line retreat in the late Holocene

The West Antarctic Ice Sheet (WAIS) holds a substantial volume of ice, and its potential instability poses a significant threat to global sea levels. While the Holocene is generally considered a period of climate stability, geological evidence indicates that the WAIS was smaller during the middle Holocene (8.2–4.2 ka BP) than it is today. In the Ross Embayment, studies using radiocarbon dating of subglacial till from ice streams like the Whillans and Mercer ice streams (WIS and MIS) show a retreat of hundreds of kilometers between 7.5 and 5.3 ka BP, followed by a readvance within the last 1.7 ka. The causes of this grounding line retreat and subsequent readvance remain unclear. Two leading hypotheses are: (1) changes in isostatic rebound rates due to the Earth's response to ice sheet melting and (2) regional climate changes altering ice dynamics. Distinguishing between these hypotheses is crucial for understanding WAIS vulnerability to future climate change. The Ross Embayment, while relatively stable in recent times, is projected to be vulnerable to future ocean warming. The Siple Coast grounding line is currently buttressed by the Ross Ice Shelf, which overlays a cold ocean cavity, limiting basal melting. However, the possibility exists that warmer waters have interacted with the WAIS ice streams in the past, particularly when the bed topography was lower or polynya activity reduced. Previous studies have linked transitions from warm to cold ocean cavities to grounding line retreat slowing, but whether such a shift explains the Holocene grounding line reversal remains unexplored. An alternative explanation for WAIS retreat and advance, first suggested for the Weddell Embayment, proposes that lower modern bed uplift rates result from extensive grounding line retreat during the last deglaciation, with subsequent isostatic rebound leading to regrounding. While this mechanism has been explored using ice sheet models, prior simulations lacked the precise age constraints now available from marine exposure dating of subglacial till, leading to discrepancies in timing. This study aims to test these competing hypotheses by using a coupled ice sheet-GIA modeling approach to analyze the interplay between the ice sheet, solid Earth, and ocean during the Ross Ice Shelf's development. The study focuses on the Ross Embayment to assess the relative contributions of GIA and ocean thermal forcing to Holocene grounding line migration.

Existing research points to a significant retreat of the West Antarctic Ice Sheet (WAIS) grounding line in the Ross Embayment during the mid-Holocene, followed by a readvance in the late Holocene. Radiocarbon dating of subglacial sediments provides age constraints on this retreat and advance, indicating a substantial shift in grounding line position. However, the driving mechanisms behind these changes remain debated. Some studies attribute the retreat to changing rates of isostatic rebound, where the Earth's crust slowly rises in response to ice sheet unloading. Others emphasize the role of regional climate change, particularly variations in ocean temperature and the extent of sea ice. Previous ice sheet modeling efforts have attempted to simulate these changes, with varied success. The accuracy of these models is often limited by uncertainties in Earth's rheological parameters, the complexity of ice sheet dynamics, and the limited understanding of past ocean conditions. These previous studies highlighted the difficulty of definitively isolating the relative contributions of isostatic rebound and climate change. The goal of this current study is to improve upon previous attempts by utilizing a more comprehensive modeling approach and incorporating recently refined age constraints from geological data.

This study uses an ensemble of ice sheet model (ISM) simulations coupled with global glacioisostatic adjustment (GIA) model simulations to investigate the relative contributions of ocean thermal forcing and GIA to Holocene grounding line migration in the Ross Embayment. The Parallel Ice Sheet Model (PISM) is employed for ISM simulations, run from 40 to 0 ka BP. These simulations incorporate varying mantle viscosity, lithosphere flexural rigidity, and mantle density to represent different Earth structures. Surface climate forcing is derived from WAIS Divide ice cores, while standard ocean forcing is derived from the TraCE-21ka climate model. However, due to limitations in TraCE-21ka's resolution of ice shelf cavities, modified ocean forcing experiments are also conducted, imposing temperature anomalies in the Ross Sea region during the Holocene. These experiments test the grounding line response to plausible changes in ocean forcing, considering the timing and magnitude of temperature changes suggested by proxy data. The ice thickness outputs from the ISM simulations serve as input for a 1D GIA model, incorporating a range of radially varying Earth structures to capture GIA and local sea-level change. An iterative coupling between the ice sheet and GIA models considers the feedback of GIA on the late Holocene ice-sheet readvance. The study focuses on two end-member cases of Earth structure: GIAWE (relatively weak) and GIASE (relatively strong). These different Earth structures allow for the examination of sensitivity of model results to variations in Earth's rheological properties. The model predictions are then compared to available geological proxy data, such as radiocarbon dating of subglacial sediments, to assess the plausibility of the different scenarios. The methodology also examines the influence of bed topographic changes on ice sheet response to oceanic cooling by conducting additional ice sheet advance experiments with fixed beds and beds derived from the GIA models. These advance experiments help to evaluate the contribution of bed uplift to the readvance.

The study's ice sheet model simulations demonstrate a strong sensitivity of grounding line migration to both mantle viscosity and ocean thermal forcing. Higher mantle viscosity leads to earlier and more extensive retreat for a given ocean forcing. Simulations with low mid-Holocene ocean temperature anomalies show regrounding in areas of raised bed topography. Higher ocean temperature anomalies delay the minimum grounding line extent. Comparison of modeled grounding line position to radiocarbon age constraints from subglacial sediments reveals that simulations with ocean temperature anomalies of at least +0.5°C show sufficient retreat at various ice stream locations (Whillans, Mercer, Kamb, and Bindschadler). Simulations with lower mid-Holocene ocean temperature anomalies and low mantle viscosity do not accurately reproduce the timing and extent of observed retreat and readvance. The inclusion of gravitational, rotational, and deformational (GRD) effects using a global GIA model improves the accuracy of bed topography change predictions compared to a simplified viscoelastic deformation model. The GIA model simulations show that local bed subsidence occurs as the ice sheet regrounds, suggesting isostatic rebound is not the primary driver of ice sheet advance. Analysis of ice stream responses to changes in ocean forcing reveals that basal melt rates and horizontal ice flux increase as the ice sheet thins and retreats. The subsequent cooling leads to reduced basal melt rates, ice shelf thickening, regrounding, and grounding line advance. Experiments examining the influence of bed topographic changes show that the ice sheet response to ocean cooling is robust and consistent with geological age constraints, regardless of bed topography changes over the past 2.0 ka. This confirms the hypothesis that modest oceanic cooling is responsible for the late Holocene ice sheet readvance. The study found that a transition from a warm to cold ocean cavity is the more plausible explanation for the grounding line reversal, while acknowledging GIA's significant influence on the ice sheet's response to oceanic changes. The paleoclimate reconstructions also support the hypothesis, showing a warm ocean cavity during the retreat phase and a transition to a cold cavity in the late Holocene, coinciding with changes in sea ice extent, polynya activity, and wind patterns. The simulations demonstrate the grounding line's sensitivity to even temporary intrusions of warmer water.

The findings strongly suggest that a shift in ocean cavity regime from warm to cold is the primary driver of the observed Holocene grounding line reversal in the Ross Embayment. While isostatic rebound plays a role, its influence is secondary to the effect of ocean temperature changes. The rapid readvance following ocean cooling emphasizes the ice sheet's responsiveness to relatively modest changes in oceanic conditions. The results highlight the importance of accurately representing past ocean conditions in ice sheet models to accurately predict future WAIS behavior. The relatively high mantle viscosity and lithosphere flexural rigidity of the Siple Coast region contribute to the grounding line's sensitivity to increases in ocean thermal forcing, making this area particularly vulnerable to future changes in ocean circulation. Differences in the timing of readvance among various ice streams may be attributed to variations in shelf water mass composition and the isolation of different sections of the cavity from warmer waters. Future research could explore the role of smaller-scale climate variations, such as the El Niño Southern Oscillation and Southern Annular Mode, on grounding line oscillations. Furthermore, incorporating improved representations of sub-ice shelf ocean circulation within higher-resolution regional climate models is needed to better understand interactions between the ocean and ice sheet.

This study demonstrates that a warm-to-cold ocean cavity regime shift, rather than isostatic rebound, is the most likely cause of the late Holocene West Antarctic Ice Sheet grounding line readvance in the Ross Embayment. The findings emphasize the significant impact of ocean thermal forcing on grounding line dynamics and highlight the vulnerability of this region to future changes in sub-ice shelf ocean circulation. Future research should focus on improving the representation of sub-ice shelf ocean processes and incorporating higher-resolution regional climate models to enhance the accuracy of future predictions.

The study's use of a 1D GIA model simplifies the complex three-dimensional nature of the Earth's response to ice sheet loading. The representation of past ocean conditions, while improved upon previous work, still relies on proxy data and climate model outputs with inherent uncertainties. The model's parameterization of ice shelf basal melting might not fully capture all the relevant processes, and the coarse resolution of the ice sheet model may not fully represent small-scale variations in grounding line behavior. Furthermore, the study focuses primarily on the Ross Embayment, and the findings may not be directly generalizable to other sectors of the WAIS.