Groundwater Exfiltration’s Feedback on Ice Flow
Insights from recent study in Science Advances by Robel et al. (2023)
The uncertainties surrounding sea-level projections have long been influenced by the complex dynamics of ice sheets in Greenland and Antarctica1,2. A pivotal factor in these projections is ice dynamics, driven by bed properties and water presence at the ice-bed interface3,4. Despite extensive research, the precise influence of these factors remains inadequately understood.
Significant portions of both ice sheets are underlain by thick sediment layers that can contain saturated water throughout their thickness 5,6. These water sources include basal ice melt, groundwater, and paleo seawater. The movement of ice and the deformation of sediment, both influenced by water, shape the ice dynamics of sedimentary beds. Changes in the water budget at the base can consequently impact ice dynamics.
Under changing ice loading, groundwater from sediment layers can discharge to the ice-bed interface—a phenomenon termed “exfiltration.” This process not only affects ice dynamics but also amplifies ice melt rates at the ice-ocean interface through the entrainment of warmer ocean water. Additionally, the exfiltrated groundwater carries warmth and nutrients that can influence marine biogeochemistry.
Robel et al.7 study focuses on Antarctica and its ice sheet’s thinning in relation to potential groundwater exfiltration. By leveraging the ice sheet thickness change data from satellite remote sensing spanning 2003 to 2019, they estimate the extent of exfiltration and infiltration in the Antarctic basal water budget. Their findings highlight:
- Exfiltration contributes 10% of the total Antarctic basal water budget.
- For West Antarctica, it constitutes 21%, and for Thwaites and Pine Island glaciers, which are rapidly retreating, it accounts for 38%.
- Exfiltration in Antarctica accounts for half of previously predicted basal meltwater in places, while in other regions with observed thinning but no anticipated basal melt, the source of subglacial water is solely attributed to exfiltration.
Robel et al. predict that under present thinning rates, the exfiltration rate could double, potentially quadrupling with higher thinning rates projected by credible models by 2100. This suggests a dynamic relationship between ice sheet thinning and exfiltration.
Despite the study’s well-established theoretical framework, lingering uncertainties are attributed to constrained understanding of subglacial sediment properties. This includes aspects such as permeability, specific storage capacity, and groundwater measurement, all of which are hindered by the inaccessibility of the subglacial environment.
The study postulates that the cumulative water flux resulting from exfiltration could significantly contribute to subglacial discharge into the ocean. Particularly in the most rapidly thinning region of Antarctica, exfiltration might amplify the rate of subglacial discharge, potentially affecting the melting rate at the ice-ocean junction. As our climate warms, the Antarctic ice sheet is projected to undergo rapid thinning, intensifying exfiltration rates. However, the precise extent of the influence on ice dynamics remains uncertain due to the current lack of a comprehensive understanding of subglacial hydrology.
Exfiltration’s influence on ice dynamics holds implications for both Greenland and Antarctica. While Greenland’s subglacial water is dominated by surface melt in summer, exfiltration could play a pivotal role in sustaining subglacial drainage in winter and explaining observed discrepancies in winter subglacial water pressure. Sophisticated subglacial observations and the inclusion of groundwater in subglacial hydrology models are crucial for accurate ice sheet projections.
Robel et al.’s study underscores the significance of exfiltration’s impact on ice dynamics and the importance of thorough subglacial observations. The research sheds light on a novel avenue of inquiry, exploring the effects of exfiltration through numerical ice sheet modelling. Ultimately, this study contributes to advancing our understanding of ice sheet behaviour and its implications in the context of a changing climate.
1 Goelzer, H. et al. The future sea-level contribution of the Greenland ice sheet: a multi-model ensemble study of ISMIP6. The Cryosphere 14, 3071-3096, doi:10.5194/tc-14-3071-2020 (2020).
2 Seroussi, H. et al. ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century. The Cryosphere 14, 3033-3070, doi:10.5194/tc-14-3033-2020 (2020).
3 Kazmierczak, E., Sun, S., Coulon, V. & Pattyn, F. Subglacial hydrology modulates basal sliding response of the Antarctic ice sheet to climate forcing. The Cryosphere 16, 4537-4552, doi:10.5194/tc-16-4537-2022 (2022).
4 Maier, N., Gimbert, F. & Gillet-Chaulet, F. Threshold response to melt drives large-scale bed weakening in Greenland. Nature 607, 714-720, doi:10.1038/s41586-022-04927-3 (2022).
5 Walter, F., Chaput, J. & Luethi, M. P. Thick sediments beneath Greenland’s ablation zone and their potential role in future ice sheet dynamics. Geology 42, 487-490, doi:10.1130/g35492.1 (2014).
6 Gustafson, C. D. et al. A dynamic saline groundwater system mapped beneath an Antarctic ice stream. 376, 640-644, doi:doi:10.1126/science.abm3301 (2022).
7 Robel, A. A., Sim, S. J., Meyer, C., Siegfried, M. R. & Gustafson, C. D. Contemporary ice sheet thinning drives subglacial groundwater exfiltration with potential feedbacks on glacier flow. 9, eadh3693, doi:doi:10.1126/sciadv.adh3693 (2023).
