Numerical modelling of permafrost spring discharge and open-system pingo formation induced by basal permafrost aggradation

• Main Authors: Hornum, Mikkel T., Hodson, Andrew J., Jessen, Søren, Bense, Victor, Senger, Kim
• Format: Text
• Language: English
• Published: 2020
• Subjects: Adventdalen; Arctic; Svalbard; open system Pingo; permafrost; wedge*
• Online Access: https://doi.org/10.5194/tc-2020-7; https://tc.copernicus.org/preprints/tc-2020-7/

In the high Arctic valley of Adventdalen, Svalbard, sub-permafrost groundwater feeds several pingo springs distributed along the valley axis. The driving mechanism for groundwater discharge and associated pingo formation is enigmatic because wet-based glaciers in the adjacent highlands and the presence of continuous permafrost seem to preclude recharge of the sub-permafrost groundwater system by either a sub-glacial source or a precipitation surplus. Since the pingo springs enable methane that has accumulated underneath the permafrost to escape directly to the atmosphere, our limited understanding of the groundwater system brings significant uncertainty to the understanding of how methane emissions will respond to changing climate. We address this problem with a new conceptual model for open-system pingo formation wherein pingo growth is sustained by sub-permafrost pressure effects during millennial scale basal permafrost aggradation. We test the viability of this mechanism for generating groundwater flow with decoupled heat (1D-transient) and groundwater (2D-steady-state) transport modelling experiments. Our results show that the pingos in lower Adventdalen easily conform to this conceptual model. Simulations suggest that the generally low-permeability hydrogeological units cause groundwater residence times that exceed the duration of the Holocene. The likelihood of such pre-Holocene groundwater ages is also supported by the hydrogeochemistry of the pingo springs, which demonstrate a sea-wards freshening of groundwater, potentially supplied by paleo-subglacial melting during the Weichselian. Such waters form a sub-permafrost fresh water wedge that progressively thins inland, where the duration of permafrost aggradation is longest. The mixing ratio of the underlying marine waters therefore increases in this direction because less unfrozen freshwater is available for mixing. Although this unusual hydraulic system is most likely governed by permafrost aggradation, the potential for additional pressurization is also explored. We conclude that methane production and methane clathrate formation may also affect hydraulic the pressure in sub-permafrost aquifers, but additional research is needed to fully establish their influence.