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

Abstract

Abstract. 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.

Sub-permafrost methane seepage from open-system pingos in Svalbard

“…Figure 4 shows that all measured δ 13 C-CH 4 values in the pingo springs compare well with the results of the pore gas extractions (range −53 ‰ VPDB to −69 ‰ VPDB) from the upper core sections at the CO 2 Well Park (Well Site B in Fig. 2a; data from Huq et al, 2017). Here, the methane in the permafrost and underlying host rocks of the subpermafrost aquifer has been attributed to a biogenic source because the δ 13 C-CH 4 values are moderately 13 C-depleted (i.e.…”

“…These values were assumed to indicate the deeper geogenic methane source because ethane and propane were also detected at significant concentrations relative to the methane (see also Ohm et al, 2019). Figure 4 also shows that the δ 13 C-DIC values (range −8.5 ‰ to +26 ‰ VPDB) observed in the pingo springs do not compare well with the values from the lower shalerich units of the rock cores either (range −26 ‰ to +21 ‰ VPDB; Huq et al, 2017). This difference cannot be attributed to differences in the DIC speciation among our water samples (containing CO 2(aq) , H 2 CO 3 , HCO − 3 and CO 2− 3 ) and the published rock pore gas samples (CO 2(g) only).…”

“…> 300 m; see Fig. 4) at Well Site B by Huq et al (2017). These values were assumed to indicate the deeper geogenic methane source because ethane and propane were also detected at significant concentrations relative to the methane (see also Ohm et al, 2019).…”

“…There are no taliks beneath the river because river discharge volumes drop rapidly in late August which allows freezing to commence early in the winter. Although the typically fine-grained, frozen marine sediment infill in the valley has a low hydraulic conductivity, the underlying glacial tills, and in particular the upper (unfrozen) geological strata beneath that, seem to support important sub-permafrost fluid migration pathways (Huq et al, 2017;Hornum et al, 2020: Fig. 2b).…”

“…Earlier work has shown that the fractured sandstones host an important biogenic methane-rich aquifer, whilst the mudstones form an effective flow boundary that seems to suppress the upward migration of its own geogenic methane resource (Huq et al, 2017). The gas-rich upper sandstone aquifer therefore contains few hydrocarbons other than methane, whilst in the lower mudstone successions, ethane and propane have been detected at levels indicative of a geogenic gas source (Huq et al, 2017;Ohm et al, 2019). Fluid migration through the outcropping mudstones to the Innerhytte and Riverbed pingos is therefore likely to exploit faults (shown conceptually in Fig.…”