Comment on "exceptionally high heat flux needed to sustain the Northeast Greenland Ice Stream" by Smith-Johnsen et al. (2020)

. Smith-Johnsen et al. (The Cryosphere, 14, 841– 854, https://doi.org/10.5194/tc-14-841-2020, 2020) model the effect of a potential hotspot on the Northeast Green-land Ice Stream (NEGIS). They argue that a heat ﬂux of at least 970 mW m − 2 is required to have initiated or to con-trol NEGIS. Such an exceptionally high heat ﬂux would be unique in the world and is incompatible with known geological processes that can raise the heat ﬂux. Fast ﬂow at NEGIS must thus be possible without the extraordinary melt rates invoked in Smith-Johnsen et al. (2020).

away from the central Iceland hotspot, especially underneath Greenland, as shown by, for example, the temperature at 80-150 km depth beneath Iceland and the adjacent Atlantic Ocean (Fig. 8 in Lebedev et al., 2017). Fahnestock et al. (2001) base their inferred high heat flux on the depths of stratigraphic ice layers up to 9000 yr in age, 40 suggesting that the heat flux has at least been so high for the last few thousands of years. A steady-state 970 mW m -2 heat flux would imply a local geothermal gradient close to a staggering ca. 400 °C km -1 at which felsic rocks would melt at about 2 km depth. Although Fahnestock et al. (2001) suggest that the local bedrock topography is consistent with volcanism, there is no independent evidence for volcanism that is expected above such shallow melting. intrusions. This is in line with Stevens et al. (2016), who conclude, regarding melt, that "ice-age cycling could help it migrate upward to shallow depth or erupt, contributing to the high observed geothermal heat flux", but with the caveat "if melt occurs at depth". The conclusion is based on the vug-wave magma-transport model of Morgan and Holtzman (2005), which is similar to the mobile-hydrofracture transport model of Bons (2001) and Bons et al. (2001). Magma transport in vug waves or mobile hydrofractures may be enhanced by ice-age cycling or tectonic events, but this will only have effect if 65 magma is present in the source region. The question remains if and why this would be the case underneath the upstream area of NEGIS. Furthermore, the same magma-transport mechanism also applies to igneous activity in hotspots such as Iceland.
If the geothermal heat flux there is only raised locally to <350 mW m -2 , it is unlikely that it would be raised three times more in the Greenland crust where there is no obvious evidence or reason for significant igneous activity.
Another potential cause for the high heat flux that is invoked by Smith-Johnson et al. (2020) (and others e.g., Artemieva, 70 2019) is hydrothermal fluid flow, which is the flux of hot fluids from deeper levels in the crust that typically leave mineral deposits (Oliver et al., 2006). An indication of the fluid flux required to achieve 0.1 m/yr basal melting can be obtained by assuming that the melting is achieved by 100 °C aqueous fluids that melt basal ice at 0 °C while themselves cooling down to 0 °C. Using a heat capacity of 4.2 kJ kg -1 K -1 and a latent heat of 334 kJ kg -1 for melting ice, we obtain a required fluid flux of ~2·10 -6 kg m -2 s -1 (or ~0.07 m 3 m -2 yr -1 ). This is more than three orders of magnitude more than the 2-7·10 -10 kg m -2 s -1 75 expected for metamorphic fluid fluxes (Connolly and Thompson, 1989) that could potentially provide the hot fluids. Even the much lower estimated melting rate of 6.1 mm/yr of Buchardt and Dahl-Jensen (2007) would require >10 times the mass of hot fluid than expected. Hydrothermal fluid flow can therefore not produce all the heat required for a significantly elevated basal melting rate.
Uranium enrichments are known in southern Greenland in the Gardar Province (e.g., Bartels et al., 2016), and their 80 https://doi.org/10.5194/tc-2020-339 Preprint. Discussion started: 17 December 2020 c Author(s) 2020. CC BY 4.0 License. radiogenic heat production can add to the geothermal heat flux directly, and indirectly through enhanced hydrothermal fluid flow, as is the case in the uranium-rich Mt. Painter Inlier in South Australia (Weisheit et al., 2013) where the geothermal heat flux is raised to about 120 mW m -2 (Sandiford et al., 1998). In the sediments above the world's largest known U-deposit, Olympic Dam in South Australia, the geothermal heat flux is raised by only 43 mW m -2 from a background value of 73 mW m -2 (Houseman et al., 1989). 85

Conclusions
In summary, a heat flux of 970 mW m -2 is geologically unfeasible. Any heat flux above about 100-150 mW m -2 should be treated with caution in the absence of other evidence, such as volcanic or tectonic activity. Most other studies actually do propose much more moderate and realistic geothermal heat flux values below the Greenland Ice sheet (e.g. Buchardt and Dahl-Jensen, 2007;Rogozhina et al., 2016;Rezvanbehbahani et al., 2017, Artemieva, 2019