Physical properties of gneiss and amphibolite in central west Greenland: Implications for thermal regime in permafrost

Understanding the ground thermal regime in permafrost regions is important because seasonal and climatic temperature variations are related to human risks. Small local variations in the ground thermal regime can be accelerated by positive feedback loops leading to accelerated permafrost degradation. In central West Greenland, we hypothesize that mafic Kangâmiut dykes and their gneissic host rocks have contrasting thermal properties, and that it can lead to differences in the ground thermal regime across very small distances. To test this, we measure the thermal properties of gneiss and a Kangâmiut dyke as well as minor pegmatite and supracrustal rock in a drill core (GIOS2023-01) near Kangerlussuaq. Temperature measurements from the corresponding borehole allow us to calculate a local heat flux of 48 ± 4 to 51 ± 4 mW m–2. To understand the thermal conductivity under in situ conditions, we use ultrasonic P- and S-wave velocity as a proxy to evaluate its dependence on pore fluid and lithostatic pressure. Together, thermal properties and heat flux provide input to transient numerical thermal models that explore the ground thermal regime under different scenarios. The thermal properties of the gneiss (k = 3.1–3.4 W m–1 K–1, ϱ = 2603–2703 kg m–3, cP = 754–825 J kg–1 K–1) is clearly different from those of the amphibolitic dyke (k = 2.5–4.6 W m–1 K–1, ϱ = 3049–3088 kg m–3, cP = 660–672 J kg–1 K–1). One supracrustal sample (k = 3.03 W m–1 K–1, ϱ = 2926 kg m–3, cP = 695 J kg–1 K–1) is somewhat similar to the dyke, whereas a quartz-rich pegmatite is highly conductive (k = 6.9 W m–1 K–1, ϱ = 2622 kg m–3, cP = 893 J kg–1 K–1). Although ultrasonic velocities indicate that elastic moduli increase with axial stress and vary between gneiss and dyke, and between wet and dry state, these mechanical effects translate into negligible changes in thermal conductivity. For exposed bedrock, transient numerical thermal models suggest that the active layer thickness is several decimetres thicker in gneiss than in the dyke, and that the difference is in the order of metres when comparing gneiss or dyke to the pegmatite. With increasing sediment thickness overlying the bedrock, the difference in active layer thickness decreases. At a certain sediment thickness (~0.75 m with the chosen sediment properties), the active layers thicknesses reverse, i.e. active layers in the dyke are thicker than in gneiss (and pegmatite). This shift reflects two opposite effects: with no or little sediment cover, the relatively dense dyke is resistant to annual temperature variations. As sediment thickness increase, more of the temperature variations occurs in the sediment, and the dyke act as a heat sink, causing high temperature gradients and fast heat diffusion. As the sediment thickness exceeds the active layer thickness, the difference in active layer thickness between lithologies decreases to few centimetres. This small difference persists for several metres of sediment cover (~4 m with the chosen sediment properties). Metre-scale differences in active layer thickness have direct consequences for stability of rock slopes. Although centimetre-scale differences in active layer thickness will have limited influence on the stability of the sediment, it can accelerate by positive feedback loops. This insight could be validated in future field studies and provide input to predictive models of permafrost instability.