Ground surface-temperature reconstruction based on data from a deep borehole in permafrost at Janssonhaugen, Svalbard

Analyses of the geothermal gradient in permafrost areas constitute a key signal of the ground-surface temperature history. Permafrost temperatures in selected areas are particularly well suited to reconstructing past surface-temperature changes, mainly because there is no thermal disturbance due to circulating groundwater. One year of temperature data from an instrumented 102 m deep borehole in permafrost on Janssonhaugen, Svalbard is presented. Ground thermal properties are calculated. The average value for the thermal conductivity is 1.85 +/- 0.05 52 Wm(-1)K(-1), and the average value for the thermal diffusivity is 1.1 m(2) s(-1), which gives a phase speed for the annual wave of 5.65 x 10(-2) m d(-1). The depth of zero annual amplitude is 18 m. The permafrost thickness is estimated as approximately 220 m. Analysis of the temperatures reveals an increasing temperature gradient with depth. Using a heat-conduction inversion model, a palaeoclimatic reconstruction is presented, showing a warming of the surface temperature over the last 60-80 years. The temperature profile represents a regional signal on Svalbard, which shows an inflection associated with near-surface warming of 1.5 +/- 0.5 degreesC in the 20th century.