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Abstract

A 1.7 km long equilibrium temperature log from borehole Litoměřice, Czechia, measured 13 years after drilling provided a detailed knowledge of temperature gradient. Its most conspicuous feature is a gradual increase with depth from 22.0 K/km to 25.6 K/km within a lithologically homogeneous depth section 900–1700 m occupied by mica-schist. Conductivity and diffusivity was measured on drill-core samples from the upper part of this borehole section. Due to (i) the homogeneous lithology of mica-schist, (ii) a small scatter of the measured conductivity values and (iii) no trend in the dip angle of foliation, we assumed no depth trend of conductivity in the section 900–1700 m other than that resulting from pressure-temperature corrections. However, they introduce only a slight trend with values decreasing from 3.34 W/(m.K) at 900 m to 3.29 W/(m.K) at 1700 m, i.e. about 1.5 %, that cannot explain the observed 16% increase of the temperature gradient. We interpreted the increase as a transient feature generated by long-term ground surface temperature changes during the last glacial cycle. The inversion technique applied to the temperature log section 900–1700 m indicated the magnitude of the last glacial–Holocene warming of 13–15 K and the occurrence of a temperature minimum 15–20 ka. The long-term mean ground surface temperature of 1–2 °C suggests that the borehole site was permafrost-free for most of the last glacial cycle. The existence of about 100 m deep permafrost is possible in the coldest part of the last glacial.