Anomalously low heat flow density in eastern Karelia, Baltic Shield: a possible palaeoclimatic signature

Abstract

We report new heat flow density (HFD) values in seven drill holes in the Kamennye Lakes area in eastern Karelia, Russia, approximately at latitude 63°15′N, longitude 36°10′E. The investigated holes are 250–750 m deep and they intersect Archaean ultrabasic serpentinites and talc-carbonate rocks. Measured gradients range from 0.8 to 3.7 mK m −1 and the apparent HFD values from 2.4 to 11.6 mW m −2. The holes are not technically disturbed by fluid flow or any drilling effects. Average heat production of the rocks as analysed in the core samples of the deepest measured hole is 0.25 μ W m −3, but the low heat production is not a critical factor in producing the low HFD values. This is due to refraction of heat as shown with 2-D conductive simulations of heat transfer in a low heat-production formation surrounded by higher heat production. Hydrogeological disturbances can be ruled out by the presence of saline groundwater in the sections deeper than 150–400 m, and low topographic variation in the area, as well as Peclet number estimates, which suggest negligible convective heat transfer in the bedrock. All the temperature profiles are curved indicating recent palaeoclimatic disturbances. Inversion studies with singular value decomposition techniques yielded a climatic warming of about 1.0–1.5 K which started 150–200 years ago and was preceded by a cool period which lasted about 100 years. Nevertheless, recent climatic changes cannot explain the very low apparent HFD values, but long-period effects of the Weichselian glaciation are sufficient to decrease the HFD values to the levels measured. These effects were investigated with forward simulations and suggest that present temperature gradients in the range of 1–4 mK m −1 in the uppermost 1 km can be created by a very cold ground temperature (−10 to −15°C) during the glaciation time (60-11 ka ago).