ANOMALOUSLY LOW HEAT-FLOW DENSITY IN EASTERN KARELIA, BALTIC SHIELD -A POSSIBLE PALEOCLIMATIC SIGNATURE

We report new heat flow density (HFD) values in seven drill holes in t he Kamennye Lakes area in eastern Karelia, Russia, approximately at la titude 63 degrees 15'N, longitude 36 degrees 10'E. The investigated ho les are 250-750 m deep and they intersect Archaean ultrabasic serpenti nites 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 eff ects. Average heat production of the rocks as analysed in the core sam ples of the deepest measured hole is 0.25 mu 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 m heat as shown with 2-D conductive simul ations of heat transfer in a low heat-production formation surrounded by higher heat production. Hydrogeological disturbances can be ruled o ut by the presence of saline groundwater in the sections deeper than 1 50-400 m, and low topographic variation in the area, as well as Peclet number estimates, which suggest negligible convective heat transfer i n the bedrock. All the temperature profiles are curved indicating rece nt 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 whi ch lasted about 100 years. Nevertheless, recent climatic changes canno t 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 s imulations 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 g round temperature (-10 to -15 degrees C) during the glaciation time (6 0-11 ka ago). (C) 1998 Elsevier Science B.V. All rights reserved.