80,000 years ground surface temperature history inferred from the temperature–depth log measured in the superdeep hole SG-4 (the Urals, Russia)

Temperature and thermal conductivity data obtained from the Ural superdeep borehole SG-4 (58°24′N Lat., 59°44′E Long., Middle Urals, Russia) is used to assess ground surface temperature history (GSTH) over the past 80,000 years. GSTH is estimated by solving the one-dimensional heat conduction equation for a homogeneous semi-infinite medium and by comparing the solution with the measured temperatures by means of least square method. The resolution power of paleoclimate signal in the underground temperature field is theoretically studied and a parameter “relative duration” of the climatic event has been introduced. Relative duration is equal to η=(ts−tf)/ts, where ts and tf are the times before temperature logging of the beginning and end of a climatic event. Only events with η≥0.5 can be resolved from the temperature field. The reconstructed history reveals a number of past climatic events. These include the end of the Wurm glaciation and warming at the beginning of Holocene about 10,000 years ago, Holocene Optimum 4000–6000 years ago, Medieval Warm Period with a culmination about 1000 years ago and Little Ice Age 200–500 years ago. The amplitude of postglacial warming (12–13 K) inferred from the underground temperatures is greater than previously estimated. It is assumed that surface temperature variation amplitude is a function of the combined effects of surface air temperature changes and snow thickness changes. Statistical relations between mean annual soil–air temperature difference and snow cover thickness are proposed.