Not only research on ice cores from Antarctica or Greenland can be a source of valuable information about the climate on the Earth hundreds of thousands of years ago. It appeared quite recently that a unique geothermal-hydrochemical anomaly observed in north-eastern Poland, within the so-called Suwałki Anorthosite Massif, may significantly broaden our knowledge about the palaeoclimate. Research results will also help to identify factors influencing the recent thermal regime of the Earth.

Climatic changes and related temperature variations of the Earth’s surface result in changes in lithosphere temperature – both within near-surface strata and in deep zones of sedimentary and crystalline rocks resting several kilometres beneath the ground surface.
How is it possible?
The reason is very low velocity of thermal energy transfer in rocks. It enables preservation of information about climatic changes on Earth in the distant past, recorded at depths of several hundreds to thousands of metres.
The last glaciation, that ended approximately 14,000 years ago, even now influences the thermal regime of outer zones of the Earth’s crust.

The Suwałki anomaly

In the mid-1970s, a distinct temperature inversion was recorded in a number of deep boreholes drilled in crystalline rocks of north-eastern Poland in search for polymetallic (iron-vanadium-titanium) ores.
Borehole temperatures, instead of increase with depth... showed a decreasing trend!
The minimal temperature was observed in Lower Cretaceous sandstones whose top surface occurs here at depths of 380-440 m.

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Temperature inversion in boreholes drilled in the Suwałki Anorthosite Massif 

At that time, a hypothesis was proposed that this phenomenon might have been related to the last glaciation. It was claimed that the temperature inversion was caused by deep infiltration of cooled postglacial waters originating from melting of an ice sheet (Majorowicz, 1976, 1982). Scientific considerations, associated with an analysis of groundwaters in Mesozoic deposits, allowed to put forward a hypothesis, innovative of its time, that the very deep permafrost occurs in this area (Michalski, 1985). It was stated that the observed types of groundwater are the result of cryogenic alterations. Estimates were made that the permafrost reached a depth of nearly 600 m.

It was quite recently that scientists, based on analyses of heat flow variations, discovered that thermal disturbances related to Pleistocene-Holocene climatic changes are recorded in boreholes practically throughout the entire area of Poland.

Renewed investigations of the Suwałki geothermal-hydrochemical anomaly, conducted in association with the Geophysical Institute of the Academy of Sciences of the Czech Republic, confirmed that the anomaly is related to the deep permafrost that occurred in this area in the past. This is the only site yet recognized in Central Europe where signs of so deeply seated permafrost has been directly detected. It also proves that mean annual temperatures were very low in this area during most of the late Pleistocene.

A warming – but how strong?

Although a lot of information about climatic conditions during the last glaciation has lately been collected, the average value of mean annual temperatures for that period still remains a very controversial issue. Research methods such as pollen analysis, measurements of oxygen (18O/16O) and hydrogen (D/H) isotopic ratios, as well as tree-ring dating and investigations of deep-water sediments are the methods of indirect determination of palaeotemperature – they require calibration to the temperature scale. The precision of these methods dramatically decreases for stadial (cool) periods, dominant in the climatic interval considered.

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Relative temperature changes during last 160,000 years determined based on ice cores from Vostok station, Antarctica, (Petit et al. 1999). The periods when the Vistulian Glaciation ice-sheet probably covered north-eastern Poland are also shown (Behre K.E. 1989 - Biostratigraphy in the last glacial period in Europe. Quatern. Sc. Res. 8) Eemian Vistulian Glaciation ice-sheet cover in north-eastern Poland Pleistocene Holocene relative temperature changes time (thousands of years BP)

Geothermal modelling, performed based on the newest data from the Suwałki anomalous area, has shown that the rapid increase in temperature during ice-sheet retreat in north-eastern Poland was in excess of 18 Celsius degrees! (Šafanda, Szewczyk, Majorowicz, 2004, Geophysical Research Letters, no. 31).  Thus, the previous opinion that the average temperature increase did not exceed 8 Celsius degrees, must be revised.

It should be stressed that, in contrast to polar areas where climatic changes during the Vistulian Glaciation were accompanied by considerable increase in ice thickness (e.g. >1500 m in central Antarctica - Petit et al. Nature 1999, vol. 399), climatic changes in the Suwałki region during the same period were not associated with any change in deposit thickness – the total thickness of Vistulian Glaciation deposits is merely 6-10 m in this area. Reduced thicknesses significantly facilitate palaeotemperature calculation, so the modelling becomes more reliable.

Specific conditions

Precise temperature measurements, performed in hydrogeological boreholes of this region in 2003-2004, unambiguously confirmed the occurrence of both temperature inversion and the trend of increasing the inversion area towards the centre of the massif.

Analysis of factors that might have influenced the formation, and then preservation of such deep permafrost in the Suwałki region indicates that the simultaneous and independent occurrence of low heat-flow value and very porous highly water-saturated sedimentary overburden were of essential importance.

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A preliminary model of permafrost deterioration in the Udryń region calculated in cooperation with the Geophysical Institute of the Academy of Sciences of the Czech Republic

Low heat-flow value, resulting from very low natural radioactivity of anorthosites composing the Suwałki Massif, was the reason for the occurrence of subzero temperatures down to a depth of nearly 600 m during the last glaciation. The overburden played a role of an insulating layer, contributing to the prolongation of the process of permafrost formation. However, the layer also retarded the process of permafrost degradation initiated when the climate rapidly got warmer.

Correcting the map

A significant change in surface temperatures resulted in substantial disturbance of thermal regime in subsurface layers, even to depths of 2000 m. The disturbances highly affected the Earth's heat flow. Previously constructed maps of heat flow did not take the disturbances into account and hence the maps are incorrect.

The newest map of heat flow in the Polish Lowlands (yet unpublished), constructed at the Polish Geological Institute in 2004, for the first time involves the influence of the palaeoclimatic factor, due to, among others, results of analyses of geothermal data from the Suwałki Massif.

The Szypliszki project

It is planned to drill a 450 m-deep borehole to thoroughly examine the thermal profile in the area of the Suwałki geothermal-hydrochemical anomaly. The optimum site for such a borehole is a locality at the village of Szypliszki in the center of the anomaly.

The borehole will be drilled within the framework of a PGI research project entitled integrated program of shallow drilling experiments aimed at solving substantial geological problems of Poland, under the leadership of Dr Jerzy Nawrocki. The project is financed by the National Fund for Environmental Protection and Water Management.

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Topography of north-eastern Poland with the study area marked. Location of the projected Szypliszki PIG-1 borehole is shown against the background of the Bouguer gravity anomaly (isolines) delimiting the area of the Suwałki Anorthosite Massif. 

The primary task of the drilling project is to determine the extent to what the remains of permafrost have been preserved in the central part of the Suwałki Massif. In particular, it is intended to examine the temperature profile under a stable thermal regime, the type and chemical composition of groundwater, the amount and age of alterations, and the degree of preservation of frost structures in drilling cores, and to determine petrophysical parameters, including thermal ones. Results of these investigations will be a base, among others, for a palaeoclimate modelling allowing a reconstruction of climatic conditions during the late Pleistocene.

It is expected that the borehole will reach a total depth of 450 m, piercing Lower Cretaceous deposits in which the best-preserved remains of permafrost are supposed to be encountered. Crystalline basement rocks occur at a depth of about 840 m below ground level in this area. Depth to the top of Lower Cretaceous deposits, which are the main objective of the project, is estimated at approximately 391 m.

There is evidence that signs of permafrost had remained preserved here less disturbed by the Holocene warming, and locally the permafrost might have even survived into recent times.

The projected borehole will enable a direct verification of research hypothesis. Geophysical, lithological and hydrogeological investigations will elucidate to what extent permafrost deterioration processes are advanced and will allow more accurate evaluation of the mean effective temperature in the late Pleistocene. The scientific result of this unique experiment will carry an essential significance for the recognition of climatic conditions during that period not only in the area of Europe.

Dr Jan Szewczyk
Polish Geological Institute - Hydrogeology and Engineering Geology Department

translated by Krzysztof Leszczyński