Jan Šafanda

The thermal regime of the crystalline continental crust: Implications from the KTB

An extensive geothermal research program within the German Continental Deep Drilling Program (KTB) covered an almost complete spectrum of experimental and theoretical aspects. The main results and conclusions are as follows: (1) Equilibrium temperature is 118.6°C at 4000 m in the KTB pilot borehole (KTB-VB) and will be around 260°C at 9100 m in the KTB main borehole (KTB-HB). Time required for thermal equilibration of the KTB-HB to within 1% of the initial perturbation will be about 13–16 years. (2) The failure to predict temperature correctly for the KTB was mainly due to an unaccounted perturbation by a transient ground surface temperature history. (3) Pleistocene surface temperature variations affect the present-day crustal temperature between 1.3 to 2.7 K up to a depth of 4000 m. Accordingly, present-day heat flow density is systematically too low down to approximately 1500 m. Model simulations indicate that groundwater flow does not eliminate paleoclimatic signals, even though it may translate them to a depth incompatible with both their diffusive age and their amplitude. These results emphasize the importance of an adequate consideration of paleoclimatic effects for the interpretation of thermal data. (4) Lateral heat transport is significant when steep inclination and folding of the rock formations coincide with contrasts in thermal conductivity. This is indicated by typical variations in the vertical components of temperature gradient and heat flow density, such as in the KTB-HB. In contrast, thermally relevant advection of heat is confined to the top 500–1000 m. In the vincinity of the KTB, this is about twice the maximum difference in elevation. (5) In the deeper crust, free convection systems require permeabilities greater than 10−17 m2 for large rock volumes, but simple numerical models indicate that the associated temperature regimes are imcompatible with KTB borehole data. (6) Heat production rate shows no systematic variation with depth and is related to lithology at the KTB as in other deep boreholes in crystalline rock. Numerical models, using heat production rate derived from seismic velocities, yield temperatures compatible with KTB borehole data.

Air‐ground temperature coupling and subsurface propagation of annual temperature signals

[1] Borehole-based reconstructions of ground surface temperature (GST) have been widely used as indicators of paleoclimate. These reconstructions assume that heat transport within the subsurface is conductive. Climatic interpretations of GST reconstructions also assume that GST is strongly coupled to surface air temperature (SAT) on timescales of decades and longer. We examine these two assumptions using records of SAT and subsurface temperature time series from Fargo, North Dakota; Prague, Czech Republic; Cape Henlopen State Park, Delaware; and Cape Hatteras National Seashore, North Carolina. The characteristics of downward propagating annual temperature signals at each site clearly indicate that heat transport can be described as one-dimensional conduction in a homogeneous medium. Extrapolations of subsurface observations to the ground surface yield estimates of annual GST signals and allow comparisons to annual SAT signals. All annual GST signals are modestly attenuated and negligibly phase shifted relative to SAT. The four sites collectively demonstrate that differences between annual GST and SAT signals arise in both summer and winter seasons, in amounts dependent on the climatic setting of each site. INDEX TERMS: 1645 Global Change: Solid Earth; 1875 Hydrology: Unsaturated zone; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3367 Meteorology and Atmospheric Dynamics: Theoretical modeling; KEYWORDS: heat transport, air-ground termperature coupling, paleoclimate

Daily, seasonal, and annual relationships between air and subsurface temperatures

[1] Inversions of borehole temperature profiles that reconstruct past ground surface temperature (GST) changes have been used to estimate historical changes in surface air temperature (SAT). Paleoclimatic interpretations of GST reconstructions are based on the assumption that GST and SAT changes are closely coupled over decades, centuries, and longer. This assumption has been the subject of some debate because of known differences between GST and SAT at timescales of hours, days, seasons, and years. We investigate GST and SAT relationships on daily, seasonal, and annual timescales to identify and characterize the principal meteorological changes that lead to short-term differences between GST and SAT and consider the effects of those differences on coupling between the two temperatures over much longer time periods. We use observational SAT and subsurface data from Fargo, North Dakota; Prague, Czech Republic; Cape Henlopen State Park, Delaware; and Cape Hatteras National Seashore, North Carolina. These records comprise intradaily observations that span parts of one or two decades. We compare subsurface temperature observations to calculations from a conductive subsurface model driven with daily SAT as the surface boundary condition and show that daily differences exist between observed and modeled subsurface temperatures. We also analyze year-to-year spectral decompositions of daily SAT and subsurface temperature time series and show that dissimilarities between mean annual GST and SAT are attributable to differences in annual amplitudes of the two temperature signals. The seasonal partitioning of these amplitude differences varies from year to year and from site to site, responding to variable evapotranspiration and cryogenic effects. Variable year-to-year differences between mean annual GST and SAT are closely estimated using results from a multivariate regression model that associates the partial influences of seasonal meteorological conditions with the attenuation of annual GST amplitudes.

Heat flow models across the Trans-European Suture Zone in the area of the POLONAISE’97 seismic experiment

Abstract Heat flow data from the Polish basin show a sharp change in the transition from the East European Craton (EEC) and Teisseyre–Tornquist Zone (TTZ) in the north-east to the accreted terranes in the south west (Paleozoic Platform). The analysis of this data and numerical modelling of the crustal temperatures show evidence of extensive crustal–mantle warming in the area between the Sudetes to the south and the Trans-European Suture Zone to the north. The change in heat flow is 100% when compared with values for the EEC. Heat flow in the anomalous zone is also higher than in the Sudetes. The axis of the anomaly is aligned with the Dolsk Fault and Variscan deformation front. Low crustal/mantle temperatures derived from the relationship between temperature and P n velocities (more than 8.2 and as high as 8.4 km/s) are at odds with high crustal temperatures calculated from surface heat flow, seismic velocity based heat generation models and thermal conductivity. High heat flow (Variscan platform) and related high temperatures of the crust coincide with small crustal thickness (30–35 km). The opposite is the case for the low heat flow EEC (45–50 km). High heat flow above thin crust and low heat flow above thick crust with no major variation in elevation is supported by a simple isostatic balance model. Crustal heat generation explains part of the high heat flow within the zone with thick meta-sediments reaching down to 20 km depth, however, it is far from explaining high heat flow in Variscan crust and in the transition zone into a cold EEC. 2D numerical models of heat flow based on new seismic data require a contrast of 15 mW/m 2 in mantle heat flow. High mantle heat flow (35–40 mW/m 2 ) is likely to occur in the high heat flow zone while cold crust and cold and high-density mantle (mantle heat flow of 20–30 mW/m 2 ) is typical of the EEC. Thermal lithosphere thickness for the craton is 200 km while it is only 100 km in the accreted terranes to the southwest of the TTZ. The TTZ in Poland appears as a relatively cold area.

Deep temperature distribution along three profiles crossing the Teisseyre-Tornquist tectonic zone in Poland

Abstract The Teisseyre-Tornquist tectonic zone is a large deep fracture zone separating the Precambrian East European craton from the Variscan system of Central Europe. With the use of ample deep seismic sounding data, the detailed crustal temperature distribution was calculated for three profiles crossing this zone in its northern, central and southern segments. The conversion technique of transforming the reported seismic velocities ( υ p ) into heat production values ( A ) was used together with the temperature-dependent thermal conductivity structure and the observed surface heat flow pattern. In addition to the obtained 2-D temperature distribution, this work allowed a more reliable estimate of the crustal contribution to the surface heat flow, and thus the characteristic Moho heat flow ( Q M ~ 26 mW m −2 ), to be assessed in this region. The results can also serve as an independent confirmation of the necessity of applying the υ p − A conversion separately for Phanerozoic and for Precambrian rocks.

Signature of the last ice age in the present subsurface temperatures in the Czech Republic and Slovenia

Abstract Ground surface temperature (GST) history can be evaluated by analysing the present-day temperature-depth profiles measured in boreholes. Due to the diffusive character of the process, however, the resolution of the method decreases quickly for the more remote events. The reconstructed GST at a given moment in the past is a weighted average of temperature over a certain period of time. The present study shows that because the cold climate of the last (Weichselian) glacial prevailed in the period of 75–10 ka, there is a chance to obtain its mean GST, despite the large averaging intervals, from temperature profiles measured in deep boreholes. This fact is demonstrated on the GST inversions of carefully selected profiles, three from the Czech Republic and two from Slovenia, the depth of which ranges between 1.5 and 2.4 km. They yield GST histories with a minimum between 19 and 10 ka followed by a warming of 6–15 K. In order to suppress the nonclimatic noise and to extract the common signal, the joint inversions were carried out for the Czech and the Slovenian boreholes, respectively. The Czech data show the minimum at 17 ka and the warming of 8 K. The Slovenian data have the minimum at 16 ka and the warming amounts to 7 K. These results agree well with information extracted earlier from the German KTB super deep borehole, where the inversion of the temperature log indicated 10 K warming since the glacial minimum, and represent an independent estimate of the difference between glacial and interglacial conditions typical for the region of Central Europe.

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).

Numerical modelling of permafrost in bedrock in northern Fennoscandia during the Holocene

Abstract The occurrence of permafrost in bedrock in northern Fennoscandia and its dependence on past and presently ongoing climatic variations was investigated with one- (1D) and two-dimensional (2D) numerical models by solving the transient heat conduction equation with latent heat effects included. The study area is characterized by discontinuous permafrost occurrences such as palsa mires and local mountain permafrost. The ground temperature changes during the Holocene were constructed using climatic proxy data. This variation was used as a forcing function at the ground surface in the calculations. Several versions of the present ground temperature were applied, resulting in different subsurface freezing–thawing conditions in the past depending on the assumed porosity and geothermal conditions. Our results suggest that in high altitude areas with a cold climate (present mean annual ground temperature between 0°C and −3°C), there may have been considerable variations in permafrost thickness (ranging from 0 to 150 m), as well as periods of no permafrost at all. The higher is the porosity of bedrock filled with ice, the stronger is the retarding effect of permafrost against climatic variations. Two-dimensional models including topographic effects with altitude-dependent ground temperatures and slope orientation and inclination dependent solar radiation were applied to a case of mountain permafrost in Yllas, western Finnish Lapland, where bedrock permafrost is known to occur in boreholes to a depth of about 60 m. Modelling suggests complicated changes in permafrost thickness with time as well as contrasting situations on southern and northern slopes of the mountain. Extrapolating the climatic warming of the last 200 years to the end of the next century when the anticipated increase in the annual average air temperature is expected to be about 2 K indicates that the permafrost occurrences in bedrock in northern Fennoscandia would be thawing rapidly in low-porosity formations. However, already a porosity of 5% filled with ice would retard the thawing considerably.

Dispersed organic matter from Silurian shales of the Barrandian Basin, Czech Republic: optical properties, chemical composition and thermal maturity

Abstract Samples of lower Silurian (Liten and Kopanina formations) dark shales from the Barrandian basin, Czech Republic, were examined using reflected light microscopy, Fourier-Transform infrared spectroscopy (FT-IR) and gas chromatography-mass spectrometry (GC-MS). The samples contained uniform assemblage of organic material that included dominant graptolites, chitinozoa, bitumens and minor anthracite to meta-anthracite rank particle of unknown origin. Two types of graptolite morphologies were recognized: a nongranular blocky type and a granular type. Bitumen is also common in these rocks and forms discrete, oval bodies resembling droplets or angular particles with granular internal fabrics. The thermal maturity of the sediments was examined by measuring random reflectance of organic particles. The variations in graptolite reflectance in between three localities in the western part of the basin were relatively minor, suggesting a similar level of diagenetic transformation. The values range from 0.78% to 1.53% R r . The reflectance of graptolites was correlated with the reflectance of chitinozoa and bitumen, and also indirectly with illite crystallinity data, which indicates thermal maturation levels in the high-temperature part of the oil window to the onset of wet gas/condensate zone. Mature petroleum that commonly impregnates veins crosscutting the sequence further supports a maturation level within the oil window. Graptolite and bitumen reflectance values markedly elevated above the regional diagenetic background were found in several centimeter-wide contact zones immediately adjacent to basalt sills that locally penetrate the Silurian strata. As shown by computer modeling, heating in the range of 600–800 °C, which lasted only several years, was sufficient to promote an increase in reflectance (up to 2–2.5% R r ) in the contact samples. With increasing thermal stress, the graptolite periderm undergoes progressive optical and structural changes being gradually converted into a highly condensed aromatic residuum structurally and chemically similar to highly matured kerogen. On a basin-wide scale, however, the overall impact of basalt intrusions on organic maturity of enclosing shales was minimal.

Large ground surface temperature changes of the last three centuries inferred from borehole temperatures in the Southern Canadian Prairies, Saskatchewan

Abstract New temperature logs in wells located in the grassland ecozone in the Southern Canadian Prairies in Saskatchewan, where surface disturbance is considered minor, show a large curvature in the upper 100 m. The character of this curvature is consistent with ground surface temperature (GST) warming in the 20th century. Repetition of precise temperature logs in southern Saskatchewan (years 1986 and 1997) shows the conductive nature of warming of the subsurface sediments. The magnitude of surface temperature change during that time (11 years) is high (0.3–0.4°C). To assess the conductive nature of temperature variations at the grassland surface interface, several precise air and soil temperature time series in the southern Canadian Prairies (1965–1995) were analyzed. The combined anomalies correlated at 0.85. Application of the functional space inversion (FSI) technique with the borehole temperature logs and site-specific lithology indicates a warming to date of approximately 2.5°C since a minimum in the late 18th century to mid 19th century. This warming represents an approximate increase from 4°C around 1850 to 6.5°C today. The significance of this record is that it suggests almost half of the warming occurred prior to 1900, before dramatic build up of atmospheric green house gases. This result correlates well with the proxy record of climatic change further to the north, beyond the Arctic Circle [Overpeck, J., Hughen, K., Hardy, D., Bradley, R., Case, R., Douglas, M., Finney, B., Gajewski, K., Jacoby, G., Jennings, A., Lamourex, S., Lasca, A., MacDonald, G., Moore, J., Retelle, M., Smith, S., Wolfe, A., Zielinski, G., 1997. Arctic environmental change of the last four centuries, Science 278, 1251–1256.].