Contact: Will Gosnold
Geology and Geological Engineering
University of North Dakota
Grand Forks, ND 58201-8358
willgosnold@mail.und.nodak.edu
Has Northern Hemisphere Heat Flow Been Underestimated?
Will Gosnold - University of North Dakota, Jacek Majorowicz - University of North Dakota, Jan Safanda - Geophysical Institute of the Czech Academy of Sciences, Jan Szewczyk - Polish Geological Institute
Point Two. Two recently published surface heat flow maps show anomalously low heat flow in the Canadian Shield in a
pattern that is coincident with the Wisconsinan ice sheet. The coincidence of low heat flow and ice accumulation has no
geophysical basis, thus the coincidence may suggest the existence of a transient signal caused by a warming event.
Recent studies of heat flow in North America indicate that in several sites, the ice base temperature was close to the
pressure melting point. We hypothesize that there may have been cold ice-free periods during the Pleistocene that would
account for the apparent colder surface temperatures.
Figure 4. T-z profiles from Poland showing
large near-surface warming effects.
50
30
10
Deg C
70
90
Figure 8. Heat Flow Map of the Canadian Shield
from Mareschal and Jaupart (2004) with
permission of J-C. Mareschal. Low heat flow
values coincident with glaciated regions suggest
that the thermal gradient may be perturbed by
significant post-glacial warming. Heat flow sites
are shown as white dots and the labels refer to
geologic provinces delineated by white lines.
1800
1600
1400
1200
1000
800
600
400
200
0
meters
140
120
40
Heat flow
100
35
80
30
60
25
Deg C
20
0
1000
2000
3000
4000
5000
6000
7000
Depth (m)
20
15
10
Observed
Steady state model
0
-5
0
100
200
300
400
Depth m
500
600
700
Figure 7. Geothermal Map of North America from Blackwell
and Richards (2004) with permission of M. Richards. The low
heat flow region in the Canadian Shield may be a remnant of
cold ice-free periods during the Pleistocene.
90
Heat Flow (mW/m^2)
70
60
50
40
30
20
10
0
0
1000
2000
3000
4000
5000
6000
7000
Depth (m)
Figure 3. There are significantly fewer deep boreholes in
North America, but the increase in heat flow with depth
appears in a suite of 759 sites in the IHFC Global Heat
Flow Database for the region east of the Rocky Mountains
and north of latitude 40 N. The red line is a linear least
squares fit to the data.
y = 0.0102x + 13.899
R2 = 0.9998
20
y = 0.0084x + 14.695
R2 = 0.9996
y = 0.0073x + 14.976
R2 = 0.9998
y = 0.0112x + 13.268
R2 = 0.9997
y = 0.0092x + 14.355
R2 = 0.9997
15
y = 0.0078x + 14.876
2
R = 0.9997
10
y = 0.022x
R2 = 1
0
0
Figures 5,6. T-z profiles from two highprecision temperature logs in deep boreholes
in the Williston Basin. The blue line is the
observed temperature and the red line is a
least squares fit to the bottom 100 m of the
data. Note that if the warming event was
3°C, the steady-state gradient would still be
about 10 percent low. However, until a
reliable thermal conductivity model for the
basin is available such profiles as this are
inconclusive.
35 - 40 mW m-2
Point 4. The signal is subtle and is not likely to be detected with
typical heat flow methods.
35
200
300
400
Depth (m)
500
600
700
800
Figure 11. A T-z profile resulting from a 15 K warming
event 10 ka would reveal the warming event only if the
entire curve was analyzed. The boxes in the figure above
give the equation of a line and the correlation coefficient
(R2) for a least-squares linear fit to the perturbed curve.
Temperature gradients measured in 100 m segments would
be accepted as linear even though a large post-glacial
warming signal is present. The only way such a subtle
signal can be recognized is to use multiple heat flow
determinations in deep (z > 2km) boreholes.
80%
0
400
800
Depth (m)
1200
60%
Kukkonen, I. T., Gosnold, W.D., Jr., and Safanda, J., Anomalously low
heat flow density in eastern Karelia, Baltic Shield: A possible
palaeoclimatic signature, Tectonophysics 291, p. 235-249, 1998.
0
1600
Figure 10. The subtly of the signal can be seen in synthetic T-z profiles
based on rapid warming of the surface at 10 ka. The effect of post-glacial
warming on subsurface temperatures is greatest near the surface and
diminishes with depth. The black curve with a surface intercept of zero
degrees is the steady-state condition and was the initial condition for the
models.
Clauser, C., P. Giese, E, Huenges, T. Kohl, H. Lehmann, L. Rybach,
J. Safanda, H. Wilhem, K. Windloff, and Zothe, G., The thermal
regime of the crystalline continental crust-implications from the KTB,
J. Geophys. Res., 102, 18417-18441, 1997.
Kukkonen, I. T., and A. Joeleht, Weichselian temperatures from
geothermal heat flow data, J. Geopys. Res., 108, B3, 2163,
doi:10.1029/2001JB001579, 2003.
30%
0
Coster, H.P., Terrestrial heat flow in Persia, M.N.R.A.S. Geophys.
Supp. 5, 131-146, 1947.
Jessop, A.M., The distribution of glacial perturbation of heat flow in
Canada, Can. J. of Earth Sciences, 8, 162-166, 1971.
15 deg
10 deg
5 deg
3 deg
70%
40%
5
Birch, F., The effects of Pleistocene climatic variations upon
geothermal gradients, Amer. J. Sci., 246, 729-760, 1948.
Huang, S, H.N. Pollack, and P.Y. Shen, Late Quaternary temperature
changes seen in world-wide continental heat flow measurements,
Geophys. Res. Lett., 24, 1947-1950, 1997.
90%
25
Steady-state T-z
3 Deg T-z
5 Deg T-z
10 Deg T-z
15 Deg T-z
Birch, F., Crustal structure and surface heat flow near the Colorado
Front Range, Trans. Am. Geophys. Union, 28(5), 792-797, 1947.
Dahl-Jensen, D., K. Mosegaard, N. Gundestrip, G.D. Clow, S.J.
Johnsen, A.W. Hansen and N. Balling, Past temperatures directly
from the Greenland ice sheet, Science, 282, 28—271, 1998.
100%
50%
10
Anderson, E.M., Earth contraction and mountain building, Beitr. Z.
Geophys., 42, 133-159, 1934.
Crain, I., The influence of Post-Wisconsin climatic changes on
thermal gradients in the St. Lawrence Lowland, M.Sc. thesis. McGill
University, Montréal, Quebec, 1967.
30
15
Figure 9. Seismic velocity anomaly slice maps at 130 km
and 170 km depths with permission of Suzan Van der Lee.
The lower velocity areas do not coincide with the lowest
heat flow regions depicted in Figs. 7 and 8. Of course
shields have low heat flow, but why do northern hemisphere
shields have lower heat flow than southern hemisphere
shields?
100
Benfield, A.E., Terrestrial heat flow in Great Britain, Proc. Roy. Soc.
A173, 428-450, 1939.
20
80
25
References
40
5
Figure 2. An increase in heat flow with depth has been
observed by analysis of more than 1500 deep boreholes
located throughout the Fennoscandian Shield, East
European Platform, Danish Basin, Germany, Czech
Republic, and Poland. The red lines are linear and
exponential least squares fits to the data.
64.8 mW m-2 (86)
52.3 mW m-2 (145)
68.1 mW m-2 (157)
33.1 mW m-2 (315)
Deg C
0
Brazil
Africa
Australia
N. America
Fennoscandia and
East European Craton
(1,352)
The accepted practice in determining heat flow is to use the temperature
gradient measured in the segment of a borehole from which core samples
or drill cuttings are available for thermal conductivity measurements. This
means that in most instances the length of the T-z profile used for
calculations is only a few tens of meters. Of special significance to the
problem we are addressing is the fact that two-thirds of all terrestrial heat
flow determinations have been made in boreholes less than 1000 m deep.
This fact becomes more critical in North America where 87 percent of heat
flow determinations have been made in boreholes less than 500 m deep.
The significance of this fact is that the effect of post-glacial warming is
greatest near the surface and diminishes with depth. Consequently, heat
flow determinations in shallow boreholes have not provided the data
necessary for heat flow researchers to detect the subtle effect of climatic
warming following the last glacial epoch.
160
40
A number of workers have cautioned that corrections to
geothermal gradient measurements may be required to
account for a transient perturbation from post-glacial
warming (Anderson, 1934; Benfield, 1939; Coster, 1947,
Birch; 1948; Crain, 1968); Jessop, 1971; and Beck, 1977).
However, the amplitude of warming has been generally
considered to be small and within the margin or error for
most continental heat flow measurements. Consequently,
only about 14 percent of all heat flow measurements were
corrected for the transient signal (Jessop, 1971). Our
hypothesis is that warming at the end of the last glaciation
was greater by a factor of 5 than is generally thought.
Consequently, heat flow values determined from boreholes
less that 2 km deep could be underestimated by up to 60
percent.
5
-1
0
Point One. Temperature vs. depth (T-z) measurements
in parts of Europe and North America show a
systematic increase in heat flow with depth. This
phenomenon is best recognized in analyses of deep (>
2km) boreholes in non-tectonic regions with normal to low
background heat flow.
Discussion.
percent Q
The amplitude of post-glacial warming has been generally
accepted to be about 3 to 5K. However, recent research
in northern Europe and Greenland suggest that the
amplitude of warming was may be closer to 15 K. Present
seasonal land surface temperature patterns in Eurasia
and North America are comparable in absolute values
when compared by latitude, and there is no reason that
North America should have been warmer than Eurasia
during the Pleistocene. Based on these observations, we
present four points to advance the hypothesis that heat
flow in the northern hemisphere may have been
significantly underestimated.
Point 3. Surface heat flow values in southern hemisphere shields
average approximately 50 mWm-2, but surface heat flow values in
northern hemisphere shields average 33 mWm-2. surface heat flow values
in southern hemisphere shields average approximately 61 mWm -2, but
surface heat flow values in northern hemisphere shields average
approximately 37 mWm-2. There must be a physical or chemical reason for
northern and southern shield areas of similar ages to have different heat flow
values. The northern hemisphere post-glacial warming signal may be part of
a combination of several possible explanations. There is some evidence that
crustal thicknesses of southern continents are less than that of northern
continents. Also, crustal radioactivity has been suggested to be greater at
sites where heat flow has been measured in southern hemisphere continents.
Both of these factors could contribute to higher heat flow in the southern
continents. Separation of these signals requires analysis beyond the scope of
this project, but we call attention to the overall problem since its solution
would be a significant contribution in our quest to understand Earth’s thermal
regime.
Deg C
Figure 1. Warming of
the ground surface
since the Pleistocene
caused a transient
reduction of the nearsurface geothermal
gradient. The signal is
greatest at shallow
depths and persists to a
depth of about 2 km.
400
800
1200
1600
2000
Depth (m)
2400
2800
Figure 12. The modeled effect on heat flow for 3, 5, 10
and 15 K warming events at 10 ka shows that values
determined from depths less than 2000 m could require
significant corrections depending on the amplitude of the
warming event.
3200
Rolandone, F., J.C. Mareschal, C. Jaupart, Temperatures at the base
of the Laurentide Ice Sheet inferred from borehole temperature data,
Geophys. Res. Lett. 30, (18) 1944 (doi:10.1029/2003GL018046),
2003.
Safanda, J., and D. Rajver, Signature of the last ice age in the
present subsurface temperatures in the Czech Republic and
Slovenia, Global and Planetary Change, 29, 241-257, 2003.
Safanda, J., J. Szewczyk, and J. Majorowicz, Geothermal evidence
of very low glacial temperatures on a rim of the Fennoscandian ice
sheet. Geophys. Res. Lett., 31, L07211,
doe:10.1029/2004GL019547, 2004.
Sass, J.H., A.H. Lachenbruch, and A. M. Jessop, Uniform Heat Flow
in a Deep Hole in the Canadian Shield and its Paleoclimatic
Implications, J. Geophys. Res.,76; 35, 8586-8596, 1971.
Szewczyk J., Heat flow density determination by thermal parameter
modeling, P. Geolog.vol. 49, no.11, 2001.
Szewczyk J., Evidences for the Pleistocene-Holocene Climatic
Changes from the deep well temperature profiles from the Polish
LowLands. In Int. Conf., The Earth’s Thermal Field and Related
Research Methods, Moscow, 2002.