Has Northern Hemisphere Heat Flow Been Underestimated?

1
Has Northern Hemisphere Heat Flow Been
Underestimated?
Contact Will Gosnold Geology and Geological
Engineering University of North Dakota Grand
Forks, ND 58201-8358 willgosnold_at_mail.und.nodak.ed
u
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
Figure 1. Warming of the ground surface since the
Pleistocene caused a transient reduction of the
near-surface geothermal gradient. The signal is
greatest at shallow depths and persists to a
depth of about 2 km.
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 Earths thermal regime.
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.
Discussion. 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.
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.
Figure 4. T-z profiles from Poland showing large
near-surface warming effects.
Brazil 64.8 mW m-2 (86) Africa 52.3 mW m-2
(145) Australia 68.1 mW m-2 (157) N. America
33.1 mW m-2 (315) Fennoscandia and East
European Craton 35 - 40 mW m-2 (1,352)
References Anderson, E.M., Earth contraction and
mountain building, Beitr. Z. Geophys., 42,
133-159, 1934. Benfield, A.E., Terrestrial heat
flow in Great Britain, Proc. Roy. Soc. A173,
428-450, 1939. Birch, F., Crustal structure and
surface heat flow near the Colorado Front Range,
Trans. Am. Geophys. Union, 28(5), 792-797,
1947. Birch, F., The effects of Pleistocene
climatic variations upon geothermal gradients,
Amer. J. Sci., 246, 729-760, 1948. Coster, H.P.,
Terrestrial heat flow in Persia, M.N.R.A.S.
Geophys. Supp. 5, 131-146, 1947. 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. 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. 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, 28271,
1998. 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. Jessop, A.M., The
distribution of glacial perturbation of heat flow
in Canada, Can. J. of Earth Sciences, 8, 162-166,
1971. Kukkonen, I. T., and A. Joeleht,
Weichselian temperatures from geothermal heat
flow data, J. Geopys. Res., 108, B3, 2163,
doi10.1029/2001JB001579, 2003. 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. 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 (doi10.1029/2003GL018046),
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, doe10.1029/2004GL019547, 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 Earths Thermal
Field and Related Research Methods, Moscow, 2002.
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.
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
gt 2km) boreholes.
Point 4. The signal is subtle and is not likely
to be detected with typical heat flow
methods. 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.
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 (gt 2km) boreholes in
non-tectonic regions with normal to low
background heat flow.
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.
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.
Figures 5,6. T-z profiles from two
high-precision 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 3C, 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.
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.
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?
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.
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.