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| Titel |
Past surface temperature changes as estimated from the Outokumpu deep borehole, Southern Finland: methods and results |
| VerfasserIn |
Volker Rath, Ilmo T. Kukkonen, Liisa Kivekaes, Jan Safanda, Vladimír Cermák |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250050600
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| Zusammenfassung |
| The 2516 m deep Outokumpu Deep Drill Hole in eastern Finland was drilled in 2004-2005
into a Palaeoproterozoic formation with metasedimentary rocks, ophiolite-derived altered
ultramafic rocks and pegmatitic granite. The down-hole temperatures have been logged five
times after end of drilling and extend to day 948 after drilling. The hole is completely cored
(79% core coverage) and thermal conductivity measurements were done at 1 m
intervals, complemented by numerous determinations of heat production and wireline
logging. The geothermal results on temperature gradient, thermal conductivity and
heat flow density yield an exceptionally detailed data set, and indicate a significant
vertical variation in gradient and heat flow density. Heat flow density increases
from about 28-32 mW m-2 in the uppermost 1000 m to 40-45 mW m-2 at depths
exceeding 2000 m. The estimated undisturbed surface heat flow value is 42 mW
m-2.
We present paleoclimatic results from forward and inverse transient conductive models,
which suggest that the vertical variation in heat flow can mostly be attributed to a
palaeoclimatic effect due to ground surface temperature (GST) variations during the last
100,000 years. The modeling suggests that the average GST was about -3 to -4°C during the
Weichselian glaciation. Holocene GST values are within ±2 degree from the present average
GST in Outokumpu (5°C). Nearly independent of the parametrization chosen, the
post-holocene temperature history appears rather smooth, with neither a significant medieval
optimum nor little ice age. In most cases, a reasonable fit could be found for depths 1500 m, (2)
heterogeneity effects not consistent with 1D assumptions, and (3) a possible residual
contribution of post drilling equilibration of temperatures. All these effects are not easily
coped with in the framework of 1D inversion, and will be investigated in future
studies.
The Outokumpu area was covered by ice sheets perhaps only during the Middle (about 60
kyrs B.P.) and Late Weichselian (about 20 kyrs B.P.) glacial maxima. Paleoclimate proxies as
oxygen isotopes in mammoth teeth suggest that most of southeastern Fennoscandia
was probably ice-free du in between. The paleotemperatures in the range of -1
... -3 °C thus obtained are in a good agreement with the Outokumpu geothermal
result on Weichselian average GST. The temperatures obtained for the last few
thousand years are systematically higher than paleotemperature reconstructions
interpolated from nearby locations. Apart from the distance to the proxy sites involved,
this could be due to changes in land use and snow cover in the Outokumpu area.
The reconstruction finally shows a strong recent warming of about 0.5 K in the
last 50 years. The paleoclimatic interpretation of these results reveals many open
questions, and up till now only preliminary or even speculative answers can be
given.
The results of the current study may be improved by further development of method, e.g.
by changing the constraints of the current inversion algorithm. In particular, the far too
smooth postglacial warmimg, and in consequence the late occurrence of the Holocene
temperature maximum is at least partly an artifact of the smoothing redularization employed.
Numerical experiments with method minimizing the total variation of the GSTH will be
presented. Uncertainty in the results is dealt with using a MCMC algorithm in a Bayesian
framework. |
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