| Both
theory and numerical models speculate that the state of the geodynamo during
Superchrons (long periods without polarity reversals) is different from
its state during (even longer) periods of mixed polarity. Robust observational
data, however, are still conspicuously lacking.
A recently acquired
high-quality red bed record in the Dome de Barrot (right) showed
non-reduced secular variation during the Permo-Carboniferous Reversed Superchron
(PCRS, ~50 Myr). The secular variation is comparable to recent times, a
result that contrasts with model predictions.
A recent analysis of
paleointensities during the Cretaceous Normal Superchron (CNS, ~35 Myr),
however, provided tentative support that secular variation, indeed, may
have been reduced during this Superchron. This discrepancy may indicate
that the geodynamo was fundamentally different during these two Superchrons.The
observational data base, however, is still too small and its quality often
inadequate, thus prohibiting firm conclusions
Right:
Gorges de Daluis, Dôme de Barrot (France). |
|
This proposal aims to
provide high-quality secular variation records during the PCRS, a period
for which observations are most critical. We intend to analyse red bed
sediments because they contain a high-quality magnetic signal. Our recent
advance in understanding the natural remanent magnetisation (NRM) in red
beds permits to distinguish the various modes of NRM acquisition in these
rocks.
The primary nature of
the NRM will be assessed by an array of paleomagnetic and rock magnetic
tests, including recently developed and promising new laboratory techniques.
In addition, we aim to develop a suitable method to obtain – for the first
time - relative paleointensity records for hematite-bearing rocks. The
required precise time constraints will be obtained through cyclostratigraphic
(Milankovitch) chronology.
Cyclical
section of Permian redbeds on Lac du Salagou, Lodève (France)
Finally, we aim to compare
our observations to both (new) results from the CNS, the Triassic (intermediate
interval between two superchrons) and to predictions from theory and models.
Cyclical
section of Triassic red (brown ?) beds from the Argana Basin, Morocco
|
2009
-
Haldan, M.M.,
Langereis, C.G., Biggin, A.J., Dekkers, M.J. and Evans, M.E. (2009). A
comparison of detailed equatorial red bed records of secular variation
during the Permo-Carboniferous Reversed Superchron, Geophys. J. Int,
in press
|
