Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmgp11b0829s&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #GP11B-0829
Astronomy and Astrophysics
Astronomy
1035 Geochronology, 1507 Core Processes (8115), 1521 Paleointensity, 1535 Reversals (Process, Timescale, Magnetostratigraphy)
Scientific paper
40Ar/39Ar dating of lavas on Tahiti, long thought to record the primary part of the Matuyama-Bruhnes (M-B) reversa1, gives an age of 795+/- 7 ka, indistinguishable from that of transitional lavas in Chile and La Palma, but older than the accepted age for the reversal. Only the transitional lavas on Maui and one from La Palma (dated at 776 +/- 2 ka), agree with the astronomical age for the M-B reversal. Virtual geomagnetic poles (VGPs) associated with the Tahitian and Chilean lavas cluster near Australia, as do VGPs recorded on Tahiti during the Big Lost and Punaruu events, two apparently unsuccessful reversals. These findings, suggestive of a recurring, mantle-held flux pattern at the outer core surface during reversal attempts, are also theoretically equivalent to the situation that would arise today if the axial dipole were to continue to weaken and vanish. Hence, we propose that the 795 ka lavas record the onset of a dynamo process--one which only on occasion would result in polarity change. This initial instability, associated with the first of two decreases in field intensity, began 18 kyrs prior to the actual polarity switch. These data may provide the first observational support to the claim that complete reversals require a significant interval of time for magnetic flux to escape from the solid inner core and sufficiently weaken its stabilizing effect.
Brown Laurel
Chauvin Annick
Coe Robert S.
Hoffman Kenneth A.
Jicha Brian R.
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