Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp33a..01b&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP33A-01
Other
1521 Paleointensity, 1522 Paleomagnetic Secular Variation, 1535 Reversals: Process, Timescale, Magnetostratigraphy, 1560 Time Variations: Secular And Longer
Scientific paper
The excessive length of superchrons relative to other observed periods of stable polarity makes them highly visible features in the palaeomagnetic record. Here we ask: how anomalous is the geomagnetic field's behaviour in other respects during these periods? Studies of palaeosecular variation (PSV) made using the magnetization recorded in lavas or sediments provide an insight into the extent of "normal" geomagnetic variation of internal origin. The difference in PSV recorded in rocks formed during the most recent superchron (84-125 Myr ago) and rocks from the last 5 Myr has recently been shown to be smaller than previously reported but still significant. Specifically, the angular dispersion of virtual geomagnetic poles (VGPs) recorded in rocks formed at low palaeolatitudes appears to be slightly lower in the Cretaceous superchron than at other times and the magnitude of this dispersion also appears to be more constant through time. These differences are qualitatively similar to those displayed by the Glatzmaier-Roberts dynamo model when made to exhibit the relevant degrees of stability by changes in the heat flux across the core-mantle boundary. The results of new and published PSV studies made using rocks formed during the Permo-Carboniferous Reversed Superchron (262-318 Ma) will also be discussed. Producing reliable measurements of the geomagnetic palaeointensity from igneous rocks is extremely challenging and consequently, significant uncertainty remains over the average geomagnetic intensity during superchrons. That said, there is a growing body of evidence that the field, as observed at the Earth's surface, may have been stronger during the two most recent superchrons than at most other times. This is also consistent with the behaviour of certain numerical dynamo models. However, it should be noted that the potential increase in geomagnetic intensity which occurred during the Cretaceous superchron at least, does not appear larger than the total range of variation observed within the rest of the record away from superchrons. That is, average geomagnetic intensity varied at least as much during times outside of superchrons as it did at times of superchron initiation and termination. This, when taken together with other empirical evidence, does not strongly support the idea that superchrons represent periods when the geodynamo was in a fundamentally different state to that which it was in at all other times during the last few hundreds of Myr. Should superchrons simply be seen as one end-member on a continuous scale representing the geodynamo's tendency to produce polarity reversals? Regardless of the answer to this question, the specific cause(s) of the indisputable time dependence of this tendency for reversal remains as elusive and intriguing as ever.
Biggin Andrew J.
Haldan Marcela M.
Langereis Cor G.
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