Geomagnetic secular variation in the Cretaceous Normal Superchron and in the Jurassic

It is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered. If this is the case, then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation. Two such periods were the Cretaceous Normal Superchron (CNS: 84-125 Ma) when the field was dominantly of a single polarity for 40 Myr and the Jurassic period (145-200 Ma) when reversals occurred at an average rate of as much as 4.6 Myr(-1). Here, we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first-order descriptions of the palaeosecular variation (PSV) during these times. We then compare these records with one another and with that produced for the period 0-5 Ma (with average reversal frequency of 4.0 Myr(-1)). Our results are more equivocal than those obtained in a previous similar study [McFadden, P.L., Merrill, R.T., McElhinny, M.W., Lee, S.H., 1991. Reversals of the Earths magnetic-field and temporal variations of the dynamo families. journal of Geophysical Research-Solid Earth and Planets 96, 3923-3933]. We demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within-site scatter. The usefulness of our Jurassic record is severely limited by the restricted palaeolatitudinal span of the available data. However, our record for the CNS is sufficient to allow us to conclude that it was likely chat secular variation then was different from that in the 0-5 Ma period. This supports the hypothesis of a link between PSV and reversal frequency and therefore endorses PSV analysis as a first-order tool for determining geomagnetic stability in the past. (C) 2008 Elsevier B.V. All rights reserved.