Comment on the Lau Basin Cobb Mountain records by Abrahamsen and Sager

The Cobb Mountain Subchron is one of the few short polarity intervals for which transition records have been obtained both from a wide geographical distribution and from different types of paleomagnetic recorders. These records exhibit VGP paths that are remarkably similar. Given the wide geographic distribution of these sites, the similarity in VGP paths is most simply interpreted as the presence of very large scale, if not dipolar, symmetries in the transitional fields. Only the records from western California and the Lau Basin exhibit VGP paths that are markedly different from the others. New dates for the Punaruu lavas suggest that they recorded a new geomagnetic cryptochron rather than the Cobb Mountain [Singer, B.S., Hoffman, K.A., Chauvin, A., Coe, R.S., 1999. Dating transitionally magnetized lavas of the late Matuyama Chron: Toward a new 40Ar/39Ar timescale of reversals and events. J. Geophys, Res. 104, 679-693]. This new age indicates a revised correlation of some of the other California records whose ages agree better with the Punaruu cryptochron. A re-examination of the Lau Basin records reveals that much of the difference between these and the other records results from a bias introduced when the paleomagnetic data were smoothed. Reanalysis of these records shows that the Lau Basin VGP paths an very similar to those obtained from western Pacific, and the North Atlantic. (C) 2000 Elsevier Science B.V. All rights reserved.