Paleointensity variation across the Matuyama-Brunhes polarity transition: Observations from lavas at Punaruu Valley, Tahiti

Abstract

[1] We have conducted a paleointensity study of the Matuyama-Brunhes (M-B) polarity transition recorded in 34 successive lava flows of Punaruu Valley on Tahiti. A reversed polarity is obtained from the lower part of the record, major directional changes are derived from the middle part of the record, and a normal polarity is recorded in the upper part of the record. These paleomagnetic directions and five 40 Ar/ 39 Ar ages yielding a weighted mean of 771 ± 8 (1s) ka indicate that 30 lava flows recorded the geomagnetic field across the M-B transition. The 215 specimens from 32 flows were subjected to the double-heating technique of the Shaw method combined with low-temperature demagnetization (LTD-DHT Shaw method), yielding 73 successful results from 18 flows. For the reversed polarity period just prior to the major directional changes, paleointensity shows an oscillation-like variation between 3 and 38 mT corresponding to virtual dipole moments (VDMs) between 0.9 × 10 22 and 9.6 × 10 22 Am 2 . For the major directional changes, a weak paleointensity of 5 mT is obtained, which gives a VDM of 1.0 × 10 22 Am 2 . For the normal polarity period, paleointensities are 14-21 mT, giving VDMs of 3.5-5.2 × 10 22 Am 2 . For the reversed polarity period just prior to the major directional changes, a linear relationship with a correlation coefficient of 0.96 is recognized on the diagram of VDM versus virtual geomagnetic pole latitude. This linear relationship may be a precursory feature of the geodynamo at the onset of the M-B transition.

Evidence of a partitioned dynamo reversal process from paleomagnetic recordings in Tahitian lavas

“…Remarkably, the average paleodirection for these flows (Dec = 229.8, Inc = 1.5, a 95 = 6.3) is almost precisely antipodal to that for the Mochizuki et al [2011] rebound cluster flows (Dec = 43.3, Inc = 3.8, a 95 = 9.1) (Figure 3). …”

“…Note, however, that the lavas involved (DGs 18, 21 and 23) are associated with flows that were not erupted in succession. We now consider this finding by Mochizuki et al [2011] in more detail.…”

“…DGs 25 through 28 record the completion of the polarity transition with increased values of paleointensity 14-21 mT yielding VADMs of 3.1-4.8 Â 10 22 Am 2 [Mochizuki et al, 2011].…”

“…Notwithstanding the significant advances that have been provided through these analyses, the near impossibility of determining temporally synchronous behavior among globally scattered recording sites is one of many obstacles to a complete understanding of the dynamo process during this reversal. With this in mind we focus our attention on the published M-B paleomagnetic recordings obtained from one site, the Society hotspot, from where there are now available two distinct sections of a succession of lava flows exposed in the Punaruu Valley on the island of Tahiti, French Polynesia [Chauvin et al, 1990;Mochizuki et al, 2011].…”

“…Recently, Mochizuki et al [2011] reported a far more detailed account of field behavior during the M-B reversal, including paleointensity variation, obtained from a distinct section of lavas exposed on the north face of the same valley.…”