Paleosecular variation recorded by Quaternary lava flows from Guadeloupe Island

Paleomagnetic directional data were obtained from fourteen 0 to 2 Ma old lava flows at Basse-Terre Island (Guadeloupe, French West Indies). Five reversed polarity flows are consistent with their Matuyama age between 1.6-1.5 Ma and 875-790 ka while the ages of the other nine normal polarity units tie them to the Olduvai subchron and the Brunhes Chron. These directions have been combined with previous results obtained from Basse-Terre Island. The overall mean direction (D = -1.2°, I = 31.4°, α₉₅ = 3.3°) obtained from the 39 non-transitional flows from Basse-Terre Island is indistinguishable from the expected geocentric axial dipole value (D = 0°, I = 29.8°). The dispersion measured from the angular standard deviation of the Virtual Geomagnetic Poles (VGPs) was found to be close to, but smaller than the predictions of geomagnetic models. Together with further directions from the nearby Martinique Island, the 45 directions obtained within the Brunhes chron provide the most robust estimate of the statistical distribution of paleosecular variation (PSV) at this latitude. The sequence of directions shows episodes of high amplitude secular variation that are coeval with several geomagnetic events including the last reversal documented by five transitional directions. Finally, three lava flows have recorded a transitional behavior which could be link to two excursions, the Laguna del Sello (at ~340 ka) and the Pringle Falls (at ~210 ka) events.

Authigenic 10Be/9Be ratio signatures of the cosmogenic nuclide production linked to geomagnetic dipole moment variation since the Brunhes/Matuyama boundary

Geomagnetic dipole moment variations associated with polarity reversals and excursions are expressed by large changes of the cosmogenic nuclide beryllium-10 (10Be) production rates. Authigenic 10Be/9Be ratios (proxy of atmospheric 10Be production) from oceanic cores therefore complete the classical information derived from relative paleointensity (RPI) records. This study presents new authigenic 10Be/9Be ratio results obtained from cores MD05-2920 and MD05-2930 collected in the west equatorial Pacific Ocean. Be ratios from cores MD05-2920, MD05-2930 and MD90-0961 have been stacked and averaged. Variations of the authigenic 10Be/9Be ratio are analyzed and compared with the geomagnetic dipole low series reported from global RPI stacks. The largest 10Be overproduction episodes are related to dipole field collapses (below a threshold of 2 × 1022 Am2) associated with the Brunhes/Matuyama reversal, the Laschamp (41 ka) excursion, and the Iceland Basin event (190 ka). Other significant 10Be production peaks are correlated to geomagnetic excursions reported in literature. The record was then calibrated by using absolute dipole moment values drawn from the Geomagia and Pint paleointensity value databases. The 10Be-derived geomagnetic dipole moment record, independent from sedimentary paleomagnetic data, covers the Brunhes-Matuyama transition and the whole Brunhes Chron. It provides new and complementary data on the amplitude and timing of millennial-scale geomagnetic dipole moment variations and particularly on dipole moment collapses triggering polarity instabilities.