The Laschamp geomagnetic dipole low expressed as a cosmogenic 10 Be atmospheric overproduction at ~41 ka

L L Ménabréaz ⁎ (1) , Nicolas Thouveny (1) , D L Bourlès (1) , P Deschamps (1) , B Hamelin (1) , F Demory (1)

1 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement

Authigenic 10Be/9Be ratiomeasurementswere performed at high resolution along a Portuguese Margin deep-sea core (37°48 N; 10°09W) spanning the 20–50 ka time interval, in order to reconstruct variations in atmospheric cosmogenic 10Be production rates and derive the related geomagnetic dipole moment modulation. A complementary approach consisting in 230Thxs-normalized 10Be deposition rate determination on selected samples confirmed the reliability of the authigenic 10Be/9Be record. This study constitutes the first successful comparison of the two widely-used normalization techniques of 10Be concentrations. For both methods, the presented results herein evidence a factor of ~2 cosmogenic nuclide overproduction linked to a minimum dipole moment associated with the Laschamp Excursion. The latter is stratigraphically constrained beneath Heinrich Event 4. Its age is estimated at ~41 ka on the basis of direct correlation between the series of rapid paleoclimatic events recorded in the Portuguese Margin sediments and in the Greenland ice sheet, and is confirmed by calibrated radiocarbon dating carried out on the same sediments. The remarkable agreement between the authigenic 10Be/9Be and the Greenland Ice cores 10Be deposition rate records attests to their global significance. This new authigenic 10Be/9Be record has been combined with that previously obtained at the same site to produce a stacked record that is calibrated using absolute values of Virtual Dipole Moment determined on lava flows. This provides a reconstruction of dipole geomagnetic moment variations over the 20–50 ka interval, independent frompaleomagnetically-constrainedmethods, that documents the Laschamp dipole low but fails to express any dipole low related to t e Mono Lake Excursion. This high resolution record responds to the necessity of supplementing the knowledge of the atmospheric Δ14C variations in the 30–45 ka interval during which the 14C calibration curve suffers from a lack of accurate data, and during which a discrepancy of about 5500 yr between the 14C and U–Th ages is due to the Laschamp geomagnetic dipole low. Such new high resolution datasets from records obtained from different latitudes will be required to make significant advances in understanding the causes of atmospheric Δ14C variations.

Domaines: Géophysique [physics.geo-ph]

L L Ménabréaz ⁎, Nicolas Thouveny, D L Bourlès, P Deschamps, B Hamelin, et al.. The Laschamp geomagnetic dipole low expressed as a cosmogenic 10 Be atmospheric overproduction at ~41 ka. Earth and Planetary Science Letters, 2011, 312 (3-4), pp.305-317. ⟨10.1016/j.epsl.2011.10.037⟩. ⟨hal-01420387⟩