| Summary: | Authigenic Be-10/Be-9 ratio measurements were performed at high resolution along a Portuguese Margin deep-sea core (37 degrees 48 N; 10 degrees 09 W) spanning the 20-50 ka time interval, in order to reconstruct variations in atmospheric cosmogenic Be-10 production rates and derive the related geomagnetic dipole moment modulation. A complementary approach consisting in Th-230(xs)-normalized Be-10 deposition rate determination on selected samples confirmed the reliability of the authigenic Be-10/Be-9 record. This study constitutes the first successful comparison of the two widely-used normalization techniques of Be-10 concentrations. For both methods, the presented results herein evidence a factor of similar to 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 similar to 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 Be-10/Be-9 and the Greenland Ice cores Be-10 deposition rate records attests to their global significance. This new authigenic Be-10/Be-9 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 from paleomagnetically-constrained methods, that documents the Laschamp dipole low but fails to express any dipole low related to the Mono Lake Excursion. This high resolution record responds to the necessity of supplementing the knowledge of the atmospheric Delta C-14 variations in the 30-45 ka interval during which the C-14 calibration curve suffers from a lack of accurate data, and during which a discrepancy of about 5500 yr between the C-14 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 Delta C-14 variations.
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