Authigenic 10Be/9Be Ratio Signatures of the Cosmogenic Nuclide Production Linked to Geomagnetic Dipole Moment Variation During and Since the Brunhes/Matuyama Boundary

Abstract:

The atmospheric production rate of cosmogenic nuclides is linked to the geomagnetic dipole moment (GDM) by a non-linear inverse relationship. Large amplitude GDM variations associated with reversals and excursions can potentially be reconstructed using time variation of the cosmogenic beryllium-10 (¹⁰Be) production recorded in ocean sediments. Downcore profiles of authigenic ¹⁰Be/⁹Be ratios (proxy of atmospheric ¹⁰Be production) in oceanic cores provide independent and additional records of the evolution of the geomagnetic intensity and complete previous information derived from relative paleointensity (RPI). Here are presented new authigenic ¹⁰Be/⁹Be results obtained from cores MD05-2920 and from the top of core MD05-2930 collected in the West Equatorial Pacific Ocean. Completing data of Ménabréaz et al. (2012, 2014), these results provide the first continuous ¹⁰Be production rate sedimentary record covering the last 800 ka. Along these cores, authigenic ¹⁰Be/⁹Be ratio peaks are recorded – within methodological errors - at the stratigraphic level of RPI lows. High-resolution chronologies (δ¹⁸O-derived) lead to interpret these peaks as successive global ¹⁰Be overproduction events triggered by geomagnetic dipole lows present in the PISO-1500 and Sint-2000 stacks. The largest amplitude ¹⁰Be production enhancement is synchronous to the very large decrease of the dipole field associated with the last polarity reversal (772 ka). It is consistent in shape and duration with the peak recorded in core MD90-0961 from the Maldive area (Indian Ocean) (Valet et al. 2014). Two significant ¹⁰Be production enhancements are coeval with the Laschamp (41 ka) and Icelandic basin (190 ka) excursions, while ¹⁰Be production peaks of lower amplitude correlate to other recognized excursions such as the Blake (120 ka), Pringle-Falls (215 ka), Portuguese Margin (290 ka), Big Lost (540 ka) among others. This study provides new data on the amplitude and timing of dipole field variations, helping to understand the difference between paleosecular variation, excursions, aborted reversals and reversals regimes.