Amplitude and timing of the Laschamp geomagnetic dipole low from the global atmospheric 10 Be overproduction: Contribution of authigenic 10 Be/ 9 Be ratios in west equatorial Pacific sediments

International audience[1] Authigenic 10 Be/ 9 Be ratios were measured along a sediment core collected in the west equatorial Pacific in order to reconstruct cosmogenic 10 Be production variations near the equator, where the geomagnetic modulation is maximum. From 60 to 20 ka, the single significant 10 Be production impulse recorded at 41 ka results from the geomagnetic dipole low that triggered the Laschamp excursion. No significant 10 Be overproduction signature is recorded at the age of the Mono Lake excursion ($34 ka). A compilation of authigenic 10 Be/ 9 Be records obtained from sediments was averaged over a 1 kyr window and compared with the 1 kyr averaged 10 Be flux record of Greenland ice cores. Their remarkable similarity demonstrates that 10 Be production is globally modulated by geomagnetic dipole variations and redistributed by atmosphere dynamics. After calibration using absolute values of the virtual dipole moment drawn from paleomagnetic database, the authigenic 10 Be/ 9 Be stack allows reconstructing the geomagnetic dipole moment variations over the 20–50 ka time interval. Between 48 and 41 ka, the dipole moment collapsed at a rate of À1.5 Â 10 22 A m 2 kyr À1 , which will be an interesting criterion for the assessment of the loss rate of the historical field and the comparison of dipole moment loss prior to excursions and reversals. After a 2 kyr duration of the minimum dipole moment ($1 Â 10 22 A m 2), a slow increase started at 39 ka, progressively reaching 5 Â 10 22 A m 2 at 20 ka. The absence of a significant dipole moment drop at 34 ka, the age of the Mono lake excursion, suggests that the duration and amplitude of the dipole weakening cannot be compared with that of the Laschamp. This study provides a reliable basis to model the production of radiocarbon and in situ cosmogenic nuclides and to improve the calibration of these dating methods

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

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

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

International audienceAuthigenic 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 inthe 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

Increased production of cosmogenic 10 Be recorded in oceanic sediment sequences: Information on the age, duration, and amplitude of the geomagnetic dipole moment minimum over the Matuyama–Brunhes transition

International audienceNew high-resolution authigenic Be-10/Be-9 ratio (Be-ratio) records covering the last geomagnetic reversal, i.e. the Matuyama-Brushes transition (MBT), have been obtained and set on a time scale using benthic delta O-18 (Cibicides wuellerstorfi) records. The geographic distribution of the four studied sites allows global comparison between the North Atlantic, Indian and Pacific Oceans. All Be-ratio records contain a twofold increase triggered by the geomagnetic dipole moment (GDM) collapse associated with the MBT. The stratigraphic position of the Be-ratio spike, relative to marine isotope stages, allows establishment of a robust astrochronological framework for the MBT, anchoring its age between 778 and 766 ka (average mid-peaks at 772 ka), which is consistent with all other available Be-10-proxy records from marine, ice and loess archives. The global Be-10 atmospheric production doubling represents an increase of more than 300 atoms m(-2) s(-1) that is compatible with the increased magnitude of atmospheric Be-10 production obtained by simulations between the present GDM and a null-GDM. The minimum Be-10-derived GDM average computed for the 776-771 ka interval is 1.7 +/- 0.4 x 10(22) Am-2, in agreement with model simulations and absolute paleointensities of transitional lava flow

Cosmogenic signature of geomagnetic reversals and excursions from the Réunion event to the Matuyama–Brunhes transition (0.7–2.14 Ma interval)

International audienceLong-term variations of the geomagnetic dipole moment (GDM) during periods of stable polarity and in transitional states (reversals and excursions) provide key information for understanding the geodynamo regime. Following several studies dealing with the Brunhes chron and the Matuyama–Brunhes transition, this study presents a new authigenic 10 Be/ 9 Be ratio (Be-ratio) record obtained from the MD97-2143 core (western Pacific Ocean). This new Be-ratio series yields a record of GDM variations covering the early Brunhes and mid to late Matuyama time period (i.e. 700–2140 ka), independently from the relative paleointensity (RPI) record obtained from the same core, that can be compared with available RPI records and stacks. Stratigraphic offsets measured between the Be-ratio peaks and the corresponding RPI minima reach 2 to 14 cm. They can be assigned to (post-) detrital remanent magnetization (pDRM) effects leading to magnetization locking-in delays varying from 2 to 12 ka in the studied core. 10 Be overproduction episodes triggered by geomagnetic dipole moment lows (GDL) linked to polarity reversals and excursions confirm the global control exerted by the GDM on cosmogenic radionuclides production. A dipole moment reconstruction derived from the Beryllium-10 (BeDiMo) was compiled and calibrated using absolute paleointensity data. This independent record complements the available paleomagnetic RPI records, permitting 1) to overcome the pDRM lock-in offsets induced below the mixing layer, 2) to confront and increase the robustness and precision of GDM reconstructions and, 3) to better constrain the chronology of geomagnetic field instabilities during the mid to late Matuyama chron. Our new 10 Be derived inventory is fully compatible with the GDL series linked to geomagnetic polarity reversals and events (Matuyama–Brunhes transition, Jaramillo and Olduvai subchron boundaries, Cobb Mountain, Réunion) and it strengthens the occurrence of several excursions (Kamikatsura, Santa Rosa, Punaruu, Bjorn, Gilsa, Gardar) that were until now reported from only sparse locations

Stable runoff and weathering fluxes into the oceans over Quaternary climate cycles

International audienc