L Ménabréaz

N Thouveny

D L Bourlès

B. Hamelin P Deschamps

F Demory

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

Keywords: Cosmogenic nuclides, atmospheric 10 Be production rate, geomagnetic dipole moment, Laschamp Excursion, marine sediments

Authigenic 10 Be/ 9 Be ratio measurements were performed at high resolution along a Portuguese Margin deep-sea core (37°48 N; 10°09 W) spanning the 20-50 ka time interval, in order to reconstruct variations in atmospheric cosmogenic 10 Be production rates and derive the related geomagnetic dipole moment modulation. A complementary approach consisting in 230 Th xs -normalized 10 Be deposition rate determination on selected samples confirmed the reliability of the authigenic 10 Be/ 9 Be record. This study constitutes the first successful comparison of the two widely-used normalization techniques of 10 Be 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 10 Be/ 9 Be and the Greenland Ice cores 10 Be deposition rate records attest to their global significance. This new authigenic 10 Be/ 9 Be 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 Δ 14 C variations in the 30-45 ka interval during which the 14 C calibration curve suffers from a lack of accurate data, and during which a discrepancy of about 5500 yr between the 14 C 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 Δ 14 C variations.

Introduction

The cosmogenic nuclide Beryllium-10 ( 10 Be, half-life: 1.387 ± 0.012 Ma [START_REF] Chmeleff | Determination of the 10 Be half-life by multicollector ICP-MS and liquid scintillation counting[END_REF][START_REF] Korschinek | A new value for the half-life of 10 Be by Heavy-Ion Elastic Recoil Detection and liquid scintillation counting[END_REF]) is produced in the Earth's upper atmosphere by nuclear interactions between energetic primary and secondary cosmic ray particles with target elements O and N. As shown in the early work of [START_REF] Elsasser | Cosmic-ray intensity and geomagnetism[END_REF], and later refined by [START_REF] Lal | Theoretically expected variations in the terrestrial cosmic-ray production rates of isotopes[END_REF], 10 Be atmospheric production is directly modulated by the geomagnetic field strength over millennial time scales, following a negative power law. This relationship between the dipole moment and the atmospheric production rate of cosmogenic nuclides was initially inferred from archeomagnetic absolute paleointensities and neutron monitor data comparison [START_REF] Elsasser | Cosmic-ray intensity and geomagnetism[END_REF]. More recently, a physical model of cosmic ray particle interactions with atmospheric targets yielded compatible simulations [START_REF] Masarik | Simulation of particle fluxes and cosmogenic nuclide production in the Earth's atmosphere[END_REF]Wagner et al., 2000). Regarding the classically-used paleomagnetic reconstructions, 10 Be-derived paleointensity records can therefore constitute an alternative global and independent reading of the dipole moment variations, and more particularly those that accompany geomagnetic excursions and polarity events. During the last few years, efforts have been made to extract a geomagnetic signal from both single and stacked 10 Be records in natural archives such as ice sheets (e.g. [START_REF] Muscheler | Geomagnetic field intensity during the last 60,000 years based on 10 Be and 36 Cl from the Summit ice cores and 14[END_REF] and marine sediments (e.g. [START_REF] Frank | A 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10 Be in globally stacked deep-sea sediments[END_REF]Carcaillet et al., 2004a;[START_REF] Christl | Highly resolved Beryllium-10 record from ODP Site 1089--A global signal? Earth Planet[END_REF]. For these latter archives, two correction techniques were used to account for oceanic transport: the authigenic (adsorbed from the water column) 10 Be/ 9 Be ratio [START_REF] Bourlès | 10 Be and 9 Be in marine sediments and their potential for dating[END_REF][START_REF] Henken-Mellies | 10 Be and 9 Be in South Atlantic DSDP Site 519: Relation to geomagnetic reversals and to sediment composition[END_REF][START_REF] Robinson | The relationship between 10 Be and geomagnetic field strength records in central North Atlantic sediments during the last 80 ka[END_REF][START_REF] Carcaillet | Geomagnetic moment instability between 0.6 and 1.3 Ma from cosmonuclide evidence[END_REF]Carcaillet et al., 2004a;Carcaillet et al., 2004b;[START_REF] Leduc | Authigenic 10 Be/ 9 Be signature of the Laschamp excursion: A tool for global synchronisation of paleoclimatic archives[END_REF] and 230 Th xs -normalisation (e.g. [START_REF] Frank | A 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10 Be in globally stacked deep-sea sediments[END_REF][START_REF] Christl | Beryllium-10 in deep-sea sediments: a tracer for the Earth's magnetic field intensity during the last 200,000 years[END_REF][START_REF] Christl | Highly resolved Beryllium-10 record from ODP Site 1089--A global signal? Earth Planet[END_REF][START_REF] Christl | Reconstruction of global 10 Be production over the past 250 ka from highly accumulating Atlantic drift sediments[END_REF]. The sole attempt to compare both correction techniques was presented by [START_REF] Knudsen | In-phase anomalies in Beryllium-10 production and palaeomagnetic field behaviour during the Iceland Basin geomagnetic excursion[END_REF], but large uncertainties and environmental complications prevented them from isolating the 10 Be production component from the 10 Be/ 230 Th xs signal.

The last 50 ka is a key period for the study and calibration of geomagnetic excursions and the associated cosmogenic responses, because this period contains the best-documented and best-dated geomagnetic instability, the Laschamp Excursion (LE). This excursion was first identified in the Laschamp and Olby lava flows of the Chaîne des Puys, France [START_REF] Bonhommet | Sur la présence d'aimantations inverses dans la Chaîne des Puys -Massif Central, France[END_REF], and was later described in lava flows of Iceland (e.g. [START_REF] Kristjansson | Geomagnetic excursions in late-glacial basalt outcrop in south-western Iceland[END_REF][START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavas: Confirmation of the Laschamp excursion[END_REF] and New Zealand [START_REF] Cassata | Laschamp and Mono Lake geomagnetic excursions recorded in New Zealand[END_REF] notably. Other records of the Laschamp Excursion and/or associated dipole low were obtained from lacustrine and marine sediments (e.g. [START_REF] Thouveny | On the brievity of the Laschamp Excursion[END_REF][START_REF] Vlag | Evidence for a geomagnetic excursion recorded in the sediments of Lac St. Front, France: a link with the Laschamp Excursion?[END_REF][START_REF] Lund | Deep-sea sediment records of the Laschamp geomagnetic field excursion (<41,000 calendar years before present[END_REF][START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF]. The most recent chronological constraints are provided by Ar-Ar, K-Ar, and 230 Th-U dating of lava flows from France (Laschamp, Olby and Louchadière), and from New Zealand (McLennans Hill) [START_REF] Guillou | On the age of the Laschamp geomagnetic excursion[END_REF][START_REF] Plenier | Origin and age of the directions recorded during the Laschamp event in the Chaîne des Puys (France)[END_REF][START_REF] Singer | 40 Ar/ 39 Ar, K-Ar and 230 Th-238 U dating of the Laschamp Excursion: A radioisotopic tie-point for ice core and climate chronologies[END_REF]. The statistical analyses of these age populations have led to the following average ages: 40.4±2.0 ka [START_REF] Guillou | On the age of the Laschamp geomagnetic excursion[END_REF] 40.7±1.0 ka [START_REF] Singer | 40 Ar/ 39 Ar, K-Ar and 230 Th-238 U dating of the Laschamp Excursion: A radioisotopic tie-point for ice core and climate chronologies[END_REF], and 37.0±0.7 ka [START_REF] Plenier | Origin and age of the directions recorded during the Laschamp event in the Chaîne des Puys (France)[END_REF], the third thus introducing a significantly younger age estimation.

Another younger, but more controversial, excursion has also been documented: the Mono Lake (ML) Excursion that was first detected in the Wilson Creek formation, California, by [START_REF] Denham | Evidence that the Laschamp polarity event did not occur 13300-30400 years ago[END_REF] as they were seeking for a record of the Laschamp Excursion. It was later described in other northwestern American lacustrine (e.g. [START_REF] Negrini | Mono Lake geomagnetic excursion found at Summer Lake, Oregon[END_REF][START_REF] Liddicoat | Mono Lake Excursion in Mono Basin, California, and at Carson Sink and Pyramid Lake, Nevada[END_REF][START_REF] Coe | Overprinting of natural magnetic remanence in lake sediments by subsequent high intensity field[END_REF] as well as in marine sediments (e.g. [START_REF] Nowaczyk | Magnetostratigraphic results from the eastern Arctic ocean: AMS 14 C ages and relative paleointensity data of the mono lake and Laschamp geomagnetic reversal excursions[END_REF][START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF]. The radiometric age of the ML Excursion has been revised from ~24 ka [START_REF] Denham | Evidence that the Laschamp polarity event did not occur 13300-30400 years ago[END_REF]) to ~32 ka [START_REF] Negrini | A paleoclimate record for the past 250,000 years from Summer Lake, Oregon, USA: I. Age control and magnetic lake level proxies[END_REF][START_REF] Benson | Age of the Mono Lake excursion and associated tephra[END_REF].

Surprisingly, other 39 Ar-40 Ar dating of ash layers surrounding the excursion at Wilson Creek [START_REF] Kent | Laschamp Excursion at Mono Lake? Earth Planet[END_REF] has yielded ages between 38 and 41 ka, which suggests that the excursion recorded at Mono Lake could be the Laschamp Excursion. The correlation of the Mono Lake RPI stack [START_REF] Zimmerman | Revised chronology for late Pleistocene Mono Lake sediments based on paleointensity correlation to the global reference curve[END_REF] to the GLOPIS record of [START_REF] Laj | High resolution global paleointensity stack since 75 kyr (GLOPIS-75) calibrated to absolute values[END_REF] supported the assignment of this excursion to the Laschamp. It remains true that most RPI records exhibit two successive RPI lows at 32-34 ka and at 41 ka (e.g. Channel et al., 2006;[START_REF] Lund | Deep-sea sediment records of the Laschamp geomagnetic field excursion (<41,000 calendar years before present[END_REF], which is also supported by recent reports of excursional directions and low absolute paleointensities in lava flows at ages near 32 ka, i.e. distinct from the Laschamp age [START_REF] Cassata | Laschamp and Mono Lake geomagnetic excursions recorded in New Zealand[END_REF][START_REF] Singer | 40 Ar/ 39 Ar, K-Ar and 230 Th-238 U dating of the Laschamp Excursion: A radioisotopic tie-point for ice core and climate chronologies[END_REF]Kissel et al., 2011).

The dipole field reduction linked to the Laschamp Excursion has been recorded as enhancements of 10 Be deposition rate in ice cores (e.g. [START_REF] Yiou | Beryllium 10 in the Greenland Ice Core Project ice core at Summit, Greenland[END_REF][START_REF] Muscheler | Changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10 Be and 14 C records[END_REF][START_REF] Raisbeck | Direct north-south synchronization of abrupt climate change record in ice cores using Beryllium 10[END_REF] and marine sediments (e.g. [START_REF] Frank | A 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10 Be in globally stacked deep-sea sediments[END_REF][START_REF] Christl | Beryllium-10 in deep-sea sediments: a tracer for the Earth's magnetic field intensity during the last 200,000 years[END_REF]Carcaillet et al., 2004b;[START_REF] Leduc | Authigenic 10 Be/ 9 Be signature of the Laschamp excursion: A tool for global synchronisation of paleoclimatic archives[END_REF]. Regarding the ML Excursion, a Chlorine-36 atmospheric production enhancement detected at around 32 ka in the GRIP ice core (Wagner et al., 2000) constitutes the only documented evidence for a significant increase of the cosmogenic production. Atmospheric cosmogenic nuclide production variations induced by these geomagnetic instabilities need to be well constrained, notably because they impact 14 C age calibration methods.

Here we present a highly resolved reconstruction of the 10 Be atmospheric production variations over the 20-50 ka interval, obtained from a well-dated 10 m sediment sequence deposited off the coast of Portugal. Authigenic 10 Be/ 9 Be ratios were measured, and compared to 230 Th xs -derived 10 Be fluxes which were investigated on the same samples. The 10 Be production record was then transformed into a 10 Be-derived geomagnetic dipole moment record.

Material and methods

Coring site and sediment description

Climate and oceanographic settings as well as sedimentation processes within the area have been described notably in [START_REF] Baas | Late Quaternary sedimentation on the Portuguese continental margin: climate-related processes and products[END_REF] and [START_REF] Pailler | High frequency palaeoceanographic changes during the past 140 000 yr recorded by the organic matter in sediments of the Iberian Margin[END_REF]. The sediment deposition regime is related to glacial-interglacial sea-level variations and to the Tagus River inputs. The contribution of large and variable continental material gave rise to a sediment carbonate content of ~10% during glacials and ~50% during interglacials [START_REF] Thomson | Implications for sedimentation changes on the Iberian margin over the last two glacial/interglacial transitions from ( 230 Th excess ) 0 systematics[END_REF][START_REF] Hall | Palaeocurrent reconstruction, sediment and thorium focussing on the Iberian margin over the last 140 ka[END_REF], and with less than 1% organic matter [START_REF] Pailler | High frequency palaeoceanographic changes during the past 140 000 yr recorded by the organic matter in sediments of the Iberian Margin[END_REF]. Hemipelagic sediments along the Portuguese Margin also contain scarce and thin Ice Rafted Debris (IRD) material diluted in the clay matrix (feldspars, quartz, magnetite and hematite-coated grains). These layers detected by their IRD abundance and Magnetic Susceptibility (MS) signatures have been shown to be contemporaneous with Heinrich Events [START_REF] Bard | Hydrological Impact of Heinrich Events in the Subtropical Northeast Atlantic[END_REF][START_REF] Thouveny | Rock magnetic detection of distal ice-rafted debries: clue for the identification of Heinrich layers on the Portuguese margin[END_REF][START_REF] Moreno | Climatic and oceanographic changes in the Northeast Atlantic reflected by magnetic properties of sediments deposited on the Portuguese Margin during the last 340 ka[END_REF].

Using different methodologies, several studies have demonstrated that the upper part of the giant cores collected with the Calypso corer of the R/V Marion Dufresne are affected by significant oversampling (e.g. [START_REF] Thouveny | Rock magnetic detection of distal ice-rafted debries: clue for the identification of Heinrich layers on the Portuguese margin[END_REF][START_REF] Skinner | Analysis and modelling of gravity-and piston coring based on soil mechanics[END_REF][START_REF] Skinner | Analysis and modelling of gravity-and piston coring based on soil mechanics[END_REF][START_REF] Széréméta | Oversampling of sedimentary series collected by giant piston corer: Evidence and corrections based on 3.5-kHz chirp profiles[END_REF]. This artefact has led to overestimations of apparent sedimentation rates (e.g. Thompson et al., 1999). Despite their perfect stratigraphic preservation, the upper 12 m of core MD95-2042 are affected by this artefact, which hampered the evaluation of accurate sedimentation rates and the interpretation of paleomagnetic and cosmogenic nuclides results.

During the MD-140 PRIVILEGE campaign of the R/V Marion Dufresne (2004) on the Portuguese Margin, a giant box corer CASQ (0.25m 2 x12m) allowed to collect a high resolution, undisturbed sediment sequence (Core MD04-2811; Lat.: 37°48 N; Long.: 10°09 W; 3162m water-depth) at the site of the previously studied SU81-18 and MD95-2042 cores (Fig. 1; see references in caption). These cores were extensively studied for radiocarbon calibration (section 3.2) and therefore benefits from an excellent chronological control. Core MD04-2811 was thus selected in order to document a lack of the MD95-2042 record in the critical time interval (20-35 ka) and to assess accurate sediment fluxes using the 230 Th xs normalization.

MD04-2811 sediments are composed of oxidized carbonate-rich silty clays in the upper Holocene section (1.5m), and homogeneous hemipelagic silty clays deposited during the last glacial period.

Paleomagnetic record and sampling strategy

Paleomagnetic investigations were performed on 1.5 m long U-channels collected along the MD04-2811 core. Low field Magnetic Susceptibility (MS) was measured using a Bartington MS2C probe. As described in former studies (e.g. [START_REF] Thouveny | Rock magnetic detection of distal ice-rafted debries: clue for the identification of Heinrich layers on the Portuguese margin[END_REF], the major structures of the MS profile consist in a succession of peaks corresponding to the ferrimagnetic responses of IRD layers deposited during Heinrich Events 1 to 5 superimposed on the paramagnetic contribution of the clayey fraction and a diamagnetic contribution of carbonates (Fig. 2).

Natural Remanent Magnetization (NRM) and artificially induced magnetizations were measured using a 2G cryogenic magnetometer 760-SRM model and were demagnetized by Alternating Fields (AF). The Relative Paleointensity (RPI) curve was established by normalizing the NRM intensity to the Anhysteretic Remanent Magnetization (ARM) intensity. The NRM/ARM ratio obtained for the 30mT AF demagnetization step (NRM 30mT /ARM 30mT ) represents the best estimate of the RPI. Three main RPI lows are recorded in the 435-520 cm, 575-630 cm and 740-810 cm intervals, the latter displaying the lowermost values (Fig. 2). This structure is classically attributed to the dipole low related to the Laschamp Excursion (e.g. [START_REF] Laj | High resolution global paleointensity stack since 75 kyr (GLOPIS-75) calibrated to absolute values[END_REF][START_REF] Thouveny | Geomagnetic moment variation and paleomagnetic excursions since 400 kyr BP: a stacked record from sedimentary sequences of the Portuguese margin[END_REF]. The mean inclination of the Characteristic Remanent Magnetizations equals ~ 50°, i.e. it is weaker than the inclination (56.8°) of the field created by a geomagnetic axial dipole field (GAD) at the site latitude.

Unlike other records of the area [START_REF] Thouveny | Geomagnetic moment variation and paleomagnetic excursions since 400 kyr BP: a stacked record from sedimentary sequences of the Portuguese margin[END_REF], there is no anomalous direction related to the RPI low.

Based on the interpretation of the RPI profile, 52 sediment samples were taken at a 30 cm resolution, increased to 10 cm within paleointensity lows intervals, for 10 Be and 9 Be measurements.

Analytical techniques

Leaching procedure: dissolution of the authigenic phase.

In this study, authigenic 10 Be/ 9 Be ratios, interpreted as a proxy of 10 Be fluxes, are presented. Following the procedure thoroughly described in [START_REF] Bourlès | 10 Be and 9 Be in marine sediments and their potential for dating[END_REF], the authigenic phase is extracted by leaching ~1g of dried and homogenized sediment using a 0.04M hydroxylamine in 25% acetic acid solution. A 2ml aliquot of the resulting leaching solution is separated for natural 9 Be measurements. The remaining solution, spiked with 300µl of a 10 -3 g/g 9 Be-carrier, is used for chemical extraction and Accelerator Mass Spectrometry (AMS) measurements of the resulting 10 Be/ 9 Be ratio. This procedure permits to overcome changes in extraction efficiency.

Total sample digestion and bulk sediment 10 Be content. 230 Th xs -normalized 10 Be flux quantification requires a total dissolution of sediment samples, assuming that lattice-bound 10 Be (from in-situ production) concentrations are negligible with respect to authigenic content. Sample dissolution was achieved at 70°C in a mixture of HF, HNO 3 and HCl [START_REF] Bourlès | 10 Be and 9 Be in marine sediments and their potential for dating[END_REF], and spiked with 300µl of a 10 -3 g/g 9 Becarrier to perform chemical extraction and AMS measurements.

Beryllium isotopes chemical extraction and measurements.

Once the sample is in solution, whatever the method used, Beryllium is then extracted in the form of Be acetylacetonate using organic solvent. After decomposition of these complexes, Be oxy-hydroxides are precipitated at pH 8 and oxidized to perform AMS measurements at ASTER, the French AMS national facility (installed at the CEREGE) which operates at 5MV. 10 Be concentrations are deduced from spiked 10 Be/ 9 Be measured ratios calibrated against the NIST 4325 Standard Reference Material with an assigned 10 Be/ 9 Be value of 2.79x10 -11 [START_REF] Nishiizumi | Absolute calibration of 10 Be AMS standards[END_REF], and are decay-corrected using the 10 Be half-life of 1.387 ± 0.012 Ma [START_REF] Chmeleff | Determination of the 10 Be half-life by multicollector ICP-MS and liquid scintillation counting[END_REF][START_REF] Korschinek | A new value for the half-life of 10 Be by Heavy-Ion Elastic Recoil Detection and liquid scintillation counting[END_REF]:

 [ 10 Be] g/ g sample  10 Be 9 Be       M  m spike  9 Be   spike m sample  e  10 t
where ( 10 Be/ 9 Be) M is the measured Be ratio,  10 is the decay constant of 10 Be and m sample is the weight of the leached sediment powder sample.

Measured ratios as well as the resulting 10 Be concentrations are listed in Table 1.

Chemistry blanks cluster around 9x10 -15 , up to 1000 times lower than the 10 Be/ 9 Be ratio of the samples.

The external reproducibility obtained on the NIST SRM 4325 standard (estimated from 1 standard deviation of long-term repeated measurements and integrating all effects contributing to the variability of ASTER) is limited to 0.6% [START_REF] Lebatard | Application of the authigenic 10 Be/ 9 Be dating method to continental sediments: Reconstruction of the Mio-Pleistocene sedimentary sequence in the early hominid fossiliferous areas of the northern Chad Basin[END_REF].

Uncertainties in the measured 10 Be/ 9 Be ratios and 10 Be concentrations were calculated from counting statistics and instrumental error propagation, according to the standard equation (e.g. [START_REF] Taylor | An Introduction to Error Analysis[END_REF].

Natural 9 Be concentrations were measured on the assigned 2ml aliquot using a graphite-furnace Atomic Absorption Spectrophotometer with a Zeeman effect background correction (Thermo Scientific ICE 3400) and the standard additions method (Table 1).

Uncertainties in 9 Be concentrations depend on: (1) the reproducibility of the measured absorbances at each added 9 Be concentrations (standard deviation less than 1%), and (2) the least-square fitting between measured absorbances and added 9 Be concentrations (R 2 >0.999).

Uncertainties (1) in the 9 Be concentrations are generally significantly lower than 10%

(average value: 4.3%).

The final 10 Be/ 9 Be uncertainty results from standard propagation of the above-cited Replicate authigenic 10 Be and 9 Be analyses were performed in order to check the reproducibility of the Be isotopes chemical extraction (Table 1).

Uranium and Thorium isotopic analyses: chemical procedure and TIMS measurements.

U-series measurements were performed by Thermo-ionisation Mass Spectrometry (TIMS) at the CEREGE) by using a 236 U-233 U-229 Th mixed spike calibrated according to the procedure described in [START_REF] Deschamps | Further investigations on optimized tail correction and high-precision measurement of Uranium isotopic ratios using Multi-Collector ICP-MS[END_REF].

The chemical procedure used to separate the uranium and thorium fractions for determination of 230 Th xs is similar to that described in [START_REF] Deschamps | 234 U/ 238 U Disequilibrium along stylolitic discontinuities in deep Mesozoic limestone formations of the Eastern Paris basin: evidence for discrete uranium mobility over the last 1-2 million years[END_REF]. Before total digestion, samples were spiked. Total digestion was achieved by sequential treatments with aqua regia and concentrated nitric and hydrofluoric acids. Following co-precipitation on iron oxyhydroxides, U and Th fractions were chemically separated and purified using standard ion exchange resins (AG-1X8 and U-Teva) and separation protocols modified from [START_REF] Deschamps | 234 U/ 238 U Disequilibrium along stylolitic discontinuities in deep Mesozoic limestone formations of the Eastern Paris basin: evidence for discrete uranium mobility over the last 1-2 million years[END_REF]. Typical chemical blanks attained over the course of this study were about 0.25 ppb for uranium and 0.28 ppb for thorium.

U and Th analyses were performed using a VG-Sector 54-30 mass spectrometer equipped with a 30-cm electrostatic analyzer and an ion-counting Daly detector. The instrumental abundance sensitivity is greater than 0.15 ppm at 1 amu (proportion of the 238 U ion beam measured at mass 237). The detailed procedure is described in [START_REF] Bard | U/Th and 14 C ages of corals from Barbados and their use for calibrating the 14 C time scale beyond 9000 years BP[END_REF] with further modifications described in Deschamps et al. (2011). U and Th fraction was loaded on the side filament of a triple zone-refined Re filament assembly. Reported errors (2) for the 234 U/ 238 U and 230 Th/ 232 Th ratios are about 0.14 ‰ and 0.9 %, respectively. The internal analytical reproducibility achieved in the course of this study on replicate measurements of the NBS-960 international standard yielded a mean δ 234 U value of 36.5±0.8‰ (2σ, n=23) which is in excellent agreement with values reported in the literature [START_REF] Deschamps | Further investigations on optimized tail correction and high-precision measurement of Uranium isotopic ratios using Multi-Collector ICP-MS[END_REF][START_REF] Andersen | Toward epsilon levels of measurement precision on 234 U/ 238 U by using MC-ICPMS[END_REF].

To The preserved, decay-corrected, 230 Th xs -normalized 10 Be deposition rates are then calculated following equations ( 11) and ( 12) as given in [START_REF] François | 230 Th normalization: An essential tool for interpreting sedimentary fluxes during the late Quaternary[END_REF], using a water depth of 3162 m.

Analytical errors and uncertainty related to assumption concerning the lithogenic 238 U/ 232 Th activity ratio, R, were properly propagated through 230 Th xs and flux calculations.

Note that owing to small analytical errors achieved by the TIMS method, most of the final errors on 230 Th xs and flux calculations are related to the arbitrary fixed uncertainty on R.

Using authigenic 10 Be/ 9 Be and 230 Th xs -normalized 10 Be fluxes as tracers of geomagnetic moment variations

The particle-reactive 10 Be is mainly produced in the atmosphere through nuclear reactions (spallation reactions) on oxygen (O) and nitrogen (N), and is transferred to the Earth's surface in soluble form by precipitation [START_REF] Raisbeck | Cosmogenic Be-10/Be-7 as a probe of atmospheric transport processes[END_REF] within ~3 yrs [START_REF] Baroni | Volcanic and solar activity, and atmospheric circulation influences on cosmogenic 10 Be fallout at Vostok and Concordia (Antarctica) over the last 60 years[END_REF]. Atmospheric 10 Be is ultimately removed from water via settling particles and is either deposited in marine and lacustrine sediments, or is efficiently retained by continental sediment components, and finally enters the oceans through various paths and deposits in deep sea sediments. Due to its short atmospheric residence time and chemical reactivity, 10 Be records of production enhancements should be synchronous in the geophysical reservoirs. In marine sediments, the 10 Be concentration results from a complex interplay between several processes: e.g. cosmogenic production, redistribution by atmospheric, riverine and oceanic transport, adsorption and deposition processes. Considering the adsorption processes, for example, the absolute 10 Be concentration is controlled by the scavenging efficiency and the specific surface of the settling sedimentary particles and thus depends on environmental conditions affecting their chemical and grain size composition. Consequently, 10 Be concentrations are meaningless and in order to account for these dependencies, a correction procedure is required. Soluble forms of 10 Be and 9 Be have different sources: while 10 Be is cosmogenic, the stable isotope 9 Be originates from partial dissolution of detrital, aeolian and riverine inputs [START_REF] Brown | Continental inputs of beryllium to the oceans[END_REF]. Once homogenized in seawater, both isotopes are scavenged with the same efficiency. Authigenic 10 Be/ 9 Be ratio of marine surface sediments reflects the Be isotope composition of the overlying deep waters, and its spatial variability is mainly controlled by the proximity to continental 9 Be inputs [START_REF] Bourlès | 10 Be and 9 Be in marine sediments and their potential for dating[END_REF]. Although the authigenic 9 Be normalization method has provided promising results in specific environments [START_REF] Bourlès | 10 Be and 9 Be in marine sediments and their potential for dating[END_REF][START_REF] Carcaillet | Geomagnetic moment instability between 0.6 and 1.3 Ma from cosmonuclide evidence[END_REF]Carcaillet et al., 2004a;Carcaillet et al., 2004b;[START_REF] Lebatard | Application of the authigenic 10 Be/ 9 Be dating method to continental sediments: Reconstruction of the Mio-Pleistocene sedimentary sequence in the early hominid fossiliferous areas of the northern Chad Basin[END_REF], quantitative reconstructions of the 10 Be fluxes to the sediment requires taking into account syndepositional lateral transport of adsorbed Be.

Because residence time of Be in the water column is about 500-1000 yrs, boundary scavenging and deepwater circulation must also contribute to the removal of dissolved Be and thus influence the 10 Be/ 9 Be ratio. Moreover, lateral transport of adsorbed Be can be significant in sediments heavily affected by focusing, such as drift deposits. However, at ocean margins where particle flux is high, the short Be residence time allows to record the 10 Be atmospheric flux variations with almost no signal attenuation.

Assuming a scavenged flux equivalent to its known production rate in the overlying water column [START_REF] Bacon | Glacial to interglacial changes in carbonate and clay sedimentation in the Atlantic Ocean estimated from 230 Th measurements[END_REF], 230 Th can be used to quantify rates of particle rain to the seafloor and to correct for syndepositional sediment redistribution by bottom currents. The principles and limitations of this method are fully reviewed in [START_REF] Henderson | The U-series toolbox for paleoceanography[END_REF] and [START_REF] François | 230 Th normalization: An essential tool for interpreting sedimentary fluxes during the late Quaternary[END_REF]. Like authigenic 10 Be/ 9 Be, the 10 Be/ 230 Th xs ratio may also be potentially influenced by advection of dissolved Be. Moreover, biases resulting from nuclide characteristics (different chemical affinities) are introduced: one illustration of this problem is reported by [START_REF] Chase | The influence of particle composition and particle flux on scavenging of Th, Pa and Be in the ocean[END_REF], who showed a composition-dependant differential scavenging of Be and Th, suggesting that opal/carbonate ratio of settling particles may partly explain the sedimentary 10 Be/ 230 Th xs variability.

Here, authigenic 10 Be/ 9 Be ratios were measured for 51 sampled layers, and an additional 230 Th xs -normalization was performed for 9 of them, which were selected using the Be isotopes ratio profile. This allows direct comparison and cross-calibration on the same homogenized material.

Results

3.1

The authigenic 10 Be/ 9 Be ratios and RPI on the same sequence Authigenic 9 Be concentrations vary from 4.35x10 -7 g/g to 7.22x10 -7 g/g (Table 1), refining and confirming the previous range obtained on adjacent core MD95 2042 by Carcaillet et al. (2004). Decay-corrected authigenic 10 Be concentrations vary around a mean value of 7.32x10 -15 g/g with a prominent increase to 13.6x10 -15 g/g at 712 cm. The range of variation (5.33 to 13.6x10 -15 g/g) and the average value are identical to those of the adjacent core MD95 2042 (Carcaillet et al., 2004b). The resulting authigenic 10 Be/ 9 Be evolution is presented in Figure 2, and compared to the RPI and MS profiles. The main feature of the 10 Be/ 9 Be record is a unique significant enhancement at 712 cm, with 10 Be concentrations leading to a near doubling of the long-term average 10 Be/ 9 Be value of 1.44x10 -8 to 2.69x10 -8 .

Along this sequence, both records of the 10 Be atmospheric production and its geomagnetic modulation display inverse long-term trends. While the RPI record exhibits three minima, only one significant enhancement appears in the 10 Be/ 9 Be record, with values above the -mean+1‖ occurring between 662 and 722 cm depth. This interval overlaps the depth interval recording the lowest RPI values located between ~690 and ~820 cm. The depth shift thus evidenced between these two expressions of the dipole low is further discussed in section 4.1

Chronostratigraphy

The first step in establishing the chronostratigraphy of core MD04-2811 is to construct a correlation matrix based on the MS records (Fig. 3) with the dated neighboring core MD95 2042 (Lat.: 37°45'N; Long.: 10°10'W; 3146 m water depth) in order to translate the available chronological data onto the MD04-2811 depth scale (Fig. 4).

Two major studies have established radiocarbon dates series on MD95-2042 (Bard et al., 2004;[START_REF] Shackleton | Absolute calibration of the Greenland time scale: implications for Antarctic time scales and for δ 14 C. Quat[END_REF], Electronic Appendix, Table A.1) that were compared with ages obtained from correlations with annually laminated Greenland ice cores (GRIP and GISP2). Among the paleoclimatic proxies measured along the MD95-2042 sequence, sea-surface temperature proxies (alkenones and planktonic δ 18 O) show marked variations which are in robust mutual stratigraphic agreement, and which correlate to Dansgaard-Oeschger cycles and Heinrich Events. For this study, the age control on the core MD95-2042 was updated by tuning the alkenone-derived sea-surface temperature record with the NGRIP oxygen isotopes record using a minimum number of tie points (R= 0.876, Fig. 3 and4).

Uncertainties affecting the age model are derived from the two-sigma GICC05 timescale uncertainties associated with multi parameter counting of annual layers [START_REF] Andersen | The Greenland Ice Core Chronology 2005, 15-42 ka. Part 1: constructing the time scale[END_REF][START_REF] Rasmussen | NGRIP, GRIP, and GISP2 Synchronization Match Points[END_REF]: the glacial part of this time scale has an estimated associated error of 2% back to 40 ka and of 5-10% back to 57 ka. Although multiple correlation procedures -limited by precision and resolution of the individual time series -and the accuracy of the cross-correlation procedure represent a source of uncertainty which is difficult to quantify using simple statistical parameters, the computed age model is validated by the fact that the ages assigned to the Heinrich Events (Fig. 4) are indistinguishable from the calibrated 14 C ages obtained on North Atlantic basin sediment sequences [START_REF] Thouveny | Rock magnetic detection of distal ice-rafted debries: clue for the identification of Heinrich layers on the Portuguese margin[END_REF]. Moreover, the 14 C ages obtained on the studied sequence calibrated using the INTCAL09 dataset [START_REF] Reimer | IntCal09 and Marine09 radiocarbon age calibration curves, 0-50000 Years cal BP[END_REF] relying mainly on 234 U/ 230 Th absolute dating of Hulu Cave stalagmites (China, [START_REF] Wang | A High-Resolution Absolute-Dated Late Pleistocene Monsoon Record from Hulu Cave[END_REF] over the 35-45 ka period (Electronic Appendix, Table A.1) are fully compatible with this ice core-derived chronology.

10 Be deposition rates (excess 230 Th-normalized 10 Be fluxes)

Bulk 10 Be concentrations obtained after total sample dissolution are comprised between 1.1x10 -14 g/g and 1.8x10 -14 g/g (see Table 2). Total mass flux ranges between 4.79 and 6.09 g/cm 2 /ka, and is fully compatible with results obtained further northward on the Portuguese Margin by [START_REF] Thomson | Implications for sedimentation changes on the Iberian margin over the last two glacial/interglacial transitions from ( 230 Th excess ) 0 systematics[END_REF], who determine a total sediment accumulation flux of ~5 g/cm 2 /ka during glacial times over the last 140 ka. Preserved vertical 10 Be deposition rates vary from 3.18x10 9 to 6.59x10 9 atoms/cm 2 /ka (Fig. 5a). The 10 Be deposition rate downcore evolution is strictly similar to that of the authigenic 10 Be/ 9 Be ratios measured on the same samples: the characteristic increase with a maximum at 712 cm depth is indeed reflected in the 10 Be deposition rates record. The comparison between authigenic 10 Be/ 9 Be ratios and 230 Th xs -normalized 10 Be fluxes is illustrated in Figure 5b. The linear relationship between these two variables is highly significant (R = 0.912), albeit mainly carried by the maximum variability recorded during the main enhancement.

Discussion

Identification of the Laschamp dipole low and chronological implications

During low dipole field intervals, the magnetic torqueses are insufficient to align magnetic grains and so ensure acquisition of a well-defined magnetization. Nevertheless, after recovery of the field strength, re-alignments of magnetic grains and acquisition of viscous magnetization (VRM) result in a biased record of directions and intensities (e.g. [START_REF] Coe | Overprinting of natural magnetic remanence in lake sediments by subsequent high intensity field[END_REF]. In core MD04-2811, as in other neighboring cores (see Figures 4 and9 in [START_REF] Thouveny | Geomagnetic moment variation and paleomagnetic excursions since 400 kyr BP: a stacked record from sedimentary sequences of the Portuguese margin[END_REF], the structure of paleomagnetic records (directions and RPI) does not allow an accurate and precise stratigraphic positioning of the dipole field minimum.

Comparison of the positions of the respective boundaries of the main RPI low (at ~700 and ~820 cm) with the main 10 Be/ 9 Be enhancement (at ~660 cm and ~725 cm) calls attention to a ~50 cm lag between the two signals (Fig. 2). This lag integrates a maximum 10 cm shift due to the residence time with respect to scavenging of Be, given a ~1 cm/50 yrs average sedimentation rate. The remaining 40 cm lag is greater than the 15-30 cm lags revealed from neighboring cores by Carcaillet et al. (2004); however given the sources of distortions cited above, it is not possible to assign this lag to the magnetization lock-in depth only.

In core MD95-2042 core, the declination anomaly and the RPI low related to the Laschamp excursion are recorded 50 cm beneath the MS peak associated with the Heinrich Event 4 [START_REF] Thouveny | Geomagnetic moment variation and paleomagnetic excursions since 400 kyr BP: a stacked record from sedimentary sequences of the Portuguese margin[END_REF]. In North Atlantic Ocean cores [START_REF] Kissel | Rapid climatic variations during marine isotopic stage 3: magnetic analysis of sediments from Nordic Seas and North Atlantic[END_REF], excursional inclination peaks related to the Laschamp are recorded at the same stratigraphic position as rock-magnetic signatures of the Dansgaard-Oeschger (D-O) Interstadial 10, i.e prior to H4. Nevertheless, the best stratigraphic indicator of the occurrence of the dipole low related to the Laschamp is the cosmogenic nuclide signature which is not affected by delays and distortions imparted by post-depositional processes.

Along core MD04-2811 the highest 10 Be/ 9 Be ratio peak -recorded ~40 cm beneath the MS peak corresponding to H4 -can thus undoubtedly be interpreted as the 10 Be atmospheric overproduction due to the geomagnetic dipole reduction associated with the Laschamp Excursion (LE). This 40 cm depth interval suggests that the LE occurred ~2 ka before Heinrich Event 4, which is dated at ~39 ka in North Atlantic sedimentary sequences (e.g. [START_REF] Thouveny | Rock magnetic detection of distal ice-rafted debries: clue for the identification of Heinrich layers on the Portuguese margin[END_REF]. Indeed, the chronological reconstruction positions the 10 Be/ 9 Be Laschamp signature at 41.2 ± 1.6 ka (minimal uncertainty derived from GICC05 chronology).

This age is coherent with the most recent independent radiometric age determinations obtained on volcanic material (40.4  2.0 ka, [START_REF] Guillou | On the age of the Laschamp geomagnetic excursion[END_REF]40.7  0.9 ka, Singer et al., 2009). It also coincides tightly with the age of the maximum 10 Be flux in Summit ice cores [START_REF] Muscheler | Geomagnetic field intensity during the last 60,000 years based on 10 Be and 36 Cl from the Summit ice cores and 14[END_REF][START_REF] Svensson | A 60 000 year Greenland stratigraphic ice core chronology[END_REF]. This suggests that the 10 Be residence time in the water column in such settings is short enough to preserve the simultaneity of the record in both geological reservoirs, considering age uncertainties and resolution.

The 10 Be production and recording processes in sedimentary sequences that are totally independent from: 1) the local geomagnetic vector, and 2) magnetization processes in weak and highly variable local fields, enable to evaluate the duration of the dipole field anomaly.

Both the high resolution and the accurate dating of this record highlight the persistence of a weak dipole field defined by authigenic 10 Be/ 9 Be values above -mean+1‖ during ~2.5 ka.

The minimum field intensity phase defined by authigenic 10 Be/ 9 Be values above -mean+2‖ lasted about 1 ka. This duration estimate is similar to that provided by the ice core record of [START_REF] Muscheler | Geomagnetic field intensity during the last 60,000 years based on 10 Be and 36 Cl from the Summit ice cores and 14[END_REF].

Significance of the 10 Be/ 9 Be record

10 Be/ 9 Be ratios and 230 Th xs -normalized 10 Be fluxes: a cross evaluation

In this study, 10 Be deposition rates reinforce the authigenic 10 Be/ 9 Be signal validity.

Comparison between the amplitude of 10 Be/ 230 Th xs and authigenic 10 Be/ 9 Be variations shows that both records agree within uncertainties (Fig. 6a) and document an increase by a factor of ~2 coincidental with the Laschamp geomagnetic dipole low. This is the first time that such a match has been reported, demonstrating that both normalization methods provide equivalent results in this case, and that normalizing authigenic 10 Be concentrations by authigenic 9 Be concentrations allows a reliable correction for total particle flux variations. Moreover, quantification of the vertical 10 Be fluxes allows to underline the effects of boundary scavenging: the 10 Be deposition rates determined in this study (mean value: 4.09 ± 0.31x10 9 at./cm 2 /ka) are (1) both slightly higher than an estimate of the present global production rate of 1.21 ± 0.26x10 9 at./cm 2 /ka [START_REF] Monaghan | The global-average production rate of 10 Be[END_REF] and higher than Greenland 10 Be fluxes (0.25 to 0.6x10 9 at./cm 2 /ka [START_REF] Muscheler | Changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10 Be and 14 C records[END_REF])); and (2) within the range reported for Atlantic drift deposits of the last 75 ka (~1 to 7x10 9 at./cm 2 /ka, [START_REF] Christl | Highly resolved Beryllium-10 record from ODP Site 1089--A global signal? Earth Planet[END_REF][START_REF] Knudsen | In-phase anomalies in Beryllium-10 production and palaeomagnetic field behaviour during the Iceland Basin geomagnetic excursion[END_REF][START_REF] Christl | Reconstruction of global 10 Be production over the past 250 ka from highly accumulating Atlantic drift sediments[END_REF]. Although the resolution of the 10 Be/ 230 Th xs record must be improved, the observed close agreement suggests that the effects of rapid millennial-scale changes in deep water circulation (advection of dissolved Be and Th) could be either similar in both approaches, or insignificant.

Comparison with 10 Be production rate ice core records and with relative geomagnetic field intensity reconstructed from paleomagnetic records

Over the 20-50 ka interval, there is a remarkable match between the MD04-2811 authigenic 10 Be/ 9 Be record and the 10 Be flux variations recorded along the GRIP and GISP2

ice cores (Fig. 6b; [START_REF] Finkel | Beryllium 10 concentrations in the Greenland Ice Sheet Project 2 ice core from 3-40 ka[END_REF][START_REF] Yiou | Beryllium 10 in the Greenland Ice Core Project ice core at Summit, Greenland[END_REF][START_REF] Muscheler | Geomagnetic field intensity during the last 60,000 years based on 10 Be and 36 Cl from the Summit ice cores and 14[END_REF]. It must be emphasized that even small variations of the two records match which strongly supports the high quality of the authigenic 10 Be/ 9 Be record presented. Observed for such different geophysical reservoirs as marine sediments and polar ice, this agreement demonstrates that this ratio most likely mainly reflects the changes in global atmospheric 10 Be production rate. This confirms that the normalization of authigenic 10 Be concentrations to authigenic 9 Be concentrations minimizes the impact of secondary mechanisms, among which ocean system effects (see section 2.4). In addition, the fact that these variations are recorded with the same amplitude in both archives, indicates that the 10 Be residence time in the atmosphere-ocean reservoirs should be short enough to minimize the attenuation of the production signal. The Laschamp signature corresponds to the near-doubling of the global atmospheric 10 Be production expected from the physically constrained algorithm [START_REF] Elsasser | Cosmic-ray intensity and geomagnetism[END_REF][START_REF] Lal | Theoretically expected variations in the terrestrial cosmic-ray production rates of isotopes[END_REF] which expresses the control of global atmospheric 10 Be production rate exerted by the magnetospheric shielding as modulated by geomagnetic moment variations,.

This also suggests that a significant component of the Greenland 10 Be flux signal results from an atmospheric transport from lower latitudes. Another important result is that these marine and ice core records of the atmospheric 10 Be production rates both fail to display any other significant 10 Be deposition enhancement. Wagner et al. (2000a) identified in the GRIP ice core an increased 36 Cl flux between D-O events 6 and 7, which they attributed to the RPI minimum observed at ~34 ka in the NAPIS-75 stack (based on the GISP-2 chronology; [START_REF] Wagner | a. Chlorine-36 evidence for the Mono Lake event in the Summit GRIP ice core[END_REF], and referred to as the Mono Lake Excursion. After smoothing of the dataset with a 1/2000 yrs -1 low-pass filter, the 36 Cl peak amplitude appears equivalent to that of the Laschamp. Surprisingly, in Wagner et al. (2000b), smoothing of the dataset with a 1/3000 yrs - 1 low-pass filter reduces the 36 Cl Mono Lake peak previously noted in Wagner et al. (2000a) to the level of an insignificant variation. Together with this observation, the 10 Be fluxes in Summit ice cores and the presented high-resolution 10 Be/ 9 Be record question the validity of a 36 Cl signature linked to the Mono Lake dipole field low.

Lal's algorithm (1988) has been used to convert the authigenic 10 Be/ 9 Be record into a relative Virtual Dipole Moment (VDM) record that is compared to the GLOPIS-75 stacked paleomagnetic record (Fig. 6c). This reconstruction and the GLOPIS-75 curve agree relatively well over the 30-48 ka time interval, although the cosmogenic record points out possible higher intensities for the 20-30 ka period and beyond 48 ka than indicated in the paleomagnetic records of the GLOPIS-75 stack.

Reconstruction of the geomagnetic dipole moment variations from the 10 Be/ 9 Be ratios

A stack of the Portuguese Margin 10 Be/ 9 Be records

In order to achieve a statistically significant record of the authigenic 10 Be/ 9 Be ratio, the MD04-2811 record was combined with the record previously obtained by Carcaillet et al. (2004) on core MD95-2042, recovered at the same site. These data were relocated on the depth scale of core MD04-2811 using the MS correlation matrix (reciprocal to the one used to transfer chronological data (Fig. 7a)). Be ratios measured on both cores vary in a similar range and present the same structure characterized both by a sharp maximum linked to the LE dipole low and by the absence of any other significant peak. Differences between MD95-2042 and MD04-2811 data sets result from local sedimentary processes, differential postdepositional processes (i.e. compaction), sampling and analytical uncertainties. After stacking the two 10 Be/ 9 Be ratios records, a smoothed composite record was produced by computing weighted moving averages over a 1000 years sliding window offset by 500 years (except for the 20-25 ka interval, for which MD04-2811 data are directly reported). Associated uncertainties combine both the sample analytical uncertainty and standard deviation around each weighted average, and are calculated using the unbiased weighted estimator of the sample variance (see Electronic Appendix Table A.2). Fig. 7b shows this composite record and the associated standard error (1). The 1000 yrs-smoothed authigenic 10 Be/ 9 Be composite record displays a ~1.45 time increase during the LE time interval compared to the long-term average (1.32x10 -8 ). Although the studied time interval covers the age range of the Mono Lake Excursion, the slight increase of the 10 Be/ 9 Be composite curve at ~33-34 ka is not significant regarding the uncertainties associated with the adjacent data points. This reveals the absence of any 10 Be overproduction linked to a dipole reduction associated with the Mono lake Excursion. This observation -if confirmed at other sites-would have strong implications on the interpretation of the magnitude and duration of the -geomagnetic dipole low‖ associated with the Mono lake Excursion.

Calibrating to virtual dipole moments

The previously described 10 Be-stack was converted into Virtual Dipole Moment (VDM) variations using absolute paleointensities reconstructed from lava flows. In order to reproduce the full range of the 10 Be production variation, the 10 Be/ 9 Be ratios measured along core MD04-2811 were associated with VDM values selected from the GEOMAGIA-50 [START_REF] Korhonen | GEOMAGIA50: an archeointensity database with PHP and MySQL[END_REF] The relationship thus obtained between authigenic 10 Be/ 9 Be ratios and VDMs is, at best, fitted by a polynomial of order 2 (Fig. A.3). When applied to the 10 Be/ 9 Be stack this polynomial produces a -10 Be-derived VDM‖ record for the 20-50 ka interval, which can be compared to GLOPIS-75 and SINT-800 paleointensity stacks (Fig. 8). This 10 Be based VDM record provides a reconstruction in better agreement with GLOPIS-75 than does Lal's algorithm conversion (Fig. 6c). It documents a stronger geomagnetic moment before (maximum ~10.5x10 22 A.m 2 ) than after (maximum ~7.0x10 22 A.m 2 ) the LE, and allows to estimate a decreasing rate of ~1x10 22 A.m 2 /ka for the dipole collapse which initiates the Laschamp (minimum ~2.6x10 22 A.m 2 reached at ~ 41 ka). Although this record exhibits the same main variations as GLOPIS-75, it fails to reproduce a sharp VDM reduction at the age of the Mono Lake Excursion: at 34-32 ka relatively low VDM values (~ 4.7x10 22 A.m 2 ) are in opposition with the sharp increase recorded at 32-33 ka in the GLOPIS-75.

Conclusion

Authigenic 10 Be/ 9 Be ratios measured along a rapidly accumulating sedimentary sequence from the North-East Atlantic (core MD04-2811) provide a new record of dipole geomagnetic moment variations over the 20-50 ka time interval, independent from paleomagnetic methodological constraints.

This first successful geomagnetic reconstruction using both authigenic 10 Be/ 9 Be and 10 Be/ 230 Th xs normalization techniques of 10 Be concentrations in marine sediments evidence an almost doubling of the atmospheric 10 Be production which documents the occurrence of the Laschamp geomagnetic dipole low prior to the Heinrich Event 4, at an age of ~ 41 ka. These ratios are fully compatible with the Summit ice core 10 Be record, which further supports their significance in terms of 10 Be atmospheric production. Identical 10 Be flux responses recorded in a polar ice core and in a mid-latitude marine core strongly suggest that the cosmogenic nuclide overproduction signal in Greenland ice results from the global geomagnetic moment modulation.

The authigenic 10 Be/ 9 Be ratio records of neighboring cores MD04-2811 and MD95-2042 cores were compiled and averaged over a 1 ka sliding window. The obtained stack was calibrated using absolute virtual dipole moment (VDM) values, determined from lava flow paleointensities, to provide a 10 Be derived VDM record. This allows to characterize the Laschamp dipole low by a decrease from ~10.5x10 22 A.m 2 at ~49 ka to ~2.6x10 22 A.m 2 at ~41 ka. The latter estimate is concordant with radiometric ages obtained on excursional lava flows. This record fails to document a significant VDM reduction at the age of the Mono Lake Excursion (32-34 ka).

By providing a new high-resolution 10 Be dataset at mid-latitude, the results presented here are of great interest for better constraining the influence of geomagnetic field intensity variations on cosmogenic nuclide production rates. Among these cosmogenic nuclides, atmospheric 14 C is of particular importance considering that the paleointensity (NRM/ARM ratios at 30mT alternating field step). (c) Authigenic 10 Be/ 9 Be ratios (10 -8 ) with 1 uncertainty. Solid and dotted lines correspond to the mean value over the 20-50 ka time period and the associated standard deviation (1), respectively (see Table 1). A.1), are from Bard et al., 2004 and[START_REF] Shackleton | Absolute calibration of the Greenland time scale: implications for Antarctic time scales and for δ 14 C. Quat[END_REF] alkenone index and NGRIP  18 O (see Fig. 3) was applied to core MD04-2811 by correlation of MS profiles using the Analyserie software [START_REF] Paillard | Macintosh program performs time-series analysis[END_REF]. Ages of the MD04-2811 susceptibility peaks (grey bands, ages reported on the left) are undistinguishable from ages of Heinrich layers computed by [START_REF] Thouveny | Rock magnetic detection of distal ice-rafted debries: clue for the identification of Heinrich layers on the Portuguese margin[END_REF]: H2: 24778±297 yr BP; H3: 31163±509 yr BP; H4: 39379±948 yr BP. dots represent the authigenic 10 Be/ 9 Be record and the associated 1 uncertainty, and grey line represents the Greenland 10 Be flux in Summit ice cores on GICC05 time scale [START_REF] Finkel | Beryllium 10 concentrations in the Greenland Ice Sheet Project 2 ice core from 3-40 ka[END_REF][START_REF] Yiou | Beryllium 10 in the Greenland Ice Core Project ice core at Summit, Greenland[END_REF]composite GRIP-GISP2: Muscheler et al., 2005). (c) Relative temporal variations of geomagnetic field intensity over the 20-50 ka interval. Solid lines represents the 1 uncertainty envelop of the GLOPIS-75 stack: raw data from [START_REF] Laj | High resolution global paleointensity stack since 75 kyr (GLOPIS-75) calibrated to absolute values[END_REF] are normalized to 8x10 22 A.m 2 (most recent GLOPIS VADM value) and plotted on the GICC05 time scale using the NGRIP, GRIP and GISP2 synchronization match points dataset [START_REF] Rasmussen | NGRIP, GRIP, and GISP2 Synchronization Match Points[END_REF]. Dots are the authigenic 10 Be/ 9 Be-derived VDM record (M/M 0 ) calculated using the Lal's algorithm (1988): P/P 0 = (M/M 0 ) -0.5 , where P/P 0 is the authigenic 10 Be/ 9 Be ratio divided by the surface sample value of (1.360.35)x10 -8 , undistinguishable within uncertainties from the long-term average (1.44  0.043)x10 -8 . [START_REF] Guyodo | Global changes in intensity of the Earth's magnetic field during the past 800 kyr[END_REF]. The grey zone is the 1 envelop of the GLOPIS-75 based VADM reconstruction [START_REF] Laj | High resolution global paleointensity stack since 75 kyr (GLOPIS-75) calibrated to absolute values[END_REF]. Bold black curve is the authigenic 10 Be/ 9 Be-based VDM (Virtual Dipole Moment) record and 1 associated uncertainties.

Highligths to the manuscript: -The Laschamp geomagnetic dipole low expressed as a cosmogenic 10 Be atmospheric overproduction at ~41 ka.‖ by L. Ménabréaz *, N. Thouveny, D.L. Bourlès, P. Deschamps, B. Hamelin and F. Demory.

Earth and Planetary Science Letters  Cosmogenic 10 Be concentration was measured along a marine sediment core (20 -50ka)  Authigenic 9 Be and excess 230 Th were used to normalize 10 Be concentrations  A doubling of the cosmogenic production is revealed at ~41ka (NGRIP age)  Geomagnetic field intensity variations are reconstructed from 10 Be production rates  The dipole low of Laschamp excursion is confirmed at 41 ka.

  234 e  234 t  e  230 t   e  230 t

Fig. 1 .

 1 Figure Captions :"The Laschamp geomagnetic dipole low expressed as a cosmogenic 10 Be atmospheric overproduction at ~41 ka."

Fig. 2 .

 2 Fig. 2. Magnetic properties and cosmogenic nuclide 10 Be record as a function of depth in core MD04-2811. (a) Magnetic susceptibility; susceptibility peaks identify distal ferrimagnetic IRD layers deposited during Heinrich Events 1 to 5 (labeled H1 to H5). (b) Relative

Fig. 3 .

 3 Fig. 3. (a-b) Paleoclimatic variations over Greenland and the Portuguese margin: (a) variations of the air paleotemperature proxy over Greenland:  18 O of the NGRIP ice core (NGRIP-Members, 2004 and references therein) plotted on the GICC05 time-scale (NGRIP dating group, 2006 and references therein); (b) variations of the Portuguese margin seasurface paleotemperature proxy: UK'37 alkenone index in core MD95-2042 (raw data of Pailler and[START_REF] Pailler | High frequency palaeoceanographic changes during the past 140 000 yr recorded by the organic matter in sediments of the Iberian Margin[END_REF] on a time scale tuned to NGRIP  18 O using the Analyserie software[START_REF] Paillard | Macintosh program performs time-series analysis[END_REF]. Correlation tie-points are marked by vertical dashes. Triangles indicate the position of the 23 samples used for AMS dating: raw data (Electronic Appendix, Table

  ; (c-d) Magnetic Susceptibility (MS) records of Portuguese margin cores as a function of depth: (c) core MD95-2042 (specific MS in 10 -9 m 3 /kg) (data from Thouveny et al., 2000 transferred on the NGRIP chronology; (d) core MD04-2811 (volume susceptibility 10 -5 SI). Susceptibility peaks (S0 to S5) mark IRD layers deposited during the Younger Dryas (YD) and Heinrich events (H1 to H5) events.

Fig. 4 .

 4 Fig. 4. Age-depth relationship in core MD04-2811. Open circles stand for the age model derived from the GICC05 chronology: the MD95-2042 age control obtained tuning UK'37

Fig. 5 .

 5 Fig. 5. (a) Downcore variation of the 230 Th xs -normalized 10 Be fluxes and (b) relation with authigenic 10 Be/ 9 Be ratios measured on the same homogenized sediment samples.

Fig. 6 .

 6 Fig. 6. (a and b) Relative temporal variations of 10 Be atmospheric production rate proxies over the 20-50 ka interval. All records are normalized to their respective mean values over the studied time interval. (a) Black dots represent the authigenic 10 Be/ 9 Be record and the associated 1 uncertainty. Grey dots represent the 230 Th xs -normalized 10 Be fluxes. (b) Black

Fig. 7 .Fig. 8 .

 78 Fig. 7. (a) Authigenic 10 Be/ 9 Be ratios measured along MD04-2811 (solid circles) and MD95-2042 cores (open circle, Carcaillet et al., 2004) on the MD04-2811 depth scale. (b) Calculated authigenic 10 Be/ 9 Be composite record over the 20-50 ka interval. This record is low-pass filtered (1/1000 yr -1 ). Associated uncertainties are 1.

  calculate excess230 Th activity ( 230 Th xs ), measured230 Th concentration is corrected for detrital and authigenic 230 Th components. The fraction of 230 Th supported by the decay of detrital 238 U is estimated from the 232 Th sample concentration and an assumed average lithogenic 238 U/ 232 Th activity ratio of R = 0.6  0.1 for the Atlantic Basin[START_REF] Henderson | The U-series toolbox for paleoceanography[END_REF]. Ingrowth of 230 Th from decay of authigenic uranium is estimated by assuming that authigenic uranium has an initial 234 U/ 238 U activity ratio equivalent to that ofsea-water (i.e. 1.147, Andersen et al., 2010).230 Th xs activities were calculated as: Th Total , 232 Th Total and 238 U Total are the measured 230Th , 232 Th and 238 U activities, respectively, and 238 U Total , is the sedimentary 238 U activity. ( 234 U/ 238 U) 0 stands for the initial

	248
	249
	250
	251
	where 230

(decay-corrected) authigenic activity ratio and the decay constants of radioisotopes i are expressed by  i .

  absolute paleointensity database (see Electronic Appendix Fig.A.3 and Table A.3). 10 Be production rates are inversely proportional to VDM values, thus (1) the 10 Be/ 9 Be maximum value was assigned to the minimum VDM value linked to the LE, (2) the intermediate 10 Be/ 9 Be ratios were assigned to the intermediate VDM values, and (3) the minimum 10 Be/ 9 Be values were assigned to the maximum VDM values.

Acknowledgements:

This study was funded by the MAG-ORB project of the ANR (Agence Nationale de la Recherche) program BLANC #ANR-09-BLAN-0053-01. The ASTER French AMS national facility (CEREGE, Aix en Provence) is supported by the INSU/CNRS, the French Ministry of Research and Higher Education, IRD and CEA. We gratefully thank Maurice Arnold, Georges Aumaître and Karim Keddadouche for their invaluable help and support during the AMS measurements, and Wulfran Barthelemy for his assistance during the TIMS measurements. We are also grateful to Eva Moreno, for helping in U-channel sampling and in preliminary paleomagnetic measurements, as well as R. Muscheler who kindly provided the 10 Be ice cores data. We acknowledge the scientific and technical crew of R.V. Marion Dufresne cruise 140 who collected core MD04-2811. We thank Professor Martin Frank and an anonymous reviewer who helped to improve this manuscript.