Patrizia Macrì
email: macri@ingv.it.

Leonardo Sagnotti

Jaume Dinarès-Turell

Andrea Caburlotto

Relative geomagnetic paleointensity of the Brunhes Chron and the Matuyama-Brunhes precursor as recorded in sediment core from Wilkes Land Basin (Antarctica)

Keywords: paleomagnetism, relative paleointensity, Brunhes Chron, Matuyama-Brunhes precursor, Antarctica

Paleomagnetic and rock magnetic investigation was performed on the 35-m long MD03-2595 CADO (Coring Adélie Diatom Oozes) piston core recovered on the continental rise of the Wilkes Land Basin (East Antarctica). Analysis of the Characteristic Remanent Magnetization (ChRM) inclination record indicates a normal magnetic polarity for the uppermost 34 m of the sequence and a distinctive abrupt polarity change at the bottom of the core. This polarity change, which spans a 27 cm thick stratigraphic interval, represents a detailed record of the Matuyama-Brunhes (M-B) transition and it is preceded by a sharp oscillation in paleomagnetic directions that may correlate to the M-B precursor event. Paleomagnetic measurements enable reconstruction of geomagnetic relative paleointensity (RPI) variations, and a high-resolution age model was established by correlating the CADO RPI curve to the available global reference RPI stack, indicating that the studied sequence reaches back to ca 800 ka with an average sedimentation rate of 4.4 cm/ka. Orbital periodicities (100 ka and 41 ka) were found in the ChRM inclination record, and a significant coherence of ChRM inclination and RPI record around 100 ka suggests that long-term geomagnetic secular variation in inclination is controlled by changes in the relative strength of the geocentric axial dipole and persistent non-dipole components. Moreover, even if the relatively homogeneous rock magnetic parameters and A c c e p t e d M a n u s c r i p t lithofacies throughout the recovered sequence indicates a substantial stability of the East Antarctic Ice Sheet during the middle and late Pleistocene, influence of the 100 ka and 41 ka orbital periodicities has been detected in some rock magnetic parameters, indicating subtle variations in the concentration and grain-size of the magnetic minerals linked to orbital forcing of the global climate.

Introduction

Pleistocene sedimentary sequences from peri-Antarctic margins have great potential for studies of paleoenvironmental changes and past geomagnetic field behavior at high latitudes: the Antarctic continental rises are very sensitive to past environmental and climatic changes, and their marine sedimentary sequences may preserve detailed information about past climatic variations. Well resolved paleomagnetic data from high latitudes are also critically important for global reconstruction of past geomagnetic field behavior and can provide critical constraints for geodynamo modelling.

The QUASAR (QUAternary Sedimentary processes on the east Antarctic continental Rise) project of the Italian Programma Nazionale Ricerche in Antartide-PNRA, focus on a multiproxy analysis of the MD03-2595 CADO (Coring Adélie Diatom Oozes) piston core recovered on the continental rise of the Wilkes Land Basin (WLB) which consists of a continuous fine-grained sedimentary sequence with a high chronostratigraphic potential. The QUASAR project aims to reconstruct sedimentary processes offshore of the east Antarctic continental margin and to infer climatic fluctuations of the continental East Antarctic Ice Sheet.

We present a paleomagnetic and rock magnetic study of the 35-m long CADO core, with the reconstruction of geomagnetic relative paleointensity (RPI) and Characteristic Remanent Magnetization (ChRM) direction stratigraphic records. The obtained paleomagnetic data allow A c c e p t e d M a n u s c r i p t to establish of a high-resolution age model and to quantify the sedimentation rate for the recovered sequence. This is particularly valuable because the corrosive character of Antarctic deep waters with respect to carbonate implies that peri-Antarctic sediments are commonly deprived of age constraints derived from micropaleontology or stable isotope stratigraphy.

The MD03-2595 CADO core is located just outside the inner core tangent cylinder, an imaginary cylinder parallel to the Earth's spin axis that circumscribes the equator of the inner core and intersects the surface of the Earth in the polar regions at a latitude of ca 69.6° in both hemispheres. Different models indicate that geodynamo action occurs mainly by polar vortices within the tangent cylinder [e.g., Glatzmaier and Roberts, 1995a;[START_REF] Aurnou | Experiments on convection in Earth's core tangent cylinder[END_REF] or by differential rotation outside the tangent cylinder [e.g., [START_REF] Kuang | An Earth-like numerical dynamo model[END_REF]. Glatzmaier and Roberts [1995b] showed that there were strong toroidal fields and differential rotation within the tangent cylinder, with high density of magnetic flux patches that strengthen the nondipolar component of the geomagnetic field over the polar regions. This might cause a larger dynamics, complexity and variability of the geomagnetic field within the tangent cylinder with respect to the intermediate and low latitudes. The available paleomagnetic study of several cores from the Arctic region indicates that the magnetic field has been strongly variable during at least the last 300 ka [e.g. [START_REF] Nowaczyk | High resolution magnetostratigraphy of four sediment cores from the Greenland Sea -I. Identification of the Mono Lake excursion, Laschamp and Biwa I/Jamaica geomagnetic polarity events[END_REF][START_REF] Nowaczyk | Geomagnetic events and relative paleointensity variations during the last 300 ka as recorded in Kolbeinsey Ridge sediments, Iceland Sea, indication for a strongly variable geomagnetic field[END_REF], with geomagnetic excursions more frequent and of longer duration than elsewhere. In this context, only additional studies on sedimentary sequences from southern and northern high latitudes can provide the necessary experimental evidence to understand past geomagnetic variability within the polar regions, inside or close to the surface projection of the tangent cylinder.

In this context, since there are only few relative paleointensity determinations from high southern latitudes, along the peri-Antarctic margins [e.g. [START_REF] Brachfeld | High-resolution paleomagnetic records from Holocene sediments from the Palmer Deep, western Antarctic Peninsula[END_REF]2003;[START_REF] Sagnotti | Environmental magnetism of late Pleistocene sediments from the pacific margin of the Antarctic Peninsula and interhemispheric correlation of climatic events[END_REF][START_REF] Macrì | A composite record of Late Pleistocene relative geomagnetic paleointensity from the Wilkes Land Basin (Antarctica)[END_REF]2006], the reconstruction of a high-resolution paleomagnetic record from the CADO sediments represents a valuable input for understanding geomagnetic field dynamics in the Antarctic region during the past few kyrs.

A c c e p t e d M a n u s c r i p t 2 Geological setting and lithostratigraphy

The MD03-2595 CADO core was recovered during the MD130 cruise of the R/V "Marion Dufresne II" from the continental rise of the WLB on a sediment wave field in front of the Mertz Glacier (Lat. 64° 54.04 S, Long. 144° 46.37 E) (Fig. 1A). The depositional system of the WLB was formerly investigated by the joint Australian and Italian WEGA (WilkEs land GlAcial history) Project, with about 600 km of multichannel seismic reflection and 3.5 kHz acoustic profiles and 11 piston cores triggered [see [START_REF] Brancolini | Post Cruise Report AGSO Survey 217: Joint Italian/Australian Marine Geoscienze Expedition Aboard the R.V. Tangaroa to the Geotge Vth Land Region of East Antartica during February[END_REF]. The morphology of this area is characterized by several submarine canyons incising the slope, and a ridge-channel system perpendicular to the continental margin that originated from the interplay of turbidites and bottom currents [START_REF] Escutia | Cenozoic Sedimentation on the Wilkes Land Continental Rise, Antarctica[END_REF][START_REF] Rebesco | The history of sedimentation on the continental rise west of the Antarctic Peninsula[END_REF][START_REF] Busetti | Plio-Quaternary sedimentation on the Wilkes land continental rise: preliminary results[END_REF].

The 35-m long CADO core was collected along the transect "Line W39" (Fig. 1A) crossing the depositional flank of the sediment ridge B and the Buffon Canyon (Fig. 1B), and it recovered a continuous sedimentary sequence characterized by a higher chronostratigraphic potential with respect to the already studied and relatively short WEGA cores (each ca 4 meter long) collected in the adjacent area (see Fig. 1A). The sedimentary sequence consists of very fine-grained terrigenous sediments including intervals of massive homogeneous mud, with some sparse finegrained ice rafted debris (IRD), alternated to laminated mud intervals interpreted as the result of traction currents and\or distal turbidities [START_REF] Escutia | Cenozoic Sedimentation on the Wilkes Land Continental Rise, Antarctica[END_REF][START_REF] Busetti | Plio-Quaternary sedimentation on the Wilkes land continental rise: preliminary results[END_REF]Caburlotto, 2003]. The massive mud facies appears bioturbated, particularly in the first 6 m at the top of the core, and are characterized by a relatively higher content of organic matter and well preserved open-ocean diatoms. The laminated intervals are composed of mm-to cm-thick muddy and silty layers with lack of bioturbation and general lower content of organic matter.

Sampling and methods

A c c e p t e d M a n u s c r i p t

In December 2004, 1.5 m long u-channels were sampled from the 24 archive sections of the MD03-2595 core at the core repository of the Antarctic Climate and Ecosystems Cooperative Research Centre-CRC in Canberra, Australia. Because of empty sections, u-channel samples were not collected for 640-660 cm, 790-820 cm, 1007-1050 cm, 1209-1221 cm, and 1450-1519 cm depth intervals of core.

The u-channels were then measured in a magnetically shielded paleomagnetic laboratory at the Istituto Nazionale di Geofisica e Vulcanologia, Rome, using an automated pass through 2-G Enterprises DC SQUID cryogenic magnetometer system with a 4.5 cm pass through diameter access and an in-line Bartington instrument MS2C susceptibility loop sensor, a set of three orthogonal alternating field (AF) demagnetizing coils with optional anhysteretic remanent magnetization (ARM) capabilities and an isothermal remanent magnetization (IRM) pulse magnetizer. High-resolution (1-cm spacing) records of natural remanent magnetization (NRM), ARM and low-field volume specific magnetic susceptibility (κ) were obtained for all u-channels from the MD03-2595 core.

The raw magnetic moment data measured by the three orthogonal SQUID sensors of the cryogenic magnetometer were corrected, directly by the measuring software, taking into account the different areas under the SQUID response curves. This correction compensates for the effects of the negative regions on the edge of the response functions for the transverse axes and for the broader width of the response function along the long axis of the u-channel. It therefore ensures that the data are free from artefacts that may result from an inadequate consideration of the different widths and shapes of the response functions for the three SQUID pick-up coils, which could result in fictitious inclination shallowing (or steepening) of the paleomagnetic data [see [START_REF] Roberts | High-resolution magnetic analysis of sediment cores: Strengths, limitations and strategies for maximizing the value of long-core magnetic data[END_REF]. Moreover, we took particular care in avoiding artefacts eventually due to deformation effects introduced during sampling, which could result in remanence deflections. We adopted a conservative approach in this study, disregarding the paleomagnetic data for ~5cm at both ends of each u-channel and stratigraphic gap.

A c c e p t e d M a n u s c r i p t

After measuring κ, the NRM was progressively subjected to alternating field (AF) demagnetization in nine steps up to a maximum peak field of 100 mT. Then, an ARM was imparted by translating the u-channels at a speed of 10 cm/s in a constant symmetric AF of 100 mT with a superimposed direct current (DC) bias field of 0.1 mT, which was subsequently stepwise demagnetized using the same sequence of AF steps applied to the NRM. An IRM was then imparted in a steady DC field of 0.9 T. However the intensity of the IRM exceeded the dynamic range of the SQUIDs, so we do not report these data below. In order to investigate the magnetic mineralogy of the sediments, specific rock magnetic investigations were also performed on eight representative powder samples. Magnetic susceptibility variations in heating-cooling cycles from room temperature up to 700°C were obtained using a Kappabridge KLY-3 AGICO magnetic susceptibility meter, coupled with a CS3 furnace. Hysteresis properties, including coercive force (B C ), saturation remanent magnetization (M RS ) and saturation magnetization (M S ) were measured on a MicroMag alternating gradient magnetometer (AGM model 2900, Princeton Measurements Corporation) with a maximum applied field of 1 T. The acquisition of an IRM, and subsequent back-field remagnetization, both in a succession of fields up to 1 T, was obtained with the same AGM instrument. The remanent coercive force (B CR ) was computed from the back-field remagnetization curves.

Results

Paleomagnetism

Demagnetization diagrams (Fig. 2) indicate stable paleomagnetic behavior throughout most of the core, with the data aligned along linear demagnetization paths toward the origin, after removal of a viscous low coercivity remanence component at 10-20 mT. The characteristic remanent magnetization (ChRM) was computed by principal component analysis on the individual linear demagnetization paths [START_REF] Kirschvink | The least-squares line and plane and the analysis of paleomagnetic data[END_REF], measured at 1 cm spacing, and was generally isolated in the 20-80 mT, or rarely 40-80 mT, step intervals. The ChRM was not A c c e p t e d M a n u s c r i p t identified in a few short intervals, that are characterized by unstable paleomagnetic behavior, and for the depth interval 1955-2095 cm, because a technical trouble occurred during demagnetization steps higher than 20mT for u-channel 14. Fig. 3 shows the downcore stratigraphic trends of the ChRM declination and inclination, the maximum angular deviation (MAD) and the RPI curves NRM 20mT /κ and NRM 20mT /ARM 20mT . The high latitude of the study site guarantees that the ChRM inclination is representative enough of the paleomagnetic vectors, and can provide unambiguous identification of the paleomagnetic polarity. The ChRM inclination trend shows that the core is mostly of normal polarity. A distinctive abrupt polarity change is recorded at the bottom of the core, across ca 27 cm of section, with the 0° inclination mid-point positioned at a depth of ca 3400 cm. About 45 cm below the polarity transition, another sharp inclination swing, spanning ca 12 cm of section, is recorded: the inclinations pass again through the horizontal, approach full normal polarity (-73°), and then quickly progress to shallow values and finally return back to the reverse polarity mean values. With regards to the ChRM declination, we point out that even if the absolute azimuthal orientation is not granted for each u-channel and relative azimuthal displacements are possible at each u-channel and stratigraphic break, it generally consistently oscillates around 360°. A few sharp, and stratigraphically limited, ChRM declination oscillations mostly correspond to intervals of nearly vertical ChRM or to the polarity transition. MAD angles are usually less than 2° along the core, indicating that the ChRM directions are generally very well defined; higher values (MAD up to 15°) were obtained for a few intervals at the bottom of the core in correspondence to the polarity transition (Fig. 3). The RPI records, computed by normalizing the NRM 20mT by κ and ARM 20mT , show a similar pattern supporting a general coherency between the two normalization procedures. RPI records will be discussed in closer detail in section 4.3.

Rock magnetism

A c c e p t e d M a n u s c r i p t

In Fig. 4 we show the downcore stratigraphic trends of the main rock magnetic parameters measured for the MD03-2595 CADO core. Oscillations of the magnetic susceptibility κ and of the ARM 0mT intensities throughout the core are limited, with values comprised within the same order of magnitude, which is one of the requisites to obtain reliable RPI estimates (e.g., Tauxe, 1993). In particular, κ fluctuates around a mean value of 35 x 10 -5 SI (with a full range of variability from ca 5 x 10 -5 SI to 60 x 10 -5 SI), with few sharp peaks corresponding to sparse pebbles or mm thick silty layers. ARM 0mT mostly oscillates around 2 x 10 -1 A/m (with a full range of variability from 0.05 to 5 x 10 -1 A/m). As a result of the limited variation of both κ and ARM 0mT , the ARM 0mT /κ ratio is also rather constant, with values fluctuating between 0.5 and 1.5 x 10 3 A/m. The median destructive field of the NRM (MDF NRM ) is generally comprised between 10 and 40 mT; several swings to lower and higher MDF NRM values (from 5 mT to 50 mT, respectively) are evident in the bottom part of the core (last 3-4 m) where the intensity of the NRM is lower. The MDF ARM values show a substantial uniformity as well, with a mean value of ca 28 mT. The ranges of variability for all the rock magnetic parameters are very similar to the those observed in the nearby WEGA cores [START_REF] Macrì | A composite record of Late Pleistocene relative geomagnetic paleointensity from the Wilkes Land Basin (Antarctica)[END_REF].

In order to perform rock magnetic analysis, 8 sub-samples were directly picked from the uchannels at different selected levels corresponding to high/low MDF NRM or ARM 0mT /κ values (Fig. 4). All thermomagnetic curves (Fig. 5A,B,C) show an abrupt decrease of the magnetic susceptibility value at a temperature of 550°-580°C, corresponding to the Curie temperature of magnetite (Fe 3 O 4 ). Several samples are also characterized by a peak at ca 280°C followed by a distinctive decrease between 280°C and 430°C (fig 5A, B), that may be due to the thermally induced conversion of maghaemite (γ-Fe 2 O 3 ) to haematite (α-Fe 2 O 3 ) [START_REF] Stacey | The physical principles of rock magnetism[END_REF]. The presence of hematite is indicated by the decreasing trend of the heating curves above 580°C, which is observed in some samples. A thermally induced production of newly formed magnetite is indicate by the cooling curves which are always well above the heating curves (e.g. mT (Fig. 6A), only two samples are characterized by lower coercivity (B c of ca 10 mT) (Fig. 6B). The hysteresis ratios mostly fall in a restricted region of the "Day plot" [START_REF] Day | Hysteresis properties of titanomagnetites. Grainsize and compositional dependence[END_REF], typical of pseudo-single domain (PSD) magnetite grains (Fig. 6C). The κARM vs. κ plot can be used to infer grain size variations of ferrimagnetic particles [e.g. [START_REF] King | A new rock-magnetic approach to selecting sediments for geomagnetic paleointensity for the last 4000 years[END_REF], even if we recognize that, in practice, it cannot be rigorously applied to determine the actual grain size of magnetite particles because the ARM intensity varies significantly depending on the experimental procedures, not yet fully standardized between individual laboratory [see [START_REF] Sagnotti | Inter-laboratory calibration of low field magnetic and anhysteretic susceptibility measurements[END_REF], and because magnetostatic interaction may affect the efficiency of ARM acquisition [START_REF] Sugiura | ARM, TRM and magnetic interactions: concentration dependence[END_REF]Yamazaki and Ioka, 1997]. However, the κARM vs. κ plot indicates a substantial uniformity in the overall magnetic mineral grain size of the sediments (Fig. 6D), with few, but pronounced, departures from the main cluster of the data corresponding to distinctive peaks in the magnetic susceptibility linked to rare pebbles or silty layers (i.e. 30-33 cm, 1792-1795 cm, 2826-2829 cm, 2934-2937 cm and 3181-3193 cm depth intervals, see Fig. 3). These intervals, which provided rock magnetic data out of the typical range of variability, were not taken into account for the reconstruction of the RPI records.

Further rock magnetic measurements are available on 70 discrete samples selected from the WEGA cores [see [START_REF] Macrì | A composite record of Late Pleistocene relative geomagnetic paleointensity from the Wilkes Land Basin (Antarctica)[END_REF], which show hysteresis parameters and thermomagnetic curves that are very similar to those measured for the CADO samples.

We conclude that all the rock magnetic parameters of the WEGA and the CADO cores indicate that these sediments share the same magnetic carriers, which consists of low-coercivity minerals, most likely represented by a mixture of PSD magnetite and maghemite grains.

Relative paleointensity and age model

As the CADO core shows a well defined ChRM and fulfils the basic requirements of a substantial uniformity in lithology and in concentration, composition and grain size of the A c c e p t e d M a n u s c r i p t magnetic minerals [START_REF] King | A new rock-magnetic approach to selecting sediments for geomagnetic paleointensity for the last 4000 years[END_REF][START_REF] Meynadier | Relative geomagnetic intensity of the field during the last 140 ka[END_REF]Tauxe, 1993;Valet and Meynadier, 1998], we used the obtained paleomagnetic and rock magnetic data to reconstruct variation in relative paleointensity of the geomagnetic field. RPI curves were computed by normalizing the NRM demagnetized at 20 mT with different concentration-dependent rock magnetic parameters (NRM 20mT /κ and NRM 20mT /ARM 20mT ) (see Fig. 3). All the data far from the uniformity criteria, and that could affect the normalized intensities, were not included in the reconstruction of the RPI records. The two RPI curves generally match closely, showing the same stratigraphic patterns throughout the core and only different amplitude in some levels.

We considered the NRM 20mT /κ record as the best RPI proxy (as discussed below) and then we correlated it to the available global RPI stack SINT-800 of [START_REF] Guyodo | Global changes in geomagnetic intensity during the past 800 thousand years[END_REF] (Fig. 7A).

Ages from the dated reference curve were transferred to the CADO RPI record on the basis of visual correlation of the main paleointensity features using the Analyseries 1.2 software of [START_REF] Paillard | Macintosh program performs time-series analysis[END_REF], which allows adjustment and dating between selected pairs of depth and age tie points of two numerical series. Result of correlation indicates that the CADO core extends back to about 800 ka across the Matuyama-Brunhes (M-B) polarity transition (Fig. 7B).

The reconstructed average sediment accumulation rate ranges from 1.6 to 8.6 cm/ka (as calculated from the ages assigned at the individual tie-points; see the step-like line in Fig. 7C).

The mean sedimentation rate for the entire core is 4.4 cm/ka, which is compatible with its position within the WLB, close to the depositional flank of the sediment ridge B. By comparison, the average sedimentation rates for six short cores collected in an adjacent area varied between 19 cm/ka for core PC19 along the WEGA channel and 0.6 cm/ka for core PC20 which was recovered from a ridge crest [START_REF] Macrì | A composite record of Late Pleistocene relative geomagnetic paleointensity from the Wilkes Land Basin (Antarctica)[END_REF].

The CADO RPI record was also compared with the PISO-1500 global RPI stack recently published by [START_REF] Channell | Stacking paleointensity and oxygen isotope data for the last 1.5 Myr (PISO-1500)[END_REF] and with other southern high latitude records compiled by Tauxe and Yamazaki [2007] and available from the database of the Magnetics Information A c c e p t e d M a n u s c r i p t Consortium (MagIC) website (Fig. 8). This comparison indicates that the major RPI lows may be considered global in nature and serve as a target for age control.

In order to confirm the efficiency of the RPI normalization procedure, we tested whether the normalized records are coherent with respect to each other and with respect to the normalization parameters, using the coherence function (Fig. 9). Times series were at first prepared by removing any linear trend using a linear function (integration method), after that the spectral analysis was performed using the software package Analyseries 1.2 [START_REF] Paillard | Macintosh program performs time-series analysis[END_REF] with the Blackman-Tukey method, a Bartlett window and a band-width of ca 0.006 kyr -1 .

The coherence function analysis suggests that the normalized intensity curves are coherent with each other over most of the relevant frequency range, but they are generally not coherent with the normalization parameters, especially for NRM 20mT /κ series (showing significant coherence above the 95% confidence level only in frequencies from 0.005 to 0.011, roughly around the 100 kyr eccentricity cycle) (Fig. 9A). A coherence extending also to higher and lower frequencies was obtained for the NRM 20mT /ARM 20mT and ARM 20mT parameters. Usually the efficiency of ARM acquisition may be decreased by magnetostatic interaction between SD grains [START_REF] Sugiura | ARM, TRM and magnetic interactions: concentration dependence[END_REF]. This effect could bring to an erroneous evaluation of the magnetizability of the sediment when using ARM as the normalizer for relative paleointensity estimations and may result in a significant coherence between the NRM/ARM and ARM [Yamazaki, 2008].

However, magnetostatic interaction is not likely to produce important effects in the CADO sediments, which are mostly characterized by PSD ferromagnetic grains. In any case, we conclude that the magnetic susceptibility κ is the best choice for RPI normalization of the sampled sequence, although we recognize that a climatic overprint at eccentricity frequency may affect the reconstructed RPI curve.

A significant coherence appears between ChRM inclination variations and paleointensity (NRM 20mT /κ), though no such coherency was observed between ChRM inclination and ARM 0mT /κ confirming that shallow inclinations are not influenced by magnetic grain size (Fig.

A c c e p t e d

M a n u s c r i p t 9B). As expected, there is some coherency and an in-phase relationship between NRM 20mT /κ and ARM 0mT /κ records in specific intervals of the time series, indicating a magnetic grain size dependency of the RPI record. However, even if the efficiency of the remanence acquisition seems to be influenced by magnetic grain size, the coherency of the computed NRM 20mT /κ with the SINT-800 reference curve of [START_REF] Guyodo | Global changes in geomagnetic intensity during the past 800 thousand years[END_REF] (Fig. 9B) and with the PISO-1500 stack of [START_REF] Channell | Stacking paleointensity and oxygen isotope data for the last 1.5 Myr (PISO-1500)[END_REF] (Fig. 9C), over most of the frequency spectrum indicates that the reconstructed geomagnetic RPI variations are significant and can be globally correlated. With the aim to detect the presence of possible periodicities in the time series, standard Fourier spectra were calculated for normalization parameters (ARM 20mT and κ), the normalized RPI records (NRM 20mT /κ and NRM 20mT /ARM 20mT ) and for ChRM inclination and ARM 0mT /κ ratio (Fig. 10). Spectral analysis did not point out any periodicity corresponding to the Milankovitch orbital precession (19-23 ka), obliquity (41 ka) or eccentricity (100 ka) cycles for the normalized RPI curves (NRM 20mT /κ and NRM 20mT /ARM 20mT ), but reveals a significant 100 ka periodicity for both the normalizing parameters κ and ARM 20mT , with a minor influence of the 41 ka periodicity for the magnetic susceptibility κ. Orbital eccentricity and obliquity periodicities also appear in the ChRM inclination and ARM 0mT /κ parameters.

ChRM inclination, VGP directions and the M-B polarity reversal

Above the reverse to normal polarity transition, the ChRM inclination values fluctuate around a mean value of ca -68°, which is ca 8° shallower than the geomagnetic inclination value of -76.8° expected at the high latitude of the coring site (Fig. 11). Paleomagnetic inclination shallowing is a well known process linked to deposition of magnetic particles and subsequent sediment compaction [START_REF] Deamer | Compaction-induced inclination shallowing in synthetic and natural clay-rich sediments[END_REF][START_REF] Arason | Compaction and inclination shallowing in deep-sea sediments from the Pacific Ocean[END_REF]. Shallow bias, or inclination error, has been found in laboratory re-deposition experiments and in some modern natural sediments [see Tauxe and Kent, 1984;[START_REF] Clement | Dependence of the duration of geomagnetic polarity reversals on site latitude[END_REF]Tan et al., 2003]. The inclination shallowing A c c e p t e d M a n u s c r i p t observed in sediments deposited in saline waters could also be a function of the process of flocculation, which in turn relates to clay content [van Vreumingen, 1993a;b]. A recent numerical modelling of the relationship between inclination shallowing and relative paleointensity in sediments indicates that in absence of inclination shallowing the relative paleointensity recording mechanism may be non-linear [START_REF] Mitra | Full vector model for magnetization in sediments[END_REF]. The occurrence of inclination shallowing appears therefore as a prerequisite for reliable relative paleointensity studies. However, the CADO ChRM inclination record in the interval of constant normal polarity shows some short-lived swings to anomalously low values (i.e. <45° see Fig. 3), that cannot be justified by a sedimentary inclination shallowing only. The frequency of anomalously shallow ChRM inclination data (see histogram of Fig. 11A) is similar to that found on Antarctic lava flows of the Erebus volcanic province [START_REF] Lawrence | Paleomagnetic field properties at high southern latitude[END_REF], which supports the geomagnetic origin of these features. In the following, we will try to verify if, according to the RPI age model, such shallow ChRM inclinations in the CADO core may correlate to age of known global geomagnetic excursions of the Brunhes Chron [for comprehensive reviews see [START_REF] Langereis | Magnetostratigraphy and astronomical calibration of the last 1.1 Myr from an eastern Mediterranean piston core and dating of short events in the Brunhes[END_REF][START_REF] Lund | Geomagnetic field excursions occurred often during the last million years[END_REF][START_REF] Singer | Ar/Ar ages from transitionally magnetized lavas on La Palma, Canary Islands, and the geomagnetic instability timescale[END_REF][START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF]Thouveny et al., 2004;[START_REF] Roberts | Geomagnetic excursions: knowns and unknowns[END_REF][START_REF] Roberts | Geomagnetic excursions: knowns and unknowns[END_REF].

In Fig. 12 the ChRM inclination and the virtual geomagnetic poles (VGP) latitude of the CADO core are plotted versus age and compared to the CADO RPI and the SINT-800 RPI stack, with vertical grey bars drawn to indicate the main intervals of low RPI in the SINT-800 stack of [START_REF] Guyodo | Global changes in geomagnetic intensity during the past 800 thousand years[END_REF]. The position of the main established geomagnetic excursions for the Brunhes Chron is indicated by arrows, with their estimated age range mostly derived from the Geomagnetic Instability Time Scale (GITS) of [START_REF] Singer | Ar/Ar ages from transitionally magnetized lavas on La Palma, Canary Islands, and the geomagnetic instability timescale[END_REF][START_REF] Roberts | Geomagnetic excursions: knowns and unknowns[END_REF], and the review of Thouveny et al. [2008]. The validated geomagnetic excursions, sensu [START_REF] Roberts | Geomagnetic excursions: knowns and unknowns[END_REF], are indicated in bold.

Starting from the younger RPI minimum, we observe that a long interval of shallow inclination (up to ca -30°) occurs around 20-35 ka (MAD<1°). This age range includes the occurrence of A c c e p t e d M a n u s c r i p t the Mono Lake excursion whose age has been established at 32 ka by [START_REF] Singer | 40Ar/39Ar dating links Albuquerque Volcanoes to the Pringle Falls excursion and the Geomagnetic Instability Time Scale[END_REF].

According to our reconstructed age model, a data gap occurs around 41 ka, the age of the Laschamp excursion [START_REF] Guillou | On the age of the Laschamp geomagnetic excursion[END_REF]. Lower in the sequence, a distinct sharp shallow inclination peak (up to -42°), corresponding to a broad RPI minimum, occurs at 108 ka, close the age interval assigned to the Blake excursion (ca 110-120 ka, Tric et al. [1991]; Zhu et al.

[1994]). All the mentioned shallow ChRM inclination values correspond to intervals with average ARM/κ and MDF NRM values, and are characterized by well defined ChRM with MAD<2°, therefore they do not seem to represent zones of local unusual magnetic mineralogy.

On the contrary, other oscillations to anomalously shallow ChRM values (up to ca -35° MAD<1.5°), observed at ca 62 ka, 279 ka and 404 ka, that do not correlate to any of the established Brunhes geomagnetic excursions, correspond to a bioturbated layer with darker mottles, a silty layer and a layer with sparse pebbles, respectively.

Several smoothing processes affect the ability of a sedimentary sequence to record transient geomagnetic features, including smoothing of the geomagnetic signal due to lock-in processes or magnetic mineral reduction related to diagenesis [i.e., deMenocal et al. 1990;[START_REF] Lund | Measurement of the degree of smoothing in sediment paleomagnetic secular variation records: an example from Late Quaternary deep-sea sediments of the Bermuda Rise, western North Atlantic Ocean[END_REF]Tarduno and Wilkison, 1996;[START_REF] Roberts | Why are geomagnetic excursions not always recorded in sediments? Constraints from post-depositional remanent magnetization lock-in modelling[END_REF][START_REF] Sagnotti | Evidence for a variable paleomagnetic lock-in depth in the Holocene sequence from the Salerno Gulf (Italy): implications for "high-resolution" paleomagnetic dating[END_REF], or due to the instrumental response function of the pick-up coils in the magnetometer [START_REF] Oda | Deconvolution of long-core paleomagnetic data of Ocean Drilling Program by Akaike's Bayesian Information Criterion minimization[END_REF][START_REF] Guyodo | Deconvolution of u-channel paleomagnetic data near geomagnetic reversals and short events[END_REF]. In the CADO core the absence of clear records of many known geomagnetic excursions may probably results from smoothing associated to the relatively low sedimentation rate (on average ca 4.4 cm/kyr) that could be unsuitable to preserve the details of transient geomagnetic field variations [see [START_REF] Lund | Measurement of the degree of smoothing in sediment paleomagnetic secular variation records: an example from Late Quaternary deep-sea sediments of the Bermuda Rise, western North Atlantic Ocean[END_REF][START_REF] Kent | Correlation of paleointensity variation records in the Brunhes/Matuyama polarity transition interval[END_REF][START_REF] Roberts | Why are geomagnetic excursions not always recorded in sediments? Constraints from post-depositional remanent magnetization lock-in modelling[END_REF][START_REF] Roberts | Geomagnetic excursions: knowns and unknowns[END_REF]. An alternative explanation is that geomagnetic excursions were not globally manifested and they did not produce large deviations from the average geomagnetic field directions at the location of the CADO core. In fact, there is a counter argument in the literature against the significance of post-depositional sedimentary smoothing [e.g. [START_REF] Hartl | A precursor to the Matuyama/Brunhes transition-field instability as recorded in pelagic sediments[END_REF]Tauxe, 2006], which supports the hypothesis Fig. 13 shows the ChRM inclination, declination, MAD and VGP computed for the bottom (last 100 ka) of the core. The polarity reversal recorded at ca 780 ka can be correlated to the Matuyama-Brunhes (M-B) transition (whose mid-point was dated at ca 789 ka by [START_REF] Quidelleur | The age and duration of the Matuyama-Brunhes transition from new K-Ar data from La Palma (Canary Islands) and revisited 40Ar/39Ar ages[END_REF], at ca 774 ka by [START_REF] Channell | The Matuyama-Brunhes boundary interval (500-900 ka) in North Atlantic drift sediments[END_REF], at ca 776 ka by [START_REF] Coe | Matuyama-Brunhes reversal and Kamikatsura event on Maui: paleomagnetic directions, 40Ar/39Ar ages and implications[END_REF] and [START_REF] Singer | 40Ar/39Ar dating links Albuquerque Volcanoes to the Pringle Falls excursion and the Geomagnetic Instability Time Scale[END_REF]). The M-B transition feature is defined by a reverse-to normal polarity change, taking place within a 27 cm interval, which occurs in a single u-channel and correlates to a RPI minimum [see [START_REF] Kent | Correlation of paleointensity variation records in the Brunhes/Matuyama polarity transition interval[END_REF][START_REF] Channell | The Matuyama-Brunhes boundary interval (500-900 ka) in North Atlantic drift sediments[END_REF]. Taking into account the average sedimentation rate of 4.4 cm/ka calculated by the RPI correlation for this interval, the time it takes for the full directional change corresponds to 6.8 ka, which is comparable to the duration for the M-B reversal proposed in the literature [START_REF] Hartl | A precursor to the Matuyama/Brunhes transition-field instability as recorded in pelagic sediments[END_REF][START_REF] Coe | Matuyama-Brunhes reversal and Kamikatsura event on Maui: paleomagnetic directions, 40Ar/39Ar ages and implications[END_REF][START_REF] Clement | Dependence of the duration of geomagnetic polarity reversals on site latitude[END_REF][START_REF] Hyodo | Millennial-to submillennial-scale features of the Matuyama-Brunhes geomagnetic polarity transition from Osaka Bay, southwestern Japan[END_REF].

Below the transition, at ca 790 ka, an abrupt inclination swing, reaching values of the full normal polarity mean (-74°), could be correlated to a globally recognized precursor of the M-B reversal which has been previously reported in deep-sea cores from different oceans and dated from 10 to 17 ka prior to the M-B reversal, with a duration estimated from 2 to 5 ka [START_REF] Kent | Correlation of paleointensity variation records in the Brunhes/Matuyama polarity transition interval[END_REF][START_REF] Hartl | A precursor to the Matuyama/Brunhes transition-field instability as recorded in pelagic sediments[END_REF]Yamazaki andOda, 2001, Dinarès-Turell et al., 2002].

The precursor was also radiometrically dated in various volcanic sites with an age range spanning from ca 791-793 to 798 ka [START_REF] Singer | Ar/Ar ages from transitionally magnetized lavas on La Palma, Canary Islands, and the geomagnetic instability timescale[END_REF][START_REF] Coe | Matuyama-Brunhes reversal and Kamikatsura event on Maui: paleomagnetic directions, 40Ar/39Ar ages and implications[END_REF][START_REF] Brown | Paleomagnetic directions and 40Ar//39Ar ages from the Tatara-San Pedro volcanic complex, Chilean Andes: Lava record of a Matuyama-Brunhes precursor?[END_REF].

Considering again a mean sedimentation rate of 4.4 cm/ka for this part of the MD03-2595 CADO core, the duration of the precursor (spanning ca 12 cm of stratigraphic interval) should be ca 2.7 ka, with a time interval between the mid-point of the reversal and the precursor of ca 10 ka. Both are estimates in good agreement with the values reported in the literature [see [START_REF] Channell | The Matuyama-Brunhes boundary interval (500-900 ka) in North Atlantic drift sediments[END_REF][START_REF] Hyodo | Millennial-to submillennial-scale features of the Matuyama-Brunhes geomagnetic polarity transition from Osaka Bay, southwestern Japan[END_REF].

A c c e p t e d M a n u s c r i p t

In principle, a delayed lock-in of the remanent natural magnetization in the stratigraphic intervals slightly older than a geomagnetic reversal may cause a record of apparent multiple polarity changes of the ChRM component in a sedimentary sequence [see [START_REF] Spassov | A lock-in model for the complex Matuyama-Brunhes boundary record of the loess/palaeosol sequence at Lingtai (Central Chinese Loess Plateau)[END_REF]. This process is caused by variable relative contributions of quite different remanence carriers and components during the reversal, and could gives rise to an irregular polarity lock-in at different stratigraphic depths. In the CADO core, however, the homogeneity of the lithology and of all the rock magnetic parameters through the whole core, indicates that a geomagnetic origin of the ChRM oscillation recorded before the M-B transition is by far most likely and we conclude that the CADO core preserves a detailed record of the geomagnetic M-B precursor.

Discussion and conclusion

The collection of well defined paleomagnetic data at high-resolution throughout the study sequence, and the consequent compilation of detailed ChRM directions and RPI record, brought important contributions to the reconstruction of the geomagnetic field variability from high latitudes of the southern hemisphere, and therefore to the understanding of the key general features of the Earth's magnetic field within the polar regions.

The CADO RPI curve shows that paleointensity oscillations with periods longer than a few ka can be matched to the global RPI stacks and that a few sharp oscillations to anomalously low ChRM values, occurring during intervals of RPI lows, may represent the smoothed record of geomagnetic excursions or, alternatively, document the occurrence of only limited deviations from average paleomagnetic direction at the site location during the whole Brunhes Chron. In any case, on the whole the paleomagnetic record of the Brunhes Chron does not indicate a substantially larger variability of the geomagnetic field with respect to coeval records from intermediate and low latitudes with comparable sedimentation rates. This is in contrast with the larger variability reported for coeval sediment cores from the Arctic region [e.g. [START_REF] Nowaczyk | High resolution magnetostratigraphy of four sediment cores from the Greenland Sea -I. Identification of the Mono Lake excursion, Laschamp and Biwa I/Jamaica geomagnetic polarity events[END_REF][START_REF] Nowaczyk | Geomagnetic events and relative paleointensity variations during the last 300 ka as recorded in Kolbeinsey Ridge sediments, Iceland Sea, indication for a strongly variable geomagnetic field[END_REF].

A c c e p t e d M a n u s c r i p t

Though the relatively low geomagnetic variability and the relatively modest record of known geomagnetic excursions, the paleomagnetic record of the CADO core preserves a relatively detailed registration of the Matuyama-Brunhes reversal and supports the global existence of a precursor preceding by a few ka the full major reversal. As a result, this record represents the southernmost registration of the M-B transition and its precursor and indicates that the timing, rate and dynamics of such reversal and event, at high southern latitudes, are not substantially different from those reconstructed elsewhere.

In addition, orbital periodicities (100 ka and 41 ka) are found for the ChRM paleomagnetic inclination record, which are similar to those reported for other sedimentary cores at lowlatitudes [Yamazaki and Oda 2002;[START_REF] Clement | Dependence of the duration of geomagnetic polarity reversals on site latitude[END_REF]. These data support the model of Yamazaki and Oda [2002], which suggests a connection between the geodynamo and the orbital eccentricity, indicating that long-term geomagnetic secular variation in inclination are controlled by changes in the relative strength of the geocentric axial dipole and persistent non-dipole components. The significant coherence between ChRM inclination variations and paleointensity (NRM 20mT /κ) are also similar to those obtained on a sediment core from the western equatorial Pacific by Yamazaki and Oda [2002;[START_REF] Clement | Dependence of the duration of geomagnetic polarity reversals on site latitude[END_REF] and indicates that shallower (steeper) inclinations may correlate with paleointensity minima (maxima).

Finally, rock magnetic data from the CADO core indicate a substantial uniformity of concentration, composition and grain size of the magnetic minerals contained in the study sequence. On the one hand, this uniformity is consistent with the observed homogeneity of sedimentary facies and indicates that the alternation of paleoclimatic phases that characterized the last ca 800 ka did not produce pronounced oscillations in the environmental magnetic proxies at the core location. This result matches the previous findings of [START_REF] Macrì | A composite record of Late Pleistocene relative geomagnetic paleointensity from the Wilkes Land Basin (Antarctica)[END_REF] for shorter cores from the WLB and indicates a substantial stability of the East Antarctic Ice Sheet during the middle and late Pleistocene. On the other hand, a climatic periodicity of 100 ka is detectable in the κ and ARM 20mT parameters, together with a minor contribution of the 41 ka van Vreumingen, M.J., 1993b. The Magnetization Intensity of Some Artificial Suspensions While Flocculating in a Magnetic-Field. Geophys. J. Int. 114 (3), 601-606. Zhu, R.X., Zhou, L.P., Laj, C., Mazaud, A., and Ding, Z.L., 1994. The Blake geomagnetic episode recorded in Chinese loess. Geophys. Res. Lett., 21: 697-700. Yamazaki, T., and Ioka, N., 1997. Cautionary note on magnetic grain-size estimation using the ratio of ARM to magnetic susceptibility. Geophys. Res. Lett., 24: 751-754.

Figure captions

Fig. 1 -A) Bathymetric map of the continental rise of the Wilkes Land Basin: the morphology of the rise is characterized by a channel-ridge system elongated perpendicular to the margin.

Points aligned on acoustic Line 26 shows the location of the previously studied WEGA cores (see text). B) 3.5 kHz acoustic profile crossing the Buffon Canyon (Line W39) with location of the MD03-2595 core. Redrawn from [START_REF] Busetti | Plio-Quaternary sedimentation on the Wilkes land continental rise: preliminary results[END_REF]. Fig. 5 -Thermomagnetic curves of the samples selected for rock magnetic analyses, showing the variation in the magnetic susceptibility (κ) with temperature in heating-cooling cycles from room temperature to 700°C. The sharp drop at ca 580°C in all the heating curves indicates the presence of magnetite, whereas the thermal instability between 250 and 350°C suggests the presence of maghemite which is converted to hematite during the thermal treatment. The cooling path is always well above of the heating path and indicates that new magnetite is produced by thermal alteration during heating. Fig. 6 -A) and B) Representative hysteresis loops and C) B cr /B c vs M r /M s plot [START_REF] Day | Hysteresis properties of titanomagnetites. Grainsize and compositional dependence[END_REF] for the selected specimens of the CADO core. B cr : remanent coercive force, B c : coercive force, M r : saturation remanent magnetization, M s : saturation magnetization. Hysteresis parameters were computed after correction for the high-field paramagnetic slope. SD= single domain, PSD= pseudo single domain, and MD= multidomain grains.. D) The ARM/κ ratio, indicates a substantial uniformity in magnetic mineral grain size, apart from a few data from distinct intervals, which were disregarded for RPI analyses Fig. 7 -SINT-800 reference curve of [START_REF] Guyodo | Global changes in geomagnetic intensity during the past 800 thousand years[END_REF] and the CADO RPI record (NRM 20mT /k). Vertical lines indicate tie points used for correlation. B) After correlation the CADO RPI curve was scaled to its maximum value and plotted together with the SINT-800.

The correlation indicates that the CADO core reaches back to ca 800 ka. C) Downcore variation of the sedimentation rates obtained with the RPI-based age model for the CADO core.

Unbroken line is age versus depth, broken line is depth versus sedimentation rate calculated between the RPI tie points. [START_REF] Guyodo | Global changes in geomagnetic intensity during the past 800 thousand years[END_REF], PISO-1500 from [START_REF] Channell | Stacking paleointensity and oxygen isotope data for the last 1.5 Myr (PISO-1500)[END_REF], Core 1089 from [START_REF] Stoner | A ~570-kyr geomagnetic paleosecular variation record from the sub-Antarctic South Atlantic (ODP Site 1089)[END_REF]], Ks87-752 from Valet et al. [1994], 5-pc01 from [START_REF] Stoner | South Atlantic and North Atlantic geomagnetic paleointensity stacks (0-80 ka): implications for inter-hemispheric correlation[END_REF], Core 1101 from [START_REF] Guyodo | A sedimentary paleomagnetic record of the Matuyama chron from the Western Antarctic Margin (ODP Site 1101)[END_REF], Core PC20 from [START_REF] Macrì | A composite record of Late Pleistocene relative geomagnetic paleointensity from the Wilkes Land Basin (Antarctica)[END_REF].

A c c e p t e d M a n u s c r i p t Fig. 9 -Spectral analysis performed using Blackman-Tukey method with a Bartlett window. A) Square coherence and phase for the RPI curves with respect to each other and with respect to the normalization parameters. B) Square coherence and phase for the ChRM inclination with respect to the NRM 20mT /κ RPI record and the ARM 0mT /κ ratio, and for the NRM 20mT /κ RPI record with respect to the ARM 0mT /κ ratio and the reference SINT-800 stack. B) Square coherence and phase between CADO and PISO-1500 RPI records. See text for explanation. Expected normal and reverse inclinations at the CADO latitude site are shown as a vertical dashed lines. B) Equal-area projections of paleomagnetic directions. D and I are mean declination and inclination calculated using Fisher's statistics [START_REF] Fisher | Dispersion on a sphere[END_REF]] on the nottransitional and not-excursional normal and reverse ChRM directions (i.e. directions providing a VGP latitude greater than ± 45°), with uncertainties estimated by the α 95 parameter. Fig. 12 -CADO RPI record and SINT-800 of [START_REF] Guyodo | Global changes in geomagnetic intensity during the past 800 thousand years[END_REF], compared with ChRM inclination record and VGP latitude of the CADO core. The black arrows indicate geomagnetic excursions recognized in the literature, with the validated excursions, sensu [START_REF] Roberts | Geomagnetic excursions: knowns and unknowns[END_REF], indicated in bold (see text). The grey vertical areas correspond to the main RPI minima in the SINT-800 record of [START_REF] Guyodo | Global changes in geomagnetic intensity during the past 800 thousand years[END_REF].

  fig 5A). Hysteresis loops from the same samples generally indicate a coercivity (B c ) of ca 20

  s c r i p t that the sediment magnetization is locked within the top few centimeters (see the comprehensive review paper by Tauxe and Yamazaki, 2007].

  s c r i p t obliquity periodicity for the κ and κARM 0mT /κ parameters, indicating that subtle variations linked to orbital cycle affect the concentration and grain-size of the magnetic particles in the study sequence.

  Tan, X.D., Kodama, K.P., Chen, H.L., Fang, D.J.,Sun, D.J., and Li, Y.A., 2003. Paleomagnetism and magnetic anisotropy of Cretaceous red beds from the Tarim basin, northwest China: Evidence for a rock magnetic cause of anomalously shallow paleomagnetic inclinations from central Asia. J. Geophys.Res.,108, 2107, doi:10.1029/2001JB001608. Tarduno, J.A., and Wilkison, S.L., 1996. Non-steady state magnetic mineral reduction, chemical lock-in, and delayed remanence acquisition in pelagic sediments. Earth Planet. Sci. Lett., 144: 315-326. Tauxe, L., and Kent, D.V., 1984. Properties of a detrital remanence carried by haematite from study of modern river deposits and laboratory redeposition experiments. Geophys. J. R. Astr. Soc., 77: 543-561. Tauxe, L., 1993. Sedimentary records of relative paleointensity of the geomagnetic field: theory and practice. Rev. Geophys., 31: 319-354. Tauxe, L., Herbert, T., Shackleton, N.J., and Kok, Y.S., 1996. Astronomical calibration of the Matuyama-Brunhes boundary: Consequences for magnetic remanence acquisition in marine carbonates and the Asian loess sequences. Earth Planet. Sci. Lett., 140: 133-146. Tauxe, L., and Kent, D.V., 2004. A simplified statistical model for the geomagnetic field and the detection of shallow bias in paleomagnetic inclinations: Was the ancient magnetic field dipolar? In Timescales of the Paleomagnetic field, Channell, J.E.T. et al., eds., Geophysical Monograph, 145: 101-116. Thouveny, N., Carcaillet, J., Moreno, E., Leduc, G., and Nérini, D., 2004. Geomagnetic moment variation and paleomagnetic excursions since 400 ka BP: a stacked record of sedimentary sequences of the Portuguese margin. Earth Planet.Sci.Lett., 219: 377-396. Tauxe L (2006) Long term trends in paleointensity: The contribution of DSDP/ODP submarine basaltic glass collections. Physics of the Earth and Planetary Interiors, 156: 223-241.

  Tauxe, L., and Yamazaki, T., 2007. Paleointensities. In Schubert, G., (Ed.), Treatise on Geophysics, vol.5, Geomagnetism. Elsevier Ltd, Oxford: 509-564. Thouveny, N., Bourlès, D.L., Saracco, G., Carcaillet, J.T., and Bassinot, F., 2008. Paleoclimatic context of geomagnetic dipole lows and excursions in the Brunhes, clue for an orbital influence on the geodynamo? Earth Planet. Sci. Lett., 275: 269-284. Tric, E., Laj, C., Valet, J.-P., Tucholka, P., Paterne, M., and Gichard, F., 1991. The Blake geomagnetic event: transition geometry, dynamical characteristics and geomagnetic significance. Earth Planet. Sci. Lett., 102: 1-13. Valet, J.P., Meynadier, L., Bassinot, F.C., and Garnier, F., 1994. Relative paleointensity across the last geomagnetic reversal from sediments of the Atlantic, Indian and Pacific Oceans. Geoph. Res. Lett., 21(6): 485-488. Valet, J.-P., and Meynadier, L., 1998. A comparison of different techniques for relative paleointensity. Geophys. Res. Lett., 25: 89-92. van Vreumingen, M.J., 1993a. The Influence of Salinity and Flocculation Upon the Acquisition of Remanent Magnetization in Some Artificial Sediments. Geophys. J. Int. 114 (3), 607-614.

  Yamazaki, T., and Oda, H., 2001. A Brunhes-Matuyama polarity transition record from anoxic sediments in the South Atlantic (Ocean Drilling Program Hole 1082C). Earth Planets Space, 53: 817-827.

  ., and Oda, H., 2002. Orbital influence on Earth's magnetic field: 100,000-year periodicity in inclination. Science, 295: 2435-2438. Yamazaki, T., and Oda H., 2004. Intensity-Inclination Correlation on Long-Term Secular Variation of the Geomagnetic Field and its Relevance to Persistent Non-Dipole Component. AGU Monograph 145 "Timescales of the Internal Geomagnetic Field": 287-298. Yamazaki, T., 2008. Magnetostatic interactions in deep-sea sediments inferred from first-order reversal curve diagrams: Implications for relative paleointensity normalization. Geochem. Geophys. Geosyst., 9, doi: Q02005, doi:10.1029/2007GC001797.

Fig. 2 -

 2 Fig. 2 -Typical alternating field (AF) demagnetization behavior at selected depths of the MD03-2593 core. Paleomagnetic data indicate that, after removal of a viscous component at AF < 10-20 mT, a characteristic remanent magnetization (ChRM) can be easily identified. Orthogonal vector diagrams: open and closed symbols represent projections onto vertical and horizontal planes, respectively. Equal area projections: open (close) symbols represent projection onto the upper (lower) hemisphere.

Fig. 3 -

 3 Fig. 3 -Paleomagnetic directions, maximum angular deviation (MAD) [Kirschvink, 1980] and the normalized relative paleointensity curves NRM 20mT /κ and NRM 20mT /ARM 20mT for the CADO core. The u-channel sections (1 to 24) are indicated to the right of the lithostratigraphic column. The dashed lines on the ChRM declination and inclination plots indicate the expected value of the declination (360°) and inclination (-76.8°) at the core location.

Fig. 4 -

 4 Fig. 4 -Downcore variation of the main rock magnetic parameters measured for the CADO core. The little squares on the right of the lithostratigraphic column indicate the position of the 8

Fig. 8 -

 8 Fig. 8 -Comparison of the CADO RPI curve with the SINT-800 and the PISO-1500 global RPIstacks and with other southern high-latitude RPI records extracted from the MagIC database, in the interval 0-800 ka. Sint-800 from[START_REF] Guyodo | Global changes in geomagnetic intensity during the past 800 thousand years[END_REF], PISO-1500 fromChannell et

Fig. 10 -

 10 Fig. 10 -Standard spectra of the parameters used for the RPI normalisation (κ and ARM), the normalized relative paleointensity curves (NRM 20mT /κ and NRM 20mT /ARM 20mT ), ARM 0mT /κ and the ChRM inclination.

Fig. 11

 11 Fig.11 -A) Histograms of ChRM inclinations and declinations. N is the number of data.

Fig. 13 -

 13 Fig. 13 -ChRM directions, MAD and VGP latitude for the bottom part of the CADO core (interval dated at 750-800 ka). Vertical dashed lines correspond to the u-channel breaks. The gray area shows the interval that was correlated to the uppermost part of the reverse Matuyama Chron preserving a record of the Matuyama-Brunhes transition and of its precursor. See text for discussion.

  Fig. 1

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  Fig.3

  Fig.4

  Fig. 5

  Fig.7

  Fig. 8

Acknowledgments

The MD03-2595 CADO core was studied as part of the QUASAR (QUAternary Sedimentary processes on the east Antarctic continental Rise) project that was funded by the PNRA (Programma Nazionale di Ricerche in Antartide). Members of the QUASAR working group are acknowledge for help with continued discussion on core stratigraphy and sedimentology. We thank two anonymous reviewers for their comments and suggestions, which allowed to improve the manuscript. . . . . . . .. . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . .. .. . . . . .. .. . . .


Maria Xiridou
email: maria.xiridou@rivm.nl

Maaike Van Veen

Maria Prins

Roel Coutinho

How patterns of migration can influence the heterosexual transmission of HIV in the Netherlands

Keywords: Mathematical model, HIV incidence, heterosexuals, migrants

Objectives Migrants originating from countries with high HIV prevalence account for a large number of heterosexually acquired HIV infections in Western European countries. These migrants may be infected before migrating and they may engage in unsafe sexual practices both in the country of residence and in their country of origin. We investigate how patterns of migration may affect the heterosexual HIV epidemic in the Netherlands.

Methods A mathematical model was used that describes the transmission of HIV in heterosexual partnerships between African migrants, Caribbean migrants, and the local Dutch population. Infection of migrants before migrating to the Netherlands and during trips to their country of origin was also accounted for.

Results

The incidence of HIV infection among adult heterosexuals in 2010 was 1.50 new infections per 100,000 individuals per year. If the number of migrants entering the country increases, then the incidence of HIV will increase, although this change among Dutch will be negligible. Moreover, if HIV prevalence among those migrating to the Netherlands (at the time of entry to the country) is higher, then the incidence in the respective ethnic group will increase; among the other ethnic groups, the increase will be very small.

Conclusions Heterosexual transmission of HIV in the Netherlands occurs mostly within migrant communities. Limiting migration and introducing travel restrictions would likely have no effect on HIV incidence in countries with low HIV prevalence among heterosexuals. Policy making should focus on targeted interventions, to reduce the burden of disease in migrant communities in Europe.

INTRODUCTION

In many western European countries, heterosexually acquired HIV infections are frequently found among migrants from countries with high HIV prevalence. [START_REF]HIV/AIDS surveillance in Europe 2007[END_REF][START_REF] Van Den Broek | Sexually transmitted infections, including HIV, in the Netherlands in 2007[END_REF][START_REF]The UK Collaborative Group for HIV and STI Surveillance. A complex picture. HIV and other sexually transmitted infections in the United Kingdom: 2006[END_REF] Migrants may already be infected when they enter the country. Also, they may engage in unsafe sexual practices in the country of residence and in their country of origin, while visiting family and friends. [START_REF] Sadler | Sexual behaviour and HIV infection in black-Africans in England: results from the Mayisha II survey of sexual attitudes and lifestyles[END_REF][START_REF] Kramer | Migrants travelling to their country of origin: a bridge population for HIV transmission?[END_REF] Mathematical modelling indicates that HIV infection during visits of migrants to their home country is very rare and that after migration transmission occurs mostly within migrant communities. [START_REF] Xiridou | Can migrants from high-endemic countries cause new HIV outbreaks among heterosexuals in low-endemic countries?[END_REF] However, patterns of migration may change, for instance due to war, unstable political situations, or due to changing policy regulations about immigration, resulting in restricting or broadening flows of migration. To investigate how such changes may affect HIV incidence, we use a mathematical model that describes the spread of HIV via heterosexual partnerships between migrants and locals. The model is parameterised using data from the Netherlands, serving as example for high income countries with migration from HIV endemic areas.

METHODS

The model is described elsewhere in detail. [START_REF] Xiridou | Can migrants from high-endemic countries cause new HIV outbreaks among heterosexuals in low-endemic countries?[END_REF] The modelled population includes all sexually active adult heterosexuals, 15-69 years old, stratified into three ethnic subpopulations: African migrants, Caribbean migrants, and the remaining "general" Dutch population (here called 'Dutch'). The group of migrants comprises of first and second generation migrants. Individuals get infected via sexual contacts in the Netherlands; migrants may also get infected during trips to their country of origin or they may be already infected at the time of entry to the Netherlands. Parameters relating to sexual behaviour were estimated from several Dutch studies. [START_REF] Van Veen | Disassortative sexual mixing among migrant populations in the Netherlands: a potential for HIV/STI transmission?[END_REF][START_REF] Van Der Klis | Second national serum bank for population-based seroprevalence studies in the Netherlands[END_REF][START_REF] Nisso | Sexual health in the Netherlands[END_REF] In the results, the initial prevalence of HIV was 3.5% for Africans, 0.5% for Caribbeans, and 0.05% for Dutch natives. [START_REF] Van Veen | National estimate of HIV prevalence in the Netherlands: comparison and applicability of different estimation tools[END_REF] Complete description of the data sources and the parameter values is given in [START_REF] Xiridou | Can migrants from high-endemic countries cause new HIV outbreaks among heterosexuals in low-endemic countries?[END_REF].

RESULTS

From the model, the incidence of HIV among heterosexuals is estimated at 1.50 new infections per 100,000 individuals per year in 2010 (infections occurring via sexual contacts in the Netherlands or during trips of migrants to their home country). [START_REF] Xiridou | Can migrants from high-endemic countries cause new HIV outbreaks among heterosexuals in low-endemic countries?[END_REF] Within each ethnic subpopulation, the incidence is 67.18 new infections per 100,000 African migrants, 12.12 per 100,000 Caribbean migrants, and 0.47 per 100,000 Dutch individuals.

Figure 1a shows the incidence of HIV in 2020 if the number of migrants entering the country changes (assuming that the number of second generation migrants remains constant). HIV incidence among migrants increases with the number of new migrants entering the country, although the increase is not large; for instance, among African migrants incidence increases from 65 to 66.5 new infections per 100,000, if the number of new migrants increases from 3000 to 6000. Among Dutch residents, the increase is very small (from 0.392 to 0.405 per 100,000, if the number of migrants entering the country increases from 0 to 6000 -Figure S1 in Appendix). Figure 1b shows that if the prevalence of HIV among those migrating to the Netherlands increases, then the incidence in the respective ethnic group will be considerably higher; in the other ethnic groups, however, there will be only a minor increase (Figure 1b and Figure S2 in Appendix).

In Figure 1c,d it is shown how the incidence of HIV is affected by changes in the duration of stay of African migrants. The incidence among Africans (black line) declines with increasing duration of stay. This is rather surprising because one might expect that the longer migrants stay in the country, the more (different) partners they have and hence more chances to get infected and/or to infect others. Indeed, the actual number of new infections (grey columns) does increase with increasing duration of stay. However, the size of the African community also increases with increasing duration of stay (black columns) and faster than the number of new HIV infections, resulting in a decreasing incidence. In contrast, in the non-African subpopulations, the incidence increases with the duration of stay of African migrants (Figure 1d), since the size of these communities remains constant but there are more new infections. Similar results are obtained by changing the duration of stay of Caribbean migrants: the incidence in their own subpopulation decreases with increasing duration of stay, while the incidence in the other two subpopulations increases; the size of the Caribbean community also increases (results not shown here).

DISCUSSION

We show that changes in patterns of migration can have a considerable impact within the migrant communities living in high income countries like the Netherlands, but they hardly affect the incidence of HIV among heterosexuals in the local population. Consequently, the impact on the overall heterosexual population is minor, due to the small size of the migrant communities compared to that of the local population. Higher numbers of migrants entering the country or higher prevalence among those migrating will result in higher incidence within migrant communities. If migrants stay longer (before returning to their country or moving to another country), then there will be more new infections within their own community, but the incidence will decline, due to the increasing size of the respective migrant community. It should be stressed that these results were obtained using the same duration of stay for first and second generation migrants, because these two subgroups are not distinguished in the model (after entering the population). However, differences between first and second generation migrants have been documented in the Netherlands; for instance, second generation migrants report more sexual mixing with the Dutch than first generation migrants. [START_REF] Xiridou | Can migrants from high-endemic countries cause new HIV outbreaks among heterosexuals in low-endemic countries?[END_REF] The model can be extended to include such differences and investigate their impact on the spread of HIV. Also it would be interesting to examine the impact of re-entrance of migrants (departing from their country of residence and returning after several years) and the importance of co-infections with other sexually transmitted infections.

In conclusion, migrants originating from high-endemic countries will not easily cause considerable increases in HIV incidence in low-endemic countries. Unstable political conditions in a country with high HIV prevalence resulting in new flows of refugees or migrants to Europe are unlikely to pose a threat for an increasing HIV incidence among the local population.

Figure 1 :

 1 Figure 1: The impact of migration on HIV incidence, shown as number of new infections in 2020 per 100,000 individuals of the respective ethnic group. (a) The incidence of HIV among African migrants (solid line; values shown on left vertical axis) and among Caribbean migrants (dashed line; values shown on right vertical axis), calculated with different numbers of first generation migrants entering the population, as shown on the horizontal axis. (b) HIV incidence among African migrants, calculated with different levels of HIV prevalence (%) among those migrating to the Netherlands (at the time of entry to the country): horizontal axis shows prevalence among Africans at entry; the three lines correspond to different prevalence among Caribbeans at entry, 6% (solid line), 3% (dashed line), 0.5% (dotted line). The incidence among Caribbean migrants, among Dutch, and in the whole heterosexual population is shown in the Appendix. (c) HIV incidence among Africans (black line); number of African migrants living in the Netherlands, in thousands (black columns); and number of newly infected Africans (grey columns). (d) HIV incidence among Dutch (dashed line) and among Caribbean migrants (solid line). The horizontal axes in (c) and (d) show the duration of stay (in years) of African migrants in the Netherlands.

Acknowledgements

The authors would like to thank the anonymous referees and the WHO Regional Office for Europe and in particular Martin Donoghoe and Annemarie Rinder Stengaard for their assistance and suggestions.

Competing interests

None.

Author contributions

Maria Xiridou developed and analysed the model and drafted the paper. Maaike van Veen analysed the epidemiological data and obtained parameter estimates for sexual behaviour. Roel Coutinho and Maria Prins contributed to the study design and the interpretation of the results. All authors contributed to the preparation of the manuscript.

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