Anne-Laure Daniau
email: al.daniau@ipgq.u-bordeaux1.fr

Maria Fernanda

Sánchez Goñi
email: mf.sanchezgoni@epoc.u-bordeaux1.fr

Josette Duprat
email: jm.duprat@orange.fr

Last glacial fire regime variability in western France inferred from microcharcoal preserved in core MD04-2845, Bay of Biscay

Keywords:

Dansgaard-^Oeschger events Heinrich events

High resolution multiproxy analysis (microcharcoal, pollen, organic carbon, Neogloboquadrina ^pachyderma (s), ice rafted debris) of the deep-sea record MD04-2845 (Bay of Biscay) provides new insights for understanding mechanisms of fire regime variability of the last glacial period in western France. Fire regime of western France closely follows Dansgaard-^Oeschger climatic variability and presents the same pattern than that of southwestern Iberia, namely low fire regime associated with open vegetation during stadials including Heinrich events, and high fire regime associated with open forest during interstadials. This supports a regional climatic control on fire regime for western Europe through fuel availability for the last glacial period. Additionally, each of Heinrich events 6, 5 and 4 ^is characterised by three episodes of fire regime, with a high regime bracketed by lower fire regime episodes, related to vegetational succession and complex environmental condition changes.

Introduction

Beyond man-made fires, biomass burning depends on climate and also appears to be a significant driver for climate change through the release of greenhouse gases, namely carbon dioxide, methane, nitric oxide and aerosols [START_REF] Crutzen | Biomass burning as a source of atmospheric gases CO, H2, N2O, NO, CH3Cl and COS[END_REF]Lobert et al., 1990;[START_REF] Andreae | Emission of trace gases and aerosols from biomass 715 burning[END_REF][START_REF] Van Aardenne | A 1 X 1 resolution data set of historical anthropogenic trace gas emissions for the period 1890-^1990[END_REF][START_REF] Van Der Werf | Continental-scale partitioning of fire emissions during the 1997 to 2001 El Nino/La Nina Period[END_REF][START_REF] Thonicke | Modeling glacial-^interglacial changes in global fire regimes and trace gas emissions[END_REF]. Variations of atmospheric greenhouse gas concentration in concert with the millenial-scale Dansgaard-^Oeschger (D-^O) climatic variability have been identified in Greenland and Antarctic ice cores [START_REF] Petit | Climate 857 and atmospheric history of the past 420,000 ^years from the Vostok ice core, 858 Antarctica[END_REF][START_REF] Flückiger | N2O and CH4 variations during the last glacial epoch: insight into global processes[END_REF]. However, events in the ocean and on land [START_REF] Sánchez Goñi | Contrasting impacts of Dansgaard-^Oeschger events over a western European latitudinal transect modulated by orbital parameters[END_REF]. The direct 85 correlation between past fire regime and these climatic proxies, in 86 particular vegetation shifts, allows the discussion of the complex 87 relationships between fire and climate. [START_REF] Trigo | The North Atlantic Oscillation influence on Europe: climate impacts and associated physical mechanisms[END_REF][START_REF] Dupuis | Wave climate evolution in the Bay of Biscay over two decades[END_REF]. Annual precipitation of the 95 Loire-^Britany basin is 500-^700 mm and 600-^1000 mm for the Adour-96 ^Garonne basin. The specific region covered by mountains (the Massif 97 Central and the Pyrenean) receive more than 1500 mm. The mean 98 temperatures in winter vary from 0 to more than 8 ^°C on the Atlantic 99 coast, and from 15 to more than 22 ^°C in summer [START_REF] Serryn | Atlas Bordas Géographique[END_REF]. The 100 region is ^colonised today by Atlantic vegetation (Fig. 1) characterised by deciduous forests (mainly oaks, ash, beech, birch) and coniferous trees restricted to montane and some southern maritime areas, and by semi-natural grasslands [START_REF] Polunin | A Guide to the Vegetation of Britain and Europe[END_REF].

Compared to Spain, Portugal, Italy and Greece, France does not experience a high fire activity. The average of 30,738 ha of burnt wooded area and 5218 forest fires for the period AD 1980-^2000 represent only 4% and 10%, respectively, of that of the other countries (European Commission, 2001). Fire activity in France is mainly concentrated in its Mediterranean part, during summer months (July and August). The western part of France is, on the whole, not affected by large fires, but local fires caused by lightning do occur (http://www.feudeforet.org/). However, as the present-day incidence and spatial distribution of fires resulting from lighting strikes is restricted by human activities, this type of ignition may have been more important in the past.

Oceanic surface circulation and wind patterns

The current circulation pattern in the Bay of Biscay (Fig. 2) is characterised by a mean southward surface circulation during summer, the formation of eddies near the shelf break and a weak flow of the slope current. During winter, the mean surface circulation is dominated by a strong poleward intrusion of the slope current along the Iberian Peninsula and the Armorican and Celtic slopes, with a branch of westward orientation inside the Bay of Biscay, generating a particular cyclonic cell circulation centred at 46°N, 6.5°W (Durrieu de [START_REF] Durrieu De Madron | Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay[END_REF][START_REF] Colas | Circulation et dispersion lagrangiennes en Atlantique Nord-Est[END_REF]. This poleward slope current (the Navidad current) in the winter season brings warm surface water into the Bay of Biscay and may affect biological productivity [START_REF] Garcia-Soto | Navidad development in the southern Bay of Biscay: climate change and swoddy structure from remote sensing and in situ measurements[END_REF]. Its strength depends on the wind pattern over the Bay The mean wind direction over the Bay (Fig. 1) is from northwest in summer and from southwest in the winter season. However, a southward direction towards northern Iberia prevailed inside the bay for both situations [START_REF] Colas | Circulation et dispersion lagrangiennes en Atlantique Nord-Est[END_REF]. Upwellings appear during spring and summer period along the shelf area in relation to wind blowing to the coast [START_REF] Heaps | A mechanism for local upwelling along the European continental slope[END_REF]. Northwesterly to northerly wind events create coastal upwellings off southern Brittany, Vendée and Landes. Westerly winds generate upwelling to the north of the Loire [START_REF] Froidefond | Distribution of suspended matter in a coastal upwelling area. Satellite data and in situ measurements[END_REF][START_REF] Lazure | 3D modelling of seasonal evolution of Loire and Gironde plumes on Biscay Bay continental shelf[END_REF][START_REF] Puillat | Hydrographical variability 877 on the French continental shelf in the Bay of Biscay, during the 1990s[END_REF]. Core MD04-2845 is outside of these upwelling areas.

Morphology and recent sedimentation

The Bay of Biscay is characterised by a continental shelf oriented NW-^SE along the French coast and E-^W along the Spanish coast, with a platform relatively flat. Its large width along the French coast prevents direct feeding on the shelf break by bottom nepheloid layers [START_REF] Jouanneau | Fine-grained sediment budget on the continental margin of the Bay of Biscay[END_REF] except for the region of the Cap-Ferret Canyon [START_REF] Ruch | Sediment budget and transfer of suspended sediment from the Gironde estuary to Cap Ferret Canyon[END_REF]. Two sedimentary systems characterise the continental shelf of this bay, an inner shelf (b100-^120 m) and an outer shelf (N120 m) showing a general grain-size fining southward [START_REF] Allen | Carte de répartition des sédiments superficiels sur le 712 plateau continental du Golfe de Gascogne[END_REF]. Pre-and early Holocene offshore mud and sands occupy the zone between 75 and 120 m depth, and coarse sands and gravels carried by rivers between 30-^75 m, probably related to the influence of the Gironde paleoriver detected 50 km seaward of the estuary mouth [START_REF] Lericolais | Seaward pinching out and internal stratigraphy of the Gironde incised valley on the shelf (Bay of Biscay)[END_REF]. Rivers (Fig. 1) are the main sources of fine sediments (including microcharcoal and pollen) to the Bay of Biscay, taking into account that westerlies are the dominant winds in the Bay. The main rivers, the Garonne and the Dordogne feeding the Gironde estuary, the Loire, the Vilaine, the Charente and the Adour rivers, deliver about 2.5 × 10 6 ^t yr - ^1 of continental fine sediments, with the Gironde estuary accounting for 60% of this amount [START_REF] Jouanneau | Fine-grained sediment budget on the continental margin of the Bay of Biscay[END_REF].

The continental suspended matter which spreads out from the Gironde estuary on the shelf is transported to the north along the coast by a surface turbid plume [START_REF] Castaing | Le transfert à l'océan des suspensions estuariennes[END_REF] and may be carried to the Cap-Ferret Canyon in a bottom nepheloid layer [START_REF] Ruch | Sediment budget and transfer of suspended sediment from the Gironde estuary to Cap Ferret Canyon[END_REF]. However, an estimated 0.9 × 10 6 ^t yr - ^1 of fine sediments could reach the slope and eventually the open ocean [START_REF] Jouanneau | Fine-grained sediment budget on the continental margin of the Bay of Biscay[END_REF].

Experimental studies on the present-day pollen-^vegetation relationship confirms that marine pollen assemblages from the south-

171
western French margin represent an integrated image of the regional U

N C O R R E C T E D P R O O F
2845 in the interval HE3/GI4 to HE2/GI2 (mean age difference b400 ^yr ^). The samples presenting conventional AMS 14 C younger than 21,786 14 C yr BP were calibrated by using CALIB Rev 5.0 program and "global" marine calibration data set (marine 04.14c) [START_REF] Stuiver | Extended 14 C database and revised CALIB radiocarbon calibration program[END_REF][START_REF] Hughen | Marine04 marine radiocarbon age calibration, 0-^26 cal kyr BP[END_REF][START_REF] Stuiver | CALIB 5.0[END_REF]. 14 C radiometric ages older than 21,786 Q1 ^14 C yr BP were calibrated by matching the obtained conventional AMS 14 C with the calendar ages estimated for MD95-2042 deep-sea core by Bard et al. (2004).

The core was sampled every 5 cm between 2245 cm and 760 cm for microcharcoal and organic carbon content analyses; this interval encompasses the end of MIS 6 to MIS 2.

Image analysis for microcharcoal counting

A total of 294 samples were analysed for microcharcoal using automated image analysis in transmitted light. Microcharcoal preparation technique followed the work of [START_REF] Daniau | Dansgaard-^Oeschger climatic variability revealed by fire emissions in southwestern Iberia[END_REF].

Chemical treatment was performed over 24 h on 0.2 g of dried sediment and involved the addition of 5 mL of 37% HCl; 5 mL of 68% HNO 3 ; 10 mL of 33% H 2 O 2 . This chemical treatment is used to remove pyrites, humic material, labil or less refractory organic matter (OM)

and to bleach non-oxidised OM. Dilution of 0.1 was then applied to this residue and this suspension was filtered onto a cellulose acetate membrane containing nitrocellulose of 0.45 μm porosity and 47 mm in diameter. A portion of the membrane was dissolved onto a plexiglass slide with ethyl acetate before gentle polishing with alumin powder of 200 ^. The use of polish slide (used classically in petrographic analysis) for image analysis is better than the previously assembling technique consisting of mounting the portion of the membrane onto a slide with Canadian balsam [START_REF] Daniau | Dansgaard-^Oeschger climatic variability revealed by fire emissions in southwestern Iberia[END_REF] as the focus on the edge of microcharcoal is improved. The slides were scanned with an automated Leica DM6000M microscope at × ^500 magnification. In order to have a good statistical representation of each sample, 200 view-fields (200 images) of 0.0614 mm 2 were ^"^g rabbed" in color with a 1044 × 772 ^pixels digitising camera

(1 ^pixel = 0.276 μm). The surface scanned by the microscope represents a surface area of 12.279 mm 2 .

Microcharcoal recognition was performed by using a program of image analysis developed in this study with LeicaQwin software.

The subroutine written for microcharcoal identification during image analysis procedure was established by a simultaneous visual identification of microcharcoal in transmitted light following criteria from [START_REF] Boulter | An approach to a standard terminology for palynodebris[END_REF], namely black debris, opaque, with sharp edges (see also [START_REF] Enache | Tracking recorded fires using charcoal morphology from the sedimentary sequence of Prosser Lake, British Columbia (Canada)[END_REF] and petrographic criteria in reflected light, that is with visible plant structures characterised by thin cell walls and empty cellular 256 cavities, or particles without plant structure but of similar 257 reflectance than that of previous ones [START_REF] Noël | Caractérisation et calibration des flux organiques sédimentaires dérivant 846 du bassin versant et de la production aquatique (Annecy, le Petit lac). Rôles 847 respectifs de l'Homme et du climat sur[END_REF]. In a controlled The three-point running mean "MCCnb" and "MCCsurf" was also 271 calculated for CCnb and CCsurf respectively to be able to compare fire 272 regime with vegetation history of the same core (resolution of 10 cm).

273

The term MCC will be used as global mean average microcharcoal

s = ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 1 n-1 X n i = 1 x i -x ð Þ 2 v u u t 283 284
The minimum of variation of MCC discussed below is higher than the 285 widen confidence interval so that variations observed in MCC curve

286

are not related to an artefact of sampling bias. A mean age (ka) has been calculated for consecutive levels (8.6-8.7 and 8.9-9 m) before applying linear interpolation to estimate age difference between control points (3) and ages based on calibrated AMS 14 C (1,2) . t1:18

a Levels presenting small AMS 14 C inversions. t1:19

U N C O R R E C T E D P R O O F
CO 2 formed was determined quantitatively by infrared absorption.

Replicate analyses were performed on 10 selected samples reanalysed 6 times each (greatest lower and least upper bound are indicated ^in Fig. 3). Last glacial productivity changes revealed by a core located off Lisboa were related to variations in Portugal's upwelling system [START_REF] Pailler | High frequency palaeoceanographic changes during the past 852 140,000 yr recorded by the organic matter in sediments of the Iberian margin. 853 Palaeogeography[END_REF]. However, the Gascogne seamount is far from regions affected today by upwellings in the Bay of Biscay, namely the shelf break or river plumes. This site was also outside the direct influence of upwelling during the LGP because the shelf break was still drowned at the maximum lowstand sea level (maximum of -^120 m depth for the last glacial maximum). Therefore, a different mechanism to that explaining the OC content variation in the Iberian margin must be responsible for the observed shifts in OC between GS and GI in our core.

Results

Microcharcoal analysis

Concentration of

Forest development detected in core MD04-2845 during GI (Fig. 5) suggests an increase of temperatures and precipitation. At present, the increase of humidity as well as river run-off in southwestern France is related with the negative mode of the NAO [START_REF] Trigo | The North Atlantic Oscillation influence on Europe: climate impacts and associated physical mechanisms[END_REF][START_REF] Dupuis | Wave climate evolution in the Bay of Biscay over two decades[END_REF]. Climatic conditions in this region during GI might be related, as suggested for southern Iberia [START_REF] Combourieu Nebout | Enhanced aridity and atmospheric high-pressure stability over the western Mediterranean during the North Atlantic cold events of the past 50 ^ky[END_REF][START_REF] Moreno | Saharan dust transport and high-latitude glacial climate variability: the 831 Alboran sea record[END_REF][START_REF] Sánchez Goñi | Synchroneity between marine and terrestrial responses to millennial scale climatic variability during the last glacial period in the Mediterranean region[END_REF], to a prevailing negative NAO-like situation. Under this climatic regime, SST warming might also be linked to enhanced influence of a warm paleo Navidad current, as occurs today during negative NAO index [START_REF] Garcia-Soto | Navidad development in the southern Bay of Biscay: climate change and swoddy structure from remote sensing and in situ measurements[END_REF], favouring productivity blooms and OC increase during the GI. Moreover, our hypothesis of the paleo-Navidad current strengthening under a negative NAO-like mode during GI is consistent with the observed weakening of Mediterranean Outflow Water (MOW) at that time [START_REF] Voelker | Mediterranean outflow strengthening during northern hemisphere coolings: a salt source for the glacial Atlantic?[END_REF][START_REF] Toucanne | Contourites of the Gulf of Cadiz: a high-resolution record of the paleocirculation of the Mediterranean outflow water during the last 50,000 ^years[END_REF]. The formation of western Mediterranean Deep Water, which likely was a major source of the MOW during the last glacial [START_REF] Myers | Modeling the paleocirculation of the 836 Mediterranean: the last glacial maximum and the Holocene with emphasis on the 837 formation of sapropel S1[END_REF][START_REF] Voelker | Mediterranean outflow strengthening during northern hemisphere coolings: a salt source for the glacial Atlantic?[END_REF], is at present positively correlated to the NAO [START_REF] Rixen | The Western Mediterranean Deep Water: a proxy for climate change[END_REF].

The comparison between this past oceanic scenario pattern with the modern oceanic and atmospheric circulations (Fig. 2) suggests that GI would be, as in the modern situation, more influenced by southwesterly winds (winter situation) than GS periods that were characterised by prevailing northwesterlies (summer situation). As both wind systems come from the ocean, we do not expect therefore a substantial aeolian contribution of microcharcoal to the Gascogne [START_REF] Bradshaw | Holocene records of fire from the boreal and temperate zones of Europe[END_REF]Mouillot et 470 al., 2002;[START_REF] Calvo | Regeneration patterns in a Calluna vulgaris heathland in the Cantabrian mountains (NW Spain): effect of burning, cutting and ploughing[END_REF]. Beyond the influence of the type and 471 density of the vegetation on the fire regime through changes in fire 472 frequency or severity, increase of microcharcoal concentration during 473 periods of afforestation suggests the increase of microcharcoal 474 production related to increase of woody fuel and biomass accumula-475 tion during GI as total plant biomass in forested areas is generally 476 higher than open ground formations (Magri, 1994). This increase of woody fuel during GI indicates an increase of humidity and associated sea and air surface temperature warming as likely the result of a prevailing negative NAO-like index.

U N C O R R E C T E D P R O O F

Increase of fire regime related to woody fuel availability for the LGP has been already suggested by [START_REF] Daniau | Dansgaard-^Oeschger climatic variability revealed by fire emissions in southwestern Iberia[END_REF] for southwestern Iberia and inferred from the strong correlation between MCC and forest percentage curves. This hypothesis is supported by the simulation of [START_REF] Ni | Impact of climate 843 variability on present and Holocene vegetation: a model-based study[END_REF] for semi-arid regions where lack of fuel, related to drought, often limits the incidence of fire. Although the relationship between fire regime and fuel changes occurs also in western France during the LGP, the tight correlation found in Iberia is less evident in western France. This suggests that in addition to fuel, a direct climatic control may have operated on the fire regime variability of the latter region during the LGP. The reason may be that D-^O climatic variability has a lesser impact on vegetation shift between GS and GI which allows the development of a mosaic of vegetation with a dominance of grasses through both GS and GI. At present, a sharp contrast of precipitable water exists in Iberia between negative NAO (high precipitation) and positive NAO (marked drought) situations. Although the same pattern occurs in southwestern France, the contrast of precipitable water is less marked in this region [START_REF] Trigo | The North Atlantic Oscillation influence on Europe: climate impacts and associated physical mechanisms[END_REF]. Sánchez [START_REF] Sánchez Goñi | Contrasting impacts of Dansgaard-^Oeschger events over a western European latitudinal transect modulated by orbital parameters[END_REF] show that the boundary of the Mediterranean region was similar to that of the present-day during GI. This would imply that a similar gradient of precipitable water than that observed at present-day between the two regions may occur during GI and suggests that during GS (positive NAO-like situation), moisture content in western France is sufficient to develop a continuous vegetation cover, mainly grasses, while lack of winter moisture in southwestern Iberia triggered the development of semidesert vegetation.

Zooming in on Heinrich events 6, 5 and 4

The vegetation of HE 6, 5 and 4 (Fig. 6), mainly composed of Pinus woodlands, some Picea (well developed during HE6), Sphagnum, Calluna, and Cyperaceae species, is typical of present-day Boreal or central European vegetation, characterised by evergreen coniferous forest and mires which indicate high rainfall areas [START_REF] Polunin | A Guide to the Vegetation of Britain and Europe[END_REF]. The relative high percentages of Artemisia during HE indicates this species occupies drier soils.

Heinrich event 4. Fire regime during HE4 presents a three-phase structure (Fig. 6 ^a). Decrease of fire regime at the beginning of HE4, phase (a), is contemporaneous with a large reduction by ∼ 50% ^of Pinus percentages contemporaneous with the succession of Cyperaceae, Sphagnum and Calluna progressively dominating the pollen assemblage. Succession of Cyperaceae and Sphagnum indicates progressive colonisation of water-filled depressions by fens (minerotrophic peatland) then by bogs (ombrotrophic), which is the typical ecological succession of peatland (mire) [START_REF] Mcnamara | Hydrogeologic controls on 827 peatland development in the Malloryville Wetland, New York (USA)[END_REF]. This plant succession suggests also a vegetation of transitional mire which forms a mosaic of bog and fen, or occurs where bog and fen succeed each other [START_REF] Polunin | A Guide to the Vegetation of Britain and Europe[END_REF]. Whereas the development of Sphagnum indicates moist or waterlogged environments, this species colonises the upper part of the water table [START_REF] Rochefort | Does prolonged flooding prevent or enhance regeneration and growth of Sphagnum?[END_REF].

Most Sphagnum species are absent or poorly represented in habitats with an extremely fluctuating water table compared to Calluna vulgaris which prefers considerable fluctuations [START_REF] Laitinen | Community and species responses to water level fluctuations with reference to soil layers in different habitats of mid-boreal mire complexes[END_REF]. As for HE4 and 5, the low fire regime during the first and last phases may be related to low fuel content and relative moist conditions limiting fire ignition and spread, respectively. [START_REF] Greisman | Fire, forest and cultural landscape history during the last 11 ^000 ^years in Småland -^a case study at Stavsåkra[END_REF]. Paleosols testifying to short occurrence of permafrost have been detected in this region during the youngest HE2 and 1 (Bertran et al., 2008). In addition, evidence of permafrost formation during the HE6 time interval is suggested from growth cessation of the Grotte de Villars's speleothem located in southwestern France [START_REF] Genty | Precise dating of Dansgaard-^Oeschger climate oscillations in western Europe from stalagmite data[END_REF].

Fire regime stays at a high level during (b2) although this period is contemporaneous with woody fuel decrease (Pinus and Picea population decline) and increase in humidity (Pediastrum and Cyperaceae increase). However, Pinus forest decline was smaller than in HE4 and 5. At the same time, Betula forest-steppe expanded at the expenses of Picea (Fig. 5). This environmental succession might indicate thawing of permafrost under the onset of particular warming and drying trend, similar to that reported by Blyakharchuk and Sulerzhitsky (1999) for the western Siberia forest zone during the Holocene. This particular warming coincides with the start of obliquity increase (Fig. 5) and, therefore, enhancement of seasonal contrast at northern latitudes

U

  N C O R R E C T E D P R O O F 74 examine whether the fuel amount is, as in southwestern Iberia, the 75 main factor controlling the millennial-scale fire regime variability in 76 this region. For this, we analysed microcharcoal particles preserved in 77 deep-sea core MD04-2845 in the Bay of Biscay at the latitude of 78 Bordeaux. Complementary analysis of organic carbon on this core has 79 been carried out in order to understand better the microcharcoal 80 signal. Preliminary δ 8 O isotopic measurements and previous studies 81 of this core (carbonates, ice rafted debris, planktonic foraminifera, 82 pollen) have shown that it covers Marine Isotope Stage (MIS) 7 to 1 83 and revealed the impact of D-^O climatic variability and Heinrich84

  vegetation, fire and climate 90Western France extends from 42°N to 48°N (Fig.1) and comprises 91 essentially the Loire-^Britany and the Adour-^Garonne river basins. This

  92 region is characterised by a temperate and oceanic climate. Winter 93 precipitation in its southern part is particularly influenced by the NAO 94

Figure 1 .

 1 Figure 1. Map of France showing the location of core MD04-2845 (filled star). Black dotted lines represent the division of the Atlantic and Sub-atlantic vegetation domains simplified from Ozenda (1982). Area 1: ^Southwestern France is characterised by vegetation composed of Quercus robur, Q. pyrenaica, Castanea sativa, Pinus pinaster with some Q. suber. Area 2: This region presents a mosaic of Quercus pubescens, Q. robur and Q. petrae. Some Q. ilex is found near the coast. Area 3: ^Vegetation of the central-western part of France is mainly characterised by a mosaic of Q. robur on alluvial soils and Q. petraea associated with hornbeam (Carpinus betulus) on slope and drained areas. Area 4: ^Vegetation of the north-western part, characterised by a high level of precipitation, is a mosaic of three main floristic associations including Fagus forest, Fagus-Quercus robur forest and heathland community dominated by Calluna and Ulex. Near the coast, Fagus forest is replaced by Quercus-Fraxinus forest. Area 5: ^The Massif Central is colonised mainly by Q. robur, Q. petraea, Q. pubescens and Castanea sativa between 600 ^and 800 m. Fagus, Fagus-Pinus forest, heathland, Ulex, and Pinus sylvestris forest are found above. Between 1500 and 1600 m the vegetation is composed of heathland of Calluna and Juniperus sibirica.

  U N C O R R E C T E D P R O O F related to the NAO, with a low NAO index leading to an intensified Navidad current (Garcia-Soto et al., 2002).

Figure 2 .

 2 Figure 2. Map of oceanic circulation and wind directions in the Bay of Biscay during summer and winter situations (modified from Durrieu de Madron et al., 1999 and Koutsikopoulos and Le Cann, 1996). Below 200 m depth, oceanic circulation during winter is represented by black lines (with a strong poleward of the slope current corresponding to the Navidad current). Black dotted lines represent oceanic circulation during summer. Shelf residual current (along the coast) and wind induced currents (grey double arrows) are found on the shelf. Arrows represent dominant wind directions. WD w (black arrow): wind direction in winter. WD s (grey dotted arrow): wind direction in summer.

  258 light adjustment, a threshold value in red, green, blue (RGB) color 259 was defined to identify microcharcoal based on several observa-260 tions of particle on different slides. 261 From measured variables of microcharcoal (surface area, length, 262 width, elongation, number) we calculated for each sample: 263 a) the concentration of microcharcoal (CCnb): number of micro-264 charcoal per gram (nb g - ^1) 265 b) the concentration of microcharcoal surface (CCsurf), which is the sum 266 of all surfaces of microcharcoal in one sample per gram (μm 2 g - ^1). 267 This is to avoid the overrepresentation of microcharcoal concentra-268 tion as the result of potential fragmentation during production 269 (Théry-Parisot, 1998) or transport.270

287

  Particulate ^organic ^carbon (POC) analysis288The organic carbon (OC) content was determined directly on dry 289 weight material sediment by combustion in an LECO CS 200 analyzer 290 (Etcheber et al., 1999) on the same sample depth as that used for 291 microcharcoal analysis. Around 50 mg of powdered and homo-292 genized samples were acidified in crucibles with 2 N HCl to destroy 293 carbonates and then dried at 60 ^°C to remove inorganic carbon and 294 most of the remaining acid and water. The OC analysis was 295 performed by direct combustion in an induction furnace, and the

Figure 3 .

 3 Figure 3. Results from microcharcoal and organic carbon content and comparison with other marine proxies of core MD04-2845. All records are plotted versus depth. Limits of Marine Isotopic Stages (MIS) and the Eemian Interglacial are also indicated. From left to right: (a) the carbonate percentages (CaCO 3 ), (b) the ice rafted debris (IRD) concentration, (c) the polar foraminifera Neogloboquadrina pachyderma (s) abundance percentages, (d) the organic carbon content (OC) percentage, (e) the microcharcoal number concentration curve (CCnb) and the three-point running mean (MCCnb), (f) the microcharcoal surface area concentration (CCsurf) and the three-point running mean (MCCsurf). Grey rectangle shown on the OC curve represents the greatest lower and least upper bound of replicates. Dotted bar on microcharcoal concentration curve (CCnb and CCsurf) indicate the 95% confidence interval. Grey bands indicate Heinrich events and other Dansgaard-Oeschger stadials. Heinrich events are identified on the basis of peaks in ice rafted debris (IRD), high polar foraminifera (N. pachyderma s.) percentages and AMS 14 C ages. Greenland interstadial numbers (GI) are also shown. 1714-1740 cm: sedimentary hiatus.

Figure 4 .

 4 Figure 4. Spectral analysis applied to linear detrended OC content and microcharcoal concentration (CCsurf and CCnb) before applying the Lomb periodogram algorithm (Past software). Periodicities (ka) have been selected for a 0.01 (99%) significance level. Peaks in the spectrum are presented with frequency (X-axis) and power value (Y-axis) (units proportional to the square of the amplitudes of the sinusoids present in the data).

Figure 5 .

 5 Figure 5. Comparison between microcharcoal trends and climatic proxies from core MD04-2845. All records are plotted versus age. From left to right: (a) sea surface temperature derived from foraminifera assemblages for February (SSTFeb) and August (SSTAug), (b) three-point running mean microcharcoal number concentration curve (MCCnb), (c) three point-running mean microcharcoal surface area concentration curve (MCCsurf), (d) pollen concentration, (e) to (p) pollen group percentage curves, (q) obliquity curve from Laskar et al. (2004). Boreal forest: at 90% Picea, Alnus and Salix; pioneer trees: mainly Betula, Cupressaceae and Hippophäe; temperate deciduous forest: deciduous Quercus, Carpinus and Corylus; grasses: Poaceae; semi-desert: Chenopodiacaeae, Ephedra and Artemisia. Although Betula can also belong to boreal forest, percentage variation in our record does not follow the Picea pattern, which suggests its inclusion in the Atlantic forest. 68-70 ka: sedimentary hiatus.

Figure 6 .

 6 Figure 6. Comparison between microcharcoal concentration and climatic proxies during Heinrich events (HE). (a) HE4, (b) HE5, (c) HE6. All records are plotted versus age. (a) Sea surface temperature of February (SSTFeb) and August (SSTAug), (b) three-point running mean.

  traced in our record by several well-marked GI characterised by the highest development of Betula associated with summer SST above 15 ^°C. This particular warming could explain the maintenance of strong fire regime during (b2) likely resulted from particularly high summer temperatures, despite of the decline of Pinus and Picea forest. Conclusion Microcharcoal and organic carbon analyses were applied to deepsea core MD04-2845 recovered in the Bay of Biscay, at 45°N. Marine productivity variability revealed by organic carbon content parallels D-^O climatic variability and is interpreted as a proxy of paleo-Navidad current strength. This current appears to have been reinforced during GI as the result of the prevailing negative NAO-like mode, and inversely during GS. Both GI and GS climatic situations were marked by dominant westerlies and microcharcoal concentration variation is independent of changes in fluvial sedimentary input implying that D-U N C O R R E C T E D P R O O F 669 ^O microcharcoal concentration variability reflects fire regime variation 670 in western France during the last glacial period. 671 The highest fire regimes are observed during GI related to high-672 fuel coniferous-mixed wooded vegetation. Decreasing fire regime is 673 associated to fuel reduction in a steppe dominated environment 674 during HE and the other GS. This fire regime pattern is similar to that 675 observed in southwestern Iberia for the same time period and 676 supports a regional climatic control of fire regime, through biomass 677 changes, in western Europe. 678 Contrary to the continuous low fire regime during HE in south-679 western Iberia (Daniau et al., 2007), a three-phase structure 680 characterises fire regime within HE6, 5 and 4 in western France. The 681 highest fire regime episode is bracketed by two low fire regime 682 phases. The first low fire regime phase is related to the Pinus forest 683 decline and wet conditions, while the fire increase is contempora-684 neous with an afforestation. The late fire decrease is, paradoxically, 685 associated with the maintenance of substantial woody fuel and dryer 686 conditions than those of the previous phase. We explain this late 687 decrease as the result of a new phase of mire expansion. This would 688 indicate climatic conditions sufficiently moist to limit fire spread. 689 During HE6, the strong relationship between the highest fire regime 690 and the expansion of Picea and Sphagnum communities would reflect 691 the formation of discontinuous permafrost. 692 Q2 Uncited reference Association of Stratigraphic Palynologists Foundation, pp. 1011-1064.

  The high fire regime during (b1) may be explained by the development of highly fire sensitive Boreal forest system(Bergeron et al., 2004) Picea mariana trees and Sphagnum dry peat, suggesting that cold temperature is not the unique factor involved in permafrost formation. Alternation between Picea and Sphagnum species in this region is triggered by fire. Picea trees diminish the solar radiation at the surface soil and permafrost begins to form. Fire alters Picea forest, the permafrost layer begins to melt and Sphagnum develops due to unfrozen conditions. Over time, peat accumulates and succession leads to favourable conditions for tree colonisation, which may promote permafrost formation.

	associated with particularly dry climate as indicated by the strong
	development of semi-desert vegetation.
	Beyond climatic conditions favourable to Picea and Sphagnum
	establishment during this period, high fire regime may have also
	contributed to the resulting vegetation assemblage. Permafrost forms
	in peatlands when mean annual temperature (MAT) is below 0 ^°C and becomes continuous when MAT is under -. However, Camill ^6^°C (2000) reports permafrost formation in Canadian boreal peatlands
	related to dense

We assume as a working hypothesis that Picea and Sphagnum grew together in western France during HE6. If we apply the present-day model of P. mariana in North America to Picea in western Europe, at longer time scale, the development of Picea-Sphagnum association during (b1) concomitant with high fire regime suggests that discontinuous permafrost occurred in western France during the transition MIS 4/MIS 3. Moreover, forest fires of this period were probably intense because the expansion of Sphagnum populations is favoured by high-temperature forest fires

Please cite this article as: Daniau, A.-L. et al., Last glacial fire regime variability in western France inferred from microcharcoal preserved in core MD04-2845, Bay of Biscay, Quaternary Research (2009), doi:10.1016/j.yqres.2009.01.007

Batten, D.J., 1996. Palynofacies and palaeoenvironmental interpretation. In: Jansonius, J.,

McGregor, D.C. (Eds.), Palynology: Principles and Applications, 3. American

Journal of Quaternary Science, doi:10.1002/jqs.1220.

Acknowledgments 695 The work of A.-L. D. was supported by a BDI CNRS-région Aquitaine

fellowship. This study was funded by the RESOLuTION (ESF-EURO-697 CORES-EUROCLIMATE) and ARTEMIS projects. We gratefully acknowl-

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Bertran, P., Allenet, G., Gé, T., Naughton, F., Poirier, P., Sanchez Goni, M.F., 2008. Blyakharchuk, T.A., Sulerzhitsky, L.D., 1999. Holocene