Manuela Capano
email: capano@cerege.fr

Cécile Miramont

Lisa Shindo

Frédéric Guibal

Christian Marschal

Bernd Kromer

Thibaut Tuna

Edouard Bard
email: bard@cerege.fr.

• Edouard Bard

Onset of the Younger Dryas Recorded with 14 C at Annual Resolution in French Subfossil Trees

Keywords: 14 C calibration, carbon cycle, dendrochronology, interhemispheric gradient, Younger Dryas

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Radiocarbon ( 14 C) is commonly used to date biogenic samples sourcing from over the past 50,000 yr. While alive, organisms equilibrate their 14 C/ 12 C ratio with that of the atmospheric pool and, when they die, this ratio begins to decrease as 14 C decays. The 14 C/ 12 C ratio of a biological sample correlates with the time that has elapsed since its death [START_REF] Libby | Chicago radiocarbon dates V[END_REF]. However, the atmospheric 14 C/ 12 C ratio varies through time and space due to fluctuations of 14 C production rate (originating mainly via changes in solar activity and/or in geomagnetic field intensity) and to changes in the carbon cycle, making it necessary to correct atmospheric 14 C fluctuations in order to calculate accurate ages. For this purpose, a calibration curve is needed, obtained by comparing raw 14 C ages with true calendar ages derived from independent dating methods (e.g. [START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF].

Various archives are used to construct the 14 C calibration curve, but the best is based on dendrochronologically dated tree-ring series. For the Holocene (≈ the past 11,600 yr), subfossil trees are abundant, allowing the construction of a calibration curve dated absolutely with annual resolution. For times before the Holocene, the availability of trees is limited to some areas and periods [START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF]. While subfossil trees that lived during warm periods (e.g. Bølling and Allerød phases) are fairly common [START_REF] Miramont | L'apport des séries dendrochronologiques françaises au prolongement de la chronologie européenne absolue et à la calibration du radiocarbone[END_REF][START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF][START_REF] Adolphi | Radiocarbon calibration uncertainties during the last deglaciation: Insights from new floating tree-ring chronologies[END_REF][START_REF] Reinig | New tree-ring evidence for the Late Glacial period from the northern pre-Alps in eastern Switzerland[END_REF], trees corresponding to the Younger Dryas cold phase are exceptional [START_REF] Miramont | L'étalonnage du temps du radiocarbon par les cernes d'arbre. L'apport des series dendrochronologiques du gisement de bois subfossiles du torrent des Barbiers (Alpes Françaises du sud)[END_REF]. Indeed, the Younger Dryas (YD) gap prevents the Bølling-Allerød floating sequences from being connected with the absolute chronology [START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF][START_REF] Reinig | New tree-ring evidence for the Late Glacial period from the northern pre-Alps in eastern Switzerland[END_REF]. [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF] analyzed 14 C in two trees of the Barbiers chronology from the southeast French Alps and reconstructed a continuous 240-yr chronology that was used to tentatively link the absolute calibration curve [START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF] with the floating CELM (Central European Lateglacial Master) chronology for the Allerød period. In this follow-up study, we present an augmented 416-yr tree-ring chronology based on five trees from the Barbiers site, annually 14 C dated to the onset of the YD period.

The 14 C content measured in French subfossil trees is also compared to kauri records from the Southern Hemisphere (Hogg et al. 2016a(Hogg et al. , 2016b) ) and employed to study the atmospheric 14 C gradient between the Northern and Southern Hemispheres (NH and SH). Indeed, a systematic 14 C difference exists between wood from trees that lived at the exact same time in the NH and SH (SH wood being generally older than NH wood). Precise measurement of this 14 C interhemispheric gradient ( 14 C-IHG) and its variations over the Allerød-YD transition is of prime interest for identifying changes in the global carbon cycle during the last deglaciation. The chronological accuracy of the tree-ring record allows comparison of the 14 C-IHG variations with other carbon cycle changes evidenced in the atmospheric CO 2 and CH 4 records measured in bubbles occluded in polar ice cores over the Late Glacial period [START_REF] Marcott | Centennialscale changes in the global carbon cycle during the last deglaciation[END_REF].

C Chronologies from Tree Rings over the Younger Dryas Event

The synchronization of the German HOC (Holocene Oak Chronology) and PPC (Preboreal Pine Chronology) sequences gave rise to a long dendrochronological series, starting (i.e. oldest year) at 12,410 cal. BP [START_REF] Friedrich | The 12,460-year Hohenheim oak and pine tree-ring chronology from Central Europe; a unique annual record for radiocarbon calibration and paleoenvironment reconstructions[END_REF]). This chronology was then extended into the Younger Dryas period with the Swiss YD sequences, leading to an absolute chronology starting at 12,594 cal BP [START_REF] Schaub | Environmental change during the Allerød and Younger Dryas reconstructed from tree-ring data[END_REF][START_REF] Hua | Atmospheric 14 C variations derived from tree rings during the early Younger Dryas[END_REF][START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF]. However, [START_REF] Hogg | The New Zealand kauri (Agathis australis) research project: A radiocarbon dating intercomparison of Younger Dryas wood and implications for IntCal13[END_REF] highlighted problems in the dendrochronological synchronization of the Ollon505 larch tree with the PPC. Indeed, the 14 C data of decadal tree-ring samples from the absolute chronology were included in the IntCal13 calibration curve, but the Ollon505 larch tree was excluded [START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF]).

In the framework of the 2015 IntCal Workshop in Zurich, Michael Friedrich corrected the matching of sequences and validated the chronology up to 12,325 cal BP [START_REF] Kromer | Progress report on dendrochronology and 14C of the Hohenheim Preboreal/YD and Late Glacial pine chronologies[END_REF]. New dendrochronological and 14 C analyses performed in Zurich have been dedicated to filling the gap between the absolute chronology and the YD floating chronologies [START_REF] Reinig | New tree-ring evidence for the Late Glacial period from the northern pre-Alps in eastern Switzerland[END_REF]. In addition, floating tree-ring chronologies also exist for the Allerød period: the German LGP (Late Glacial Pine) and the Swiss SWILM (Swiss Late-glacial Master) chronologies, synchronized in the CELM chronology; Italian chronologies from Carmagnola and Avigliana; and French chronologies from the Drouzet site [START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF][START_REF] Adolphi | Radiocarbon calibration uncertainties during the last deglaciation: Insights from new floating tree-ring chronologies[END_REF].

For the YD onset, only a few floating chronologies are available for the NH:

• The Barb12-17 sequence, a 240-yr chronology belonging to the French Barbiers chronology that has been sampled at annual resolution for 14 C measurements [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF]. Preliminary 14 C dating of the Barb12-17 sequence between ca. 12,836 to 12,594 cal BP [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF]) was based on visual matching of wiggles against the kauri sequence from New Zealand (Hogg et al. 2016a(Hogg et al. , 2016b)).

• The youngest part of the CELM chronology, a 1606-yr chronology composed of Swiss and German pines from the Danube [START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF]. The last ring of the youngest tree of the sequence (G5) has been dated in several works to between ca. 12,500 and 12,844 ± 32 cal BP [START_REF] Hughen | Synchronous radiocarbon and climate shifts during the last deglaciation[END_REF][START_REF] Muscheler | Tree rings and ice cores reveal 14 C calibration uncertainties during the Younger Dryas[END_REF][START_REF] Hua | Atmospheric 14 C variations derived from tree rings during the early Younger Dryas[END_REF][START_REF] Bronk Ramsey | A complete terrestrial radiocarbon record for 11[END_REF]Hogg et al. 2016b;[START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF]).

• The Swiss YD chronologies. YDB (Younger Dryas B) was the oldest series included in the absolute dendrochronological sequence starting at 12,594 cal BP [START_REF] Schaub | Environmental change during the Allerød and Younger Dryas reconstructed from tree-ring data[END_REF]). The 14 C measurements have been complemented with measurements at annual resolution of the original YD series and of the new floating Late Glacial subfossil pine chronologies from Zurich, Switzerland, the so-called "Binz" find [START_REF] Reinig | New tree-ring evidence for the Late Glacial period from the northern pre-Alps in eastern Switzerland[END_REF][START_REF] Sookdeo | Quality dating: A well-defined protocol for quality highprecision 14 C-dates tested on Late Glacial wood[END_REF].

For the SH, the YD period is recorded in two floating chronologies. As is the case for NH records, these chronologies have been dated through 14 C measurements:

• The huon pine [Lagarostrobos franklinii (Hook.f.) Quinn] chronology from Tasmania (Australia), a floating 617-yr chronology dated through wiggle-matching to ca. 12,760-12,070 cal BP [START_REF] Hua | Atmospheric 14 C variations derived from tree rings during the early Younger Dryas[END_REF]). The LGP chronology reaching ca. 14,200 cal BP, was linked to the absolute chronology by using the huon pine bridge [START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF]).

• The kauri [Agathis australis (D. Don) Steudel] chronology, a 1451-yr floating chronology from Towai (New Zealand), which spans the period between ca. 13,134 and 11,694 cal BP based on the 14 C wiggle-matching with the absolute IntCal13 curve overlapping between 11,874 and 11,694 cal BP (Hogg et al. 2016a(Hogg et al. , 2016b;;[START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF]. Hogg et al. first revised the dating of the YDB chronology, the beginning of which was positioned at 12,615 cal BP, and then used the kauri chronology to connect the YDB and the CELM sequences, thus extending the tree-ring record back to 14,174 ± 3 cal BP (Hogg et al. 2016a(Hogg et al. , 2016b)). [START_REF] Sookdeo | Quality dating: A well-defined protocol for quality highprecision 14 C-dates tested on Late Glacial wood[END_REF] measured 14 C at annual to 5-yr resolution in Cottbus and Breintenthal woods over the period 12,324-11,880 cal BP, allowing to recalibrate the kauri sequence over a longer period (i.e. ca. 400 yr), thus refining the dating of the kauri chronology to 13,144-11,704 ± 6 cal BP.

For this new study, we further analyzed the 14 C concentration in tree rings of subfossil pines from the French Barbiers chronology, creating an annual-resolution 14 C sequence spanning more than 400 yr. Because the new data from Swiss trees are still floating and not yet published, we compare our new record only with chronologies from the SH [START_REF] Hua | Atmospheric 14 C variations derived from tree rings during the early Younger Dryas[END_REF]Hogg et al. 2016aHogg et al. , 2016b)), allowing to date the Barbiers chronology.

SITE AND MATERIALS

The Barbiers River site (44.354899N, 5.830316E) is located at an altitude of 660 m at the foot of Mount Saint Genis, which culminates at 1260 m in the Middle Durance area (southern French Alps). Several hundred subfossil trees (Pinus sylvestris L.) from the Late Glacial and Holocene periods have been discovered there [START_REF] Miramont | Subfossil tree deposits in the middle Durance (Southern Alps, France); environmental changes from Allerød to Atlantic[END_REF][START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF], and their noteworthy fossilization and preservation can be explained by a combination of climatic, geologic and topographic factors: (1) this region is submitted to stormy rainfall influenced by both Mediterranean and mountain climates [START_REF] Frei | A precipitation climatology of the Alps from high-resolution rain-gauge observations[END_REF][START_REF] Kieffer-Weisse | Estimation de paramètres statistiques des précipitations extrèmes dans les Alpes françaises[END_REF]; (2) bedrock consists of highly erodible calcareous marl [START_REF] Gidon | Carte géologique au 50/000, Laragne[END_REF]; and (3) steep slopes are widely developed in this alpine area.

During the Late Glacial Maximum (LGM) the Durance glacier, which had reached its maximal extent [START_REF] Jorda | Données nouvelles sur l'âge du dernier maximum glaciaire dans les Alpes méridionales françaises[END_REF], was near the Barbiers River site. A dry and cold climate prevailed and the flanks of Mount Saint Genis were covered by scree slopes. After the LGM, rapid warming accompanied by increased precipitation led to the reactivation of the hydrological cycle in unglaciated rivers such as Barbiers River. With increased runoff came intense erosion of steep and erodible slopes, leading to silty and coarse riverine sediment deposition, which formed wide and deep alluvial fans, the so-called "Main Postglacial Infilling" (MPI), which dates to 14,500-6,000 cal BP [START_REF] Miramont | Le cycle de sédimentation « postglaciaire principal » des bassins marneux sub-alpins : état des lieux[END_REF]). In the Middle Durance area, the MPI resulted in vast and flat plains, which have been used for agriculture since Neolithic times. With the occurrence of high sedimentation rates throughout the building of MPI, many trees were buried (mainly Pinus sylvestris L.), which represents Lateglacial and early Holocene pioneer vegetation. These trees have remained well preserved and buried until their recent exposure following the vertical incision of the rivers.

Most of the subfossil trunks discovered in the Middle Durance area are 0.2-2 m high, are still rooted in standing position and are well preserved with pieces of bark remaining. Tree-Ring Width series (TRW) show frequent abrupt growth changes, wedging, and missing rings due to sediment deposition and flooding of the rivers [START_REF] Sivan | Rythmes de la sédimentation et interprétations paléoclimatiques lors du Postglaciaire dans les Alpes du Sud. 14C et dendro-géomorphologie, deux chronomètres complémentaires[END_REF]. Alluvial sediments gradually buried the stumps while the trees were alive, reducing oxygen exchange between the atmosphere and the root system. Trees that were buried in alluvial deposit survived for several years or several decades after flooding, but formed very narrow growth rings, before dying. Because of high sedimentation rates, these trees failed to develop epitrophic or adventive roots.

At the Barbiers River site, 18 trees were found in 2000 [START_REF] Miramont | L'étalonnage du temps du radiocarbon par les cernes d'arbre. L'apport des series dendrochronologiques du gisement de bois subfossiles du torrent des Barbiers (Alpes Françaises du sud)[END_REF]. New sampling campaigns performed in autumn 2017 revealed 4 new trees from the Barbiers site, and also permitted the resampling of previously analyzed trees, with better-preserved wood and longer series.

METHODS

Dendrochronological Analysis

Whenever possible, 2 or 3 disks of 5-10-cm thickness were sampled from each subfossil pine: one just above the root to estimate the germination date, and one higher on the stem to avoid reaction wood and to measure TRW. Disks were air-dried and sanded flat using progressively finer sandpaper up to 400 grain size. Standard dendrochronological techniques were employed in chronology development [START_REF] Fritts | Tree rings and climate[END_REF][START_REF] Cook | Methods of dendrochronology[END_REF]. TRW was measured on at least 3 radii using a LINTAB measuring system with a precision of 0.01mm and the TSAPWin software [START_REF] Rinn | TSAP e, time series analyses presentation[END_REF][START_REF] Rinn | TSAP-Win, time series analysis and presentation for dendrochronology and related applications[END_REF].

We observed 2 groups of trees belonging to distinct sediment layers: the older one dates from the Allerød-Younger Dryas transition and the younger one dates to around 10,400 BP on the 14 C plateau. In this study we explore only the first group of trees.

The dendrochronological match of sequences was challenging due to growth anomalies resulting from geomorphological stress. Some trees had grown slowly and produced very narrow rings at the beginning or the end of their life. In addition, we identified several missing rings, thanks to the new 2017 disks taken in the lower part of the trunks that helped improving some TRW series correlations (Table 1).

Previously, 9 trees were grouped into 2 floating sequences BARBA (6 trees) and BARBB (3 trees) [START_REF] Miramont | L'étalonnage du temps du radiocarbon par les cernes d'arbre. L'apport des series dendrochronologiques du gisement de bois subfossiles du torrent des Barbiers (Alpes Françaises du sud)[END_REF][START_REF] Kaiser | Challenging process to make the Lateglacial tree-ring chronologies from Europe absolute[END_REF]. On the basis of annual radiocarbon measurements, [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF] revised the position of tree Barb17 in the BARBA chronology. 14 C dating confirms the dendro-match between BARBA and BARBB, allowing to merge them in a single chronology BARB (see Figure 1 for the position of the trees composing the BARB chronology). Tree Barb16 has been removed from BARB because the dendro-match is too weak. Trees Barb31 and Barb30 have been added to BARB, thus extending the chronology to 416 yr. The irregular growth of Barb18, Barb32, Barb33, Barb36 and weak correlation coefficients prevent any dendro-matching with the other trees. Table 1 provides the values of the statistical tests (overlap, Gleichläufigkeit and t-value) performed on the different tree sequences grouped in BARB chronology.

Radiocarbon Methods

We analyzed the trees Barb13, Barb14, and Barb30 from the BARB chronology. Clear and mostly regular ring growth in Barb13 and Barb14 allowed systematic sampling of the sequences at annual resolution. Barb30 tree-ring series showed quite different ring patterns.

Very narrow rings at the beginning and at the end of the Barb30 sequence led us to mix the wood of 2 rings (3 for sample 1-3) in these parts of the sequence (Table S1).

Sampling was performed on a selection of rings from each tree: rings number #3 to 54 of the 108-yr sequence of Barb13; rings number #43 to 142 of the 173-yr sequence of Barb14; rings number #1 to 155 of the 196-yr sequence of Barb30 (Figure 1; Table S1). Then, every third ring of the three sequences was measured for 14 C. Rings number #72 to 108 of the tree Barb17 were further measured at full annual resolution, with replicated 14 C measurements from the same cellulose extracts for rings number #80 to 105. The average absolute value of the differences between 26 replicates pairs is 36 yr with a standard deviation (SD) of 20 yr, which is less than twice the average 1 sigma uncertainty (2•29 = 68 yr) of these measurements (Table S1). These higher resolution analyses were performed to assess the possible presence of a solar event at around 12,670 cal BP, as hypothesized by [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF].

The wood samples were sliced into small pieces using a scalpel under a binocular microscope, before being treated chemically for cellulose extraction using the ABA-B method. This method consists in a classical ABA treatment, with solutions of HCl and NaOH at 4% concentration, followed by the bleaching step, performed with 60 g of NaClO 2 in 1 L of ultrapure water in acid solution (HCl) at pH 3 [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF]). After chemical pretreatment, the dried residues were weighed in tin capsules, combusted with the elemental analyzer, and the evolved CO 2 was finally transformed into graphite with the AGE III system. The graphite target was then analyzed for its 14 C/ 12 C and 13 C/ 12 C ratios using the AixMICADAS system [START_REF] Bard | AixMICADAS, the accelerator mass spectrometer dedicated to 14C recently installed in Aix-en-Provence, France[END_REF]. Standards (OxA2 NIST SRM4990c) and blanks (VIRI-K, chemically treated like the other wood samples) were processed together with samples and used for normalization and blank correction, respectively. In addition, IAEA-C3 (cellulose) and IAEA-C8 (oxalic acid) standards were pretreated (IAEA-C3 underwent also the chemical pretreatment) and measured for 14 C in the same batches, serving as control standards.

High precision 14 C measurements were performed with long AMS runs to reach at least 800,000 ion counts for each OxA2 standard targets on the same magazine. An additional uncertainty of 1.6‰ was propagated in the 14 C analytical errors and background correction following the convention described in [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF]. The data are reported in terms of conventional 14 C age in yr BP and Δ 14 C in ‰, which is the 14 C/ 12 C ratio after correction for fractionation and decay [START_REF] Stuiver | Discussion: reporting of 14 C data[END_REF]. In order to certify our highprecision in wood dating, we successfully participated in the International Intercomparison on wood dating organized by ETH (Zurich) in 2017-2018. Indeed, two wood blanks and three wooden sequences, dated to different periods, were annually dated and the results were compared with those of other laboratories [START_REF] Wacker | First radiocarbon inter-comparison on annual tree-ring samples[END_REF].

In order to date the Barbiers chronology, we compared it with the available chronologies from the SH: the kauri chronology from New Zealand (Hogg et al. 2016a(Hogg et al. , 2016b) ) and the huon pine chronology from Tasmania [START_REF] Hua | Atmospheric 14 C variations derived from tree rings during the early Younger Dryas[END_REF]). The kauri chronology has two advantages compared to the huon pine sequence: (1) it spans the entire period covered by the Barbiers chronology and (2) it exhibits less scatter than the huon pine record. Moreover, Hogg et al. (2016b) suggested that synchronization problems still persist between the individual trees of the huon record. For these reasons, we did not tune the Barbiers record to the huon pine record, comparing the Barbiers data just with the kauri record. Kauri sequence is dated by 14 C wiggle matching against the absolute tree-ring-based part of the calibration curve [START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF]Hogg et al. 2016aHogg et al. , 2016b)). The dating for kauri proposed by Hogg et al. (2016aHogg et al. ( , 2016b) ) was shifted by 10 yr towards older ages following [START_REF] Sookdeo | Quality dating: A well-defined protocol for quality highprecision 14 C-dates tested on Late Glacial wood[END_REF].

In [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF], the dating of the Barbiers sequence was a preliminary determination based on a visual match with the kauri chronology (Hogg et al. 2016a(Hogg et al. , 2016b)). For the present study, we used statistical analyses to compare and match the Barbiers and kauri series (i.e. R-cross-correlation coefficient, χ 2 minimization and the Bayesian statistics tool of the OxCal program). The χ 2 value varies with both the placement year and the 14 C-IHG value, allowing χ 2 minimization with both unknown variables (resulting in a surface in 3D plots such as in Figure 4).

In order to compare our annual-resolution data with decadal-average data from kauri, we compared both records by averaging the Barbiers record to mimic the resolution of the kauri record. For each decade, we calculated the weighted average of the Barbiers 14 C data included in the studied decade and the weighted error of the 14 C data as the uncertainty for the decadal estimate. The starting year is arbitrary when calculating the Barbiers decadal average. We thus considered all possibilities leading to 10 averaged Barbiers curves, which were then compared with the kauri sequence (the first option starting from rings number #1 to 10 in the sequence, while other options differ by one ring numbers #2-11, #3-12 etc.; Figure 2).

RESULTS

Results of 14 C Dating of the Barbiers Series

The 14 C results for the Barb14 tree confirm its dendrochronological match with the Barb12 series, 14 C-dated in Capano et al. ( 2018; Figure 1). The dendrochronological position of the well-synchronized trees Barb13 and Barb14 is also confirmed with the new 14 C results (Figure 1). The 14 C dating confirms the dendro-match between Barb30 and BARB, attested by a high correlation coefficient, but short overlap (Table 1). Overall, this confirms the usefulness of considering 14 C data at annual resolution for a more precise synchronization of series characterized by the irregular ring growth typical of the YD event (Figure 1).

Optimization of the cross-correlation coefficient between the Barbiers decadal average curves and the kauri decadal curve shows slightly different optimum results, with the highest correlation observed when the Barbiers series starts at 13,008 cal BP with a decadal average starting with rings number #5 to 14 and so forth (Figures 2 and3). In fact, all 10 options for averaging Barbiers exhibit high R values (>0.9) and we took this 10-yr range as a conservative error on the Barbiers chronology starting at 13,008 ± 5 and ending at 12,594 ± 5 cal BP (Figure 5).

The 10 decadal average series for Barbiers and the kauri chronology have been used to calculate the χ 2 distance, which is minimum when the Barbiers series starts at 13,008 cal BP (option starting with the rings number #5-14 average; Figures 2 and4). As for the optimization of the correlation coefficient, we take the 10-yr range of the different decadal averages as a conservative uncertainty for the placement of the Barbiers series: 13,008-12,594 ± 5 cal BP (Figure 5). As descibed below, the χ 2 minimization also provides an optimal value for the IHG.

Finally, we compared the Barbiers and kauri chronologies by means of the Oxcal statistical program (Bronk [START_REF] Bronk Ramsey | Wiggle matching" radiocarbon dates[END_REF]) (Figure 6). The comparison was performed using the OxCal "Delta_R" uniform function (U(-120,120); Bronk Ramsey 2009) in order to account for the IHG. The 10 average options for Barbiers were compared with the kauri chronology, leading to an optimal placement of 13,001-12,586 ± 6 Cal BP (2σ error; Table 2). As for the two previous techniques, we kept all 10 possible solutions as they show similar levels of agreement (Table 2). The youngest and oldest years of the 10 dating solutions lead to a range of 20 yr. This allows to date the Barbiers sequence to 13,006-12,591 ± 10 cal BP.

The OxCal Offset function also allows to compare the non-averaged series of Barbiers directly with the decadal record of Kauri. This leads to a chronology date of 13,003-12,588 ±25 cal BP (2σ error range). This dating option agrees with previous estimations, but has a somewhat larger uncertainty, which is linked to the different resolutions of the Barbiers and Kauri datasets.

Overall, the three statistical techniques (cross-correlograms, χ 2 minimization and OxCal Delta_R) provide very similar results for the dating of Barbiers chronology. Taking the largest uncertainty, we can date our sequence to 13,008-12,594 ±10 cal BP. A higher precision for the dating of Barbiers chronology will become possible only when an annual-resolution 14 C series of kauri chronology or any other absolutely dated 14 C series for the YD period become available.

The existence of a brief Δ 14 C excursion at ca. 12,670 cal BP (12,680 cal BP with the new kauri placement by [START_REF] Sookdeo | Quality dating: A well-defined protocol for quality highprecision 14 C-dates tested on Late Glacial wood[END_REF]) was suggested in our previous work [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF]. To evaluate the possibility of a solar event, we have improved the Barbiers 14 C record specifically with the Barb17 tree: 14 C was measured for every year from rings number #72 to 108 with at least two replicates for rings number #80 to 100 and 102, 105 (Table S1, Figure 1). The high resolution 14 C record is characterized by rather noisy 14 C values before and during the 14 C peak, which makes it difficult to maintain the hypothesis of a sharp solar event.

Results on the Atmospheric 14 C IHG [START_REF] Lerman | C14 in tree rings from different localities[END_REF] were the first to identify the presence of a small but systematic 14 C difference between wood from trees that lived at the exact same time in the NH and SH (SH wood being 36 ± 8 yr older than NH wood). This 14 C-IHG is maintained principally by air-sea CO 2 exchange with the ocean surface that is typically much older than the atmosphere (from 400 to 1200 yr, [START_REF] Bard | Correction of accelerator mass spectrometry 14 C ages measured in planktonic foraminifera: Paleoceanographic implications[END_REF]. Indeed, there is proportionally more ocean surface in the SH than in the NH, and the Southern Ocean is also characterized by generalized upwelling of old water and intense CO 2 piston velocity due to extreme winds in the 40°to 60°S latitude belt [START_REF] Levin | Carbon isotope measurements of atmospheric CO 2 at a coastal station in Antarctica[END_REF][START_REF] Braziunas | The preindustrial atmospheric 14 CO 2 latitudinal gradient as related to exchanges among atmospheric oceanic and terrestrial reservoirs[END_REF]. The 14 C-IHG is even more complex for trees grown after the Industrial Revolution due to anthropogenic fossil CO 2 which has since been emitted mainly in the NH [START_REF] Stuiver | Anthropogenic and solar component of hemispheric 14 C[END_REF][START_REF] Mccormac | Variations of radiocarbon in tree rings: Southern Hemisphere offset preliminary results[END_REF], thus disturbing and reversing the gradient.

Following [START_REF] Lerman | C14 in tree rings from different localities[END_REF], several other authors have further quantified the IHG. [START_REF] Hogg | High-precision radiocarbon measurements of contemporaneous tree-ring dated wood from the British Isles and New Zealand: AD 1850-950[END_REF] measured the 14 C-IHG over the last millennium, deriving an average value of 40 ± 13 yr and suggesting a 130-yr periodicity for 14 C-IHG variations. Taking into account the variability of the 14 C-IHG, [START_REF] Mccormac | SHCAL04 Southern Hemisphere calibration, 0-11.0 cal. kyr BP[END_REF] proposed using a modeled 14 C-IHG Figure 5 Comparison between the Barbiers and the kauri chronology for the Younger Dryas period. The published kauri chronology (Hogg et al. 2016a(Hogg et al. , 2016b) is shifted by 10 yr towards older ages [START_REF] Sookdeo | Quality dating: A well-defined protocol for quality highprecision 14 C-dates tested on Late Glacial wood[END_REF].

of 56-58 yr for the SH calibration curve (SHCal04), with a variability of ±8 yr at 1000 cal BP and ±25 yr at 11,000 cal BP.

Hogg et al. (2016a) calculated the 14 C-IHG for most of the YD event. They found variable values (averaged to 38 ± 6 yr), which are generally in agreement with the data for the last two millennia (44 ± 17 yr; [START_REF] Hogg | High-precision radiocarbon measurements of tree-ring dated wood from New Zealand: 195 BC-AD 995[END_REF]. The same authors also described a collapse of the 14 C-IHG during the period between 13,100 and 12,660 cal BP (Hogg et al. 2016a). However, annual-resolution 14 C data from French trees spanning the early part of the YD around 12,660 cal BP indicate that the 14 C-IHG was similar to the modeled value calculated by [START_REF] Mccormac | SHCAL04 Southern Hemisphere calibration, 0-11.0 cal. kyr BP[END_REF], with no evidence of a collapse [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF].

For this study we can evaluate the 14 C-IHG with the Barbiers record, extended to 415-yr with improved resolution with respect to our previous publication [START_REF] Capano | Wood 14 C dating with AixMICADAS: methods and application to tree-ring sequences from the Younger Dryas event in the southern French Alps[END_REF]). To calculate the 14 C-IHG, the Barbiers data from the NH were compared to the kauri data from the SH. As explained in the previous section dealing with chronological placement, Barbiers is averaged 10 times at decadal resolution and the 14 C-IHG calculation was performed at this same resolution, leading to 42-43 estimations, depending on the averaged curve. Considering the optimum correlation coefficient R for each of the 10 options, the 14 C-IHG was then calculated by subtracting the Barbiers 14 C age and Δ 14 C ‰ from the corresponding kauri decadal 14 C-age and Δ 14 C values (Δt = ( 14 C age) kauri -( 14 C age) Barb , ΔΔ 14 C = Δ 14 C kauri -Δ 14 C Barb ). Note that the ratio between the 14 C-IHG in age and in ‰ is not strictly constant. This is due to the fact that both Δ 14 C values are elevated and vary through time (N.B. if the Δ 14 C values were close to 0, the Taylor series approximation of Ln(1x) = x at first order would lead to Δt = -8•ΔΔ 14 C).

The 14 C-IHG shows mean values ranging between 31 yr with a SD of 23 yr and 43 yr with a SD of 23 yr, depending on the chronological placement (-5 to -7‰). Minimizing the χ 2 distance provides a direct estimation of the 14 C-IHG (Figure 4). Depending on the chronological placement, the optimum IHG value varies from 34 to 46 yr (-5 to -7‰). Matching the Barbiers and kauri decadal records with the Oxcal program, also provides a direct 14 C-IHG estimation, which ranges between 37 and 40 yr (-6‰; Table 2). Overall, the three statistical techniques provide similar 14 C-IHG values.

Considering only the best chronological placement, 13,008-12,594 ±10 cal BP, the mean 14 C-IHG value is 37 yr with a SD of 21 yr (n = 43), -6‰ with a SD of 3‰ (Figure 3). The 14 C-IHG time variations in Figure 3 indicate two periods with minimal values close to 0 from 12,960 to 12,900 cal BP and from 12,860 to 12,840 cal BP, where the 14 C-IHG oscillates between -4 and 24 yr (between 1 and -4‰):

• During the first period (12,960-12,900 cal BP) the mean 14 C-IHG is 10 yr with a SD of 12 yr (n = 7), -2‰ with a SD of 2‰.

• During the second period (12,860-12,840 cal BP), the mean 14 C-IHG is 6 yr with a SD of 12 yr (n = 3), -1‰ with a SD of 2‰. The period in-between (12,870 cal BP) shows a value of 39 yr with a SD of 4 yr (n = 3), -6‰ with a SD of 1‰, in agreement with the overall mean 14 C-IHG of 37 yr.

These intervals with minimum 14 C-IHG correspond in time to the beginning of the YD event at 12,846 ± 138 cal BP [START_REF] Rasmussen | A new Greenland ice core chronology for the last glacial termination[END_REF].

By excluding the two short periods with minimal 14 C-IHG, it is possible to recalculate a mean 14 C-IHG of 45 yr with a SD of 14 yr (n = 33), -7‰ with a SD of 2‰. These values are indistinguishable from the 14 C-IHG observed over the last two millennia (44 yr with a SD of 17 yr; [START_REF] Hogg | High-precision radiocarbon measurements of tree-ring dated wood from New Zealand: 195 BC-AD 995[END_REF].

The plot of 14 C age versus calendar ages (Figures 1,5) exhibits three "age plateaux" during the periods 13,010-12,970 cal BP, at the beginning of the Barbiers chronology, 12,820-12,770 cal BP, immediately before the drastic Δ 14 C rise, and 12,720-12,690 cal BP, immediately after the Δ 14 C rise. The 14 C-IHG determination is expected to be particularly robust during these periods. Indeed, a slight inaccuracy in the chronological placement of the Barbiers sequence with respect to the kauri record would not affect the 14 C difference over an "age plateau."

The IHG during the three "age plateaux" is the following:

• Over the 13,010-12,970 cal BP "plateau" the 14 C-IHG average is 37 yr with a SD of 14 yr (-6 with a SD of 2‰; n = 5).

• For the 12,820-12,770 cal BP "plateau" the 14 C-IHG average is 49 yr with a SD of 9 yr (-7‰ with a SD of 1‰; n = 6).

• During the 12,720-12,690 cal BP plateau the 14 C-IHG average is 53 yr with a SD of 9 yr (-8‰ with a SD of 1‰; n = 4).

During each of the three "plateaux," the individual decadal 14 C-IHG values are fully compatible with each other. It is thus reasonable to assume that the 14 C-IHG was stable during these periods. Under this hypothesis, we can calculate one sigma errors of the mean (SE = SD/ p n) for the 14 C-IHG determinations: 37 ± 6 yr, 49 ± 4 and 53 ± 5 yr, respectively for the three 14 C "age plateaux" in chronological order. The data suggest that the first 14 C-IHG value is thus statistically lower than the two other values. This may indicate that the IHG increased during the transition between the Allerød and the YD periods at around 12,846 cal BP, but, more 14 C data will obviously be needed to verify this suggestion. An additional problem is that the uncertainty on the climatic transition is large (±138 yr in the Greenland ice GICC05 chronology). Identification of this transition directly in the trees would certainly help to resolve the issue.

The period between 12,760 and 12,730 cal BP is characterized by a drastic Δ 14 C rise of 25‰. However, the 14 C-IHG remains high, with a mean of 55 yr, and relatively stable, with a SD of 19 yr based on 4 values (-8‰ and a SD of 3‰). This translates to an 14 C-IHG estimate of 55 yr and SE of 10 yr, which agrees with the overall mean and SE of 45 ± 2 yr (excluding the two brief periods of minimal IHG).

The 14 C-IHG in subfossil trees reflects the asymmetry between the 14 C signatures of CO 2 fluxes to the atmosphere in the SH and NH. Variations of the 14 C-IHG are thus to be expected when the N-S balance of CO 2 fluxes is perturbed in both ocean and land sources. For example, [START_REF] Rodgers | Interhemispheric gradient of atmospheric radiocarbon reveals natural variability of Southern Ocean winds[END_REF] used an ocean-atmosphere model to study the 14 C-IHG variations over the last millennium and attributed an abrupt 14 C-IHG decrease by 16 yr (2‰ in Δ 14 C) to a weakening of the Westerlies in the Southern Ocean.

The new 14 C record of Barbiers trees suggests a 14 C-IHG increase between the end of the Allerød (IHG = 37 ± 6 yr) and the early YD (IHG = 48 ± 3 yr). This is compatible with ocean circulation changes, namely: a decrease in deep-water convection in the North-Atlantic and an associated increase in wind-driven upwelling in the Southern Ocean.

The new 14 C-IHG record also shows a 120-yr period (12,960-12,840 cal BP) characterized by two brief phases of minimal values followed by the YD increase: the first of these phases lasted 6 decades, while the second was shorter, spanning 2 decades only. It is reasonable to hypothesize that these century-long anomalies in 14 C-IHG are in fact synchronous with the start of the YD event.

It is also important to note that the 14 C-IHG minima are also synchronous with Δ 14 C minima in both the NH and SH. This suggests that this century-long period was characterized by a strong release of relatively old CO 2 , preferably into the NH atmosphere. The timing of the Δ 14 C decrease would thus correspond to the start of the rise in atmospheric CO 2 that occurred at the beginning of the YD (Figure 6).

Further analyses are needed to confirm our observations, together with carbon cycle modeling to simulate the triple isotope signatures and the 14 C-IHG. Between 13,000 and 12,800 cal BP, atmospheric Δ 14 C is rather stable, but exhibits century-long cycles with minima corresponding to the two 14 C-IHG anomalies. It will be important to evaluate the possible variations of the 14 C production during this period by comparing the observed Δ 14 C variations with those in the 10 Be record from ice cores (e.g. [START_REF] Bard | Solar modulation of cosmogenic nuclide production over the last millennium: comparison between 14 C and 10 Be records[END_REF][START_REF] Adolphi | Connecting the Greenland ice-core and U/Th timescales via cosmogenic radionuclides: Testing the synchronicity of Dansgaard-Oeschger events[END_REF]). Nevertheless, cosmogenic production changes through solar modulation should affect both hemispheres in a rather symmetric way. Hence, CO 2 fluxes in the NH from old carbon oxidation can be seen as a possible mechanism to explain the observed variations.

CONCLUSIONS

We have analyzed a 416-yr-long tree-ring chronology from subfossil trees (Pinus sylvestris L.) which grew in the southern French Alps during the Younger Dryas cold period. 14 C measurements at annual resolution of this Barbiers chronology were performed at least every third year of the sequence, establishing a high-resolution 14 C sequence in a period characterized by tree growth recession.

We date the new Barbiers record to 13,008-12,594 ±10 cal BP by matching the 14 C patterns with those of the kauri chronology. Consequently, the Barbiers record should include the transition between the Allerød and YD events at 12,846 cal BP, although the exact position of the boundary is still uncertain due to the error of the ice core chronology (138 yr).

Our new data allow to calculate the 14 C-IHG between the Northern Hemisphere (Barbiers pines from France) and the Southern Hemisphere (kauri trees from New Zealand). The mean 14 C-IHG is 37 yr with a SD of 21 yr based on 43 decadal estimations.

Over the 415 yr, the 14 C-IHG exhibit minimal values of between 12,960-12,840 cal BP. Excluding these minima from the average, leads to a mean 14 C-IHG of 45 yr with a SD of 14 yr based on 33 decadal values, in agreement with observations for the last two millennia.

The Barbiers record suggests a 14 C-IHG increase between the end of the Allerød period (IHG = 37 ± 6 yr) and the early part of the YD (IHG = 48 ± 3 yr). This change is compatible with a previously reported drop of deep-water convection in the North-Atlantic and an associated increase in wind-driven upwelling in the Southern Ocean.

Between 12,960 and 12,840 cal BP the 14 C-IHG minima are accompanied by Δ 14 C minima in both the NH and SH. These data have been compared with the records of CO 2 and CH 4 mixing ratios measured in air bubbles occluded in Antarctic ice. The concomitant 14 C-IHG and Δ 14 C minima seem to correspond to the start of the rise in atmospheric CO 2 and the decrease in CH 4 at the beginning of the YD.

Further work is needed using carbon cycle modeling to simulate the various carbon isotope signatures, so as to take into account possible variations of 14 C production during this period by considering variations in the 10 Be ice core record.

Figure 1

 1 Figure 1 Comparison between the trees included in the Barbiers chronology on the relative Aix scale. The bar diagram of the upper part of the figure reports all cross-dated trees (the gray bars refer to trees that were not 14 C dated). In the lower part of the figure, the 14 C record for each analyzed tree is shown in its dendrochronological position on the relative Aix scale. (Please see electronic version for color figures.)

Figure 2

 2 Figure2Top: results of the cross-correlation coefficient for the 10 averaged groups of Barbiers series compared against the kauri record. Bottom: the averaging system used for obtaining the 10 groups.

Figure 3 Δ

 3 Figure 3 Δ 14 C ‰ of Barbiers averaged groups compared against kauri record.The best dating position of Barbiers chronology obtained from the crosscorrelation coefficient is given for each group. The best R value is obtained for the group 5-14. The IHG is shown for all groups compared with kauri (lower part of each graph).

Figure 4

 4 Figure 4 3D representation of the best dating option obtained by comparing Barbiers averaged series with kauri record by means of a χ 2 test. The obtained combination of results suggests that the Barbiers chronology starts at 13,008 cal BP and that the mean IHG value is 40 yr.

Figure 6

 6 Figure6Comparison between Antarctica WDC CO 2 data (red curve, top of graph) on the WD2014 age-scale[START_REF] Marcott | Centennialscale changes in the global carbon cycle during the last deglaciation[END_REF][START_REF] Buizert | The WAIS Divide deep ice core WD2014 chronology. Part 1: Methane synchronization (68-31 ka BP) and the gas ageice age difference[END_REF], Antarctica WDC CH 4 data (purple curve, middle of graph) on the WD2014 age-scale[START_REF] Marcott | Centennialscale changes in the global carbon cycle during the last deglaciation[END_REF][START_REF] Buizert | The WAIS Divide deep ice core WD2014 chronology. Part 1: Methane synchronization (68-31 ka BP) and the gas ageice age difference[END_REF], and Δ 14 C ‰ from New Zealand kauri trees and French Barbiers trees (green and pink curves, respectively, bottom of graph; fromHogg et al. 2016a and the present study). The gray band highlights the collapse of the IHG between 12,960-12,840 cal BP, which corresponds to the rise of pCO 2 and the drastic drop of CH 4 at the onset of the Younger Dryas cold period.

Table 1

 1 Correlation matrix of BARB: overlap, Gleichläufigkeit and t-value (BP and H), respectively for each matching position. In bold: Gleichläufigkeit and t-values above 60 and 4.0 which are usually the respective minima required to accept dendro-match.

	Tree/
	chronology	Barb30	Barb13	Barb14	Barb4	Barb31	Barb9	Barb12	Barb5	Barb7	Barb17
	Barb13	92/60/5.3/3.3
	Barb14	88/65/3.4/3.1 104/77/8.4/9.0
	Barb4	71/57/2.3/2.5 87/69/4.5/3.3 156/58/4.5/5.1
	Barb31	58/70/4.0/3.3 74/68/6.4/4.4 89/60/5.9/5.5 89/62/5.1/3.9
	Barb9	ovl<60 yrs ovl<60 yrs	80/70/3.0/5.4 80/71/4.1/4.9 79/61/2.7/2.7
	Barb12	ovl<60 yrs ovl<60 yrs	115/58/3.0/3.0 139/71/5.7/5.3 61/60/2.4/2.0 62/57/2.3/1.8
	Barb5	ovl<60 yrs ovl<60 yrs	86/55/0.0/0.3 95/61/2.9/3.0 ovl<60 yrs ovl<60 yrs 95/65/3.2/3.4
	Barb7	ovl<60 yrs ovl<60 yrs	83/55/0.5/0.3 107/48/2.0/1.0 ovl<60 yrs ovl<60 yrs 131/57/3.8/2.6 92/66/3.9/3.6
	Barb17	ovl<60 yrs ovl<60 yrs	ovl<60 yrs	ovl<60 yrs	ovl<60 yrs ovl<60 yrs 126/56/3.5/5.1 ovl<60 yrs 79/57/0.6/0.8
	BARB*	92/60/4.8/4.1 108/70/8.9/6.9 173/64/7.1/8.2 180/66/7.1/6.6 89/61/8.2/6.4 80/67/3.8/4.7 210/60/6.9/9.4 95/74/4.8/4.7 96/62/4.1/4.4 126/60/3.2/5.1

*Without the corresponding tree.

Table 2

 2 Results of the comparison between the 10 averaged groups of Barbiers series and kauri record obtained by using OxCal program. IHG values in 14 C yr are obtained by using a uniform Delta_R function (U(-120,120)). Calibrated ranges refer to the first and last years of the chronology for each group.

		Agreement %	IHG yr ±2σ	Calibrated range
	Groups	Acomb	(modeled Delta_R)	(cal BP ±2σ)
	1-10	432	40 ± 6	13,012-12,597 ± 5
	2-11	433	40 ± 6	13,002-12,588 ± 5.5
	3-12	429	40 ± 7	13,003-12,588 ± 6
	4-13	435	37 ± 7	13,001-12,586 ± 6
	5-14	457	37 ± 6	13,001-12,586 ± 5
	6-15	463	37 ± 6	13,001-12,586 ± 6
	7-16	439	37 ± 6	13,010-12,596 ± 4.5
	8-17	440	38 ± 6	13,011-12,596 ± 4
	9-18	431	39 ± 6	13,012-12,597 ± 4
	10-19	430	40 ± 6	13,013-12,598 ± 4

ACKNOWLEDGMENTS

AixMICADAS and its operation are funded by the Collège de France, the EQUIPEX ASTER-CEREGE and the ANR project CARBOTRYDH (PI E.B.).

SUPPLEMENTARY MATERIAL

To view supplementary material for this article, please visit https://doi.org/10.1017/RDC. 2019.116


Mathieu Métifiot

Marie-Line Andreola

Frédéric Becquart

Wahbi Jomaa

Paul O Verhoeven

Gerard Baldacchino

Simon Hemour
email: simon.hemour@u-bordeaux.fr

Fréderic Becquart

Gérard Baldacchino

D-Mask Consortium

Frederic Becquart

Gérard Baldacchino

S H Performed

The properties of hot household hygroscopic materials and their potential use for non-medical facemask decontamination

Main Text Introduction

Facemasks are widely used amidst the global COVID-19 pandemic to reduce the airborne transmission in the context of social interactions [START_REF] Who | Water , sanitation , hygiene and waste management for the COVID-19 virus[END_REF], as high viral loads of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) might be found in asymptomatic [START_REF] Wölfel | Virological assessment of hospitalized patients with COVID-2019[END_REF] and patients positive to the disease [START_REF] Chan | Improved molecular diagnosis of COVID-19 by the novel, highly sensitive and specific COVID-19-RdRp/Hel real-time reverse transcription-PCR assay validated in vitro and with clinical specimens[END_REF]. Decontamination of used facemasks is a common practice to mitigate the shortage risk (4) and the potential ecological impact of disposable units worn by billions of daily users [START_REF] Who | Water , sanitation , hygiene and waste management for the COVID-19 virus[END_REF]. However, the recommended household decontamination procedures are time-and energyintensive [START_REF] Yan | Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community[END_REF], which can potentially lead to low public acceptance. Meanwhile, it has been shown that SARS-CoV-2 can be remaining on cloth and a surgical mask for up to seven days. The infrequent decontamination that could result can then be considered as a possible indirect vector of contamination. Quick and easy decontamination methods is therefore required. Moist heat, a combination of heat and humidity is a known treatment method of inactivating some pathogens. For example influenza viruses on stainless steel surfaces have been inactivated after being elevated during 15 minutes at the temperature of 65°C associated to a 25% relative humidity (RH) [START_REF] Mcdevitt | Role of absolute humidity in the inactivation of influenza viruses on stainless steel surfaces at elevated temperatures[END_REF], while equivalent level of inactivation have been reached at 55°C and 75% RH. However, dry heat of 70°C may not be effective enough to inactivate the virus, even over a larger exposition time of an hour [START_REF] Fischer | Assessment of N95 respirator decontamination and re-use for SARS-CoV-2[END_REF]. While RH is the most reported parameter [START_REF] Chan | The Effects of Temperature and Relative Humidity on the Viability of the SARS Coronavirus[END_REF], absolute humidity (AH) seems to be more appropriate for predicting the influence of humidity on virus deactivation [START_REF] Lin | Humidity-Dependent Decay of Viruses, but Not Bacteria, in Aerosols and Droplets Follows Disinfection Kinetics[END_REF]. During the combined heat & high absolute humidity condition, the arrangement of the lipid bilayer of the virus as well as the interactions involved in the envelope proteins could be affected [START_REF] Prussin | Survival of the Enveloped Virus Phi6 in Droplets as a Function of Relative Humidity, Absolute Humidity, and Temperature[END_REF][START_REF] Tang | The effect of environmental parameters on the survival of airborne infectious agents[END_REF][START_REF] Weber | Inactivation of influenza A viruses in the environment and modes of transmission: a critical review[END_REF]. In droplets, the humidity rate and the evaporation kinetics of water can be matched [START_REF] Lin | Humidity-Dependent Decay of Viruses, but Not Bacteria, in Aerosols and Droplets Follows Disinfection Kinetics[END_REF] towards an intermediate evaporation rates and high concentration of salts, leading to virus inactivation. Therefore, aiming at benefiting from the temperature-humidity synergy, we are reporting here the SARS-CoV-2 virus inactivation from facemask in an enclosure filled with hot hygroscopic materials available in household settings.

Results and discussion

The following three steps procedure is tested (Figure 1-A): Two 1L polypropylene containers, each filled with 250g of hygroscopic material, are first heated in a microwave oven for 2 minutes (i). The containers are taken out of the oven, and the mask is placed in one of them, lying on the hot hygroscopic material, while the remaining material is deposited on the mask. This step has been tested with a handling time below 1:30min (ii). The container is then hermetically sealed for 15-20 min to set a high humidity atmosphere (iii). During the first phase, the porous hygroscopic medium is exposed to micro-wave power, which results in a volumetric heat generation. Heating efficiency depends on wave interactions with polar molecules or clusters [START_REF] Angoy | 17 -Microwave technology for food applications[END_REF] described by the loss factor which is responsible for the wave attenuation and the conversion of electrical field energy into heat. In the case of dry food products, they are strongly linked to residual moisture content and starch composition [START_REF] Peng | Microwave foaming of starch-based materials (I) dielectric performance[END_REF]. As a matter of fact, wheat, corn and rice starches are typically used in microwave food products formulations [START_REF] Ndife | Dielectric properties of six different species of starch at 2450 MHz[END_REF] since dielectric loss factor of these starches are commonly above 14 as compared to about 10 for water (data given at 20°C, ( 16)). Those materials are considered as holding potential for this decontamination method. More specifically, the process evaluated in this work is using short-cut pasta. During the decontamination of stage (iii), the mask carrying the virus is "sandwiched" between the non-consolidated hygroscopic porous media. The relative low thickness of the mask and its low heat capacity compared to the heat capacity of the porous stack (675 J K-1 for 500 g of the chosen media, compared to 30 J K-1 for a cloth mask) ensure a fast setting of a local thermal equilibrium. The evolution of mask temperature is then governed by the evolution of porous hygroscopic media temperature. As the container is sealed during this period, heat and mass transfer with the environment are reduced leading to a slow cooling process and thus maintaining a high temperature during this phase. Due to the hygroscopic nature of the chosen material [START_REF] Yoshino | Properties and Water Sorption Characteristics of Spaghetti Prepared Using Various Dies[END_REF], temperature elevation necessarily involves a vapor flux from the core of the material to its surface. This desorption rate ensures a high relative humidity in the vicinity of the mask during the entire protocol duration (Figure 1-B). For a typical case of using a 1L sealed box filled with 500 g pasta at 70°C, a desorption amount as low as 0.13 g is enough to generate a 100% relative humidity. The proposed method is thus not only able to rise the temperature of the mask to a high value (Figure 1-A), but also to develop high humidity (Figure 1-B) conditions towards virus inactivation. In the hours following of the experiment, the cooled hygroscopic porous medium will smoothly reabsorbs humidity from the environment. This can be seen as a self-reactivation mechanism which allows using the material for many further decontaminations cycles. It is expected that over the decontamination cycles, heat-induced water loss from the hygroscopic medium would reduce, leading to a higher temperature. To evaluate this effect, the inactivation tests were performed with "new" material (Test I) and material used for the 3rd time (Test II). Besides, the two tests were carried out with different heating power in different BSL3 laboratories, leading to a truly two independent validations of the decontamination process. Altogether, our results indicate that the hot hygroscopic protocol described in this work provides reliable and stable heat and humidity conditions which are efficient for inactivation of viral infectivity. This protocol holds wide potential to initiate new research in "decaying" temperature and humidity decontamination technics that are critical for the concept of frugal and reliable operation using household settings.

Materials and Methods

For each inactivation study, 15 000 VERO E6 cells were seeded in 24 well plates for 24 hours to reach 80% confluence. A 25 µl sample of viral solution (5.13 Log TCID50/ml for test I and 4.84 Log TCID50/ml for test II) was spotted on paper patches such as antibiotic disks. Three infections were carried on in parallel (samples 1, 2 and 3) for each mask. Paper disks spotted with 25 µl of viral solution, either treated with hygroscopic materials or untreated, were used as negative control. A positive control corresponding to the spotted but untreated virus was also performed. Patches containing the virus were settled in a whole mask folds (to mimic real conditions of decontamination of an entire mask). The mask was then subject to the above described decontamination procedure. After 20 minutes for test I and 15 minutes for test II, the patches were recovered and incubated in cell culture medium for another 15 minutes, and the eluates were used to infect VERO cells. The cells were observed at 2 days, 5, 6, 7 days and 9 days (not shown) post-infection to follow the appearance of cytopathic effect. While cells with untreated virus (Fig. 2, positive control) were lysed, cells incubated with treated virus continued growing and did not show any cytopathic effect even 9 days post infection, showing the efficacy of the treatment. No toxic product was released after treatment with hygroscopic materials (negative treated control). RNA from inactivation test II were also extracted and amplified by qRT-PCR. Three regions of SARS CoV 2 genome were targeted (N, and two regions of Orf 1ab). While the viral RNA was detected in positive sample as expected (Ct around 15,9)), samples treated with the hygroscopic process were undetected as for the negative control and for the three amplified regions, which correspond to a loss of infectivity above 5 log.

Figures (for initial submission)

Figure 1. (A) Temperature evolution inside the mask under the same steaming conditions that are used for the biologic trial as pertains to Fig. 2. (B) Temperature, Water loss, and Water vapor desorption rate from initial 500g of hygroscopic material during and after a household-compatible heat and humidity decontamination process. The desorption rate is linked to the concentration of water at the surface of the medium, and thus, its pattern is a direct image of the material temperature. The fast heating process does not lead the hygroscopic medium to release all the water it contains, because the mass loss is limited by the slow material water mobility

Figure 2 .

 2 Figure 2. CPE at Day #6 (A), and at day #7 (B). Cells are growing in every case but for the positive control where cells are lysed. (Excerpt from the four independent inactivation tests with a total of 18 samples available in the supporting information)

NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice.

Acknowledgments

We thank the other faculty, staff, and undergraduate research assistants, listed in the Supporting Information, who contributed many hours to support this study.

This study has received financial support from the French State in the Frame of "Investments fof the future" program. It is partly funded by University of Bordeaux Idex OPE 2020-0208-C19 funds