Benoit Thibodeau

Christophe Migon

Aurélie Dufour

André Poirier

Xavier Mari

Bassam Ghaleb

Louis Legendre

Low sedimentary accumulation of lead caused by weak downward export of organic matter in Hudson Bay, northern Canada

Keywords: Vertical transfer, particulate matter, oligotrophy, ocean, climate change; carbon budget 2

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

Introduction

Atmospheric transportation of anthropogenic contaminants over thousands of kilometers was reported in the late 1970s as the main mechanism explaining the presence of contaminants at high latitude [START_REF] Rahn | The Asian source of Arctic haze bands[END_REF][START_REF] Barrie | The influence of mid-latitudinal pollution sources on haze in the Canadian arctic[END_REF]. However, the precise mechanisms by which insoluble contaminants deposited at the water surface are exported to depth are still not completely resolved. The settling velocity of individual atmospheric particles with a diameter <5 µm (e.g. dust, sea spray, volcanic ash, anthropogenic material (De Angelis and Gaudichet 1991)) is null or very low in seawater, based on Stokes's law [START_REF] Buat-Ménard | Non-steady-state biological removal of atmospheric particles from Mediterranean surface waters[END_REF], and their removal from the surface ocean depends on their aggregation with larger particulate biogenic material. The adsorption/aggregation of lithogenic atmospheric material (mostly dust) on/with organic particles are likely to increase the density, and thus the settling velocity of the resulting aggregates [START_REF] Deuser | Biological control of the removal of abiogenic particles from the surface ocean[END_REF][START_REF] Fowler | Rapid removal of Chernobyl fallout from Mediterranean surface waters by biological activity[END_REF]; [START_REF] Alldredge | Characteristics, dynamics and significance of marine snow[END_REF][START_REF] Jackson | Aggregation in the marine environment[END_REF][START_REF] Armstrong | A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals[END_REF][START_REF] Francois | Factors controlling the flux of organic carbon to the bathypelagic zone of the ocean[END_REF]Turner 2002;[START_REF] Burd | Particle aggregation[END_REF].

However, recent studies indicate that, even if this 'ballast effect' probably increases the settling velocity of the sinking material, it may not be the main determinant of the downward export flux [START_REF] Passow | Switching perspectives: Do mineral fluxes determine particulate organic carbon fluxes or vice versa?[END_REF][START_REF] Heimbürger | Vertical export flux of metals in the Mediterranean Sea[END_REF].

Tight coupling between primary productivity and downward fluxes of particulate organic carbon (POC) has been observed in various marine regions [START_REF] Gačić | The open-ocean convection in the Southern Adriatic: A controlling mechanism of the spring phytoplankton bloom[END_REF][START_REF] Migon | Transfer of atmospheric matter through the euphotic layer in the northwestern Mediterranean: Seasonal pattern and driving forces[END_REF][START_REF] Boyce | Global phytoplankton decline over the past century[END_REF][START_REF] Passow | The biological pump in a high CO2 world[END_REF][START_REF] Yool | Climate change and ocean acidification impacts on lower trophic levels and the export of organic carbon to the deep ocean[END_REF][START_REF] Turner | Zooplankton fecal pellets, marine snow, phytodetritus and the ocean's biological pump[END_REF]. This indicates that biological productivity can lead to efficient export of atmospheric material to the sediment by inclusion of biogenic material in mineral-organic aggregates. Given that phytoplankton dynamics is controlled by nutrient availability, the environmental conditions that control limiting nutrients ultimately control the downward export of POC [START_REF] Lampitt | Long-term variability of downward particle flux in the deep northeast Atlantic: Causes and trends[END_REF][START_REF] Heimbürger | Temporal variability of vertical export flux at the DYFAMED time-series station (Northwestern Mediterranean Sea)[END_REF].

Climate in the Arctic and more generally at high latitudes underwent dramatic changes during the past decades [START_REF] Macdonald | Recent climate change in the Arctic and its impact on contaminant pathways and interpretation of temporal trend data[END_REF]. The 20 th century was the warmest period in the Arctic in at least 44,000 years [START_REF] Miller | Unprecedented recent summer warmth in Arctic Canada[END_REF], and environmental changes included increases in precipitation and river discharge as well as declines in snow cover and sea-ice extent (ACIA 2004). While the effects of such changes on primary productivity could be variable over Arctic shelves [START_REF] Michel | Arctic Ocean outflow shelves in the changing Arctic: A review and perspectives[END_REF], they may have important consequences on the rates of contaminant scavenging and export from surface to deep water. For example, [START_REF] Outridge | Evidence for control of mercury accumulation rates in Canadian High Arctic lake sediments by variations of aquatic primary productivity[END_REF] suggested that the 20 th century increase in the accumulation of Hg in the sediment of a Canadian high Arctic lake had largely been driven by an increase in autochthonous primary productivity since 1854.

In the present study, we hypothesized that low primary productivity was the main forcing factor that determined the sedimentation of atmospherically-deposited matter in Hudson Bay during the last centuries. To test this hypothesis, we investigated the concentration and isotopic signature of Pb in two sediment cores from Hudson Bay, and compared these values with already published sedimentary records from nearby high-productivity lakes [START_REF] Outridge | Regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the Canadian Arctic[END_REF]. We used Pb because its multiple isotopes allow the identification of sources. Moreover, the pollution history of Pb is well documented, especially around Hudson Bay where Pb was measured in lake sediments. Our hypothesis would be rejected if (1) the sedimentary accumulation of Pb during the last century were the same in the sedimentary records of both low-productivity Hudson Bay and high-productivity nearby lakes, thus indicating that primary productivity did not play a key role in the vertical export of Pb, or (2) the recent isotopic composition of records from Hudson Bay and surrounding lakes were not recording the same anthropogenic signal.

Materials and Methods

Study area

Hudson Bay has an area of about 841,000 km 2 , an average depth of 125 m (maximum depth of 250 m), and slopes generally less than 2 degrees [START_REF] Prinsenberg | Chapter 10 The Circulation Pattern and Current Structure of Hudson Bay[END_REF]. Dense cold saline water enters the Bay from the northwest (Hudson Strait and Foxe Basin, located in the north of Hudson Bay; Fig. 1). Circulation inside the Bay is cyclonic, with surface currents averaging 5 cm s -1 in summer, and 2-3 cm s -1 in winter when the Bay is ice-covered [START_REF] Saucier | A 3-D coupled ice-ocean model applied to Hudson Bay, Canada: The seasonal cycle and time-dependent climate response to atmospheric forcing and runoff[END_REF]. There is a surface outflow of relatively warm and fresh water to the northeast of the Bay towards Hudson Strait. Estimates of the residence time of surface (fresh) water are quite variable in the literature, i.e. they range from the order of one month to more than 6 years [START_REF] Prinsenberg | Freshwater contents and heat budgets of James Bay and Hudson Bay[END_REF][START_REF] Jones | Northern Hudson Bay and Foxe Basin: Water masses, circulation and productivity[END_REF]Granskog et al. 2009). Many rivers discharge freshwater in the southern part of Hudson Bay, causing a strong latitudinal surface salinity gradient. The annual discharge (710 km 3 yr -1 ) is equivalent to an annual freshwater yield of about 65 cm over the whole bay [START_REF] Prinsenberg | Chapter 10 The Circulation Pattern and Current Structure of Hudson Bay[END_REF]). This freshwater inflow has a profound influence on the physical, chemical and biological properties of the Bay because it fosters stratification of the water column, which usually reduces vertical mixing and thus upward transport of nutrients [START_REF] Prinsenberg | Chapter 10 The Circulation Pattern and Current Structure of Hudson Bay[END_REF].

Although the intense stratification may suggest that Hudson Bay is oligotrophic, a recent study indicates that even if the riverine inputs are relatively minor sources of nitrate, the inputs of freshwater favor rather than impede primary productivity inshore by indirectly fostering the entrainment and upwelling of deeper water to the surface (Kuzyk et al. 2010a). Because Hudson Bay is semi-enclosed within the Canadian Shield, its geochemical characteristics are strongly influenced by local factors such as the geological substrates that are drained by river runoff, wet and dry atmospheric depositions, and seasonal sea-ice formation and melt. Despite its remote location, Hudson Bay is subject to anthropogenic contamination through medium to long-range atmospheric transport, as is most of Northern Canada [START_REF] Barrie | Arctic contaminants: sources, occurrence and pathways[END_REF][START_REF] Outridge | Regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the Canadian Arctic[END_REF][START_REF] Outridge | Evidence for control of mercury accumulation rates in Canadian High Arctic lake sediments by variations of aquatic primary productivity[END_REF]Kuzyk et al. 2010a;[START_REF] Outridge | Comparison of mercury and zinc profiles in peat and lake sediment archives with historical changes in emissions from the Flin Flon metal smelter, Manitoba, Canada[END_REF]. The different potential sources of anthropogenic lead contamination in the Arctic were traced using lead isotopes, as each source is characterized by a specific isotopic signature linked essentially to the mined ore deposits [START_REF] Sturges | Lead 206/207 isotope ratios in the atmosphere of North America as tracers of US and Canadian emissions[END_REF][START_REF] Sturges | Arctic air, snow and ice chemistryStable lead isotope ratios in Alaskan arctic aerosols[END_REF]. The sources thus identified were located in Canada, the USA, Europe and Russia [START_REF] Sturges | Lead 206/207 isotope ratios in the atmosphere of North America as tracers of US and Canadian emissions[END_REF][START_REF] Sturges | Stable lead isotope ratios in arctic aerosols: evidence for the origin of arctic air pollution[END_REF]).

Sediment cores

The two sediment cores used for this study (Table 1) were chosen to capture the characteristics of water masses that enter Hudson Bay (station HB-6), and those that are at the end of the cyclonic gyre (station A). The two cores were collected using a box-corer during the MERICA cruise (étude des MERs Intérieures du Canada) in summers 2003 (station A) and 2004 (station HB-6). The sediment cores were collected and provided to us by Michel Starr (Maurice Lamontagne Institute, Fisheries and Oceans Canada). Cores were stored in a cold room at 4°C until 2006, when sub-samples were dried, crushed and stored at room temperature in the laboratory. Analyses reported in this paper were performed in 2007 and 2010. Due to the respective geographic locations of the two coring stations, the allochtonous material that reached stations HB-6 and A originated from the western and eastern coasts of the Bay, respectively. The proportion of marine organic carbon in the surface sediment in the vicinity of cores A and HB-6 was 80 to 85% of the total organic carbon, respectively [START_REF] Kuzyk | Towards a sediment and organic carbon budget for Hudson Bay[END_REF], stressing the importance of autochthonous organic matter in the total organic sediment load of the two areas.

Station

Lat

Chronostratigraphy

The activity of 210 Pb of dried and ground sediment samples was obtained indirectly by measuring the decay rate of its daughter isotope 210 Po (t 1/2 = 138.4 days; a = 5.30 MeV) by alpha spectrometry. Measurements were carried out more than 2 years after sampling to ensure that secular equilibrium was reached. A 209 Po spike was added to the samples to determine the extraction efficiency. Polonium was extracted and purified by chemical treatments (reacted sequentially with HCl, HNO 3 , HF and H 2 O 2 ) and deposited on a silver disk [START_REF] Flynn | The determination of low levels of polonium-210 in environmental materials[END_REF]). The 209 Po and 210 Po activities were measured with a silicon surface barrier a-spectrometer (EGG&ORTEC type 576A).

Cesium-137 was measured on dried and ground sediment samples (1 cm 3 ) by g-ray spectrometry at 661.6 keV (g-ray yield = 85 %) using a low-background high-purity Ge well detector (Canberra). Standard sediment (IAEA-300) was used to calibrate the yield of the detector. Uncertainties were estimated for counting errors following the protocol of [START_REF] Not | 210 Pb-226 Ra-230 Th systematics in very low sedimentation rate sediments from the Mendeleev Ridge (Arctic Ocean)[END_REF].

Sedimentation accumulation rates (SAR) were calculated using the radioactive decay constant (λ = 0.03114 yr -1 ) of 210 Pb and the slope of the linear regression of the logarithm of over depth. The supported 210 Pb was estimated using the asymptotic value of 210 Pb data at the bottom of the core. The linear regression for sedimentation rates was applied to the middle part of the core to avoid potential modern disturbance by bioturbation in the upper part of the core. We considered that the topmost part of the core corresponded to the present time, as no sediment had been lost during sampling. For the bottom part of the core, where no excess 210 Pb was measured, we assumed a constant sedimentation rate.

Isotope geochemistry

In a clean-room (class 100) environment, about 50 mg of dry sedimentary material were dissolved in a Teflon bomb with a mixture of HF-HNO 3 (10:1 ratio) on a hot plate at 110°C for 48 h, and then evaporated to dryness with a drop of HBr to help conversion to PbBr 2 . In order to ensure high purity of the separated Pb, double-pass ion chromatography was performed on AG1-X8 resin in dilute HBr medium to remove matrix elements, followed by elution of Pb phase in 6M HCl (similar to [START_REF] Manhes | Comparative uranium-thorium-lead and rubidium-strontium study of the Saint Sèverin amphoterite: consequences for early solar system chronology[END_REF]. The procedural blank value was negligible (i.e. average value of 42 pg Pb for 1,250,000 pg Pb of sample). Mass spectrometry was done on an IsoProbe multi-collector ICP-MS, with an Aridus desolvating membrane as the introduction system. A transmission of 480 V/ppm was achieved using this set-up. All isotopes of Pb were measured on Faraday detectors with 10 11 W resistors, with amplifiers cross-calibrated in the morning during the plasma warm-up time. Mass bias of samples was obtained using the NBS-991 Tl doping technique (with a Pb/Tl = 10 to 12), and the correlation between Pb and Tl mass biases was calculated from the repeated analysis of NBS-981 [START_REF] Belshaw | A new variable dispersion double-focusing plasma mass spectrometer with performance illustrated for Pb isotopes[END_REF]). Long-term reproducibility of the internal standard was better than 0.03% on isotopic ratios normalized to 204 Pb. The external analytical uncertainty on the isotopic ratios normalized to 204 Pb is on the third digit (4 th digit for the 206 Pb/ 207 Pb ratio). Any larger change in isotopic composition was thus considered as significant. All reagents used were distilled in sub-boiling stills, and subsequently diluted with Milli-Q water.

Lead and aluminium analysis

Lead concentrations were measured on sediment core samples, and aluminum (Al) concentrations were used to normalize Pb. Aluminum was considered to be a purely lithogenic element representative of the input of the detrital component into the sediment. Indeed, very little or no Al of anthropogenic origin is thought to reach the study region, as the Hudson Bay is extremely remote and there is no significant industrial release of Al in these water. Moreover, aluminum is commonly used to normalize various elements because its natural sources highly exceed its anthropogenic sources [START_REF] Daskalakis | Normalization and elemental sediment contamination in the coastal United States[END_REF][START_REF] Heimbürger | Natural and anthropogenic trace metals in sediments of the Ligurian Sea (Northwestern Mediterranean)[END_REF][START_REF] Heimbürger | Vertical export flux of metals in the Mediterranean Sea[END_REF][START_REF] St | Importance of Open Marine Waters to the Enrichment of Total Mercury and Monomethylmercury in Lichens in the Canadian High Arctic[END_REF]. All reagents were certified Suprapur ® and provided by Merck (Darmstadt, Germany). All samples were handled under laminar airflow in a class-100 clean room. Dry bulk sediments were ground using an agate mortar. Thirty milligrams of ground sediment were weighted using a precision balance (Sartorius MC 1, accuracy 0.01 mg), and transferred to a Teflon flask. The organic and carbonate matrices were destroyed as follows:

2 mL HCl 37% and 1 mL HNO 3 65% were added to the flask and heated at 130°C for 4 h. Next, 1 mL of HF 40% and 2 mL HNO 3 65% were added to the flask to dissolve the silicate material, which was then heated at 130°C for 4 h or until complete evaporation. The residue was ultrasonically dissolved in 1 mL HNO 3 1N, and made up to 9 mL with Milli-Q water. Trace metal concentrations were measured using an Inductively Coupled Plasma Optical Emission Spectrometer (SPECTRO ARCOS™) equipped with an autosampler (CETAC ASX-260™) and an ultrasonic nebulizer (CETAC U5000AT™). Analytical procedures were validated using international certified reference material (CRM) for sediment (NCS DC 75305 and IAEA-433), aerosol (B3-0562). Replicates of CRM were always within the quoted confidence intervals (values corresponding to "Certified" in Table 2). Detection limits (0.05 µg L -1 for Al, 0.01 µg L -1

for Pb) were defined as three times the standard deviation of blank measurements for each metal, and relative standard deviations were always <10% (results of the validation procedure are given in Table 2). The anthropogenic enrichment of samples was estimated based on the Pb/Al ratio of each sample divided by the Pb/Al ratio of the oldest samples in each core, which represent the closest to natural background value available for our region. As Al indicates terrestrial input, this index was used as a qualitative index of the sedimentary accumulation trend of non-terrestrial Pb.

NCS-DC 75305

(µg g -1 )
IAEA 433

(µg g -1 )
B3-0562

(

Results

Chronostratigraphy

The chronology of all cores was based on 210 Pb (Fig. 2, blue curves). At station A, the surface sample was lost, and the first sample was thus at 2 cm. The 210 Pb values decreased almost linearly from 523 Bq g -1 at 2 cm to 88 Bq g -1 at 8 cm, after which they decreased slowly until they reached the supported value of ~50 Bq g -1 at 27.5 cm. At station HB-6, 210 Pb was about 123 Bq g -1 at the surface of the core, and progressively increased to 176 Bq g -1 at 6 cm, after which it decreased linearly to reach the supported value of ~73 Bq g -1 at 15 cm.

We estimated the SAR using two approaches. Firstly, the CFCS model estimate yielded sedimentation rates between 0.141 ± 0.032 cm yr -1 (r 2 = 0.86) and 0.114 ± 0.024 cm yr -1 (r 2 = 0.92) for cores A and HB-6, respectively. Secondly, 137 Cs values peaked at 4.5 and 5.5 cm for cores A and HB-6, respectively (Fig. 2). We calculated the SAR by dividing the depth of the peak depth (4.5 and 5.5 cm) by the number of years between the time of sampling and 1963.

These 137 Cs-derived sedimentation rates were consistent with the CFCS-derived rate (0.113 and 0.134 cm yr -1 for cores A and HB-6 respectively).

We hypothesized that the sedimentation rate was constant over the whole period covered by each core, and estimated the age of the sediment by dividing its depth in the core (cm) using constant sedimentation rates of 0.094 to 0.141 and 0.115 to 0.192 cm yr -1 for cores HB-6 and A, respectively. The estimated years corresponding to different depths in each core are indicated in Fig. 2 (dashed horizontal lines, corresponding to the range of sedimentation rate estimates for each core).

Our estimated sedimentation rates are based on the assumptions that the topmost part of the cores corresponded to the present time, and we used a constant sedimentation rate for the bottom part. Despite the resulting uncertainties, our estimates of 0.094 to 0.141 and 0.115 to 0.192 cm yr -1 for cores HB-6 and A, respectively are consistent with values previously estimated for these marine areas of Hudson Bay, i.e. 0.05 to 0.17 cm yr -1 [START_REF] Kuzyk | Towards a sediment and organic carbon budget for Hudson Bay[END_REF][START_REF] Hare | Natural and anthropogenic mercury distribution in marine sediments from Hudson Bay, Canada[END_REF]). We also observed a 206 Pb/ 207 Pb profile shift from the natural background to anthropogenic value in the 1800's, a sharper shift during the 1900's, and a strong dominance of anthropogenic lead in the late 1900's (Fig. 3). However, because the resulting age estimates must be taken with caution, we do not discuss here the precise timing of past events, but we examine instead the general temporal trends of the lead concentrations and the isotope signature.

Lead geochemistry

Bottom-to-top variations in concentrations of Al and Pb were different in the two cores (Fig. 2, left and middle panels). At station A, Al and Pb increased by ∼30% from 10 to 4 cm (i.e.

from the 1800's to the early 1900's), after which they both decreased in the topmost samples. At station HB-6, there was no strong vertical variation in either the Al or Pb profile. In both cores, variations in the enrichment ratio normalized to Al were small (Fig. 2, right panel). There was a strong positive correlation between Pb and Al in the two cores (station A: r = 0.95, Prob <0.0005; station HB-6: r = 0.83, Prob <0.005). Concerning the isotopes, there was a shift in 206 Pb/ 207 Pb in core A from 1.09 at the bottom to about 1.16 in the topmost centimeters (Fig. 3). In core HB-6, the pattern was opposite, with the bottom-to-top ratio shifting from 1.22 to 1.17. The isotopic values of the two cores converged at the top. We also investigated the variations in 204 Pb, 206 Pb and 208 Pb as it had been suggested that normalizing 206 Pb and 208 Pb to 204 Pb generally allows to distinguish between three mixed endmembers [START_REF] Ellam | The graphical presentation of lead isotope data for environmental source apportionment[END_REF]). In the scatter diagram of 208/204 Pb vs 206/204 Pb (Fig. 4; values in the supplemental material), the ratios from the two cores are very far apart for the oldest samples and progressively converge toward similar values for the most recent samples. [START_REF] Guiot | Reconstruction of seasonal temperatures in Central Canada since A.D. 1700 and detection of the 18.6and 22-year signals[END_REF][START_REF] Graney | Coastal conduit in southwestern Hudson Bay (Canada) in summer: Rapid transit of freshwater and significant loss of colored dissolved organic matter[END_REF][START_REF] Poirier | Re-Os and Pb isotope systematics in reduced fjord sediments from Saanich Inlet (Western Canada)[END_REF]. Each sample is labelled with its depth (in cm) in its core to highlight the temporal convergence of the isotope ratios from the two cores toward the same values.

Discussion

Historical variations in the sources of Pb

The Hudson Bay is characterized by intense sediment resuspension due to postglacial isostatic rebound [START_REF] Hare | Contemporary and preindustrial mass budgets of mercury in the Hudson Bay Marine System: The role of sediment recycling[END_REF][START_REF] Kuzyk | Towards a sediment and organic carbon budget for Hudson Bay[END_REF], which can dilute anthropogenic inputs in surface sediments [START_REF] Hare | Natural and anthropogenic mercury distribution in marine sediments from Hudson Bay, Canada[END_REF]. This could partly mask the sedimentary record of anthropogenic Pb, and thus prevent the use of our sediment records to investigate the vertical export of Pb during the last centuries. However, our cores clearly recorded Pb of anthropogenic origin, as shown by the convergent trend toward the anthropogenic value of 1.16 [START_REF] Sturges | Lead 206/207 isotope ratios in the atmosphere of North America as tracers of US and Canadian emissions[END_REF] in Figure 3, and are thus suitable for investigating the vertical export dynamics of anthropogenic Pb as shown by the 210 Pb chronology, which was corroborated by a secondary stratigraphic marker ( 137 Cs).

Because Al in sediments traces the terrestrial inputs, the correlation between Pb and Al provides information on the importance of lithogenic inputs in the accumulation of Pb [START_REF] Brumsack | The trace metal content of recent organic carbon-rich sediments: Implications for Cretaceous black shale formation[END_REF].

Effect of primary productivity on the sedimentary Pb record

Local anthropogenic inputs of metals are low in the Hudson Bay area because of its remote location and the scarcity of industries. Hence, medium to long-range atmospheric transport was presumably the main source of anthropogenic contaminants. A small increase from 25 to 30 mg kg -1 in Pb concentration was observed in sediment core A around 6 and 7 cm (Fig. 2).

However, because a similar increase was also observed in Al, Pb normalized to Al was mostly constant, which suggested that Pb was of terrestrial origin. This is consistent with the slight reduction in 206 Pb/ 207 Pb near 6 and 7 cm in core A indicating a transient return to more terrestrial (background) values. This transient increase in terrestrial input could be related to increased precipitation due to the climatic variability of Hudson Bay [START_REF] Guiot | Reconstruction of seasonal temperatures in Central Canada since A.D. 1700 and detection of the 18.6and 22-year signals[END_REF]. In core HB-6, the record showed no sign of increase in either Pb concentration or Pb/Al, suggesting relatively constant Pb input (terrestrial and atmospheric) during the last two centuries in this part of the bay. The ~20% increase in Pb concentration in core A is much smaller than the three-to five-fold increase in Pb concentration in the recent sediments (last century) of two lakes in the Hudson Bay [START_REF] Outridge | Regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the Canadian Arctic[END_REF]. The observed increase in Pb in lake sediments was attributed by [START_REF] Outridge | Regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the Canadian Arctic[END_REF] to medium to long-range atmospheric inputs of anthropogenic Pb originating from Eurasia and North America. Because waters of the lakes and Hudson Bay should have been both exposed to similar inputs of atmospheric materials, the difference in Pb accumulation between the two environments indicates that the transfer mechanisms of Pb to the sediment where different in the two environments, assuming no diagenetic or post-sampling effect on Pb concentrations. The 206 Pb/ 207 Pb isotopic values (Fig. 3) show, for the cores on the two sides of the Hudson Bay, an apparent progressive overprint of anthropogenic lead (from bottom to top of cores) during the 1900's, whose isotopic ratio reflects a mixed Canada-USA aerosol origin ( 206 Pb/ 207 Pb = 1.16-1.17; [START_REF] Sturges | Lead 206/207 isotope ratios in the atmosphere of North America as tracers of US and Canadian emissions[END_REF]. In other words, despite a globally increased atmospheric import of anthropogenic lead [START_REF] Nriagu | A History of Global Metal Pollution[END_REF], the amount of Pb deposited in Hudson Bay's sediment did not change dramatically during the last century, but the sources of emission changed as shown by changes in the isotope ratios. This indicates that sedimentary accumulation of anthropogenic Pb was limited in Hudson Bay by a factor that acted differently in the bay than in surrounding lakes.

Assuming a typical sinking velocity of 20 to 200 m d -1 for Pb when packaged within biogenic aggregates [START_REF] Alldredge | In situ settling behavior of marine snow[END_REF][START_REF] Armstrong | Settling velocity spectra and the ballast ratio hypothesis[END_REF] Our results and previously published data show that the sedimentary accumulation of Pb in the last century was not the same in the records of low-productivity Hudson Bay as in the highproductivity nearby lakes, whereas the recent isotopic composition was the same in sediments from both Hudson Bay and the surrounding lakes [START_REF] Outridge | Regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the Canadian Arctic[END_REF]. Hence, we cannot reject our initial hypothesis that primary productivity was the main forcing factor determining the sedimentation of airborne matter. This hypothesis is consistent with the suggestion from previous studies that sedimentary sequences in Hudson Bay did not always record directly the atmospheric deposition of allochthonous matter, but could be affected by low primary productivity. For example and similarly to our Pb observations, it was recently suggested that the sedimentation of polychlorinated biphenyl (PCBs) was exceptionally low in Hudson Bay, because the very low productivity in the Bay and the resulting weak downward flux of organic matter inhibited the transfer of PCBs from surface to depth and, therefore, their sedimentation (Kuzyk et al. 2010a).

Such a relationship between primary productivity and the downward flux of aggregates may be a typical feature of many environments in the world ocean [START_REF] Passow | Switching perspectives: Do mineral fluxes determine particulate organic carbon fluxes or vice versa?[END_REF][START_REF] Heimbürger | Vertical export flux of metals in the Mediterranean Sea[END_REF]).

The conceptual model in Fig. 5 summarizes the above ideas, illustrating the earlyindustrial and post-industrial situations in Hudson Bay. In the early-industrial model, atmospheric inputs of Pb were low, and this Pb was characterized by the local isotopic signature a. Most Pb, perhaps all, was exported downwards and accumulated in the sediment after being scavenged by organic particles, thus transferring the isotopic signature of Pb from surface waters to the sediment. The post-industrialization model is characterized by a higher rate of Pb input from the atmosphere to the surface water due to the enhanced medium to long-range transport of anthropogenic Pb [START_REF] Outridge | Regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the Canadian Arctic[END_REF]. This Pb is characterized by the different isotopic signature b, which reflects its mixed local and anthropogenic origins. The similar concentration of Pb accumulated in the sediment is explained by a limitation of its export by the low primary productivity of Hudson Bay, which restricts the availability of organic particles to scavenge Pb and export it to the sediment. As a consequence, despite a likely increase in atmospheric deposition of Pb with increased industrialization, its accumulation was similar after and before industrialization, but its isotopic signature was different in the two periods. Enhanced atmospheric inputs of contaminants in the last century were not significantly recorded in Hudson Bay sediments, presumably because of the exceptionally low productivity of the Bay [START_REF] Kuzyk | Towards a sediment and organic carbon budget for Hudson Bay[END_REF], which prevented the efficient transfer of chemical elements from the water column to the sediment. However, the signature of the Pb isotopes suggests a shift from local input to medium to long-range anthropogenic inputs during the 20 th century, which thus recorded the signature of the last century. This is consistent with our hypothesis that low primary productivity was the main forcing factor that determined the sedimentation of atmosphericallydeposited matter in Hudson Bay during the last century. This observation is globally significant as Hudson Bay is extremely sensitive to warming, with an increase of 0.47°C per decade over the last 50 years [START_REF] Mulder | Increased variance in temperature and lag effects alter phenological responses to rapid warming in a subarctic plant community[END_REF]). Warming will lead to increased river discharge, which could enhance primary productivity in the Hudson Bay because river discharge promotes the upwelling of deep waters (Kuzyk et al. 2010a;Kuzyk et al. 2010b). If Arctic regions are currently warming up, one may expect increasing fluxes of nutrients to Hudson Bay, in the northern part of Canada.

It may therefore also be expected that the magnitude of primary productivity in Hudson Bay will increase as well, which would lead to an increase in export fluxes of atmospherically-deposited contaminants, once packaged with particulate biogenic matter.

Possible effect of transparent exopolymer particles (TEP)

It was mentioned above that one of the possible explanations to our observations was that the Pb-containing aggregates sank at very low velocity. This could have occurred if the aggregates formed in Hudson Bay had been neutrally buoyant, and had thus remained in surface waters long enough to be flushed out of the Bay. While there are multiple factors that can influence the particle settling velocity [START_REF] Maggi | The settling velocity of mineral, biomineral, and biological particles and aggregates in water[END_REF], the accumulation of positively buoyant transparent exopolymer particles (TEP) in oligotrophic surface waters is hypothesized to contribute to slowing down the downward export flux (Azetsu-Scott and [START_REF] Passow | Switching perspectives: Do mineral fluxes determine particulate organic carbon fluxes or vice versa?[END_REF][START_REF] Mari | Transparent exopolymer particles: Effects on carbon cycling in the ocean[END_REF].

The increase of TEP volume concentration in surface waters is a significant feature in some oligotrophic waters, e.g. the Mediterranean Sea [START_REF] Mari | Non-Redfield C:N ratio of transparent exopolymeric particles in the northwestern Mediterranean Sea[END_REF][START_REF] Bar-Zeev | Transparent exopolymer particle (TEP) dynamics in the eastern Mediterranean Sea[END_REF], the Pacific Ocean [START_REF] Wurl | Production and fate of transparent exopolymer particles in the ocean[END_REF][START_REF] Kodama | Vertical distribution of transparent exopolymer particle (TEP) concentration in the oligotrophic western tropical North Pacific[END_REF], and Hudson Bay [START_REF] Michel | Transparent expopolymeric substances (TEP) in Hudson Bay during fall: significance and potential roles[END_REF]). The limited TEP dataset for Hudson Bay is characterized by a sub-surface maximum around 50 m where it co-occurs with the chlorophyll a maximum, suggesting a link between the two [START_REF] Michel | Transparent expopolymeric substances (TEP) in Hudson Bay during fall: significance and potential roles[END_REF]. Nutrient limitation increases the production of TEP by phytoplankton and lowers their bacterial degradation, and the two processes contribute to TEP accumulation in surface waters [START_REF] Mari | Non-Redfield C:N ratio of transparent exopolymeric particles in the northwestern Mediterranean Sea[END_REF][START_REF] Mari | Transparent exopolymer particles: Effects on carbon cycling in the ocean[END_REF]. Owing to their high stickiness, TEP are often seen as a catalyst of aggregation, and thus a key component for the formation of fast-sinking aggregates [START_REF] Passow | The origin of transparent exopolymer particles (TEP) and their role in the sedimentation of particulate matter[END_REF]), but [START_REF] Mari | Transparent exopolymer particles: Effects on carbon cycling in the ocean[END_REF]) proposed that a system with high volume concentration of lowdensity TEP and low concentration of dense particles in surface waters should be characterized by low downward POC export [START_REF] Mari | Transparent exopolymer particles: Effects on carbon cycling in the ocean[END_REF]. The latter conditions are those observed during periods of severe oligotrophy in Hudson Bay, where the accumulation of TEP in surface waters could have enhanced the residence time of TEP-associated elements (e.g. Pb) and particles

Fig. 1 :

 1 Fig. 1: Map of Hudson Bay with major currents. The two red dots indicate the locations of our two sediment cores at about 60°N (Stations HB-6 and A), the yellow dots indicate the locations of the two lakes to which our data are compared in the discussion and the two straight arrows on the eastern side of James Bay represent major river inputs.

  excess 210 Pb (i.e. 210 Pb scavenged from the water column) following the constant flux and constant sedimentation model (CFCS), previously described by Sanchez-Cabeza and Ruiz-Fernández (2012). Excess 210 Pb was estimated from the 210 Pb activity (Fig 2; data in the online supplement) minus the supported 210 Pb (i.e. the 210 Pb produced locally from 226 Ra disintegration)

Fig. 2 :

 2 Fig. 2: Vertical profiles of 210 Pb, 137 Cs, Al and Pb in cores A (top) and HB-6 (bottom). Left panels. 210 Pb (Bq g -1 ; blue, upper scale) and natural log of excess lead-210 (red, lower scale); different scales are used for the two stations. Red dashed lines: linear regressions of natural log of excess lead-210 on depth and their 95% confidence intervals. Middle panels. Profiles of Al (pink, mg kg -1 ). Right panels. Pb concentration (green, mg kg -1 , upper scale), and enrichment ratio of Pb normalized to Al (blue, bottom scale). The dashed horizontal lines correspond to increasing age with depth (years are indicated), and the intervals indicate the uncertainties in our estimates. In the right panel, the enrichment ratio of Pb normalized to Al is provided on a 0-to-2 scale, where 1 indicates no enrichment compared to the oldest value recorded at the site, 2 represents a 2-time enrichment, and a value between 0 and 1 represents a decrease of Pb/Al compared to the oldest recorded value.

Fig. 3 :

 3 Fig. 3: 206 Pb/ 207 Pb ratio in cores HB-6 and A. The dashed vertical line represents the isotopic value of the Arctic anthropogenic Pb (1.16; Sturges et al. 1993). The error bars are smaller than the dots (see text for details).

Fig. 4 .

 4 Fig. 4. Plot of the isotopic ratios 208/204 Pb on 206/204 Pb, along with national aerosol averaged values for two different potential source regions (grey boxes), i.e. Canada and the USA in the 1980's (Sturges and Barrie

  The strong positive correlations of Pb and Al in the two cores (Section 3.2) indicate that the historical Pb accumulation was strongly controlled by terrestrial inputs. In addition, the vertical export of terrestrial or atmospheric Pb from surface waters to the sediment would have different effects on Pb concentration normalized to Al, i.e. terrestrial Pb would be deposited together with terrestrial Al, hence constant Pb normalized to Al, whereas atmospheric Pb would not be deposited together with terrestrial Al, hence higher Pb normalized to Al[START_REF] Daskalakis | Normalization and elemental sediment contamination in the coastal United States[END_REF][START_REF] Heimbürger | Natural and anthropogenic trace metals in sediments of the Ligurian Sea (Northwestern Mediterranean)[END_REF][START_REF] Heimbürger | Vertical export flux of metals in the Mediterranean Sea[END_REF]. Moreover, enrichment factors of mercury relative to aluminium have been similarly used to identify the source of mercury (atmosphere against underlying soils) by[START_REF] St | Importance of Open Marine Waters to the Enrichment of Total Mercury and Monomethylmercury in Lichens in the Canadian High Arctic[END_REF]. At our two stations, there were no strong variations in the enrichment ratio of Pb normalized to Al along the cores, indicating that the sedimentary accumulation of airborne Pb remained constant during the last 200 years in the two cores.Irrespective of the isotope considered, the two cores were characterized by a trend (i.e. older to recent) that converged toward a value half way between typical historical Canadian and USA lead emissions (Figs.3 and 4). The oldest samples in the two cores were characterized by opposite 206 Pb/ 207 Pb, 206 Pb/ 204 Pb and 208 Pb/ 204 Pb isotopic signatures (Figs.3 and 4), meaning that early-industrial Pb inputs (i.e. between 1800 and 1900) originated from different sources in the western and eastern parts of Hudson Bay. The temporal trends of 206 Pb/ 204 Pb and 208 Pb/ 204 Pb in core HB-6 are similar to those observed in two Hudson Bay lakes[START_REF] Outridge | Regional variations in atmospheric deposition and sources of anthropogenic lead in lake sediments across the Canadian Arctic[END_REF], suggesting similar early-industrial sources of lead on the western side of the Bay. The earlyindustrial 206 Pb/ 207 Pb isotopic signature was different in cores A and HB-6 (1.10 and 1.22, respectively), on the eastern and western sides of the Bay, respectively. This probably reflected the spatial heterogeneity that exists in the different potential sources of Pb (with different isotopic signatures) in the Canadian shield, which surrounds most of Hudson Bay(GEOROC 2003). The isotopic ratio values toward which the two cores converged were the same as in surrounding lakes on the two sides of HudsonBay (Fig. 3 in this study, and Figs. 6 and 7 in Outridge et al. 2002). Irrespective of the early-industrial sources, the fact that the isotopic composition in the two cores started with different values and converged toward a single value indicates that both cores recorded the imprint of medium to long-range anthropogenic Pb deposition during the last century.

  ; McDonnell and Buesseler 2010), i.e. when sedimentation is driven by biological productivity, surface Pb would reach 250 m (i.e. the maximum depth of Hudson Bay) within less than 15 days (the depths of coring sites HB-6 and A were 120 and 130 m, respectively). Hence even if the estimates of the residence time of Hudson surface waters vary over a wide range in the literature (i.e. from one month to 6 years, Section 2.1), Pb packaged within biogenic aggregates should sediment within the Bay. As a result, the only ways by which Pb would have not reached the sediment should have been either a lack of aggregation due to very low biological productivity, and/or a very low sinking velocity of the Pb-containing aggregates, i.e. <<20 m d -1 . These two conditions would have led Pb to be flushed out of the bay.

Fig. 5 .

 5 Fig. 5. Schematic representation of our hypothesis on the atmospheric and water-column fluxes of contaminants before and after industrialization in Hudson Bay. The illustrated mechanisms explain both the absence of difference in Pb sedimentary accumulation, and the change in the isotopic signatures of sediments from the early-industrial to the post-industrial period.

Table 1 :

 1 Characteristics of the two coring stations and cores.

		. (°N)	Long. (°W)	Depth (m)	Core length (cm)
	HB-6	60.94°	91.78°	120	24
	A	60.17°	79.00°	130	30

  µg g -1 )

	Aluminium
	Measured (mean)	4.03 ± 0.297	80.39 ± 3.030	108.56 ± 1.37
	Certified	4.08 ± 0.16	78.30 ± 4.30	107.91 ± 0.98
	Lead
	Measured (mean)	22.2 ± 0.622	26.5 ± 0.329	16.94 ± 0.334
	Certified	22.0 ± 1.1	26.0 ± 0.6	17.055 ± 0.195
	Table 2: Certified reference material (CRM) validation results. Measurements were averaged
	from 3 replicates.

Acknowledgements

All data used in this paper are available in the online supplement. Funding was provided by the Fonds Québécois de la Recherche sur la Nature et les Technologies through an international doctoral internship grant awarded to BT. We thank M. Starr, and the monitoring program for Canadian interior seas (MERICA) funded by the Canadian Department of Fisheries and Oceans, Natural Resources Canada, and the Canadian Coast Guard for providing the cores and the ship time. We are grateful to R. Losno (Institut de physique du globe de Paris) for his assistance with trace metal analyses. We are thankful to the two anonymous reviewers for their constructive input.


Andrej Jančařík
email: andrej.jancarik@cemes.fr

Jan Holec

Yuuya Nagata

Michal Šámal

André Gourdon

André Gourdon

Preparative-Scale

Andre Gourdon
email: andre.gourdon@cemes.fr

Preparative-scale synthesis of nonacene

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

Over the last decade, particular attention has been brought to long unsubstituted acenes (longer than pentacene) from both experimental and theoretical points of view. [START_REF] Dorel | Strategies for the Synthesis of Higher Acenes[END_REF] The nature of their electronic structure such as gap stabilization [START_REF] Korytár | Signature of the Dirac cone in the properties of linear oligoacenes[END_REF][START_REF] Schmitteckert | Incommensurate quantum-size oscillations in acene-based molecular wires-Effects of quantum fluctuations[END_REF][START_REF] Krüger | Electronic Resonances and Gap Stabilization of Higher Acenes on a Gold Surface[END_REF] and open shell singlet ground state for longer acenes is still actively discussed. [START_REF] Plasser | The Multiradical Character of One-and Two-Dimensional Graphene Nanoribbons[END_REF][START_REF] Hachmann | The radical character of the acenes: A density matrix renormalization group study[END_REF][START_REF] Jiang | Electronic Ground State of Higher Acenes[END_REF][START_REF] Malrieu | Can a Topological Approach Predict Spin-Symmetry Breaking in Conjugated Hydrocarbons?[END_REF][START_REF] Yang | Nature of ground and electronic excited states of higher acenes[END_REF] Furthermore, longer acenes can be seen as the narrowest zig-zag graphene nanoribbons (ZGNR) and could display spin-polarized edge-states of interest for carbon-based spin electronics. [START_REF] Fujita | Peculiar Localized State at Zigzag Graphite Edge[END_REF][START_REF] Ruffieux | On-surface synthesis of graphene nanoribbons with zigzag edge topology[END_REF] However, acenes longer than pentacene are challenging to prepare and to handle. First, intermolecular - stacking between these planar and rigid molecules rapidly limits their solubility as their sizes increase. Second, acenes possess only one Clar aromatic sextet spread over the whole skeleton, leading to a decrease of the HOMO-LUMO gap with an increasing number of benzene rings, and therefore to an increase of the chemical reactivity. Although photooxidation with molecular oxygen can be avoided by working under argon, rapid dimerization in solution even at low concentration might become a problematic limitation.

Several strategies have been pursued to prepare long acenes, all relying on the same concept: masked stable and soluble precursors are prepared and purified by standard in-solution chemistry techniques and, in a final step, the masking groups are removed in the solid state or at low temperature in very dilute conditions on surfaces or in matrices. In particular Neckers, Bettinger and coworkers have explored the photogeneration, in stabilizing matrices, of hexacene, 12 heptacene [START_REF] Mondal | Photogeneration of Heptacene in a Polymer Matrix[END_REF][START_REF] Bettinger | Stable photoinduced charge separation in heptacene[END_REF][START_REF] Einholz | Heptacene: Characterization in Solution, in the Solid State, and in Films[END_REF][START_REF] Mondal | Stability, and Photochemistry of Pentacene, Hexacene, and Heptacene: A Matrix Isolation Study[END_REF] , octacene, nonacene [START_REF] Christina | Photogeneration of Octacene and Nonacene[END_REF] , and undecacene [START_REF] Shen | Evolution of the Optical Gap in the Acene Series: Undecacene[END_REF] by photodecarbonylation of precursors comprising two bridging -diketone groups, following Yamada's concept [START_REF] Yamada | Photochemical Synthesis of Pentacene and its Derivatives[END_REF] using the Strating-Zwanenburg 20 reaction. Long acenes can also be prepared by on-surface synthesis in Ultra High Vacuum (UHV) and observed at liquid helium temperature and their electronic structures mapped by Scanning Tunneling Spectroscopy (STS). Higher acenes up to undecacene have been obtained by deoxygenation of epoxides, [START_REF] Krüger | Electronic Resonances and Gap Stabilization of Higher Acenes on a Gold Surface[END_REF][START_REF] Krüger | Imaging the electronic structure of on-surface generated hexacene[END_REF][START_REF] Krüger | Decacene: On-Surface Generation[END_REF] dehydrogenation of partially saturated precursors, [START_REF] Zuzak | Nonacene Generated by On-Surface Dehydrogenation[END_REF][START_REF] Zuzak | Higher Acenes by On-Surface Dehydrogenation: From Heptacene to Undecacene[END_REF] or thermal or photo decarbonylation of diketone adducts. 25- 27 However, both types of generation, in stabilizing matrices or on surfaces in UHV only give minute amounts of materials and cannot be used for macroscopic amounts of materials needed for applications. Indeed, the preparation of acenes longer than pentacene in a pure state is very recent despite 70 years of claims, [START_REF] Sanjio | Heptacene and Beyond: The Longest Characterized Acenes[END_REF] and so far limited to hexacene and heptacene. In 2012, Chow and coworkers isolated hexacene by decarbonylation of a monoketone precursor in the solid state and its structure was determined by X-ray diffraction. [START_REF] Watanabe | The synthesis, crystal structure and charge-transport properties of hexacene[END_REF] They demonstrated that a field-effect transistor made with a single crystal of hexacene showed a hole mobility significantly higher than pentacene. In 2017, Bettinger et al. have reported the formation of heptacene in the solid state by thermal cycloreversion from a mixture of diheptacenes obtained in solution. [START_REF] Einholz | Heptacene: Characterization in Solution, in the Solid State, and in Films[END_REF] More recently, we have also obtained heptacene and benzohexacene by cheletropic decarbonylation at moderate temperature, confirming the thermal stability of these higher acenes. [START_REF] Jancarik | A Practical General Method for the Preparation of Long Acenes[END_REF] The preparation of even longer acenes in bulk form has been indeed an attracting challenge since Clar's prediction in 196 claiming that the synthesis of octacene (and beyond) was a remote target. [START_REF] Clar | Polycyclic Hydrocarbons[END_REF] And in a recent review, C. Tönshoff and H. F. Bettinger conclude that "It is not even clear if acenes larger than that of heptacene can exist outside the special environment provided by matrix isolation or on-surface synthesis". [START_REF] Tönshoff | Pushing the Limits of Acene Chemistry: The Recent Surge of Large Acenes[END_REF] To answer this question, we present here the synthesis of nonacene 1 and demonstrate its surprising stability.

Results and discussion

As nonacene was expected to be highly insoluble and reactive, our strategy is based on pure soluble and chemically stable masked nonacene, that could be deprotected quantitatively by heating at medium temperatures in the solid state. In previous contributions, we have shown [START_REF] Jancarik | A Practical General Method for the Preparation of Long Acenes[END_REF][START_REF] Holec | A Large Starphene Comprising Pentacene Branches[END_REF] that 7,7-dimethoxy-2,3,5,6-tetramethylenebicyclo[2.2.1]-heptane (further in the text as tetraene) (scheme 1), which can be prepared at the tens of grams scale, 35 can undergo successive Diels-Alder reaction with arynes to provide non-planar, soluble, and not fully delocalized acene precursors. In these compounds, one of the benzene rings is bridged by a dimethyl ketal group.

Scheme 1. Schematic strategy of the preparation of various acenes starting from 7,7-dimethoxy-2,3,5,6tetramethylenebicyclo[2.2.1]-heptane. Diels-Alder addition (a) with arynes, followed by aromatization gives a non-planar bridging dimethylketal, which can be deprotected (b) to yield a polyaromatic precursor bridged by a carbonyl group. (c) Solid state thermal or photochemical decarbonylation gives the acene with only carbon monoxide as by-product.

Cleavage of this ketal yields the corresponding polyaromatic acene precursors comprising norbornadiene-7-one moieties, mentioned simply as ketone precursors in this report. These compounds are at least partially soluble, chemically stable and can be purified by standard insolution techniques such as chromatography or recrystallization. They are stable enough to be stored in the dark for long period of time. The final step is a thermal or photochemical cheletropic decarbonylation in the solid state yielding the corresponding acenes in quantitative yields without any non-volatile by-products. It has been shown that this method of decarbonylation was very effective to prepare sensitive high quality materials, such as pentacene and hexacene, [START_REF] Watanabe | Acenes Generated from Precursors and Their Semiconducting Properties[END_REF] for opto-electronic devices.

Our exploration of nonacene 1 commenced with the synthesis of such a carbonylated precursor 7a/7b which can be easily transformed in the solid-state form to the nonacene by simple heating.

The synthesis of the precursors 7a/7b leading to nonacene 1 follows the route shown in scheme 2 with four synthetic steps starting from the diene 2, which can be prepared in two steps from the tetraene shown in scheme 1 above. 34 The key reaction of the synthetic sequence is the double Diels-Alder reaction of the diene 2 and the in situ generated bis(aryne) obtained by fluoride-induced decomposition of 2,5bis(trimethylsilyl)-1,4-phenylene bis(trifluoromethanesulfonate) 3. [START_REF] Yoshida | The Renaissance and Bright Future of Synthetic Aryne Chemistry[END_REF][START_REF] Pe˜na | Bottom-up Approaches to Nanographenes through Organic Synthesis[END_REF][START_REF] Rodríguez-Lojo | Straightforward Synthesis of Novel Acene-Based Aryne Precursors[END_REF] Alternatively, the Diels-Alder reaction can be carried out from the aryne precursor 4 (synthesized from the same tetraene in three steps in 33% yield), with the diene 2. The formed mixture of these two isomers syn and anti 5a/5b can be easily separated on silica gel. As expected the NMR spectra of the two isomers are almost identical and owing to their symmetry and to the distance between the ketal groups, it is not possible to assign their structures by proton and carbon NMR spectroscopy. Fortunately, slow evaporation of a solution of the isomer 5b in a mixture of solvents (hexane/EtOAc) provided suitable crystals for X-ray analysis (Fig. 1).

Fig. 1. Ortep representation of isomer 5b

It shows that the two naphthalene ends of the anti-isomer 5b are perfectly parallel to each other and the angle between naphthalene and anthracene units is 105.5°.

Surprisingly, these products 5a and 5b were formed in the ratio 1:2 in favour of synisomer 5a. Indeed, considering the distance between the reactive sites, a statistical ratio 1:1 was initially expected. In order to investigate the stereoselectivity of the Diels-Alder reactions between the diene 2 and the benzyne compounds in acetonitrile, their transition states (TS) were determined by using density functional theory (DFT). 40 By using Gaussian 16, Revision C.01, 40 the geometries of TS were optimized with QST3 method at the B3LYP/6-31+G(d,p) level of theory with the polarizable continuum model (PCM) to include solvent effects (acetonitrile). Subsequently, single point energy calculations of TS were calculated at the M06-2X/6-31+G(d,p) level of theory with PCM (acetonitrile). [START_REF] Pieniazek | Sources of Error in DFT Computations of C C Bond Formation Thermochemistries: π→σ Transformations and Error Cancellation by DFT Methods[END_REF] Based on the activation energy determined by the calculations, anti/syn ratio was estimated to 1/1.64, which is consistent with the observed stereoselectivity in the experiment (anti/syn = 1/2) (See supporting information for details).

The isomers 5a and 5b underwent smooth aromatization by DDQ at room temperature in almost quantitative yield. Then, the two dimethylketal groups of 6a/6b were cleaved by trimethylsilyl iodide, which afforded the corresponding carbonyl isomers 7a and 7b in 94% and 95% yield, respectively. The anti-isomer 7b is less soluble in many organic solvents than its counterpart 7a, likely due to the ability to pack in quasi one-dimensional chains with efficient π-π stacking, which is not the case for U-shape isomer 7a. Both isomers are colourless chemically stable compounds.

Decarbonylation of 7a/7b in the solid state can be followed by thermal gravimetric analysis (TGA) as shown in Fig. 2. A weight loss of 11.9% for syn-isomer and 13.0% for anti-isomer (calcd 10.5%) correspond to the loss of two carbonyl groups per molecule. The full decarbonylation occurred below 190°C in a one step process for both isomers. However, in the case of syn-isomer 7a, the TGA thermogram consists of a gradual weight loss starting at about 60°C. This can be explained by a lower thermal stability compared to the anti-isomer (starting around 180°C). The decarbonylation is accompanied by a colour change from white to anthracite and the formation of nonacene. Under these conditions, nonacene is surprisingly thermally stable up to almost 500°C. The formation of nonacene 1 by loss of two carbonyl groups is also evident during the high-resolution ESI MS measurement of 7a/7b in which only a peak at m/z 478.1731 from 7a or 478.1727 from 7b corresponding to the formula C38H22 (calcd m/z: 478.1722) was recorded.

The decarbonylation process was also followed by FTIR experiments (Fig. 3), where the stretching vibration of carbonyl peak at 1780 cm -1 disappear after heating the carbonyl precursor in KBr pellet.

Fig. 3. FTIR spectra (KBr pellets) of the carbonylated precursors 7b (in blue) with a strong CO peak at 1781 cm -1 and of the resulting nonacene 1 (in red) after 1 min. heating at 350°C in glovebox.

This transformation has been also followed by solid state cross-polarization magic angle spinning (CP-MAS) NMR spectroscopy. The spectrum of the precursor 7a shows (Fig. 4) three groups of signals, one at 57 ppm (bridgehead sp 3 carbons), a complex peak at 120-137 ppm (aromatic carbons) and the carbonyl carbons at 193 and 198 ppm. Despite the symmetry of the molecule, the different environments in the solid state of the two carbonyl groups is a cause of these two signals. After heating the sample for 20 minutes at 200°C under inert atmosphere, the carbonyl signals disappear and aromatic region get narrower (Fig. 4). However, a smaller and broader sp 3 signal at 54 ppm remains. We attribute this peak to partial dimerization/polymerization by of the decarbonylated compound. Upon heating at higher temperatures, this peak decreases whereas the peak attributed to the aromatic carbons gets narrower with a decrease of the shoulder at 137 ppm. The evolution of the CPMAS spectrum of the isomer 7b is very similar (see SI). This behaviour is very reminiscent to that of heptacene. [START_REF] Einholz | Heptacene: Characterization in Solution, in the Solid State, and in Films[END_REF] The latter molecule dimerizes in solution during its preparation, but cycloreversion to heptacene of the two dimers has been observed by heating for 12 min at 300°C in the solid state. However, after several weeks, some of the heptacene reacted back to the dimers.

Based on the TGA experiment, the formed nonacene is surprisingly stable almost up to 500°C, which allows to realize the extrusion of carbonyl groups at much higher temperature. In a new experiment we carried out the decarbonylation of the precursors 7a/7b at 350°C for 20 min (Fig. 6). Gratifyingly, the decarbonylation process was much cleaner with only a sharp doublet in aromatic region without any signs of dimerization. Keeping these samples in the NMR rotor at room temperature in a glove box for 2 months did not lead to any degradation or dimerization of nonacene 1 suggesting that the nonacene prepared under these conditions is stable. Alternatively, nonacene 1 can also be obtained by decarbonylation in a solvent at high temperature. For instance, the soluble precursor 7a was dissolved in chlorobenzene and the solution was thoroughly degassed. This solution was heated at 200°C for 10 minutes and the formation of a dark violet precipitate was observed. This suspension was drop-casted on an indium-tin oxide (ITO) glass slide without any matrix and dried. In a parallel experiment, the suspension was mixed with 2,5-dihydroxy benzoic acid as a matrix and then drop-casted on an ITO glass slide and dried. These ITO glass slides were introduced in a MALDI chamber, and the high-resolution mass spectra (HRMS) were recorded. Both spectra showed only the parent peak corresponding to the nonacene 1, and no trace of a dimer as was observed for heptacene. [START_REF] Einholz | Heptacene: Characterization in Solution, in the Solid State, and in Films[END_REF] These results suggest that, in contrast with heptacene, nonacene 1 is so highly insoluble that just after decarbonylation, immediate precipitation of the monomer prevents the dimerization in solution.

Summary and outlook

In summary, pure nonacene can be prepared by thermal bis-decarbonylation of precursors either in the solid state, or in high boiling point solvents. This long acene is surprisingly thermally stable up to 450°C and does not decompose at room temperature under dry argon. Our preparation procedure could as well be applied for the construction of substituted nonacenes and to even longer acenes, opening the way to OFETs and molecular spintronics applications.

Methods

Direct synthesis of 6a/6b. A well-dried Schlenk flask was charged with diene 2 (200 mg, 0.72 mmol) and CsF (480 mg, 3.16 mmol, 4.0 equiv.) under argon and then anhydrous acetonitrile (16 mL) was added. The heterogenous mixture was cooled to 0°C and then solution of aryne precursor bis(trimethylsilyl)-1,4-phenylene bis(trifluoromethanesulfonate) (298 mg, 0.58 mmol, 0.8 equiv.) in anhydrous THF (4 mL) was added dropwise. The reaction was allowed to warm to room temperature overnight. Progress of the reaction was controlled by TLC, eluent (hexane -EtOAc 3:2). Reaction time depending on a scale of the reaction (1-3 days). After the evaporation of the solvent, the residue was chromatographed on silica gel (hexane : acetone 3:1) to get the desired product as a mixture of two regioisomers as a colourless solid. This mixture (146 mg, 0.23 mmol) was dissolved in anhydrous toluene (15 mL) under argon. The solution was cooled to 0°C and then DDQ (53 mg, 0.231 mmol, 2 equiv.) was added in one portion. The reaction mixture was stirred at 0°C for 10 min. and then 6 hours at room temperature. The volume of the reaction mixture was reduced to a half and the mixture was filtered over a frita S4. The solid was washed with toluene and finally with methanol to get the first isomer 6b (40 mg) as a white solid. The mother liquor was evaporated and the residue was purified by chromatography on silica gel (hexane : acetone 3:1) to get the second isomer 6a (82 mg) as a white solid. The ratio of isomers is 1:2 and combined yield is 54 % after two synthetic steps. 6a: [START_REF] Dorel | Strategies for the Synthesis of Higher Acenes[END_REF]

Synthesis of 7a:

In a well dried Schlenk flask, 6a (160 mg, 0.255 mmol) was dissolved in anhydrous dichloromethane (10 mL) under argon. Then trimethylsilyl iodide (109 μL, 0.766 mmol, 3 equiv.) was added dropwise and the homogeneous reaction mixture was stirred overnight at room temperature. Next day the heterogeneous reaction mixture was stirred on air at room temperature for 6 hours to complete the hydrolysis of formed iodo-methoxy intermediate. The product was collected by filtration over a glass frit filter S4, washed with mixture of solvents (hexane : dichloromethane, 4:1) to afford first portion of pure compound 7a as a white solid. The mother liquor was evaporated and the residue was purified by chromatography on silica gel (hexane : acetone 3:1) to get the second portion of the product as a white solid. The combined yield was (130 mg, 95%). [START_REF] Dorel | Strategies for the Synthesis of Higher Acenes[END_REF] H NMR (500 MHz, CD2Cl2): 4.99 (4H, s), 7.41 -7.44 (4H, m), 7.78 -7.81 (4H, m), 7.93 (4H, s), 8.04 (4H, s), 8.36 (2H, s). [START_REF] Mondal | Photogeneration of Heptacene in a Polymer Matrix[END_REF] C NMR (126 MHz, CD2Cl2): 57.55, 120.88, 121. 22, 126.57, 126.76, 128.34, 131.61, 133.31, 137.61, 137.93, 194.46

Fig. 1 .

 1 Fig. 1. Molecular structure of nonacene 1.

Scheme 2 .

 2 Scheme 2. Synthesis of nonacene 1: a) CsF, acetonitrile/THF (4:1), room temperature, 16 h., 62 % (anti : syn 1:2); b) CsF, acetonitrile/THF (5:1), room temperature, 16 h., 98 % (anti : syn 1:2); c) DDQ, toluene, room temperature, 4 h. 94 %; d) TMSI, room temperature, 24 h., 94% for 7a and 48 h., 95% for 7b; e) neat 350 °C, 20 min., quant.

Fig. 2 .

 2 Fig. 2. TGA thermograms of 7a/7bshowing the weight loss of two CO groups (ca 12 and 13%; calcd. 10.5 %).

Fig. 4 :

 4 Fig. 4 : Evolution of the CPMAS 13 C NMR spectra of 7a (left) in function of the temperature.

Fig. 6 .

 6 Fig. 6. Evolution of the CPMAS spectra of 7a/7b by decarbonylation at 350°C for 20 min to form the nonacene 1.

  ppm. CP MAS: 56.99 (bridgehead), 120.20-137.12 (aromatic), 193.26 and 197.73 (C=O) ppm. DCI MS: 478 ([M -2 x CO] + ). HR DCI MS: calcd for C38H22 478.1722 (M -2xCO); found 478.1727.Synthesis of 7b:In a well dried Schlenk flask, 6b (25 mg, 39.89 μmol) was suspended in anhydrous dichloromethane (4 mL) under argon. Then trimethylsilyl iodide (23 μL, 159.6 μmol, 4 equiv.) was added dropwise and the heterogenous reaction mixture was stirred overnight at room temperature. Next day the heterogeneous reaction mixture was stirred on air at room temperature for 6 hours to complete the hydrolysis of formed iodo-methoxy intermediate. The product was collected by filtration over a glass frit filter S4, washed with mixture of solvents (dichloromethane : acetone, 4:1) to afford pure compound 7b(20 mg, 94 %) as a white solid. CP MAS: 55.83 (bridgehead), 119.23-136.01 (aromatic), 195.55 (C=O) ppm. DCI MS: 478 ([M -2 x CO] + ). HR DCI MS: calcd for C38H22 478.1722 (M -2xCO); found 478.1731. Preparation of Nonacene 1 Nonacene was obtained by heating 7a or 7b in the solid state at 350°C under vacuum for 15 min or as a suspension by heating a purged solution of 7a in chlorobenzene 10 min. at 200°C CP MAS: 125.5 -128.8 ppm

Acknowledgements:

The authors thank Yannick Coppel (LCC-Toulouse) for recording CP-MAS spectra, David Neumeyer (CEMES) for recording TGA spectra, Christian Bourgerette (CEMES) and Isabelle Seguy (LAAS) for help in spin-coating experiments. We gratefully acknowledge Nathalie Saffon, from the Institut de Chimie de Toulouse, who solved the X-ray structure. Vladimír Vrkoslav (IOCB, Prague) is gratefully acknowledged for the MALDI experiments. Colin Martin (NAIST-CEMES) is thanked for comments and corrections of the manuscript. A. J. acknowledges funding from the Foundation EXPERIENTIA and from ERDF/ESF "UOCHB MSCA Mobility" (No. CZ.02.2.69/0.0/ 0.0/17 050/0008490). This research has received funding from the EraNET Cofund Initiatives QuantERA under the European Union's Horizon 2020 research and innovation programme grant agreement ORQUID. This project has received financial support from the CNRS through the MITI interdisciplinary programs and JST-ERATO (No. JPMJER1903) and JSPS-WPI.

Author contributions

A.J. and J.H. performed the syntheses and characterization of all products. M.S realized all the MALDI experiments. Y.N. did the simulations and calculations. A.J. and A.G designed and supervised the project.

Competing interests

The authors declare no competing interests.