Baltzer Agnès
email: agnes.baltzer@univ-nantes.fr

Reynaud Marine

Ehrhold Axel

Fournier Jérôme

Cordier Céline

Clouet Hélène

Agnès Baltzer

Space-time evolution of a large field of pockmarks in the Bay of Concarneau (NW Brittany) Évolution spatio-temporelle d'un champ de pockmarks dans la Baie de Concarneau (Nord-Ouest de la Bretagne)

Keywords: geophysics, pockmarks, tide, turbidity, gas, chimneys pipes, Concarneau géophysique, marée, failles, gaz

About a decade ago, a large field of pockmarks, covering an overall area of 36 km(2) was discovered in water depths of < 30m in the central part of the Bay of Concarneau (Southern Brittany, France). This field, composed of features from 5 m to 35 m in diameter and < 1 m in depth, is characterized by unusual high densities of pockmarks, up to 5840 per square kilometre. Geophysical data correlated with sedimentary samples acquired in 2005 and 2009 show that pockmarks and their immediate surroundings are associated with dense tubes benches cover, built by a filter-feeding amphipod: Haploops nirae, forming original benthic communities. Two complementary surveys were carried out in April 2011 (Pock and Ploops) and April 2014 (Pock and Tide), on the Oceanographic Vedette (O/V) Haliotis (Ifremer/Genavir), to map the limit of the pockmarks and Haploops fields with the maximum accuracy. The link between the presence of the Haploops nirae communities and the occurrence of pockmarks / gas was then established and the proposed hypothesis was that tidal cycles may provide a good candidate for a short-term (monthly) triggering mechanism of fluid expulsion (Baltzer A, Ehrhold A, Rigolet C, Souron A, Cordier C, Clouet H. 2014. Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders. Geo-Marine Letters, 34,[215][216][217][218][219][220][221][222][223][224][225][226][227][228][229][230]. Due to the high-level precision (50 cm) of the positioning system (Magellan Aquarius Ixsea Hydrins) coupled with the RTK attitude system, these new bathymetric and imagery maps together with the sub-bottom Chirp profiles, allow us to compare the data sets from April 2011 and April 2014. The superimposition of the two data sets shows that the distribution of the Please note that this is an author-produced PDF of an

Introduction

Pockmarks are crater-like features found on the seabed, which occur in deep marine environments [START_REF] King | -Pockmarks on the Scotian shelf[END_REF][START_REF] Josenhans | -Side scan sonar mosaic of pockmarks on Scotian shelf[END_REF][START_REF] Fader | Gas related sedimentary features from the eastern Canadian continent shelf[END_REF] in lakes, shallow bays, estuaries and fjords [START_REF] Hovland | Seabed pockmarks and seepages: impact on geology, biology, and the marine environment[END_REF][START_REF] Kelley | Giant seabed pockmarks evidence for gas escape from Belfast Bay, Maine[END_REF][START_REF] Kelley | Shallow water pockmarks formation in temperate estuaries: a consideration of origins in the western gulf of Maine with special focus on Belfast Bay[END_REF]). They may be classified according to three criteria: (1) their morphology: circular, elliptical, asymmetrical, composite [START_REF] Hovland | Seabed pockmarks and seepages: impact on geology, biology, and the marine environment[END_REF][START_REF] Judd | Seabed Fluid Flow: The Impact on Geology, Biology and the Marine Environment[END_REF]; (2) their state of development: new, growing, decaying [START_REF] Pickrill | Shallow seismic stratigraphy and pockmarks of a hydrothermally influenced lake, Lake Rototoiti, New Zealand[END_REF]; and (3) their mechanisms of formation: e.g. fault-strike pockmarks, icebergscour pockmarks, current modified pockmarks [START_REF] Pilcher R | -Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF]. They can also be induced by anthropogenic activities such as trawling and ship anchoring [START_REF] Harrington | Formation of pockmarks by pore-water escape[END_REF][START_REF] Fader | Gas related sedimentary features from the eastern Canadian continent shelf[END_REF]. They often form in muddy sediments, large deltas, around small basins and along irregular topographic highs (Hovland and Judd 2009). Deep sources exist for thermogenic hydrocarbons or gas hydrates [START_REF] Scanlon | Pockmarks in the floor of Penobscot Bay, Maine[END_REF][START_REF] Kelley | Shallow water pockmarks formation in temperate estuaries: a consideration of origins in the western gulf of Maine with special focus on Belfast Bay[END_REF][START_REF] Miller | -Natural gas hydrates in the Rio Grande Cone (Brazil): A new province in the western South Atlantic[END_REF], but they may be related to pore water expulsion by compaction [START_REF] Harrington | Formation of pockmarks by pore-water escape[END_REF] or may develop above buried paleo-channels that are rich sources of organic material [START_REF] Lopez | Seafloor facies related to upward methane flux within a Giant Pockmark of the Lower Congo Basin[END_REF]Pilcher et al. 2017;Leon et al. 2009;Leon et al. 2014). However, as active fluid flux is rarely observed in pockmarks, the trigger mechanisms can be difficult to identify and may result from natural disasters (earthquakes, tsunamis, storms) or occur more continuously over longer-term events [START_REF] Kelley | Giant seabed pockmarks evidence for gas escape from Belfast Bay, Maine[END_REF]Hovland et al. 2002). Judd and Hovland (2009) hypothesized that most pockmarks discovered are most of the time inactive, which was confirmed by [START_REF] Brothers | Shallow stratigraphic control on pockmark distribution in north temperate estuaries[END_REF]. This paper presents the results of a study conducted on a field of shallow-water pockmarks (< 30 m of water depth) located in the eastern part of the Bay of Concarneau, Southern Brittany, France [START_REF] Ehrhold | The REBENT monitoring network, a spatially integrated, acoustic approach to surveying nearshore macrobenthic habitats: application to the Bay of Concarneau (South Brittany, France)[END_REF][START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF][START_REF] Reynaud | Une association remarquable entre des crustacés amphipodes (Haploops nirae) et des pockmarks (cratères de dégazage naturels) en Baie de Concarneau[END_REF]). This area is characterised by a high density of pockmarks (an average of 3500 structures/km 2 ) associated with dense Haploops tube benches covering muddy environments. The limits of the pockmark field coincide with the expansion of the Haploops communities [START_REF] Souron | Sédimentologie des vasières subtidales à Haploops spp. sur les fonds à pockmarks de Bretagne Sud[END_REF], revealing specific link(s) between them. [START_REF] Reynaud | Une association remarquable entre des crustacés amphipodes (Haploops nirae) et des pockmarks (cratères de dégazage naturels) en Baie de Concarneau[END_REF] proposed 3 hypotheses for this link, a food link, a constructional link (fine sediments supplied by re-suspension is used for tubes building) or a reproduction/dispersion link (the pockmark activity may keep the larvaes inside a limited area with specific geochemical conditions. Each of these links supposed that pockmarks are active, at least sometimes.

In order to explore the potential activity of this pockmark field, this paper presents the results of two surveys, Pock and Ploops (2011) and Ploops and Tide (2014), conducted at the same season (April), and using the interferometric sidescan sonar (I2S) and chirp tools of the O/V Haliotis, characterised by a very accurate positioning system, coupled to a RTK beacon. Thus it was possible to compare the state and the space-time evolution of this pockmark field between April 2011 and April 2014 using these high-level resolution tools. The analyses of the data sets from 2014 allow to determine the evolution of the acoustic clues of ascending gas, such as blank columns and gas chimneys, and thus will indicate the state of the vertical connections between the sources and the pockmarks (surface gas expression) over a 3 year time-frame. These observations better constrain the gas availability of the system and eventually provide some clues for the preferential pathways used for gas recharge and gas expulsion. The only known fact is that gas is biogenic methane [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF][START_REF] Dubois | -Role of pockmarks in diversity and species assemblages of coastal macrobenthic communities[END_REF]. The questions addressed here are: 1) Is there a continuous activity for these pockmarks? 2) If not, what is the rhythm of expulsion? 3) Where does the gas come from?

1 General setting Situated in Southern Brittany (Fig. 1), the Bay of Concarneau covers an area of approximately 200 km 2 above the 50 m isobath. It corresponds to a tectonic depression with a width of 5-6 km and a length of about 15 km. The bay is bounded by major structural features associated with the Kerforn fault system (N160) (Béchennec et al. 1997) which controls the incised valley of Concarneau [START_REF] Proust | Les vallées fossiles de la baie de Vilaine: nature et évolution du prisme sédimentaire côtier du Pleistocène armoricain[END_REF]. Middle Eocene sandstone and calcareous formation unconformably overlap the crystalline basement rocks. These Eocene series (e 5-6), faulted and folded, are incised by paleo-valleys that preserve sparse marine calcareous infillings of the upper Oligocene [START_REF] Bouysse | -Présence d'Ypresien, niveau transgressif et taux de sédimentation flandriens (Bretagne méridionale) en baie de Vilaine[END_REF]. Two paleovalley systems, oriented N120, parallel to the south Armorican fault system and N160 (Kerforn system) respectively, were deepened from the end of the Eocene to the beginning of the Oligocene [START_REF] Pinot | Le pré-continent breton, entre Penmarc'h, Belle-île et l'escarpement continental[END_REF]. These relatively narrow and sinuous palaeo-valleys (200-500 m in width for lengths of 20 and 18 km) converge to form a large and unique drainage system (4 km wide) in the South of the Pointe de Trévignon (Fig. 2). Fluviatile and estuarine pleistocene deposits infill this drainage system.

An holocene sedimentary layer, characterised by a thickness of 3 to 5 m, covers the entire area [START_REF] Menier | Morphologie et remplissage des vallées fossiles sud-armoricaines: apport de la stratigraphie sismique[END_REF], except for some plateaus like the Maërl Terrasse between Concarneau and Trévignon [START_REF] Ehrhold | The REBENT monitoring network, a spatially integrated, acoustic approach to surveying nearshore macrobenthic habitats: application to the Bay of Concarneau (South Brittany, France)[END_REF].

The hydrodynamic conditions of the study area are characterized by their close dependence on the geomorphological context. The strongest swells are generally associated with westerly gales. To the South of Belle-Ile, waves are of a height ranging from 4 to 6 m (Hs), a mean period of 6 to 8 s -1 , and mainly come from the W-NW [START_REF] Tessier | Caractérisation et dynamique des turbidités en zone côtière: l'exemple de la région marine Bretagne Sud[END_REF]. The Bay of Concarneau is relatively protected from swell dynamics, due to the presence of shallow waters and rocky platforms like the Pourceaux, and the Glénan Islands [START_REF] Pinot | Le pré-continent breton, entre Penmarc'h, Belle-île et l'escarpement continental[END_REF]). The tide is semi-diurnal with a range of 5.5 m for a tidal coefficient of 120. Tidal currents are moderate with a mean value of 0.25 m/s -1 , integrated on the water column, for a tidal coefficient of 90 (unpublished data from P. Bailly du Bois, IRSN). Surface currents can reach a velocity of 15 -30 cm/s -1 whereas under-water velocities range from 0 -5 cm/s -1 for a coefficient of 100 [START_REF] Tessier | Caractérisation et dynamique des turbidités en zone côtière: l'exemple de la région marine Bretagne Sud[END_REF]. Tidal currents describe a clock wise movement, from South to South-West during ebb to North to North-East during floods. Locally, the intensity of the current is lower than 30 cm/s -1 between Concarneau and Trévignon.

Data and methods

2-1 Acoustic measurements acquisition

The data used in this study is the result of two oceanographic sea surveys. The Pock and Ploops survey was conducted in April 2011 aboard the Oceanographiv Vedette O/V Haliotis (Ifremer/Génavir). This vessel is equipped with a hull-mounted interferometric sonar (GeoAcoustics GeoSwath, 250 kHz) which allowed simultaneous acquisition of bathymetric and sonar images. 1250 km of profiles were acquired, with 96 profiles obtained in four non-contiguous areas, area 1, area 2, area 3, area 4 (Fig. 1 & 2). The Pock and Tide survey in April 2014 aboard the O/V Haliotis complemented the 2011 data set with the acquisition of 205 additional profiles (shaded lines on figure. 1); additionally, some profiles acquired in 2011 were exactly replicated in 2014 (shaded lines within the areas 1, 2 , 3 and 4 on figure 1).

2-2 Acoustic measurements processing

The 2011 and 2014 survey sidescan sonar data was processed with GeoTexture (GeoAcoustic) and bathymetry data with CARAIBES software, developed by Ifremer. The bathymetric and sidescan sonar resolutions are respectively 80 and 50 cm. This acoustic data was exported and interpreted in ESRI ArcGIS. Sidescan sonar data gives a picture of acoustic backscatter properties of the seabed which is illustrated by a grayscale image [START_REF] Edy | Sonar mosaic computation from multibeam echosounder[END_REF]. Acoustic backscatter varies depending on the angle of incidence of the acoustic wave front to the seabed, the impedance contrast across the sediment-water interface, surface roughness, and bathymetry and volume reverberation. Grey tones of the sonar image will depend on the nature and the compaction of the sediment, as well as on the topography of the seabed. Light tones generally represent low backscatter, such as relatively finergrained and underconsolidated sediments and dark tones represent high backscatter, such as relatively coarser-grained or compacted sediments, while a high slope gradient will appear darker. Seventeen Shipeck core boxes were sampled during the second survey to identify and confirm the Haploops acoustic facies on sonar images.

The picking of the pockmarks was realised manually, on ArcGIS [START_REF] Reynaud | Une association remarquable entre des crustacés amphipodes (Haploops nirae) et des pockmarks (cratères de dégazage naturels) en Baie de Concarneau[END_REF] providing an accurate map (resolution of 50 cm) of the pockmark distribution. Tidal corrections and centimeter precision positioning of data were made possible thanks to an Aquarius Thalès GPS, an RTK beacon located on the coast and a tide-gauge-buoy.

Results

3-1 Bathymetry maps

For each of the areas (large black frame), of the bathymetry map, 1 focus zone (small white frame) was selected within areas that were surveyed both in 2011 and 2014 (Fig. 1). The global bathymetry map, grouping data from 2011 and 2014 acquisitions, is shown on Figure 2. The exceptional density of the pockmarks (5 840 features/km 2 ) is much higher than the usual descriptions found in the literature (Garcia-Garcia et al. 1999;[START_REF] Ferrin | Shallow gas features in the Galician Rías Baixas (NW Spain)[END_REF]. Presenting a sub-circular to circular shape, in particular aalong the palaeo-valleys, and the diameter of the pockmarks varies from 5 m (exceptionnally less than 1 m-limit of the sonar resolution) to 35 m with only a few decimetres in depth. Pockmark depths are difficult to measure as chirp profiles do not often exactly pass through pockmarks. The comparison of bathymetry sets for the 4 focus zones (Fig. 3) clearly shows that almost all the pockmarks occurred at the same location in 2011 and 2014, without any substantial change between these two dates, except a trend towards more marked features in 2014. This may be explained by the data processing : the same resolution was used for both dates but not the same colour code, which explains the different aspect of the 2011 maps (Fig. 3). On all the focus zones, the global distribution remain similar with some very few changes in pockmarks locations: the black circle or oval covers the same area sample, and reveals a new pockmark distribution On focus 1, trawl-scour pockmarks appear, perfectly visible and aligned, in 2011 and in 2014.

On focus 2, a third pockmark seems to have occurred in the right circle, and two pockmarks appear more marked in the black oval, on the left.

On focus 3, a new pockmark occurred in 2014, underlined by the circle on the right (Fig. 3).

On focus 4, the black circle on the right designates the occurrence of new pockmarks.

Eventually, Focus 5 shows an extract of the bathymetry acquired in 2011 (location on Figure 2), where it is possible to distinguish small pockmarks (around 8 meters), large pockmarks (around 25 meters) and within the large ones, some deeper pockmarks (alpha type), with a net crater shape or shallower pockmarks (beta type) with a smoothy crater shape.

3-2 Sidescan imagery

Sidescan imagery has revealed that the limits of the settlements of Haploops colonies (Fig. 2) coincide with the presence of a field (36 km 2 ) of pockmarks [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF]. This field extends to the South of Trévignon, up to the nort-east of the Glénan Islands (Fig. 2). The main result of the comparison of the sonar images from 2014, confirmed by direct-field observations with scuba-diving, is that the distribution of the Haploops colonies is related to the pockmark field limits all around the studied area. To the south, tubes benches have become discontinuous and have begun to be more and more fragmented, to finally disappear just after the pockmark field boundaries [START_REF] Ehrhold | The REBENT monitoring network, a spatially integrated, acoustic approach to surveying nearshore macrobenthic habitats: application to the Bay of Concarneau (South Brittany, France)[END_REF][START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF] The comparison of the two sonar mosaics reveals that for some pockmarks, located along the east edge of the area 2, there is some thin layer of mud deposit in 2014 which was not there in 2011.

3-3 Seismic profiles

A total of 98 seismic chirp profiles were acquired during the sea surveys realised in 2011 in 4 areas (Fig. 1 & Fig. 2), and 205 profiles were acquired in 2014 to complete the survey. In each area, some profiles acquired in 2011 were exactly replicated in 2014. Figures 4,5, 6 and 7 present one characteristic profile realised for each area. The sedimentary infilling of the Bay of Concarneau has been studied for decades and the interpretation is based on the four seismic units described by [START_REF] Menier | Morphologie et remplissage des vallées fossiles sud-armoricaines: apport de la stratigraphie sismique[END_REF] and used by [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF]. Above the bedrock composed of paleozoic granite and oligocene sandy limestone (Unit 0), three seismic units infill the incised palaeo-valley (Figs. 4,5,6,& 7). Unit 1, not observed in the area because of gas occurrence, is a relict unit, only preserved as patches in the deepest parts of the valley, correlated to braided fluvial sediments aged from middle Pleistocene by Laurent (1993),.. Unit 2 is composed of low amplitude flat parallel reflectors, which onlap the valley sides and locally downlap the base unit, interpreted as inner mud flat facies. Unit 3 occurs above Unit 2 and presents a surface characterised by a smooth sided U-shaped channel. The top of this Unit corresponds to an erosional surface and represents tidal flat deposits with coarser lag deposits within tidal channels. Unit 4 extends over the whole of the valley system, except on the terrace situated between Concarneau and Trévignon, which is covered by rocky shoals and maerl beds (Fig. 2) [START_REF] Ehrhold | Réseau de surveillance benthique (REBENT) -Région Bretagne. Approche sectorielle subtidale: identification et caractérisation des habitats benthiques du secteur de Concarneau[END_REF]). Most of this unit is composed of flat-bedded agrading deposits, characterised by low seismic amplitude reflectors and corresponds to unconsolidated Holocene muds, rich in turritelids [START_REF] Bouysse | -Présence d'Ypresien, niveau transgressif et taux de sédimentation flandriens (Bretagne méridionale) en baie de Vilaine[END_REF][START_REF] Ehrhold | The REBENT monitoring network, a spatially integrated, acoustic approach to surveying nearshore macrobenthic habitats: application to the Bay of Concarneau (South Brittany, France)[END_REF].

The limits of the pockmark field correspond to the lateral extension of seismic Unit 3 [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF]). This unit is cut by different acoustic anomalies, called "acoustic turbidity", "acoustic blanket", "acoustic curtain", "acoustic column" or "pillar shaped" and chimney pipesdepending on their lateral extension, [START_REF] Hovland | Seabed pockmarks and seepages: impact on geology, biology, and the marine environment[END_REF][START_REF] Garcia- | -Quaternary sedimentation and thermal diapirism in the Ria de Pontevedra (Galicia), Northwest Spain[END_REF][START_REF] Baltzer | -Seistec Seismic Profiles: A Tool to Differentiate Gas Signatures[END_REF]Akulichev et al. 2015).

While the pockmark distribution is related to the existence of Unit 3, the size of the pockmarks tends to correspond to the thickness of this unit. The small pockmarks are correlated with a maximum thickness of 3 ms (4,5m) for Unit 3, with a gassy reflector situated around 2 ms (1,5m) below the seabed. The larger pockmarks correspond to a thickness of up to 12 ms (9 m) for Unit 3, with a gassy reflector observed from 4 ms (3,5 m) to 8 ms (6 m). Within this large pockmark group, some of them appear to be aligned along the palaeo-valley edges, related to the sedimentary infilling of these paleovalleys [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF].

Figure 4 presents profile 82 (2011) and profile 72 (2014), revealing 3 branches of the palaeo-river along the Kerforne fault (Fig. 3). Gas facies occur in the two eastern branches and cross through units 2 and 3 up to the surface. The seabed surface corresponds to a serrated (saw-shaped) reflector, due to the presence of Haploops tubes and pockmarks, clearly visible between shots 400 and 1600. It is framed by a continuous sea bottom reflector, with no Haploops benches, visible from shot 0 to 100 and from 1650 to 2100. Profile 72, (the poor quality is due to the weather), shows that gas was still present in 2014, confined exactly to the same places, reaching the surface via the same acoustic columns.

Figure 5 shows profiles 68 (2011) and 32 (2014) which cross the Kerforne paleo-valley and the western branch (Fig. 3). Bench tubes colonize the whole seabed surface (serrated seabed reflector) and some large pockmarks occur above the main palaeo-valley, especially along the valley edges, connected to the gas ascension seen in the acoustic columns or chimney pipes. The blanket acoustic facies correspond to the infilling of the western branch, characterized by smaller pockmarks issued from the interfluve. The same pathways crossing units 2 and 3 were used, with three chimney pipes (red circles) reaching the seabed.

Figure 6 reveals large pockmarks on the surface connected with subjacent acoustic blanket facies. The seabed topography underlines the interfluve between the palaeo-valleys. The top of Unit 2 presents a discontinuous surface, slightly hummocky, with large and irregular depressions. These depressions are connected with the acoustic blanket underneath and resemble to the actual surface pockmarks, but larger in size. Note that the he density of surface pockmarks reflects the proximity of the sub-bottom gas blanket.

Figure 7 shows a clear bulge corresponding to the actual interfluve superimposed on a paleo bulge, together with depressions (similar to large pockmarks?). The occurrence of large pockmarks on the left part of the bulge seems to be related to the presence of a gas "diapir", which is connected to reflectors underneath. Does this reflector represent a preferential conduit for gas circulation, downlapping the bulge? Each time gas is close to the surface, large pockmarks occur.

All the seismic profiles, within each of the different areas, present very similar profiles in 2011 and 2014, with the same visible gas column ascension pathways (chimney pipes), occurring at the same places with the same connections to the surface towards the pockmark features.

Discussion

The first question is whether these pockmarks are still active, just dormant or fossil features? [START_REF] Judd | Seabed Fluid Flow: The Impact on Geology, Biology and the Marine Environment[END_REF] hypothesized that most pockmarks discovered are indeed inactive and Brothers et al.

(2011), ( 2013) have shown that pockmarks may be self-scouring features after initial formation.

In the Concarneau's pockmarks field, bathymetry maps show that most of the pockmarks seem to be empty craters like the pockmark Alpha (Fig. 3 -Focus 5), except some pockmarks with unclear crater limits like the pockmark Beta (Fig. 3 -focus 5). Reflectivity images present white backscattering inside some of these pockmarks, attesting that they are infilled with muddy deposits, whereas most of them appear to be free of infilling [START_REF] Ehrhold | The REBENT monitoring network, a spatially integrated, acoustic approach to surveying nearshore macrobenthic habitats: application to the Bay of Concarneau (South Brittany, France)[END_REF][START_REF] Souron | Sédimentologie des vasières subtidales à Haploops spp. sur les fonds à pockmarks de Bretagne Sud[END_REF]. Video images confirm the presence of fluid mud inside some pockmarks, whereas most of them are empty and surrounded by Haploops colonies [START_REF] Reynaud | Une association remarquable entre des crustacés amphipodes (Haploops nirae) et des pockmarks (cratères de dégazage naturels) en Baie de Concarneau[END_REF]). The occurrence of several "flares" on the chirp profiles (Figs. 4, 5 and 6) together with flares observed in the water column during mid-current flowing tides for spring tides [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF]) indicate the activity of, at least, some pockmarks. Moreover, turbidity was observed by divers during 2011 explorations for biologists works [START_REF] Dubois | -Role of pockmarks in diversity and species assemblages of coastal macrobenthic communities[END_REF] and significant turbidity was observed in the global area in summer 2015 during a SHOM (Service Hydrographique et Océanographique de la Marine) oceanographic cruise (com. pers. from E. Marshes). Two more direct observations tend to confirm the activity of this pockmark field: 1) the persistance of "nude" pockmarks, without any haploops tubes inside, in contrast to the density of the surrounding colonies and 2) the existence of fragmented tubes in some pockmarks, revealing recent gas/fluid expulsion. All these arguments attest the presence of active pockmarks in this field but with a discontinuous activity. Eventually, the presence itself of Haploops settlements confirm this observation.

Thus, the second question addressed here where does the gas come from ? Chirp profiles analyses have highlighted the availability and depth of gas sources [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF]), which will, in turn, control the pockmark distribution. Then, the pockmark distribution on the seabed represents the spatial arrangement of individual upward-seepage drainage-conduits connected to the deeper sediment layers and sources. Gas migration and seepage to the surface are usually related to the existence of two geological features, a gas migration source and a preferential route for gas motion (fewer resistance pathways). The comparison of the data sets of 2011 and 2014 reveals that the global distribution of the pockmarks remains similar, and that only a few new pockmarks occurred in 2014.

The permanence of this pockmark pattern observed in the 2014 bathymetry (Fig. 3) tend to attest the presence of permanent preferential pathways.

Chirp profiles show these preferential pathways, revealed by enhanced reflectors and chimneys pipes (Figs. 4,5,6 and 7). Moreover, the comparison of the chirp profiles, from April 2011 to April 2014, revealed that the gas pathways occurred at the same depth, in the same location, without any or very few vertical or horizontal fluctuations. This observation confirms the stability of the system which supplies, at least for 3 years, gas from palaeo-valley floor sources to the surface.

The distribution of the pockmarks related to the palaeo-valleys is in agreement with several scientific papers [START_REF] Garcia- | -Quaternary sedimentation and thermal diapirism in the Ria de Pontevedra (Galicia), Northwest Spain[END_REF][START_REF] Ussler | Submarine pockmarks: a case study from Belfast May, Maine[END_REF][START_REF] Kelley | Shallow water pockmarks formation in temperate estuaries: a consideration of origins in the western gulf of Maine with special focus on Belfast Bay[END_REF][START_REF] Brothers | Shallow stratigraphic control on pockmark distribution in north temperate estuaries[END_REF]Weschenfelder et al. 2016 ). Enhanced Gas Reflector identified on chirp profiles (Fig. 4, 5, 6 and 7) correspond to gassy sediment levels issued from biogenic gas, methane [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF][START_REF] Dubois | -Role of pockmarks in diversity and species assemblages of coastal macrobenthic communities[END_REF], generated by bacterial activity buried under a few metres in sediment rich in organic matter along the paleo rivers (at least 7% from [START_REF] Floodgate | -The origin of shallow gas[END_REF]; [START_REF] Garcia- | -Quaternary sedimentation and thermal diapirism in the Ria de Pontevedra (Galicia), Northwest Spain[END_REF]. Different authors describe such gassy sediment levels at the Pleistocene/Holocene interface [START_REF] Chronis | Late Quaternary evolution of the Gulf of Patras[END_REF][START_REF] Gontz | Seafloor features and characteristics of the Black Ledges area, Penobscot Bay, Maine, USA[END_REF]Weshenfelder et al. 2016) as being related to the significant organic matter degradation in the infilling transgressive deposits, sealed by thin Holocene muds. The combination of the permeability of this sealing layer and its resistance to stress build-up controls the rate of gas/fluid leakage from the reservoir to the surface. Once the upward pressure decreases, a pockmark will form a circular shape. Cathles et al. (2011) have shown that when a gas chimney reaches halfway from the reservoir to the seabed, liquefaction of overlaying sediments initiate pockmark formation. [START_REF] Kluesner | High density of structurally controlled, shallow to deep water fluid seep indicators imaged offshore Costa Rica[END_REF] shows that gas migrates upwards through conduits (e.g. faults) and intra sedimentary discontinuities, along the valley edges, (e.g. until gas is blocked by a sealing layer). The figure 8 shows on the left-hand side, the pockmark distribution obtained by hand made plotting [START_REF] Reynaud | Une association remarquable entre des crustacés amphipodes (Haploops nirae) et des pockmarks (cratères de dégazage naturels) en Baie de Concarneau[END_REF]. On the right-hand side, the superimposition of the pockmark density map with the geological BRGM map, shows that the limits of the pockmark field is restricted almost exactly to the eocene calcareous outcrop in (e 5-6), a nummulitic limestone (Lutetian to Bartonian age). This formation is strongly faulted, faults belonging to the Kerforne faults system, and underlined by black lines on the Figure 8. The pockmark density appears to be exceptional, with densities up to 5840 features/ km 2 , when faults are combined with the presence of palaeo-valleys junctions. Patches without any (or very few) pockmarks (<290) do not show specific distribution, probably related to the sedimentary cover itself and the different permeability and resistance to stress of the intrasedimentary sealing layer. Thus, the preferential pathways supplying large pockmarks, situated along the palaeovalley edges and within the palaeo-valley infilling itself, were initiated during seismic events which have shaken the faulted Eocene, creating migration pathways into the sedimentary column (U2, U3,U4) for escaping methane. [START_REF] Cathles | -The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration[END_REF] pointed out that once a pockmark forms, depletion of the gas reserve can occur as quickly as within one year. After this period, the pockmark will become dormant and during dormancy, shallow gas reservoirs can recharge. [START_REF] Judd | Seabed Fluid Flow: The Impact on Geology, Biology and the Marine Environment[END_REF] noted that vertical stacking of pockmarks in seismic data points to episodic reoccurrence of discharge.

Then, the last question is related to the dormancy period of the pockmarks of Concarneau and the rhythm of the gas/fluid/ expulsion. [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF] proposed as triggering mechanisms that short term pressure controlled by the tide could be superimposed on longer term temperature controlled processes and episodically on earthquake occurrence.

The results presented in this paper reveal that pockmarks activity is discontinuous but sufficiently frequent to keep chimneys pipes and intra sedimentary pathways permanent (visible) for 3 years into the sedimentary columns. Combining these observations, two models are proposed in Figure 9, illustrating two cases: the "bottom-up" and the "top-down" preferential pathways for gas migration.

The first one illustrates the natural migration of the gas stored in the valley sedimentary infilling, through the units U2, U3, and U4 and the second one shows the anthropogenic induced migration, through the trawl-scours pathways. For both models, the gas (biogenic methane) is issued from the bottom of a paleo-valley, deep enough to allow the maturation of buried organic matter. If the thickness of the sedimentary infilling is not sufficient (palaeo-valley on the left of the block diagram), there is no clue of any gas discharge (no pockmarks).

The "bottom-up" model supposes that the initial pockmark craters were formed during seismic events which have shaken the faulted Eocene, creating preferential pathways in the sedimentary column (U2, U3,U4). The shifting of the gas horizon (EGR), materialised by red arrows on Figure 9, illustrates the shifting of methane solubility due to the spring tidal pressure [START_REF] Diez R | Gas accumulations and their association with particle size distribution patterns in the Ria de Arouza seabed (Galicia, NW Spain) an application of discriminante analyses[END_REF]). This shifting would enable the gas horizon (EGR) to reach the chimneys pipes, possibly at different levels, like at the base of estuarine muds of U3 or at the base of intercalated storm deposits of U4. Thus, each time the tidal pressure is high enough, during spring tides or storms, the tidal pumping effect connects the "bottomup" pathways to the chimney pipes and pockmarks are re-activated: flares may be observed in the water column.

The "top-down" model illustrates the case of a preferential pathway originated from the seafloor and descending to reach the gaz horizon (ENG). In this case, these preferential patways have been created by the trawling action and result in trawl-scour pockmarks, a process similar to the iceberg-scour pockmarks one . As for the first model, the connection is realised and the pockmark is active when spring tide overpressure is sufficient.

These observations give an intermittent pattern of pockmarks activity, which will be a function of the EGR depth, the water depth and the tidal (or storms) overpressure conditions. The proposed hypothesis is that during each spring tide, some permanent pockmarks are re-activated, depending on their water depth and the spring tide coefficent. New pockmarks may be created only during storms or earthquake events, which would explain their small numer.

These observations imply that the quantity of gas supplied in the water column may be substantial and that the gas recharge is continuous. Such observations were made by [START_REF] Hasiotis | -A pockmark field in the Patras Gulf (Greece) and its activation during the 14/7/93 seismic event[END_REF] who described such acoustic turbidity in the Gulf of Patras and suggested that there is a continuous supply of gas towards the pockmarks from below, and that the sediments within the migration path are loaded with gas.

To undertake further study in this area, the objective will be to record gas/fluid expulsion in the water column, and to understand the rhythm of these expulsions. To do so, one long piezometer and a special monitoring tool for gas/fluids and turbidity measurements (from SHOM/IFREMER), will be implanted for longer term observations in this pockmark field.

Conclusion

The pockmarks of the Bay of Concarneau appear to be active, probably due to combined trigger processes occuring at different time scales. Earthquake movement (common in this area) may be relayed or added to short-term controlled tidal pressure (spring tide) processes, thereby triggering gas escape along preferential pathways. These pockmarks are connected to the paleo-valley infillings, along intra-sedimentary and structural discontinuities, continuously supplying gas to the upper sedimentary layer. The expulsion process is activated when connection between the gas horizon (ENG) and preferential pathways is realised. This system appears to be stable, at least for a short time scale (decades) revealed by the constant distribution of pockmarks. An intermittant regular pattern of expulsions is proposed here, which is function of the tidal overpressure, the water depth, and the depth of the ENG. It is suggested that exceptionnal overpressure induced by storms or earthquakes may create new pockmarks. Neverthless, the rhythm of expulsion together with the nature itself of the expulsed gas/fluid need to be explored. Future work will focus on the above water column study with the implantation during at least 6 months of a camera and a piezometer. This next research project will allow the measurement of the volume of gas produced and injected into the water column of this bay.

Special thanks are due to the captains of the Oceanographic Vedette Haliotis Arnaud Gillet and the informatic engineer Renaud Cagna. The first sea survey (2011) was partly funded by the Total Foundation for Biodiversity and the Sea in the framework of an IFREMER/TOTAL project. This article benefitted from rich and constructive feedback from the two reviewers. The sea-bottom reflector appears to be continuous and highly reflective in the absence of Haploops tubes, whereas it presents a serrated shape when Haploops tubes are present.

Figure 9A:

The "bottom-up" model shows the gas, issued from the buried organic matter at the bottom of the buried valley, migrating along preferential pathways in U2. Gas is stopped and stored just under the EGR (Enhanced Gas Reflector) at the top of U2. A second network of pathways may conduct gas through U3 up to the base of U4. Consequently, gas is able to migrate up to the sea floor surface, via tidal pumping processes, creating the large pockmarks. Note the presence of chimney pipes within the unit U4.

Figure 1 :

 1 Figure captions:

Figure 2 :

 2 Figure 2: Bathymetric map of the area compiled from data acquired in 2011 and 2014. Palaeo-valleys, determined from chirp profiles, have been superimposed on the map (dark blue). Red lines show the location of the profiles selected in figures 4, 5, 6 and 7. The white line represents the Haploops area limits: it has been drawn on the side scan imagery, where black grey facies corresponds to Haploops tubes (see photos) and light grey facies corresponds to mud with no Haploops tubes present. The white frames represent the focus zones from 1 to 5.

Figure 3 :

 3 Figure 3: For Focus 1, Focus 2, Focus 3 and Focus 4 zones, (white frames in figure 1), an extraction of bathymetric maps was realised in 2011 and 2014. The comparison of the two bathymetric extractions reveals the almost perfect superimposition of pockmarks from 2014 and 2011. Focus 5 shows a detailed bathymetry map with large and small pockmarks. Large pockmarks (> 20 m) usually occur very clearly on the bathymetry, such as the Alpha pockmark indicated, but may sometimes

Figure 4 :

 4 Figure 4: Area 1: chirp profile 82 was realised in 2011and was replicated in 2014 (profile 72), exactly at the same location (see location on figure 2). Interpretation of Profile 82 reveals the presence of gas intercalated in the different valley branches. Gas pathways issued from the bottom of the palaeovalleys occurred at the same places in 2011 and 2014 (whereas the 2014 data quality is poor, because of the weather).

Figure 5 :

 5 Figure 5: Area 2: chirp profile 68 was realised in 2011 and was replicated in 2014 (profile 32), exactly at the same location (see location on figure 2). Interpretation of Profile 68 reveals the presence of gas within the main valley. Gas issued from the bottom of this paleo-valley followed the same pathways in 2011 and 2014, materialised by chimney pipes (red circles), and creating pockmarks (red circles) on the sea floor. A flare occurs on profile 68, which was not present in 2014.

Figure 6 :

 6 Figure 6: Area 3: chirp profile 41 was realised in 2011 and was replicated in 2014 (profile 178),exactly at the same location (see location on figure2). Interpretation of Profile 41 reveals the presence of gas within the main valley, and the absence of gas within the small buried valley on the left. There are no Haplooops tubes above the small buried valley. Gas issued from the bottom of the paleo-valley followed the same pathways in 2011 and 2014, materialised by chimney pipes (red circles), and creating pockmarks on the sea floor surface.

Figure 7 :

 7 Figure 7: Area 4: chirp profile 30 was realised in 2011 and was replicated in 2014 (profile 122),exactly at the same location (see location on figure2). Interpretation of Profile 30 reveals the presence of gas within the main valley, and the absence of gas within the small buried valley on the left. There are no Haplooops tubes above the small buried valley. Gas issued from the bottom of the paleo-valley followed the same pathways in 2011 and 2014, materialised by chimney pipes (red circles), and creating pockmarks on the sea floor surface.

Figure 8A :

 8A Figure 8A: Pockmark distribution merged from 2011 and 2014 data. All the pockmarks have been manually plotted (resolution of 50 cm).

Figure 8B :

 8B Figure 8B: Superimposition of the geological map (1:250 000 vectorised map from BRGM) with the map of pockmark density. The black lines delimit the Haploops nirae area. The light brown line delimits the eocene outcrop (e5-6), coloured in yellow on the geological map. Faults are underlined by a black line and the paleo-valleys are represented by blue lines.

Figure 9 :

 9 Figure 9: Two sketches illustrating the possible pathways for gas migration are presented in this paper: the upper block diagram shows a "bottom-up" model and the second diagram shows a "topdown" model for gas along preferential pathways.

Figure 9B :

 9B Figure 9B:The "top-down" model illustrates the specific case of the trawl-scour pockmarks. These pockmarks have been triggered by the trawls of fishing boats on the sea floor. The digging action initiates a preferential pathway for gas escape, resembling iceberg-scours.

  Figure 1

Figure 9

 9 Figure 9

Acknowledgements


Damien Tocqueville

Francesco Crisanti

Julian Guerrero

Esther Nubret

Marc Robert

David Milstein

Niklas Von Wolff

Electrification of a Milstein-type Catalyst for Alcohol Reformation

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

Novel energy and atom efficiency processes will be keys to develop the sustainable chemical industry of the future. Electrification could play an important role, by allowing to fine-tune energy input and using the ideal redox agent: the electron. Here we demonstrate that a commercially available Milstein ruthenium cata-lyst (1) can be used to promote the electrochemical oxidation of ethanol to ethyl acetate and acetate, thus demonstrating the four electron oxidation under preparative conditions. Cyclic voltammetry and DFT-calculations are used to devise a possible catalytic cycle based on a thermal chemical step generating the key hydride intermediate. Successful electrification of Milstein-type catalysts opens pathway to use alcohols as renewable feedstock for the generation of esters and other key building blocks in organic chemistry, thus contributing to increase energy efficiency in organic redox chemistry.

In order to achieve the goals of the Sustainable Development Scenario (SDS) of the International Energy Agency, the chemical industry's emission should decline by around 10 % before 2030. 1,[START_REF] Wolff | [END_REF] This could be achieved by increasing energy efficiency and the usage of renewable feedstocks. In this respect, molecular electrocatalytic alcohol oxidation could be powerful tool by potentially providing energy and atom efficiency for organic synthesis and energy applications. 3,4,[START_REF] Wolff | [END_REF][5][6][7] Besides the use of aminoxyl-derivatives, [8][9][10][11][12][13] especially the seminal work of Vizza, Bianchini and Grützmacher demonstrated that (transfer)hydrogenation (TH) catalysts could be activated electrochemically and used in a so-called "organometallic fuel cell". 14 Other TH systems are however mostly limited to two electron oxidations of secondary or benzylic alcohols (Scheme 1A). [15][16][17][18][19][20][21] Scheme 1. A: Advantages/limitation of electrochemical homogeneous alcohol oxidation using well-defined catalysts. B: Current efforts to electrify Acceptor-less Alcohol Dehydrogenation (AAD) systems due to their large range of application in thermal catalysis.

As an exception, Waymouth et al. recently reported an example of the intramolecular coupling of vicinal benzylic alcohols to the corresponding esters. 19,22 In order to extend the range of Please do not adjust margins Please do not adjust margins possible catalysts candidates, the Waymouth group recently also explored the possibility to use an iron-based acceptor-less alcohol dehydrogenation (AAD) catalysts 23 for electrocatalytic alcohol oxidation (Scheme 1B). 24 The stability under electrochemical conditions in this case is limited to <2 turnovers, but it opens the door to explore a wide range of AAD reactions under electrochemical conditions. Here, we demonstrate that a commercially available Milstein-type AAD catalyst (1) 25 is competent for the electrocatalytic alcohol oxidation of ethanol to ethyl acetate and acetate (Scheme 1B). Increasing the base loading gradually from 5 to 20 mM yields a stark increase of current at this new wave at ca. -0.25 V (Figure 3). Using (TBA)PF6 instead of NaPF6 (used to avoid Hofmannelimination 26 ) gave similar results (see SI, section 2.2-2. Noteworthy, rinse-test CPE and a CPE using a simple Ruprecursor, RuCl3, did not show any ethyl acetate formation and gave similar results to blank experiments, indicating that Runanoparticles are probably not the active catalyst species and that catalyst instability could be responsible for low FE. Further studies are underway to fully understand catalyst speciation under preparative conditions (see SI section 4.7) The observed catalytic activity of 1 compares well in terms of TON and product selectivity with other molecular homogeneous TH systems, with most systems being limited to the two-electron oxidation of secondary or benzylic alcohols. The Waymouth group reported a NNC ruthenium pincer for the oxidation of isopropanol to acetone with a TON of 4. 18 The same group reported on the usage of phenoxy mediators with an iridium pincer complex, Please do not adjust margins Please do not adjust margins reaching a TON of 8 for the same reaction. 22 Bonitatibus and coworkers demonstrated the activity of an iridium-based systems with a TON of 32 for the formation of p-benzaldehyde. 17 Appel and co-workers reported on a nickel (TON = 3.1) 15 and a cobalt triphos systems (TON = 19.9) 16 for benzaldehyde formation from benzyl alcohol. To the best of our knowledge, there is only one acceptor-less alcohol dehydrogenation (AAD) catalyst that has been activated electrochemically so-far, 24 generating acetone with a TON < 2. Only a handful of molecular systems are known to catalyze the electrochemical four electron alcohol reformation to esters, however at significantly higher potentials (1.15 V vs Fc + /Fc 0 ). 2,27,28 Thus, although not designed for electrochemical applications, 1 shows high activity for the challenging 4 electron oxidation of aliphatic substrates. Please do not adjust margins

Please do not adjust margins

To achieve the transposition from thermal to electrochemical TH, both Grützmacher et al. and Waymouth took advantage of a fast equilibrium between the alcohol substrate and a metal hydride intermediate that could be readily oxidized. The chemistry of ruthenium pincer AAD systems is well studied (Scheme 2) 25,[29][30][31][32][33] and allows for a putative assignment of the observed CV-behavior. In the presence of excess base and alcohol (Figures 2 and3), 1 is expected to yield dearomatized complex 2, 25 as well as the alkoxide species 3 25,32 . We might therefore assign the first wave at -0.5 V to the oxidation of dearomatized complex 2 and the wave around 0 V to the oxidation of the alkoxide complex 3. Indeed, independently synthesized samples of 2 and 3 (in the presence of excess ethanol) give rise to oxidation half-waves at -0.45 V and -0.1 V respectively (see SI, section 3 and 5.2). This is also in agreement with the observed behavior upon increasing the alcohol concentration with the expected consumption of dearomatized species 2 and concomitant disappearance of the first oxidation wave at -0.5 V. The equilibrium between 2, 3 and 4 has been reported 32 and addition of excess ethanol to 2 is thus not only generating 3, but also is expected to deliver 4 (Scheme 2). The appearance of a new anodic wave at ca. -0.25 V (Figure 2) is thus attributed to the increasing formation of 4 upon addition of larger amounts of EtOH. Complex 4 is relatively unstable in solution, 25,32,33 and decomposes in the presence of electrolyte (see SI section 3.1). DFT calculations were thus used to predict its oxidation potential (see SI, section 6), which was in reasonable agreement with the observed wave (-0.19 V). The DFT calculations also confirmed the assignment of the other waves related to the dearomatized complex 2 (-0.33 V) and the ethoxide species 3 (-0.1 V). A more detailed mechanistic analysis remains currently hampered by the chemical instability of 4 under the employed reaction conditions, as well as difficulties to isolate 3 in the solid state (limiting kinetic measurements). DFT calculations were thus used to get a better view on possible reaction pathways (Scheme 2, 3 and SI section 6.3). The oxidation of 4 at -0.19 V (DFT) yields the radical cation 5, with a calculated pKa in THF of 8.2. In the presence of NaOEt, 5 should thus deprotonate readily to give radical 6, which has an extremely negative oxidation potential of -2.1 V. At the potential it is generated, 6 should thus directly be oxidized to cationic complex 7. This cationic species 7 has a calculated pKa of 22.7 in THF, which is in good agreement with experimental data from the Saouma group on a similar system. 26 The high pKa of 7 in THF also validates the need for a strong base (e.g. NaOEt) to reform dearomatized 2. Both Grützmacher and co-workers, 14 as well as Waymouth 24 have noted that the accelerating effect during electrocatalysis stems from the oxidation of a metal hydride intermediate that is generated by fast chemical steps.

In order to verify this hypothesis and to exclude an electrochemical activation of this hydride formation step, transition state barriers were computed (Scheme 3). Taking the dearomatized complex 2 as a reference point, a first step will form the alkoxide species 3 (TS0 = 21.2 kcal/mol). Oxidizing 2 to 8 slows down the formation of the alkoxide species (TS0 ox = 27.5 kcal/mol), most-likely due to decreased basicity of the ligand. From the alkoxide species 3 dihydride 4 is formed via a linear, charge-separated transition state TS1 (15.7 kcal/mol).

The role of such linear transition states was highlighted recently in the case of ruthenium pincer catalysis for alcohol oxidation. [34][35][36][37] In principle, it might be envisioned that the oxidation of the metal center could be an additional driving force for this hydride abstraction step. However, after oxidation, the energy span 38,39 rises by about 11 kcal/mol (TS1 ox = 24.7 kcal/mol). Likewise, a beta-hydride elimination via sidearm opening is not accelerated either by oxidation (TS2ox = 37.5 kcal/mol, see SI section 6.4). It thus seems that the generation of 4 is not accelerated by electron transfer steps and relies on a thermally activated chemical step. Importantly, alkoxide solutions were shown to be excellent hydride donors electrochemically, further corroborating that under the employed basic conditions, generation of 4 from 3 should be fast. 40 Oxidation of 4 to 5 also doesn't accelerate thermal intramolecular release of H2 (TS3B ox = 37.5 kcal/mol), which is significantly higher than neutral thermal H2-releasing states (TS3A and TS3B). The experimentally observed acceleration via electron-transfer is thus proposed to follow a classical ECEC mechanism initiated by the oxidation of 4 to 5 (at roughly -0.09 V (DFT)), followed by deprotonation and re-oxidation as described above, finally delivering 2 at the electrode surface. Importantly, at the electrode surface 2 and 3 should be oxidized at the employed potentials, but based on DFT-calculations, these pathways are thought to be non-productive (Scheme 3) and could explain the low catalyst life-time and degradation under electrochemical conditions.

Conclusions

In conclusion, a Milstein-type catalyst (1) was activated electrochemically for the dehydrogenation of a primary alcohol to the corresponding ester. CV and DFT studies were used to propose a putative catalytic cycle, based on the chemical generation of dihydride intermediate 4 and its electrochemical conversion. It was shown that 1 can be used under preparative conditions yielding the four-electron oxidation products ethyl acetate and acetate, thus confirming successful electrification. We are currently exploring the electrochemical activation to other AAD systems and substrates, as well as to investigate the mechanism in more details. This will hopefully allow to address adverse deactivation pathways, as well as to design more active catalysts.

Figure 1 .

 1 Figure 1. Scan rate dependence of a 1 mM solution of 1 in in 2:1 THF/DFB + 0.2 M NaPF6 (from light to dark green: 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5 V/s, 3 mm GC electrode). Inset: evolution of the peak current as a function of the square root of the scan rate.

Figure 2 .

 2 Figure 2. CVs of 10 mM NaOEt (grey) and of 5 mM 1 + 5 mM NaOEt with increasing concentrations of EtOH (from light to dark green: 200, 400, 600, 800 and 1000 mM) in 2:1 THF/DFB + 0.2 M NaPF6. Scan rate 0.1 V/s, electrode: 3 mm diameter GC electrode.The cyclic voltammogram (CV) of complex 1 (Figure1) shows a quasi-reversible diffusive one electron oxidation wave at 0.2 V (all potentials are referenced vs Fc + /Fc 0 ) in 0.2 M NaPF6 THF/DFB (2:1) (DFB = 1,2 Difluoro Benzene) assigned to the Ru(II)-Ru(III) couple (see SI, section 2.2). The addition of 1 to a 10 mM sodium ethoxide (NaOEt) solution in 200 mM ethanol (EtOH) in 0.1 M NaPF6 (2:1 THF/DFB) gives rise to several waves at ca. -0.5, 0.0 and 0.2 V with currents significantly higher than in the absence of catalysts or substrate, indicative of possible

Figure 3 .

 3 Figure 3. CV of 5 mM NaOEt (grey), 5 mM of 1 + 1M EtOH with varying concentrations of base (5, 10, 15, and 20 mM NaOEt, light to dark green) in 2:1 THF/DFB + 0.2 M NaPF6. Scan rate 0.1 V/s, electrode: 3 mm diameter GC electrode.

  5 and section 4). In order to assess catalytic turnover under preparative conditions, controlled potential electrolysis (CPE) was performed. CPE experiments were run in pure ethanol (to reduce cell resistance) in the presence of 0.1 M electrolyte of well soluble bases (e.g. NaOEt, LiOH, see SI section 4). CPE in 0.1 M LiOH with 1 mM 1 at E = 0 V vs Fc 0/+ delivered ca. 15 mM of acetate and 6 mM of ethyl acetate, corresponding to 21 turnovers (per 4 electrons, or 42 turnovers per two electrons) and a Faradaic efficiency (FE) of ca. 62 % (see SI section 4.3). In the absence of applied potential (OCP, open circuit potential), no ethyl acetate was formed (see SI, section 4.4). Likewise, in the absence of catalyst, the passed charge was significantly lower (7 C vs 40 C) with no detected formation of ethyl acetate. The low FE could be due to catalyst degradation, as Runanoparticle formation is observed on the electrode post CPE (confirmed by SEM/Elemental mapping, see SI section 5).

Scheme 2 . 7 :

 27 Scheme 2. Reactivity of pyridine-based ruthenium complexes via dearomatization/aromatization, as well as DFT-based

-oxidation Very low potential low TON II) This work: 4e -alcohol reformation N Fe ( i Pr) 2 P P( i Pr) 2 H H CO H cat.

		A Homogeneous Electrocatalytic Alcohol Oxidation
		OH			cat.	O		Cheap redox agent in principle
				-2H + , -2e -				Atom/Energy efficient
		I) HAT-systems			II) TH-catalysts
		Stability/tuneable limited to 2° and benzyl alc. often high potentials 2e -oxidation		Tuneable Lower potentials limited to 2° or activated alc. mostly 2e -oxidation
		e.g. TEMPO, Ru=O based systems	e.g. RuNNC, Ir-MACHO
		B Electrification of AAD-Catalysts
		OH	cat.	NuH	O	⚡		OH	cat.	NuH	O
			-2 H 2	NuH			-4H + ,4e -	NuH
		I) Fe-MACHO
		Tuneable
		O limited to 2°alc./ 2e NuH		OH		cat.	O
		CH 3 CH 2 OH	-2H + , -2e -	NuH:			Nu -2H + , -2e -	NuH
		cat.				EtOH, H 2 O
			N		H	O	=	O	or	O
		Et 2 N	Ru		P(tBu) 2		NuH		O	OH
		Cl	CO 1
		Tuneable	Low potential	Primary alcohol	4e -oxidation
	a. Laboratoire d'Electrochimie Moléculaire	higher TON
	Université Paris Cité, CNRS
	F-75006 Paris, France.
	b. Institut Universitaire de France (IUF),
	F-75005 Paris, France.
	c. Department of Molecular Chemistry and Materials Science, The Weizmann
	Institute of Science
	Rehovot 7610001, Israel.
	Electronic Supplementary Information (ESI) available including experimental
	procedures, CV-characterization of key intermediates, CPE results, DFT-calculated
	redox potentials and BDFEs, See DOI: 10.1039/D2SC04533H
	2 | J. Name., 2012, 00, 1-3				This journal is © The Royal Society of Chemistry 20xx

# These authors contributed equally.

Acknowledgements

NvW is thankful for funding from the IdEx Université de Paris 2019, ANR-18-IDEX-0001. Computations were performed using HPC resources from GENCI-CINES (Grant 2019AP010811227 and AD010812061R1).

Data availability

Author Contributions

Conflicts of interest

There are no conflicts to declare.