Chuxian Li
email: chuxian.li@ensat.fr

Gaël Le Roux

Jeroen E Sonke

Pieter Van Beek

Marc Souhaut

Nathalie Van Der Putten

François De Vleeschouwer

Ga € El Le Roux

Ecolab

Recent 210 Pb, 137 Cs and 241 Am accumulation in an ombrotrophic peatland from Amsterdam Island (Southern Indian Ocean)

Keywords:

come

Introduction

Lead 210 ( 210 Pb, T ½ 22.3 years) dating is the most common method employed to estimate short term (from years to decades) chronologies in peat, estuarine, fluvial, and lacustrine environ ments (Le [START_REF] Le Roux | Constructing recent peat accumulation chronologies using atmospheric fall-out radionuclides[END_REF][START_REF] Robbins | Determination of recent sedimentation rates in Lake Michigan using Pb-210 and Cs-137[END_REF][START_REF] Benoit | 210Pb and 137Cs dating methods in lakes: a retrospective study[END_REF][START_REF] Humphries | 137 Cs and 210 Pb derived sediment accumulation rates and their role in the long-term development of the Mkuze River floodplain, South Africa[END_REF]. 210 Pb originates from the decay of gaseous 222 Rn, which escapes from the Earth's continental crust to the atmosphere [START_REF] Graustein | Radon fluxes from soils to the atmosphere measured by 210Pb 226Ra disequilibrium in soils[END_REF]. 210 Pb adsorbs strongly to the surface of aerosols in the 0.1e0.5 mm diameter size range as soon as it is produced in the air [START_REF] Knuth | Size distribution of atmospheric Pb and 210 Pb in rural New Jersey: implications for wet and dry deposition[END_REF]. 210 Pb bearing aerosols are distributed globally by general atmospheric circulation and can be deposited on the Earth's surface mainly by precipitation, but also by dry fallout as well as convective updrafts [START_REF] Knuth | Size distribution of atmospheric Pb and 210 Pb in rural New Jersey: implications for wet and dry deposition[END_REF][START_REF] Baskaran | Po-210 and Pb-210 as atmospheric tracers and global atmospheric Pb-210 fallout: a review[END_REF]. The 210 Pb deposited from the atmosphere is called "unsupported 210 Pb" or "excess 210 Pb" (denoted 210 Pb ex ), which should be distinguished from the 210 Pb produced inside the matrix (e.g. lake sediment) and which is, named "supported 210 Pb" [START_REF] Guevara | 210Pb fluxes in sediment layers sampled from Northern Patagonia lakes[END_REF].

The Constant Rate of Supply (CRS) model based on 210 Pb ex flux, which could be validated by nuclear fallout studies (e.g. 137 Cs, 241 Am), is by now widely used (e.g. [START_REF] Appleby | Radiometrically determined dates and sedimentation rates for recent sediments in nine North African wetland lakes (the CASSARINA Project)[END_REF][START_REF] Appleby | Three decades of dating recent sediments by fallout radionuclides: a review[END_REF].

Another radionuclide that is widely used to derive ages is 137 Cs. With a half life of 30.2 years, 137 Cs is considered as one of the important radionuclides among those from nuclear emissions (e.g. atmospheric nuclear weapon tests in the 1950s 1970s with the peak in 1963 in the Northern Hemisphere and the fallout from the Chernobyl accident in 1986), with respect to being a persistent tracer and an indicator of single event chronology [START_REF] Aoyama | Re-construction and updating our understanding on the global weapons tests 137 Cs fallout[END_REF][START_REF] Rodway-Dyer | The use of 137 Cs to establish longer-term soil erosion rates on footpaths in the UK[END_REF]. In contrast, 241 Am another artificial radionuclide is strictly related to nuclear bomb testing in remote areas.

Up to now, most studies about the inventory of sediment ra dionuclides and radiochronology have been conducted in the Northern Hemisphere. Limited work has been carried out in the Southern Hemisphere, especially in the Indian Ocean. The scarcity of studies conducted in the Southern Hemisphere is partly due to the lower fallout of 210 Pb and 137 Cs, fewer continental surfaces and fewer man made radionuclide emissions and fallout, which generally result in lower activities bordering on analytical detection limits [START_REF] Owens | Spatial variability of caesium-137 inventories at reference sites: an example from two contrasting sites in England and Zimbabwe[END_REF][START_REF] Bonotto | 210Pb-derived chronology in sediment cores evidencing the anthropogenic occupation history at Corumbataí River basin, Brazil[END_REF]. In the Southern Hemisphere nuclear weapons fallout is about three times lower than that in the Northern Hemisphere. Consequently, the 137 Cs fallout peak is usually more difficult to identify in cores due to the relatively high measurement uncertainties associated with the low 137 Cs concentrations [START_REF] Hancock | Plutonium as a chronomarker in Australian and New Zealand sediments: a comparison with 137 Cs[END_REF]. Southern Hemisphere investigations on 210 Pb and 137 Cs have been confined primarily to large land masses, such as South America [START_REF] Sanders | Mercury flux to estuarine sediments, derived from Pb-210 and Cs-137 geochronologies (Guaratuba Bay, Brazil)[END_REF][START_REF] Guevara | 210Pb fluxes in sediment layers sampled from Northern Patagonia lakes[END_REF][START_REF] Cisternas | Effects of historical land use on sediment yield from a lacustrine watershed in central Chile[END_REF], Australia and New Zealand [START_REF] Pfitzner | 137 Cs and excess 210 Pb deposition patterns in estuarine and marine sediment in the central region of the Great Barrier Reef Lagoon, north-eastern Australia[END_REF][START_REF] Hancock | Plutonium as a chronomarker in Australian and New Zealand sediments: a comparison with 137 Cs[END_REF] and South Africa and Madagascar [START_REF] Humphries | 137 Cs and 210 Pb derived sediment accumulation rates and their role in the long-term development of the Mkuze River floodplain, South Africa[END_REF][START_REF] Kading | Mercury contamination history of an estuarine floodplain reconstructed from a 210 Pb-dated sediment core (Berg River, South Africa)[END_REF]Iva novitch and Harmon, 1992;[START_REF] Rabesiranana | Assessment of soil redistribution rates by 137 Cs and 210 Pb ex in a typical Malagasy agricultural field[END_REF].

No studies on terrestrial sediment radionuclides and radio chronology exist for the Southern Indian Ocean, although the area is an important part of the global atmospheric and oceanic circu lation patterns. Amsterdam Island (AMS, 37 S) is located just north of the Subtropical Front (at approximately 40 S, [START_REF] Orsi | On the meridional extent and fronts of the Antarctic Circumpolar Current[END_REF], where cool, low salinity subpolar water submerges beneath warm, saline subtropical water [START_REF] Prell | The subtropical convergence and late Quaternary circulation in the southern Indian Ocean[END_REF]. The island is located at the northern edge of both the Southern Westerly wind belt and the Antarctic Circumpolar Current.

The main objectives of this study were to investigate (1) to what extent observations in the Southern Indian Ocean could define Southern Hemisphere mid latitude 210 Pb, 137 Cs and 241 Am back ground conditions for the last 100 years and (2) how 210 Pb levels are comparable between different matrices (e.g. wetland, sediment, glacier and atmospheric deposition) at different latitudes of the world, which could allow us to draw a new global sketch of 210 Pb flux.

In this paper, for the very first time the inventories and fluxes of 210 Pb and 137 Cs together with a 210 Pb based peat accumulation rate for AMS are reported. This island is located in Southern Indian Ocean at 37 S and at 3400 km from the nearest land mass. The atmospheric conditions at this location offer the possibility to potentially define 210 Pb background concentrations, in a place with minimal perturbation from anthropogenic influences [START_REF] Gaudichet | Mineralogical composition of aerosols at Amsterdam island[END_REF][START_REF] Angot | A 2-year record of atmospheric mercury species at a background Southern Hemisphere station on Amsterdam Island[END_REF]. AMS is therefore an ideal site to investigate the background levels of 210 Pb, 137 Cs and 241 Am in the Southern Hemisphere, as well as to detect long range transport of anthropogenic radionuclides.

Materials and methods

Study area

AMS (37 50 0 S, 77 32 0 E) is a small volcanic island with a surface of 55 km 2 and a maximum elevation of 881 m above sea level (a.s.l.). The center of the island is formed by a volcanic caldera in which an ombrotrophic peatland develops. The island is located at the northern edge of the westerly wind belt in the South Indian Ocean at a minimum distance of 3400 km and 5000 km upwind from the nearest land masses, respectively, Madagascar and South Africa. The climate in AMS is mild oceanic, with frequent clouds [START_REF] Angot | A 2-year record of atmospheric mercury species at a background Southern Hemisphere station on Amsterdam Island[END_REF]. The annual precipitation is 1124 mm y À1 based on 40 year annual average data from 1951 to 1990 (Meteo France data reported in [START_REF] Miller | A 10-year trajectory flow climatology for Amsterdam island, 1980[END_REF]. The weather station is located at 29 m above sea level. However, although no record exists, the cloud accumulation at the top of the island makes the precipitation level much higher in the caldera than that recorded at the weather station. The orographic effect [START_REF] Roe | Orographic precipitation[END_REF][START_REF] Roux | Aerosol deposition and origin in French mountains estimated with soil inventories of 210 Pb and artificial radionuclides[END_REF] would enhance precipitation and therefore, 210 Pb bearing aerosols scavenging. Relatively higher volume of rainfall above 600 m a.s.l. also favors moss growth and peat accumulation in the caldera of AMS. The annual temperature is 13.8 C at the weather station, while the average annual humidity is 80% with little seasonal var iations. Located in the middle of the Indian Ocean, AMS is also at the crossroads of African, Australian and Southern American dust tra jectories [START_REF] Li | Distribution, transport, and deposition of mineral dust in the Southern Ocean and Antarctica: contribution of major sources[END_REF][START_REF] Lamy | Increased dust deposition in the Pacific Southern Ocean during glacial periods[END_REF].

Core sampling and sub sampling

One 5 m long peat sequence (AMS14 PB01A, 37 50.742 0 S, 77 32.898 0 E) was collected from the center of a raised bog at 738 m a.s.l. in December 2014 using a stainless steel Russian D corer of 10 cm internal diameter and 50 cm length [START_REF] Belokopytov | Giktorf's peat borers[END_REF][START_REF] De | Emissions from pre-Hispanic metallurgy in the South American atmosphere[END_REF][START_REF] Vanneste | Elevated dust deposition in Tierra del Fuego (Chile) resulting from Neoglacial Darwin Cordillera glacier fluctuations[END_REF] (Fig. 1). A second (AMS14 PB01B, 4.15 m length) and a third core (AMS14 PB01C, top 1 m length) at the same site were collected and stored as archives. All cores were photographed, described and wrapped in plastic film and PVC tubes before being shipped by boat to France in þ4 C fridges. Cores were frozen and subsequently sliced at roughly 1 cm resolution using a clean sub sampling pro cedure described in [START_REF] De | Coring and sub-sampling of peatlands for palaeoenvironmental research[END_REF]. After being cleaned with MilliQ water and the edges removed [START_REF] Givelet | Suggested protocol for collecting, handling and preparing peat cores and peat samples for physical, chemical, mineralogical and isotopic analyses[END_REF], the subsamples were dried using an ALPHA 1e4 LD plus freeze dryer. Prior to freeze drying, the dimensions of each sub sample were measured using a vernier caliper in order to i) obtain the volume for calculating the dry bulk density and ii) to estimate the cut loss between each slice by comparing the cumulative slices thickness and the total length of the core. In this paper, we focus on last 100e150 years of peat accumulation representing the top 12 cm of the master core.

Radiometric measurements

Fifteen freeze dried sample aliquots (approximately 0.3 g) were analyzed at the LAFARA underground laboratory located in Ferri eres in the French Pyr en ees [START_REF] Van Beek | LAFARA: a new underground laboratory in the French Pyrenees for ultra low-level gamma-ray spectrometry[END_REF]. Prior to analysis, samples were sealed to prevent any 222 Rn loss and stored for a period of 3 weeks to ensure radioactive equilibrium between 226 Ra, 214 Pb and 214 Bi. The 210 Pb, 226 Ra, 137 Cs and 241 Am activities were determined using a well type germanium detector that was protected from cosmic rays by 85 m of rock, thus yielding a very low background [START_REF] Van Beek | LAFARA: a new underground laboratory in the French Pyrenees for ultra low-level gamma-ray spectrometry[END_REF]. The 210 Pb, 137 Cs and 241 Am were measured using the gamma lines at 46.5 keV, 661.7 keV and 59.5 keV, respectively. The 226 Ra was determined using the 295 keV, 351.9 keV and 609.3 keV gamma emissions of its decay chain descendants ( 214 Pb and 214 Bi). Because of the very low ac tivities present in the peat samples and because the volume of material to be analyzed was small, each sample was analyzed for at least four days. We used RGU1, RGTH1 and IAEA 375 standards provided by IAEA to calibrate the detector.

Excess 210 Pb activities are calculated by correcting the total 210 Pb for the 210 Pb supported by 226 Ra. Since the 226 Ra activities were below the detection limit in the core, the 210 Pb ex activities are equivalent to the total 210 Pb activities (Supplementary Table S1). In the following, we report 210 Pb ex activities. The detection limits achieved in this study (considering the low amount of material that was analyzed) were 20 Bq.kg À1 for 210 Pb, 0.4 Bq.kg À1 for 137 Cs and 0.6 Bq.kg À1 for 241 Am.

Calculating decays, fluxes and inventories

Atmospheric 210 Pb fluxes F (Bq$m À2 $yr À1 ) were calculated using:

F l I 210Pb (1)
where l is the 210 Pb day constant (0.03114 yr À1 , l ln2/T 1/2 ) and I is the 210 Pb ex inventory (Bq$m À2 ) in peat calculated from [START_REF] Appleby | In Dating recent sediments by 210Pb: problems and solutions[END_REF]:

I X i ∞ i 0 rðxÞ,CðxÞ,dx (2)
where rðxÞ (g$cm À3 ) is the dry bulk density, dx is the soil thickness (cm) and CðxÞ is the excess 210 Pb activity (Bq$kg À1 ). [START_REF] Sanchez-Cabeza | Regional calibration of erosion radiotracers (210Pb and 137Cs): atmospheric fluxes to soils (northern Spain)[END_REF] The 137 Cs inventory was also calculated using equation (2).

The formula for calculating 137 Cs cumulative decay corrected fallout at year 1969 (maximum fallout, according to [START_REF] Aoyama | Re-construction and updating our understanding on the global weapons tests 137 Cs fallout[END_REF]

Results and discussion

3.1. Downcore distribution of 210 Pb, 137 Cs and 241 Am activities

Given that ombrotrophic peatlands are only depending on precipitation for their water balance, 210 Pb flux is assumed to be exclusively of atmospheric origin at AMS. This is confirmed by the absence of supported 210 Pb because no 226 Ra was detected. Except for the first sample that contained living Sphagnum moss, the 210 Pb ex activities decrease with increasing depth down to approx imately 12 cm. (Fig. 2 and Supplementary Table S1). As shown in Fig. 2, the activities of 137 Cs and 241 Am displayed peaks at the same depth (4.4e5.8 cm), with values of 22 ± 2 Bq$kg À1 and 7 ± 1 Bq$kg À1 , respectively, indicating that these peaks can be related to nuclear bomb testing [START_REF] Spalding | Forensics: age written in teeth by nuclear tests[END_REF].

The total 210 Pb ex inventory was 3160 ± 200 Bq$m À2 with a corresponding 210 Pb ex flux of 98 ± 6 Bq$m À2 $y À1 . According to equation (3), the corrected value for 137 Cs inventory on AMS in 1969 was 144 ± 13 Bq$m À2 , which is two times lower than the modeled average inventory of 580 Bq$m À2 (range from 150 to 1430 Bq$m À2 ) at 35 S [START_REF] Aoyama | Re-construction and updating our understanding on the global weapons tests 137 Cs fallout[END_REF], but higher than the activity reported at 45 S in the same paper. Our observed 137 Cs inventory for AMS is thus close to the lower limit of these estimates at 35 S. 241 Am ac tivity is relatively high showing potentially the immobility of this radioelement in the peat column compared to cesium. Despite this, the activity inventory ratio 137 Cs/ 241 Pu 1969 1.2, derived from 137 Cs and 241 Am activities respectively, is on the same order of magnitude than what can be found in Antarctica and sub Antarctica islands [START_REF] Pourchet | Radionuclides deposition over Antarctica[END_REF][START_REF] Roos | Deposition of 210Pb, 137Cs, 239þ 240Pu, 238Pu, and 241Am in the antarctic peninsula area[END_REF].

210 Pb chronology

Well dated peat profiles are valuable archives of past environ mental changes. The Constant Rate of Supply (CRS) model (Appleby, 1997; [START_REF] Binford | Calculation and uncertainty analysis of 210Pb dates for PIRLA project lake sediment cores[END_REF] was applied to 210 Pb inventories calculated from the 210 Pb ex data to generate ages. The peat core presented here spans a period of about 157 years, indicating a peat accumulation rate of about 0.75 mm yr À1 . [START_REF] Oldfield | Radiometric dating (210Pb, 137Cs, 241Am) of recent ombrotrophic peat accumulation and evidence for changes in mass balance[END_REF] suggested that 210 Pb measurements alone cannot result in an accurate chronology of peat accumulation. However, when constrained by 241 Am and 137 Cs activity profiles in the upper part of a sequence, 210 Pb can provide good chronologies of peat accumulation. The 210 Pb ex dates from the peat profile in AMS calculated using the CRS model were independently validated by 137 Cs and 241 Am that displayed highest activities between 1960 and 1981 (Fig. 3), corresponding to the period of nuclear weapon tests (i.e. in the 1960s). Moreover the detection of 241 Am excludes the possibility that 137 Cs post depositional mobility processes would have shifted the 137 Cs activity maximum (Schettler et al., 2006 Part B).

210 Pb flux in AMS compared with global 210 Pb depositional flux

We updated the database from [START_REF] Turekian | Geochemistry of atmospheric radon and radon products[END_REF] and [START_REF] Preiss | Etude du 210 Pb d'origine atmosph erique dans l'air, la neige, les sols et les s ediments: Mesures, inventaires et interpr etation a l' echelle globale[END_REF] by compiling 47 new entries from the last 20 years. Global estimates of 210 Pb flux from the literature, integrated over 30 latitudinal belts within different matrices, are shown in Fig. 4 and Supplementary Table S2. Only terrestrial data, i.e. from wetlands (peatlands, salt marshes and swamps), sediments (lakes, estuarine and soil profiles), glaciers (ice core and firn) as well as atmospheric deposition (snow, precipitation plus dry fallout) are included.

Globally, the 210 Pb flux measured in wetland sequences show smaller uncertainties compared to fluxes measured from atmo spheric deposition and sediment sequences (Fig. 4). The dominant 210 Pb input to wetlands is through the atmosphere in contrast to lake basins and the data are integrated over several years (1 cm layer represents a period of more than 10 years). The relatively larger uncertainty observed in atmospheric deposition could be explained by short term measurements (i.e. annual) that can enhance different tropospheric contributions. Many factors (e.g., adjoining drainage areas) could alter the 210 Pb flux in sediment sequences.

In general, the 210 Pb flux is lower at high latitudes (>60 lati tude) (Fig. 4), especially when measured in glaciers and atmo spheric deposition, where the low density of land masses results in lower 222 Rn emissions. According to the existing data, the 210 Pb fluxes measured in sediments are higher than those measured in the other three categories in the Northern Hemisphere, while in the Southern Hemisphere the values detected in wetlands are the highest (except between 60 and 90 S where no measurement ex ists). The 210 Pb fluxes from atmospheric deposition are similar to those from wetlands in the Northern Hemisphere (Fig. 4). Wetlands are found to be the appropriate archives for a good estimation of atmospheric flux, due to the absence of in wash of sediments, and the low energy budget (absence of streams) [START_REF] Preiss | A compilation of data on lead 210 concentration in surface air and fluxes at the air-surface and water-sediment interfaces[END_REF]. However in the Southern Hemisphere, 210 Pb flux from wetlands and atmospheric deposition are not similar, which might be partially due to the scarcity of data (only 7 for wetlands) resulting in a non representative dataset. Compared to the Northern Hemi sphere, fewer data are available for the Southern Hemisphere, and thus more effort should be made in the future to fill this gap.

In the peat core from AMS, the 210 Pb flux was 98 ± 6 Bq$m À2 $y À1 , which is higher than most values reported at around 40 S. For example, [START_REF] Baskaran | Po-210 and Pb-210 as atmospheric tracers and global atmospheric Pb-210 fallout: a review[END_REF] S2).

deposition in Tasmania is 41.5 Bq$m À2 $y À1 (42.5 S, 147.5 E) [START_REF] Preiss | Etude du 210 Pb d'origine atmosph erique dans l'air, la neige, les sols et les s ediments: Mesures, inventaires et interpr etation a l' echelle globale[END_REF]. [START_REF] Rosen | Note on some observations of radon and thoron exhalation from the ground[END_REF] found a much lower value (1.24 Bq$m À2 $y À1 ) in Wellington (41 17 0 S), New Zealand based on the calculation of the 222 Rn flux. The estimated unsupported 210 Pb fluxes values from the global model made by [START_REF] Turekian | Geochemistry of atmospheric radon and radon products[END_REF][START_REF] El-Daoushy | A summary on the lead-210 cycle in nature and related applications in Scandinavia[END_REF] were 74 (in terrestrial settings) and 58.4 Bq$m À2 $y À1 , respec tively. Unsupported 210 Pb flux from lake sediments in Northern Patagonia (from 40 30 0 S to 41 10 0 S) showed very low values, ranging between 4 and 48 Bq$m À2 $y À1 [START_REF] Guevara | 210Pb fluxes in sediment layers sampled from Northern Patagonia lakes[END_REF]. The average 210 Pb flux from lake sediment in central Chile (36 51 0 S, 73 05 0 W) was 23.6 Bq$m À2 $y À1 [START_REF] Cisternas | Effects of historical land use on sediment yield from a lacustrine watershed in central Chile[END_REF]. However, when compared with a soil profile from Madagascar, we find almost the same value (95.8 Bq$m À2 $y À1 , [START_REF] Rabesiranana | Assessment of soil redistribution rates by 137 Cs and 210 Pb ex in a typical Malagasy agricultural field[END_REF] as in AMS. Our value is also quite similar to other values obtained from swamps or salt marshes found in South Africa (139 ± 37 Bq$m À2 $y À1 , n 4, [START_REF] Humphries | 137 Cs and 210 Pb derived sediment accumulation rates and their role in the long-term development of the Mkuze River floodplain, South Africa[END_REF][START_REF] Kading | Mercury contamination history of an estuarine floodplain reconstructed from a 210 Pb-dated sediment core (Berg River, South Africa)[END_REF][START_REF] Ivanovich | Uranium-series Disequilibrium: Applications to Earth[END_REF]. The 210 Pb flux in AMS is lower than some measurements conducted in wetlands between 30 and 60 N, e.g., Romania (between 133 and 277 Bq$m À2 $y À1 , [START_REF] Begy | Atmospheric flux, transport and mass balance of 210 Pb and 137 Cs radiotracers in different regions of Romania[END_REF], China (254 and 421 Bq$m À2 $y À1 , [START_REF] Bao | Recent atmospheric lead deposition recorded in an ombrotrophic peat bog of Great Hinggan Mountains, Northeast China, from 210 Pb and 137 Cs dating[END_REF].

Depositional flux of 210 Pb at any given site depends on the local 222 Rn emanation rates and the relative proportion of maritime and continental air masses along with the differences in the amount and frequency of precipitation [START_REF] Baskaran | Po-210 and Pb-210 as atmospheric tracers and global atmospheric Pb-210 fallout: a review[END_REF]. The highly variable 210 Pb depositional fluxes give insight into the sources and sinks of aerosols. The study sites in Argentina and Chile [START_REF] Cisternas | Effects of historical land use on sediment yield from a lacustrine watershed in central Chile[END_REF][START_REF] Guevara | 210Pb fluxes in sediment layers sampled from Northern Patagonia lakes[END_REF], showing much lower 210 Pb fluxes, are located at the coast near the Pacific Ocean, and are influenced by westerly winds bringing oceanic air masses. The contribution of sea salt for 210 Pb from oceanic 222 Rn is negligible. Global 222 Rn flux from continents is estimated to be around 1300e1800 Bq$m À2 $d À1 , which is around 2 orders of magnitude higher than that from the oceans [START_REF] Samuelsson | 222Rn and 210Pb in the Arctic summer air[END_REF][START_REF] Nazaroff | Radon transport from soil to air[END_REF]. The 210 Pb flux at AMS is relatively high (discussed above). Despite the remoteness of the island, [START_REF] Angot | A 2-year record of atmospheric mercury species at a background Southern Hemisphere station on Amsterdam Island[END_REF] found that air masses passing over the southern African continent and/or Madagascar could enhance 222 Rn activity in AMS based on the study of back trajectory. Con tinental air masses originating from South Africa and/or Madagascar are enriched in 210 Pb that is then scavenged to clouds over the Indian Ocean. The regular orographic heavy rainfalls in the caldera at the top of AMS promote the deposition of 210 Pb from the South Africa and/or Madagascar air masses into the peatland, resulting in a 210 Pb flux on the island similar to those in South Africa and Madagascar.

Conclusion

For the very first time, radionuclide ( 210 Pb, 137 Cs, 241 Am) mea surements conducted in a peat core taken from an ombrotrophic peatland on AMS are presented. The chronology based on 210 Pb using a CRS model is consistent with other chronomarkers ( 137 Cs and 241 Am), which allows the reconstruction of a peat mass accu mulation rate of 0.75 mm yr À1 for a period of 157 years. The 210 Pb flux of 98 ± 6 Bq$m À2 $y À1 measured in the peat core is relatively high compared to many observations at around 40 S. This pattern might be explained by the fact that, most studies conducted in the mid latitudes in Southern Hemisphere are located on the west side of the continents, close to the coast, and are highly influenced by westerly winds bringing air masses with low 222 Rn activities. However, the 210 Pb flux at AMS is comparable to the ones in South Africa and Madagascar. This may be explained by the continental air mass enriched in 222 Rn originating from South Africa and/or Madagascar influencing AMS together with the frequent and heavy rainfalls at the top of the island which would enhance deposition. The 137 Cs inventory was 144 ± 13 Bq$m À2 (corrected to 1969). Since no terrestrial studies were conducted in the south Indian Ocean, the data reported here from AMS ( 210 Pb, 137 Cs and 241 Am) are of value for the global database.

Fig. 1 .

 1 Fig. 1. Sampling site in Amsterdam Island.

  Fig.2. Vertical profiles of 210 Pb ex ,137 Cs and 241 Am activities in the peat core collected in Amsterdam Island.

Fig. 3 .

 3 Fig. 3. 210 Pb ex inferred chronology based on CRS model.

  is shown below,

	CD 1969	1969 X	D t e Àlð1969ÀtÞ	(3)
		t

CD 1969 : cumulative decay corrected fallout at year 1969 (Bq$m À2 ); l: radioactive decay constant of 137 Cs (0.023 yr À1 , l ln2/T 1/2 ); D t : annual deposition of 137 Cs at year t (Bq$m À2 ).

Acknowledgements

We are grateful to Svante Bjorck, Bart Klink and Elisabeth Michel for their help during fieldwork. These results would never have been obtained if we had not had the incredible support of the Mission 66 of Amsterdam Island. Very special thanks to Alain Quivoron and Hubert Launay. The field expedition of this project was supported by IPEV project 1066 PARAD to FDV. We are very grateful to Nina Marchand (IPEV) for the incredible logistical sup port and C edric Marteau for making the sampling possible in the protected areas of the TAAF Nature Reserve. Chuxian Li's PhD is supported by a scholarship from the Chinese Scholarship Council (No. 201506990002). We are grateful to EDF (Electricit e De France) for allowing us to run our germanium detectors in the tunnel of Ferri eres. We thank the European Union and R egion Occitanie Pyr en ees M editerran ee for supporting the LAFARA underground laboratory through a FEDER funding (SELECT project). We finally thank the 4 anonymous reviewers for their useful comments that greatly improved the quality of this manuscript.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvrad.2017.05.004.


Anaële Lefeuvre

Xavier Yerro

Alan Jean-Marie

Phuong Anh

Vo Dong

Catherine Azzaro-Pantel
email: catherine.azzaropantel@ensiacet.fr

Modelling pyrolysis process for CFRP recycling in a closed-loop supply chain approach

Keywords: Pyrolysis, Recycling, Carbon Fibre Reinforced Polymers, Life Cycle Assessment

Several pathways have been investigated to recycle Carbon Fibre Reinforced Polymers (CFRP), that have been increasingly used in a wide range of applications such as automotive, aerospace and renewable energy. Pyrolysis is currently considered to be the only process that is available on an industrial scale. Yet to our knowledge, no study based on process systems engineering concepts has been performed in order to evaluate the environmental gain that can be obtained from the production of the very energyintensive production of virgin carbon fibres. In this work, a pyrolysis recycling process of a capacity of 1500 tonnes/year has been modelled by using Aspen Hysys v8.6 simulation software to evaluate the avoided environmental impacts using a Life Cycle Assessment approach, and in particular, those related to Global Warming Potential (expressed in kg CO 2 eq.). A functional unit based on the recycling of 1 kg of carbon fibre reinforced polymers has been considered. An economic evaluation has also been carried. The objective is to use these results in order to build the process echelon of the CFRP recycling supply chain based on reliable data.

Introduction

In the last 30 years, Carbon Fibre Reinforced Polymers (CFRP) have been increasingly used in a wide range of applications (Pimenta and Pinho, 2011) such as automotive, aerospace and renewable energy due to their high mechanical properties, stiffness to weight ratio and damage tolerance. In that context, CFRP disposing will become a big issue as today's low-weight CFRP structures reach the end of their service lives. Under upcoming economic, environmental and regulatory pressure, the development of viable CFRP waste recycling pathways is thus a strategic issue. Among CFRP recycling processes, pyrolysis appears as a rather mature process available on an industrial scale due to several advantages such as a high retention of mechanical potential, a recovery potential of chemical feedstock from the resin, and no use of chemical solvents. Despite these advantages, the pyrolysis process has only been developed from an empirical approach and a global modelling of the process based on process flowsheeting concepts has not been reported in the dedicated literature. The lack or imprecise nature of economic and environmental information about CFRP recycling processes at industrial scale may thus hamper the development of reliable recycling strategies for the deployment of CFRP recycling supply chains that are particularly relevant to promote the concepts of circular economy. The aim of this study is to use the well-known potential of process flowsheeting to evaluate design parameters of the pyrolysis process that can be further used for the design of closed-loop supply chain networks and for the determination of the best recycling strategies. For this purpose, a process model for CFRP pyrolysis that captures the essence of a plant to scale up the technology at various scales has been developed based on Aspen Hysys v8.6 simulation software (AspenTech: Optimizing Process Manufacturing 2017). The estimation of capital and operating costs as well as the evaluation of environmental impact is an integral component of such assessment. This study highlights the link between process flowsheeting tools and LCA tools as well as the interest of modelling some specific specific steps such as pyrolysis in global process modelling.

Modelling methodology

Pyrolysis is a thermal recycling process of FRP that decomposes the matrix at around 400 to 750 °C [START_REF] Oliveux | Current Status of Recycling of Fibre Reinforced Polymers: Review of Technologies, Reuse and Resulting Properties[END_REF] depending on the thermal properties of resin in order to recover fibres. The main characteristic of this process is the thermal decomposition in an inert environment or in a controlled atmosphere with a low proportion of oxygen to avoid the oxidation of fibres. Classically, the procedures for process simulation mainly involve defining chemical components, selecting a thermodynamic model, determining plant capacity, choosing, proper operating units and setting up input conditions (flowrate, temperature, pressure, and other conditions). The studied pyrolysis plant involves a thermoset epoxy resin matrix (40% vol) reinforced with polyacrylonitrile-based carbon fibre (60% vol). A capacity of 1500 t/y of recycled CFRP waste has been considered for design. Pyrolysis conditions have been set in Aspen Hysys v8.6 according to the results reported in the literature. Temperature, pressure, atmospheric composition are the three important parameters governing the pyrolysis process. Recently, [START_REF] Yang | Recycling of Carbon Fibre Reinforced Epoxy Resin Composites under Various Oxygen Concentrations in Nitrogen-oxygen Atmosphere[END_REF] have studied the impact of various oxygen concentrations in nitrogen-oxygen atmosphere and temperature reaction on the recycling carbon fibre (rCF) mechanical properties. It has been shown that rCF mechanical properties decrease with the increase in oxygen concentration and temperature. From these results, the pyrolysis process has been modelled according to the following conditions. The operating conditions in a fixed-bed reactor have been set at 550°C under N 2 atmosphere, to achieve full fibre/matrix separation and prevent fibre degradation. Then, the proportion of gases and liquids coming from the pyrolysis reaction has been determined from the work of [START_REF] Cunliffe | Pyrolysis of Composite Plastic Waste[END_REF]. The product yields of oil, gas and solid residue have been determined together with a detailed composition of the derived gases from the degradation of the epoxy resin in relation to final pyrolysis temperature (400°C to 800°C). Moreover, the thermodynamic model chosen for the simulations is the NRTL activity coefficient model [START_REF] Fakhrhoseini | Predicting Pyrolysis Products of PE, PP, and PET Using NRTL Activity Coefficient Model[END_REF]. Figure 1 shows the flowsheet of the pyrolysis process developed with Aspen Hysys v8.6. First, CFRP waste, gaseous nitrogen and propane, respectively, streams 1, 13 and 14 are introduced into the pyrolysis reactor modelled by a boiler and a reactor (FH-100 and CRV-100). The CFRP waste is modelled by carbon and Diglycidyl Ether of Bisphenol A (DGEBA) in relation with the proportion of carbon fibres and epoxy matrix. Energy is provided to the reactor (Q-100) to reach a temperature of 550°C. The output of the reactor is composed of nitrogen, propane and CO (stream 15) as well as of the carbon fibre (stream 3), and of the decomposed epoxy matrix (stream 2). The carbon fibre and the decomposed epoxy matrix are then mixed together (MIX-100) (stream 4). A separator unit (X-100) enables the recovery of the rCF, (stream 7) whereas the decomposed matrix can be found in streams 6, 8 and 18. A heat exchanger (E-100) cools the mixture of the decomposed matrix (stream 5). The pretreatment of effluents is composed of a gas/liquid separator (V-100). Then, liquid (stream 9) is heated to 45°C (stream 11). A first distillation column (T-100) allows the separation of E-Acetate. From the second distillation columns (T-101), methanol can be obtained (stream 20). In this study, the functional unit involves 1 kg of CFRP waste to be recycled by a pyrolysis process; the LCI has been performed from the results obtained from Aspen Hysys v8.6. A specific focus will be paid on Global Warming Potential (GWP) impact which is known as a typical indicator for assessment of energy-related performance. Concerning cost evaluation, the Aspen Process Economic Analyzer has also been used to estimate the cost of major capital expenditure items (CAPEX) as well as operating costs (OPEX).

Results

Environmental impact

The results obtained by Aspen Hysys v8.6 have been aggregated to identify the main inputs and outputs of the process in order to build Life Cycle Inventory data (Table 1). In this study, the rCF has been modelled in Simapro by virgin glass fibre (vGF) to take into account a possible decrease in their capacity to reinforced polymers due to a contamination of their surface, and a decrease in their mechanical properties, and their length. Consequently, the recycling of carbon fibre avoids the manufacturing of virgin glass fibre, thus leading to a negative environmental impact with a calculated total GWP for 1 kg of recycled CFRP waste being equal to -0.53 kg CO2 eq. (Figure 2). A negative impact is also calculated due to the recovery of both Methanol (-0.015 kg CO2 eq) and Ethyl acetate (-0.431 kg CO2 eq) that has been taken into account in a similar way since their recovery avoids their production in an extended boundary of the system. This result thus confirms the benefit to recycle CFRP waste. A substantial gain is also obtained for the other categories of impacts as highlighted in Figure 2. These results are in agreement with those obtained from a literature analysis: pyrolysis can be simply modelled as a combustion process of the matrix (35wt% of CFRP waste) (see Figure 3). No energy recovery from thermal decomposition of matrix has been assumed. The total energy used in pyrolysis has been estimated at about 30 MJ/kg composite [START_REF] Witik | Carbon Fibre Reinforced Composite Waste: An Environmental Assessment of Recycling, Energy Recovery and Landfilling[END_REF]. The environmental impact of virgin glass fibre has been calculated by [START_REF] Das | Life Cycle Assessment of Carbon Fiber-Reinforced Polymer Composites[END_REF]: the manufacturing of 1 kg of vGF leads to a GWP value equal to 2.6 kg CO2 eq. Under these conditions, a total GWP equal to -0.08 kg CO2 eq has been obtained.

Economic evaluation

Table 2 presents the results concerning both capital and operational expenditures computed from Aspen Hysys v8.6. An order of magnitude obtained from the review of [START_REF] Krawczak | Recyclage Des Composites[END_REF] after actualization and capacity extrapolation is also mentioned. The comparison is yet difficult since only the pyrolysis pilot (without mentioning the separation train) is considered, thus justifying the use of process systems engineering approach to a more accurate evaluation.

Conclusions

In this study, the modelling of the pyrolysis recycling process has been carried out by using Aspen Hysys v8.6 simulation software. The environmental assessment has been implemented via SimaPro. The results obtained from flowsheeting simulation have thus been used to build the Life Cycle Inventory data using Ecoinvent database. The focus on Global Warming Potential has shown that an environmental gain is obtained from the production of virgin fibres with a functional unit involving the recycling of 1 kg of Carbon Fibre Reinforced Polymers (CFRP) waste. An economic evaluation has also been carried. The objective is to use these results in order to build the corresponding pyrolysis process echelon of the CRFP recycling supply chain with the appropriate precision level at this step of project development. The perspective is to extend the approach to the other pathways of CFRP recycling that gain increasing attention (supercritical water process, microwave for instance). For this purpose, PSE concepts are of tremendous importance to generalize the link between process simulation and LCA software tools [START_REF] Morales-Mendoza | An integrated approach combining process simulation and life cycle assessment for ecoefficient process design[END_REF] and to optimise the recycling strategy taking into multiple criteria at earlier design stage of the supply chain.

Figure 1 :

 1 Figure 1: Flowsheet of the pyrolysis process modelled using Aspen Hysys v8.6 (DGEBA*: Diglycidyl ether Bisphenol A) Following the guidelines of Life Cycle Assessment methodology (Guinée 2001) based on the leading standards of ISO 14040, the environmental impact of the process has been evaluated using the software Simapro 8.0 (Péchenart and Roquesalane 2014) with Ecoinvent database and the calculation methods ReCIPe Midpoint (H) 1.10 (Methodology -ReCiPe, 2017). Life Cycle Assessment (LCA) is divided into four steps: goal and scope definition, Life Cycle Inventory (LCI), Life Cycle Impact Assessment (LCIA) and Interpretation.In this study, the functional unit involves 1 kg of CFRP waste to be recycled by a pyrolysis process; the LCI has been performed from the results obtained from Aspen Hysys v8.6. A specific focus will be paid on Global Warming Potential (GWP) impact which is known as a typical indicator for assessment of energy-related performance. Concerning cost evaluation, the Aspen Process Economic Analyzer has also been used to estimate the cost of major capital expenditure items (CAPEX) as well as operating costs (OPEX).

Figure 2 :

 2 Figure 2: Environmental impact of the recycling of 1 kg of CFRP by pyrolysis calculated from SimaPro

Figure 3 :

 3 Figure 3: Simplified modelling of the CFRP pyrolysis recycling process

Table 1 :

 1 Aggregation of components for life cycle inventory design

	Inputs	Outputs
	Stream 13 : Nitrogen (0.667 kg/kg CFRP	Stream 20 : Methanol {GLO} (0.025
	recycled)	kg/kg CFRP recycled)
	Stream 14 : Propane {GLO} (0.994 kg/kg	Stream 17 : Ethyl acetate {GLO} (0.164
	CFRP recycled)	kg/kg CFRP recycled)
	Steam, in chemical industry {GLO}	Stream 7 : PAN carbon fibre (Das 2011)
	(0.228 kg/kg CFRP recycled)	(0.602 kg/kg CFRP recycled)
	Water, process and cooling (3.675 l/kg	Stream 16 : Hazardous waste, for
	CFRP recycled)	incineration {RoW} (0.189 kg/kg CFRP
		recycled)
	Electricity, low voltage {FR} (0.552
	kWh/kg CFRP recycled)

Table 2 :

 2 Cost calculated by Aspen Hysys v8.6 and comparison with literature data[START_REF] Krawczak | Recyclage Des Composites[END_REF]

	Aspen Hysys v8.6	Literature


Parya Zolfaghari

Joel Ortiz
email: joel.ortiz-sosa@inria.fr

Cedric Killian
email: cedric.killian@irisa.fr

Sébastien Le Beux
email: slebeux@encs.concordia.ca

Non-Volatile Phase Change Material based Nanophotonic Interconnect

Keywords: Phase change Material, ONOC, Design Method

Integrated optics is a promising technology to take advantage of light propagation for high throughput chip-scale interconnects in many core architectures. A key challenge for the deployment of nanophotonic interconnects is their high static power, which is induced by signal losses and devices calibration. To tackle this challenge, we propose to use Phase Change Material (PCM) to configure optical paths between writers and readers. The non-volatility of PCM elements and the high contrast between crystalline and amorphous phase states allow to bypass unused readers, thus reducing losses and calibration requirements. We evaluate the efficiency of the proposed PCM-based interconnects using system level simulations carried out with SNIPER manycore simulator. For this purpose, we have modified the simulator to partition clusters according to executed applications. Simulation results show that bypassing readers using PCM leads up to 52% communication power saving.

I. INTRODUCTION

Advances in fabrication process enabled the use of manycore architectures for High Performance Computing (HPC) [START_REF] Vivet | IntAct: A 96-Core Processor With Six Chiplets 3D-Stacked on an Active Interposer With Distributed Interconnects and Integrated Power Management[END_REF]. Due to their increasing complexity and the heterogeneity of targeted applications, task mapping became critical to optimize execution time, energy efficiency and reliability [START_REF] Singh | Mapping on multi/many-core systems: Survey of current and emerging trends[END_REF]. Considering the large number of diversity of cores, memories, and accelerators, Network on Chips (NoCs) naturally became the backbones of manycore architectures. This further complexifies task allocation [START_REF] Chen | MARCO: A High-performance Task Mapping and Routing Co-optimization Framework for Point-to-Point NoC-based Heterogeneous Computing Systems[END_REF], which now require complex algorithms involving, for instance, neural networks [START_REF] Reza | Neuro-NoC: Energy Optimization in Heterogeneous Many-Core NoC using Neural Networks in Dark Silicon Era[END_REF]. On the performance side, nanophotonic interconnects, also called Optical Networks-on-Chip (ONoCs), have the potential to deliver the bandwidth required by data intensive applications in HPC [START_REF] Wu | SUOR: Sectioned Undirectional Optical Ring for Chip Multiprocessor[END_REF]. However, they suffer from high static power consumption, which is mostly due to losses experienced by optical signals and devices calibration requirements. Indeed, optical devices are sensitive to manufacturing process and thermal variations, which call for constant calibration of resonating devices such as ring resonators [START_REF] Narayan | PROWAVES: Proactive Runtime Wavelength Selection for Energy-efficient Photonic NoCs[END_REF]. To reduce the static power consumption of nanophotonic interconnects, we investigate the use of Phase Change Material (PCM) elements to bypass unused optical devices. Indeed, the high contrast between crystalline (cr) and amorphous (am) phase states has enabled the design of non-volatile directional couplers [START_REF] Xu | Low-Loss and Broadband Nonvolatile Phase-Change Directional Coupler Switches[END_REF]. A key limitation of PCM elements is the slow phase state changes compared to the required nanosecond scale latency communication requirements in manycore. We tackle this challenge by partitioning the manycore to execute different applications and by reconfiguring the interconnect only when new applications are executed. While recent studies have demonstrated the efficiency of PCMs to design multi-level optical memories [START_REF] Ríos | Integrated all-photonic nonvolatile multi-level memory[END_REF] and weighting functions in spiking neural networks [START_REF] Chakraborty | Toward Fast Neural Computing using All-Photonic Phase Change Spiking Neurons[END_REF], this work is the first attempt to use the technology in nanophotonic interconnects. The contributions are the following. We propose a non-volatile Single Writer Multiple Reader (SWMR) optical channel. The channel is reconfigurable and allows to bypass unused readers using PCM-based directional couplers. We develop a method to configure the interconnect according to the mapping of applications on the cores. To do this, we modified SNIPER simulator to enable the thread distribution of multi-applications from Splash2 and PARSEC benchmarks. We evaluate the power consumption of the network using a model we developed. We show that an average of 21% power saving is obtained compared to channels without PCM, and up 52% saving is reached for mapping involving 4 clusters. Finally, we simulate the execution of the applications in parallel by considering the proposed reconfigurable interconnect. This allows us to divide the architecture to execute two applications independently and simultaneously. On average, we obtain a 21.6% reduction in the execution time. The paper is organized as follows. Section II discusses related works. In Section III, we present the proposed PCM based optical interconnect. Section IV describes the power model. Section V presents the experimental setups and Section VI is results section. Section VII concludes the work.

II. RELATED WORK

A. Optical Network on Chip

The static power consumption of nanophotonic interconnects is mostly due i) losses experienced by optical signals, which call for high laser power, and ii) optical devices calibration [START_REF] Sun | DSENT -A Tool Connecting Emerging Photonics with Electronics for Opto-Electronic Networks-on-Chip Modeling[END_REF]. At the link level, PID controllers are designed to compensate the effect of temperature variation on the wavelength shift of ring resonator resonance [START_REF] Dang | PID controlled thermal management in photonic networkon-chip[END_REF]. While the design in [START_REF] Dang | PID controlled thermal management in photonic networkon-chip[END_REF] involves to monitor received optical signals, the controlled proposed in [START_REF] Chen | Simultaneously Tolerate Thermal and Process Variations Through Indirect Feedback Tuning for Silicon Photonic Networks[END_REF] relies on sensors monitoring the temperature of ring resonators. At the network level, reconfigurable interconnects were proposed to optimize the use of optical channels [START_REF] Wu | SUOR: Sectioned Undirectional Optical Ring for Chip Multiprocessor[END_REF], thus allowing to reduce the resources overhead. At system level, application-specific mapping methods allow to reduce crosstalk noise [START_REF] Fusella | Crosstalk-Aware Automated Mapping for Optical Networks-on-Chip[END_REF]. Run-time methods enabling to select the number of used wavelengths according to bandwidth requirement was proposed in [START_REF] Narayan | PROWAVES: Proactive Runtime Wavelength Selection for Energy-efficient Photonic NoCs[END_REF]. Approximate computing techniques were also investigated to reduce bandwidth requirement [START_REF] Lee | Approximate nanophotonic interconnects[END_REF]. All these approaches are complementary to this work, which aims at using PCM to reduce the static power.

B. Phase Change Material (PCM)

PCM elements are characterized by femtojoule-scale phase transition energy consumption, 10 15 switching cycle endurance and years long state retention [START_REF] Zheng | GST-on-silicon hybrid nanophotonic integrated circuits: a non-volatile quasi-continuously reprogrammable platform[END_REF]. These properties led to the usage of PCMs in numerous optical application such as spiking neural networks [START_REF] Chakraborty | Toward Fast Neural Computing using All-Photonic Phase Change Spiking Neurons[END_REF], optical memories [START_REF] Ríos | Integrated all-photonic nonvolatile multi-level memory[END_REF] and directional couplers [START_REF] Xu | Low-Loss and Broadband Nonvolatile Phase-Change Directional Coupler Switches[END_REF]. The design proposed in [START_REF] Xu | Low-Loss and Broadband Nonvolatile Phase-Change Directional Coupler Switches[END_REF] relies on a PCM element inserted between the branches of a directional coupler. The signal transmission depends on the phase state of the material: amorphous (am) and crystalline (cr) states lead to cross and bar transmission respectively. This work involves such a directional coupler, which we use to bypass disconnected readers in SWMR channels.

III. PCM BASED SWMR CHANNEL

This section presents the proposed PCM-based nanophotonic interconnect. It involves SWMR links onto which PCM elements are added to control the routing of the optical signals.

We first present an overview of the proposed interconnect and we then present use-case scenarios.

A. Overview

Figure 1 represents the proposed configurable SWMR channel assuming one waveguide and N wavelengths. As in conventional SWMR channel, the writer modulates the optical signal, which propagates towards the destination receiver where Opto-Electronic (OE) conversions are carried out. However, unlike conventional SWMR channels where all intermediate receivers must be crossed before reaching the destination, the configuration ability of the proposed SWMR channel allows to connect only a selected set of readers. By disconnecting readers from the optical path, we aim at reducing the power consumption as follows: i) MRRs in disconnected receivers do not require, power hungry, calibration and ii) optical losses can be reduced since signals do not propagate through all the MRRs. To achieve this, we introduce a bypass waveguide at each reader, and we use a Directional Coupler (DC) between each reader to configure the optical path as represented in Figure 1. For each DC, the path can be configured to transmit signals towards the bypass waveguide or towards the receiver. The couplers are large band, thus allowing transmitting all modulated signals to a same output waveguide. The DC is configured according to the state of an embedded PCM element, as shown in Figure 1.c: amorphous (am) and crystalline (cr) phases lead to cross and bar configuration respectively. The PCM is non-volatile, thus allowing maintaining the DC configuration without consuming static energy.

B. Configuration Method and Use Case Scenarios

Considering the rather slow state change of PCM elements (around 100ns [START_REF] Yamada | Rapid, phase transitions of GeTe-Sb2Te3 pseudobinary amorphous thin films for an optical disk memory[END_REF]) and their limited endurance, the SWMR channels are configured according to application specific or architecture specific connectivity requirements. The execution times of the targeted benchmarks applications, which typically range from 100ms to 10s, will ensure a low reconfiguration frequency of the SWMR channel (<<1Hz). The connectivity requirements depend on the number of writers (i.e. the number of SWMR channels) and number of readers per channel. In the context of manycore architectures where multiple SWMR channels are used, each channel is configured according to the list of readers to be reached by the writer. To connect a reader, the preceding DC is configured either to cross if the previous reader is disconnected (i.e. signals coming from bypass) or to bar if the previous reader is also connected. Since we assume that a SWMR configuration does not change during the execution of an application, the MRRs of disconnected readers are not calibrated. On the contrary, the MRRs of connected readers are calibrated as follows: MRRs of the destination receiver drop the signals from the waveguides, while the others MRRs let the signals propagating (through). Maintaining the calibration of all connected MRRs allows minimizing the communication latency overhead [START_REF] Chen | Simultaneously Tolerate Thermal and Process Variations Through Indirect Feedback Tuning for Silicon Photonic Networks[END_REF]. To further save energy, the injected laser power is adapted to losses experienced by the optical signal for each configuration. Figure 2 illustrates various connectivity scenarios for a 4readers SWMR channel. We assume three wavelengths, thus leading to three MRRs per reader. The scenarios are:  all readers connected (Figure 2.a) leads to a regular SWMR channel for which all the MRRs require calibration. To achieve the channel configurations, all the PCM elements are set to cr state, thus leading to bar transmission for the directional couplers. As previously mentioned, any of the connected receivers can be reached by the writer by calibrating the corresponding MRRs to the drop transmission . Regarding the optical losses, this configuration leads to the highest losses, and hence highest laser power requirements, since the optical signal can propagate through all MRRs.  r2, r3 and r4 connected (Figure 2 IV. POWER MODEL In this section, we first describe the method to configure the SWMR channels according to connectivity requirements, then we present the network power model.

A. Method

We estimate the total network power consumption by considering the contribution of each SWMR channel, as defined in Eq. 1. 𝑃 𝑆𝑊𝑀𝑅 𝑖 corresponds to the power consumption of channel i, which depends on its configuration. In Eq. 2, 𝑃 𝑙𝑎𝑠𝑒𝑟 𝑖 is the laser power required to reach the photo detector sensitivity on the last connected reader. 𝑃 𝑀𝑅-𝐶 𝑖 is the power required to calibrate the rings in connected readers and 𝑃 𝑐𝑜𝑛𝑓𝑖𝑔 𝑖 is the power needed to configure the PCM elements in the directional couplers. PT is the power required to serialize and modulate optical signals in the writer. PR is the receiver power consumption, which corresponds to photo detection, amplification, comparison and deserialization.

P total = ∑ P SWMRi N-1 i=0
(1)

P SWMRi = P laseri + P T + P R + P config i + P MR-Ci

(2)

To evaluate laser, calibration and reconfiguration powers, we consider the following key parameters from the connectivity requirement: i) number of connected readers n, ii) number of PCM elements in the crystalline state A, iii) number of PCM elements in the amorphous state B and iv) position of the last connected reader 𝑝 𝑟 𝑖 .

B. Laser Power

For each channel, we define the required laser power according to the photodetector sensitivity (typically -8 dBm for 10 -12 BER [START_REF] Lee | Approximate nanophotonic interconnects[END_REF]), the lasing efficiency and the worst-case losses experienced by the signal to reach the last connected reader (Eq. 3). The worst-case losses in a channel is defined by Eq. 4

P laser i = (P received i + Loss wc i )/eff (3)
Loss wc i = L MR-t × N ω ×n + L w ×p r i ×d+ L MR-d + ∑ IL cr bar + ∑ IL am cross +X talk B-1 b=0 A-1 a=0 (4)
The propagation of the signals towards the last connected reader involves crossing all rings in the n connected interfaces. 𝐿 𝑀𝑅-𝑡 is the through loss per ring and 𝑁 𝜔 is the number of wavelengths, which also corresponds to the number of rings per reader. The waveguide propagation loss experienced by the signals depends on the position of the last connected reader, the distance between interfaces (𝑑) and the waveguide loss ( 𝐿 𝑤 in dB/cm). 𝐿 𝑀𝑅-𝑑 is the drop loss in the receiver. The insertion loss induced by the directional couplers depends on the number of crossed PCM elements and their state. 𝑋 𝑡𝑎𝑙𝑘 is crosstalk power penalty which depends on the number of wavelengths and the ring position in the reader interface [START_REF] Bahadori | Crosstalk penalty in microring-based silicon photonic interconnect systems[END_REF].

C. Ring Calibration

MRRs are sensitive to temperature variation, which calls for calibration to compensate the wavelength detuning. To evaluate the calibration power, we consider the Thermal Tuning Efficiency (𝑇𝑇 𝑒 ) of the closed loop feedback system proposed in [START_REF] Narayan | PROWAVES: Proactive Runtime Wavelength Selection for Energy-efficient Photonic NoCs[END_REF]. Only the rings in connected readers require calibration, which leads to Eq. 5. In the equation, ∆ 𝑀𝑅-𝐶 𝑘𝑗 corresponds to the required wavelength shift of ring at position j in reader k. Assuming a homogeneous distribution of the signal wavelengths among the FSR, the maximum shift for each ring is FSR/N. Therefore, the required shift for each MRR is defined by Eq. 6 where ∆ 𝑠ℎ𝑖𝑓𝑡 𝑘𝑗 is the wavelength shift of MRR j located at reader k. It is obtained with Eq. 7 where ∆𝑇 is the temperature variation and 𝑑 𝑑𝑇 is the MRR thermal sensitivity.

P MR-C = ∑ ∑ ∆ MR-C kj N  j=1 n k=1
*TT e

D. Channel Configuration

The following presents a model to evaluate the power needed to reconfigure SWMR channels. Indeed, when executing a new set of applications, the interface connectivity might change, thus leading to a different set of connected readers. This requires changing the state of PCMs from cr to am or from am to cr, which requires energy. By assuming PCM already configured for a given connectivity requirement, we obtain reconfiguration power using Eq. 8. It is worth mentioning that the reconfiguration time of the PCMs is not critical in our system since SWMR channels are reconfigured only when new applications are executed. Hence, the design of circuits to change PCM state is out of the scope of the paper but details related to implementation involving electrical pulses, optical pulses or external heaters can be found in [START_REF] Abdollahramezani | Tunable nanophotonics enabled by chalcogenide phase-change materials[END_REF].

V. EXPERIMENTAL SETUP A. Architecture

We consider the 3D architecture illustrated in Figure 3: bottom layer implements processing units and memories, while top layer implements the optical interconnect. It is composed of 16 clusters of 4 cores. Each cluster includes a shared last level cache (L3). Each core has a private L1 data (L1d), instructions (L1i) caches, of 32KB each, and a private L2 cache of 512KB. MESI protocol ensures coherency between the distributed caches. All clusters are connected with ONIs through TSVs. Each ONI has 1 transmitting and 15 receiving waveguides which are featured with SWMR and WDM, and include PCM based bypassing paths, as depicted in Figure 3. Each waveguide transmits 8 optical signals at different wavelengths.

B. Simulator and Benchmarks

We consider PARSEC and SPLASH2 benchmark suites [START_REF] Bienia | PARSEC vs. SPLASH-2: A quantitative comparison of two multithreaded benchmark suites on Chip-Multiprocessors[END_REF] which are commonly used to study the performance of manycore architectures. They contain applications from different domains, which leads to various processing and communication requirements. We have selected six representative applications according to their parallelization level: i) FFT and Raytrace from Splash2 and ii) x264, Blackscholes, Barnes and Dedup from PARSEC. The execution of these applications on the architecture are simulated using the SNIPER manycore simulator [START_REF] Carlson | An Evaluation of High-Level Mechanistic Core Models[END_REF]. We modified the Splash2 applications to allow their parallel executions and we modified SNIPER to handle the distribution of threads. It is possible to distribute applications on any number of clusters, which is suitable to explore the design space (see results section). Although the application mapping and the cluster partitioning lead to numerous design options, the results will only refer to scenario below due to space limitation. 4.f-g illustrate cluster partitioning for the parallel execution of two applications. From the simulation results, we extract the communication traces between L2 caches and the distributed L3, which corresponds to the traffic on the SWMR channels. Table 1 summarizes the architectural and technological parameters. Phase transition energy from am to cr 2nj [START_REF] Xu | Low-Loss and Broadband Nonvolatile Phase-Change Directional Coupler Switches[END_REF] VI. RESULTS

A. Losses and Power Analysis

We investigate the power consumption of the proposed interconnect and we compare it with a baseline interconnect without PCM elements. For this purpose, we first evaluate the loss of each channel used for 1x4 SWMR configuration and we report results in Figure 5. For SWMR0, the waveguide loss is as small as 0.28 dB due to the short distance between the writer (cluster 0) and the last connected reader (cluster 3). Since readers located after the last connected reader are not used, the MR through loss for ONoC with and without PCM is the same (1.41dB). The need to cross three directional couplers for the PCM-based channel leads to 0.48dB overhead compared to the channel without PCM. Channels 1 to 3 demonstrate higher waveguide propagation losses since the signals propagate through the entire waveguide. Since only three readers are connected, MR through loss for channels with PCM is the same as for SWMR0 (i.e. 1.41dB) while they reach 2.4 dB without PCM due to the need to cross all the MRRs in the intermediate readers. Finally, the losses induced by the directional couplers (3.56dB) lead to 1.4dB additional losses for the PCM based channel compared to channel without PCMs. To summarize, although PCMs allow reducing the MRR through loss when bypassing disconnected readers, the rather high insertion loss they involve lead to higher total losses. While PCM-based interconnect requires higher laser power, we will show in the following that significant power saving can be achieved thanks to significant reduction in the ring calibration power. Figure 6.a shows the power breakdown for previously discussed configuration and channels. For SWMR0, the power consumption is 1mW higher with PCM due to laser power overhead. Calibration power is the same with and without PCM since no reader is bypassed. For channels 1, 2 and 3, we obtain x5 reduction in the calibration power since only the connected readers require calibration. Overall, despite the 8mW increase in laser power, our approach leads to up to 52% power saving. As shown in .b, we obtain an average of 45% power saving per used channel power for 1x4 configuration. As the number of connected readers increases, lower power saving is obtained due to ring calibration requirements. Although configuration 4x4 leads to 6% power increase, our approach demonstrates a 21% power reduction on average. The higher energy efficiency of our approach for lower connectivity scenario ideally complements with the mapping of independent applications on the clusters, which we study in the following.

B. Benchmark Analysis

We simulate the execution of benchmark applications using the environment defined in Section V.B, as reported in Figure 7.

The mapping of Blackscholes (Figure 7.a) on 4 clusters (resp. 16 clusters) leads to 1376ms (resp. 1188ms) execution time and 28W (resp. 120W) power consumption. By using all clusters in the architecture, a 15% speedup is achieved at the cost of x4.2 power increase. Configuration involving 4 clusters and 8 clusters lead to intermediate speedups and powers, which we will use to optimize the multi-applications mapping and the ONoC configuration. While all tested applications follow a similar trend, the actual speedup and power consumption highly depends on the parallelization level and the communication patterns. Using 4 clusters for FFT is preferable since 16 clusters lead to 3% speedup and 3.5x power overhead (Figure 7.e). On the other hand, the mapping of x264 on 16 clusters allows 60% speedup. To conclude, clusters partitioning, which is achieved by reconfiguring SWMR channels, can be optimized depending on the executed applications.

C. Multi-Application Mapping and ONoC Configuration

We consider the execution of pairs of applications on the architecture. The baseline is a sequential execution of the two applications on all the clusters (denoted 1616 in Figure 8) connected using an ONoC without PCM. As reported in Figure 8.b, the baseline execution time for x264 and Blacksholes is 3663ms. We now consider the proposed PCM-based ONoC. PCMs allow to partition the architecture in order to allocate a dedicated number of clusters per application (see Figure 4.f for mapping details). Since readers which are not part of a SWMR channel partition are disconnected, the application tasks can be executed in parallel without any resources sharing, which contribute to reduce the execution time. As illustrated in Figure 8.b (case denoted 8/8 in the figure), the execution time is significantly reduced and reaches 2962ms. We then consider an uneven mapping of the application on the clusters (illustrated in Figure 4.g): 4/12 (resp. 12/4) allocates 4 (resp. 12) clusters to Blacksholes (resp. x264) and 12 (resp. 4) clusters to x264 (resp. Blacksholes). Mapping 12/4 leads to the best improvement in the execution time (26.8%). For the interconnect, SWMR channels are configured to connect only readers of interface executing the same application as the writer. Considering the 1.2Tb/s aggregated bandwidth of the optical interconnect (16 channels with 8 wavelengths at 10Gbps each), the baseline scenario would lead to a 2.7pJ/bit energy consumption under the assumption that all channels permanently transmit data. By considering the actual number of bits transmitted, we obtain a 47.7% energy per-bit reduction for the proposed ONoC compared to a network without PCM. The significant Figure 8.a summarizes the best mapping we obtain for each application pair. Reductions in execution time and energy per bit reach up to 42% and 68.8% respectively for FFT /Blacksholes. Mappings involving Barnes lead to the lowest improvement since it is the least computation intensive application. No improvement is obtained when Barnes is combined with x264, which is the most data intensive application. This calls for finer grain mapping we will explore in our future works. On average, we obtain 21.6% execution time reduction and 41.2% per-bit energy reduction for the PCM-based ONoC. This validates the potential for PCMs to improve the on-chip optical communication energy efficiency and to facilitate partitioning of manycore.

In our last study, we evaluate the power required to change the state of PCMs during network reconfigurations. For this purpose, we first consider a pessimistic scenario in which the two applications with the smallest execution time, i.e. Barnes and Dedup, require full reconfiguration of the PCMs after each execution. The resulting 1.3Hz reconfiguration frequency would lead to 5µW, which is negligible considering the total system power consumption (100W). Nevertheless, the PCMs reconfiguration power and endurance can be improved by optimizing the remapping to take into account the current state of PCMs. This will be explored in our future works.

VII. CONCLUSION This paper investigates the potential of PCMs elements for reducing static power in nanophotonic interconnects. We propose a configurable non-volatile SWMR channel capable of bypassing unused readers, thus avoiding unnecessary calibration of optical devices. To efficiently use the non-volatile capability of our interconnect, we develop a method to map applications on dedicated clusters. Configurability property of PCM allows to partition the architecture for parallel execution of applications which leads to reduction in execution time compared with sequential execution of applications on all 16 clusters. Results show that up to 52% static power reduction of the interconnect and 42% execution time reduction can be reached. Since this work is the first attempt to use PCM elements in nanophotonic interconnects, the perspectives are numerous and include the bypassing of group of readers to reduce insertion losses and the fine grain mapping of applications on manycore.

Figure 1 :

 1 Figure 1: a) proposed SWMR channel with PCM-based directional couplers to configure the optical path through readers and bypass waveguides, b) MRRs states, c) signal transmission (for all wavelengths) through the directional coupler according to the state of the PCM and d) signal transmission to connected reader or use bypass path for disconnected reader.

Figure 2 .

 2 Figure 2. PCM elements configuration and ring calibration for various scenario: a) all interfaces connected, leading to a regular SWMR channel, b) r1 disconnected, which allows to not calibrate its MRRs, c) r2 and r4 disconnected d) r2 only connected leading to SWSR channel.

P

  config =f ×( ∑ E cr i →am i + ∑ E am j →cr j )

Figure 3 .

 3 Figure 3. Considered 3D hardware architecture.

Figure 4 :

 4 Figure 4: mapping example: a) application 1 mapped on Cluster 0 to 3, which leads to the use of Channel 0 to 3 only ; b-e) mapping of one application; f-g) mapping of two applications.

Figure 4 .

 4 Figure 4.a illustrates the mappings of an application on four clusters, which leads to the use of four SWMR channels. Each used channel is configured to bypass disconnected readers (not illustrated in the figure for sake of clarity).Figure 4.b-e illustrate mappings on 6, 8, 9 and 12 clusters.Figure4.f-g illustrate cluster partitioning for the parallel execution of two applications. From the simulation results, we extract the communication traces between L2 caches and the distributed L3, which corresponds to the traffic on the SWMR channels. Table1summarizes the architectural and technological parameters.

Figure 4 .

 4 Figure 4.a illustrates the mappings of an application on four clusters, which leads to the use of four SWMR channels. Each used channel is configured to bypass disconnected readers (not illustrated in the figure for sake of clarity).Figure 4.b-e illustrate mappings on 6, 8, 9 and 12 clusters.Figure4.f-g illustrate cluster partitioning for the parallel execution of two applications. From the simulation results, we extract the communication traces between L2 caches and the distributed L3, which corresponds to the traffic on the SWMR channels. Table1summarizes the architectural and technological parameters.

Figure

  Figure 4.a illustrates the mappings of an application on four clusters, which leads to the use of four SWMR channels. Each used channel is configured to bypass disconnected readers (not illustrated in the figure for sake of clarity).Figure 4.b-e illustrate mappings on 6, 8, 9 and 12 clusters.Figure4.f-g illustrate cluster partitioning for the parallel execution of two applications. From the simulation results, we extract the communication traces between L2 caches and the distributed L3, which corresponds to the traffic on the SWMR channels. Table1summarizes the architectural and technological parameters.

Figure 5 :

 5 Figure 5: Loss breakdown on each channel for 1x4 configuration.

Figure 6 :

 6 Figure 6: Power consumption results: a) power breakdown per channel for 1x4 configuration and b) average power consumption per channel according to the network configurations.

Figure 7 :

 7 Figure 7: Execution time and power wrt. number of used clusters.

Figure 8 :

 8 Figure 8: a) Best mapping results and improvements compared to execution on 16 clusters, b-c) execution time for different mappings and ONoC configurations for Blacksholes/x264 and Blacksholes/Raytraces.

  .b) require to bypass r1, which is achieved by configuring DC1 to the am state. Since r2 is part of the connected readers, DC2 is also configured to am. The rest of the PCMs are set to cr state, thus allowing to transmit the signals to r3 and r4. Since one reader is bypassed, the MRR through losses are reduced, thus leading to laser power saving. Furthermore, reduction of calibration power is also achieved since signals do not propagate through r1.  r1 and r3 connected (Figure2.c) involves bypassing r2. PCM element in DC1 is configured to am, thus allowing the signal propagation through r1. To bypass r2, PCM elements in DC2 and DC3 are also configured to am. Since r4 is also disconnected and is beyond the last connected reader, DC4 does not require any specific configuration. In term of power, further reduction can be achieved compared to previous scenario due to lower MRRs losses, waveguide propagation losses and MRR calibration.  r2 connected (Figure2.d) leads to SWSR channel. To achieve this, DC1 and DC2 are set to cross state while DC3 and DC4 can be in any state (i.e. no reconfiguration needed).

Table 1 :

 1 Hardware and Technological Parameters.

	Parameter Definition	Value
		Architecture
		# 𝑐𝑙𝑢𝑠𝑡𝑒𝑟𝑠	16
		# 𝑐𝑜𝑟𝑒𝑠 𝑝𝑒𝑟 𝑐𝑙𝑢𝑠𝑡𝑒𝑟	4
		𝐿1 𝐼/𝐷 𝑐𝑎𝑐ℎ𝑒𝑠	32 KB each
		𝐿2 𝑐𝑎𝑐ℎ𝑒	512 KB
		𝐶𝑎𝑐ℎ𝑒 𝑝𝑟𝑜𝑡𝑜𝑐𝑜𝑙	MESI
		ONI
	𝑑	Distance between interfaces	0.376cm [14]
	𝑁 	Number of wavelengths	8
	𝐵 𝑅	Bit-rate	10Gb/s
	𝑃 𝑇	Transmitter power	24mw [6]
	𝑃 𝑅	Receiver power	24mw [6]
	𝐿 𝑤	Waveguide loss in db/cm	0.25 [14]
	𝐿 𝑀𝑅-𝑑	MRR drop loss	0.7 [14]
	𝐿 𝑀𝑅-𝑡	MRR through loss	0.02 [14]
	𝑄	MRR Quality Factor	20,000 [17]
	𝑇𝑇 𝑒	Thermal tuning efficiency	120pm/mw [6]
	𝑋 𝑡𝑎𝑙𝑘	crosstalk power penalty	0.0494 dB
		Directional Coupler
	𝑏𝑎𝑟 𝐼𝐿 𝑐𝑟 𝐼𝐿 𝑎𝑚 𝑐𝑟𝑜𝑠𝑠	Bar output Insertion loss for PCM in cr state cross output Insertion loss for PCM in am state 0.72dB [7] 0.16dB [7]
	𝐸 𝑐𝑟->𝑎𝑚	Phase transition energy from cr to am	2nj [7]
	𝐸 𝑎𝑚->𝑐𝑟

  improvement in energy is due to i) the reduced static power induced by bypassed readers and ii) the higher data rate induced by shortened execution time. We carried out the same study for Blacksholes/Raytrace application pair, as reported in Figure8.c. Interestingly, the best configuration is obtained by allocating 4 clusters to Blacksholes (against 12 in previous case), which is due to a lower computation load of Raytrace. Mapping 4/12 leads to 40.3% execution time reduction and 42.4% energy per bit reduction.

ACKNOWLEDGMENT

This work benefited from the Fonds de recherche du Québec -Nature et technologies (FRQNT) under grant 2021-NC-286424 and the support of the SHNoC ANR18-CE25-0006 and OpticALL2 ANR-18-CE24-0027 projects of the French National Research Agency (ANR).