Guillaume Bertrand
email: guillaume.bertrand@email.com

Ricardo Hirata
email: rhirata@usp.br

Augusto Auler
email: aauler@gmail.com

Francisco Cruz
email: cruz@usp.br

Lise Cary
email: l.cary@brgm.fr

Emmanuelle Petelet-Giraud

Eliot Chatton
email: eliot.chatton@etudiant.univ-rennes1.fr

Luc Aquilina
email: luc.aquilina@univ-rennes1.fr

Jean-Sébastien Moquet
email: js.moquet@gmail.com

Maria Gracia Bustamante
email: bustamanterosell@gmail.com

Christian Millo
email: millo@usp.br

Veridiana Martins
email: veridian@usp.br

Suzanna Montenegro
email: suzanam@ufpe.br

Hélène Celle-Jeanton
email: helene.jeanton@univ-fcomte.fr

Groundwater isotopic data as potential proxy for Holocene paleohydroclimatic and paleoecological models in NE Brazil

Keywords: Groundwater, δ 18 O, d-excess, Rain forest, Holocene, Brazil

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

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Introduction

Over the last two decades, the Holocene climate reconstructions for South-America Monsoon System (SASM) historically poor, have been subject to increasing investigation (Vimieux et al., 2009;[START_REF] Vuille | A review of the South American monsoon history as recorded in stable isotopic proxies over the past two millennia[END_REF]Prado et al., 2013a;[START_REF] Baker | Nature and causes of Quaternary climate variation of tropical South America[END_REF]. These reconstructions are based on combination of large-scale numerical models (e.g., IPSL-CM5A-LR, Kageyama et al., [2012] and references therein; MIROC-ESM, [START_REF] Watanabe | MIROC-ESM 2010: Model description and basic results of CMIP5-20c3m experiments[END_REF]; CNRM-CM5, [START_REF] Voldoire | The CNRMCM5.1 global climate model: Description and basic evaluation[END_REF], FGOALS-s2, [START_REF] Bao | The Flexible Global Ocean-Atmosphere-Land System Model, spectral version 2: FGOALS-s2[END_REF]; BCC-CSMI.I, [START_REF] Xin | Introduction of CMIP5 experiments carried out with the climate system models of Beijing Climate Center[END_REF]) calibrated and controlled by various local proxy data.

These proxies include dated pollens records (e.g., [START_REF] Oliveira | Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River[END_REF][START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF], lake sediment geochemical analyses (e.g., [START_REF] Viana | A late Holocene paleoclimate reconstruction from Boqueirão Lake sediments, northeastern Brazil[END_REF], terrigenous pulses in ocean cores (e.g., [START_REF] Arz | Climatic changes during the last deglaciation recorded in sediment cores from the northeasternBrazilian Continental Margin[END_REF], travertine deposits (e.g., [START_REF] Wang | Wet periods in northeastern Brazil over the past 210 kyr linked to distant climate anomalies[END_REF], tree rings (e.g., [START_REF] Ferrero | Tree-ring based reconstruction of Río Bermejo streamflow in subtropical South America[END_REF], ice core δ 18 O variability investigations (mainly in the Andes; e.g., [START_REF] Thompson | Annually Resolved Ice Core Records of Tropical Climate Variability over the Past ~1800 Years[END_REF] and speleothem δ 18 O analyses (e.g., [START_REF] Cruz | Stable isotope study of cave percolation waters in subtropical Brazil: implications for paleoclimate inferences from speleothems[END_REF][START_REF] Sylvestre | Moisture Pattern During the Last Glacial Maximum in South America[END_REF][START_REF] Novello | Multidecadal climate variability in Brazil's Nordeste during the last 3000 years based on speleothem isotope records[END_REF]2016;[START_REF] Cheng | Climate change patterns in Amazonia and biodiversity[END_REF]. However, despite the current increasing interest in the South America

A C C E P T E D M

A N U S C R I P T (SA) paleoclimatic reconstruction, available proxies are still considered to be sparse for the area if they are compared to records from Northern Hemisphere [START_REF] Vuille | A review of the South American monsoon history as recorded in stable isotopic proxies over the past two millennia[END_REF]Prado et al., 2013a;b). Moreover, at the continent scale, the repartition of the proxies is heterogeneous. Multi-proxy reconstruction syntheses (e.g., [START_REF] Villalba | Long-term multi-proxy climate reconstructions and dynamics in South America(LOTRED-SA): State of the art and perspectives[END_REF]Prado et al., 2013a;b), have highlighted that the Brazilian North-East (referred to "Nordeste" hereafter) is still very poorly documented. This scarcity of paleodata for Nordeste is probably due to the current geologic and climatic contexts of the area. From a climatic point of view, Nordeste is nowadays mainly an arid region featuring caatinga vegetation and potential Holocene lacustrine/riverine sediment/pollen data accumulation and conservation are not in optimal conditions. In this area, only few data over the 3 last millenia from lacustrine sediments were investigated through geochemical proxies next to the seashore (Siffedine et al., 2011;Zocatelli et al., 2012). In parallel, investigation of speleothem isotopic records necessarily implies to find carbonated contexts with access to developed and unflooded karstic network, such geological conditions being not frequently found in Nordeste.

The few available data for the Nordeste for the whole or portions of the Holocene are nevertheless considered of good quality. Although calibration (against modern hydrometeorological data) and corrections (of long-term isotopic effects on moisture sources) are still discussed [START_REF] Baker | Nature and causes of Quaternary climate variation of tropical South America[END_REF][START_REF] Moquet | Calibration of speleothem δ 18 O records against hydroclimate instrumental records in Central Brazil[END_REF], these proxies are relevant, in term of time resolution, for detailed multidecadal and millenial variations of the SASM over the Nordeste [START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF][START_REF] Novello | Multidecadal climate variability in Brazil's Nordeste during the last 3000 years based on speleothem isotope records[END_REF][START_REF] Cheng | Climate change patterns in Amazonia and biodiversity[END_REF], and contributed to describe the large scale drivers of SASM changes during the Holocene and the complexity of their interactions.

These indicators have allowed for rainfall intensity patterns to be delineated and forcing typologies to be produced which explain the "mosaic response" of the SA throughout the Holocene [START_REF] Sylvestre | Moisture Pattern During the Last Glacial Maximum in South America[END_REF]. The major driver of the SAMS circulation is the land-sea temperature gradient [START_REF] Vera | Toward a unified view of the American Monsoon Systems[END_REF]Marengo et al., 2012;[START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF]Prado et al., 2013a;b). During the Early-Mid Holocene, lower austral insolation in the Southern Hemisphere induced a less effective land-sea contrast than nowadays, combined with a lower moisture ascendant motion and rain cloud formation leading to a drier climate in the SAMS area. In addition, the latent heat released by the cloud formation was lower, and as this controls the high level circulation through the Bolivian High-Northeast Trough linkage, the subsidence over the Nordeste was lower than nowadays. This allowed for incursions in the area of both the Inter Tropical Convergence Zone (ITCZ) from the North and of the South Atlantic Convergence Zone

A C C E P T E D M

A N U S C R I P T (SACZ) from the South (Silvia- [START_REF] Dias | Mid-Holocene climate of tropical South America: a model-data approach[END_REF], producing more precipitation in this zone while the rest of the SAMS zone was drier [START_REF] Valdes | South American paleoclimate model simulations: How reliable are the models?[END_REF][START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF][START_REF] Dias | Mid-Holocene climate of tropical South America: a model-data approach[END_REF][START_REF] Cheng | Climate change patterns in Amazonia and biodiversity[END_REF]. Numerical simulations with general circulation models that incorporate basic atmospheric water isotope physics ( 18 O and/or 2 H) support this pattern by simulating the amount effect, as well as the link to insolation-driven monsoon activity [START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF].

In parallel, a temperature cooling of the region during Early-Mid Holocene has been documented, although at a coarser time resolution, through pollen, oceanographic and noble gas data (e.g., [START_REF] Behling | Late Quaternary vegetational and climate dynamics in northeastern Brazil, inferences from marine core GeoB 3104-1[END_REF][START_REF] Bush | Climate Change in the Lowlands of the Amazon Basin[END_REF]Ledru et al., 1996;[START_REF] Sylvestre | Moisture Pattern During the Last Glacial Maximum in South America[END_REF][START_REF] Stute | Cooling of Tropical Brazil (5 degrees C) during the Last Glacial Maximum[END_REF][START_REF] Wang | Wet periods in northeastern Brazil over the past 210 kyr linked to distant climate anomalies[END_REF][START_REF] Santos | Paleoceanographic reconstruction of the western equatorial Atlantic during the last 40 kyr[END_REF]. Despite of remaining uncertainties, it is now broadly accepted that this cooling ranged between 4 and 8°C over SA in comparison to the Late Holocene, and around 5° in Nordeste [START_REF] Farrera | Tropical climates at the Last Glacial Maximum: a new synthesis of terrestrial paleoclimate data. I. Vegetation, lake levels, and geochemistry[END_REF][START_REF] Stute | Cooling of Tropical Brazil (5 degrees C) during the Last Glacial Maximum[END_REF][START_REF] Chatton | Glacial recharge, salinization and anthropogenic contamination in the coastal aquifers of Recife (Brazil)[END_REF].

Finally, palynological and paleobotanical records argued for the existence of ecological corridors between the Amazonian Forest and the coastal Atlantic Forest of NE Brazil, an area nowadays featuring an arid caatinga phytogeographical domain [START_REF] Oliveira | Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River[END_REF][START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF]Cheng et al., 2013 and references therein). These resulting vegetation changes were partially modeled in climatic simulations by Silva [START_REF] Dias | Mid-Holocene climate of tropical South America: a model-data approach[END_REF].

Nevertheless, two main gaps have been identified in the understanding of the Holocene climatic trends of Nordeste, which still preclude a comprehensive scheme of the SA response to insolation driven patterns. First, more proxy data are still necessary for evaluating the magnitude and impacts of the SAMS variability over the Nordeste precipitation patterns. Second, the possible vegetation changes and their dynamics during this period still need to be clarified, as vegetation-based modeling requires proxies of vegetation impacts on hydroclimatological processes [START_REF] Dias | Mid-Holocene climate of tropical South America: a model-data approach[END_REF]. In addition, although the ecological corridor theory is supported by paleobotanical clues, the hypothesis of a more widespread hydro-ecological connection between Amazonian basin and NE Brazil [START_REF] Oliveira | Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River[END_REF][START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF], which would have strong consequences on rainfall nature and distribution, has yet to be strengthened.

To address this, our work aims to present insights about the potential for water isotope proxies (deuterium, oxygen-18 and deuterium excess referred to d-excess in the following) from 14 C dated groundwater sampled in Nordeste in the perspective of evaluating the vegetation impacts on hydroclimatological processes and reciprocally, the hydrological changes to Holocene ecological variability. Our findings are based on dated paleogroundwater

A C C E P T E D M A N U S C R I P T
isotopic data primarily acquired in the purpose of hydrogeological investigations (2012( -2015 Coqueiral project ; Coqueiral project ;[START_REF] Cary | Origins and mechanisms of groundwater salinization of coastal aquifers in Recife (Brazil)[END_REF][START_REF] Chatton | Assessment of natural dynamics and anthropogenic impacts on residence times in the urban aquifers of Recife (Brazil) using a multi-tracer approach (noble gases[END_REF]2016;[START_REF] Bertrand | Groundwater contamination in coastal urban areas: anthropogenic pressure and natural attenuation processes. Example of Recife (PE State, NE Brazil)[END_REF] but whose the patterns were significantly different from the expected trends, in comparison to Holocene paleogroundwater isotopic records found elsewhere in subtropical conditions. On the basis of these observations, we present new hypotheses regarding Holocene potential paleoecological and paleohydrological trends of the Nordeste.

Site description

The studied system, known as the Recife sedimentary aquifer system (Figure 1), is located in the Pernambuco state in Brazil (8º04'03"S, 34º55'00" W, z mean =5 masl). The studied area is urbanized and contains numerous surficial channels. The upstream area corresponding to the regional substratum, is, as the whole continental Nordeste, an arid to semi-arid area similar to Caatinga or Cerrado systems [START_REF] Powell | Biogeography of C3 and C4 vegetation on South America[END_REF][START_REF] Cheng | Climate change patterns in Amazonia and biodiversity[END_REF], comprised of a low arboreal deciduous scrubland where the vegetation is dormant and leafless for much of the year [START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF]. On the seashore, Atlantic forest and mangrove ecosystems stands are found. The aquifer system lies over the Pernambuco and Paraíba basins, separated by the regional transverse structure of the Pernambuco Lineament (Figure 1). It is composed by the deep systems Cabo and Beberibe of Cretaceous age, reaching a maximum depth of 250 m in the studied area [START_REF] Cary | Origins and mechanisms of groundwater salinization of coastal aquifers in Recife (Brazil)[END_REF][START_REF] Bertrand | Groundwater contamination in coastal urban areas: anthropogenic pressure and natural attenuation processes. Example of Recife (PE State, NE Brazil)[END_REF].

These two systems are overlaid by Tertiary and Quaternary sediments forming the superficial aquifers Boa Viagem and Barreiras. Details on the geological and environmental settings may be found in [START_REF] Cary | Origins and mechanisms of groundwater salinization of coastal aquifers in Recife (Brazil)[END_REF].

The hydrogeological pattern consists in flowlines following the general hydrographic gradient from the western part of the area to the coastline on the east [START_REF] Cary | Origins and mechanisms of groundwater salinization of coastal aquifers in Recife (Brazil)[END_REF]Chatton et al.,2016, Figure 2). The recharge of the modern groundwater mainly occur both vertically from the surface and horizontally from hydraulic connection with the regional substratum. Deep aquifer are mainly fed through to the recharge over the regional substratum, although punctual hydraulic connections occur with the shallow systems next to the seashore due to intensive exploitation pumping. For the deep groundwater systems (Cabo and Beberibe), investigations on recharge temperatures (reported in Table 1) through noble gas (Ar/Ne) analyses suggest a mean recharge temperature around 5 degrees below the current mean air temperature [START_REF] Chatton | Glacial recharge, salinization and anthropogenic contamination in the coastal aquifers of Recife (Brazil)[END_REF]. According to [START_REF] Stute | Cooling of Tropical Brazil (5 degrees C) during the Last Glacial Maximum[END_REF], such

A C C E P T E D M A N U S C R I P T
recharge temperatures are in agreement with a recharge during the Last Glacial Maximum or Early Holocene. These authors found similar recharge temperature for 7-20 Kyrs old groundwater in the Maranhão Basin, Piaui state (7°S, 41,5°W). In contrast, the shallow groundwater system of Recife (Boa Viagem and Barreiras) showed an inferred recharge temperature similar to the current mean annual air temperature of 25°C in the area (WMO, 2013;[START_REF] Chatton | Glacial recharge, salinization and anthropogenic contamination in the coastal aquifers of Recife (Brazil)[END_REF].

Methodology

Significant differences in recharge temperature suggest distinct recharge periods between the deep and the superficial systems [START_REF] Chatton | Glacial recharge, salinization and anthropogenic contamination in the coastal aquifers of Recife (Brazil)[END_REF], and motivated further dating and analyses of stable isotopes described in the rest of this paper. We performed sampling and data acquisition in March 2013 and March 2014, at the end of the rainy period (September-March), in production wells of the 4 main aquifers (deep aquifers Cabo and Beberibe; shallow aquifers Barreiras and Boa Viagem). Surface water was sampled upstream and downstream in the Beberibe River, close to the estuary, in the Capibaribe river, as in the COMPESA (Pernambuco State Water and Sanitation Agency) drinking water treatment plants (Table 1).

Deuterium and oxygen-18 of water were measured on raw water using a Picarro Laser spectrometer L2130i at USP and reported on the usual δ-scale in ‰ relative to V-SMOW standard, according to Eq. 1:

δ sample (‰)=(R sample /R standard )*1000, Eq. 1.
where R is the 2 H/ 1 H and 18 O/ 16 O isotope ratios. Our analytical uncertainty is ±0.1 ‰ for δ 18 O and ± 0.9‰ for δ 2 H.

We post-processed all the measurements according to IAEA and USGS (2013).

For 14 C dating, we collected a 1 L aliquot in high-density plastic bottles and we analyzed the sample by accelerator mass spectrometry at the BRGM (French Geological Service, Orléans, France). We measured δ 13 C TDIC according to the procedure of Kroopnick et al. (1970). We obtained a carbonate precipitation by adding NaOH (pH[12) and BaCl 2 in water samples. A precipitate of BaCO 3 was then obtained and dried. We added phosphoric acid to the dry BaCO 3 sample inside a vacuum line, and we purified and trapped the evolved CO 2 with liquid nitrogen in a glass tube. Then, we measured the isotopic composition of this gaseous CO 2 with a dual inlet mass spectrometer.

A C C E P T E D M A N U S C R I P T

The 14 C activity was measured relative to the modern carbon reference and expressed in % of modern carbon (pMC). The calibration of 14 C age of groundwater is usually subject to various assumptions depending on the studied groundwater system geological characteristics (see e.g., Fontes and Garnier, 1979;Cheng, 1992). These assumptions may imply possible influence of dead carbon through to carbonated matrix dissolution, carbon isotope exchanges between the various C phases along the groundwater flowpath in open and/or closed system (both estimated measuring 13 C and dissolved inorganic carbon speciation), initial activity soil 14 CO 2 which entered the aquifer at the time of recharge. In the context of this study, we hypothesized a very limited influence from carbonated matrix i.e. no 14 C dilution, as the deep aquifers mainly consist in siliciclastic sediments [START_REF] Cary | Origins and mechanisms of groundwater salinization of coastal aquifers in Recife (Brazil)[END_REF][START_REF] Bertrand | Groundwater contamination in coastal urban areas: anthropogenic pressure and natural attenuation processes. Example of Recife (PE State, NE Brazil)[END_REF], and that groundwater flows mostly in closed conditions, similarly to the empirical Vogel´s approach (1970). In such a conceptual framework, the 13 C variability of dissolved inorganic carbon is only attributed to changes in soil 13 CO 2 , depending on the plant type over the recharge area and C speciation (see e.g., Bertrand et al., 2013 and references therein). For this study, considering the suspected changes in vegetation type during the Holocene [START_REF] Powell | Biogeography of C3 and C4 vegetation on South America[END_REF][START_REF] Oliveira | Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River[END_REF], the equilibrium 13 CO 2 variability may be resulting from changes from C4 plants ( 13 C CO2 = -10 to -15‰ vs PDB) to C3 plants (around -25‰ vs PDB) respiration, and therefore has not been used for 14 C age calibration. This does not impact age calculation given that matrix carbonate influence is neglected considering the geological context of the aquifers.

In contrast, we took into account the specific correction for South Hemisphere atmosphere 14 C production SHcal13 (Hogg et al., 2013) through the Calib program [START_REF] Stuiver | CALIB 5.0[END_REF]. The uncertainties ζ calculated according to Scott et al., (2007) and the SHcal13 curve allowed for an evaluation of age ranges with a 2ζ uncertainty (95% confidence; Table 2).

Results and discussion

Isotopic patterns of groundwater in the Recife aquifer system: differences between modern and paleo recharges

The 14 C dating performed for the deep groundwater samples in Recife allowed for a recharge period to be estimated ranging from -4.6 kyrs to -20 kyrs BP, with the greatest proportion corresponding to -6 to -13 Kyrs B.P,

A C C E P T E D M A N U S C R I P T
i.e. largely during the Early-Mid Holocene (Table 2). Conversely, the shallow groundwater systems (Boa Viagem and Barreiras) contain CFC´s and SF6 concentrations suggesting a modern components of their recharge (Supplementary material Table S1). At a first insight, the large range of ages in paleogroundwater highlight their potentiality for Holocene climate reconstruction, although the coarse time resolution still preclude a detailed evaluation of paleoclimatic processes, and that only low frequency processes should be addressed through these data.

Concerning the stable isotopes of water, shallow groundwater plot along a "modern groundwater line"

(MGL; δ 2 H= 4,5δ 18 O +5,7) indicating recharge by modern precipitation, represented by the Local Meteoric Water Line built from 1 year sampling (LMWL; δ 2 H= 7,4δ 18 O +11,1; Figure 3A; Table 3) and the Ceará Mirim (located 300 km northern Recife) MWL built from 3 years monitoring (between 1973 and 1975 through the IAEA/WMO-GNIP network, 2016), followed by secondary evaporation processes. This indicates infiltration of the numerous channels featuring the area [START_REF] Cary | Origins and mechanisms of groundwater salinization of coastal aquifers in Recife (Brazil)[END_REF]. In contrast, the deep groundwater tends to slightly plot above the LMWL and presents richer signatures than modern groundwater, with a trend suggestive of recharge similar to the current Manaus meteoric water line (LMWL; δ 2 H= 8,1δ 18 O + 13,5; [START_REF] Gat | Atmospheric water balance in the Amazon basin: an isotopic evapotranspiration model[END_REF] within the Amazonian basin (03°06′S 60°01′W) (Figure 3A), and argues, in addition, for the absence of significant evaporation of the water entering the deep aquifers.

These differences can be further examined through the analyses of the deuterium excess ("d-excess") [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF]Eq. 2). This parameter is primarily, in atmospheric waters, inversely proportional to the relative humidity (RH) in the evaporation area further providing the moisture leading to cloud formation and rainfall [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF][START_REF] Clark | Environmental Isotopes in Hydrology[END_REF]. d-excess can then be altered by evaporation [START_REF] Rozanski | Deuterium and oxygen-18 in European groundwaters-links to atmospheric circulation in the past[END_REF].

d = δ 2 H -8δ 18 O Eq. 2
In precipitation, the d-excess mean at Recife (March 2013-Feb 2014) is 11.8‰. This value is a bit higher than the range from 10.3 to 11.2‰ observed by [START_REF] Gat | Atmospheric water balance in the Amazon basin: an isotopic evapotranspiration model[END_REF], through to IAEA-GNIP network, for rainy season on the coastal cities of Nordeste around Recife (Fortaleza, Natal, Salvador). This latter is however probably more representative of infiltrating waters over the last decades in the area. Similarly the average of d-excess in precipitation for 3 entire hydrological years (1973)(1974)(1975) at Ceara Mirim is lower with a mean value of 9.8‰.

A C C E P T E D M A N U S C R I P T

In aquifers, d-excess for paleogroundwater is higher (d-excess mean =12,4‰) than for modern groundwater (d-excess mean = 9‰) and recharge (Figure 3B). This is a pattern that contrasts with the usual paleowaters recharged in colder and wetter conditions as reported in many other areas throughout the world, especially in subtropical environments [START_REF] Clark | Environmental Isotopes in Hydrology[END_REF][START_REF] Rozanski | Deuterium and oxygen-18 in European groundwaters-links to atmospheric circulation in the past[END_REF][START_REF] Rozanski | Reconstruction of past climates from stable isotope records of palaeo-precipitation preserved in continental archives[END_REF]. For instance, in North-Africa, Holocene paleogroundwaters present a mean d-excess of about 5‰ lower than modern precipitations, related to higher RH over moisture source (colder conditions) during that period [START_REF] Rozanski | Deuterium and oxygen-18 in European groundwaters-links to atmospheric circulation in the past[END_REF]. This suggests that the dexcess-shift observed between shallow (modern) and deep (Early-Mid Holocene) groundwater in Recife cannot be attributed to the only increase of rainfall amount and insolation/temperature decreases [START_REF] Stute | Cooling of Tropical Brazil (5 degrees C) during the Last Glacial Maximum[END_REF][START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF][START_REF] Chatton | Glacial recharge, salinization and anthropogenic contamination in the coastal aquifers of Recife (Brazil)[END_REF] which should favor depleted δ 18 O, δ 2 H (amount and temperature effects, Clar and Fritz, 1997), higher regional RH and therefore lower d-excess, and suggests that additional processes altering moisture sources occurred between the Early-Mid Holocene and the present day.

These preliminary observations highlight the interest of the groundwater isotopic signatures as a possible complementary tool for paleoclimate studies over the Nordeste region. In this context, a control of the sensitivity of these parameters in comparison to other available and well constrained proxy data is required. In addition the observed d-excess need to be understood in the paleoclimatic context of the area.

Comparison of regional speleothem δ 18 O and 14 C-dated groundwater isotopic fingerprints : rainfall amount and moisture source pattern changes during Holocene

In NE Brazil, it was demonstrated that the main factor affecting the temporal isotopic signatures variability of precipitation is the amount effect [START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF]Baker and Fritz, 2015, and references therein). On this basis, speleothem δ 18 O carbonate which follows the average δ 18 O variability of drip water entering the karstic systems (Rainha, Furna Nova and Abissal caves, located at 05°36' S, 37°44' W at 100 masl and at 100 km distance from the Atlantic coast in the Rio Grande do Norte; Figure 3) was extensively used as a proxy of precipitation amount variation in the area [START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF][START_REF] Cheng | Climate change patterns in Amazonia and biodiversity[END_REF]. This proxy was shown, especially for the Early-Mid Holocene period, to capture shifts in the location of the ITCZ related to austral summer insolation (Figure 4A).

A C C E P T E D M A N U S C R I P T

By assuming isotopic equilibrium between the drip water supplying the speleothem and carbonate at the time of precipitation (using Eq. 3;Böhm et al., 2000;[START_REF] Affek | Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by -clumped isotope‖ thermometry[END_REF]Kluge et al., 2013), we compared δ 18 O H2O inferred from δ 18 O carbonate and the isotopic signatures of our groundwater samples.

δ 18 O carbonate -δ 18 O H2O = 1000lnα carbonates-H2O = 18.03 (1000T -1 ) -32.17 Eq. 3 Calculated δ 18 O H2O range (from -7 to -0,9 ‰ vs VSMOW) is significantly different (usually lower) from the measured δ 18 O groundwater (varying from -1,5 to -1,1 ‰ vs VSMOW) (Figure 4B). These differences may be the result of hydrogeological and/or physical and chemical processes. From a hydrogeological point of view, the more depleted signature of drip water fed by karstic system can be explained by a rapid infiltration of water within large fissures featuring karst in some places, limiting previous isotopic enrichment through to evaporation effect near the surface, which would conversely give increased δ 18 O values in deposited calcite [START_REF] Fairchild | Modification and preservation of environmental signals in speleothems[END_REF][START_REF] Baker | Modern stalagmite δ 18 O: Instrumental calibration and forward modeling[END_REF]. Depending on the geometry of the system and the degree of connection between the surface and the percolation pathway feeding the speleothem, huge δ 18 O H2O differences from a speleothem to another may occur, even in the same network. This leads to high variability of the equilibrated δ 18 O carbonates between speleothem from neighboring or even identical caves [START_REF] Ford | Karst Hydrogeology and Geomorphology[END_REF], and references therein). Consistently, Serefiddin et al.

(2004) pointed out a difference of 5 ‰ for δ 18 O carbonates for speleothems spanning the same periods and from neighboring caves fed by two distinct karstic networks. In addition, beyond the likely different recharge conditions occurring in Recife (featured by crystalline and sedimentary lithologies) isotope ratios in groundwater we measured at our site may be buffered by hydrogeological processes. Groundwater circulation is constrained by potentiometric heads organized as equipotentials in aquifer, resulting in a nested organization of parallel flowlines between which mixing is limited [START_REF] Toth | A theoretical analysis of groundwater flow in small drainage basins[END_REF] : local flow systems occur relatively close to the surface, i.e. from a higher elevation recharge area to a directly adjacent discharge area such as a stream or spring, whereas intermediate and regional flows reach to a greater depth in the aquifer. However, the recharge processes (e.g., mixing in the soil zone) buffer precipitation variability (including δ 18 O), and limits time resolution. Therefore the resulting δ 18 O of the sampled groundwater could show a lower sensitivity than δ 18 O carbonate to high frequency isotopic variability of precipitation.

From a physical and chemical perspective, it is also possible that isotopic equilibrium and carbonate saturation within the drip water supplying the speleothem are only reached after the latest fractions of rain events, usually more depleted than the bulk rain water reaching the groundwater system. In spite of these differences whose specific reasons are out the scope of this paper, a co-variation of δ 18 O carbonate and δ 18 O groundwater is observable for the Early-Mid

A C C E P T E D M A N U S C R I P T

Holocene (Figure 4B). This suggests that dated groundwater could be a possible amount effect proxy in complement to speleothems, although it is buffered and of lower resolution.

In parallel and although the number of dated groundwater data is limited to 13, d-excess appears to be related to the insolation (Figure 4C). The insolation decrease, while associated with increase in precipitation in the area, seems to be followed by a d-excess increase trend, after a time-lag. This trend strengthens the idea, as mentioned previously, that regional moisture origin has changed between the Early-Mid Holocene and today. This can reflect a temporal modification of the RH over a unique moisture source, spatial modifications of moisture origin and/or a higher recycling of local precipitations, providing a kinetically fractionated vapour [START_REF] Salati | Recycling of Water in the Amazon Basin: An Isotopic Study[END_REF][START_REF] Rozanski | Deuterium and oxygen-18 in European groundwaters-links to atmospheric circulation in the past[END_REF][START_REF] Rozanski | Isotopic patterns in modem global precipitation[END_REF][START_REF] Gat | Atmospheric water balance in the Amazon basin: an isotopic evapotranspiration model[END_REF][START_REF] Clark | Environmental Isotopes in Hydrology[END_REF]. We discuss these possibilities below.

Spatio-temporal changes in moisture sources: Paleoecological shifts over Nordeste during the Holocene

The current spatial moisture pattern over northern tropical SA is mainly driven by the SAMS circulation which transports, during austral summer, tropical Atlantic moisture that is largely recycled over the Amazon basin westward toward the Andes. The warm and humid air masses are then deflected southward by the Andes through to the Andean low-level jet, transporting moisture towards southeastern SA [START_REF] Cheng | Climate change patterns in Amazonia and biodiversity[END_REF].

The westward Atlantic-Amazon basin moisture transport trend was firstly assessed by [START_REF] Salati | Recycling of Water in the Amazon Basin: An Isotopic Study[END_REF] and analyzed by [START_REF] Gat | Atmospheric water balance in the Amazon basin: an isotopic evapotranspiration model[END_REF] along a transect across northern Brazil, from Belem to Santarem and Manaus (Figure 3A), yielding an overview of isotopic variability in precipitation. These authors primarily highlighted a shift to more negative δ data values toward inland stands. This is related to the expected continental effect, describing the impoverishment trend in heavy isotopes in precipitation between littoral and continental stands resulting from rainout advancement [START_REF] Clark | Environmental Isotopes in Hydrology[END_REF]. However, the most prominent characteristic of rainfall precipitation signatures was the increase in d-excess from 7,5 ‰ near the coast in Belem to about 13 ‰ at Manaus, as in other inland cities such as Porto Velho, Cuiaba and Brasilia. This spatial pattern was attributed to evaporation from open water bodies and from the large rainforest canopy interception (Figure 5). While transpiration by plants does not lead to isotopic fractionation (Zimmerman et al., 1967), re-evaporation from open water bodies and

A C C E P T E D M A N U S C R I P T

interception leads to the production (F evap ) of a kinetically fractionated vapor (δ evap ). This re-evaporated flux mixes with the long-range moisture transport corresponding, after a progressive Rayleigh's distillation equilibrium fractionation of the Atlantic moisture (F vap1 ; δ vap1 ), to the remaining cloud water (F vap2 ; δ vap2 ) (Figure 5A). Such a mixing leads to an increase of the d-excess in the resulting moisture and precipitation (δ vap3, δ P2 ) (Figure 5B). In the Amazonian basin, open water and canopy interception re-evaporation (F evap ) potentially accounts for 35 % of the total evapotranspiration (F evap + F transp ) [START_REF] Gat | Atmospheric water balance in the Amazon basin: an isotopic evapotranspiration model[END_REF]. Various estimations of the portion of the total evapotranspiration then participating to the local precipitation (F evap_rec + F transp_rec ) were proposed (reviewed in Rocha et al. [2015] and Eltahir and Bras [1994]) and range from 16 % in eastern border of the Amazon basin (Lettau et al., 1979) to 56 % (Marques et al., 1956).

To account for this additional processes, an analytical steady-state evapotranspiration isotopic model was proposed and tested by [START_REF] Gat | Atmospheric water balance in the Amazon basin: an isotopic evapotranspiration model[END_REF]. to evaluate the sensitivity of d to the RH and runoff ratios vs rainout ratio. According to these tests, the 3 ‰ difference between coastal and inland d-excesses would be consistent with a [START_REF] Andrade-Lima | Contribuicão ao estudo do paralelismo da flora amazônico-nordestina [Contribution to the study of the paralelism of the amazonian-Nordeste flora[END_REF][START_REF] Andrade-Lima | Present-day forest refuges in northeastern Brazil[END_REF][START_REF] Rizzini | Delimitação, caracterização e relaçoes da flora silvestre hileiana [Delimitation, characteriation and relationshiphs of the wild hileiana flora[END_REF][START_REF] Oliveira | Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River[END_REF]. These studies show that Tropical forest taxa are well represented throughout the Holocene in Nordeste such as in Icatu River Valley (10°24' S 43°13' W, z= 400 m) which was dominated by gallery forest taxa from 10,060 to 6790 yrs B.P.

Hence, the reported d-excesses in Early-Mid Holocene paleogroundwaters in Recife (Figure 3B and4C)

strengthen the hypothesis of the existence of such a paleoecological shift in NE Brazil during the Early-Mid

Holocene. The rainfall increase coupled with a mean temperature cooling alone does not explain the recycling signature of the Recife´s paleogroundwater,. Our observation rather involves an alternative explanation: the Gat and Matsui scheme (1991) (Figure 5C) could have occurred closer to the Nordeste coast (Figure 5D). Such a hypothesis is strengthened by the time lag of temporal d-excess variation in paleogroundwater compared to insolation and precipitation changes (Figure 4C). This lag could result from the dynamics of ecological processes, i.e. the time required for the rain forest ecosystem settlement, necessary to trigger significant moisture recycling, a process that seems to be optimal after the maximum insolation phase. Our data suggest that such shift could have occurred, in Recife area earlier than e.g., Icatu River valley , as highest d-excess values are found at about 13100 yrs B.P in Recife. These findings need however to be further comfirmed by more detailed investigations.

In this context, it should be noted that although the concept of a more forested Nordeste and the existence of migration routes have been hypothesized for decades, the location of such routes and the timing of forest expansion phases remained unclear [START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF]. Our hypothesis is in agreement with at least one of the hypothesized routes [START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF][START_REF] Cheng | Climate change patterns in Amazonia and biodiversity[END_REF][START_REF] Oliveira | Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River[END_REF], connecting hydro-ecologically the Recife area to the Amazon basin (Figure 5C). Nevertheless, the significant d-excess shift observed in the long-term for the deep groundwater system rather argues for more widespread ecological changes leading to the formation of an extended regional rain forest ecosystem (Figure 5D), as suggested by [START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF]. Such widespread shift provided the conditions to deeply recycle local moisture, portion of which then entered the aquifers.

Concluding remarks on future challenges and use of isotopic proxy data in

Nordeste groundwater

ACCEPTED MANUSCRIPT

A C C E P T E D M A N U S C R I P T

The use of d-excess is already largely implemented in dated ice cores to evaluate past moisture source relative humidity conditions (e.g., [START_REF] Steffensen | Highresolution Greenland Ice Core data show abrupt climate change happens in few years[END_REF] [START_REF] Dee | SPEEDY-IER: A fast atmospheric GCM with water isotope physics[END_REF] and highlight the importance of the knowledge of the ecosystem type to better constrain proportion of evaporation versus transpiration recycling processes through d-excess (Jaseckco et al., 2013;[START_REF] Dee | SPEEDY-IER: A fast atmospheric GCM with water isotope physics[END_REF]. This is yet challenging for present day modeling but can be constrained thanks to observational network (e.g., IAEA/WMO, 2016). For paleoclimatologic studies, such calibration is difficult as d-excess, requiring combined 18 O and 2 H data, is only sparsely available over glacial-interglacial time scales, and mainly in ice cores [START_REF] Werner | Glacial-interglacial changes in H 2 18 O, HDO and deuterium excessresults from the fully coupled ECHAM5/MPI-OM Earth system model[END_REF]. As it is illustrated in this study, paleogroundwater data could provide a valuable help to integrate the dynamic behavior of ecosystem settlements through modifying land/soil modules over large time scales and calibrate these models in mid-latitude areas.

Nevertheless, it should be noted that isotope variability, as evaluated in groundwater, suffers limitations.

Due to the integrative reservoir function of such large aquifers, and although nested organization of flow lines following equipotential downgradients [START_REF] Toth | A theoretical analysis of groundwater flow in small drainage basins[END_REF] rather ensures temporal segregation from shallow to deeper aquifer layers, time resolution can be lower than for other proxies, e.g., speleothems, lake sediment cores [START_REF] Rozanski | Deuterium and oxygen-18 in European groundwaters-links to atmospheric circulation in the past[END_REF][START_REF] Alvarado | Reconstruction of past climate conditions over central Europe from groundwater data[END_REF]Jiráková et al., 2011;Jasecko et al., 2015). In addition, groundwater exploitation through to extensively screened wells (over several meters), such as in Recife may lead to mixing of water of various ages and alter the resolution of the recorded isotopic signatures.

Despite these limitations, however, the presented results illustrate the usefulness of our approach through further strategic sampling. This sampling should be performed on specifically designed low pumping rate wells to limit mixing processes potentially altering isotopic signals and time resolution. Another advantage is that d-excess

A C C E P T E D M A N U S C R I P T
data in groundwater can be simultaneously investigated with recharge temperature inferred from noble gas (e.g., [START_REF] Stute | Cooling of Tropical Brazil (5 degrees C) during the Last Glacial Maximum[END_REF][START_REF] Chatton | Glacial recharge, salinization and anthropogenic contamination in the coastal aquifers of Recife (Brazil)[END_REF] to complete rainfall amount information obtained from e.g., speleothems and obtain a more comprehensive overview of climatological parameters at the moment of the recharge. In addition, It should be noted as well that the potential timescale for paleoclimate reconstruction in the Recife area is much higher in the south of the city than in the zone investigated for the present study. The depth of the Pernambuco basin Piedade´s Graben much less exploited for groundwater pumping, is deeper than 3000 m [START_REF] Cary | Origins and mechanisms of groundwater salinization of coastal aquifers in Recife (Brazil)[END_REF], and therefore has the potential to contain older paleowaters, especially next to the seashore according to the regional general flowpath (Figure 2). Consequently, both better adapted sampling and investigations at greater depths should provide a valuable support for our findings regarding paleoclimatological patterns in Nordeste and NE South-America, especially by tracing the recycling effects in regional moisture patterns. This should help to reciprocally diagnose a more detailed spatio-temporal typology of Holocene ecological changes, which appear, thanks to the reported new isotopic proxies, to be likely much more widespread than narrow ecological corridors between Amazonian basin and the Atlantic forest. are reported as well as The Global Meteoric Water line [START_REF] Craig | Isotopic variations in meteoric waters[END_REF], the Local Meteoric Water Line performed during the hydrogeological study [START_REF] Cary | Origins and mechanisms of groundwater salinization of coastal aquifers in Recife (Brazil)[END_REF] and the Manaus water line reported by [START_REF] Gat | Atmospheric water balance in the Amazon basin: an isotopic evapotranspiration model[END_REF].

Modern Groundwater Line corresponds to the shallow groundwater affected by kinetically evaporated source which are attributed to the local channel infiltration. Note that the deep groundwater plot above the LMWL. B. Deuterium excess d vs well screening depth.

Figure 4: Comparison of δ 18 O trends between Rio Grande do Norte cave calcite [START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF] [START_REF] Coplen | Discontinuance of SMOW and PDB[END_REF]. C. Corresponding dexcess for the 14 C dated Recife´s deep groundwater. Note that although the number of data is limited and that an outlier is identified, possibly linked with high frequency and temporary ecological shifts [START_REF] Oliveira | Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River[END_REF], dexcess seems to reversely follow the insolation pattern with a time-lag. More data would be necessary to confirm this general trend. H1 refers to the Heinrich event 1 characterized by depleted δ 18 O in the Rio Grande do Norte cave calcite.

Figure 5 A. Conceptual scheme of moisture and isotopic patterns for northern tropical SA, and potentially occurring during early-Mid Holocene period (modified after [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF][START_REF] Gat | Atmospheric water balance in the Amazon basin: an isotopic evapotranspiration model[END_REF]. from [START_REF] Cruz | Orbitally driven east-west antiphasing of South American precipitation[END_REF][START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF]. In a context of increased rainfall due to ITCZ location shift, the setting of transitional ecosystems (firstly hypothesized as narrow corridors represented by lime green arrows, but possibly widespread according to [START_REF] Auler | Quaternary ecological and geomorphic changes associated with rainfall events in presently semiarid northeastern Brazil[END_REF]) between Amazonian Forest and Atlantic Forest, favoring open water bodies and interception evaporation, could have promoted recycling of water, what leads to a modified meteoric water line and higher d-excess. Symbols: ε: equilibrium enrichment factor; F: fraction of contribution from evaporation or transpiration to the clouds. f: remaining fraction of water within the clouds according to the Rayleigh distillation model (cf. [START_REF] Clark | Environmental Isotopes in Hydrology[END_REF].

A C C E P T E D M A N U S C R I P T

  recycling of half the precipitation, of which 20 to 40 % would be from open water bodies in an environment featured by a mean RH of 75 % In this context, the d-excess differences between deep paleogroundwaters and modern recharge and groundwater in Recife could be explained either by the past influence of a distant moisture source similar to the present central Amazon basin, or by a local change in ecosystem type. Indeed, a tropical rain forest like in the Amazon basin does not exist in Nordeste nowadays, this area being arid to semi-arid. Forest stands are limited, and exist only from the seashore to the coastal Atlantic forest. The only relatively large water bodies are located along the seashore and consist in estuary and mangroves, i.e. presenting similar isotopic features as seawater. Nevertheless, Silva-Dias et al. (2009) suggest, through modeling approaches that Early-Mid Holocene rainfall increase in Nordeste and related processes such as ITCZ southward shift and SACZ northward displacement, could be accompanied by a positive vegetation feedback, implying a potential role for paleoecological conditions at the continental scale (Figure 5C;D). Consistently, the paleobotanical and palynological investigations in Nordeste for the Early-Mid-Holocene argued for the existence of transitional ecological corridors between the present Amazonian rain forest and the Coastal Atlantic Forest. This hypothesis was first suggested by floristic disjunctions revealed by studies of humid A C C E P T E D M A N U S C R I P T forests enclaves in high elevations within the semi-arid region of the caatinga phytogeographical domain (Andrade-

Figure 1 :

 1 Figure 1

Figure 2 :

 2 Figure 2: Conceptual sketch of the aquifer systems.

Figure 3 :

 3 Figure 3: A.δ 18 O vsδ 2 H diagram. Shallow (Boa Viagem/Barreiras) and deep (Cabo/Beberibe) groundwater samples

  α represent fractionation factor between vapor and liquid. f represent the remaining fraction of the initial content of vapor in the air-mass (Rayleigh distillation model) B. Same processes represented in the δ 18 O vs δ 2 H domain. C and D. Conceptual schemes of (paleo)climatological and ecological process during Late and Early-Mid Holocene (adapted

  and/or location changes due to modification in atmospheric circulations (e.g.[START_REF] Masson-Delmotte | GRIP deuterium excess reveals rapid and orbitalscale changes in Greenland moisture origin[END_REF] and is increasingly monitored in global precipitation and groundwater to validate Global Climate Model simulations(Pfhal and Sodemann, 2014). Our findings should encourage new validation exercises for isotope-enabled GCMs or Earth System Models of Intermediate Complexity (EMICs). Specifically, the modules related to soil-atmosphere interactions could be better constrained by integrating changes in recycling processes from continental surface water evaporation, which seem of great importance in intertropical zones like in Nordeste. Usually, incorporation of water isotopes in GCMs or EMICS (see the Stable Water Isotope Intercomparison Group projects, e.g.,Kurita et al. [2011];[START_REF] Risi | Water-stable isotopes in the LMDZ4 general circulation model: Model evaluation for present-day and past climates and applications to climatic interpretations of tropical isotopic records[END_REF]) implies to incorporate soil hydrology and atmospheric postcondensation exchanges in the model physics. Such efforts are currently made (e.g.,

  and Recife dated deep groundwaters (Cabo/Beberibe). A. Original Cruz et al.´s δ 18 O speleothem records in Rio Grande do Norte (note that δ 18 O are expressed vs PDB). B. Calculated δ 18 O of drip water assuming an isotopic equilibrium between carbonate and drip water (using equation reported in Kluge et al. [2013] and assuming an equilibrium temperature of 25°C between 0 and 12,5 kyrs B.P (corresponding to the mean annual temperature in Rio Grande do Norte; Cruz et

	al., 2009). and of 20°C between 12,5 and 26 kyrs accordingly with the 5°C difference between these two period
	suggested by Stute et al. (1995). Comparison with 14 C dated Recife´s deep groundwater (note that all δ 18 O are
	expressed vs SMOW; δ 18 O(vs VSMOW)= 1.03092[δ 18 O (VPDB)] + 30.92;

Table 2

 2

	Well	O 2	Con duct ivity	pH T°C Ca	Mg	Na	K	Cl	HC O3	SO4 NO3 NO2	δ 13 C (TDI C) (‰)	Calc ulat ed δ 13 C (eqC O2) (‰)	a_C 14 (pm C)	σ a_C 14	lowe r rang e age C14 cal age (Kyr s BP)	upp er rang e C14 cal age (Kyr s BP)
	BEB-057	2. 2	824	5.9 31.3	41.3 3	28.6 2	57.3 3	24.5 3	204. 58	35.2 8	32.5 2	0.17 0.08 -19.7 -21.0	30.3 0	0.19	10.6 8	11.1 4
	BEB-059	2. 43	945 6.09 29.7	19.9 7	25.3 4	115. 47	30.8 2	210. 93	133. 77	11.4 1	1.03 0.09 -7.2 -9.3	59.1 0	0.21 4.57 4.85
	BEB-062	3. 03	239 5.86 32.8 3.94 3.60	19.6 6	11.5 1	28.7 2	35.2 8	11.4 3	0.16 0.01 -15.6 -16.7	24.4 0	0.15	13.0 5	13.2 8
	CAB-003	1. 01	262 6.25 31.3 3.39 3.31	43.6 4	6.74	23.4 9	91.1 5	8.22 0.18 0.02 -18.8 -14.0	12.8 0	0.11	19.6 2	20.1 0
	CAB-005	1. 06	5063 6.48 30.4	130. 55	119. 38	589. 44	23.6 6	1402 .67	111. 73	170. 89	0.77 0.11 -14.9 -21.6	27.9 0	0.10	11.7 2	12.0 5
	CAB-012	0. 6	224. 3	6	30.5 1.48 1.67	34.8 0	7.83	17.6 7	49.9 9	15.6 4	0.28 0.03 -11.4 -18.8	34.4 0	0.10 9.47 9.55
	CAB-017	1. 1	2560 6.1 30.8	60.4 7	55.3 4	298. 50	20.1 8	724. 13	41.1 6	88.5 0	0.58 0.06 -19.5 -13.1	28.9 0	0.22 7.33 7.57
	CAB-024	2. 68	950 5.99 31.2	25.7 1	22.7 1	120. 43	19.8 4	237. 65	47.7 8	41.7 7	0.33 1.10 -17.7 -21.6	44.0 0	0.15	10.2 4	10.5 2
	CAB-027	1. 09	237	6.3 32.1 3.70 3.47	39.5 0	6.20	19.5 7	77.9 2	10.3 3	0.15 0.03 -17.1 -19.3	19.3 0	0.10	15.6 2	16.0 5
	CAB-036	1. 25	818	6.2	30	26.5 4	22.5 4	84.5 2	16.5 8	186. 14	47.0 4	28.9 1	0.56 0.06 -11.4 -20.1	26.0 0	0.10	12.6 6	12.7 5
	TTD94 n.a 2191 6.9 30.5	47.3 0	61.5 0	323. 00	13.3 0	537. 10	70.8 0	169. 10	n.a	n.a -19.5 -25.1	28.9 0	0.10	10.7 0	11.0 9
	TTD11 0	n.a 1945 6.6 30.6	47.3 0	56.4 0	281. 20	13.3 0	468. 00	65.3 0	142. 20	n.a	n.a -18.1 -22.6	20.9 0	0.10	14.3 9	15.1 1
	ALM7 3	n.a 846	6.8 30.2	44.1 0	39.9 0	164. 80	21.5 0	0.00	308. 00	2.90 n.a	n.a -10.1 -15.4	46.2 0	0.30 6.89 7.25

Table captions Table 1 :

 captions1 Groundwater data: Stable isotopes of water, d-excess and recharge temperature

Table 2 :

 2 Geochemical and isotopic data for the dated paleogroundwater and calculated 14 C age ranges.

Table 3 :

 3 Stable isotopes data of precipitations in Recife(Monthly sampling, 2013(Monthly sampling, -2014 period) period). Comparison with the average of regional IAEA stations (IAEA/WMO, 2016). The continental effect on isotopic signatures appears to be mostly true all the year long. Highlights δ 18 O, δ 2 H and d-excess are analyzed in NE Brazil modern and Mid Holocene groundwater They are compared to speleothem δ 18 O rainfall amount proxy, available for this period d-excess shows that rainfall increase was accompanied with moisture recycling Recycling argues for rain forest settlement during Early-Mid Holocene d-excess seems then to be a promising tool for paleoecological studies of the area.

	ACCEPTED MANUSCRIPT

Acknowledgements

This communication was possible thanks to a former study focusing on the Recife area hydrogeological functioning, part of the French-Brazilian COQUEIRAL project, financed by ANR CEP&S (ANR-11-CEPL-012) / FACEPE (APQ-0077-3.07/11) / FAPESP (2011/50553-0). It is accredited by the French competitiveness cluster DREAM. Consortium: BRGM, CeRIES Lille 3 University, CAREN Rennes 1 University, GEOHYD, UFPE, USP, APAC, CPRM, INPE. The authors thank local authorities and owners for allowing access to wells and all colleagues and students involved in the sampling campaigns. G. Bertrand thanks O. Samperez, M. Bertrand, D. Bertrand for the material support during the elaboration of the manuscript. The authors greatly thank the two anonymous reviewers and the editor which did important remarks and provided very detailed comments for the improvement of the manuscript, and A. Merayan for the language checking.

A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T

Hogg H.G., Hua Q., Blackwell P.G. Niu M., Buck C.E., Guilderson T.P. Heaton T.J.., Palmer5 J.G. Reimer P.J., Reimer R.W., Turney C.S.M., Zimmerman S.R.H., 2013. SHcal13 Southern Hemisphere Calibration, 0-50,000 Years Cal BP. Radiocarbon 55(4), 1889-1903. IAEA/USGS, 2013. A Laboratory information management system for stable hydrogen and oxygen isotopes in water samples by laser absorption spectroscopy: user manual and tutorial. Revision v 1.5. IAEA/WMO, 2016. Global network of isotopes in precipitation: The GNIP database. Available via http://wwwnaweb.iaea.org/napc/ih/IHS_resources_gnip.html Jasechko S., Sharp Z.D., Gibson J.J., Birks S.J., Yi Y.,


Olivier Barais * - †

Johann Bourcier
email: johann.bourcier@irisa.fr

Yérom-David Bromberg
email: david.bromberg@irisa.fr

Christophe Dion
email: christophe.dion@b-com.com

Towards microservices architecture to transcode videos in the large at low costs

The increasing popularity of videos over Internet, combined with the wide heterogeneity of various kinds of end users' devices, imposes strong requirements on the underlying infrastructure and computing resources to meet the users expectations. In particular, designing an adequate transcoding workflow in the cloud to stream videos at large scale is: (i) costly, and (ii) complex. By inheriting key concepts from the software engineering domain, such as separation of concerns and microservice architecture style, we are giving our experience feedbacks of building both a low cost and efficient transcoding platform over an ad hoc computing cloud built around a rack of Raspberry Pis.

I. INTRODUCTION

In the last decades, we have witnessed a spectacular rise in popularity of video streaming over Internet. Industrials, such as Cisco or Ericsson, predict that video traffic will dominate other types of traffic by 2019, and will reach an astonishing 80% of the world's total internet traffic. The concomitant raising popularity of Twich, YouTube and Netflix, correlated with the emergence of various kinds of mobile devices, drives the massive increase in video consumption. As a result, we live in a video-oriented world. Such a phenomenon changes both our video consumption experience and habits. Video streaming is becoming a key benefit for any digital business. However, delivering video content to users imposes heavy constraints on the underlying infrastructure and computing resources to meet the needs and the end users' expectations. In fact, streaming videos to consumers' devices, requires to encode each video in various formats to meet devices' specificities. In practice, it is not realistic, nor possible to store videos in all possible formats at the server side. To alleviate this burden, HLS and MPEG-DASH, gain momentum to become the de facto standards to deliver a dynamic adaptive video streaming [START_REF] Grafl | Scalable video coding guidelines and performance evaluations for adaptive media delivery of high definition content[END_REF], [START_REF] Sodagar | The MPEG-DASH standard for multimedia streaming over the internet[END_REF]. Videos are splited into a sequence of segments, made available at a number of different bitrates (i.e. at different quality levels), so that clients can automatically download the adequate next segment to be played, based on both their characteristics and their network conditions. However, still, video transcoding is cumbersome: it requires a massive amount of server side resources, and thus an infrastructure that scales adequately.

The emergence of cloud computing, combined with adaptive bitrate delivery, paves the way for new solutions that scale better. The fact that segments from the splited video are independent from each other enables to setup various patterns to spread over a set of servers the segments to be transcoded. Hence, many research works have been done to provide, as much as possible, the most adequate algorithms, from different optimization strategies, to schedule tasks, to transcode a sequence of segments, on a group of interconnected servers [START_REF] Guo | Load balancing in a cluster-based web server for multimedia applications[END_REF], [START_REF] Ashraf | Cost-efficient virtual machine provisioning for multi-tier web applications and video transcoding[END_REF], [START_REF] Boutin | Apollo: Scalable and coordinated scheduling for cloudscale computing[END_REF]. As a result, according to the transcoding workflow, algorithms optimize either CPUs consumption, network bandwidth consumption, transcoding time, customers' quality of experience or a mix of these aforementioned properties though the use of heuristics or empirical studies. Most of the existing solutions (from either academia or industry such as Netflix), leverage on Infrastructures as a Service (IaaS) clouds, such as Amazon Elastic Compute Cloud (EC2), to provision and instantiate on the fly the adequate numbers of Virtual Machines (VMs), dedicated to the transcoding of video segments depending on the incoming load. From this context, whatever the chosen scheduling algorithms, it appears clearly that streaming video in the large comes with an inherent significant cost to build a video transcoding cloud computing platform whatever the considered scaling methodology: vertical scaling to increase/decrease instance capacity, and/or horizontal scaling to add/remove instances [START_REF] Tak | Cloudy with a chance of cost savings[END_REF], [START_REF] Walker | The real cost of a cpu hour[END_REF].

In this paper, we provide feedback experiences about the use of an ad hoc cloud computing platform built from a farm of Raspberry Pis to promote low cost but efficient video transcoding. The inherent resources constraints of Raspberry Pis give us the opportunity to revisit how to design and implement the commonly used transcoding workflows. First, using VMs to scale video transcoding is not the most adequate approach as it requires the use of hypervisors, or virtual machine monitors (VMMs), to virtualize hardware resources. VMMs are well known to be heavyweight with both boot and run time overheads [START_REF] Harter | Slacker: Fast distribution with lazy docker containers[END_REF], [START_REF] Zhu | Twinkle: A fast resource provisioning mechanism for internet services[END_REF] that may have a stronger impact on small bare metal servers such as Raspberry Pis. Consequently, we promote the use of containers, such as Docker, as a lightweight alternative, that startup as fast as normal processes. Further, the transcoding workflow involves complex interleaved operations such as splitting, merging, scheduling, storing, transcoding, and streaming. To cope with this inherent complexity while fostering and increasing evolutivity, maintainability, scalability, we are designing our approach arounds the best practices in software engineering, learnt, in particular, from the experience of microservices architectural style 1 . As a result, we apply a separation of concerns to isolate each operations into a graph of microservices isolated into lightweight containers. Since, microservices have the inherent ability to be isolated from each other, we are able to spawn as much as expected containers to provide an agile development and continuous delivery. Consequently, our approach enables us to experiment in an easy manner how different transcoding workflow may scale depending on the deployed microservice architecture, and the underlying strategies to split, merge, schedule, store, and transcode video streams.

Our contributions are as following:

• We have designed an ad hoc cloud computing platform built around a rack of 16 interconnected Raspberry Pis. The remainder of this paper is structured as follows. Section II provides a general overview of the proposed approach. Section III details the technologies that are integrated to implement this approach. Section IV evaluates the proposed approach regarding the performances and the complexity of the solution. Section V discusses a research roadmap and concludes this experiment.

II. APPROACH

Each operation involved in our transcoding workflow is instantiated as a microservice that is deployed on a lightweight container (See Figure 1). Further, a microservice can be spawned several times across the cloud, i.e on different machine for either reliability and/or scalability. For instance, a splitter microservice splits a video files into a set of chunks; the latter are then forwarded to a scheduler microservice to distribute the chunks among transcoder microservices spawned multiple times across the cloud. The aim of the scheduler microservice is to perform an accurate load balancing of transcoding jobs among the multiple instances of transcoder microservices. There exists a huge set of different strategies to schedule jobs dedicated or not to multimedia streaming [START_REF] Guo | Load balancing in a cluster-based web server for multimedia applications[END_REF].

Providing a new kind of scheduler is not currently, in this paper, our key concern. However, in our approach, as each operation is isolated from each other, we can change in a smooth manner the scheduling strategy.

Currently, our default scheduler microservice implements an enhanced round robin strategy, also named first fit [START_REF] Guo | Load balancing in a cluster-based web server for multimedia applications[END_REF]. According to the incoming chunks, the scheduler selects the first and/or the most available transcoder microservices to dispatch the chunk. In fact, each transcoder microservice has a FIFO buffer to store incoming chunks. Chunks are poped one by one by the transcoder to be transcoded. If no transcoders are available, the cloud is then in a saturated state, and the scheduler microservice will not dispatch any new chunks until one of the transcoder's buffer is drained. The scheduler microservice is aware of the availability state of the different transcoders through the storage microservice that acts as a distributed key/value storage, which uses a gossip protocol to propagate, for instance, updates of availability states among microservices of the cloud. Further, the scheduler microservice may interact with a monitor microservice to get resources usage such as CPU, RAM and network statistics, related to each transcoder, to refine its load balancing policy. Once a transcoding job is finished, the merger microservice is notified, and then reassembles incrementally the encoded chunks that are finally streamed from the streamer microservice.

We have setup an automated deployment process. From a bench of tools and scripts, all the aforementioned microservices are continuously and smoothly deployed onto our ad hoc cloud computing platform (See Figure 1 ,). Microservices are thus automatically instantiated in lightweight containers on the underlaying 16 bare metal servers (Figure 1 ) without requiring any specific configuration efforts. Consequently, as soon as a user sends a request to get a specific movie from the cloud (Figure 1 ), the transcoding workflow is started.

Currently, we are not doing any caching as our main objective is to evaluate the on the fly transcoding feature of our approach. However, adding a caching mechanism is as easy as adding a new caching microservice in our architecture.

Finally, our approach may be seen as a particular implementation of a map/reduce model [START_REF] Garcia | A study of transcoding on cloud environments for video content delivery[END_REF]. In fact, the splitter microservice acts as a preliminary phase to feed the available mappers. The map operation consists then in transcoding chunks, just as the transcoder microservices do. Finally, the reduce operation is similar to the merge microservices jobs. However, our approach enables a greater flexibility in the workflow complexity as promoted by the microservice architectural style.

III. IMPLEMENTATION

Our implementation relies on lightweight container technologies and low cost hardware, namely Docker and Raspberry Pi 2 (RPi). We have setup an ad doc cloud computing platform to take benefits from a farm of RPi. One key characteristics of our specific cloud is to enable us to both deploy and run containers in a smooth and easy manner whatever the number of RPi involved. Moreover, we have built a dedicated workflow that is able to take benefits from the underlying cloud to transcode videos in a distributed manner. To reach this aim, we have containerized various video transcoding software commonly used. Finally, we have

A. Setting up the ad hoc cloud computing platform

Our cloud is in charge of scheduling, deploying and executing all tasks relative to video transcoding in a distributed way. The key challenge is to optimize the cloud in such a way that its global resource usage is uniformly saturated. The following tools are used to setup our clouds.

Docker: In this work, Docker2 is used as our underlying lightweight container system. It provides us a common way to package and deploy microservices. Particularly, it provides a high level API to manage hardware resource allocated to each microservice. It enables us to place and isolate each process on a specific core. We leverage this particular feature to indicate placement constraint for each task. For instance, the hardware encoding task must run on a GPU, and not on one core of a classical CPU.

Docker Swarm: Each microservice is containerized; it is encapsulated inside a docker image ready to be deployed and run on top of our Raspberry Pi farm. To deploy microservices, Docker Swarm3 is used as it provides native clustering facilities. Its related API, for managing deployment of containers, enables us to control how to schedule the deployment of microservices over the pool of Raspberry Pis. In particular, Docker Swarm may follow different strategies to schedule the deployment of docker images on docker hosts: (i) random, which randomly allocates a host, (ii) spread, which chooses the host containing the minimum number of containers, and (iii) binpack which places as much container as possible on the minimum number of host. In addition, Docker Swarm uses filter to select eligible hosts for running a specific docker image. In our context, we use the spread strategy to choose a specific host, and we use filters to put constraint on the devices capable of hosting specific transcoding task.

Go-docker: One key shortcoming of Docker Swarm is that it does not enable to express precedence among tasks. To overcome this issue, Go-Docker4 is used to schedule all tasks over the Docker Swarm, while respecting precedence constraints and optimizing resource usage. Go-Docker can be seen as a batch cluster management tool. It uses inherently Docker Swarm to actually dispatch docker images on remote nodes. Hence, all the tasks (such as chunk copy, chunk software encoding, chunck hardware encoding, merge, . . . ) that have to be executed with precedence and location constraints are managed by Go-Docker.

Consul: Consul serves a triple purpose. First, it acts as a service discovery for the Docker Swarm cluster itself. Second, it provides failure detection, service registration and service discovery functionality for all containers launched on the cluster. Thirdly, it provides a flexible distributed key/value storage service for dynamic configuration, feature flagging, coordination, leader election. For instance, in our scenario, Consul allows us to know when a task is over.

B. Video transcoding workflow

Our video transcoding workflow (depicted in Figure 2) is composed of several tasks:

1) a splitter to split the video in several small video chunk.

2) a chunk transfer task to transfer each chunk to a set of targeted host 3) a scheduler takes the decision to start the transcoding process on a specific node based on runtime information. 4) a video encoding task with two different implementations: one for software encoding and another one for hardware encoding. 5) an encoded chunk transfer to transfer the encoded chunk back. 6) a merger task which gather all encoded video chunk and assemble them incrementally. 7) a streamer task which takes the output of the merger task to stream the newly encoded video. The RPi has an under-powered CPU but a powerful GPU. Consequently for encoding the videos chunk, we uses both in parallel: the software encoding on top of the CPU and the hardware encoding on top of the GPU. Consequently, we rely on the following software libraries that are packages as docker containers to implement all the above mentioned tasks: a) FFmpeg: FFmpeg is a project that groups libraries and programs for handling multimedia data. FFmpeg includes h264 software encoder. We use it to enable chunk software Fig. 2: Transcoding flow chart encoding on available core of the RPi cluster. We also use it for splitting and merging videos chunks.

b) MKVToolNix: MKVToolNix is a set of tools to create, alter and inspect Matroska files. We use it to split (create chunks) and merge Matroska videos files (From our experiment, FFmpeg does not correctly split and merge Matroska videos files).

c) OpenMAX: The RPi has a very under-powered CPU but a powerful GPU. Consequently, it is interesting to use also the GPU for encoding some videos chunks. To access to the GPU, it exists an open standard (OpenMAX) that are designed for low capability devices. Broadcom have implemented one of the specifications (OpenMAX IL), and their chip is used in the RPi. Even if OpenMAX is an extremely difficult API to work with, it exists a GStreamer module that provides an implementation of an h264 encoder. d) GStreamer: GStreamer is a multimedia library for constructing graphs of media-handling components. It supports range from simple Ogg/Vorbis playback, audio/video streaming to complex audio (mixing) and video (non-linear editing) processing. In our experiment, Gstreamer is used to manage the h264 hardware encoding based on OpenMax librairies. e) UDPCast: . For spreading the videos chunks on all the RPi, we use a multicast file-transfer tools nammed udpcast. UDPcast sends data simultaneously to many destinations on a LAN. In out case, we send each chunk to p nodes where p is upper than 1 and lower than n (the number of RPi) in order to provide fault tolorance and improve the freedom where we could place the encoding task.

Based on this technical stack, our workflow is as follow. We use M KV T oolN ix or F F mpeg to create chunks. Then, we use udpcast to spread a first set of chunks to the nodes and we send all tasks to execute to go-docker that will decide when the task can be started. Next, we send a first set of task to encode chunk. We use in parallel 3 core of each RPi with F F mpeg to use a software encoder, and gstreamer to use in parallel an hardware encoder for encoding another chunk. During this first, set of chunk encoding, we send the next set of chunk to the nodes. When a chunk is encoded, we deploy a task to copy back the encoded chunk to the master nodes. When all the chunks are encoded and copied back to the master nodes, a last task performs the merging using M KV T oolN ix. During all the process, go-docker is used to batch the tasks.

C. Analysing performance

In the context of this work, we also use a couple of tools to understand and optimize the performance of our distributed video transcoding infrastructure. We therefore used the 3 following tools.

cAdvisor: cAdvisor (Container Advisor) provides container users an understanding of the resource usage and performance characteristics of their running containers. It is a running daemon that collects, aggregates, processes, and exports information about running containers. Specifically, for each container it keeps resource isolation parameters, historical resource usage, histograms of complete historical resource usage and network statistics. CAdvisor allows us to precisely monitor the current execution of our workflow. In this work, we used cAdvisor to improve and optimize the performance of our scheduler.

InfluxDB: InfluxDB is a distributed time series database. cAdvisor only displays realtime information and doesn't store the metrics. We need to store the monitoring information which cAdvisor provides in order to display a time range other than realtime.

Grafana Metrics Dashboard:

The Grafana Dashboard enables to pull all the pieces together visually. InfluxDB and Grafana are used for post mortem analysis of our workflow execution.

IV. EVALUATION

The goal of this evaluation section is to provide feedback experiences about the use of an ad hoc cloud computing platform built from a farm of Raspberry Pis to promote low cost but efficient video transcoding. To this end, we have evaluated three main axis regarding our implementation and deployment: • the cost of our solution, both regarding financial investment and energy consumption. • the intrinsic encoding performance of our specific deployment setup.

• the development effort required to setup such platform for video transcoding.

A. Cost of the solution

Our specific experimental setup is composed of a farm of 16 Raspberry Pis 2, a 24 ports Gigabit switch, a set of ethernet cables and the corresponding power cables to provide the required energy. This setup is shown on the figure 3. Our experimental setup offers a computing infrastructure of 16 GPU and 64 ARM cores. As shown in Table I, the hardware cost of our small cluster is 776$. This cost is comparable to the price of a working station configured with a state of the art CPU : an intel I7 CPU.

To evaluate the power consumption of our cluster, we have measured the consumption of each hardware device separately using a smart plug which monitor energy consumption. Each raspberry Pi in our setup consumes roughly 4W during the experiment. The switch which connects all raspberry pi together consumes itself 15 watts during the experiment. In total, our farm of 16 Raspberry Pi with the switch consumes about 80 Watts when used at full power. This power consumption is comparable to the power consumption of an i7 5775C which consumes on average 99 watts when performing a x264 encoding task 5 .

Thus the price and the power consumption of our small scale cluster of raspberry pis is comparable to a typical workstation 5 taken from the hardware test conducted here http://techreport.com/review/28751/intel-core-i7-6700k-skylake-processorreviewed/5 Fig. 3: Our experimental setup using a high performance processor with a comparable amount of memory.

B. Encoding Performance

In this section, we compare the performance of encoding a video both using a workstation (featuring an Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz, 16 Gb of memory, running on Linux Ubuntu) and our cluster of 16 Raspberry Pi 2. Our experiment consists in encoding a H264 video file into another H264 video file with the "High profile". The input video and output video have a resolution of 1280*688 with 24 images/seconds each. In this section, each experiment was repeated 10 times, and we present average value of these 10 runs.

Our first experiment compares the time needed to encode a small video chunk of 2 min and 30 seconds both on the workstation and a single raspberry pi 2 to setup a base line comparison. On the raspberry pi 2, we performed two different experiments. The first one performs software encoding leveraging the 4 available cores on the ARM processor. This experiment is done using the ffmpeg software. The second experiment performs hardware encoding leveraging the GPU available on the raspberry Pi. This experiment is done using the Gstreamer omx software. Table II presents the result of this experiment. These results shows that using software encoding the Raspberry Pi2 takes 12 times longer than the workstation used in our experiment, while the hardware encoding takes 4.3 times longer. Our second experiment evaluate the performance of the full farm of raspberry Pi2 against the workstation. In this experiment, we leverage the full power of the raspberry pi 2 cluster, thus using both hardware and software encoding in parallel. This experiment uses exactly the same setup as the first experiment, but the input video is longer : 25 minutes. Tables III shows the results of this experiment. These results show that our cluster of Raspberry pi 2 outperforms the workstation by achieving this task 2.4 times quicker. The results shown in this section highlights the very good performance of our approach to provide a low cost, low power efficient video encoding infrastructure. Indeed, the farm of 16 raspberry Pi 2 is 2.4 times quicker for encoding a video than the same task on our comparison workstation.

C. Setup complexity

To complement our performance, cost and energy evaluation, we have evaluated the development effort which was required to setup such an infrastructure.

The first part was to setup the infrastructure on each raspberry Pi and the docker swarm. Therefore all the following commands have to be repeated on each raspberry (16 times for our setup): 1 command to flash the system, 1 command to copy ssh public key, 1 command to upgrade , 1 command to copy the configuration file for the cluster (cluster-lab configuration file), 1 command to change the docker configuration to provide privileged mode for docker (require to access to the GPU from the docker), 1 command to restart docker daemon and the cluster lab daemon.

After setting up the docker swarm, we had to prepare all docker images for each task of our video encoding infrastructure. Several tools used in our setup are mainstream and therefore docker images was available: consul, cAdvisor 6 and dockerui. Some images that were needed for our experiment were not available. We have thus created the docker image for go-docker and a template image for all our video encoding tasks. Finally, we completed our experimental with a shell script which discusses with Go-docker to run the batch of command. This shell script comprises 28 line of codes7 .

V. CONLUSION AND RESEARCH ROADMAP

We have designed our experiment to setup a video encoding infrastructure on a farm of low cost, low energy devices, through a bunch of open source, off the shelf software libraries. We show that the use of a micro-services approach ease the design of an ad-hoc map-reduce transcoding process that can leverage the computation power of low-cost board like RPi with good performance results. However, we have noticed several lacks, or limitations of current off the shelf tooling to simplify the deployment and optimize the operations of this video encoding infrastructure. This section therefore identifies opportunities for improving state of the art software components.

A. Tooling for non-expert

Our long term goal is to offer a framework which leverages existing tooling to easily deploy a specific video encoding process on a farm of low cost, low power computing devices. To reach this goal, we notice the lack of suitable tooling for non-expert to design the video encoding process and how this process can be automatically parallelized and deployed on a distributed system.

We also noticed a lack of specific tooling for non-expert to add specific treatment on demand during the video encoding process. For example, one may want to add a specific indexation mechanism during the video encoding process, or incrust a second video in the first one depending on user request. These specific examples show the need for an architecture which can automatically plug third parties software working on the video encoding.

B. ensure correctness of the resulting video

When automatically designing and deploying such a video encoding infrastructure, it is really important to perform test to ensure that the resulting video conforms to the expectations. We notice a lack of ready to use, off the shelf software libraries to automatically verify that the resulting video is conform to the user expectation.

C. Optimization of the distributed video encoding system

We noticed several opportunities for improving the performance of our distributed system to saturate the resource usage of each computing device involved in our cluster. On one hand, it would be beneficial to predict the duration for encoding each video chunk in order to optimize placement of each video task and chunk data on RPi. On the other hand, it would also be beneficial to be able to start video encoding before the first chunk is entirely delivered to the first computing device.

5 MonitorFig. 1 :

 51 Fig. 1: Approach fine-tuned scripts to use a set of tools to both understand how to optimize our video transcoding workflow, and get good performances that scale.

TABLE I :

 I Cost to build an ad hoc personal cloud

	Device	Number	Unit Price Total
	Rapsberry Pi 2	16	35$	560$
	Switch	1	80$	80$
	Alimentation	4	10$	40$
	8Gb SDCARD class 10	16	5$	80$
	Ethernet cables	16$	1$	16$
	Total for a small cluster			776$

TABLE II :

 II Time required to encode small video chunks

TABLE III :

 III Time required to encode long video chunks

http://martinfowler.com/articles/microservices.html

https://www.docker.com/

https://docs.docker.com/swarm/

http://www.genouest.org/godocker/

https://github.com/RobinThrift/raspbian-cadvisor

All the scripts are available online http://olivier.barais.fr/blog/

ACKNOWLEDGEMENTS

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreements No. 611337, the HEADS project, FP7-ICT-2013-10, from the CHIST-ERA under project DIONASYS, and from the French National Research Agency (ANR) under grant ANR-14-CHR2-0004.