Evgeny Dafner
email: dafnere@uncwil.edu

Melchor Gonzah Lez-Dah Vila

J Magdalena Santana-Casiano

Richard Sempeh

Total organic and inorganic carbon exchange through the Strait of Gibraltar in September 1997

Keywords: Water exchange, Chemical oceanography, Carbon

The total organic carbon (TOC) and total inorganic carbon (C 2 ) exchange between the Atlantic Ocean and the Mediterranean Sea was studied in the Strait of Gibraltar in September 1997. Samples were taken at eight stations from western and eastern entrances of the Strait and at the middle of the Strait (Tarifa Narrows). TOC was analyzed by a high-temperature catalytic oxidation method, and C 2 was calculated from alkalinity}pH 2 pairs and appropriate thermodynamic relationships. The results are used in a two-layer model of water mass exchange through the Strait, which includes the Atlantic in#ow, the Mediterranean out#ow and the interface layer in between. Our observations show a decrease of TOC and an increase of C 2 concentrations from the surface to the bottom: 71}132 M C and 2068}2150 mol kg\ in the Surface Atlantic Water, 74 }95 M C and 2119}214 8 mol kg\ in the North Atlantic Central Water, 63}116 M C and 2123}2312 mol kg\ in the interface layer, and 61}78 M C and 2307}2325 mol kg\ in the Mediterranean waters. However, within the Mediterranean out#ow, we found that the concentrations of carbon were higher at the western side of the Strait (75}78 M C, 2068}2318 mol kg\) than at the eastern side (61}69 M C, 2082}2324 mol kg\). This di!erence is due to the mixing between the Atlantic in#ow and the Mediterranean out#ow on the west of the Strait, which results in a #ux of organic carbon from the in#ow to the out#ow and an opposite #ux of inorganic carbon. We estimate that the TOC input from the Atlantic Ocean to the Mediterranean Sea through the Strait of Gibraltar varies from (0.97$0.8)10 to (1.81$0.90)10 mol C s\ (0.3;10 to 0.56;10 mol C yr\), while out#ow of inorganic carbon ranges from (12.5$0.4 )10 to (15.6$0.4 )10 mol C s\ (3.99}4 .90;10 mol C yr\). The high variability of carbon exchange within the Strait is due to the variability of vertical mixing between in#ow and out#ow along the Strait. The prevalence of organic carbon in#ow and inorganic carbon out#ow shows the

Introduction

The average annual increase of CO in the atmosphere has been estimated to be about 1.4 ppm yr\, with a year-to-year #uctuation about the mean of about $0.6 ppm yr\ [START_REF] Gammon | History of carbon dioxide in the atmosphere[END_REF][START_REF] Conway | Evidence for interannual variability of the carbon cycle from NOAA/CMDL Global air sampling network[END_REF]. It has long been recognized that the oceans are the most intense sink of carbon dioxide in the global carbon cycle, absorbing about half of the carbon of anthropogenic origin released to the atmosphere [START_REF] Siegenthaler | Atmospheric carbon dioxide and the ocean[END_REF]. The carbon dioxide sink in the ocean is dominated by the North Atlantic, despite its small area in comparison with the North Paci"c [START_REF] Watson | Modelling atmosphere}ocean CO transfer[END_REF]. To improve the determination of regional or global carbon #uxes (and uncertainties), it is clear that more carbon observations are needed.

Estimates of the meridional total carbon dioxide #ux have been presented for the whole North Atlantic [START_REF] Martel | The North Atlantic circulation in the early 1980s * an estimate from inversion of a "ne di!erence model[END_REF] and for di!erent parts of this ocean: the subtropical North Atlantic [START_REF] Brewer | Carbon dioxide transport by ocean currents at 253N latitude in the Atlantic Ocean[END_REF], the tropical Atlantic [START_REF] Broecker | Interhemispheric transport of carbon dioxide by ocean circulation[END_REF][START_REF] Keeling | Transport of heat, CO , and O by the Atlantic's thermohaline circulation[END_REF], and the temperate North Atlantic Ocean [START_REF] Tans | Observational constrains on the global atmospheric CO budget[END_REF]. Meridional #uxes of dissolved organic matter in the North Atlantic have been calculated by [START_REF] Walsh | Meridional #uxes of dissolved organic matter in the North Atlantic Ocean[END_REF]. Recently total inorganic carbon (C 2 ) and dissolved organic carbon (DOC) transports have been evaluated by Stoll et al. (1996a, b) for the northern North Atlantic Ocean. These authors have pointed out that the meridional transport of DOC cannot be ignored in the total carbon #ux.

One step toward better interpretation of the role of the North Atlantic in the global carbon cycle is to estimate the organic and inorganic carbon exchanges between di!erent marine basins. The Strait of Gibraltar is of special interest in this regard, because it is a single connection between the Atlantic and the Mediterranean Sea, and a comparatively well-studied region from a physical point of view. Despite this, there exists only one indirect estimate of organic and inorganic carbon exchange through this Strait (Copin-MonteH gut, 1993).

In response to increased interest in global carbon change and greenhouse warming, measurements of the marine carbon system (i.e., total organic carbon, titration alkalinity, pH) were completed in the CANIGO project (Canary Islands Azores Gibraltar Observations, MAST III Programme). This program included a synoptic survey of spatial variability within the Strait with an emphasis on seasonal monitoring, primarily designed to examine temporal variability and the mechanisms controlling the carbon exchange between the Atlantic and the Mediterranean basins. In this paper, we describe the results of total organic and inorganic carbon exchange through the Strait of Gibraltar measured in September 1997.

Study area

The Strait of Gibraltar is a narrow and shallow connection between the Atlantic Ocean and the Mediterranean Sea (Fig. 1). It is in an east}west orientation about 60 km long with a minimum (1988), the section made up of stations 1}3 is called the Spartel Sill section, between stations 4 and 5 the Tarifa Narrows section, and between Gibraltar and Ceuta the Gates of Hercules section.

width of about 12 km at the Tarifa Narrows and an average depth of about 600 m. Thirteen kilometers west of Tarifa Narrows lies the main bathymetric sill of the Strait, the Camarinal sill, with a maximum depth of about 285 m and an area of about 7.86 km [START_REF] Guimerans | Contribution to the knowledge of the Gibraltar Sill[END_REF]. Another sill (the Spartel sill) is situated 21 km west of the main sill and has a maximum depth of more than 350 m (La [START_REF] Violette | Tidal-induced pulses in the #ow through the Strait of Gibraltar[END_REF]. Between these two sills, is located the Tangier Basin, with a maximum depth of about 64 0 m.

Materials and methods

Sampling

The seawater samples were taken at eight stations within the Strait of Gibraltar from the RV Thalassa with a CTD rosette system equipped with 10 l Niskin bottles between 2 and 9 September 1997 (Fig. 1). The bottles were "red at the depths of maximum or minimum distribution of temperature, salinity, oxygen and #uorescence, within and between di!erent water masses and at the interface layer between the Atlantic in#ow and Mediterranean out#ow. Samples were taken to within a few meters of the bottom. The positions of the stations are listed in Table 1.

Total organic carbon measurements

The 15 ml samples were collected in glass tubes, precombusted prior to the cruise (4 503C, at least 6 h) and closed with Te#on PFA-backed screw caps that had been rinsed with Milli-Q water and dried. Samples were not "ltered and were drawn as soon as possible after the rosette sampler was on the deck of the ship (either "rst or immediately following the gas samples). During sampling, seawater was allowed to #ow directly from the Niskin stopcock without the stopcock touching the tubes to reduce the chance of contamination. Samples were taken in duplicate, poisoned by addition of HgCl (10 mg l\ "nal concentration), stored in the dark and analyzed in the Laboratory of Marine Microbiology, CNRS, Marseille.

The Shimadzu instrument used in this study was the commercially available Model TOC-5000 total carbon analyzer with quartz combustion column in the vertical position "lled with 1.2% Pt silica pillows [START_REF] Cauwet | HTCO method for dissolved organic carbon analysis in seawater: in#uence of catalyst on blank estimation[END_REF]. A sample volume of 100 l was used. Subsamples were acidi"ed with 10 l of 85% H PO and sparged for 10 min at a #ow rate of 50 ml min\ with CO -free pure air to purge inorganic carbon. Each sample, standard or blank was injected 3}4 times. The nominal analytical precision of the procedure was within 2%, but some variability in results from two di!erent vials gives rise to an actual precision that is lower (4 }6%). The catalyst bed was conditioned by injecting 100 l of acidi"ed and sparged Milli-Q water until the lowest stable integrated area was obtained. Standardization was carried out every day with potassium hydrogen phthalate diluted in Milli-Q water (three concentrations).

The instrument response factor, measured as the slope of the standard addition to Milli-Q water (r '0.999 for 11 runs), remained relatively constant and reproducible over the time of analysis. In general, calibration curves did not exhibit di!erences in slope (average slope: 5917$184 , N"11), and the bias between slopes was within approximately 3%. The system blank was determined by analyzing low-carbon water (LCW) from ampoules provided by J. Sharp (University of Delaware) and was found to be about 5 M C (the average DOC concentration in the Deep Paci"c Water reference was 4 2$5 M C). The total organic carbon (TOC) concentrations in samples were calculated by averaging all replicate sample injections, subtracting the average LCW value as the total blank and dividing by the slope of the standard curve.

Intercalibration exercises in 1995, organized by Dr. J.H. Sharp, indicated that our instrument was suitable for marine DOC studies.

Total inorganic carbon

Total inorganic carbon (C 2 ) was computed from experimental values of pH 2 and total alkalinity, the carbonic acid dissociation constants of [START_REF] Roy | Determination of the ionization constants of carbonic acid in seawater[END_REF], and the boric acid dissociation constant of [START_REF] Dickson | Thermodynamics of the dissociation of boric acid in synthetic sea water from to 273.15 to 318[END_REF]. Several authors have examined the thermodynamic consistency of the measured components of the carbonate system (e.g., [START_REF] Millero | Titration alkalinity of seawater[END_REF]Clayton et al., 1995;[START_REF] Lee | The carbon dioxide system in the Atlantic Ocean[END_REF][START_REF] Mcelligott | Discrete water column measurements of CO fugacity and pH 2 in seawater: a comparison of direct measurements and thermodynamic calculations[END_REF]. From these studies it appears that at present it is not possible to draw a "rm conclusion regarding the best set of constants to use. Considering the accuracy of photometric pH ($0.002) and potentiometric total alkalinity ($1 mol kg\) determinations [START_REF] Mintrop | Alkalinity determination by potentiometry}intercalibration using three di!erent methods[END_REF], [START_REF] Lee | The carbon dioxide system in the Atlantic Ocean[END_REF] estimated that the probable errors in C 2 determined with the [START_REF] Roy | Determination of the ionization constants of carbonic acid in seawater[END_REF] constants is around 4 mol kg\. This is slightly higher than the accuracy of the experimental value ($2 mol kg\, [START_REF] Johnson | Coulometric total carbon dioxide analysis for marine studies: maximazing the performance of an automated gas extraction system and coullometric detector[END_REF]. Recent determinations of pH 2 , A 2 and C 2 in the Azores area have shown internal consistency among these paramenters. C 2 measured was only, on average, 0.54 mol kg\ higher than C 2 calculated, with a standard deviation of $2.6 mol kg\, when the [START_REF] Roy | Determination of the ionization constants of carbonic acid in seawater[END_REF] constants were used (Mintrop and GonzaH lez-DaH vila, unpublished observations).

The pH on the total scale was measured following the spectrophotometric technique of Clayton and Byrne (1993) using the m-cresol purple indicator (DOE, 1994 ). A system similar to that described by [START_REF] Bellerby | Shipboard #ow injection determination of sea water pH with spectrophotometric detection[END_REF] was developed in our lab. The pH measurements were done with a Hewlett Packard Diode Array spectrophotometer in a 253C-thermostated 1-cm #ow-cell by using a Peltier system. Using a stopped-#ow protocol, seawater previously thermostated to 253C was analyzed for blank determinations at 730, 578 and 4 34 nm. The #ow was restarted and the indicator injection valve switched on to inject 10 l dye through a mixing coil (2 m). Three photometric measurements were carried out for each injection in order to remove any dye e!ect on the seawater pH measurement.

Total alkalinity of seawater was determined by titration with HCl to the carbonic acid end point using two potentiometric systems [START_REF] Millero | Titration alkalinity of seawater[END_REF] described previously [START_REF] Mintrop | Alkalinity determination by potentiometry}intercalibration using three di!erent methods[END_REF]. In brief, the titration systems consisted of a Titrino 702SM titrator (Metrohm, Herisau, Switzerland) and a Titrino 719S, both interfaced to personal computers. All measurements were made in thermostated plastic cells provided by F. Millero of the Rosenstiel School of Marine and Atmospheric Sciences (RSMAS), Miami, Florida. Both the acid, in a water-jacketed burette, and the seawater sample, in a water-jacketed cell, were maintained at 253C with a constant temperature bath. The titration was performed by adding HCl to the seawater past the carbonic acid end point. A computer program was used to run the titration, record the volume of the acid added and the EMF readings of the electrodes. The HCl solution (25 l, 0.25 M) was made from concentrated analytical grade HCl (Merck, Darmstadt, Germany) in 0.4 5 M NaCl, in order to yield an ionic strength similar to open ocean seawater. The acid was standardized by the titration of weighed amounts of Na CO dissolved in 0.7 M NaCl solution resulting in a value of 0.2505$0.0001 M. The acid concentration was also determined by coulometry at RSMAS (F. Millero). The results of both methods agree within $0.0001. The total alkalinity of seawater was evaluated from the proton balance at the alkalinity equivalence point, pH "4 .5, according to the exact de"nition of total alkalinity [START_REF] Dickson | An exact de"nition of total alkalinity and a procedure for the estimation of alkalinity and total inorganic carbon from titration data[END_REF]. A FORTRAN computer program (provided by F. Millero) was used to calculate the carbonate parameters [START_REF] Millero | Titration alkalinity of seawater[END_REF]. The precision of the "t (s value) is better than 0.4 mol kg\ for the samples analyzed. The performance of the titration systems was monitored by titration of di!erent batches of CRM (C35) that have known C 2 and A 2 . The agreement of our alkalinity values with the CRM data was within $1.5 mol kg\.

Results and discussion

Hydrography

Over the past decade it has been shown that &the present dynamical understanding of the exchange through the Strait of Gibraltar is that it is hydraulically controlled so that the physical dimensions of the Strait combined with the overall evaporation over the Mediterranean basin determine how much #ow can get through the Strait and how much saltier the Mediterranean is than the Atlantic' [START_REF] Bryden | Steady two-layer exchange through the Strait of Gibraltar[END_REF][START_REF] Kinder | Hydraulic control in the Strait of Gibraltar[END_REF]. A classical model of water mass exchange through the Strait of Gibraltar suggests that two water layers interact in the Strait: the Atlantic in#ow, which spreads to the Mediterranean Sea, and the Mediterranean out#ow, which out#ows from the Mediterranean Sea. In#ow slightly exceeds out#ow to balance the net loss from the excess of evaporation over the Mediterranean Sea. The interaction of these water bodies within the Strait produces an extremely strong halocline, which allows identi"cation of a third transition or interface layer between them [START_REF] Wesson | Mixing at Camarinal Sill in the Strait of Gibraltar[END_REF][START_REF] Bray | The role of the interface in exchange through the Strait of Gibraltar[END_REF]. Table 2 gives some characteristics of the water bodies found within the Strait in September 1997.

The Atlantic in#ow forms in the Gulf of CaH diz and is a mixture of Surface Atlantic Water (SAW; salinity of about 36.4 6), North Atlantic Central Water (NACW, salinity less than 36.00), and Spanish Coastal Water (SCW, salinity of about 36.10 (Van Geen et al., 1988[START_REF] Cauwet | Distribution of organic carbon in the Gulf of Lions: the in#uence of Rho( ne inputs[END_REF]). The thickness of the Atlantic in#ow is highly variable, and in September 1997 it decreased from about 120 m above the Southern Spartel sill to 4 0}50 m along the Gates of Hercules section (Table 2).

The interface layer is de"ned by a sharp halocline with a change in salinity of about 1.50, occurring at a depth between 4 0 and 230 m (Table 2). [START_REF] Lacombe | The regime of the Strait of Gibraltar[END_REF] used the salinity values 37.00}37.50, and [START_REF] Bryden | Flow through the Strait of Gibraltar[END_REF] the values 36.50}38.00 to characterize the interface layer between the two #ows. Following [START_REF] Bryden | Exchange through the Strait of Gibraltar[END_REF] and [START_REF] Bray | The role of the interface in exchange through the Strait of Gibraltar[END_REF], we have divided the interface layer at its mid-depth and incorporated the upper part of the interface into the upper layer and the lower part into the lower layer. Salinity values for the mid-depth are 37.00 for the Spartel sill section, 37.32 for the Tarifa Narrows and 37.50 for the Gates of Hercules section. The interface layer was found to be deeper, thicker, fresher and colder on the west of the Strait. As seen from Table 2, along the Spartel sill section, there is evident slope of the interface layer from the north (4 6 m) to the south (125 m). The shallowest position of this layer was found along the Gates of Hercules section (4 0}50 m), where it continued to get saltier and warmer.

The high-density Mediterranean out#ow is marked by high salinity and temperature. It forms by a mixing of Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW). The LIW originates in the eastern Mediterranean and is characterized by intermediate maxima in temperature (about 13.003C) and salinity (about 38.50) [START_REF] Kinder | Aspiration of deep waters through straits[END_REF]. The residual LIW is found preferentially in the northwestern part of the Alboran Sea in a depth band of were observed at the Tarifa Narrows and along the Gates of Hercules section (Table 2). WMDW is formed by the winter convection south of France (Medoc [START_REF] Group | Observation of formation of Deep Water in the Mediterranean Sea[END_REF][START_REF] Stommel | Deep winter-time convection in the western Mediterranean Sea[END_REF] and is characterized by decreasing salinity and temperature (38.4 2}38.4 4 and (12.903C). The salinity distribution in Fig. 2 demonstrates that at Tarifa Narrows and along the Gates of Hercules, values of salinity in the near-bottom layer were about 38.4 5, always higher than those de"ning WMDW.

A similar situation, which indicates a small contribution of WMDW to the out#ow, has been described previously by a number of other investigators [START_REF] Roether | On the formation of the out#ow through the Strait of Gibraltar[END_REF][START_REF] Measures | Aluminium as a tracer of the deep out#ow from the Mediterranean Sea[END_REF][START_REF] Minas | Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the Western Alboran Sea[END_REF].

The water exchange through the Strait is under the in#uence of periodic forces, such as tides (La Violette [START_REF] Violette | Tidal-induced pulses in the #ow through the Strait of Gibraltar[END_REF], and aperiodic forces, such as atmospheric pressure [START_REF] Stanton | Low frequency variability in the Mediterranean Out#ow west of Gibraltar[END_REF], wind stress over the western Mediterranean Sea and the Gulf of CaH diz [START_REF] Grukndlich | On the observation of a solitary event in the Mediterranean out#ow west of Gibraltar[END_REF], formation of the WMDW during winter convection, and features of the Alboran Gyre circulation [START_REF] Bryden | Origins of the Mediterranean out#ow[END_REF]. As a result, the water #ow in the Strait is not in the form of continuous currents but rather in tidally induced pulses, which vary in the amount of water they contain according to the daily and monthly variation in the above-mentioned factors. Additionally, tide pulses are strongly a!ected by the bathymetric constriction (the small cross section and the presence of the sills). These short-period pulses are able to trigger very short internal waves and current fronts in the upper layer, which propagate eastward into the Alboran Sea during the #ood tide [START_REF] Boyce | Internal waves in the Strait of Gibraltar[END_REF]LaViolette and Lacombe, 1988;[START_REF] Alpers | Recent contributions of remote sensing to the study of internal waves in the Strait of Gibraltar and Messina[END_REF].

TOC and C 2 vertical distributions

Because of the great variability of the dynamic environment, it is di$cult to present any maps or vertical sections from direct observations from the Strait of Gibraltar. Moreover, our presentation here is limited to discussion of vertical pro"les of TOC and C 2 distributions (Fig. 2). For a general impression of spatial variability of both organic and inorganic carbon, the weighted concentrations of TOC and C 2 for each water layer were also calculated (Table 2). The main feature of the TOC vertical distribution in Fig. 2 is the presence of several maxima, located in the SAW (almost at all stations) and in the interface layer (St. 2, 4 , 6, 7). The C 2 vertical distribution shows an increasing concentration from the surface to the base of the interface layer and homogeneous values within the Mediterranean waters. An additional feature of the C 2 vertical distribution is a subsurface maximum located in the NACW (St. 2, 3). 2307 mol kg\ (St. 3, 301 m) to 2325 mol kg\ (St. 7, 398 m). However, in these waters, the Atlantic side in general shows higher TOC (75}78 M C) and lower C 2 (2068}2318 mol kg\) concentration than the Mediterranean side (65}69 M C and 2082}2324 mol kg\). By contrast, the integrated values show (Table 2) that the di!erence between TOC concentrations in the Mediterranean waters above the Spartel sill and between Gibraltar and Ceuta is at the limit of TOC determination ($5 M C).

TOC values measured in September 1997 are similar to those reported by various investigators in the di!erent seas of the Mediterranean basin [START_REF] Cauwet | Le carbone organique dissous dans de mer: con"rmation de la sous-estimation anteH rieure[END_REF][START_REF] Cauwet | Distribution of organic carbon in the Gulf of Lions: the in#uence of Rho( ne inputs[END_REF]Avril and Copin-MonteH gut, 1992;Copin-MonteH gut and Avril, 1993a, b;[START_REF] Cauwet | Dissolved and particulate organic carbon in the western Mediterranean Sea[END_REF][START_REF] Yoro | Cross-slope variations of organic carbon and bacteria in the Gulf of Lions in relation to water dynamics (northwestern Mediterranean)[END_REF] and in the Atlantic Ocean [START_REF] Carlson | Annual #ux of dissolved organic carbon from the euphotic zone in the northwestern Sargasso Sea[END_REF][START_REF] Thomas | Dissolved organic carbon in the equatorial Atlantic Ocean[END_REF][START_REF] Doval | Dissolved organic matter in a temperate embayment a!ected by coastal local upwelling[END_REF]. Similar TOC concentrations were also found in the Strait of Gibraltar in June 1997 [START_REF] Dafner | Seasonal and interannual variations of organic carbon exchange through the Strait of Gibraltar[END_REF]. By contrast, [START_REF] Gasol | Diel variations in bacterial heterotrophic activity and growth in the north-western Mediterranean Sea[END_REF] and [START_REF] Doval | Dissolved and particulate organic carbon in the northwestern Mediterranean Sea[END_REF] presented DOC concentrations in deep waters of the Catalan-Balearic Sea as low as 4 4 }52 M C. In April 1998, lower TOC concentrations (38}52 M C) were also measured in the LIW in the western Alboran Sea [START_REF] Dafner | Total organic carbon distribution and budget through the Strait of Gibraltar in April 1998[END_REF]. Comparison of TOC values measured near the Gates of Hercules section in June and September 1997 with observations from April 1998 suggests a seasonal cycle of organic carbon within the LIW, implying signi"cant bacterial turnover of the organic material in this water mass [START_REF] Packard | Formation of the Alboran oxygen minimum zone[END_REF].

In the Mediterranean Sea, no directly measured C 2 values were found in the literature; all of them are based on pH}alkalinity data. The values reported by [START_REF] Peh Rez | Sistema del carboH nico, oxmH geno y nutrientes en el MediterraH neo occidental[END_REF]), Copin-MonteH gut (1993) and [START_REF] Frankingnoulle | Utilisation de l'alcaliniteH totale comme traccur de masses d'eau[END_REF] for LIW in the Mediterranean Sea (2320}2330 mol kg\) are similar to those reported here. Directly measured C 2 values for the North}East Atlantic Central Water ranged from 2128 to 2135 mol kg\ (Mintrop and GonzaH lez-DaH vila, 1999, unpublished data;[START_REF] Rios | The carbonic system distribution and #uxes in the NE Atlantic during Spring 1991[END_REF], consistent with the values reported here.

Relationships of TOC and C 2 to salinity

The description of vertical distribution of TOC and C 2 presented previously does not take explicit account of tidal variability in the exchange through the Strait. Most of the out#ow occurs in bursts during the semidiurnal tidal cycle: when the out#owing tide is strongest, the interface is relatively shallow and so there is a thick layer of Mediterranean water #owing swiftly over the Camarinal sill. Conversely, on the in#owing tide, the interface is relatively deep and so there is a thick layer of Atlantic water #owing swiftly into the Mediterranean. Such bursts account for more than half of the exchange across the Camarinal sill between the Atlantic and Mediterranean basins [START_REF] Bryden | Flow through the Strait of Gibraltar[END_REF][START_REF] Bray | The role of the interface in exchange through the Strait of Gibraltar[END_REF].

To remove discrepancies caused by tidal #uctuations, relationships of TOC and C 2 to salinity were developed (Fig. 3). Fig. 3 shows large departures from linearity in both TOC and C 2 concentrations in the Atlantic in#ow and in the upper interface layer (salinity lower than 37.00), but a linear decrease of TOC and increase of C 2 values with salinity higher than 37.00. The departures are due either to biological processes or variability of salinity, which a!ects C 2 content in seawater. Although the correlation between TOC and salinity is weak (r "0.56, n"25), it is still signi"cant. By contrast, a strong correlation is found between C 2 and salinity greater than 37.00 (r "0.97, n"96). Fig. 3 also suggests that the relationship between C 2 and salinity is more signi"cant when data from the euphotic layer are removed (r "0.99, n"131); a lower C 2 content Fig. 3. Scatter plots of TOC and C 2 pools vs. salinity. Strong Atlantic in#ow is marked by salinity less than 36.50, the Mediterranean out#ow by salinity greater than 38.00. Intermediate salinities depict the interface layer between the two layers, a zone where substantial periods of both in#ow and out#ow are typical. The 37.00 isohaline approximates an average position of the interface layer above the Spartell sill section, 37.32 at the Tarifa Narrows and 37.50 along the Gates of Hercules section.

is found there due to consumption of inorganic carbon by photosynthesis. The regression line is C 2 "2120($1.5)#79.24 ($0.83)(S!36.00), which gives C 2 values similar to those obtained using only the Mediterranean out#ow. No C 2 variation is discernible at the computed precision between waters with the same salinity but from di!erent depths. This indicates that biological production does not have any appreciable in#uence on C 2 in this region and that there is only a binary mixing between the Atlantic and Mediterranean waters. [START_REF] Brunet | AlkaliniteH totale}carbone inorganique}calcium}densiteH . In: Propri-eH tieH teH s Hydrologiques et Climatiques des Eaux du Basin Occidental de la MediterraneH e. Compagne Mediprod IV*15 Octobre}17[END_REF] and Copin-MonteH gut (1993) have presented for the Alboran Sea similar relationships between C 2 and salinity. The C 2 values from them are slightly lower than those reported here, but these di!erences could be due to the systematic di!erences in the pH measurements (photometric pH vs. NBS potentiometric pH) and the error in the equilibrium constants [START_REF] Roy | Determination of the ionization constants of carbonic acid in seawater[END_REF] vs. [START_REF] Dickson | A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media[END_REF].

The speci"c total inorganic carbon (C 2 /S) shows a linear relationship with salinity below 100 m. A similar relationship is found for the equivalent speci"c total organic carbon (TOC/S) with salinity in the whole pro"le except for the chlorophyll maximum (Fig. 4 ). The regression lines between the speci"c C 2 (in mol kg\) and speci"c TOC (in M C) and salinity C 2 /S"4 0.39(0.75)#0.516(0.020)S, TOC/S"12.4 6(0.92)!0.279(0.025)S, do not pass through the origin. The speci"c C 2 of the Mediterranean waters is higher than that of the less saline Atlantic waters. A substantial quantity of inorganic carbon is added to seawater during its residence time in the Mediterranean Sea. This enrichment may be explained by inputs from the rivers, the Black Sea, the atmosphere and the remineralization of organic carbon. The remineralization is clearly shown in the speci"c TOC vs. salinity plot, where the Mediterranean in#ow shows a signi"cant reduction in TOC content with respect to the Atlantic in#ow.

According to [START_REF] Bryden | Flow through the Strait of Gibraltar[END_REF], at salinities less than 36.50 there is a strong in#ow, which we show is characterized by highest TOC and lowest C 2 concentrations (64 }132 M C and 2068}2170 mol kg\). For the Mediterranean water salinities greater than 38.00, there is strong out#ow, and the lowest TOC and highest C 2 contents (75}78 M C and 2274 }2314 mol kg\ above the Spartel sill, and 65}69 M C and 2278}2325 mol kg\ along the Gates of Hercules section) are found in the out#owing waters. For intermediate salinities, there are substantial periods of both in#ow and out#ow with intermediate values of TOC and C 2 .

TOC and C 2 balance through the Strait of Gibraltar

In reviewing di!erent balance estimations, [START_REF] Bryden | Gibraltar experiment: a plan for dynamic and kinematic investigations of strait mixing, exchange and turbulence[END_REF] have concluded that the #ows through the Strait are apparently limited, from 1 to 2;10 m s\. Later estimates suggest that transports through the Strait of Gibraltar are 30% smaller than historical estimates and, furthermore, tidal oscillations contribute nearly half of the total transport [START_REF] Bryden | Steady two-layer exchange through the Strait of Gibraltar[END_REF]. Because of the great variability of the dynamic environment, it is di$cult to assign, from direct observations, mean values of water transports for TOC and C 2 balance calculations. For our calculations, we use the two-layer model of water mass exchange through the Strait and present TOC and C 2 budgets calculated according to the water transport estimates measured Table 3 Water transport (Sv), salinity, TOC ( M C) and C 2 ( mol kg\) values applied for carbon balance estimations (Fig. 5) through the Strait of Gibraltar. According to [START_REF] Bray | The role of the interface in exchange through the Strait of Gibraltar[END_REF] and [START_REF] Tsimplis | Estimation of transport through the Strait of Gibraltar[END_REF] during the Gibraltar experiment [START_REF] Bryden | Exchange through the Strait of Gibraltar[END_REF][START_REF] Bray | The role of the interface in exchange through the Strait of Gibraltar[END_REF]. Recent determinations in the framework of the CANIGO project have shown volume transport values for the eastern entrance of the Strait of 0.81 and !0.76 Sv with an accuracy of 0.06 Sv. These estimates are in good agreement with the measurements from [START_REF] Bryden | Exchange through the Strait of Gibraltar[END_REF] (0.72$0.16, !0.68$0.15 Sv). For the western entrance, the average out#ow is }0.67 Sv and the average in#ow 0.78 Sv [START_REF] Tsimplis | Estimation of transport through the Strait of Gibraltar[END_REF]. The in#ow and out#ow are in reasonable balance, with a slightly higher in#ow, as expected, to balance the net evaporation over the Mediterranean basin. Table 3 shows volume transport values used in this study for each section. In turn, TOC and C 2 transports were estimated by dividing the interface layer according to the salinity values (37.00, 37.32 and 37.50 from west, center and east of the Strait), and incorporating the upper part of the interface into the upper layer (Atlantic in#ow) and the lower part into the lower layer (Mediterranean out#ow). For budget estimations, integrated values of carbon for out#ow and in#ow at each station were averaged along longitudinal sections. Table 3 also shows the calculated average values of salinity, TOC and C 2 for these layers, taken for the carbon #uxes estimates. Multiplying the average TOC and C 2 concentrations by water transport of the upper and lower layers yields TOC and C 2 transports. Summarizing all carbon transports along longitudinal sections gives the carbon balance. Results of these calculations are presented in Fig. 5.

The dominant feature of carbon exchange through the Strait of Gibraltar is organic carbon in#ow to the Mediterranean Sea and inorganic carbon out#ow from the Mediterranean Sea. We estimate that the TOC transport from the Atlantic to the Mediterranean Sea varies from (0.97$0.87)10 to (1.81$0.90)10 mol C s\ (0.30;10 to 0.57;10 mol C yr\). Recently, we found that the TOC balance through the Strait was 0.4 5;10 mol C yr\ in June 1997 and 0.38;10 mol C yr\ in April 1998, on average [START_REF] Dafner | Seasonal and interannual variations of organic carbon exchange through the Strait of Gibraltar[END_REF]. This estimate is similar to the average value of TOC in#ow presented here for September 1997 (0.4 7;10 mol C yr\). The C 2 balance from the Mediterranean Sea to the Atlantic Ocean varies from (12.5$0.4 )10 to (15.6$0.4 )10 mol C s\ for the di!erent longitudinal sections, which gives annual estimates of 3.8;10 }4 .9;10 mol C yr\.

The interaction between in#ow and out#ow results in strong shear, which induces vertical exchange between layers [START_REF] Wesson | Mixing at Camarinal Sill in the Strait of Gibraltar[END_REF][START_REF] Bray | The role of the interface in exchange through the Strait of Gibraltar[END_REF]. In order to balance the salinity exchange in the Strait, and considering the input of water and the evaporation are negligible on this area, we estimate that the out#ow increases volume transport by the entrainment at 0.13 Sv from the western entrance. By contrast, the Atlantic in#ow increases its volume only by 0.05 Sv from the eastern entrance of the Strait by mixing with out#ow (Fig. 5). The di!erence in vertical mixing between in#ow and out#ow from the west and east of the Strait is }0.08 Sv, in agreement with recent observation of Tsimpis and [START_REF] Tsimplis | Estimation of transport through the Strait of Gibraltar[END_REF]. In terms of carbon, these vertical exchanges correspond to 0.87;10 and !0.4 8;10 mol C s\ for TOC, and 0.11;10 and !0.3;10 mol C s\ for C 2 . The vertical mixing on the Atlantic side results in an input of organic carbon compounds from the in#ow to the out#ow and in the opposite direction for inorganic carbon (Tables 2 and3).

Indirect estimates of organic and inorganic carbon balances were recently published by Copin-MonteH gut (1993). For DOC, she presented a value of 1.25;10 mol C yr\, which is twice as high as our estimations for each longitudinal section. This di!erence is due to the TOC concentrations used (in her calculation she used DOC values of 67 M C for the in#owing and 92 M C for the out#owing waters), rather than di!erent water transport estimates. By contrast, C 2 balances obtained by Copin-MonteH gut and Brunet et al. (1984 ) (!4 .6;10 and }6.1;10 mol C yr\) are the same as we present here despite their use of di!erent water transport estimates. It was estimated that in the Gulf of CaH diz, entrainment increases the observed amount of carbon out#ow, to about 8.6;10 mol C s\ for C 2 and 2.1;10 mol C s\ for TOC on leaving the outer Gulf of CaH diz (Dafner et al., 2000a). It gives the net carbon transport with the Mediterranean out#ow to the Stoll et al. (1996b) open ocean of about 8.8;10 mol C s\. This carbon bulk is about 61.5% of the amount of fossil fuel CO released into the atmosphere currently (ca. 14 .3;10 mol s\) [START_REF] Anderson | The transport of carbon dioxide into the Weddel Sea[END_REF]. The estimates of carbon #uxes through the Strait of Gibraltar are similar to those presented recently for the meridional TCO and DOC transports in the northern North Atlantic (Table 4 ). [START_REF] Polat | Chemical exchange between the Mediterranean and the Black Sea via the Turkish straits[END_REF] obtained a value of 1.79;10 ton C yr\ (or 0.15;10 mol C yr\) for the TOC export from the Sea of Marmora to the Aegean Sea and of about 0.4 1;10 ton C yr\ (or 0.34 ;10 mol C yr\) for the TOC out#ow in the opposite direction. As we see, TOC in#ow and out#ow through the Strait of Gibraltar are two and three orders of magnitude higher than through the Turkish Straits. Several authors have evaluated the budget of dissolved organic nitrogen and phosphorus through the Strait [START_REF] Beh Thoux | Biological "xation of atmospheric nitrogen in the Mediterranean Sea[END_REF][START_REF] Coste | Re-evalution of the nutrient exchange in the Strait of Gibraltar[END_REF]. They have found that in#ow from the Atlantic Ocean exceeds out#ow from the Mediterranean Sea. Our data, together with the results quoted above, indicate that in#ow of organic material to the Mediterranean Sea through the Strait of Gibraltar is considerably higher than out#ow. The Mediterranean Sea acts as a sink of organic carbon and source of inorganic carbon for the Atlantic Ocean.

Fig. 1 .

 1 Fig. 1. Bathymetry in the Strait of Gibraltar and station grid during CANIGO 2 cruise of the RV Thalassa (2}9 September 1997). To the east of the Strait lies the Alboran Sea, the western-most basin of the Mediterranean Sea. To the west of the Strait lies the Gulf of Cadiz, an embayment in the northeast Atlantic Ocean. After La Violette and Lacombe(1988), the section made up of stations 1}3 is called the Spartel Sill section, between stations 4 and 5 the Tarifa Narrows section, and between Gibraltar and Ceuta the Gates of Hercules section.

  Fig. 2 shows that the TOC concentrations in the SAW ranged from 71 M C (St. 7, 30 m) to 132 M C (St. 2, 60 m) while C 2 ranged from 2068 mol kg\ (St. 1, 17 m) to 2150 mol kg\ (St. 4 , 108 m). Fig. 2 also shows that the upper maximum of TOC was related to the maximum of chlorophyll a.

Fig. 2 .

 2 Fig. 2. Vertical distributions of salinity, chlorophyll a ( g l\), TOC ( M C) and C 2 ( mol kg\) in the di!erent parts of the Strait of Gibraltar. Stations 1}3 were situated above Spartell Sill, stations 4 }5 at Tarifa Narrows, and stations 6}7 at Gates of Hercules.

Fig. 4 .

 4 Fig. 4 . Scatter plot of speci"c TOC and C 2 vs. salinity. Linear regressions are found for C 2 values below 100 m and TOC except for TOC values in the chlorophyll a maximum.

Fig. 5 .

 5 Fig. 5. Two-layer model of water mass exchange through the Strait of Gibraltar with values of TOC and C 2 transports (lower panel) and balances (upper panel) along the Strait (2}9 September 1997).

Table 1

 1 Location and depth of the stations with date and time of sampling during the cruise of the RV Thalassa, 2}9 September 1997. Time of sampling corresponds to Greenwich Meridian Time and coincides with the beginning of the CTD cast

	St. no.	Stations locations	Depth (m)	Sampling date	Time of sampling
	1	5 354 W, 36302N	160	2.09.1997	13 h 20 min
	2	5 355W, 35358N	278	3.09.1997	9 h 00 min
	3	5 355W, 35353N	4 17	4 .09.1997	9 h 06 min
	4	5 336W, 35358N	527	5.09.1997	9 h 10 min
	5	5 336W, 35355N	550	6.09.1997	10 h 11 min
	6	5 318W, 36305N	732	7.09.1997	9 h 35 min
	7	5 318W, 36302N	827	8.09.1997	7 h 50 min
	8	5 318W, 35358N	575	9.09.1997	9 h 22 min

Table 2

 2 Some characteristics of water masses in the Strait of Gibraltar, as observed in September 1997. TOC and C 200}600 m and is essentially absent near the African coast. The maximum salinity values(ca. 38.50)

	represent weighted average concentrations. NACW was found
	2

*

Table 2

 2

	shows the

  , errors in the in#ow and out#ow transports are considered to be 0.06 Sv

	Variables	Spartel sill	Tarifa Narrows	Gates of Hercules
	Atlantic inyow
	Transport	0.74	0.69	0.82
	Salinity	36.30$0.06	36.4 2$0.02	36.67$0.17
	TOC	95$9	80$8	89$7
	C 2	2116$12	2128$12	2129$15
	Mediterranean outyow
	Transport	0.74	0.69	0.82
	Salinity	37.65$0.50	38.28$0.02	38.36$0.02
	TOC	71$1	66$3	67$3
	C 2	2277$10	2301$8	2310$2

Table 4

 4 Historical estimates of meridional TCO and DOC #uxes in the North Atlantic Ocean. Negative values correspond to southward transport

	Areas	Carbon transport (mol C s\)	References
	Section at 25N and through the Florida Straits !0.68;10 Temperate North Atlantic !1.60;10 Tropical Atlantic !1.05;10 $0.4 8;10 Section &583N, North Atlantic !0.16;10 Section &583N, North Atlantic (DOC) 0.04 ;10 }0.16;10	Brewer et al. (1989) Broecker and Peng (1992) Keeling and Peng (1995) Stoll et al. (1996a)

E.Dafner et al. / Deep-Sea Research I 48 (2001) 1217}1235

Acknowledgements

The authors gratefully acknowledge F. Abrahansen and M. Goutx for their help with cruise preparation. We thank the crew of the RV Thalassa for their help and support, and L.M. Laglera for his help with sample collection and alkalinity determination. Our special thanks go to J. Sharp, who kindly provided the deep Paci"c and carbon-free water reference standards. We thank C. Goyet for critical reading and constructive comments on the developing manuscript. This research was funded by the European Commission, MAST III Programme (Contract MAS3-CT96-0060). Financial support for E.V. Dafner came from Ministere A!aires Etrange`res Francais and Conseil GeH neH ral des Bouches du Rho( ne, France.


Evgeny V Dafner'

Richard ~empere','

Nicolas Gonzalez2

Fernando ~o M E Z ~

Madeleine Goutx'

Cross-slope variations of dissolved organic carbon in the Gulf of Cadiz, NE Atlantic Ocean (February 1998)

ABSTRACT: Our observations at a traverse of Ca&z in February 1998 indicated dissolved organic carbon (DOC) concentrations ranging from 61 to 89 pM C within the Spanish Shelf Water, from 48 to 64 pM C within the North Atlantic Surface Water, and from 49 to 55 pM C within the North Atlantic Central Water. Moreover, according to temperature and salinity data, the shallow core of the Mediterranean outflow was also identified deeper than 350 m depth with DOC concentrations between 52 and 58 pM C. The data suggest that DOC distribution at the traverse of the Gulf of Cadiz was controlled by outputs of degraded particles transported within alongslope descending shelf waters and also by features of the circulation in the Gulf of Cadiz. An estimate of organic carbon transport suggests that ca 1.34 to 2.68 X 102 m01 C S-' were transported within the shallow core of Mediterranean outflow. This amount of DOC was 2 orders of magnitude less than that calculated within the Mediterranean outflow in the Strait of Gibraltar for the summer and autumn seasons. These processes might partly regulate the biogeochemistry of the shallow core of the Mediterranean outflow along the Iberian coast. KEY WORDS: DOC . Transport . Shallow core of Mediterranean outflow. Gulf of Cadiz . NE Atlantic Ocean Previous investigations have emphasised the importance of the Gulf of Cadiz surface waters in the transport of nutrients (Cano 1978, Minas & Minas 1993), trace metals [START_REF] Van Geen | Trace metals enrichments in waters of Gulf of Cadiz, Spain[END_REF][START_REF] Morley | Distribution and behaviour of some dissolved trace metals in the western Mediterranean Sea[END_REF]) mesozooplankton and ichthyoplankton [START_REF] Greze | Zooplankton transfer through the Gibraltar Strait and peculiarities of its taxonomic composition and distribution in adjacent areas[END_REF][START_REF] Rubin | El ictioplankton, el mesozooplankton y la hidrologia en el Golfo de Cadiz, estrecho de Gibraltar y sector noroeste del mar Alboran, en julio de 1994[END_REF] to the Western Mediterranean Sea through the Strait of Gibraltar. More recently, we found that the high productivity of waters along the Iberian Coast and the continental shelf of the Gulf of Cadiz was likely the ecological source of the high con-centrations (> 100 pM C) of dissolved organic carbon (DOC) observed in the Atlantic inflow in the northwestern part of the Strait of Gibraltar in summer and in autumn (Anonymous 1998).

By contrast, little is known about the organic carbon content of the deep-water masses in the Gulf of Cadiz and its possible export toward adjacent seas. The high density Mediterranean outflow leaves the Strait and flows around the northern side of the Gulf of Cadiz in contact with the bottom before reaching its compensating level at ca 1200 m depth and spreading throughout the North Atlantic Ocean [START_REF] Stanton | Low frequency variability in the Mediterranean Outflow west of Gibraltar Deep-Sea Res[END_REF]). It has now been documented that heavy erosion in the Gulf of Cadiz causes advection and resuspension of sediments from the continental slope [START_REF] Eittreim | Turbidity distribution in the Atlantic Ocean Deep[END_REF]). As intensive biological production occurs in coastal areas such as the Gulf of Cadiz, part of this production might be exported as continental shelf phytodetritus [START_REF] Walsh | Importance of continental margins In the marine biogeochemical cycling of carbon and nitrogen[END_REF]. Although these processes are likely to be potential sources of organic carbon inputs within the Mediterranean outflow, there are currently no reports dealing with organic carbon observations in the Gulf of Cadiz. In the present paper, we describe the distribution of the DOC pool in the Gulf of Cadiz during winter and we give an estimation of its transport to the north-west from the Strait of Gibraltar through a shallow core of Mediterranean outflow found in the investigated area.

Materials and methods. The seawater samples were taken from the RV 'Cornide de Saavedra' with a CTD rosette system equipped with 10 1 Niskin bottles between 10 and l l February 1998 at 5 stations along a section in the Gulf of Cadiz (Fig. 1). Samples for DOC analysis were withdrawn from Niskin bottles into precombusted (450°C, 6 h) 0.5 l glass flasks (Schotta). They were filtered through precombusted (450°C, 6 h) GF/F O Inter-Research 1999 Resale of full article not permitted glass fibre filters, collected in duplicate in 10 m1 precombusted glass ampoules, poisoned by addition of HgC12 (10 mg I-' final conc.), sealed, and stored in the dark. Samples were analysed by using a commercially available Shimadzu TOC-5000 analyser according to the method described in Yoro et al. (1997). The filtration (m) the Mediterranean outflow (15.30°C and 37.07%0). The H. Sharp indicated that our instrument was suitable for bottom contour in Fig. 2 suggests that this section was marine DOC studies.

("C) 4 (PM) (PM) (PM) g 1 ' ) chl a
Some samples for particulate organic carbon and nitrogen (POC and PON) analyses were precombusted (450°C, 6 h) GF/F filters, which were dried on silica gel and frozen to -20°C until analysis in the laboratory. Measurements were carried out with a Perkin Elmer 2400 CHN analyser. To measure phytopigments, 0.5 1 of seawater were filtered through GF/F filters at low pressure (<l00 mm Hg). Pigments were extracted in 90% acetone for 24 h in cold and dark conditions. After this time, chlorophyll a (chl a) was estimated fluorimetrically by using a Turner Design-l0 fluorimeter calibrated with pure chl a (Sigma Co.) (UNESCO 1994). All data are presented in Table 1.

Results and discussion. DOC content in the different water bodies: Physical parameters including temperature and salinity (Fig. 2) indicate a homogeneous water body from the surface to the bottom in the upper slope and down to 135 m offshore (T = 16.14 to 17.12"C, S = 36.06 to 36.47 % o ) . At Stn l , salinity values lower than 36.10 characterised the Spanish shelf water [START_REF] Van Geen | Entrainment of tracemetal-enriched Atlantic-shelf water in the inflow to the Mediterranean Sea[END_REF][START_REF] Walsh | Importance of continental margins In the marine biogeochemical cycling of carbon and nitrogen[END_REF] with the highest DOC value (89 FM C , Table 1, Fig. 3). The salinity values higher than 36.10 very likely correspond to those of the North Atlantic Surface Water (NASW). As shown in Fig. 3, the DOC concentrations within the NASW ranged from 64 pM C inshore down to 48 pM C in the middle of the section (Stn 3). The North Atlantic Central Water (NACW) was found between 250 and 400 m depth (Fig. 2) and was defined by minima of both salinity (136.00 %o) and temperature (<13.8OoC). The colder and less saline NACW overlays the Mediterranean outflow and underlays NASW, producing thermohaline and DOC gradients. NACW contained the lowest DOC concentrations, with nearly homogeneous values in the range of 48 to 49 pM C. [START_REF] Doval | Dissolved organic matter in a temperate embayment affected by coastal local upwelling[END_REF], with reference to unpub- Physical evidence of shelf waters descending: An additional feature in this section is the existence of an along-slope DOC gradient which suggests the alongslope export of organic material likely related to winter convection. To prove its existence and to explain some additional features of DOC distribution at the traverse of Cadiz we have examined the nature of the physical environment with regard to the density (figure not shown), density gradient, which may be interpreted as a buoyancy frequency (Fig. 4A), and geostrophic velocity (Fig. 4B). The curvature of Brunt-Vaisala frequencies near the slope between 150 to 250 m (Fig. 4A, isolines 1.5, 2.0 and 2.5 cycles [c] h-') suggests the tion (65 pM C, Fig. 3). This suggests that this DOC accumulation within this undercurrent was due to hydrodynamic concentrations of along-slope descending material, particle resuspension and subsequent breakdown (see next section). A similar hypothesis was presented by [START_REF] Milliman | Organic matter carbon in US Atlantic continental slope sediments: decoupling the grain-size factor[END_REF], who assumed that the Gulf Stream a n d o r storm events in the Cape Fear and Cape Hatteras areas may induce resuspension of sediment and organic matter from the shelf and transport it offshore.

Potential sources of DOC along the Gulf of Cadiz transecf: Particle fragmentation and dissolution might be a significant source of DOC in estuarine (Sempkre et al. 1994, Sempere & Cauwet 1995) marine environments [START_REF] Smith | Intense hydrolytic enzyme activlty on marine aggregates and implication for rapid particle dissolution[END_REF]). Thus, sources for DOC in the Gulf of Cadiz may be sought from the distribution of suspended matter and its origin along the transect. During filtration of shelf water samples for DOC analysis at Stn 1, filters had a distinct green-brown colour, suggesting a large amount of suspended material in those waters. As for DOC, concentrations of POC and PON (Table 1) throughout the water column at this station were almost homogeneous (19 to 22 pM C and 1.4 to 1.6 PM N respectively). The POC/PON ratio was higher than 10, showing that one source for this suspended material probably derives from river outputs through the shelf [START_REF] Parsons | Biolopcal oceanographic processes[END_REF]. Only at Stns 2 and 3 were the POC/PON values (5.7 to 8.0) close to the classical Redfield ratio of 6.6, indicating a major contribution of organic matter from plankton, faecal pellets and other compounds of biological origin easily degradable with a high nutritive value [START_REF] Buscail | F l u e s and budget of organic matter in the benthic boundary layer over the north-western Mediterranean margin[END_REF]. At the other stations, C/N ratios were as much as or more than 2-or 3-fold the Redfield values, suggest- phaeo/chl a ratio as an indicator of inputs of material from the shelf to the stations along the transect, we observed that such material was concentrated at the depths of DOC accumulation (Fig. 3 Portugal [START_REF] Madelaine | Calculs dynamiques au large de la Peninsula Iberique[END_REF] and to be 20 cm S-' for the upper core (800 to 1000 m) of ing that the particulate organic material was already Mediterranean outflow at the traverse of Cadiz by partly degraded. However, a contribution of terrige- Ambar & Howe (1979b). Hence, the range 10 to 20 cm neous inputs to the suspended material at these S-' most likely represents the shallow core velocity in stations cannot be put aside.

this area. In contrast to the POC/PON pattern, the algal pig-From the triangle delimited by bottom contours we ments (Table 1) presented a homogeneous distribution estimated the shallow core area to 25 800 m2 with an of chl a in the well-mixed layer (0.30 to 0.50 pg 1-l) average DOC value of 52 pM C. Thus, the DOC flux while phaeopigments (hereafter phaeo), the degraded could range from 1.34 to 2.68 X 102 m01 C S-'. For comforms of chlorophyll, decreased from Stn 1 to Stn 5 parison, the average estimates of organic carbon out-(1.12 to 0.07 pg l-'). In particular, Stn 1 exhibited high flow through the Strait of Gibraltar were estimated to phaeo concentrations (up to 0.95 pg l-', on average) ca 5.27 X 104 m01 C S-' in June and 4.94 X 104 m01 C S-' and a high phaeo/chl a ratio (21.6), indicating that in in September (Anonymous 1998) by using an average addition to river inputs, phytodetritus contributed to water transports reported by [START_REF] Bray | The role of the interface in exchange through the Strait of Gibraltar[END_REF]. Assumthe suspended material in the shelf waters. Using the ing that the shallow core represents only a small component of the Mediterranean outflow, it is not surprising to find such low values for organic carbon transports within this water mass. Notwithstanding, because such transport is important in terms of DOC of phytodetritus origin and presumably includes potentially important geochemical markers of the Gulf of Cadiz Shelf, it should be considered when studying the biogeochernistry and the dynamics of the shallow core of Mediterranean outflow along the Iberian coast.

Fig. 1 .

 1 Fig. 1. Bathymetry of the Gulf of Cadiz and veins of Mediterranean outflow (hatched areas) in the Gulf of Cadiz; flow separation is directly controlled by the local bottom topography with its numerous canyons, after Kase & Zenk (1996). The hydrographic section sampled during cruise of the RV 'Cornide de Saavedra' (10-1 1 February, 1998) is shown by the line Stn l-Stn 5

-

  blank, which included blanks of the filter and glass fil-lished data by Alvarez-Salgado & Miller, have reportration system, was estimated to 5 pM C by filtering ted that DOC concentrations in the East NACW (300 to Milli-Q plus@ water. The system blank was determined 400 m) were about 60 pM C. A previous investigation by analysing low carbon water from ampoules provided in the north-western part of the Strait of Gibraltar has by J. H. Sharp (ca 5 pM C). This value was subtracted shown that in the modified NACW (S = 36.05 %o, depth from all measured values. DOC contamination from the 158 m) the total organic carbon (TOC) concentration preservation reagent and from the phosphoric acid was was 76 pM C during the productive summer period below the detection lirmt. Ogawa & Ogura (1992) &d not (Anonymous 1998) flnd significant changes in DOC concentrations between At Stn 4, we observed the well-known high temperfresh sanlples and those preserved with HgC12for about ature -high salinity layer at the bottom associated with 3 mo. Intercalibration exercises in 1995 organised by J.

Fig. 2 .

 2 Fig. 2. Cross-slope distributions of (A) temperature ("C) and (B) salinity (%o) along the section at the traverse of Cadlz (10-11 February 1998, Gulf of C a d ~z ) .Note that the bottom represents the depth observed at each station and is not a typical along-slope bathymetric profile

Fig. 3 .

 3 Fig. 3. Cross-slope distributions of DOC (PM C) along the section at the traverse of Cadiz (10-11 February 1998, Gulf of Cadiz)

  Fig. 4. Cross-slope distribution of (A) Brunt-Vaisala frequencies (c h-') and

Acknowledgements. We thank the crew of the RV 'Cornide d e Saavedra' for their co-operation at sea. Our thanks to M. Tsimplits and S. Bacon for help with calculation of physical properties and A. Bianchi for helpful comments on a draft manuscript. Our speclal thanks also go to J. H. Sharp, who kindly provided the Deep Pacific and carbon-free water reference standards and P. Wangersky for English correction of the manuscript. This research was funded by the European Commission, MAST 111 Program.me (Contract MAS3 -CT96 -0060). Financial support for E. Dafner came from Ministere des Affaires BtrangPres francais and Conseil general des Bouches du RhBne, France.


R Lomba

P Laboureur

C Dumand

C Chauveau

F Halter

Determination of aluminum-air burning velocities using PIV and Laser sheet tomography

Keywords: Aluminum, Burning velocity, Flame temperature, Dust flame

In a context of growing concerns over climate change, aluminum has the potential to serve as a dense energy carrier in order to replace fossil fuels and reduce greenhouse gases emissions. Indeed, its combustion in air may provide carbonfree energy for applications in which a high-energy storage capacity is required.

However, attempts of designing a metal-fueled combustor will conflict with a relatively large dispersion of the burning velocity values reported in the literature,

even when similar powders are used. This uncertainty is partially due to the range of experimental conditions and techniques used on those previous studies. In the present work, an experimental Bunsen-type aluminum-air burner is introduced.

It is shown that the setup is capable of generating stable dust suspensions under well-controlled conditions. The stabilized aluminum-air flames are studied using emission spectroscopy, Particle Image Velocimetry, laser sheet tomography, and direct visualization of the AlO(g) emissions. The measured burning velocities are then compared to previous results obtained for similar powders as a function of dust concentration. A reasonable agreement is obtained, and it is shown that

Introduction

Aluminum powders present an interesting potential as a carbon-free energy carrier [1]. Indeed, they are highly energetic, with gravimetric and volumetric energy contents that are comparable to hydrocarbon fuels. Its main combustion product in atmospheric air is alumina, which can be reduced back to aluminum through low-carbon pathways [2], provided that the energy mix used is mainly composed of renewable or nuclear energy. For the transportation sector, burning aluminum powders directly with atmospheric air could provide the specific energy and power density that batteries and hydrogen storage materials are still struggling to achieve. In this context, this work focuses on fine micron-sized aluminum powders, since they represent an interesting compromise regarding fast reaction rates associated with small (high surface-to-volume ratio) particles, while avoiding the safety issues and lower energy content (due to alumina layer) characteristic of aluminum nanoparticles.

In order to better evaluate this potential, it is necessary to know the aluminumair aerosol laminar burning velocity, which is a fundamental property describing the mixture's reactivity, heat production, and heat transfer. However, experimental data on the burning velocity of metal-air aerosols are still scarce and scattered.

The first of these burning velocity measurements were made by Cassel [START_REF] Cassel | Report of Investigations 6551[END_REF] in his pioneering work on aluminum dust Bunsen flames. Aslanov et al. [START_REF] Aslanov | [END_REF] developed a particle dispersion system and studied the flame propagation in a tube. This dispersion system was also used under similar conditions by Boichuk et al. [5],

and to feed a stabilized dust flame burner introduced by Goroshin et al. [6]. Risha et al. [START_REF] Risha | 43rd AIAA Aerospace Sciences Meeting and Exhibit[END_REF] later developed a similar dust burner and measured the burning velocities of Al-air flames. Julien et al. [START_REF] Julien | [END_REF] measured the flame speed of a spherical Al-O 2 /Ar flame propagating at constant pressure and extracted the burning velocity from the expansion coefficient calculated by chemical equilibrium. Despite the fact that most of those studies used a similar aluminum powder and dispersion system, the burning velocity values presented range from 10 cm/s to 32 cm/s.

In addition, some unique aspects regarding the propagation of metallic flames were observed, which increases the complexity of the phenomenon. The instabilities linked to the discreet nature of solid fuels were observed [9,10], and the very luminous nature of the aluminum flame in a non-transparent medium, which is the dust cloud, lead to interrogations concerning the role of radiation on the flame propagation [START_REF] Julien | [END_REF]. These effects raise even the question of whether a nonstretched laminar burning velocity can be properly defined for metal dust clouds [11]. Therefore, in order to reduce such uncertainties on the burning velocity measurements, it is necessary to work under well-controlled conditions.

In this work, the development and characterization of a laminar dust burner inspired by previous works in the literature [6,12] are presented. Various optical diagnostics, such as high-speed tomography, direct visualization of AlO(g) emissions and spectroscopy, were conducted in order to measure the flame properties. The results are then compared with the existing literature, and analyzed in a context of zero-carbon power generation.

Experimental setup

The aluminum powder used in this work is pure (> 99.8 %) and spherical, with a Sauter mean diameter of 7.1 µm and a narrow size distribution. The powder characteristics are further described in a previous work [13]. A schematic of the experimental apparatus used is shown in Fig. 1. It consists of a stirred powder reservoir, which is placed over a scale and connected to one end of an ejector nozzle. An air flow is supplied to the other end of the ejector nozzle, creating a depressurization inside thereof. This depressurization generates a suction flow of aluminum particles and atmospheric air, and disperses the powder into the main body of the burner, forming a turbulent flow. This flow is then laminarized and finally contracted at the exit. The contraction profile is such that boundary layer is crushed and a flat velocity profile is obtained at the burner exit.

Around the burner rim, an annular methane-air pilot flame of 1.5 mm of thickness is stabilized in order to improve the flame stabilization. Ignition is then achieved by supplying a methane flow rate to the main flow of the burner, which is ignited by the annular pilot flame, creating a hybrid Al/CH 4 -air flame structure.

Once the hybrid flame is stable, the methane flow rate is removed, thus allowing an aluminum-air flame to stabilize itself at the burner exit. The flame is then characterized using a spectrometer, a tomography setup, a camera and filter set in order to visualize flame emissions.

The aerosol flow at the burner exit is characterized by a particle image velocimetry (PIV) setup, using the aluminum particles themselves as trackers. An example of PIV image used for studying the aerosol generation is given in Fig. The total air flow rate Q T is calibrated as a function of the powder mass flow rate ṁ, thus allowing the definition of a nominal dust concentration at the burner entrance, ṁ/Q T . The powder mass flow rate is obtained through the scale measurements of the dust reservoir mass, as illustrated in Fig. 3(a), in which m T is the total mass of powder dispersed in a given run. However, in order to obtain the dust concentration at the burner exit B, it is still necessary to take into account the powder accumulation at the walls inside the experimental device.

For such, two distinct methods were used. The first one consists of comparing the weighing measurements described above with the mass difference of the burner itself before and after a run, dM : B = (1 -dM /m T ) ( ṁ/Q T ). The second method consists of studying the attenuation of a laser beam crossing the dust flow at the burner exit (Fig. 1) with Beer-Lambert's law and the refractive indexes given in [14] in order to obtain the instantaneous dust concentration. The laser attenuation measurements have shown that the concentration is temporally stable.

A calibration campaign comprised of several measurement points with distinct mass flow rates and run duration was undertaken, and the resulting calibration law is shown on Fig. 3(b). These results show that the weighing and laser attenuation measurements are in good agreement. This serves as an indicator that the mean particle size at the burner exit corresponds to the powder's nominal mean particle size. The resulting calibration law is able to determine the dust concentration within a margin of ±25.3 g/cm 3 with a confidence level of 95%. This precision is acceptable when compared to the 310 g/cm 3 stoichimetric equivalence ratio.

Results

Once stabilized at the burner exit, the metal dust flame produces a very intense light that can be analyzed through emission spectroscopy. An optical fiber connected to a spectrometer (Ocean Optics HR4000) is focused on the flame zone.

The light signal emitted by the combustion of the metal particles is composed of two distinct contributions. A first part is due to the thermal excitation of the condensed combustion products, and corresponds to a continuous emission background. It was observed that this emission background can be well represented by an increasing straight line between 400 nm and 600 nm. Therefore, a fitting algorithm is adjusted to the condensed phase emissions and thereafter subtracted from the flame spectrum. The resulting spectrum after this subtraction corresponds to the emission lines originating from the gaseous species. The main features of the gaseous part of the aluminum flame spectrum are the aluminum vapor lines at 394.3 nm and 396 nm, and the B 2 Σ + -X 2 Σ + (blue-green) band system of gaseous aluminum monoxide.

It is possible to determine the temperature which better describes the observed AlO emissions. By assuming a thermal equilibrium between all the species in the reaction zone, the calculated AlO temperature can be associated to the gaseous phase temperature. The fitting process used to determine this flame temperature is described by [15], and consists of a line-by-line simulation of the AlO spectrum based on the spectroscopic constants and relative band strengths from [16,17].

Figure 4(a) illustrates an example of an emission spectrum fitting result, obtained for an aluminum-air equivalence ratio of φ = 1.3. The emission line at 519.3 nm observed experimentally is not described in the fitting algorithm, and therefore is not taken into account for temperature calculation. Figure 4(b) shows the results obtained as a function of the dust concentration of the aerosol. Each point in this figure corresponds to an average of 100 spectra. The gaseous phase temperature is observed to be constant for all the dust concentrations studied, and is about 3150 K ± 180 K. This value is 400 K below the adiabatic flame temperature, and is in good agreement with the temperature measurements of the condensed phase carried out in previous experiments in a constant volume chamber [13].

The burning velocity S L is defined as the velocity upstream of the preheating zone at which fresh gases move through the combustion wave in the direction normal to the wave surface [START_REF] Glassman | Combustion[END_REF]. This interface is defined in the following as the flame surface. In the dust burner setup, the mass conservation principle yields the relationship S L = Q T /A, where Q T is the total airflow at the burner exit, and A is the surface area of the flame. The latter can be obtained by the numerical integration of the rotational figure formed by the flame contour. Two distinct methods for contour detection were used in this study. The first one is a laser sheet tomography of the flame, which constitutes an instantaneous planar visualization of the Mie-scattered laser light (532 nm) by the condensed phase of the flow. An example of a tomography image is given in Fig. 5(a). In this image, three different regions can be observed: a fresh mixture region, where the laser scattering is identical to the cold-flow PIV images (Fig. 2(a)); a dark region; and a burned mixture region. Another example is given in Fig. 5(b), with the distinction that the laser power is weaker and that it mainly illuminates the lower part of the flame. Camera exposure was set accordingly. As a result, flame emissions at 532 nm are no longer drowned in the laser intensity, and are still discernible.

The second method is the direct visualization of the AlO(g) emissions, illustrated in Fig. 5(c).

The burned gases expansion and the streamlines deviation at the reaction zone can be observed in the mean velocity field obtained by PIV measurements of the high-speed tomography images, as shown in Fig. 6. The analysis of each velocity field is used for the interpretation of the tomography images and for calibrating the image processing algorithm. Indeed, the contour detection was empirically calibrated in order to match the minimum normal velocity isoline. The calibration procedure is briefly described hereafter. First, the dark inner zone contour is detected by an image processing algorithm. A set of 10 points is then defined along this contour's lateral borders, and the orthogonal velocity profiles are extracted at these points. For determining the orthogonal direction, the raw, unfiltered contour is locally approximated by a straight line. Indeed, the presence of inter-particle spacing is responsible for a non-physical roughening of this contour, and this approximation becomes a necessary step for determining the combustion wave orientation.

The binary tomography image is then thickened using a Gaussian filter, and subsequently binarized again. The threshold for this last operation minimizes the error between the detected contour position and the minimum aerosol velocity for the defined set of points. On average, the contour position is displaced of 1.21 mm, and the mean standard deviation of the error between the calibrated contour and the position of minimum normal velocity is of 0.24 mm. Consequently, and also taking into account the error when determining the total air flow rate, the estimated error when obtaining burning velocities by rotating the calibrated contour is of ± 1.4 cm/s. Figure 7(a) illustrates the results of the empirical contour calibration procedure. For the sake of visibility, only the rectangular section in Fig. 5(a) is represented.

Figure 7(b) shows the spatial evolution of the velocity normal to the combustion wave along the dashed line in Fig. 7(a). The observed behavior of the aerosol velocity is typical of Bunsen-type flames. In the center of the fresh mixture, the streamlines expansion results in a reduction of the aerosol velocity normal to the flame front. The fresh mixture then starts accelerating upstream of the preheating zone, defining the burning velocity as the minimum value in Fig. 7(b). Upon entering the flame, the aluminum particles accelerate for a total length of about 4 mm. This length is defined as the thermal , and is composed by the preheating zone, the reaction zone, and the product condensation zone. The time needed for an aluminum particle to this thickness may be evaluated using velocity profiles. This time encompasses both heating time and a reaction time. A residence time of the order of 4 ms is obtained.

The tangential strain rate K is evaluated based on instantaneous flow field and mean value of K rms = 57 s -1 was obtained. Therefore, the study of the velocity field of the reacting flow shows that the flame strain rate is weak, and that the dark inner zone contour may be used as an indicator of the flame front position.

The projection of the aerosol velocity in the orthogonal direction of the flame contour provides a method for evaluating the flame structure and the tomography contour detection algorithm. While the exact nature of the observed dark region is unclear, some insight can be obtained from these results. In Fig. 7(b), the dot dashed vertical line represents the position of the detected dark inner zone contour obtained through tomography. It is shown that this frontier is located after the beginning of the preheating zone. In Fig. 5(b), the flame emissions at 532 nm can be attributed to product condensation, since there are no emission bands from gaseous species in the flame at this wavelength (Fig. 4(a)). Therefore, Fig. 5(b) qualitatively indicates a spatial separation between the dark region and the region where the products condensation is primarily occurring. Therefore, the absence of Mie-scattering in the dark zone can be explained by a drop in particle density after the beginning of the preheating zone coupled with the fact that condensed

Al 2 O 3 is not yet formed.
Burning velocities were also obtained through direct visualizations of the AlO(g) emissions. Indeed, aluminum monoxide is a well-known combustion intermediate. It is often used as an indicator of the reaction zone position [START_REF] Glorian | [END_REF] and for estimating burning velocities. In order to isolate the AlO(g) emissions, the flame was observed through a band-pass filter centered at 488 nm and with a FWHM of 10 nm. Typically, the mean image for an acquisition time between 5s to 10s is obtained, such as in Fig. 5(c), and is then Abel-deconvoluted using a Fourier-Hankel procedure [20]. Figure 8 shows a comparison of the burning velocities results yielded by tomography and AlO(g) direct visualization. For both methods, the burning velocities obtained were observed to be roughly constant for dust concentrations between 0.8 < φ < 1.5, with mean values of 34 cm/s when using tomography and 32 cm/s when using AlO(g) visualization. Although a reasonable agreement is obtained, the tomography method is considered to be more reliable. The AlO(g) method integrates temporal instabilities of the flame position and artificially thickens the flame, which introduces inaccuracies in the determination of the burning velocity. Moreover, combustion of metal suspensions can be optically dense, causing multiple light scatter which makes the spatial determination of the emissions source at a specific wavelength unreliable [21]. The above-mentioned reasons might explain the data scatter from the burning velocities measurements conducted with the AlO(g) method. Estimated measurement errors for tomographyobtained data are represented by the vertical and horizontal error bars in Fig. 8, and are coherent with the observed data scatter.

Figure 9 shows the burning velocity results obtained with flame tomography and compares them with existing literature for aluminum powder at about the same mean diameter. The results are observed to agree the most with a recent study in which burning velocities were obtained from particle velocity profiles and under low-stretch conditions [11]. The stability of the burning velocity as a function of the equivalence ratio was previously observed in [6]. The authors attributed this behavior to the large density difference between air and aluminum particles. Indeed, the addition of aluminum to the aerosol doesn't dilute the oxygen concentration, and the burning rate at each individual particle remains roughly constant. It can also be observed in Fig. 9 that the available data is dispersed over a wide range of values, despite similar powder sizes and, in most cases, similar aerosol generation devices. The results obtained in this work also present a lower reproducibility error when compared to previous works, highlighting the importance of working under well-controlled conditions and using optical diagnostics suited for multiphase metal combustion.

Conclusions

The dust burner presented in this work is capable of generating a dust aerosol with homogeneous dust concentration and a flat velocity profile. As a result, the experimental setup is capable of stabilizing aluminum-air flames at low strain rates and extracting reliable burning velocity measurements through the use of instantaneous flames contour detection using high-speed tomography. Regarding the use of aluminum-air combustion for power generation, it is interesting to note that the burner device has an estimated power output of about 8 kW. Indeed, at a mean powder diameter of 7 µm, the results presented in this work show that aluminum should be as reactive as classical hydrocarbon fuels, presenting similar specific energies and similar burning velocities. Therefore, metals, and in particular aluminum, can be considered as a potential candidate for a clean, renewable and dense energy carrier.

Figure 1 :

 1 Figure 1: Schematic of the experimental setup.

  2(a), where a good qualitative spatial homogeneity of the dust concentration afar from the boundary layers can be inferred. Under typical operating conditions, the Stoke's number associated with the particles is Stk ≈ 10 -3 1, implying that aluminum particles closely follow the air streamlines and are stationary with respect to the gaseous phase. The mean velocity profile extracted exactly at the burner exit is shown in Fig. 2(b). The profile is flat and symmetric, and the total gaseous flow rate is obtained by integrating the mean velocity profile.

  .9 l/min (b) Mean velocity profile extracted at the burner exit.

Figure 2 :

 2 Figure 2: PIV results characterizing the aerosol flow.

  band @ 95% (± 25.3 g/m 3 ) (b) Dust concentration calibration law.

Figure 3 :

 3 Figure 3: PIV results characterizing the aerosol flow.

  spectrum, T = 3152 K (a) Example of a simulated spectrum superposed to the experimental AlO.

  Aluminum-air flame temperatures as a function of the aluminum dust concentration.

Figure 4 :

 4 Figure 4: Temperature fitting procedure and results.

  (a) Tomography image of an aluminum-air flame. (b) Tomography image with lower laser power. (c) Mean image of the AlO emissions of an aluminumair flame.

Figure 5 :

 5 Figure 5: Different visualization methods for an aluminum-air (φ = 1.22).

Figure 6 :

 6 Figure 6: Mean velocity field.

  Contour detection for the rectangular section in Fig.5(a).

  Aerosol velocity in the direction of the dashed line in Fig.7(a) and along its length (from the innermost extremity to the outermost).

Figure 7 :

 7 Figure 7: Contour detection results and evolution of aerosol velocity normal to the combustion wave.

Figure 8 :

 8 Figure 8: Comparison of the burning velocities obtained with the tomography and direct visualization techniques, as a function of aluminum dust concentration.

Acknowledgments

Support for this work was provided by Groupe PSA, as part of the OpenLab-Energetics collaboration between Groupe PSA and Université d'Orléans for clean energy technologies.

Burning velocity, cm/s

This study [11]-counterflow [START_REF] Julien | [END_REF] -spherical [START_REF] Risha | 43rd AIAA Aerospace Sciences Meeting and Exhibit[END_REF] -burner [5] -tube [6] -burner [START_REF] Aslanov | [END_REF] -tube [3] -burner