R Arraes-Mescoff

M Roy-Barman

L Coppola

M Souhaut

K Tachikawa

C Jeandel

R Sempere

C Yoro

The behavior of Al, Mn, Ba, Sr, REE and Th isotopes during in vitro degradation of large marine particles

Keywords: Marine particles, Dissolution, Trace elements, Thorium isotopes, Western Mediterranean Sea

The extent and the time constant of dissolution of a set of inorganic tracers during the decomposition of large marine particles are estimated through in vitro experiments. Large marine particles were collected with in situ pumps at 30 m and 200 m in the Ligurian Sea at the end of summer. They were subsequently incubated under laboratory conditions with their own bacterial assemblage for 20 days in batches under oxic conditions in the dark. Some samples were initially sterilized in Ž . Ž . order to observe possible differences between biotic and abiotic samples. Particulate ) 0.2 mm and dissolved -0.2 mm Ž . concentrations of Al, Sr, Ba, Mn, Rare Earth Elements REE and Th isotopes were determined over time. We obtain percentages of dissolution in agreement with the general knowledge about the solubility of these tracers: Th f Al -Heavy REE -Light REE -Mn -Ba -Sr. For Mn and Ce, precipitationradsorption occurs at the end of the experiment probably due to their oxidation as insoluble oxides. Particulate residence time of the tracers ranged from less than 1 day to 10-14 days. During the experiment, biological activity has a control on the dissolution process through the remineralization of particulate organic carbon. In the 30 m experiment, the observed dissolution of aragonite indicates that the pH of the incubation solution significantly decreases in response to the CO respiration. Speciation calculations suggest that this pH 2 shift leads to a decrease of the complexation of dissolved REE by carbonate ions. Th isotope data are consistent with an irreversible dissolution of Th and they do not support a rapid particle-solution chemical equilibrium.

Introduction

Ž

. Large ) 50 mm marine particles play a key role in the biogeochemical cycle of the elements in the ocean. These particles have mainly a biological ori-Ž . gin organic flocculate, fecal pellets and they represent the main transport pathway between the surface water and the sediment. When they sink toward the sediment, they interact with seawater and small suspended particles through physical, chemical and biological processes such as aggregation-disaggregation, dissolution-precipitation, biodegradation, ad-Ž . sorption-desorption [START_REF] Lal | The oceanic microcosm of particles[END_REF] . These processes modify the chemical composition of the particles and Ž therefore the fluxes of settling matter Schaffer, . 1996 . Various types of models have been developed Ž to quantify these processes [START_REF] Mccave | Vertical fluxes of particles in the ocean[END_REF][START_REF] Bacon | Distribution of thorium isotopes between dissolved and particulate forms in the Deep-Sea[END_REF][START_REF] Murnane | Estimate of particle-and thorium-cycling rates in the northwest Atlantic Ocean[END_REF]Ruiz-. Pino, 1994 . They can be calibrated with thorium Ž . data [START_REF] Bacon | Distribution of thorium isotopes between dissolved and particulate forms in the Deep-Sea[END_REF] or laboratory ex-Ž . periments [START_REF] Balistrieri | Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean[END_REF] . However, they are still poorly constrained due to the large number of processes involved and the lack of data for many of them.

This work is a part of the Etude Integree des ´Ž . Metaux En Trace dans l'Ocean EIMETO project to ´śtudy the interactions between the various phases in Ž the oceanic water column seawater, colloids, small . and large particles . Our aim is to use in vitro experiments to constrain the dissolution of large marine particles in response to bacterial activity. These experiments were accomplished by a collaboration between marine geochemists and marine biologists to study the possible coupling between biological activity and the behavior of organic and inorganic tracers. In this paper, we present the results obtained for the inorganic tracers. Results obtained for organic tracers and bacterial production are pre-Ž . sented elsewhere [START_REF] Yoro | Decomposition de la matiere organique et du ´flux de carbone a travers le compartiment bacterien en milieu `ḿarin. these[END_REF] To estimate the extent and the time constant of dissolution of a set of inorganic tracers, we have collected marine particles at 30 m and 200 m in the Ligurian Sea. Aliquots of these particles were incubated for different times at constant temperature and then the incubation solutions and the residual particles were analyzed. We have selected a set of tracers that are useful to understand the transport of matter Ž . in the ocean. Aluminum Al is a tracer of the Ž lithogenic fraction in the particulate matter [START_REF] Davies | Impact of atmospheric depo-śition on particulate manganese and aluminum distribution in northern Mediterranean surface water[END_REF][START_REF] Guerzoni | Saharan dust inputs to the western Mediterranean Sea: depositional patterns, geochemistry and sedimentological implications[END_REF] Ž .

Strontium Sr is a tracer of biogenic particulate phases. It is found in carbonates and in acantharian Ž . planktonic protozoan tests that are made of ce-Ž . Ž. lestite SrSO , [START_REF] Bernstein | Morphologies and transformations of celestite in seawater: the role of acantharians in strontium and barium geochemistry[END_REF] . Barium Ba 4 Ž is a tracer of exported production Dehairs and . Goeyens, 1987a;[START_REF] Francois | Biogenic barium fluxes to the deep sea: implications for paleoproductivity reconstruction[END_REF] The behav-iors of Sr and Ba have similarities, but celestite is Ž . much more soluble than barite BaSO . Manganese 4 Ž .

Ž Mn is sensitive to redox conditions [START_REF] Sunda | Photoreduction of manganese oxides in seawater and its geochemical and biological implications[END_REF][START_REF] Moffett | The importance of microbial oxidation in the upper ocean: a comparison between the Sargasso Sea and equatorial Pacific[END_REF] . Formation of Mn oxyhydroxides induces the scavenging of other trace metals Ž . [START_REF] Balistrieri | Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean[END_REF]Tachikawa et al., 1999b and Ž . is mediated by bacterial activity [START_REF] Moffett | A radiotracer study of cerium and manganese up take onto suspended particles in Chesapeake Bay[END_REF] Ž . Rare Earth Elements REE and the isotopic compo-Ž . sition of Neodymium Nd are used as tracers of water masses and seawaterrparticle interactions Ž Jeandel et al., 1995;[START_REF] Henry | Particulate and dissolved Nd in the Western Mediterranean Sea: sources, fates and budget[END_REF]Tachikawa . Ž . et al., 1999a . Thorium Th is an insoluble element Ž with isotopes of distinct origins Roy- [START_REF] Roy-Barman | 230 Th-232 Th systematics in the Central Pacific Ocean: the sources and the fates of thorium[END_REF][START_REF] Roy-Barman | Thorium isotopes in the Western Mediterranean Sea: an insight into the marine particle dynamics[END_REF] In situ produced Th isotopes are commonly used to calibrate particle related processes Ž . [START_REF] Bacon | Distribution of thorium isotopes between dissolved and particulate forms in the Deep-Sea[END_REF] .

Sampling and analytical procedure

Sampling

The samples were collected at the end of summer at the ADYnamique des Flux Atmospheriques en The particulate matter was shared approximately equally between organic and inorganic analyses. We also sampled seawater at 200 m with Niskin bottles. This water was filtered through 0.2 mm filters to remove bacteria and grazers. Counting by epifluo-Ž rescence microscopy after DAPI staining Porter and . Feig, 1980 indicated that 0.2 mm filtration was an efficient protocol to remove living microorganisms such as flagellates and bacteria. This filtered water was used for particle removal and dilution. Therefore, we assume that marine particles have been degraded mainly by their own bacterial assemblage. For the 30 m samples, optical observations indicated the presence of pteropods and Scanning Electron Ž . Microscope SEM observations indicated the presence of a majority of diatoms and of some coccolithophorids. For the 200 m samples, optical observations indicated the presence of small crustaceans and SEM observations indicated the presence of diatoms and silicoflagellates.

Incubation procedure

A detailed description of the protocol is given Ž . elsewhere Sempere et al., 2000 . Immediately after ´śampling, the largest swimmers were removed from the filters. Then filters were maintained wet by adding few cubic centimeters of 0.2 mm filtered seawater and stored for 24 h in the dark at 48C to minimize bacterial activity until the beginning of the incubation experiments in the laboratory. In Marseille laboratory, the particles were removed from the filters by rinsing the filters with 0.2 mm filtered seawater. Particles were distributed in 500-cm 3 polypropylene bottles using a high precision peri-Ž . staltic pump Perfifill IQ 2000, Jencons while the After separation, the bottles were placed in incu-Ž bators at in situ temperature 23 " 18C for the 30 m . samples and 13 " 18C for the 200 m samples and in the dark to prevent photosynthesis. The bottles were gently agitated upside down once a day. Available oxygen in the incubation bottles was calculated from Ž . the total organic carbon TOC consumption and from the volume of available air in the incubation bottle. It indicates that the experiments were conducted under aerobic conditions. The bacterial growth was measured as bacterial production using leucine Ž . incorporation technique [START_REF] Kirchman | Biomass and production of heterotrophic bacterioplankton in the oceanic subarctic Pacific[END_REF] . The total bacterial production in the incubation solutions was much larger than in seawater due to the strong particle enrichment. Each bacterium has the activity Ž . organic matter consumption, reproduction of a typical marine bacterium whose development is not lim-Ž ited by food at least at the beginning of the incuba-. tion . Some samples were initially sterilized in order to observe possible differences between biotic and Ž abiotic samples C7 was poisoned with sodium azide; D5 and D9 were sterilized by 60 Co-gamma irradia-. tion of 16 kGy for 2 h . In these bottles, bacterial growth was negligible. Incubations lasted up to 21 days and were stopped by separating the incubation solution from the residual particles. This was done by filtration on 0.2 mm Durapore filters. The incubation time A t B represents the time between the end of the particle sub-sampling with the particle separator and the end of the incubation. The maximum incubation time roughly corresponds to the large particle transit time in the water column of the Western basin of the Mediterranean Sea: large particles have a sinking speed of 100 mrday for a depth of 2000 m Ž yielding to a transit time of 20 days [START_REF] Fowler | Rapid removal of Cher-nobyl fallout from Mediterranean surface waters by biological activity[END_REF]Henry et al., 1994 . 2.3. Chemical analysis 2.3.1. Incubation solutions Ž 3 . For Al, Ba, Mn and Sr, a small aliquot 1.4 cm of the incubation solution was diluted in 20 cm 3 of 2% HNO and an Indium-Rhenium internal stan-3 dard was added to the diluted solution. The diluted Ž solution was analyzed by ICP-MS ELAN 5000, . Perkin Elmer, see [START_REF] Valladon | ICP-MS chemical ´ánalysis of geological samples: a new method for interferences and shifts corrections. Application to REE determination[END_REF] The remaining solution was used for Th and REE analysis. 229 Th, 150 Nd and 174 Yb spikes and Fe carrier were added to the solution. After a week of isotopic equilibration, the pH was raised to 8-9 Ž . using NH to produce Fe OH . The precipitate was 3 3 recovered by centrifugation, dissolved in HNO and 3 processed through anionic ion exchange column Ž . AG1X8 to separate Th from Fe q REE. Th was Ž further purified [START_REF] Chen | 238 U-234 U and 232 Th in seawater[END_REF][START_REF] Roy-Barman | 230 Th-232 Th systematics in the Central Pacific Ocean: the sources and the fates of thorium[END_REF] and run by Thermal Ionization Mass Spec-Ž trometry Finnigan Mat 261 equipped with a Spec-. tromat post-counting system . The Fe q REE fraction was dried and then dissolved in 0.5 cm 3 of 6 N HCl. This solution was passed through an anionic Ž 3 . ion exchange column 0.5 cm of AG1X8 that fixed Fe and let the REE pass through. REE were eluted completely with 2.5 cm 3 of 6 N HCl. The yield of the REE chemistry was ) 80%. Purified REE were diluted in 10 cm 3 of 2% HNO , Indium-Rhenium 3 internal standard was added to the solution and the Ž sample was analyzed by ICP-MS ELAN 6000, . Perkin Elmer .

Particles

The 0.2 mm filters were placed in a hot 5% HCl solution for a day to detach the particles. The particles were washed out from the filter with distilled water. Then, the particles were dissolved in 1 cm 3 of aqua regia. The solution was dried and the residue was dissolved again in 0.3 cm 3 of 16 N HNO . The HNO . When complete dissolution of these solids 3 Ž . was achieved no residual solid after centrifugation , the supernatant solution and the sink solution were mixed back together. This procedure allowed to use HF for the residual solid dissolution without producing CaF precipitates with the Ca released during the 2 5% HCl and aqua regia dissolution steps. The final solution was separated in two splits. One aliquot was dried, diluted in 20 cm 3 of 2% HNO , Indium-3 Rhenium internal standard was added and the sample was analyzed for Al, Ba, Mn, Sr and REE by Ž . ICP-MS ELAN 6000, Perkin Elmer . The second aliquot was dried, 229 Th spike was added and the sample was processed through anionic ion exchange Ž resin to separate Th from the matrix [START_REF] Chen | 238 U-234 U and 232 Th in seawater[END_REF]Roy-Barman et al., 1996 . Purified Th was run by Thermal Ionization Mass Spectrometry.

Blanks

In order to estimate the reagent contribution and the possible contamination during the incubation, Ž . 0.2 mm filtered seawater 200 m was incubated with no prior addition of particles. This water was filtered after incubation and both filtered solutions Ž . later referred as Aseawater blanksB and filters Ž . later referred as Aparticle blanksB were analyzed. For particles, the blank contribution is F 1% for Al at 30 m and F 23% for Al at 200 m, F 4% for Ba, below detection limit for Mn, below detection limit Ž . Ž . for REE except for La F 30% and Ce F 40% at 200 m, F 7% for Th at 30 m and F 44% for Th at 200 m. For Sr it ranges from 5% to 40%. The particulate concentrations presented in this study are blank-corrected. Concentrations in seawater blanks were similar to Mediterranean seawater for all elements indicating no significant contamination except Ž . Al and Th see discussion below .

Accuracy

Accuracy of the ICP-MS analysis was checked by Ž running the SLRS standard Institute for Environ-. mental Chemistry, Ottawa and a standard solution Ž . ICP-MS grade . Agreement between ICP-MS measurements and recommended values of Al, Ba, Sr, Mn, REE and Th is typically better than 7%. Accuracy for Th measurement by TIMS was checked by running Th standards. Agreement between measurements and the recommended 230 Thr 232 Th value Ž . [START_REF] Banner | U-238 U-230 Th-232 Th systematics in saline groundwaters from central Missouri[END_REF] is typically better than 2%.

Results

Unless stated otherwise, the concentrations of elements in the filtered incubation solutions and in the residual particles are expressed in 10 y 9 g of element Ž . per gram of incubation solution ngrg . Results are given in Tables 123.

For both series, concentrations in the initial incu-Ž . bation solutions D0-D1 and C0 are systematically higher than in the seawater blanks indicating that dissolution had started before the beginning of the incubation during the 24 h between the particle pumping and the separation of the different aliquots.

Therefore, in the following, we will consider the evolution of concentrations since the time t s y 24 h Ž . that is when the particles have been collected . This allows us to maintain a timescale which is consistent Ž . with the organic tracer study [START_REF] Yoro | Decomposition de la matiere organique et du ´flux de carbone a travers le compartiment bacterien en milieu `ḿarin. these[END_REF] [START_REF] Henry | Particulate and dissolved Nd in the Western Mediterranean Sea: sources, fates and budget[END_REF] Mn concentrations 1.9-. 2.1 ngrg in seawater blanks are higher than values Ž reported for Mediterranean seawater 0.03-0.11 . ngrg, [START_REF] Morley | Distribution and behaviour of some dissolved trace metals in the Western Mediterranean Sea[END_REF] . However, our samples were collected only 9 km from the shore whereas Morley's samples were collected at offshore sites. Strong concentration gradients are commonly ob-Ž served between coastal area and open waters Land-. 232 Ž ing and Bruland, 1987 . Th concentrations 0.27-. 0.33 pgrg of seawater blanks are higher than the concentrations measured on the same seawater that Ž has not followed the incubation procedure 0.206 . pgrg . This suggests that a slight contamination has occurred during sampling or preparation. However, since concentrations in incubation solutions are systematically higher than in seawater blanks, we consider that contamination cannot account for the high Th concentrations in the incubation solutions. Data given in Table 3 Concentrations and isotopic ratios of Thorium isotopes Ž .

Sample

Discussion

Particulate matter characteristics

The particulate matter is more abundant at 30 m than at 200 m. This is consistent with most biological activity occurring in the surface waters and eolian inputs. The elements in the particulate matter are enriched to various extents compared to crustal material. This is due to the addition of an authigenic fraction such as organic matter, biominerals, Mn oxyhydroxides precipitates or trace metal adsorption to the lithogenic material. Refractory elements such as Al and 232 Th are used as tracers of the lithogenic matter because they are expected to be insoluble in seawater and to present no authigenic enrichment. Nevertheless, Th isotopes raise the possibility of a 232

Ž

. small authigenic fraction of Th see Section 4.7 .

Similarly, the suggestion that Al is present in diatoms raises the possibility of nonlithogenic Al in Ž . the particulate matter [START_REF] Hydes | Dissolved aluminum in the Mediterranean[END_REF] . Still, we consider that at this shallow coastal site, Al and Th should be mainly lithogenic and that using the bulk Al concentration to estimate the authigenic fraction of soluble elements will have a minor influence on the final result. The percentage of authigenic matter contributing to the total particulate matter, Fa, is given by: X Al . 92-94% for Ba, 50-70% for Mn, 20-37% for Nd Ž than at 30 m 57-97% for Sr, 49-68% for Ba, . F 30% for Mn, F 8% for Nd . Lithogenic material of eolian origin accumulates in the surface waters before being carried through the water column by Ž . rapidly sinking particles Ruiz-Pino et al., 1990 . Therefore, its residence time at 200 m is much shorter than in the surface waters. This is why we collected less lithogenic material at 200 m compared to 30 m and why the relative contribution of the AlocalB authigenic matter is larger at 200 m compared to 30 m. In addition, aggregation between 30 m and 200 m of small particles enriched in authigenic component will dilute the lithogenic material. Specific processes may enhance this general trend: e.g. in surface waters, photo-reduction pre-Ž vents the formation of Mn oxyhydroxides [START_REF] Sunda | Photoreduction of manganese oxides in seawater and its geochemical and biological implications[END_REF] . Lateral advection of particulate material at the sampling site is also possible but we cannot quantify it.

It is often assumed that the authigenic fraction is Ž . easily soluble labile while the lithogenic fraction is supposed to be refractory. However, recent works suggest that a substantial fraction of the lithogenic Ž matter may be dissolved [START_REF] Henry | Particulate and dissolved Nd in the Western Mediterranean Sea: sources, fates and budget[END_REF] . Tachikawa et al., 1999a . We observe significant authigenic fractions on the residual particles for Mn and Ba. This suggests that a fraction of the authigenic material is not soluble under the conditions and on the timescale of the experiment.

Dissolution of tracers from the particulate matter

In order to remove the effect of heterogeneous distribution of particles among the different aliquots, Ž . we consider the percentage of dissolution D of an X Ž . element X in the particulate matter rather than the concentration of this element in the incubation solutions or in residual particles, respectively. D is X obtained by dividing the increase of concentration in solution by the total amount of element initially present on particles:

X y X s s w D s 2 Ž . X ž / X y X q X s s w p
where X , X and X are the concentrations of the 3 is used as a proxy in many seawater-particle Ž interaction models [START_REF] Bacon | Distribution of thorium isotopes between dissolved and particulate forms in the Deep-Sea[END_REF] . Ž. Ruiz-Pino, 1994 . From Eq. 3 , we derive D as a X function of time:

Ž . tq24 t X max D t s D 1 y e 4 Ž . Ž . X X ž / and X init py labile max D s 5 Ž . X init ž / X p
where t is the incubation time as defined previously, t s 1rk is the residence time of element X on X X particles, D max is the maximum dissolution fraction X of X and the superscript init refers to the time of sample collection. We note that X init s X y X q p s s w

-6% -Th -13% f Ž . Ž . Ž .
Light REE 6-12% -Mn 80% f Ba 80% . For all the elements, D max values are higher at 200 m X than at 30 m because the proportion of insoluble lithogenic matter is lower at 200 m than at 30 m. This is in agreement with the higher 230 Thr 232 Th ratio in the particulate matter from 200 m compared Ž . to 30 m see Section 4.7 . We obtain values for t X ranging from less than a day to more than 10 days. For Ba and Mn, the dissolution is faster at 30 m than at 200 m. In the case of Ce, the dissolution seems Ž slower at 30 m than at 200 m but the time constants . are poorly constrained .

Refractory elements

Ž

. Our study Fig. 2 and Table 4 confirms the Ž refractory nature of Al, Th and REE [START_REF] Maring | The impact of atmospheric aerosols on trace metal geochemistry in open ocean surface seawater: I. Aluminium[END_REF][START_REF] Fisher | Accumulation of Th, Pb, U and Ra in marine phytoplankton and its geochemical significance[END_REF][START_REF] Greaves | Rare earth element mobilization from marine atmospheric dust into seawater[END_REF] and it justifies the use of Al and 232 Th as tracers of the particulate lithogenic matter. Compared to previous studies, the 30 m results demonstrate that Al, Th and REE, which are mainly lithogenic, remain insoluble even when strong bacterial activity occurs. At 200 m, the percentage of dissolution increases as a larger authigenic fraction is present on particulate matter.

Manganese

At 30 m and 200 m, 53% and 80% of the total particulate Mn is dissolved under biotic conditions Ž . Ž . Fig. 2 andTable 4 . The residence time t is of Mn the order of 3 days at 30 m and of the order of 5 days at 200 m. For both depths, the concentrations in A negative value corresponds to a net adsorptionrprecipitation. solution are higher in the sterilized samples than in the nonsterilized samples after 20 days of incubation Ž . Table 1 and Fig. 2 . At 30 m, we observe a decrease of D between 10 and 20 days whereas at 200 m Mn dissolution continues over the same period. Under oxic conditions, the behavior of Mn is controlled by Ž .

2q the balance between 1 oxidation of dissolved Mn 4q Ž . to Mn as insoluble Mn oxides Moffett, 1997 and Ž . 2q 2 reduction of Mn oxides to Mn by dissolved Ž . organic compounds [START_REF] Sunda | Photoreduction of manganese oxides in seawater and its geochemical and biological implications[END_REF] . The specific rate of Mn 2q bacterial oxidation in seawater Ž . k is of the order of 1-2% per day Moffett, 1997 . ox The specific rate of Mn 4q reduction in seawater k red is of the order of 20% per day but the value of k red increases to 40% per day when 5 mgrl of marine Ž . humic substances where added [START_REF] Sunda | Photoreduction of manganese oxides in seawater and its geochemical and biological implications[END_REF] Reduction is strongly enhanced by light but we do not consider this effect because our samples were kept in the dark during the incubation. It appears that the residence time of Mn on particles with respect to reduction of Mn 4q is 1rk s 2.5-5 days which is red close to our t value.

Mn

What is the implication of this reversible process? If an element is subject to both dissolution and scavenging and if these processes both follow first order kinetics:

dX s s k X y k X 6 Ž . y 1 py labile 1 s ž / d t d X py labile s y k X q k X 7 Ž . y 1 py labile 1 s ž / d t
where k is the scavenging rate, k is the dissolu-1 y 1 tion rate, X , X and X the concentrations in solus l p tion, of the labile particulate and of the total particulate. Then, we obtain by integration: X init rX init . In that case, Mn is constantly added py labile p to the particles so that a labile authigenic fraction remains on particles even after a long incubation Ž time this may be an alternative explanation for the significant Ba, Sr and Mn authigenic fraction found . in residual particles . At steady state, we expect to obtain a ratio Mn rMn s k rk f 10-40. . t s 0.3 day agree with our values. These authors Mn stressed the possible effect of filtration in breaking phytoplankton cells and the resulting artificial release of trace metals. Direct dissolution of aerosols Ž .

D t s D max 1 y e y Ž k 1 qk y 1 .Ž tq24. 8 Ž . Ž . Ž . X X with k y 1 init X q X y X Ž . sw py labile sw ž / k q k 1 y 1 max D s 9 Ž . X init X p Ž . Ž . If k < k , Eqs.
max Saharan dust in seawater yields comparable D Mn Ž . Ž 30% but the process is much more rapid t f Mn . Ž . 0.003 day [START_REF] Guieu | Dissolved input of manganese in the ocean: the aerosol source[END_REF] This large timescale difference suggests that Mn is not present under the same chemical forms in the aerosols arriving in seawater and in the marine particles.

Barium and strontium

Particulate Sr is dissolved rapidly and extensively Ž as expected if it is present as celestite Bernstein et . Ž . al., 1992 . A significant fraction 20-30% of the authigenic Ba is not dissolved during the experiment. This authigenic refractory fraction may be well crys-Ž . tallized barite BaSO that has a slow dissolution [START_REF] Dehairs | Discrete suspended particles of barite and the barium cycle in the open ocean[END_REF] This discrepancy suggests that in our samples, particulate Ba was not present as well-crystallized barite or that in the natural environment rapid particulate Ba dissolution is balanced by a rapid particle formation that did not occur in our experiment.

The faster Ba dissolution in the 30 m experiment compared to the 200 m experiment is also consistent with the faster mineralization of POC and the larger Ž . bacterial production [START_REF] Yoro | Decomposition de la matiere organique et du ´flux de carbone a travers le compartiment bacterien en milieu `ḿarin. these[END_REF] . For the 30 m samples, small pteropods were present on the first Ž . Ž . filters D0-D4 and for irradiated samples D5-D9 Ž . and were absent on the last ones D6-D8 . Dissolu-Ž . tion of the pteropod shells CaCO may provide a 3 substantial source of labile Ba and Sr. Additional release of Ba and Sr may come from the dissolution of coccoliths. The pH and the inorganic carbon of the solutions were not measured so that it is not possible to determine precisely the influence of CO 2 release on the dissolution of carbonates. However, Ž . Ž about 460 mM C at 200 m and 720 mM C at 30 . m of TOC were consumed by bacteria in the incuba-Ž . tion bottles Yoro, 1998 . Then, 244 The effect of CaCO dissolution can be estimated 3 from the observation of the incubated material in the 30-m experiment where the largest carbon mineral-Ž . ization occurs Table 5 . After 20 days of incubation pteropod shells were completely dissolved whereas some coccoliths were still visible. This suggests that the final incubation solutions are undersaturated with respect to aragonite but that calcite still buffers the [START_REF] Byrne | Marine chemistry and geochemistry of the lanthanides[END_REF] 7.2-7.4. This unusually low pH is due to the combi-Ž nation of the strong carbon mineralization generally . found in cold deep waters and of the high temperature. Therefore, the high POC content of the incubation solutions may have controlled the dissolution process. Such a control may not be as efficient in real seawater. However, such conditions may exist in micro-environments when particulate matter is being degraded.

Fractionation between REE

Dissolved REE are scavenged by oxygen-donor substance such as Mn-Fe oxyhydroxides and partic-Ž ulate organic matter [START_REF] Balistrieri | Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean[END_REF] . [START_REF] Sholkovitz | Ocean particle chemistry: the fractionation of Rare Earth Elements between suspended particles and seawater[END_REF]Tachikawa, 1999 . The scavenged REE are redissolved at greater depths by the remineralization of carrier particles. These processes induce fractionations between the different REE in Ž both dissolved and particulate fractions Tachikawa . et al., 1999a . Fig. 3 presents shale-normalized REE patterns of the incubation solutions at 30 m. During the incubation, the concentrations of dissolved Light Ž . REE LREE: La, Ce, Pr, Nd increase whereas those Ž of dissolved Heavy REE HREE: Dy, Ho, Er, Tm, . Ž . Yb, Lu slightly decrease Fig. 3a . In natural environments, authigenic LREE are enriched on particles Ž compared to HREE Byrne and Sholkovitz, 1996; . Tachikawa et al., 1999b . Therefore, when dissolution of particulate REE occurs, LREE are more abundantly released to the solution than HREE. Our results are in good agreement with field observations of the preferential LREE release by river particles Ž . Sholkovitz, 1992 and with the REE patterns of Ž sequentially leached marine particles [START_REF] Sholkovitz | Ocean particle chemistry: the fractionation of Rare Earth Elements between suspended particles and seawater[END_REF][START_REF] Tachikawa | Distribution of Rare Éarth Elements and neodymium isotopes in settling particulate Ž . material of the tropical Atlantic Ocean EUMELI site[END_REF] [START_REF] Elderfield | The oceanic chemistry of the Rare Earth Elements[END_REF][START_REF] Moffett | Microbially mediated cerium oxidation in sea water[END_REF] and 2 preferential dissolution of Ce neighbors which increases a relative depletion of Ce. The higher dissolution of La, Pr or Nd compared to Ce observed during the incuba- tion confirms this mechanism. For the 30 m samples, there is a simultaneous precipitation of Ce and Mn in the biotic samples that is not observed for Nd. Our results are consistent with the mechanism suggested Ž . by Moffett 1990 : the oxidation of Ce III to Ce IV is coupled to the microbially mediated oxidation of Mn II to Mn IV. This coupling does not affect Nd Ž . present only as Nd III .

The behavior of REE in seawater is strongly controlled by REE-CO 2y complexes that stabilize 3 REE in solution with a stronger effect on HREE Ž . compared to LREE Byrne and Sholkovitz, 1996 . We estimated the speciation of La for the initial ( ) R. Arraes-Mescoff et al.r Marine Chemistry 73 2001 1-19 15 Ž . seawater and for D8 Table 5 . Due to the strong decrease in CO 2y during the incubation, carbonate 3 complexes represent only 53% of the dissolved La in the final incubation solution whereas 91% of the La was present as carbonate complexes in the initial seawater. The increase of dissolved REE when the pH decreases is consistent with previous observa-Ž . tions in natural waters [START_REF] Gaillardet | Chemical ´ànd physical denudation in the Amazon River Basin[END_REF] The REE pattern in the final incubation solution depends on the REE pattern of the initial seawater and of the labile REE on the incubated particles, on the REE fractionation during sorption on Fe-Mn oxyhydroxides and organic matter and during phosphate precipitation if it occurred. Due to the large number of unconstrained parameters, we did not try to reconstruct it.

In the 30 m incubation solutions, the increase of LREE concentrations is much smaller for the irradi-Ž . ated samples D5c and D9c in Fig. 3b than for the biotic samples. It suggests that the LREE dissolution is enhanced by biotic processes. By contrast, the HREE precipitation may be related to an abiotic process since both irradiated and non-irradiated samples show similar decreases of concentration.

Dissolution of Th isotopes

Thorium is a very insoluble element. 232 Th is derived from continental material. 230 Th is present in lithogenic matter but in the marine environment, it is also produced by the in situ radioactive decay of 234 U in seawater and rapidly scavenged by particulate matter. The production of 230 Th at a known rate is used to calibrate scavenging Ž and marine particle transport models Bacon and . Anderson, 1982 . One of the most important outcomes of these 230 Th studies is that the scavenging of particle reactive metals is reversible.

The 230 Thr 232 Th ratio can be used to estimate the fraction a of authigenic Th in the particles:

R y R p l i t h o a s 10 Ž . R y R sw litho
where R represents the 230 Thr 232 Th ratio, the subscript p, litho and sw correspond to particles, lithogenic material and seawater, respectively, R litho 13.6 = 10 Table 3 . At 30 m, R F 4.8 = 10 p so that a F 4%. At 200 m, R f 5-6 = 10 y 6 so that p a f 6-17%. This increase of a with depth is consistent with the uptake of dissolved Th on the particles Ž during their transit from 30 m to 200 m Roy-Bar-. man et al., 2000 .

During the incubations, there is a significant increase of dissolved Th. This increase may represent the total quantity of Th that can be potentially released from the particles or it may only represent the net effect between dissolution of particulate matter and scavenging by the residual particles. We can use Th isotopes to investigate the details of these processes. We take advantage of the fact that marine particles and the seawater in which they are diluted have distinct 230 Thr 232 Th ratios: particulate Th are the 230 Thr 232 Th ratios in the incubation solution, Ž seawater blank and particles we consider that dissolution of particulate matter does not change signifi-. cantly the mass of the incubation solution . Therefore, the incubation solutions should lie on a straight line between marine particles and seawater blanks Ž . Fig. 4a . If readsorption occurs, concentration will decrease but there will be no change of 230 Thr 232 Th ratio so that the sample will be shifted horizontally toward the right. If there is a rapid reversible equilibrium between seawater and particles, we expect no change of concentrations and the isotopic ratio of the incubation solution will be buffered by the 230 Thr 232 Th ratio of marine particles because they contain the majority of Th. The data can be explained by simple dissolution or simple dissolution Ž . q readsorption Fig. 4b andc but rapid reversible equilibrium between seawater and particles can be ruled out.

The 230 Thr 232 Th ratio of the incubation solutions points towards the 230 Thr 232 Th ratio of the labile fraction of the particles. On Fig. 4b andc, the straight lines passing through the incubation solutions, the initial seawater and the bulk particles indicates that the bulk particles represent a possible Th source during the simple dissolution of the particulate matter in seawater. In this case, labile and bulk Th cannot be distinguished isotopically. However, it is also possible to draw straight lines passing through the incubation solutions and the initial seawater, ending up significantly above the bulk particles. This leaves open the possibility that the 230 Thr 232 Th ratio of the labile fraction is distinct from the 230 Thr 232 Th ratio of the bulk particles.

The small shift between the seawater blanks and the 200 m seawater is consistent with a small amount of contamination of the blanks by crustal material, but it will not change the global correlation obtained for the incubation solutions and the 200 m seawater. It is surprising to obtain an irreversible dissolution for Th because it is a very insoluble element and there is a high particle concentration. However, the degradation of particulate matter may produce a large amount of colloidal matter. These colloids Ž . scavenge Th [START_REF] Baskaran | Scavenging of thorium isotopes by colloids in seawater of the Gulf of Mexico[END_REF] but due to their small size they remain in AsolutionB during the filtration, producing these apparently high dissolved Th concentrations.

Is there a link between inorganic tracers and organic matter dissolution?

In principle, the influence of biological activity on inorganic tracer behavior should lead to distinct tracer concentrations in sterilized and non sterilized dissolved samples. However, since the samples were Ž . Ž sterilized 24 h 200 m samples to 48 h 30 m . samples after particle collection at sea, significant concentration changes under biotic conditions may have occurred before the sterilization. For Ba and Sr at 30 m, dissolution occurred before sterilization so that we have no clue whether it is biotic or abiotic. For Ba at 200 m, the lack of significant difference Ž . Ž . between samples C6 biotic and C7 poisoned suggests that an abiotic process controls dissolution. For Mn at 30 and 200 m, both concentrations and D Mn are significantly higher for poisoned or irradiated Ž . Ž samples C7, D9 compared to biotic samples C5-. C6, D7-D8 , suggesting a biotic control on the reprecipitation of Mn. This process may also control the simultaneous precipitation of Ce. We noted in Section 4.6 that LREE dissolution is enhanced by biotic processes and that HREE precipitation may be related to abiotic processes. At 200 m, REE and Th Ž . concentrations are much higher in C7 poisoned Ž . than in C5 and C6 not poisoned . This is not due to sample contamination by sodium azide addition. Still, it is not clear if this difference is just due to the lack Ž of biological activity in C7 this would imply that biological activity has a very strong influence on the scavenging of these elements and this was not ob-. served at 30 m or to sodium azide inducing a larger Ž extent of REE and Th release for an unknown . reason . We noted that during these experiments, POC oxidation occurs on timescales of the same Ž . order as inorganic tracers Yoro, 1998 and we suggested earlier that POC oxidation to CO may trig-2 ger the dissolution of some inorganic phases. The high enrichment of POC in the incubation bottles compared to natural environments enhanced the amount of CO released and the rate of pH change.

2 Therefore, it may have produced artificially high D X and t values compared to seawater. However, both X D and t derived from the incubation may be X X relevant for micro-environment in which particulate matter is degraded. This is particularly interesting with regards to the recent claim that a biologically mediated dissolution of calcium carbonate occurs Ž . above the chemical lysocline [START_REF] Milliman | Biologically mediated dissolution of calcium carbonate above the chemical lysocline?[END_REF] It was recently suggested that bacterial dissolution of the organic coating of diatom frustules can enhance Ž . the dissolution of silica [START_REF] Bidle | Accelerated dissolution of diatom silica in marine bacterial assemblages[END_REF] Bacterial dissolution of organic coating may also enhance dissolution of some of the tracers presented Ž . in this work: 1 it removes a protective coating from Ž . mineral phases and 2 organic coatings are potential carriers of particle reactive metals such as REE Ž Balistrieri et al., 1981;[START_REF] Sholkovitz | Ocean particle chemistry: the fractionation of Rare Earth Elements between suspended particles and seawater[END_REF]. Tachikawa et al., 1999a .

Conclusion

Inorganic tracers display a wide range of solubilities during the degradation of large marine particles. These variations are in agreement with the general knowledge concerning these tracers in the ocean. There are also significant variations in the timescales necessary to achieve this dissolution. These timescales range from less than one day to 14 days. For Mn, the timescale that we obtain is grossly consistent with previous estimates whereas for Ba, the timescale that we obtain is much shorter than what is inferred from dissolution kinetics considerations. We suspect that during the experiment, biological activity has a significant control on the dissolution process. This control may be due in part to the high particulate matter content of the incubation solutions. Therefore, in future studies, an effort should be made to work with less particle-enriched environments. However, this work stresses the possibility of rapid dissolution of inorganic tracers in confined environments.

  Nice in the Ligurian Sea Fig. 1 . The site is located in a coastal environment and it receives strong eolian inputs. Large particles were collected Ž 3 3 by filtration of seawater 17 m at 30 m and 23 m at . Ž 200 m on Teflon grids diameter: 142 mm, pore size . Ž . 60 mm with in situ pumps Challenger Oceanic .

Fig. 1 .

 1 Fig. 1. Sampling site location.

  initial solution was continuously stirred. Each sub-Ž . sample is formed by a large number ) 100 of 3 Ž 2 cm doses supplied by a silicone tubing Heussner . et al., 1990 . Experimental error calculated from the Ž . variability n s 6 of GFrF-particulate organic car-Ž . bon POC at the initial time in different bottles after particle dispatching were within 4%. The volume of the solution was adjusted to 200-250 cm 3 with 0.2-mm filtered seawater. During the separation procedure, the bottles were maintained in ice in order to reduce bacterial activity. Particles from 200 m were Ž separated in eight aliquots hereafter referred as C . series : each aliquot contains particles filtered from approximately 1.4 m 3 of seawater. Particles from 30 m were separated in 10 aliquots: each aliquot contains particles filtered from approximately 0.79 m 3 Ž . of seawater hereafter referred as D series .

3

  sample was centrifuged to separate the solution from the residual solids. Residual solids were dissolved in HF-HNO -HClO , dried and dissolved in 16 N 3 4

  0.05 0.11 0.01 b.d. b.d b.d b.b.d. b.d. 0.003 0.01 b.d b.d b.0.01 0.15 b.d. b.d. b.d. b.d. b.d. -Mescoff et al.r Marine Chemistry 73 2001 1-19 8 Table 3

  and Al in the continental crust Taylor . and McLennan, 1985 , X and Al are the concenp p trations of the elements X and Al in the particulate Ž sample. Fa is larger at 200 m 95-99% for Sr,

  -Mescoff et al.r Marine Chemistry 73 2001 1-19 10

  the incubation solution, on residual particles and in seawater blanks. Evolution of D as X a function of time is shown on Fig.2. We assume that the dissolution of the labile fraction of the element X on particles follows a first order kinetic lawin the incubation sample. Eq. Ž .

  -Mescoff et al.r Marine Chemistry 73 2001 1-19 11 Ž . Fig. 2. Evolution of the percentage of dissolution D of Sr, Ba, Mn, Nd, Ce and Th as a function of time. The percentage of dissolution is x Ž . given by Eq. 2 . The curves represent the theoretical evolution of D with time assuming that dissolution follows a first order kinetics law x Ž Ž .. see Eq. 4 . ( ) R. Arraes-Mescoff et al.r Marine Chemistry 73 2001 1-19

  5 and the adjustment of the residence time based on our data will not change significantly when considering irreversible or reversible dissolution. This is the case for Mn where k s k < k

  than 90% of the non-refractory Mn should be present in solution: this is consistent with our observations. Our experiments suggests dominance of reduction at least at the begining of the experiments. At 30 m, the decrease of D between Mn 10 and 20 days suggests that oxidation becomes dominant over reduction at the end of the incubation.Other experimental dissolutions of Mn have been Ž . proposed.[START_REF] Collier | The trace element geochemistry of marine biogenic particulate matter[END_REF] studied partic-Ž . ulate matter mainly phytoplankton collected from Ž max the Pacific Ocean. Their results D s 50%; Mn ( ) R. Arraes-Mescoff et al.r Marine Chemistry 73 2001 1-19 13

  al., 1980 . At 30 m, the dissolution of Ba is less extensive but faster than at 200 m. The lower percentage of Ba dissolution of the 30 m particles can be related to their larger lithogenic fraction because this Ba may be present in poorly soluble minerals such as barite or silicates. The slower dissolution rate may be related to the more extensive crystallization of Ba at 200 m compared to 30 m: in the ocean barite is formed at depths around Ž . and below 200 m Dehairs et al., 1980 . The values Ž . obtained for t F 1 day at 30 m, 3 days at 200 m Ba are very short compared to the particulate Ba residence time of 24 years based on barite dissolution Ž . kinetics

  mM of CO at 2 200 m and 421 mM of CO at 30 m could have been 2 Ž produced during the experiment corresponding to . bacterial growth efficiency of 40-50% and could have acidified the solution. Dissolution of CaCO 3 will tend to buffer the pH of the solution.

.

  2y Ž solution. It implies that CO ; 40-60 mM Copin-3 Montegut, 1996 . Considering that at the end of the íncubation S CO ; 2400 mM, it follows that pH ;2

  Fig. 2 presents D and D as a function of time. At 200 m, both Nd Ce Ž D and D are twice higher than at 30 m

Fig. 3 .

 3 Fig. 3. Shale normalized REE patterns of the incubation solutions Ž . Ž . at 30 m: a nonirradiated samples and b irradiated samples. The trends of concentration variations of LREE and HREE are indicated by upwards and downwards arrows.

Th and 230 Ž

 230 Th have distinct sources in the ocean Roy-Bar-. 232 man et al., 1996 .

.

  Th s 5-6 = 10 at 200 m contains a large frac-Ž 230 232 y 6 . tion of lithogenic Th Thr Th s 4-5 = 10 and dissolved Th is enriched in 230 Th due to the in Ž 230 232 y 5 . situ production Thr Th s 1.36 = 10 . Fig. 4 presents the 230 Thr 232 Th ratio versus 1r 232 Th for the incubation solutions. In the case of a simple Ž binary mixing Th in incubation solutions seawater . Th q particle derived Th due to dissolution , 230 Thr 232 Th ratio as a function of 1r 232 Th for the incubation solutions is given by: C and C are the 232 Th concentrations in the s s w incubation solution and seawater blank, C is amount p of 232 Th dissolved from particles divided by the mass of the incubation solution and R , R and R s sw p

Fig. 4 .

 4 Fig. 4. 230 Thr 232 Th ratio versus 1r 232 Th for the incubation Ž . solutions. a Theoretical evolution of the incubations solutions: Ž . 1 for simple dissolution, incubation solutions lie on a straight Ž . line between labile marine particles and seawater blanks; 2 if readsorption occurs the sample will be shifted horizontally toward Ž . the right; 3 for rapid reversible equilibrium between seawater and particles, incubation solutions lie on a straight vertical line Ž .Ž . Ž . see text for details . b 30 m data. c 200 m data. The 30 m and 200 m data can be explained by simple dissolution or simple dissolutionqreadsorption but rapid reversible equilibrium between seawater and particles can be ruled out. The straight lines passing through the incubation solutions, the initial seawater and the bulk particles indicate that the bulk particles represent a possible Th source during the simple dissolution of the particulate matter in seawater. However, it is also possible to draw straight lines passing through the incubation solutions, the initial seawater and ending up significantly above the bulk particles, leaving open the possibility that the 230 Thr 232 Th ratio of the labile fraction is distinct from the 230 Thr 232 Th ratio of the bulk particles.

  Mescoff et al.r Marine Chemistry 73 2001 1-19 seawater blanks. Therefore, we suspect a strong con-Ž tamination of the solutions with respect to Al microbiologists wrapped their material and samples in Al . foil to protect them from carbon contamination .To compare our particulate matter concentrations with the literature, we express the quantity of ele-

	7.4"1.6 R. Arraes-Ž n.d. 5.6"4.6 25"18 . ment per mass of seawater ngrg SW . Particulate 6.5"2.5 1.8"6 10"6 5.6"1.7 11"3 Ž Al concentrations in this work 0.16-0.42 ngrg SW . at 30 m and 0.004-0.02 ngrg SW at 200 m are of		9.3"1.6	9.0"8.4	15"4	8.6"2.6	10"1.9 (	9.2"2.0	9.0"1.3 )	8.3"0.7		12"3	10"9	13.6"1.8
	13.1"3 the same order of magnitude or slightly lower than n.d. 4. 7"3.8 10.8"7.7 3.8"1.4 1.5"5.1 2.7"1.6 7.1"2.1 9.3"2.7 Ž . previous measurements 0.05-0.6 ngrg SW at the Ž . DYFAMED site Sarthou and Jeandel, 2000 . Partic-Ž ulate Ba concentrations 0.003-0.006 ngrg SW at . 30 m and 0.0009-0.002 ngrg SW at 200 m are		6. 8"1.1	4.3"4.0	7.1"1.9	3.6"1.0	8.6"1.5	4.1"0.9	8.8"1.1	22.4"1.8		3.19"0.7	3.4"3.1	3.38"0.4
	1.78"0.05 lower than those determined in the Mediterranean n.d. 0.83"0.04 0.43"0.04 0.59"0.03 0.86"0.04 0.26"0.03 1.27"0.04 0.89"0.04 Ž Sea 0.169 ngrg SW at 50 m and 0.04-0.06 ngrg . Ž . SW at 100-300 m by Dehairs et al., 1987b . In all cases, comparison is made with particles collected on Ž . filters with a much smaller pore size f 1mm com-Ž . pared to our work 60 mm , so that it is not surpris-Ž ing that we obtain different concentrations there is		0.73"0.04	0.48"0.04	0.48"0.03	0.42"0.03	0.83"0.04	0.45"0.03	0.99"0.05	2.74"0.1		0.27" 0.03	0.33"0.02	0.206"0.008
	4.58"0.22 no previously published data for these elements on 4.45"0.85 4.82"0.39 4.69"0.34 4.39"2.7 4.59"0.24 4.26"0.27 4.22"0.48 4.74"0.19 . large particles . The Nd concentrations in filtered Ž . large particles ) 50 mm reported for the Sargasso Ž Sea 0.03 pgrg SW at 40 m and 0.01 pgrg SW at . 200 m, Jeandel et al., 1995 are in agreement with Ž our values 0.035 pgrg SW at 30 m and 0.015 pgrg . SW at 200 m .		7.30"2.2	4.40"1.0	n.d.	n.d.	5.30"1.8	6.40"1.9	5.90"4.0	7.70"5.1
	550"26	576"110	517"41	575"40	385"240	412"21	400"24	250"28	537"21		108"32	62"15	n.d.	n.d.	50"17	26"8	27"19	14"9
		D2 12 131"1	D3 24 108"1	D4 48 124"2	a D5 68.2 89"5	D6 240 91"1	D7 504 95"1	D8 504 60"1	a D9 504 114"1	200 m	C0 0 14.8"0.3	C1 25 14.2"0.2	C2 25 n.d.	C3 49 8.9"0.3	C4 241 9.5"0.3	C5 481 4.08"0.08	C6 481 4.7"0.3	b C7 481 1.87"0.06	Seawater Blanks	BL1 y 24 y 24 BL2 Ž . 200 m filtered water analysis of 3 l	n.d.s Not determined.

a Irradiated sample. b Poisoned with sodium azide.

Table 5

 5

	Initial seawater and final incubation solution estimated composi-
	tions
				Initial	D7 and D8
				seawater
	Ž ÝCO mM . 2 y Ž HCO mM . 3 2y Ž . CO mM 3 Ž . pH SWS		2000 1650 345 8.2	a 40-60 2400 2260 7.2-7.4
	3y 4 Total PO Fraction of La as La -CO -Ž . mM 3q 2y 3 c complexes	0.1 91%	b 53% 3.3
	Fraction of La as free La	3q c	4%	21%
	3q Fraction of La as La -SO -3y 4 c complexes	3%	17%
	3q Fraction of La as La -PO 4 3y c complexes	0.3%	2%
	a	Ž Estimated assuming that ÝCO s ÝCO seawater q 400 . 2 2
	mM.
	b Estimated from TOC mineralization assuming a PrC Red-
	field ratio of 1r120.
	c Speciation of the dissolved REE was calculated by taking
	into account free REE as well as chloride, fluoride, sulfate,
	Ž carbonate, bicarbonate and phosphate complexes Byrne and
		.

Table Nd Ce

 Nd . 4 . At both depths, t is estimated around 10 days.

	Nd t is shorter at 200 m 4 days than at 30 m 14 Ž . Ž Ce . Ž . days . Recently, Tachikawa et al. 1999b demon-
	strated that the negative Ce anomaly of seawater Ž . samples is formed by two processes: 1 microbially Ž mediated in situ Ce oxidation Ce oxide is insoluble, . Ž .

Acknowledgements

We are grateful to J.C. Marty for his support to our project at the DYFAMED station. We thank the captain and the crew of the R.V. Professeur Georges Petit for their assistance and hospitality during the cruise. We thank C. Bournot-Marrec, M. Goutx, F. Van Wambeke, P. Brunet, M. Valladon, B. Reynier, L. Guidi for their help at different stages of the project. We are grateful to F. Dehairs, R. Sherrell and two anonymous reviewers for their fruitful comments. The thesis of R.A.-M. was partly financed by the AConselho Nacional de Pesquisa do Brasil Ž . CNPq B and by the ASociete de Secours des Amis ´des SciencesB.


Evgeny V Dafner
email: dafnere@uncwil.edu

Richard Sempere

Harry L Bryden

Total organic carbon distribution and budget through the Strait of Gibraltar in April 1998

Keywords: TOC, Distribution, Balance, Apparent oxygen utilisation, The Strait of Gibraltar

In order to investigate total organic carbon TOC exchange through the Strait of Gibraltar, samples were taken along two Ž . Ž . sections from the western Gulf of Cadiz and eastern Western Alboran Sea entrances of the Strait and at the middle of the Śtrait in April 1998. TOC was measured by using a high-temperature catalytic oxidation method. The results referenced here are based on a three-layer model of water mass exchange through the Strait, which includes the Atlantic inflow, Mediterranean outflow and an interface layer in between. All layers were characterised by a decrease of TOC concentrations from the Gulf of Cadiz to the Western Alboran Sea: from 60-79 to 59-66 mM C in the Atlantic inflow and from 40-60 to 38-52 mM C in the Mediterranean waters, respectively. TOC concentrations in the modified North Atlantic Central Water Ž . varied from 43 to 55 mM C. Intermediate TOC values were measured in the interface layer 43-60 mM C . TOC concentrations increased from the middle of the Strait towards continents indicating a contribution of organic carbon of photosynthetic origin along Spain and Morocco coasts or TOC accumulation due to upwelling in the northeastern part of the Strait. Our results indicate that the short-term variability caused by the tide greatly impacts the TOC distribution, particularly in the Gulf of Cadiz. The TOC input from the Atlantic Ocean to the Mediterranean Sea through the Strait of Gibraltar varies 4 4 y 1 Ž 12 12 y 1

. from 0.9 = 10 to 1.0 = 10 mol C s or 0.28 = 10 to 0.35 = 10 mol C year , respectively . This estimate suggests that the TOC inflow and outflow through the Strait of Gibraltar are two and three orders of magnitude higher than reported via the Turkish Straits and Mediterranean River inputs.

Introduction

The exchange between the Mediterranean and Atlantic through the Strait of Gibraltar is a key issue both for the Mediterranean environment and for global ocean circulation and climate variability Ž . [START_REF] Bryden | Recent progress in strait dynamics[END_REF] It is well known that the Surface Atlantic Water, which flows into the Mediterranean Sea, causes an increase of productiv-Ž ity in the Alboran Sea Packard et al., 1988;[START_REF] Minas | Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the Ž . Western Alboran Sea[END_REF] It has also been shown that in the Atlantic inflow concentrations of the organic forms of nutri-Ž . ents are higher Coste et al., 1988 and mineral Ž . forms of nutrients are lower McGill, 1961 than 0304-4203r01r$ -see front matter q 2001 Elsevier Science B.V. All rights reserved.

Ž . PII: S 0 3 0 4 -4 2 0 3 0 0 0 0 1 0 9 -2 ( ) E.V. Dafner et al.r Marine Chemistry 73 2001 233-252 234 within the Mediterranean outflow. Estimates by Ž . Greze et al. 1985 suggest that the annual zooplankton inflow from the Atlantic Ocean is twice as high as its outflow from the Mediterranean Sea.

By contrast, little is known yet about the influence of the Mediterranean outflow on the biogeochemistry of the eastern North Atlantic although the physical environment of this phenomenon has received more attention. Over the last decade, it has been found that the salinity and temperature of the Ž Deep Mediterranean waters have increased Bethoux ét al., 1990;[START_REF] Rohling | Man-induced salinity and temperature increases in Western Mediterranean Deep Water[END_REF][START_REF] Francour | Are the Mediterranean waters becoming warmer? Information from biological indica[END_REF] There are some simulations of an increase of its influence on the dynamic of the North Atlantic Ž . Ocean [START_REF] Rahmstorf | Influence of Mediterranean outflow on cli-Ž . mate[END_REF] and on the climate, in Ž . general [START_REF] Johnson | Climate control requires a dam at the Strait Ž . of Gibraltar[END_REF] To predict an influence of the climate warming on the cycling of the major organic nutrients in the North Atlantic and the Mediterranean Sea, we need to know in what quantity these materials are exchanged between both basins.

Ž . Polat and Tugrul 1996 estimated an organic carbon budget for the eastern part of the Mediterranean Sea through the Turkish Straits. In contrast, only indirect estimates of the organic carbon budget Ž exist for the Strait of Gibraltar Copin-Montegut, .

1993 . No reports of direct observations of organic carbon distribution and exchange through the Strait of Gibraltar have been presented up to the present. In this paper, we describe the results of total organic Ž . carbon TOC measurements, in terms of distribution and budget, that were estimated from the samples collected during April 1998 in the Strait of Gibraltar area.

Study area

The Strait of Gibraltar is a narrow and shallow connection between the Atlantic Ocean and the Ž . Mediterranean Sea Fig. 1 . The Strait has an eastwest orientation, a 60-km-long passage with a minimum width of about 12 km on the Tarifa Narrows, and an average depth of about 600 m. Thirteen kilometres westward of the Tarifa Narrows lies the main bathymetric sill of the Strait, the Camarinal sill, with a maximum depth of about 285 m and a surface 2 Ž . of about 7.86 km [START_REF] Guimerans | Contribution to the knowledge of the Gibraltar sill[END_REF] Another Ž . sill the Spartel sill is situated 21 km west of the ( ) E.V. Dafner et al.r Marine Chemistry 73 2001 233-252 235 main sill and has a maximum depth of more than Ž . 350 m La [START_REF] Violette | Tidal-induced pulses in the flow through the Strait of Gibraltar[END_REF] . Between these two sills, is the Tangier Basin, with a maximum depth of approximately 640 m.

Materials and methods

Field sampling

Discrete seawater samples were taken from the Ž . RRV Discovery cruise 232 at a series of stations across the Strait of Gibraltar between 12 and 18 of Ž . Ž . April 1998 Fig. 1 . A CTD rosette system Seabird equipped with 10 l Niskin bottles was used. Stations were chosen to cover a variety of waters from west-Ž . ern Stn. W1-W11, in the Spartel sill area , and Ž from eastern Stn. E1-E11, in the Western Alboran . Sea entrances of the Strait, and in the middle of the Ž . Strait Tarifa Narrows, Stn. 13386 and 13399 . The bottles were fired at the depths of maximum or minimum distribution of temperature, salinity, oxygen and fluorescence, within and between different water bodies and at the interface layer between the Atlantic inflow and Mediterranean outflow. Samples were taken to within a few meters of the bottom. The positions of the stations are listed in Table 1.

Samples were not filtered and were drawn as soon as possible after the rosette sampler was on the deck Ž of the ship either first or immediately following the . gas samples . The 10 ml samples were collected in Ž . duplicate in precombusted 4508C, at least 6 h glass Ž w . ampoules Wheaton . During sampling, seawater was allowed to flow directly from the Niskin stopcock without the stopcock touching the tubes, to reduce the chance of contamination. All ampoules were rinsed three times with the respective sample water before filling to appropriate volumes, poisoned Ž y 1

. by addition of HgCl 10 mg l final concentration 2 and immediately after filling the ampoules were flame-sealed and stored in the dark.

HTCO analysis

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 filled with 1.2% Pt on silica Ž pillows with approximate diameter of 2 mm Cauwet, . 1994 . The furnace temperature was maintained at 6808C and the effluents passed through a mercury Ž . Ž trap gold wire to remove mercury Ogawa and . Ogura, 1992 . A magnesium perchlorate water trap has been added to the system located before the halogen scrubber, and an in-line membrane filter and the nondispersive infrared CO detector. Prior to 2 analysis, subsamples were acidified with 10 ml of 85% H PO to a pH ; 2 and sparged for 10 min 3 4

with CO -free pure air to remove inorganic carbon 2 as CO . TOC contamination from the preservation 2 reagent and from the phosphoric acid was below the detection limit. During the sparging procedure, we Ž followed the recommendations of J.H. Sharp Uni-. versity of Delaware , i.e., care was taken such that during sparging of each sample, seawater did not overflow from the ampoules. This condition was possible at a gas flow rate of 40 ml min y 1 . One hundred ml injections were repeated three to four times for each sample, the analytical precision of the procedure being within 3%, on average. Some variability in values from two different vials gives rise to Ž . a lower overall precision 14% . Prior to analyses of standards and samples, the catalyst bed was 'conditioned' by injecting 100 ml of acidified and sparged water from a high-quality water purifier, a Millipore w

Ž

. Milli-Q Plus System hereafter, DW , until the lowest stable integrated area was obtained. To bring the blank down, the catalyst was pretreated by washing in 1% HCl and gently rinsed with DW, and dried in a furnace at a temperature of about 4508C for 10-15 min. This protocol is different from that used previously, as, for example, heating the catalyst to Ž . 5008C for 6 h in a muffle [START_REF] Skoog | Methodical investigations on DOC determinations by the HTCO method[END_REF] . units, n s 19 . The bias between slopes was approximately within 4%.

Calibration of the instrument

The accuracy and the system blank of our instrument were determined by analysis of the Deep At-Ž . lantic Water DAW reference and low carbon water Ž . LCW from ampoules provided by J.H. Sharp. The Ž . average dissolved organic carbon DOC concentration in the DAW reference and in the LCW were 45 " 1 mM C, n s 19 and 3 " 1 mM C, n s 3, respectively. Generally, there was no significant difference in peak areas between DW samples and the LCW standards. Individual DW blanks were rechecked after every two to four sea sample injections and were found to be about of 685 " 198 area Ž . units, n s 153. Statistical analysis ANOVA showed no difference in the peak areas between DW blanks Ž and LCW standards Student's t-test was 0.13, p -. 0.05, critical value of t at p s 0.05 was 4.3 . TOC concentrations in sea samples were calculated on the basis of the DW blank by averaging all replicate injections of samples, subtracting the average DW value as the total blank and dividing by the slope of the standardisation curve. Our results in the most recent DOC method intercalibration, conducted by Ž . Ž . Sharp 1994 ; [START_REF] Sharp | Analyses of dissolved organic carbon in seawater: the JGOFS EqPac methods comparison[END_REF] , were in agreement Ž with other analysts J.H. Sharp, personal communi-. cation . Therefore, we have confidence that TOC concentrations presented in this report have been determined within an acceptable error of 3 " 1 mM C.

Results and discussion

Hydrography

A classical model of water masses exchange through the Strait suggests that two water masses flow through the Strait: Surface Atlantic Water Ž . SAW , which spreads to the Mediterranean Sea, and ( ) E.V. Dafner et al.r Marine Chemistry 73 2001 233-252 238 Mediterranean Water, which outflows from the Mediterranean Sea. Inflow slightly exceeds outflow to balance the net loss from the excess of evaporation in the Mediterranean Sea over the sum of precipitation and river discharges. The interaction of these water masses produces an extremely strong Ž . halocline Fig. 2 , which allows identification of a Ž third, a transition or interface layer in between Wes-. son and [START_REF] Wesson | Mixing at Camarinal Sill in the Ž . Strait of Gibraltar[END_REF][START_REF] Bray | The role of the interface in exchange through the Strait of Gibraltar[END_REF] Tables 2 and3 give some characteristics of the water masses found within the Strait in April of 1998, and ( ) E.V. Dafner et al.r Marine Chemistry 73 2001 233-252 239 ( ) E.V. Dafner et al.r Marine Chemistry 73 2001 233-252 240 Ž -36.10 Van Geen and[START_REF] Van Geen | Atlantic water masses in the Strait of Gibraltar: inversion of trace metal data[END_REF][START_REF] Van Geen | Entrainment of trace-metal-enriched Atlantic-shelf water in the inflow to the[END_REF], 1991

TOC sources in the Atlantic inflow

The increase of TOC concentrations from the middle of the sections towards Spain and Morocco Ž .

Ž . Ž . Fig. 5. Integrated TOC profile from Tarifa Narrows q , western `and eastern v entrances of the Strait of Gibraltar.

( ) E.V. Dafner et al.r Marine Chemistry 73 2001 233-252 243 indicate an accumulation of organic carbon along Ž continents. Having only oxygen and fluorescence in . Volts data, we can only hypothesize that the sources of TOC accumulation are either production of organic carbon in situ, or Atlantic inflow already enriched in organic carbon during its flow to the Strait.

In situ production can be induced by upwelling since such phenomenon, already reported between Ž Gibraltar and the Bay of Malaga including north-. eastern part of the strait , may affect the northwest-Ž ern Alboran Sea [START_REF] Cano | Resultados de la campana 'Alboran-76[END_REF][START_REF] Gil | Circulacion geostrofica, dinamica de ´´ḿ esoescala y fertilizacion de aguas someras en el sector notre del Mar de Alboran[END_REF] . Ž . [START_REF] Rodrıguez | Patterns in the size structure of the phytoplankton community in the deep fluorescence maximum of the Alboran[END_REF]. Minas et al. 1991 used diagram analysis for study of water exchange through the Strait and demonstrated that in the upwelling area along the Spanish coast, nutrients are largely transported to the photic layer. They concluded that this process is responsible for bioproductivity increase along the Spanish coast and for formation of the Alboran oxygen minimum. As seen from distribution of salinity in Fig. 3, this upwelling is found in the northern part of the eastern section between stations E11 and E8. At these stations, we did not Ž . observe oxygen oversaturation Fig. 6 , but we found increased TOC concentrations up to 73 mM C. This suggests that higher TOC concentrations in proxim-Ž . ity of Gibraltar Stn. E11-E8 were due to organic material accumulation very likely produced in the northeastern part of the strait rather than local phytoplankton production. Fig. 6 shows also a slight oxygen oversaturation of the surface layer along the Ž .

Ž

. western section up to 4% and in the southern part Ž . of the eastern section Stn. E1-E5, up to 5% . As indicated by Fig. 4, these areas are also characterised by high TOC concentrations that allow us to hypothesize on production of organic carbon by phytoplankton.

However, Atlantic inflow may also explain some high TOC concentrations observed in the Strait. In their paper on trace metal studies in the Atlantic Ž . inflow, [START_REF] Van Geen | Entrainment of trace-metal-enriched Atlantic-shelf water in the inflow to the[END_REF] Dafner et al., 1999 . This high DOC concentration level might be due to resuspension of fresh bottom fluff settled during the last productive cycle or terrestrial input. In this case, the hypothesis of TOC enrichment in the Gulf of Cadiz due to TOC-rich Atlantic waters cannot be ṕrecluded.

TOC pool in the Mediterranean outflow

The origin of the Mediterranean outflow is an unresolved question among physical oceanographers. There is the evidence that at the time of our observa-Ž . tions, LIW contributed to the outflow Section 3.1 Ž . although at the southern stations E2-E5 we have observed diluted WMDW with salinity values of about 38. 45-38.46. According to extensive hydrological observations from the literature, the outflow seems to fluctuate between waters rich in LIW and waters rich in WMDW. One permanent characteristic Ž . of LIW is an oxygen minimum Fig. 4, Stn. E7 , which is located mainly in the northern sector of Alboran anticyclonic gyre and related to increased Ž productivity in the Atlantic inflow Packard et al., . 1988;[START_REF] Minas | Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the Ž . Western Alboran Sea[END_REF] . The upwelling in the northwestern part of the Alboran Sea and the resulting plankton growth prove that a potential source of organic matter, which is required to sustain an enhanced oxygen utilisation rate.

The lowest TOC concentrations observed in the Western Alboran Sea suggest that the refractory pool of organic carbon in the LIW is likely to range from 38 to 42 mM C. This estimate is slightly lower than the 44-52 mM C that was recently presented for this Ž water in the Catalan-Balearic Sea Doval et al., . Ž 1999 and the Alboran Sea Sempere et al., unpub-´. lished observation . The renewal time-scale of water masses, which form the Mediterranean outflow Ž ranges from 7-10 Andrie and Merlivat, 1988;

. Ž [START_REF] Christensen | Carbon oxidation in the deep Mediterranean Sea: evidence for dissolved organic Ž . carbon source[END_REF]to 100 years Lacombe et . al., 1981;[START_REF] Gascard | Water masses and circulation in the western Alboran Sea and in the Strait of Gibraltar[END_REF] for the WMDW, Ž and from 10 to 20 years for the LIW Stratford and . Williams, 1997 . Although LIW is younger than deep oceanic waters, the TOC values reported here are close to those measured for the very old deep Ž . ocean waters [START_REF] Hansell | Deep ocean gradient in concentrations of dissolved organic carbon[END_REF] . These findings argue for the participation of bacteria turnover of the organic material inside the LIW on its pathway to the Strait of Gibraltar from the Cata-Ž . lan-Balearic Sea. More recently, Packard et al. 1988 have emphasised that metabolic activity, bacterial biomass, and particulate organic material should be higher in the LIW of the Western Alboran Sea than farther to the east, and especially higher than in the LIW of the Balearic Sea.

High TOC concentrations within the Mediter-Ž . ranean outflow ca. 60 mM C were found in the Gulf of Cadiz. . Benabdeljelil, personal communication, . 1998 . This feature suggests that before overflowing through the Spartel sill, the Mediterranean outflow could delay for some time within the Tangier basin. As a working hypothesis, we assume that enrichment of the Mediterranean outflow by organic carbon in this basin occurs either via resuspension of bottom sediments or via input of particulate organic carbon from SAW and its deeper resuspension. To check this hypothesis, additional research in the Tangier basin is required.

TOC Õs. salinity relationship

The analysis presented in Section 3.2 does not take explicit account of tidal variability in exchange ( ) E.V. Dafner et al.r Marine Chemistry 73 2001 233-252 245 Fig. 7. Scatter plot of TOC concentrations vs. salinity. In the Gulf of Cadiz and at Tarifa Narrows, strong Atlantic inflow is marked by śalinity lower than 36.50, Mediterranean outflow is traced by salinity greater than 38.00 and intermediate salinities depict interface between two layers for which substantial periods of both inflow and outflow. The 37.00 isohaline approximated an average position of the interface Ž . layer above the Spartel sill, after [START_REF] Bryden | Flow through the Strait of Gibraltar[END_REF] In the Western Alboran Sea, the average location of interface layer is shown by the Ž 2 . isohaline 37.50. The equation of the regression line in the Western Alboran Sea is y sy 0.085 x q 42.04 r s 0.69, n s 120 . through the Strait. Most of the outflow occurs in bursts during the semidiurnal tide cycle: when the outflowing tide is strongest, the interface is relatively shallow, so there is a thick layer of the Mediter-Table 4 Comparison of the slopes of DOC vs. AOU relationship by historical data ranean waters flowing swiftly over the Camarinal sill. Conversely, on the inflowing tide, the interface is relatively deep, so there is a thick layer of Atlantic water flowing swiftly into the Mediterranean. Such bursts account for more than half of the exchange across the sill between the Atlantic and Mediter-Ž ranean 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 the tide fluctuations, a relationship between TOC and salinity was developed. Fig. 7 shows several features of this TOC vs. salinity relationship. Firstly, in the Atlantic inflow and Mediterranean outflow, the salinity values were higher in the Western Alboran Sea than in the Gulf of Cadiz. Secondly, a larger scatter of TOC 2 . This water mass is usually observed only above the Spartel sill and is not involved in water mass exchange through the Strait of Gibraltar, but NACW influences the properties of underlying and overlaying waters Ž . due to entrainment [START_REF] Price | Mediterranean outflow mixing and dynamics[END_REF] . TOC concen- 3 , we observed a lower Ž . scatter of both parameters Fig. 7 ; consequently, the Ž 2 TOC pool correlates with salinity r s 0.69, n s . 120 .

For the Mediterranean waters with salinities greater than 38.00, there is strong outflow. Indeed, Ž the lowest TOC content was found there 42-60 mM C from the Atlantic Ocean and 38-52 mM C from . the Mediterranean Sea sides . For intermediate salinities, there are substantial periods of both inflow and outflow. This transition zone shows a coincidence of intermediate values of salinity as well as TOC. An average inflow is marked by a salinity lower than 37.00 along the western section and about 37.50 along the eastern section. The transition between inflow and outflow occurs, on average, at a salinity Ž . of 37.00 and 37.50, respectively Fig. 7 . In the Western Alboran Sea, TOC exhibits a Ž positive linear correlation with fluorescence Fig. 8; . r s 0.73, n s 120 and increased fluorescence values Ž in the SAW are inversely related to AOU Fig. 8; . r s 0.79, n s 120 . In the Gulf of Cadiz, the rela- . nas, 1993a,b;[START_REF] Antoine | Algal pigment distri-bution and primary production in the eastern Mediterranean as derived from coastal zone colour scanner observations[END_REF] Historical data on the DOC vs. AOU relationship, summarised in Table 4, suggest a large variability of ( ) E.V. Dafner et al.r Marine Chemistry 73 2001 233-252 248 the slope in different areas of the ocean. One of the problems comparing the historical data in Table 4 is Ž . that prior to [START_REF] Peltzer | Spatial and temporal variability of total organic carbon along 1408W in the equatorial Ž . Pacific Ocean in 1992[END_REF] all authors used a model I regression causing a negative bias in their slopes in addition to the natural variability. Since then, most authors have used the geometric Ž . mean regression a model II regression . Thus, some of the scatter in the numbers shown in Table 4 is due to the different statistical treatment and we cannot make corrections without the original data.

Relationships of TOC

The major difficulty in interpretation of the relationship of TOC vs. AOU arises from an estimate of the preformed TOC concentrations, while the preformed AOU is at equilibrium with the atmosphere Ž . Ž . [START_REF] Toggweiler | Is the downward dissolved organic matter Ž . DOM flux important in carbon transport?[END_REF], 1992[START_REF] Tanoue | Vertical distribution of dissolved organic carbon in the North Pacific as determined by the high-temperature catalytic oxidation method[END_REF] have shown that a single linear regression analysis throughout the entire water column was inappropriate for estimations of the contribution of DOC to the utilisation of oxygen in the water column. In general, correlation was found when subsurface and intermediate levels are considered separately, i.e., variations of both Ž . AOU and probably DOC were as much due to a Ž mixing as to biogeochemical processes [START_REF] Thomas | Dissolved organic carbon in the equatorial Atlantic Ocean[END_REF] From both sides of the Strait of Gibraltar, we have obtained similar correlations and slopes throughout the entire water column, which lay approximately in middle of the range presented in Table 4. The inverse relationship from the Strait of Gibraltar was linear and the slope of y 0.25 was lower than the y 0.77 reported by Redfield et al.

Ž

. 1963 for organic matter of average marine composition. From only the vertical profile of both property distributions, it is impossible to conclude if these correlations are due to DOC consumption by bacteria or mixing. To evaluate the contribution of TOC to AOU, an analysis of both these properties along Ž isopycnal surfaces is needed Doval and Hansell, . 2000 , which we could not perform in this work due to a poor spatial resolution of sampling and a large variability of the dynamical environments. Theoretically, the slope, which we found in the Strait of Gibraltar area, suggests that bacterial remineralization of organic matter accounts for approximately one-third of the oxygen deficit in surface and deep waters. This value is higher than that reported by Ž . Ogawa and Ogura 1992 for the Northwestern Pa-Ž . cific, by [START_REF] Peltzer | Spatial and temporal variability of total organic carbon along 1408W in the equatorial Ž . Pacific Ocean in 1992[END_REF] for the equato-

Ž

. rial Pacific and by [START_REF] Wiebinga | Determination of the distribution of dissolved organic carbon in the Indian sector of the Southern Ocean[END_REF] for the Indian sector of the Southern Ocean.

Organic carbon exchange through the Strait of Gibraltar

Because of the great variability of the dynamic environment, it is difficult to assign from direct observations mean values of typical water transports for a TOC balance computation. Lacombe and Richez Ž . 1982 have pointed out that with the complex spatial and temporal variations of the flow, only approximate values of transport are available. Bryden and Ž . Kinder 1988 , reviewing historical balance estimations, have concluded that the flows through the Strait are apparently limited 1 to 2 = 10 6 m 3 s y 1 . The new transports through the Strait of Gibraltar are 30% smaller than historical estimates and, furthermore, tidal oscillations contribute nearly half of the Ž . total transport [START_REF] Bryden | Recent progress in strait dynamics[END_REF] . Consequently, for a TOC pool budget calculation, we cannot use general schemes of organic carbon esti-Ž mation as designed for the open ocean e.g., [START_REF] Walsh | Meridional fluxes of dissolved organic matter in the North Atlantic Ocean[END_REF][START_REF] Peltzer | Spatial and temporal variability of total organic carbon along 1408W in the equatorial Ž . Pacific Ocean in 1992[END_REF] Here, we present a TOC budget, which was calculated according to the water transport estimates based Ž on the data of Gibraltar experiment [START_REF] Bryden | Exchange through the Strait of Gibraltar[END_REF]Bray et al., 1995 and CANIGO observations summarised in Fig. 9. The estimate of the TOC transport was made by simply dividing the interface Ž layer according to the salinity values 37.00 and 37.50 from western and eastern entrances of the . Strait, respectively and incorporating the upper part Ž . of the interface into the upper layer Atlantic inflow Ž and the lower part into the lower layer Mediter-. ranean outflow . TOC transport in the Atlantic inflow and Mediterranean outflow was calculated as the product of the water transport and the average TOC concentration in each water layer along western and eastern sections and at Tarifa Narrows.

Fig. 9 shows that TOC transport through the Strait varies from 3.7 = 10 4 to 4.8 = 10 4 mol C s y 1 for the Atlantic inflow and from 2.8 = 10 4 to 3.8 = 10 4 mol C s y 1 for the Mediterranean outflow. Differences of TOC transport between Atlantic inflow and Mediterranean outflow give an estimate of TOC balance over the water column, which are in the 4 4 y 1 Ž range of 0.9 = 10 to 1.0 = 10 mol C s or ´. al., 1988 ; they have found that inflow of these organic compounds exceeded their outflow from the Mediterranean Sea. Our data together with results quoted above indicate that inflow of organic material to the Mediterranean Sea through the Strait of Gibraltar is considerably higher than outflow. We think that this feature is mostly due to the high biological productivity of the Gulf of Cadiz, where Ž . we observed high DOC concentrations 89 mM C Ž . even in the winter season Dafner et al., 1999 . An indirect estimate of the organic carbon budget through the Strait of Gibraltar was also recently Ž . evaluated by Copin-Montegut 1993 . She presented á value as high as 1.25 = 10 12 mol C year y 1 , that is, threefold to fourfold higher than our estimates. Polat Gibraltar is two orders of magnitude higher than through the Turkish Straits and than Mediterranean River inputs. The Mediterranean outflow through the Strait of Gibraltar is also higher by three orders of magnitude than through the Turkish Straits. It is likely that these budgets are reflections of the productivity of the seas adjoining to the straits-the mesotrophic Alboran Sea to the east from the Strait of Gibraltar, and the oligotrophic Aegean Sea to the west from the Turkish Straits.

Conclusions

TOC concentrations found in the surface layer of the Strait of Gibraltar area are in agreement with those previously reported for the Atlantic Ocean and Ž . Mediterranean Sea ca. 60-70 mM C . Our estimates of the TOC pool in the deep waters of the Western Ž . Alboran Sea 38-52 mM C differ slightly from those published recently for the Mediterranean Sea, suggesting on a participation of bacteria on turnover of the organic material inside the LIW on its way to the Strait of Gibraltar. The correlation between TOC and salinity was found in the Western Alboran Sea Ž . r s 0.69, n s 120 where the water column was characterised by a thicker layer of SAW and by a thinner layer of the Mediterranean waters. In the Gulf of Cadiz, the water column was under the ínfluence of the short-term variability caused by tides, SAW was thicker, Mediterranean waters was thinner and an additional water body was found Ž . modified NACW . Accordingly, this relationship was characterised by a large bias. The correlations between TOC and AOU, and TOC and fluorescence over the entire water column are observed. These correlations are due to the decrease of both properties with depth rather than biological processes.

For calculation of the TOC transport, the two-layer model of water mass exchange through that Strait was used. Estimates of the TOC budget ranged from 0.28 = 10 12 to 0.35 = 10 12 mol C year y 1 . Our computations of the TOC budget suggest that TOC inflow is two-and outflow is three orders of magnitude higher through the Strait of Gibraltar then via the Turkish Straits. The amount of TOC transporting throughout the Strait of Gibraltar show that the TOC flux by Atlantic inflow and the Mediterranean outflow cannot be ignored in the modelling of carbon cycle in the Mediterranean Sea and the eastern North Atlantic Ocean.

  Bathymetry of the Strait of Gibraltar and stations grid during cruise 232 onboard of the RRV Discovery 12-18 April 1998 . To the east of the Strait lies the Alboran Sea, the westernmost basin of the Mediterranean Sea. To the west of the Strait lies the Gulf of Cadiz, an émbayment of the Northeast Atlantic Ocean.

  Typical four vertical profiles of salinity, oxygen mM kg and TOC mM C from the western Sts. W3, W4 and eastern Sts. E4, . E7 sides of the Strait of Gibraltar. Minimum salinity from the western entrance of the Strait correspond to NACW. Minimum oxygen at Stn. E7 is characterised the residual core of LIW.

  Fig. 3 depicts distributions of salinity along both sections. The Atlantic inflow in the Gulf of Cadiz is formed Ž . by the mixing of the SAW salinity about of 36.46 , Ž North Atlantic Central Water NACW, salinity less . than 36.06 , and Spanish Coastal Water with salinity Ž . Ž . Fig. 3. Distribution of salinity on western Gulf of Cadiz and eastern Western Alboran Sea entrances of the Strait of Gibraltar. Hereafter, the bottom represents the depth observed at each station and is not typical bathymetric profiles.

  .Dafner et al.r Marine Chemistry 73 2001 233-252 241 stress over the west Mediterranean Sea and the Gulf Ž . of Cadiz[START_REF] Grundlich | On the observation of a solitary event in ¨the Mediterranean outflow west of Gibraltar. AMeteorB Forschungsergeb[END_REF] , formation of the WMDW ´during the winter convection, and features of the Ž Alboran Gyre circulation[START_REF] Bryden | Origins of the Mediterranean outflow[END_REF] . As a result, the water flow in the Strait does not move in the form of continuous currents but as tidal-induced pulses, which vary in the amount of water they contain according to the daily and monthly variation in the above-mentioned factors.3.2. TOC Õertical distributionAll data from the Strait of Gibraltar are presented Ž . along two sections in Fig. 4: western Gulf of Cadiz Ž . and eastern Western Alboran Sea . Both these sections have been characterised by similar features of Ž . TOC distribution: i a decrease of TOC content Ž . from the Gulf of Cadiz towards the Alboran Sea; ii án increase of TOC concentrations in the Atlantic Ž . Ž . Ž . Fig. 4. TOC contour plots mM C along western Gulf of Cadiz and eastern Western Alboran Sea entrances of the Strait of Gibraltar.´ ( ) E.V.Dafner et al.r Marine Chemistry 73 2001 233-252 242 inflow from the middle of the sections towards the Ž . continents; iii a decrease of TOC concentrations from the Atlantic inflow to the Mediterranean out-Ž . Ž flow, and iv a patchiness of TOC inventory mM . C below the Atlantic inflow. This patchiness was due either to sampling during different times of the tide, particularly in the Gulf of Cadiz, or to different ćomposition of the water layers from both sides of the Strait. In the Western Alboran Sea, water column was composed only by SAW, Mediterranean waters and the interface layer in between. Figs.2 and 3, and Table2show that at several stations above the Ž . Spartel sill Stn. W2-W6 additionally modified NACW was observed.We summarize all measurements in one vertical Ž . profile Fig.5, which shows a large scatter of TOC Ž . concentrations in the upper 200 m 50-79 mM C including SAW, NACW, interface layer and upper Mediterranean waters, and a decrease down to 38-48 mM C in deep waters. At several depths, the scatter Ž . is due to joining into one profile near-bottom measurements from the Gulf of Cadiz and pelagic obser-vations from the same depth from the Western Alboran Sea. Some of the stations from both sides of the Ž . Strait W4, W5, W11 and E2, E3, E4, E5, E7, E10 were characterised by an increase of TOC concentra-Ž tions in the near-bottom layer Stn. W4 and E4 in . Fig. 4 , which could be a result of bottom sediment resuspension.

Fig. 6 .

 6 Oxygen saturation % in the upper 100 m layer. Note that the area between Stn. E8-E10 was characterised by upwelling.( ) E.V.Dafner et al.r Marine Chemistry 73 2001 233-252 244

  Figs. 2 and 4 and show that at Stn. Ẃ4, TOC concentrations increased from 51 mM C at the depth of 325 m to 60 mM C in the near-bottom Ž . Ž . layer 415 m . Armi and Farmer 1989 have observed that the Tangier basin acts as a reservoir of the Mediterranean waters that fills and drains on each tidal cycle. Acoustic Doppler Current Profiler data from our cruise have shown that different layers of the Mediterranean waters in this basin could flow to the west as well as to the east, depending on the Ž tide A

  ánd salinity values was found in the Gulf of Cadiz than in the Western Alboran Sea. According to Ž . Bryden et al. 1989 , east of the Camarinal sill salinities lower than 36.50 indicate a strong inflow. This layer is characterised by the largest scatter of Ž . TOC concentrations 48-79 mM C . Note that modified NACW was found between SAW and the inter-Ž . face layer or within SAW Fig. 2, Table

Fig. 8 .

 8 Fig. 8. Composite of TOC vs. AOU, TOC vs. fluorescence, and AOU vs. fluorescence for the western and eastern entrances of the Strait of Gibraltar.

  Õs. AOU, TOC Õs. fluorescence and AOU Õs. fluorescence Recent studies have shown an inverse relation Ž between DOC and apparent oxygen utilisation AOU, . Table 4 , since both DOC and oxygen are used by bacteria, which in turn produce CO through respira-2 Ž . tion. In contrast, Martin and Fitzwater 1992 and Ž . Hansell et al. 1993 have reported weak or insignificant correlation between DOC and AOU. Our observations, pooling Atlantic inflow and Mediterranean outflow together, show the former correlation. Fig. 8 shows plots of TOC vs. AOU, AOU vs. fluorescence Ž . in Volts , and TOC vs. fluorescence from the eastern and western entrances of the Strait. The slopes and intersects of lines drawn through the data points Ž . of TOC vs. AOU Fig. 8 and TOC vs. fluorescence Ž . Fig. 8 at both sections are quite similar and the differences in correlation, presumably, are due to the different number of observations.

  tionship of AOU vs. fluorescence, by contrast, ex-Ž . hibits a great scatter Fig. 8 , but the TOC vs. Ž . fluorescence relationship is still significant Fig. 8 . A simple comparison between AOU and fluores-Ž . cence from both sections Fig. 8 suggests that variability of fluorescence values in the Gulf of Cadiz ẃas lower than in the Eastern Alboran Sea. Large fluorescence scatter from the eastern entrance of the Ž Strait is due primarily to deeper sampling down to 800 m to the east and down to 340 m to the west of . the Strait that explains the scatter of low fluores-Ž . cence values ; 0.5 V . Secondly, in the Eastern Alboran Sea, higher photosynthetic activities could Ž explain the scatter of high fluorescence values ) 1.5 . V . SAW is almost always easily discernible in the Gulf of Cadiz and in the Alboran Sea in satellite ´Ž images of chlorophyll a distribution Minas and Mi-

  Fig. 9. Two layers model of water masses exchange through the Strait of Gibraltar with average values of salinity, mass transports in the Ž . Atlantic inflow and Mediterranean outflow based on the data of the Gibraltar experiment Bryden et al., 1994; Bray et al., 1995 and CANIGO observations. These values of mass transport and average TOC concentration in each water layer were used for calculations of Ž . Ž . TOC transports lower panel and balances upper panel along the Strait.

  15 = 10 mol C year for organic carbon export from the Marmara to the Aegean Sea and about of 0.41 = 10 4 tons C year y 1 Ž 9 y 1 . 0.34 = 10 mol C year for the outflow from the Aegean Sea to the Marmara Sea. River TOC inputs have been estimated to 20-83 = 10 10 mol C year 1 Ž . Sempere et al., 1999 and references therein . These ´data indicate that TOC inflow through the Strait of

Table 1

 1 Position and depth of the stations, date and time of sampling during the cruise 232 of the RRV Discovery in the Strait of Gibraltar area

	Standardisation was carried out every day using Ž . potassium hydrogen phthalate Kanto Chemical dis-Ž . solved in DW 3 concentrations prepared just before
	sample analyses. The instrument response factor,
	measured as the slope of the standard addition to Ž 2 . DW r ) 0.999 for 19 runs , remained relatively
	constant and reproducible over the time of analysis.
	Calibration curves have exhibited little difference in Ž the slope ave. slope: 6045 " 242 arearmM C, n s . Ž 19 and intersect ave. intersect: 800 " 164 area

Table 2

 2 Characteristics of the different water bodies observed above the Spartel sill in April 1998

	Mediterranean deep water	Depth, Salinity TOC, mM	m average			249-343 38.05-38.37 49	313-419 38.02-38.15 55
	Interface layer	Depth, Salinity TOC, mM	m average			226-249 36.37-38.05 50	207-313 36.52-38.02 47			173-271 36.50-37.99 48	121-189 36.51-37.60	137-167 36.50-36.85 53
	North Atlantic central water	Depth, Salinity TOC, mM	m average		113-197 36.06-35.93 54	143-215 36.06-35.96 51	125-171 36.06-35.97 46	93-115 36.06-35.98 50	105-111 36.06-36.05 48
	N stations Surface Atlantic water	Depth, Salinity TOC, mM	m average	W1 0-101 36.38-36.06 60	W2 0-113 36.38-36.12 63	W3 0-226 36.42-36.37 61	W4 0-207 36.31-36.21 60	W5 0-91 36.30-36.18 59	W6 0-149 36.26-36.12 58	W7 0-173 36.24-36.50 61	W8 0-121 36.27-36.51 63	W9 0-137 36.40-36.50 62	W10 0-107 36.28-36.44 65	W11 0-177 36.35-36.45 63

Position of the interface layer was determined by the range of salinity between 36.50 and 38.00; modified NACW by salinity lower than 36.06. Stn. W8 was sampled only down to 51 m depth. Note that all stations were sampled during different tides.

  . The thickness of SAW in the Strait is highly variable and in April 1998 it decreased Ž . from about 100 m south of the Spartel sill Table2to about 40-55 m at the Tarifa Narrows and to about[START_REF] Parrilla | The physical oceanography of the[END_REF] . The residual LIW is found preferentially in the northwestern part of the Alboran Sea in a depth band of 200-600 m and is essentially absent near the African coast. As shown in Fig.3and Table3, we observed the salinity values close to those listed above in April 1998 at the eastern section at Stn. E6 to E10, and at the Tarifa Narrows. WMDW is characterised by a de-

		Ž	.
	Ž 15-40 m in the Western Alboran Sea Table 3 . .
	The interface layer is defined by a well-developed
	halocline with a change in salinity of about 2.00 Ž . Fig. 2 . For identification of the interface layer, Ž . Lacombe and Richez 1982 have suggested a range of salinity values between 37.00 and 37.50. Bryden Ž . et al. 1989 have characterised this layer in the	Ž crease in salinity and temperature 38.42-38.44 and . -12.908C . WMDW is formed by the winter con-Ž vection in the south of France Medoc Group, 1970; . Stommel, 1972 . The salinity distribution in Fig. 3
	southern part of the Camarinal sill by a salinity range	demonstrates that in the Western Alboran Sea, values
	of 36.50-38.00. The interface layer was found to be	of salinity in the near-bottom layer were always
	deeper, thicker, fresher and colder on the west of the Ž . Strait Table 2 . The shallowest position of this layer	higher than those observed for WMDW. Diluted Ž . WMDW salinity of about 38.45 was only found at
	was found in the Western Alboran Sea, where it Ž . continued to get saltier and warmer Table 3 .	the southern most stations E5 to E2. A similar situation, which indicates a small contribution of
	The Mediterranean outflow is marked by a high salinity and temperature, and is formed by the mix-Ž . ing of Levantine Intermediate Water LIW and Ž . Western Mediterranean Deep Water WMDW . The LIW originates in the Eastern Mediterranean and is characterised by an intermediate maximum in tem-Ž . Ž . perature about 13.158C and salinity about 38.47	WMDW to the outflow, has been described previ-Ž . ously by Roether and Weiss 1975 , Measures and Ž . Ž . Edmond 1988 and Minas et al. 1991 . The water exchange through the Strait is under Ž the influence of periodic forces, such as tides La . Violette and Lacombe, 1988 , and nonperiodic forces, Ž . such as atmospheric pressure Stanton, 1983 , wind

Table 3 Ž

 3 .Characteristics of the different water bodies observed at the Tarifa Narrows Stn. 13386 and 13399 , and at the Western Alboran Sea in Positions of the basis of SAW and the interface layer were determined by the phenomenological criteria vertical distribution of salinity individually for each station. Note that all stations were sampled during different tides.

	April 1998
	N stations Surface Atlantic water		Interface layer		Mediterranean deep water
		Depth, Salinity	TOC, mM Depth,	Salinity	TOC, mM average Depth,	Salinity	TOC, mM
		m		average	m		average	m	average
	13386	0-55	36.29-36.51 54	55-110 36.51-37.98 46	110-549 37.98-38.47 39
	13399	0-41	36.34-36.54 61	41-100 36.54-38.00 48	100-611 38.00-38.48 41
	E1	0-63	36.51-36.80 64	63-85	36.80-37.01 60
	E2	0-29	36.35-36.51 63	29-127 36.51-38.01 59	127-435 38.01-38.45 47
	E3	0-45	36.51-36.60 60	45-133 36.60-38.02 56	133-459 38.02-38.46 44
	E4	0-31	36.50-36.60 63	31-129 36.60-38.01 55	129-451 38.01-38.46 45
	E5	0-45	36.59-36.61 61	45-135 36.61-38.02 54	135-519 38.02-38.47 45
	E6	0-67	36.46-36.54 54	67-113 36.54-38.01 50	113-557 38.01-38.47 41
	E7	0-33	36.28-36.51 59	33-135 36.51-38.02 51	135-831 38.02-38.45 45
	E8	0-17	36.96-37.10 66	17-77	37.10-38.01 52	77-789	38.01-38.46 42
	E9	0-15	37.59-37.60 58	15-85	37.60-38.00 53	85-675	38.00-38.48 44
	E10	0-37	37.54-38.00 65	37-135 38.00-38.40 50	135-353 38.40-38.48 46
	E11	0-37	37.84-38.10 55	37-71	38.10-38.33 47	71-111	38.33-38.40 46
								Ž	.

  have shown that high trace-metal concentrations in the surface water of the Alboran Sea may be due to enriched Atlantic waters flowing into the Mediterranean Sea through the Gulf of Cadiz. Our recent results indicate high DOC

	´Ž concentrations up to 89 mM C in the Gulf of Cadiz . Ž . even during the winter season

Acknowledgements

We thank the crew of the RRV Discovery for help and support. We acknowledge R. Boscolo and E. Oikonomou for allowing us to use their unpublished data for oxygen. Our special thanks go to J.H. Sharp, who kindly provided the Deep Atlantic and low carbon water reference standards. We thank P.J. Wangersky, D.A. Hansell and G. Cauwet for critical reading and constructive comments on the development of the manuscript. This research was funded by the European Commission, MAST III Programme Ž . Contract MAS3-CT96-0060 . Financial support for E.V. Dafner came from Ministere Affaires Etrangeres Francais and Conseil General des Bouches du Rhone, ´´F rance. Some of the ideas presented here reflect discussions held at several CANIGO Workshops.

 Dafner et al.r Marine Chemistry 73 2001 233-252