Carbon dioxide system in the Canary region during October 1995*

SUMMARY: During the cruise F/S Poseidon 212/3 (September 30-October 8, 1995) determination of carbon system vari- ables was carried out over the section of La Palma-La Graciosa and at the ESTOC station in the Canary Island area. Total alkalinity and pH in the total scale at 25ºC were determined at 24 stations from surface to bottom. In this area, the presence of different water masses can be traced by the carbon system variables. NACW is defined by a strong gradient of A T and pH from 150 to 750 m. MW is characterised by high values of A T and pH between 1000 to 1200 m and AAIW signals are found at around 900 m in the strait between Gran Canaria and Fuerteventura with low A T , low pH and a maximum of f CO 2 . Assuming an atmospheric mean value of f CO 2 of 360 µatm and an average surface value of 393±7 µatm, we can conclude that during this cruise this oceanic area tends to release CO 2 into the atmosphere, acting as a weak source with a carbon flux towards the atmosphere of +8.0±1.8 mmol·m -2 d -1 . The saturation levels in the Canary Island area have been found to be high- er than 3600 m for calcite and 2700 m for aragonite. The inorganic carbon/organic carbon ratio (IC/OC) varies from 0.07 at 300 m to 0.5 at 3000 m. The IC/OC ratio shows that about a 34% increase in the C T of the deep water is contributed by the inorganic CaCO 3 dissolution. The IC at 300 m is around 7 µmol kg -1 , increasing with depth to 37.5 µmol kg -1 at 3700 m.


INTRODUCTION
The carbonate system in seawater is one of the most complex topics in oceanography.The system has !mg inkreste:!mazy oceancg:aphers fium va:ious fields since it plays ari iniportarit rvle in the biogeochemical cycles of three sub-spheres of the Earth, the biosphere.the lithosphere, and the hydrosphere (2hen anci Wang i995j.More recentiy, the fate or fossil fue1 CO, has promoted interest in the study of carbonate chemistry in the oceans, because of the *Received October  ---------.---------atrnosphere due to the buming of fossil fuels is absorbed by the oceans (Post et al., 1990;Hougton et al., 1995).The capacity of the oceans for uptake of SG, depends on tkc inorganic carbon chemistry and also depends greatly on many factors such as Iiydrography, circulation uf water masses, mixedlayer dynamics, wind stresses and the biological processes in the ocean (Broecker and Peng, 1982).The North Atlantic, with oceanic high latitude regions of deep water formation, mid-latitude sites of mode water forniation and subtropical oligotrophic oceans, is thought to be a large sink for atmospheric CO, (Tans et al., 1990; Takahashi et al.,  1993, 1995).The Canary Oceanic region (Fig. 1) is situated in a peculiar area 100-600 km west of the NW African coast in the eastern extensions of the subtropical North Atlantic gyre at a latitude of 27-28"N.It is part of the recirculation regime linking the Gulf Stream with the North Equatorial Current via the Azores and Canary Currents (Stramma andSiedler, 1988, Klein andSiedler, 1989).The structure of the Canary Current System is strongly influenced by the seasonally varying trade winds and the  (Stramma and Siedler, 1988).The thermohaline properties of water masses involved in the water column in the Eastern North Atlantic have been extensively described in previous studies (Broecker and  Takahashi, 198 1, Arhan er al., 1994, Measures et al,  1995).Below North-Atlantic Central Water (NACW), the penetration and influence of both Nriíarciic iniermediaic Water (AAIW) ai around 900 m and Mediterranean Water (MW) between 900 and 1500 m are clearly ohserved in the North Atlantic (Fig. 2a).
Euring the Poseicion 2 i 2i3 cruise (36 Septem~er-8 October 1995), the determination of carbon system variables was carried out north of the Canary Islands between La Palma and La Graciosa.Total alkalinity and pH at total scale and at 25°C were determined at 24 stations from surface to bottom.The main objective of this work was to study for the first time the carbon cycle in this area in order to evaluate the capacity of the zone for removing anthropogenic carbon dioxide from the atmosphere and to calculate both the saturated state of CaCO, and the ratio of in situ inorganic and organic carbon decomposition (ICIOC).
EXPERIMENTAL -Thr pH in tetu! sca!e (m=! kg.') was measwed at 25°C using the potentiometric technique.The electrodes used to measure the e.m.f. of the sample consisted of a ROSS glass pH electrode and an Orion double junction AgIAgCl reference electrode, connected to an Orion 720A pH meter.The electrodes were calibrated using a TRISIHCI buffer in synthetic seawater with a salinity of 35 and corrected foliowing ihc iast recommenáaiions by Uei'vaiis anci Dickson (1998) andLee et al. (2000).The tris buffer and the seawater samples were measured at 25OC, which allows the e.m.f. of the pH cell to be measureci, ñrst in the tris buffer anci then in the seawater sample.The pH of the unknown seawater samples was determined according to standard operating procedures (Dickson and Goyet, 1994).

Total alkalinity
Tlie lotal alkaliriity uf xawater (A,) wah deterrriiried by titration with HCI to the carbonic acid end point using two puteri~iornelric systenis and described in more detail in Mintrop et al. (2000).The HC1 solution (25 1,0.25 M) was made from concentrated analytical grade HC1 (Mercko, Darmstadt, Gennany) in 0.45 M NaC1, in order to yield an ionic strength similar to open ocean seawater.The acid was standardised by titrating weighed amounts of Na,CO, dissolved in 0.7 M NaCl solutions.The total alkalinity of seawater was evaluated from the proton hz!.cnce the a!k:i!inity equivi.!ence p ~i n t , pFTequ,\ = 4.5, according to the exact definition of total alkalinity (Dickson, 1 Y8 1).The performance of the titration systems was monitored by titrating different samples of certified reference material (CRM, batch #35) that have known inorganic carbon and A, values.The agreement betwecn our data and certificd valucs was within k1.5 pmol kg-'.

pH and alkalinity vertical profiles
Seawater pH reflects the status of the carbon dioxide system, which provides the major shortterm pH buffer.In turn, the carbonate system is intimately linked with biological productivity through the processes of photosynthesis and respiratioii.Biotic production and decomposition affcct thc pH in line with the equation (Anderson, 1983; Fraga uní! Percz, !??V; Frugu et u/., 1998) The vertical profiles of pII on a total sc, ,i l e at 25°C are shown in Figure 2b.The pH reaches a rriaxiinuni iri surface waters (8.035 k0.005) tlue to photosyrithesis.The pH tlieri decreases due to the oxidation of plant material arid exhibits a sharp decrease with depth tu approximately 1000 m coincident with a minimum in 0, and maximum in apparent oxygen ulilisation (AOU) (Llinás er al., 1999)  (Fig. 2b), minimum alkalinity (Fig. 3) and maxiinum P O , (592 yatm) (Fig. 4) between stations 864 and 866, which correspond with the inflow of AAIW through the strait between Gran Canaria and Fuerteventura.This inaxirnuni fugacity is also observed at station 846 (La Graciosa).This northern salinity minimum is the result of the northward and eastward advection of AAIW within the Gulf Stream-North Atlantic Current systern (Measures et al., 1995).The influence of AAJW in this area has also been traced by minimum values of oxygen and slightly higher nutrient concentrations (mainly silicate) by Llinás et al. (1999).The maximum salinity at around 1200 m with high values of alkalinity and total dissolved inorganic carbon is due to Mediterranean water (Fig. 2d).At station 871, a liigh maxirnuni salinity denotes the presence of a Meddy.This Meddy also shows a relative maximum of pH (Fig. 2b) and a sharp increase in alkalinity (Figs.2c and 3).

State of saturation
The precipitation or formation of solid CaCO, in surface waters and the dissolution of solid CaCO, in deep water is very important iii transferring CO, from surface waters to deep waters.A close coupling of seasonal phytoplankton maxima and particle flux peaks at thc ESTOC station has been found (Neuer et d., 1994).This characteristic of inany areas of the Atlantic has been dernonstrated in the Sargasso Sea by Deuer et al. (1990) and in the North Atlantic Bloom Experiment (NABE , 47"N, 20°W) by Newton et al. (1994).It was mediated by rapid transfer of surface water production to deep ocean.The saturation state of seawater with respect to a carbonate phase (Ri) can b e determined by calculating the ratio of the measured total ion concentration product to the apparent solubility product of carbonate i (KBP,J according to Equation 4. The thermodynamic index of the apparent solubility product from which the degree of saturation is calculated depends oriflO, and pH, pressure, T and niineralogy (aragonite or calcite).
The surface waters in this area are well bupersatiirated with respect to both calcite and aragonite.The surface value of C2 for calcite is 5.6 k0.2 and 3.6 k0.2 for aragonite, both of which decrease in deep water.Aragonite from pteropodos is more soluble than calcite from foraminifera and coccolithophorids at a given T, p and S.During this cruise total coccolithophore cell densities showed a strong gradient from open ocean localization (station 870) with !8.0@3 ~p !!1-1 tg [he nezr-shere !~cutin!! (!?p:ir the African coast) with 45.000 cell 1.'.Maximuin cell densities occurred in the upper photic zone above the deep chlorophyll rriaxirnurn, which was The satiiratioii lcvel Ioi.:irngonite is 3700 i i i aiid for calcite 1s higher ttiaii 3500 ni.The greaier wluhiiity of ihese niinerais in cieep waters reiates to thc cffect of pressure ori the solrihility of CaCO i,,,.

Inorganic and organic derived carbon
The processes i-elated to the desp di\soliitioti o f CaCO, and decornposition of organic carbon provide a direct incchanism f o r thc rcnewal o f carhon and r-elaLeci elernents in the sea and are of f'iiiida-mental interest in the biogeochemical cycles 01' these elements.Studying the nature of the particulate matter in traps in this area, Fischer p t al, (1996) found that coccolithophorids (coccoliths and coccospheres) constitute a dominant part of the particulate matter.The close correlation of organic carbon and carbonate sedimentation at 1000 m and 3000 m trap depths founded by Neuer et al. (1997) confirms the important role of coccolithophorids as primary producers and in the export flux in the Canary Island region during al1 seasons.These authors found a considerable increase in particle flux with depth, probably caused by the interaction of fast sinking particles originating from a primary source region close to the area with those advected laterally from closer to the NW African upwelling margin.
To calculate the quantity of calcium carbonate that has dissolved in the water column, the change in salinity normalised A, and C, values must be determined.It must be considered that the water masses present in this area may have had different A, and C, values at the time when the water masses were formed.Therefore, to calculate properly the ANA, and ANC, values, the preformed values (NATO and NCTO) must be determined.Given the preformed values in a body of water and its present values, it is possible to determine the ratio of in situ inorganic and organic carbon decomposition.Under the assumption that A, is not affected by the invasion of anthropogenic CO,, we do not have to differentiate between historical and contemporary N A O va!upl.H~wever, t h p Nc O l r ~l l i ~c m i i r t T T .uAuVo l"uoL corrected for the injection of anthropogenic CO,.Failure to correct the NC," values for anthropogenic CO, may result in an error of 0.05-0.07 in the inorganic carbon to organic carbon ratio (Chen, 1990).The estimated amount of excess CO, in this area is 162 pmollkg (Kortzinger et al., 1998).Several authors (Edmon, 1974; Chen and Pytkowicz,  1979; Kortzinger et al., 1998) have reported linear correlation between the potential temperature 8 and salinity normalized values of surface A, and C,.The linear regressions calculated from our data set (this cruise and unpublished data, n =42) in order to calculate the preformed values for the deep waters in this area are given by Using Equations 5 and 6, the ratio of carbon contributed to the waters from inorganic sources (IC) to carbon derived from the decomposition of organic matter (OC) can be calculated from Equations 7 to 9 (Chen et al., 1982;1987;Chen, 1990)  The ICIOC was calculated for waters deeper than 300 m and is shown in Figure 5.The ICIOC ratio increuses frorr.G.G? u: ?GG : = 9.5 u: ?GQG n?.These values are higher than the ratio of 0.17 found in the Sea of Japan (Chen et al., 1995) and 0.36 found in the North Pacific (Chen, 1990), and similar to the value of 0.52 found in the Bering Sea (Chen, 1993).An ICIOC value of 0.5 indicates that approximately 34% of the carbon in the deep Canary area is contributed by the dissolution of carbonate particles.It should be pointed out that an error of 15% in the amount of excess CO, in this area only contributes in an error of 2% in the ICIOC ratio.Our results demonstrate that far more carbon is added to the deep ocean from the decomposition of organic matter than from the dissolution of carbonates.Neuer et  al. (1997) obtained that the flux of organic carbon explained more than 70% of the variability of the sedimentation of carbonate and lithogenic material.The increase in ICIOC with depth indicates that carbonate dissolution increases as a function of depth relative to the rate of decomposition.This is consistent with saturation state calculations, which demonstrate that deep waters are more undersaturated with respect to carbonate than shallow waters.
The quantity of inorganic carbon in the water column contributed by the dissolution of carbonate particles may be calculated from Equation 10 1. 1999.Accepted May 15, 2000 meenhoiise effwt of rarhon dinxide nn t h ~ global c!io--..--.-..-.--.--.- .mate.Oceanic uptake is a key part of the global budget of tlie CO, released into the atmosphere by human activitiés.Present estimates indicate that .ihr\..+ nnol, ,f * FIG. 1. -The Northeast Atlantic Ocean at the Canary Islands and stations gnd for POSEIDON 21213 cruise (September 30 -October 8, 1995).
The surface values of computed 1170, for this time of the year clearly show that this area is acting as a source of CO,.Assuming a mean atmosphetic value of.cO, of 360 patm and surface values of 385 to 400 Ciatmr we found that d u h g this cruise this oceanic area tends to release CO, into the atmosphere.The air-sea CO, exchange(inmo1.m-'.d.') is calculated using the foilowing equation FCO, = 0.24.k.S.wO, -..\u -S O 2,,, , > during this cruise was 10 + 3 111 S-'.The Schmidt nuinber, Sc, is determined from a third-order polytiomial fit for the temperature dependence (Wanninkhof, í99.2).The resulting air-sea CO, flux was +8.0 11.8 iiiiiiol.m~?.d l .Considering the observed variability oS,cO, in this area in 1996-1997 (González-Dávila ct ni., 2000), this positive CO, flux can be considered as ri maxirnuni value on an annual scale.Hnwever, on the same scale this value is clearly compensated, showing this arca as a slight sink of CO,(Takahashi P I crl., 1997)