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Physical, chemical and sedimentological aspects of the Mediterranean outflow off Iberia
Deep-Sea Research II 49 (2002) 4163–4177
Physical, chemical and sedimentological aspects of the Mediterranean outflow off Iberia I. Ambara,*, N. Serraa, M.J. Brogueirab, G. Cabec-adasb, F. Abrantesc, P. Freitasc, C. Gonc-alvesb, N. Gonzalezd a
Instituto de Oceanografia, Faculdade de Ci#encias, Universidade de Lisboa, 1749-016 Lisboa, Portugal b Instituto de Investigac-ao * das Pescas e do Mar, Av. de Bras!ılia, 1400 Lisboa, Portugal c ! Instituto Geologico e Mineiro, Estrada da Portela, 2720 Alfragide, Portugal d ! Instituto Espanol * de Oceanograf!ıa, Centro Oceanografico da Coruna, * 15080 La Coruna, * Spain Received 24 April 2000; received in revised form 2 November 2000; accepted 20 November 2000
Abstract A multidisciplinary study of the Mediterranean outflow in the region west of the Strait of Gibraltar and off the southern and southwestern coasts of the Iberian Peninsula was developed in the frame of the Canary Islands Azores Gibraltar Observations Project. Two high-resolution CTD surveys, which included water sampling for chemical (nutrients and dissolved oxygen) and sedimentological analyses, took place in September 1997 (summer cruise) and January 1998 (winter cruise) in the study region. The correspondence between the high-salinity Mediterranean Water (MW) layer and the low-nutrient content and relatively high abundance of particles was a general result. Further details of the thermohaline analysis and of the geostrophic computations, especially for the layer of MW, are compared with the results obtained for the chemical properties and the sedimentological characteristics. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction The intermediate layers of the Northeast Atlantic are mainly occupied by a water mass originating in the Mediterranean Sea, which has a very strong signature both in the thermal and the salinity fields at depths between 500 and 1300 m. It constitutes not only an important salt and heat source to the North Atlantic Ocean, but also has far-reaching consequences for the thermohaline circulation of the Atlantic Ocean (Reid, 1978) and, therefore, its biogeochemical characteristics. The *Corresponding author. Fax: +351-21-7500009. E-mail address: [email protected] (I. Ambar).
water exchanges through the Strait of Gibraltar correspond to a mean surface inflow of Atlantic water and a denser outflow of Mediterranean origin. As the Mediterranean Undercurrent progresses along the continental slope in the northern part of the Gulf of Cadiz, the entrainment of lesssalty Atlantic water results in a gradual decrease in its density. Downstream from about 81W, the Mediterranean Undercurrent density range corresponds to intermediate levels. The analysis of the thermohaline properties of this water mass has shown two main maxima in the T=S diagrams corresponding to upper and lower cores with distinct densities (respectively, gy ¼ 27:5 and 27.8 kg m3 in terms of potential density anomaly),
0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 1 4 8 - 0
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centered at about 800 and 1200 m (Madelain, 1970; Zenk, 1970; Ambar and Howe, 1979a). In addition to these two main cores, a shallower core of Mediterranean Water (MW) has been identified in the Gulf of Cadiz off the southern coast of Iberia (Zenk, 1975; Ambar, 1983) and also off the southwestern coast of Portugal (Ambar, 1983; Hinrichsen et al., 1993), with a potential density anomaly of about gy ¼ 27:4 kg m3 and centered at depths around 500 m. This core is well detected in the region close to the upper continental slope and is characterized by higher temperatures as compared with the two ‘‘classical’’ cores. The chemical properties—nutrients, dissolved oxygen—of the Mediterranean Outflow constitute good tracers for its spreading in the North Atlantic since their concentrations are characteristically lower than those found at similar levels of the Atlantic water (Howe et al., 1974; Ambar et al., 1976; Rhein and Hinrichsen, 1993). R!ıos et al. (1992) and Perez et al. (1993) have characterized the water masses present along the Iberian Peninsula in terms of nutrients and dissolved oxygen. According to these authors, the MW nutrient and oxygen anomalies persist along the Iberian continental slope, although the influence of the MW decreases rapidly to the north and east of Cape Finisterre. Minimum values of nitrate, silicate, phosphate and, in some cases, dissolved oxygen also have been associated with meddies— submesoscale eddies shed by the Mediterranean Undercurrent (Pingree and LeCann, 1993; Shapiro et al., 1995). Pingree (1995), in his measurements across meddy Pinball off the west coast of Portugal (at about 38.21N, 10.01W), was able to distinguish the two main cores within the meddy, the upper core revealing more clearly a minimum of silicate and nitrate than the lower core. The vertical distribution of suspended particulate matter (SPM) concentration in the ocean is generally described in terms of a three-layer model in which the main features are: an upper maximum in the surface layer, an intermediate minimum at around 2000–3000 m depth, and a deep maximum in the waters close to the sea floor (Eittreim et al., 1976; Biscaye and Eittreim, 1977; McCave, 1986). Bottom currents have been shown to originate
bottom and intermediate nepheloid (turbid) layers, from resuspension and subsequent advection of bottom sediments (Biscaye and Eittreim, 1977; McCave, 1986). In spite of the small amount of information available regarding the SPM in the MW layer, evidence that high suspended particulate loads are carried by it into the North Atlantic has been found in the past (Heezen and Johnson, 1969; Thorpe, 1972; Pierce and Stanley, 1975; Eittreim et al., 1976; Abrantes et al., 1994). Canary Islands Azores Gibraltar Observations (CANIGO) was a European-funded (MAST III Programme) project that began in August 1996 and whose observational phase took place mainly during 1997–98. In this paper, a preliminary analysis is presented of the results obtained in two field surveys within the frame of CANIGO, giving special emphasis to the multidisciplinary aspects of the MW intrusion in the Atlantic.
2. Field measurements—materials and methods 2.1. Location of the surveys The field work—two high-resolution surveys— was undertaken within CANIGO Task 4.3.1. (Spatial Distribution and Seasonal Variability of Physical and Geochemical Properties—LargeScale Surveys). The focus of this task was the hydrological structure and the geochemical fields of the Gulf of Cadiz and off the south and southwest coasts of Portugal, particularly at the levels influenced by the MW. The estimate of the transport of physical, chemical (nutrients, dissolved oxygen), and geological (SPM) properties associated with the Mediterranean Outflow and the evaluation of seasonal changes in them were within the main objectives of the field work. The two surveys took place in the Gulf of Cadiz and in the Eastern Iberian Basin, extending from 6.01W (Strait of Gibraltar) to 11.01W and from 35.61N to 38.61N. The a priori design for the cruises consisted of 12 approximately shorenormal sections closed at the offshore ends by shore-parallel sections. The distances between stations varied from around 5 km close to the continental slope to around 10 km offshore. The
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first cruise (summer) took place from 11 to 27 September 1997 using the N/O Thalassa (Fig. 1a), and the second cruise (winter) between 15 and 31 January 1998 aboard B/O Cornide de Saavedra (Fig. 1b). Technical problems and adverse weather conditions during the winter cruise forced its interruption for some days (19–22 January) and prevented the occupation of many of the stations that had been made during the summer cruise (namely on sections L11–L15 and L20). 2.2. CTD data collection and calibration The high-resolution CTD surveys extended from the surface to a maximum depth of 2000 m or, in case of shallower stations, to about 10 m from the bottom. The vertical profiles of temperature and salinity were obtained from data collected with a Neil-Brown Mark III CTD coupled to a General Oceanics rosette with 22 Niskin bottles to collect water samples at several depths in each station. The salinity value of each of these water samples was measured on board with a Guildline (Portasal 8410A) salinometer. For each sampling depth, this salinity value and the temperature measured by the CTD at that depth was used to obtain the corresponding conductivity value. A plot was drawn with these conductivities against the conductivities obtained by the CTD at the
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corresponding stations and depths. The resulting regression line was then used to calibrate all CTD conductivity data. The total number of CTD stations was 192 for the summer survey and 118 for the winter survey. 2.3. Nutrient and dissolved oxygen: sampling and analytical procedures Water samples for determination of the concentrations of dissolved oxygen (O2) and nutrients (nitrate+nitrite referred to as NO3, phosphate as PO4, and silicate as Si(OH)4), were collected in both surveys with the rosette sampler in selected stations, totaling 112 and 63, respectively, in the summer and in the winter cruises (Fig. 1). The sampling depths, up to a maximum of 12, were selected on board in real time making use of the CTD data and with the purpose of covering most of the water column influenced by the MW. O2 was analyzed on board according to the Winkler method, modified by Carrit and Carpenter (1966). Samples for determination of nutrients were poured into acid-washed polyethylene cups and deep-frozen on board. Analyses were performed in the laboratory within 2 weeks, using an Autoanalyzer Alliance Integral Plus, following Tre! guer and Le Corre (1975). Accuracy was maintained through the use of Sagami CSK Standards (Ambe,
Fig. 1. Map showing the sections and the position of the respective stations during the CANIGO surveys (a) in September 1997, aboard N/O Thalassa and (b) in January 1998, aboard B/O Cornide de Saavedra. CTD stations where samples were collected for chemical (Nut) and suspended matter (SPM) analysis are marked.
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1978). The precision, estimated by replicate analysis (n ¼ 10) for concentrations of 5.0 mM NO3, 1.0 mM PO4, and 5.0 mM Si(OH)4, was, respectively, 70.8%, 71.9%, and 72.5%. A loss of precision might have occurred due to the storage freezing method, especially in the case of Si(OH)4 determinations. 2.4. Suspended particulate matter: sampling and analytical procedures Water samples were collected during both summer and winter cruises (Fig. 1) for the characterization of the MW and surrounding water masses in terms of SPM. Based on real-time CTD data, up to 12 different depth levels were sampled in each station using a rosette for water collection. The water was vacuum-filtered through pre-weighed cellulose filters, which were then used for the analysis of total suspended particulate mass (TSM), particulate organic/inorganic matter ratio (POM/PIM), and particle composition by visual analysis (VA). At some chosen stations, pre-combusted glassfiber filters were used to estimate the Carbon13/ Carbon12 ratio (d13C(PDB)) of the particulate organic matter. A detailed description of the analytical procedures can be found in Freitas and Abrantes (2002). TSM was obtained by subtracting the prefiltration filter weight from the post-filtration weight. After the TSM determination, the inorganic matter content (PIM) of the TSM was obtained by weighing before and after combustion. The POM was calculated as the difference between TSM and PIM. This method gives a rapid but approximate measure of the organic matter since not all organic matter is combusted and some inorganic matter is lost at the combustion temperature used. Particulate organic matter d13C(PDB) was determined by using mass spectrometry, after carbonate removal by acidification of the glass-fiber filters. VA of particle composition, was performed at selected stations using light microscopy. The percentage of the filter area occupied by the particles was measured as an indicator of their relative abundance. Four different particle cate-
gories were considered: minerals (lithogenic particles originated in the continental lithosphere), aggregates (irregular agglomerations of different types of particles), particles o10 mm (fine organic and inorganic particles), and biogenic particles (identifiable organisms and their parts). Biogenic particles were then further divided into carbonate, siliceous, and other particles. Identifiable organisms, such as diatoms, coccoliths (calcareous plates that cover coccolithophorids), and discoasters were counted and their number per litre was calculated.
3. Data analysis and comparison 3.1. Hydrology The CTD data collected during the summer (Fig. 1a) and winter (Fig. 1b) cruises allowed the identification of the main water masses present in the study region. The potential temperature versus salinity (y=S) multiplots of the summer (Fig. 2a) and winter (Fig. 2b) cruises showed the following hydrological structure: (i) the upper layer; (ii) the permanent thermo-halocline, corresponding to the North Atlantic Central Water (NACW); (iii) the layers of warm and salty MW; and (iv) the presence of North Atlantic Deep Water (NADW) associated with a depth-decreasing thermohaline properties. The comparison between the two y=S multiplots in Fig. 2 indicates seasonal differences in the surface layer, as expected, and also in the MW layer. The sea-surface temperatures in summer reach values of almost 24.01C, whereas in winter they do not exceed 18.01C. In summer, a subsurface layer almost homogeneous in salinity but with a depth-decreasing temperature (probably a result of the surface heating of the upwelled waters off the Portuguese coast) also contrasts with the winter mixed layer. In some stations off the southern coast of Spain, the surface salinities in winter reached very low values (o35), the result of the strong river discharges (Rivers Guadiana, Tinto, and Guadalquivir). Underneath the surface layer, the NACW curve is clearly defined by a regular decrease of both y
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Fig. 2. y=S multiplots of the CTD data collected (a) in September 1997, aboard N/O Thalassa and (b) in January 1998, aboard B/O Cornide de Saavedra. Lines of constant potential density anomaly gy ¼ 27:50 and g1 ¼ 32:25 kg m3 (g1 —potential density anomaly referred to the 1000-dbar level) are shown within the density range of the MW.
and S values until the levels of MW influence, where the range of salinities spans from values of about 35.7 to almost 38.0. The upper limit of this range (S > 37:5) was found at the bottom in some of the stations located between 6.01W and 6.51W, very close to the Strait of Gibraltar, where almost no mixing with water of Atlantic origin has yet occurred. Further downstream, the MW layer shows clearly the two y=S peaks corresponding to the main cores centered at about gy ¼ 27:50 and g1 ¼ 32:25 kg m3 (g1 is the potential density anomaly referred to the 1000 dbar level). There was not much difference in the temperature range within the MW layer between the summer and the winter data, but the salinity values were, in general, higher during winter. This is also illustrated in Fig. 5 for the salinity vertical distribution along section L9 in summer (Fig. 5a) and in winter (Fig. 5b). This apparent seasonal difference also had been detected in the temperature field during an observational programme in 1993–94 (Ambar et al., 1999) involving the repetition of a temperature section crossing the MW undercurrent off Southern Portugal. If seasonal fluctuations actually exist in the Mediterranean Outflow, a hypothesis raised by Bormans et al. (1986), could
help to explain them. In fact, these authors refer to the existence of a seasonal control of the depth of the interface between the Atlantic Water and the MW in the Strait of Gibraltar associated with the replenishment of the reservoir of outflowing water in late February–early March. In what concerns the description of the main hydrological features, the data obtained with the Thalassa cruise in summer will be the main reference here, due to the better geographical coverage compared with those of the Cornide cruise in winter. Only in comparisons connected with seasonal differences, will the data of the latter cruise be specifically referenced. The 12 sections (Fig. 1a) crossing the Mediterranean flow at right angles show a general downstream decrease of the MW salinity values, mainly resulting from entrainment of the fresher NACW. For instance, the maximum salinity values found for the near slope MW at the level of the lower core, in the summer cruise, were 37.98 (y¼ 13:251C) in the vicinity of the Strait (section L2, Fig. 3a and b), 36.54 (y¼ 11:631C) off Cape St. Vincent (section L11, Fig. 3c and d) and 36.48 (y¼ 11:801C) off the southwest coast of Portugal (section L15).
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Fig. 3. Vertical distributions of salinity (upper row) and potential temperature (lower row) at two Thalassa sections: (a), (b) section L2; (c), (d) section L11. See Fig. 1 for location of sections.
The temperature maximum found within the MW layers at depths between 400 and 600 m (in the potential density anomaly range 27:20ogy o27:40 kg m3) can be identified with the shallow core of MW. The Thalassa data allow the tracking of this core from the vicinity of the Strait of Gibraltar (e.g., second inner station of section L2), to the region off the western coast of Portugal. In a general way, it remained close to the upper slope in the northern Gulf of Cadiz, generally within the 500 m bathymetric contour (in sections L3, L5, L6, L8, L9 and L11 it was centered around the second or third inshore station). Off the western coast of Portugal, it was
found further offshore, especially near St. Vincent canyon (the eighth station of section L12) and the ! canyon of Lisboa/Setubal (the offshore edge of section L19). The downstream route of the two salinity maxima corresponding to the MW main cores (upper and lower) diverged from that of the temperature maximum, corresponding to the shallow core. In the sections of the Gulf of Cadiz, where the MW constitutes the bottom layer (i.e. until about 7.51W), the respective horizontal separation was about 15 km (in sections L1–L3) but it increased to 35 km at about 7.01W (section L5) and 60 km at about 7.51W (section L6). This
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reflects the cross-bathymetric path followed by the two main cores in their first stages in the Gulf of Cadiz. These two MW main cores (upper and lower) were more clearly separated from each other west of section L5 of the Thalassa cruise. Fig. 4 shows the distribution of salinity and temperature on the potential density anomaly surfaces gy ¼ 27:50 and g1 ¼ 32:25 kg m3, corresponding to the upper and the lower cores. The geostrophic field at 800 and 1200 dbar relative to 2000 dbar is superimposed on these distributions in order to represent the dynamical field at the level where the cores are centered. The comparison between the thermohaline distribution and the circulation patterns at 800 and 1200 dbar levels shows some aspects that are common to both cores and some that are distinct. An undercurrent was present at both levels, with
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the higher temperature and salinity values concentrated against the continental slope. There was a tendency for the lower core to meander offshorewards after its turn around Cape St. Vincent, whereas the upper core maintained its path closer to the slope, as has been observed previously (Ambar and Howe, 1979a, b; Zenk and Armi, 1990). Eddy-like structures were present in the offshore region, some of them with a corresponding thermohaline signature; examples are an anticyclonic eddy centered at about 36.31N, 10.51W, and a cyclonic eddy centered at about 36.01N, 9.51W (section L9). In the winter cruise, there was also evidence of cyclonic eddies in the thermohaline and geostrophic fields, some of them confirmed by the RAFOS float data (Serra and Ambar, 2002). Another interesting feature shown by these data was the relatively large amount of
Fig. 4. Salinity and temperature distributions (Thalassa cruise, September 1997) on potential density anomaly surfaces corresponding to the upper and lower cores: (a) salinity, gy ¼ 27:50 kg m3 (b) salinity, g1 ¼ 32:25 kg m3; (c) temperature, gy ¼ 27:50 kg m3; (d) temperature, g1 ¼ 32:25 kg m3. Superimposed are the geostrophic fields at, respectively, 800 dbar (for gy ¼ 27:50 kg m3) and 1200 dbar (for g1 ¼ 32:25 kg m3), computed relative to the 2000-dbar level.
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MW found in the southern limit of the study region, as evidenced by the relatively high-salinity patches present in this border mainly at the level of the lower core (Fig. 3c). The geostrophic field seems to indicate a local recirculation, and this could explain the presence of these patches as blobs or eddies, probably detached from the slopetrapped undercurrent and carried eastwards. Usually, it is assumed that the depth at which the lower core stabilizes, once it reaches neutral buoyancy, is around 1200 m. However, the data collected during the Thalassa cruise showed, in some of the hydrological sections (downstream from section L6), the existence of a layer of high salinity at far deeper levels, namely around 1400 m. This denser lower core was found in the middle of section L8 where 8th and 9th stations (from the inshore edge of the section) show, respectively, salinity maxima at 1467 m (S ¼ 36:53) and 1391 m (S ¼ 36:42), in section L9 (the 8th station with 36.57 at 1347 m, and the 9th station with 36.51 at 1357 m) and in section
L11 (the 5th station with 36.54 at 1381 m, and the 6th station with 36.27 at 1425 m). The geostrophic velocities (relative to 2000 dbar) computed for each of these three sections, have shown a deep eastward flow in the layers beneath the ‘‘classical’’ level of the lower core (i.e., 1200–1300 m). This would imply the existence of a countercurrent flowing along the continental slope and carrying with it dense MW, which could have plunged in the region of the Portim*ao Canyon. Fig. 5 exemplifies the presence of this countercurrent in the geostrophic velocity field normal to section L9, both for the summer and the winter cruises. The hypothesis of the presence of a counterflow in this region was raised by Ambar and Howe (1979b) based on geostrophic calculations in hydrological sections across the main stream between 8.51W and 9.51W. In the same region, Zenk (1980) also found evidence of episodes of a deep countercurrent in direct measurements at a depth of 1568 m. Within the frame of the CANIGO project, two current-meter moorings were deployed at
Fig. 5. Vertical distribution of salinity and of geostrophic velocity (relative to 2000 dbar) perpendicular to section L9 for (a) the summer cruise and (b) the winter cruise.
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about 8.81W (S1 and S2 at latitudes of 36.51N and 36.41N, respectively) in the vicinity of the abovementioned stations of Thalassa section L9 (Fig. 1). These moorings had instruments recording for a period of over 1 year (15 June 1997–20 August 1998) at the levels of the North Atlantic Central Water (B300 m), of the MW upper (B800 m) and lower (B1200 m) cores and at a level of about 1800 m. Although the two moorings were relatively close to each other (about 11 km apart), the regime differed strongly between them, the current at the site closer to the slope (S1) being generally much more polarized by the bottom topography. At the deeper level (1800 m) in this mooring S1, there was evidence of a slow countercurrent, in a general northeastward direction and with a mean velocity just above 2 cm s1, which is in agreement with the results obtained for the geostrophic field obtained with the CANIGO data. 3.2. Nutrients and dissolved oxygen The mean chemical properties of the Mediterranean outflow measured in the vicinity of the strait (section L2) during the summer and the winter cruises were, respectively, 7.9 and 8.4 mmol kg1 for NO3, 0.64 and 0.44 mmol kg1 for PO4, and 6.7 and 6.0 mmol kg1 for Si(OH)4. These concentrations are comparable with those reported in other studies (Coste et al., 1988; Minas et al., 1991; Rhein and Hinrichsen, 1993). Farther downstream, the Mediterranean outflow did not show significant differences in O2 and nutrient (NO3, PO4 and Si(OH)4) mean values between the two CANIGO cruises (Table 1). The comparison of the nutrient data has been undertaken, in spite of inherent problems with precision due to the freezing of the nutrient samples, since the errors were below the 2.5% level. A comparative analysis was made between the thermohaline structure and the nutrient concentration patterns. Fig. 6 shows the horizontal distributions of nutrients for each core of MW and was constructed from mean values of nutrients and using objective analysis (covariance scale=25 km) methods. The mean values for each nutrient were calculated averaging the corresponding available data within each core layer (defined
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Table 1 Winter and summer mean values of dissolved oxygen (O2) and nutrients (NO3, PO4 and Si(OH)4) obtained within the MW layer along the Southern and Western Iberian Peninsula Parameter (mmol kg1) Winter O2 NO3 PO4 Si(OH)4
174 (SD=9; n ¼ 203) 13.3 (SD=3.2; n ¼ 206) 0.9 (SD=0.3; n ¼ 206) 8.9 (SD=2.5; n ¼ 206)
Summer 178 (SD=8; n ¼ 178) 13.9 (SD=3.1; n ¼ 154) 1.1 (SD=0.3; n ¼ 154) 7.6 (SD=2.5; n ¼ 154)
Standard deviation (SD) and number of samples (n) are also indicated.
by the following range of potential density anomalies: upper core, from gy ¼ 27:25 to g1 ¼ 32:00 kg m3, and lower core, from g1 ¼ 32:00 to 32:35 kg m3). A downstream gradual modification of the nutrient concentrations was observed along the southern and southwestern Iberian coasts (Fig. 6). The characteristic high salinity and temperature values of the MW were associated with the lowest levels of nutrients compared with the surrounding Atlantic water. There were, however, differences in the chemical characteristics between the upper and the lower MW cores, the upper core exhibiting slightly higher O2 and lower nutrient concentrations (Fig. 6). In the upper core O2 ranged from 170 to 200 mmol kg1 (170–180 mmol kg1 in the lower core), while NO3 ranged from 6.9 to 17.4 mmol kg1 (8.5–21.8 mmol kg1 in the lower core), PO4 from 0.6 to 1.4 mmol kg1 (0.8– 2.4 mmol kg1 in the lower core) and Si(OH)4 from 3.4 to 10.9 mmol kg1 (4.8–13.2 mmol kg1 in the lower core). The eddy-like structures evidenced by the thermohaline distribution in the offshore region, also were identified by their chemical properties. For instance, the cyclonic eddy centered at about 36.01N, 9.51W (section L9) presented, at about 700-m depth, nutrient concentrations lying in the upper core range. These findings are similar to those reported by other authors (Rhein and Hinrichsen, 1993; Hinrichsen et al., 1993; Pingree and LeCann, 1993; Pingree, 1995) who have
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Fig. 6. Horizontal distributions of nutrients within the MW layers off Southern and Southwestern Iberia (data from Thalassa cruise): (a) NO3, upper core; (b) NO3, lower core; (c) PO4, upper core; (d) PO4, lower core; (e) Si(OH)4, upper core; and (f) Si(OH)4, lower core.
differentiated, from the point of view of the chemistry, one or both cores within the meddies that they detected off the Portuguese western coast. 3.3. Suspended particulate matter Two indicators of particle abundance, TSM (in mg/l) and FAO (% of filter area occupied), were measured in order to characterize the different water masses. Both indicators showed similar vertical variations between water masses although with different variances (Fig. 7). A maximum in the surface layer, coincident with the largest variability, is followed by a decrease in the NACW layer, down to a relative maximum corresponding to the MW layer. This maximum was not always coincident with the maximum in temperature or in
salinity that characterizes, respectively, the upper and the lower MW cores. In order to assess the differences in the SPM composition between water masses, several parameters (POM/PIM, d13C(PDB), and VA) were evaluated. A large variance was observed in POM and PIM values within each water mass, and the only consistent feature was the systematically higher POM/PIM ratio observed in the summer cruise (Fig. 7e). At the level of the MW, this ratio was generally o1, which means that inorganic matter was dominating. Conversely, Abrantes et al. (1994) found that the SPM in meddies off the southwestern coast of Portugal was dominated by the organic component. Much of the particulate organic carbon in the open ocean comes from the detrital remains of
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(a) SURFACE
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(b)
NACW MIN SAL MW NADW
35
35.5
36
36.5
4 6 8 10 12 14 16 18 Temperature - °C
Salinity (d)
(c) SURFACE NACW MIN SAL MW NADW
0 0.1 0.2 0.3 0.4 0.5
-2
0
2
(e) SURFACE
4
6
8 10
FAO - o/o
TSM - mg/l (f)
NACW MIN SAL MW NADW
-0.5 0 0.5 1 1.5 2 POM/PIM
-27
-26
-25
-24
13
δ C
(PDB)
-
-23
o
/oo
Fig. 7. Vertical profiles of average values near the sea surface, within the NACW, the MW (the salinity minimum between NACW and MW is also indicated), and the NADW of: (a) salinity; (b) temperature; (c) TSM; (d) FAO; (e) POM/PIM; (f) d13C(PDB). Summer survey (black) and winter survey (gray).
plankton from surface waters (Suess, 1980), and this is reflected in its isotopic composition (Eadie et al., 1978; Fischer, 1991). In these data, the surface layer showed the highest d13C(PDB) of the whole water column, while the MW corresponded to an intermediate maximum (Fig. 7f), clearer in the winter survey and in the region near the Strait of Gibraltar and of the Gulf of Cadiz. The
d13C(PDB) values observed in this study, 21– 28%, are within the range 18–30% reported for plankton and particulate organic carbon in the ocean (Eadie and Jeffrey, 1973; Rau et al., 1989; Fischer, 1991; Libes, 1992). Visual analysis of particle composition (Fig. 8) showed that particles o10 mm were predominant in the whole water column except in the surface
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(c)
(b)
(a) SURFACE NACW MIN SAL MW NADW
-10 0 10 20 30 40 50 % Minerals (d)
0
20
40 60 80 100 % < 10 µm
0
10 20 30 40 % Aggregates
50
(f)
(e)
SURFACE NACW MIN SAL MW NADW
0
20
40 60 80 100 % Biogenics
(g)
0
20 40 60 80 100 % Siliceous bio.
-10 10 30 50 70 90 % Carbonate bio. (i)
(h)
SURFACE NACW MIN SAL MW NADW
-25
25 75 125 175 Diatoms valves/l
-400 100 600 1100 1600 Coccoliths/l
-3 -2 -1 0 1 2 3 4 5 6 Discoasters/l
Fig. 8. Average values obtained by visual microscopic analysis of particle composition for each water mass of: (a) % of mineral particles; (b) % of o10 mm particles; (c) % of aggregate particles; (d) % of biogenic particles; (e) % of siliceous biogenic particles; (f) % of carbonate biogenic particles; (g) diatom valves/l; (h) coccoliths/l; and (i) discoasters/l. Summer survey (black) and winter survey (gray).
layer (Fig. 8b). In this layer, biogenic particles (including particles o10 mm) were the most abundant group. NACW had similar values for the biogenic particles but showed a vertical trend in SPM composition with increasing number of mineral particles (Fig. 8a) and decreasing number of particles o10 mm (Fig. 8b) with depth. The MW showed a clear difference in particle composition relative to the other water masses, with higher abundance of mineral and biogenic particles (except in comparison with the surface layer) and lower abundance of particles o10 mm. Within the NADW layer, the SPM was characterized by the
highest values of particles o10 mm and the lowest values of aggregate (Fig. 8c) and biogenic (Fig. 8d) particles, when compared with the other water masses, and by relatively high values of mineral particles. The only noticeable difference in the abundance of aggregates between water masses was the minimum values found in the NADW. The variation of biogenic particles between water masses was also marked by the variation of diatoms, coccoliths and discoasters, with the highest values occurring at the surface, a decrease with depth within the NACW, a maximum at the MW and a decrease again in NADW.
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Large differences were observed in the composition of SPM between the MW that flows close to the continental slope (slope-MW) and the MW encountered farther offshore (offshore-MW). The slope-MW showed higher average values of TSM, FAO, d13C(PDB), minerals, aggregates and biogenics, and lower abundance of particles o10 mm, when compared with the offshore-MW. Most of the biogenic particulate material in the slope-MW was carbonate, while in the offshore-MW it was siliceous. Slope-MW also showed higher abundance of diatoms, coccoliths and discoasters than offshore-MW. In terms of SPM, the offshore-MW was more similar to NACW and NADW than the slope-MW.
4. Summary and conclusions This work presents some preliminary results of a multidisciplinary study of the Mediterranean outflow in the region west of the Strait of Gibraltar and off the southern and southwestern coasts of the Iberian Peninsula, in the framework of the CANIGO Project. Two high-resolution CTD surveys took place in September 1997 (summer cruise) and January 1998 (winter cruise) in the study region. The field work included the collection of water samples at selected stations and depths, with the objective of characterizing the MW and comparing it with the adjacent water masses from the point of view of the chemistry (nutrient and dissolved oxygen concentrations) and of the SPM. A general correspondence was found between the high-salinity values of the MW layer and the low-nutrient and oxygen contents and relatively high abundance of particles. The progressive mixing of the MW with the entrained NACW resulted in a downstream decrease of the temperature and salinity accompanied by a slight modification in nutrient concentrations. Particle abundance in the MW was characterized by a maximum, which was not always found at exactly the same level as its temperature and salinity maxima. Most of the parameters used to characterize the SPM have shown a local maximum (of TSM, FAO, d13C(PDB)) at the depths of the MW.
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In terms of the visual analysis of the particle composition, there were also extreme values associated with the MW layer. There were some differences between the summer and the winter cruises with respect to the thermohaline characteristics, the winter salinity values being higher than those found in summer. In terms of the nutrient and the dissolved oxygen concentrations, there were no significant differences between the two data sets. The thermohaline analysis has shown the presence of the two main cores of MW—upper and lower cores—corresponding to temperature and/or salinity maxima centered at about 800 and 1200 m. The upper core is better characterized by the temperature profile and the lower core by the salinity profile. In terms of the chemistry, the upper core contained lower nutrient and higher oxygen concentrations than the lower core. However, from the point of view of the suspended matter, the distinction between the two cores was not evident. Besides the two main cores, there was also evidence of a shallower one, at depths of about 500 m, which was characterized especially by high temperatures and remained close to the upper continental slope all along Southern Iberia, but was found farther away from the upper slope in the region off Southwestern Portugal. The chemical characterization of this shallow core suggested nutrient values similar to those found for the upper core. The joint analysis of the geostrophic velocity field and the thermohaline distribution patterns, has shown some interesting aspects of the Mediterranean undercurrent. A counterflow (i.e. eastward), beneath the westward undercurrent, in the region of higher bathymetric gradient within the Portim*ao Canyon (off Southern Portugal), transported high-salinity water (in the range 36.4–36.6) at depths between about 1300 and 1500 m. The existence of a counterflow was also apparent from the analysis of the direct current measurements made at 1800 m in the same region, under CANIGO. Another important aspect shown by the geostrophic and hydrological fields in both cruises, was the existence of eddies of MW, rotating either
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anticyclonically, the so-called meddies, or cyclonically. The chemical signature of these eddies corresponded to that ascribed to the MW at the corresponding depths. This physical, chemical and sedimentological analysis in a joint approach has proved to be useful for characterizing water masses and facilitates their differentiation, which sometimes is difficult based only on the thermohaline distributions.
Acknowledgements This work was supported by the European Union MAST III project CANIGO (Contract MAS3-CT96-0060). The observational work was done from aboard the R/V Thalassa and the B/O Cornide de Saavedra. The authors thank all the cruise participants and the crew members who made possible the realization of this work. The participation of NS was supported by the Fundac-a* o para a Ci#encia e a Tecnologia (Grant BD/ 19535/99).
References Abrantes, F., Ambar, I., Zenk, W., Hinrichsen, H., Zahn, R., 1994. Suspended matter of the Mediterranean Water Outflow off Portugal. In: Proceedings of the Oceans Sciences Meeting, American Geophysical Union, San Diego. Ambar, I., 1983. A shallow core of Mediterranean Water off Western Portugal. Deep-Sea Research I 30, 677–680. Ambar, I., Howe, M.R., 1979a. Observations of the Mediterranean outflow—I. mixing in the Mediterranean outflow. Deep-Sea Research I 26A, 535–554. Ambar, I., Howe, M., 1979b. Observations of the Mediterranean Outflow—II. the deep circulation in the vicinity of the Gulf of Cadiz. Deep-Sea Research I 26A, 555–568. Ambar, I., Howe, M.R., Abdullah, M.I., 1976. A physical and chemical description of the Mediterranean outflow in the Gulf of Cadiz. Deutsche Hydrographische Zeitschrift 29, 58–68. Ambar, I., Armi, L., Bower, A., Ferreira, T., 1999. Some aspects of time variability of the Mediterranean Water off South Portugal. Deep-Sea Research I 46, 1109–1136. Ambe, M., 1978. Note of the experience in the preparation of CSK Standard Solutions and the ICES-SCOR intercalibration experiment, 1969–1970. Marine Chemistry 6, 171–178.
Biscaye, P.E., Eittreim, S.T., 1977. Suspended particulate loads and transports in the nepheloid layer of the abyssal Atlantic Ocean. Marine Geology 23, 155–172. Bormans, M., Garrett, C., Thompson, K.R., 1986. Seasonal variability of the surface inflow through the Strait of Gibraltar. Oceanologica Acta 9 (4), 403–414. Carrit, D.E., Carpenter, J.H., 1966. Comparison and evaluation of currently employed modifications of the Winkler method for determining oxygen in sea water. A NASCO Report. Journal of Marine Research 24 (3), 286–318. Coste, B., Le Corre, P., Minas, H.J., 1988. Re-evaluation of the nutrient exchanges in the Strait of Gibraltar. Deep-Sea Research I 35 (5), 767–775. Eadie, B., Jeffrey, A., 1973. d13c analyses of oceanic particulate organic matter. Marine Chemistry 1, 199–209. Eadie, B.J., Jeffrey, L.M., Sackett, W.M., 1978. Some observations on the stable carbon isotope composition of dissolved and particulate organic carbon in the marine environment. Geochimica et Cosmochimica Acta 42, 1265–1269. Eittreim, S., Thorndike, E., Sullivan, L., 1976. Turbidity distribution in the Atlantic Ocean. Deep-Sea Research I 23, 1115–1127. Fischer, G., 1991. Stable carbon isotope ratios of plankton carbon and sinking organic matter from the Atlantic sector of the southern ocean. Marine Chemistry 35, 581–596. Freitas, P., Abrantes, F., 2002. Suspended particulate matter in the Mediterranean Water at the Gulf of Cadiz and off the SW coast of the Iberian Peninsula. Deep-Sea Research II 49 (19), 4245–4261. Heezen, B., Johnson, L., 1969. Mediterranean undercurrent and microphysiography west of Gibraltar. Bulletin de l’Institut Oc!eanographique du Monaco 67, 1–49. Hinrichsen, H.-H., Rhein, M., K.ase, R.H., Zenk, W., 1993. The Mediterranean water tongue and its chlorofluoromethane signal in the Iberian basin in early summer 1989. Journal of Geophysical Research 98 (C5), 8405–8412. Howe, M.R., Abdullah, M.I., Deetae, S., 1974. An interpretation of the double T–S maxima in the Mediterranean outflow using chemical tracers. Journal of Marine Research 32 (3), 377–386. Libes, S., 1992. The use of stable isotopes in the study of paleoceanography. In: Libes, S. (Ed.), Introduction to Marine Biogeochemistry. John Wiley and Sons, New York, pp. 557–593. Madelain, F., 1970. Influence de la topographie du fond sur l’!ecoulement Mediterran!een entre le D!etroit de Gibraltar et le Cap Saint-Vincent. Cahiers Oc!eanographiques 22, 43–61. McCave, I.N., 1986. Local and global aspects of the bottom nepheloid layers in the world ocean. Netherlands Journal of Sea Research 20 (2/3), 167–181. Minas, H., Coste, B., Le Corre, P., Minas, M., Raimbault, P., 1991. Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the Western Alboran Sea. Journal of Geophysical Research 96 (C5), 8755–8771. * C., Fraga, F., Rios, A., 1993. DisplacePerez, F.F., Mourino, ment of water masses and remineralization rates off the
I. Ambar et al. / Deep-Sea Research II 49 (2002) 4163–4177 Iberian Peninsula by nutrient anomalies. Journal of Marine Research 51, 869–892. Pierce, J., Stanley, D., 1975. Suspended sediment concentration and mineralogy in the Central and Western Mediterranean and mineralogical comparison with bottom sediment. Marine Geology 19, M15–M25. Pingree, R.D., 1995. The droguing of meddy Pinball and seeding with Alace floats. Journal of the Marine Biological Association of the United Kingdom 75, 235–252. Pingree, R.D., LeCann, B., 1993. A shallow meddy (a smeddy) from the secondary Mediterranean salinity maximum. Journal of Geophysical Research 98 (C11), 20169–20185. Rau, G.H., Takahashi, T., Des Marais, D.J., 1989. Latitudinal variations in plankton d13C: implications for CO2 and productivity in past oceans. Nature 341, 516–518. Reid, J.L., 1978. On the mid-depth circulation and salinity field in the North Atlantic Ocean. Journal of Geophysical Research 83 (C10), 5063–5067. Rhein, M., Hinrichsen, H.H., 1993. Modification of Mediterranean Water in the Gulf of Cadiz, studied with hydrographic, nutrient and chlorofluoromethane data. Deep-Sea Research I 40 (2), 267–291. R!ıos, A.F., P!erez, F.F., Fraga, F., 1992. Water masses in upper and middle Atlantic Ocean east of Azores. Deep-Sea Research I 39, 645–658.
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Serra, N., Ambar, I., 2002. Eddy generation in the Mediterranean Undercurrent. Deep-Sea Research II 49 (19), 4225–4243. Shapiro, G.I., Meschanov, S.L., Emelianov, M.V., 1995. Mediterranean lens ‘‘Irving’’ after its collision with seamounts. Oceanologica Acta 18, 309–318. Suess, E., 1980. Particulate organic carbon flux in the oceansurface productivity and oxygen utilization. Nature 288, 260–263. Thorpe, S., 1972. A sediment cloud below the Mediterranean Outflow. Nature 239, 326–327. Tr!eguer, P., Le Corre, P., 1975. Manuel d’analyse des sels nutritifs dans l’eau de mer. Utilization de l’auto-analyser II. Technicon Lab. Oceanogr. Chim., Universit!e de Bretagne Occidentale, Brest, France, 2 e! me e! dition, 110pp. Zenk, W., 1970. On the temperature and salinity structure of the Mediterranean Water in the Northeast Atlantic. DeepSea Research 17, 627–631. Zenk, W., 1975. On the origin of the intermediate doublemaxima in T/S profiles from the North Atlantic. Meteor Forschungsergebnisse A 16, 35–43. Zenk, W., 1980. The sub-Mediterranean undercurrent. DeepSea Research 27, 97–98. Zenk, W., Armi, L., 1990. The complex spreading pattern of Mediterranean Water off the Portuguese continental slope. Deep-Sea Research 37, 1805–1823.
Physical, chemical and sedimentological aspects of the Mediterranean outflow off Iberia I. Ambara,*, N. Serraa, M.J. Brogueirab, G. Cabec-adasb, F. Abrantesc, P. Freitasc, C. Gonc-alvesb, N. Gonzalezd a
Instituto de Oceanografia, Faculdade de Ci#encias, Universidade de Lisboa, 1749-016 Lisboa, Portugal b Instituto de Investigac-ao * das Pescas e do Mar, Av. de Bras!ılia, 1400 Lisboa, Portugal c ! Instituto Geologico e Mineiro, Estrada da Portela, 2720 Alfragide, Portugal d ! Instituto Espanol * de Oceanograf!ıa, Centro Oceanografico da Coruna, * 15080 La Coruna, * Spain Received 24 April 2000; received in revised form 2 November 2000; accepted 20 November 2000
Abstract A multidisciplinary study of the Mediterranean outflow in the region west of the Strait of Gibraltar and off the southern and southwestern coasts of the Iberian Peninsula was developed in the frame of the Canary Islands Azores Gibraltar Observations Project. Two high-resolution CTD surveys, which included water sampling for chemical (nutrients and dissolved oxygen) and sedimentological analyses, took place in September 1997 (summer cruise) and January 1998 (winter cruise) in the study region. The correspondence between the high-salinity Mediterranean Water (MW) layer and the low-nutrient content and relatively high abundance of particles was a general result. Further details of the thermohaline analysis and of the geostrophic computations, especially for the layer of MW, are compared with the results obtained for the chemical properties and the sedimentological characteristics. r 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction The intermediate layers of the Northeast Atlantic are mainly occupied by a water mass originating in the Mediterranean Sea, which has a very strong signature both in the thermal and the salinity fields at depths between 500 and 1300 m. It constitutes not only an important salt and heat source to the North Atlantic Ocean, but also has far-reaching consequences for the thermohaline circulation of the Atlantic Ocean (Reid, 1978) and, therefore, its biogeochemical characteristics. The *Corresponding author. Fax: +351-21-7500009. E-mail address: [email protected] (I. Ambar).
water exchanges through the Strait of Gibraltar correspond to a mean surface inflow of Atlantic water and a denser outflow of Mediterranean origin. As the Mediterranean Undercurrent progresses along the continental slope in the northern part of the Gulf of Cadiz, the entrainment of lesssalty Atlantic water results in a gradual decrease in its density. Downstream from about 81W, the Mediterranean Undercurrent density range corresponds to intermediate levels. The analysis of the thermohaline properties of this water mass has shown two main maxima in the T=S diagrams corresponding to upper and lower cores with distinct densities (respectively, gy ¼ 27:5 and 27.8 kg m3 in terms of potential density anomaly),
0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 1 4 8 - 0
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centered at about 800 and 1200 m (Madelain, 1970; Zenk, 1970; Ambar and Howe, 1979a). In addition to these two main cores, a shallower core of Mediterranean Water (MW) has been identified in the Gulf of Cadiz off the southern coast of Iberia (Zenk, 1975; Ambar, 1983) and also off the southwestern coast of Portugal (Ambar, 1983; Hinrichsen et al., 1993), with a potential density anomaly of about gy ¼ 27:4 kg m3 and centered at depths around 500 m. This core is well detected in the region close to the upper continental slope and is characterized by higher temperatures as compared with the two ‘‘classical’’ cores. The chemical properties—nutrients, dissolved oxygen—of the Mediterranean Outflow constitute good tracers for its spreading in the North Atlantic since their concentrations are characteristically lower than those found at similar levels of the Atlantic water (Howe et al., 1974; Ambar et al., 1976; Rhein and Hinrichsen, 1993). R!ıos et al. (1992) and Perez et al. (1993) have characterized the water masses present along the Iberian Peninsula in terms of nutrients and dissolved oxygen. According to these authors, the MW nutrient and oxygen anomalies persist along the Iberian continental slope, although the influence of the MW decreases rapidly to the north and east of Cape Finisterre. Minimum values of nitrate, silicate, phosphate and, in some cases, dissolved oxygen also have been associated with meddies— submesoscale eddies shed by the Mediterranean Undercurrent (Pingree and LeCann, 1993; Shapiro et al., 1995). Pingree (1995), in his measurements across meddy Pinball off the west coast of Portugal (at about 38.21N, 10.01W), was able to distinguish the two main cores within the meddy, the upper core revealing more clearly a minimum of silicate and nitrate than the lower core. The vertical distribution of suspended particulate matter (SPM) concentration in the ocean is generally described in terms of a three-layer model in which the main features are: an upper maximum in the surface layer, an intermediate minimum at around 2000–3000 m depth, and a deep maximum in the waters close to the sea floor (Eittreim et al., 1976; Biscaye and Eittreim, 1977; McCave, 1986). Bottom currents have been shown to originate
bottom and intermediate nepheloid (turbid) layers, from resuspension and subsequent advection of bottom sediments (Biscaye and Eittreim, 1977; McCave, 1986). In spite of the small amount of information available regarding the SPM in the MW layer, evidence that high suspended particulate loads are carried by it into the North Atlantic has been found in the past (Heezen and Johnson, 1969; Thorpe, 1972; Pierce and Stanley, 1975; Eittreim et al., 1976; Abrantes et al., 1994). Canary Islands Azores Gibraltar Observations (CANIGO) was a European-funded (MAST III Programme) project that began in August 1996 and whose observational phase took place mainly during 1997–98. In this paper, a preliminary analysis is presented of the results obtained in two field surveys within the frame of CANIGO, giving special emphasis to the multidisciplinary aspects of the MW intrusion in the Atlantic.
2. Field measurements—materials and methods 2.1. Location of the surveys The field work—two high-resolution surveys— was undertaken within CANIGO Task 4.3.1. (Spatial Distribution and Seasonal Variability of Physical and Geochemical Properties—LargeScale Surveys). The focus of this task was the hydrological structure and the geochemical fields of the Gulf of Cadiz and off the south and southwest coasts of Portugal, particularly at the levels influenced by the MW. The estimate of the transport of physical, chemical (nutrients, dissolved oxygen), and geological (SPM) properties associated with the Mediterranean Outflow and the evaluation of seasonal changes in them were within the main objectives of the field work. The two surveys took place in the Gulf of Cadiz and in the Eastern Iberian Basin, extending from 6.01W (Strait of Gibraltar) to 11.01W and from 35.61N to 38.61N. The a priori design for the cruises consisted of 12 approximately shorenormal sections closed at the offshore ends by shore-parallel sections. The distances between stations varied from around 5 km close to the continental slope to around 10 km offshore. The
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first cruise (summer) took place from 11 to 27 September 1997 using the N/O Thalassa (Fig. 1a), and the second cruise (winter) between 15 and 31 January 1998 aboard B/O Cornide de Saavedra (Fig. 1b). Technical problems and adverse weather conditions during the winter cruise forced its interruption for some days (19–22 January) and prevented the occupation of many of the stations that had been made during the summer cruise (namely on sections L11–L15 and L20). 2.2. CTD data collection and calibration The high-resolution CTD surveys extended from the surface to a maximum depth of 2000 m or, in case of shallower stations, to about 10 m from the bottom. The vertical profiles of temperature and salinity were obtained from data collected with a Neil-Brown Mark III CTD coupled to a General Oceanics rosette with 22 Niskin bottles to collect water samples at several depths in each station. The salinity value of each of these water samples was measured on board with a Guildline (Portasal 8410A) salinometer. For each sampling depth, this salinity value and the temperature measured by the CTD at that depth was used to obtain the corresponding conductivity value. A plot was drawn with these conductivities against the conductivities obtained by the CTD at the
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corresponding stations and depths. The resulting regression line was then used to calibrate all CTD conductivity data. The total number of CTD stations was 192 for the summer survey and 118 for the winter survey. 2.3. Nutrient and dissolved oxygen: sampling and analytical procedures Water samples for determination of the concentrations of dissolved oxygen (O2) and nutrients (nitrate+nitrite referred to as NO3, phosphate as PO4, and silicate as Si(OH)4), were collected in both surveys with the rosette sampler in selected stations, totaling 112 and 63, respectively, in the summer and in the winter cruises (Fig. 1). The sampling depths, up to a maximum of 12, were selected on board in real time making use of the CTD data and with the purpose of covering most of the water column influenced by the MW. O2 was analyzed on board according to the Winkler method, modified by Carrit and Carpenter (1966). Samples for determination of nutrients were poured into acid-washed polyethylene cups and deep-frozen on board. Analyses were performed in the laboratory within 2 weeks, using an Autoanalyzer Alliance Integral Plus, following Tre! guer and Le Corre (1975). Accuracy was maintained through the use of Sagami CSK Standards (Ambe,
Fig. 1. Map showing the sections and the position of the respective stations during the CANIGO surveys (a) in September 1997, aboard N/O Thalassa and (b) in January 1998, aboard B/O Cornide de Saavedra. CTD stations where samples were collected for chemical (Nut) and suspended matter (SPM) analysis are marked.
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1978). The precision, estimated by replicate analysis (n ¼ 10) for concentrations of 5.0 mM NO3, 1.0 mM PO4, and 5.0 mM Si(OH)4, was, respectively, 70.8%, 71.9%, and 72.5%. A loss of precision might have occurred due to the storage freezing method, especially in the case of Si(OH)4 determinations. 2.4. Suspended particulate matter: sampling and analytical procedures Water samples were collected during both summer and winter cruises (Fig. 1) for the characterization of the MW and surrounding water masses in terms of SPM. Based on real-time CTD data, up to 12 different depth levels were sampled in each station using a rosette for water collection. The water was vacuum-filtered through pre-weighed cellulose filters, which were then used for the analysis of total suspended particulate mass (TSM), particulate organic/inorganic matter ratio (POM/PIM), and particle composition by visual analysis (VA). At some chosen stations, pre-combusted glassfiber filters were used to estimate the Carbon13/ Carbon12 ratio (d13C(PDB)) of the particulate organic matter. A detailed description of the analytical procedures can be found in Freitas and Abrantes (2002). TSM was obtained by subtracting the prefiltration filter weight from the post-filtration weight. After the TSM determination, the inorganic matter content (PIM) of the TSM was obtained by weighing before and after combustion. The POM was calculated as the difference between TSM and PIM. This method gives a rapid but approximate measure of the organic matter since not all organic matter is combusted and some inorganic matter is lost at the combustion temperature used. Particulate organic matter d13C(PDB) was determined by using mass spectrometry, after carbonate removal by acidification of the glass-fiber filters. VA of particle composition, was performed at selected stations using light microscopy. The percentage of the filter area occupied by the particles was measured as an indicator of their relative abundance. Four different particle cate-
gories were considered: minerals (lithogenic particles originated in the continental lithosphere), aggregates (irregular agglomerations of different types of particles), particles o10 mm (fine organic and inorganic particles), and biogenic particles (identifiable organisms and their parts). Biogenic particles were then further divided into carbonate, siliceous, and other particles. Identifiable organisms, such as diatoms, coccoliths (calcareous plates that cover coccolithophorids), and discoasters were counted and their number per litre was calculated.
3. Data analysis and comparison 3.1. Hydrology The CTD data collected during the summer (Fig. 1a) and winter (Fig. 1b) cruises allowed the identification of the main water masses present in the study region. The potential temperature versus salinity (y=S) multiplots of the summer (Fig. 2a) and winter (Fig. 2b) cruises showed the following hydrological structure: (i) the upper layer; (ii) the permanent thermo-halocline, corresponding to the North Atlantic Central Water (NACW); (iii) the layers of warm and salty MW; and (iv) the presence of North Atlantic Deep Water (NADW) associated with a depth-decreasing thermohaline properties. The comparison between the two y=S multiplots in Fig. 2 indicates seasonal differences in the surface layer, as expected, and also in the MW layer. The sea-surface temperatures in summer reach values of almost 24.01C, whereas in winter they do not exceed 18.01C. In summer, a subsurface layer almost homogeneous in salinity but with a depth-decreasing temperature (probably a result of the surface heating of the upwelled waters off the Portuguese coast) also contrasts with the winter mixed layer. In some stations off the southern coast of Spain, the surface salinities in winter reached very low values (o35), the result of the strong river discharges (Rivers Guadiana, Tinto, and Guadalquivir). Underneath the surface layer, the NACW curve is clearly defined by a regular decrease of both y
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Fig. 2. y=S multiplots of the CTD data collected (a) in September 1997, aboard N/O Thalassa and (b) in January 1998, aboard B/O Cornide de Saavedra. Lines of constant potential density anomaly gy ¼ 27:50 and g1 ¼ 32:25 kg m3 (g1 —potential density anomaly referred to the 1000-dbar level) are shown within the density range of the MW.
and S values until the levels of MW influence, where the range of salinities spans from values of about 35.7 to almost 38.0. The upper limit of this range (S > 37:5) was found at the bottom in some of the stations located between 6.01W and 6.51W, very close to the Strait of Gibraltar, where almost no mixing with water of Atlantic origin has yet occurred. Further downstream, the MW layer shows clearly the two y=S peaks corresponding to the main cores centered at about gy ¼ 27:50 and g1 ¼ 32:25 kg m3 (g1 is the potential density anomaly referred to the 1000 dbar level). There was not much difference in the temperature range within the MW layer between the summer and the winter data, but the salinity values were, in general, higher during winter. This is also illustrated in Fig. 5 for the salinity vertical distribution along section L9 in summer (Fig. 5a) and in winter (Fig. 5b). This apparent seasonal difference also had been detected in the temperature field during an observational programme in 1993–94 (Ambar et al., 1999) involving the repetition of a temperature section crossing the MW undercurrent off Southern Portugal. If seasonal fluctuations actually exist in the Mediterranean Outflow, a hypothesis raised by Bormans et al. (1986), could
help to explain them. In fact, these authors refer to the existence of a seasonal control of the depth of the interface between the Atlantic Water and the MW in the Strait of Gibraltar associated with the replenishment of the reservoir of outflowing water in late February–early March. In what concerns the description of the main hydrological features, the data obtained with the Thalassa cruise in summer will be the main reference here, due to the better geographical coverage compared with those of the Cornide cruise in winter. Only in comparisons connected with seasonal differences, will the data of the latter cruise be specifically referenced. The 12 sections (Fig. 1a) crossing the Mediterranean flow at right angles show a general downstream decrease of the MW salinity values, mainly resulting from entrainment of the fresher NACW. For instance, the maximum salinity values found for the near slope MW at the level of the lower core, in the summer cruise, were 37.98 (y¼ 13:251C) in the vicinity of the Strait (section L2, Fig. 3a and b), 36.54 (y¼ 11:631C) off Cape St. Vincent (section L11, Fig. 3c and d) and 36.48 (y¼ 11:801C) off the southwest coast of Portugal (section L15).
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Fig. 3. Vertical distributions of salinity (upper row) and potential temperature (lower row) at two Thalassa sections: (a), (b) section L2; (c), (d) section L11. See Fig. 1 for location of sections.
The temperature maximum found within the MW layers at depths between 400 and 600 m (in the potential density anomaly range 27:20ogy o27:40 kg m3) can be identified with the shallow core of MW. The Thalassa data allow the tracking of this core from the vicinity of the Strait of Gibraltar (e.g., second inner station of section L2), to the region off the western coast of Portugal. In a general way, it remained close to the upper slope in the northern Gulf of Cadiz, generally within the 500 m bathymetric contour (in sections L3, L5, L6, L8, L9 and L11 it was centered around the second or third inshore station). Off the western coast of Portugal, it was
found further offshore, especially near St. Vincent canyon (the eighth station of section L12) and the ! canyon of Lisboa/Setubal (the offshore edge of section L19). The downstream route of the two salinity maxima corresponding to the MW main cores (upper and lower) diverged from that of the temperature maximum, corresponding to the shallow core. In the sections of the Gulf of Cadiz, where the MW constitutes the bottom layer (i.e. until about 7.51W), the respective horizontal separation was about 15 km (in sections L1–L3) but it increased to 35 km at about 7.01W (section L5) and 60 km at about 7.51W (section L6). This
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reflects the cross-bathymetric path followed by the two main cores in their first stages in the Gulf of Cadiz. These two MW main cores (upper and lower) were more clearly separated from each other west of section L5 of the Thalassa cruise. Fig. 4 shows the distribution of salinity and temperature on the potential density anomaly surfaces gy ¼ 27:50 and g1 ¼ 32:25 kg m3, corresponding to the upper and the lower cores. The geostrophic field at 800 and 1200 dbar relative to 2000 dbar is superimposed on these distributions in order to represent the dynamical field at the level where the cores are centered. The comparison between the thermohaline distribution and the circulation patterns at 800 and 1200 dbar levels shows some aspects that are common to both cores and some that are distinct. An undercurrent was present at both levels, with
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the higher temperature and salinity values concentrated against the continental slope. There was a tendency for the lower core to meander offshorewards after its turn around Cape St. Vincent, whereas the upper core maintained its path closer to the slope, as has been observed previously (Ambar and Howe, 1979a, b; Zenk and Armi, 1990). Eddy-like structures were present in the offshore region, some of them with a corresponding thermohaline signature; examples are an anticyclonic eddy centered at about 36.31N, 10.51W, and a cyclonic eddy centered at about 36.01N, 9.51W (section L9). In the winter cruise, there was also evidence of cyclonic eddies in the thermohaline and geostrophic fields, some of them confirmed by the RAFOS float data (Serra and Ambar, 2002). Another interesting feature shown by these data was the relatively large amount of
Fig. 4. Salinity and temperature distributions (Thalassa cruise, September 1997) on potential density anomaly surfaces corresponding to the upper and lower cores: (a) salinity, gy ¼ 27:50 kg m3 (b) salinity, g1 ¼ 32:25 kg m3; (c) temperature, gy ¼ 27:50 kg m3; (d) temperature, g1 ¼ 32:25 kg m3. Superimposed are the geostrophic fields at, respectively, 800 dbar (for gy ¼ 27:50 kg m3) and 1200 dbar (for g1 ¼ 32:25 kg m3), computed relative to the 2000-dbar level.
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MW found in the southern limit of the study region, as evidenced by the relatively high-salinity patches present in this border mainly at the level of the lower core (Fig. 3c). The geostrophic field seems to indicate a local recirculation, and this could explain the presence of these patches as blobs or eddies, probably detached from the slopetrapped undercurrent and carried eastwards. Usually, it is assumed that the depth at which the lower core stabilizes, once it reaches neutral buoyancy, is around 1200 m. However, the data collected during the Thalassa cruise showed, in some of the hydrological sections (downstream from section L6), the existence of a layer of high salinity at far deeper levels, namely around 1400 m. This denser lower core was found in the middle of section L8 where 8th and 9th stations (from the inshore edge of the section) show, respectively, salinity maxima at 1467 m (S ¼ 36:53) and 1391 m (S ¼ 36:42), in section L9 (the 8th station with 36.57 at 1347 m, and the 9th station with 36.51 at 1357 m) and in section
L11 (the 5th station with 36.54 at 1381 m, and the 6th station with 36.27 at 1425 m). The geostrophic velocities (relative to 2000 dbar) computed for each of these three sections, have shown a deep eastward flow in the layers beneath the ‘‘classical’’ level of the lower core (i.e., 1200–1300 m). This would imply the existence of a countercurrent flowing along the continental slope and carrying with it dense MW, which could have plunged in the region of the Portim*ao Canyon. Fig. 5 exemplifies the presence of this countercurrent in the geostrophic velocity field normal to section L9, both for the summer and the winter cruises. The hypothesis of the presence of a counterflow in this region was raised by Ambar and Howe (1979b) based on geostrophic calculations in hydrological sections across the main stream between 8.51W and 9.51W. In the same region, Zenk (1980) also found evidence of episodes of a deep countercurrent in direct measurements at a depth of 1568 m. Within the frame of the CANIGO project, two current-meter moorings were deployed at
Fig. 5. Vertical distribution of salinity and of geostrophic velocity (relative to 2000 dbar) perpendicular to section L9 for (a) the summer cruise and (b) the winter cruise.
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about 8.81W (S1 and S2 at latitudes of 36.51N and 36.41N, respectively) in the vicinity of the abovementioned stations of Thalassa section L9 (Fig. 1). These moorings had instruments recording for a period of over 1 year (15 June 1997–20 August 1998) at the levels of the North Atlantic Central Water (B300 m), of the MW upper (B800 m) and lower (B1200 m) cores and at a level of about 1800 m. Although the two moorings were relatively close to each other (about 11 km apart), the regime differed strongly between them, the current at the site closer to the slope (S1) being generally much more polarized by the bottom topography. At the deeper level (1800 m) in this mooring S1, there was evidence of a slow countercurrent, in a general northeastward direction and with a mean velocity just above 2 cm s1, which is in agreement with the results obtained for the geostrophic field obtained with the CANIGO data. 3.2. Nutrients and dissolved oxygen The mean chemical properties of the Mediterranean outflow measured in the vicinity of the strait (section L2) during the summer and the winter cruises were, respectively, 7.9 and 8.4 mmol kg1 for NO3, 0.64 and 0.44 mmol kg1 for PO4, and 6.7 and 6.0 mmol kg1 for Si(OH)4. These concentrations are comparable with those reported in other studies (Coste et al., 1988; Minas et al., 1991; Rhein and Hinrichsen, 1993). Farther downstream, the Mediterranean outflow did not show significant differences in O2 and nutrient (NO3, PO4 and Si(OH)4) mean values between the two CANIGO cruises (Table 1). The comparison of the nutrient data has been undertaken, in spite of inherent problems with precision due to the freezing of the nutrient samples, since the errors were below the 2.5% level. A comparative analysis was made between the thermohaline structure and the nutrient concentration patterns. Fig. 6 shows the horizontal distributions of nutrients for each core of MW and was constructed from mean values of nutrients and using objective analysis (covariance scale=25 km) methods. The mean values for each nutrient were calculated averaging the corresponding available data within each core layer (defined
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Table 1 Winter and summer mean values of dissolved oxygen (O2) and nutrients (NO3, PO4 and Si(OH)4) obtained within the MW layer along the Southern and Western Iberian Peninsula Parameter (mmol kg1) Winter O2 NO3 PO4 Si(OH)4
174 (SD=9; n ¼ 203) 13.3 (SD=3.2; n ¼ 206) 0.9 (SD=0.3; n ¼ 206) 8.9 (SD=2.5; n ¼ 206)
Summer 178 (SD=8; n ¼ 178) 13.9 (SD=3.1; n ¼ 154) 1.1 (SD=0.3; n ¼ 154) 7.6 (SD=2.5; n ¼ 154)
Standard deviation (SD) and number of samples (n) are also indicated.
by the following range of potential density anomalies: upper core, from gy ¼ 27:25 to g1 ¼ 32:00 kg m3, and lower core, from g1 ¼ 32:00 to 32:35 kg m3). A downstream gradual modification of the nutrient concentrations was observed along the southern and southwestern Iberian coasts (Fig. 6). The characteristic high salinity and temperature values of the MW were associated with the lowest levels of nutrients compared with the surrounding Atlantic water. There were, however, differences in the chemical characteristics between the upper and the lower MW cores, the upper core exhibiting slightly higher O2 and lower nutrient concentrations (Fig. 6). In the upper core O2 ranged from 170 to 200 mmol kg1 (170–180 mmol kg1 in the lower core), while NO3 ranged from 6.9 to 17.4 mmol kg1 (8.5–21.8 mmol kg1 in the lower core), PO4 from 0.6 to 1.4 mmol kg1 (0.8– 2.4 mmol kg1 in the lower core) and Si(OH)4 from 3.4 to 10.9 mmol kg1 (4.8–13.2 mmol kg1 in the lower core). The eddy-like structures evidenced by the thermohaline distribution in the offshore region, also were identified by their chemical properties. For instance, the cyclonic eddy centered at about 36.01N, 9.51W (section L9) presented, at about 700-m depth, nutrient concentrations lying in the upper core range. These findings are similar to those reported by other authors (Rhein and Hinrichsen, 1993; Hinrichsen et al., 1993; Pingree and LeCann, 1993; Pingree, 1995) who have
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Fig. 6. Horizontal distributions of nutrients within the MW layers off Southern and Southwestern Iberia (data from Thalassa cruise): (a) NO3, upper core; (b) NO3, lower core; (c) PO4, upper core; (d) PO4, lower core; (e) Si(OH)4, upper core; and (f) Si(OH)4, lower core.
differentiated, from the point of view of the chemistry, one or both cores within the meddies that they detected off the Portuguese western coast. 3.3. Suspended particulate matter Two indicators of particle abundance, TSM (in mg/l) and FAO (% of filter area occupied), were measured in order to characterize the different water masses. Both indicators showed similar vertical variations between water masses although with different variances (Fig. 7). A maximum in the surface layer, coincident with the largest variability, is followed by a decrease in the NACW layer, down to a relative maximum corresponding to the MW layer. This maximum was not always coincident with the maximum in temperature or in
salinity that characterizes, respectively, the upper and the lower MW cores. In order to assess the differences in the SPM composition between water masses, several parameters (POM/PIM, d13C(PDB), and VA) were evaluated. A large variance was observed in POM and PIM values within each water mass, and the only consistent feature was the systematically higher POM/PIM ratio observed in the summer cruise (Fig. 7e). At the level of the MW, this ratio was generally o1, which means that inorganic matter was dominating. Conversely, Abrantes et al. (1994) found that the SPM in meddies off the southwestern coast of Portugal was dominated by the organic component. Much of the particulate organic carbon in the open ocean comes from the detrital remains of
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(a) SURFACE
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(b)
NACW MIN SAL MW NADW
35
35.5
36
36.5
4 6 8 10 12 14 16 18 Temperature - °C
Salinity (d)
(c) SURFACE NACW MIN SAL MW NADW
0 0.1 0.2 0.3 0.4 0.5
-2
0
2
(e) SURFACE
4
6
8 10
FAO - o/o
TSM - mg/l (f)
NACW MIN SAL MW NADW
-0.5 0 0.5 1 1.5 2 POM/PIM
-27
-26
-25
-24
13
δ C
(PDB)
-
-23
o
/oo
Fig. 7. Vertical profiles of average values near the sea surface, within the NACW, the MW (the salinity minimum between NACW and MW is also indicated), and the NADW of: (a) salinity; (b) temperature; (c) TSM; (d) FAO; (e) POM/PIM; (f) d13C(PDB). Summer survey (black) and winter survey (gray).
plankton from surface waters (Suess, 1980), and this is reflected in its isotopic composition (Eadie et al., 1978; Fischer, 1991). In these data, the surface layer showed the highest d13C(PDB) of the whole water column, while the MW corresponded to an intermediate maximum (Fig. 7f), clearer in the winter survey and in the region near the Strait of Gibraltar and of the Gulf of Cadiz. The
d13C(PDB) values observed in this study, 21– 28%, are within the range 18–30% reported for plankton and particulate organic carbon in the ocean (Eadie and Jeffrey, 1973; Rau et al., 1989; Fischer, 1991; Libes, 1992). Visual analysis of particle composition (Fig. 8) showed that particles o10 mm were predominant in the whole water column except in the surface
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(c)
(b)
(a) SURFACE NACW MIN SAL MW NADW
-10 0 10 20 30 40 50 % Minerals (d)
0
20
40 60 80 100 % < 10 µm
0
10 20 30 40 % Aggregates
50
(f)
(e)
SURFACE NACW MIN SAL MW NADW
0
20
40 60 80 100 % Biogenics
(g)
0
20 40 60 80 100 % Siliceous bio.
-10 10 30 50 70 90 % Carbonate bio. (i)
(h)
SURFACE NACW MIN SAL MW NADW
-25
25 75 125 175 Diatoms valves/l
-400 100 600 1100 1600 Coccoliths/l
-3 -2 -1 0 1 2 3 4 5 6 Discoasters/l
Fig. 8. Average values obtained by visual microscopic analysis of particle composition for each water mass of: (a) % of mineral particles; (b) % of o10 mm particles; (c) % of aggregate particles; (d) % of biogenic particles; (e) % of siliceous biogenic particles; (f) % of carbonate biogenic particles; (g) diatom valves/l; (h) coccoliths/l; and (i) discoasters/l. Summer survey (black) and winter survey (gray).
layer (Fig. 8b). In this layer, biogenic particles (including particles o10 mm) were the most abundant group. NACW had similar values for the biogenic particles but showed a vertical trend in SPM composition with increasing number of mineral particles (Fig. 8a) and decreasing number of particles o10 mm (Fig. 8b) with depth. The MW showed a clear difference in particle composition relative to the other water masses, with higher abundance of mineral and biogenic particles (except in comparison with the surface layer) and lower abundance of particles o10 mm. Within the NADW layer, the SPM was characterized by the
highest values of particles o10 mm and the lowest values of aggregate (Fig. 8c) and biogenic (Fig. 8d) particles, when compared with the other water masses, and by relatively high values of mineral particles. The only noticeable difference in the abundance of aggregates between water masses was the minimum values found in the NADW. The variation of biogenic particles between water masses was also marked by the variation of diatoms, coccoliths and discoasters, with the highest values occurring at the surface, a decrease with depth within the NACW, a maximum at the MW and a decrease again in NADW.
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Large differences were observed in the composition of SPM between the MW that flows close to the continental slope (slope-MW) and the MW encountered farther offshore (offshore-MW). The slope-MW showed higher average values of TSM, FAO, d13C(PDB), minerals, aggregates and biogenics, and lower abundance of particles o10 mm, when compared with the offshore-MW. Most of the biogenic particulate material in the slope-MW was carbonate, while in the offshore-MW it was siliceous. Slope-MW also showed higher abundance of diatoms, coccoliths and discoasters than offshore-MW. In terms of SPM, the offshore-MW was more similar to NACW and NADW than the slope-MW.
4. Summary and conclusions This work presents some preliminary results of a multidisciplinary study of the Mediterranean outflow in the region west of the Strait of Gibraltar and off the southern and southwestern coasts of the Iberian Peninsula, in the framework of the CANIGO Project. Two high-resolution CTD surveys took place in September 1997 (summer cruise) and January 1998 (winter cruise) in the study region. The field work included the collection of water samples at selected stations and depths, with the objective of characterizing the MW and comparing it with the adjacent water masses from the point of view of the chemistry (nutrient and dissolved oxygen concentrations) and of the SPM. A general correspondence was found between the high-salinity values of the MW layer and the low-nutrient and oxygen contents and relatively high abundance of particles. The progressive mixing of the MW with the entrained NACW resulted in a downstream decrease of the temperature and salinity accompanied by a slight modification in nutrient concentrations. Particle abundance in the MW was characterized by a maximum, which was not always found at exactly the same level as its temperature and salinity maxima. Most of the parameters used to characterize the SPM have shown a local maximum (of TSM, FAO, d13C(PDB)) at the depths of the MW.
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In terms of the visual analysis of the particle composition, there were also extreme values associated with the MW layer. There were some differences between the summer and the winter cruises with respect to the thermohaline characteristics, the winter salinity values being higher than those found in summer. In terms of the nutrient and the dissolved oxygen concentrations, there were no significant differences between the two data sets. The thermohaline analysis has shown the presence of the two main cores of MW—upper and lower cores—corresponding to temperature and/or salinity maxima centered at about 800 and 1200 m. The upper core is better characterized by the temperature profile and the lower core by the salinity profile. In terms of the chemistry, the upper core contained lower nutrient and higher oxygen concentrations than the lower core. However, from the point of view of the suspended matter, the distinction between the two cores was not evident. Besides the two main cores, there was also evidence of a shallower one, at depths of about 500 m, which was characterized especially by high temperatures and remained close to the upper continental slope all along Southern Iberia, but was found farther away from the upper slope in the region off Southwestern Portugal. The chemical characterization of this shallow core suggested nutrient values similar to those found for the upper core. The joint analysis of the geostrophic velocity field and the thermohaline distribution patterns, has shown some interesting aspects of the Mediterranean undercurrent. A counterflow (i.e. eastward), beneath the westward undercurrent, in the region of higher bathymetric gradient within the Portim*ao Canyon (off Southern Portugal), transported high-salinity water (in the range 36.4–36.6) at depths between about 1300 and 1500 m. The existence of a counterflow was also apparent from the analysis of the direct current measurements made at 1800 m in the same region, under CANIGO. Another important aspect shown by the geostrophic and hydrological fields in both cruises, was the existence of eddies of MW, rotating either
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anticyclonically, the so-called meddies, or cyclonically. The chemical signature of these eddies corresponded to that ascribed to the MW at the corresponding depths. This physical, chemical and sedimentological analysis in a joint approach has proved to be useful for characterizing water masses and facilitates their differentiation, which sometimes is difficult based only on the thermohaline distributions.
Acknowledgements This work was supported by the European Union MAST III project CANIGO (Contract MAS3-CT96-0060). The observational work was done from aboard the R/V Thalassa and the B/O Cornide de Saavedra. The authors thank all the cruise participants and the crew members who made possible the realization of this work. The participation of NS was supported by the Fundac-a* o para a Ci#encia e a Tecnologia (Grant BD/ 19535/99).
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