Particle fluxes dynamics in Blanes submarine canyon (Northwestern Mediterranean)

Progress in Oceanography 82 (2009) 239–251

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Particle fluxes dynamics in Blanes submarine canyon (Northwestern Mediterranean) Diana Zúñiga a,*,2, M. Mar Flexas b,2, Anna Sanchez-Vidal a, Johan Coenjaerts c, Antoni Calafat a, Gabriel Jordà b,d, Jordi García-Orellana e,1, Joan Puigdefàbregas d, Miquel Canals a, Manuel Espino d, Francesc Sardà f, Joan B. Company f a

GRC Geociències Marines, Dept. d’Estratigrafia, Paleontologia i Geociències Marines, Universitat de Barcelona, E-08028 Barcelona, Spain Institut Mediterrani d’Estudis Avançats (IMEDEA), UIB-CSIC, E-07190 Mallorca, Spain Centre d’Estudis Avaçats de Blanes (CEAB), CSIC, E-17300 Barcelona, Spain d Laboratori d’Enginyeria Maritima (LIM), UPC, E-08034 Barcelona, Spain e Departament de Física, Institut de Ciència i Tecnologia Ambientals (ICTA), Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain f Institut de Ciències del Mar (ICM), CSIC, E-08003 Barcelona, Spain b c

a r t i c l e

i n f o

Article history: Received 3 February 2009 Received in revised form 19 June 2009 Accepted 8 July 2009 Available online 12 July 2009

a b s t r a c t Within the framework of the multidisciplinary RECS project and with the aim of describing the particle flux transfer from the continental shelf to the deep basin, an array of five mooring lines equipped with a total of five pairs of PPS3/3 sequential-sampling sediment traps and RCM-7/8 current meters were deployed 30 m above the bottom from March 2003 to March 2004 inside and outside the Blanes Canyon. One mooring line was located in the upper canyon at 600 m depth, one in the canyon axis at 1700 m depth and other two close to the canyon walls at 900 m depth. A fifth mooring line was deployed in the continental open slope at 1500 m water depth. The highest near-bottom downward particle flux (14.50 g m 2 d 1) was recorded at the trap located in the upper canyon (M1), where continental inputs associated with the presence of the Tordera River are most relevant. On the other hand, the downward fluxes (4.35 g m 2 d 1) in the canyon axis (M2) were of the same order as those found in the western flank (M3) of the canyon. Both values were clearly higher than the value (1.95 g m 2 d 1) recorded at the eastern canyon wall (M4). The open slope (M5) mass flux (5.42 mg m 2 d 1) recorded by the sediment trap located outside the canyon system was three orders of magnitude lower than the other values registered by the inner canyon stations. The relevance of our data is that it explains how the transport pathway in the canyon occurs through its western flank, where a more active and persistent current toward the open ocean was recorded over the entire year of the experiment. Off-shelf sediment transport along the canyon axis showed clear differences during the period of the study, with some important events leading to strong intensifications of the current coupled with large transport of particle fluxes to the deepest parts of the canyon. Such events are primarily related to increases in river discharge and the occurrence of strong storms and cascading events during the winter. In summary, in this study it is shown that the dynamics of the water masses and the currents in the study area convert the sharp western flank of the Blanes Canyon in a more active region that favors erosion processes than the eastern flank, which has a smoother topography and where the absence of erosional conditions yields to steadier sedimentary processes. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The increasing interest of the research community in submarine canyons owes to the fact that these geological features are hot spots of biodiversity and biological productivity (Gili et al., 1999; Granata et al., 2004). Local modifications of the incoming flow by * Corresponding author. Actual address: Instituto de Investigacións Mariñas (IIM), CSIC, E-36208 Vigo, Spain. Tel.: +34 986231930; fax: +34 986292762. E-mail address: [email protected] (D. Zúñiga). 1 School of Marine and Atmospheric Sciences, State University of New York, Stony Brook, NY 11794-5000, USA. 2 These two authors contributed equally to this work. 0079-6611/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2009.07.002

these abrupt discontinuities induce different shelf-slope exchanges (Alvarez et al., 1996) that in most cases convert the submarine canyons in preferential conduits for matter transfer from the continental shelf toward the deep basin (Heussner et al., 1999; Hung et al., 2003; Liu and Lin, 2004; Palanques et al., 2005; Turchetto et al., 2007). All of these specific conditions contribute to creation of a special habitat for the recruitment and maintenance of stocks of living resources (Sardà and Cartes, 1993; Cartes et al., 1994). Several studies of particle fluxes in the western Mediterranean margin have revealed that the quantities and compositions of the fluxes are strongly related to cross-slope exchange mechanisms, with particle sources dominated by intermittent fluvial discharges

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and surface biological production (Monaco et al., 1990; Puig and Palanques, 1998; Palanques et al., 2005; Martín et al., 2006; Bonnin et al., 2008; Fabrés et al., 2008). Heussner et al. (2006) revealed that in the Gulf of Lions (NW Mediterranean) and on an annual basis, the total mass flux increases with depth and decreases seaward at an equivalent depth, indicating lateral transport of particulate matter from the adjacent shelf and upper-slope waters to the deep basin. Additionally, recent studies have determined that the existence of seasonal dense shelf-water cascading (DSWC) in the Gulf of Lions also constitutes an important mechanism for the transport high sediment fluxes through submarine canyons (Durrieu de Madron et al., 2005; Canals et al., 2006; Heussner et al., 2006; Palanques et al., 2006, 2009). Numerical simulations by Ulses et al. (2008) showed that on the Catalan margin, the Blanes Canyon, which deeply incises the shelf, could be an efficient pathway for DSWC down to the basin. Going further, Company et al. (2008) remarked that the intense cascading events recorded at the Catalan margin affect the distribution of the rose shrimp Aristeus antennatus, a target species in Mediterranean fisheries. The multidisciplinary RECS (Estudio Integral de un cañón submarino en el Mediterráneo occidental (Cañón de Blanes): Aplicación a la explotación de la gamba roja, A. antennatus) project focused on understanding how geological and physical interactions with topography may lead to differentiated ecological spots. In this framework, using sediment traps coupled with current meters inside and outside the Blanes submarine canyon, we aimed to understand the particle flux transfer between the shelf and the deep basin. Furthermore, we hoped to contribute to an understanding of how physical and biogeochemical factors may control the distribution of a benthic species such as deep-sea the rose shrimp A. antennatus (Sardà et al., in press). 1.1. Regional setting The Blanes Canyon is located on the Catalan continental margin (northwestern Mediterranean), which extends from the Cap de Creus Canyon (northern limit) to the Ebro delta river (southern limit) (Fig. 1). This margin has a narrow continental shelf, with the shelf break located at around 150 m depth and the continental

slope characterized by the presence of numerous incisions at depths up to 2000 m. The Blanes Canyon deeply cuts the continental slope in a north-to-south direction. The upper canyon is located at around 60 m depth and at a distance of less than 4 km from the coastline (Díaz and Maldonado, 1990), where the tortuous Tordera River bed and the sometimes torrential Lloret and Tossa streams reach the sea (Rovira and Batalla, 2006). The width of the canyon increases with depth until it reaches a maximum of 20 km at its deepest part (Canals et al., 2004). The canyon walls show various bathymetric characteristics (Fig. 1): whereas the east canyon wall is smooth with a step at 41.30°N, the west canyon wall is sharp and extremely abrupt. The northwest Mediterranean circulation is characterized by a general cyclonic circulation along the continental slope (the socalled Northern Current, hereafter NC), forced by the entrance of Atlantic Water (AW) through the Gibraltar Strait (Millot, 1999). In the Catalan continental margin, the AW extends from the surface down to 100–200 m depth (Millot, 1999). Below this water mass and down to approximately 600 m depth, the basin is filled by the more saline Levantine Intermediate Water (LIW) originating in the eastern Mediterranean basin. At greater depths and extending down to the seafloor, the water column is occupied by the Western Mediterranean Deep Water (WMDW), formed during the winter in the Gulf of Lions (Millot, 1999). The NC involves AW, LIW and WMDW, following the western Mediterranean continental slope in the same anticlockwise direction (Millot, 1999). The current has a baroclinic component from surface to approximately 400 m depth, associated with a shelf-slope density front that separates cold, fresh waters over the continental shelf from saltier waters from the open sea (Font et al., 1995). In addition, the NC has a barotropic component that involves the entire water column (Flexas et al., 2002). 2. Material and methods 2.1. Design of the experiment and treatment of sediment trap samples Five mooring lines were deployed during one year in and around the Blanes submarine canyon in the northwestern

Fig. 1. General bathymetric map of the study area (left) and detailed location of the mooring lines (M1–M5) within the Blanes Canyon. Individual CTD stations (white crosses) and the two CTD transects (across- and along-canyon) are also shown.

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D. Zúñiga et al. / Progress in Oceanography 82 (2009) 239–251 Table 1 Location of the sediment traps in the Blanes Canyon. Station

Latitude

Longitude

Trap depth (m)

Seafloor depth (m)

Num. samples

M1 M2 M3 M4 M5

41.50556°N 41.37028°N 41.36250°N 41.32806°N 41.34389°N

02.90750°E 02.86806°E 02.80530°E 02.95472°E 03.23528°E

570 1670 870 870 1470

600 1700 900 900 1500

19 24 12 23 5

Mediterranean (Fig. 1 and Table 1). One mooring line was located in the upper canyon (M1) (600 m depth) in a depression that connects the adjacent shelf with the canyon axis (Fig. 1), one in the canyon axis (M2) at 1700 m depth, two (M3 and M4) close to the canyon walls at 900 m depth, and a fifth mooring (M5) was located outside the canyon, at 1500 m depth. The objective of the latter was to describe the open-slope particle flux reference. At each mooring line a Technicap PPS3/3 sequential-sampling sediment trap was placed 30 m above the bottom (Table 1). The employed sediment trap has a cylindrico-conical shape with a height/diameter ratio of 2.5 and a collecting area of 0.125 m2 (Heussner et al., 1990). The rotating carousel bears 12 receiving cups, and the sample collection interval ranged from 15 days to 1 month. Unfortunately, no sediment trap samples were recovered at station M1 from August 2003 to October 2003 due to interference with fishing trawlers, and at station M3 from April 2003 to September 2003 and station M5 from May 2003 to March 2004 because of failure of the rotating motor (Fig. 2). Before deployment, all sediment traps were thoroughly cleaned. In the laboratory, the entire rotary collector was cleaned with a detergent, soaked in HCl 0.5 N overnight and rinsed several times with distilled water. Once on board, traps were rinsed with seawater. Receiving cups were filled with a 5% formaldehyde

solution in 0.45 lm-filtered seawater (buffered with sodium borate) for the purpose of slowing particle degradation and preventing swimming organisms from grazing and disrupting the collected particles. Upon recovery, the receiving cups were stored in the dark at 2–4 °C until they were processed. Following Heussner et al. (1990), swimmers (including all organisms that do not fall gravitationally through the water column) were removed to avoid error in the measured fluxes using a 1 mm nylon mesh to retain the largest organisms and to dislodge any trapped fine particles. Large aggregates, as part of the passive fluxes, were returned to the sample. The <1 mm swimmers were removed by hand under a dissecting microscope with fine tweezers. Then, to obtain several sub-samples of equal volume, a high-precision peristaltic pump was used. The total mass was determined gravimetrically. The replicates were filtered using pre-weighed Millipore cellulose acetate membrane filters (0.45 lm pore size, 47 mm diameter), rinsed with distilled water to remove salts and excess formalin and dried to a constant weight at 40 °C over 24 h. Samples for total and organic carbon and nitrogen were filtered through glass-fiber pre-filters (GFF). Total and organic carbon and nitrogen were measured following Fabrés et al. (2002). Assuming that all inorganic carbon was constituted by calcium carbonate, organic matter (organic carbon  2) and inorganic carbon (total carbon–

Fig. 2. Timetable of both the sediment traps and current meters data gathered during RECS project. Several oceanographic cruises performed during the experimental period are also noted in the figure.

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Fig. 3. Daily Tordera River discharge during the year of the experiment.

organic carbon  8.33) contents were calculated. Corg/Norg is expressed as an atomic ratio. 2.2. Fluvial discharge, hydrographic and hydrodynamic data Daily fluvial discharges (m3 s 1) corresponding to the study period were obtained from ‘‘Agència Catalana de ´lAigua, Generalitat Catalunya, Catalonia, Spain” for the Tordera River. This river deflects southward from the Blanes Canyon mouth, but it constitutes the nearest fluvial input to the study area (Fig. 1). Hydrographic data were obtained during three research cruises in June 2003, November 2003 and March 2004 (Fig. 2) by using a CTD profiler. Two CTD transects (one along the canyon axis, and one across the canyon) with individual stations spaced 4 km in longitude and 8 km in latitude were carried out during each cruise in order to assess seasonal hydrographic variability (Fig. 1). An Aanderaa vector averaging rotor current meter RCM 7 and 8 at 250 m, 600 m and close to the bottom (excepting M2, which had no device at 250 m, and M5, which had no device at 600 m) (Fig. 2) was deployed to measure current speed and direction. Mooring M1 placed the upper current meter at 375 m. Bottom current meters were located 2 m below the trap. Main statistical data are shown in Table 2. 2.3. Sediment cores collection Five sediment cores (C1, C2, C3, C4 and C5) were collected at the same locations where the mooring lines were deployed. The cores were recovered with a multi-corer sampler during cruises carried out in April 2003 and July 2003 aboard the R/V Garcia del Cid. Immediately after retrieval, cores were visually described and sliced every 0.5 cm (from surface to 5 cm), 1 cm (from 5 to 20 cm) and 2 cm (from 20 cm to bottom) and stored in sealed plastic bags at 4 °C until processing in the laboratory. To avoid crosscontamination during the coring operation, the outer part of each section was removed. All analyses reported here were performed on samples from this single replicate.

2.3.1. Sediment analysis Once in the laboratory samples were dried at 50 °C until constant weight was achieved. Determination of 210Pb activities was accomplished through the measurement of its granddaughter nuclide 210Po after roughly one year of collection time. 210Pb analyses of the sediment samples were performed following the methodology described by Sanchez-Cabeza et al. (1999), by total digestion of 200–300 mg sample aliquots. 209Po was added to each sample before digestion as an internal tracer. Polonium isotopes were counted with a-spectrometers equipped with low-background silicon surface barrier (SSB) detectors (EGpG Ortec). Uncertainties were calculated by standard propagation of the one sigma counting errors of samples and blanks. Excess 210Pb activities (210Pbxs) were determined by subtracting the 226Ra activity (assumed to equal supported 210Pb activity) from the total 210Pb activity. 210Pbderived sediment accumulation rates were calculated based on a one-dimensional, steady-state constant 210Pb flux/constant sedimentation model (CF:CS) constrained by the 137Cs concentration profiles in cores C4 and C5 (Krishnaswami et al., 1971). The activities of 137Cs and also 226Ra were determined by c-spectrometry using a coaxial high-purity Ge detector (Canberra and EG&G Ortec). A non-homogenized fraction of approximately 1 g was used for grain-size analysis conducted with a Coulter Counter LS100. For that analysis, samples were treated with both 10% H2O2 and 1 M HCl to remove organic matter and carbonates, respectively. Results are expressed in percentages and three main categories were considered: (i) sand (>63 lm), (ii) silt (4–63 lm), and (iii) clay (<4 lm). Finally, total and organic carbon (and nitrogen) analyses were carried out over aliquots (20–30 mg) of non-treated and 25% HCl-treated (to remove carbonates) homogenized sediment. 3. Results 3.1. Oceanographic conditions during the studied period Major floods events of the Tordera River occurred in October 2003 and at the end of March 2004 when maximum river daily discharge reached 78 m3 s 1 and 97 m3 s 1, respectively. Smaller increases in river discharge were also recorded in early December 2003 (up to 53 m3 s 1) and February 2004 (up to 41 m3 s 1). Temperature and transmissivity profiles along the canyon showed that the water column was clearly stratified in June 2003 and in November 2003, with higher temperatures at the surface (potential temperature >14 °C) and a representative decrease pattern with depth (Fig. 4a and b). At the cross-canyon section, it is important to note that light transmission was lower at the eastern canyon wall in both June 2003 and November 2003 (Fig. 4a and b). In March 2004 the upper water column was occupied by fresh, cold

Table 2 Statistical parameters from bottom current meters. S.D. refers to the standard deviation of the current speed. Considering the variant ellipse of the different devices, the velocity vectors were decomposed in the along- (X) and across- (Y) main axis directions. Unfortunately, data from bottom current meters M3 and M4 (the second half of the study period) were not recorded. Code (water depth)

Recording period

Days

Average speed (cm s 1) ± S.D.

Maximum speed (cm s 1)

M1 (600 m)

I (29/03/03–30/08/03) II (25/10/03–20/03/04) I (29/03/03–30/08/03) II (10/09/03–29/03/04) I (29/03/03–30/08/03) II (10/09/03–29/03/04) I (29/03/03–30/08/03) II (10/09/03–29/03/04) I (29/03/03–30/08/03) I (29/03/03–30/08/03) II (10/09/03–29/03/04)

124 195 159 249 172 248 159 230 171 71 249

5.5 ± 3.1 3.2 ± 2.9 3.2 ± 2.1 3.8 ± 2.8 4.8 ± 3.5 4.4 ± 3.2 4.5 ± 2.8 5.1 ± 2.6 5.4 ± 3.6 4.8 ± 3.4 3.7 ± 2.7

18.5 38.7 13.8 22.6 18.0 20.7 15.3 14.9 24.3 18.5 21.2

M2 (1700 m) M3 (600 m) M4 (600 m) M4 (900) M5 (1500 m)

Average X speed (cm s 1 ± S.D.) 1.8 ± 5.7 1.2 ± 3.9 0.0 ± 3.6 1.9 ± 3.9 4.6 ± 3.5 3.0 ± 4.1 4.0 ± 3.0 3.0 ± 3.6 4.4 ± 4.2 4.1 ± 4.0 2.4 ± 3.6

Average Y speed (cm s 0.7 ± 2.0 0.1 ± 1.6 0.1 ± 1.6 0.3 ± 1.6 0.3 ± 1.2 0.9 ± 1.7 0.8 ± 1.3 1.9 ± 2.7 1.2 ± 2.0 0.2 ± 1.0 0.0 ± 1.5

1

± S.D.)

Residual direction (degrees from N) 29.4 3.4 10.5 35.6 36.1 62.4 82.6 70.7 84.3 71.6 101.6

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243

Fig. 4. Temperature and transmissivity diagrams along and across the Blanes Canyon (as indicated in Fig. 1) for (a) June 2003, (b) November 2003 and (c) March 2004.

shelf water. During that time, differences in light transmission between the eastern and western flanks were slightly lower (Fig. 4c). In all profiles an intermediate bottom nepheloid layer was registered at 600–800 m depth. This permanent feature is probably associated with the density gradient at the boundary of the LIW and the WMDW, which favors internal wave activity and sediment re-suspension. Variance ellipses from currents (Fig. 5) showed that the current was constrained to flow along the local topography at intermediate depths (Fig. 5b) and close to the bottom (Fig. 5c).

Intermediate and upper layers flowed following the bathymetry over the upper canyon (at M1 at all depths) and over the western middle canyon wall (at M3 at intermediate depths), whereas over the eastern middle canyon wall this topographic control occurred only below the intermediate levels (at 600 m and 900 m). Such bathymetric constraint is explained by the proximity of the flow to the seafloor and to the canyon walls owing to potential vorticity conservation (Pedlosky, 1979). Instead, when the flow is above abrupt density changes such as the thermocline (between 150 and 300 m, Fig. 4), it is less influenced by bathymetric changes,

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Fig. 5. Variance ellipses from currents at (a) (top) 250 m and 375 m, (b) (middle) 600 m and (c) (bottom) 900 m, 1500 m and 1700 m.

which would explain the isotropicity observed at M3 and M4 at 250 m (Fig. 5a). Maximum average mean speeds of 5.5 cm s 1 and 5.4 cm s 1 were recorded at M1 and M4, respectively. At these stations the flow was oriented toward the N–NW. On the other hand, the lowest average mean speeds were registered at M2 (the deepest station), with values always below 4 cm s 1. The time series of both near-bottom-current intensities showed that intense several-day current pulses occurred during the study period. On occasion (e.g. from December 2 to 12), intensifications registered at M1 did not affect the near-bottom current at M2 (Fig. 6a). In other cases, the intensifications affected all depths and sites, as, for example, during a particularly energetic event that occurred during March 2004. In this case, maximum current speeds close to 20 cm s 1 were recorded at M1 and M2 stations. Unfortunately, we have no data for bottom-current intensities at M3 and M4 (in the second half of the study period) (Fig. 2). For that reason, we have included time series of 600 m depth at both sites (Fig. 6a) where this abrupt increase in the current speed was also recorded. In any case, it is important to note the high correlation (0.73) between intermediate and near-bottom currents at the M4

site during the first half of the experimental year. Furthermore, this event was also described by an abrupt decrease in water temperature at all stations (Fig. 6b). 3.2. Spatial variation of particle fluxes Due to the loss of some sediment trap samples, main statistical parameters time-weighted fluxes (TWF) of the Blanes Canyon were recalculated for periods with complete series in some of the mooring lines (Table 3). Then, the period from October 2003 to March 2004 was used to compare canyon stations (M1, M2, M3 and M4) and the period from March 2003 to May 2003 to compare the canyon samples with the outside canyon samples (M5). Considering the whole experimental year, the highest particle fluxes were recorded at the upper canyon (station M1), with a TWF up to 14.50 g m 2 d 1 (Table 3). Downward fluxes in both the canyon axis (station M2, 1700 m) (4.35 g m 2 d 1) and the west canyon flank (station M3, 900 m) (5.95 g m 2 d 1) were of the same order of magnitude and clearly higher than the value (1.95 g m 2 d 1) recorded at the eastern flank (station M4, 900 m). In contrast, the TWF recorded at the adjacent slope,

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245

Fig. 6. (a) Time series of current intensity at M1 (600 m), M2 (1700 m), M3 (600 m) and M4 (600 m and 900 m). (b) Time series of water temperature at M1 (600 m), M2 (600 m and 1700 m), M3 (600 m) and M4 (900 m). Unfortunately, we have no temperature data from the current meter M4 at 600 m. Shaded areas indicate the major Tordera River flood events.

outside the canyon (station M5, 1500 m), was three orders of magnitude lower (5.42 mg m 2 d 1) than values recorded at the canyon stations. 3.3. Temporal variability of bottom particle fluxes Near bottom downward fluxes at the upper canyon (station M1) showed relatively low values during the spring 2003 with a mean value of 5.89 ± 3.51 g m 2 d 1 (Fig. 7). Two clear peaks in the first half of December 2003 (up to 40.78 g m 2 d 1) and in March 2004 (up to 29.58 g m 2 d 1) were recorded, and were the highest values registered during the whole experimental year.

Deeper in the canyon axis (station M2) the fluxes have decreased substantially, with highest values of 8.66 and 9.78 g m 2 d 1 recorded during February 2004 and March 2004, respectively. The rest of the year particle fluxes remained quite steady, varying around a mean value of 3.33 g m 2 d 1. The western canyon wall particle fluxes (M3) at 900 m depth (unfortunately, sediment samples have been lost for the first half of the year) had a similar variability, but values were slightly higher than those recorded at station M2 at 1700 m depth. TWF oscillated between a minimum value of 1.68 g m 2 d 1 in September 2003 and a value up to 10 g m 2 d 1 in March 2004. At a similar depth, the eastern canyon wall (M4) particle fluxes were clearly

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Table 3 Time weight fluxes (TWF) and total mass flux statistics (mean, maximum, minimum and standard deviation S.D.) of the sediment traps located in the Blanes Canyon. Data are P P expressed in g m 2 d 1. TWF is calculated as Fi/(collection area (0.125)  di) being Fi the total mass flux in period i and di, the number of collection days of period i. This parameter will eliminate the disturbances due to the loss of samples. M1 (g m March 2003–March 2004

October 2003–March 2004

March 2003–May 2003

2

d

1

)

M2

M3

M4

5.23 0.38 1.07 2.42

5.42  10

3

2.62 2.20 0.16

7.88  10 4.53  10 1.25  10

3

3.82 9.78 0.86 4.21 4.35 3.82

5.18 10.76 1.68 2.28 5.95 5.18

1.91 5.23 0.38 1.07 1.95 1.91

Max. Min. S.D. Mean

40.78 8.97 9.32 8.23

9.78 0.86 4.21 4.09

10.76 1.68 2.28

Max. Min. S.D.

11.57 3.84 3.03

5.56 2.24 1.35

1

d 1. One recorded

3.4. Composition of sinking particles: organic matter and carbonates Organic matter percentages were almost always higher on the eastern canyon wall (M4), with two maximums of 5.48% and 5.34% in April 2003 and November 2003, respectively (Fig. 8a). Similar values were reached only at the upper canyon station M1, where a value of 5.05% was registered at the end of April 2003. On the other hand, at both M2 and M3 stations temporal variability of this constituent was lower, varying around a mean value of 2.67% and 2.28%, respectively. One exception was the relative maximum (5.14%) registered at M2 (1700 m depth) during August 2003. The organic matter particle fluxes showed the highest values at the upper canyon, with two strong peaks in December 2003 and

Fig. 7. Time series of total mass fluxes (g m

2

d

1

d

5.42  10 7.88  10 4.53  10 1.25  10 5.42  10

13.98 40.78 3.16 10.53 14.50 20.60

2

2

3

Mean Max. Min. S.D. TWF Mean

lower, with values always remaining below 4 g m exception was the absolute maximum of 5.23 g m 2 d at the end of October 2003.

M5 (g m

1

)

3 3 3 3

3 3

March 2004 when values of 660 mg m 2 d 1 and 914 mg m 2 d 1 were recorded. Following the canyon axis, organic matter fluxes at M2 were clearly lower. Values oscillated around a mean value of 100 mg m 2 d 1 and clearly follow the pattern of the total mass fluxes described in the previous section. One exception was the peak found in August 2003 where a value of 291 mg m 2 d 1 was registered. Similarly to the total particle fluxes data, organic matter fluxes at the western flank (M3) were similar to those found at M2, ranging between a minimum value of 51 mg m 2 d 1 in September 2003 and a maximum value of 259 mg m 2 d 1 in March 2004. On the other hand, values at M4 were the lowest of the whole canyon. The temporal series showed two strong peaks in April 2003 and November 2003 with values of 143 mg m 2 d 1 and 156 mg m 2 d 1, respectively. Highest values of carbonates were always recorded at the eastern canyon wall station (M4), where profiles showed low variation during the whole experimental year (Fig. 8b). At the upper canyon (M1) calcium carbonate percentage showed the widest

) for the sediment traps (M1, M2, M3 and M4) moored in the Blanes Canyon.

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247

Fig. 8. (a) Time series of organic matter (OM) fluxes (line) and percentages (bars) of the bottom sediment traps. (b) Time series of calcium carbonate (CaCO3) fluxes (line) and percentages (bars) of the bottom sediment traps.

range of variation, ranging from a minimum value of 9.39% in July 2003 up to a maximum of 24% in March 2003. At the canyon axis station M2 (1700 m depth), values remained lower during spring 2003 (mean value of 14.98%) and increased at the end of summer until reaching a maximum value of 25% at the end of September 2003. The calcium carbonate time series fluxes presented a similar trend than the total mass fluxes with the highest values recorded

at M1 (maximum value of 7016 mg m 2 d 1 in March 2004) and the lowest at M4 (minimum value of 70 mg m 2 d 1 in November 2003). The highest mean Corg/Norg atomic ratio (11.7) was found at M2, while lower values of the ratio characterized both the eastern and western canyon walls, where mean values of 8.2 and 9.0 were, respectively, registered (Table 4). 3.5. Description of sediment cores

Table 4 Time-averaged fluxes (mg m 2 d 1) and relative contributions (%) of organic matter, organic nitrogen (Norg) and calcium carbonate (CaCO3) for both inside and outside canyon mooring lines. C/N atomic ratio was calculated as Corg/Norg. Code

M1 M2 M3 M4 M5

Organic matter Flux

(%)

329.51 113.75 138.75 66.46 0.22

2.50 2.67 2.28 3.53 3.88

Norg flux

(%)

C/N

CaCO3 flux

(%)

17.49 6.02 8.88 6.45 0.01

0.14 0.14 0.15 0.30 0.17

10.9 11.7 9.0 8.2 9.3

2479.62 772.45 1041.35 367.89 1.24

17.93 19.03 17.22 20.89 23.48

The retrieved sediments were described as fine-grained hemipelagic muds with higher contents in silty fraction. Silt percentages ranged between a maximum of 73% in core C1 and a minimum of 64% in core C4. An exception was the core C3, characterized by the presence of around 10% sand along the whole core (Table 5). In contrast, cores C2, C4 and C5 were dominated by clays fraction with values of 30%, 35% and 35%, respectively. Mean grain size averaged values for cores C2, C4 and C5 ranged between 8.8 lm and 9.6 lm, being clearly lower than those found in both core C1 (15.3 lm), located in the upper canyon, and core C3 (24.3 lm), located in the western flank.

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Table 5 Sediment accumulation rates (SAR) and sedimentation rates (SR) obtained from 210Pb analyses and mean values of both mean grain size (MGS) and granulometric composition (percentage of clay, silt and sand) are given for the cores. Mean values of organic matter (OM) contents, Corg/Norg atomic ratios and calcium carbonate (CaCO3) contents are also shown for each core. Core code

C1

Length (cm)

44

C2

40

C3 C4 C5

30 45 45

Sedimentation data 2

y

Depth (cm)

SAR (g cm

0–25 25–44 0–15 15–40 0–30 0–45 0–45

Mixing 0.26 ± 0.04 Erosion 0.28 ± 0.03 Disturbed 0.082 ± 0.003 0.078 ± 0.008

1

)

SR (cm y

Silt (%)

15.3

25

73

9.6

30

24.3 9.0 8.8

24 35 35

Sand (%)

OM (%)

Corg/Norg

CaCO3 (%)

2

1.92

10.9

22.92

68

2

1.64

11.2

22.13

67 64 65

10 0 0

1.12 1.76 1.38

12.1 10.5 9.7

23.71 23.66 26.86

0.37 ± 0.05 0.47 ± 0.06 0.125 ± 0.004 0.091 ± 0.013

3.5.1. Sediment accumulation rates (SAR) derived from 210Pb analyses The study of sediment accumulation rates (SAR) and sedimentation rates (SR) of the sediment cores revealed that in core C1 the upper 25 cm were affected by mixing processes (Fig. 9), in congru-

210

Clays (%)

)

The organic matter content in superficial sediments was higher in the inside canyon cores (1.64–1.96%), especially in the one located in the vicinity of the coast. On the contrary, the lowest value (1.12%) was recorded at the western sand-dominated deposit C3. Calcium carbonate content was the main constituent of the total carbon in all cores (Table 5). Carbonate content mean values ranged from 22.13% in core C2 to 26.86% in core C5.

Fig. 9. figure.

MGS (lm) 1

ence with the presence of numerous burrows and biological indications observed during visual description of the sediment record. Down core, a SAR of 0.26 ± 0.04 g cm 2 y 1 (SR = 0.37 ± 0.05 cm y 1) was obtained. Cores C2 and C3 were clearly affected by erosion processes. While in core C2 the erosion affected only the upper 15 cm, showing a SAR for the lower part of the core of 0.28 ± 0.03 g cm 2 y 1 (SR = 0.47 ± 0.06 cm y 1), in core C3 the disruption affected the whole sediment record, so its SAR was unobtainable. In both C4 and C5 the cores were unaltered and the obtained SAR’s were 0.082 ± 0.003 g cm 2 y 1 (SR = 0.125 ± 0.004 cm y 1) and 0.078 ± 0.008 g cm 2 y 1 (SR = 0.091 ± 0.013 cm y 1), respectively.

Pb activity profiles of cores C1, C2, C3, C4 and C5, recovered at the same positions of moored sediment traps. Linear accumulation rates (r) are also shown in the

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4. Discussion 4.1. Spatial variation of particle fluxes: the existence of two main domains Particle fluxes within the Blanes Canyon were of the same order as those recorded by Martín et al. (2006) in the Palamós Canyon, located north of our experimental site. The decreased occurrence of the time-weighted-flux (TWF) with increasing distance from the coast (Table 3 and Fig. 7) reflects the high influence of terrigenous inputs linked to the outflow of the Tordera River (Fig. 3). Otherwise, the TWF differences between the inside and outside canyon stations (Table 3) agreed with previous observations in similar settings, highlighting the role of this and many other submarine canyons as traps and offshore conduits of continental shelf sediments (Heussner et al., 1999; Monaco et al., 1999; Palanques et al., 2005, 2006; Martín et al., 2006). Certainly, the concomitant increase of both down-canyon current speed and particle fluxes (Fig. 10) observed at M2 demonstrates the enormous capability of the canyon to channel energetic events along its axis, which consequently induces the transport of particulate material from the continental shelf to the deepest parts of the canyon (Durrieu de Madron et al., 1999). The role of the Blanes submarine canyon as a trap for sediment has been also observed by the high sedimentation rates (SR) recorded at the inside canyon cores C1 (0.37 cm y 1) and C2 (0.47 cm y 1), which were slightly higher than the range of SR presented by Martín et al. (2008) for the Palamós Canyon (0.05– 0.35 cm y 1). These two Catalan margin canyons showed higher SR than those found in the adjacent open slope (0.07–0.14 cm y 1) (Sanchez-Cabeza et al., 1999) and in other northwestern Mediterranean canyons such as the Lacaze-Duthiers Canyon (0.06– 0.2 cm y 1), the Grand Rhône Canyon (0.07–0.17 cm y 1) (Miralles et al., 2005) or the Planier Canyon (0.09 cm y 1) (Masqué, 1999). On the other hand, it is worth noting the significantly higher TWF recorded at the western flank of the Blanes Canyon when compared to the eastern flank at similar depths (Fig. 7), which may be explained by the existence of lateral processes close to the bottom that contribute to the transport of particulate material along the western flank. As observed in other submarine canyons, (e.g. Turchetto et al. (2007)), current meter data showed that in the Blanes Canyon the circulation close to the bottom was strongly

Fig. 10. Time series of Eddy Kinetic Energy (EKE) on the mesoscale band (which describes the energetic variability within the canyon) and sediment trap data (total mass fluxes) of M2 near-bottom devices.

249

constrained by the local bathymetry, yielding to different current regimes (Fig. 5c). The deep current coming from the northern open slope flows into the canyon along the shallow and wide eastern canyon wall (M4). Here, the current showed a high variability in direction (Fig. 5) favoring the vertical transport of hemipelagic particulate matter. In this flank, superficial sediments were characterized by a high content of fine material (Table 5) that has been sedimented at a relatively low rate (SR = 0.125 ± 0.004 cm y 1) (Table 5). The exponential decrease with depth of 210Pbxs profile, similar to that found in the adjacent open slope (C5) (Fig. 9), reveals that sediments are unaltered. This fact corroborates the notion that fewer energetic processes are affecting the eastern flank of the Blanes Canyon and thus hemipelagic sedimentation is favored. Later, the route of the bottom current along the canyon isobaths (Fig. 5c) will favor the capture of suspended sediment from the near-shore zone by the upper canyon. Indeed, the existence of higher numbers of suspended particulate matter in the eastern canyon wall (Fig. 4), being the sediments detached from the bottom as shelf waters coming from the north cross the canyon, is an indicator of lateral advection from the shelf edge toward the canyon. All this particulate material collected by the canyon will be transported to the open sea through the steep western canyon wall (Figs. 1 and 7), which acts as a physical barrier that prevents the escape of material toward the west. In this flank, the fact that sediments were totally disturbed (as observed by the 210Pb profile, avoiding to obtain SAR, Fig. 9) and the presence of coarse particles confirm that more energetic processes are affecting the western flank of the canyon. Certainly, it seems that the persistence of the current can erode the seafloor transporting the re-suspended material to the deep basin, and thus convert the western flank of the canyon to a potential additional source of sediment to the deep basin. 4.2. Temporal variability of particle fluxes: controlling factors Temporal evolution of particle fluxes showed abrupt changes throughout the year, thus revealing that the transfer of particulate matter through the canyon is clearly event-dominated. This is similar to what occurs in other submarine canyons (Walsh and Nittrouer, 1999; Palanques et al., 2006; Fain et al., 2007; Bonnin et al., 2008) where the variability of particle fluxes is a consequence of a combination of factors involving river flood events, high wave energy processes associated with local meteorological forcing and margin circulation. In the upper Blanes Canyon (M1), particle fluxes showed two maximum peaks during the first half of December 2003 and in March 2004 (Fig. 7), which were clearly related to two major river flood events (Fig. 3) that induced significant increases in the current velocities at this site (Fig. 6). A third important discharge from the Tordera River occurred in October 2003 (Fig. 3), but unfortunately, the sediment trap record at M1 was lost during that period due to trawling activities. The peak in particle fluxes observed at M1 during the early part of December 2003 was related to an important E-SE storm affecting not only the Catalan margin but also the Gulf of Lions (Bonnin et al., 2008; Fabrés et al., 2008). In fact, the Tordera River discharge in December 2003 occurred at the same time as a Rhone River flood with a 75-year recurrence period (Palanques et al., 2006). This river flood event leads to a sporadic intensification of the current speed (Fig. 6a) and an increase in water temperature along the canyon axis (Fig. 6b). However, in terms of mass fluxes it affected only the upper Blanes Canyon (M1 site) as a consequence of the still warmer surface water under stratification conditions and the short duration of the storm, thus avoiding the off-shelf transport of the sediment re-suspended along the coast (Fabrés et al., 2008; Palanques et al., 2009).

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Furthermore, the transference of material from the upper canyon to the deep sea can also occur as a consequence of dense shelf-water cascading (Canals et al., 2006). Recent observations from the nearby Gulf of Lions revealed that dense shelf water formed during dry and cold winters escapes from the shelf by cascading through the westernmost canyons (Cap de Creus and Lacaze-Duthiers Canyons) (Durrieu de Madron et al., 2005), re-suspending and transporting material toward the deep sea (Canals et al., 2006; Heussner et al., 2006; Palanques et al., 2006; Sanchez-Vidal et al., 2008). Although the precondition for cascading events in the Blanes Canyon is different from those in the Gulf of Lions (narrower continental shelf, different wind regimes due to the presence of the Pyrenees range, etc.), in the numerical model presented by Ulses et al. (2008), the authors predicted that dense shelf waters from the Gulf of Lions might be able to reach the Blanes Canyon. Indeed, in March 2004 the upper 300 m of the water column over the Blanes Canyon were occupied by cold, fresh shelf water where the transmissivity profiles showed the down-welling of shelf turbid water (Fig. 4c) that reflected the down-canyon transport of dense shelf water. Moreover, the peak in total mass flux (even at 1700 m depth) recorded during this event (Fig. 7) agrees with an intensification of the current (Fig. 6a) and an abrupt decrease of water temperature in the whole water column (Fig. 6b). This fact also supports the idea that a cascading event that occurred simultaneously with the major eastern storm occurring in winter 2004 (Fabrés et al., 2008) could be responsible for the rapid off-shelf transport of particulate material re-suspended from the inner and mid-continental shelf. Furthermore, we would like to point out that the TWF peak observed in March 2004 took place relatively close to a dense shelf-water cascading event in the Gulf of Lions, which was strong enough to re-suspend and transport all of the material stored on the shelf along the Cap de Creus Canyon toward the deep basin (Palanques et al., 2006, 2009). 4.3. Composition of the settling particles Near seafloor particulate fluxes in canyon systems have different sources. Biological production, terrestrial input and re-suspension of bottom sediments may follow a seasonal pattern. Biogenic particles can also have various origins, exhibiting seasonal changes from both a quantitative and a qualitative point of view (Rossi et al., 2003). Indeed, surface primary production in the CatalanoBalear Sea is characterized by an intense seasonality due to the winter–spring phytoplankton bloom (Estrada, 1996). Even if our sediment trap samples are clearly dominated by the lithogenic material associated with the river flood events, the winter–spring biological signal was recognized in the concentrations of particulate organic matter registered at both M1 and M4 (Fig. 8a). This confirms the existence of two main domains in the canyon. The eastern flank of the canyon, characterized as a more stable area, favors the detection of the biological signal that occurred at the surface of the water column. In this case, the lower Corg/Norg atomic ratio (8) revealed that organic material is composed of more labile organic matter, similar to that found in other submarine canyons of the western Mediterranean Sea (Buscail et al., 1990; Sanchez-Vidal et al., 2005; Fabrés et al., 2008). In contrast, during late fall and winter, the intense Tordera River inputs constitute an important source of more refractory particulate organic matter in the canyon. These energetic events, jointly with the action of waves, bottom currents or internal waves, can also be responsible for the re-suspension of older and degraded organic matter from the sediment record as observed in the C3 eroded core, which shows the highest Corg/Norg atomic ratio (12.1) (Table 5). On the other hand, calcium carbonate contents show values within the same order of magnitude as other values found in and

near submarine canyons (Martín et al., 2006), varying between a minimum value of 17.22% recorded at the western canyon wall (station M3) and a maximum value of 23.48% recorded outside the canyon (station M5) (Table 4). Therefore, biogenic material in the open slope is less diluted by the lithogenic fraction associated with the river discharge. This assumption is again corroborated by the Corg/Norg atomic ratio (Table 4). The higher values were found in the near-shore sediment trap (M1) (Corg/Norg = 10.9) likely due to the coastal input of terrestrial organic matter, and at the canyon axis (M2) (Corg/Norg = 11.7), which suggests that organic matter is being degraded and mixed with older organic matter during its transfer to the deepest parts of the canyon. The different composition of the settling particles associated with the existence of two main domains in the Blanes Canyon has relevant biological consequences. The topographic features of the canyon and the variations in both currents and particle fluxes yield to important oscillations in the meiobenthic community, as explained by Sardà et al. (in press), which revealed the importance of the local currents in the deep rose shrimp A. antennatus distribution near the Blanes submarine canyon.

5. Conclusions The study of downward particle flux in the Blanes Canyon reveals two main finds: (i) the canyon acts as a preferential conduit for the transport of particulate material from the continental shelf to the open ocean, with fluxes three orders of magnitude higher than in the adjacent open slope and (ii) two main domains within the canyon are clearly distinguished from the hydrodynamic conditions that control the transport of the particles. Whereas in the sharp western flank the unidirectional southwestward direction of the current favors the transport of the particulate material from the upper canyon to the deep basin, the smoother eastern flank, characterized by non-erosional hydrodynamic conditions, recovers less particulate material, mainly that associated with hemipelagic sedimentation processes. The present study clearly found that transport through the canyon is event-dominated. The maximum downward particle flux episodes were identified with processes involving increases in river discharge during the winter, intensification of the Northern Current that transferred energy along the canyon axis and dense shelf-water cascading. In any case, it is important to consider that the influence of the bathymetric characteristics in both current and particle fluxes are specific of the Blanes Canyon. Thus, caution is required when extrapolating specific results of the Blanes submarine canyon to other systems.

Acknowledgments The authors would like to thank the officers and crew of the R/V García del Cid and the technicians at Unidad de Tecnología Marina (UTM-CSIC). We also thank the Laboratori de Radioactivitat Ambiental from ‘‘Universitat Autònoma de Barcelona” for their help with the 210Pb analysis. D.Z. benefited from a Spanish FPU Grant (AP2000-0524), and MMF was aided by a Juan de la Cierva grant from the Spanish Ministry of Science. Support from the government of Spain and the Fulbright Commission for a post-doctoral fellowship to J.G-O. (ref 2007-0516) is gratefully acknowledged by J. García-Orellana. GRC Geociències Marines was additionally funded by the ‘‘Generalitat de Catalunya’’ through its excellency research groups program (ref. 2005 SGR-00152). This study was also supported by the Spanish national research project RECS (REN2002-04556-C02-01).

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