ARTICLE IN PRESS

Continental Shelf Research 28 (2008) 1971– 1983

Contents lists available at ScienceDirect

Continental Shelf Research

0278-43

doi:10.1

� Corr

N-4802

E-m

journal homepage: www.elsevier.com/locate/csr

Seasonal and event-controlled export of organic matter from the shelftowards the Gulf of Lions continental slope

Joan Fabres a,b,�, Tommaso Tesi c, Jose Velez b,d, Fabian Batista b, Cindy Lee b, Antoni Calafat a,Serge Heussner e, Albert Palanques f, Stefano Miserocchi c

a GRC Geociencies Marines, Dept. Estratigrafia, Paleontologia i Geociencies Marines, Facultat de Geologia, Universitat de Barcelona, c/ Martı Franques s/n, Barcelona E-08028, Spainb Marine Sciences Research Center, Stony Brook University, Stony Brook, NY 11794-5000, USAc Istituto Scienze Marine CNR, Sezione di Geologia Marina, Bologna, Italyd University of Puerto Rico-Cayey, Cayey, PR 00736, Puerto Ricoe CEFREM, UMR 5110 CNRS, Universite de Perpignan, Perpignan F-66860, Francef Institut de Ciencies del Mar (CSIC), Passeig Marıtim de la Barceloneta, 37-49, Barcelona E-08003, Spain

a r t i c l e i n f o

Article history:

Received 9 September 2007

Received in revised form

29 February 2008

Accepted 20 April 2008Available online 10 May 2008

Keywords:

Particulate flux

Organic matter

Biomarkers

Seasonality

Storms

Cascading

Continental slope

NW Mediterranean Sea

Gulf of Lions

43/$ - see front matter & 2008 Elsevier Ltd. A

016/j.csr.2008.04.010

esponding author. Present Address: GRID-Ar

, Norway. Tel.: +47 97040308; fax: +47 370350

ail address: Joan.Fabres@grida.no (J. Fabres).

a b s t r a c t

To investigate the role of coastal canyons in the transfer of organic matter from the shelf to the slope and

basin, we deployed sediment trap/current meter pairs at the head of five canyons in the Gulf of Lions

(GoL) between November 2003 and May 2004. Analysis of organic carbon, biogenic silica, Corg isotopic

composition, Corg/total nitrogen, chloropigments, and amino acids clearly shows the seasonal influence

and effect of extreme meteorological events on the composition of collected particles. The sampling

period was divided into three ‘‘scenarios’’. The first corresponded to a large easterly storm and flood of

the Rhone river during stratified water column conditions; the composition of material collected during

this event was influenced by increased transfer of riverine and coastal particulate matter, with a lower

Corg content. During the second ‘‘fall-winter’’ scenario, northern and northwestern winds blowing over

the shelf caused cooling and homogenization of the shelf water column; particles collected at this time

reflected the homogeneous source of particulate matter transported through canyons; particles sitting

in the vicinity of canyon heads are most likely swept downslope by the general south-westward

circulation. Organic tracers indicate a degraded origin for organic matter transported during this period.

A third ‘‘spring’’ scenario corresponded to northern winds alternating with eastward windstorms that

triggered and/or enhanced the cascading of dense waters accumulated on the bottom of the shelf due to

previous cooling. These conditions occurred in conjunction with increased phytoplankton productivity

in shelf surface waters. Organic matter advected mainly by dense shelf water cascading was fresher due

to the transport of newly produced particles and a variable terrestrial fraction; this fraction depended

on the proportion of resuspended material accumulated during previous high discharge periods that

was involved in each transport pulse. The tight link shown between meteorological conditions and

organic matter transport is important for continental margin geochemical studies as future changes in

climatic conditions may lead to dramatic changes in carbon sequestration capability and in the

ecosystems of deep margin environments.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Continental margins are the location of interactions betweenthe continent, the open ocean and the atmosphere; they are themarine systems that are most exposed to the effects of humanactivities and climate change. For these reasons, they are a key

ll rights reserved.

endal. Postboks 183, Arendal

50.

area for understanding the physicochemical and biogeochemicalprocesses associated with downward particle flux and the role ofthe interaction of organic–inorganic for the transportation oforganic matter (OM) (Monaco et al., 1987). Particulate OMsupplied by continental inputs (via fluvial and aeolian supply)and autochthonous biological production is received, accumu-lated, and/or transferred to the adjacent open ocean by or throughcontinental margins. The rate of exchange with both the continentand the open ocean determines the role of continental margins asa complimentary biological pump in the global carbon cycle andwe must know this rate to be able close the global carbon budget(Liu et al., 2000).

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Despite being a small fraction of the world ocean floors (only20%), continental margins are quantitatively as important as thedeep sea in carbon and nitrogen biogeochemical cycles (Walsh,1991), and their dynamic character allows for a significantlygreater amount of carbon processing than other systems (Aller,1998). In a recent study Muller-Karger et al. (2005) showed that10–15% of the ocean’s production occurs on continental marginsand argued more than 40% of ocean C sequestration occurs onmargins. Certain regions of the coastal ocean produce moreorganic carbon than the amount respired or effectively seques-tered; this excess is therefore available for export from the coastalto the open ocean (Bauer and Druffel, 1998). This transport acrosscontinental margins is an important energy supply to deep-seaecosystems that rely on the ‘‘rain’’ of detritus from remote surfacewaters (e.g., Billett et al., 1983; Smith et al., 1996). Submarinecanyons represent one of the few exceptions to the low-energydeep-sea rule by focussing flow and OM flux that can yield highbiomass communities compared to other areas in the deep sea(Vetter and Dayton, 1998).

The heterogeneous nature of OM present on continentalmargins, where land-derived inputs and primary productivityco-exist (Hedges et al., 1988), makes dealing with the processesaffecting its accumulation, transfer and degradation difficult, aseach of the different kinds of OM play a different role in diageneticreactions and are affected differently by these processes (Aller andBlair, 2006). It is therefore important to characterize theproportion of OM originating from each source. In continentalmargins terrigenous material, delivered via rivers, characterizeneritic environments whereas the marine fraction generallydominates in the deeper areas (Prahl et al., 1994; Goni et al.,1998; Gordon and Goni, 2003; Aller and Blair, 2006; Miserocchiet al., 2007).

A recent study was carried out within the EUROSTRATAFORMproject in the Gulf of Lions (GoL). The GoL is a good example ofriver-dominated margins where OM can originate from continen-tal sources when rivers are active (especially during autumnfloods), from resuspended shelf sediments, and from marinebiological production in the euphotic zone (Buscail et al., 1990).The margin of the GoL where our study was carried out isindented by several canyons running down to the continental rise,hence with a high potential for transporting particulate matter tothe deep sea. The sedimentary dynamics controlling the transportand export of particulate matter from the shelf towards the slopeand deep basin during the study period have been thoroughlydiscussed by Palanques et al. (2006), Canals et al. (2006) andBonnin et al. (2008). Palanques et al. (2006) and Bonnin et al.(2008) include thorough reviews of the progress done on under-standing the transport of particulate matter on this margin beforethe EUROSTRATAFORM project.

Part of the organic carbon in the GoL is terrestrial in nature.Buscail et al. (1990) and Gadel et al. (1990) identified lignin-derived phenolic compounds that are characteristic of terrestrialplants as being abundant in the GoL benthic nepheloid layer,decreasing offshore especially during autumn and winter. Theseasonality in the input of lignins, with maximum input duringautumnal floods and funnelling along canyon axes down to thedeepest parts of the slope is also shown in studies by Pocklingtonet al. (1987) and Buscail et al. (1990) at the mouth of Tet River andin the Lacaze-Duthiers canyon. Marine-derived OM also reachesthe sediments. In the GoL particulate matter of marine origingenerated in surface waters is characterized by high contents oforganic carbon, nitrogen and hydrolyzable carbon (Buscail et al.,1990). Comparison of fluxes of organic carbon at 50 m depth withprimary productivity estimates led to the conclusion that asignificant fraction of the primary production is exported out ofthe euphotic zone. The objective of the present contribution is to

analyze how the seasonal and event-controlled sedimentarydynamics of the slope and canyons of the GoL influences thetransport and composition of OM from the shelf towards theslope.

2. Materials and methods

2.1. Experimental design

The design for the EUROSTRATAFORM experiment, describedalso in detail in Bonnin et al. (2008), consisted of the instrumen-tation of six major GoL canyon heads with sediment trap arraysmoored at 300 m depth (see Fig. 1). Of these arrays only data fromthe five westernmost canyons under study, Petit Rhone, Herault,Aude, Lacaze-Duthiers and Cap de Creus will be discussed here. Inall cases the arrays were deployed within the section of thecanyon that is deeply incised in the outer continental shelf andtherefore potentially receiving material exported from the shelfalong the canyon axis and from both flanks (Fig. 1). Moorings weredeployed between November 2003 and April 2004 with amaintenance turnaround at the end of January 2004. Eachmooring was equipped with one sequentially sampling trap(Technicap PPS-3, Heussner et al., 1990) deployed at 30 m abovethe bottom (mab). Time-series sampling frequency was onesample per 7 days. Trap motor malfunctions resulted in no datafor the first period (P1: November 2003–January 2004) at Cap deCreus canyon and the second period (P2: January 2004–April2004) at Aude Canyon, while battery problems hindered acquisi-tion of the last samples of P1 at the Lacaze Duthier and Heraultcanyon sites.

An Aanderaa RCM-7/8 current meter was deployed 2 m beloweach sediment trap, and a second Aanderaa current meter–turbi-dimeter RCM-9/11 was deployed at 4 mab. The current meterslocated 2 m below the trap recorded hourly temperature, currentspeed and current direction data for all periods with trap dataexcept for P1 in the Petit Rhone canyon for which 4-mab currentmeter will be used. Factory calibrations were used to convert rawdata into engineering units. Hourly temperature and currentspeed data have been averaged over the same 7-day periodscorresponding to the trap time sequence to make comparisoneasier. Data from current meters at 4 mab have been analyzed anddiscussed by Palanques et al. (2006) and Bonnin et al. (2008).

2.2. Analytical methods

A detailed description of the trap sample processing prior tochemical analyses can be found in Bonnin et al. (2008) based onthe methods described by Heussner et al. (1990). Sub-samples(40–60 mg) for mass measurement and biogenic silica analysiswere filtered onto Millipore 0.45mm cellulose filters (HA) whilethose destined for total and organic carbon and for nitrogen andd13C analyses were filtered onto Millipore fiberglass filters (APFF).Filters were then oven-dried at 40 1C.

Organic carbon (Corg) and total carbon (values not shown)contents were analyzed using on-line combustion-reduction-gaschromatography in 25% HCl treated and non-treated samples,respectively (Nieuwenhuize et al., 1994), at the Scientific-Technical Services of the University of Barcelona using anelemental analyzer (models NA 2100 and EA Flash series 1112(Thermo, Milan, Italy)) with TCD detection, set at providerrecommended parameters. Data acquisition and calculations usedEager 200/300 software.

When acidifying to remove calcium carbonate, repeatedadditions of 100 ml 25% HCl with 60 1C drying steps in between

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3 4 542

43

BallastSediment trap array

Trap PPS330 mab

RCM 7/828 mab

Floats

Release

RCM9/114 mab

Floats

Fig. 1. Bathymetric map of the Gulf of Lions margin and schematic representation of the instrument distribution in the mooring arrays. The location of the sediment trap

arrays that collected the samples used in this study are shown in the map (%) (Petit Rhone canyon, Herault canyon, Aude Canyon, Lacaze-Duthiers canyon, Cap de Creus

canyon). Main rivers debouching to the Gulf of Lions are also shown for reference. Modified from Bonnin et al. (2008).

J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–1983 1973

were conducted until no further effervescence occurred. Short-term precision of carbon (n ¼ 9) averaged 71.7% of the measuredvalue. Long-term precision over the period between June 2004and July 2005 (n ¼ 31) was 73.0%. Accuracy of the carbonmeasurements was checked against the Canadian NationalResearch Council certified estuarine sediment MESS-1 (Berman,1990). The accuracy of the method is excellent as indicated by thecorrespondence between the measured carbon values,2.9870.03%, and the certified carbon values of the standard,2.9970.09%. The uncertainties in the accuracy lie within 95%confidence limits.

Total nitrogen (TN) contents and d13C were measured atIstituto Scienze Marine CNR (Bologna) using a FISONS NA2000Elemental Analyzer after removal of the carbonate fraction fromsamples in silver capsules using 1.5 N HCl. The average standarddeviation (SD) of each measurement, determined by replicateanalyses of the same sample, was 70.009%. Atomic C/N ratioswere calculated from Corg and TN values.

Stable isotopic analyses of Corg were carried out on the samesamples used for TN determination using a FINNIGAN Delta Plusmass spectrometer, which was directly coupled to the FISONSNA2000 EA by means of a CONFLO interface for continuous flowmeasurements. IAEA standard NBS19 was used to calibrate Corg.Uncertainties were less than 70.2%, as determined from routinereplicate measurements of the reference sample CH-7 (IAEA).13C/12C is expressed relative to the PDB standard by conventional‘‘del’’ notation:

d13C ¼ ðð13C=12CÞsample � ð13C=12CÞstandardÞ � 1000z

Concentrations of chloropigments (chlorophyll and its degrada-tion products, pheophorbide-a, pheophytin-a and pyropheophor-bide-a) were determined by ion-pairing reverse-phase high-pressure liquid chromatography (HPLC) after extraction in acetone(Mantoura and Llewellyn, 1983; Bidigare et al., 1985; Sun et al.,1991). Dried sediment trap samples on GFF filters were sonicatedin HPLC-grade acetone to extract pigments. Two successive 5-mlextracts were combined and filtered through a 0.2-mm Phenom-enex nylon membrane; the extract was diluted 1:1.25 with MilliQwater. Pigments in acetone extracts were separated with a 5-mm

Alltech Adsorbosphere C-18 column. Chloropigments were de-tected by fluorescence (excitation l ¼ 410 nm, emission l ¼ 660nm). Chlorophyll-a (Chl) of known concentration (Turner Designs)was used as a standard. Authentic standards of Chl degradationproducts were synthesized from purified Chl, and their concen-trations determined spectrophotometrically using known extinc-tion coefficients (King, 1993).

Concentrations of total hydrolyzed amino acids (THAA) wereanalyzed by HPLC using pre-column OPA derivatization after acidhydrolysis (Lee and Cronin, 1982; Lee et al., 2000; Ingalls et al.,2003). Acetone-extracted filters from the pigment analyses weresealed in glass tubes under N2 with 6 N HCl and 0.25 wt% phenoladded and hydrolyzed at 150 1C for 90 min. Hydrolyzates werecentrifuged to remove particles, and supernatants transferred tocombusted glass vials, evaporated, and dissolved in MilliQ water.Hydrolysis releases amino acids in peptide bonds and thoseadsorbed onto surfaces. Amino acids were quantified using amodification of Lindroth and Mopper (1979). A Waters 4 mm Nova-Pak C-18 150-mm�3.9-mm HPLC column equipped with a guardcolumn was eluted at a flow rate of 0.95 ml min�1. A binarygradient of 0.04 M sodium acetate (pH 7.0), 2% THF, and 10%methanol (eluant A) and 90% methanol and 10% 0.04 M sodiumacetate (eluant B) was used, ramping from 0% B to 74% B in 25 minand holding for 6 min. OPA-derivatized amino acids were detectedby fluorescence and identified by comparison to retention times ofauthentic standards. An amino acid mixture (Pierce Chemical,Standard H) was used as the standard. The non-protein aminoacids, b-alanine and g-aminobutyric acid (b-Ala and g-aba) wereadded individually to the standard mixture. Aspartic acid andglutamic acid measurements include Asp and Glu from thehydrolysis of asparagine and glutamine. Where lysine is notreported, this peak was not detected. Hydrolyzates of some of theindividual samples were analyzed in duplicate, and agreed within710%.

Since trap samples collected near the bottom in continentalmargins have particularly low biogenic silica (Sibio) contents, weused the method described in detail in Fabres et al. (2002) thataddresses interference by lithogenic silica. The method is inspiredby Mortlock and Froelich (1989) who used Na2CO3 solution toextract amorphous silica but is modified by: (i) incorporating two

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J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–19831974

0.5 M Na2CO3 leaching steps (2.5 h each), (ii) using inductivecoupled plasma atomic emission spectroscopy (ICP-AES) for theanalysis of Si and Al content in leachates form both steps, and (iii)correcting the Si content of the first leachate using the Si/Alcontent of the second (Kamatani and Oku, 2000). Corrected Siconcentrations are converted into biogenic opal concentrations bymultiplying obtained values by 2.4 (Mortlock and Froelich, 1989).This method makes three basic assumptions: (i) all the Almeasured during the first leach is derived from silicate minerals,(ii) all the Sibio has been digested during the first leach, so thesecond leach addresses the Si:Al ratio of silicate minerals only, and(iii) silicate minerals dissolve with a similar Si:Al ratio in bothleaching steps. Recently Ragueneau et al. (2005) published asimilar method and demonstrated that the above assumptions arereasonable. Replicate (n ¼ 7) Sibio analyses of one samplebelonging to the lower part of the concentration range(0.17–6.04%) gave an average value of 0.26% with a SD of 0.11%.Assuming 3 SD of the lowest values, the detection limit of thismethod is 0.3%. Therefore Sibio values falling on the lower part ofthe range should be regarded cautiously, but fluctuations ofseveral percentage points are significant.

3. Results

3.1. Temporal evolution of wind and current regime

Fig. 2a shows the time-series of daily averaged wind speed anddirection recorded in Toreilles, a location on the coast onshorefrom the head of the Lacaze-Duthiers canyon site. Winds duringNovember and beginning of December were variable, with thehighest magnitude winds recorded during a very importanteasterly storm on December 4. December and January werecharacterized by constant and intense NNW winds, while theremaining sampling period was characterized by 1–2 weekperiods of NNW winds separated by easterly storms lastingseveral days.

Fig. 2b corresponds to half hourly significant wave height atSete, a location in the central GoL, and illustrate that easterly windpulses, some of them not clearly reflected in the averaged windspeed, cause much larger swell due to the fetch distancecompared to the predominant northerly winds.

Time-series of temperature and current speed (Fig. 2c)recorded by current meters placed immediately under the trapwere very similar to those described in Palanques et al. (2006)that had been recorded by the current meter placed 4 mab.Temperature and current records can be divided in two clearlydistinct periods. ‘‘Fall-winter’’ lasted until the end of January andwas characterized by constant temperatures around 13.4 1C, witha few positive anomalies around December 4. Current recordsduring this time show high frequency fluctuations with the mostintense currents recorded in the Petit Rhone and the Lacaze-Duthiers canyon. The largest current fluctuation was recorded atthe same time as the positive temperature anomaly in the Lacaze-Duthiers canyon where the current reached values above 50 cm s�1.The ‘‘spring’’ period from the end of January onwards wascharacterized by fluctuating temperatures, starting with short-lasting pulses and progressing with time towards permanentlylower temperatures with low frequency variability. Current speedsin spring were characterized by high frequency speed fluctuationslasting 1–2 weeks, separated by several days of intensified currentspeed reaching values of 50–60 cm s�1 in the southwesterncanyons and 20–40 cm s�1 in the easternmost ones. Fig. 2dsynthesizes the current variability for all the canyons by usingaveraged currents for the traps collection period and allows cleardifferentiation between the scenarios presented in Section 4.1.

3.2. Temporal evolution of mass flux

The temporal pattern of particulate matter fluxes at all siteswas highly variable and characterized by several peaks lasting 1–2weeks (Fig. 3a). Maximum fluxes occurred at all sites during thefirst week of December, reaching 110 g m�2 d�1 in Lacaze-Duthierscanyon and 24–67 g m�2 d�1 at other sites. Fluxes decreased afterthis remarkable event at all sites except at the Petit Rhone canyonwhere after 2 weeks of decreasing flux there was another peak atthe end of December (18 g m�2 d�1). Background values werereached at the beginning of January at all sites, and fluxesremained low until the end of February when another peak inparticulate flux was recorded at the westernmost sites, reaching31 g m�2 d�1 at Cap de Creus canyon and 21 g m�2 d�1 at Lacaze-Duthiers canyon. Values remained below 10 g m�2 d�1 with lowfrequency fluctuations (3 or more weeks between peak values)until the beginning of April when a period of higher frequencyfluctuations (2 weeks between peak values) started and lasteduntil the end of our record. The last major peak within that periodof high frequency fluctuations was recorded in the central(23 g m�2 d�1 in Herault canyon) and western sectors (13 g m�2 d�1

in Cap de Creus canyon) during mid-April.

3.3. Temporal evolution of composition

The composition of particles transported through canyonheads follows a general seasonal pattern that allows theseparation of fluxes into the two periods identified in thetemperature and current time series. Organic carbon concentra-tion (Fig. 3b) was relatively constant, around 1.5%, during fall-winter. Values were low during the first week of December at thetime when maximum fluxes were recorded, 0.9% in Heraultcanyon and about 1.2% in Lacaze-Duthiers, Aude and Petit Rhonecanyons. During the 2 weeks following this minimum Corgconcentration, values slowly increased towards the 1.5% back-ground values recorded before the maximum flux event. Thewinter-spring period was in contrast characterized by fluctuatingCorg values that peaked towards the end of March and beginningof April. Maximum values were about 3.1% for Lacaze-Duthierscanyon and 2.2% for Herault canyon. The straightforward relationbetween mass flux and Corg concentration recognized during thefall-winter period is less evident during spring. Neverthelessrelative minimum values were recorded during the increased fluxperiods in late February and mid-April.

Biogenic silica concentration during the fall-winter period isremarkably low and constant at all sites, ranging from 0.2% to 1.1%(Fig. 3c). Sibio concentrations during the spring period are markedby an absolute maximum reached simultaneously at all sites atthe very end of February. The highest values of 6% and 5.6%occurred in the westernmost sites of Lacaze-Duthiers and Cap deCreus canyon, respectively. For the central and eastern sites valueswere 3.7% and 3.5% for the Herault and Petit Rhone canyons,respectively. Sibio concentrations decreased progressively afterthe peak never reaching the lower background values recordedduring the fall-winter period, but generally ranging between1% and 3%.

Seasonality is somewhat less evident in carbon isotopiccomposition, with values generally between �22.5 and �24(Fig. 3d). The homogeneity of the values within each site duringthe fall-winter period is disrupted by the large decrease in d13Cvalue in the Lacaze-Duthiers canyon during early December whenparticle flux increases. This minimum in d13C was also recorded atthe Aude (�23.3) and Herault canyons (�23.1). In spring the sitesare more similar to each other showing a broad maximum in lateMarch/early April. The difference between absolute minimum and

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Fig. 2. (a) Daily averaged wind speed and direction recorded at Toreilles, a location on the coast onshore from the head of the Lacaze-Duthiers canyon, during the study

period. The X-axis corresponds to time in calendar days and each of the 7 days periods shown corresponds to the time covered by one trap sample. Note that the patterns in

the background patterns mark the different wind scenarios highlighting the easterly storms separated by predominantly northerly wind periods. (b) Half hourly significant

wave height at Sete, a central location in the Gulf of Lions, clearly reflecting the impact on wave height of the easterly storms. (c) Temperature and current speed recorded

by current meters placed immediately under the trap in each of the canyon heads. The distinction between fall-winter and spring is based on the recording of the first

sudden drops in water temperature and the repeated occurrence of easterly storms separated by northern wind dominated periods. (d) Temporal series of averaged current

speed using the sampling frequency of the sediment traps. Averaged current speeds for each site highlight the impact of easterly storms in the water circulation at the

canyon heads.

J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–1983 1975

maximum values at each location in spring was in all cases morethan 1.3 d13C units while values during the fall-winter periodoscillated within a smaller (0.25–0.5%) range (except for Lacaze-Duthiers canyon).

As for other compositional variables, C/N ratios (Fig. 3e) aremore homogenous during the fall-winter period with valuesbetween 8.5 and 9.5 for all sites. During the spring period absolutemaximum (9.5–10) and minimum (7.6–8.4) values were reachedat all sites. The temporal pattern is marked by a decrease of 1.5from mid-February until the end of March.

Total concentrations of chloropigments (chlorophyll, pheo-phorbide, pyropheophorbide and pheophytin) are very low during

the fall-winter period with background values below 5 nmol g�1 atall sites (Fig. 4a), except for the maximum of 13 nmol g�1 duringearly December at Lacaze-Duthiers canyon when particle fluxpeaks there. The spring pattern mimics that of Sibio concentra-tion. The first substantial increase in chloropigment concentrationwas recorded at the end of January in the Petit Rhone canyon witha relative maximum reaching 20 nmol g�1. The absolute maximumrecorded at the very end of February is clearly the mostremarkable feature with values of 79 and 68 nmol g�1 at Cap deCreus and Lacaze-Duthiers canyons, respectively. At Petit Rhoneand Herault canyons this peak is lower, 40 and 33 nmol g�1,respectively, and was recorded 1 week earlier than at the other

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Fig. 3. Mass flux (a), % organic carbon (b), % biogenic silica (c), d13C (d) and C/N ratio (e) time-series for the particles collected by the sediment traps in the different sites.

Easterly storms, periods dominated by northerly winds and the span of fall-winter and spring are also shown like in Fig. 2.

J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–19831976

two sites. Following this peak, chloropigments decreased quicklyto values around 10 nmol g�1, and then increased again 39 nmolg�1 at Lacaze-Duthiers and Petit Rhone canyons and 27 nmol g�1

at Cap de Creus canyon. This second increase was muchless pronounced at Herault canyon with a maximum of only22 nmol g�1.

Pheophytin and pyropheophorbide were the major pigments inmaterial reaching the canyon heads. Together they accounted formore than 70 mole% at all sites during the entire sampling period(Fig. 4b–f). Pheophytin contents were always between 50 and70 mole% during the fall-winter period while they were highlyvariable (10–80 mole%) during the spring period. The lowest

pheophytin values were at the end of February and beginning ofMarch at all sites. Chlorophyll relative concentrations wereslightly higher, ranging between 4 and 12 mole% during fall-winter compared to 2–8 mole% during spring. Pheophorbide wasbelow detection levels at all sites during fall-winter except at theLacaze-Duthiers canyon at the beginning of December where itwas 10 mole%. The appearance of pheophorbide clearly markedthe transition between fall-winter and spring; it first occurred atthe beginning of January and was detected discontinuously at allsites until the end of the sampling period, reaching maximumrelative concentrations of 25 mole%. The highest prolongedoccurrence of pheophorbide was detected at the Petit Rhone

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Fig. 4. Organic marker concentration and composition for the particles collected by the sediment traps in the different sites. Chloropigments concentration (a) and

composition (b)–(f), total hydrolysable amino acids and the degradation index is shown for each of the sites. As for Fig. 3 the different meteorological events and seasons are

indicated.

J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–1983 1977

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J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–19831978

canyon between early April and early May with contents higherthan 10 mole%.

Total hydrolyzable amino acid concentrations (THAA) (mmol g�1)displayed generally similar temporal trends to the organic carboncontent in samples from the canyons during fall-winter and spring(Fig. 4g). Concentrations were low during the fall-winter peakparticle flux in Lacaze-Duthiers canyon, and increased late in thespring period. The range of total amino acid concentrations was0.05–7.17 mmol g�1. Temporal and spatial variability of amino acidcomposition in the sediment trap material was also observed(data not shown). The most abundant amino acids were asparticacid, glutamic acid, glycine, serine, alanine and threonine. Theseare the most common amino acids generally found in sedimenttrap samples (Lee and Cronin, 1982, 1984; Ittekkot et al., 1984a, b).Principal component analysis (PCA) was used to analyze a data setcontaining amino acid mole fraction of individual amino acids ateach location and time (e.g., Sheridan et al., 2002; Ingalls et al.,2006). PC2, the second principal component of amino acidPCA, explained 17% of the total variance, and was considered adegradation index (DI) of OM (sensu Dauwe et al., 1999) based onthe relative position of amino acid degradation tracers (Fig. 4h).Positive site scores along PC2 represent fresher material relativeto negative site scores; PC2 site scores ranged from �5.1 to 3.7.These site scores generally increased, and thus freshness in-creased, from fall-winter through spring; an along-shelf trend ortrend with proximity to the Grand Rhone is not apparent.Immediately prior to the large storm in early December, OM inthe 300 m traps was remarkably degraded (Fig. 4h). Neither PC1nor PC2 demonstrated a consistent relationship with respect tod13C or C/N ratio.

4. Discussion

4.1. Seasonal and event control on the transport of organic matter

The dynamic processes and forcing factors controlling sus-pended sediment transport across the GoL canyon heads betweenNovember 2003 and April 2004 have been discussed in detail byPalanques et al. (2006) and Bonnin et al. (2008). These authorshighlight the role in sediment transport played by storm-induceddownwelling during the strong and sporadic E-SE storms anddense shelf water cascading (DSWC) occurring in winter (theperiod to which we refer to here as ‘‘spring’’). DSWC occurs withvarying intensity during spring and is sporadically intensifiedduring E-SE storm events.

In discussing the control that weather conditions and forcingdynamics have on the amount and composition of particulate OMtransferred through canyon heads, we focus on the composition-ally different oceanographic seasons (‘‘fall-winter’’ and ‘‘spring’’)and the three scenarios defined by Palanques et al. (2006).

4.1.1. First scenario—downwelling during stratified conditions

The first scenario reflects the E-SE storm recorded in earlyDecember (pink band in Figs. 2–4) during the fall-winter periodwhen the water column is stratified. E-SE storms can cause largerwaves on the shelf and occasionally heavy rainfall and flash floods(Rhone river discharge reached almost 10000 m3 s�1 on December4), leading to the export of river-borne and shelf-resuspendedmaterial through the southwestern end of the GoL. E-SE intensewinds also produce convergence at the southwestern end of theGulf; this in turn induces an intense downwelling, down theupper slope of lighter and turbid shelf water that cannot escapealongshore (Ulses et al., 2008) as well as sediment transport downthe western canyon heads. Due to the stratified water columnduring fall, buoyancy limits the intrusion of shelf water to the

canyon heads (Ulses et al., 2008). Such an intrusion was reflectedby increases in temperature, current intensity (Fig. 2c) andsuspended sediment concentration (SSC) during our study(Palanques et al., 2006).

Dramatically increased particle fluxes recorded by the samplecollected December 2–8 clearly reflects the increased delivery ofparticulate matter to canyon heads during this event, peakingnoticeably more in the westernmost canyons. Lower organiccarbon concentration reflects the increased proportion of materialrich in lithogenic material and carbonate (see Bonnin et al., 2008)of riverine and/or resuspended origin reaching the canyon heads.A decrease in THAA at all sites similarly suggests an introductionor resuspension of more degraded material (Fig. 4g). The inverserelationship between flux and organic carbon concentration hasalready been linked to increased riverine and resuspendedparticle fluxes in the southwestern GoL shelf (Buscail et al.,1990). After the storm, the slow return to average organic carbonconcentrations similar to those recorded at the beginning of thesampling period highlights the storm’s influence on the composi-tion of the material available for transport even after the energeticevent had ended. Isotopic signatures of OM during this eventshow absolute minimum values for the westernmost canyons(Lacaze-Duthiers and Aude), reflecting the increase in theproportion of river-borne and coastal-derived OM (Tesi et al.,2006, 2007).

The shorter distance between the inner and outer shelf at thewestern end of the Gulf probably enhances the transfer of thematerial being transported south-westward along the coastalplume. The erosion–deposition model for this event (Ulses et al.,2008) clearly depicts a mid-shelf sediment trapping area thatresults from the preferential deposition of material exported fromthe inner shelf that does not reach the outer shelf. The areaonshore from the Lacaze-Duthiers and Cap de Creus canyons,where the model predicts sediment accumulation, narrows andcoalesces against the coast, suggesting that sediments resus-pended on the inner shelf may potentially reach the shelf breakand upper slope. In addition to organic carbon concentration andisotopic signature, pigment and THAA concentrations and com-position also reveal the enhanced arrival of coastal-derivedmaterial to the head of the westernmost canyons during the earlyDecember event. Following this storm event, increases in THAA,total pigment concentration, and the isolated presence ofpheophorbide in Lacaze-Duthiers canyon potentially reflect thetransport of pigments resulting from the increased primaryproductivity that occurs in nutrient-loaded coastal plumes(Redalje et al., 1994). Pheophorbide is a zooplankton-derivedpigment (Shuman and Lorenzen, 1975; Hallegraeff, 1981; Prahlet al., 1984) that points to the development of a zooplanktoncommunity thriving in the productive plume waters. Materialreaching the traps following this event exhibits a trend towardsfresher OM production (Fig. 4h).

4.1.2. Second scenario—dense water formation

The second scenario was established from mid-Decemberonwards within fall-winter (dark blue shaded area in Figs. 2–4).During periods when dominant northern and northwestern winds(Mistral and Tramontana, respectively) blow persistently over theGoL’ shallow inner shelf, surface waters are cooled and mixed toform dense shelf waters that sink, and spread on the mid- andouter shelf as a bottom layer (Palanques et al., 2006). Northernwinds blew continuously between mid-December and end ofJanuary (Fig. 2a), and fluxes decreased to background valuesexcept for Petit Rhone canyon (Fig. 3a). Channelized northernwinds along the Rhone valley often maintain an offshore jet thatinfluences shelf and upper slope circulation (Estournel et al.,

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J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–1983 1979

2003), as reflected by increased current intensity at the PetitRhone canyon (Fig. 2c) and southward spreading of the chlor-ophyll rich waters associated with the Rhone plume (Fig. 5). Inaddition in mid-January there was a large sudden increase inRhone river discharge up to 4000 m3 s�1 that decreased graduallytowards the end of February. All these factors favor enhanced

Fig. 5. Sequence of 10-day averaged SeaWiFS images covering the s

offshore export of riverine OM originating in the Rhone asindicated by the reduction, from mid-December onwards, of thealready low d13C values recorded in the Petit Rhone canyon andthat reached an absolute minimum value at the end of January.

Although this flood did not impact the amount of materialtransported to the upper slope, it did affect the concentration of

tudied period with the location of the mooring sites in white.

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J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–19831980

chloropigments and their composition in the Petit Rhone canyon(Fig. 4a and g)). The overall increase in d13C values from thenortheastern to the southwestern canyons, with the clearly lowervalues for the Petit Rhone canyon (Fig. 3d), reflect the dominanceof the Rhone River with regard to the input of terrestrial materialand the dilution of this contribution as the distance from itssource increases. An increase in freshness seen in the THAA PCAanalysis (Fig. 4h) is consistent with Rhone river discharge at thistime which certainly contributed nutrients that led to fresh OMproduction in late January at the Petit Rhone canyon that is notobserved in the southwestern canyon, Aude canyon. Thisterrestrial contribution may also be reflected in the decrease inTHAA at the eastern Petit Rhone canyon and undiluted back-ground of THAA at the western Aude Canyon (Fig. 4g), which isfurther from the Rhone River. The low and constant values oforganic carbon and biogenic silica during that period reflect thehomogeneous sources of particulate matter transferred throughcanyons, probably corresponding to the pool of particles sitting inthe vicinity of canyon heads that are swept downslope by thegeneral south-westward circulation. The good clustering, highatomic C/N index and almost zero intercept on the carbon y-axisrevealed by regression analysis (Fig. 6a) highlight the overallhomogeneity and degraded nature of the transferred organicmaterial (Tesi et al., 2006, 2007). In this regard the predominanceof pheophytin, a bacterially degraded pigment most common insediments (Sun et al., 1993; Ingalls et al., 2006), also pointstowards the resuspended origin of the transported OM.

4.1.3. Third scenario—dense shelf water cascading

From the end of January onwards spring conditions areestablished in the GoL. Spring is characterized by highervariability in the wind regime (Fig. 2a) that induces highervariability in the circulation on the shelf and upper slope (Fig. 2cand d). Palanques et al. (2006) and Ulses et al. (2008) recognizedthat waters cooled by predominantly northern winds thataccumulate on the shelf, ultimately cascade down the slope. Atthe end of January the first signals of DSWC were detected aswater temperature decreased at the southwestern canyon heads(Lacaze-Duthiers and Aude) (Fig. 1b). The third scenario condi-tions, recorded during spring, correspond to day- to week-longevents during which DSWC was triggered and/or intensifiedby downwelling induced by easterly storms (light blue bands inFigs. 2–4). This intensification was reflected by a noticeable

0

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Fig. 6. Corg TN regression analysis fo

increase in current speed (especially in the southwesterncanyons) and further decrease in water temperature (Fig. 2).DSWC events produced increases in particle fluxes, with those atthe end of February and in mid-April being the most remarkable.The response in particle fluxes recorded during periods of similarforcing conditions was tentatively related to the differentavailability of loose particulate matter, supplied by simultaneousor previous river discharge events and accumulated ephemerallyon the shelf (Palanques et al., 2006).

Particulate matter composition does not follow an event-likepattern as did the mass fluxes (with the exception of the eventrecorded at the very end of February), but a much more seasonalone characterized by gradual changes. The overall higherabundances and variability of Corg and Sibio during spring likelyresult from the export of (i) biogenic particles generated duringwidespread phytoplankton blooms occurring from end of Januaryonwards (Fig. 5) and/or (ii) riverine-borne particles accumulatedon the shelf during the previous flood season. The higherdispersion, lower atomic C/N regression index (very close toRedfield values) and the positive y-intercept obtained from theC/N regression analysis (Fig. 6b; indicative of carbohydrate-richmaterial common in phytoplankton blooms) points to a morevariable and fresher origin of the OM transported through canyonheads during the spring period.

The abundance of Corg and Sibio and the isotopic signature(d13C) for each sample result, ultimately, from the mixture, indifferent proportions, of particles originating from these differentsources. The maximum in Sibio and chloropigments, high Corgand high (more marine) d13C recorded at the end of Februaryresult from the direct transfer of particles derived from an intensephytoplankton bloom during a period of decreased bottomresuspension just after the first strong DSWC event. This strongbloom is clearly illustrated by the widely spread high concentra-tions of Chl-a in the SeaWiFS image corresponding to the lastweek of February (Fig. 5). Overall high Corg, low Sibio and low(more terrestrial) d13C values between mid-March and mid-Aprilmay result from selective transfer of organic-rich particles with alarger terrestrial contribution during a period of enhanced near-bottom transport by frequent DSWC events. Ulses et al. (2008)showed that episodes of northern winds produced a cooling of theshelf waters and consequently the formation of dense water alongthe coast and on the mid-shelf region that were directed, bygravity and wind-driven circulation, towards the slope andcascaded down through the Cap de Creus, Lacaze-Duthiers, Aude

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r fall-winter (a) and spring (b).

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Fig. 7. Stable isotopic compositions (d13C) of Corg vs. TN:Corg. Both parameters are

carbon-normalized. The compositional range of terrestrial and marine Corg are

plotted to show the relative contribution of allochthonous and autochthonous OM.

Open triangles represent surface sediments collected in the prodelta Rhone area

(Tesi et al., 2006, 2007). Open circles represent surface sediments collected in the

J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–1983 1981

and Herault canyons. The temperature and averaged currentspeed series depicted in Figs. 2c and d clearly show that betweenmid-March and mid-April the intensified DSWC was more regularand therefore could better contribute to enhanced export of OM ofterrestrial origin entrained by rivers in the coastal zone orpreviously deposited in the mid-shelf region.

The higher mole percentages of chlorophyll-a, pheophorbideand pyropheophorbide (the latter are chlorophyll degradationproducts resulting from zooplankton digestion of phytoplankton)relative to pheophytin during the spring (Figs. 4b–f) also highlightthe link to phytoplankton blooms for particles reaching thecanyon heads during spring. It is worth noting that the slightlyincreased fractions of chlorophyll and especially pheophorbidereflect episodically enhanced export of freshly generated OMduring DSWC events, especially in the southwestern canyons (Capde Creus and Lacaze-Duthiers canyons). Dense shelf waterformation on the coastal zone and mid-shelf and its episodicexport towards the outer shelf constitute a conveyor belt for thechlorophyll- and pheophorbide-enriched particles generated nearthe coast where productivity is maximum (Fig. 5) and supportszooplankton-rich communities. These results agree with andprovide a detailed process interpretation for the large advectiveinputs of fresh OM during late-winter early spring recorded byBuscail et al. (1990). On the other hand, the overall increasedabundance of pheophytin from mid-March to mid-April occurs atthe same time as the lowest values of d13C, which points to theresuspended origin of riverine derived OM transported duringthat period. The arrival of more degraded OM of resuspendedorigin is also reflected by drops in the overall tendency towardsfresher material reflected by the THAA PCA (Fig. 4h).

Cap de Creus area and western shelf (Garcıa-Garcıa et al., submitted). Crosses and

open squares represent the sediment trap samples collected in fall-winter and

spring, respectively.

4.2. Assessment of the Corg exported via canyons in the GoL

Elemental and Corg isotopic compositions (d13C) have beenwidely used in geochemical studies to investigate the origin ofparticulate material along the continental margin (Middelburgand Nieuwenhuize, 1998; Goni et al., 1998; Leithold and Hope,1999; Gordon and Goni, 2003; Miserocchi et al., 2007). Fig. 7shows elemental and isotopic compositions for the sediment trapsamples. The OM in the GoL samples exhibited a broad range ofTN/Corg and d13C values, indicating both marine and terrestrialsources (Tesi et al., 2006, 2007). The composition of OM fromsurface sediments in different regions of the GoL, are also shownin Fig. 7 to visually illustrate their contribution to the mass fluxes.In particular we focused on the composition of surface sedimentscollected in the Rhone prodelta, western shelf as well as the headof Cap de Creus canyon (Tesi et al., 2006, 2007; Garcıa-Garcıaet al., submitted). High variability is observed in biogeochemicaldistributions over the course of the deployment, suggesting bothallochthonous and autochthonous contribution. Although thiscomparison has been made over a limited portion of the GoL,based on our values the mass fluxes are strongly influenced frommaterial resuspended from the walls of the canyon as well as fromthe shelf. Therefore even if terrestrial and marine pulses, in theform of both floods and phytoplantonic blooms, had a clearimpact on the geochemical signature of the captured particles, themajor Corg source for the sediment traps derives from the innerand outer shelf for all three scenarios.

In order to assess the OM contribution to the particulate flux ofboth allochthonous and autochthonous sources, we applied a twoend-members mixing model using isotopically depleted soil-derived Corg and marine Corg characterized by ‘‘heavier’’ isotopiccomposition (Hedges et al., 1997). The precision of this modeldepends on the end-member values, and therefore the contribu-tions of each source can vary based on end-member values

(Gordon and Goni, 2003). However, this model is useful to quicklysummarize the data set and to understand the major seasonalpatterns. The soil-derived Corg end-member was defined on thebasis of the average of the fine material collected in Rhoneproximal prodelta stations right after a flood event(d13CT ¼ �27.6%; Tesi et al., 2006, 2007). Unfortunately, pub-lished data regarding the stable carbon isotopic composition ofsuspended particulate material is not available for the study area.The marine end-member was chosen based on published valuesfor another Mediterranean basin (d13CM ¼ �20.7%; Adriatic Sea,Tesi et al., 2006) that agrees with the typical isotopic value formid-latitude (Hedges et al., 1997) and it is consistent withunpublished data for the GoL (Harmelin-Vivien et al., 2008).Marine and terrestrial contributions (FM and FT, respectively) canbe estimated solving the following system of linear equations:

d13CSample ¼ FT � d13CT þ FM � d13CM

1 ¼ FT þ FM

where d13CSample is the isotopic composition for the sample,whereas d13CT and d13CM are the isotopic compositions forterrestrial and marine end-member in that order.

Fig. 8 summarizes the results of the two end-member mixingmodel. The assessment for the surficial sediments (Tesi et al.,2006, 2007) is also included in this graph and is used to provide aqualitative context to our observations. Based on the model, thereare no marked geographical differences in the Corg sources. It isworth mentioning that the lithic particles moving toward theouter shelf have partially lost their original terrestrial signature(Tesi et al., 2006, 2007). The input of these particles from the shelfdoes not necessarily enhance the terrestrial contribution to thematerial collected in the sediment traps. Nevertheless, the effects

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Fig. 8. Assessment of terrigenous vs. marine contribution in the sediment trap

samples. The values have been obtained applying a two end-member mixing

model with a depleted terrestrial Corg and a ‘‘heavier’’ marine Corg. Values for the

surface sediments (Tesi et al., 2006, 2007) are also included to provide a qualitative

context to our observations.

J. Fabres et al. / Continental Shelf Research 28 (2008) 1971–19831982

of flood, water dense water cascading and phytoplankton bloomwere observable. The highest terrigenous contributions (�50%)corresponded to the highest mass flux recorded in fall-winter.Since the Rhone, Herault, Aude and Tet river were flooding in thatperiod, it is likely that a fraction of the terrestrial materialdelivered by the rivers could have reached the head of the canyon.These data strongly suggest that the highest mass flux measuredin this study is a mixture of terrestrial material delivered by theflood and resuspended material from the inner shelf and/or sweptfrom the canyon head as suggested by Fig. 8. In brief, in spite ofthe large flood event, the particulate matter transferred throughthe head of the canyon was not entirely derived directly from therecent flood.

The second peak in terrigenous Corg (�45%) at the end ofJanuary corresponded to dense water cascading events. In thiscase only the Herault was flooding to any degree (Palanques et al.,2006) and probably the combination of flood with resuspension,due to cascading, enhanced the terrestrial contribution to particleflux. Finally, at the end of February 2004, just after the storm, thehighest marine Corg contribution (�75%) was observed in allsediment traps due to the important phytoplankton bloomdeveloped and the decreased arrival of particles fromother sources (i.e. fluvial or bottom resuspended input). Theseresults are consistent with the Sibio data, suggesting a significantmarine contribution to particle flux from phytoplankton such asdiatoms.

4.3. Implications for the export of organic matter to the deep basin

From the sequence of scenarios described above it is clear thatthe temporal sequence of the different processes, floods, phyto-plankton blooms and dense water formation and cascadinggreatly enhances export of OM. The fact that the lag betweenthe floods and the DSWC events is on the order of weeks, and thebloom season co-occurs with the DSWC season allows enhancedexport of relatively fresh OM to the canyon heads and the slope.This sequence of processes occurs yearly, and therefore it isimportant to the long-term geochemical budget of the margin andprovides a regular and ‘‘reliable’’ food source for the deepecosystems of the margin. Furthermore it is clear that thesequence of processes described is of paramount importance forthe sequestration of carbon from the upper layers of the ocean andits isolation from the atmosphere in the deep sea.

5. Conclusions

It is clear from our study that seasonal meteorologicalconditions have a profound influence on the amount andcomposition of particulate OM exported from the shelf towardsthe basin through canyon heads in the GoL. During ‘‘fall-winter’’the transported OM is more degraded, mostly of resuspendedorigin, despite the elevated productivity maintained in the innershelf. In contrast, during the ‘‘spring’’ period the OM reaching thecanyon heads has a more mixed origin with a larger contributionof fresher OM resulting from enhanced productivity, and thereforetransported more quickly from surface waters. The temporalcoincidence of spring phytoplankton blooms and DSWC events islikely responsible for the effective transfer of fresh OM towardsdeeper parts of the basin. Despite the temporal variations in theproportion of material originating from autochthonous andallochthonous sources, the autochthonous marine source,whether resuspension of particles sitting on the shelf or canyonheads or surface produced, predominates. The tight link betweenmeteorological conditions and transfer processes deserves furtherstudy, as future changes in atmospheric circulation patterns maylead to dramatic changes in carbon sequestration capability andecosystems of deep margin environments.

Acknowledgments

J. Fabres was supported by the European Community’s SixtFramework Program through the Marie Curie Outgoing Interna-tional Fellowship. J. Velez was supported by Stony Brook’s NSF-Alliance for Graduate Education and the Professoriate (AGEP)program. C. Lee and F. Batista were supported by the US NSF’sMedFlux program. This study was supported by the EUROSTRATA-FORM Project funded by the EU (EVK3-CT-2002-00079, EU FifthFramework Program: Energy, Environment and SustainableDevelopment). This is contribution no. 1602 of ISMAR-CNR Sededi Bologna. J. Fabres would like to thank the invaluable technicaland moral support received from L. Abramson and T. School-meester during his stay in Stony Brook University.

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