Sediment accumulation in the western Gulf of Lions, France ... et al (CSR 2008).pdf(Cap Bear). Estimates of a sediment budget indicate that 6–8% of the sediment input to the Gulf

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doi:10.1016/j.cs

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Continental Shelf Research 28 (2008) 2031–2047

Sediment accumulation in the western Gulf of Lions, France:The role of Cap de Creus Canyon in linking shelf and slope sediment

dispersal systems

A.L. DeGeesta,1, B.L. Mullenbacha,�, P. Puigb, C.A. Nittrouerc, T.M. Drexlerc,X. Durrieu de Madrond, D.L. Orangee,f

aDepartment of Oceanography, Texas A&M University, College Station, TX 77843-3146, USAbInstitut de Ciencies del Mar (CMIMA-CSIC), Passeig Marıtim de la Barceloneta, 37-49. E-08003 Barcelona, Spain

cSchool of Oceanography, University of Washington, Seattle, WA 98195-7940, USAdCEFREM, CNRS—University of Perpignan, 66860 Perpignan, FranceeDepartment of Earth Sciences, UCSC, Santa Cruz, CA 95064, USA

fAOA Geophysics Inc., 7532 Sandholdt Rd., Suite 6, Moss Landing, CA 94039, USA

Received 1 June 2007; received in revised form 10 December 2007; accepted 19 February 2008

Available online 4 March 2008

Abstract

Previous work in the Gulf of Lions (western Mediterranean Sea) has suggested that significant amounts of sediment escape through the

western part of this tectonically passive margin, despite it being far removed from the primary sediment source (the Rhone River,

�160 km to the NE). The primary mechanism behind this export is hypothesized to be the interaction of a regional, southwestward

sediment-transport path with a canyon deeply incising the southwestern part of the shelf, Cap de Creus Canyon.

To understand the pattern of off-shelf sediment export from the western Gulf of Lions, and more specifically, the role of Cap de Creus

Canyon in this transport, box cores were collected within the canyon and on the adjacent shelf during five cruises from November 2003 to

April 2005. Geochronology (210Pb-derived accumulation rates), grain-size distributions, and sedimentary structures (X-radiography)

were analyzed to assess temporal and spatial sedimentation patterns. Results indicate two mid-shelf depocenters (30–90m water depth) in

the northern and southern portions of the study area, separated by a zone of bypassing due to current acceleration around a headland

(Cap Bear). Estimates of a sediment budget indicate that �6–8% of the sediment input to the Gulf is sequestered on the shelf region.

Within the Cap de Creus Canyon, there is a significant spatial asymmetry in both grain size and accumulation rates. The northern

flank is a modern depocenter of fine-grained sediments, while the southern flank is primarily non-depositional for mud and includes

locations of apparent erosion. This suggests the influence of multiple oceanographic processes supplying sediment to the canyon:

advection of nepheloid layers from the northern rim that provide a relatively continuous sediment supply (over decadal timescales), and

episodic strong currents affecting the southern rim, which can scour sediment from the southern flank. The mid-depth thalweg has an

ephemeral mud layer, overlying sand and consolidated mud. The mud layer appears to be flushed down canyon periodically. The canyon

head contains coarse material, suggesting reworked sands may be entering.

The 100-year sediment budget, based on accumulation rates for the fine-grained fraction in the upper canyon, indicates that �1% of

the total sediment input to the Gulf is accumulating in upper Cap de Creus Canyon. However, this number may significantly

underestimate the total sediment entering the canyon because water-column measurements show that sediment is likely moving through

the upper canyon during major dense-water cascading events from the shelf and being deposited deeper in the canyon system. The

ephemeral mud layer also indicates rapid deposition and frequent flushing of sediment through the upper canyon. Overall, this study

e front matter r 2008 Elsevier Ltd. All rights reserved.

r.2008.02.008

ing author.

ess: [email protected] (B.L. Mullenbach).

ress: Chevron Energy Technology Company, 1500 Louisiana Street, Houston, TX 77006, USA.

dx.doi.org/10.1016/j.csr.2008.02.008

mailto:[email protected]

ARTICLE IN PRESSA.L. DeGeest et al. / Continental Shelf Research 28 (2008) 2031–20472032

shows that Cap de Creus Canyon is an active conduit of sediment past the shelf break, despite its location distal to the primary sediment

source to the Gulf.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: Submarine canyon; Gulf of Lions

Fig. 1. Map of the GOL, including inset to show location of the Gulf and

box to show study area. The approximate location of the Liguro-

Provencal-Catalan Current is shown by the gray arrow, with the main

canyons discussed in this paper marked. Dashed boxes include accumula-

tion rate ranges for different margin areas as determined by Zuo et al.

(1997). Cap de Creus (marked by a P for promontory), the cape on the

northern coast of Spain, critically influences the regional currents.

1. Introduction

Submarine canyons are common bathymetric featuresthat can strongly impact across-margin sediment dispersalpatterns (Shepard and Dill, 1966; May et al., 1983). Manycanyons incise far onto the continental shelf, effectivelynarrowing the distance between fluvial sediment sourcesand the shelf break and slope. This property allowscanyons to intersect along-margin transport and serve aspreferential conduits of sediment past the shelf break(Mullenbach et al., 2004). Off-shelf transport is particularlyenhanced in canyons along many modern collisionalmargins where narrow shelves and high fluvial sedimentinput co-exist (e.g. Milliman and Meade, 1983; Paull et al.,2003; Walsh and Nittrouer, 2003). In contrast, canyons ontectonically passive margins can be less effective atexporting sediment due to greater shelf width and presenceof estuaries, which can trap sediment (e.g. Gardner, 1989).

A necessary component for off-shelf transport viacanyons is the interception of active sediment-transportpathways. For some collisional margins, a canyon may benearly connected to a river mouth, directly funneling thefluvial sediment to the deep sea (e.g. the Sepik andMonterey) (Kineke et al., 2000; Paull et al., 2003). Onactive margins, canyons also have been shown to interceptsignificant along-shelf fluxes associated with proximal riverdischarge (e.g. the Eel and Quinault) (Hickey et al., 1986;Mullenbach, 2002; Puig et al., 2003). The interception ofalong-margin sediment dispersal can create significant off-shelf sediment escape, which may not be directly related tothe distance between a fluvial sediment source and acanyon head.

Along-margin variability, such as changes in shelf widthand shoreline irregularity (e.g. headland intrusion), cancontrol the proximity of canyons to large sediment fluxeson the shelf and potentially create off-shelf steering farfrom a fluvial source. Oceanographic conditions, such asfavorable along- and across-shelf currents, also play a rolein the primary location of off-shelf transport within asystem (i.e. distal vs. proximal to the source). Determiningthe main area of sediment export from a shelf can haveimplications for identifying long-term depocenters. If distalcanyons are active, the specific processes allowing sedimentto escape need to be further defined.

The western Gulf of Lions (GOL) is a wide, passivemargin in which the primary area of off-shelf sedimentescape is hypothesized to be at a location distal to the mainsediment source (Durrieu de Madron et al., 1990; Courpand Monaco, 1990; Got and Aloisi, 1990; Durrieu deMadron, 1994; Frignani et al., 2002) (Fig. 1). Previous

studies have shown that significant quantities of sedimentdo not reach the shelf break adjacent to the Rhone Rivermouth (Got and Aloisi, 1990; Arnau et al., 2004). Rather,hydrodynamical modeling and canyon mooring studiessuggest that sediment is funneled towards the narrowing,southwestern portion of the Gulf and the westernmostcanyon, Cap de Creus Canyon (Palanques et al., 2006;Heussner et al., 2006; Ulses et al., this issue). This canyon,although far from the primary sediment source (�160 km),may directly intercept the regional transport pathway andtherefore serve as a primary conduit of sediment escape forthe entire GOL.The overall goal of this project is to understand the

processes of sediment export from the western GOL and,more specifically, the role of Cap de Creus Canyon in thistransport. The objectives are to: (1) identify moderndepocenters on the western shelf and within Cap de CreusCanyon, (2) infer transport mechanisms creating theobserved depositional patterns, and (3) calculate a sedi-ment budget for upper Cap de Creus Canyon and the

ARTICLE IN PRESSA.L. DeGeest et al. / Continental Shelf Research 28 (2008) 2031–2047 2033

western shelf. This paper will provide insight into themodern processes moving sediment through a submarinecanyon distal to a fluvial source and the importance ofmargin characteristics and regional currents in creating off-shelf conduits.

2. Background

2.1. Geologic setting

Extending from the Pyreanean Mountains of northernSpain to the Alps of eastern France, the GOL is atectonically passive margin whose shelf reaches a maximumwidth of 72 km (near the mouth of the Rhone) and narrowswestward to less than 15 km near Cap de Creus Canyon(Berne et al., 2004) (Fig. 1). The shelf break is incised by atleast 12 submarine canyons that coalesce into two primarychannels on the deep slope (Canals et al., 2004). Althoughthe origin of these canyons is still debated, recent studiessuggest that the canyons were formed from halokinesis-derived depressions above Messinian evaporates, whichwere then enhanced by turbidity current erosion and masswasting (Canals et al., 2004). Some of these canyons extendrelatively far onto the shelf (e.g. Cap de Creus and Lacaze-Duthiers Canyons in the western Gulf), while othersinitiate much farther from shore (e.g. Grand and PetitRhone Canyons in the eastern Gulf). Previous seabedstudies have identified a mid-shelf mud deposit (MSMD)on most of the shelf (Jago and Barusseau, 1981). Modernsands extend from the shoreline to �30m water depth (thebeginning of the MSMD), and reworked Pleistocene sandsare exposed on the outer shelf (Courp and Monaco, 1990;Certain et al., 2005).

The main source of sediment to the GOL is the RhoneRiver, which provides 7.4–10.1� 106 tons of sedimentannually and accounts for approximately 94% of the totalsediment input to the Gulf (Pont et al., 2002; Bourrin andDurrieu de Madron, 2006). Six smaller, western rivers(Herault, Orb, Aude, Agly, Tech, Tet) supply most of theremainder of the Holocene sediment to the GOL (Fig. 1).These rivers, which flow primarily from the Pyrenees,respond quickly to climatic variations and have episodicdischarge that is difficult to quantify (Certain et al., 2005).Consequently, little data exist on the annual sediment andwater discharge of these rivers. In their compilation,Miralles et al. (2005) reported 210Pb-derived sedimentationrates of 0.02–0.65 cm/yr in the GOL. The highest rates werelocated just west of the Rhone River mouth, while thelowest were found on the deep-slope interfluves and in thedeep sea.

Previous studies have shown that sediment is escapingthe shelf via submarine canyons (Durrieu de Madron et al.,1999; Heussner et al., 2006; Palanques et al., 2006).Enhanced suspended-sediment concentrations within theboundaries of the Grand Rhone Canyon indicate afunneling of sediment, suggesting canyons are preferentialconduits of sediment off the shelf (Durrieu de Madron,

1994; Durrieu de Madron et al., 1999). Further, miner-alogical evidence indicates that Rhone sediments arereaching Lacaze-Duthiers Canyon (western GOL) (Frigna-ni et al., 2002). Previous studies by Durrieu de Madron etal. (1990), Heussner et al. (2006), and Palanques et al.(2006) show that suspended-sediment concentrations andparticulate fluxes within the canyons increase westward,away from the Rhone sediment source, and that thewestern Gulf is likely the primary outlet of sediment for theentire GOL.

2.2. Oceanographic setting

The GOL is a wave-dominated, microtidal environment(Certain et al., 2005). The general circulation pattern isdominated by the Liguro-Provencal-Catalan (LPC) orNorthern current, which flows southwestward along theslope, and creates an overall east-to-west circulation alongthe outer shelf (Durrieu de Madron et al., 1990; Millot,1990) (Fig. 1). The LPC enters the Gulf via the Liguran Seato the east and moves at speeds of up to 50 cm/s towardsthe Catalan margin (Millot, 1990), transporting particleswestward along the shelf and slope.Episodic Marine (southeasterly) wind can create sig-

nificant wave energy within the GOL and therebyresuspend sediments. The limited fetch associated withcontinental winds (Tramontane-northwesterly; Mistral-northerly) reduces their impact on sediment resuspension(Ferre et al., 2005; Guillen et al., 2006). Both Marine andTramontane wind regimes transport coastal water andsuspended sediment towards the southwest (Ulses et al.,this issue).Continental winds also are responsible for winter dense-

water formation on the GOL shelf. Density changesinduced by wind cooling of shallow shelf water cause thewater to sink to a level of neutral buoyancy, establishingunique thermohaline circulation within the Gulf. Ulses etal. (2008) reported that these waters tend to accumulate onthe outer shelf until they eventually cascade down the slopeas a near-bottom, cold-water density flow that could act asa possible mechanism of escape for suspended sediment.These cascades have been identified in several submarinecanyon heads along the entire GOL margin (Palanqueset al., 2006), being particularly intense in the Cap de CreusCanyon, and have been recognized as a mechanism movingsediment into the GOL canyons (Palanques et al., 2006;Canals et al., 2006; Heussner et al., 2006).

3. Methods

3.1. Field methods

Sediment samples were collected using a box corer(20� 30 cm cross section, 60 cm length) during five cruisesin the western GOL from November 2003 to April 2005(Fig. 2). Upon recovery, cores were subsampled forradionuclide and textural analysis using push-core tubes

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Fig. 2. Map showing core locations and designated physiographic regions.

The shelf has three distinct regions (as distinguished by sedimentary

characteristics): the northwestern, middle, and southwestern shelf. The

canyon has four regions: the canyon head (1), the thalweg (excluding the

canyon head) (2), the northern flank (3), and the southern flank (4). Inset

shows the location of across-canyon coring transect A–A0 (Fig. 8).

A.L. DeGeest et al. / Continental Shelf Research 28 (2008) 2031–20472034

(15 cm ID, thin-walled PVC tubing). Sediment within thesub-cores was extruded, sampled in 1-cm depth intervals,and stored for laboratory analysis. Plexiglass trays wereused to extract vertical slabs of sediment, which wereX-rayed shipboard for estimating density variations,stratigraphy and sedimentary structures.

3.2. Radioisotopic analysis

Down-core changes in radioisotopic 210Pb activities(t1/2 ¼ 22.3 years) were used to examine the decadal to100-year sediment records by a technique modified fromNittrouer et al. (1979). Dry samples were spiked with a209Po tracer (used for yield determination), sediments wereleached with 16N HNO3 and 6N HCl, 209Po and 210Powere plated onto silver planchets by spontaneous electro-deposition, and planchets were counted for relative alphadecay (disintegrations per minute, dpm) over a 24-h periodon a Canberra Alpha Analyst detector. Total 210Pb activitywas determined by assuming 210Po is in secular equilibriumwith 210Pb (Nittrouer et al., 1979). Samples that had largefractions of sand or shell fragments were treated similarlyto muddy samples; however very large, articulated shells orwood fragments (anything that would not behave similarlyto the surrounding sediments) were removed prior toanalysis. Supported 210Pb activity (derived from the decayof its effective parent, 226Ra, in the seabed) is determinedby measuring 210Pb activity at the bottom of a core with

low accumulation rates (sediment4100 years old). Excess210Pb activity (210Pbxs, the activity particles attain whilesinking through the water column) was calculated bysubtracting supported from total activity. In order toaccount for variability in activity due to down-core grain-size variations, activities were normalized to the percent ofclay in each sample interval (Fig. 3).Accumulation rates over decadal to 100-year timescales

were calculated using the decrease of 210Pbxs activity withdepth in the seabed. Assuming the rate of biological mixingis negligible relative to the rate of sediment accumulation,the exponential decrease of 210Pbxs activity with depth givesan accumulation rate:

A ¼�lz

lnðCz=CoÞ,

where A is the accumulation rate, l is the decay constantfor 210Pb (0.693/half life), z is the depth in the seabed, Co isthe excess activity of sediment accumulating at thesediment–water interface (assumed constant over 100-yeartimescale), and Cz is the excess activity at depth z

(Nittrouer et al., 1979). These are average accumulationrates based on the assumption of steady-state accumulationover a 100-year timescale. Because data on biologicalmixing rates were not collected as part of this study, theinfluence of bioturbation on the observed 210Pb profilescannot be ascertained. It is unknown if the assumption ofnegligible biological mixing used for the accumulation ratecalculation is valid. Therefore, all references to accumula-tion rates and sediment budgets herein are reported asmaximum apparent rates and upper-bound budget values,respectively. Sandy layers that exhibited little down-corevariation in 210Pb activity were simply classified as recent(within the last 100 years) or relict (older than 100 years)based on the presence or absence of 210Pbxs activities,respectively.

3.3. Sedimentological analysis

Sedimentological characteristics of each core wereassessed by grain-size analysis. Grain-size profiles for eachcore were created by wet sieving each depth increment at63 mm to isolate sand and coarser materials from finerfractions. All material passing through the sieve wasanalyzed using a Sedigraph 5100 particle size analyzer todetermine the distributions of silt- and clay-sized particles.Profiles of % mud were created for comparison with andcorrection of radioisotopic data.

3.4. Ancillary data

High-resolution multibeam bathymetry of Cap de CreusCanyon was acquired, processed, and provided by FugroSurvey Ltd. and AOA Geophysics Inc. GOL regionalmultibeam bathymetry was supplied by S. Berne (IFRE-MER) (Berne et al., 2002). Additionally, hydrographictime series (currents, suspended-sediment concentrations,

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0 20 40 60 80 100% Sand

0 20 40 60 80 100% Sand

0 20 40 60 80 100% Sand

0 20 40 60 80 100% Sand

0

5

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Dep

th (c

m)

0

0 20 40 60 80 100% Sand

0 20 40 60 80 100% Sand

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th (c

m)

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m)

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Dep

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m)

Dep

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m)

Dep

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m)

0.1 1 10 100Excess Pb-210 Activity

(dpm/g of clay)


(dpm/g of clay)


(dpm/g of clay)


(dpm/g of clay)


(dpm/g of clay)


(dpm/g of clay)

1.6 mm/yr

0.7 mm/yr

1.6 mm/yr

2.3 mm/yr

0.9 mm/yr

1.3 mm/yr

CSD58

CST135

CFM138 CFM440

# 20

XX50

Shelf Cores

Rim Cores

Canyon Cores

0

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40

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40

A.L. DeGeest et al. / Continental Shelf Research 28 (2008) 2031–2047 2035

temperature) from moorings and tripods located at145–750m water depth in the canyon (see Puig et al., thisissue for details) have been used in this study to interpretcanyon sedimentation processes.

4. Results

4.1. Definition of regions

For the purpose of this study, the western GOL shelf andslope is divided into regions based on sedimentarycharacteristics, oceanographic conditions, and seafloormorphology. The shelf is separated into three regions: (1)the northwestern shelf (from the northern limit of the studyarea to Cap Bear), (2) the middle shelf (from Cap Bear tothe Spanish-French border), and (3) the southwestern shelf(from the border to Cap de Creus headland) (Fig. 2). Theportion of the shelf near Cap de Creus Canyon is classifiedas the canyon rim. Cap de Creus Canyon is divided intofour morphological regions: (1) canyon head, (2) thalweg(excluding the canyon head), (3) northern flank, and (4)southern flank (Fig. 2).

4.2. Western Gulf of Lions continental shelf

Grain-size results from all shelf regions studied showpatterns similar to those defined by previous studies for theentire GOL region (Fig. 4). A MSMD, composed primarilyof silty clays or clayey silts (all o40% sand), is located in�30–85m water depth. Cores collected deeper on the shelfwere generally composed of coarse sands. No cores werecollected in water depths shallower than 30m, but previousresearch indicates that nearshore sands exist out to�20–30m water depth and therefore this value is used asthe inner boundary for this study (Martin et al., 1981;Courp and Monaco, 1990; Got and Aloisi, 1990; Certainet al., 2005).Where the shelf begins to narrow due to a coastal

promontory (Cap Bear; middle shelf), the across-shelfextent of the mud begins to narrow. Core U85 isuncharacteristically coarse grained for its water depth(silty sand) and therefore marks the narrowest extent of theMSMD (Fig. 4). South of this line (southwestern shelf), themud deposit again extends seaward to form a crescentshape parallel to the coastline. This deposit tapers east-ward, giving way to sands and gravels near the headland.In general, grain size tends to coarsen southward on the

shelf. The finest sample collected was a silty clay taken atsite XX80 (the most northern core transect), and the

Fig. 3. Excess 210Pb activity and sand content from different areas of the

study. Circles represent excess 210Pb values, ‘‘X’’s represent percent of

sand. Shelf cores tend to have steady-state accumulation (as much as

2.5mm/yr) of fine sediment, while cores collected near the canyon rim had

much lower accumulation rates and coarser grain sizes. Cores from the

canyon have significant variability, but two cores from the northern flank/

rim show some areas of fine-grained sediment accumulation.

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Fig. 4. Surficial grain-size distributions on the GOL shelf. All cores collected from �50 to 85m water were primarily muddy, whereas those collected

farther seaward on the shelf tend to be sandy (some with significant shell hash) or gravelly. Categories are distinguished by the predominant grain size

within the sample: gravel: sample contained particles 42mm in diameter (excluding shell hash); sand: 475% sand; sand with mud: sample composed of

mixed grain sizes, but the highest proportion was sand-sized particles; silt: sample dominated by mud (o63 microns) and contained more silt than clay;

and clay: sample composed of o20% sand with more clay than silt. The dashed line denotes the region where the MSMD would be expected based on

previous work. The down-core grain-size profile of core XX50 is included as an example of down-core variability. Inset shows near-bottom current

velocities from an ADCP survey in February 2005. Note the acceleration around headlands and the near-shore anticyclonic eddy just north of Cap de

Creus headland.


coarsest samples were composed of sand and gravelcollected at sites near Cap de Creus (cores J01, #3, andNCC80, Fig. 4). The R-line (south of Cap Bear) is anexception to this generality. Although still classified asmuddy samples, these cores (R55 and R87) have greateramounts of sand (430%) than those collected at similardepths elsewhere on the shelf (o30%). There does notappear to be significant down-core variability in grain sizefor most cores collected on the western shelf.

Accumulation rates on the shelf reach as much as2.5mm/yr. The highest rates are generally associated withthe MSMD (Figs. 3 and 5). However, cores collected alongthe R-line (R55 and R87) reveal uncharacteristically lowrates at the same location where grain size was coarser thanother parts of the MSMD (at the middle shelf). These datasuggest that there are two distinct ‘‘cells’’ of accumulationon the shelf: the northwestern-shelf MSMD (north of CapBear) and the southwestern shelf deposit (Fig. 5). Cores in

the northwestern region tend to be finer grained and have lessvariability than those in the southwestern region. Accumula-tion rates are 1.0–2.0mm/yr on the northwestern region ofthe shelf, with the highest rates occurring at 80-m waterdepth. In the southwestern region, cores were generallysandier and the highest accumulation rate was 2.5mm/yr atsite U50. Accumulation rates on the southwestern shelfdecrease southwestward, creating the crescent shaped depositdefined previously by grain-size characteristics.Many cores collected on the shelf at the canyon rim are

sandy and tend to coarsen seaward, creating deposits of470% sand at locations adjacent to the canyon head (Figs.3 and 4). Accumulation rates vary with location (Fig. 5).Comparison of core characteristics from the canyon rim tothe north and south of Cap de Creus Canyon revealsdistinct differences. To the north, the shelf is predomi-nantly clayey sand (o70% sand with more clay than silt)with appreciable accumulation rates (i.e. 1.6mm/yr at

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Fig. 5. Accumulation rates on the western shelf. All rates are assumed to

be maximum apparent accumulation rates due to uncertainty of the

influence from biological mixing on 210Pb profiles. Rates reach as much as

2.5mm/yr, with the highest rates generally occurring within the MSMD on

the shelf (from 30 to 80m water depth). Some cores were too coarse-

grained to obtain valid accumulation rates and are indicated by the black

circles. Three main regions were evident from the accumulation pattern:

the northwestern shelf (with a mid-shelf mud deposit), southwestern shelf

(with a mid-shelf mud deposit), and middle shelf (bypassing).

Fig. 6. Cap de Creus Canyon grain-size and accumulation rate distribu-

tion. The canyon head and rim are clearly dominated by coarse-grained

material and low accumulation rates. The northern flank and thalweg tend

to be dominated by finer sediments with higher accumulation rates. The

canyon also shows some erosional points evidenced by consolidated muds

with no excess 210Pb activity, including cores CFM230 and CTM571.


CFM138, Fig. 6). To the south, canyon rim cores aregenerally very coarse grained (sand and gravel up to 4 cmdiameter at site J01) and show no evidence of fine-sedimentaccumulation (actual rates could not be determined due tothe large proportion of coarse material in these samples)(Fig. 6). Box core attempts in this area returned only a fewcentimeters of coarse-grained and/or consolidated muddespite high fall rates of the corer. This indicates strong,resistive material not easily removed from its presentsetting (the southern rim of the canyon).

4.3. Cap de Creus Canyon

4.3.1. Canyon head and thalweg

Surficial grain-size patterns and accumulation rateswithin Cap de Creus Canyon are more complex than thoseon the shelf. Surface sediments in the canyon head arecoarse grained (sand and shell hash) down to �400m waterdepth (Fig. 6). The coarse-grained nature of these coresinhibit determination of accumulation rates, but the sandlayer in the canyon head was found to have 210Pb activityabove supported levels, which indicates recent accumula-tion (within the last 100 years)(Fig. 7A). This surficial sandlayer unconformably overlies a stiff, consolidated graymud with supported levels of 210Pb activity, indicating thatthe basal muds are older than 100 years. This distinctlayering of coarse sediment overlying consolidated mudsbecomes less recognizable down the thalweg until it

disappears at 300–400m. At this point, there is a distinctswitch to soft, unconsolidated muds overlying coarsermaterial in the thalweg region (Fig. 7B). This patternextends through portions of the thalweg to the base of thestudy area (Fig. 7C).The distinct mud layer (4–22 cm in thickness) is primarily

contained within the thalweg (Fig. 7C). It is characterizedby: (1) very fine grain size (primarily silty clays, coarseningslightly down canyon), (2) low dry bulk density (average�0.75 g/cm3), (3) a lack of sedimentary structures, (4)relatively high 210Pbxs activities (425 dpm/g of clay), and(5) little or no evidence of a down-core decrease in 210Pbxsactivity within the layer.Minimum accumulation rates, determined by assuming

that this layer had to be deposited within one half life (22.3years; based on the lack of down-core decrease in 210Pbactivity), are between 5.0 and 10mm/yr. Many of thethalweg coarse basal layers have 210Pb activity above

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Fig. 7. Example cores from the canyon head and thalweg mud layer (A and B), revealing the extent of the mud layer (C). Core CC2-150 shows an example

from the canyon head, where relatively clean sand unconformably overlies consolidated muds (A). A mud layer, 4–22 cm thick, drapes the thalweg at

�400–780m water depth (the end of the study area) (B and C). This mud layer has high water content and consistent levels of excess 210Pb. It overlies a

sand layer and consolidated mud layer (B).


supported levels, indicating that they are also less than 100years old. One thalweg core, CTM492, reveals consolidatedmud unconformably underlying the sands seen elsewhere atthe base of cores (Fig. 7B).

4.3.2. Northern and southern flanks

Accumulation rates on the flanks are variable, rangingfrom 0 to 44.1mm/yr (Fig. 6). An across-canyon profilefrom transect A–A0 shows a clear asymmetry between thecanyon flanks: significantly more accumulation occurs onthe northern flank than the southern (Fig. 8). Most corescollected on the northern flank have accumulation ratesgreater than 1.5mm/yr (Fig. 3); in contrast, those on thesouthern side are either erosional (as evidenced by thepresence of only consolidated muds or coarse grains) orhave very low accumulation rates (Fig. 6). The cores takenwithin the thalweg have high accumulation rates associatedwith the thalweg mud layer discussed previously (e.g. coreCTM571).

Reoccupation of sites within the canyon showed littleevidence of seasonal variation from March 2004 toFebruary 2005. Within the thalweg mud layer, thicknessesvaried slightly but spatial resolution was too low to know ifthis was simply an artifact of sampling variability. One core

(CTM571a) collected in April 2005, a re-occupation of siteCTM571 from October 2004, did not resemble the thalwegmud layer. However, the ability to exactly reoccupy a site isdifficult in canyons so it is unknown if the absence of themud layer was due to erosion of the previously observedmud layer, or simply due to sampling variability.

4.4. Construction of the sediment budget

In order to determine the relative importance of thewestern GOL in sequestering sediments, a semi-quantita-tive budget was created for the shelf and Cap de Creusupper canyon. The annual amount of sediment depositedwas calculated based on the following equation:

Mt ¼XðAiÞðM i � riÞ,

where Mt is the total sediment mass, Ai is the surface area fora specific region, ri is the average bulk density (based onmodern sediments) for that region, and Mi is the averagesediment thickness for that region. This is defined as:

M i ¼ ðRiÞð1 yearÞ,

where Ri is the mean 210Pb-derived accumulation rate for thatregion. For calculations involving the thalweg mud layer, thethickness used was the average of the mud-layer thicknesses

ARTICLE IN PRESS

Table 1

Shelf budget calculations

Area (m2) Mean accum

ratea (mm/yr)

# of cores

used

Standard

deviation

Mean bulk

density (g/cm3)

Total sediment (kg/yr)

Southwestern shelfb

3.27� 107 1.3 9 0.5 1.0670.15 (4.671.8)� 107

(4.671.8)� 104 ((metric tons sed)/yr)

Northwestern shelfb

3.85� 108 1.6 4 0.5 0.93070.16 (5.772.0)� 108


aMaximum apparent accumulation rates because of uncertainty in the influence of biological mixing on 210Pb profiles.bSee Fig. 5 for shelf regions.

Fig. 8. Transect A–A0 across the canyon at �571m thalweg water depth (see Fig. 2 for transect location). The canyon profile shows X-radiographs of

cores taken from both flanks. Cores from the southern flank show coarse-grained material, consolidated muds, and low accumulation rates. Cores from

the northern flank are finer grained and actively accumulating. Thalweg cores (e.g. CTM 571) show the expected mud layer overlying coarse-grained

material.


in all cores collected in that region. In regions withoutcomplex bathymetry (such as the continental shelf), theaccumulation rates and bulk densities will likely be similarover broad areas. However, within the canyon, where thereare dramatic bathymetric changes over smaller scales, theremay be significant variability at the canyon scale. Mt isdefined for each physiographic region (see inset of Fig. 2),using mean accumulation rates and bulk densities for eachregion individually. A rough estimate of the total amount ofsediment accumulating within Cap de Creus upper canyonand on the western GOL continental shelf is then calculatedby summing the mass of all regions (Tables 1 and 2) andcompared to Rhone River discharge (Tables 3 and 4).

Regions are defined by the depocenters previouslydiscussed in this study. All calculations are based on fine-grained sediment only (this assumes that most coarse-grained material would be re-worked sediment that is notapplicable to this budget). On the shelf, boundaries ofregions are set as the extent of the MSMD (�30–85m

water depth) and the Zone of Bypassing (Fig. 5). Withinthe canyon, the four main regions used in calculations aresimilar to those defined previously: the canyon head, themud layer (in the thalweg), the northern flank, and thesouthern flank (Fig. 2).

5. Discussion

5.1. Shelf sedimentation processes

Grain-size patterns and accumulation rates on the shelfindicate that fine grains are primarily accumulating at mid-shelf depths (30–85m), which is in agreement with previousstudies (Martin et al., 1981; Got and Aloisi, 1990, Certainet al., 2005) (Figs. 4 and 5). Got and Aloisi (1990) describehow across-shelf transport of a benthic nepheloid layer(BNL) can create this feature. Sediments are transportedbeyond wave base, leaving the inner shelf bare of finegrains. Those sediments reaching the outer shelf remain in

ARTICLE IN PRESS

Table 2

Canyon budget calculations

Area (m2) Mean accum.

ratea (mm/yr)

# of cores

used

Standard

deviation

Mean bulk

density (g/cm3)

Total sediment (kg/yr)

North Flankb

3.33� 107 2.0 5 1.4 0.89270.18 (6.074.4)� 107 ((kg sed)/yr)


Area (m2) Mean thickness

(m)

# of cores

used

Standard

deviation

Mean bulk

density (g/cm3)

Total sediment (kg/22 yr)

Mud Layerb

9.57� 106 0.10 10 0.007 0.74270.14 (6.771.4)� 108 ((kg sed)/(22 yr))

(6.771.4)� 105 ((metric tons sed)/22 yr))

(3.070.6)� 104 (metric tons sed)/yr))

aMaximum apparent accumulation rates because of uncertainty in the influence of biological mixing on 210Pb profiles.bSee Fig. 2 for canyon regions: north flank—Region 3, mud layer—Region 2.

Table 3

Comparison of shelf budget data with Rhone discharge

Rhone Sed discharge

(tons/yr)

Southwestern shelfb,c

(% of value)

Northwestern shelfb,c

(% of value)

Total western shelfb,c

(% of value)

Low estimatea 7.4� 106 0.6 7.7 8.3

High estimatea 10.1� 106 0.5 5.6 6.1

Total sediment (assuming Rhone is 94%)

Low estimatea 7.9� 106 0.6 7.2 7.8

High estimatea 10.7� 106 0.4 5.3 5.7

aEstimates from Pont et al. (2002) and Bourrin and Durrieu de Madron (2006).bSee Fig. 5 for shelf regions.cUpper-bound budget values based on maximum apparent accumulation rates.

Table 4

Comparison of canyon budget data with Rhone discharge

Rhone Sed discharge

(tons/yr)

North flankb,c

(% of value)

Mud layerb,c

(% of value)

Upper canyonb,c

(% of value)

Low estimatea 7.4� 106 0.8 0.4 1.2

High estimatea 10.1� 106 0.6 0.3 0.9

Total sediment (assuming Rhone is 94%)

Low estimatea 7.9� 106 0.7 0.4 1.1

High estimatea 10.7� 106 0.5 0.3 0.8

aEstimates from Pont et al. (2002) and Bourrin and Durrieu de Madron (2006).bSee Fig. 2 for canyon regions: north flank—Region 3, mud layer—Region 2.cUpper-bound budget values based on maximum apparent accumulation rates.


suspension and move southwestward in the regionalcurrents, leaving the outer shelf bare of modern sedimentdeposits (Got and Aloisi, 1990). The across-shelf samplingin this study is somewhat limited, but the results supportthis model: coarser sediments are deposited at shallowerdepths (e.g. XX50) but finer sediments are held insuspension until deeper depths (e.g. XX80). There, mudaccumulation rates are elevated and only relict sandswith low mud accumulation rates exist on the outer shelf

(Fig. 4). This pattern of mid-shelf deposition, due to theinability of surface waves to impact the seafloor underincreasing water depths and the lack of mechanisms totransport sediment across the shelf at greater depths,is observed on shelves worldwide, including the US westcoast, the Amazon shelf, and other locations within theMediterranean (Nittrouer and Sternberg, 1981; Got andAloisi, 1990; Nittrouer and Wright, 1994; Palaques et al.,2002).


Along-shelf sedimentation patterns within the westernGOL are more complex (Figs. 3–5). The primary forcing isa regional southward flow that moves along the entireshelf. This flow, which can be enhanced by meanders of theLPC current onto the shelf, transports particles southwardfor a long distance before they accumulate on the bed.However, small-scale flow variations can regulate both thelocation and characteristics of deposits (i.e. faster flowstend to leave coarser grains) (Davies et al., 1995; Pawlakand MacCready, 2002).

The northwestern shelf has a smooth coastline andexperiences a general southward flow throughout the yearwith little flow disturbance. The similarity between thisregion and other parts of the Gulf is reflected by similarsedimentation processes and depositional patterns, i.e. theMSMD (Got and Aloisi, 1990; Courp and Monaco, 1990;Certain et al., 2005).

Near Cap Bear, the seabed is sandier than the northernshelf (36% sand at R55 relative to less than 20% sand atK50) (Fig. 4). Combined with low accumulation rates(o1.0mm/yr) this suggests inhibited deposition in a zoneof likely sediment bypassing. ADCP data recorded nearCap Bear in November 2003 and Cap Creus in February2005 (Fig. 4 inset) capture significant near-bottom currentsmoving towards the southeast, suggesting current accel-eration around the headlands. This acceleration ishypothesized to be caused by the Venturi effect, whichdictates that when a confined flow encounters a restriction,it must speed up to compensate for reduced pressurecreated by the formation of a pressure gradient behind therestriction (i.e. acceleration due to conservation of massthrough a constriction). In the western GOL, the flowrestrictions are Cap Bear and Cap de Creus. The offshoreflow boundary (which is required for the Venturi effect tobe valid) is likely the intrusion of the LPC current on theouter shelf. Analogous acceleration around a headland hasbeen documented by Geyer and Signell (1990) and Geyer(1993). The variability of these strong flows around CapBear and Cap de Creus is poorly known. However, thegeographic correlation between the seabed data andcurrent information suggests that this acceleration aroundthe headland likely controls the deposition of finesediments.

The increase in accumulation rates (2.5mm/yr at U50)and fine-grain sediment (o25% sand at R55) on thesouthwestern shelf suggest that flow separation (associatedwith current deflection around Cap Bear) may allow thedevelopment of an eddy that causes increased depositionsouth of the cape. Such eddies were observed during theFebruary 2005 ADCP survey (Fig. 4 inset). Geyer (1993)documented the creation of eddies on the down-flow side ofcoastal headlands on a smaller scale, while Davies et al.(1995) modeled eddy formation at the lee side of a cape(assuming flows were sufficiently fast) at larger scales. Thistype of separation has been shown to erode fine sedimentsand move sands in shallow water (o20m), suggesting thatthey can have significant impacts on the seabed (Signell and

Harris, 2000). In deeper water, the strength of the eddy onthe bottom is reduced, allowing deposition. The conceptualmodel presented here is that the coastal current is deflectedby Cap Bear, part of the flow moves off the shelf aroundCap de Creus, while another part circulates as an eddy onthe southwestern shelf. It is hypothesized that such eddiesenhance accumulation near their center, and may beresponsible for enhanced rates on the southwestern shelf.Seismic profiles also show a ‘‘mound’’ at this location,which points to similar processes operating over theHolocene highstand (Ercilla et al., 1995).Near Cap de Creus Canyon, accumulation rates and

grain-size distributions on the northern and southern rimare drastically different (Figs. 5 and 6). Northern rim coreswere composed of a muddy, fine sand (35% mud) and hadappreciable steady-state accumulation rates (1.6mm/yr atCFM138; Fig. 3), indicating that sediment reaches this arearegularly over decadal timescales. In contrast, the southernrim is composed of very coarse sediments (gravel up to4 cm at J01) and consolidated material, which is indicativeof currents scouring the shelf and preventing fine sedimentdeposition. This comparison suggests that the north rimrepresents a potential sediment pathway to the uppercanyon; in contrast, the southern rim is an area of sedimentbypassing that may provide a pathway to the deeper partsof the canyon.

5.2. Canyon asymmetry

Within Cap de Creus Canyon, each flank is classified asdepositional or non-depositional with respect to finesediment. The northern flank has relatively high accumula-tion rates (up to 4.1mm/yr) and fine-grain sizes (primarilysilty clays), indicating an area of fine-sediment depositionover a 100-year timescale (Figs. 6 and 8). The southernflank, however, has erosional features (i.e. CFM560), lowaccumulation rates (0.5mm/yr at CFM500), and thepresence of gravel (J01), suggesting that it is an area ofbypassing. As such, it is classified as non-depositional forfine grains on a 100-year timescale.Enhanced deposition of mud on the northern flank

suggests that fine sediments are entering Cap de CreusCanyon from the northern rim (Fig. 6). The significantasymmetry of accumulation rates within the canyon (highrates on the northern flank, lower rates or erosion on thesouthern), as well as modern sediment accumulation on therim adjacent to the northern flank (1.6mm/yr at siteCFM138), provides evidence of this preferential deposi-tion, as well as a source for the sediment. It is hypothesizedthat the south-flowing regional currents transport the shelfBNL over the canyon rim, where it splits into a canyonBNL and an intermediate nepheloid layer (INL), supplyingsediment to the northern (and southern) flank.The strongest evidence for this conceptual model is

mooring data collected within the canyon. Current metersat 200 and 500m (both 5mab) in the canyon show adistinct southward flow (maximum speed of �50 cm/s)

ARTICLE IN PRESS

Fig. 9. Current meter readings from 2004 to 2005 within Cap de Creus Canyon (unfiltered, recorded every 20min; Puig et al., this issue). Currents flow to

the SE in the upper canyon, where the thalweg is narrow. However, deeper within the canyon, currents travel directly east/northeast at speeds up to

80 cm/s at 5mab, suggesting that these currents may be capable of eroding sediments. When the currents turn east/northeast, water temperature decreases

(shown by grayscale change in polar plots). These currents are caused by dense-water cascading off the southern rim of Cap de Creus Canyon, likely

causing the significant asymmetry in accumulation within the canyon. The current meter at 500m water depth also shows a distinct flow to the south,

confirming the likelihood of hemipelagic sediment transport from the northern rim that can supply sediment to the canyon.


within the canyon (Fig. 9). These southward flows haveappreciable suspended-sediment concentrations (generally�5mg/l), indicating that they are probable sources ofsediment to the canyon (Puig et al., this issue). Durrieu deMadron and Panouse (1996) and Palanques et al. (2006)have reported the presence of a well defined BNL(concentrations, �1mg/l) extending to the shelf break.Nepheloid-layer advection of sediment into Lacaze-Duthiers Canyon (located directly to the northeast ofCap de Creus Canyon) and Grand-Rhone canyon also hasbeen observed, indicating this type of transport is active inthe GOL (Durrieu de Madron et al., 1990; Durrieu deMadron et al., 1999; Frignani et al., 2002).

Advection of nepheloid layers off the adjoining shelf hasbeen seen on other margins, such as Quinault canyon onthe Washington coast (Hickey et al., 1986; Baker andHickey, 1986; Carson et al., 1986; Snyder and Carson,

1986). Similar to Quinault canyon, it is hypothesized thatas the regional currents move over the Cap de CreusCanyon, the increase in depth and deflection of the isobarscause them to slow, allowing enhanced deposition relativeto the shelf (Carson et al., 1986; Hickey et al., 1986). Whilethe specific dynamics of sediment movement vary betweenmargins, nepheloid-layer advection facilitating depositionin canyons is a well-documented process.If advective transport of nepheloid layers were the only

process occurring within Cap de Creus Canyon, one wouldexpect fine-grained deposition across the entire canyon,including the southern flank. However, data from thesouthern flank indicate that sediments were generallyconsolidated muds or coarse-grained material devoid ofmodern sediment. These types of deposits are generallyrepresentative of a high-energy environment, not hemi-pelagic sedimentation from nepheloid layers. In addition,


data collected within the canyon at 750m water depth (and toa lesser degree at the 500m site) show currents that flow dueeast (along the thalweg) with frequent and significant cold-water currents (up to 80 cm/s) directed towards the northeast(Fig. 9). These are opposite to the southward-flowing across-canyon currents observed at shallower depths in the canyon.These data point to fundamentally different environmentalconditions on the southern flank, which (1) prohibit sedimentdeposition in this part of the canyon, and/or (2) removesrecent sediment after it is deposited.

Mooring data from within the canyon and modelingwork show that such strong currents occur on the southernrim and flank of the canyon during the cascading of cold,dense water off the GOL continental shelf (Fig. 9)(Palanques et al., 2006; Ulses et al., 2008). These mooringdata clearly indicate intense cascading events in Cap deCreus Canyon: from January to April, a distinct correla-tion (particularly at the 750-m mooring site) between fastcurrents (up to 80 cm/s) and decreased temperature(�10–12 1C, relative to the 13.2 1C in the ambient slopewater; Fig. 9) was recorded, which is consistent withcharacteristics of dense-water formation (Durrieu deMadron et al., 2005, Puig et al., this issue). Previoushydrographic studies have identified the western Gulf as aprimary location of annual dense-water formation andcascading off the shelf (Millot, 1990; Durrieu de Madronet al., 2005; Canals et al., 2006). Recent work has suggestedthat after formation, much of this dense water is pushedsouthward along the shelf by Tramontane winds and thegeneral current regime within the Gulf (Ulses et al., 2008;Bourrin et al., this issue). Once it encounters Cap de Creusheadland, the flow is impeded and the dense water beginsto pool on the outer southwestern shelf until it spills intoCap de Creus Canyon from the southern rim. These near-bottom flows inhibit sediment deposition and erodesediments that may have been deposited on the southernflank. This conceptual model is consistent with theerosional seabed observed on the southern rim (Figs. 4and 5).

Moorings also recorded a strong correlation betweendense-water flows and elevated suspended-sediment con-centrations (up to 40mg/l at 5mab), suggesting that theseflows are able to move large amounts of sediment into andthrough the upper canyon (Puig et al., this issue). Canals etal. (2006) found that cascading events can be strongenough to resuspend sand (shear stresses �0.7N/m2) andare thought to be responsible for creating over-consoli-dated furrows on deep portions of the southern flank.Moreover, because the measured currents and concentra-tions are still very large at 750-m water depth, sedimentsmust be carried past the upper canyon and depositeddeeper in the canyon, seaward of the present study area(Fig. 9). These data combined with the seabed observationsindicate that large amounts of sediment are moving off theshelf and through the upper canyon, making Cap de CreusCanyon an important conduit for sediment export from theGOL shelf.

5.3. Canyon Thalweg processes

There are two regions within the canyon thalweg thatcan be distinguished by surface sediment characteristics.The mid-depth thalweg (400–780m) during March2004–January 2005 was characterized by the presence ofa mud layer, which has uniform excess 210Pb activity,overlying sand with detectable excess 210Pb activity. Thisindicates rapid sediment accumulation in this region(4–22 cm in less than one half life, 22 years) (Fig. 7B). Incontrast, the canyon head (o400m water depth) containssurface sediments composed of recent sands with negligiblefine sediment (generally 490% sand). Therefore, thecanyon head is classified as non-depositional for modernmuds. It is, however, likely an area of coarse-grainedtransport and deposition.The mud layer that overlies the thalweg sands at�400–780m water depth is indicative of rapid, non-steadystate deposition within the canyon (Fig. 7). One of the mostdistinguishing characteristics of this layer is the minimalamount of 210Pbxs decay with depth in the core. If thisdeposit had accumulated slowly over time (even over onehalf life), the activity at the base of the layer should be onlyhalf of the surface activity (Fig. 7B). Rather, there is adistinct break between relatively constant excess activitylevels and the underlying supported levels. This suggeststhat the layer was deposited very rapidly (i.e. non-steadystate) (Mullenbach, 2002). A conservative estimate foraccumulation rates can be calculated assuming the entiredeposit was laid down within one half life (over 22.3 years).This estimate gives accumulation rates up to 1.0 cm/yr forthe mud layer. However, the low bulk density throughoutthe layer also suggests that the layer was deposited in atime frame much shorter than the half life.The lack of sedimentary structure within this mud layer

opens the possibility that these mud deposits haveaccumulated steadily over a 100-year timescale, but areso biologically mixed that they appear to be episodic layers.If the initial 210Pbxs activity of sediment reaching theseabed is consistent across the canyon, then an estimate ofthe excess activity of a biologically homogenized sedimentlayer can be calculated using steady-state profiles from thenorthern flanks. The assumption is that the thalwegdeposits could have looked similar to the northern-flankcores prior to homogenization (e.g. steady-state deposi-tion). This calculation shows that 210Pbxs activities wouldbe lower (10–18 dpm/g of clay) throughout the homo-genized layer (if biological mixing were to produce theuniform activity with depth) than is actually found in thethalweg mud layer (�25 dpm/g of clay). This suggests thatthe mud layer is likely not the result of biological mixing ofsteady-state deposits.The high excess 210Pb activities recorded in both the

sands and mud layer, combined with the abrupt shift tosupported 210Pb activities at depth (CTM492, Fig. 7B)indicate that the mud layer is periodically flushed (i.e. thereis a loss of stratigraphic time in the sediment column).

ARTICLE IN PRESS

Fig. 10. General processes affecting Cap de Creus Canyon as defined by

core data. The dominant processes are along-shelf movement of sediment

to create mid-shelf mud deposits, acceleration of currents around Cap

Bear to create a zone of bypassing, advection of nepheloid layers from the

northern flank of the canyon, dense-water cascading onto the southern

flank and scouring sediments, and possible entrance of coarse sediments

down the canyon head.


Probable mechanisms are (1) dense-water cascadingthrough the thalweg that erodes the mud layer or (2)gravity-driven sediment flows down the main thalweg.While neither can be completely discounted, results fromprevious studies and this work strongly point to dense-water cascading as the most likely mechanism for flushingthe mud layer. Monitoring of Lacaze-Duthiers Canyon(located NE of Cap de Creus Canyon) since 1993 hasshown that dense-water cascading occurs annually, withsome variation in the magnitude of events (Canals et al.,2006). Near-bottom flows driven by cold-water densityanomalies appear to be an annual event in the canyons ofthe western GOL; in contrast, there was no evidence offlows driven by sediment-induced density anomalies (i.e.turbidity currents) during this record. Cascading eventsalso can induce strong shear stresses (�0.7N/m2) that arecapable of eroding sand and can be sustained for days(Canals et al., 2006); this suggests they are competentenough to flush the unconsolidated mud layer. Because thedense-water cascades in Cap de Creus Canyon have beenshown to carry significant suspended-sediment loads (Puiget al., this issue), it is reasonable to assume that sedimentcan be deposited following their passage. In addition,sediment-trap data from Cap de Creus Canyon at 500-mwater depth (5mab; Fig. 9) indicate that sand can betransported in suspension during cascading events (Puiget al., this issue). This suggests that the cascades canremove unconsolidated material and transport/emplace thecoarse-grained material followed by a new ephemeral mudlayer. All of these data point to dense-water cascading asan important mechanism for removal and formation of theephemeral mud layer. Regardless of the supply andflushing mechanisms, the rapid deposition and removal isgood evidence that significant amounts of material aremoving through Cap de Creus Canyon over short (annualto decadal) timescales.

Based on the prominence of sand in the canyon head andits general confinement within the thalweg, the canyonhead is the most probable site of entry for these coarsegrains. It is hypothesized that dense-water cascading also isresponsible for depositing sand in the canyon head. Inaddition to observations of coarse material being trans-ported in the dense water, Gaudin et al. (2006) foundsimilar sandy deposits in the head of the Boucart (Aude)Canyon (next canyon east of Lacaze-Duthiers Canyon),which were attributed to reworked sand deposited bydense-water flows. In summary, the dense-water cascadingappears to flush material down canyon and leave a rapidlyemplaced deposit in the canyon thalweg.

5.4. Sediment budgets

Budget calculations for the shelf reveal that(57720)� 104metric tons of sediment are accumulatingeach year on the northwestern shelf and (4.671.8)� 104 tons accumulate yearly on the southwestern shelf(Fig. 10, Table 1). Within the Cap de Creus Canyon, the

canyon head (o400m water depth) and most of thesouthern flank were classified as non-depositional areasand were given values of 0 tons of sediment (consideringfine-sediment accumulation only). The northern flank wasclassified as a depositional area and was calculated toaccrue (6.074.4)� 104 tons of sediment annually (Table 2).The thalweg (from 400 to 780m water depth), consideringonly the mud layer, was found to have (67714)� 104 tonsof sediment (Table 1). Because it is only definitively knownthat this layer accumulated in less than 22.3 years, for thepurpose of this annual budget, the total mass will bedivided by the half life to determine an average annualinput value. Using this method, a minimum value of(3.070.60)� 104 tons of sediment is deposited annually inthis layer. In total, an average of (9.074.4)� 104 tons ofsediment are annually deposited in Cap de Creus uppercanyon.Comparing these values with the total sediment input to

the GOL provides a better understanding for the impor-tance of the western region. Because good discharge data


were not available for the western rivers, only the RhoneRiver sediment influx (7.4–10.1� 106 tons/yr) was used toestimate the total influx to the Gulf, assuming the Rhonemakes up 94% of the total (Pont et al., 2002; Bourrin andDurrieu de Madron, 2006). It is also important to reiteratethat the sediment mass estimates for the canyon are basedon maximum apparent accumulation rates; thereforebudget values must be assumed to be upper-boundestimates. Based on the range of sediment input valuesto the GOL, this study indicates that 6.1–8.3% of theRhone input (5.7–7.8% of the total sediment influx) issequestered on the western portion of the GOL shelf eachyear (Table 3). This verifies that there is significantsediment transport to and accumulation in the westernportion of the GOL continental shelf, especially whenconsidering the large distance from the primary sedimentsource.

This study also indicates that 0.9–1.2% of the Rhoneinput (0.8–1.1% of the total sediment input) is sequesteredin upper Cap de Creus Canyon each year. This is anaverage over a 100-year timescale and does not account forshorter-term variability (Table 4). While these numbersseem small, it is important to note that they only accountfor what is currently deposited within the thalweg andnorthern flank, and do not include sediment that passesrapidly through the canyon (e.g. from periodic flushing bydense-water cascading). Based on the high SSCs recordedby mooring instruments (Puig et al., this issue) andmodeling results (Ulses et al., this issue, Ferre et al., thisissue), it is suggested that much more sediment (few� 106 tons based on model results) actually passes throughthe canyon annually, but is not included in the sedimentbudget for the upper canyon. Studies by Drexler et al.(2008) at locations deeper in the canyon help to constrainthe total amount of sediment moving through Cap deCreus Canyon.

5.5. Implications for sediment export during sea-level

highstands

Cap de Creus Canyon appears to be an importantconduit for the transport of sediment to the deeper canyonand slope based on seabed data from this study and water-column data from Puig et al. (this issue). Little sediment isaccumulating on 100-year timescales within the uppercanyon, but significant amounts of sediment appear to bepassing through the canyon due to oceanographic (dense-water cascading, current interactions with coastal mor-phology/bathymetry) and geologic (narrow shelf, canyonincision) conditions. Seabed and mooring data indicatescouring/sediment bypassing on the southern flank andperiodic flushing of an ephemeral mud layer in the thalweg,likely the result of sediment-laden cascading events (Figs. 6and 9) (Puig et al., this issue). These data support previousstudies that clearly showed high sediment fluxes throughCap de Creus Canyon, relative to the eastern canyons.Specifically, Palanques et al. (2006) and Ulses et al. (this

issue) found that sediment fluxes were an order ofmagnitude higher in Cap de Creus than any other canyon.Cap de Creus Canyon is ideally situated to move sedimentoff the continental shelf, despite its presence on a passivemargin during a sea-level highstand.This margin has implications for the study of sediment

export from passive margins during sea-level highstands.The assumption that sediments are primarily trapped onbroad shelves can be complicated by oceanographicconditions facilitating the movement of sediment overspecific pathways. As such, an understanding of theseconditions also affects our interpretation of depositssuspected of occurring during sea-level lowstands, whichmay have been influenced by a combination of factorsallowing off-shelf sediment export during high stands insea level.

6. Conclusions

(1)
Accumulation rates and grain-size patterns in thewestern GOL reveal distinct depocenters on the shelf,similar to those defined in previous research. Surficialgrain sizes show a sharp transition from mud at mid-water depths (30–85m) to sand near the slope andcanyon head. Primary deposition on the shelf occurs atmid-shelf depths on the MSMD; however, these datashow a discontinuity in the MSMD, with two primarydepocenters separated by a zone of bypassing. De-creased accumulation in the bypassing zone is appar-ently due to acceleration around a headland, pointingto the importance of the interaction between regionalflow and coastal morphology in longer-term sedimentaccumulation.
(2)
Within Cap de Creus Canyon, distinct grain size andaccumulation patterns clearly show localized zones inthe canyon that experience different transport processesand strata formation. Accumulation rates are asym-metric across the canyon, with higher accumulationrates on the northern flank compared to little or noaccumulation on the southern flank. Grain-size dis-tributions have even more dramatic asymmetry, withfiner grain sizes on northern flanks and adjacent shelfcompared to gravel and sand to the south. A sharptransition in surficial grain size also occurs in the mainthalweg, with sandy deposits unconformably overlyingconsolidated mud in the upper canyon (o300m), andunconsolidated muddy layers unconformably overlyingsand in the mid canyon (400–700m). 210Pb data fromthe mud layer indicate non-steady state accumulationover decadal timescales, which suggests rapid deposi-tion and periodic flushing. Core data show that part ofthe canyon is acting as a long-term depocenter (north-ern flank), whereas other parts appear to be moreephemeral (thalweg muds).
(3)
These seabed data, along with time-series data withinthe canyon, suggest the following off-shelf transportmechanisms: (a) sediment is preferentially supplied to

ARTICLE IN PRESSA.L. DeGeest et al. / Continental Shelf Research 28 (2008) 2031–20472046

the canyon from the shelf/slope adjacent to northernflanks via nepheloid transport by the regional current,and (b) strong currents are scouring fines from thesouthern canyon flanks due to swift dense-watercascading from the southern canyon head, and thisprocess carries sediment to deeper parts of the canyon.

(4)
Despite the significant distance from the primary fluvialsource and the passive margin, there is sedimentescaping from the shelf via Cap de Creus Canyon.However, the volumes are difficult to quantify due tothe suspected volume of sediment bypassing throughthe canyon and lack of information on sediment inputentering the GOL from the smaller rivers. It isestimated that �1% of the Rhone River sedimentbudget is accumulating in the upper canyon over 100-year timescales.
Overall, these results suggest that regional geology andoceanographic setting must both be considered whendetermining off-shelf sedimentation patterns.

Acknowledgments

We would like to thank the Office of Naval Research forfunding this work (award numbers N00014-04-1-0082,N00014-99-1-0028 and N00014-04-1-0379). We also thankthe EuroSTRATAFORM participants for their help in thefield and for their insightful discussions throughout thecourse of the program that have greatly contributed to thisresearch. We also thank the anonymous reviewers andeditor for suggestions that significantly improved the finalversion of this paper.

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Sediment accumulation in the western Gulf of Lions, France ... et al (CSR 2008).pdf(Cap Bear). Estimates of a sediment budget indicate that 6–8% of the sediment input to the Gulf

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