Submarine canyons are narrow but deepsubmarine valleys that extend for hundreds ofmeters.They represent the most impressivestructures that shape the present morphologyof passive continental margins.They can occuroff the mouth of rivers: the Tagus,Zaire,Amazon,and Orinoco in the Atlantic; the Indus in the

Indian Ocean; and the Var,Rhone, and Ebro inthe Mediterranean. Some are at times discon-nected from any stream mouth such as theNazaré canyon,off Portugal,despite the fact thatit is close to the coast. Some were connectedto a river mouth during lowstands of sea level,such as the Wilmington canyon in the north-west Atlantic, or the Blackmud canyon in thenortheast Atlantic.

This article discusses the morphology andrecent activity of the Capbreton Canyon offthe coast of France and Spain, and discussessome theories about its formation.

The margin of the Bay of Biscay is incisedby numerous canyons, but Capbreton Canyonis a unique feature due to its morphology andlocation [Cirac et al., 2001].The canyon ofCapbreton is a 300-km-long meandering sub-marine structure running approximately parallelto the north coast of Spain.The canyon headincises deeply into the continental shelf and islocated only 250 m away from the coastline ata water depth of 30 m.Along the Spanish shelf,the south wall reaches up to 3000 m at 133km from the canyon head [Shepard and Dill,1966], making this canyon the deepest onEarth. Recent surveys [Cirac et al., 2001] willhelp improve our understanding of the mor-phology.

The seismic data base used in this articleincludes ten seismic lines representing a totallength of 60 km.The canyon is 32 km wide at

Understanding Continent-OceanSediment Transfer

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Fig.1.The bathymorphology and the three-dimensional view of the canyon of Capbreton.Processing of EM1000 and EM300 data sets werepreformed by Ifremer/Direction des Recherches Océaniques/Géosciences Marines from the Itsas 1 (1998), Itsas2 (2001), Itsas5 (2001),and Itsas6(2002) cruises.Contours are in meters; isobaths are at 50-m intervals. Arrow on the three-dimensional view indicates north.

BY T. MULDER, P. CIRAC, M. GAUDIN, J.-F. BOURILLET,J.TRANIER,A. NORMAND, O.WEBER, R. GRIBOULARD,J.-M. JOUANNEAU, P. ANSCHUTZ,AND F. J. JORISSEN

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its widest point and has a typical V-shapedsection.The average slope of the thalweg is1.3% for the first 90 km.The canyon was directlyconnected with the Adour River until 1578A.D. and was then artificially disconnected.The Adour River mouth is now located 15 kmsouth of the canyon head.The upper part ofthe canyon is globally east-west, but it is com-prised of several segments (Figure 1). Maindirections of the segments are east-northeast-west-southwest and southeast-northwest fol-lowing regional lineaments that developedduring the opening of the Bay of Biscay andthe Pyrenean thrust [Boillot et al., 1972].Thelower part of the canyon is south-north andmerges with the deep-sea fan of Cap-Ferret.

Canyon Morphology and Terraces

The canyon clearly shows active and aban-doned meanders associated with terraces.Ter-races are flat structures that stand up to severaltens of meters above the thalweg.The bathy-metric map and seismic profiles through thecanyon clearly show the presence of superposedterraces,suggesting that the canyon could formby several phases of incision.All of the terraces,whatever the type,are draped by stratified faciesthat resulted from the spilling of the top ofthe turbidity currents.The finest particlestransported in the top part of the turbidity current settle and form the stratified facies.

Tranier [2002] described three types of ter-races.The horseshoe terraces that form whenan abandoned meander of the thalweg isfilled is the first, and the terraces limited by atopographic high with a relief a few metersthick bordering the terrace in the thalweg direc-tion is the second (Figure 2). In some cases,onlap structures on the topographic high sug-gest this topographic high is older than thestratified facies that form the filling of the ter-race. In addition, the seismic facies inside thiskind of terrace are chaotic at the base and

layered at the top,suggesting that these terracesare initially due to slump failures.Turbiditycurrents moving into the canyon spill over theinitial slump deposit.This kind of terrace isconfined by the canyon walls,and correspondstypically to nested levees.When no onlap isvisible, the topographic high could only bethe location of the highest rate of depositionby turbidity current spilling along the side ofthe thalweg.

The third type identified by Tranier is a flatterrace showing only the layered seismic facies.They act as a levee that forms by lateral migra-tion of the thalweg and the associated levees.This kind of terrace is located on the convex(erosional) side of meanders.As a consequence,they could form through erosion of previousdeposits, rather than through depositionalprocesses as terraces with a topographic high.

Terraces could also be interpreted as pointbars that form through lateral accretion; forexample, sub-aerial fluvial systems [Abreu etal., 2003]. However, dipping foresets inside ter-races are infrequently observed, but could bemasked by seismic hyperbols. However, thefine material usually cored in the canyon ter-races suggests another method of formation.

Canyon Activity

Eleven interface gravity cores were recoveredwithin the thalweg of the Capbreton Canyonranging from 40 to 800 m water depth.Thesecores are composed of turbidites or debrites,with 75% of the sediment made of silt orcoarser-grained material.

In 2000,a 33-cm-long Barnett interface gravitycore was taken in the thalweg of the canyonof Capbreton at a water depth of 647 m (Fig-ure 1). According to facies interpretation (Fig-ure 3), it shows a succession of sedimentaryfacies that can be interpreted as three super-imposed turbidite sequences (Figure 3) [Mulderet al., 2001].An oxidized layer that contains

live benthic foraminiferal faunas indicative ofa reprisal of hemipelagic deposition coversthe topmost turbidite sequence.Activities of234Th (half-life = 24.1 days) in excess suggestthat the most recent turbidite was depositedbetween 5 December 1999 and 14 January2000.A diagenetic model based on manganeseoxide distribution also agrees with this assump-tion [Anschutz et al., 2002]. No earthquakecapable of generating a slope failure in thearea has been recorded during this period.There was no major flood at the mouth of theAdour River,which is the nearest river.The onlynatural phenomenon with the potential totrigger significant sediment motion,and finally,a turbidity current, during this period was theviolent “Martin”storm that affected the Bay ofBiscay on 27 December 1999.

The turbidity current could be caused bythree phenomena.The first is a sediment fail-ure due to excess pore pressure generated by the storm waves or the swell. During theDecember 1999 storm, waves reached 12 m in the Bay of Biscay. In that case, the shearstrength of the sediment could be reduceddown to failure.The second is an intensifica-tion of the coastal drift and shelf currents dur-ing the storm.A large amount of particles canthen be captured at the canyon head andprogressively transformed into a turbulent,unsteady flow.The third is dissipation throughthe canyon of the 1- to 2-m-high along-coaststorm set-up through the canyon.The resultingdownwelling currents may cause intensiveparticle erosion from the canyon head anddown the canyon that generated a particle-laden bottom layer that progressively transformedinto turbidity current.This recent turbiditeshows that active gravity processes exist in thecanyon at present. However, the calculatedfrequency for events similar to the 1999turbidite, which have a periodicity of oneevent every decade, suggests that such eventscan help maintain the freshness of the canyonwall and thalweg,the mature phase of a canyon[Farre et al.,1983]; but they are neither frequentnor energetic enough to explain the initialincision.

Canyon Formation

The cause of canyon formation and persist-ence is a key question, as canyons are themain pathways for sediments from continentto ocean. It has been proposed that most ofthe Mediterranean canyons began to form bysub-aerial river erosion during the drastic fallin sea level that occurred during the Messinian.For canyons located in the Atlantic Ocean,this assumption would suggest an origin dur-ing the phases of rifting and ocean opening;that is, the Cretaceous for the north Atlantic,andthe Jurassic for the south Atlantic.

A second hypothesis is autocyclic origin,with processes such as sediment failure to setup an initial depression followed by retrogres-sive erosion to maintain, extend, and enlargethe canyon.The canyon head would moveprogressively landward.This process fits thenumerous slump scars that are observed on

Fig.2.This sparker seismic profile transverse to the canyon shows superposed terraces,both ofwhich have a topographic high (TH) (See Figure 1 for location).The source used for this profilewas a 12–1500 Joules SIG® sparker.The resolution of sparker seismic profiles is about 1 mwith a penetration of 500 ms.Data were post-processed with the “SITHERE”software developedat IFREMER.

both sides of the canyon walls.Additional evi-dence supporting this hypothesis is the abun-dance of pockmarks lined up with the headof valleys in a landward direction on the southside (Figure 1).These pockmarks suggestupward fluid motion and deep disorganiza-tion of the sedimentary pile.They could eitherbe the result of sediment disorganization inthe direction of retrogression, or indicateweakness lineaments that direct erosion andretrogression.This would be consistent withthe fact that canyons such as Capbreton followregional, tectonic directions [Cirac et al., 2001].

In some cases, both sub-aerial and subma-rine processes can contribute to the presentcanyon morphology.The major canyon of thesubmarine drainage basin of “La Petite Sole”in the northern part of the Bay of Biscay showsa downstream part linked to sub-aerial erosionduring the synrift period, when the tributariesof the upper part were controlled by autocyclicslides during the set-up of the Neogen prograd-ing prism [Bourillet et al., 2003].

Another hypothesis suggests that canyonscould be “constructed structures.” In fact, theywould be “bypass”areas in a globally prograd-ing margin.They would represent the narrowareas where margins do not prograde [Pratsonet al., 1994] or prograde at a lesser rate.Thishypothesis can be considered as valid formargins with a high sediment load, which was the case for the south part of the Bay of

Biscay during the Tertiary, but not during theHolocene.

A fourth hypothesis suggests that canyonswould result from intense and nearly continu-ous erosion by downslope eroding sedimentflows [Pratson et al., 1994]. Frequent in-situmeasurements of a sporadic or continuousactivity of particle-laden flows in canyons[Shepard and Dill, 1966; Inman, 1970] supportthis hypothesis. Because major canyons areusually connected to a river, the canyon headis located in an area where sediment loadand sedimentation rate are very high.Thissuggests that canyons are formed by eithervery frequent, slide-induced turbulent surges,or by long-duration, hyperpycnal flows.Theseflows form during floods when the river flowcarries sufficient suspended load.This hypoth-esis is consistent with the recent history of thecanyon of Capbreton that was connected tothe Adour River until 1578 A.D. The Adour is a small- to medium-sized river with a highcapacity to trigger hyperpycnal flows,as definedby Mulder and Syvitski [1995]. During Quater-nary ice-melting periods, the water and sedi-ment discharge from the Adour was probablysubstantially higher than during the Holocene,draining the northwestern part of the Pyreneanpiedmont supplied by the glaciers.

The Capbreton example shows that canyonactivity is not permanent through geologicaltime. Periods of high sediment transfer that

are responsible for the incision alternate withperiods of sporadic activity. During these periods of lesser activity,canyons act as bypasszones on slightly prograding margins with lowsediment load, or as fill on highly progradingmargins with high sediment load. Using astudy on Mediterranean canyons where thecanyon head is located close to the shelfbreak, Baztan et al. [2004] related periods ofincision to the end of sea level fall and sealevel lowstands, during which rivers are con-nected to canyons and periods of filling (orbypassing), to periods when river mouths areclose to the canyon head, but disconnectedfrom the river system.

In conclusion, it appears that canyons repre-sent a negative topography that can be orientedalong tectonic lineaments.The shape of acanyon results from the combination betweenthe global margin progradation beside thecanyon and the frequency, intensity, and per-sistence of erosional processes through thecanyon. Several kinds of gravity processescontribute to canyon formation and persist-ence. Long-duration hyperpycnal flows mightsubstantially contribute to the initial canyonerosion and to the formation of the meander-ing shape. Short-duration, slide-triggered, tur-bulent surges might explain the retrogressiveslump scars that can be observed at canyonheads and the freshness and steepness ofcanyon walls.

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Fig.3. Sedimentology and stratigraphy of the core. (a) X-ray image; (b) processed X-ray image (Scopix) and sedimentary facies; (c) grain size;(d) curve of corrected 234Thexc.activity.Note that 234Thexc.adsorbs on particles with a large specific area such as clay and fine silt.Values of

234Thexc. can only be interpreted in the finest top part of the turbidite.

Some scientists are optimistic about whatthey say will be new opportunities and cloutthat the Earth and space sciences will haveunder the organizational restructuring thatNASA announced on 24 June,while others areconcerned that these sciences may be buriedbureaucratically and suffer fiscally.

Under the restructuring, which takes effecton 1 August, Goddard Space Flight Centerdirector Alphonso Diaz will become the Sci-ence associate administrator.Associate admin-istrator for space science Edward Weiler willbecome the new head of Goddard. Associateadministrator for Earth Science, GhassemAsrar, will be science deputy associate admin-istrator and chief scientist for exploration.

The principle NASA centers focusing on science will be Goddard, the Ames ResearchCenter, and the Jet Propulsion Laboratory. Theagency’s restructuring includes the establish-ment of three other mission directorates: forexploration systems, space operations, andaeronautics research.

NASA is also creating a strategic planningcouncil and taking other steps to improve itsdecision-making process.

NASA administrator Sean O’Keefe said thatthe restructuring would support the re-orien-tation of NASA toward exploration of theMoon, Mars, and other destinations that wasannounced by President George W. Bush on14 January. O’Keefe said the changes also wouldpromote synergy across scientific disciplines.

Reaction in the Earth Sciences Community

Within the Earth sciences community, opin-ions varied about the impact the restructuringwould have on NASA science programs.

Larry Smarr, chairman of the Earth SystemScience and Applications Advisory Committee(ESSAAC), and director of the California Insti-tute for Telecommunications and InformationTechnology, said the restructuring is “a strongendorsement”of the Earth sciences; and thatscience as one of just four agency missiondirectorates will now receive more attention.

“If the exploration initiative sees Earth scienceas a prototype system for space technologyaround the other planets, then the Earth sciencemission is not just off by itself. It becomes acritical success factor for NASA’s missions as awhole.That gives Earth science a much greaterlongevity as a funded part of NASA,”Smarr said.

Charles Kennel, chair of the NASA AdvisoryCouncil,and director of the Scripps Institution,San Diego, said the idea that Earth sciencewill not prosper within an integrated sciencedivision is belied by the fact that it had donewell in the past when Earth and space sciencewere in the same division.

However, he said the restructuring wouldhave important interagency ramifications,because the associate administrator for EarthScience had been at the same administrativelevel as other key government officials associ-ated with federal agencies with large Earthsciences components, and was able to holddirect and regular discussion with his coun-terparts in these other agencies.“The big loss

to Earth science with the diminution of its vis-ibility within NASA is that it won’t have accessat the appropriate levels of government out-side NASA,”he said.

Kennel said the restructuring primarily wasintended to deal with fixing the agency’sproblem with its human space flight mission,and that any affected Earth science programswere “road kill.”He added,“Because of thepolitical softness of Earth science within the[Bush] administration, nobody stood up andsaid,‘No, you can’t do it.’”

Rafael Bras, professor of civil and environ-mental engineering and Earth, atmosphericand planetary Sciences at MIT, and formerESSAAC chair, said,“I worry about the dangerof the Earth sciences getting lost in the myriadof activities of a large science directorate.There will certainly be the possibility of dam-aging internal competition for increasinglylimited resources, although theoretically theopportunity of beneficial synergism exists.”

ESSAAC member Timothy Killeen, directorof the National Center for AtmosphericResearch (NCAR) and AGU president-elect,said that while the new agency mission iscompelling and organizational changes lookgood, the question is how the program ismanaged. He said “potential distortions”withthe mission could undercut NASA’s long-termcommitment to Earth science, and might alsohave an impact on some space science pro-grams not directly related to that mission.

Killeen said that “it is a somewhat second-order derivative motivation to study the Earth[in the new mission] than it has been in thepast, and that is a great concern.”He said sci-entists should pay attention to how the plan

Eos,Vol. 85, No. 27, 6 July 2004

These processes can be active on differenttime periods (e.g., lowstands, highstands ofsea level) controlled by global tectonics oreustatism, and can be active simultaneouslyand depend on the regional setting.In all cases,canyons such as Capbreton represent animportant pathway for sediment transfer fromcontinent to ocean, either directly throughhyperpycnal flows, or with residence timealong the continental shelf through slide-induced surges.

Acknowledgments

We thank the crews of the R/V Côtes de laManche (INSU/CNRS), R/V Thalia (IFREMER),and R/V Suroît (IFREMER) for the survey car-ried out within the framework of the Unité deRecherche Marine 17.We are also indebted toEliane Le Drezen,Alain Normand, and BenoitLoubrieu of the Groupe Cartographie-Traite-ment de données of IFREMER for the multi-beam data processing.This is contributionDGO Université Bordeaux 1/UMR-CNRS EPOCno. 1510.

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Author Information

T. Mulder, P. Cirac, M. Gaudin, J.-F. Bourillet,J.Tranier,A. Normand, O.Weber, R. Griboulard,J.-M. Jouanneau, P.Anschutz, and F. J. Jorissen

For additional information, contact Thierry Mul-der, Université Bordeaux 1, Département de Géolo-gie et Océanographie, UMR CNRS 5805 EPOC, 33405Talence Cedex, France; E-mail: tmulder@epoc.u-bordeaux1.fr

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NASA Restructuring Draws Mixed Reactions