Shelf-to-canyon sediment-transport processes on the Eel continental margin (northern California)

Marine Geology 193 (2003) 129^149 www.elsevier.com/locate/margeo

Shelf-to-canyon sediment-transport processes on the Eel continental margin (northern California) P. Puig
, A.S. Ogston, B.L. Mullenbach, C.A. Nittrouer, R.W. Sternberg School of Oceanography, University of Washington, Box 357940, Seattle, WA 98195, USA Received 18 January 2002; received in revised form 29 October 2002; accepted 1 November 2002

Abstract To investigate the processes by which sediment is supplied to the head of a submarine canyon, an instrumented tripod and a mooring were deployed in the northern thalweg of the Eel Canyon during autumn and winter 1999^2000. This was done as part of the STRATAFORM program, and in combination with a long time-series benthic-tripod data collection on the Eel continental shelf. Sediment-resuspension events on the shelf were forced by waves, and near-bottom suspended-sediment concentrations (SSC) were enhanced during the Eel River flood season. Periodic SSC fluctuations in intermediate waters (corresponding to water depths equal to the shelf-break depth) were predominantly recorded at semidiurnal tidal frequencies, associated with decreases of water salinity and increases of temperature. Within the Eel Canyon, increases of water turbidity were not directly related to the Eel River discharge, but they were linked to the occurrence of storms. This relationship was evident in the bottom-boundary-layer measurements at 120 m depth in the canyon head, although farther down-thalweg (280 m depth), significant increases of near-bottom SSC associated with storm events were recorded also. The highest SSC measured within the canyon coincided with a highly energetic storm on 28 October 1999, in the absence of any river flood event, but associated with a down-canyon density-driven flow. On the shelf at 60 m depth, near-bottom SSC during this storm event reached extremely high concentrations ( s 10 g l31 ), characteristic of fluid-mud suspensions. The across-shelf sediment transport near the bottom showed a persistent off-shelf direction through the entire recording period, while the alongshelf transport fluctuated in direction, but resulted in net transport toward the Eel Canyon head. Within the canyon, near-bottom sediment fluxes were continuously directed down-canyon, while the across-canyon flux was negligible. Sediment fluxes through intermediate slope waters (above the canyon rims) were directed toward the north, following the orientation of the adjacent shelf-break. Results from this field study have identified some of the major processes controlling the off-shelf sediment export in the Eel continental margin, and corroborate previous findings that a substantial portion of the Eel River sediment discharged on the shelf can be exported into the Eel submarine canyon. 6 2002 Elsevier Science B.V. All rights reserved. Keywords: continental shelf; submarine canyon; sediment transport; bottom-boundary layer; Eel Canyon

1. Introduction * Corresponding author. Present address: Institut de Cie'ncies del Mar (CSIC), Passeig Mar|¤tim de la Barceloneta, 37-49, 08003 Barcelona, Spain. Tel.: +34-93-2309518; Fax: +34-93-2309555. E-mail address: [email protected] (P. Puig).

The dynamics and the fate of sediment transferred from the continents to the marine environment have been studied throughout the world

0025-3227 / 02 / $ ^ see front matter 6 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 ( 0 2 ) 0 0 6 4 1 - 2

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during the last few decades. Integrated research projects have been carried out to describe and quantify o¡-shelf sediment export on various continental margins (e.g. Carson et al., 1986; Walsh et al., 1988; Monaco et al., 1990; Biscaye et al., 1994; van Weering et al., 1998). Recently, as part of the STRATA FORMation on Margins (STRATAFORM) program (Nittrouer and Kravitz, 1996; Nittrouer, 1999), the Eel continental margin in northern California was chosen as one of the two ‘natural laboratories’ to examine how modern sediment-transport processes in£uence the formation of the stratigraphic record on continental margins at a wide range of temporal and spatial scales. A primary goal of this program was to identify the principal mechanisms controlling the shelf^slope sediment exchange, and to investigate the role of the Eel Canyon as a preferential pathway for exporting sediment from the shelf to the deep slope. During Plio^Pleistocene lowstands of sea level, gravity-driven mechanisms (i.e. turbidity currents, debris £ows) dominated sediment transport through submarine canyons, funneling large volumes of terrigenous sediment to deeper parts of the continental margins (Shanmugam et al., 1985). Although Holocene sea-level rise has reduced drastically the supply of coarse-grained sediments to submarine canyons, it is widely recognized that canyons continue to be preferential conduits for the transfer of sediments from the shelf to the deep ocean. Numerous studies indicate that submarine canyons have greater suspended-sediment concentrations (SSC; e.g. Drake and Gorsline, 1973; Gardner, 1989a; Durrieu de Madron, 1994), downward particle £uxes (e.g. Monaco et al., 1990; Puig and Palanques, 1998a; Hung and Chung, 1998) and sediment accumulation rates (e.g. Carpenter et al., 1982; Thorbjarnarson et al., 1986; Sa¤nchez-Cabeza et al., 1999; Schmidt et al., 2001) than the adjacent open-slope areas. However, few studies have provided information about contemporary sedimenttransport processes acting within canyons by means of analysis of combined currents and SSC data. The most complete and recent studies were conducted in Quinault Canyon (Hickey et al., 1986; Baker and Hickey, 1986), Baltimore Can-

yon (Gardner, 1989a,b), Foix Canyon (Puig et al., 2000a,b) and Monterey Canyon (Xu et al., 2002), although none included bottom-boundary-layer measurements and therefore, the ability to observe near-bed SSC was limited. The study conducted in Quinault Canyon determined that the dominant mode of o¡-shelf sediment transport was episodic formation of an intermediate nepheloid layer (INL) moving out from the shelf-break, from which particles settled to the canyon £oor (Baker and Hickey, 1986; Hickey et al., 1986). These authors also concluded that down-slope transport along the canyon axis and resuspension within the canyon were negligible. In contrast, in Baltimore Canyon, Gardner (1989a,b) found little evidence of resuspension by storms and advection of shelf sediment to the canyon head, mainly due to the coarse-grain sizes of bottom sediment (nearly 100% sand and gravel). However, resuspension within the canyon occurred regularly between 200 and 600 m depth, when energy from internal tides was focused along the canyon axis. Sediment resuspended by internal tides became detached from the canyon £oor and moved along isopycnals, generating an INL in the upper canyon section. Similarly, in the Foix Canyon (which also has relict sandy deposits on the adjacent shelf), no clear relationship was observed between SSC and storm events or river discharge at depths greater than 600 m (Puig et al., 2000a), despite detachment of a persistent INL around 400 m depth (Puig and Palanques, 1998b). Resuspension by internal waves was suggested to explain the formation of this INL, although no clear evidence of that mechanism could be determined from the SSC records at 600 m depth. Recently, measurements from northern California in the Monterey Canyon at 1450 m depth (Xu et al., 2002) observed that near-bottom SSC £uctuated on a tidal cycle with higher concentrations related to down-canyon £ows, although short-term SSC spikes were frequent in the records. The most dramatic one was recorded in early February, when an unusually large increase of water turbidity lasted almost a week. The authors interpreted this anomalous increase of SSC as a turbidity current induced by £ushing of sediment accumulated over the previous year at

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20

50

10

41° 00'

Study area

Mad River

S 60

700

6

8

Humboldt Bay

40° 50'

2

4

40° 40'

Fig. 2

Eel River

Eel Canyon

40° 30' 124° 40'

124° 30'

124° 20'

124° 10'

Fig. 1. Bathymetric map of the Eel continental margin showing the location of the Eel Canyon head and the position of the S-60 benthic tripod. Dashed lines represent the distribution and thickness (in cm) of the 1997 £ood deposit (after Wheatcroft and Borgeld, 2000). Note that the 2-cm isoline of the £ood deposit reaches near the northern thawleg of the Eel Canyon, suggesting that part of the sediment discharged by the Eel River could be delivered to the head of the canyon.

the canyon head, triggered by the ¢rst large storm of the fall/winter season. This paper presents simultaneous measurements of combined currents and SSC collected on the Eel continental shelf and within the Eel Canyon. Observational data includes time series obtained by the long-term bottom-boundary-layer tripod deployed on the Eel shelf at the S-60 site (60 m depth), and by a second tripod and an instrumented mooring deployed in the northern thalweg of the Eel Canyon, at 120 and 280 m depths, respectively. The main objective of this study was to characterize the major processes controlling the o¡-shelf sediment export in the Eel continental margin, and identify the dominant mode

of sediment transport that occurs in the head of the Eel submarine canyon.

2. Regional background The study region is located in the northern California continental margin, o¡ the Eel River mouth (Fig. 1). The adjacent continental shelf is V20 km in width and extends seaward to the 150-m isobath in the open-slope region. South of the Eel River mouth, where the Eel Canyon incises the slope, the continental shelf is narrower (V10 km), and the shelf-break is located at 90 m depth. The head of the Eel submarine canyon ex-

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tends more than 10 km in width, is bowl-shaped and has four distinct entrants, which converge to a main canyon axis at about 700 m depth. The northern California rivers supply large amounts of sediment to the marine environment due to the high rainfall of this region and the drainage-basin characteristics (i.e. steep slopes, erodible soils, and extensive mass wasting), which produce extremely large sediment yields (Griggs and Hein, 1980). The primary source of sediments to the study area is the Eel River, which has a mean suspended-sediment discharge of 1^3U107 ton yr31 (Brown and Ritter, 1971; Wheatcroft et al., 1997; Syvitski and Morehead, 1999; Sommer¢eld and Nittrouer, 1999) and a sediment yield of 2.7U103 ton km32 yr31 (Janda and Nolan, 1979). The Mad River also discharges onto the Eel margin, and although it has similar sediment yields (V2U103 ton km32 yr31 ), its suspended-sediment load is one order of magnitude lower, V2.5U106 ton yr31 (Janda and Nolan, 1979). The Eel River discharge is strongly a¡ected by winter storms, which move into California from the northwest producing intense rains (as much as 80% of the annual precipitation) and transports about six orders of magnitude more sediment in the winter than during the dry summer periods (Syvitski and Morehead, 1999). The Eel continental shelf is covered by modern sediments supplied primarily from the Eel and Mad rivers and is characterized by a wave-dominated inner-shelf sand deposit that grades into muds, within a transition zone located between the 50-m and 60-m isobaths (Leithold, 1989). Boundary-layer sediment-transport measurements on the Eel shelf at depths between 50 and 70 m revealed that the highest suspended-sediment events and sediment £uxes were associated with wave resuspension and increased £uvial supply during storms (Ogston and Sternberg, 1999; Cacchione et al., 1999; Wright et al., 1999). During major £oods in the winter, southerly winds from the leading edge of eastward-moving low-pressure systems favor northward and seaward transport of suspended-sediment concurrent with high river discharge. As the low-pressure cell passes over the shelf, the winds become northerly, causing south-

ward and also seaward transport of resuspended seabed sediment. Wind forcing also in£uences the position and direction of the Eel River plume during £oods. Water-column studies found that southerly winds prevalent during Eel River £oods trap the sediment-laden plume against the coast (Geyer et al., 2000; Hill et al., 2000). Suspended sediment settled rapidly from the plume and is potentially deposited in the inner shelf and subsequently transported seaward due to the wave action in the bottom-boundary layer. During £ood events with large waves, this sediment transport toward mid-shelf depths occurs as £uid-mud £ows (nearbottom thin layers of dense sediment suspensions s 10 g l31 ) (Ogston et al., 2000a; Traykovski et al., 2000). As a result of seaward transport as £uid mud, a £ood deposit of several centimeters (up to 8.5 cm) is formed over the mid- and outershelf (i.e. water depths of 50^110 m), north of the Eel Canyon (Wheatcroft et al., 1996; Sommer¢eld et al., 1999; Drake, 1999). This deposit is clearly identi¢ed by X-radiographs, 7 Be (t1=2 = 53.3 days) signatures and grain-size distribution. Mass accumulations for recent £ood events (1995 and 1997) suggest that as much as 75% of Eel River sediment was exported from the shelf to adjacent regions during and immediately following the £ood events (Wheatcroft et al., 1997; Sommer¢eld et al., 1999). On a V100-year time-scale, £uvial sediment input combined with marine dispersal processes have produced a mid-shelf depocenter in the Eel shelf deposits, evident by both the spatial distribution of 210 Pb accumulation rates and by partially preserved clay-rich £ood layers (Sommer¢eld and Nittrouer, 1999). This mid-shelf depocenter coincides with the spatial distribution of the most recent £ood deposits (Wheatcroft and Borgeld, 2000). Integration of 210 Pb mass accumulation rates on the shelf indicates that only V20% of the annual sediment supply to the margin is trapped between the 50-m and 150-m isobaths and that most of the sediment escapes from the shelf (Sommer¢eld and Nittrouer, 1999). O¡-shelf sediment transport to the open slope is controlled by a combination of river discharge, shelf sediment resuspension, and margin circulation. De-

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tachment of INLs at the shelf-break supplies much of the sediment accumulating on the slope (Walsh and Nittrouer, 1999). The upper-slope sediment budget (from 210 Pb geochronology) accounts for a maximum 20% of the annual sediment discharge from the Eel River (Alexander and Simoneau, 1999). This sediment combined with the V20% retention on the shelf (Sommer¢eld and Nittrouer, 1999) demonstrates that V40% of the Eel sediment can be documented on the shelf and open slope, and the fate of V60% of the discharge remains unknown. 7 Be measurements in sediment-trap samples at 450 m depth and in box cores collected at several locations in the upper continental slope also indicate that signi¢cant amounts of ¢ne-grained £uvial sediment are exported o¡-shelf over the course of the winter £ood/storm season (Sommer¢eld et al., 1999). In addition, the presence of 7 Be in surface sediment at the head of the Eel Canyon following the winter £ood season suggests a rapid transport of £uvial sediment toward the canyon head, and may account for a substantial portion of the Eel sediment budget (Mullenbach and Nittrouer, 2000). Box-core signatures suggest that ¢ne-grained £uvial sediment is deposited in the head of the canyon after being resuspended from the adjacent shelf and mixed with older sediment, although the shelf-to-canyon sediment delivery mechanisms are poorly understood.

3. Methods 3.1. Shelf measurements A benthic tripod was maintained on the midshelf mud deposit of the Eel continental shelf at 60 m depth (S-60 site) as part of the STRATAFORM long-term monitoring study from 1995 to 2000 (Fig. 1). Data presented in this paper include measurements from two consecutive 3-month deployments during autumn and winter 1999^2000, when a focused sediment-transport study in the Eel Canyon was performed. The instrumented shelf tripod was initially deployed on 20 October 1999, retrieved on 10 January 2000, and redeployed until 4 March 2000. Instruments mounted

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on the tripod included two Marsh McBirney electromagnetic current meters (EMCMs) and two Optical Backscatter Sensors (OBSs, DpA Instruments) placed at 30 and 100 cm above the bottom (cmab), as well as a pressure sensor located at 140 cmab. All these instruments were programmed to sample every hour and collect 470 samples at 1 Hz (7.5 min of data per burst). During the ¢rst tripod deployment, an autonomous OBS (AOBS, DpA Instruments) was installed at 13 cmab and data was collected hourly. Additionally, an upwardlooking 300-KHz acoustic-doppler current pro¢ler (ADCP, RD Instruments) was mounted on the tripod and the current velocity was measured every hour in 1-m vertical bins, pro¢ling from 5 m above the bottom (mab) (i.e. 55 m water depth) to the sea surface. During the ¢rst deployment, currents in the bottom-boundary layer were obtained from the 30 cmab EMCM. On 28 October 1999, coinciding with the occurrence of a high energetic storm, the tripod changed its orientation by 11‡ and the 100 cmab EMCM started malfunctioning. It is unclear what caused this change in orientation, but the fact that the EMCM was bent suggests that the tripod was hit by a heavy object, such as a log or a crab pot, moved by the action of the waves. During the second deployment, currents in the bottom-boundary layer were obtained from extrapolation of the upward-looking ADCP currents into the bottom-boundary layer due to an electronic failure that a¡ected both EMCMs. The upward-looking ADCP performed excellently in both deployments and provided near-bottom currents at 5 mab during the entire study. OBSs were calibrated in the laboratory using surface sediment collected at the tripod location prior to each deployment, and the AOBS was calibrated at DpA Instruments facilities also using surface sediment collected at the S-60 site. Signals from these instruments were then converted to SSC. The north and east current components from the EMCM and the ADCP were rotated to obtain the along-shelf and across-shelf current components, taking 25‡ clockwise from North as the orientation of the local bathymetry (negative southward and seaward). The 11‡ rotation in the

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Fig. 2. High-resolution bathymetry for the northern thalweg and main axis of the Eel submarine canyon, showing the location of the benthic tripod and moored instruments. The tripod was placed at 120 m depth. The mooring was at 280 m depth, with moored current meter and transmissometer pairs located at 15 mab and at 115 mab (over the canyon rims).

¢rst deployment was restored before changing the current reference system. 3.2. Canyon measurements During the same period that the shelf tripod was maintained at the S-60 site, as part of a focused sediment-transport study in the Eel Canyon, an instrumented mooring was deployed in the northern thalweg of the Eel Canyon at 280 m water depth from 21 October 1999 to 29 March 2000 (Fig. 2). The mooring included two electromagnetic current meters (InterOcean S-4A) equipped with temperature, conductivity and pressure sensors, coupled with transmissometers

(10 cm path length). One instrument package was placed on the mooring near the seabed, at 15 mab, and the other one in mid-waters, at 115 mab (i.e. 165 m water depth). This later set of instruments was located above the canyon rims and approximately at the same water depth as the shelf-break adjacent to the Eel Canyon. The recording interval was set at 1 h, and all instruments worked, although those located in midwaters only recorded data until 5 January 2000. Sequential sediment traps also were mounted on the instrumented mooring at the same sampling levels. Transmissometer data were corrected for slow drift, presumably caused by fouling of the optical

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surfaces. As fouling is cumulative in time, a piecewise baseline of reference voltages was constructed, linking readings of the highest water clarity measurements, and the o¡set was then proportionally subtracted from the raw data. Transmissometers were calibrated separately in the laboratory using surface sediment from the mooring location, and the recorded transmissivity signal was converted to SSC. During the study period, a second tripod (with the same characteristics as the one maintained on the shelf) was deployed in the northern thalweg of the Eel Canyon at 120 m depth (Fig. 2). The tripod deployment began on 12 January 2000 and ended on 3 April 2000, covering the second half of the shelf-tripod and canyon-mooring deployments, and the entire 1999^2000 £ood season (see Figs. 3^5). All canyon tripod instruments worked, except the OBSs, and, therefore, absolute measurements of SSC are not available. However, the tripod also was equipped with a downward-looking video system placed at 180 cmab that recorded clips of 7 s every 4 h for observations of seabed roughness. The qualitative analysis of the water turbidity, using the opacity of the video images, provided temporal evolution of SSC estimates during the entire deployment. Qualitative turbidity units ranged from 100 when the monitor screen was black, due to large amounts of suspended sediment in the water, to 5 when the image of the seabed was perfectly clear, during periods of high visibility. The north and east current components at the tripod (EMCMs and ADCP) and mooring sites (S-4s) were transformed to along-canyon and across-canyon current components, respectively, taking 47‡ clockwise from North as the orientation of the canyon axis (negative down-canyon and north-westward). 3.3. Sediment £uxes Sediment £uxes were calculated on the shelf at the S-60 location and also at the mooring sites in the canyon. Instantaneous sediment £uxes (q(t)) were obtained by multiplying the current speed by the SSC at each sampling level and location.

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The instantaneous £uxes were also calculated for both current components. Averaged over time, these give the net across- ( 6 qu s ) and alongshelf ( 6 qv s ) (or across- and along-canyon), suspended-sediment £uxes. From the resultant vector of those £ux components ! q = ( 6 qu , 6 qv s ), the estimated magnitude of the net horizontal £ux of suspended sediment and the £ux direction at each site were obtained. Sediment £uxes in the canyon were calculated at each mooring sampling level (15 and 115 mab) using currents and SSC measurements obtained by the S-4 current meter^transmissometer pairs. On the shelf, sediment £uxes were calculated at 30 cmab during the ¢rst deployment, using the EMCM current velocity and the SSC measured at that level. EMCM current velocities at 100 cmab were not available for most of the study period, but near-bottom currents (5 mab) were measured continuously by the ADCP. Therefore, assuming that the ¢rst ADCP bin was within the logarithmic current pro¢le and using 1 cm as the average z0 value for the Eel continental shelf at 60 m depth (Wright et al., 1999), currents from 5 mab were extrapolated to 1 mab. Estimates of sediment £uxes on the shelf were then calculated at 100 cmab using the measured SSC and the inferred current velocities. Currents from 5 mab were also extrapolated to 30 cmab for comparison with measured values at that level during the ¢rst 1deployment. The coe⁄cient of determination (R2 ) between inferred and measured current speeds at 30 cmab was 0.86, which suggests a good correlation between both parameters and validates the extrapolation of currents from 5 to 1 mab. 3.4. Riverine and oceanographic conditions The Eel River water discharge during the study period was obtained from USGS gauging station 11477000, located at Scotia, CA, V25 km upstream of the river mouth. Wind and wave conditions during the entire study period were recorded by NOAA Buoy 46030, located on Blunts Reef (40‡25P22QN, 124‡31P31QW) at 82 m water depth about 25 km south of the Eel Canyon. NOAA Buoy 46022, located on the continen-

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tal slope north of the Eel Canyon at 275 m depth (40‡43P12QN, 124‡31P12QW) also provided wind and wave conditions from the study area until late January 2000. Wind speed and direction, signi¢cant wave height and average wave period were obtained directly from the standard meteorological data ¢le. Near-bed wave-orbital velocities were estimated from the NOAA buoy surface wave spectra that is collected hourly using linear wave theory. Calculated wave-orbital velocities have been compared to measured data and found to have a good correlation.

4. Results 4.1. River discharge and wave climate The Eel River discharge (Fig. 3a), from October 1999 to early January 2000, was usually below 100 m3 s31 and it only increased twice to values of approximately 500 m3 s31 in mid-November, associated with a period of southwesterly winds (Fig. 3b). The £ood period in early 2000, which also coincided with a persistent period of southwesterly winds, began on 11 January 2000 and ended in mid-March, when the river discharge

Fig. 3. Temporal evolution of the Eel River discharge at Scotia, CA, and of wind velocity vectors, signi¢cant wave height and average wave period recorded by NOAA Buoy 46030 at the time of the study. Note the occurrence of a large storm event on 28 October 1999. The wind vectors indicate the direction from which the wind blows.

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Fig. 4. Time series of the Eel River discharge and wave-orbital velocity (at 60 m depth, 100 cmab), combined with SSC measurements from the shelf-tripod deployment (30 and 100 cmab).

dropped below 500 m3 s31 . During this period, there were several £oods ( s 1000 m3 s31 ) that lasted 3^6 days. The largest event occurred on 14 February 2000, when the hourly river discharge peaked at 4810 m3 s31 . Based on the return-frequency distribution of the Eel River discharge (Syvitski and Morehead, 1999), this event corresponded to a moderate £ood with a recurrence period of V4 years. During the study several storms occurred related to either dominantly southerly or northerly winds. Based on NOAA Buoy 46030 data, 14 storms occurred with signi¢cant wave heights above 5 m, and in four events they reached

more than 6 m (Fig. 3c). Average wave periods during storms £uctuated between values of around 11 s and maximum periods of 15.8 s (Fig. 3d). The maximum signi¢cant wave height during the study period (9.3 m) was recorded on 28 October 1999, and average wave periods during this particular storm ranged between 13 and 14 s. Maximum signi¢cant wave height during this storm measured by NOAA Buoy 46022 was 10.7 m. Based on the return periods for annual maximum signi¢cant heights recorded by this buoy (Wiberg, 2000), the 28 October 1999 storm corresponded to a storm event with a return period of V6 years.

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Fig. 5. Time series of the Eel River discharge and wave-orbital velocity (at 60 m depth, 100 cmab), combined with SSC measurements from the trasmissometers deployed in the canyon (at 280 m depth, 115 and 15 mab), and with estimates of camera opacity measured by the benthic tripod in the canyon head at 120 m depth. Note that the increases of the camera opacity are correlated to intensi¢cations of the orbital velocity due to storm activity and do not appear to be in direct response to the Eel River discharge.

4.2. Shelf resuspension events The temporal evolution of the orbital velocity at 60 m depth is shown in Fig. 4b. Orbital velocities at 1 mab during moderate storms were above 30 cm s31 , and during energetic storms reached values up to 50 cm s31 . The greatest orbital velocities during the deployment occurred during the 28 October 1999 storm, which reached a peak of 88 cm s31 . Increases in near-bottom SSC were associated with intensi¢cations of the orbital velocities, although SSC over shorter periods £uctuated mainly at diurnal and semidiurnal tidal frequen-

cies (Fig. 4c). The temporal variation of SSC at 100 cmab during the deployment showed minimum values V0.02 g l31 and maximum values V1 g l31 (during the periods dominated by storms). The temporal mean SSC at 100 cmab for the entire deployment was 0.14 g l31 . During and after the £ood season, SSC was greater, showing a mean value of 0.20 g l31 from 11 January 2000 until the end of the deployment, re£ecting the input of freshly delivered river sediments. For most of the recording period, SSC at 30 cmab displayed the same temporal variability as at 100 cmab, although concentrations were slightly higher (Fig. 4d). A notable exception oc-

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Fig. 6. Detail of the temporal evolution of salinity, temperature, SSC and along-canyon current component at mid-waters 115 mab. Note the increases of SSC associated with decreases in salinity and increases of temperature. The largest concentrations coincide with the intensi¢cation of the o¡-shelf (i.e. down-canyon) current component.

curred during the 28 October 1999 storm (when SSC at 100 cmab was V0.6 g l31 ), SSC at 30 cmab showed extremely high values of about 8 g l31 . SSC at 13 cmab during that event was s 10 g l31 , reaching concentrations considered £uid mud. Values for mean SSC at 30 cmab during the entire recording period and at 13 cmab during the ¢rst deployment were 0.29 and 0.35 g l31 , respectively. 4.3. Canyon resuspension events Increases of water turbidity within the canyon recorded by the mooring transmissometers at 280 m depth (at 15 and 115 mab) and by the tripod camera at 120 m depth (2 mab) re£ect a clear correlation with storm events (i.e. large waves), but lack a direct relationship with the Eel River

discharge (Fig. 5). The storm correlation was more evident in the bottom-boundary layer (i.e. 2 mab) at the canyon head, where increases of the camera opacity matched perfectly with the intensi¢cation of the wave-orbital velocity. However, signi¢cant increases of the SSC near the bottom (15 mab) at 280 m depth were observed only when the camera opacity reached 100 units (‘blackscreen’) for several hours. Mean SSC at 280 m depth was 2.08 and 1.55 mg l31 for 15 and 115 mab, respectively. The greatest SSC measured within the canyon coincided with the 28 October 1999 storm, in absence of any £ood event. During this period SSC at 15 mab peaked at 103 mg l31 while SSC at 115 mab reached maximum values around 30 mg l31 (Fig. 5).

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Periodic £uctuation of SSC measured by the moored instruments also occurred with semidiurnal tidal frequencies, which were more accentuated in mid-waters than near the bottom. In the absence of storm events, the 15-mab SSC oscillated between baseline values V0.6 mg l31 and concentration peaks V5 mg l31 , whereas at mid-waters, SCC £uctuated between baseline concentrations of V0.2 mg l31 and peaks up to 15 mg l31 (Fig. 5). Increases of SSC recorded at 115 mab corresponded with increases of temperature and decreases of salinity, and the largest concentrations coincided with the intensi¢cation of the o¡-shelf (i.e. down-canyon) current component (Fig. 6). SSC £uctuations at 15 mab were more sporadic than at 115 mab and no clear relationship was found between SSC peaks and the current regime measured at 15 mab. 4.4. Shelf and canyon currents Shelf currents were highly variable, with signi¢cant spectral energy at low frequencies and at storm and tidal components. Near-bottom currents at 5 mab reached maximum hourly mean velocities of 67 cm s31 ; while at 30 cmab, currents measured only during the ¢rst deployment reached 31 cm s31 . At the canyon head (120 m depth), maximum current velocities at 5 mab were slightly greater than on the shelf, reaching 78 cm s31 . Closer to the bottom, the highest velocities at 100 and 30 cmab were 60 and 51 cm s31 , respectively. Farther down-thalweg (280 m depth), near-bottom (15 mab) currents were of a similar magnitude to those at the shallower canyon location, showing maximum velocities of 61 cm s31 . In contrast, higher in the water column, above the canyon rims (115 mab), maximum current velocities were weaker and only reached 37 cm s31 .

Fig. 7. Progressive vector plots of the currents measured during the study. Note the veering of the currents at the head of the canyon (120 m depth) as they approach the seabed. Crosses represent the origin of measurements in each sampling location and the small arrow indicates when the canyon tripod deployment began.

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The progressive vectors of the hourly currents measured at all sampling sites are shown in Fig. 7. Near-bottom (5 mab) shelf currents displayed a persistent o¡-shelf and SSW £ow direction that was disrupted by several current reversals toward NNE and SSW, following the orientation of the bathymetry (Fig. 7a). These current reversals were mainly induced by changes in the wind direction, associated with the passage of atmospheric pressure cells. The temporal progression of the current vectors in the canyon head (120 m depth) at several heights above the bottom during the second half of the deployment (Fig. 7b) indicated that currents in mid-waters (60 mab) were oriented N^S, with their direction changing dominantly based on the coastal wind regime. Closer to the seabed, changes in current direction were gradually less pronounced and the current £ow veered o¡-shelf and towards SSW, due to in£uence from the canyon topography. Where currents were constrained within the canyon walls ( 6 15 mab), the current £ow was dramatically directed down-canyon during all the deployment, regardless of the current regime operating on the shelf or in mid-waters over the canyon. The temporal evolution of near-bottom currents at the canyon head showed a well-de¢ned oscillation of the current velocity associated with changes in the up- and down-canyon direction, predominantly at semidiurnal tidal frequencies, although the down-canyon component was clearly stronger than the up-canyon component. This near-bottom current regime also was observed at the mooring site (15 mab), where the current £ow was strongly constrained by the canyon topography, and the down-canyon component also dominated over the up-canyon component (Fig. 7c). Currents in intermediate waters (115 mab) were less in£uenced by the canyon topography and were oriented toward NNE, following the orientation of the adjacent shelfbreak and in agreement with the dominant coastal wind direction at that time (Fig. 7c). 4.5. Sediment £uxes The temporal evolution of suspended-sediment

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£uxes on the shelf are shown in Fig. 8. The magnitude of the instantaneous suspended-sediment £uxes at 1 mab £uctuated between minimum values V0.1 g m32 s31 and maximum £uxes up to 400 g m32 s31 (recorded during the £ood period, Fig. 8a). Fluxes at 30 cmab during the ¢rst deployment were dominated by the 28 October 1999 storm, when instantaneous £uxes reached values up to 1800 g m32 s31 (Fig. 8b). The net £ux of suspended sediment along the shelf at 1 mab was 2.26 g m32 s31 to SSW, while the net £ux across the shelf was 1.72 g m32 s31 seaward. The resultant of both components generated a net suspended-sediment £ux of 2.85 g m32 s31 to 243‡. The cumulative transport of sediment at 1 mab across the shelf during winter 1999^2000 was 24.9 ton m32 o¡-shelf, while the along-shelf cumulative transport was 32.7 ton m32 toward SSW (Fig. 8c). The across-shelf transport at 1 mab was directed persistently o¡-shelf throughout the entire recording period, with some sporadic events directed onshore, while the along-shelf transport £uctuated more. It was initially directed toward NNE (especially during the £ood season), but due to the change in coastal winds and near-bottom current £ow, resulted in the net along-shelf transport to SSW. Sediment transport at 30 cmab during the 28 October 1999 storm was directed o¡shelf and toward NNE, and that single-eventdominated cumulative sediment transport for the entire deployment (Fig. 8d). The instantaneous sediment £uxes in the canyon reached maximum values during the 28 October 1999 storm (Fig. 9). Maximum £uxes during this particular event were 27.8 and 4.07 g m32 s31 at 15 and 115 mab, respectively (Fig. 9a,b). The resultant of the net suspended-sediment £ux in mid-waters was 15.8 mg m32 s31 toward 10‡, following the direction of the main £ow above the canyon walls. Canyon £uxes at 15 mab were approximately ¢ve times greater than those at 115 mab and were clearly directed down-canyon, providing a cumulative transport of 1.1 ton m32 (Fig. 9d), and resulting in a net suspended-sediment £ux of 80.7 mg m32 s31 toward 228‡.

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Fig. 8. Temporal evolution of the instantaneous suspended-sediment £ux magnitude and cumulative suspended-sediment transport along-shelf and cross-shelf at the S-60 site during the entire study.

5. Discussion 5.1. Shelf-to-canyon sediment delivery Observations on the Eel shelf during this study corroborate previous ¢ndings at S-60 site obtained by Ogston and Sternberg (1999) : sediment-suspension events on the shelf are principally forced by waves and are enhanced during periods of large river discharges, while tidal currents and low-frequency currents (associated with the coastal wind regime) determine the direction of sediment £uxes. The combination of these processes during autumn/winter 1999^2000 produced a net sediment transport at the S-60 site (1 mab)

of 2.85 g m32 s31 to 243‡, which was clearly directed o¡-shelf and toward the region of the canyon head. Previous deployments at the S-60 site during the last 5 years documented a large interannual variability of £ux magnitude and direction, apparently related to El Nin‹o^La Nin‹a events. In some years the net sediment £ux was also towards the south, but other years it was northward (Ogston et al., 2002b). Besides this interannual variability, on a shorter time-scale, the passage of eastward-moving cyclonic storms over the Eel shelf tends to cause southerly winds that favor northward transport of suspended sediment, but as the low-pressure cell passes over the shelf, the winds become north-

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Fig. 9. Temporal evolution of the instantaneous suspended-sediment £ux magnitude and cumulative suspended-sediment transport along-canyon and cross-canyon at the canyon-mooring site during the entire study.

erly, causing southward transport (Cacchione et al., 1999). This can be observed clearly in the temporal evolution of the cumulative sediment transport on the shelf (Fig. 8c), when the alongshelf current £uctuated repeatedly, being directed ¢rst toward NNE and then SSW. Assuming that the sediment-transport conditions at di¡erent locations along the Eel shelf are similar, during the SSW transport situation, a signi¢cant amount of the Eel sediment deposited temporarily on the mid-shelf can be resuspended and transported toward the Eel Canyon. This result is supported by the distribution of the 1995 and 1997 Eel £ood deposits (Wheatcroft and Borgeld, 2000) that extend to areas near the canyon head (Fig. 1). In

addition, the presence of 7 Be in the surface sediments from the upper canyon also demonstrates transport to the canyon head of sediment recently discharged to the shelf by the Eel River (Mullenbach and Nittrouer, 2000). On the longer timescale (V100 year), the redistribution of shelf sediment by southward transport toward the Eel Canyon region is supported by the high 210 Pb accumulation rates (0.6^0.8 g cm32 yr31 ) of bottom sediments at the outer shelf surrounding the head of Eel Canyon (Sommer¢eld and Nittrouer, 1999). After suspended-sediment particles encounter the canyon, near-bottom currents are favorable for their transport toward deeper regions. Progressive vector plots at the canyon head (120 m

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depth) at di¡erent heights above the seabed (Fig. 7b) indicate that currents over the canyon tend to veer o¡-shelf, as the canyon axis is approached and that close to the bottom they follow the canyon orientation. Once currents are constrained within the canyon walls ( 6 15 mab), they are completely aligned with the canyon orientation, and the down-canyon transport component dominates over the up-canyon component. This predominantly down-canyon £ow near the bottom occurs regardless of the current regime operating on the shelf or in intermediate waters. The nearbottom transport pattern is maintained farther down-thalweg (280 m depth), where the progressive current vector 15 mab is also directed downcanyon (Fig. 7c), generating a net sediment £ux at 15 mab of 80.7 mg m32 s31 . 5.2. Suspended-sediment variability in intermediate slope waters In intermediate waters of the continental slope, just above the canyon rims, the progressive current vector during the recording period was not in£uenced by the canyon topography and was directed toward the north (Fig. 7c), following the orientation of the adjacent shelf-break. The net sediment £ux in intermediate waters (115 mab) was approximately ¢ve times less than near the canyon bottom (15 mab), and was oriented approximately parallel to the shelf-break along the continental slope (15.8 mg m32 s31 to 10‡). Currents and sediment £ux direction were in agreement with the southerly wind regime blowing at the beginning of the deployment, when the current meter in mid-waters was operative. However, currents may reverse over the year due to wind forcing (Largier et al., 1993), inducing southward suspended-sediment transport during periods typically dominated by northerly winds. During most of the time, periodic £uctuations of SSC recorded at shelf-break depths were more accentuated than those observed near the bottom, suggesting a potential o¡-shelf sediment-transport mechanism occurring mainly at intermediate water depths. This continuous process seems to contribute to the development and maintenance of the INL detached from the seabed at the

shelf-break, which has been observed periodically on the Eel margin during hydrographic surveys (Walsh and Nittrouer, 1999; Mullenbach et al., 2000). SSC in intermediate waters £uctuated principally with semidiurnal tidal frequencies, with peaks of SSC corresponding to increases of temperature and lowering of water salinity (Fig. 6). From the data of a single instrument, the cause of these SSC £uctuations is not clear, although the strong changes in temperature and salinity suggest the presence of internal tides, which may cause sediment resuspension at shelf-break depths. 5.3. Storm-induced sediment transport at the canyon head Increases of water turbidity (SSC and camera opacity) measured within the canyon were not directly linked to the Eel River discharge, but they were associated with the occurrence of storms (Fig. 5). The high correlation between wave-orbital velocity and the camera opacity in the bottom-boundary layer at the canyon head suggests that wave resuspension on the shelf is the principal mechanism supplying sediment to the canyon. Farther down-canyon, increases of near-bottom (15 mab) SSC recorded at 280 m depth also re£ect a clear link with major storm events and a lack of direct relationship to the Eel River discharge. Near-bottom SSC £uctuations within the canyon also occurred in some parts of the record at tidal frequencies, although no clear relationship was observed with lower-frequency current regime. Resuspension by internal tides focused along the canyon axis might account for these periodic SSC increases, but the sampling frequency of the current meter and transmissometer pairs (1 h) was probably too small to clearly depict internal-tide resuspension e¡ects. Detailed analysis of the canyon-head tripod data revealed that during energetic storms sediment transport at the head of the canyon takes place as density-driven currents £owing downcanyon (Puig et al., 2000b). Although absolute SSC measurements during the canyon tripod deployment were not directly available, di¡erences between current velocities from EMCMs at 30 and 100 cmab revealed that down-slope currents

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Fig. 10. Detail of the measurements during the 28 October 1999 storm, showing the SSC records obtained on the shelf at the S-60 site and within the canyon at the mooring site (280 m depth) and the resultant instantaneous sediment £uxes.

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at 30 cmab were much greater (V15 cm s31 ) than currents at 100 cmab, when camera opacity values reached 100 units for several hours. These inverse near-bottom current pro¢les combined with high estimates of SSC reveal the presence of densitydriven £ows induced by storms which can carry large amounts of sediment towards deeper parts of the canyon. These events occurred in late January 2000 and early March 2000 when 15-mab SSC within the canyon at 280 m depth sharply increased, reaching maximum concentrations around 20 and 50 mg l31 , respectively (Fig. 5). However, the greatest SSC and sediment £uxes at the mooring site were recorded during the 28 October 1999 storm, before the Eel River £ood season. At that time, boundary-layer measurements at the head of the canyon were not available (tripod not deployed yet), although comparison with major storms that occurred during the second half of the study period indicates that during the 28 October 1999 storm event a large density-driven £ow may have occurred. Details of the measurements during the 28 October 1999 storm are shown in Fig. 10. On the shelf, at 60 m depth, wave-orbital velocities peaked at 88 cm s31 and SSC at 30 cmab was V8 g l31 , while at 13 cmab it reached concentrations in excess of 10 g l31 , considered £uid mud. Toward the end of the event, when wave-orbital velocities decreased and mean current speed increase, SSC at 13 cmab reached values V30 g l31 probably due to suspension into the water column of highly concentrated suspended sediment previously trapped in the wave-boundary layer. Towards the end of the 28 October 1999 storm, the sudden reduction of SSC at 13 cmab (from V27 g l31 to 0.3 g l31 in 2 h) seems to be associated to the formation of a lutocline, generated by the slow settling of the ¢ner resuspended particles when wave and current energy ceased (Ross and Metha, 1989; Metha, 1991). The rapid change in SSC could indicate when the lutocline dropped below the OBS level. Previous £uid-mud £ows observed on the shelf during the STRATAFORM study were associated with combinations of large wave-orbital velocities (storms on the shelf) and high sediment input (£oods in the Eel River) (Ogston et al., 2000a; Traykovski et al.,

2000). However, data collected by the S-60 tripod during 28 October 1999 storm indicates that £uidmud concentrations can be reached by resuspension of consolidated shelf sediments (i.e. V7 months since the previous £ood) under extreme wave conditions. This data also corroborates numerical model results from Wiberg (2000), who calculated sediment concentration pro¢les using bed sediment characteristics from the Eel shelf at depths of 50^60 m. Concentration pro¢les calculated showed that SSC can reach values above 10 g l31 at V10 cmab when wave-orbital velocities are higher than 75 cm s31 , even though a weak response in concentration occurs higher in the water column (Wiberg, 2000 ; Fig. 4a). In the canyon, near-bottom (15 mab) SSC at 280 m depth suddenly reached 103 mg l31 at the beginning of the 28 October 1999 storm, and SSC remained high for almost a day. In intermediate waters (115 mab), SSC increased 3 h later than at 15 mab, reaching maximum values of approximately 30 mg l31 (Fig. 10d). This time lag could re£ect a rapid, highly concentrated transport of sediment near the bottom, induced by the density-driven £ow, and a more dilute advective sediment transport in intermediate waters. The intermediate-water transport mechanism is probably related to the detachment of sediment particles along isopycnals at shelf-break depths and could largely contribute to the development of the observed INL (Walsh and Nittrouer, 1999; Mullenbach et al., 2000). Sediment traps mounted on the mooring near the bottom and at mid-depth, at 15 and 115 mab, respectively, were over¢lled due to extreme collection rates during this event. Sediment particles transported o¡-shelf and downcanyon ¢lled the receiving cup and the lower part of the collecting funnel of both traps, caused blockage of subsequent receiving cups, and limited time-series observations of the downward £ux. 5.4. Comparison with other submarine canyons Few studies have provided information about contemporary sediment-transport processes acting in submarine canyons (Baker and Hickey, 1986; Hickey et al., 1986; Gardner, 1989a,b; Puig et al.,

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2000a; Xu et al., 2002). These previous studies suggest di¡erent modes of dominant sedimenttransport processes within canyons, although some observations ^ particularly those from canyons located on the west coast of the USA ^ are consistent with results obtained at the head of the Eel Canyon. The most signi¢cant similarity found in these previous studies is the large increase of SSC measured in the Monterey Canyon during February 1994 by Xu et al. (2002), which showed a comparable behavior to the Eel 28 October 1999 storm event, although the Monterey event was measured at 1450 m depth (100 mab). Both events were potentially down-canyon density-driven £ows induced by large storms, and transported signi¢cant amounts of sediment toward deeper parts of the canyon. Both, Quinault and Eel canyons exhibit the presence of an INL detached at shelf-break depths that extends over the canyon, which contributes to the o¡-shelf sediment transport. Resuspension within Quinault Canyon and down-slope transport along the canyon axis were considered negligible (Baker and Hickey, 1986; Hickey et al., 1986). However, in the two 3-month deployments of this study (conducted during two consecutive winter seasons), some shelf and canyon SSC variations were clearly correlated. This relationship occurred during storms in early November. In winter 1980^81, it corresponded to the ‘R’ event, which was interpreted as a possible local resuspension in the deep canyon (Hickey et al., 1986), while in winter 1981^82, it corresponded to a large storm that created a signi¢cant increase of water turbidity on the shelf and in mid-waters above the canyon, and generated the largest nearbottom and mid-water sediment-trap £uxes observed during the entire study in the upper canyon section (Baker and Hickey, 1986). Data from these two events (particularly the later one) also resemble the observational pattern found in the Eel Canyon during the 28 October 1999 storm, suggesting that down-canyon density-driven £ows may have also occurred in Quinault Canyon. Observational data suggest that in some submarine canyons, density-driven £ows or turbidity currents induced by storms can occur periodically, commonly during the ¢rst major storm of the

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autumn/winter season. These canyons are typically located on continental margins with narrow shelves, with signi¢cant sediment sources from rivers, and high energetic wave regimes (e.g. Quinault, Monterey and Eel canyons). Under these conditions, modern sediment can easily reach the outer continental shelf and be temporarily stored at the head of the canyon, therefore, providing unconsolidated material for generation of density-driven £ows. The lack of previous bottom-boundary-layer measurements at the head of submarine canyons may account for the fact that relatively frequent density-driven transport through canyons has not been observed or clearly identi¢ed before. On the other hand, in submarine canyons located on continental margins with relict or coarsegrained sediments on the shelf edge around the canyon head (e.g. Baltimore and Foix canyons), contemporary sediment-transport mechanisms within the upper canyon section appear to be mainly dominated by internal wave resuspension and INL detachments (Gardner, 1989a,b; Puig et al., 2000a).

6. Summary Analyses of simultaneous time-series data collected on the Eel continental shelf and within the Eel Canyon during autumn/winter 1999^2000 support the following conclusions. (1) Observational data presented in this paper clearly indicate that the Eel Canyon acts as a preferential conduit of sediment from the Eel shelf to the deep sea. (2) Sediment-resuspension events on the Eel shelf are dominantly forced by waves, and nearbottom SSC are enhanced during the Eel River £ood period. Shelf sediment-transport processes during autumn and winter 1999^2000 generated a net sediment £ux at 1 mab directed seaward and southwards, toward the Eel Canyon region. Bottom-boundary-layer data during this study indicate that sediment resuspension during highly energetic storms (Hs of V10 m, recurrence period V6 years) can create £uid-mud concentrations on the shelf, in absence of any recent £ood event.

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(3) Sediment resuspended on the Eel shelf is transported to the open slope, and moves seaward at intermediate depths by suspended-sediment detachments induced by tidal and storm activity. This continuous o¡-shelf transport mechanism may contribute to the development and maintenance of a shelf-break INL. (4) Near-bottom currents within the Eel Canyon head (120 m depth) are strongly constrained by the canyon morphology and predominantly directed down-canyon, regardless of the current regime operating on the shelf or in intermediate waters. This near-bottom current pattern is also maintained at 280 m depth, resulting in continuous down-canyon sediment transport. (5) Increases of sediment concentration and £uxes within the canyon are not directly related to the Eel River discharge, but they are linked to the occurrence of major storms that generate down-canyon density-driven £ows, carrying large amounts of sediment toward deeper parts of the margin. Comparison with previous studies suggest that the occurrence of storm-induced density-driven £ows at the heads of submarine canyons could be more frequent that previously expected.

Acknowledgements This work was funded by the O⁄ce of Naval Research, Marine Geology and Geophysics Program, Grants No. N00014-95-1-0418 and N00014-99-1-0028 as part of the STRATAFORM program. P. Puig also bene¢ted from a Fulbright scholarship supported by the Spanish Ministry of Education and Culture. The authors wish to thank Josefa Varela Guerra, Dale Ripley and the crew of the RV Wecoma for their assistance and support during surveys. The manuscript bene¢ted from the reviews made by P. Traykovski and an anonymous reviewer, to whom the authors are especially grateful. References Alexander, C.R., Simoneau, A.M., 1999. Spatial variability in sedimentary processes on the Eel continental slope. Mar. Geol. 154, 243^254.

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