Contemporary sedimentation processes in and around an active West Coast submarine canyon

Marine Geology, 71 (1986) 15--34

15

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

CONTEMPORARY SEDIMENTATION PROCESSES IN AND AROUND AN ACTIVE WEST COAST SUBMARINE CANYON*

EDWARD T. BAKER and B A R B A R A M. HICKEY

Pacific Marine Environmental Laboratory, 7600 Sand Point Way N.E., Seattle, WA 98115 (U.S.A.) School o f Oceanography, University o f Washington, Seattle, WA 98195 (U.S.A.) (Received November 8, 1984; revised and accepted April 16, 1985)

ABSTRACT

Baker, E.T. and Hickey, B.M., 1986. Contemporary sedimentation processes in and around an active West Coast submarine canyon. Mar. Geol., 71: 15--34. Contemporary processes controlling the off-shelf transport of suspended particles in the vicinity of Quinault submarine canyon on the Washington continental shelf were identified using CTD/transmissometer surveys and a three-month deployment of current meter/transmissometer/sequentially-sampling sediment trap arrays. Resuspension events on the shelf create a shelf-break-depth nepheloid layer that transports sediment off the shelf. Fluctuations of the particle concentration in this layer were positively and significantly correlated with the short-term (9.5 days) temporal variation of trap deposition rates in the canyon and on the adjacent open slope. Contributions to the deposition rate from downslope transport along the canyon floor, from local resuspension of canyon sediments, and from particles in the ambient b o t t o m nepheloid layer were negligible. These observations indicate that deposition on the slope is controlled by fast-sinking (100--200 m day-') amorphous aggregates that scavenge fine-grained particles from the shelf-break-depth nepheloid layer. Areal variations in the deposition rate result from interaction between the regional flow pattern and the canyon geometry. Rates were high (~60 g m -2 d a y - ' ) in the canyon upper head because it indents the shelf and thus underlies the shelf-break-depth nepheloid layer created by across-isobath advective transport of resuspended mid-shelf silty sediments. Rates were low (3--6 g m -2 day-' ) elsewhere in the canyon and on the open slope because the transport is much less rapid and direct, occurring primarily by horizontal diffusion rather than across-isobath advection of particles, and because the availability of erodable fine-grained particles is reduced in the relatively coarser outer-shelf sediments. Trap deposition rate, extrapolated to an annual rate based on a historical seasonal pattern of shelf storms, was consistent with published radiometric bottom sediment accumulation rates throughout the study area. INTRODUCTION S u b m a r i n e c a n y o n s a p p e a r to be n a t u r a l c o n d u i t s for the t r a n s f e r o f partic u l a t e m a t t e r f r o m t h e s h e l f t o t h e d e e p sea. " C a t a s t r o p h i c " g r a v i t y - c o n t r o l l e d *Contribution number 653 from Pacific Marine Environmental Laboratory and number 1615 from the School of Oceanography, University of Washington. 0025-3227/86/$03.50

© 1986 Elsevier Science Publishers B.V.

16 processes (turbidity currents, debris flows, slumps) transported large volumes of both coarse and fine-grained shelf-derived sediments seaward through canyons during the Pleistocene (e.g., Shepard and Einsele, 1962; Shepard et al., 1969; Emery et al., 1970; Nelson and Kulm, 1973; Kelling and Stanley, 1976). Although the Holocene rise in sea level abruptly reduced both the frequency of large-scale transport events and the supply of coarse-grained sediments (Nelson, 1976; Barnard, 1978), canyons continue to be favorable sites for the concentration and accumulation of fine-grained sediments. For example, near-bottom enhancement of turbidity in canyons relative to the adjacent slope is well d o c u m e n t e d (Beer and Gorsline, 1971; Drake and Gorsline, 1973; Plank et al., 1974; Baker, 1976; Biscaye and Olsen, 1976; Drake et al., 1978). Also, recent ( ~ 1 0 0 yr) sediment accumulation rates can be three to four times higher in canyons than on the adjacent slope (Carpenter et al., 1982). Bottom sediment accumulation rates and static descriptions of particle distributions, however, are inadequate to determine the p r o c e s s e s by which suspended sediment is dispersed from the shelf to the deep sea. In this paper, for the first time, information on time- and space-variable velocity fields, particle concentrations, and deposition rates is integrated to investigate contemporary sedimentation processes in and adjacent to a submarine canyon. Data from Quinault submarine c a n y o n on the Washington continental slope are used to compare contemporary (~ 1 yr) deposition rates to recent ( - 1 0 0 yr) sediment accumulation rates, to estimate the vertical transport rate of particles through the water column over the slope, and to explain why sediment accumulation rates are higher in Quinault Canyon than on the adjacent open slope. STUDY AREA The continental slope off Washington consists of a steep and highly incised upper portion extending from the shelf break at ~ 2 0 0 m down to ~ 1500 m, and a more gently sloping lower portion which grades into Cascadia Basin (Fig.l). Several major canyons indent the slope. Canyon heads border about 20% of the 200-m (shelf-break) contour and account for about 40% of the surface area of the slope. Quinault Canyon is ~ 3 0 km at its broadest and indents the shelf by ~ 1 5 km relative to the regional trend of the 200-m isobath. Slopes in the canyon head (200--600 m depth range) are ~ 6 to 30 °, decreasing to 0.3 ° at 1600 m, approximately 30 km down the principal canyon axis from the shelf break. The Columbia River is the predominant source of sediment to the Washington shelf, annually delivering at least 2.1 X 1013 g of silt and clay (Nittrouer, 1978). About two-thirds of this supply accumulates on the middle to outer Washington shelf in a well-defined band originating at the m o u t h of the Columbia River and extending north-northwest under the influence of the prevailing shelf currents (Fig.l) (Gross and Nelson, 1966; Smith and Hopkins, 1972; Nittrouer, 1978). Locations where this transport avenue intercepts the

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Fig. 1. B a t h y m e t r y o f the c o n t i n e n t a l shelf and slope o f f Washington. The l o c a t i o n o f t h e m i d - s h e l f silt d e p o s i t is e x p r e s s e d here as a b a n d o f elevated s e d i m e n t a c c u m u l a t i o n rate ( N i t t r o u e r , 1978) t r e n d i n g n o r t h - n o r t h w e s t across t h e shelf. C o n t o u r interval is 400 m.

shelf break (Astoria, Quinault, and, to a lesser extent, Willapa Canyons) are areas of favored sediment supply and thus high accumulation rates on the slope (Carpenter et al., 1982). The distribution of suspended particles over the Washington shelf and slope has been described by several investigators (Plank et al., 1974; Baker, 1976; Pak et al., 1980). A b o t t o m nepheloid layer ( B N L ) o f varying thickness and intensity is ubiquitous over the shelf. Seaward of the shelf break, the BNL thickness and particle concentration decreases. Intermediate nepheloid layers (INL), separated from the sea floor by a zone of less-turbid water, are also c o m m o n seaward of the shelf break. METHODS A variety of instruments and sampling schemes were utilized to quantify particle concentration and flux. The concentration of fine-grained suspended

18 particles was measured with transmissometers of 0.25-m path length (Bartz et al., 1978). Light transmission (T) values were converted to attenuation (c) by the expression: in T 0.25

C-

Spatial distributions of attenuation were compiled from CTD/transmissometer sections made typically along the Quinault Canyon axis, along an open slope "control" line south of the canyon, and across the upper canyon head (Fig.2). Light attenuation can be converted to approximate particle mass concentration (C, mg 1-1) by the calibration equation: c = 0.50C + 0.48 derived from 177 membrane-filtered (0.4 pm pore size) water samples collected on all cruises. The correlation coefficient (r) was 0.98 and the standard error of the estimate was 0.07 m -1 for a given value of C. We note, however, that the efficiency of a suspended particle in attenuating light decreases with increasing particle size (Baker and Lavelle, 1984). Thus the above calibration may underestimate the actual particle concentration during periods of local resuspension, if the mean particle size increases substantially. Temporal changes in the fine-particle concentration roughly 5 m and 200 m above bottom (mab) at specific locations (Fig.2)were measured using transmissometers coupled to Aanderaa current meters on taut wire subsurface

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19

moorings. Deployments were made for three m o n t h s during two successive winter seasons, 1980--81 and 1981--82 (Table 1). Current speed, direction, and beam transmission were recorded at intervals of 20 min. These data series were filtered first with a five-point binomial filter, then with a cosine-Lancotz filter (half-power point of 2.5 h) to suppress high-frequency noise. The data were next decimated to hourly intervals. Lastly they were filtered with another cosine-Lancotz filter (half power point of 40 h) to suppress tidal and inertial frequencies. Accumulation of particles on the lens during long-term immersion of the transmissometer resulted in unavoidable instrument drift. This effect, deduced from the observation that attenuation values from moored and CTD-mounted transmissometers were consistent at the beginning of a d e p l o y m e n t but not at the end, was most apparent in deep-water records where attenuation fluctuations were relatively small. In order to measure the depositional response to changes in the velocity and particle concentration fields, sequentially-sampling sediment traps (Baker and Milburn, 1983) were deployed in conjunction with the current meter/transmissometers during the 1981--82 field season {Table 1). The traps were cylindrical with a height/width ratio of three and a diameter of 0.20 m, thereby satisfying the geometrical requirements for representative vertical particle flux measurements in moderate ( ~ 1 5 cm s -~) current speeds {Gardner, 1980) typical of the slope environment in and around Quinault Canyon. Trap efficiency in higher current speeds (e.g., storm events at the shelf break) is probably < 100% (Bloesch and Burns, 1980). Each trap collected ten consecutive samples by means of a self-contained rosette system which brought a new sample jar under a funnel at the b o t t o m of the trap every 9.5 days. Samples were preserved with a solution of sodium azide in filtered seawater placed in each jar before deployment. Trap samples

TABLE

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N o v e m b e r 22, 1 9 8 0 - - J a n u a r y 22, 1981 Q~, shelf 156 151 Q~, c a n y o n u p p e r h e a d 405 150/400 Q], c a n y o n l o w e r h e a d 1197 142/392/892/1192 Q~, c a n y o n n e c k 1639 384/689/1334/1634 Q~, c a n y o n n e c k 1446 Q~, o p e n slope 393 138/288/388 a

151 400 892/1192 1334/1634 1446 288/388

-

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147 252/452 1039/1189/1239 a 1337/1577 a 395 a

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a Instrument

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20 were returned to the laboratory where each was split into fractions for total mass flux and textural analyses. The mass percentage of fecal pellets in each sample was determined by a separation technique described in Baker et al. (1985). Mooring data are denoted Qi, where j is the mooring number and i indicates whether the mooring was deployed during the second ( 1 9 8 0 - - 8 1 ) o r third (1981--82) Quinault Canyon experiment (the two deployments discussed in this paper; Q~, a summer experiment, is not considered here). Specific instrument depths are given in parentheses. FINE-PARTICLE DISTRIBUTIONS Particle distributions observed in January 1981, October 1981, and January 1982 (Figs.3--5) formed a consistent pattern even though absolute turbidity values differed substantially among cruises. Three features were always present: a turbid BNL over the shelf, an INL extending horizontally seaward at the shelf-break depth, and a weak BNL over the slope apparently feeding deep INLs. Both the attenuation and horizontal extent of the INL varied in response to the intensity of shelf resuspension events, consistent with the hypothesis that shelf-break INLs are off-shelf extensions of the shelf BNL (Pak et al., 1980). The most prominent shelf-break INL was observed during January 23--24, 1981 (Fig.3), a few days after a remarkably strong resuspension event on the shelf (Fig.6A). Reduced INLs were observed during periods of moderate (October 4--9, 1981, Fig. 4) and minimal (January 13--15, 1982, Fig.5) resuspension at the shelf edge (Fig.6). The formation of INLs over the canyon and open slope is discussed in detail by Hickey et al. (this volume). Briefly, material is resuspended on the shelf (to depths ~ 2 0 0 m) by gravity waves and currents and is subsequently advected by the regional flow. Above the b o t t o m boundary layer, regional flow on the shelf generally follows the large-scale (~10 km) direction of the isobaths; within the b o t t o m boundary layer, the flow is directed slightly to the left of the direction of the isobaths. Northward flow that generally occurs during storms is thus modified by northeastward flow along the south rim of the canyon and northwestward flow along the north rim. Material resuspended into the regional flow on a particular isobath on the shelf will thus be advected along that isobath except in locations where the regional flow c r o s s e s small-scale topographic features. Over the upper reaches of the canyon head, for example, an INL is formed by northeastward flow off the east--west trending wall along the south rim of Quinault canyon. The upper canyon head is also a particularly favorable location for the creation of intense INLs because it indents the mid-shelf deposit of Columbia River silts (Fig.l). In regions of deeper b o t t o m depth, such as the canyon lower head to canyon neck, stratification weakens the effect of the canyon topography on the flow field so that the regional flow at INL depths crosses the isobaths (i.e., the currents d o n ' t quite " m a k e the turn"), thereby producing overhead

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smectite and plagioclase, whereas material from all other locations was compositionally similar to terrigenous sources south of the Columbia River (Kraft, 1 9 8 4 ) . Material in the shelf and c a n y o n upper head traps was also significantly coarser and more poorly sorted than from other locations (Snyder and Carson, this volume). For a symmetrical c a n y o n with no small-scale features where inertial effects would allow the flow to cross isobaths, we could expect symmetric development of INLs on both north and south c a n y o n rims. However, Quinault c a n y o n does have a number of small-scale features, and indeed, observations suggest that INL development over Quinault c a n y o n is not symmetric. On October 4--5, 1981 (Fig.4), during a period of northward b o t t o m flow (Fig.6B), the INL from the southern rim of the c a n y o n was prominent. On January 15, 1982, however, the INL from the northern rim along this same transect extended southward well past the c a n y o n axis and there was no INL from the southern rim (Fig.5). Near-bottom flow during

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(B) Fig.6. ( A ) Low-passed 6-h attenuation records (m -~) 5 mab during the Q2 experiment. The data are plotted as fluctuations about the mean o£ each record (note scale differences between records). Record means are given on the right-hand side of each series. (B) Lowpassed 6-h velocity vectors (cm s-') and attenuation records (m ~) during the Q3 experiment. Records at Q~ ( l & 7 m) and Q~ (452 m) are 5 mab. Records at Q~ (1189 m) and Q: are 55 and 198 mab, respectively. Attenuation is plotted as in the upper figure except for Q~ (452 m) which has not been low-passed because of the shortness of the record. Sediment trap collection intervals (1--10) are shown by vertica] dashed lines.

25

this period was consistently southward for m ore than two weeks prior to the transect o c c u p a t i o n (Fig.6B). [Note that although the shelf record ends on January 9, the open slope record indicates southward flow during January 10--16 (Fig.6S).] Unlike the shelf-break INL, the slope BNL was unaffected by resuspension events on the shelf (Fig.6). A t t e n u a t i o n levels 5 mab at 1200 m in the c a n y o n were 0.56 m -1 on January 24, 1981, 0.58 m -1 on O ct ober 8, 1981, and 0.60 m -~ on January 14, 1982 (see also Fig.6). TIME-DEPENDENT DEPOSITION

Trap deposition rates, like a t t e n u a t i o n in the shelf BNL and shelf-break INL, were highly t i m e - d e p e n d e n t (Fig.7). The most remarkable result was that each location displayed a deposition m a x i m u m during collection interval 5 coincident with the major resuspension event on the shelf (Fig.6B). Other deposition maxima were not regionally uniform. Only the shelf edge trap showed p r o m i n e n t maxima in intervals 7 and 8, and only the traps in the lower c a n y o n head (Q]) showed a m a x i m u m in interval 9. The mean deposition rate for a given location decreased with distance from the mid-shelf accumulation m a x i m u m (Fig.l) rather than from the shelf break. Processes

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26 that could influence the observed deposition patterns include particle advection in the slope BNL, local resuspension, settling of the ambient fine-grained particle population at the trap depths, and settling of large particles and aggregates from shelf-break INLs.

Transport in the slope BNL Down-canyon transport of particles in the BNL, the process this experiment was designed to investigate, was found to be insignificant. Net transport measured by the current meters was up-canyon during each d e p l o y m e n t (Hickey, in prep.). Also, no substantial down-canyon flow was observed during or just subsequent to shelf resuspension events. Consistent with these results, no significant attenuation increase was observed on the canyon floor during the major resuspension events of January 11, 1981 (Fig.6A) or November 15, 1981 (Fig.6B).

Local resuspension The importance of local resuspension can be evaluated by estimating the erosive ability of the observed near-bottom currents and by examining the degree of vertical similarity in the deposition rates at individual mooring sites. The results of several investigations summarized by Nowell et al. (1981) suggest that "observable" erosion of fine-grained ( ~ 2 0 pm diameter)sediments requires a friction velocity (u.) of at least 0.7 cm s-'. An approximate value for the free-stream velocity (u) necessary to cause a u. = 0.7 cm s-1 is given by: u 2 = u2,/CD

The drag coefficient (CD) for steady flow is about I × 10 -3 (Nowell et al., 1981; Lavelle and Mofjeld, 1983), yielding u ~ 18 cm s -1. This estimate is probably conservative; Rhoads et al. (1978) observed a "critical erosion velocity" 1 m off the bed for sediment in the range 5--53 pm of 30--50 cm s -1. Near-bottom speeds (based on 20-min estimates) indicate that only at the shelf edge could resuspension have been significant. Near-bottom speeds exceeded 25 cm s-' during every interval on the shelf but never at the lower canyon (Q~) or canyon head (Q~ or Q~) locations (Hickey et al., this volume). If resuspension did occur in the canyon or over the open slope, perhaps as a result of bed processes that are poorly understood at present, we would expect to find a vertical gradient in the trap deposition rate. At both locations with multiple traps, however, no vertical gradients were observed. The average deposition ratio between traps in the canyon upper head [Q~(450)/Q~(250)] was 1.0, and in the canyon lower head was 1.1 [Q~(1237)/Q~(1037)] and 1.2 [ Q 33(1187)/Q~(1037) ], implying that the principal particle source at each location was above the uppermost trap. The average least-squares regression coefficient for all trap pairs was 0.94 -+ 0.02.

27

Vertical settling o f fine-grained particles

A high correlation between deposition and fine-grained particle concentration at the trap depth would indicate that deposition is controlled by vertical settling of these particles. A least-squares regression between normalized deposition rates [(D i -- D ) / D , where D is the deployment deposition mean and i is the trap interval] and normalized attenuation [(c i -- ~)/~] yields a high correlation (r = 0.77, significant at the 90% level) only at the shelf edge (Fig.8A). In the canyon lower head (Q~) the correlation is not significant at

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Fig.8. Comparison of normalized deposition rate (dots) and normalized attenuation (circles and triangles) for each trap interval. The temporal deposition pattern in the canyon and over the open slope is significantly correlated with the temporal pattern o f attenuation in the overlying INL, as measured by the transmissometer at Q] (252 m).

28 either 1039 or 1189 m (r = 0.30 and 0.26, respectively) (Figs.8C and D). With the assumption that the attenuation pattern in the canyon neck (Q~) was the same as that at the nearby lower canyon head site (Q~), the normalized deposition in the canyon neck is likewise not significantly correlated (r = 0.46) with the local particle concentration. This assumption is not unreasonable because attenuation fluctuations within 200 m of the b o t t o m in the deep canyon are advectively controlled (Hickey et al., this volume) and the horizontal velocity components at those depths are coherent.

Vertical transport of large particles from the shelf-break INL The inability of the first three processes to account for the observed timedependent deposition suggests that shelf-break INLs control slope deposition via the rapid settling of large particles and aggregates. This inference is supported by the positive and significant (at the 95% level) correlation between fluctuations in INL particle concentration [normalized attenuation at 252 m in the canyon upper head (Q~)] and near-bottom normalized deposition in the canyon upper head (r = 0.71, Fig.8B), in the narrow neck of the lower canyon (Q~, r = 0.79, Fig.8E), and over the open slope (r = 0.83, Fig.8F). The correlation is not significant at the canyon lower head location (Q~) due to a second, stronger peak in intervals 8 and 9 that is not present in the INL attenuation record and cannot be fully explained with the available data. This peak may be related to the small shelf resuspension events that occurred during intervals 7 and 8 (Fig.6B), causing substantial deposition increases in the shelf trap; minor deposition increases also occurred at all other slope locations during interval 8 (Fig.7), but none were as large as at Q~. This disparity could be related to the significant and time variable vertical structure in horizontal currents observed at that location (Hickey, in prep.). COMPARISON OF TRAP DEPOSITION RATES AND BOTTOM SEDIMENT ACCUMULATION RATES Agreement between contemporary deposition rates from sediment traps and recent (10--100 yr) b o t t o m accumulation rates obtained by 21°Pb-dating of cores (Nittrouer, 1978; Carpenter et al., 1982; Thorbjarnarson et al., this volume) generally improved with distance from the mid-shelf accumulation m a x i m u m (Table 2). At the shelf edge, the mean trap deposition rate exceeded the sediment accumulation rate by a factor of ten. This disparity is attributable to the frequent shelf resuspension events that recycle surficial b o t t o m sediments within the b o t t o m boundary layer and cause an artificially high trapping rate (Gardner et al., 1983). During intervals 9 and 10, when resuspension was weak and current speeds rarely exceeded 15 cm s -~ (Fig.6B; Hickey et al., this volume), the deposition rate was consistent with the sediment accumulation rate (7.6, cf. 5.2 g m -2 day-l). Off the shelf, where resuspension is minimal, the difference between mean trap deposition rates and

29 TABLE 2 Comparison of trap deposition rates (mean value for all ten collection intervals) with radiometrically dated net b o t t o m sediment accumulation rates in the vicinity of each mooring Trap deposition rate (g m -2 day -1) Deployment mean Shelf edge: Q~, 145 m

54 ÷ 58

Canyon upper head: Q~, 250 m Q~, 450 m

58 ± 71 60 ± 65

Hypothetical annual mean

Sediment accumulation rate (g m ~ day -1)

10.9 ± 2.2 a (n = 6) 5.2 _+ 2.3 b (n = 4)

20 14

14.0, 9.9 b

Canyon lower head: Q~, 1037 m Q~, 1187 m Q], 1237 m

5.57 ± 3.00 6.70 ± 3.08 6.28 ± 2.19

3.2 3.6 3.8

4.8 ± 2.1 b (n = 3) 3.3 ÷ 0.5 c (n = 5)

Canyon neck: Q~, 1575 m

3.10 ± 1.71

1.8

3.6, 3.0 b

2.2 ÷ 0.3 c (n = 3) Open slope: Q~, 393 m

3.04 ± 2.97

2.2

1.5 ± 0.7 b (n = 3)

a F r o m Nittrouer (1978). b F r o m Carpenter et al. (1982). CFrom Thorbjarnarson et al. (this volume). s e d i m e n t a c c u m u l a t i o n r a t e s d e c r e a s e d f r o m a f a c t o r o f 5 in t h e c a n y o n u p p e r h e a d t o a f a c t o r o f o n l y 1.2 a t t h e c a n y o n n e c k . The observed differences between sediment trap and sediment core deposit i o n r a t e s o f f t h e s h e l f is a n a r t i f a c t o f w i n t e r t i m e - o n l y t r a p d e p l o y m e n t s . Agreement improves from the canyon upper head to the canyon neck because of the seaward decline of seasonal storm effects on the deposition fluctuations (Fig.7). In the canyon upper head the deposition rate was extremely sensitive to shelf current conditions, changing by a factor of >50 between energetic periods with strong northward flow and quiescent periods when flow was w e a k o r s o u t h w a r d . D e p o s i t i o n in t h e c a n y o n n e c k , t h e m o s t s e a w a r d l o c a t i o n , d i f f e r e d b y a f a c t o r o f o n l y five b e t w e e n e n e r g e t i c a n d q u i e s c e n t p e r i o d s . Results from year-long sediment trap deployments should be consistent w i t h l o n g - t e r m s e d i m e n t a c c u m u l a t i o n r a t e s if d e p o s i t i o n o n t h e s l o p e is in fact controlled by settling from the INL rather than by along-bottom transport. In the absence of such data, we calculated hypothetical annual d e p o s i t i o n r a t e s (DA) b a s e d o n t h e e x p e c t e d a n n u a l d u r a t i o n o f o b s e r v e d deposition rates resulting from intervals of low (<30 cm s-l), moderate ( 3 0 - - 7 0 c m s - l ) , a n d h i g h ( > 7 0 c m s -1) s h e l f c u r r e n t s p e e d s . ( T h i s w e i g h t i n g

30 m e t h o d makes the implicit assumption that wave resuspension on an annual basis is related to current speed exactly as during the period of observation.) Thus: D A = D1F1 + D m F m + D h F h ,

(1)

where D1, Dm, and D h are the off-shelf deposition rates characteristic of low, medium, and high current speed categories found on the shelf for fractions F~, Fro, and F h of a year. Analysis of long-term, n e a r - b o t t o m current records fr o m a mid-shelf site south of Quinault Canyon gave F~ = 0.89, Fm = 0.11, and F h = 0.003 (Sternberg and McManus, 1972). The measured shelf currents (Q1, Fig.6B) show that intervals 7--10 comprised a period of low speeds, intervals 3 and 4 comprised a period of m o d e r a t e speeds, and interval 5 was d o m i n a t e d by high speeds*. Thus, for example, D A for the 450-m trap in the c a n y o n upper head is estimated by inserting the mean deposition rate for each o f the appropriate current speed periods into eqn. (1): DA = 2.5 g m - : d a y - l ( 0 . 8 9 ) + 1 0 4 g m -2 da y -~(0. 11)+ 1 8 9 g m -2 d a y - l ( 0 . 0 0 3 ) = 1 4 g m - 2 d a y -1 The close agreement of the D A values from all locations to the long-term sediment accumulation rates (Table 2) again emphasizes the i m port ance of vertical transport from the INL and the minor role t hat the BNL of this c a n y o n plays in seaward sediment transport. A few major storms in each year co n tr o l the seaward sediment dispersal o f f the shelf and resultant deposition on the slope. VERTICAL TRANSPORT PROCESSES Knowledge of the timing of shelf resuspension events and the short-term deposition patterns p r o d u c e d over the slope allows us to estimate the vertical transport rate of sedimentary particles, as well as speculate on the transport processes. Th e vertical transport rate must be rapid enough to have delivered a significant fraction of the particles in an INL created by the N ovem ber 11--15 resuspension event to the c a n y o n neck trap (1575 m) by November 23 (end o f interval 5). This schedule requires a settling velocity of 100--200 m day -1. Fecal pellets sink at rates of from 10 to > 2 0 0 m day -1 (Fowler and Small, 1972; Small et al., 1979) and are natural suspects for the rapid vertical transport of particles. Discrete, intact pellets, however, comprised only 2.4 to 11.3% by weight of the trapped particles, similar to the results of several o t h e r deep-water trap studies (Honjo, 1980; Honjo et al., 1982a; Pilskaln, 1984).

*At the lower canyon head location, the low-speed deposition rate was obtained from interval 10 data only. High deposition rates in intervals 7, 8, and 9 are thought to be related to periods of energetic resuspension on the shelf.

31 The scarcity of pellets in the trap samples implies that am orphous detritus such as marine snow (Silver et al., 1978) and unpellatized fecal m a t t e r (Bishop et al., 1978) cont r i but e substantially to the observed flux. Bishop et al. (1978) concluded that a m o r p h o u s fecal m a t t e r sinking faster than 100 m day -1 d o m i n a t e d the vertical flux at several stations in the Atlantic; Shanks and T r e n t (1980) measured marine snow sinking rates of 43--95 m day 1. Honjo et al. (1982b) suggested that a m o r p h o u s organic detritus would scavenge fine-grained lithogenic particles as it .sunk. It appears that such detritus sinking through a c o n c e n t r a t e d INL may play an i m p o r t a n t role in the sedimentation of lithogenic particles on the continental slope. CONCLUSION The first-order c ont r ol on c o n t e m p o r a r y deposition on the Washington continental slope is the f o r m a t i o n of shelf-break dept h intermediate nepheloid layers by shelf resuspension events. Seaward extension and particle conc e n tr atio n in observed INLs were positively and significantly correlated with the magnitude of resuspension events on the adjacent shelf and with the short-term (9.5 days) t e m por a l variation of deposition rates on the slope. Deposition rates were not related to b o t t o m resuspension or advection in the b o t t o m nepheloid layer. The episodic injections of large am oun t s of particulate m a t t e r into the slope waters by shelf processes were registered on the underlying sea floor within a few days, before t h e y could be dispersed by the regional circulation. Estimated vertical transport rates ( 1 0 0 - - 2 0 0 m day -1) and the lack of correlation between n e a r - b o t t o m transmissometer and sediment trap data suggests that particles are packaged for vertical transport into fast-settling but rare aggregates. Evidence from this study and others suggests that a m o r p h o u s aggregates such as marine snow and fecal m a t t e r account for a far larger share o f the transport than intact fecal pellets. C o n t e m p o r a r y deposition rates over the slope were consistent with recent (100-yr) accumulation rates based on 21°Pb-dating (Carpenter et al., 1982; Thorbjarnarson et al., this volume), when deposition rates were annually weighted by the e xpe c t e d magnitude and f r e q u e n c y of current-induced resuspension events at the shelf edge. In addition, the spatial distribution of c o n t e m p o r a r y deposition rates was consistent with that for 100-yr scales, indicating that deposition rates are higher in the upper head of Quinault Canyon than on the open slope or in the c a n y o n below a b o t t o m dept h of 1200 m. Deposition rates are high in the c a n y o n upper head because it underlies the INL created by resuspension and across-isobath advection of the mid-shelf deposit o f Columbia River silts. The shape of the head of Quinault c a n y o n on its south side -- a broad bank on the shelf to the west of a narrow valley (see Fig.2) -- is particularly favorable to INL form at i on over the head. Deposition rates are lower elsewhere in the c a n y o n and on the open slope because particles are supplied primarily by horizontal diffusion rather than

32 b y across-isobath (east--west) a d v e c t i o n , and because o u t e r shelf s e d i m e n t s are a relatively p o o r s o u r c e o f e r o d a b l e fine-grained particles t h a t c o u l d diffuse seaward to s u p p l y t h e slope INL. C o m p a r i s o n o f o u r results with t h o s e f r o m east c o a s t c a n y o n s ( B u t m a n , 1 9 8 3 ; Gardner, 1 9 8 3 ; H u n k i n s , 1 9 8 3 ) e m p h a s i z e s the individuality of the s e d i m e n t t r a n s p o r t e n v i r o n m e n t a r o u n d d i f f e r e n t c a n y o n s . S e d i m e n t s as d e e p as 6 0 0 m in these east c o a s t c a n y o n s were actively r e s u s p e n d e d and a p p a r e n t l y dispersed seaward w i t h i n a deep I N L created b y converging upand d o w n - c a n y o n currents. Shelf-break I N L s did n o t result in significant s e d i m e n t r e d i s t r i b u t i o n processes. D i f f e r e n t s e d i m e n t t r a n s p o r t environm e n t s b e t w e e n c a n y o n s m a y result f r o m d i f f e r e n t c a n y o n geometries, d i f f e r e n t shelf w i d t h and b a t h y m e t r y (e.g., a shelf valley), d i f f e r e n t s e d i m e n t source f u n c t i o n s , d i f f e r e n t d e n s i t y stratification, a n d / o r d i f f e r e n t c u r r e n t ( b o t h subtidal and tidal) and wave fields over the shelf and slope. Whatever the specific details o f a particular c a n y o n e n v i r o n m e n t , h o w e v e r , it seems clear f r o m this increasing b o d y of k n o w l e d g e a b o u t s u b m a r i n e c a n y o n s t h a t episodic f o r m a t i o n o f I N L s plays an i m p o r t a n t role in c o n t e m p o r a r y shelfslope particle e x c h a n g e processes. ACKNOWLEDGEMENTS Financial s u p p o r t for this research was p r o v i d e d t h r o u g h N S F G r a n t s O C E 8 0 - 0 9 2 2 7 and O C E 8 3 - 0 8 1 9 6 and D O E C o n t r a c t D E - A T 0 6 - 7 6 - E V - 7 1 0 2 5 . REFERENCES Baker, E.T., 1976. Distribution, composition, and transport of suspended particulate matter in the vicinity of Willapa submarine canyon, Washington. Geol. Soc. Am. Bull., 87 : 625--632. Baker, E.T. and Lavelle, J.W., 1984. The effect of particle size on the light attenuation coefficients of natural suspensions. J. Geophys. Res., 89: 8197--8203. Baker, E.T. and Milburn, H.B., 1983. An instrument system for the investigation of particle fluxes. Cont. Shelf Res., 1: 425--435. Baker, E.T., Feely, R.A., Landry, M.R. and Lamb, M., 1985. Temporal variations in the concentration and settling flux of carbon and phytoplankton pigments in a deep fiordlike estuary. Estuarine Coastal Shelf Sci., 21: 859--877. Barnard, W.D., 1978. The Washington continental slope: Quaternary tectonics and sedimentation. Mar. Geol., 27: 79--114. Bartz, R., Zaneveld, J.R.V. and Pak, H., 1978. A transmissometer for profiling and moored observations in water. Soc. Photo-Opt. Instrum. Eng. J., 160(V): 102--108. Beer, R.M. and Gorsline, D.S., 1971. Distribution, composition, and transport of suspended sediment in Redondo submarine canyon and vicinity (California). Mar. Geol., 10: 153--175. Biscaye, P.E. and Olsen, C.R., 1976. Suspended particulate concentrations and compositions in the New York Bight. In: M.G. Gross (Editor), Middle Atlantic Continental Shelf and The New York Bight. Am. Soc. Limnol. Oceanogr. Spec. Syrup., 2: 45--57. Bishop, J.K.B., Ketten, D.R. and Edmond, J.M., 1978. The chemistry, biology, and vertical flux of particulate matter from the upper 400 m of the Cape Basin in the southeast Atlantic Ocean. Deep-Sea Res., 25: 1121--1161. Bloesch, J. and Burns, N.M., 1980. A critical review of sedimentation trap technique. Schweiz. Z. Hydrol., 42: 15--55.

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