Analysis of suspended-particle-size distributions over the Nova Scotian Continental Rise

Marine Geology, 66 (1985) 189--203

189

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

ANALYSIS OF SUSPENDED-PARTICLE-SIZE THE NOVA SCOTIAN CONTINENTAL RISE

DISTRIBUTIONS

OVER

MARY JO RICHARDSON and WILFORD D. GARDNER

City University of New York, Lehman College, Bronx, N Y 10468 (U.S.A.) Lamont-Doherty Geological Observatory of Columbia University, Palisades, N Y 10964 (U.S.A.) (Accepted for publication June 15, 1984)

ABSTRACT

Richardson, M.J. and Gardner, W.D., 1985. Analysis of suspended-particle-size distributions over the Nova Scotian Continental Rise. In: A.R.M. Nowell and C.D. Hollister (Editors), Deep Ocean Sediment Transport -- Preliminary Results of the High Energy Benthic Boundary Layer Experiment. Mar. Geol., 66: 189--203. The relationships of particle-size distributions to particle concentrations, silicate content and process of b o t t o m boundary layer formation on the Nova Scotian Continental Rise were examined to analyze the dynamics of the benthic nepheloid layer. High particle concentrations (>400 ~g 1-') in the intense benthic nepheloid layers are nearly always associated with a coarse (-~ 8 urn) particle mode. These coarse particles must be renewed on a time scale of weeks in order to be maintained in suspension. No unique type of particle-size distribution in the benthic nepheloid layer is associated with water masses distinguished on the basis of silicate content. However, the method of formation of the b o t t o m boundary layer does influence the particle-size distributions. Locally mixed b o t t o m boundary layers are dominated by a coarse particle mode (-~8 urn). Bottom boundary layers composed of hydrographically distinct filaments of b o t t o m water mainly contain fine particles (-~ 2--3 ~m) obtained upstream. Where the filament has extremely high particle concentrations (>1 mg 1-'), the fine particle mode is masked by a locally resuspended coarse mode indicating that the filament must occasionally (within 200-300 kin) be accelerated by some external energy to the point that the critical shear stress for erosion is exceeded. INTRODUCTION A p r i m a r y s o u r c e o f s u s p e n d e d p a r t i c l e s in t h e d e e p o c e a n is t h e p r o d u c tive surface waters, where plankton dominate the particles present. The skeletons, pellets and biogenic debris sink, comprising a large fraction of the m a t e r i a l in t r a n s i t t h r o u g h t h e w a t e r c o l u m n . I n o r g a n i c d e t r i t u s a l s o e n t e r s the ocean via rivers, wind and glaciers. Ultimately this material becomes the seafloor sediments. Particles settling from the surface waters are constantly subjected to the physical, biological and chemical processes of aggregation, disaggregation, decomposition and dissolution. These processes alter the particles during 0025-3227/85/$03.30

© 1985 Elsevier Science Publishers B.V.

190

their descent through the water column resulting in a sharp decrease in the concentration of suspended particles in the surface 100--200 m and a more gradual decrease through the rest of the water column. In many areas of the oceans, as the sea floor is approached, there is an increase in the concentration of suspended particles. This deep-ocean increase in the concentration of suspended particles is the benthic nepheloid layer. Particles in the benthic nepheloid layer consist of primary particles settling from the surface waters and particles from the seafloor resuspended either by deep-sea biota or eroded by deep-sea currents. Resuspended particles are usually the dominant c o m p o n e n t of the nepheloid layer. The size distribution of particles in the nepheloid layer is affected by the source of resuspended particles, gravitational settling, and the dynamics of the bottom boundary layer, i.e. eddy diffusion (usually upward) caused by vertical turbulent diffusion of sediment against the local vertical gradient in particle concentration, and horizontal advection. The Nova Scotian Continental Rise is the region of detailed investigation in the High Energy Benthic Boundary Layer Experiment (HEBBLE). This location was chosen because it is along a western boundary of the Atlantic where deep equatorward currents flow, where strong benthic nepheloid layers exist, and where large topographic features are minimal. During the initial stages of the project in 1979 on R/V "Knorr" Cruise 74, general survey measurements were made in a 2 6 0 , 0 0 0 km 2 area {Fig.l). Hydrographic 66"

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profiles were made with a CTD, nephelometer, transmissometer and water bottles (Spinrad et al., 1983; Gardner et al., 1983). Strong benthic nepheloid layers were found with near-bottom particle concentrations up to several mg 1-' {Gardner et al., 1983). Short-term (-~2 weeks) and long-term (~-8 months) current velocity measurements generally show a southwestward flow over the rise with mean velocities from 8 to 32 cm s-1 and an exceptionally high m a x i m u m short-term velocity of 73 cm s-1 (Richardson et al., 1981). Temperature and silicate measurements show that this equatorward flow usually consists of southern-source (Antarctic) b o t t o m water typical of the North American Basin (Richardson et al., 1981; Weatherly and Kelley, 1982). However, there was at least one period of time when northern-source (Denmark Straits) b o t t o m water was f o u n d in the region (Richardson et al., 1981). In examining CTD data during the study, Weatherly and Kelley (1982) observed that some of the b o t t o m layers are " t o o cold" to be locally mixed turbulent boundary layers. They have inferred the intrusion of a continuous "Cold Filament" b o t t o m layer -~ 100 km wide and -~60 m thick flowing equatorward at ---9 cm s-1. In this paper we examine the characteristics of the particles within and outside of the benthic nepheloid layers in relationship to the intensity of the nepheloid layers, the sources of the b o t t o m water, and the process of formation of the b o t t o m boundary layer to see if we can shed further light on the dynamics of the b o t t o m boundary layer in terms of resuspension and development of nepheloid layers. METHODS

In September--October 1979, forty-five stations were made at which suspended particle concentrations and particle-size distributions were measured (Fig.l). Water samples were taken throughout the water column with ten 10-1 Niskin bottles m o u n t e d on the CTD rosette and with seven 30-1 Niskin bottles in the b o t t o m 250 m of the water column but with occasional samples up to 400 m above the bottom. Particle concentrations were determined for the 30-1 Niskins by filtering the water through 0.4 pm nuclepore filters (Gardner et al., 1983). Few particle concentration measurements were made using the 10-I bottles. Particle-size distributions in water samples were determined with a model T A I I Coulter Counter. Special precautions must be observed in using particle counters at sea to combat problems of electronic noise, vibration, and continuous ship motion n o t encountered in the laboratory. Electronic noise was reduced by: (1) building a wire and aluminum foil Faraday cage around the sensing stand; (2) connecting the cage, stirring m o t o r housing, sensing stand housing, and electronics housing to c o m m o n ground; and (3) using a constant voltage supply and noise filter to compensate for voltage changes frequently experienced on ships, although the internal electronics of the Coulter Counter is usually sufficient to overcome this problem. Radio transmission can also interfere with counters so samples were n o t processed during the

192 daffy ship radio communication. Vibration was reduced by mounting the Coulter Counter with sufficient padding. The biggest problems with particle counters at sea arises from the fluctuations in the mercury level of the manometer caused by rolling and pitching of the ship. One suggested solution has been to count in the TIME mode with the mercury moving up and down and assume that the average count is correct, but this m e t h o d is erroneous for two reasons. First, the fluctuations cause changes in the flow rate through the sensing zone. Since there is a short dead time after each particle is counted, increase in the flow rate could reduce the number of particles counted. Second, and more important, is the possibility t h a t the oscillation of mercury will force fluid back through the orifice, thus recounting some particles. The surging of mercury was damped by partially clamping the tubing connecting the aperature stopcock control and the manometer. This simple modification allows use of the counter n o t only in t i m e and p r e s e t c o u n t modes, but also in the m a n o m e t e r mode. The restriction was adjusted so the flow rate through the clamped tubing is only slightly greater than that through the orifice. When the clamp is too tight the flow is blocked and the signal becomes noisy. In practical terms this condition is met when ship motion moves the mercury meniscus less than 1--2 mm in the reset mode. A further check was made by comparing counting time for a known volume in the lab and at sea at several temperatures since flow rates are affected by viscosity differences due to changes in temperature. When the time to count 2 ml of sample as measured by the manometer differed by more than 10% from the time taken in the laboratory, the results were discarded and the sample was rerun. When operating in the t i m e mode there is the risk that the clamp is too tight and the volume counted is smaller than expected. A good alternative to a clamp is to use a glass stopcock as done by workers at Halifax (C. Boyd, pers. commun., 1980}. Analyses were made with a 50 pm aperture. The two lowest channels were deleted in the reduction of data since these channels are frequently contaminated with electronic noise. The uppermost channel contains all particles larger than 20 p m, however, there were often no counts recorded in that channel. Since the sample was n o t stirred during the counting time (to avoid additional electronic noise) particles larger than 24 ~m with a density of 2.5 g cm -3 would settle below the aperture tube orifice and n o t be counted. So nominally the range of particles measured is 1--24 um. RESULTS The conditions on the Nova Scotian Continental Rise of swift and variable currents, high and low particle concentrations, different water sources, and different processes of b o t t o m boundary layer formation provide a setting for examination of the relationship of these water and particle properties with particle-size distribution to understand the dynamics of the benthic nepheloid layer.

193 Particle concentration

The concentrations of suspended particles in the benthic nepheloid layers varied from a few tens of t~g y1 to several mg 1-1 during the period of this study (Gardner et al., 1983). The high concentration values exceed any previously reported in the deep ocean. A few samples had such high concentrations that the samples would have to be diluted to insure that particle size distributions were n o t being biased by coincidence of particles during passage through the orifice of the aperture tube. Since these samples had large, rapidly settling particles (deposits of silt-sized material were observed at the b o t t o m of 20-1 water samples} that would settle rapidly without stirring, these samples were n o t counted. To examine the relationship between concentration and particle-size distributions the stations were grouped into fifteen low concentration stations ( < 4 0 0 pg 1-1) and twenty-one high concentration stations (> 400 ~ g 1-1 ; Table 1) with representative examples shown in Fig.2. The low concentration stations have a m o d e at 2--3 p m that does not shift with depth. Many of these distributions show bimodality with a second mode at 20 /~m which decreases or disappears with depth. It must be rem e m b e r e d that the distributions are shown as percent total volume vs. logarithmically increasing equivalent spherical diameter. This allows comparison of distributions with markedly different concentrations. However, at the low concentrations reported here the second mode at 20 p m is composed only of one or t w o particles and may n o t be significant. The high concentration stations are markedly different from the low concentration stations. Above the intense benthic nepheloid layer where concentrations are less than 400 p g 1-I the distributions are similar to those seen in the low concentration stations. They have a predominant mode at 2--3 pm. Some have a secondary mode at 20 pm. However, as particle concentrations increase, with closer proximity to the bed, the distributions change character. The distributions in the lower portion of the water column have a larger predominant mode (-~8 tim), demonstrating a coarsening of the suspended particle mode with depth. This feature is seen at almost all stations where concentrations exceed 400 #g 1-1 (Table 1). The only stations which have high particle concentrations and no coarsening with depth are stations 21, 40 and 43 (Table 1). These exceptions are discussed later and are explained in terms of b o t t o m boundary layer formation. Therefore, samples with high particle concentrations from intense benthic nepheloid layers have a coarse mode (-~8 p m) of suspended particles representative of local resuspension, whereas samples from low concentration nepheloid layers have a fine mode (-~2--3 t~m) indicative of having less intense local erosion.

194

TABLE 1 T a b u l a t i o n s o f KN 74 particle data for the b o t t o m b o u n d a r y layer. Particle c o n c e n t r a t i o n s are given for t h e w a t e r b o t t l e closest t o t h e s e a f l o o r (usually w i t h i n 20 m). High c o n c e n t r a t i o n s (H) are > 4 0 0 pg 1-~. L o w c o n c e n t r a t i o n s (L) are < 4 0 0 ug 1-~. Silicate values are given for a w a t e r s a m p l e w i t h i n 20 m of the sea floor. High values are > 25 uga t o m l -~. L o w values are 1 0 - - 2 0 u g - a t o m 1-~. The b o u n d a r y layer is classified as being either in t h e " C o l d F i l a m e n t " (I) or o u t s i d e the " C o l d F i l a m e n t " (O). The particle-sized i s t r i b u t i o n m o d e s are classified as either c o a r s e n i n g w i t h d e p t h (C) ( f r o m 2--3 t o 8 u m ) or r e m a i n i n g fine (F; 2--3 p m ) . A dash indicates n o data or inconclusive data. N o t e t h a t the c o n c e n t r a t i o n s are mass c o n c e n t r a t i o n s d e t e r m i n e d by filtration w h e r e a s c o n c e n t r a t i o n s in Figs.2, 4 a n d 6 are v o l u m e c o n c e n t r a t i o n s Station

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18.7 13.6 12.8 19.9 13.6 19.8 25.1 34.0 33.5 39.4 37.3 32.4 25.1 39.8 37.0 25.1 33,0 33,0 34.9 -26.9 23.0 34.2 30.3 26.2 24.9 27.4 39,8 35.6 -33.1 -34.5 33.9 ---

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Fig.2. P a r t i c l e - s i z e d i s t r i b u t i o n s f o r s e l e c t e d s t a t i o n s . D i s t r i b u t i o n s are p l o t t e d as p e r c e n t o f t h e t o t a l v o l u m e o f p a r t i c l e s c o u n t e d in l o g a r i t h m i c a l l y i n c r e a s i n g s i z e r a n g e s vs. e q u i v a l e n t s p h e r i c a l d i a m e t e r . N u m b e r s a l o n g t h e l e f t s i d e o f t h e d i s t r i b u t i o n s are m e t e r s a b o v e b o t t o m . N u m b e r s o n t h e d i s t r i b u t i o n s are t o t a l v o l u m e o f p a r t i c l e s m e a s u r e d in p p b . Dist r i b u t i o n s are s e g r e g a t e d b y p a r t i c l e c o n c e n t r a t i o n ( h i g h > 4 0 0 /~g l-Z; l o w < 4 0 0 u g 1-z) a n d t h e s i l i c a t e c o n t e n t ( h i g h > 2 5 u g - a t o m l-Z; l o w 1 0 - - 2 0 u g - a t o m l -z) o f t h e w a t e r . A c o a r s e n i n g in t h e p a r t i c l e - s i z e m o d e t o w a r d s t h e b e d is o b s e r v e d in t h e s a m p l e s w i t h h i g h p a r t i c l e c o n c e n t r a t i o n s as d i s c u s s e d f u r t h e r in t h e t e x t .

Silica te

The silicate content of the bottom water in the benthic nepheloid layers varied from 10 to 40/~g-atom 1-I during the period of this study (Richardson et al., 1981). The low silicate values (10--20 pg-atom 1-1) represent advection of northern-source water into the region which more normally has high silicate ( > 2 5 #g-atom 1-1) southern-source water. We examined the particle-size distributions to determine whether these different water masses had distinctive particle characteristics representative of their origin. The silicate signal was n o t related to current direction or speed variation (Richardson et al., 1981). No correlation between silicate values and suspendend particle concentrations was seen (see Fig.2). Also no distinctive relationship is seen between the particle-size distributions and the silicate content of the water masses (Fig.2). The low silicate stations (sta.9, 10, 13 and 16) are found along one transect made at the beginning of the cruise (Fig.l). The high silicate stations (sta.30, 31, 36 and 39; Fig.l) were taken later. Since the silicate content is not related to current direction, speed, suspended particle concentration or

196 suspended-particle-size distribution, we infer that this anomalous intrusion of northern-source water is an isolated parcel of water which was pulsed with sufficient energy to generate bed-shear stresses that locally eroded material in the isolated instances of high particle concentrations.

Bottom boundary layers Two types of b o t t o m mixed layers are observed in the HEBBLE region. One t y p e is the "Cold Filament" of Weatherly and Kelley {1982). These b o t t o m mixed layers {Fig.3, sta.19, 20 and 21) are " t o o cold" to have resulted from vertical mixing of the b o t t o m layer given the gradient in potential temperature above the thermocline capping the layers. Weatherly and Kelley (1982) interpret this "Cold Filament" to be a continuous feature that is -~100 km wide and ~ 6 0 m thick. This feature was observed both during the collection of this data and the following year as well. The other type of b o t t o m mixed layer is a homogeneous turbulent boundary layer caused by vertical mixing of the b o t t o m tens of meters of the water column {Fig.3, sta.18 and 22). This type of boundary layer was also observed during both years of study. We examined the particle characteristics in the t w o different types of boundary layers to determine whether the dynamics of the boundary layers were sufficient to cause differences in the size of particles resuspended or held in suspension. To make a reasonable assessment of potential boundary layer effects on the particle characteristics, two stations were chosen that had similar suspended particle concentrations and silicate contents to avoid the influence of these other variables. Stations 21 and 22 both have high concentrations of suspended particles and high silicate contents (Table 1). The particle-size distributions for station 21 (taken in the "Cold Filament"; Fig.3) have a primary mode at -~3 ~m in the water above the "Cold Filament" [ > 5 0 m above b o t t o m (m.a.b.), Fig.4]. Within the "Cold Filament" ( < 5 0 m.a.b., Fig.4), the particle-size distributions are broader and the modal peak is over a larger size range (2--5 p m ) but the peak does n o t shift significantly. Since station 21 has high particle concentrations we would have expected a coarser particle m o d e in the b o t t o m mixed layer as reported above. Since the coarser particles are n o t present in as great an abundance they must have been deposited upstream. The particle-size distributions for station 22 (taken outside the "Cold Filament"; Fig.3) does show features similar to those stations that have high suspended particle concentrations (Fig.4, cf. Fig.2). Above the mixed layer (>70 m.a.b.) the particle size distribution shows a mode at 2-3 #m. As the sea floor is approached the shape of the distributions change. Within the b o t t o m mixed layer (the b o t t o m 70 m), the distributions show a coarsening in the predominant particle mode from 2--3 to 7--9 pm. Of the thirteen stations which were taken in the "Cold Filament" all b u t four stations show particle size distributions similar to that of station 21 (i.e. the particle-size-distribution mode remains at 2--3 ~m throughout the b o t t o m boundary layer; Table 1). The four anomalous stations (stations 24,

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25, 31 and 44) all have exceptionally high particle concentrations whose size distributions do show a coarsening of the mode in the bottom boundary layer. Such exceptionally high particle concentrations of coarse particles are unlikely to be passively transported in the "Cold Filament" estimated to be traveling at 9 cm s -1 (Weatherly and Kelley, 1982). These coarser particles must then be locally derived. Thus, differences in the particle-size distributions were observed depending

198 on the process of formation of the b o t t o m boundary layer. The "Cold Filam e n t " boundary layer generally had fin e particles which would travel along with the layer. The turbulent mixed-layer, boundary layers had coarser particles which would be locally derived (Fig.4).

Apparent density One of the characteristics of suspended particles most difficult to determine is their in-situ bulk density, which is essential to accurately calculate particle settling velocities. However, calculations of an apparent density may be useful in providing insight into the nature and source of the suspended material (Richardson, 1980). Organic-rich matter has a density close to that of seawater, whereas mineral grains have a density approaching 2.5 g cm -3. However, few marine particles are c o m p o s e d solely of either organic matter or mineral grains. Organic-rich matter, fecal pellets, and aggregates are often c o m p o s e d of varying percentages of skeletons of plankton, organic matter, and clays. Contents of organic-matter in suspended matter drop substantially from 25--50% in the water column to < 2 % in surface sediments (Baker et al., 1979; R o w e and Gardner, 1979). The apparent density is the ratio between mass and volume concentrations of a sample of suspended particles. The mass concentration was determined b y filtration, and the volume concentration by the Coulter Counter analysis. Filtration gives a measurement of dry weight of particles per volume of seawater. The Coulter Counter gives a measurement of w e t volume of particles per volume of seawater. The ratio of the t w o gives a dry weight of material per w e t volume of material. It is evident that these calculations of apparent density cannot be thought of as actual bulk densities since the m e t h o d of obtaining the volume of particles uses only a subset of the total particle population. For example, the Coulter Counter was used to measure only particles of a given size range, nominally 1--24 um, whereas the filter pore size of 0.4 u m w o u l d trap most particles greater than 0.4 ~m, along with some smaller particles as well. For this reason, the apparent density calculations are used for comparative purposes only. Apparent densities for the samples taken in the stations reported in the previous sections ranged from 0.61 to 3.12 g cm -3. However, generally there is a linear relationship between particle volume measured with the Coulter Counter and particle concentration measured b y filtration (Fig.5). The apparent densities of the particles increases as total particle concentration increases. The samples which have high concentrations ( > 4 0 0 ug 1-1) of suspended particles have high apparent densities (2.22 + 0.36, n = 93) while those with low concentrations (< 400 p g 1-I ) have lower apparent densities (1.77 +- 0.53, n = 139). Therefore the mineral content of the suspended particles in high concentration samples is greater than that in low concentration samples. No difference in apparent density of particles was seen between particles within the "Cold Filament" and those outside, other than that which could be attributed to particle concentration.

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CONCENTRATION (p.g/I ) Fig.5. Plot of particle concentration determined by filtration vs. particle v o l u m e determ i n e d by Coulter Counter analysis. An apparent density o f 1 . 7 7 g c m -3 which is the mean apparent density f o u n d for l o w concentration ( < 4 0 0 #g l -z ) s a m p l e s . A n apparent density of 2.22 g c m -3 is the mean apparent density found for high concentration ( > 4 0 0 #g 1-z) samples. DISCUSSION

Our results have related particle-size distributions to particle concentration, water mass type and process of b o t t o m boundary layer formation, and particl~ concentration to apparent density. From these results we can infer that the dynamics involved in producing the b o t t o m boundary layers directly affects the characteristics of the particles in the benthic nepheloid layer. In the HEBBLE region many of the b o t t o m boundary layers are locally mixed turbulent boundary layers (Weatherly and Kelley, 1982; and Table 1). Boundary layers of this type (Fig.3, sta.22) have associated with them both intense benthic nepheloid layers, and weak nepheloid layers, in terms of particle concentration. Particles in the intense benthic nepheloid layers have a high apparent density and particle-size distributions show a coarsening of the m o d e with depth. Because the particles are relatively coarse and dense in these layers we infer that the particles are locally derived by erosion of the seabed. Based on Stokes settling and a density difference o f 1.5 g cm -3, the coarser m o d e (8 # m ) particles would settle at a rate o f 2.5 m/day. Therefore,

200 as a first-order approximation, over a period of a few weeks these particles would settle out of suspension if n o t continually replenished. So, we conclude that these benthic nepheloid layers are locally derived and maintained by the same processes which form the locally mixed b o t t o m boundary layers as observed in the potential temperature profiles. Our distributions show a coarser mode (~8 pm) in the intense benthic nepheloid layers than those of McCave (1983) taken a year later (----5 pm; Fig.6). Particle concentrations and current velocities were also exceptionally high when our data were collected and were much lower the following year. Therefore, the coarser material which we sampled was resuspended from the seafloor, but the dynamics of the b o t t o m boundary layer were n o t sufficient to keep the coarse material in suspension. So, the benthic nepheloid layers sampled during Knorr 74 are more recent features than those sampled by McCave (1983) which perhaps have lost their coarse particles. The other type of b o t t o m boundary layer f o u n d in the HEBBLE region is the ~ 1 0 0 km wide, -~60 m thick "Cold Filament" (Weatherly and Kelley, 1982). This b o t t o m mixed layer being " t o o cold" to be locally mixed is advected into and flows through the region as a distinct layer (Weatherly and Kelley, 1982). The characteristics of the suspended particles found in the "Cold Filament" are usually different from those found in the locally mixed b o t t o m boundary layer adjacent to the filament. The particle size distribution in the benthic nepheloid layer within the filament seldom exhibit the same coarsening of the particle mode with depth as those in the locally mixed boundary layers. From this we infer that, unlike the particles in the locally mixed benthic nepheloid layers, the particles in the "Cold Filam e n t " were resuspended sufficiently far upstream for the coarse particles to settle out before entering the HEBBLE region. The fine particles are then carried passively in the filament through the region. Based on Stokes settling with a density difference of 0.5 g cm -3, these fine particles carried in the "Cold Filament" would settle at a rate of only 20 m yr -1. It would take years for the fine material to settle out of suspension w i t h o u t biogenic (filterfeeders), organic (adhesion by organic bonding) or mechanical (fiocculation and differential settling) aggregation. Whereas, with similar calculations, the coarse and dense particles are likely to settle out in a matter of weeks. This difference in the particle-size distributions with depth inside and outside the "cold filament" is also seen, but to a lesser degree, in the data of McCave (1983). His station 7 outside the "Cold Filament" shows a coarsening of the particle mode with depth {from 3 to 5 pm) whereas for station 8, adjacent to station 7 but inside the "Cold Filament" the predominant mode remains at 3 um (Fig.6). The concentrations reported by McCave (1983) are volume concentrations which are lower than mass concentrations by approximately a factor of two (the apparent density). So in comparing his data to ours, both of these stations consist of high concentration samples, on the order of 400--700 pg 1-~. We would expect t o find a coarsening with depth attributed to concentration effects for both of these stations. Since this feature is n o t found for Station 7

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Fig.6. Particle-size distributions for stations 7 and 8 o f McCave ( 1 9 8 3 ) . A coarsening in the particle m o d e w i t h depth is seen for s t a t i o n 7 ( o u t s i d e the "Cold F i l a m e n t " ) b u t n o t 8 (inside the "Cold F i l a m e n t " ) . The distributions s h o w n at the left for s t a t i o n 7 are taken from 30-1 samplers and s h o w s this feature m o r e clearly. The other distributions were taken from 5-1 samplers. F r o m McCave ( 1 9 8 3 ) .

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(Fig.6), inside the "Cold Filament", McCave's (1983) data further support our conclusion that the coarser particles have been removed from suspension before the filament reached the HEBBLE area. From our data presented here and that reported by Weatherly and Kelley (1982) and our interpretation of that of McCave {1983), the "Cold Filam e n t " is a distinct b o t t o m boundary layer in terms of its formation and suspended particle characteristics. Since the "Cold Filament" is " t o o cold" to be locally mixed, it must be mixed further upstream to the Northeast. When the b o t t o m boundary layer is formed, particles are resuspended from the seafloor to form the benthic nepheloid layer. As the "Cold Filament" flows southwestward the coarser particles are not able to remain in suspension. The high concentrations of coarse particles are estimated to settle out of the boundary layer on a time scale of a few weeks, during which time the filament, traveling at an average rate of 9 cm s-1 (Weatherly and Kelley, 1982) would flow 200--300 km. The highest concentrations of suspended particles in the filament are found downstream in the southwestern portion of the region {Table 1 and Fig.2). From this we conclude that the "Cold Filament" must locally derive sufficient energy from external sources as it flows southwestward to substantially resuspend sediment to reach the high {2.7 mg 1-1) concentrations of suspended particles present in the southwestern portion of the region. CONCLUSIONS

An examination of the particle-size distributions in the benthic nepheloid layer and their relationships to particle concentration, water mass and process of formation of the b o t t o m boundary layer lead t o these conclusions: (1) Locally mixed b o t t o m boundary layers with high particle concentrations are associated with a coarse mode (-~8 p m ) of particles which must be renewed on a time scale of weeks in order to maintain these layers. (2) The benthic nepheloid layer is n o t a function of different water masses as determined b y temperature and silicate, but is influenced by the process of b o t t o m boundary layer formation. (3) B o t t o m boundary layers which are locally mixed turbulent layers are associal~ed with benthic nepheloid layers caused by local resuspension of sediment. A b o t t o m boundary layer which is a distinct and separate ribbon of water flowing along the b o t t o m such as the "Cold Filament" has associated benthic nepheloid layers generally composed of fine suspended particles carried in the b o t t o m layer. (4) The "Cold Filament" boundary layer occasionally has exceptionally high particle concentrations and coarser particles than usual. Therefore, the "Cold Filament" must be locally (within 200--300 kin)accelerated b y some external energy source to the point that the critical shear stress for e~osion is exceeded.

203 ACKNOWLEDGEMENTS

We thank all those who participated in the successful collection of these data aboard Knorr 74. In particular, we thank Dr. Charles Hollister for his support and organization of the entire HEBBLE project. Cindy Wigley aided in the transfer of the data to the computer. This research was supported by Office of Naval Research Contacts N000-14-79-C-00-71 and N000-14-80-C0098. Lamont-Doherty Geological Observatory contribution number 3839. REFERENCES Baker, E.T., Feely, R.A. and Takahashi, K., 1979. Chemical composition, size distribution and particle morphology of suspended particulate matter at DOMES sites A, B and C: Relationships with local sediment composition. In: J.L. Bischoff and D.Z. Piper (Editors), Marine Geology and Oceanography of the Pacific Manganese Nodule Province. Plenum, New York, N.Y., pp.163--201. Gardner, W.D., Richardson, M.J., Hinga, K.R. and Biscaye, P.E., 1983. Resuspension measured with sediment traps in a high-energy environment. Earth Planet. Sci. Lett., 66: 262--278. McCave, I.N., 1983. Particulate size spectra, behavior and origin of nepheloid layers over the Nova Scotian Continental Rise. J. Geophys. Res., 88: 7647--7666. Richardson, M.J., 1980. Composition and characteristics of particles in the ocean: Evidence for present day resuspension. Ph.D. thesis, WHOI-80-52, M.I.T. Cambridge, Mass., W.H.O.I., Woods Hole, Mass. Richardson, M.J., Wimbush, M. and Mayer, L., 1981. Exceptionally strong near-bottom flows on the continental rise of Nova Scotia. Science, 213: 887--888. Rowe, G.T. and Gardner, W.D., 1979. Sedimentation rates in the slope water of the Northwest Atlantic Ocean measured directly with sediment traps. J. Mar. Res., 37: 581--600. Spinrad, R.W., Zaneveld, J.R.V. and Kitchen, J.C., 1983. A study of the optical characteristics of the suspended particles in the benthic nepheloid layer of the Scotian Rise. J. Geophys. Res., 88: 7641--7645. Weatherly, G. and Kelley, E.A., 1982. "Too cold" b o t t o m layers in the HEBBLE area. J. Mar. Res., 40: 985--1012.