Nepheloid layers on the continental slope west of Porcupine Bank

Deep-SeaResearch,Vol. 33, No. 6, pp. 791 818, 1986.

0198-0149/86$3.11tl+I).[R~ PergamonJournalsLtd.

Printed in Great Britain.

N e p h e l o i d layers on the continental slope west o f P o r c u p i n e B a n k R. R. DICKSON* and I. N. M C C A V E t (Received for publication 21 January 1986) Abstract--Bottom nepheloid layers (BNL) are well developed on the west slope of Porcupine Bank, particularly in a narrow zone on the slope lying between the 400 and 600 m isobaths. However, these layers may become detached and spread along isopycnal surfaces to form intermediate nepheloid layers (INL) further off- and along-slope. Vertical sections of properties normal to the slope show that the main zone of BNL formation is marked by a doming of temperature and density isopleths and by a 'chimney' in salinity in the overlying watercolumn. The detailed hydrographic structure, however, is one of individually mixed but discrete layers in T, S and nephels suggesting boundary mixing followed by detachment, lateral injection and hence interleaving of layers. Plots of particle volume vs size within these turbid plumes show a persistent peak at 13.5 lain: subsidiary peaks at larger particle diameters, found close to the bed, consist of aggregated smaller material. The layers are believed to be formed by bottom erosion under internal tides and waves. Under northerly (long-slope) winds the 9°C isotherm is drawn up the slope, the density gradient steepens close to the bed and the slope of the characteristic for M2 tidal frequency, a = [(~2]e)l/2/(N2 - 1%2)1/2], comes close to the bottom slope 13 at the point where maximum intensity BNL's are observed. Temperature spectra under these conditions show increased total energy and it is thought that high-frequency internal waves also propagate along the sharpened density surface to impinge on the slope.

INTRODUCTION

ON 13-17 June 1980, R.V. Cirolana worked a widely spaced exploratory grid of 10 stations on the continental slope west of Porcupine Bank using a SeaMarTech nephelometer and Bissett-Berman 9040 STD (Fig. 1). On the first stations occupied (50, 52) a conspicuous intermediate nepheloid layer (INL) was encountered at 450--600 m in -900 m water depth (Fig. 2). Attempts to trace the possible upstream origin of this feature were successful; at Sta. 94, 90 km to the south in 450 m water depth, an intense bottom nepheloid layer (BNL) was discovered which far exceeded the amplitude of any BNL encountered in the stations to date (Fig. 2). None of the short transects worked in 1980 suggested that the INL's encountered penetrated far from the slope into the interior (e.g. Stas 55 and 92, Fig. 2). In 1981, 1982 and 1983 we examined the properties, distribution and physical processes responsible for these nepheloid layers, the results of which are presented here. Several studies of sediment dispersal from the shelf have noted intermediate nepheloid layers (i.e. separated from the bottom) off the continental slope (DRAKE, 1971, 1974); * Ministry of Agriculture, Fisheries and Food, Directorate of Fisheries Research, Fisheries Laboratory, Lowestoft, Suffolk NR33 OHT, U.K. ? Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K. 791

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DRAKE and GORSLINE(1973) showed many such layers in the Santa Barbara channel and in submarine canyons off southern California. BAKER(1976) and BAKERet al. (1974) also demonstrated INL's in the waters of Willapa Canyon and over Nitinat fan off Washington. CONNARY and EWlNG (1972) showed several INL's along the slope and rise between the Congo and the Niger rivers and suggested a possible relationship with water and suspended matter from the Congo. The most recent work in the field has been off Oregon and Washington and off Peru by the Oregon State University group (PAK and ZANEVELD, 1978; PAK et al., 1980a, b). They observed tongues of turbid water (their type I INL's) up to a few tens of metres thick and pools of turbid water up to 150 m thick (type II INL's), the former both shallow (120 m) and deep (400 m), the latter only shallow (centred on 150 m) (PAK and ZANEVELD, 1978). The type I INL's followed isopycnal (isothermal) surfaces and were related to high light scattering on the shelf edge or slope where they were presumed to originate via resuspension of bottom sediment and lateral advection of turbid water. The resuspension of bottom sediment probably arose from interaction of a current with rough slope topography, but internal wave action was also thought by Pak and Zaneveld to be a possible mechanism. Water motions of several origins may be responsible for sediment erosion on the continental slope. Thermohaline currents, high-frequency internal waves and internal tides are all possibilities (HUTHNANCE, 1981). In particular, regions where the bottom slope is close to the slope of the characteristic ~t = [(~2 _f2)/(N2_ a2)]1/2 favour intensification of motions with frequency ~. HORN and MEINCKE (1976) demonstrate this for internal tides on the slope off West Africa, while WUNSCH (1969), WuNscn and HENDRY (1972), CACCHIONEand WUNSCH(1974) and FAHRBACHand ME1NCKE(1978) have examined in the field, the laboratory and in theory, intensification of progressive internal waves of high frequency on the slope. In addition, HOTCHKISSand WUNSCH(1982) have shown the focusing of high-frequency internal wave energy in submarine canyons and have suggested that this may have geological effects. The possibility of such motion being able to move sediment has been shown by SOUTHARD and CACCmONE (1972) and CACCH1ONEand SOUTHARD(1974). A photographic traverse by LONSDALEand HOLLISTER (1979) on the north slope of Porcupine Bank reveals bedforms suggesting zones of faster bottom current at depths of 2600-2960 and 1400-1600 m but not in the vicinity of 450600 m where our nepheloid layers are thought to originate, though the latter lies 250 km further south along the slope. METHODS

In this study we have made particle size distribution and concentration measurements using a HIAC particle counter with a CMH-90 sensor (Pacific Scientific Inc.). This instrument has not been used previously for study of nepheloid layers so some correlations are given here to demonstrate comparability with other methods. The HIAC counter senses and counts particles passing through a light beam by blockage of light which leads to reduction in the output of a photodiode. Signal amplitude is related to particle size through calibration experiments. The sensor covers a 60-fold range, from about 1.5 to 90 Ixm. The agreement between the HIAC and Coulter counters is good over the range >6 ~tm but below that size the HIAC appears to give lower particle counts, possibly because of the proximity of that size to the wavelength of light. Spherical particles are assumed as the basis for relating sensed particle area to size. The particle

794

R . R . DICKSON and I. N. MCCAVE

size distributions can be represented in terms of number or volume• The latter has been used here with 12 points on a X/2 progression between 1.41 and 88 txm. Total particle volume concentration in the range 1•41-88 jam also has been calculated. In many cases the samples were first counted raw, then subjected to 5 min ultrasonic disaggregation and recounted• The nephelometer results are plotted against raw (particle volume) 2/3 in Fig. 3a. The correlation is fairly good. A few values of attenuation coefficient were also available from a 1 m path-length transmissometer using light at 660 lam (PAK et al., 1979), loaned by Dr J. R. V. Zaneveld of Oregon State University (Fig. 3b). The correlation here too is quite good on only 15 points, and the intercept of 0.444 is acceptably close to the theoretical 0.40 for pure seawater--for example, SPINRADand ZANEVELD(1982) found an intercept of 0.466 on a similar plot using Coulter Counter data. HIAC data show a lower slope but this is to be expected because (a) the HIAC size range is greater and, more importantly (b) the HIAC sees the projected particle area and presumes a particle volume equivalent, whereas the Coulter Counter takes as particles that material which does not conduct electricity. For porous aggregates the HIAC volume will thus be larger. The difference in the two lines reinforces the notion of particle aggregation in nepheloid layers (cf. McCAvE, 1983). At sea, samples for HIAC analysis were collected from the rosette of 30 l Niskin bottles immediately on recovery and transferred to 250 ml glass conical flasks which had previously been well-cleaned and oven-dried in the laboratory. These flasks were sealed with cling-film immediately after transfer of the sample. As soon as possible thereafter (normally within an hour or two of sampling, but never >6 h) a 15 ml wash followed by V 50

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three 15 ml replicate samples were passed through the CMH-90 sensor for counting at 15 ml min -1, with continuous mechanical stirring of the 250 ml 'reservoir' throughout. The remainder of the sample was then agitated by partial immersion of the conical flask in an ultrasonic bath for 5 min, to disaggregate any flocculated material. After a further 15 ml wash, three replicate 15 ml aliquots of this shaken sample were recounted as before. P H Y S I C A L O C E A N O G R A P H I C C O N T R O L S ON N E P H E L O I D L A Y E R D I S T R I B U T I O N

Nepheloid layer west of Porcupine Bank Following the discovery of intermediate nepheloid layers in 1980, five transects of 4-5 closely spaced stations were worked on 15-18 June 1981 in the same area of slope to determine the detailed configuration of the plume at near-bottom and intermediate depths (Fig. 1). In this case the SeaMarTec nephelometer was coupled with a Guildline 8705 CTD on each of the 5 transects, and the southernmost transect was repeated using an Oregon State University 1 m path-length transmissometer (PAK et al., 1979) to provide a quantitative measure of turbidity (Stas 87-91, Fig. lb). As on the 1980 cruise, discrete water samples were taken for particle size analysis and scanning electron microscopy. The detailed 1981 station grid showed the presence of a well-developed bottom nepheloid layer over the whole area of slope covered by the survey, but with some tendency towards a local concentration maximum in the zone from 400 to 600 m water depth (stipple tone, Fig. 4). At almost all stations the turbidity of the BNL was in sharp contrast to that of the intermediate clear water minimum. The repetition of the southern section using nephelometer and transmissometer showed, despite a 3-day time sepa15°10 '

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796

R . R . DICKSON and I. N. M¢CAVE

ration, that the same general features were common to both instruments and to the findings of the previous year. Specifically the bottom nepheloid layer was developed to a very much greater (180-200 m) thickness at around the 460 m depth contour (e.g. Sta. 48, Fig. 5) than at stations immediately higher or lower on the slope (Stas 47 and 51, respectively, Fig. 5). Second, intermediate nepheloid layers occurred at the offshore end of the transect, most notably around 600 m depth which appear to originate in bottom nepheloid layers further upslope and spread along constant density surfaces over the slope. In Fig. 6, for example, where the distributions of turbidity, temperature, salinity and ~t are compared for the two repeats of the southern transect in 1981, the offshore INL at Stas 49 and 50 or 90 and 91 is shown to lie deeper (at 500-700 m) than the bottom 60

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layers closer inshore, but along roughly the same density surface (c, = 27.327.35 kg m-3). In the zone of generation, the BNL has a structure that in most cases includes a detached bottom layer probably provided by lateral injection into the lower water column at some point upstream--to the south and possibly upslope [see MCCAVE (1983) for further discussion]. There is another set of INL's in water depths shallower than 350 m. As there is scarcely any water in contact with the bottom at these depths on southern Porcupine Bank, these layers may have a different origin, related perhaps t o surface suspended matter that has been injected in layers through frontal interleaving. Hydrographic structure

The general watermass structure at the southern approaches to the Rockall Trough has been described by ELLETr and MARTIN(1973). The upper 1000 m of the water column, in which our transects were taken, is composed of an upper layer of North Atlantic Central Water (NACW) about 500 m thick at around 20°W, narrowing to -200 m thickness against the slope itself. With increasing depth below this layer the watermass properties depart progressively from those of the NACW T-S 'envelope'. In particular, salinities for water of 7-10°C show an increasing positive anomaly with depth down to the level at which Gulf of Gibraltar Water exerts its greatest influence (i.e. at 900-1000 m depth; HARVEY, 1982). Past deployments of long-term current meter moorings by MAFF on the Celtic Sea Slope at 10°W (observations from 330 to 3995 m), Goban Spur at 49°N 13°W (2049 m) and recently (with SMBA) on the Porcupine Bank Slope along the 52°30'N and 51°42'N transects (276-2505 m) leave little doubt that the slope water participates in a steady northward residual flow (DlcKsoN et al., 1985). The distribution of temperature, salinity, Gt and nephels (or attenuation coefficient) along the two southern transects in June 1981 (Fig. 6) provides more detailed information Nephels (Relative U n i t s )

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on the hydrographic structure in the area. On each of these transects (and indeed on the other sections worked) the key features shown are the doming of the isopleths of temperature and ~t and the 'chimney' in salinity of 35.40-35.45%o above the point on the slope where the bottom nepheloid layer is most strongly developed. Evidently, the process or processes responsible for enhanced resuspension of sediment at this zone are also responsible for these enhanced vertical excursions in all properties, from the seabed into the overlying water column. This is not necessarily synonymous with a more effective vertical mixing, however; in detail, the vertical distribution of properties at any given station in this zone (e.g. Sta. 76, Fig. 7) is one of well-mixed but distinct layers in temperature and salinity, frequently associated, as in this case, with distinct nephel inversions. The preservation of these layers argues against effective vertical mixing and reinforces the suggestion that the structure originates in boundary mixing followed by detachment and injection of th~ layers as suggested by AR~I (1978) and MCCAVE(1983). Generation mechanism While the 1980 and 1981 surveys described the distribution and properties of nepheloid layers west of Porcupine Bank, they provided little information on the cause of the layers, or on the reason for the local enhancement of erosion at the slope-shelf break around 400-4500 m depth, and the local geostrophic or tidal currents seem equally unable to explain the latter point. Long-term current meter records of 90-213 days duration~are available from repeated deployments on this slope between October 1981 and January 1984 and cover both the 'enhanced erosion zone' at -500 m and three other depth zones (750, 1500 and 2500 m nominal). Table 1 compares results from the near-bottom instruments of each mooring, set 29-54 m above the bed. Though the maximum speeds attained in all of these records are capable of suspending the fine fraction of the local sediment (for which an erosion velocity of ~20 __ 5 c m s -1 is assumed following SOUTHARD et al., 1971), the main point is that the records from the 'erosion zone' show no evidence of enhanced mean current speeds, higher maximum current speeds or a higher incidence of currents >20 cm s-1 than we observe at other depths on this slope. The inference is that the more intense current speeds (which must be the ultimate cause of localised slope erosion) evidently are too intermittent, of too high a frequency or lay too close to the bed to be observed in these long-term records. The 'intermittency~problem arises from the fact that nephelometers are particularly sensitive to the smallest suspended particles (<5 lam) which are very slow to settle out. Quite clearly then a nepheloid layer may be recorded by the nephelometer several weeks after the erosion event which gave rise to it has ceased. In his review of waves and currents near the continental slope edge HUTHNANCE(1981) notes that if the continental slope below the shelf break is near to or a little steeper than the characteristic slope of the internal tide (a) then both models and observations suggest increased motion there. Specifically he shows (p. 216) that regions of characteristic slope a = (~2 _ f2)l/2](N2 _ ~2)1/2 (where f is the coriolis.parameter and N 2 is the BruntVaisala frequency) favour strong near-bottom currents at frequency a in various ways: internal tide generation, incident internal wave intensification, bottom-trapped wave accumulation and intensification of motion in a thickened viscous boundary layer. Though CTD profiles from the surveys of July 1980 and 1981 showed that a was then always steeper than the local bottom slope (13), a third brief survey of this area on 25 July 1982 captured what is thought to be the intermittent erosion event in progress. On that

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occasion a CTD transect normal to the slope through the erosion zone at 51°42'N showed the presence of a thin cold layer, 0.5-0.75°C cooler than the overlying watercolumn, within 30 m of the seabed at the erosion site but not higher or lower on the slope (Fig. 8a). The resulting density step at the bottom at this point (Fig. 8b, Stas 15 and 16), hence increased N 2, resulted in a temporary reduction of (~ to match the local bottom slope ([3) at the slope break for the first time in the three surveys (Fig.. 9), while higher and lower on the slope, bottom slopes (dashed line, Fig. 9) were either too shallow or too steep to produce the required match with the characteristic slope of the internal tide. The accompanying nephelometer transect (Fig. 10) confirmed the spreading of the BNL offslope along the 27.35-27.45 kg m -3 ~t surface, separating from the bottom to form an INL at 650-750 m depth in 1782 m water depth at Sta. 18. These observations suggest that erosion at the slope break in this sector is an intermittent process, requiring the development of a thin, cold, high-density 'step' close ~)

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to the bed to support vigorous water movements in the near-bottom layer. Such a hypothesis poses three important questions. First, why does the cold layer develop, second how often (and for how long) does it occur, and third, can more vigorous water movements be demonstrated during its periodic recurrence? We have evidence from the Nova Scotian slope (P. C. SMITH, personal communication) that such a cold deep layer may develop intermittently through classical upwelling; strong and sustained long-slope winds drive the integrated Ekman transport offshore and colder water from deeper levels on the slope rises to replace it, washing onto the upper slope as a thin, dense, near-bottom layer. A similar process is thought to be evident in several of the standard CTD sections that are worked 4--6 times per year across the slope west of Scotland (-57°N) by the Dunstaffnage Laboratory of the Scottish Marine Biological Association (Fig. 11). Additional evidence for the link b,etween long-slope winds and near-bottom cooling at the shelf break has been found in several of the thermistor records from the Continental Slope Experiment (CONSLEX) of 1982-1983. Thermistors located 1 m from the bed in bottom-standing tripods were deployed on the shelf break at the inshore end of three sections to the northwest of the Orkneys and Shetlands where the continental slope trends northeast-southwest in water depths of 236,200 and 185 m, respectively. On days 280--290 of 1982, a protracted northeasterly (i.e. long-slope) gale was accompanied by a rapid cooling of up to 1.25°C, simultaneously at all three of these near-bottom thermistors (information kindly supplied by J. Huthnance, IOS Bidston). Since the near-bottom cooling observed at Porcupine Bank in July 1982 had occurred after a period of protracted northerly winds (the long-slope wind direction in that sector

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369

S 311

..'"''" ~".

10 ~2

'/'"'- ,""

,

,"""~"

2000

-"

',,

h

ST18

.. 20.

10

:

~

15

,- / ",,

5 L00

:,,,,,

"'

52

lO BNL

10 151

"

-

0

S 999

"

"" "''

5

'.,

~,"

/"

o

S 6tlS 0

,,

-

o]

5 M2 ;%,

"

.

20

lO

....

z~

~, 50~

"\,

O" ,,,"'

~i

5 1~

o ~

01 5 071

S 1"/67

k....

2

8 16 32 ~,

6

100553

,, 2

oe, /

i

,,,

z, 8 1 6 3 2 ~ JJm

2

~] z~ 0 1 6 3 2 G ~ um

0 2

~, 8 , 3 2 6 k

2

~

8 1632~,

.um

Fig. 10. Nephelometer profiles from the 1982 transect, and corresponding to the 7~C and or, profiles shown in Fig. 8. Note the well-developed BNL at Stas 14--16 which detaches to appear as an INL at Stas 17 and 18, following the same o, range as the BNL at the shallower stations. Particle size distributions are shown below with sample depths and layer category (i.e. BNL, INL, CW) indicated.

of the slope) an attempt was made to measure the effect of northerlies on the nearbottom thermal structure during a fourth visit to the site in July 1983. For a period of 6.5 days from 0100 h 3 July to 1200 h 9 July, an ll-sensor thermistor chain (Aanderaa TR2), 50 m in length and logging at 2 min intervals from each thermistor, was deployed at the slope-erosion site (51°41'N 14°39'W) in 464 m water depth. The mooring was designed so that the deepest thermistor lay 5 m from the bed. During 6 and 7 July a passing depression brought a northerly gale to the site, with winds gusting to >40 kn. From 6 July a cooling of up to 0.25°C was observed, spreading from the seabed to the upper part of the thermistor chain before abruptly reverting to normal temperatures on the evening of 7 July as the weather moderated (Fig. 12). Though this northerly episode

ADr 1977

~,~

6-20

A p t 1978

kls

i k;; P

0 <94

P

Ct

R

0

<1

P

0

R

<9-2

0

0

100.0

1000 m.

-/

P

G

R

0 <10"0

P

lg-20 Apt '76 lg-20:pr.'7,

d

2000

Fig. 11. Temperature sections across the eastern slope of the Rockall Trough at 57°N (D. J. ELLETT, personal communication), compared with progressive vector diagrams of wind speed at Tiree for the 15-day period prior to the working of each section• Uptilting of the 9°C isotherm at the shelf break occurs following periods of steady northwest (long-slope) wind. Pr¢- storm spectra 3 July 0105

4 0000

5 0000

Storm

6

0000

I

spectra

7 0000

8 0000 I

0

9 00(30

V, lv, I ,

,,

E ,~5

ox o 3s 9.7oc uJ

25 iii

9"6"C

Fig. 12. ll-sensor thermistor chain record from the slope erosion site west of Porcupine Bank between 0100 h 3 July and 1200 h 9 July 1983. Thermistors were located between 409 and 459 m in 464 m water depth, and data interval is 2 min; note the rapid cooling by 0.25°C close to the bed during gale force northerly (long-slope) winds on 6 and 7 July.

R

1= B

B

A t:

%

,.-i

Ct

b

o" ....... ~o"

.......

~o' ........

~O' . . . . . . . .

FREQUENCY

5o'

~

.......

~0"

.......

~O' ........

~0' ........

~O'

FREQUENCY

THERMISTOR CHI~|N LEVEL I

THERMISTOR CHRIN LEVEL 4

1=

i= .1

B

•~

%

A

1= ..1

w,-=

t= -t ~,

E

JJ ...... ~0' ........

FREQUENCY THERRISTOR CHfllN LEVEL 7

~0' ........

50'

d

....... IlO-f ,,,,,,, FREOUENCY

, , ...... ~O'

THERMISTOR CHAIN LEVEL 10

Fig. 13. (a)-(d) Comparison of temperature spectra before (A) and during (B) the storm of 6-7 July 1983 at thermistors 1 (+55 m), 4 (+40 m), 7 (+25 m) and 10 (+10 m). The spectra refer to equal 1200-term pieces of record for the periods 1701 h 4 July-0859 h 6 July (pre-storm) and 0901 h 6 July-0059 h 8 July (during storm). There is a broad-band, order of magnitude increase in spectral energy during the storm close to the seabed but no change 55 m above bottom.

806

R . R . DICKSON and I. N. McCAVE

was brief, it confirms the type of response expected from more protracted periods of long-slope wind; more important, a comparison of temperature spectra for equal timeperiods before and during the northerly gale confirms that spectral energy increased over almost the entire accessible frequency range ( - 4 x 10-2 to 101 cpd) for thermistors close to the bed (e.g. Fig. 13d, level 10 at a bottom separation of 10 m). This excess spectral energy during the period of near-bottom cooling diminished rapidly with height above the bed so that at level 1 on the chain, 55 m above the seabed, there was essentially no difference between the spectra (Fig. 13a). Contemporaneous long-term current measurements from the MAFF/SMBA array at a height of 25 m above the bed had too long a sampling interval to provide spectra during this brief event, but indicate at least, from their steady northward residuals during this period, that the observed cooling was not due to eddy interactions with the continental slope.

NEPHELOID LAYER PROPERTIES AND BEHAVIOUR

General

Nepheloid layers are distinguished by their higher particle concentrations than the surrounding water. Values of volume concentrations (C) measured by HIAC counter are given in Table 2 and summarised in Table 3. Here the bottom nepheloid layers have been separated into those in the zone of BNL generation (for which (~ = 306 ppb) and other BNL's outside this zone (for which C = 136 ppb). The groups all differ significantly at the 99% level except the INL's and clear water (CW) which differ only at the 95% level. These values are fairly typical for deep-sea nepheloid layers in active areas (e.g. BlSCAYE and EI~REIM, 1974; MCCAVE, 1983) but the clear water is turbid ((~ = 53 ppb) relative to oceanic values. No high values (>1 ppm), such as are found on the shelf or deep-sea areas of intense resuspension (SPINRADand ZANEVELO,1982; PAK and ZANEVELD,1978; M C C A V E , 1979, 1983), were measured. For each of the water types, several samples were sized and filtered (e.g. Fig. 10). For some samples we have concentrations by mass as well as volume from the HIAC counter. The quotient of these values, an apparent density, lies in the range 0.64-1.52 mg mm -3 with a mean of 1.10 for 15 values (Table 4). This may be used to estimate mass concentrations from the volume concentrations of Tables 2 and 3. Portions of the filters have also been examined under the scanning electron microscope (Figs 14 and 15). These data are considered for each water type. Bottom sediments

Five short gravity cores were taken for us by Dr P. Barnett (SMBA, Dunstaffnage Laboratory, Oban) from depths of 380, 400, 460, 600 and 750 m on the slope along a zonal transect at 51°41.6'N which crosses the zone of BNL generation. Mud (<63 lam) percentage decreases progressively with increasing depth along this transect (42-18% by weight). Very little of the abundant suspended amorphous silica from diatoms, radiolarians and sponges survives in the bottom sediments, these components averaging ~<5% in visual estimates on smear slides. Coccoliths, on the other hand, are abundant in all these sediments at the surface and down-core.

CW/SNL INL BNL

SNL CW/INL BNL

220 466 504 559

352 755 919

294 374 402 497

225 295 385

107 371 436

67 440 521

49

50

51

56

57

58

CW INL BNL

CW CW CW BNL

CW B/INL BNL

CW CW B~NL BNL

CW I/BNL BNL

242 415 449

48

SNL CW B/INL BNL

Type

66 293 376 413

Depth (m)

348 48 119

148 101 318

281? 74? 422

59 76 88 388

99 51 149

49 155 76 163

104 182 406

139 208 212 388

Before (mm 3 m -3)

After (ram 3 m -3) A:B

278 453 310 489

88*

364 562 418 644

90

91

89

272 329 424

7 26 144 303 358 402 434

85*

87

22 76 149 247 328 435 446

Depth (m)

84*

Station (cruise 81-6)

INL INL

CW BNL

CW BNL CW BNL

CW INL BNL

SNL SNL CW/SNL CW INL B/INL BNL

SNL SNL SNL/CW CW CW BNL BNL

Type

69 98

39 176

35 257 78 190

27 49 179

269 231 59 42 126 316 520

303 142 145 23 60 242 320

Before (ram 3 m 3)

97 96

55 144

30 205 52 190

1.41 0.98

1.41 0.82

0.86 0.80 0.66 1.00

0.81 1.10 0.75

0.85 0.83

268 434

22 54 135

1.07

0.87 0.83

2.13

A:B

45

211 267

49

After (mm 3 m 3)

Particulate material volumes in the size range 1.5-90 ~m before and after ultrasonic disaggregation

47

Station (cruise 81-6)

HIAC volume

Table 2.

182 286 382

329 380

319 439

341 584

65

66

70

71

456 539

314 466

CW/INL BNL

193 384

63

64

CW/INL CW

5(~ 830 1216 1502

62

CW BNL

CW BNL

INL BNL

INL INL BNL

CW BNL

CW CW/INL CW/INL CBW

S/INL CW INL CBW

149 548 719 11~8

61

CW CBW

Type

394 7(14

Depth (m)

59

Station (cruise 81-6)

HIAC volume

54 205

59 160

62 581

97 61 256

31 287

47 203

57 42

67 76 75 71

57 34 47 59

1.g7? 50?

Before (mm 3 m 3)

62 204

58 155

99 411

190

268

After (mm 3 m 3)

1.15 1.00

(1.98 0.97

1.60 (I.71

0.74

0.93

A:B

Table 2

Depth (m)

311 338 362 383 4(J6 52 100 151 2112 247 3(X) 35(} 169 267 367 414 461 505 553 488 685 835 871

14

15

16

17

Cruise 82-6

Station (cruise 81-6)

continued

CW INL B/INL B/INL

CW CW CW CW BNL BNL BNL

SNL CW CW CW CW CW CW

CW CW BNL BNL BNL

Type

68 32 48 42

24 30 26 40 79 239 346

107 28 70 101 48 29 64

20 22 32 72 252

Before (mm 3 m 3 )

74 47 57 43

45 37 39 60 68 237 332

97 37 45 63 34 41 65

41 45 80 120 498

After (mm 3 m 3 )

1.09 1.47 1.19 1.02

1.88 1.23 1.50 1.50 0.86 0.99 0.96

0.92 1.32 0.66 0.62 0.71 1.41 1.02

2.05 2.05 2.50 1.67 1.97

A:B

357 1(127

681 938 1149

650 755 854

334 605 635 715

73

74

75

76

CW INL INL BNL

CW INL CW

INL CW CBW

INL BNL

INL BNL

Type

40 97 131 126

40 43 45

43 37 67

60 68

54 133

Before (ram 3 m 3)

34 75 153 100

39 37 49

46 73 81

54 72

33 102

After (mm ~ m 3)

0.85 0.77 1.17 0.79

0.98 0.86 1.09

1.07 1.97 1,21

0.90 1.06

0.61 0.77

A:B

Table 2

18

Station (cruise 81-61

continued

682 999 1365 1564 1767

9(13

Depth (m)

INL CW CW CW CBW

CBW

Type

55 33 10 18 29

37

Before (mm 3 m 3)

60 42 12 20 46

30

After (ram 3 m-3)

1.09 1.20 1.20 1.11 1.58

0.81

A:B

*84, 85, 88 at the same position on 18 June 1981. SNL, surface nepheloid layer; INL, intermediate nepheloid layer; BNL, bottom nepheloid layer; CW, clean water; CBW, clear bottom water. bottom water.

499 894

Depth (m)

72

Station (cruise 81--61

H1AC volume

810

R.R. DXCKSONand I. N. MCCAVE Table 3.

Volume concentrations and volume ratios before and after insonification C'

S.D.

N

A:B

S.D.

N

CW INL BNL (gen.)

53 80 306

30 38 110

54 17 21

1.24 0.922

11.44 0.287

30 9

11.852

0.103

16

BNL (other)

136

44

10

C, mean concentration; S.D., standard deviation; N, number of samples. A:B, ratio of particle volume recorded after insonification to that recorded before.

Table 4.

Volume and mass concentrations with apparent density

Sta.

Depth (m)

C,. (mm -~m 3)

C,,, (mg m 3)

A Layer (mg mm 3) type

87 88 88 89 911 91

379 453 310 489 562 418

49 258 78 190 176 69

55 311 100 217 221 77

1.12 1.21 1.28 1.14 1.26 1.11

INL BNL CW BNL BNL INL

14 14 14 14

311 362 383 4116

2/I 32 72 252

211 40 60 160

1.011 1.25 11.83 0.64

CW CW BNL BNL

15 15 15 15

2112 247 30(1 350

48 101 29 64

40 1411 44 811

0.83 1.39 1.52 1.25

CW CW CW CW

16

533

346

240

11.69

BNL

fi = 1.10, S.D. = 0.26.

B o t t o m n e p h e l o i d layers In the z o n e o f s t r o n g e s t a p p a r e n t s e d i m e n t r e s u s p e n s i o n (e.g. Stas 14-16, Fig. 10), the b o t t o m n e p h e l o i d l a y e r exhibits a b r o a d size p e a k in the r a n g e 8 - 4 0 lam. It is p o s s i b l e that this p e a k is e v e n b r o a d e r b u t is t r u n c a t e d d u e to p a r t i c l e d i s r u p t i o n by s h e a r in flow t h r o u g h the H I A C c o u n t e r ' s orifice. T h e B N L s a m p l e s t e n d e d to d e c r e a s e in p a r t i c l e v o l u m e after insonification ( T a b l e 3), w h e r e a s I N L ' s s h o w e d no significant c h a n g e a n d c l e a r - w a t e r s a m p l e s g a i n e d in v o l u m e ( m e a n v o l u m e ratios a f t e r / b e f o r e insonification a r e 0.852, 0.922 a n d 1.24, r e s p e c t i v e l y ) . This b e h a v i o u r has b e e n a t t r i b u t e d by MCCAvE (1983, 1985) to d i s r u p t i o n o f aggregates in the c o u n t e d r a n g e ( 1 . 5 - 9 0 ~tm) for B N L ' s but relatively g r e a t e r d i s r u p t i o n of large a g g r e g a t e s b e y o n d this r a n g e in the clear w a t e r . ( A n a l t e r n a t i v e possibility is that the gain s h o w n b y the l o w e r - c o n c e n t r a t i o n C W s a m p l e s is d u e to c o n t a m i n a t i o n r e l e a s e d f r o m the sides o f the b e a k e r by u l t r a s o n i c v i b r a t i o n . H o w e v e r the b e a k e r was r e p e a t e d l y insonified a n d it s e e m s u n l i k e l y that a n y such effect w o u l d be a p e r s i s t e n t s o u r c e o f e r r o r .

Fig. 14. Scanning electron micrographs of suspended material filtered onto 0.2 lain Nuclepore filters from - 2 I of seawater. Each picture carries a scale bar (5, 50 or 500 ~m). Samples are from Stas 87-91 of the repeated southern transect in 1981 (see Figs l and 6), and Stas 14-16 in 1982 (see Fig. 10). (A) 87 BNL. Aggregate >50 lam long containing clay and coccoliths in turbid bottom water. (B) 87 BNL. Abundant coccoliths (cyclococcolithus in centre'?) and clay particles in turbid bottom water. (C) 15/426 m BNL. Abundant coccoliths, diatoms and mucus on filter from nepheloid layer. (D) 16/553 m BNL. Aggregation of slipper diatoms (Naviculoid; Amphora spp). (E) 151400 m BNL. Calcite encrusting an amorphous silica rod. The angle of 114° between the branches is characteristic of the angle between the (4(/41) and (4401) crystal faces of calcite. (F) 89 CW. Scattered organic debris, mainly siliceous, from clear water filler. Note scarcity of clays and coccoliths.

Fig. 15. Locations as for Fig. 14. (A) 91 INL. Clay and coccolith aggregate plus other particles (cf. Fig. 14A from BNL). (B) 91 INL. Abundant coccoliths and clays (cf. Fig. 14B from BNL). (C) 15/247 m CW. Relatively clean filter with dark patch of mucus and remains of chaetocerid diatom* which has dark stains (?dried body mucus) associated with it. (* If Chaetoceras sociale this colonial diatom would act as an efficient ~sweepcr" of material to the seabed.) (D) 89 CW. Thin smears of presumed organic matter (darker patches) and dried out gobs of mucus on a filter with low particle loading. (E) 89 CW. Detail of dried out. flaking mucus patch in (D) showing embedded organic and mineral particles. (F) 14/338 m CW. Dark film of mucus with associated organic debris - - mainly of diatoms. Probably a mucus aggregate.

Nepheloidlayerson the continentalslope west of PorcupineBank

813

The correlation coefficient between (~ and log (A/B) is only 0.38). The only significant difference in disaggregation behaviour is that between CW and BNL (Table 3), at the 99.9% level. The aggregates in the BNL and its composite sublayers are extremely varied, ranging from tightly bound clay/coccolith structures (Fig. 14A) to diatom aggregates (Fig. 14D), calcareous growths (Fig. 14E) and patches of mucus (Fig. 14C). The scanning electron microscope reveals abundant dispersed coccoliths (Fig. 14C), a feature lacking in the clear-water samples. The bed sediment also contains abundant coccoliths and the surface waters over the upper slope are known to be characterised by intermittent upwelling and dense coccolith blooms, as revealed by satellite imagery (DICKSONet al., 1980; HOLLIGAN et al., 1983). The high concentration of coccoliths in the bottom nepheloid layer coupled with an abundant source in bottom sediments suggests that they have been resuspended from the bed. Their settling rate of - 1 cm h-1 makes it unlikely that they owe their concentration to variation in sinking rate. It is probable that much of the initial vertical flux of coccoliths is in faecal pellets, as in HONJO'S (1976) picture for the Pacific. Clear water The relatively clear water between the surface, bottom and intermediate nepheloid layers has the typically 'flat' size distribution of oceanic particle size spectra (Fig. 10; cf. MCCAVE, 1984). All nepheloid layer samples (SNL, INL and BNL) show a pronounced peak at 10-20 I~m, and a subdued remnant of this is found in the clear-water samples also. The coarser peaks from the SNL's also persist in some clear-water samples (Fig. 10). These larger particles appear to be aggregates, often including mucus (Fig. 15F). The associated smaller particles are chiefly of organic origin (Fig. 14F). However, very few coccoliths are encountered in clear-water samples. A marked feature of the clear water is the presence of patches of mucus (Fig. 15C-E) which may be responsible for scavenging particles and sweeping them down to the bed. The mucus appears to be of two types, that forming thin, dark films, which also appears to emanate from the disintegrated bodies of diatoms (Fig. 15C, F) and the type forming thicker flaking patches (Fig. 15D, E). The latter type has also been seen in SEM as a dried body exudate surrounding a ruptured copepod nauplius. Similar material has been examined by EMERY and HONJO (1979) and EMERY et al. (1984), who show it to be directly related to the particulate organic carbon content of the sample, and to be more abundant at the surface in upwelling areas of high productivity. Intermediate nepheloid layers Individual nephelometer profiles through the BNL frequently suggest that it is made up of a stack of discrete sublayers, all of high volume concentration and thus hard to differentiate, in which the upper sublayers have probably already become detached from the seabed. In Fig. 10, for example, we see the possible build-up of such a stack in moving from Sta. 14 to Sta. 16. These sublayers, however, are all classified as part of the BNL for present purposes since it is unclear whether they represent changes in resuspension locally or recently. The layers designated as INL's are those which are clearly separated from the BNL by a layer of less turbid water. INL's have particle size distributions which are intermediate in character between those of the BNL and CW. They contain the 8-40 ~tm peak of the typical BNL, but it is relatively subdued. Their mean volume concentration is also

814

R . R . DICKSON and I. N. MCCAVE

intermediate between those of the BNL's and CW, and the volume concentration shows little change after insonification (Table 3). The detached turbid layers farther from the bottom and (laterally), away from the slope have lost the bulk of their suspended sediment (t~ = 80 ppb compared with 306 ppb for freshly generated BNL's) in what appears to be a relatively short distance from their source. Presuming that they were generated at some point along the 120 km stretch of slope from the southern end of Porcupine Bank (and that they did not originate in the seabight or cross its mouth from further south which is by no means certain!) a mean long-slope drift of 8 cm s-I (Table 1) would indicate an age of up to 2 weeks for the INL's observed. We suspect that INL's lose their larger particles rapidly after detachment, dropping them onto the slope. The composition of the particles in the INL's strongly supports an origin as BNL's which have spread from their point of origin and detached from the slope along isopycnal surfaces. They contain an assemblage of abundant coccoliths and aggregates (Fig. 15A, B) which is similar to the BNL but is lacking in the CW. It is therefore most unlikely that the INL's described here can have originated through the concentration of material at particular density interfaces by settling. D I S C U S S I O N AND C O N C L U S I O N S

To the southeast of the present study area, the tidal model of PINGREE and MARDELL (1981) identifies a particularly energetic sector of the European continental slope, lying along the southern margin of the Celtic Sea between longitudes 5°30'W and 8°30'W. Across this narrow sector, where the vertically integrated M2 current strength exceeds 40 cm s-1, the model suggests that - 6 0 % of the tidal energy flux onto the shelf takes place; the large M2 tidal currents, coupled with a large cross-slope tidal current gradient concentrates the generation of internal waves and tides at the shelf break in this sector, promoting intense bottom mixing, a spreading of the seasonal thermocline and hence shelf-break cooling throughout the summer. To the northwest, along the Celtic Sea slope, where ME tidal currents weaken to 20 cm s-~ or less, internal-wave and -tide effects are much less frequent, perhaps occurring only during spring tides. In the case of the present study area along the west slope and 'shelf break' of Porcupine Bank, tidal streaming is still further reduced. The break of slope encountered here is analagous to the shelf break but lies 300 m deeper and the vertically integrated M2 tide predicted by Pingree's model (R. D. PINGREE, personal communication) does not exceed 5-6 cm s-1 for this site. In this paper we have inferred an intermittent mechanism for the resuspension of bottom sediments at particular locations on the shelf-slope profile and during periods of particular wind forcing. The mechanism involves the intensification of near-bottom water movements through the development of a critical match between the characteristic slope of the internal M2 tide and the bottom slope which arises through upwelling-induced changes in near-bottom density gradient. Long-slope northerly winds drive the integrated Ekman transport off-slope and raise the ~t ~ 27.3-27.4 kg m -3 density surfaces from a position against the slope to a position which overlies the shelf break. The resulting increase in near-bottom N 2 in a region of rapidly changing bottom gradient permits the temporary match between M 2 characteristic--and b o t t o m - slopes referred to above. Short-term high resolution thermistor-chain observations

Nepheloid layers on the continental slope west of Porcupine Bank

815

suggest that this mechanism may even develop to some extent during the brief periods of northerly wind which accompany the passage of atmospheric depressions; the spectral response in near-bottom thermistors suggests increased energy over a broad band from semidiurnal (or greater) periods down to 4-min periods, suggesting high-frequency motion superimposed on internal tides. The Rockall Trough is not an area of persistent upwelling, but if this mechanism can develop even during brief episodes of long-slope northerly wind, then clearly many shortterm periods of upwelling may occur in different locations along the entire eastern margin of the Rockall Trough whenever and wherever the northerly wind component is aligned with the local slope contours for an adequate period of time. This may occur even at the frequency of passing depressions. The generation of nepheloid layers at the shelf break through resuspension by internal waves and tides would be similarly intermittent and variable in location in areas of weak M2 tide. The resuspended material is likely to remain in suspension for a much longer period than the event which gave rise to it. Existing current measurements along the European continental margin show that the upper slope is blanketed by a persistent northwardflowing slope current. Thus the resuspended material will be transported mainly to the north along the slope but with a minor component mixing offshore along isopycnal surfaces (e.g. Fig. 10). During its long-slope and off-slope movement, the suspension is depleted through the settling-out of the larger particles, and particles of all sizes are additionally scavenged out by rapidly sinking mucus aggregates (HoNJo, 1982) originating in the zone of high primary productivity which overlies the shelf break and upper slope in this region (Fig. 16). The finest component is liable to remain in suspension for a long time, however, and it is these particles, ~<2 ~tm in size, which provide most of the signal sensed by nephelometers. Equally, since a particular set of c~,surfaces will intersect the shelf break over long stretches of the slope, the likelihood is that offshore spreading of resuspended fines will take place along these particular surfaces and ultimately to large distances off-slope. This is the most probable explanation for many records of increased turbidity associated with pycnoclines. Figure 17 shows a nephelometer profile worked by R.V. Knorr in 1975 in the centre of the Rockall Channel, and is the only off-slope profile of which we are aware; despite the fact that this station lies ~125 km from the nearest sector of the continental slope (the northwest tip of Porcupine Bank), the profile shows a well-marked INL lying on the same ~, surface (27.38 kg m -3) as that identified in Fig. 10, suggesting that this INL is both persistent and of widespread occurrence in the Rockall Channel, through its association with the shelf break over long distances. The process which we describe is probably general at the shelf break or on continental slopes exhibiting multiple breaks of gradient (Fig. 16b). It may not even be necessary for the upwelling mechanism to occur, as in some locations density gradients in the water column may intersect the slope at a point favouring internal wave amplification (e.g. FAHRBACH and MEINCKE, 1978) and indeed a suitable pycnocline may be made to intersect the shelf break by upwelling. The data shown by PAK and ZANEVELD(1978) from the Oregon slope in November, outside the upwelling season, reveal internal nepheloid layers extending from the shelf break at -160 m and from a deeper break in slope at -400 m depth. Hydrographic data are not reported, however, although they are needed together with optical and sedimentological data to identify the 'process'. We have not yet determined the near-bed velocities associated with resuspension events at Porcupine

816

R . R . DICKSONand I. N. McCAVE

N' LY

O

O

WEST

WIND

.

,

O

| H i g h prc~l_uctivity

I

EAST

200 m

........................

~

.............

~

.

.

~

.

'27"4 d t

.

tOO

=" ~

SPECIFIC MODEL --

l

" l..

••

t, ~ "

~..~'~-

a.

350

~ . , ~ . ~ . . . . . .,,-.~-

I .

---

~

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Fig. 17. Nephelometer profile taken with the GEOSECS laser instrument in 1975. Profile originally published in LONSDALEand HOLL1STER(1979, their Fig. 7) where the apparently errant points of the INL do not appear.

Nepheloid layers on the continental slope west of Porcupine Bank

817

Bank, but now that their location and timing can be predicted, deployment of an eventtriggered acoustic current meter array of the type used in the High Energy Benthic Boundary Layer Experiment (GRANTet al., 1986) would yield the necessary data. Acknowledgements--The authors gratefully acknowledge the help of Peter Barnett (SMBA, Oban) for collecting sediment cores across the west slope of Porcupine Bank, Arthur Folkard (MAFF Lowestoft) for HIAC calibration work, David Ellett (SMBA, Oban) for permitting the use of unpublished temperature sections from the eastern slope of Rockall Trough at 57°N, Ron Zaneveld (OSU, Corvallis Ore) for permitting the use of his 1-m transmissometer, and John Huthnance (lOS, Bidston) for sharing with us his expert knowledge of slope processes.

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R . R . DICKSON and I. N. MCCAVE

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