Turbidity of waters over the Northwest Iberian continental margin

Progress in Oceanography 52 (2002) 299–313 www.elsevier.com/locate/pocean

Turbidity of waters over the Northwest Iberian continental margin I.N. McCave a,∗, I.R. Hall b a

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK b Department of Earth Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, Wales, UK

Abstract In OMEX-II-II, 9 cruises gathered optical data, principally by transmissometer. The distribution of optical turbidity caused by concentration of particulate matter (PMC) in the water column over the northern Iberian margin shows several features related to hydrography. It is concluded that a signal of PMC seen in Mediterranean Water (MW) found north of 42°N is not carried from its source at the Gibraltar Sill and Gulf of Cadiz because it is shown, using intermediate stations, that this turbid plume decays, mainly by fall out but also partly by mixing, to very low levels around southern Portugal. PMC maxima sometimes seen in MW on the northern Iberian margin are thus most likely to result from intermittent local resuspension by MW interacting with slope sediments. The highest turbidity is found over the upper slope and is the result of (i) shelf edge resuspension and off-shelf flow of turbid plumes, mainly between 100 and 300 m depth, and (ii) resuspension under the slope current aided by internal waves, in the depth range 500–800 m where the density gradient between ENACW and MW is maximal. Below the MW, flows are generally slow, and turbidity is low. The bottom nepheloid layer in deep water is also weak with PMC values ⬍100 mg m-3. The focus of resuspension activity on the upper slope means that the region is an efficient exporter to the ocean of sediment that either escapes from the shelf or sinks to the bed from surface production. This accounts for upper slope sediments recorded in other studies as sandy or in places as rocky bottom.  2002 Elsevier Science Ltd. All rights reserved

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

2.

Cruises and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

3. Results and Discussion . . . . . . . . . . . . . 3.1. Water masses . . . . . . . . . . . . . . . . 3.2. Contribution from mediterranean outflow 3.2.1. Decay of the MW outflow plume . .

∗

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

Corresponding author. Tel: +44-1233-333-450. E-mail address: [email protected] (I.N. McCave).

0079-6611/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 2 ) 0 0 0 1 2 - 5

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

302 302 305 305

300

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

3.2.2. MW turbidity off northwestern Iberia 3.3. Turbidity of upper slope waters . . . . . 3.4. Contributions from the Outer Shelf . . . 3.5. Deep water bottom nepheloid layers . . .

4.

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

306 306 307 310

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310

1. Introduction

The distribution of turbidity caused by fine-grained particulate matter suspended in the water column over continental margins is frequently a good guide to the processes and pathways by which sediment escapes the shelf and makes its way to the deep sea. As part of the OMEX II-II project, we have examined the records of turbidity from the Iberian Margin in order to elucidate particulate material transfer mechanisms in the slope region. The main study area was between 41°30⬘ and 43°N off the coast of northern Portugal and Spain, but with several observations off the central and southern Portuguese coast. Patterns of turbidity further north on the European Margin have suggested resuspension is caused by internal waves on open continental slopes (Dickson & McCave, 1986; Thorpe & White, 1988; McCave, Hall, Antia, Chou, Dehairs, Lampitt et al., 2001). Elsewhere in the world the role of submarine canyons has been emphasised as conduits for sediment into the deep sea and as the sites where focussing of internal waves produces intermediate nepheloid layers by resuspension of sea bed sediments (Baker & Hickey, 1986; Gardner, 1989a,b). In addition the data of Drake (1971, 1974); Harlett and Kulm (1973); McCave (1979); Baker and Hickey (1986) and Sherwood, Butman, Cacchione, Drake, Gross, Sternberg et al. (1994) all showed clearly that under the intense wave activity off the US west coast from California to Washington, a similar latitude to northern Iberia, sediments are resuspended over the outer to mid-shelf and spread seawards to rain out over the continental slope. The northern Iberian Margin has a shelf about 40 km wide and a relatively steep slope down to about 2000 m below which there is a gentler continental rise out to a trough between the shelf and Galicia Bank. The depth of the base of this trough is ⬍3000 m, and thus the base of the rise is not reached here, but is farther to the south where the rise merges with the Iberia Abyssal Plain at about 4800 m. Major canyons are only found in the south of the area, principally the Nazare´ , Lisbon and Setubal Canyons (Fig. 1). These have little influence on the sedimentary regime in the northern Iberian region. Sediments of the area comprise mainly calcareous muds on the lower slope, but with increasing terrigenous content of increasingly coarse grain size on the upper slope where some areas lack modern sediment altogether (van Weering, de Stigter, Boer, & de Haas, 2002, this volume; Flach, Muthumbi, & Heip, 2002, this volume). The outer shelf is mainly covered with fine sands with a mud content of 5–25%, but there are also significant areas of rock outcrop testifying to the vigour with which the outer shelf is stirred preventing deposition (Dias & Nittrouer, 1984; Dias, Garcia, Rodrigues, Martins, Carapito, & Jouanneau, 2002). The present paper gives the best optical transmission and CTD data obtained over the NW Iberian margin and uses them to show the distribution of water masses and nepheloid layers, and relationships between them. These are used to infer the mechanisms controlling the distribution of particulate material. In the absence of long time series of turbidity, and noting the great seasonal variability and uncertainty of water flux estimates (Huthnance, van Aken, White, Barton, Le Cann, Coelho et al., 2002), it has not been possible to estimate sediment fluxes. Data from the outer shelf allow inference of the resuspension and transport processes occurring there also.

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

301

Fig. 1. Location map of West Iberian Margin showing the location of stations and lines of CTD/Transmissometer casts examined for this work. RRS Charles Darwin, 䊏 RV Meteor, 䉬 RV Pelagia. N. Can = Nazare Canyon, L. Can = Lisbon Canyon, S. Can = Setubal Canyon, CSV = Cape St. Vincent.

302

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

2. Cruises and Methods We occupied 66 CTD stations (43 in water over 200 m deep) providing hydrographic data on temperature, salinity, light transmission (SeaTech, 0.25 m path-length) and scattering during cruise 105 of RRS Charles Darwin (CD-105; 29 May–22 June 1997, pre-upwelling). A further 28 stations were occupied on CD-110 (winter1998/9) (Hall et al., 2000). Six cruises to the area have been made by RV Pelagia in 1997–99 (60 stations, 50 off-shelf). Of these PLG-109 (July 1997, summer upwelling) and PLG-138 (May 1999, spring pre-upwelling) yielded several repetitions of lines P and R (see Fig. 1) with CTD and 0.25 m SeaTech transmissometer data (archived as BODC records 253864–3924; 255990–6014; 257164–7202). A further 14 usable stations were occupied during RV Meteor cruise 42 (M-42) in winter (28 Dec 1998–14 Jan 1999) with a CTD and SeaTech transmissometer, providing moderate temporal coverage. During CD- 105 a grid of CTD stations, typically 10 km apart, was occupied, which included cross-slope sections at latitudes 43°N, 42°50⬘N, 42°40⬘N, 42°30⬘N, 42°20⬘N, 42°09⬘N, 42°N, 41°48⬘N and 41°25⬘N, (sections labelled N– V from north to south respectively, with stations identified by water depth). On Cruise M-42 a line of stations along 42°09⬘N (line S) plus stations near Nazare´ Canyon and in the Gulf of Cadiz have been used (denoted as M- on Fig. 1). Aspects of part of the data set, that from CD-105 and CD-110, have been presented by Hall et al. (2000), where full analytical details will be found. All optical beam attenuation measurements (on CD, M & PLG cruises) were made with 0.25m path-length SeaTech transmissometers. However, only during the acquisition of the data set of Hall et al. (2000) on cruises CD-105 and CD-110 was sufficiently stringent attention paid to the zero/blank procedures to yield consistent data adequate to assign an attenuation coefficient to the clear water minimum and for calibration with an acceptable zero intercept. This gives some problems in trying to apply the Hall et al. (2000) calibration to the Pelagia and Meteor cruise data, so some of these data have had to be given herein simply as attenuation coefficient without standardised clear water minima. The difficulty is that in a continental margin setting the minimum increases in an unpredictable manner going from the open ocean up the slope towards the shelf. Deployment protocols on Charles Darwin included measurement of blanked and air values before each deployment. Light scattering was determined (only on CD-105) with a SeaTech Light Scattering Sensor (LSS) which measures infra-red light (880 nm) back-scattered from particles in the sample volume using a solar-blind silicon detector. The full-scale of 5 Volts corresponds approximately to 10 mg m⫺3. (Note mg m⫺3 is the correct SI equivalent to the commonly used µg/l). As the size, shape and refractive index of the particulate matter in the water column vary from place to place, no single literature equation can be used to relate either beam attenuation or scattering to particulate matter concentration (PMC) (Baker & Lavelle, 1984; Gardner, Biscaye, Zaneveld, & Richardson, 1985; Moody, Butman, & Bothner, 1986; Gardner, 1989a). We have applied the calibration of Hall et al. (2000) determined on CD-105 to all the Darwin attenuation data here. Our calibration, taken in layers of relatively high optical turbidity (Intermediate and Bottom Nepheloid Layers; INL and BNL respectively) and clear water, was PMC = 1596c – 573 with r = 0.96, where PMC is in mg m⫺3 and c is attenuation coefficient (m⫺1). Pure (particle free) sea water has a beam attenuation coefficient of cw = 0.359 according to the SeaTech Manual, and cw is 0.358 according to Bishop’s (1986) empirical determination for seawater in situ. These figures are close to the cw of 0.361 indicated by the zero intercept of our calibration. 3. Results and Discussion 3.1. Water masses Below the surface mixed layer of 50–100 m thickness, the salinity decreases from 35.8–36.0 to a minimum of ~35.6 at a potential temperature θ ~11°C at depths of 400–500 m (Fig. 2). This marks Eastern

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

303

Fig. 2. Potential temperature (θ) versus salinity (solid lines) and attenuation coefficient (dashed lines for different station depths) for the 5 stations on the 43°N transect ‘N’ on CD- 105. Note the surface mixed layer, lower salinity ENACW, high salinity MW core, mixing line down to the bend at 3.5°C marking LSW, and ISOW below that. The turbidity shows a high at the chlorophyll maximum at the base of the mixed layer (~15°C), at the deeper stations (3100 and 3300 m), and turbid plumes in NACW (level of maximum B–V frequency) at the shallower stations (1600-2300 m). All profiles also show a bottom nepheloid layer and relatively low turbidity in the lower MW.

N. Atlantic Central Water (ENACW), which is formed partly to the south in subtropical waters and partly in Porcupine SeaBight and the northern Bay of Biscay (van Aken, 2000a). Below this there is a strong salinity gradient to the core of the Mediterranean Water (MW) lying between about 800 and 1400 m. The salinity gradient from ~450 to 850 m confers a strong density gradient of ~0.88 kg m⫺3 km⫺1 giving a buoyancy Brunt-Va¨ isa¨ la¨ N2 of ~9 x 10⫺6 s⫺2 with peaks of ~11 x 10⫺6 s⫺2 corresponding to an internal wave period of ~30–35 minutes (Daniault, Maze´ , & Arhan, 1994). The maximum salinity of the MW core of ~36.2 at this latitude (41–43°N) is significantly reduced from ⬎36.7 in the Gulf of Cadiz (Fig. 3). This is accomplished by mixing with the underlying Labrador Sea Water (LSW) which at this latitude no longer displays the salinity minimum found farther north over Porcupine Abyssal Plain. Only on the northernmost transect N did our casts go deep enough to get below the LSW into the Iceland–Scotland Overflow Water (ISOW) (Fig. 2) which, with LSW, forms Northeast Atlantic Deep Water (NEADW) (Harvey, 1982; van Aken, 2000b). Maze´ , Arhan, & Mercier (1997) define water masses on θ–S plots with density-defined lower bounding surfaces for ENACW of σo = 27.25 (~550 m), MW of σ1 = 32.35 (~1500 m), LSW of

304

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

Fig. 3. PMC and salinity for Meteor stations, (b) M42-46 in the Gulf of Cadiz and (a) M42-02 west of southern Portugal (for positions see Fig. 1). In (a) note the close match between salinity (dashed line) and turbidity at the base and top of the plume, and the high PMC maximum. In (b), about 250 km down the MW flow path from (a), the base of the plume is still apparent but the concentration is ⱕ50 mg m⫺3 (approximate value due to optical transmission zero problems) and the overlying ENACW region has increased turbidity relative to the Gulf of Cadiz. (The concentration scale for (a) was achieved by using the slope of the calibration of Hall et al. (2000) and assuming a similar concentration beneath the plume, ~35 mg m⫺3 as the minimum at M42-46. (As a check, this gives a reasonable value for the surface maximum of 150 mg m⫺3).

σ2 = 36.96 (~2150 m) and ISOW of σ4 = 45.85 (~3500 m). Hydrographic, dynamical and sedimentary data indicate northward flow of all the water masses, ENACW, MW, LSW and ISOW (Gardner & Kidd, 1987; Daniault, Maze´ , & Arhan, 1994; Maze´ et al., 1997; Fiuza, Hamann, Ambar, del Rio, Gonzalez, & Cabanas, 1998) (but see below for an alternative view for LSW). The two flavours of ENACW, subtropical and subpolar, are both involved in a slope current with seasonally varying importance off N.W. Spain (Alvarez-Salgado, Perez, Figueiras, Castro, Borges, Moncoiffe, et al., 2000). This poleward flowing current is at a minimum in spring and summer and a maximum in winter when its speed may be 15 cm s⫺1 or more (Huthnance et al., 2002). The geostrophic estimates of Maze´ et al. (1997) indicate northward transport at all levels, with an increase in flux from south to north fed by inflow from offshore to the west. Northward flow in the ENACW slope current (with seasonal reversals) and in MW is generally agreed, but Huthnance et al. (2002) suggested that the LSW is moving south through the region, based primarily on hydrographic data. Sedimentary topography recorded by 3.5 kHz profiler suggests long-term (105-year scale) northward flow at the 2000–2100 m depth of LSW. Over the shelf, water movements are complex and seasonal, but net northward sediment transport is clearly indicated by current meters and turbidity patterns (Drago, Oliveira, Magalha˜ es, Cascalho, Jouanneau, & Vitorino 1998).

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

305

3.2. Contribution from mediterranean outflow 3.2.1. Decay of the MW outflow plume The turbidity of the MW at 42–43°N is low and variable. Hall et al. (2000) concluded that the MW is not actively resuspending sediment in the N. Iberian area as it corresponded to a turbidity minimum (see Fig. 2). However, this is not the case on all surveys in the more extensive data set considered here. Certainly the MW enters the region with a huge turbidity signal, up to 250 mg m⫺3 in the Gulf of Cadiz (Fig. 3b), a feature noted long ago by Thorpe (1972). The lower core at ~1300 m has the highest turbidity, a similar level to that observed by Thorpe (1972). This turbid signal disappears quite rapidly to the north. At Nazare´ Canyon (39°35⬘N), some 370 km to the north it is barely detectable (Fig. 4), while about 250 km along flow from the Gulf of Cadiz at 37°35⬘N (station M42-02) the base of the plume is still apparent but the maximum is reduced from 250 to ~50 mg m⫺3 (Fig. 3a). We assume that the stations are fairly representative of concentrations in the outflow and downstream regions and that the system is sufficiently steady to make valid comparisons between sites. Temporally there is a long history of known high turbidity close to the source of MW outflow, and spatially the chosen stations lie on the main route of ‘meddies’ shed at Cape St Vincent into the Atlantic. Some of the reduction in turbidity is accomplished by dilution above and below with clearer ENACW and LSW and some by particle fall-out. Dilution of Gulf of Cadiz MW (S = 36.56) with Labrador Sea Water (taken as S = 35.2 in this region, van Aken, 2000a) yields a 70/30 MW/LSW mix contributing to S = 36.16 at downstream station M42-02 (Fig. 4b). Given an initial mean MW PMC of ~150 mg m⫺3, and underlying LSW PMC of 34 mg m⫺3 (Fig. 4), this dilution alone would yield a substantial plume with PMC = 115 mg m⫺3 at station M42-02. But the weak plume concentration there actually averages ~50 mg m⫺3. Thus 65 mg m⫺3 or more of the decline in concentration is attributable to particle fall out. If the mixing in this region has

Fig. 4. Nazare´ Canyon axis stations occupied in winter ’98/’99 (M42-43, -44) and spring ‘99 (May) (PLG 138-14). Note (a) the thick nepheloid layers and multiple INL’s in the Canyon, and (b) the subdued turbidity signature associated with MW. Although these stations are in the Canyon, the adjacent walls reach up only to ~2100 m and thus do not affect the MW turbidity.

306

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

been partly with higher salinity ENACW (also having a PMC of 40 mg m⫺3), as is suggested by Baringer and Price (1999), then the dilution would be less and the fall-out loss of concentration closer to 100 mg m⫺3. Flow velocities near Cape St. Vincent where the outflow starts to separate completely from the bottom are about 0.1–0.3 m s⫺1 (Prater & Sanford, 1994) and so this fall-out occurs in the order of 10 days to 1 month at minimum. However, the paths of Meddies are dominated by eddy motion and have a translation speed of 0.02–0.06 m s⫺1 (Richardson, Bower, & Zenk, 2000), giving a more likely transit time of 1.5– 5 months. The faster speeds imply fall velocities of 6–18 metres per day (0.07–0.21 mm/s), equivalent to primary mineral particles of 6–17 mm in diameter. However, it is expected that the particles will be aggregates of smaller entities. A similar situation, but on the shelf, was examined by Hill, Milligan and Geyer (2000) where effective settling velocities in the Eel River plume inferred from particle loss-rate were 0.06 to 0.10 mm s⫺1. Measured aggregate settling velocities in that area were 0.09–8.1 mm s⫺1 (Sternberg, Berhane, & Ogston, 1999), but these higher speeds do not predict loss-rates because a kinetic rate of aggregation control is also involved. Thus the inferred behaviour of the MW plume and its particle properties are consistent with those of other investigated cases. By the latitude of Nazare´ Canyon (Fig. 4) the MW turbidity has reduced to ⬍25 mg m⫺3, again mainly by fall-out because its salinity is 36.1, indicating that little dilution has occurred during its movement from station M42-02. Thus the MW flowing north of Nazare´ Canyon carries scarcely any remnant signal from the intense erosion in the outflow region south of Cadiz. 3.2.2. MW turbidity off northwestern Iberia The absence of a clear MW turbidity signal at the outer stations of Nazare´ Canyon at 39°34⬘N suggests that any PMC signature seen farther north must be locally generated by interaction between the MW plume and the continental slope. In winter 98/99 along line S a small peak at the MW level was apparent out to the deepest station (Fig. 5). In July 1997 along line P there was again a small MW turbidity signature at the outermost station, but there was a fairly clear maximum with a spike at the 2000 m station, a situation repeated in spring 1999 on PLG 138 (Fig. 6). Meanwhile in 1997 along line R in May, there was scarcely anything to discern in CD105 data, but two months later at the 1200 m station on PLG 109 there was a turbidity increase at MW level, which declined rapidly offshore (Fig. 7). These data are consistent with intermittent removal of material from the slope in suspension in the MW flow, though this is never very strong. It is clearly far less strong than the resuspension that occurs over the Gibraltar Sill and downstream in the Gulf of Cadiz where the turbidity signal in MW is very large. The trajectory of eddies of MW (meddies) is away from the slope (Richardson, Bower, & Zenk, 2000), and the MW flow scheme of Maze´ et al. (1997) also shows a significant flow away from the margin at ~41°N to flow around Galicia Bank. We suggest these flows leaving the margin carry with them the turbidity signal generated intermittently by faster flows into the Iberian Basin, thereby producing a small particle export into the ocean from the margin. 3.3. Turbidity of upper slope waters Many profiles show increases in turbidity at specific levels, as seen for example in CD-105 line T, 1600 m station (Fig. 8). The principal features are a bottom nepheloid layer, a spatially and temporally variable MW maximum plume at 600–800 m and a shelf-edge plume. Inspection of the profiles in Figs. 5–8 shows that these features occur in several profiles, though not with any lateral consistency in an East–West sense. At the shallower stations, i.e. in ⬍1500 m water depth, there is significant turbidity over the upper continental slope, (Fig. 5, the inshore three profiles; Fig. 6, 300–500 m plume at the 2600 m station; Fig. 7 CD105, the inshore 3 profiles). This maximum appears to be related to (i) export of high turbidity from the shelf edge (see below), and (ii) resuspension at depths of 500–800 m. The latter depth range also coincides with the maximum in the Brunt-Va¨ isa¨ la¨ frequency of ⱖ2.10-6 s⫺2. It is not clear whether the resuspension

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

307

Fig. 5. PMC (upper) and salinity (lower) profiles from winter ’98/’99 cruise Meteor 42 along 42°09⬘N (Line S). A slight peak in turbidity between 900 and 1100 m is marked by black dots and lies in MW. Two abrupt shifts in turbidity just off the slope numbered 1 and 2 in the 1600 m and 2000 m profiles and are seen to correspond to salinity interfaces similarly marked on the lower panel.

at this level is caused by the slope current or internal waves. However, singly or combined they lead to pronounced turbidity at this level. The frequency of resuspension is not available from moored transmissometers, but Huthnance et al. (2002) remark on the internal waves and M2 tides of 10–20 m amplitude and presence of variability at periods of 2–3 and 10–25 days. 3.4. Contributions from the Outer Shelf The clear origin of material from the outer shelf can be seen at 100–250 m depth in the plume that is most pronounced in the winter data from Meteor (Fig. 5), and also at other times. A transect over the outer shelf by Meteor in winter 98/99 shown in Fig. 9 reveals an intense seaward-thickening bottom mixed-

308

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

Fig. 6. Repeated PMC profiles for line P (~42°40⬘N) from Pelagia cruises 109 (July 1997) and 138 (May 1999) showing a spatially and temporally variable MW signature, a common but relatively weak BNL, and in profile 109-05 a remarkable peak in NACW at ~400 m which is not found at inshore stations and thus probably was derived from the slope to the south. (Vertical lines with arrows mark the 0.35 m⫺1 baseline for attenuation for the three profiles to the right.)

and nepheloid-layer. The seaward thickening of this layer occurs over a very short east–west distance (~7 km). All data suggest that the principal component of flow is not across the shelf parallel to the line of section but along the shelf edge with the greatest flow speeds over the most seaward sections (Huthnance et al., 2002). This may be coupled with resuspension by the long period (~15 s) North Atlantic winter waves. The high turbidity in these layers would be carried offshore in the eddies seen in surface water

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

309

Fig. 7. Repeated PMC profiles for line R (~42°20⬘N) from Darwin 105 (May 1997) (upper) and Pelagia 109 (July 1997) (lower). (Vertical lines with arrows mark the 0.35 m⫺1 baseline for attenuation for the three profiles to the right.)

patterns from satellites (Alvarez-Salgado et al., 2000). Profiles in spring and summer on the outer shelf and uppermost slope (above 300 m) also all show strong BNLs of ~1000 mg m⫺3. Oliveira, Vitorino, Rodrigues, Jouanneau, Dias, & Weber (this volume) show that transport of shelf BNLs offshore to form INLs occurs along isopycnal surfaces below σθ = 26.9 kg m⫺3. Under spring/summer upwelling conditions

310

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

Fig. 8. PMC profile from CD-105 station T1600 (position on Fig. 1) showing turbidity peaks indicative of particular water masses and associated processes.

transport is to the south whereas in winter it is to the north with downwelling assisting the off-shelf transport. 3.5. Deep water bottom nepheloid layers The increase in turbidity close to the bed in deep water comprising the BNL is frequently slight (a few 10s of mg m⫺3) and sometimes undetectable (Figs. 5–7). From this one would deduce relatively tranquil conditions in depths below about 2500 m, which is consistent with the very slow geostrophic flow estimates of Maze´ et al. (1997). Nevertheless there may be tidal flows, which generate the BNLs seen on the lower slope. Only in the Nazare´ Canyon was there evidence of much stronger sediment transport in the bottom nepheloid layer (and associated INLs) (Fig. 4).

4. Conclusions The data presented have shown that the major activity and particle export on the northern Iberian Margin is on the upper slope, principally under ENACW with strong shelf edge export. The flow of MW through the region offer a nice example of the rapidity with which a water mass will loose a heavy load of suspended

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

311

Fig. 9. Marked thickening of the outer shelf nepheloid layer on Meteor-42 at 42°09⬘N (upper), is seen to be associated with thickening of the mixed layer shown by salinity (lower). The ‘top of BNL’ line is at the same level in both panels.

particles, in this case along a track length of ~250 km (1.5–5 months) some 200 mg m⫺3 are lost, of which over 150 mg m⫺3 by fall-out. Thereafter the MW interacts intermittently with the slope to acquire a weak turbidity signal of ⬍50 mg m⫺3. The overlying ENACW appears to gain its load in the depth range of 500–800 m, which corresponds to the steep density gradient of the mixing zone between ENACW and MW having a local maximum in buoyancy frequency. This leads to the suspicion that internal waves are implicated in causing resuspension at this level, as well as the slope current, which also occurs there. This activity means that the Iberian Margin is an efficient exporter of fine sediment to the adjacent ocean

312

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

basin. Material shed from the shelf edge out over the slope at low sea-level continues to be winnowed at high sea-level, and yields the sandy texture of the sediments on the slope recorded by Flach et al. (2002) and van Weering et al. (2002). The deeper slope and rise is a zone of much weaker activity, which additionally receives material transported from injection points at the mouths of canyons farther south.

Acknowledgements We thank Tjeerd van Weering and Henko de Stigter for Pelagia data and Laurenz Thomsen for Meteor data, and the staff of BODC for cleaning it up. This paper has benefited from review by Jean-Claude BrunCottan and Xavier Durrieu de Madron. Cambridge Earth Sciences No. 6398.

References Alvarez-Salgardo, X.A., Perez, F.F., Figueiras, F.G., Castro, G.G., Borges, A., Moncoiffe, G., Milller, A.E.J., Frankignoulle, M., & Savidge, G. (2000). Thermohaline, chemical and biological characterisation of the poleward flowing slope current of the N.W. Iberian upwelling systems. In, 32nd International Liege Colloquium on Ocean Hydrodynamics, Abstracts, p.4. Baker, E. T., & Hickey, B. M. (1986). Contemporary sedimentation processes in and around an active West Coast submarine canyon. Marine Geology, 71, 15–35. Baker, E. T., & Lavelle, J. W. (1984). The effects of particle size on the light beam attentuation coefficient of natural suspensions. Journal of Geophysical Research, 89, 8197–8203. Baringer, M.O. & Price J.F. (1999). A review of the physical oceanography of the Mediterranean outflow. Marine Geology, 63-82. Bishop, J. K. B. (1986). The correction and suspended particulate matter calibration of SeaTech transmissometer data. Deep-Sea Research, 33, 121–134. Daniault, N., Maze´ , J. P., & Arhan, M. (1994). Circulation and mixing of Mediterranean Water off the Iberian Peninsula. Deep-Sea Research I, 41, 1685–1714. Dias, J. M. A., & Nittrouer, C. A. (1984). Continental shelf sediments of northern Portugal. Continental Shelf Research, 3, 147–165. Dias, J. M. A., Jouanneau, J. M., Arau´ jo, M. F., Drago, T., Garcia, C., Gonzalez, R., Oliveira, A., Rodrigues, A., Vitorino, J., & Weber, O. (2002). Present day sedimentary processes on the northern Iberian shelf. Progress in Oceanography, 52(2-4), 249–259. Dickson, R. R., & McCave, I. N. (1986). Nepheloid layers on the continental slope west of Porcupine Bank. Deep-Sea Research, 33, 791–818. Drago, T., Oliveira, A., Magalha˜ es, F., Cascalho, A., Jouanneau, J. M., & Vitorino, J. (1998). Some evidences of northward fine sediment in the northern Portuguese continental shelf. Oceanologica Acta, 21, 223–231. Drake, D. E. (1971). Suspended sediment and thermal stratification in Santa Barbara Channel, California. Deep-Sea Research, 18, 763–769. Drake, D. E. (1974). Distribution and transport of suspended particulate matter in submarine canyons off southern California. In R. J. Gibbs (Ed.), Suspended solids in water (pp. 133–153). New York: Plenum Press. Fiuza, A. F. G., Hamann, M., Ambar, I., del Rio, G. D., Gonzalez, N., & Cabanas, J. M. (1998). Water masses and their circulation off western Iberia during May 1993. Deep-Sea Research I, 45, 1127–1160. Flach, E., Muthumbi, A., & Heip, C. (2002). Meiofauna and macrofauna community structure in relation to sediment composition at the Iberian Margin compared to the Goban Spur (NE Atlantic). Progress in Oceanography, 52(2-4), 433–457. Gardner, J. V., & Kidd, R. B. (1987). Sedimentary processes on the northwestern Iberian continental margin viewed by long-range side-scan sonar and seismic data. Journal of Sedimentary Petrology, 57, 397–407. Gardner, W. D. (1989a). Baltimore Canyon as a modern conduit of sediment to the deep sea. Deep-Sea Research, 36, 323–358. Gardner, W. D. (1989b). Periodic resuspension in Baltimore Canyon by focussing of internal waves. Journal of Geophysical Research, 94, 18185–18194. Gardner, W. D., Biscaye, P. E., Zaneveld, J. R. V., & Richardson, M. J. (1985). Calibration and composition of the LDGO nephelometer and the OSU transmissometer on the Nova Scotian Rise. Marine Geology, 66, 323–344. Hall, I. R., Schmidt, S., McCave, I. N., & Reyss, J-L. (2000). Particulate matter distribution and 234Th/238U disequilibrium along the northern Iberian Margin: Implications for particulate organic carbon export. Deep-Sea Research I, 47, 557–582. Harlett, J. C., & Kulm, L. D. (1973). Suspended sediment transport on the northern Oregon continental shelf. Bulletin of the Geological Society of America, 84, 3815–3826. Harvey, J. (1982). θ-S relationships and water masses in the eastern North Atlantic. Deep-Sea Research, 29, 1021–1033.

I.N. McCave, I.R. Hall / Progress in Oceanography 52 (2002) 299–313

313

Hill, P. S., Milligan, T. G., & Geyer, W. R. (2000). Controls on effective settling velocity of suspended sediment in the Eel River flood plume. Continental Shelf Research, 20, 2095–2112. Huthnance, J.M., vanAken, H.M., White, M., Barton, E.D., LeCann, B., Coelho, E.F., Fanjul, E.A., Miller, P. & Vitorino, J. (2002). Ocean Margin Exchange - Water flux estimates. Journal of Marine Systems. (in press). Maze´ , J. P., Arhan, M., & Mercier, H. (1997). Volume budget of the eastern boundary layer off the Iberian Peninsula. Deep-Sea Research I, 44, 1543–1574. McCave, I. N. (1979). Suspended material over the central Oregon continental shelf in May 1974, I: Concentration of organic and inorganic components. Journal of Sedimentary Petrology, 49, 1181–1194. McCave, I. N., Hall, I. R., Antia, A. N., Chou, L., Dehairs, F., Lampitt, R. S., Thomsen, L., van Weering, T. C. E., & Wollast, R. (2001). Sources, composition and flux of suspended particulate material on the European margin 47°–50° N: A synthesis of results from the OMEX I programme. Deep-Sea Research II, 48, 3107–3139. Moody, J. A., Butman, B., & Bothner, M. H. (1986). Near-bottom suspended matter concentrations during storms. Continental Shelf Research, 7, 609–628. Oliveira, A., Vitorino, J., Rodrigues, A., Jouanneau, J.M., Dias, J.A., & Weber, O. (2002). Nepheloid layer dynamics in the northern Portuguese shelf. Progress in Oceanography, 52(2-4). Prater, M. D., & Sanford, T. B. (1994). A Meddy off Cape St Vincent: 1. Description. Journal of Physical Oceanography, 24, 1572–1586. Richardson, P. L., Bower, A. S., & Zenk, W. (2000). A census of Meddies tracked by floats. Progress in Oceanography, 45, 209–250. Sherwood, C. R., Butman, B., Cacchione, D. A., Drake, D. E., Gross, T. F., Sternberg, R. W., Wiberg, P. L., & Williams, A. J. (1994). Sediment-transport events on the northern California continental shelf during the 1990-1991 STRESS experiment. Continental Shelf Research, 14, 1063–1099. Sternberg, R. W., Berhane, I., & Ogston, A. S. (1999). Measurement of size and settling velocity of suspended aggregates on the Northern California continental shelf. Marine Geology, 154, 43–53. Thorpe, S. A. (1972). A sediment cloud below the Mediterranean Outflow. Nature, London, 239, 326–327. Thorpe, S. A., & White, M. (1988). An intermediate nepheloid layer. Deep-Sea Research, 35, 1655–1671. van Aken, H. M. (2000a). The hydrography of the mid-latitude Northeast Atlantic Ocean II: The intermediate water masses. DeepSea Research I, 47, 789–824. van Aken, H. M. (2000b). The Hydrography of the mid-latitude northeast Atlantic Ocean I: The deep water masses. Deep-Sea Research I, 47, 757–788. van Weering, Tj.C.E., de Stigter, H.C., Boer W., & de Haas, H. (2002). Recent sediment accumulation and carbon burial along the NW Iberian margin. Progress in Oceanography, 52(2-4).