Evidence for aggregate formation in a nepheloid layer and its possible role in the sedimentation of particulate matter

Marine Geology, 20 (1976)M7--M13

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

Letter Section E V I D E N C E F O R A G G R E G A T E F O R M A T I O N IN A N E P H E L O I D L A Y E R A N D ITS P O S S I B L E R O L E IN T H E S E D I M E N T A T I O N O F P A R T I C U L A T E MATTER

RICHARD A. FEELY Pacific Marine Environmental Laboratory, Seattle, Wash. (U.S.A.)

(Received August 12, 1975; revised and accepted October 10, 1975)

ABSTRACT Feely, R.A., 1976. Evidence for aggregate formation in a nepheloid layer and its possible role in the sedimentation of particulate matter. Mar. Geol., 20:M7--M13. During cruise 73-A-3 of the R/V "Alaminos", seawater samples from the nepheloid layer and overlying water were collected and filtered through 0.4 um Millipore@ Ultra Thin filters for microscopic analysis with a Jeol JSM-U3 scanning electron microscope. The results of the studies of the suspended matter with the scanning electron microscope indicate that there is a significant increase in the percentage of the total number of particles that are associated as aggregates for the nepheloid layer relative to the overlying water. This suggests that once nepheloid layers are formed, they might represent regions of high sedimentation due to aggregate formation.

INTRODUCTION

In r e c e n t years a considerable a m o u n t o f research e f f o r t has been d e v o t e d t o u n d e r s t a n d i n g t h e m e c h a n i s m s o f t r a n s p o r t o f particulate m a t t e r in deep ocean basins via n e p h e l o i d layers. Many investigators have suggested t h a t the increased c o n c e n t r a t i o n s o f particulate m a t t e r in n e p h e l o i d layers r e f l e c t such d y n a m i c processes as a d v e c t i o n ( E i t t r e i m et al., 1972), erosion b y b o t t o m c u r r e n t s (Betzer and Pilson, 1971) and dissolution-advection (Lal and L e r m a n , 1973). This p a p e r presents the p r o p o s i t i o n t h a t o n c e n e p h e l o i d layers are f o r m e d t h e y might r e p r e s e n t regions o f m a x i m u m s e d i m e n t a t i o n due t o t h e f o r m a t i o n o f aggregates. Studies o f f l o c c u l a t i o n o f s u s p e n d e d m a t t e r have primarily been c o n f i n e d t o estuarine e n v i r o n m e n t s (Schubel, 1 9 6 8 ; Edzwald, 1 9 7 2 ) , where f l o c c u l a t i o n o f river-derived s u s p e n d e d m a t t e r is the d o m i n a n t m e c h a n i s m o f s e d i m e n t a t i o n . Only a few studies have b e e n c o n c e r n e d with the f o r m a t i o n o f aggregates in near-shore and d e e p - o c e a n e n v i r o n m e n t s (Riley, 1 9 6 3 ; S h e l d o n et al., 1967; G o r d o n , 1 9 7 0 ; Kranck, 1973; and Rhoads, 1973).

M8 The mechanism of formation of aggregates in the pelagic environment is not well understood. Baylor and Sutcliffe (1963) have shown that aggregates can form in the presence of bubbles. Riley et al. (1963) showed that aggregates can form in the absence of bubbles, Sheldon et al. (1967) showed that particles will form in membrane-filtered seawater in the presence of bacteria. The aforementioned observations were made using surface seawater as the sample media. Since biological activity is maximized near the surface, one might expect biological mechanisms to play an important role in the formation of aggregates in near-surface regions of the oceans. On the other hand, Hahn and Stumm (1970) have demonstrated that, under model experiments with clay particles in solutions of varying ionic strength, coagulation of particles to form aggregates follows a second-order kinetic reaction (1): dN --

dt

- 4k T =

fN

2

(1)

3n

w h e r e N is the number of particles per unit volume, k is the Boltzmann constant, T is temperature in degrees Kelvin, n is the viscosity of seawater, t is time, and f is a stability factor which represents the fraction of particle collisions which are successful. Using a modified form of this equation, Edzwald (1972) determined that for the clay particle, kaolinite, illite and montmorillonite, the stability factor, f, is within the range of 0.02--0.14. EXPERIMENTAL METHODS In order to determine the physical, chemical and mineralogical characteristics of particulate matter in the near-bottom nepheloid layer of the Gulf of Mexico, a variety of sampling procedures were employed during cruises 71-A-12 and 73-A-3 of the R/V "Alaminos" to insure optimum conditions for sample analysis. The chemical and mineralogical aspects of this study have been reported elsewhere (Feely et al., 1974; and Feely, 1975). For the microscopic study, seawater samples from the nepheloid layer and overlying water were collected in 30 1 Niskin bottles and 1--3 1 aliquots were filtered through 0.4 ~m Millipore® Ultra Thin Type HA 19 X 42 mm rectangular filters. The filters were then washed with three 10 ml portions of filtered deionized triply distilled water to remove the dissolved salts and stored in plastic petri dishes for shipment to the laboratory. In the laboratory, the filters were removed, cut into 5 mm squares and mounted onto aluminium stubs. The stubs were placed into a vacuum evaporator and coated first with carbon and then with metal containing 40% Pd and 60% Au by weight. Five stubs were prepared from each filter. The stubs were placed into a Jeol JSM-U3 Scanning Electron Microscope and were observed at a magnification of 3000 X. The absolute magnification was determined by introducing a standard calibration grid (2,160 lines/mm, prepared b y Ernest F. Fullam, Inc.) into the scanning electron microscope and subjecting it to the

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same conditions as the samples. Scanning electron micrograph negatives were made directly from the image on the cathode ray tube using Kodak® Tri-X Pan film. Of the thirty to forty scanning electron micrograph negatives that were prepared from each filter, twenty negatives were chosen (the choice was based entirely upon photographic consideration such as contrast and resolution) as fields for particle counting and sizing. Particle counts were made for three size ranges: 0.3--2 pm, 2--20 ~m, and >t 20 pm. RESULTS

The results of the studies of the suspended matter with the scanning electron microscope are presented in Table I. The data shows that the total number of particles {per 20 fields) in the nepheloid layer is significantly greater than the total number of particles in the overlying water (1.3--2.7 times the value for the region above the nepheloid layer). These data are supported by the suspended-matter concentrations which also show concentrations ( in units of pg/1) which are 1.2--2.3 times the concentration for the region above the nepheloid layer. Furthermore, the percentage of the total number of particles that are associated as aggregates is significantly greater for the nepheloid layer relative to the overlying water (27.4 vs. 19.6).

Fig. 1. Scanning electron micrograph of suspended particles randomly distributed on a Millipore ® filter.

Mll Fig. 1 shows a scanning electron micrograph of suspended matter collected from 1000 m at station 3, cruise 73-A-3. In order to insure that particle overlap on the filters was kept to a minimum, very small quantities of seawater (1--3 l) were filtered through the 19 X 42 mm filters. In the scanning electron micrograph in Fig. 1, the particles appear to be randomly distributed on the filter and seem to have settled as discrete entities. In contrast, aggregates appear in the scanning electron micrograph as complex associations of biologic fragments and mineral grains grouped together in clumps (Fig. 2). Fine-grained particles appear on the surfaces of the coarse particles and seem to have been scavenged by the larger particles.

Fig. 2. Scanning electron micrograph of an aggregate of suspended particles on a Millipore® filter. DISCUSSION Since the rate of formation of aggregates is proportional to the square of the number of particles per unit volume (eq. 1), then a three-fold increase in the number of particles in the nepheloid layer relative to the overlying water would increase the rate of aggregate formation by a factor of 9. The increase in the percentage of particles associated as aggregates in the nepheloid layer relative to the overlying water indicates that aggregates are probably being formed within the nepheloid layer. If it is assumed, as a first approximation,

M12

that once aggregates are formed their sinking rates are very nearly the same as whole particles with equivalent diameters, then one would expect from Stokes law* that the formation of aggregates in a nepheloid layer would increase the flux of suspended matter downward and decrease the concentration of suspended matter over a given time interval.

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Fig. 3. Variations of total suspended m a t t e r at the same location for time intervals of 11~ years and 7 days.

Fig. 3 shows the vertical distribution of suspended matter within the near-bottom nepheloid layer at station 3 (26°23.2'N, 91o56.8'W) in the Gulf of Mexico over time periods of 11/~ years and 7 days (a com. plete description of the station locations and sample collection and filtration procedures is given in Feely, 1975). The figure shows significant variations in the vertical distribution of suspended matter within the near-bottom nepheloid layer which appears to indicate a slow dissipation with respect to time. Although it has been shown that vertical eddy diffusion and advection play a major role in controlling temporal variations of suspended matter within the nepheloid layer (Feely, 1975), once a nepheloid layer is formed its slow dissipation may be due, in part, to the formation of aggregates. Thus, the conclusion is that nepheloid layers might represent regions of high sedimentation due to the formation and increased settling rates of aggregates. *Stokes Law states that the settling velocity of fine particles in a liquid m e d i u m is directly proportional to the square of the radius of the particle and inversely p r o p o r t i o n a l to the viscosity of the liquid medium.

M13

ACKNOWLEDGEMENTS

The financial support for this project was provided by the Office of Naval Research Grants NOOO 1468-K-0308(0002) and NOOO 14-67-0108(0004) and Atomic Energy Commission Grant AT{40-1)3852.

REFERENCES Baylor, E.R. and Sutcliffe, W.H., 1963. Dissolved organic matter in seawater as a source of particulate food. Limnol. Oceanogr., 8: 369--371. Betzer, P.R. and Pilson, M.E.Q., 1971. Particulate iron and the nepheloid layer in the western North Atlantic, Caribbean and the Gulf of Mexico. Deep-Sea Res., 18: 783--761. Edzwald, J.K., 1972. Coagulation in Estuaries. Sea Grant Publ. UNC-36-72-06. University of North Carolina, 204 pp. Eittreim, S., Gordon, A.L., Ewing, M., Thorndike, E.M. and Bruchhausen, P., 1972. The nepheloid layer and observed bottom currents in the Indian--Pacific Antarctic Sea. In: A.L. Gordon (Editor), Studies in Physical Oceanography. Gordon and Breach, London, pp. 19--36. Feely, R.A., Sullivan, L., and Sackett, W.M., 1974. Light scattering measurements and chemical analysis of suspended matter in the near-bottom nepheloid layer of the Gulf of Mexico. In: R.J. Gibbs (Editor), Suspended Solids in Water. Plenum., New York, N.Y., pp. 281--294. Feely, R.A., 1975. Major element composition of the particulate matter in the near-bottom nepheloid layer of the Gulf of Mexico. Mar. Chem., 3: 121--156. Gordon, Jr., D.C., 1970. A microscopic study of organic particles in the North Atlantic Ocean. Deep-Sea Res., 17: 175--185. Hahn, H.H. and Stature, W., 1970. The role of coagulation in natural waters. Am. J. Sci., 768: 354--368. Kranck, K., 1973. Flocculation of suspended sediment in the sea. Nature, 246: 348--350. Lal, D. and Lerman, A., 1973. Dissolution and behavior of particulate biogenic matter in the ocean: some theoretical considerations. J. Geophys. Res., 78: 7100--7111. Rhoads, D.C., 1973. The influence of deposit-feeding benthos on water turbidity and nutrient recycling. Am. J. Sci., 273: 1--22. Riley, G.A., 1963. Organic aggregates in seawater and the dynamics of their formation and utilization. Limnol. Oceanogr., 8: 372--381. Schubel, J.R., 1968. Suspended sediment of the northern Chesapeake Bay, Tech, Rep. Chesapeake Bay Inst. 35. (Ref No. 68-2). Sheldon, R.W., Evelyn, T.P.T. and Parsons, T.R., 1967. On the occurrence and formation of small particles in seawater. Limnol. Oceanogr. 12(3): 367--375.