The formation of sedimentary strata in an allochthonous shelf environment: The Washington continental shelf

Marine Geology, 42 (1981) 201--232

201

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

THE FORMATION OF SEDIMENTARY STRATA IN AN ALLOCHTHONOUS SHELF ENVIRONMENT: THE WASHINGTON CONTINENTAL SHELF

C.A. N I T T R O U E R * and R.W. S T E R N B E R G

Department of Marine, Earth and Atmospheric Sciences, North Carolina State University,Raleigh, N.C. 27650 (U.S.A.) Department of Oceanography, University of Washington, Seattle, Washington 98195 (U.S.A.) (Received and accepted January 23, 1981)

ABSTRACT Nittrouer, C.A. and Sternberg, R.W., 1981. The formation of sedimentary strata in an allochthonous shelf environment: the Washington continental shelf. Mar. Geol., 42: 201--232. Models o f fine-scale (<10 ° m, <102 yr) strata formation can enhance geological interpretations of the stratigraphic record. The Washington continental shelf represent~ a good locality to develop such a model for shelf strata, because it is undergoing sedimentation o f material supplied b y the Columbia River. Modern sand accumulates on the inner (<40---60 m water depth) and mid-shelf regions, and modern m u d (mostly silt) accumulates on the mid and o u t e r (>120 m) shelf regions. The inner shelf sand and mid-shelf sandy silt are the predominant accretionary deposits on the shelf. I m p o r t a n t sedimentological observations obtained by boxcoring the upper ½ m o f these deposits are: (a) progressive decrease in grain size with distance from the Columbia, (b) downward coarsening within the seabed, (c) loss o f distinct sedimentary structure (homogenization o f sediment) with distance from the Columbia. Non~iimensional parameters based on sediment transport and benthic biological studies can be used to relate rates o f sediment mixing (reworking) to rates o f accumulation. These parameters have the potential for quantitative prediction o f strata formation, from measurements o f active processes. Physical and biological mixing can be assumed to remain relatively constant in an alongshelf direction on the Washington shelf, and accumulation rate is known (from Pb-210 geochronology) to decrease away from the Columbia. Together these factors tend to increase the ratio o f mixing to accumulation with distance from the Columbia, and predict the sedimentological observations listed above. INTRODUCTION

Consideration of time is fundamental to the understanding of stratigraphy. Sedimentary strata are separated into a hierarchy of units (e.g., formations, members, beds, laminae) which reflect sediment accumulation over different time scales. The geological processes which dominate strata formation *Previous address: Department o f Oceanography, University of Washington, Seattle, Washington 98195 (U.S.A.). 0025-3227/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

202

change with the time scales. For example, in the continental shelf environment, eustatic sea level change can control strata formation over millenia, but storm events can dominate on time scales of months. After the establishm e n t of a time scale of interest, stratigraphic studies of modern environments require the measurement o f time-related parameters in order to characterize important processes (e.g., rate of sea level rise, frequency of storms). A time-related parameter of particular significance to stratigraphic studies is sediment accumulation rate, which is here defined as the sum of local deposition and erosion over a given time scale. Positive accumulation (net deposition) has the u t m o s t significance, because it creates the sedimentary strata from which paleoenvironmental interpretations are made. Negative accumulation (net erosion) has less significance because it leaves behind no stratigraphic record, except where its existence is marked b y unconformities. Strata that form as the result of positive accumulation represent a fraction of the total time required for their creation. The total time includes periods of non-deposition and of deposition which was subsequently removed by erosion. The sediment that is preserved m a y have only a limited relationship to mean environmental conditions, and therefore an understanding of the particle transfer mechanism from m o d e m environment to preserved strata is critical to paleoenvironmental interpretations. Further mention of sediment accumulation will refer to positive accumulation. This paper will examine strata formation over a time scale of a century on a continental shelf undergoing allochthonous sedimentation. The time scale was chosen to allow direct comparison with independent studies of active oceanic processes. Allochthonous sediment refers to material derived from a source b e y o n d the boundaries of the shelf, and is usually supplied by rivers (Swift, 1974). An understanding of allochthonous sedimentation is important for predicting the fates of sedimentary components (including anthropogenic pollutants) on m o d e m shelves, and for interpreting strata formed on ancient shelves. The primary objectives of this paper are: (a) to evaluate factors affecting strata formation on the Washington continental shelf, (b) to determine h o w local strata formation affects intra~shelf sedimentation, and (c) to suggest predictive models for strata formation. BACKGROUND

Strata formation on continental shelves Sedimentary structures. Strata formed on the time scale of interest of this paper, a century, involve a vertical length scale o f about 50 cm (i.e., assuming an accumulation rate of 0.5 cm/yr). Stratification on this scale is usually referred to as sedimentary structure, and has been used extensively as a diagnostic tool to interpret sediment cores and outcrops (Pettijohn and Potter, 1964; Reineck and Singh, 1975). The m o d e m continental shelf environment has been relatively inaccessible to studies o f sed'__uv~ntary s t ~ e t u r e , and m o s t detailed studies have been restricted to shallow waters of the inner shelf

203 (e.g., Clifton et al., 1971; Howard and Reineck, 1972). The two general causes for primary sedimentary structures in the shelf environment are biological (bioturbation) and physical (wave and current activity) processes. Although the dominant process is generally assumed to characterize the sedimentary structure of the seabed, most studies have recognized an additional, independent parameter -- sediment accumulation rate -- to be critical to the nature of strata (Moore and Scruton, 1957; Rhoads and Stanley, 1965; Howard, 1975; Nelson et al., 1980). Scientists investigating shelf strata in modern environments strive for predictive models of strata emplacement, and such models require an understanding of the mechanisms by which biological and physical processes operate.

Biological processes. Benthic organisms displace sediment particles during the completion of normal bodily functions, such as feeding and locomotion (Rhoads, 1974). A significant effect on strata occurs when particles are moved vertically, because they cross depositional gradients left by physical processes (see next section). Deposit-feeding infauna usually dominate sediment reworking (Aller, 1977), and an especially important group for strata alteration are "conveyor-belt" species which feed at depth and defecate at the surface (Rhoads, 1974). Activity of the benthic community decreases rapidly with depth below the surface of the seabed; Myers (1977) estimates the turnover time for a lagoonal sediment to differ from about two days to two years between depths of 1 and 10 cm below the surface. On continental shelves relatively few organisms operate below the upper 10 cm of the seabed {Mare, 1942), but even slow reworking of deep sediment might have significant geological effects over a hundred-year time scale. Although studies have estimated sediment processing rates for individual organisms which can be found on shelves (e.g., Rhoads, 1963, 1967; Gordon, 1966; Nichols, 1974), vertical variation of community reworking is more difficult to evaluate, especially for deep sediment (below 10 cm) where rates are slow. An alternate means for estimating rates of biological activity comes from profiles of radioisotopes (which will be described in a following section) and assumes that sediment reworking resembles a random-walk, eddy diffusion process. In fact, on time scales of feeding by benthic organisms, particles are moved advectively; conveyor-belt species transport particles from depth to the surface, many other species displace particles laterally, and some may even rework sediment downward (PoweU, 1977). Aller and Dodge (1974), however, suggest that deposit feeding can resemble a diffusive process when viewed over a time scale significantly greater (by a factor of three or more) than the turnover time of the sediment. Another common assumption for random displacement of sediment is that organisms are non-selective in their choice of particles. Recent studies have shown that many deposit feeders are selective (both passively and actively), choosing particles according to size (Whitlach, 1974; Fenchel et al., 1975), density or surface texture (Self and Jumars, 1978). Some organisms have been shown to use specific shapes and sizes of particles to line tubes and burrows (Fager, 1964; AUer,

204 1977). In the absence of physical processes, selectivity tends to increase the residence time within the zone of biological reworking for selected particles, and to cause the preferential accumulation of non-selected particles (Jumars et al., 1981). For the case of grain-size selection, the net result can be biological grading (downward coarsening) as described by Rhoads and Stanley (1965).

Physical processes. Shear stresses exerted on the seabed by fluid motion can cause particles to be eroded and transported; when flow is no longer competent for motion, the particles are deposited. On continental shelves the most c o m m o n causes for the initiation of transport are surface gravity waves and unidirectional b o t t o m currents (Komar, 1976). Sternberg and Larsen (1976) estimate that on the Washington continental shelf (at 75 m water depth) the seabed is eroded a b o u t 75 days each year, 2/3 of the time due to wave action and 1/3 due to currents. Shear stresses exerted by waves and currents often act together to affect sediment transport, and althoug h the initiation of transport m a y be due to the presence of waves, the direction of transport is usually controlled by currents (Komar, 1976; Grant and Madsen, 1979). Lateral m o v e m e n t o f sediment is an important result of physical transport, b u t there is also a significant amount of vertical sorting associated with erosion and deposition. During major transport events (i.e., storms), the finest sediment is carried in suspension and diffuses upward into the flow, whereas the coarsest sediment is transported as bedload near the seafloor (Smith, 1977); (fine and coarse sediment here refer to the relative hydraulic equivalent grain size of particles, and segregation can be according to other particle characteristics such as grain shape and density). With the cessation of motion the bedload material becomes the base of a layer that fines upward into the suspended load sediment (Smith, 1977), thus forming a graded storm layer similar to those in the North Sea reported by Reineck and Singh (1972). They observed layers several millimeters thick, b u t ephemeral layers could reach a maximum thickness of several centimeters (or more) depending on the erosion depth. Once deposited, a layer is again subject to partial or complete erosion and redeposition. Because fine sediment is last deposited and first eroded, its probability for net accumulation is less than for coarser sediment at the same locality on the shelf. A similar result can occur in areas of sandy sediment where the seabed is deformed into small scale bedforms and sediment transport is mainly from migration of ripples. In this case the laminations are predominantly cross-bedded instead of planar, but coarse sediment is enriched near the base of the unit (Brush, 1965) and would be accumulated preferentially. Evidence of physical transport (sedimentary structures) can be destroyed by bioturbation between periods of transport, b u t physical sorting m a y still leave an impact on the strata formed. Sediment mixing and accumulation. For consideration of continental shelf strata formation, a simplified view of the seabed would divide ~t vertically into t w o regions: a zone of mixing and a zone of accumulation (Fig.l). Mix-

205

'~~'~"

mixing

?

zone of OCCumulotion

Fig.1. A simplified view o f the seabed for consideration o f strata formation. Particles are actively displaced by biological and physical processes in the zone of mixing. With net accumulation, particles are preserved below, in the zone o f accumulation.

ing refers to the displacement of particles by biological and physical processes. The zone of mixing is the portion of the seabed where sediment is churned by benthic organisms, and is eroded and redeposited by physical transport events (including bedform migration). With net accumulation, sediment near the base of this surface layer is removed from active mixing and becomes part of the preserved strata. This synopsis represents a simplified view of the seabed because one depth does not accurately characterize mixing; the benthic community reworks sediment at rates which differ with depth, and physical processes erode to various depths with different frequency. However, for this initial consideration, strata formation on a particular time scale will be assumed to be characterized by mixing to a single depth within the seabed. A residence time for particles within the surface mixed layer can be determined from mixed layer thickness (L) and accumulation rate (A). For a thick ness of 10 em and an accumulation rate of 0.5 cm/yr, residence time (L/A) would be 20 years. That is, physical and biological processes would operate on sediment particles for an average of 20 years before they were preserved by net accumulation. Selective feeding by organisms and hydraulic sorting by physical processes would cause the residence time for specific types of particles to differ from the average. For example, particles which were selectively ingested by organisms and/or which had a relatively small hydraulic equivalent grain size would have a residence time greater than 20 years. The concept of particle residence time within the mixed layer emphasizes that strata formation involves the integration of sedimentary processes over time. The length of time depends on sediment mixing and accumulation. These

206 same two factors control the nature of strata which are preserved, and the representation o f each factor is critical to predictive models of strata formation. Several models of sediment mixing and accumulation have been developed to explain the vertical distribution of radioisotopes and conservative components within the seabed (Goldberg and Koide, 1962; Berger and Heath, 1968; Guinasso and Schink, 1975). Radiochemical studies can be especially important because some isotopes are irreversibly adsorbed to particle surfaces, have a known or measurable input rate to the seabed (source function), and can therefore be used with an appropriate model to estimate rates of mixing and accumulation. The steady-state profile (ignoring consolidation) for excess activity (i.e., above levels supported by the parent isotope) of a nonexchangeable (i.e., chemically immobile) radioisotope is given by the advection-diffusion equation. 02C D Oz2

A 0C ~z

mixing

~nulation

XC

0

(1)

decay

w h e r e C = activity of radioisotope in sediment ( d p m / g ; d p m = disintegrations per minute); D = particle mixing coefficient (cm2/yr); A = sediment accumulation rate (cm/yr); X = decay constant for radioisotope (yr -1) = 0.693(half life)-1; z = depth below sediment surface (cm). Sediment mixing is assumed to resemble eddy diffusion, and is characterized b y a mixing coefficient D. Sediment accumulation is modelled as an advectire process, with an accumulation rate A. Eq.1 assumes that the mixing coefficient is constant with depth, which follows from the previous assumption of a single mixing depth. Solutions can be derived for eq.1 in its entirety or for simplified forms (see: Berner, 1971; Aller, 1977). In general, the surface mixed layer requires the complete equation to describe the profile of a radioisotope, b u t for relatively low accumulation rates (A 2 < < kD) the accumulation term can be ignored. Below the surface mixed layer (where D = 0) the mixingterm can be ignored, and the solution to eq.1 becomes the radioactive decay equation which is used to calculate accumulation rate: C = Cr, exp ( - - ~ z )

(2)

where C m is the radioisotope activity at the base of the mixed layer. Fig. 2 shows hypothetical profiles for excess Pb-210 (half-life 22.3 yr) assuming a mixed layer thickness of 6 cm, an accumulation rate of 0.15 cm/yr, and values for the mixing coefficient as shown. The half-life of the radioisotope employed to measure rates o f mixing and accumulation must be commensurate with the process to be examined. For example, if the half life o f an isotope is t o o small (X > > ALL), the activity of the isotope will decay to zero

207

Pb-21Oex Activity (dpm/gm) ,

........

,p

.......

0

E

A =0.15 cm/yr D in cm21yr

5-

!oo

~Doo

.....

A S.~I

D =O.I

(D 0

I0-

J¢: 15¢1) "0

20-

z ~ / / , ~'/z

25

Fig.2. Hypothetical profiles for excess Pb-210, assuming a mixed layer thickness (L) of 6 cm, an accumulation rate (A) of 0.15 cm/yr, and values for the mixing coefficient (D) as shown. (After Bruland, 1974.)

within the mixed layer, and accumulation rate will be impossible to measure. If the half life is too large (X << D/L2), the slope of the activity profile will approach zero (constant with depth), and mixing rate will be impossible to resolve. Fortunately, a number of radioisotopes with differing half lives are available for continental shelf studies, so mixing and accumulation can be measured on several different time scales. Guinasso and Schink (1975) develop a time-dependent mixing model based on the advection
G = Db/Lb

(3)

A This is a ratio of mixing rate to accumulation rate. The model is designed for biological mixing, and the subscript b designates D b and L b a s the mixing coefficient and mixed layer thickness associated with benthic biological activity. Mixing (in the numerator) is represented by the volume of sediment reworked per area of seabed per time, with units of cm/yr (the same as accumulation rate). For small values of G (<0.1; i.e., weak mixing relative to accumulation) textural heterogeneities are maintained within the mixed layer, and ultimately are preserved in the geological record (Fig.3). Large values of G (>10) indicate that the mixed layer is homogenized, and consequently the resolution of environmental events within the geological record is reduced. Considering sedimentary structure, the transition from low to medium to high values of G would correspond to the change from bedded to mottled to homogeneous structure which Moore and Scruton (1957) describe as the sequence for intensifying bioturbation. Previous discussion (under biological processes) demonstrated that an eddy diffusion mixing model (with a single mixing coefficient) is not valid for cases of selective feeding. Where community feeding is strongly selective, a model which explicitly considers this process (such as that presented by Jumars et al., 1981) would be useful to explain

208 relative concentration

relative concentration

Iayer

T,

Tz

small G

Large G G = Db/Lb A

Fig.3, Profiles o f a conservative tracer following emplacement on the surface at time To, for different values o f the biological p a r ~ e t e r G. (According to the time-dependent mixing model o f Guinasso and Schink, 1975.)

strata formation. Another important limitation is that hydraulic sorting does not resemble eddy diffusion, and an additional parameter is needed to characterize strata formation under conditions dominated by physical processes. Smith (1977) suggests the form for a physical mixing parameter based on sediment transport theory. The seabed is eroded to a depth Lp with a frequency fp. After ~ecea~ation of motion, the mixed layer grad~ upward, with the coarsest sediment a t the base. Again, mixing is r e p r ~ m t e d by a reworking rate in units of cm/yr, and strata formation is dependent on the ratio of mixing rate to accumulation rate:

H

=

Lpfp A

(4)

This equation does not p ~ t e r i z e the ~ g of ~ n t , and therefore does n o t allow explicit conclusions a b u t selective preservatiom H o O v e r , as

209 Smith (1977) indicates, large values of H (intense mixing relative to accumulation) should allow only the coarsest sediment at the base of the mixed layer to accumulate, thus forming homogeneous (massive) sands (Fig.4). Small values of H should allow more of the fine-grained portions, of the graded sequence to be preserved, forming texturally heterogeneous deposits. An important similarity of biological and physical mixing is that large values of G and H both suggest that the preserved sediment will be homogeneous.

The Washington continental shelf Sediments. The Columbia River is the dominant source of sediment for the Washington continental shelf. Approximately 10 ~ metric tons of sediment (mostly fine-grained) are discharged annually to the ocean by the Columbia (Van Winkle, 1914; Judson and Ritter, 1964; Haushild et al., 1966; USGS, 1970), and at least 50% of this sediment is estimated to accumulate on the Washington shelf (Nittrouer and Sternberg, in prep.). Most sediment is supplied during the spring flood (May and June) associated with snow melt in the Cascade mountains. Between floods, the sediment in the river is temporarily deposited behind the numerous dams within the drainage system, but the dams have little effect on the overall downstream movement of fine sand, silt, and clay which can be transported as suspended load (Whetten et al., 1969). The Washington continental shelf is about 200 km long and 40 km wide (Fig.5). The bathymetry is generally featureless, with an offshore gradient which is constant or varies smoothly. This topography results from Columbia River sediment which has been transported northward, and has covered basal irregularities (Nittrouer and Sternberg, in prep.). Where the modern sediment cover is thin or absent the surface is hummocky, as observed on the outer ~--~---'---~

- -

----=---~L~

,.~_~__,__~ .

O00000OGGO000000

OoooooooooO0 -o'~o~o

_

graded storm layer

o'~o o o o o o o o o o o o o o

OOOO00000OOOO ,~oo-o o o o o 0'o b-'~6D5"o O00000000000O

0000000000000

0000000000 000 0000 000 O O O massive O o O O O O sand O O O

0000000000000

°°00° 00o0o Q O O Q bedded -g~-o-o~ ;o-~strat a _9_0 ° 0 OG ~D(30 OOOoo ooo000

small H

)000000000000000 ~ 0 0 0 0 o o o o o o 0 O0 O00000 0 O00D-5 0ooo00000000

O

Lpfp H= A

0

Q

O

0 O

Large H

Fig.4. Viewsof the seabed describing the sedimentary structure expected for different values of the physical parameter H. (From the ideas of Smith, 1977.)

210

Fig.5. Bathymetric chart of the Washington continental shelf. shelf (~120 m water depth) south of Quinault Canyon, and on the inner shelf (<:90 m) north of Grays Harbor. These areas are aL.aoc h a r ~ ~ by unusual sediment texture and composition, and are believed to be detached ~ o m the

211 modern dispersal system for Columbia River sediment (Gross et al., 1967; White, 1970; McManus, 1972; Venkatarathnam and McManus, 1973). The surficial sediment on the outer shelf (south of Quinault canyon) is mostly composed of relict sand particles, but Nittrouer and Sternberg (in prep.) suggest that the fine c o m p o n e n t of the sediment (silt and clay) is modern. This paper emphasizes modern sediment which forms two major deposits: inner shelf sand extending from shore to 40--60 m water depth, and mid-shelf silt extending from the sand to about 120 m water depth (McManus, 1972). Nittrouer and Sternberg (in prep.) have investigated these deposits by means of high-frequency (3.5 kHz) seismic reflection profiling. The inner shelf deposit is the landward (and modern) portion of a transgressive sand layer which unconformably covers Pliocene bedrock and underlies the mid~helf silt. The silt deposit trends from the Columbia toward the head of Quinault canyon (north-northwesterly), and is found on the outer shelf (> 90 m water depth) north of the canyon. The deposit becomes progressively thinner away from the Columbia River as a result of decreasing accumulation rate (Nittrouer et al., 1979). Except near the Columbia (within 15 km), the Holocene stratigraphy reveals very few internal reflectors, suggesting for much of the shelf that steady rather than catastrophic events may control sediment accumulation.

Benthic biology. The benthic community on the Washington continental shelf is poorly understood from a functional sedimentological viewpoint (i.e., h o w the organisms rework the seabed). Several studies have evaluated the distribution of macrofauna, and from these observations some general ideas about benthic mixing can be inferred. The benthic c o m m u n i t y north of the Columbia River contains a shallow (<90 m water depth) sandy-bottom assemblage and a deeper m u d d y - b o t t o m assemblage (Lie and Kisker, 1970). Near the Columbia a similar distribution with depth is observed, although the deeper assemblage differs in species composition and community structure from that farther north (Richardson et al., 1977). Polychaetes are generally the most abundant macrofauna on the Washington shelf, representing >70% (by number) of organisms belonging to macrofaunal taxa (Smethie et al., 1981). The abundance and diversity of organisms along the Oregon--Washington coast is observed to increase with water depth, probably due to increased stability of and food supply to the seabed (Carey, 1972; Richardson et al., 1977). These observations suggest that benthic mixing is greater for the mid-shelf silt than the inner shelf sand; however such a conclusion is tenuous without direct studies of mixing rates. Also lacking is an evaluation of the operational modes of the ambient organisms, which is important for understanding the mechanisms of particle reworking. To meet this latter need, the present study includes an examination of benthic fauna in a cross-shelf transect.

Physical oceanography and sediment transport. Along the Washington coast the mean surface currents flow southward during the summer and north-

212 ward during the winter, in reponse to a sloping sea surface caused by offshore and onshore (respectively) Ekman Transport (Hickey, 1979). The Columbia River effluent is carried southward and somewhat westward (due to Coriolis effect) during the summer, and northward during the winter (Duxbury et al., 1966). This circulation can transport those fine-grained particles which remain in suspension after they reach the marine environment (see Zaneveld and Pak, 1979), but of more general importance to sediment transport on the Washington shelf are flow events associated with storm systems. These events generally occur during the winter, last several days, and produce northerly b o t t o m currents sufficient to erode the seabed (Hopkins, 1971; Sternberg et al., 1977). The non-linear relationship between b o t t o m shear stress and rate of sediment transport indicates that these extreme events would cause the greatest transport and consequently the deepest physical mixing (i.e., erosion and deposition) (Smith, 1977). However, when currents are weak, oscillatory stresses exerted by surface gravity waves can initiate sediment motion and allow transport by currents. For the mid-shelf region of the Washington shelf this synergistic affect occurs twice as frequently (about 50 days each year) as erosion by currents alone (Sternberg and Larsen, 1976), and represents an additional means of physical mixing. The predominant direction for dispersal of Columbia River sediment is toward the north (Smith and Hopkins, 1972; Sternberg and McManus, 1972). Studies with seabed drifters verify the dominance of northerly b o t t o m currents, and also indicate an onshore c o m p o n e n t of motion shoreward of about 50 m water depth and an offshore c o m p o n e n t seaward of 50 m (Morse et al., 1968; Gross et al., 1969). Shoreward of the 50-m isobath, wave interaction with the seabed is predicted to increase significantly, causing the onshore c o m p o n e n t and also preventing accumulation of fine-grained sediment (Creager and Sternberg, 1972; Smith and Hopkins, 1972). The offshore component results from turning of currents due to frictional forces in the b o t t o m Ekman layer (Smith and Long, 1976), and explains the north-northwest trend of the mid-shelf silt deposit. METHODS

Core processing Cores for this study were collected on six cruises during 1976 and 1977 (W7606A, C7608A, T T t l 0 , W7611C, W7612C, TT117). Loran-A was used for positioning, and station locations for each cruise can be found in Nittrouer (1978). Coring was done with a NEL-Reineck box corer, which is designed to obtain undisturbed cores with a cross-sectional area 20 cm × 30 cm and a length of 40--50 cm. Immediately after each core was retrieved, it was placed in an inclined cradle, and one side of the core box was removed. Samples were obtained at 10 to 15 separate depths (in core) for Pb-210 and porosity measurement, using a small ptastic syringe (with t h e end cut-off) which removed a 10 cm 3 sample from a 1-cm depth interval. Approximately

213 20 cm a of sediment (i.e., two full syringes) from each depth were placed in a water-tight vial. Samples for grain size analysis were obtained from a 2--3 cm interval at the surface and at several depths. For examination of benthos, samples with a surface area of 20 cm 2 and extending to a depth of 20 cm were stored in a 10% formalin solution. Duplicate box cores were obtained at many stations, in order to evaluate within-station variability, and a large subcore (15~m diameter tube) was removed from the additional tore. Formalin was added to terminate biological activity, then the subcore was capped and carefully returned to a shore-based laboratory for the above sampling as well as for X-radiographic examination.

Laboratory analysis Pb-210 and porosity. Samples were dried at 60 °, and porosity calculated from the weight loss of water. The dried samples were used for Pb-210 analysis, following the Pb-210/Po-210 technique of Beasley (1969) and Schell et al. (1973). The same technique was used on Washington shelf cores by Nittrouer et al. (1979) and involves: spiking the sample with man-made Po-208; leaching with HNO3, HC104, and HCI; plating Po-208 and Po-210 onto a silver planchet; and measuring decay of both polonium isotopes by alpha-particle spectroscopy. All Pb-210 profiles from cores were corrected for sediment consolidation and salt content of interstitial water, according to the method described in Nittrouer (1978). Grain size. Samples were examined for grain size using standard sieve (class interval 0.25 ¢) and pipette analysis (class interval 1.00 ¢, with some detailed analyses at 0.50 ¢) (Krumbein and Pettijohn, 1938). Benthos. Samples were carefully washed through a sieve (0.35 mm mesh) and organisms were removed (under microscopic examination) for identification. Polychaetes were identified and classified according to family using Fauchald (1977). X-radiography. The large subcores (15-cm diameter) were carefully extruded into a tray and dissected for the above analyses. A medial slab (3 cm thick, 15 cm wide) was preserved and X-radiographed to examine sedimentary structures. RESULTS

Benthos Biological oceanographic processes affecting the seabed on the Washington Shelf are more poorly understood than physical oceanographic processes, and for this reason an examination of benthos was performed. In order to evaluate cross-shelf differences in biological mixing, a transect at 46°50~N was estab-

214

lished containing nine equally spaced stations, with three stations in each of the three major sediment zones (i.e., inner shelf sand, mid-shelf silt, outer shelf sand). Polychaetes are the numerically dominant macrofauna observed along this transect, and were chosen for detailed analysis of functional groups, following the approach of Fauchald and Jumars (1979}. Representative samples of large, sparsely distributed benthos (e.g., heart urchins, bivalves, holothurians) are difficult to obtain by coring, although these organisms may affect sediment mixing significantly. It is assumed in this study that functional grouping of polychaetes accurately reflects the nature of biological mixing along the experimental transect. Some stations along the transect were examined for benthos at different times over a one-year period, but no significant seasonal changes were found in the polychaete community. The observed families were classified according to their feeding mode and motility, and the communities are described in ternary diagrams (Figs.6 and 7; see Jumars and Fauchald, 1977). The communities observed within the mid-shelf silt and outer.shelf sand are very similar, and are characterized as motile burrowers. The dominant family is Capitellidae, which contains conveyer-belt species (Rhoads, 1974) known to rework the upper 15 cm of the seabed ( Fauchald and Jumars, 1979}. The polychaete community within the inner shelf sand is less motile and contains a Surface Feeding

• mid ond outer shelf samples

"

• inner shelf somples

0%

~% .

50%

Filtering

$0%

Burrowing

Fig.6. Ternary diaw~m dmcflbijag t~e feeding mode of polyehaete eom~nunities observed across a tranmeet (46~50~N) o f the Wadfington shelf.

215 ~

Discretely

Motile ~

/

• mid and outer

.\

. \

/ Sessile

~ V

"

V

/ '

== . •

Motile

V

Fig.7. Ternary diagram describingthe motility of polychaete communities observed across a transect (46°50"N) of the Washington shelf. larger fraction of filter feeders and surface deposit feeders than are found offshore. The observed density of organisms was somewhat lower within the inner shelf sands, 19 +- 11 organisms (~ ± s.d.; per 20 cm 2) versus a density of 30 +- 19 organisms offshore. The above observations of type and number of organisms suggest that biological reworking of sediment within the mixed layer would be greater for the mid-shelf silt and outer shelf sand than for the inner shelf sand.

Sediment mixing The shape of Pb-210 profiles can provide insight to sediment mixing over time scales of decades. Typical profiles for excess Pb-210 on the Washington shelf resemble Fig.2 (see Nittrouer et al., 1979, for examples), including a lower region of net accumulation which is separated by an abrupt change in slope from a near-surface region of intense mixing. The surface mixed layer is taken to be delineated by those data points which cause the correlation coefficient for the slope of the lower region to fall below about 0.85. Nittrouer et al. (in prep.) demonstrate that some mixing continues below

216 this surface layer, b u t that the rate is reduced by a b o u t two orders of magnitude. Therefore, the most intense mixing (on decade time scales) is restricted to the Pb-210 surface mixed layer. The raw data for Pb-210 measurements are presented in Nittrouer (1978), and the profiles are described here. Within the inner shelf sand the mixed layer is generally homogeneous and its thickness exceeds 15 cm. Pb-210 observations from these sands are often restricted by shortness of cores and low values of excess activity; therefore more detailed evaluation of mixing is not possible. The nature of the mixed layer within the mid-shelf silt is similar to that within the outer shelf sand. The thickness is variable, with values ranging from 0 to a b o u t 30 cm (deepest mixing found near the Columbia). Within these mid- and outer-shelf deposits the mixed layer is often (but not always) homogeneous. In some cases a reversed profile (i.e., Pb-210 activity increasing downward) is observed within the mixed layer, probably reflecting advective instead of diffusive mixing. Residence times for particles in the mixed layer range from about 2 0 - 7 0 years (mean about 35 years), with the times for the mid-shelf silt generally shorter than for the outer shelf sand, due to differences in accumulation rate.

Sediment grain size A grid pattern containing more than 100 stations was sampled for grainsize analysis. At many of these stations multiple depths within cores were analyzed, and the data are presented in Nittrouer (1978). Sediment from the Washington shelf characteristically reveals a polymodal grain-size distribution, which makes standard statistical treatment difficult (i.e., m o m e n t measures must be applied to the individual modes). Surface samples were subjected to a Q-mode factor analysis (Imbrie and Van Andel, 1964; Sternberg et al., 1977) to evaluate the spatial dominance o f various m o d a l grain sizes (Fig.8). The regions of modern sediment are dominated near the Columbia by fine sand (2.75 ¢), and north o f the Columbia by very fine sand (3.25--3.75 ¢) on the inner shelf and by coarse silt (4.5 ¢) on the mid-shelf. The region seaward of the 40-m isobath near the Columbia shows significant local variability, and at some stations the 2.75 ¢ sand is overwhelmed by very fine sand or coarse silt. The outer shelf region north of the Columbia contains a variety of modal grain sizes from medium to very fine sand (1.50--3.75 ¢), which probably reflect a complex history and multiple sediment sources. The relict inner shelf sediment north of Grays Harbor is dominated by fine sand (2.75 ~). Fine sand (2.75 ¢) occurs on the Washington shelf both as modern (near the Columbia) and as relict (north of Grays Harbor) sediment. Several systematic trends are observed within the m o d e m detmsits on the open shelf, away from textural complexities found near the Columbia River (which are described in t h e next section). The mid~d~elf silt deposit has a bimodal grain-size distribution (Fig.9), containing both sand and silt. The predominant distinction between the mid-shelf and inner~helf deposits is the absence o f the silt mode nearshore. Fig.9 shows the a l ~ a h e l f changes which

217

/

Dominant Modal Grain Sizes

(from Q-mode factor analysis)

I

/

! 30'

124"

~0'

Fig.8. The distribution of the dominant modal grain sizes as delineated by Q-mode factor analysis. The 4.5¢ and 2.75¢ modes continue northward of the region shown, and are separated by the 90-rn isobath.

218

20-77% SILT

6% SAND

WEIGHT

17% CLAY

215 km 10--

W 7606A Sta. 65 0-3

RERCENT

C

I

2

WEIGHT 20 t10

t

..,.,-,.m

I

!

4

33%

53%

S/t~O

SILT

I 8

I

I tO

14% CLAY

130 km PERCENT

J

W 7606A Sta. 47 0-3

2

6

4

S

10

20-

WEIGHT

13%

~% SI~

30% SA,.~'O

CLAY

75 km

10 PERCENT

o J 0

I

I

!

2

!

4

I

I

,=

W 6

I

W7606A Sta. 25 0-3 rl! 8

II

20 m

WEIGHT

I

44% SAND

-

43% SILT

[

13% CLAY

25 km PERCENT

W7606A

Sta. 80 0-.1 I C

I 0

i

I 2

!

GRAIN

! 4

10

SIZE (phi units)

Fig.9. Histograms o f ~min size obaerved in surface sediment along the ml'dmhelf silt depo. sit. In a n o r t h ~ d~ion, the sand mode shifts toward finer sizes and the silt mode becomes more dominant relative to the sand.

219

occur within surface sediment of the mid-shelf deposit with increased distance from the Columbia; the sand mode shifts toward finer sizes and the silt mode becomes more dominant relative to the sand. Fig.10 further demonstrates the progressive fining observed along the axis of the mid-shelf deposit, by comparing the relative concentrations of the different sediment types. This figure also shows that the mid-shelf deposit becomes a silt deposit about 50 km north of the Columbia. Besides the lateral change described above, there is also a systematic vertical change in texture. Grain size generally increases with depth in box cores obtained from the mid-shelf silt deposit. In 87% of cores examined for profiles of both Pb-210 and grain size (16 cores) surface sediment was significantly finer than the sediment preserved below the surface mixed layer. A detailed examination of cores (Fig.ll) demonstrates that the downward coarsening represents an enhancement of the sand mode relative to the silt mode. A similar examination of the inner shelf sand was not possible, because of limitations inherent to coring and using Pb-210 geochronology in sand.

Sedimentary structures Examination of internal sedimentary structures reveals a difference between the region n e a r t h e Columbia River (within about 20 km), where physical and biological structures are common, and the region farther north, where cores are characterized by relatively homogeneous sediment. Near the Columbia the inner shelf sands are planar and cross-bedded (Fig.12A), reflecting the dominance of physical mixing. Surficial silt layers (up to several centimeters thick) are sometimes present during the summer, but not during the winter. These layers probably represent suspended sediment supplied aRt

weight

percent

'

~'

'

,~o

Distance from the Columbia inl

i

I

'

,~o

River

'

~o

..........

'

(km)

i i

Fig.10. Concentrations o f sand' silt, and clay (triangles, squares, and circles, respectively observed in surface sediment along the axis of the mid-shelf deposit. This deposit becomes a silt deposit about 50 k m n o r t h o f the Columbia.

220

20-

WEIGHT

37%

49%

14%

SAND

SILT

CLAY

0 - 2 cm

10 PERCENT

W7606A Sta. 26B

r

0

2

4

I

I 6

I

'1~

i' 8

,

I

I 10

30-

20-

55"/.

33%

12%

SAND

SILT

CLAY

30-32

WEIGHT

cm

W"/fl~A Sta, 268

10PERCENT

,,

0

!

0

! 2

I

GRAIN

I 4

! 6

8

10

SIZE (ptlt units)

Fig.11. Histograms of grain size observed in a sediment core from the mid-shelf silt deposit (at 46°50~N).

during peak discharge of the Columbia (May and June), deposited temporarily, and removed with the return of winter storms. Interesting features found at the river mouth are ellipsoidal mud balls (long axis up to 8 cm), which may represent rip-up clasts from the silt layers. Seaward of the bedded sands (>40 m water depth) the sediment is characterized by a chaotic microstratigraphy of intermixed mud and sand (Fig.12B). Biological mixing is evidenced by tubes and burrows, which often represent concentrations of Fig.12. X-radiographs (negatives) o f cores from the region near the Columbia River (within 20 kin). A (left). Core from inner shelf sand showing physical sedimentary structures (note planar bedding in lower portion o f core) and surficial layers o f silt ( o n l y ohsarved during summer). Some cracks were present in slab o f c o ~ x ~ . B (right). Core obtained seaward o f inner shelf send showing b l o l o ~ m l structures ~ heterogeneous texture. (The orientation arrows are 1.25 cm from tip to tail.)

221

L

222

fine sediment ~t~hin an o ~ r w i s e sandy substrate. The heterogeneous structure in ~ R ~ p t o b a b ~ the result of a fluctuating sediment supply which is incompletely mixed. North of the Columbia River (>20 km away) the physical and biological structures become much less distinct (Fig.13). Some physical sedimentary structure m a y be observed within the inner shelf sands, b u t seldom is it found below the u p p e r 10--15 cm. Similarly, tubes and burrows are observed in the sediment farther offshore, but seldom are they found below the upper 10 cm. Absent from this region is the heterogeneity found farther south, instead there are the m o r e subtle and spatially consistent textural patterns (alongshelf f'ming and downward coarsening) described in the last section. DISCUSSION

Many studies have investigated the effects of modern oceanic processes on shelf sediment. Many other studies have investigated sedimentary strata formed under ancient shelf conditions. In one case the emphasis is on sedim e n t actively mixed and in the other case on sediment preserved by net accumulation. The approach of this paper is to evaluate the transfer mechanism from mixed t o accumulated sediment. Observations of strata formation on the Washington shelf will be considered below, in light of concepts o f sediment mixing and accumulation.

Factors affecting strata formation In the evaluation of strata formation the consideration of time is of great importance. N o t only is the measurement of time necessary in order to determine rates of contributing factors, b u t the establishment of a time scale of interest is a prerequisite to defining the contributing factors. In the present study the time scale o f interest is a century, and biological and physical oceanic processes affect strata formation on this time scale. Sedimentary structures on the Washington shelf reflect physical processes on the inner shelf and biological processes farther offshore. The change in sedimentary structures landward of a b o u t the 50-m isobath is probably due to increased b o t t o m turbulence from the shoaling waves (i.e., surface gravity waves), and to decreased abundance of burrowing infauna. The absence of a particular sedimentary structure does not necessarily reflect the absence of a process. F o r example, physical processes are known to rework the mid-shelf silt deposit a b o u t 75 days each year, b u t the deposit is characterized by biological structure. Preservation o f sedimentary structures is dependent on the relative ~ n g rate of a process, n o t just with respect to other mixing pmcemms, b u t also Fig.13. X-radiographs (negatives) o f cores from the region n o r t h o f t h e COlumbia River (along 46°502N). A (left). Core f r o m inner shelf sand showing homogeneous-sediment. B (right). Core f r o m mid-shelf silt showing s o m e biological structures and mottling. (The o r i e n t a t i o n arrows are 1.25 cm f r o m tip to tail.)

223

224

with respect to accumulation rate. As the ratio of mixing rate to accumulation rate increases, sediment tends to become homogenized. Washington shelf strata can be used as an example of the predictive possibilities for a parameter relating mixing and accumulation. Neither physical nor biological mixing on the Washington shelf would be expected to show major variation in an alongshelf direction. Tidal currents may cause local increases in physical mixing near the mouths of estuaries, and benthic organisms show some changes in c o m m u n i t y composition which may cause differences in biological mixing between the region near the Columbia River and that further north, b u t an assumption of constant alongshelf mixing rate is probably valid as a first approximation. Accumulation rate for the mid-shelf silt deposit is known to decrease northward by a factor of a b o u t 4 (Nittrouer et al., 1979), and a decrease northward also would be expected within the inner shelf sand. Combination of the above assumptions and observations would indicate an increase northward in the ratio of mixing rate to accumulation rate ~, and would predict increased homogenization of sedimentary strata in a northward direction. This effect is observed. The "natural cyclic p e r i o d " for sedimentation has been defined by Moore and Curray {1964) as the length of time necessary to average-out fluctuations in environmental conditions. In c o n t e x t of the present paper, natural cyclic period is the length of time necessary for mixing and accumulation to be considered steady. Measurement of the natural cyclic period would require detailed time-series analysis of the factors affecting mixing and accumulation. In this study the natural cyclic period for the Washington shelf is assumed to be less than a century. The residence time within the surface layer is the average period over which sediment particles are mixed before being preserved b y net accumulation. The residence time for the Washington shelf is 20 70 years (for the mid and outer shelf deposits). If the residence time exceeds the natural cyclic period for a locality, then the sedimentary structure o f preserved strata will be similar through time. If the natural cyclic period exceeds the residence time, then the structure of the preserved strata will vary with time (and vary vertically within the geologic column). Whereas the relationship between mixing rate and accumulation rate indicates the t y p e of sedimentary structure preserved, the relationship between natural cyclic period and residence time indicates the constancy of the preserved structure. Longer cores from the Washington shelf are needed to better evaluate the time-variation of sedimentary structure, b u t generally, structure appears consistent throughout box cores, and therefore residence time probably exceeds natural cyclic period for much of the shelf. 1Measurements o f sediment mixing have recently been made for the mid-shelf region using U - 2 3 8 / T h - 2 3 4 d i s e q u i l i b r i u m s t u d i e s ( N i t t r o u e r et al., in prep.). These measurements s h o w differences between the seabed near the C o l u m b i a (D b -- 95 cm2/yr, L b = 11 c m ) and t h e s e a b e d 75 k m n o r t h w a r d (D b = 47 cm~/yr, L b = 8 c m ) . Over the same distance, accumulation rate decreases b y a f a c t o r o f a b o u t 2 ( f r o m a b o u t 0.7 t o 0.3 c m / y r ; Nittrouer e t al., 1979). The above o b s e r v a t i o n s indicate t h a t G increases f r o m a b o u t 12 t o 20 bet w e e n the t w o locations.

225 The effects of local strata formation on intra-shelf sedimentation Sorting refers to the variation of sediment type according to such characteristics as grain size, shape, and density. Russell (1939) describes two kinds of sorting: local sorting which is represe~nted by vertical variation of sedim e n t at a locality, and progressive sorting which is represented by lateral trends in sediment characteristics. Biological graded bedding and graded storm layers are examples of local sorting. The c o m m o n observation within sediment dispersal systems of grain size fining away from the sediment source is an example of progressive sorting. Swift (1970) documents cross-shelf fining within modern and ancient shelf environments where sediment originates from a linear source nearshore and movement offshore is primarily the result of wave-drift currents. Crossshelf sorting is observed on the Washington shelf (i.e., the silt mode is absent from the inner shelf). In addition, the presence of a point source of sediment and the importance of unidirectional alongshelf currents produce alongshelf sorting. Within surface sediment of the mid-shelf deposit the sand mode becomes finer and the silt mode becomes more dominant with distance from the Columbia River (Figs.9 and 10). A possible cause is the more frequent transport of fine particles as suspended load, which is a more efficient mode of transport than as bedload, and would allow fine particles to travel farther than coarse particles during the same period of time. Assuming that the coarsest m o d e m sand particle (2.75 ¢) moves alongshelf an average of 0.1 km/yr (silt particles move about 80 km/yr; Smith and Hopkins, 1972), then only 2000 years would be required for the particle to travel the length of the Washington shelf. Assuming further that the shelf dispersal system has remained relatively constant for 2000 years, then the entire length of the mid-shelf deposit should possess the same texture. This is n o t the case, and therefore an additional mechanism is probably in operation to explain alongshelf fining. Progressive sorting is the result of the mixing-accumulation process of strata formation, and is inseparable from local sorting. Progressive sorting results from processes which increase the residence time within the mixed layer of certain sediment particles and cause preferential accumulation of other particles. On the Washington shelf downward coarsening within the seabed reflects preferential accumulation of coarse sediment. This accumulation causes alongshelf fining within surface sediment, because the accumulation of particles removes them from the dispersal system and eliminates their availability to other parts of the shelf.

The prediction of strata formation An important limitation of the simple predictive models for strata formation (represented in the upper parts of Figs.14 and 15) is the assumption of a single mixing deptt~. The two parameters for strata formation (G and H) can be modified ~o include multiple-mixed layers (lower parts of Figs.14 and

226

mixing coefficient

Db =

t L b = biological

mixing depth (cm)

(cm2/yr )

A = accumulation

G : Db/Lb A

rate

(cm/yr)

volume of sediment processed per unit area of seabed per time

VV V 4vvVv

net accumulation rate

D3

LI L2 L3

Dm

Lm

D2

~. Ob/Lb G= b:l

A

Fig.14. For calculation of the biological parameter G, the upper figure demonstrates a single mixed layer, and the lower figure demonstrates multiple mixed layers.

15) but this also increases the complexity of field measurements needed for evaluation of the parameters. Net accumulation rate is best measured by Pb-210 geochronology, using profiles o f bomb-produced radioisotopes Pu-239, -240 (Benninger et al., 1979) or Cs-137 (Nittrouer et al., in prep.) as independent checks. For evaluation of the biological parameter (G), mixing rates over various depths (and time scales) can be measured by a series of radioisotopes representing a range of half-lives (see Benninger et al., 1979), for example: Th-234 (half-life 24 days), Be-7 (half-life 53 days), Th-228 (half-life 1.9 years), Pb-210 (half-life 22.3 years). If biological mixing on a particular shelf deviates significantly from a diffusive mixing model, then experiments can be performed using tagged particles to calculate the mixing rate. The important task is to measure the volume of sediment reworked per area of seabed per time, because the basic concept relating this rate to accumulation rate is mechanistically sound. For evaluation of the physical parameter (H), erosion frequency can be measured by instrumented tripods which monitor the benthic boundary layer. The depth o f erosion can be measured using the Rn-222 technique (described in Smethie et al., 1981) or high-frequency acoustic systems mounted on the tripods.

227

fp = frequency or erosion

I Lp

(yr4)

A = accumulation rote (cm/yr)

=

physicol erosion depth (cm)

H=

fp Lp

A volume of sediment reworked per unit area of seabed per time net accumulation rate

f2 fn

LI L2 Ln

H .[p~l (fpLp) ]/n A

WVvVvVWVvVV~ Fig.15. For calculation of the physical parameter H, the upper figure demonstrates a single mixed layer, and the lower figure demonstrates multiple mixed layers.

The important next step is to begin field studies which can evaluate G and H as well as the nature o f the sedimentary strata. The objective of these studies should be to determine critical values for G and H which characterize strata formation. A number of complications may arise, for example, nonlinear interactions between biological and physical processes may prevent the simple summation of the two parameters under conditions of combined biological and physical mixing (e.g., Jumars et al., 1981). However, such problems can be realized only after initial studies begin. CONCLUSIONS

The Washington shelf (1) The examination of polychaete distribution suggests that burrowing infauna become more abundant and consequently that biological mixing becomes more intense seaward of the inner shelf sand deposit. (2) Pb-210 profiles indicate that the most intense mixing (on decade time scales) is found within the upper 10--15 c m of the seabed. Particles within this surface layer (on the mid and outer shelf) are reworked by biological and physical mixing for an average of about 35 years before being preserved by net accumulation.

228 (3) The mid-shelf silt deposit locally shows a downward coarsening within the seabed, and regionally shows an alongshelf fining away from the Columbia River. (4) Physical stratification dominates the inner-shelf sediment (<40--60 m water depth), and biological structures are most common farther offshore. In general, sedimentary strata become more homogeneous with distance from the Columbia River.

Strata formation (1) The structure of sedimentary strata can be predicted by parameters relating mixing rate to accumulation rate. As the ratio of these factors increases, structures become less distinct and the strata become more homogeneous. The variability of strata preserved through time is controlled by the relationship between the residence time of particles within the surface mixed layer and the natural cyclic period of sedimentation. (2) Physical and biological mixing processes can increase the residence time of fine sediment within the surface layer and can cause the preferential accumulation of coarse sediment. This results in local sorting with sediment coarsening downward, and in progressive sorting with sediment fining away from the source. (3) Instrumentation and techniques are available to evaluate the physical and biological parameters for strata formation. Field studies should investigate critical values of these parameters. ACKNOWLEDGEMENTS The studies of benthic biology were done in conjunction with R.F.L. Self and P.A. Jumars. The Q-mode factor analysis of grain size data was done in conjunction with J.C. Borgeld. The ideas expressed in this paper were improved by discussions with J.D. Smith and J.S. Creager. Many people helped with the completion of field and laboratory work, but exceptional contributions were made by J.T. Bennett, B.J. Meredith, J.D. Hoeft, and R.B. McKinney. This paper was improved by helpful comments from R.C. Aller, M.H. Bothner, D.J. DeMaster, J.D. Howard, P.A. Jumars, and R.W. Tillman. During completion of the research the first author was supported by fellowships from Texaco and Amoco Corporations. This research was sponsored by the National Science Foundation (Grants No. OCE 76-99791 and OCE 78-06820). This paper is Contribution No. 81-1, Department of Marine, Earth and Atmospheric Sciences, North Carolina State University; and Contribution No. 1205, Department of Oceanography, University of Washington.

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