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73
Marine Geology, 68 (1985) 73-106
Elsevier Science Publishers B.V., Amsterdam
-
Printed in The Netherlands
VARIABILITY IN GEOACOUSTIC AND RELATED PROPERTIES
OF SURFACE SEDIMENTS FROM THE VENEZUELA BASIN,
CARIBBEAN SEA
KEVIN B. BRIGGS, MICHAEL D. RICHARDSON and DAVID K. YOUNG
Naval Ocean Research and Development Activity, OceanographyDivision,
NSTL, MS 39529-5004 (U.S.A.)
(Received November 23, 1984; revised and accepted March 8, 1985)
ABSTRACT
Briggs, K.B., Richardson, M.D. and Young, D.K., 1985. Variability in geoacoustic and
related properties of surface sediments from the Venezuela Basin, Caribbean Sea.
In: D.K. Young and M.D. Richardson (Editors), Benthic Ecology and Sedimentary
Processes of the Venezuela Basin: Past and Present. Mar. Geol., 68: 73-106.
The spatial variability of sediment geoacoustic and related properties was determined
from replicate 0.25 m2 USNEL box-core samples collected from three sedimentary
provinces in the Venezuela Basin. The maximum variation in sediment physical, chemical
and geoacoustic properties occurred among distant sites representing pelagic, turbidite
or hemipelagic sedimentary regimes rather than within sites or within core depth gradients. Variability among sites was a function of differences in calcium carbonate dissolution among a water-depth gradient of 1550 m (3500-5050 m) and differences in the
relative contribution of particles from pelagic and terrigenous sources. The least amount
of variation was evident among subcores obtained from the same box core or site, suggesting that a single subcore was adequate to describe sediment physical and geoacoustic
properties over large areas of the sea floor. The considerable amount of downcore variability in sediment properties was a function of gradients in biological activity, biolog-
ically mediated chemical changes and the presence of ponded terrigenous sediments
deposited
origin.
by intermittent
turbidite
flows interspersed
between sediments of pelagic
For sediments with greater than 35% calcium carbonate, porosity is a good predictor
of compressional wave velocity; whereas mean grain size is a good predictor of compressional wave attenuation. Porosity-mean grain-size relationships had little predictive
value in carbonate sediments. The differences among these and previously published
empirical relationships show that particle-size distribution of larger carbonate material
and composition of the sediment matrix must be considered when predicting sediment
geoacoustic properties. Sediment shear strength did not show any correlation with other
sediment physical or acoustic properties and is primarily controlled by biological and
chemical processes.
INTRODUCTION
Physical properties of surface sediments are required inputs to most
geoacoustic models used by diverse fields of study such as underwater
acoustics, marine sedimentology, geophysics and marine geotechnique
0025-3227/85/$03.30
0 1985 Elsevier Science Publishers B.V.
74
(Hamilton, 1980). The inputs most often required include compressional
wave velocity and attenuation, shear wave velocity and attenuation, and
sediment bulk density. These sediment properties are either measured
directly, predicted from empirical relationships among other sediment
physical properties (e.g. porosity, mean grain size and percent calcium
carbonate) or determined from province-wide tabular information. Prediction of the depth gradients of geoacoustic and physical properties, which
is an important part of any geoacoustic model, requires accurate determination of surface values of those properties.
Spatial variability of values of surface sediment geoacoustic and physical
properties are controlled by the interaction of hydrodynamic, chemical,
biological and sedimentological processes (Richardson and Young, 1980;
Richardson, 1983; Richardson et al., 1983). For example, geoacoustic
properties of deep-sea carbonate sediments are profoundly affected by
dissolution, dilution, fragmentation and cementation of hollow foraminiferan tests (Morton, 1975; Johnson et al., 1977; Mayer, 1979; Hamilton
et al., 1982). In order to accurately predict these important geoacoustic
properties of surface sediments one must understand those processes which
control their spatial distribution and make relevant measurements to accurately predict their spatial variability.
In this paper we present data on the spatial variability of geoacoustic
and related properties of surface sediment collected from three sedimentary
provinces in the Venezuela Basin (3429-5065 m depth). We discuss the
effects of biological and sedimentary processes on the distribution and variability of these properties with emphasis on the processes which dissolve,
dilute or fragment pelagic foraminiferan tests in sediments. We develop new
empirical predictive relationships between physical and acoustic properties
where established relationships fail to provide heuristic value.
MATERIALS AND METHODS
Replicate sediment samples were collected with a 0.25 m2 USNEL box
corer from sites representative of the three major sedimentary provinces in
the Venezuela Basin (Fig. 1). Site 1 was located on the eastern Beata Ridge
in the pelagic carbonate province (15 0 07'N, 690 22tW; 3950 m water depth).
Site 2 was located on the central Venezuela Abyssal Plain in the turbidite
province (13'45'N, 670 45W; 5050 m). Site 3 was located on the southwestern apron of the Aves Ridge in the hemipelagic province (13 0 30'N,
640 45W; 3500 m). In addition, single 0.25 m2 box corer samples were
collected from eight sites (numbered 42-44 and 67-71) located on transects between these three primary sites. Detailed collection and analysis procedures were presented by Briggs and Richardson (1984) and will only
briefly be reported here.
The 0.25 m2 USNEL box corer was especially designed to remotely
collect relatively undisturbed, large-sized samples of the sea floor (see
Hessler and Jumars, 1974, for a description of the box corer). Recent
75
Fig.1. Map of the eastern Caribbean Sea showing the three province locations and stations
along the two transects (Site 1: pelagic; Site 2: turbidite; Site 3: hemipelagic).
modifications including the addition of vent doors to reduce the bow wave
effect and removable spades to aid in sample manipulation have improved
the quality of samples,especially at the sediment-water interface.
Subsamples of sediment and overlying water were carefully collected by
hand from each box core with 6.1 cm (inside diameter) plastic piston core
liners. Thin sections of sediment for producing X-radiographs were also
collected with (36 cm wide X 44 cm long X 3 cm thick) acrylic boxes.
Care was exercised to obtain relatively undisturbed subsamples with the
sediment-water interface preserved intact within the cores and boxes.
During 1979-1981, we collected 43 box-core samples (193 subsamples)
for analysis of sediment physical, chemical and acoustic properties. Twelve
box cores were collected from the pelagic province, fourteen from the
turbidite province, eight from the hemipelagic province and eight from
transect stations between primary sites (Fig.1). In addition to the samples
reported here, subsamples were obtained to determine the spatial distribution of meiofauna, microfauna, pelagic and benthic Foraminifera, and
various organic compounds in the sediments and to determine rates of
sedimentary and diagenic processes. Results from these subcores are presented elsewhere in this issue.
Acoustic and shear strength measurements and X-radiographs were made
aboard ship using freshly collected cores. Compressional wave velocity and
attenuation were determined at 1 cm intervals on sediment in 153 subcores.
Compressional wave velocity of a signal at 400 kHz was calculated from the
difference between time-delay measurement made on a core filled with
76
distilled water and time-delay measurement made on the sediment subcore
(Richardson et al., 1983). All sound velocities were calculated at the common temperature, salinity and pressure (230 C, 35 ppt, 1 atm) suggested by
Hamilton (1971a). Attenuation measurements were calculated as 20 log of
the ratio of received voltage through distilled water versus received voltage
through sediment. Attenuation measurements are reported as a sedimentspecific constant (k) which is independent of frequency or pathlength
(Hamilton, 1972). Sediment shear strength (kPa) was measured directly
in 41 cylindrical subcores with a Wykeham-Farrance laboratory vane apparatus. Torque required to shear sediment with a 1.26 or 2.54 cm high by
1.26 cm diameter vane rotated at 70° min' (no load) was measured and
converted to shear strength utilizing the formula of Monney (1974). Shearstrength measurements were made perpendicular to bedding at 2 cm depth
intervals in cores collected from the turbidite site and at 3 cm depth intervals in all other cores (one measurement per depth interval). All cores not
extruded for shear-strength measurements were refrigerated or frozen for
subsequent laboratory analysis of other sediment properties.
Sediment porosity (30 subcores), grain size (26), calcium carbonate
(18) and organic carbon and nitrogen (17) measurements were made on
core samples extruded at 2 cm intervals. Sediment porosity was determined
by weight loss after oven drying at 105'C for 24 h. Water content values
were converted to porosity values with the use of the tables of Lambert and
Bennett (1972). Values were not corrected for pore water salinity. Salt-free
porosity may be obtained by multiplying values by 1.012 (Hamilton, 1971a).
Sediment grain-size distribution was determined on disaggregated samples
with an ATM Sonic Sifter for sand-sized particles and with a Micromeritics
sedigraph and pipette for silt- and clay-sized particles. Grain-size statistics
were determined using the graphic formulae of Folk and Ward (1957).
Percent calcium carbonate was measured gasometrically from dried samples
with an apparatus based on the design of Hulsemann (1966). Sediment
organic carbon and nitrogen determinations were performed on previously
frozen sediment with a Perkin-Elmer Model 240 CHN analyzer after treating
the samples to remove calcium carbonate.
DISTRIBUTION AND VARIABILITY OF SEDIMENT PROPERTIES
Pelagicsite
The pelagic site was characterized by carbonate sediment composed
chiefly of pelagic foraminiferan, coccolithophoran and pteropod tests.
Color of the sediment was nearly uniform, varying from pale brown (Munsell
color code: 10YR/6/3) to light yellowish brown (1OYR/6/4). Sediment at
this site exhibited decreasing values of porosity from a mean of 78% at the
sediment surface to 71% at 16 cm depth in the sediment (Fig.2). This
gradient of decreasing porosity was probably a result of dewatering and
compaction by biological activity (Richardson et al., this volume). Below
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16 cm porosity values ranged from 72 to 75%, reflecting the effects of
changing grain size.
Mean grain-size diameter values decreased from a mean value of 5.3 0
(25.4 gm) at the sediment surface to 7.4 o (5.9 pm) at 24 cm depth. Below
that depth in the sediment mean grain size ranged between 6.45 0 (11.4 pm)
and 7.45 0 (5.7 pm). All sediments were extremely poorly sorted, strongly
fine- to coarse-skewed, platykurtic clayey sands or sandy clays (Briggs and
Richardson, 1984). The grain-size frequency distributions were bimodal
with the primary mode in the 1-2 0 (500-250 pm) interval and the secondary mode in the 10-11 0 (1-0.5 pm) interval. The trend of decreasing grainsize diameter was attributable to a diminution in amount of sand-sized
foraminiferan tests in the sediment; specifically, medium sand-sized (1-2 0)
particles.
Values of calcium carbonate (mean: 67%) varied little in the upper 22 cm
of sediment. Slightly higher calcium carbonate values (mean: 73%) from
22 to 32 cm corresponded to a zone of larger mean grain-size diameter.
Although most of the carbonate was contributed by sand-sized pelagic
foraminiferan tests, the remainder was derived from coccoliths and pteropod tests. Intact and fragmented tests of the pteropods Creseis sp., Styliola
sp., Diacra sp., Clio sp. and Cavolina sp. were most numerous between 22
and 32 cm depth. These data imply that pteropod tests were responsible
for the higher percent of calcium carbonate and larger mean grain-size
diameter found between 22 and 32 cm sediment depth.
Percent organic carbon and nitrogen decreased with depth (Fig.2). The
sediment C/N ratio also decreased from 5.5 at the surface to 4.0 at 30 cm
depth. This decrease resulted from organic carbon exhibiting a greater
decrease than organic nitrogen. This gradient of C/N ratios was indicative of
a depletion of labile organic carbon in the sediment through biological
assimilation. Although the deeper sediment was higher in nitrogen relative
to carbon, we suggest the nitrogen was largely refractory or inextricably
bound in clay minerals and of little value to deposit feeders or most microorganisms (Muller, 1977; Rice, 1982; Copin-Montegut and Copin-Montegut,
1983). Classification of organic matter from these sediments by pyrolysismass spectrometry support these suppositions (Zsolnay, unpubl.). The
amount of refractory material as deduced from fragmentation patterns in
pyrolysis-ionization mass spectra increased with depth in the cores, whereas
non-refractory material was highest near the surface.
Sediment shear strength values (kPa) increased from a minimum of 0.79
at the surface to a maximum of 4.62 at 15 cm depth (Fig.2). The increase
in values of shear strength was attributed to biogenic compaction in the
upper 15 cm (Richardson et al., this volume). The subsequent decrease
in values of shear strength below 15 cm depth is not presently explainable.
Sediment compressional wave velocity (Vp) values decreased from a mean
of 1500 m s-' at the surface to 1484 m s-' at 31 cm depth (Fig.2). Values of
sediment compressional wave attenuation (k) increased from 0.78 at the
surface to a maximum of 1.06 at 11 cm depth and then decreased to a
79
minimum of 0.55 at 37 cm depth. Richardson (1983) found compressional
wave velocity values were correlated with density and porosity at the pelagic
site. The increase in compressional wave attenuation from the surface to
14 cm may be the result of increased friction between sand-sized particles
in sediment with lower values of mean grain-size diameter. In carbonate
sediments with mean grain-size diameter less than 7.5 0 (5.5 Mm), cohesion
apparently was the dominant influence on attenuation resulting in lower
attenuation values between 22 and 37 cm. A more complete discussion of
these phenomena is given in the next section.
Sedimentary texture at the pelagic site, as evidenced in the X-radiograph,
was relatively uniform (Fig.3). The top few centimeters had a slightly
lower density (to X-rays) due to reworking by macrofauna (Richardson
et al., this volume). In general, the sediment exhibited an apparent high
density compared with other sites because of the numerous foraminiferan
tests that scatter and absorb X-rays.
Fig.3. X-radiograph of sediment from the pelagic site. Image is a positive produced from
developed X-ray transparency and thus darker areas of figure denote areas of greater
sediment density.
80
Turbiditesite
The turbidite site, which was below the lysocline, was characterized by
terrigenous turbidite layers interspersed between carbonate-depleted layers
of pelagic-derived sediment. Values of physical, chemical and acoustic
properties of sediments demarcated the alternating layers distinctly (Fig.4A
and B). Up to six layers were discerned from color descriptions, X-radiographs and physical property measurements from sediment cores. The top
pelagic-derived layer (layer 1, 0-12 cm) was dark brown (1OYR/4/2-3)
with low opacity to X-rays (Fig.5A). The shallowest turbidite layer (layer 3,
14-28 cm) was gray to very dark gray (1OYR/3-5/1) with the cross-bedded
nature of turbidite deposits undisturbed (Fig.5B). Between these two layers
was a dense, very dark brown (1OYR/3/2-3) iron-rich, chemically altered
layer (layer 2, 12-14 cm). The decrease in Eh at the interface of the pelagic
and turbidite layers as a consequence of organic carbon oxidation was
responsible for a post-depositional concentration of iron in this layer
(Colley et al., 1984; Briggs, unpubl.). Below the shallow turbidite layer was
a less dense, gray (10YR/5/1) pelagic-derived layer (layer 4, 28-36 cm).
The next layer (5) was a thin (1-2 cm) very dark gray (1OYR/3/1) turbidite
layer. The upper few centimeters of another gray (1OYR/5/1) pelagicderived layer (layer 6) were evident in X-radiographs (Fig.5B).
Porosity values decreased from 86% at the sediment surface to 70% at
14 cm depth, the base of layer 2. Richardson et al. (this volume) attributed
the gradient of decreasing porosity to compaction by sediment reworking
activities of benthic deposit feeders. Porosity values decreased from a mean
of 80% at the top of pelagic layer 4 to near 74% at the base of this layer.
This porosity profile apparently resulted from the burial of this pelagic layer
by turbidity flows (layer 3), thus preserving intact a biologically mediated
porosity profile (Richardson et al., this volume). Porosity values determined
from the shallow turbidite layer were quite variable (54-80%). Mean grainsize values were relatively uniform (approximately 10.1 0, or 0.9 gm) down
to 10 cm, below which mean grain-size diameter increased to 9.4 0 (1.5 gm)
at 14 cm depth. This slight increase in mean grain-size diameter could have
resulted from mixing sediments between the turbidite layer 3 (primary
mode: 5 0) and layer 2 (primary mode: 10 0). X-radiographs display no
visible evidence of mixing between layers 2 and 3, suggesting a change in
mean grain size due to chemical alteration. Mean grain size in layer 3 was
quite variable. The increase in mean grain-size diameter (and the concom-
itant decrease in porosity) corresponds to the graded bedding of turbidite
sediments evident in the X-radiographs (Fig.5B). Variability of grain size and
porosity is attributable to the cross-laminated depositional characteristics
also evident in X-radiographs of this sediment. Mean grain-size values in layer
4 were similar to layer 1 (approximately 10 0, or 1 ,m). Sediment in pelagic
layers (layers 1 and 4) were very poorly sorted, coarse-skewed to near-symmetrical, platykurtic clays with a mode in the 10-11 0 (1-0.5,um) interval
(Briggs and Richardson, 1984). Sediments from turbidite layers were poorly
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Fig.5. A. X-radiograph of top three layers of sediment from the -turbidite site. Darker areas
of figure denote areas of greater sediment density (1 = surface pelagic-derived layer;
2 = iron-rich, highly cohesive layer; 3 = shallower turbidite).
B. X-radiograph of layers
3-6 of sediment from the turbidite sedimentary province. Darker areas of figure denote
areas of greater sediment density (3 = shallower turbidite; 4 = pelagic layer; 5 = deeper
turbidite; 6 = portion of another pelagic layer).
84
to very poorly sorted, strongly fine-skewed, platykurtic to very leptokurtic
silts or clayey-silts with a primary mode in the 5-6 0 (31-16 pum)interval
and a secondary mode in the 10-11 0 interval. Turbidite facies of wavy,
convoluted laminae of silt-sized sediment correspond to Bouma sequence
"C" and are typical of sediments deposited at the distal end of turbidite
flows.
Percentages of calcium carbonate were low (2-6%) and varied little with
depth in the sediment (Fig.4A). Such low values result primarily from
carbonate dissolution at or near the CCD (sensu Berger, 1968). Dissolution
is also responsible for the lack of sand-sized particles (e.g. foraminiferan
tests) in the pelagic layers at this site.
Percent organic carbon was highest and most variable in the shallower
turbidite layer (0.41-1.54%) in contrast with the lower and more uniform
values of percent organic carbon in pelagic layers 1 and 4 (0.56-0.80%). Percent organic nitrogen decreased from a mean of 0.15% at the sediment surface to 0.09% at 36 cm with the greatest variability (0.05-0.13%) within the
shallow turbidite layer. High C/N ratios ranging up to 11.8 within the
shallow turbidite layer reflected the presence of refractory terrigenous
detritus (e.g. lignocellulose) which is regarded as richer in carbohydrates
than in protein. Vascular plant debris and insect exoskeletons were observed
in sieved material from this layer. Pyrolysis-mass spectrometry analysis
of the finer-sized particles in the turbidite layer by Zsolnay (unpubl.) also
demonstrates terrestrial sources of organic material occurring there.
Values of sediment shear strength increased from a mean of 0.32 kPa
in the upper 2 cm of sediment to a highly variable mean of 11.60 kPa at
14 cm. Richardson et al. (this volume) attributed this increase to compaction
by biogenic mixing and to biologically induced chemical alteration of the
sediment. Low shear strength of the shallow turbidite layer (0.80-2.08
kPa) suggests little biological or chemical alteration. Profiles of shear
strength in the second pelagic layer (28-36 cm) were similar to profiles in
the first pelagic layer, indicating in-situ preservation of the surficial shear
strength profile after burial by turbidite flows. The presence of a third
pelagic layer with a shear strength profile similar to the other pelagic layers
was suggested by data from sediment below 38 cm.
Values of sediment compressional wave velocity (1486-1501 m sl')
and attenuation (0.05-0.20) were low and varied little in pelagic layers 1
and 4 (0-14 and 28-36 cm). The turbidite layer was characterized by
higher and much more variable values of compressional wave velocity (mean
values: 1519-1566 m s-' ) and attenuation (mean values: 0.12-0.80).
Values of increased compressional wave velocity and attenuation covaried
as expected with decreased porosity and increased mean grain size (Hamilton,
1971b). High variability in sediment acoustic properties in turbidite layers
corresponded to the high variability of other sediment physical properties.
85
Hemipelagicsite
Sediments at the hemipelagic site (5-25% calcareous sand) were composed
of sand-sized foraminiferan tests and silt- and clay-sized particles originating
from the Orinoco and Amazon Rivers (Bowles and Fleischer, this volume).
Porosity values decreased from a mean of 82% at the sediment surface to a
minimum of 75% near a depth of 20 cm (Fig.6). Richardson et al. (this
volume) attributed this reduction in porosity to dewatering and compaction
by sediment reworking activities of deposit-feeding benthos.
Mean grain-size values remained rather uniform with depth in the sediment (9.1-10.3 0, or 1.8-0.8 pm), except in surface samples (Fig.6). The
sediments were very poorly sorted, strongly coarse-skewed, leptokurtic to
very leptokurtic clays with a primary mode in the 10-11 4 interval (hemipelagic material) and a secondary mode in the 1-2 0 interval (primarily
foraminiferan tests). Several surface sediments had a higher percentage of
particles in the 1-2 0 mode, yielding higher values of mean grain-size diameter. The existence of a steep gradient in mean grain size in the upper 2 cm
of sediment was confirmed in 0.5 cm interval samples for grain-size analysis
by Briggs (1985). Richardson et al. (this volume) suggested ingestion of
surface sediments by deposit-feeding megafauna over the long term may
destroy foraminiferan tests by dissolution and mechanical damage.
Percent calcium carbonate varied little with depth in the sediment (2227%). Values of mean grain size (0) and percent calcium carbonate varied
inversely, suggesting the slightly lower values of calcium carbonate were a
direct result of lower percentages of sand-sized foraminiferan tests. Sizefrequency distributions of sediments corroborate this supposition (Briggs
and Richardson, 1984).
Percent organic carbon and nitrogen ranged from 0.45 to 0.73% and 0.09
to 0.13%, respectively, with the highest values found near 16 cm (Fig.6).
Maximum values were associated with a sediment color change of yellowish
brown (1OYR/5/4) to olive gray (5Y/5/2) and the initial detection of hydrogen sulfide odor at a depth of 16 cm. The significance of these observations
to the sedimentologicalhistory of this site is not readily obvious.
Sediment shear strength values increased from a mean of 0.67 kPa at the
sediment surface to 12.51 kPa at 17 cm depth. Values below this depth
were quite variable. Compaction by extensive burrowing and tube building
by deposit feeders coupled with biologically mediated chemical changes
which bond sediments were probably responsible for the increasing gradient
of sediment shear strength (Richardson et al., this volume).
Values of sediment compressional wave velocity (1482-1528 m s'l)
and attenuation (0.14-0.31) varied little with depth compared with the
other two sites. Values were in general agreement with those of Hamilton
and Bachman (1982) for sediments with comparable values of porosity and
mean grain size.
X-radiographs showed sediment density (to X-rays) at the hemipelagic site
to be quite variable (Fig.7). A gradient of increased density with depth in the
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sediment was clearly evident with areas of lower density created by bioturbation extending to almost 20 cm into the sediment. Values of porosity
from Fig.6 and gradients of density from X-radiographs were in agreement.
Transects
Values of physical, chemical and geoacoustic properties determined from
surface sediments collected along two transects between the three sites
exhibited patterns of gradual transition from one sedimentary province to
the next (Table 1). Values of mean grain size and calcium carbonate in
surface sediments displayed the greatest spatial variation, whereas compressional wave velocity displayed the least spatial variation. Sediments
with the lowest percentages of calcium carbonate had the lowest values of
porosity, organic carbon and organic nitrogen. C/N ratios ranged from
4.0 to 5.5 with no apparent correlation with water depth or other gradients
of sediment properties.
Vertical profiles of sediment physical properties for stations between
pelagic and turbidite sites (nos. 42-44) most closely resembled pelagic site
profiles, whereas stations between turbidite and hemipelagic sites (nos.
67-71) resembled hemipelagic profiles (Figs.8 and 9).
A decrease in values of porosity and an increase in values of shear strength
with depth in the sediment were evident in all profiles. Compaction of the
sediment by benthic deposit feeders is primarily responsible for these trends.
The shallowest stations (nos. 70, 71) in the second transect had the highest
values of shear strength. These higher values of shear strength correlated with
presumed higher inputs of organic carbon to these shallower stations which
are closer to the sources of organic matter. High inputs of organic matter to
the sediment probably result in increased chemical bonding at the reduction
potential discontinuity (Richardson et al., this volume).
Deeper stations had lower percent calcium carbonate due to increased
dissolution whereas stations closer to the Lesser Antilles had lower percent
calcium carbonate because of greater contribution of hemipelagic sedimentation. Processes which increase dissolution dominated the transect between
pelagic and turbidite sites: calcium carbonate was reduced from 66% at the
pelagic site (3950 m depth) to near 40% at station 44 (4805 m depth).
Dissolution was nearly complete at the turbidite site (5050 m depth) where
sediment calcium carbonate content averaged 5%.
Mean grain-size profiles followed similar trends with the largest-sized
particles corresponding to higher percentages of calcium carbonate. Close
examination of sediment grain-size frequency histograms (Briggs and Richardson, 1984) and microscopic examination of the sediment showed that
sand-sized foraminiferan tests were responsible for these trends among mean
grain-size profiles. Differences among values of mean grain size below a sedi-
ment depth of 20 cm at stations 42-44 and below 6 cm at stations 67-71
were small. Destruction of sand-sized foraminiferan tests through repeated
ingestion by deposit-feeding benthos may account for similarities in values
89
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92
of mean grain size below 6 cm sediment depth at stations 67-71. Similarity
in mean grain-size values below 20 cm sediment depth at stations 42-44 and
the pelagic site could have resulted from both biological activity and paleoclimatic changes in bottom water calcium carbonate saturation.
Values of compressional wave velocity varied little among transect stations
with most values centered around 1490 m s-'. Compressional wave attenuation (k) values varied little among stations (nos. 67-71) on the transect
between turbidite and hemipelagic sites. Attenuation values decreased with
increasing water depth from the pelagic to turbidite provinces. These differences were related to changes in mean grain size (see next section).
In general, the variability of sediment physical properties in the Venezuela
Basin was highest among distant sites and lowest among subcores obtained
from the same box core or site. An intermediate amount of variability was
also introduced by downcore gradients in sediment physical properties.
We suggest (from Figs.2, 4A, B, and 6) that a single subcore is adequate to
characterize sediment physical properties for a rather large area of the deepsea floor in the Venezuela Basin if downcore variability is also measured.
RELATIONSHIPS AMONG SEDIMENT PROPERTIES
Empirical correlations among values of sediment physical, geotechnical,
acoustical and chemical properties provide a rapid method of predicting
values of sediment properties which are either unknown or difficult to
measure (Hamilton, 1974; Keller, 1974). Comparisons of correlations of
sediment properties among different provinces also provide insights into the
sedimentary and diagenetic history of surface sediments. In this section we
present and discuss empirical correlations among mean grain size, porosity,
percent calcium carbonate, shear strength, and compressional wave velocity
and attenuation for all sites.
In most marine sediments porosity increases with increasing phi (4)
value or decreasing grain-size diameter (Hamilton and Bachman, 1982).
Sediments from the turbidite and hemipelagic provinces and on the transect
between those sites (stations 67-71) contained less than 35% calcium carbonate and exhibited a relationship between mean grain size and porosity
(see triangle, diamond, and cross symbols in Fig.1OA) which was in agreement with that of Hamilton and Bachman (1982). Sediments from the
pelagic province and the Beata Ridge transect (stations 42-44) contained
greater than 35% calcium carbonate and exhibited porosities that were
higher than would be predicted from the mean grain-size data. Such unpredictably high porosities have also been reported for carbonate sediments
on the Ontong-Java Plateau and eastern equatorial Pacific (Johnson et al.,
1977; Mayer, 1979; Hamilton et al., 1982) and in the Whiting Basin near
Puerto Rico (Morton, 1975). The fact that foraminiferan tests are hollow
complicates the relationship between porosity and mean grain size. Bachman
(1984) divided porosity values in biogenic carbonate into porosity within
tests (intratest porosity) as well as porosity between tests (external porosity).
93
(A)
+
o
80 .
[!Po
to
a,
0
+
+
+
0
+
++
I
9 &
o
0
H
70
A
A
2
60-
6
6
7
8
Mean Grain
A
Size
2
9
)
10
A
A
A A0
A
0
!
0
60
AO
A
O
+0
40+y
5
6
7
8
9
10
Mean Grain Size (1)
Fig.10. A. Relationship of porosity (%) to mean grain size (0) for all locations sampled in
the Venezuela Basin (circles = pelagic site; squares = transect between pelagic and turbi-
dite sites; triangles = turbidite site; crosses = transect between turbidite and hemipelagic
sites; diamonds = hernipelagic site). B. Relationship of external porosity (%) to mean
grain size (0) for all locations sampled in the Venezuela Basin (r2 = 0.85). Legend is same
as in (A).
94
Using Bachman's mathematical relationships, external porosity was calculated from frequency histograms of mean grain size and values of porosity.
Figure 10B illustrates that sediment external porosity increased with increasing mean phi (0) value (decreasing mean grain-size diameter). This relationship is almost identical to that of Hamilton and Bachman (1982) for
non-carbonate sediments. It is obvious that total porosity-mean grain-size
relationships have little predictive value in most carbonate sediments -- an
important fact to take into account when predicting other sediment properties from porosity and mean grain size in carbonate sediments.
Figure 11 illustrates that in sediments with greater than 10% calcium
carbonate, mean phi (0) value decreased with increasing percent calcium carbonate (mean grain-size diameter increased with increasing %CaCO3 ). This
relationship is a direct result of increasing amounts of calcium carbonate
with a greater abundance of sand-sized foraminiferan tests. In sediments
with less than 10% calcium carbonate (primarily sediments from the turbidite site), mean grain size and percent calcium carbonate were unrelated.
The turbidite site (5050 m water depth) was located at or below the CCD
where foraminiferan and coccolithophoran tests dissolved at the same rate
as they were deposited. The higher mean grain-size values (5-8 A; 31-4 pm)
for sediments at the turbidite site were a result of the abundant non-carbonate silt-sized particles in turbidite layers at that location. At calcium carbonate percentages greater than 55%, values of mean grain size were not readily
predictable. This poor empirical relationship resulted from the changing
relative abundances of foraminiferan and coccolithophoran tests. Selective
carbonate preservation with depth and changes in depositional patterns
related to productivity cycles are factors which control abundances of the
carbonate tests in the Venezuela Basin (Showers and Margolis, this volume).
It appears that mean grain size-percent calcium carbonate relationships
should be used for predictive purposes with caution.
In the absence of direct measurement, the prediction of compressional
wave velocity and attenuation from sediment physical properties at or near
the sediment surface is required by most geoacoustic models (Hamilton,
1980). Compressional wave velocity and attenuation are usually predicted
from porosity or mean grain size - two easily measured and commonly
available sediment physical properties. Numerous empirical predictor
equations have been developed using concomitant measurements of acoustic
and physical properties (e.g. Nafe and Drake, 1963; Horn et al., 1968; Akal,
1972; Buchan et al., 1972; Anderson, 1974). The most recent and comprehensive equations for non-carbonate sediments are those of Hamilton and
Bachman (1982) for prediction of compressional wave velocity and Hamilton (1980) for prediction of compressional wave attenuation. Hamilton et
al. (1982) reviewed the relationship between acoustic and physical properties
in calcareous deep-sea sediments. Our study includes data from both calcareous and non-calcareous sediments.
Empirical relationships derived from the plot of porosity and compressional wave velocity were similar to those developed by Hamilton and
Bachman (1982) for non-carbonate sediments (Fig.12A). Plots of external
95
porosity or mean grain size versus compressional wave velocity provide no
useful relationships (Fig.12B and C). These results differ from the study by
Johnson et al. (1977) of carbonate sediments on the Ontong-Java Plateau,
western Pacific (1613-4441 m water depth), where mean grain size-compressional wave velocity plots provided useful predictive relationships but
porosity-compressional wave velocity plots did not. Hamilton et al. (1982)
found that both mean grain size and porosity were good predictors of compressional wave velocity for carbonate sediments of the eastern equatorial
Pacific (3462-4849 m water depth). Hamilton et al. (1982) suggested that
hollow foraminiferan tests transmit compressional waves as solid particles,
citing the good correlations among mean grain size, external porosity and
compressional wave velocity for sediment of the Ontong-Java Plateau. As
sand-sized forarniniferan tests lost their integrity with increasing water depth
(1613-4441 m), mean grain-size diameter decreased with little corresponding change in porosity, accounting for the poor correlation between porosity
and compressional wave velocity. We found no such relationships in our
samples (Fig.12A-C). External porosity did not correlate with compressional wave velocity and foraminiferan tests did not appear to act as solid
spheres.
It is obvious that previous explanations of the relationships between compressional wave velocity and porosity or mean grain size in carbonate sediments are inadequate to fully explain our data. The differences in these
relationships between the Ontong-Java Plateau, eastern equatorial Pacific
and Venezuela Basin may be related to factors controlling carbonate
11
-
10.<
9
A
+
05
.
8
0
W3o
C
+
7
6U
0
A
10
20
30
40
50
60
70
CaCO 3 (%)
Fig.11. Relationship of mean grain size (0) to calcium carbonate (CaCO3 , %)for all loca-
tions sampled in the Venezuela Basin (r2 = 0.88). Legend is same as in Fig.1OA.
96
deposition and accumulation in these regions. Hamilton et al. (1982) summarized these factors from previously published papers. They included:
(1) the productivity of Foraminifera and Coccolithophora in surface waters;
(2) the dissolution of carbonate material; (3) the dilution of carbonate by
non-carbonate material; and (4) winnowing, scour and erosion of bottom
sediments.
The Venezuela Basin is a low-energy environment with restricted water
renewal at depth (Kinder et al., this volume). It is therefore doubtful that
winnowing of finer material accounts for any changes in surface sediment
physical properties. Variations in mean grain size or percent calcium carbonate with depth in the sediment are potentially controlled by changes in relative productivity of different species of Foraminifera, Coccolithophora and
Pteropoda in the overlying surface waters. These changes in productivity are,
in turn, related to paleoclimatic changes. The 4000 year cycles in paleoclimate reported by Showers and Margolis (1985) from sediments collected
in this study did not correlate with any physical property we measured. It
is nevertheless possible that temporal changes in the depth of the lysocline
and CCD or changes in relative productivity may contribute to some of the
observed spatial variation of sediment physical properties with location
and depth in the cores. We believe the major factors controlling the physical
characteristics of carbonate sediments in the Venezuela Basin are dilution of
carbonate by hemipelagic sedimentation and dissolution of carbonate
(A)
1600
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A
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A
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15001E
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0
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A
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I
70
Porosity (f)
i
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97
(B)
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1500 11 13
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(C)
A
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A
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-
1550
A
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.. 18cT
1500
00
0
00
XV+
a
+D
O
000
5
6
7
8
9
10
Mean Grain Size ()
Fig.12. A. Relationship of compressional wave velocity (Vp, m s-') to porosity (n, %) for
all locations sampled in the Venezuela Basin: Vp = 2616.58-28.787n
+ 0.184n'. B. Relationship of compressional wave velocity (Vp, m s-') to external porosity for all locations
sampled in the Venezuela Basin. C. Relationship of compressional wave velocity (Vp, m
s') to mean grain size (o) for all locations sampled in the Venezuela Basin. Legends are
same as in Fig.1OA.
98
material by chemical processes accelerated by sediment ingestion, digestion
and egestion by benthic deposit feeders (Richardson et al., this volume).
Carbonate deposition on the Ontong-Java Plateau is controlled by
dissolution, winnowing and dilution (Hamilton et al., 1982). The sedimentary lysocline of this region in the Pacific, at 3500 m, is located 1 km
shallower than in the Venezuela Basin. Winnowing of finer carbonate and
non-carbonate particles from bathymetric highs results in a residual lag
deposit composed of foraminiferan tests. The finer particles are deposited
downslope contributing to a gradient of decreased grain size and percent
calcium carbonate but increased compressional wave velocity with increasing
water depth.
In the eastern equatorial Pacific most sediments are at or below the lysocline resulting in a lower percentage of sand sized foraminiferan tests and
calcium carbonate and a lower mean grain-size diameter compared with the
Ontong-Java Plateau sediments (Hamilton et al., 1982). Sediments in the
eastern equatorial Pacific Ridge are primarily diluted with biogenic silica
from radiolarian skeletons. The lower density of biogenic silica (1.90 g cm-'
vs 2.65 and 2.71 g cm-' for Venezuela Basin sediments) results in lower than
expected bulk density for a given porosity.
The major differences between these three regions of carbonate deposition
are the depth of the lysocline and the composition of finer-sized particles
surrounding sand-sized foraminiferan tests and silt-sized coccoliths. For sediments with a greater than 25% non-carbonate matrix of finer-sized particles
(Venezuela Basin sediments), 400 kHz sound waves may be transmitted
through the sediment matrix without involving the coarser carbonate
material. The low values of measured shear wave velocity (27-125 m s-')
and calculated shear modulus (0.001-0.024 X 1010 dynes cm-2) for sediment at the pelagic site support this suggestion (Richardson, 1983). It follows that porosity would be a better predictor of compressional wave
velocity than mean grain size in those sediments. In sediments with a
siliceous matrix and a low percentage of sand-sized foraminiferan tests
(eastern equatorial Pacific), 200 kHz compressional waves also are transmitted through the matrix. The lower bulk density and greater structural
rigidity of sediments containing siliceous material account for the slightly
higher velocity than found in non-carbonate sediments with similar values of
porosity and mean grain size. Compressional wave velocity in siliceous
sediments is predicted by either porosity or mean grain size. In sediments
with less than a 20% non-carbonate matrix and an abundance of sand-sized
foraminiferan tests (Ontong-Java Plateau), 200 kHz compressional waves
are transmitted by the interaction of carbonate particles. Mean grain size
should therefore be a better predictor of compressional wave velocity than
porosity. Hamilton et al. (1982) also suggest that fine-grained calcareous
sediments will have a higher compressional wave velocity at the same
porosity or grain size than non-calcareous sediments because of the higher
system bulk modulus. This accounts for the higher compressional wave
velocity of Ontong-Java Plateau sediments, which contain a low percentage
99
of sand-sized particles (and a high percentage of silt-sized particles) compared with Venezuela Basin sediments of the same mean grain size. The
range of grain sizes in eastern equatorial Pacific sediment is from 6 to 9 0
(15 to 2 Mim);Ontong-Java Plateau sediment has a range from 4 to 9 0 (60
to 2 Mm). Hamilton's calculated values of rigidity (shear modulus) ranging
from 0.02 to 0.20 X 1010 dynes cm-2 for these sediments was much higher
than measured values for Venezuela Basin sediments (Richardson, 1983).
We therefore agree with Hamilton that variations in sound velocity must be
due to variations in sediment rigidity (shear modulus).
The plot of porosity and compressional wave attenuation (k) revealed two
trends (Fig.13A). In sediments with less than 35% calcium carbonate, attenuation increased with decreasing porosity. In sediments with greater than
35% calcium carbonate, porosity and attenuation were unrelated. Plots of
attenuation versus external porosity or mean grain size (Fig.13B and C)
exhibited the same increase in attenuation which accompanies decreasing
porosity or increasing grain size in non-carbonate sediments (Hamilton,
1980). Comparison of Fig.13A and B suggest that compressional wave
attenuation is unaffected by water inside foraminiferan tests. In Venezuela
Basin sediments, mean grain size is a better predictor of compressional wave
attenuation than sediment total porosity. Our data agree with Hamilton
(1972) and McCann and McCann (1969) in that high-porosity calcareous
"oozes" have higher attenuation than non-calcareous sediments with the
same porosity and mean grain size. Differences in techniques, specifically
the frequency used, probably do not account for this difference for atten(A)
1.00
000
°
000
00
00
0.50 .3
A
0.50
70
I I, I
,
'
A
A
A
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-
70
Porosity(%
Fig.13.A. Relationship of compressional wave attenuation
tions sampled in the Venezuela Basin.
|
+
`6
A
A
60
1
+
go*000
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so
(k) to porosity (%) for all loca-
100
(B)
1.00
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c
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70
External Porosity (%)
(C)
1.00
C
0
C
S)
0.50
5
6
7
8
9
10
Mean Grain Size (0)
Fig.13 (continued). B. Relationship of compressional wave attenuation (k) to external
2
porosity (n, %) for all locations in the Venezuela Basin: k = 7.801 -0.187n + 0.0011n .
C. Relationship of compressional wave attenuation (k) to mean grain size (o) for all locations sampled in the Venezuela Basin: k = 1.962 -0.220 0 + 0.0034 02. Legends are same
as in Fig.1OA.
101
uation measurements made in shallow-water sediments (Richardson, 1984).
In non-carbonate sediments, Hamilton (1972) suggests compressional
wave attenuation is controlled by friction between individual particles in
sands and cohesion in silt- and clay-sized sediments. Both Hamilton (1972)
and McCann and McCann (1969) found a sharp inflection in plots of attenuation versus mean grain size or porosity indicating that 6 0 (16 mm) or
65% porosity are the values above which cohesive forces dominate. This
inflection point occurred at 7.5 0 (5.5 pum) and 70% external porosity
in plots of our data from carbonate sediments. The bimodal distribution of
carbonate sediment grain size may account for these discrepancies. The
presence of abundant 1-2 0 (500-250 pm) sized foraminiferan tests (mean:
25% of sediment weight at the pelagic site) may increase attenuation by
friction more than predicted from non-carbonate sediments with lower
sorting coefficients (i.e. better sorted sediments).
McCann and McCann (1969) suggest that the higher than predicted (i.e.
for non-calcareous sediments) attenuation values for calcareous sediments
are due to the abundance of non-surface active particles. The lack of
attraction between these particles, which are suspended in water, increases
the viscous dissipation of compressional waves. This may account for highporosity calcareous sediment having higher than predicted attenuation.
Because sediment compressional wave attenuation and vane shear strength
are both controlled by cohesion in silts and clays (Hamilton, 1971a), a correlation between the two was expected. However, shear strength appears
totally unrelated to attenuation as well as porosity and mean grain size in
both carbonate and non-carbonate sediments (Fig.14A-C). Sediments from
(A)
0
1.00
0.50
0 0
Shear Strength (kPa)
Fig.14.A. Relationship of compressional wave attenuation (k) to sediment shear strength
(rf, kPa) for all locations sampled in the Venezuela Basin.
102
(B)
15
0+
+
10
m
a-
-
~A
6
8
1+
T
Ag70
5
80
+
4 +
o
++
(IC
t++
Poros
(O/l)
A
A e++
(C)
5
A
°o°+$
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+6
0
A
70
60
15
++
00
A0
4+
80
o0oj%
+
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004+
+
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sz
in
+
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30
fl
0
0
130
0
0
0
00
8
+(
000
0A
A0
0
A0++
+
AAA
+0
+A++
5678
9
+
10
Mean Grain Size (40)
Fig.14 (continued). B. Relationship of shear strength (rf, kPa) to porosity for all locations
sampled in the Venezuela Basin. C. Relationship of shear strength (Ijf, kPa) to mean grain
size (o) for all locations sampled in the Venezuela Basin. Legends are same as in Fig.10A.
103
the Venezuela Basin with greater than 35% carbonate generally had lower
shear strength values (mean < 6 kPa) than reported by Keller and Bennett
(1970) for carbonates from surficial cores from the North Pacific and North
Atlantic Basins (mean: 7.6 kPa) and by Johnson et al. (1977) for the
Ontong-Java Plateau (mean: 6.4 kPa). The occurrence of biogenic silica
instead of clay as the next most abundant component in the sediment fabric
may be a factor in explaining higher shear strength values in the North
Pacific and North Atlantic Basins and the Ontong-Java Plateau. However,
it is apparent from these data and those of Richardson et al. (this volume)
that knowledge of biological and chemical processes affecting surface sediments is just as important as measurement of sediment physical properties
in order to predict sediment shear strength.
CONCLUSIONS
- Variation in geoacoustic and related properties was greatest among
distant sites representing pelagic, turbidite or hemipelagic sedimentary
regimes. Variability among sites was primarily a function of increased
calcium carbonate dissolution along a gradient of depth (3500-5050 m)
and a westward decrease in intensity of dilution of calcium carbonate by
terrigenous sediments, which originated from the Orinoco and Amazon
Rivers.
- The least amount of variation was evident between subcores obtained
from the same box core or site, suggesting a single subcore was adequate to
describe sediment physical and geoacoustic properties over a large area of
the sea floor if downcore variability is considered.
- The considerable downcore variability in sediment physical properties
was a function of gradients in biological activity, biologically mediated
chemical changes and the presence of ponded terrigenous sediments deposited by intermittent turbidite flows interspersed between sediments of pelagic
origin.
- Dewatering and compaction of sediments by biological activity resulted
in negative gradients of porosity and positive gradients of sediment bulk
density and shear strength with depth in the sediment at all sites.
-
Chemical bonding of sediments at the reduction potential discontinuity
(RPD) resulted in higher than expected sediment shear strength values near
the RPD in sediments from the hemipelagic and turbidite sites.
- Destruction of sand-sized foraminiferan tests by water depth related
dissolution and ingestion/digestion/egestion activities of benthic animals
resulted in negative gradients of mean grain size (diameter) with depth in
the sediment at the pelagicprovince.
Alternation of pelagic and turbidite sediments in the central Venezuela
Abyssal Plain resulted in sharp discontinuities in gradients of sediment
physical properties. The turbidite layers had higher values of mean grain
size (diameter), organic matter, and compressional wave velocity and attenuation but lower values of porosity and shear strength than the pelagic layers.
-
104
- Porosity-mean
grain size relationships had little predictive value in
carbonate sediments. The contribution to total porosity from water between
foraminiferan tests (external porosity) and contribution to total porosity
from water within tests (intratest porosity) over a range of grain sizes from 4
to 40 gm nearly balanced each other and thus yielded almost constant
porosity values.
- Compressional wave velocity (Vp) was predictable from porosity in the
carbonate sediments, whereas Vp-mean grain size relationships had little
predictive value. Foraminiferan tests did not appear to act as solid spheres in
the transmission of compressional waves. The nature of the sediment matrix
(detrital hemipelagic, pelagic carbonate, or biogenically siliceous) controls
compressional wave velocity.
- Compressional wave attenuation was predictable from either external
porosity or mean grain size but not total porosity. Attenuation was apparently unaffected by water inside foraminiferan tests. The higher than
predicted attenuation values for calcareous sediments (as compared to noncalcareous sediments of the same grain size) in this and other studies may
result from the non-surface active nature of the carbonate matrix which
increases viscous damping.
- Sediment shear strength did not correlate with compressional wave
attenuation, porosity or mean grain size. Sediment shear strength is apparently controlled by biological and chemical processes.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the assistance of the ships' captains and
crews of the R/V "Gyre" (cruise 79G7), USNS "Lynch" (cruise 708-80) and
USNS "Bartlett" (cruise 1301-82). We also wish to thank all of the scientific
colleagues who participated in sample collection on the aforementioned
cruises. Special thanks are extended to Skidaway Institute of Oceanography
and Steve Bishop in particular for use of the CHN Analyzer. We also thank
Larry Reynolds and Randy Rogers for assistance and cooperation in particlesize analysis and Ricky Ray for the extensive data manipulation and
graphical formatting. We thank E.L. Hamilton, G.H. Keller and L.A. Mayer
for their careful reviews of. the manuscript. This work was supported by
NORDA program element 61153N, Herb Eppert, Program Manager. Contribution 333:003:85 from the Naval Ocean Research and Development
Activity.
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