VOL. 78, NO. 30
JOURNAL
OF GEOPHYSICAL
RESEARCH
OCTOBER 20, 1973
Distribution
ofSuspended
Matterin thePanama
Basin
WILLIAMS. PLANK,J. RONALD
V. ZANEVELD,
ANDI-IASONG
PAX
Schoolo• Oceanography,OregonState University, Corvallis, Oregon 97331
The distribution of suspended matter in the Panama basin was determined by means of
light scattering and Coulter counter measurements on water samples collected at 50 hydrographic stations. The observed distribution indicates three probable sources of suspended
matter: (1) the surface waters throughout the basin;' (2) erosion and runoff from the continents; and (3) bottom erosion at two sites on the Carnegie Ridge. The distribution of
suspendedmatter also confirms in most. respectsthe pattern of abyssal circulation proposed
by Norman P. Laird (1971).
The Panama basin,boundedby the Carnegie
and Cocos ridges and the coasts of Panama,
Columbia, and Ecuador, has been the subject
of an increasingnumber of oceanographicstudies in recent years. The circulation of the surface and intermediatelayers has been described
by Stevenson[1970] and Wyrtki [1967]. Laird
[1969, 1971] has hypothesized a model for
abyssalcirculationin the basin basedon observations of hydrographic properties. Studies of
sedimentdistribution [va,n Andel et al., 1971;
Ko.wsmann, 1973; Moore et al., 1973; v.an
Andel, 1973] have also provided information
that has implicationsfor the abyssalcirculation
in the basin.
tween the Malpelo and Carnegie ridges, and
;•henspreads
northward
fillingthe subbasins
west of the Malpelo Ridge. The distribution
of oxygenand salinity in the near-bottomwater
supportsthis overall pattern of circulation.
An examination of the bathymetry of the
ridges surrounding the basin indicates that
another likely route for water entering the
basin would be through the saddle over the
Carnegie Ridge, which is at about 85øW and
has a sill depth of about 2300 meters. Laird's
studiesof hydrographicpropertiesdid not point
to this saddle as a source of bottom
water.
However, a study of the sedimentcover in the
basin [van Andel et al., 1971; Kowsmann,
1973; Moore et al., 1973] showed that this
During November and December 1971 we
conducteda program of optical measurements saddle is an area of active erosion and that
in the Panama
basin in order to delineate
the
distribution of suspendedparticulate matter in
the basin. We hoped to use this distribution,
along with hydrographic and current meter
measurements,to gain further insight into the
abyssal circulation and the sourcesand sinks
of particulatematter in the basin.
Laird [1969] hypothesizeda singlesourcefor
the bottom water in the basin: the pass over
the Carnegie ridge near the coast of Ecuador,
which has a sill depth of about 2500 meters.
The distribution of near-bottom potential temperaturesindicatesthat after enteringthe basin
through this pass some of the water continues
northward, parallel to the coast, filling several
subbasins off the coast of Columbia.
The re-
maining water turns to the west, passesbeCopyright (•) 1973 by the American Geophysical Union.
7113
productsof the erosionare found along the
north side of the ridge. Since the distribution
of suspendedparticulateshas been utilized in
several casesas an indicator of deep circulation
[Jerlov, 1959, 1968; Hunkins et al., 1969;
Ewing et al., 1970; Ichiye et al., 1972], the
implicationof bottom erosionby northwardflowingcurrentsled us to believethat a study
of suspended
matter in the basinmight be useful in evaluatingthis saddle as a sourceof
bottom water in the basin.
Severaltechniqueshave beenusedto measure
the concentrationof suspendedmatter, each of
which has its limitations. Large numbers of
measurementshave been made by filtering or
centrifugingwater samplescollectedby sampling bottlesvaryingin sizefrom 5 liters up
to hundredsof liters [Lisitzin, 1972; Ewing and
Thorndike, 1965]. The difficultieswith this
7114
PLANK ET AL..' SUSPENDED MATTER
-method
are obvious'
to obtain
accurate
mea-
90'
85'
80'
surements, especially in the extremely clear
waters found at great depths,large samplesare
necessary,but the larger the samples,the more
cumbersomeand time-consumingthey are to
ha•ndleand analyze, and the greater is the
likelihood
of contamination.
A modification
to obtain information
on the sus-
pendedmatter; measurementsof light scattering
and measurementsof the particle sizesby means
of a Coulter
counter.
/• (45),(m-err)
-•xI0-$
0 ....
•5 . ,
5o
of
the filtration method that appears to hold
promise is the techniqfie of in situ filtration.
This method is slow, but accuracy can be improved as much as necessaryby pumping for
longer times. Another techniquefor measuring
suspendedmatter that has been used a great
deal is the measurementof an optical property
such as light attenuation or scattering [Burr,
1957; Jerlov, 1955; Thorndike and Ewing,
1967; Ochakovsky, 1966]. The difficulties involved in calibrating one such technique have
been discussedby Beardsley et al..[1970]. In
general,it appearsthat suchtechniquesmay be
most useful for rapid surveys of large areas,
and that for very accurate measurementsof
the concentrationof suspendedparticles or of
the particle size distribution,other methodsmay
be preferable. In this study, two techniques
were utilized
0o
90 =
85 =
80 =
Fig. 1. B•thymetw of the Panam• basin
[genera.lizedafter van Andel et al., 1971]. Stations
are indicated by black dots. Letters A, B, C,
and D show locations of profiles in Figure 2.
Crosssectionsin Figures 3 through 6 are indicated
by lines 1-1 through 4-4. Current meter locations
are indicated by triangles. Bathymetric contours
are in kilometers.
EXPERIMENTAL
PROGRAM
Our study was conductedduring the 1971
cruise of the RV Yaquina to the Panama basin.
At each of the 50 stationsshown in Figure 1
hydrographiccastswere made, usuallyto within
15 meters of the bottom. The water samples
Scattering
Cross-Section,
( u2xI0'$perI/2 ml)
;)
0
5
0
5
I
Central
Stahon
A
I0 o
B
Basin
4-43A
C
Fig. 2. Profiles of light scatteringat 45ø and scattering crosssectionof suspendedparticles
at locations A, B, C, and D in Figure 1. Circles denote light scattering values. Triangles
denote crosssection values. Units of abscissaapply to both parameters.
PLANK ET AL.' SUSPENDEDMATTER
7115
obtainedwere analyzedfor light scatteringand direction [fi(45)] was measuredwith a Briceparticle content, in addition to temperature, Phoenix light-scattering photometer. The relaoxygen content, salinity, and the concentrations tionship between light scattering and particle
of several nutrients.
concentrationsin the deep sea has been discussedby Pak et al. [1971] and Beardsleyet al.
ographic applications has been discussedex[1970]. In general,if the suspendedmatter is
tensively [Carder, 1970; Sheldona•.d Parsons, fairly uniform in compositionand size distribu1967]. Briefly, the instrument measures the tion, the light-scattering measurementsshould
height of an electronicpulse generatedby the be linearly related to total particle concentrapassageof a suspendedparticle through a small tion. These conditionsshould apply at depths
orifice.The height of this pulsehas been shown greater than about 300 meters. At the two locato be proportional,within certain limits, to the tions'indicatedby trianglesin Figure 1, currents
volume of the particle. In this paper 'particle
STATION
NUMBER
diameter' means the diameter of a sphere
52
5:5
54
55 56
57 58
o
having the same volume as that measuredby
The
use of the
Coulter
counter
in
ocean-
the instrument.
Particles with diametersas small as 0.4 •m
can
be measured
with
the
Coulter
counter.
However, in the interests of rapidity and ease
of analysisit was decidedto utilize a 100-•m
orifice, which allows measurement of particles
down to about 2 /•m. The cumulativesize distribution of particles was determined at four
diameters: 2.22, 3.53, 5.60, and 10.22 •m. The
volume concentration of particulate matter
over this range of sizes was obtained by numerical integration, and it is this parameter
whichis plottedin Figures3, 4, 5, and 6. Several
investigators[Bader, 1970; Plank et al., 1972;
Sheldon et al., 1972] have found the relative
size frequencydistributionof particulatematter
in sea water to be almost constant, especially
if only samplescollectedbelowabout 300 meters
are considered.
The
size distributions
i.o
:•.o
:5.0
•':;i:i:!'.,!.'.::
ppmx 10-2
4.0 [] >1.4 '"•I.:•-1.4 I• I.O-I.?I•o.e-,.o [•o.•-o.e I-I< o.•
LONGITUDE (west)
STATION
0
5?-
5:5
NUMBER
54
55 56
57 58
of the
particleswe examinedwere found in almost all
cases to be nearly exponential in shape. We
thus feel that an assumption of constant relative size-frequencydistribution is a good one.
Measurementsof the concentrationof particles
between2.2 and 10.2 •m are then proportional
to the total concentrationof suspendedmatter.
The proportion of the total concentrationrep-
1.0
.
x•?-.o,:
.
:5.0
resented is difficult to determine, however, as
it dependson the upper limit of sizesthat one
assumesto exist in the sample and also on the
shapeof the size distributionin• the submicron
range.
Light-scatteringmeasurementswere made on
the same samplesthat were analyzed in the
Coulter
counter in order to obtain
an inde-
4.0
•
83
8•?-
81
LONGITUDE(west)
Fig. 3. Crosssectionsof the concentrationof
particlesbetween2.2 and 10.2#m in ppm X 10-3
pendentmeasureof relative particle concentra- by volume (upper) and light scatteringat 45ø
tion. The scattering of light having a wave- in (m str)-• X 10-3 (lower) along line 1-1 in
length of 436 nm at 45ø from the forward Figure 1.
were measured i meter from the bottom in an
concentrationparameter in Figure 2 is geo-
attempt to ga.insomeinformationabout current
speedsand directionsnear the two low points
on the Carnegie Ridge.
metric cross section (sometimesreferred to as
RESULTS
Profiles of light scattering and particle concentration
from
the
four
locations
marked
A
through •) in Figure 1 are plotted in Figure 2.
These profiles are intended to indicate the general shapeof profilesfound in the varioustypes
of topographic provinces in the area we are
studying and to indicate the degree of correlation found between measurementsof light scattering and particle concentration.The particle
STATION
56
2
:5
6
8
scatteringcrosssection),which is proportional
to the % root of particle volume. The values
on the abscissarepresentthe projected crosssectionalarea of the particulate matter between
2.22 and 10.22 /•m in diameter contained in
1/• ml of seawater (the size of the sample
analyzed in the Coulter counter). This parameter has been previo.uslyused in studiesof the
correlation of light scattering and suspended
particle concentration[Beardsleye'i al., 1970].
Note that the valueson the abscissa
in Figure 2
apply to both parameters.
Figures 3, 4, 5, and 6 are cross sectionsof
NUMBER
9
I0
II
12
0
2øS
o-
5oN
LATITUDE
Fig. 4. Cross sections of the concentration of particles between 2.2 and 10.2 gm in ppm X
10-2 by volume (upper) and light scattering at 45ø in (m str) -• X 10-• (lower) along line
2-2 in Figure 1.
PLANK ET AL.: SUSPENDEDMATTER
STATION
5-21
5-7
7117
NUMBER
26
39
48
z• 5 6 7
I.O
90
88
86
LONGITUDE
STATION
5-21
•l
5-7
I
90
26
I
88
I
84
82
(wesl)
NUMBER
5•
48
I
I
86
I
4,
5
6 7
I
84
LONGITUDE
Fig. 5. Cross sections of the coricentration of particles between 2.2 and 10.2 /zm in
ppm X 10-0 by volume(upper) and light scatteringat 45ø in (m str)-• X 10-a (lower) along
line3-3in Figure1.
light scatteringand particle concentrationalong
the lines marked 1-! through 4-4 in Figure 1.
Particle
concentrations
on these cross sections
are shown in units of ppm X 10-' by volume.
For comparisonwith other authors whose results are reported in terms of mass concentration, we should note that, for an assumedparticle density of 2.75 g/ml (the density of clay
particles), I ppm by volume equals2.75 mg/1.
Figure 7 shows contours of integrated particle volume
in the lower
of the water
100 and 500 meters
column. The values are obtained
by numerically integrating the volume of particulate
matter
from the bottom
to 100 and 500
meters above the bottom.
The
contour value
representsthe amountof particulatematter, in
cubic microns,containedin a columnof water
« cm• in cross-sectional
area and 100 meters
high (for the upper figure; 50'0metershigh for
the lower).
DISCUSSION
It {S interestingto comparethe volume
concentrationsof particles we measuredwith
concentrationsobtained by other workers who
have made measurementsover a wider range
of sizes or who have used other techniques to
obtain particle concentrations.
7118
[PLANEET AL.' SUSPENDED
MATTER
STATION
3,7 49A 3,9
$4
NUMBER
41
43,
,
17
19
about 0.012 ppm to 0.014 ppm, and the minimum concentrationsin deeper waters decreased
to less than 0.006 ppm. Consideringthe difference in the size rangesexamined,our measure-
16
0.6
ments
are then
not inconsistent
with
those of
Sheldonet al. [1972].
In a study of suspendedmatter in the Caribbean Sea utilizing a gravimetric analysis,Bassin
et al. [1972] filtered water from 5- and 30direr
1.2
:3:1.8
Niskin
2.4
I
I
I
3,_øS
o
-
3,5
3,7 49A 3,9
•
and
obtained
minimum
ma•
compareswell with the valuesobtainedby both
LATITUDE
STATION
3,3,
bottles
concentrations(at depths of about 3 km) of
less than 50 ttg/1. This is equivalent (for an
•
.
_
assumed
particle density of 2.75 g/ml) to a
'• EILo•2 i•oe-•o ....Do6
os1--1>o6
volume
concentration
of 0.018 ppm. This value
(•e I
NUMBER
41
43,
19
I?
Sheldon and us (assuming our measurements
16
,.•..• ....• -.,:..........•:•.•::•,-:..•,••f•.
85 ø
80 ø
90 ø
85 ø
80 ø
90 ø
85 ø
80 ø
90 ø
0.6
ioo
•
PARTICLEVOLUME
1.2
•'-"P
k
/I
'••'•••':•-:•
I
-
3.0
I [
i
•
I
I
[
I
[
L•TITUDE
Fig. 6. Cross sections of the concentration of
particles between 2.2 and 10.2 •m in ppm X 10-2
by volume (upper) and light scattering at 45ø
in (m str) -• X 10-a (lower) along line 4-4 in
Figure 1.
500
•. PARTICLE VOLUME
Sheldon et al. [1972] determined volume
concentrations
between diameters
of I and 100
:?[]<2 km
•>3
km
ttm by meansof a Coulter counter over a wide
range of depths and in several areas of the
world. At 500 meters the volume concentrations
they obtained varied from about 0.03 to 0.15
ppm. At 5000 meters the minimum concentrations they foufid were lessthan 0.02 ppm.
If, as Sheldonfound, particle size spectra in
deep waters show roughly equal amounts of
material in logarithmically equal-sizedintervals,
then
our measurements
between
2.2 and
9C•
85 ø
80 ø
10.2
ttm shouldamount to about 34% of their measurements between I and 100 ttm for water
with the same total concentrationof particles.
Our measurements at 500 meters averaged
Fig. ?. The integrated volume of suspended
particles between 2.2 and 10.2 gm in the lower
100 meters of the water column (upper) and the
lower 500 meters of the water column (lower).
Units of the contours are 2 /•ma X m/ml.
PLANK ET AL.: SUSPENDEDMATTER
are •quivalent to 34% of Sheldon's),although
any comparison of gravimetric and Coulter
counter results is, of course, complicatedby
the different size range of particles examined
by the two methods.These comparisonsindicate that our measurements
over a narrow
size
range are not only valid in a relative sense,but
are alsovaluablein indicatingroughlythe magnitude of the total amount of suspendedmatter
present.
The crosssectionsof Figures 3-6 show that
high concentrationsof suspendedmatter occur
primarily in four locations:
1. In the surface layers throughout the
basin.
7119
ously, sediment samples indicate that this is
the case.Current directionwas predominantly
alignedwith the axesof the two passes,and in
both cases180ø reversalsoccurred,indicating
that the currentshave a tidal component.It is
probable that these two passesare sites of
advectiveand diffusiveprocesses
of sufficiently
high energyto erodeand maintain in suspension
increasedamountsof particulatematter.
The presenceof higher concentrations
of particles throughout most of the water columnover
the CarnegieRidge that we seein Figure 4 may
be a result of the confluenceat this point of
all three sourcesof suspendedmatter. That is,
settling from the surface, erosion, and runoff
from the shelf and bottom erosionin the vicinity
2. Over the Carnegie Ridge in the two
north-south sections(Figures 4 and 6). These of the sill at about 1øS latitude all contribute
high concentrationsseem to occur throughout particles to the water column near station 3,
a greater range of depths in the section near resultingin high concentrationsfrom the surthe coast (Figure 4).
face to the bottom. High concentrationsnear
3. On the east-westsectionin Figure 5, in the bottom both to the north and south of the
the intermediate and deep waters near the Carnegie Ridge is this crosssectionmay be a
coast and between 86øW and 88øW, to the
result of the erosion of particles from the
northwest of the saddle.
saddle by the oscillatory currents mentioned
4. Near the bottom in the Peru trench
above. The particles may then be transported
(Figure 3) and in the deep coastalbasinto the to the north and south by gravity or advection
or both.
north of the CarnegieRidge (Figure 4).
Three probable sources of the suspended
The profiles in Figure 2 show that on the
matter found in the basin are thus indicated:
continentalrise (location A) and near the sad1. The •urface layers throughout the basin dle at 85øW (location C) there are wellcontributeparticlesthat may settle through the developednepheloidlayers near the bottom,
water column to the bottom.
as might be expectedif active erosionis occur2. The continentsuppliesmaterial by runoff ring. In the deepcoastalbasin (locationB) the
and erosionon the shelf. The particle 'clouds' increasein particle concentrationis smallerin
near
the
coast between
about
300
and
1300
magnitudeand is spreadover a greaterdepth.
meters in Figure 3 may indicate that the
The profiles
fromthe centraibasin(location
Guayas River, located at the same latitude as D) demonstratethe characteristics
of an area
this section,is an important sourceof suspended where there is probably little erosionor sediparticles.
mentation taking place in the size rangesof
3.
Bottom erosion in the saddle at 85øW and
probably also in the pass near the coast is injecting particlesinto the water column.The current meter measurements
taken at the locations
particleswe examined.
Valuesof light scattering
and particle concentrationare low and relatively constantbelow400 meters.
In Figure7 the contoursof integratedparti-
shown in Figure 1, although not of sufficient cle volumeshowa high gradientnear the coast.
duration (6-7 hours) to determine the long- This indicates,first, that the continent is a
term circulation, did show velocities between 7
and 10 cm/sec I meter above the bottom. Consideringthe fact that, with such a short record,
the probability of seeingmaximum velocitiesis
very small, it is likely that velocities high
enoughto erode bottom sedimentsdo occur at
these sites, and, as has been discussedprevi-
majorsourceof particulatematterand,second,
that much of this matter is being depositedin
the deepcoastalbasin.The contoursalsoshow
regions
of highparticlecontent
centered
around
the CarnegieRidgesaddleat 85øW. It appears
that material eroded from the sill is probably
beingspreadboth to the north and the south
7120
PLANK ET AL.: SVSPENDED MATTER
and is settling out of the water column on the on the compositionand index 0f refraction of
flanks of the ridge. The contoursin Figure 7 the particles.
In conclusion,the observed distribution of
also indicate tongues of suspendedparticles
extendinginto the northwest portion of the particulate matter in the Panama basin has
basin. Two explanationsfor these tongues in general confirmed the pattern of abyssal
circulationsuggested
by Laird [1971], although
seemplausible.
it
does
appear
that
the saddle at 85øW
1. The particles are erodedfrom the bottom
longitude
is
a
site
of
reasonably
high-energy
in the vicinity of the saddle at 85øW and are
dynamicprocesses.
However,we cannotevaluinjected by lateral mixing into the bottom
ate precisely,on the basisof our measurements,
water comingfrom the passnear the coast.
the contributionof theseprocesses
to the supply
2. The particles are eroded in the saddleat
of bottom water to the baSin.
85øW and are transportedto the north by
Since the work done in this study was conbottom water entering the basin over the centrated around the two saddles in the Carnesaddle.
Until more direct current meter measure-
mentsare availableit is difficultto say exactly
what the relationshipis betweenthe observed
distribution
of suspended
matterand the longterm mean abyssal circulation. One recent esti-
mate of the settlingrate of a 5-•m sphereof
density2.65 in seawater (0øC, 34%) is 1.17 X
10-• cm//sec,or about 370 meters per year
[Gibbs et al., 1971]. SinceLaird's estimateof
renewal
timefor the basinwasabout175years,
if a particleof this sizeis transportedto a region where there is no upward-directedturbulent flux of particlesfrom the bottom, it will
gieRidge,•heparticledistribution
in thenorthern part of the basinis as yet not well defined.
Acknowledgments. This investigation has been
supported by contract N00014-67-A4)369-0007
under project NR 083-102 with Oregon State
University.
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