Journal of Marine Research, 42, 1117-1145, 1984
Bioturbation processes in continental slope and rise sediments
delineated by Pb-210, microfossil and textural indicators
by J. N. Smith' and C. T. Schafer2
ABSTRACT
Measurements of Pb-210 activities, in conjunction with micropaleontological, geotechnical
and sedimentological observations, on sediment cores have been used to characterize two
distinctive bioturbation regimes on the continental slope and rise east of Newfoundland. On the
rise (2600 m), excess Pb-210 is confined to the upper few centimeters of the coarser-grained
sediments underlying the axis of the Western Boundary Undercurrent. The geological and
geochemical evidence for a low rate of bioturbation in this high bottom current regime is
consistent with a reduced population of deeper burrowing macrofauna, particularly the species
Maldane sarsi. In contrast, a higher flux of organic-rich, fine-grained particulate material to the
middle slope (1500 m water depth), and the comparatively stable sedimentological conditions
that prevail in this low bottom current regime, have led to the active colonization of the sediment
substrate by bioturbating organisms. Enhanced mixing of middle slope deposits is reflected by
comparatively lower shear strengths within the upper 30 cm of the sediment column, and by the
reduced variability of the sediment-depth distribution of the most abundant species of
foraminifera. Excess Pb-210 has been transported downward from the sediment-water interface
to depths greater than 12 cm. Some Pb-210 profiles from the middle slope can be interpreted in
terms of a diffusion mixing model for which the biological mixing coefficients are of the order of
0.10 - 1.0cm2yr-l•
Measurements of the two and three dimensional distribution of excess Pb-21 0 in one middle
slope box core indicate that the mixing process in these sediments has a pronounced heterogeneous component on time scales of the same order as the half life of Pb-210 (22.3 yr). Spatial
correlations between Pb-210 anomalies and artifacts of bioturbation observed in x-radiographs
of the core suggest that Pb-210 maxima observed at depth may be the result of an inclined
orientation of burrow structures which have introduced a significant lateral component to the
downward transport of surficial sediments.
1. Introduction
Benthic macrofauna can have a significant effect on the physical and chemical
properties of surficial sediments as a result of their feeding, burrowing and physiological activities (Gordon, 1966; Peng et al., 1979; Guinasso and Schink, 1975).
1. Atlantic Oceanographic Laboratory, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth,
Nova Scotia, Canada B2Y 4A2.
2. Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada
B2Y 4A2.
1117
1118
Journal of Marine Research
[42,4
Bioturbation by benthic invertebrates can induce perturbations in the geochemical
gradients of interstitial waters (Schink and Guinasso, 1978; Aller, 1978; 1980; 1983;
McCaffrey et al., 1980). Particle transport resulting from bioturbation can also distort
textural and paleontological signals preserved in the stratigraphic record and may alter
the geotechnical properties of the sediments (Gray, 1974; Davis, 1974; Robbins et af.,
1979; Rhoads and Boyer, 1982). In modern marine biological systems, these changes
may be accompanied by secondary feedback effects which can promote restructuring
of the original benthic community (Rhoads, 1974). Perturbations in the biogeochemical regime may cause changes in the rates and mechanisms of diagenetic reactions
leading to the enhanced remobilization of some radionuclides (e.g., Pu-239,240) at
depth in the sediment through organically-mediated
reactions (Livingston and Bowen,
1979). Bioturbation affects the stability and composition of marine sediments and
influences their role as geochemical sinks and/or sources.
Lead-210 is a useful sediment particle tracer because of its limited chemical
mobility in sediments under both oxidizing and reducing conditions (Robbins, 1978).
In addition, its half life (22.3 yr) provides a convenient time scale for the resolution of
recent sediment mixing phenonema. In continental slope environments where sedimentation rates are of the order of centimeters per thousand years, the depth distribution of
short-lived particle tracers deposited in surficial sediments is controlled principally by
the rate of bioturbation. Under certain conditions, for example, biological mixing can
be simulated by one dimensional, "diffusion" type models which imply steady-state
exchange of sediment induced by the activities of organisms distributed uniformly
throughout the mixing zone (Berger and Heath, 1968; Peng et al., 1977, 1979;
Benninger et al., 1979). However, marine sediments are frequently characterized by a
heterogeneous distribution of deposit feeding organisms which may produce a complex
time-dependent configuration of tubes and burrows. In some instances, these structures are irrigated continuously by bottom water drawn downward from the sedimentwater interface (Aller, 1980; 1982; Christensen et al., 1984). Particle reworking
phenonema in this latter type of microenvironment, which is typical of middle slope,
sedimentary regimes east of Newfoundland
(Smith and Schafer, 1979) are not
adequately characterized by one dimensional mixing models. In the present paper we
report two and three dimensional sediment distributions of Pb-21 0 which illustrate the
impact of bioturbation on the transport of lithogenic and biogenic particles in both the
lateral and vertical directions.
a. Field and laboratory methods. In 1977, the CSS Dawson (BIO Cruise 77-034)
occupied a total of 59 stations along four transeCts that extended from the continental
shelf break at 300 m to 350 m depth, to the 3000 m isobath on the upper part of the
continental rise (Fig. 1). Samples collected included water from 4 m and 100 m above
the bottom, Van Veen grabs, and Lehigh gravity cores; a box core (0.25 m2) was
collected at one of the middle slope stations (48). It was subsampled using a 10 cm
1984]
Smith & Schafer: Bioturbation processes
50·
46·
50·
49·
1119
50·
n::
'tt,% -;;,
,,0
50·
~
'b
" "
49·
'b
48·
"
46·
4B·
LABRADOR
SEA
Figure I. Location of study area and of core stations on the N.E. Newfoundland continental
slope.
diameter PVC pipe which also served as the combination barrel and liner for the longer
Lehigh gravity cores collected at adjacent localities. Seabed photographs were taken
with a UMEL 35 mm deep-sea camera.
The subsamples from the box core were stored and transported vertically at 4°C and
sampled subsequently at 1 cm depth intervals in the laboratory. Lead-210 and Ra-226
activities were measured on the samples using the techniques outlined in Smith and
Walton (1980) and Smith and Ellis (1982).
Several stations along the same transect on the slope and rise were reoccupied in
1979 (Cruise 79-017). Box cores collected at these sites were subsampled aboard the
ship using a technique that preserved the three dimensional orientation of the sampled
sediment. Two plastic sampling chambers (2 cm x 8 cm x 20 cm), open at their two
opposite ends, were inserted into the box core in the configuration shown in Figure 2.
The sampling chambers were withdrawn from the box core with their contained
1120
Journal of Marine Research
[42,4
-~---
~
I
II~
::~
,
I
:,
I
I
"()
::
I
I
,}-
,L __ Y,
II
II
II
':
I
JI)
/
Figure 2. Device used to extrude subsamples of sediment from box core. Two subcores (A-I and
A-2) were inserted into box core; these were later sectioned into smaller units using the divider
illustrated above.
sediment and x-radiographed using a Philips K-200 industrial x-ray system. One of the
facing plates of each chamber was removed (indicated by arrow, Fig. 2) and an
aluminum, 1 cm x 1 cm lattice having external dimensions of 2 cm x 7 cm x 16 cm
was inserted into it at right angles to the exposed cross-sectional surface of the
sediment (Fig. 2). The two cross-sectional slabs of sediment were separated by this
procedure into 112 subsamples, each having a known position and orientation with
respect both to the sediment surface and to the x-radiograph of the sediment slab. The
subsamples were extruded from the grid and returned to the laboratory for Pb-21O
analysis.
Whole planktonic and benthonic foraminifera tests, test fragments, diatoms, and
radiolarians were counted in sediment subsamples from associated box and gravity
cores using methods that involved (a) concentration by washing the entire subsample
through a 62 ~m sieve, (b) drying the >62 ~m fraction and then concentrating it
further by flotation in a 10:4 mixture of bromoform and acetone, (c) transferring a
split of the dried float sample to a standard 60 square microfossil slide for counting
with a Wild binocular microscope. The micropaleontological results are based on a
minimum count of 300 benthonic foraminifera specimens; the other fossil groups
considered were identified and counted from a split of the foraminifera sample that
contained at least 500 particles (Schafer et al., 1981).
Shear vane measurements were carried out on box core samples using a motorized
Pilcon Hand Vane Test that was rotated at 4 rpm (Schafer and Asprey, 1982). The
tester was fitted with a standard 33 mm Pilcon vane that was pushed into the sediment
at 5 cm increments to facilitate measurement of shear strength at four levels in the
upper 20 cm interval of each selected box core. Size analyses were performed using
standard sieving and pipette techniques; sediment samples used for water content
determinations were dried for 24 hours at 100°e.
b. Environmental setting. The continental slope at latitude 49°30'N dips gently
seaward at an angle of about 0.8 degrees to the lower slope (2000 to 2500 m) where the
1984]
Smith & Schafer: Bioturbation processes
1121
gradient decreases to less than 0.5 degrees. The lower slope and upper continental rise
(2500 to 3000 m) are swept by the Western Boundary Undercurrent (WBU) which
passes along the Labrador continental margin, through the study area, and then to the
south around Flemish Cap and the Grand Banks (Fig. 1). Bottom current measurements indicate that the WBU has a high speed core, attaining transient current
velocities of at least 18 cm sec-1 (Carter and Schafer, 1983). This core follows a depth
interval between about 2800 and 3000 m on the upper part of the continental rise
(Schafer, 1979).
The microtopography of the seabed as recorded by underwater photographs exhibits
a systematic variation with increasing water depth. The continental shelf edge and
upper slope down to 700 m is characterized by sand to sandy silts strewn with ice-rafted
subangular cobbles and boulders (Figs. 3a and b). Evidence of current activity is
restricted to the isolated occurrence of scour moats at the base of cobbles, infilled
burrows, and to a general linear alignment of biogenic remains such as worm tubes.
Silty sediments on the middle slope between 700 and 1400 m are densely populated by
burrowing macroinvertebrates. Shallow craters (4 to 8 cm in diameter) produced by
browsing echinoids, small rimless burrows « 1 cm in diameter) and short linear trails
are present down to 1000 m (Figs. 3c, d, e and f). Between 1000 m and 1400 m the
burrows become more numerous and more diverse in form and size. The most
conspicuous forms are cones, small (1 to 2 cm diameter) symmetric, rimless burrows
and large (",,5 cm diameter) irregular burrows. The excellent preservation of these
bioturbation features, in conjunction with the comparatively high percentages of
clay-size particles in the sediment, precludes strong current activity near the seabed.
Ice rafted pebbles and cobbles that have a lithologic affinity with Proterozoic and
Paleozoic rocks of Newfoundland and their offshore equivalents (Carter, 1979) again
become evident at 2000 m on the lower slope where bioturbation forms still remain well
preserved. However, as the fast flowing core of the WBU is approached near 2600 m,
bioturbation structures become rare and the seabed is strewn with manganese-coated
gravel, most of which has been reworked from underlying Holocene sediments (Carter,
1979). Together with observations of scour moats and of the linear alignment of
fragments, the manganese-coated gravel attests to the power of local bottom currents
in inhibiting fine sediment deposition (Figs. 3g and 3h). The primary modal diameter
of surficial sediment beneath the WBU increases to 0.16 mm from only 0.01 mm on the
middle slope. Bioturbation structures in sediments underlying the WBU at about
2700 m are scarce and consist of occasional burrows and short simple trails; below
3000 m, bioturbation artifacts again become increasingly evident in underwater
photographs.
In contrast to many parts of the continental slope off the east coast of North
America where sediments generally become finer-grained with increasing water depth
(e.g. Keller et af .• 1979), the slope at 49°30'N undergoes a change from relatively
coarse to fine sediments between the upper and middle slope and then reverts back to
1122
Journal of Marine Research
[42,4
b
Figure 3. Bottom photographs taken along an upper slope to upper rise transect at 49°30'N. Ice
rafted boulders and biogenic debris are evident on the upper slope; (a) 500 m water depth, (b)
700 m water depth. Burrows and trails appear in fine sediments on the middle slope; (c)
1000 m water depth, (d) 1200 m water depth, (f) 1400 m water depth. Sediment covered
pebbles are evident on the lower part of the middle slope near the edge of the WBU; (e)
2000 m water depth. Manganese-covered boulders and pebbles appear below the axis of the
WBU; (g) and (h) were taken at water depth of 2758 m. The reference dot in each photograph
is 12 em in diameter.
1984]
Smith & Schafer: Bioturbation processes
1123
46°W
500N
45'
30'
15'
49°W
47°W
49°N
46°W
Figure 4. Distribution of total organic carbon (wt %) in bottom sediments in the study area.
Note the enrichment in the total organic carbon content of sediments on the middle slope
( ~ 1500 m) compared to values for those sediments underlying the axis of the WBU
(~ 2800 m).
comparatively coarse sediments beginning at a depth of 2000 m. Upper slope sediments
have organic carbon concentrations of >0.4% whereas lower slope and upper rise
sediments are characterized by lower concentrations that average about <0.2% (Fig.
4). Suspended particulate matter (SPM) levels within the bottom nepheloid layer at
levels 4 m and 100 m above the seabed have their lowest values in the vicinity of the
shelfbreak (0.09 to 0.11 mg 1-1). Values increase to about 0.25 mg 1-1 in water depths
greater than 800 m. Sediment textural patterns, and the SPM distribution in slope
bottom water, are consistent with qualitative models that predict sediment winnowing
(and/or nondeposition) beneath the fast-flowing core of the WBU on the continental
rise. These models also predict an increase in deposition rates under the slower-moving
lateral zones of the WBU, especially on the middle slope or upslope side (e.g., Heezen
et al.. 1966; Carter et al., 1979; Balsam and Heusser, 1976).
Carter et al. (1979) identified four stratigraphic units in the slope and rise sediments
of the study area, of which two occur in cores discussed in this work. The youngest unit
(AI of Piper et al., 1978) forms a thin cover with a maximum thickness of about 25 cm
that extends from the upper slope to the lower slope (1000 m to 2000 m). It is composed
principally of muddy sands on the upper slope and grades to almost pure muds on the
lower slope; it represents deposits that accumulated during the late Holocene. This unit
comprises the upper sections of cores 48, 017-2 and 017-7. An underlying unit (A2) can
be distinguished by its high gravel and carbonate content. On the rise, in the vicinity of
cores 017-3 and 13A, A2 sediments have been reworked to yield a gravelly sand lag
deposit up to 10 cm thick that overlies gravelly mud that is typical of unit A2 elsewhere.
Foraminiferal and palynological evidence suggest that the lag deposit is a chronostrati-
1124
Journal of Marine Research
[42,4
graphic equivalent to unit Al and to the upper part of unit A2 which was deposited
during the middle Holocene climate optimum approximately 5000 to 7000 years B.P.
(Schafer et al., 1984).
2. Results and discussion
a. Macrofaunal. One element of the present study concerns the relationship between
the composition of macroinvertebrate
communities and the artifacts of bioturbation
that are evident in vertical profiles of radionuclide activity and microfossil concentration. In order to characterize these relationships, infaunal specimens were collected
and identified from sieved box core sediments at Station 2, located in a water depth of
approximately 1500 m on the middle slope, and from Station 3, located in a water
depth of 2700 m beneath the fast flowing core of the WBU (Fig. 1). In the
fine-grained, bioturbated sediments found at Station 2, the infaunal macroinvertebrate
community comprises about 74 species including, in order of numerical importance,
polychaetes (33 species), cumaceans, isopods, amphipods, bivalve molluscs, pennatulids, ophiuroids and scaphopods (D. Davis, pers. comm.). An average of 34 ± 21 living
animals was recorded in the 2 cm thick, surficial layer of each 17 cm2 sampling unit in
four replicate box core samples. Polychaete species, particularly of the families
Maldinidae and Lumbrinereidae,
as well as a diverse epifauna including gastropods
(Naticidae) and ophiuroids are probably responsible for many of the tube and burrow
structures evident in the bottom photographs of the middle slope sediments (Fig. 3).
Deep burrows (below 2 cm) are caused by larger individuals of the polychaete species
identified and possibly also by undetermined species of Crustacea and Coelenterata.
A less diverse fauna was found at Station 3 below the WBU axis, with about 44
species (19 polychaetes) recorded from box core samples. Population densities were
also relatively low, with an average of 21 ± II living animals in each 17 cm2 unit of
surficial sediment. Polychaetes comprise the dominant component of the infaunal
assemblage. Most of the species in this category differ from those observed at Station
2. The ubiquitous forms include Pholoe anoculata and Notomastus latericeus (P.
Pocklington and D. Davis, pers. comm.).
Epibenthic sled sampling showed that some invertebrate forms encountered at
Station 2 were also recorded at Station 3 but were not as abundant or as large in size. In
box core samples collected at Station 2, 50-75% of the animals encountered were
polychaetes that have a characteristic vermiform shape. At Station 3, the polychaete
percentage decreases to between 35 to 55%. Cumaceans and minute crustaceans (up to
8 mm) are of secondary importance. The lower mean concentrations and sizes of
organisms in the box core samples collected from the upper rise are consistent with
bottom photographic evidence of the reduced occurrence of burrow features compared
to that observed at the middle slope stations.
The surface layers of the relatively stable and cohesive sediments deposited under
quiescent conditions on the middle slope are enriched in organic matter compared to
1984]
Smith
& Schafer: Bioturbation
.... ......
.--- •..........
017-2
....... ........
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.
1125
processes
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RISE -3200m)
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B
CORE DEPTH
20
(em)
Figure 5. Data for shear strength (kPa), total organic carbon (%) and clay % are illustrated as
function of sediment depth for three cores collected from different locations on continental
slope and rise.
continental rise sediments (Figs. 4 and 5). This comparatively high concentration of
potential food material, in conjunction with minimal sediment reworking by bottom
currents, appears to be responsible for the dense infaunal population observed in the
middle slope regime.
The reduced numbers of bioturbating macroinvertebrates observed in the vicinity of
the WBU axis coincide with the occurrence of comparatively dense populations of
living benthonic foraminifera (Schafer and Cole, 1982). These large foraminiferal
populations occupy an occasionally mobile sediment substrate that contains less than
1% total organic carbon. Under these conditions predation pressure may be relatively
low. The foraminifera, as well as many other elements of the indigenous meiofaunal
community, are generally confined to the uppermost centimeters of the sediment; their
feeding habit is associated with minimal vertical mixing of sediment particles. This
relationship agrees with the absence of bioturbation artifacts and with a pronounced
reduction in the percentage of silt plus clay that is observed over the upper 3 em
interval of all base-of-slope and upper rise cores from the WBU axis regime (Carter et
al., 1979).
1126
Journal of Marine Research
[42,4
b. Geotechnical relationships. Geotechnical measurements can provide insight into
sediment reworking phenonema and their effect on sediment strength (Rowe, 1974;
Ruddiman et al .. 1980; Rhoads and Boyer, 1982). Average shear strengths measured
in the 0 to 20 cm intervals of four box cores from the middle slope and upper rise were
5.1 kPa and 7.4 kPa, respectively (Schafer and Asprey, 1982). The mean shear
strength values for the Newfoundland slope near 49°30'N are remarkably similar to
those observed on the slope between Hudson Canyon and Georges Bank (Keller et al ..
1979). This similarity is somewhat surprising in light of the inferred differences in
sedimentary processes between the two localities that are suggested by the precipitous
and incised character of the New England slope compared to the relatively planar and
gently-sloping macro morphology of the northeast Newfoundland slope.
An increase in mean shear strength with sediment depth on the lower slope and
upper continental rise off Newfoundland was attributed to increasing percentages of
silt and clay-size particles in the deeper intervals of these deposits. Keller et al. (1979)
reported that sediments with high shear strengths on the continental slope near the
Mid-Atlantic Bight correlated with higher concentrations of clay-size particles. They
suggested that these clay sediments could reflect the exposure of more cohesive
material on the bottom surface as a consequence of the erosion of surficial, lowshear-strength sediments by bottom currents. For upper rise cores 017-3 and 017-7, the
direct correlation (r = 0.94; 0.92, respectively) of shear strength with % clay size
particles (Fig. 5) is probably sufficient to explain the down core gradient in shear
strength. In contrast, mean values of shear strength for middle slope sediments (core
017-2) are lower, decreasing from 8.3 kPa at 17.5 cm to 0.9 kPa at 2.5 cm. This
decrease cannot be eXplained solely by the small decrease (13%) in the percentage of
clay-size particles over this depth range. Biological and photographic evidence of
bioturbation in middle slope sediments suggests that the relatively low average shear
strengths, and their comparatively high gradient with respect to depth in this core, may
be attributable in part to sediment reworking.
Decreases in shear strength as a consequence of bioturbation have been observed by
Rowe (1974) in silty mud sediments and by Myers (1977) in sandy sediments.
Considerable spatial variability has also been observed in different sets of shear
strength measurements from the same box cores collected from a middle slope locality
by Schafer and Asprey (1982). These observations may be indicative of the local
morphological heterogeneity and/or abundance of bioturbation structures that, in
turn, reflect the nature of temporal and spatial species distributions in this environment. The high variance of shear-strength measurements in Norwegian fjord
sediments have similarly been attributed by Richard and Parks (1976) to the spatial
and temporal heterogeneity of bioturbation.
c. Pb-2JO activity profiles. Sources of Pb-2l0 in the water column include atmospheric inputs and in situ production by the radioactive decay of Ra-226 and from
1984]
1127
Smith & Schafer: Bioturbation processes
Pb - 210
(dpm/g)
00
4
8
Pb - 210 (dpm/g)
4
0
8
E
~
Pb-210 (dpm/g)
4
0
B
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w
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l-
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Ci
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Pb-210 (dpm/g)
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I
4
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:
I/
(j)
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16
017-3
UPPER RISE
2600m
+
+
13-A
UPPER RISE
2600m
017-7
LOWER RISE
3200m
017-2
MIDDLE SLOPE
1500m
20
Figure 6. Total Pb-210 activity distribution in four cores from continental slope sediment
regimes. Cores (017-3 and 13-A) are from below the axis of the WBU on the upper rise and
are relatively unbioturbated, exhibiting little penetration of excess Pb-210 below the upper
few em, while the more bioturbated cores from the lower rise and middle slope locations
(017-7 and 017-2) exhibit deeper transport of Pb-210 into the sediment.
Rn-222 released from bottom sediments. Transport of Pb-2l0 to the sediments involves
scavenging by rapidly settling biogenic (e.g., fecal pellets) and inorganic particulate
matter in addition to other possible processes such as coprecipitation with Fe and Mn
oxides at the sediment-water interface (Bacon et al.. 1976, 1980). In the absence of
post-depositional redistribution of sediment, excess Pb-2l0 should be confined to the
uppermost centimeter of the sediment column with the low sedimentation rate that
prevails over most of the Newfoundland continental slope (",9 cm/1000 yr; Carter et
al.. 1979). However, bioturbation can cause downward transport of excess Pb-21 0 into
older sediment strata, a phenonemon that can be used to estimate sediment mixing
rates and to investigate specific aspects of the mixing process.
Lead-2l 0 sediment-depth profiles for three distinctive depositional settings on the
continental slope and rise are illustrated in Figure 6. Profile 13-A is from a Lehigh
gravity core collected at Station 13 at a water depth of 2600 m beneath the fast flowing
axis of the WBU (Fig. 1). The sediment in the upper 10 cm interval of this core consists
of olive gray sandy muds that have been reworked by the WBU to produce a silty-sand
lag deposit. The good agreement between the thickness of the surficial reworked
sediment interval in gravity core 13-A with reworked sediment intervals in box cores
collected near this location suggests that loss of surface material during the gravity
1128
Journal of Marine Research
[42,4
coring operation was less than 1 cm. The Ra-226 activity measured in the 21-22 cm
interval of core 13-A is 1.52 ± 0.18 dpm/g, which is in reasonable agreement with the
Pb-21O activity of 1.34 ± 0.10 dpm/g measured for this sediment interval. Similar
comparisons undertaken on other cores indicate that no significant departure from
secular equilibrium has occurred between Ra-226 and supported Pb-210 in the bottom
portions of cores from these slope regimes as a consequence of radon diffusion from the
sediments (Imboden and Stiller, 1982).
Elevated excess Pb-21O activities are confined to the upper few centimeters of core
13-A and there is little evidence of recent downward transport of surface material as
the result of bioturbation. The absence of bioturbating organisms suggested by the
Pb-21 0 profile is supported by bottom photographic evidence, faint bedding observed
in x-radiographs of the core, and by the generally low numbers of macrofaunal species
observed in box cores from this area of the rise. The WBU apparently creates an
inhospitable environment for burrowing macrofauna because: (1) bottom currents
inhibit the deposition of fine-grained, organic-rich suspended matter which serves as a
food source for benthic infauna, and (2) local, transient, bottom current activity
promotes sediment instability that is less conducive to the construction of burrows
compared to the cohesive, clay-enriched middle slope deposits.
Lead-2JO profile 017-7 is from a box core collected at Station 7 (Fig. 1), at a water
depth of 3200 m on the continental rise, downslope from the location of the fast-flowing
core of the WBU. The mean Ra-226 activity measured in sediment samples from the
2-3 cm, 7-8 cm and 13-14 cm intervals in this core is 1.85 ± 0.22 dpm/g. The almost
uniform and comparatively high Pb-21O activities (mean = 7.1 dpm/g) in the upper
4 cm of the core, followed by an abrupt decrease to Ra-226 supported levels below
5 cm, is suggestive of rapid mixing of surface sediments by bioturbating organisms.
This Pb-21O activity profile is consistent with the Berger and Heath (1968) mixing
model applied to a low-sedimentation-rate
regime in which rapid mixing occurs on the
time scale ofPb-210 decay in the 0 to 4 cm interval and negligible mixing occurs below
Scm.
In contrast, the excess Pb-21 0 distribution (Ra-226 supported Pb-21 0 equals 1.61 ±
0.19 dpm/g) in core 017-2, which was collected in 1500 m of water at Station No.2 on
the middle slope (upslope from the axis of the WBU), decreases exponentially as a
function of sediment depth between 0 and 7 cm. For the special case of random, steady
state exchange of adjacent volume elements of sediment induced by a homogeneous
distribution of organisms, the effect of biological mixing on radionuclide activity
profiles can be simulated by a diffusional type of mixing model (Guinasso and Schink,
1975). In low sedimentation rate settings, and under suitable boundary conditions
(Officer, 1982), the distribution of excess Pb-21 0, Apb-2 I 0,,' with depth, (x), may be
solely governed by constant, steady state mixing described by;
A Pb-21O" ()X
=
Ao Pb-210" e
-<.\jKb)'/'X
(1)
1984]
Smith
& Schafer: Bioturbation
processes
1129
where AOpb-210" is the excess Pb-210 activity at the sediment surface, A is the decay
constant (0.0311 yr-I) for Pb-2l0 and Kb is a biological mixing coefficient that is
analogous to a diffusion coefficient (Cochran and Aller, 1979; Krishnaswami et al.,
1980; Christensen, 1982). This equation applies to regimes in which the wet density of
sediment is constant with depth, and where V2«4AKb'
a condition which is generally
valid for bioturbated continental slope regimes where the sedimentation rate, v, is of
the order of 1-10 cm/1000 yr. A mixing coefficient of 0.170 cm2 yr-I (5.4 x 10-9
cm2sec-l)
gives a good fit (R = 0.998, n = 7; P < 0.01) to the excess Pb-210
distribution between 0 and 7 cm, a value which is in the general range of values of
1-14 x 10-9 cm2sec-1 previously measured in pelagic sediments (Nozaki et al., 1977;
Turekian et al., 1978; Peng et al., 1979; Berger and Killingley, 1982; OeM aster and
Cochran, 1982).
In contrast to the greater homogenization of surficial sediments evident in core
017-7 from the continental rise, sediment mixing in the upper strata of middle slope
sediments is slower, but extends to a greater depth in the sediment column and is
consistent with a steady-state "diffusion" analog model. However, below a depth of
8 cm in middle slope deposits, additional peaks are evident in the Pb-21O profile
through the 8-11 cm and 13-15 cm intervals. The occurrence of these peaks in the
Pb-21O distribution is suggestive of downward, "advective," transport of discrete
volume elements of surface material, promoted by the activities of deeper dwelling,
burrowing organisms such as the decapod crustacean Munidopsis or the burrowing sea
anemone Edwardsia both of which were observed in middle slope box core sediments
(D. Davis, pers. comm.).
Further evidence for this advective bioturbation process is provided by three Pb-2l0
profiles measured in box core 77-48, collected at 1500 m near Station No.2 in the
middle slope depositional regime. Pb-2l0 profile 48-C (Fig. 7) corresponds to material
subsampled from the center of the box core, while profiles 48-A and 48-B are from
subcores, 1 cm wide, collected 20 cm and 18 cm, respectively, from the center of the
box core.
Sediment in the subcores 48-A and 48-C contained numerous small (1 mm in
diameter) burrows in the upper 3 cm interval and several large burrows (1 to 5 mm in
diameter) angling down from the sediment-water interface to depths as great as 12 cm.
Pb-21O profile 48-C exhibited elevated activities near the surface and a pronounced
maximum in the 6 cm to 9 cm interval. X-radiographic
evidence of burrows
intersecting the 6 cm to 9 cm interval indicate that the Pb-21O subsurface maximum
was caused by downward transport of material from the vicinity of the sediment-water
interface, possibly due to the infilling of burrows during their excavation or subsequent
to their evacuation or as the result of burrow irrigation by the organism.
A Pb-2l0 maximum is evident in the 7 cm to 10 cm interval of profile 48-A,
suggesting that heterogeneous downward transport of surface material has also
occurred at this location, probably through inclined burrows evident in x-radiographs
1130
Journal of Marine Research
Pb-210
[42,4
IN SEDIMENT CORE
77-48
Figure 7. Three Pb-210 profiles measured in sub-cores from box core 48 exhibit high quantities
of Pb-210 at sediment depths as great as 12 cm. Downward transport of Pb-210 into the
deeper sediments as a result of bioturbation has produced maxima in these profiles at depths
of the order of 8 cm.
of the core. In contrast, profile 48-B, located only 3 cm from profile 48-A, has a
uniform Pb-2l0 gradient down to a depth of 12 cm. The 48-B sediment had a more
mottled appearance, a higher porosity and a greater abundance of burrows of all sizes
compared to subscores 48-A and 48-C. These characteristics
are consistent with a
higher and more uniform rate of mixing through the upper 12 cm of subcore 48-B
compared to cores 48-A and 48-C. This increased mixing rate has resulted in a Pb-21 0
profile which is a better approximation to a "diffusion" analog profile. The small
decrease in the Pb-21O activity, from 7.1 dpm g-I at the surface to 5.3 dpm g-I in the
11-12 cm interval of 48-B (Ra-226 supported Pb-2l0 ~ 1.50 ± 0.12 dpm g-I),
corresponds to a comparatively high biological mixing coefficient (Eq. 1) of 22 cm2
yr-1 (7.2 x 10-1 cm2 sec-\ r = 0.78) that lies in the range of mixing coefficients
(10-6 - 10-8 cm2 sec-I) measured from Th-234, Be-7 and Pb-210 profiles in estuarine
sediments (Aller et al.. 1980; Krishnaswami et al., 1980; Olsen et al., 1981). DeMaster
et al., (1982; 1984) have also measured high biological mixing coefficients (3-33
cm2jyr) in Nova Scotian continental rise sediments using Th-234 and Pb-210. Their
estimates of mixing coefficients also exhibited considerable lateral variability throughout the HEBBLE site.
The close correlation of Pb-2l0 profiles 48-A and 48-B over the 7 cm to 9 cm
interval may be the result of enhanced lateral mixing of sediment at this depth interval,
1984 ]
Smith & Schafer: Bioturbation processes
Pb-210
1131
DISTRIBUTION
CORE A-I
Figure 8. The two dimensional distribution of Pb-210 in core A-I, subsampled from box core
OI7-2-IV, exhibits significant lateral gradients which are indicative of a heterogeneous
distribution of bioturbating organisms. Anomalies in the Pb-210 distribution (Table 1)
correspond to bioturbation artifacts observed in x-radiograph of the core (see text).
a contention that is supported by the orientation of bioturbation artifacts observed in
the x-radiographs. Hence, one possible mechanism leading to the occurrence of Pb- 210
maxima at depth in a sediment column invokes rapid vertical mixing in neighboring
sediment regimes followed by lateral transport of recently derived, surface material at
depths of the order of 8-15 em. Lateral mixing of sediment could result from the
construction of horizontal burrows by deposit feeders as they mine nutritious sediment
seams (Seilacher, 1967). A more graphic illustration of the potential importance of
lateral sediment transport at depth by bioturbators is described in the following
examination of the two dimensional distribution of Pb- 210 activity in slope sediments.
d. Two dimensional Pb-210 distributions. Two dimensional Pb- 210 distributions were
measured in sediment subcores, A-I and A-2, following the procedures outlined in
Section lao Figures 8 and 9 emphasize the sampling constraints i.e., the cross sectional
area of each sample cell was 1 cm2 (Fig. 2), and depict topographical surfaces, the
height of which corresponds to the Pb-210 activity distribution. The excess Pb-210
activity distribution can be approximated by subtracting the mean Ra-226 activity of
1.60 ± 0.12 dpm/g (estimated by gamma [GeLi] analyses of 10 sediment samples)
from each Pb-210 measurement. The mean Pb-21O activities estimated for each 1 cm
depth interval in cores A-I and A-2 are given in Tables 1 and 2. The two mean Pb-21O
profiles compare favorably to the core 017-2 profile (also from the middle slope) and
1132
Journal of Marine Research
Pb-210
[42,4
DISTRIBUTION
CORE A-2
Figure 9. The two dimensional distribution of Pb-21O in core A-2 (Table 2) exhibits a ridge of
elevated Pb-210 activity between a depth of 10 to 12 cm which corresponds to a possible
infilled burrow observed at this location in the x-radiograph of the core (Fig. 10). Pb-21O
activities are generally lower and more laterally uniform in this core compared to core A-I.
Table I. Total Pb-21O activities (dpm/g) corresponding to given sediment width and depth
intervals from sub-core A-I. Mean corresponds to mean Pb-21O activity for each depth
interval and Fpb-2IOe>is the flux of excess Pb-210 (dpm/cm2 yr) estimated from the
integrated inventory of excess Pb-21O in each I cm width interval.
Depth (cm)
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
10-11
11-12
12-13
13-14
14-15
Fpb-2lOex
0-1
10.78
10.52
8.41
9.53
4.74
3.66
3.06
3.10
2.72
4.10
3.49
2.13
1.84
1.89
1.99
1-2
11.75
9.52
8.16
6.41
4.16
2.95
2.33
2.74
2.73
3.36
2.25
2.45
2.03
1.90
1.81
2-3
10.08
9.43
7.04
5.92
3.75
3.30
2.71
2.55
2.72
1.82
1.74
2.33
1.72
1.85
1.74
1.42
1.22
l.l0
Width (cm)
3-4
4-5
9.98
12.39
7.82
8.85
6.68
6.02
5.30
5.81
7.43
3.48
3.63
3.92
2.39
2.81
2.62
1.92
2.09
1.98
1.63
1.66
1.78
1.85
1.78
1.68
1.70
1.80
1.40
1.54
1.63
1.65
1.00
l.l4
Mean
5-6
13.52
10.76
7.65
6.18
4.52
4.11
3.02
2.19
2.03
1.97
1.68
2.19
2.22
1.85
1.84
6-7
15.94
14.34
8.25
7.07
5.63
4.80
3.82
2.94
2.43
2.02
1.75
3.28
4.29
1.90
1.86
12.06
10.18
7.46
6.60
4.82
3.77
2.88
2.58
2.39
2.37
2.08
2.26
2.23
1.76
1.79
1.20
1.66
1.28
1984]
Smith
& Schafer: Bioturbation
processes
1133
Table 2. Pb-2l0 results for core A-2. See Table 1 for explanation of units.
Depth (em)
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10
10-11
11-12
12-13
13-14
14-15
0-1
9.25
8.53
4.48
3.28
3.57
2.96
2.11
2.06
1.70
1.66
3.51
2.87
1.66
1.71
1.52
1-2
9.09
7.28
4.16
3.05
2.78
3.22
2.30
2.43
1.68
1.61
2.25
4.31
1.68
1.87
1.69
2-3
8.50
5.22
1.85
2.64
2.35
2.89
1.95
2.38
1.91
1.96
2.01
2.57
1.89
1.81
1.61
Width (cm)
3-4
8.57
4.69
3.70
2.64
2.12
2.80
1.86
1.70
1.89
1.80
1.74
2.76
1.79
1.76
1.55
4-5
7.98
5.61
3.64
2.77
2.18
2.67
1.74
1.66
1.79
1.87
1.81
2.12
1.55
2.23
1.82
5-6
10.01
5.47
4.40
3.56
3.32
2.45
2.01
1.78
1.85
1.78
1.77
2.87
2.18
2.77
1.83
6-7
7.66
5.65
3.89
2.99
2.29
3.00
1.96
1.89
1.70
2.12
1.81
2.93
5.32
3.30
2.13
8.72
6.06
3.73
2.99
2.66
2.86
1.99
1.99
1.79
1.83
2.13
2.92
2.30
2.21
1.74
Fpb-21O••
0.83
0.81
0.59
0.60
0.61
0.77
0.76
0.69
Mean
are consistent with a simple diffusional type of mixing model for which the biological
mixing coefficients are 0.30 cm2yr-1 (core A-I) and 0.17 cm2yr-I (core A-2).
The excess Pb-210 fluxes, FPb-210 •• in cores A-I and A-2 (Tables 1 and 2) were
determined by dividing the total measured sediment inventory of excess Pb- 210 in each
sediment column (excess Pb-210 integrated over the mass-depth, g cm-2) by the mean
lifetime of Pb-210 with respect to radioactive decay, 32.2 yr. The mean fluxes of 1.28
dpm cm-2 yr-I (core A-I) and 0.69 dpm cm-2 yr-1 (core A-2) are of the same order of
magnitude as Pb-2l0 fluxes measured in sediment traps in the deep North Atlantic
(Brewer et al., 1980). The absence of significant disequilibria between Pb-2l0 and
Ra-226 in the 500 m to 1500 m depth range in Labrador Sea water (Bacon et al., 1980)
precludes the use of these water column inventory data to estimate Pb-210 fluxes onto
the Newfoundland continental slope. An excess Pb-2l 0 flux of only 0.2 dpm cm -2 yr-1
would be predicted for a 2000 m deep water column in which the particulate Pb-2l0
activity was 0.5 dpmjl 00 kg, as measured by Bacon et al. (1980) in the Labrador Sea,
and the water residence time of the particles responsible for Pb scavenging and removal
was 5 years (Bacon and Anderson, 1982). However, considerably elevated fluxes would
be expected in slope sediments, as measured by Carpenter et al. (1982) off the coast of
Washington, owing to transport of Pb-210 off the shelf and from the atmosphere
(flux = 1.0 dpm cm-2 yr-I; Turekian et al .. 1977) followed by particle scavenging and
transport to the middle slope. In addition, horizontal transport of dissolved Pb-210 in
the ocean and uptake at the continental margins (Bacon et al., 1976; Spencer et al.,
1981) may augment Pb-210 inventories in Newfoundland slope sediments.
1134
Journal of Marine Research
[42,4
Core A-I
The Pb-2l0 activity distribution in core A-I is characterized by localized concentration gradients which represent a breakdown in the applicability of a steady-state model
to this mixing regime. The pronounced vertical and horizontal gradients in the Pb-2l0
distribution are probably artifacts of a heterogeneous distribution of bioturbating
organisms and/or nonisotropic mixing phenonema. It is also possible that the transport
of Ph-2l0 has a size-selective component, either as the result of differential physical
mixing of different particle sizes (Rhoads and Boyer, 1982) with the Pb-210 associated
with the finer-grained clays and organic matter, or as the result of size selective feeding
by deposit feeders. In this latter case, Pb-210 associated particle sizes may be
preferentially ingested by the organism leading to an increased residence time of these
particles in the sediment strata inhabited by the organisms, a process which may be
simulated by a Markov model (Jumars et al., 1981).
Most of the individual vertical Pb-2l0 profiles in the upper portion of the sediment
column are consistent with one-dimensional
box or diffusion mixing models. For
example, the Pb-21O profile for the 0-1 cm width interval in core A-I, similar to that
observed in core 017-7 (Fig.6), is consistent with a simple box model characterized bya
rapidly mixed, 4 cm deep, surficial zone of constant Pb-2l0 activity, overlying a zone of
negligible mixing (on time scales of the order of the Pb-2l0 half-life) in which the
Pb-2l0 activity approaches its Ra-226 supported, background value. The adjacent,
exponentially-decreasing
Pb-210 activity profile (1-2 cm width interval) is more
consistent with a diffusion mixing model for which mixing occurs down to the 7 cm
level at a slightly lower rate (Kb = 0.20 cm2 yr-1). In view of the comparatively stable
biological conditions that prevail in bathyal zone environments, the nature of the
bioturbation mechanism would not be expected to change so dramatically over very
small distances. Therefore the actual distributions noted above probably represent
transient perturbations
superimposed on a steady-state distribution. The effect of
nonsteady state mixing on Pb-21O distributions has also been noted by DeMaster et al.
(1984) in the HEBBLE area. The probability for the detection and possible misinterpretation of transient bioturbation artifacts will be substantially decreased if subsampIing, cross sectional areas greater than 10 cm2 are employed in bioturbated slope
sediment regimes.
Anomalies in the Pb-2l0 distribution (Fig. 8) can be compared to artifacts of
bioturbation in the X-radiograph of core A-I (Fig. 10). The cell of highest Pb-2l0
activity (6-7 cm width; 0-1 cm depth) is located at the sediment water interface and
lies immediately above a tube 5 mm in diameter. This tube angles down from the
sediment-water interface to intersect the adjacent lower sampling cell located at the
(6-7 cm width; 1-2 cm depth) interval. This lower cell has an unusually high Pb-2l0
activity compared to other cells observed at the 1-2 cm depth interval. If the upper cell
represents an undisturbed volume element of sediment containing a quantity of Pb-21O
not greatly depleted by downward mixing processes, then the high Pb-210 inventory of
Smith & Schafer: Bioturbation processes
1984]
0
,
2
,
A-I
WI DTH (em)
4
,
.
6
0
,
2
I
A-2
4
I
1135
.
6
0
o
2
2
4
4
6
6
E
E
u
u
IS
f-
s;:
Q..
Q..
W
W
0
o
10
10
12
12
14
14
16
16
Figure 10. Numerous tubes and infilled burrows are evident in x-radiographs of cores A-I and
A-2. Pebbles below 13 cm appear as opaque discs. Tubes in upper portions of the core have
retained their mucus wall structures while walls of infilled burrows (pale-colored tubular
objects in lower portion of x-radiograph) have broken down. Several artifacts of bioturbation
observed in x-radiograph are correlated with the Pb-21 0 anomalies discussed in the text.
the lower ceIl may be due to inputs from the surface transported downward as a result
of the activities of the organism which constructed the tube observed in the
radiograph. The only other tube (3 mm in diameter) that is in contact with the
sediment-water interface intersects the four surface celIs between the 0 and 4 cm width
intervals. These four cells are characterized by reduced Pb-210 inventories compared
to the remaining three surface celIs for which there is no x-radiographic evidence of
burrowing. These observations indicate once again that the small tubes (probably the
habitats of polycheate worms) are either agents or artifacts of the downward transport
of Pb-21 0 from the sediment-water interface. The small modal length of the individual
tubes precludes significant mass transport of sediment over distances of several
centimeters. However, their intersection with the sediment-water interface is signifi-
x-
1136
Journal of Marine Research
[42,4
cant because elevated Pb-210 levels are typical of the higher porosity material that
constitutes the upper few millimeters of sediment collected in these regimes. Samples
of this material, scraped or siphoned off the sediment surface of box cores, are
characterized by Pb-21O activities greater than 20 dpm/g, and may include a high
proportion of deposited nephloid layer sediment. The active biological transport of
small quantities of this material to depth in the sediment column could produce
significant perturbations in the Pb-210 distribution.
Elevated Pb-21O activities are evident at a depth of9-11 cm (0-1 cm width interval)
and at a depth of 10-12 cm (6-7 cm width interval). Both cells correspond to low
density regions in the x-radiograph of the core which probably represent infilled
burrows. However, several neighboring regions of low density observed in the
x-radiographs (Fig. 10), which also appear to be artifacts of infilled burrows, do not
exhibit elevated Pb-2l0 activities. These burrows may be older, in which case their
excess Pb-2IO inventory has been depleted by radioactive decay, or they may not have
been efficient conduits for the transport of surface material to depth in the sediment
column.
Core A-2
In general, individual profiles in the upper 10 cm interval of core A-2 conform
closely to exponentially-decreasing
profiles that are compatible with a diffusion type of
mixing model. This core contains a reduced inventory of excess Pb-2IO compared to
core A-I (Table 2). The excellent preservation of worm tube structures in the surface
layers of sediment (observed in x-radiographs of the cores) indicates that the integrity
of the core was preserved and that mass losses from the high porosity, semi-liquid
surface layer of material during the sampling operation were minimal. Rather than
being a sampling artifact, the decreased inventory of Pb-2IO in core A-2 probably
reflects the general heterogeneity of Pb-210 distributions observed throughout the
middle slope sediment regime.
The left-hand side of core A-2 (0-1 cm width interval) corresponds to the sediment
column immediately adjacent to the 2-4 cm width interval of core A-I (Fig. 2). This
latter width interval (Table 1) is characterized
by the lowest Pb-2IO inventories
observed in core A-I. The Pb-2l0 inventory in the 0-1 width interval of core A-2 is
reduced by approximately 20% compared to the 2-4 cm width interval of core A-I and
Pb-21O inventories tend to decrease further from left to right across core A-2. The
lateral decrease in Pb-2l0 inventories in core A-2 may therefore represent the spatial
extension of a systematic decrease in Pb-210 inventories that begins within core A-I.
The single most notable feature of the Pb-21O distribution in core A-2 is the
occurrence of a zone of elevated Pb-210 activity extending diagonally across the width
of the core between the 10 cm and 14 cm depth levels. This zone of high Pb-210 activity
correlates well with x-radiographic evidence of a horizontal infilled burrow, approximately 1 cm in diameter, which dips gently downward across core A-I from the 11 cm
1984]
Smith & Schafer: Bioturbation processes
1137
depth level on the left-hand side of the core to the 12 cm level on the right-hand side.
The comparatively high Pb-21O activity within this zone and its good agreement with
the outline of the infilled burrow in the x-radiograph implies that the organism
responsible for the burrow had been in comparatively recent contact with the upper
portion of the sediment column. If the source of the excess Pb-21O at the 10-14 cm
depth interval is sediment from the 0-1 cm depth interval (characterized by Pb-210
activities of 8-10 dpmj g), then a comparatively large mass transport of sediment, and
a correspondingly high mixing rate, would be required to produce the elevated levels of
the order of 3-5 dpmjg that are observed between the 10 and 14 cm depth levels.
Several other structures identified as laterally-oriented infilled burrows in the upper
portions of the core between the 7-10 cm depth interval, are not associated with
elevated Pb-210 activities, a circumstance also noted in core A-I. If the dynamics of
burrow construction are the same throughout the sediment column, these observations
suggest that the occurrence of more recently formed burrows underlying older burrows
is a common phenonema in the mixing mode of middle slope sediments. This may
denote a community succession in which the later arriving species constitute forms that
typically inhabit deeper levels of the substratum (e.g., Rhoads, 1974).
e. Bioturbation and microfossil abundance distributions. The input function governing the relative proportions of microfossil taxa observed in marine sediments is
dependent on the temporal variation of limiting ecological conditions. However, in
addition to physical sediment reworking processes, biologically-modulated sediment
mixing may produce further distortions in the originally-deposited microfossil record
(Hutson, 1980). These distortions may be especially important in regard to the
magnitude and timing of paleoecological events that are recorded by microfossil
abundance signals (eg., Berger and Heath, 1968). The impact of post-depositional
sediment mixing on microfossil distributions can be considered for the distinctive
middle slope and continental rise environments described above (Stations 48 and 13).
The average coefficient of variation (CV) in the sediment-depth distribution of the
comparatively small microfossils such as planktonic foraminifera fragments, diatoms
and radiolarians (variables 5, 6 and 7; Tables 3 and 4) is 42 ± 10% and 89 ± 29% in
cores 48A and 13A, respectively. The smaller CV value assigned to core 48A agrees
with the greater homogenization of middle slope sediment regimes predicted on the
basis of Pb-210 evidence for higher rates of bioturbation. In contrast, the mean CV
values for the sediment distributions of the comparatively large, whole specimens of
planktonic foraminifera (variables 1 and 4; Tables 3 and 4) are not statistically
different for cores 48A and 13A. These results would be consistent with a size selective
component in the sediment mixing mechanism with smaller particles (with which the
Pb-210 is associated) undergoing more rapid reworking than the larger size classes.
However, the most pertinent question arising from these microfossil concentration
measurements is whether the species percentage abundance signal is significantly
Journal of Marine Research
1138
[42,4
Table 3. Counts of Paleontological Variables for Core 48A. NICc is the number of specimens
per cc of wet sediment and % Calc. is the percentage of calcareous species in the total
benthonic population. Parameter 2 is the total number of benthonic foraminifera species.
Parameters I, 4, 5, 6 and 7 are respectively benthonic foraminifera, planktonic foraminifera,
planktonic foraminifera fragments, diatoms, and radiolarians.
2
3
Benthonic forams
Species
% Calc.
NICe
4
Plank.
5
Plank. fro
6
Diat.
7
Rads.
NICe
NICe
NICe
NICe
2-3
3-4
4-5
6-7
7-8
9-10
10--11
11-12
12-13
13-14
15-16
16-17
143
135
133
158
188
186
180
161
165
139
126
80
38
38
34
42
43
32
34
34
34
34
38
20
78
87
90
86
83
90
87
86
81
82
92
95
51
24
103
54
49
289
171
163
211
133
19
24
185
503
206
262
195
223
182
286
211
288
270
48
67
122
206
107
195
178
32
224
71
200
77
94
775
1560
1100
1635
671
935
568
1081
1080
1020
1290
838
Mean (X)
SD
CV (%)
149.5
30.5
20.3
35.1
5.9
16.7
86.4
4.9
5.6
107.6
86.7
80.5
238.3
105.5
44.3
131.1
65.9
50.2
1046.1
326.8
31.2
Variable
Core Interval
Table 4. Counts of Paleontological Variables for Core 13A. See Table 3 for explanation of
units.
Variable
Core Interval
2
3
Benthonic forams
Species
% Calc.
NICe
2-3
3-4
4-5
6-7
7-8
9-10
]0--11
11-12
12-13
13-14
15-]6
16-17
264
480
510
337
546
238
441
365
354
403
3]6
492
32
40
45
36
32
34
32
31
35
35
3]
28
Mean (X)
SD
CV (%)
395.5
99.5
25.1
34.3
4.6
13.3
89
88
80
92
92
97
98
98
98
98
100
100
94.2
6.1
6.5
4
Plank.
5
Plank. fro
6
Diat.
7
Rads.
NICe
NICc
NICc
NICc
4599
4303
5149
1662
5058
1422
6252
5989
4219
5998
6327
3350
32239
18629
27268
13331
22330
5369
]4659
7438
4755
10282
4579
3062
399
55
936
4
247
99
0
193
153
0
211
96
1595
1144
2575
869
1604
394
970
676
230
825
416
574
4527.3
]667.0
36.8
13661.8
9649.6
70.6
199.4
260.5
130.6
989.3
664.3
67.1
1984]
Smith & Schafer: Bioturbation processes
1139
altered by bioturbation. These data have traditionally provided an important source of
proxy information for statistical paleoclimatic and paleo-oceanographic analyses with
quantitative emphasis generally being placed on the comparatively abundant taxa.
Three textural facies-sedimentation rate regimes can be identified in middle slope
and upper rise cores based on their grain size characteristics. The 0-17 cm portion of
core 48A constitutes one facies characterized by relatively low sand percentages (2%3%) while the 2-8 cm and 9-17 cm intervals of core l3A constitute two additional
facies having sand percentages in the 20% - 30% and 50% - 100% ranges, respectively.
The validity of comparing the temporal species variability for specific taxa between
these different facies rests on the assumptions of (1) a constant sedimentation rate, and
(2) a constant input function for the fossil species during the period of sediment
deposition defined by each facies. Average sedimentation rates for the three textural
facies, based on the depth of a 9300 year old ash zone (Schafer et al., 1984) are 9
cm/1000 yr for core 48A and for the 9-17 cm interval of core 13A (Schafer et al.,
1984). The 2-8 cm section of core l3A constitutes material which has been recently
reworked by the overlying Western Boundary Undercurrent and may have been
deposited under substantially lower but constant, sedimentation rate conditions of the
order of 1cm/1000 yr (Carter, 1979). Comparable source functions for the indigenous
benthonic foraminifera species within the three facies are predicted on the basis of the
close proximity of the sampling sites and the absence of significant climatic fluctuations during the past few thousand years that would have had a significant impact on
the middle slope and rise sedimentation regimes.
The variances of the three most abundant benthonic foraminifera species observed
in the three textural end member intervals were compared with a Variance Ratio (F)
test;
(2)
where (J is the standard deviation of a foraminiferal species percentage abundance in
one of the three facies and n/n-l is the Bessel's correction which is usually applied in
cases where n (the number of core internal subsamples) is less than 20 (Moroney, 1964,
p. 225). For comparative purposes, values of F that are less than those given in a
Variance Ratio Table for a 5% confidence level are considered to be not significant.
The most abundant foraminiferal taxon in core l3A, Epistominella spp. has a CV
that is almost 2.5 times greater than that of its counterpart, Elphidium excavatum.
which is the most abundant taxon in core 48A. The CV for Epistominel/a spp. through
the 9-17 cm interval of core l3A also exceeds the CV for the depth distribution of that
species through the shallower 2-8 cm interval by a factor of 4. The F test is significant
at the :s 1% level for all comparisons. These, and similar results for some of the less
abundant taxon, are indicative of a greater degree of mixing in both core 48A and in
the 2-8 cm section compared to the 9-17 cm section of core l3A. Reduced variances in
the 2-8 cm section of core l3A may reflect specimen mixing associated with physical
1140
Journal of Marine Research
[42,4
sediment reworking that frequently occurs below the axis of the WBU (Carter and
Schafer, 1983) where bottom current velocities in excess of 18 cm sec-1 (Carter and
Schafer, 1983) are capable of eroding and redistributing foraminifera tests over the
upper few cm of sediment surface (Kontrovitz et al., 1978; 1979). Reworking is also
suggested by a pronounced reduction in the proportion of silt plus clay-size particles
relative to sand in the 2-8 cm interval, and by the higher concentrations of planktonic
foraminifera fragments and radiolarians compared to those in the other textural
facies.
The greater overall homogeneity of the dominant microfossil species distributions in
middle slope compared to upper rise sediments is consistent with Pb-210 evidence for
higher rates of bioturbation in middle slope environments during the past 100 years.
However, the microfossil results also suggest that sediment reworking, promoted by
the WBU, has produced some physical mixing of surficial sediments from the upper
rise regime, probably on longer time scales of the order of thousands of years.
Bioturbation processes in Holocene sediments on the middle slope can give rise to false
"paleosignals"
in those instances where tests from the sediment surface have been
mixed down to some modal sediment depth that reflects the niche dimensions and
burrowing habit of the dominant indigenous species. These paleosignals may be
expected to have a spatially-heterogeneous
distribution which is similar to that of the
indigenous bioturbating taxa. Consequently, the strategy for core sampling of Holocene sections on the slope and rise off Newfoundland should focus on those areas where
sedimentation rates are comparatively high but where the flux of total sedimentary
organic carbon is disproportionately low. These environments often tend to be devoid of
burrowing macrofauna and are usually populated by a greater proportion of meiofauna
including many fossil-forming taxa that fall within this category of organisms (Thiel,
1979; Schafer and Cole, 1982). The x-radiographic and sedimentological evidence
from the N.E. Newfoundland slope suggests that sedimentary sequences favorable for
coring Holocene sections should occur with greater frequency in areas lying immediately downslope from the edge of the WBU core. In these environments, sedimentation
rates are somewhat lower than those observed on the middle slope, but organic matter
fluxes tend to be comparatively low and intermittent thereby discouraging colonization
by dense populations of bioturbating organisms.
3. Conclusions
1. Sedimentological,
micropaleontological
and radionuclide (Pb-2l0) measurements conducted on sediment box cores have identified two distinctive bioturbation
regimes on the continental slope, east of Newfoundland. The sediments underlying the
axis of the WBU are relatively unbioturbated compared to middle slope deposits with
excess Pb-210 confined to the upper few centimeters of the sediment column. These
sediments represent an inhospitable environment for bioturbating organisms because
of the almost continuous presence of bottom currents that maintain an unstable (i.e.,
1984]
Smith & Schafer: Bioturbation processes
1141
mobile) sediment substrate. Bottom currents also inhibit the deposition of fine-grained,
organic-rich material which serves as a food source for the indigenous benthic infauna.
In contrast, the higher flux of fine particulate material to the more quiescent, middle
slope (1500 m water depth) environment has provided the requisite conditions for the
colonization of the bottom sediments by comparatively dense populations of bioturbating organisms.
2. Excess Pb-2l0 activity profiles in the bioturbated, middle slope regime are, in
some cases, consistent with a diffusion analog mixing model. Other sediment-depth
profiles exhibit maxima at depth in the sediment column which are suggestive of
downward transport of Pb-21O-enriched surface sediments by an "advective" process.
Measurements of the detailed two and three dimensional distribution of excess Pb-210
in one middle slope box core indicate that the mixing process has a heterogeneous
component on time scales of the order of the Pb-210 half life (22.3 yr). Precise
correlation of anomalies in the Pb-21 0 distribution with bioturbation artifacts observed
in x-radiographs of the core indicate that maxima in one-dimensional Pb-210 profiles
may be the result of a skewed orientation of burrow structures which introduces a
pronounced lateral component into the downward transport of surface sediments.
3. Concentration variables for comparatively small microfossil particles observed in
middle slope sediment cores have low mean coefficients of variation for their
sediment-depth distributions, compared to those determined for upper rise sediments
underlying the WBU. Further, the percentage abundance variation of the dominant
foraminiferal species in core 48-A from the middle slope is approximately 3 time lower
than that of its counterpart in parts of core 13-A from the upper rise. These results are
consistent with a relatively greater degree of biologically-modulated sediment mixing
in the middle slope deposits compared to that observed in sediments on the rise. In the
deep bathyal continental rise environment, lateral redistribution and the mixing of
microfossil tests into the upper several em of the sediment appears to be the dominant
reworking processes.
Acknowledgments. The authors are indebted to K. Ellis, M. Huh, F. J. Bishop and F. Cole for
much of the analytical work reported in this paper and to Dr. J. M. Bewers and Dr. D. Piper for
their helpful discussions and reviews of earlier versions of the manuscript.
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