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Nature of sediment accumulation on the Amazon continental shelf
(ontinentalShelfRe~ealx'h,Vol. 6, No. I/2. pp. 21Dto 225. 1986. Printed in Great Britain.
027S~-343/~ $3,11n+ iI.(10 PergamonJournals I.td.
N a t u r e o f s e d i m e n t a c c u m u l a t i o n on the A m a z o n c o n t i n e n t a l s h e l f STEVEN A . K U E H L , * t DAVID J. D E M A S T E R * a n d CHARLES A . NITTROUER*
(Received for publication 13 December 1985) Abstract--Sediment accumulation on the Brazilian continental shelf near the Amazon River is investigated using radiochemical (e.g. 21~Pb, ~4C) techniques to provide a better understanding of this major dispersal system of fine-grained sediment. 2"~Pb profiles from 57 cores collected during 1983 reveal the distribution of modern (100-y time scale) accumulation rates on the Amazon subaqueous delta. Accumulation rates increase from <(J. l cm y t (I). l g c m 2 y t) nearshorc, to rates as high as ~10 cm y ~ (6.9 g c m 2 y ~) on the outer topset and the l~)reset regions (30-50 m water depth). Reduced upward accretion nearshore (<15 m water depth), which is reflected in the limited subaerial expression of the Amazon delta, probably results from the intense activity of surface waves and tidal currents. A thick (as much as 2 m) depth). This layer probably is reworked by waves and currents, and most of the sediment is eventually transported to other parts of the dispersal system. HC dating of an anomalous area of relict (age >100 y) sediment in the northwestern portion of the subaqueous delta indicates that this sediment was deposited <1000 y ago. The absence of modern sediment in this area is not understood. A sediment budget for the Amazon shelf indicates that 6.3 + 2.0 x l0 s tons of sediment accumulate annually. Much of the remainder of Amazon River sediment ( - 6 x 10~ tons y J) probably is transported northwestward beyond the Brazilian shelf and/or is accumulating landward of the shelf as coastal accretion.
INTRODUCTION
THE NATURE of continental shelf sedimentation off the Amazon River has been the subject of much controversy during the past decade. Several conflicting models have emerged in the evolution towards a better understanding of this major sediment dispersal system. MILLIMANel al. (1975) suggest that most of the inner-shelf mud deposit off the Amazon River is relict, having formed during periods of lower sea level by coastal accretion. GIBBS (1976) contends that the entire inner shelf is presently accumulating sediment. In an attempt to resolve this controversy, KUEHL et al. (1982) examine sediment accumulation rates on the Amazon shelf using 2tt)pb geochronology, and demonstrate that much of the inner shelf is undergoing active sediment accumulation; however, many questions remain. Only first-order estimates of accumulation rate could be made by KUEHL et al. (1982), because sediment cores (i.e. box cores) used for 21°pb geochronological studies were too short ( - 0 . 5 m) to obtain complete radionuclide profiles in the seabed. Therefore, little information regarding areal trends in accumulation rate on the Amazon shelf is available. The fact that some Amazon River sediment is transported northwestward beyond the Brazilian border has been recognized for a long time (LYELL, 1830). An accurate * Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, U.S.A. t Present address: Department of Geology, University of South Carolina, Columbia, SC 29208, U.S.A. 209
210
S.A. KUEHLet al.
estimate of sediment accumulating on the Amazon shelf is needed to better estimate the magnitude of sediment bypassing. An anomalous area of relict sediment (on 100-y time scale) is observed in the northern part of the Amazon shelf (KVEHL et al., 1982). Questions remain concerning the time of formation for this relict area. The primary objectives of this study are to answer the questions outlined above. Sediment cores (3 m long) collected during a 1983 cruise to the study area are used to determine accurate estimates of accumulation rate on a 100-y time scale (with 2~°Pb geochronology), and to construct a sediment budget for modern Amazon River sediments accumulating on the continental shelf. The time of formation for the anomalous relict muds on the shelf is investigated using 14C dating techniques. These data are integrated with other research to obtain a better understanding of modern sediment accumulation on the Amazon shelf. SETTING
Based on measurements made at Obidos (700 km upstream of the river mouth) the sediment discharge of the Amazon River is estimated to be 1.2 + 1 × 109 tons y-i (MEADE et al., 1985), and is composed of about 85-95% silt and clay-sized sediment (G1B~S, 1967; MEADE, 1985). Amazon River sediments discharged to the continental shelf are advected to the northwest under the combined influence of surface waves, tidal currents, and the North Brazilian Coastal Current (NBCC) (NITTROUER et al., This Volume). Surface waves have an average height of about 1.5 m, and result from easterly trade winds (MESERVE, 1974). The tidal range for the area is as great as 6 m (NOAA, 1982), with associated current speeds of 200 cm s-1 (about 2 m above the seabed) near the river mouth (CURTIS, 1985; NITTROUERet al., This Volume). The NBCC is a branch of the South Equatorial Current, and moves northwestward with surface speeds of 35-75 cm s-I (CocttRANE, 1963). The distribution of surface (0-2 cm depth in seabed) grain size on the Amazon shelf is described in detail by NITrROUER et al. (1983). The inner shelf (<70 m water depth) is primarily clayey silt or silty clay, with some sandy areas exposed near the river mouth. Significant downcore variability (cm scale) in grain size occurs near the river mouth, and is reflected in a patchy distribution of surface grain size. The outer shelf (70-100 m water depth) is mantled with fine sand, which represents a relict transgressive sand layer (MILLIMAN et al., 1975; GIBBS, 1976; NITI'ROUERet al., 1983). High-resolution seismic-reflection profiles (3.5 kHz) reveal that the inner-shelf mud deposit (<70 m water depth) is a subaqueous delta (FIGUEIREDO e t al., 1972; NITTROUER et al., 1986). The delta contains topset, foreset, and bottomset strata characteristic of 'classic' river deltas. However, a major difference exists in that the topset beds for the Amazon delta are subaqueous. The gradients for all areas of the delta are low (< 1°), and evidence of mass movement (such as occurs on the Mississippi delta; COLEMANet al., 1974) is not observed using side-scan sonar techniques (ADAMS et al., This Volume). METHODS
Most sediment cores discussed in the present paper were collected from the Amazon shelf during May-July 1983, the annual period of peak water discharge of the Amazon River. Piston cores, gravity cores, and box cores were collected from the R.V. lselin at 77 stations on the subaqueous delta. A long, large-diameter gravity corer (3 m long, 12.7 × 12.7 cm cross section) was used to collect most samples for accumulation rate studies
Sediment accumulationon the Amazoncontinentalshelf
21 1
(KUEHL et al., 1985). Sediment samples, 5-cm vertical thickness, were taken from the gravity corer and homogenized for measurements of porosity, grain size, 2mpb, 226Ra, and 137Cs. Standard coring operations were restricted to >5 m water depth because of ship limitations. An inflatable boat (from which small gravity cores were taken) was used to extend two cross-shelf transects to the shoreline. Piston cores from a previous cruise (1979) were used for 14C age determinations. Sediment accumulation rates were determined using 21°pb geochronology, and verified by measurements of 137Cs. 2u~pb is a naturally occurring radionuclide (22.3-y half life) which can be used to investigate sedimentary processes occurring over a century time scale. 137Cs is a bomb-produced radionuclide which has been present in the marine environment for about 30 years, and can be used to determine the relative effects of sediment mixing and sediment accumulation on profiles of 2u~Pb. For a detailed description of the general approach used to resolve mixing and accumulation from profiles of 21°pb and 137Cssee NITTROUERet al. (1984) and DEMASTER et al. (1985). 2mpb activities were measured using the technique described by NITTROUER et al. (1979). 226Ra and 137Cs activities were determined by measuring gamma radiation emitted from wet sediment samples. Sediment samples were packed in 125-ml Petri dishes which were sealed with electrical tape prior to gamma counting. The samples were counted using an intrinsic germanium detector coupled to a multichannel analyzer. 22t~Ra activities were determined indirectly by measuring the 609-keV gamma peak of 214Bi, a daughter of 226Ra. 137Cs activities were measured directly by counting its 662keV gamma peak. After counting, sediment samples were dried, and activities are expressed relative to dry weight. HC ages were determined from the organic carbon fraction of piston-core samples using the general technique of NOAKESet al. (1965). Frozen core sections were returned to the laboratory and dried at 60°C. The samples were pretreated with 0.2 N HCI (at 25°C) to remove CaCO3. About 400 g of sample were needed because of the low organiccarbon content (~0.7%) of the sediments (MILLIMAN, 1975; BERNER, 1982; AI.LER and ALLER, This Volume; SttOWERSand ANOLE, This Volume). The samples were combusted to form CO2, which was reacted with molten lithium metal to form lithium carbide. Deionized water was added to the lithium carbide producing acetylene, which was then trimerized to benzene using an activated chromium catalyst. The UC activity was measured by adding 1 ml of scintillation cocktail (0.4% POP in benzene) to the synthesized benzene in a teflon scintillation vial, and counting the photons emitted using a coincidence scintillation counter. The counter was calibrated with benzene synthesized from an NBS oxalic acid standard. Grain-size measurements were made on core samples using a SediGraph 5000ET grain-size analyzer (manufactured by Micromeritics Inc.) for the silt and clay fraction, and standard sieves for the sand fraction. Sediment was dispersed with sodium metaphosphate and by placing samples in an ultrasonic bath before the analysis. The SediGraph measures equivalent grain size on the basis of settling velocity and hence is comparable to standard pipette analyses. RESULTS 2u~pb geochronology Profiles of 2H~pb in the seabed were measured from 57 cores collected during the 1983 cruise; data tabulated in KUEnL (1985). 2u~pb activities were corrected for the amount of
212
S.A. KUEHLet al. Table l.
Comparison of excess 21°pb activities (dpm g 1) measured using total- and partial-dissolution techniques
Core No.
Total dissolution (T)
Partial dissolution (P)
Difference ( T - P)
170
2.64 + 0.31 1.48 + 0.19 1.44 + /I.23
2.64 _+ 0.22 1.42 + 0.16 1.68 + 0.19
0.00 _+ I).38 0.06 _+ [).25 -0.24 + 11.31t
171
6.64 1.99 1.48 1.34 1.28
__+0.50 + 0.23 + 0.24 + 0.25 + 0.27
6.46 1.97 1.16 1.33 1.11
__+/I.31 + 0.211 + 0.211 + 11.19 __+0.11
175
5.36 5.22 4.47 2.82 1.80
+ 0.42 __+0.36 __+0.33 + 0.26 __+ 0.31
4.86 4.70 4.82 2.61 2.09
__+0.25 + 0.34 __+0.31 + 0.19 __+0.19
11.18 + 1/.59 0.02 __+ 0.30 0.32 __+0.31 0.01 + 0.31 0.17 + 0.29 0.50 0.52 -0.35 /I.21 -0.29
+ 0.49 + 0.5[) __+ 0.45 __+tl.32 + 0.36
salt in pore water, and profiles were normalized to a representative porosity of 75%. Excess 2~°pb activity (used to determine accumulation rates) was calculated by subtracting 226Ra-supported 2t°pb activity from the total zmPb activity. The supported value was estimated from 2~°Pb activities measured deep in sediment cores, where excess activity had decayed to negligible levels. This approach is reasonable for Amazon shelf sediments, because 226Ra activities are generally uniform in sediment cores (e.g. Fig. 3). A partial-dissolution technique (NITTROUER et al., 1979) was used to leach sediments, because more sample could be used (thus reducing counting time) relative to total dissolution techniques. Total dissolution (using HCI, HNO3, HCIO4, and HF) was performed on 13 samples from three cores to evaluate the partial dissolution technique. Excess 2mpb activities were calculated for the total-dissolution samples using 226Ra activities (for supported 2~°pb) measured for each of the samples. Significant differences were not observed between excess 2mpb activities determined in the total-dissolution experiment and excess activities determined using the partial-dissolution technique (Table 1). Characteristic 2mpb profiles Moderate accumulation. A common profile of 2mpb from the Amazon shelf is shown in Fig. 1. The profile contains three distinct zones including: (1) a zone of nearly uniform excess 2~°pb activity near the surface, (2) a zone of logarithmic decrease in excess 2"lPb activity, and (3) a zone of constant, low activity (226Ra-supported). The profile is similar to those from other shelf environments [e.g. the Washington continental shelf (NtTTROUERet al., 1979)], with the major exception that the thickness of the uniform layer for the Amazon shelf (-100 cm) is much greater than for other shelves ( - 1 0 cm~ NVrrROUER et al., 1979). The uniform layer of excess 21°pb activity in near-surface sediments is commonly a result of intense reworking by physical and/or biological processes (see Discussion). The accumulation rate (0.7 cm 3;-I, 0.5 gcm-: y-i) is calculated from the slope of the excess 2mpb profile in the zone of logarithmic decrease. The depth of L~TCs penetration below the surface zone ( - 2 0 cm) indicates that accumulation (and not mixing) controls the slope of the excess 2~°pb profile in the zone of logarithmic decrease
Sediment accumulation on the Amazon continental shelf
STATION Pb-210
Activity
03
(dpm/g)
10
i Accumulation 50
0.7
cm/yr
0.5
g/cm2y
118 Cs-137
1.0 n
Rate
I
Activity
.01 0
o
0.1
1,0
I
I
50-
o 100
,no.
a
o
(dpm/g)
1.1 n rn
==
213
___-__'___
._
~,
_
n0n-detectable
L)
c
150
~
150-
o n
4::
== 200
E3
0
200 " Q
250
o Io ÷
300
Total
Activity
I 250
Excess Activity 300
Fig. 1. Seabed profiles of 2l°Pb and 137Cs from a gravity core collected in an area of moderate sediment accumulation rate. The 21°pb profile displays three characteristic zones, including: (a) a near-surface zone of uniform 21°pb activity, (b) a zone where excess 2"~pb decreases logarithmi(" cally with depth, and (c) a zone of low, constant (22'Ra-supported) 2IOPb activity. The accumulation rate is calculated from the slope of the excess 2]°Pb profile in the zone of logarithmic decrease. Penetration depth of ~37Cs into the zone of logarithmic decrease confirms the calculated accumulation rate.
(Fig. 1). In this case, 137Cs penetration depth is the product of the number of years since initial input of 137Cs and the sediment accumulation rate (i.e. -3(1 y x 0.7 cm y ~ = - 2 1 cm). If diffusive mixing controls the slope of the excess 21°pb profile in the zone of logarithmic decrease, then 137Cspenetration would be deeper, and the accumulation rate calculated from the slope of the excess 2~°pb profile would be an upper limit. Rapid accumulation. In the outer topset and foreset regions north of the river mouth, accumulation rate is so rapid that even the 3-m gravity cores were too short to retrieve complete profiles of excess 21°pb. In this area, a piston core was used to determine an accurate accumulation rate (Fig. 2). Radiographs of the piston core display horizontal laminae (0.1 mm thick silty layers), which demonstrate stratigraphic integrity of the core. The penetration depth of 137Cs (130 cm) is less than would be expected ( - 3 0 0 cm) for the observed accumulation rate of 10.3 cm y-l (7.1 g c m -2 y-~), probably because piston cores commonly do not recover the upper portion of the seabed. This is supported by the fact that penetration depth of 137Cs from a gravity core obtained at the same station is >235 cm. Steady-state assumptions used in 2~°Pb geochronology apparently are not valid for some areas of the Amazon shelf. Some cores collected near the foreset region ( - 5 0 m water depth) reveal excess 21°pb activities which fluctuate throughout the 3-m gravity cores, indicating that the initial specific activity (disintegrations per minute per gram) of sediment reaching the surface of the seabed varies with time (Fig. 3). Preservation of these fluctuations over a small (<5 cm) depth scale indicates that relatively little
214
S . A . KUEHL et al.
STATION 176 Pb-210
Activity
0.1
(dpm/g)
Cs-157 I0 J
I.O
0
i
.01
7.1g/craZy
200-
.._~
I00-
n
200-
(dpm/g)
0.1 i
0
Accumulation Rate T--,- % 10.3 cm/y ~ ~
I00-
Activity
IO J
L I'"t--'-4--
r - ~ - n o n - detectab le
u 300-
300-
400-
400-
500-
500-
600-
600-
o
0
.c
700-
i
800_
700-
Total Activity Excess Activity
J
800
Fig. 2. Seabed profiles of 2mPb and 137Cs from a piston core collected in an area of rapid sediment accumulation. The penetration depth of ~37Cs(130 cm) is less than expected (-300 cm) from the 10.3 cm y i accumulation rate, because the corer probably failed to retrieve the upper portion of the seabed.
sediment reworking occurs. Intense reworking of the near-surface sediment would homogenize 2t°pb activities, resulting in a layer of uniform excess activity. Negligible accumulation. Excess 2t°pb is absent from the seabed in one area of the subaqueous delta in the northwestern portion of the study area (Fig. 4), indicating negligible sediment accumulation over a 100-y time scale. STATION 174 Pb-210
Activity I0
(dpm/g)
Cs- 137 Activity I00 r
.01 0
(dpm/9)
0,I
LO
I
,
Ro-226
0
OI
Activity I0
(dpm/g) I0
i
i
Mean Groin Size ( ~ )
0
-I
' i
3i
5i
17 ~ll •
j 1
5O
502
~
50-
*
50-
I00
I00
I00-
100
._~ 150
150
150"
150
2O0
200"
200
250
250"
250
300
500-
300
®
£ 200
250
300
I
+ Excess Activity J
Fig. 3. Seabed profiles of 2mpb, ~TCs, -~-~Ra, and grain size from a gravity core collected in an area of non-steady-state accumulation. Excess 2mPb activity fluctuates throughout the 3-m core as a result of temporal variability in the amount of -~mPb scavenged from oceanic waters.
Ii'
Ii~
Sediment accumulation on the Amazon continental shelf
2 15
STATION 184 Pb-210 0.1
(dpm/g)
1.0
Negligible Accumulation Rate
50
Cs-137 I0
I o
0
E
Activity
J
o
.01 0
(dpm/g)
0.1
1.0
i
i
non - detectable
50
I00
I00
0
150
150
~.
200
o
|
Activity
o
¢D
200
o Total
Activity
250
250
300
300
Fig. 4. Seabed profiles of 2mpb and 137Csfrom a gravity core collected in the area of negligible accumulation. The 2~°pb activities are supported by 226Ra in the sediment. The absence of excess 2'~Pb and of t37Cs in the seabed indicates that the sediment is relict (i.e. deposited >IIX) y ago).
STATION 180 Pb-210 o.t
Activity
(dpm/g)
1.0 I
Cs-137 I0 I
0
.01 0
Activity O. I l
(dpm/g) 1.0 I
0
Low
Accumulation Rate
[]
50"
50-
0 0
o
0 0
I00"
I00
0
,3
0
150
.E
150-
=----I--
0 0
al=
g
.--=1..--
0
z0o
0
200-
0 0
250.
300
250-
I°
Total
Activity 300
Fig. 5, Seabed profiles of 21°pb and 137Csfrom a gravity core collected in the area of slow accumulation/intense reworking. 137Cs and uniform levels of excess 2"~Pb activity (above 22~'Rasupported levels of ~1 dpm g ~) penetrate to significant depth in the seabed, where the -7°Pb activities abruptly drop to supported levels. The absence of a significant zone of logarithmic decrease in excess 2"~pb activity indicates that steady-state accumulation in this area is slow. However, the presence of 137Cs and excess 21'Pb indicates that the surficial sediment is modern.
216
S. A KUEHLet al.
Slow accumulation~intense reworking. In many cores collected from shallow water (< 15 m), uniform excess 2~°pb activity was observed to some depth (e.g. 150 cm at Sta. 180) where the activities abruptly drop to supported values (Fig. 5). In most cases, the 2t°pb activities drop to supported values over a vertical distance of <10 cm which indicates that the steady-state accumulation rate (on 100-y time scale) is <0.1 cm y-1 52oW
51 °
50 °
49 °
47 °
48 °
46 °
I
50
~ ~ ; O
SEDIMENT ACCUMULATION RATE (CM/Y)
4° .
• .°
3° -
N
°'
l
2° .
1o .
0.1
0
20
/ 60
0,1
1
• 2
tO0
i s o b a t h s in m e t e r s
O
Amazon
0o . •
!!'" 1° .
,
, •
Marajd
\
.
Island
::... •
".::: .~
"' Bele'm .L-'.':.. :
Fig. 6. Distribution of accumulation rate based on 2HJPb profiles from 57 cores (solid circles). The highest accumulation rates occur along a region between about the 30- and 50-m isobaths. The seaward extent of modern accumulation is defined, for most of the shelf, by the transition offshore from m u d to relict transgressive sand. Locations of non-steady-state cores (squares) and of piston cores used for ~4C analyses (stars) are shown• The dashed line shows the location of the across-shelf transect for Fig. 8.
Sediment accumulation on the A m a z o n continental shelf
217
Table 2. HC agesfor two piston cores Depth in core (cm) HC age (y)
Modern area PC 79-10
0-30 270-300
2630 + 210 31130+ 240
Relict area PC79-54
0-30 270-300
2700 + 210 3800 + 260
(0.1 g cm -2 y-l). Because excess 21°pb is observed to significant depth in cores, sediments in the surface zone are being reworked (see Discussion). Distribution o f accumulation rate
The distribution of sediment accumulation rate on the Amazon shelf is shown in Fig. 6. The highest rates ( - 1 0 cm y-~, 6.9 g cm -2 y-l) are observed in the outer topset and foreset regions (30-50 m water depth) north of the river mouth. Accumulation rates generally decrease toward shore. Low sediment accumulation rates (<1 cm y-~, 0.7 g cm -2 y-l) are observed in the bottomset region. The area of relict muds described by KVEHLet al. (1982) at about 4°N, was again observed during the 1983 cruise. This area is characterized by overconsolidated mud which contains only supported levels of 2t°pb. t4C ages of relict muds t4C ages (Table 2) were determined for two piston cores from the topset region of the subaqueous delta: (1) from an area of modern sediment accumulation near the river mouth, and (2) from the area of relict (100-y time scale) sediments at - 4 ° N (see Fig. 6 for locations). No significant difference exists between the ~4C ages for the top and bottom of the core obtained near the river mouth. This observation is reasonable because the 2t°pb accumulation rate (1.0 cm y-l, 0.7 g cm -2 y-J) predicts a difference of only 270 y between the top and bottom of the core, a difference which could not be resolved because of analytical limitations. The old 14C age observed at the surface of the piston core probably indicates that the carbon, which is primarily terrestrial material (SHOWERS and ANGLE, This Volume), is stored for a significant time in the terrestrial environment before being dispersed to the shelf. The ~4C age for the piston-core bottom in the area of relict muds is only about 1000 y older than the ages near the river mouth.
DISCUSSION
Areas o f negligible accumulation Physically reworked layer. Slow sediment accumulation (over a 100-y time scale) occurs in water depths less than about 15 m. Shallow-water areas commonly reveal uniform excess 21°Pb activities to significant depth in the seabed, below which excess activities abruptly decrease to background values (Fig. 5). A common explanation for uniform levels of 21°Pb activity near the surface of the seabed is that intense biological and/or physical mixing homogenizes sediments. Intense biological mixing cannot e~plain profiles observed on the inner Amazon shelf, because radiographic examination of cores from this area reveals interlaminated sediments with relatively little evidence of bioturbation (KUEHL et al., This Volume), and because biological sampling of this area reveals a
218
s . A . KUEHL et al.
52oW 6°
50°
51°
5° t ~"°°,~
(CM) OF
480 = . . . . . .
REWORKED
47°
46°
I
LAYER
:. •
3°
/
I
THICKNESS
"°1.
1° 1
49°
I
,,, I
0 20 i$Obaths
.iI.°,
60
100
in m e t e r s
0o
1o,
": """ " ' E
Marajd
Island
r!,' , •.
2°S , Fig. 7. Distribution and thickness of the reworked layer on the Amazon subaqueous delta based on -~°Pb profiles from 57 cores (solid circles). The layer reaches a maximum thickness in a region centered about the 15-m isobath. Sediment in this layer is reworked by waves and currents, and most is transported ultimately to other parts of the dispersal system.
Sediment accumulation on the Amazon continental shelf
219
virtual absence of macrofauna (ALLERand ALLER,This Volume). Therefore, frequent physical disturbance to significant depth in the seabed is probably responsible for observed 2~°pb profiles. The dominant physical processes operating on the inner shelf which could rework surface sedin~nts are surface waves and tidal currents (NITrROUERet al., This Volume). Sediments deposited in shallow water probably are resuspended and transported to other portions of the dispersal system. The physically reworked layer attains a maximum thickness in water depths of - 1 5 m, and thins both in seaward and landward directions (Fig. 7). The seaward thinning probably results from the reduced influence of waves and tides (in greater water depths) in reworking the upper portion of the seabed. A possible explanation for the reduced thickness nearshore is that wave and current activity are so intense that sediment remains in suspension, and hence a thick reworked layer has no opportunity to develop. Sediment cores used to determine the reworked-layer thickness were collected soon after peak sediment discharge of the Amazon River. Therefore, the thicknesses reported in this paper may represent maximum values. A thick layer of sediment deposited during high discharge could be progressively removed by physical processes during periods of reduced sediment supply. Additional observations during low discharge are needed to test this hypothesis. Relict area. A modern physically reworked layer is absent from the area of relict muds. Surface sediment is overconsolidated and contains no excess 21°pb, thus indicating that negligible accumulation (or deposition) occurs. The possibility that these areas formed during lower sea-level conditions, as suggested by MILLIMAN et al. (1975), can be evaluated from the 14C data. The data demonstrate that the relict area probably formed <1000 y ago (during sea-level conditions similar to those today). The absence of modern accumulation in this area is not well understood. Apparently, the sheet of modern sediment accumulating in the proximal portion of the dispersal system breaks up into discrete, shore-attached mud-shoals which move northwestward along the dispersal system (ALLERSMA,1971; WELLS and COLEMAN,1978; RINE, 1980). The break-up of the mud sheet might result from decreased sediment supply because of accumulation nearer to the source. A c r o s s - s h e l f trends in sediment accumulation rate
The dominant trend in accumulation rate on the Amazon subaqueous delta is in the across-shelf direction (Fig. 8). Accumulation rates increase seaward from low values nearshore, to rates as high as - 1 0 cm y-i (6.9 g cm -z y-l) on the seaward portion of the topset beds and on the foreset beds. Accumulation rates are relatively low (<1 cm y-~, 0.7 g cm 2 y-~) on the bottomset beds. This across-shelf trend in accumulation can be explained in terms of the simple model of MCCAvE (1972): the locus of sediment accumulation is a function of the relative rates of sediment supply and removal. Nearshore, high suspended-sediment concentrations (>500 mg 1-~) (GraBS, 1976; NITTROUER et al., This Volume) indicate a high potential for sediment accumulation. gtowever, this accumulation does not occur because shoaling surface waves and strong tidal currents resuspend any sediment which is temporarily deposited. In the outer topset and the foreset regions, lower suspended-sediment concentrations ( - 1 0 mgl ~) are observed. However, resuspension by waves and currents is less effective in these regions, and the net flux of sediment to the se'abed is much greater. An important observation is that the sediment accumulation rate is not directly correlated to suspended-sediment concentration (an often made assumption). Information regarding both suspended-
220
s . A . KUEHL et al.
0'
Topset
O.
Fo
50•
•
.
.
- . , , . . .
Relict Transgressive Sands
10o.
a
,o~
b o
2o
40
60
80
100
120
1,io
Distance From Shore (km)
Fig. 8. A cross-shelf transect showing relationship between (a) deltaic stratigraphy (schematic), and (b) sediment accumulation rate, from an area northwest of the river mouth. Sediment accumulation rate increases from low values nearshore, to values as high as - 1 0 cm y ~ near the outer topset and foreset regions. Accumulation rates are low in the bottomset region and are negligible in the relict sand area on the outer shelf.
sediment concentration (supply) and physical resuspension activity (removal) is needed to predict sediment accumulation rates. Seaward of the foreset region, the sediment supply is reduced, and accumulation rates are low on the bottomset beds and are negligible on the outer shelf (70-100 m water depth) where relict sands are exposed. The NBCC impinges upon the outer shelf and may be partly responsible for the absence of modern Amazon River muds. The limited subaerial expression of the Amazon delta (NITTROUER e t al., 1986) can be explained from the observed across-shelf trend in accumulation rate. The outer topset and the foreset regions are rapidly accreting upward and seaward. However, as the topset surface approaches sea level, upward accretion becomes negligible (because of the increasing effect of resuspension) and the surface remains submerged. High accumulation rates in the foreset region indicate that the subaqueous delta is prograding toward the shelfbreak. Non-steady-state accumulation Strong downcore variability in excess 21°Pb activity (Fig. 3) is observed for some stations near the foreset region (Fig. 6). Cores from these stations are characterized by relatively high 2t°pb activities which fluctuate downcore. Excess 2H~pb activities are as much as five times higher than typical activities observed nearshore. DEMASTER et al. (This Volume) demonstrate that the offshore gradient in specific 21°pb activity (at the surface of the seabed) results from an oceanic supply of dissolved 21°Pb. Nearshore, little dissolved 21°pb is available to particles, and the particles are deposited with a lower specific activity. Particles which are transported farther offshore become mixed with oceanic waters (which are rich in dissolved 21°pb), and scavenge additional 2~°Pb before being deposited.
Sediment accumulation on the Amazon continental shelf
221
The temporal variation in initial 2H~pb activity which is supplied to this area (reflected in downcore variability) has three likely explanations. The first explanation assumes that all the particles have the same affinity for 21°pb, and that the residence time of particles in suspension fluctuates through time. Then, if the particles are not saturated with respect to 21°pb, the specific activity of particles will depend on the residence time of particles in the 21°pb-rich waters. Frontal mixing of the Amazon River plume is likely to vary with seasonal changes in discharge. Seasonal or longer-term fluctuations in equatorial circulation (i.e. the NBCC) may also be important in controlling the residence time of particles in 21°pb-rich waters. The second explanation for observed downcore variability in excess 21°pb assumes that the residence time of particles in suspension is constant (at one locale), but that the nature of particles and their affinity for 21°pb varies through time. Although extensive downcore variability in grain size is not observed (Fig. 3), field observations of these cores revealed thin black laminations of apparently high organicmatter content. Because organic components are important scavenging agents (DAVIS, 1984), temporal fluctuations in these components may be responsible for the observed fluctuations in specific 21°pb activity. Changes in inorganic components important to scavenging (e.g. Fe/Mn oxides; CARPENTERet al., 1981) also may be responsible for the fluctuations in 21°pb activity. The third possible explanation for these fluctuations is related to changes in suspended-sediment concentration. High concentrations would deplete oceanic waters of dissolved 21°pb, resulting in lower specific activity of sediment reaching the seabed. Seasonal changes in discharge or in circulation could drive associated changes in suspended-sediment concentration. More detailed studies are needed to resolve the effects of residence time, scavenging efficiency, and suspendedsediment concentration on temporal fluctuations in specific 21°pb activity. Estimates of sediment accumulation rate in the non-steady-state area are difficult because assumptions used in 21°pb geochronology (i.e. uniform specific activity supplied to the zone of logarithmic decrease) are not valid. However, because excess 21°pb is observed throughout the 3-m gravity cores (i.e. all sediment younger than 100 y) and because deep physical and biological mixing are not observed, accumulation rates in this area are probably high (>3 cm y-J, 2.1 g cm -2 y-J). If the downcore fluctuations in 2J°Pb activity are controlled by seasonal variation in river discharge, then each cycle would represent 1 y of sediment accumulation. This would yield an accumulation rate of - 2 5 cm y-l (17.2 g cm -2 y-~) for Sta. 174 (Fig. 3). This is not unreasonable, because the non-steady-state cores are from areas of generally high accumulation rate ( - 1 0 cm y-J, 6.9 g cm -2 y-J). Until a better understanding for the cause of downcore variability is achieved, the 25 cm y-~ estimate is hypothetical. The absence of significant downcore variability in excess 2H~pb in cores taken from the inner shelf does not necessarily indicate that specific activities of 21°pb initially reaching the seabed are constant. For the inner shelf, the seabed is frequently reworked by physical processes which would tend to destroy evidence of variations in initial 2~°Pb activity by homogenizing near-surface 210pb profiles, resulting in a constant level of 2lOpb activity being supplied to the preserved zone below.
Sediment budget A sediment budget for the Amazon shelf is constructed from the distribution of 21°pb sediment accumulation rates (Fig. 6). The budget includes the continental shelf from the Para River to the area of relict muds at about 4°N. For budget calculations, the measured
222
S.A. KUEHLet al.
area between each contour line and the midpoint value of accumulation rate is used to estimate the mass of sediment accumulating in each of the areas. The resulting sediment budget equals 6.3 x l0 s tons y-1. In defining an error estimate for this budget, many sources of uncertainty must be considered, including: (1) the error associated with individual accumulation rate measurements, (2) the control available in contouring distribution of accumulation rates, and (3) the uncertainty of accumulation rates in the non-steady-state area. The error of individual accumulation rate measurements can be estimated from the standard error of the slopes calculated from 2~°pb profiles. The standard error in slopes results in about a 30% uncertainty in accumulation rate (average uncertainty for all profiles), and an uncertainty of 2.0 × l0 s tons y-~ in the overall sediment budget. Possible errors associated with limited control (station coverage) are difficult to quantify. Because of ship limitations, no coverage of shallow areas (<5 m) was possible. The shoal area just north of the river mouth (Fig. 6) is especially problematic because of the shoal's large areal extent. However, an important observation from this study is the absence of significant accumulation of sediment in shallow areas (because of resuspension and lateral transport). Therefore, the unsampled shallow areas probably contribute little to the overall sediment budget. Another source of error for the shelf budget results from uncertainty of accumulation rate for the non-steady-state area. The accumulation rate in this area may range from 3 to 25 cm y I (2.1-17.2 g cm -2 y-~); however, because of the limited areal extent, the effect on the budget is probably small. Based on the sediment budget, and errors associated with accumulation rate measurements, 6.3 _+ 2.0 x l0 s tons y-I of Amazon River sediment are estimated to be accumulating on the adjacent continental shelf. This budget leaves about 30-60% of the total Amazon River sediment discharge ( - 1 2 x l 0 s tons y-~; MEADE et a l . , 1985) unaccounted. The sediment discharge measurements for the Amazon River were made at Obidos (700 km upstream of the river mouth). Annual flooding of low-lying wetlands and associated flood plain accretion might result in reduced sediment supply to the ocean. Although the magnitude of sediment loss as a result of flood plain accretion is not known, the estimate of - 1 2 × l0 s tons y-~ probably is an upper limit for discharge to the Atlantic Ocean. Of the sediment supplied to the continental shelf, loss to the southeast probably is negligible because sediment transport is consistently toward the northwest, and because relict sands are observed on the shelf to the southeast. Although modern Amazon River sediments are not observed on the outer shelf (70-100 m water depth), sediment could be bypassing the outer shelf and accumulating on the adjacent continental slope. An estimate of potential sediment loss to the slope can be made from the thickness of the Holocene sediment (--1 m; DAMUTH and KUMAR, 1975), and from the area of the slope (100-2000 m water depth). Assuming the Holocene layer is entirely Amazon River sediment with a porosity of 75%, loss to the slope would account for < 5 % of Amazon River discharge. Holocene sediment on the slope is largely a foraminiferal ooze (DAMUTH and KUMAR, 1975), therefore, little, if any, Amazon sediment is presently accumulating on the slope. ALLERSMA(1971) estimates that 1-2 × 10s tons y-I of Amazon sediment are moving northwestward along the coast of the Guianas. Most of the Amazon sediment which is not accumulating in the study area probably is bypassed to the northwest, and, according to NITrROUER et al. (This Volume), most is transported landward of the 10-m isobath. Some sediment may be trapped in mangrove swamps along the shoreline: however, no estimate of shoreline accretion for northern Brazil has been reported yet.
Sediment accumulation on the Amazon continental shelf
223
CONCLUSIONS
The major conclusions of this paper are summarized below. (1) Sediment accumulation rates determined using 2"~Pb geochronology reveal the nature of sediment accumulation on the Amazon subaqueous delta. Slow accumulation (<0.1 cm y-1 0.1 g c m - 2 y-l) occurs in water depths <--15 m, probably as a result of resuspension by shoaling surface waves and strong tidal currents. Accumulation rates progressively increase offshore (because of reduced resuspension), to a maximum of - 1 0 cm y-I (6.9 g c m - 2 y-I) in the outer topset and foreset regions. Farther seaward, accumulation rates are generally <1 cm y-~ (0.7 c m -2 y-l) in the bottomset region, probably as a result of reduced sediment supply. (2) The inner shelf (<30 m water depth) is mantled with a thick (as much as 2 m) layer of sediment with uniform e x c e s s 2 m P b activity, which is reworked probably by surface waves and tidal currents. This physically reworked layer reaches a maximum thickness in - 1 5 m water depth, and thins both in the seaward and landward directions. The seaward thinning could result from decreased influence of waves and tides. Nearshore, turbulence may be so great that sediment remains in suspension, and a reworked layer does not develop. (3) The limited subaerial expression of the Amazon delta is explained from the observed distribution of accumulation rates. Upward accretion of the subaqueous topset region is relatively slow in water depths less than about 15 m. Therefore, the topset region remains submerged as the delta progrades seaward. (4) 14C dating of an anomalous area of relict (100-y time scale) mud near the northern boundary of the study area reveals that the sediment accumulated <1000 y ago. The absence of modern sediments in this area is not well understood. (5) Non-steady-state input of 2"~Pb is identified at some stations in water depths of about 50 m. Dgwncore fluctuations in excess 2~°Pb activity could result from: (a) temporal changes of particle residence time in ~l°Pb-rich oceanic waters, (b) temporal changes in chemical reactivity of particles supplied to the area, or (c) temporal changes in suspended-sediment concentration. Further study is needed to resolve the mechanisms. (6) A sediment budget indicates that 6.3 _+ 2.0 x l0 s tons y-1 of sediment accumulate on the Amazon continental shelf. The remainder of sediment discharged to the shelf by the Amazon River probably is transported northwestward beyond the Brazilian shelf, and/or is accumulating landward of the shelf as coastal accretion. Acknowledgements--The authors thank the crew and scientists aboard the R.V. lselin, whose untiring dedication allowed collection of the voluminous sediment samples used in this study. This research was supported by the National Science Foundation (grant Nos OCE-7908496, OCE-8117709 and OCE-8415413).
REFERENCES ADAMSC. E., JR, J. T. WELLS and J. M. COLEMAN(This Volume) Transverse bedforms on the Amazon shelf. Continental Shelf Research, 6, 175-187. ALLER J. Y. and R. C. ALLER (This Volume) General characteristics of benthic faunas on the Amazon inner continental shelf with comparison to the shelf off the Changjiang River, East China Sea. Continental Shelf Research, 6,291-310. ALLERSMAE. (1971) Mud on the oceanic shelf off Guyana. In: Symposium on Investigations of Resources in the Caribbean Sea and Adjacent Regions, UNESCO, Paris, pp. 193-203.
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BERNER R. A. (1982) Burial of organic carbon and pyrite sulfur in the modern ocean: its geochemical and environmental significance. American Journal of Science, 282, 451-473. CARPENTER R., J. Z. BENNETT and M. L. PETERSON (1981) 2mPb activities in and fluxes to sediments of the Washington continental slope and shelf. Geochimica et Cosmochimica Acta, 45, 1155-1172. COCHRANE J. D. (1963) Equatorial undercurrent and related currents off Brazil in March and April 1963. Science, 142, 669-671. COLEMAN J. M., N. N. SUHAYDA,T. WHELANand L. D. WRIGHT (1974) Mass movement of Mississippi River delta sediments. Gulf Coast Association of Geological Societies Transactions, 24, 49-68. CURTIN T. B. (1985) Current measurements over the continental margin off the Amazon River: 29 May-5 July 1983. Department of Marine, Earth and Atmospheric Sciences, Technical Report No. 5193, North Carolina State University, Raleigh, NC, 290 pp. DAMUTH J. E. and N. KUMAR (1975) Amazon cone: morphology, sediments, age, and growth pattern. Geological Society of America Bulletin, 86,863-878. DAVIS J. A. (1984) Complexation of trace metals by adsorbed natural organic matter. Geochimica et
Cosmochimica Acta, 48,679-692. DEMASTER D. J., B. A. MCKEE, C. A. NITTROUER, J. QIAN and G. CHENG (1985) Rates of sediment accumulation and particle reworking based on radiochemical measurements from continental shelf deposits in the East China Sea. Continental Shelf Research, 4, 143-158. DEMASTER D. J., S. A. KUEttL and C. A. NITTROUER (This Volume) Effects of suspended sediments on geochemical processes near the mouth of the Amazon River: examination of biological silica uptake and the fate of particle-reactive elements. Continental She(f Research, 6, 1(17-125. FtGUEmEDO A. G., L. A. P. GAMBOA, M. GORINI and E. COSTA ALVES (1972) Natureza da sedimentacao atual do Rio Amazonas--testemunhos e geomorfologia submarina, [canyon) Amazonas testemunhos submarinos. Congresso Brasiliero de Geologia (26th), 2, 51-56. GraBs R. J. (1967) The geochemistry of the Amazon River system: part I. The factors that control the salinity and the composition and concentration of the suspended solids. Geological Society of America Bulletin, 78, 1203-1232. GIBBS R. J. (1976) Amazon River sediment transport in the Atlantic Ocean. Geology, 4, 45-48. KUEHL S. A. (1985) Sediment accumulation and the formation of sedimentary structure on the Amazon continental shelf. Ph.D. Thesis, North Carolina State University, 213 pp. KUEHL S. A., C. A. NITFROUER and D. J. DEMASTER (1982) Modern sediment accumulation and strata formation on the Amazon continental shelf. Marine Geology, 49, 279-300. KUEHL S. A., C. A. NITFROUER, D. J. DEMASTERand T. B. CURTIN (1985) A long, square-barrel gravity corer for sedimentological and geochemical investigation of fine-grained sediments. Marine Geology, 62, 365370. KUEItE S. A., C. A. NITFROUER and D. J. DEMASTER (This Volume) Distribution of sedimentary structures in the Amazon subaqueous delta. Continental She([ Research, 6, 3 l 1-336. LYEIA, C. (183(I) Principles" of geology, being an attempt to exphtin the Jormer changes of the Earth's surface bv reference to causes now in operation, Vol. l, John Murray, London, 511 pp. MCCAVE 1. N. (1972) Transport and escape of fine-grained sediment from shelf areas. In: Shelf sediment transport: process and pattern, D. J. P. Swll-q-, D. B. DUANE and O. H. PILKEY, editors, Dowden, Hutchinson, and Ross, Stroudsburg, Pennsylvania, pp. 225-248. MEADE R. H. (1985) Suspended sediment in the Amazon River and its tributaries in Brazil during 1982-84. U.S. Geological Survey Open-File Report85-492. MEADE R. H., T. DUNNE, J. E. RICHEY, U. DE M. SANTOSand E. SALATI(1985) Storage and remobilization of suspended sediment in the lower Amazon River of Brazil. Science, 228, 488--49(I. MESERVE J. M. (1974) U.S. Navy rnarine climatic atlas of the world, Vol. 1, North Athmtic Ocean. United States Printing Office, Washington, DC, 371 pp. MIH.IMAN J. D. (1975) Projecto Remac, a synthesis. Contributions in Sedimentology, 4, 151-175. MILt,IMAN J. D., C. P. SUMMERHAYESand H. T. BARRETTO (1975) Quaternary sedimentation on the Amazon continental margin: a model. Geological Society of America Bulletin, 86, 610--614. NIq-FROUER C. A., R. W. STERNBERG, R. CARPENTER and J. T. BENNETI" (1979) The use of Pb-210 geochronology as a sedimentological tool: application to the Washington continental shelf. Marine Geology, 31,297-316. NITTROUER C. A., M. T. SHARARAand D. J. DEMASTER (1983) Variations of sediment texture on the Amazon continental shelf. Journal of Sedimentary Petrology, 53, 179-191. NITrROUER C. A., D. J. DEMASTER, B. A. MCKEE, N. H. CUTSHAti and I. L. LARSEN (1984) The effect of sediment mixing on Pb-210 accumulation rates for the Washington continental shelf. Marine Geology, 54, 2(11-221. NITTROUER C. A., T. B. CURTIN and D. J. DEMASTER (This Volume) Concentration and flux of suspended sediment on the Amazon continental shelf. Continental She!f Res'earch, 6, 151-174.
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NITTROUER C. A., S. A. KUEHL, D. J. DEMASTER and R. O. KOWSMAN(1986) The deltaic nature of Amazon shelf sedimentation. Geological Society of America Bulletin, 97, 444-458. NOAA (1982) Tide tables 1983, east coast of North and South America. United States Department of Commerce, Washington, DC, 285 pp. NOAKES J. E., S. M. KIM and J. J. ST1PP (1965) Chemical and counting advances in liquid scintillation age dating. Sixth International Conference on Radiocarbon and Tritium Dating, Pullman, Washington, pp. 68-92. RINE J. M. (1980) Depositional environments and Holocene reconstruction of an argillaceous mud belt - Suriname, South America. Ph.D. Thesis, University of Miami, 222 pp. SHOWERS W. J. and D. G. ANGLE (This Volume) Stable isotopic characterization of organic carbon accumulation on the Amazon continental shelf. Continental She!I Research, 6, 227-244. WELLS J. T. and-J. M. COLEMAN (1978) Longshore transport of mud by waves: northeastern coast of South America. Geologie en Mijnbouw, 57, 353-359.
027S~-343/~ $3,11n+ iI.(10 PergamonJournals I.td.
N a t u r e o f s e d i m e n t a c c u m u l a t i o n on the A m a z o n c o n t i n e n t a l s h e l f STEVEN A . K U E H L , * t DAVID J. D E M A S T E R * a n d CHARLES A . NITTROUER*
(Received for publication 13 December 1985) Abstract--Sediment accumulation on the Brazilian continental shelf near the Amazon River is investigated using radiochemical (e.g. 21~Pb, ~4C) techniques to provide a better understanding of this major dispersal system of fine-grained sediment. 2"~Pb profiles from 57 cores collected during 1983 reveal the distribution of modern (100-y time scale) accumulation rates on the Amazon subaqueous delta. Accumulation rates increase from <(J. l cm y t (I). l g c m 2 y t) nearshorc, to rates as high as ~10 cm y ~ (6.9 g c m 2 y ~) on the outer topset and the l~)reset regions (30-50 m water depth). Reduced upward accretion nearshore (<15 m water depth), which is reflected in the limited subaerial expression of the Amazon delta, probably results from the intense activity of surface waves and tidal currents. A thick (as much as 2 m) depth). This layer probably is reworked by waves and currents, and most of the sediment is eventually transported to other parts of the dispersal system. HC dating of an anomalous area of relict (age >100 y) sediment in the northwestern portion of the subaqueous delta indicates that this sediment was deposited <1000 y ago. The absence of modern sediment in this area is not understood. A sediment budget for the Amazon shelf indicates that 6.3 + 2.0 x l0 s tons of sediment accumulate annually. Much of the remainder of Amazon River sediment ( - 6 x 10~ tons y J) probably is transported northwestward beyond the Brazilian shelf and/or is accumulating landward of the shelf as coastal accretion.
INTRODUCTION
THE NATURE of continental shelf sedimentation off the Amazon River has been the subject of much controversy during the past decade. Several conflicting models have emerged in the evolution towards a better understanding of this major sediment dispersal system. MILLIMANel al. (1975) suggest that most of the inner-shelf mud deposit off the Amazon River is relict, having formed during periods of lower sea level by coastal accretion. GIBBS (1976) contends that the entire inner shelf is presently accumulating sediment. In an attempt to resolve this controversy, KUEHL et al. (1982) examine sediment accumulation rates on the Amazon shelf using 2tt)pb geochronology, and demonstrate that much of the inner shelf is undergoing active sediment accumulation; however, many questions remain. Only first-order estimates of accumulation rate could be made by KUEHL et al. (1982), because sediment cores (i.e. box cores) used for 21°pb geochronological studies were too short ( - 0 . 5 m) to obtain complete radionuclide profiles in the seabed. Therefore, little information regarding areal trends in accumulation rate on the Amazon shelf is available. The fact that some Amazon River sediment is transported northwestward beyond the Brazilian border has been recognized for a long time (LYELL, 1830). An accurate * Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, U.S.A. t Present address: Department of Geology, University of South Carolina, Columbia, SC 29208, U.S.A. 209
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S.A. KUEHLet al.
estimate of sediment accumulating on the Amazon shelf is needed to better estimate the magnitude of sediment bypassing. An anomalous area of relict sediment (on 100-y time scale) is observed in the northern part of the Amazon shelf (KVEHL et al., 1982). Questions remain concerning the time of formation for this relict area. The primary objectives of this study are to answer the questions outlined above. Sediment cores (3 m long) collected during a 1983 cruise to the study area are used to determine accurate estimates of accumulation rate on a 100-y time scale (with 2~°Pb geochronology), and to construct a sediment budget for modern Amazon River sediments accumulating on the continental shelf. The time of formation for the anomalous relict muds on the shelf is investigated using 14C dating techniques. These data are integrated with other research to obtain a better understanding of modern sediment accumulation on the Amazon shelf. SETTING
Based on measurements made at Obidos (700 km upstream of the river mouth) the sediment discharge of the Amazon River is estimated to be 1.2 + 1 × 109 tons y-i (MEADE et al., 1985), and is composed of about 85-95% silt and clay-sized sediment (G1B~S, 1967; MEADE, 1985). Amazon River sediments discharged to the continental shelf are advected to the northwest under the combined influence of surface waves, tidal currents, and the North Brazilian Coastal Current (NBCC) (NITTROUER et al., This Volume). Surface waves have an average height of about 1.5 m, and result from easterly trade winds (MESERVE, 1974). The tidal range for the area is as great as 6 m (NOAA, 1982), with associated current speeds of 200 cm s-1 (about 2 m above the seabed) near the river mouth (CURTIS, 1985; NITTROUERet al., This Volume). The NBCC is a branch of the South Equatorial Current, and moves northwestward with surface speeds of 35-75 cm s-I (CocttRANE, 1963). The distribution of surface (0-2 cm depth in seabed) grain size on the Amazon shelf is described in detail by NITrROUER et al. (1983). The inner shelf (<70 m water depth) is primarily clayey silt or silty clay, with some sandy areas exposed near the river mouth. Significant downcore variability (cm scale) in grain size occurs near the river mouth, and is reflected in a patchy distribution of surface grain size. The outer shelf (70-100 m water depth) is mantled with fine sand, which represents a relict transgressive sand layer (MILLIMAN et al., 1975; GIBBS, 1976; NITI'ROUERet al., 1983). High-resolution seismic-reflection profiles (3.5 kHz) reveal that the inner-shelf mud deposit (<70 m water depth) is a subaqueous delta (FIGUEIREDO e t al., 1972; NITTROUER et al., 1986). The delta contains topset, foreset, and bottomset strata characteristic of 'classic' river deltas. However, a major difference exists in that the topset beds for the Amazon delta are subaqueous. The gradients for all areas of the delta are low (< 1°), and evidence of mass movement (such as occurs on the Mississippi delta; COLEMANet al., 1974) is not observed using side-scan sonar techniques (ADAMS et al., This Volume). METHODS
Most sediment cores discussed in the present paper were collected from the Amazon shelf during May-July 1983, the annual period of peak water discharge of the Amazon River. Piston cores, gravity cores, and box cores were collected from the R.V. lselin at 77 stations on the subaqueous delta. A long, large-diameter gravity corer (3 m long, 12.7 × 12.7 cm cross section) was used to collect most samples for accumulation rate studies
Sediment accumulationon the Amazoncontinentalshelf
21 1
(KUEHL et al., 1985). Sediment samples, 5-cm vertical thickness, were taken from the gravity corer and homogenized for measurements of porosity, grain size, 2mpb, 226Ra, and 137Cs. Standard coring operations were restricted to >5 m water depth because of ship limitations. An inflatable boat (from which small gravity cores were taken) was used to extend two cross-shelf transects to the shoreline. Piston cores from a previous cruise (1979) were used for 14C age determinations. Sediment accumulation rates were determined using 21°pb geochronology, and verified by measurements of 137Cs. 2u~pb is a naturally occurring radionuclide (22.3-y half life) which can be used to investigate sedimentary processes occurring over a century time scale. 137Cs is a bomb-produced radionuclide which has been present in the marine environment for about 30 years, and can be used to determine the relative effects of sediment mixing and sediment accumulation on profiles of 2u~Pb. For a detailed description of the general approach used to resolve mixing and accumulation from profiles of 21°pb and 137Cssee NITTROUERet al. (1984) and DEMASTER et al. (1985). 2mpb activities were measured using the technique described by NITTROUER et al. (1979). 226Ra and 137Cs activities were determined by measuring gamma radiation emitted from wet sediment samples. Sediment samples were packed in 125-ml Petri dishes which were sealed with electrical tape prior to gamma counting. The samples were counted using an intrinsic germanium detector coupled to a multichannel analyzer. 22t~Ra activities were determined indirectly by measuring the 609-keV gamma peak of 214Bi, a daughter of 226Ra. 137Cs activities were measured directly by counting its 662keV gamma peak. After counting, sediment samples were dried, and activities are expressed relative to dry weight. HC ages were determined from the organic carbon fraction of piston-core samples using the general technique of NOAKESet al. (1965). Frozen core sections were returned to the laboratory and dried at 60°C. The samples were pretreated with 0.2 N HCI (at 25°C) to remove CaCO3. About 400 g of sample were needed because of the low organiccarbon content (~0.7%) of the sediments (MILLIMAN, 1975; BERNER, 1982; AI.LER and ALLER, This Volume; SttOWERSand ANOLE, This Volume). The samples were combusted to form CO2, which was reacted with molten lithium metal to form lithium carbide. Deionized water was added to the lithium carbide producing acetylene, which was then trimerized to benzene using an activated chromium catalyst. The UC activity was measured by adding 1 ml of scintillation cocktail (0.4% POP in benzene) to the synthesized benzene in a teflon scintillation vial, and counting the photons emitted using a coincidence scintillation counter. The counter was calibrated with benzene synthesized from an NBS oxalic acid standard. Grain-size measurements were made on core samples using a SediGraph 5000ET grain-size analyzer (manufactured by Micromeritics Inc.) for the silt and clay fraction, and standard sieves for the sand fraction. Sediment was dispersed with sodium metaphosphate and by placing samples in an ultrasonic bath before the analysis. The SediGraph measures equivalent grain size on the basis of settling velocity and hence is comparable to standard pipette analyses. RESULTS 2u~pb geochronology Profiles of 2H~pb in the seabed were measured from 57 cores collected during the 1983 cruise; data tabulated in KUEnL (1985). 2u~pb activities were corrected for the amount of
212
S.A. KUEHLet al. Table l.
Comparison of excess 21°pb activities (dpm g 1) measured using total- and partial-dissolution techniques
Core No.
Total dissolution (T)
Partial dissolution (P)
Difference ( T - P)
170
2.64 + 0.31 1.48 + 0.19 1.44 + /I.23
2.64 _+ 0.22 1.42 + 0.16 1.68 + 0.19
0.00 _+ I).38 0.06 _+ [).25 -0.24 + 11.31t
171
6.64 1.99 1.48 1.34 1.28
__+0.50 + 0.23 + 0.24 + 0.25 + 0.27
6.46 1.97 1.16 1.33 1.11
__+/I.31 + 0.211 + 0.211 + 11.19 __+0.11
175
5.36 5.22 4.47 2.82 1.80
+ 0.42 __+0.36 __+0.33 + 0.26 __+ 0.31
4.86 4.70 4.82 2.61 2.09
__+0.25 + 0.34 __+0.31 + 0.19 __+0.19
11.18 + 1/.59 0.02 __+ 0.30 0.32 __+0.31 0.01 + 0.31 0.17 + 0.29 0.50 0.52 -0.35 /I.21 -0.29
+ 0.49 + 0.5[) __+ 0.45 __+tl.32 + 0.36
salt in pore water, and profiles were normalized to a representative porosity of 75%. Excess 2~°pb activity (used to determine accumulation rates) was calculated by subtracting 226Ra-supported 2t°pb activity from the total zmPb activity. The supported value was estimated from 2~°Pb activities measured deep in sediment cores, where excess activity had decayed to negligible levels. This approach is reasonable for Amazon shelf sediments, because 226Ra activities are generally uniform in sediment cores (e.g. Fig. 3). A partial-dissolution technique (NITTROUER et al., 1979) was used to leach sediments, because more sample could be used (thus reducing counting time) relative to total dissolution techniques. Total dissolution (using HCI, HNO3, HCIO4, and HF) was performed on 13 samples from three cores to evaluate the partial dissolution technique. Excess 2mpb activities were calculated for the total-dissolution samples using 226Ra activities (for supported 2~°pb) measured for each of the samples. Significant differences were not observed between excess 2mpb activities determined in the total-dissolution experiment and excess activities determined using the partial-dissolution technique (Table 1). Characteristic 2mpb profiles Moderate accumulation. A common profile of 2mpb from the Amazon shelf is shown in Fig. 1. The profile contains three distinct zones including: (1) a zone of nearly uniform excess 2~°pb activity near the surface, (2) a zone of logarithmic decrease in excess 2"lPb activity, and (3) a zone of constant, low activity (226Ra-supported). The profile is similar to those from other shelf environments [e.g. the Washington continental shelf (NtTTROUERet al., 1979)], with the major exception that the thickness of the uniform layer for the Amazon shelf (-100 cm) is much greater than for other shelves ( - 1 0 cm~ NVrrROUER et al., 1979). The uniform layer of excess 21°pb activity in near-surface sediments is commonly a result of intense reworking by physical and/or biological processes (see Discussion). The accumulation rate (0.7 cm 3;-I, 0.5 gcm-: y-i) is calculated from the slope of the excess 2mpb profile in the zone of logarithmic decrease. The depth of L~TCs penetration below the surface zone ( - 2 0 cm) indicates that accumulation (and not mixing) controls the slope of the excess 2~°pb profile in the zone of logarithmic decrease
Sediment accumulation on the Amazon continental shelf
STATION Pb-210
Activity
03
(dpm/g)
10
i Accumulation 50
0.7
cm/yr
0.5
g/cm2y
118 Cs-137
1.0 n
Rate
I
Activity
.01 0
o
0.1
1,0
I
I
50-
o 100
,no.
a
o
(dpm/g)
1.1 n rn
==
213
___-__'___
._
~,
_
n0n-detectable
L)
c
150
~
150-
o n
4::
== 200
E3
0
200 " Q
250
o Io ÷
300
Total
Activity
I 250
Excess Activity 300
Fig. 1. Seabed profiles of 2l°Pb and 137Cs from a gravity core collected in an area of moderate sediment accumulation rate. The 21°pb profile displays three characteristic zones, including: (a) a near-surface zone of uniform 21°pb activity, (b) a zone where excess 2"~pb decreases logarithmi(" cally with depth, and (c) a zone of low, constant (22'Ra-supported) 2IOPb activity. The accumulation rate is calculated from the slope of the excess 2]°Pb profile in the zone of logarithmic decrease. Penetration depth of ~37Cs into the zone of logarithmic decrease confirms the calculated accumulation rate.
(Fig. 1). In this case, 137Cs penetration depth is the product of the number of years since initial input of 137Cs and the sediment accumulation rate (i.e. -3(1 y x 0.7 cm y ~ = - 2 1 cm). If diffusive mixing controls the slope of the excess 21°pb profile in the zone of logarithmic decrease, then 137Cspenetration would be deeper, and the accumulation rate calculated from the slope of the excess 2~°pb profile would be an upper limit. Rapid accumulation. In the outer topset and foreset regions north of the river mouth, accumulation rate is so rapid that even the 3-m gravity cores were too short to retrieve complete profiles of excess 21°pb. In this area, a piston core was used to determine an accurate accumulation rate (Fig. 2). Radiographs of the piston core display horizontal laminae (0.1 mm thick silty layers), which demonstrate stratigraphic integrity of the core. The penetration depth of 137Cs (130 cm) is less than would be expected ( - 3 0 0 cm) for the observed accumulation rate of 10.3 cm y-l (7.1 g c m -2 y-~), probably because piston cores commonly do not recover the upper portion of the seabed. This is supported by the fact that penetration depth of 137Cs from a gravity core obtained at the same station is >235 cm. Steady-state assumptions used in 2~°Pb geochronology apparently are not valid for some areas of the Amazon shelf. Some cores collected near the foreset region ( - 5 0 m water depth) reveal excess 21°pb activities which fluctuate throughout the 3-m gravity cores, indicating that the initial specific activity (disintegrations per minute per gram) of sediment reaching the surface of the seabed varies with time (Fig. 3). Preservation of these fluctuations over a small (<5 cm) depth scale indicates that relatively little
214
S . A . KUEHL et al.
STATION 176 Pb-210
Activity
0.1
(dpm/g)
Cs-157 I0 J
I.O
0
i
.01
7.1g/craZy
200-
.._~
I00-
n
200-
(dpm/g)
0.1 i
0
Accumulation Rate T--,- % 10.3 cm/y ~ ~
I00-
Activity
IO J
L I'"t--'-4--
r - ~ - n o n - detectab le
u 300-
300-
400-
400-
500-
500-
600-
600-
o
0
.c
700-
i
800_
700-
Total Activity Excess Activity
J
800
Fig. 2. Seabed profiles of 2mPb and 137Cs from a piston core collected in an area of rapid sediment accumulation. The penetration depth of ~37Cs(130 cm) is less than expected (-300 cm) from the 10.3 cm y i accumulation rate, because the corer probably failed to retrieve the upper portion of the seabed.
sediment reworking occurs. Intense reworking of the near-surface sediment would homogenize 2t°pb activities, resulting in a layer of uniform excess activity. Negligible accumulation. Excess 2t°pb is absent from the seabed in one area of the subaqueous delta in the northwestern portion of the study area (Fig. 4), indicating negligible sediment accumulation over a 100-y time scale. STATION 174 Pb-210
Activity I0
(dpm/g)
Cs- 137 Activity I00 r
.01 0
(dpm/9)
0,I
LO
I
,
Ro-226
0
OI
Activity I0
(dpm/g) I0
i
i
Mean Groin Size ( ~ )
0
-I
' i
3i
5i
17 ~ll •
j 1
5O
502
~
50-
*
50-
I00
I00
I00-
100
._~ 150
150
150"
150
2O0
200"
200
250
250"
250
300
500-
300
®
£ 200
250
300
I
+ Excess Activity J
Fig. 3. Seabed profiles of 2mpb, ~TCs, -~-~Ra, and grain size from a gravity core collected in an area of non-steady-state accumulation. Excess 2mPb activity fluctuates throughout the 3-m core as a result of temporal variability in the amount of -~mPb scavenged from oceanic waters.
Ii'
Ii~
Sediment accumulation on the Amazon continental shelf
2 15
STATION 184 Pb-210 0.1
(dpm/g)
1.0
Negligible Accumulation Rate
50
Cs-137 I0
I o
0
E
Activity
J
o
.01 0
(dpm/g)
0.1
1.0
i
i
non - detectable
50
I00
I00
0
150
150
~.
200
o
|
Activity
o
¢D
200
o Total
Activity
250
250
300
300
Fig. 4. Seabed profiles of 2mpb and 137Csfrom a gravity core collected in the area of negligible accumulation. The 2~°pb activities are supported by 226Ra in the sediment. The absence of excess 2'~Pb and of t37Cs in the seabed indicates that the sediment is relict (i.e. deposited >IIX) y ago).
STATION 180 Pb-210 o.t
Activity
(dpm/g)
1.0 I
Cs-137 I0 I
0
.01 0
Activity O. I l
(dpm/g) 1.0 I
0
Low
Accumulation Rate
[]
50"
50-
0 0
o
0 0
I00"
I00
0
,3
0
150
.E
150-
=----I--
0 0
al=
g
.--=1..--
0
z0o
0
200-
0 0
250.
300
250-
I°
Total
Activity 300
Fig. 5, Seabed profiles of 21°pb and 137Csfrom a gravity core collected in the area of slow accumulation/intense reworking. 137Cs and uniform levels of excess 2"~Pb activity (above 22~'Rasupported levels of ~1 dpm g ~) penetrate to significant depth in the seabed, where the -7°Pb activities abruptly drop to supported levels. The absence of a significant zone of logarithmic decrease in excess 2"~pb activity indicates that steady-state accumulation in this area is slow. However, the presence of 137Cs and excess 21'Pb indicates that the surficial sediment is modern.
216
S. A KUEHLet al.
Slow accumulation~intense reworking. In many cores collected from shallow water (< 15 m), uniform excess 2~°pb activity was observed to some depth (e.g. 150 cm at Sta. 180) where the activities abruptly drop to supported values (Fig. 5). In most cases, the 2t°pb activities drop to supported values over a vertical distance of <10 cm which indicates that the steady-state accumulation rate (on 100-y time scale) is <0.1 cm y-1 52oW
51 °
50 °
49 °
47 °
48 °
46 °
I
50
~ ~ ; O
SEDIMENT ACCUMULATION RATE (CM/Y)
4° .
• .°
3° -
N
°'
l
2° .
1o .
0.1
0
20
/ 60
0,1
1
• 2
tO0
i s o b a t h s in m e t e r s
O
Amazon
0o . •
!!'" 1° .
,
, •
Marajd
\
.
Island
::... •
".::: .~
"' Bele'm .L-'.':.. :
Fig. 6. Distribution of accumulation rate based on 2HJPb profiles from 57 cores (solid circles). The highest accumulation rates occur along a region between about the 30- and 50-m isobaths. The seaward extent of modern accumulation is defined, for most of the shelf, by the transition offshore from m u d to relict transgressive sand. Locations of non-steady-state cores (squares) and of piston cores used for ~4C analyses (stars) are shown• The dashed line shows the location of the across-shelf transect for Fig. 8.
Sediment accumulation on the A m a z o n continental shelf
217
Table 2. HC agesfor two piston cores Depth in core (cm) HC age (y)
Modern area PC 79-10
0-30 270-300
2630 + 210 31130+ 240
Relict area PC79-54
0-30 270-300
2700 + 210 3800 + 260
(0.1 g cm -2 y-l). Because excess 21°pb is observed to significant depth in cores, sediments in the surface zone are being reworked (see Discussion). Distribution o f accumulation rate
The distribution of sediment accumulation rate on the Amazon shelf is shown in Fig. 6. The highest rates ( - 1 0 cm y-~, 6.9 g cm -2 y-l) are observed in the outer topset and foreset regions (30-50 m water depth) north of the river mouth. Accumulation rates generally decrease toward shore. Low sediment accumulation rates (<1 cm y-~, 0.7 g cm -2 y-l) are observed in the bottomset region. The area of relict muds described by KVEHLet al. (1982) at about 4°N, was again observed during the 1983 cruise. This area is characterized by overconsolidated mud which contains only supported levels of 2t°pb. t4C ages of relict muds t4C ages (Table 2) were determined for two piston cores from the topset region of the subaqueous delta: (1) from an area of modern sediment accumulation near the river mouth, and (2) from the area of relict (100-y time scale) sediments at - 4 ° N (see Fig. 6 for locations). No significant difference exists between the ~4C ages for the top and bottom of the core obtained near the river mouth. This observation is reasonable because the 2t°pb accumulation rate (1.0 cm y-l, 0.7 g cm -2 y-J) predicts a difference of only 270 y between the top and bottom of the core, a difference which could not be resolved because of analytical limitations. The old 14C age observed at the surface of the piston core probably indicates that the carbon, which is primarily terrestrial material (SHOWERS and ANGLE, This Volume), is stored for a significant time in the terrestrial environment before being dispersed to the shelf. The ~4C age for the piston-core bottom in the area of relict muds is only about 1000 y older than the ages near the river mouth.
DISCUSSION
Areas o f negligible accumulation Physically reworked layer. Slow sediment accumulation (over a 100-y time scale) occurs in water depths less than about 15 m. Shallow-water areas commonly reveal uniform excess 21°Pb activities to significant depth in the seabed, below which excess activities abruptly decrease to background values (Fig. 5). A common explanation for uniform levels of 21°Pb activity near the surface of the seabed is that intense biological and/or physical mixing homogenizes sediments. Intense biological mixing cannot e~plain profiles observed on the inner Amazon shelf, because radiographic examination of cores from this area reveals interlaminated sediments with relatively little evidence of bioturbation (KUEHL et al., This Volume), and because biological sampling of this area reveals a
218
s . A . KUEHL et al.
52oW 6°
50°
51°
5° t ~"°°,~
(CM) OF
480 = . . . . . .
REWORKED
47°
46°
I
LAYER
:. •
3°
/
I
THICKNESS
"°1.
1° 1
49°
I
,,, I
0 20 i$Obaths
.iI.°,
60
100
in m e t e r s
0o
1o,
": """ " ' E
Marajd
Island
r!,' , •.
2°S , Fig. 7. Distribution and thickness of the reworked layer on the Amazon subaqueous delta based on -~°Pb profiles from 57 cores (solid circles). The layer reaches a maximum thickness in a region centered about the 15-m isobath. Sediment in this layer is reworked by waves and currents, and most is transported ultimately to other parts of the dispersal system.
Sediment accumulation on the Amazon continental shelf
219
virtual absence of macrofauna (ALLERand ALLER,This Volume). Therefore, frequent physical disturbance to significant depth in the seabed is probably responsible for observed 2~°pb profiles. The dominant physical processes operating on the inner shelf which could rework surface sedin~nts are surface waves and tidal currents (NITrROUERet al., This Volume). Sediments deposited in shallow water probably are resuspended and transported to other portions of the dispersal system. The physically reworked layer attains a maximum thickness in water depths of - 1 5 m, and thins both in seaward and landward directions (Fig. 7). The seaward thinning probably results from the reduced influence of waves and tides (in greater water depths) in reworking the upper portion of the seabed. A possible explanation for the reduced thickness nearshore is that wave and current activity are so intense that sediment remains in suspension, and hence a thick reworked layer has no opportunity to develop. Sediment cores used to determine the reworked-layer thickness were collected soon after peak sediment discharge of the Amazon River. Therefore, the thicknesses reported in this paper may represent maximum values. A thick layer of sediment deposited during high discharge could be progressively removed by physical processes during periods of reduced sediment supply. Additional observations during low discharge are needed to test this hypothesis. Relict area. A modern physically reworked layer is absent from the area of relict muds. Surface sediment is overconsolidated and contains no excess 21°pb, thus indicating that negligible accumulation (or deposition) occurs. The possibility that these areas formed during lower sea-level conditions, as suggested by MILLIMAN et al. (1975), can be evaluated from the 14C data. The data demonstrate that the relict area probably formed <1000 y ago (during sea-level conditions similar to those today). The absence of modern accumulation in this area is not well understood. Apparently, the sheet of modern sediment accumulating in the proximal portion of the dispersal system breaks up into discrete, shore-attached mud-shoals which move northwestward along the dispersal system (ALLERSMA,1971; WELLS and COLEMAN,1978; RINE, 1980). The break-up of the mud sheet might result from decreased sediment supply because of accumulation nearer to the source. A c r o s s - s h e l f trends in sediment accumulation rate
The dominant trend in accumulation rate on the Amazon subaqueous delta is in the across-shelf direction (Fig. 8). Accumulation rates increase seaward from low values nearshore, to rates as high as - 1 0 cm y-i (6.9 g cm -z y-l) on the seaward portion of the topset beds and on the foreset beds. Accumulation rates are relatively low (<1 cm y-~, 0.7 g cm 2 y-~) on the bottomset beds. This across-shelf trend in accumulation can be explained in terms of the simple model of MCCAvE (1972): the locus of sediment accumulation is a function of the relative rates of sediment supply and removal. Nearshore, high suspended-sediment concentrations (>500 mg 1-~) (GraBS, 1976; NITTROUER et al., This Volume) indicate a high potential for sediment accumulation. gtowever, this accumulation does not occur because shoaling surface waves and strong tidal currents resuspend any sediment which is temporarily deposited. In the outer topset and the foreset regions, lower suspended-sediment concentrations ( - 1 0 mgl ~) are observed. However, resuspension by waves and currents is less effective in these regions, and the net flux of sediment to the se'abed is much greater. An important observation is that the sediment accumulation rate is not directly correlated to suspended-sediment concentration (an often made assumption). Information regarding both suspended-
220
s . A . KUEHL et al.
0'
Topset
O.
Fo
50•
•
.
.
- . , , . . .
Relict Transgressive Sands
10o.
a
,o~
b o
2o
40
60
80
100
120
1,io
Distance From Shore (km)
Fig. 8. A cross-shelf transect showing relationship between (a) deltaic stratigraphy (schematic), and (b) sediment accumulation rate, from an area northwest of the river mouth. Sediment accumulation rate increases from low values nearshore, to values as high as - 1 0 cm y ~ near the outer topset and foreset regions. Accumulation rates are low in the bottomset region and are negligible in the relict sand area on the outer shelf.
sediment concentration (supply) and physical resuspension activity (removal) is needed to predict sediment accumulation rates. Seaward of the foreset region, the sediment supply is reduced, and accumulation rates are low on the bottomset beds and are negligible on the outer shelf (70-100 m water depth) where relict sands are exposed. The NBCC impinges upon the outer shelf and may be partly responsible for the absence of modern Amazon River muds. The limited subaerial expression of the Amazon delta (NITTROUER e t al., 1986) can be explained from the observed across-shelf trend in accumulation rate. The outer topset and the foreset regions are rapidly accreting upward and seaward. However, as the topset surface approaches sea level, upward accretion becomes negligible (because of the increasing effect of resuspension) and the surface remains submerged. High accumulation rates in the foreset region indicate that the subaqueous delta is prograding toward the shelfbreak. Non-steady-state accumulation Strong downcore variability in excess 21°Pb activity (Fig. 3) is observed for some stations near the foreset region (Fig. 6). Cores from these stations are characterized by relatively high 2t°pb activities which fluctuate downcore. Excess 2H~pb activities are as much as five times higher than typical activities observed nearshore. DEMASTER et al. (This Volume) demonstrate that the offshore gradient in specific 21°pb activity (at the surface of the seabed) results from an oceanic supply of dissolved 21°Pb. Nearshore, little dissolved 21°pb is available to particles, and the particles are deposited with a lower specific activity. Particles which are transported farther offshore become mixed with oceanic waters (which are rich in dissolved 21°pb), and scavenge additional 2~°Pb before being deposited.
Sediment accumulation on the Amazon continental shelf
221
The temporal variation in initial 2H~pb activity which is supplied to this area (reflected in downcore variability) has three likely explanations. The first explanation assumes that all the particles have the same affinity for 21°pb, and that the residence time of particles in suspension fluctuates through time. Then, if the particles are not saturated with respect to 21°pb, the specific activity of particles will depend on the residence time of particles in the 21°pb-rich waters. Frontal mixing of the Amazon River plume is likely to vary with seasonal changes in discharge. Seasonal or longer-term fluctuations in equatorial circulation (i.e. the NBCC) may also be important in controlling the residence time of particles in 21°pb-rich waters. The second explanation for observed downcore variability in excess 21°pb assumes that the residence time of particles in suspension is constant (at one locale), but that the nature of particles and their affinity for 21°pb varies through time. Although extensive downcore variability in grain size is not observed (Fig. 3), field observations of these cores revealed thin black laminations of apparently high organicmatter content. Because organic components are important scavenging agents (DAVIS, 1984), temporal fluctuations in these components may be responsible for the observed fluctuations in specific 21°pb activity. Changes in inorganic components important to scavenging (e.g. Fe/Mn oxides; CARPENTERet al., 1981) also may be responsible for the fluctuations in 21°pb activity. The third possible explanation for these fluctuations is related to changes in suspended-sediment concentration. High concentrations would deplete oceanic waters of dissolved 21°pb, resulting in lower specific activity of sediment reaching the seabed. Seasonal changes in discharge or in circulation could drive associated changes in suspended-sediment concentration. More detailed studies are needed to resolve the effects of residence time, scavenging efficiency, and suspendedsediment concentration on temporal fluctuations in specific 21°pb activity. Estimates of sediment accumulation rate in the non-steady-state area are difficult because assumptions used in 21°pb geochronology (i.e. uniform specific activity supplied to the zone of logarithmic decrease) are not valid. However, because excess 21°pb is observed throughout the 3-m gravity cores (i.e. all sediment younger than 100 y) and because deep physical and biological mixing are not observed, accumulation rates in this area are probably high (>3 cm y-J, 2.1 g cm -2 y-J). If the downcore fluctuations in 2J°Pb activity are controlled by seasonal variation in river discharge, then each cycle would represent 1 y of sediment accumulation. This would yield an accumulation rate of - 2 5 cm y-l (17.2 g cm -2 y-~) for Sta. 174 (Fig. 3). This is not unreasonable, because the non-steady-state cores are from areas of generally high accumulation rate ( - 1 0 cm y-J, 6.9 g cm -2 y-J). Until a better understanding for the cause of downcore variability is achieved, the 25 cm y-~ estimate is hypothetical. The absence of significant downcore variability in excess 2H~pb in cores taken from the inner shelf does not necessarily indicate that specific activities of 21°pb initially reaching the seabed are constant. For the inner shelf, the seabed is frequently reworked by physical processes which would tend to destroy evidence of variations in initial 2~°Pb activity by homogenizing near-surface 210pb profiles, resulting in a constant level of 2lOpb activity being supplied to the preserved zone below.
Sediment budget A sediment budget for the Amazon shelf is constructed from the distribution of 21°pb sediment accumulation rates (Fig. 6). The budget includes the continental shelf from the Para River to the area of relict muds at about 4°N. For budget calculations, the measured
222
S.A. KUEHLet al.
area between each contour line and the midpoint value of accumulation rate is used to estimate the mass of sediment accumulating in each of the areas. The resulting sediment budget equals 6.3 x l0 s tons y-1. In defining an error estimate for this budget, many sources of uncertainty must be considered, including: (1) the error associated with individual accumulation rate measurements, (2) the control available in contouring distribution of accumulation rates, and (3) the uncertainty of accumulation rates in the non-steady-state area. The error of individual accumulation rate measurements can be estimated from the standard error of the slopes calculated from 2~°pb profiles. The standard error in slopes results in about a 30% uncertainty in accumulation rate (average uncertainty for all profiles), and an uncertainty of 2.0 × l0 s tons y-~ in the overall sediment budget. Possible errors associated with limited control (station coverage) are difficult to quantify. Because of ship limitations, no coverage of shallow areas (<5 m) was possible. The shoal area just north of the river mouth (Fig. 6) is especially problematic because of the shoal's large areal extent. However, an important observation from this study is the absence of significant accumulation of sediment in shallow areas (because of resuspension and lateral transport). Therefore, the unsampled shallow areas probably contribute little to the overall sediment budget. Another source of error for the shelf budget results from uncertainty of accumulation rate for the non-steady-state area. The accumulation rate in this area may range from 3 to 25 cm y I (2.1-17.2 g cm -2 y-~); however, because of the limited areal extent, the effect on the budget is probably small. Based on the sediment budget, and errors associated with accumulation rate measurements, 6.3 _+ 2.0 x l0 s tons y-I of Amazon River sediment are estimated to be accumulating on the adjacent continental shelf. This budget leaves about 30-60% of the total Amazon River sediment discharge ( - 1 2 x l 0 s tons y-~; MEADE et a l . , 1985) unaccounted. The sediment discharge measurements for the Amazon River were made at Obidos (700 km upstream of the river mouth). Annual flooding of low-lying wetlands and associated flood plain accretion might result in reduced sediment supply to the ocean. Although the magnitude of sediment loss as a result of flood plain accretion is not known, the estimate of - 1 2 × l0 s tons y-~ probably is an upper limit for discharge to the Atlantic Ocean. Of the sediment supplied to the continental shelf, loss to the southeast probably is negligible because sediment transport is consistently toward the northwest, and because relict sands are observed on the shelf to the southeast. Although modern Amazon River sediments are not observed on the outer shelf (70-100 m water depth), sediment could be bypassing the outer shelf and accumulating on the adjacent continental slope. An estimate of potential sediment loss to the slope can be made from the thickness of the Holocene sediment (--1 m; DAMUTH and KUMAR, 1975), and from the area of the slope (100-2000 m water depth). Assuming the Holocene layer is entirely Amazon River sediment with a porosity of 75%, loss to the slope would account for < 5 % of Amazon River discharge. Holocene sediment on the slope is largely a foraminiferal ooze (DAMUTH and KUMAR, 1975), therefore, little, if any, Amazon sediment is presently accumulating on the slope. ALLERSMA(1971) estimates that 1-2 × 10s tons y-I of Amazon sediment are moving northwestward along the coast of the Guianas. Most of the Amazon sediment which is not accumulating in the study area probably is bypassed to the northwest, and, according to NITrROUER et al. (This Volume), most is transported landward of the 10-m isobath. Some sediment may be trapped in mangrove swamps along the shoreline: however, no estimate of shoreline accretion for northern Brazil has been reported yet.
Sediment accumulation on the Amazon continental shelf
223
CONCLUSIONS
The major conclusions of this paper are summarized below. (1) Sediment accumulation rates determined using 2"~Pb geochronology reveal the nature of sediment accumulation on the Amazon subaqueous delta. Slow accumulation (<0.1 cm y-1 0.1 g c m - 2 y-l) occurs in water depths <--15 m, probably as a result of resuspension by shoaling surface waves and strong tidal currents. Accumulation rates progressively increase offshore (because of reduced resuspension), to a maximum of - 1 0 cm y-I (6.9 g c m - 2 y-I) in the outer topset and foreset regions. Farther seaward, accumulation rates are generally <1 cm y-~ (0.7 c m -2 y-l) in the bottomset region, probably as a result of reduced sediment supply. (2) The inner shelf (<30 m water depth) is mantled with a thick (as much as 2 m) layer of sediment with uniform e x c e s s 2 m P b activity, which is reworked probably by surface waves and tidal currents. This physically reworked layer reaches a maximum thickness in - 1 5 m water depth, and thins both in the seaward and landward directions. The seaward thinning could result from decreased influence of waves and tides. Nearshore, turbulence may be so great that sediment remains in suspension, and a reworked layer does not develop. (3) The limited subaerial expression of the Amazon delta is explained from the observed distribution of accumulation rates. Upward accretion of the subaqueous topset region is relatively slow in water depths less than about 15 m. Therefore, the topset region remains submerged as the delta progrades seaward. (4) 14C dating of an anomalous area of relict (100-y time scale) mud near the northern boundary of the study area reveals that the sediment accumulated <1000 y ago. The absence of modern sediments in this area is not well understood. (5) Non-steady-state input of 2"~Pb is identified at some stations in water depths of about 50 m. Dgwncore fluctuations in excess 2~°Pb activity could result from: (a) temporal changes of particle residence time in ~l°Pb-rich oceanic waters, (b) temporal changes in chemical reactivity of particles supplied to the area, or (c) temporal changes in suspended-sediment concentration. Further study is needed to resolve the mechanisms. (6) A sediment budget indicates that 6.3 _+ 2.0 x l0 s tons y-1 of sediment accumulate on the Amazon continental shelf. The remainder of sediment discharged to the shelf by the Amazon River probably is transported northwestward beyond the Brazilian shelf, and/or is accumulating landward of the shelf as coastal accretion. Acknowledgements--The authors thank the crew and scientists aboard the R.V. lselin, whose untiring dedication allowed collection of the voluminous sediment samples used in this study. This research was supported by the National Science Foundation (grant Nos OCE-7908496, OCE-8117709 and OCE-8415413).
REFERENCES ADAMSC. E., JR, J. T. WELLS and J. M. COLEMAN(This Volume) Transverse bedforms on the Amazon shelf. Continental Shelf Research, 6, 175-187. ALLER J. Y. and R. C. ALLER (This Volume) General characteristics of benthic faunas on the Amazon inner continental shelf with comparison to the shelf off the Changjiang River, East China Sea. Continental Shelf Research, 6,291-310. ALLERSMAE. (1971) Mud on the oceanic shelf off Guyana. In: Symposium on Investigations of Resources in the Caribbean Sea and Adjacent Regions, UNESCO, Paris, pp. 193-203.
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