RECENT SEDIMENT ACCUMULATION RATES
ON THE
MONTEREY FAN
A Thesis
Presented to
The Faculty of the Department of Geology
San Jose State University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
By
William L. Hughes
December, 1988 ACKNOWLEDGMENTS
I would like to acknowledge the help and support of many individuals who made this thesis possible. Dr. Alan
Shiller provided assistance as the thesis advisor and patiently instructed me in laboratory procedures and geochemistry. Dr. David Andersen has provided suggestions and encouragement during the project. Dr. Michael Ledbetter suggested the subject area and assisted in the sampling of cores.
Tom Walsh of Scripps Institution of Oceanography provided samples from cores. Dr. William Normark and
Christina Gutmacher of the Marine Geology Branch of the u.s.
Geological Survey have been generous with their time and provided radiographs, photos and cores for analysis. Donna
Beale of u.c. Santa Cruz provided assistance in calibrating the 210Po tracer.
Figures for this research were drafted by Lynn
McMasters. I would lilce to thank Applied Earth Technology,
Inc. for the use of a computer to translate my thoughts to words on paper. Financial support for this research was provided in part by the Packard Foundation and Moss Landing
Marine Laboratories.
Finally, I thank rny wife Joanne, who provided continuous support during the duration of this project.
iii TABLE OF CONTENTS
page
ABSTRACT ~ ~ ...... • ...... viii
INTRODUCTION ...... 1 Purpose of Study . • . . . . • • • • ...... • . . . • • • • • • • • • • . . . 3
Lead-210 Geochronology . . . . • . . • • • • • • • ...... 8
Initial Assumptions .....••••••••••••••...... 11
METHODS ...... ,...... l3 RESULTS 19
Radiochemical Data ••...... •••••.....•••••••.... 19
Suprafan ...... 19
Levee ...... 29
Fan-Valley ...... 32
Sur Slide .••.••••••..... 38
DISCUSSION •••••....••••••..•...•.••••..•..•...•••.•••• 43
Supra fan...... 4 3
Levee ••••••••...... •••••.....•.•••••••...... 45
Fan-Valley . . . • • • . • ...... • • • • . . . • • • • • ...... • • 4 7
Sur Slide 48
Effect of Moss Landing Harbor Dredge Spoil ..••••• 49
Comparison with Other Deep-Sea Canyons 50
Sediment Accumulation and Depositional Processes . 51
Implications Regarding Sea-Level ....•.••••••••... 52
CONCLUSION ...... , ...... 54 REFERENCES CITED . . . • . • . • • • • • ...... • • • • ...... • . • • 57
JV APPENDIX: Total Lead-210 Activity Data ...... 62
v LIST OF ILLUSTRATIONS
Figure page
1. Location of study Area 2
2. Core Locations ...... 5
3. 238u Decay Series 9
4. I d ea1 1zed. Pro f'11 eo f 210 Pb Ac t'lVl 't y ...... 16
5 . E xcess 210 P b At'c lVl 't y P ro f'11 es ...... 20
a. Core 98G ...... 20
b. Core 100G ...... 21
c . Core 14G ...... 21
Core 15G
e. Core 16G ...... 2 2
Core 69Bx
Core 49Bx
h. Core B3Bx • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2 4
Core B4Bx
j . Core B7Bx ...... 2 5
k. Core 90Bx • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2 5
1 • Core 43Bx • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2 6
m. Core 1PG ...... • ...... • • • • • • • • • • ...... • • • 2 6
n. Core 2PG ...... 2 7
0 • Core 4PG • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2 7 6. Photograph of core 14G ...... 30 7. Radiograph of core 14G ...... 31 8. Photograph of core 15G ...... 33
vi 9. Radiograph of core 15G ...... 34 10. Photograph of core 16G ...... 35 11. Radiograph of core 16G ...... 36 12. Radiograph of core 1PG ...... 40 13. Radiograph of core 2PG ...... 41 14. Photograph of core 2PG ...... 42 15. Distribution of Sediment Accumulation Rates ...... 44
Tables
1. Cores Used in the Study ...... • 6
2. Sediment Accumulation and Sedimentation Rates ...• 28
vii ABSTRACT
Lead-210 geochronology was used to determine sediment accumulation rates between the water depths of 2550 and
4135 m on the Monterey Fan. Fifteen box and gravity cores were studied from the fan-valley, levee, suprafan, and Sur slide. Using excess 210Pb activity profiles, the sediment accumulation rates were calculated.
Slumping from the fan-valley walls and changes in the gradient result in varying rates of sediment accumulation along the channel. Upslope of the oxbow meander, sediment accumulation rates increase (0.09 to 0.61 gjcm2;yr) with increasing water depths. Within the meander the rates of sediment accumulation range from 0.17 to 0.51 gjcm2;yr.
On the suprafan, sediment accumulation rates range from
0,05 to 0.56 gjcm2jyr. The differences in the rates on the suprafan indicate that hemipelagic sedimentation is not the only active depositional process in this area. Sediment accumulation rates range between 0.12 and 2.04 gjcm2;yr on the northern levee. This would indicate that the crest of the levee is a zone of nondeposition or erosion and that the sediment is deposited farther down the flank.
Sediment accumulation rates on the Sur slide are 0.48 to 1.35 gjcm2jyr. Using 210Pb geochronology, the age of the slide was estimated to be between 140 and 600 yr B.P.
viii The dumping of dredge spoil from Moss Landing Harbor at the head of the canyon is not reflected in an increase in sediment accumulation rates at the depths studied. Dredge spoil is either trapped further up the canyon or is too widely dispersed to be detected.
The recent sedimentation rates on the Monterey Fan were compared with the Astoria Fan off the Oregon continental shelf. It was found that the modern sedimentation rates on the Monterey Fan are greater than both Late Pleistocene and
Holocene rates on the Astoria Fan. Variations in sea-level probably affect the sedimentation rates on the fan; however, the tectonic setting is the primary control of the sedimentation rate on the Monterey Fan.
ix INTRODUCTION
The Monterey Fan is the dominant physiographic feature
along the central California coast. The fan is fed by the
Ascension and Monterey Canyon systems. The Monterey Canyon
contributes more sediment to the fan (Wilde, 1965) because
it cuts across the shelf and heads at Elkhorn Slough, about
120 km south of San Francisco (fig. 1).
The Monterey Fan has a radius of 300 km and covers
approximately 100,000 km2 of the sea floor (Hess and
Normark, 1976). The Monterey Fan is thought by some workers
to be relatively inactive due to the present high stand of
sea-level and the lack of a large river transporting
sediment to the canyon (Shanmugam and Moiola, 1988).
Evidence for this is a blanket of olive green mud containing
Holocene pelagic foraminifera and radiolaria (Komar, 1969).
Other authors also have reported finding mostly marine mud
in the canyon (Shepard, 1948; Martin and Emery, 1967; Yancy,
1968; Wolf, 1970; Oliver and Slattery, 1973; Greene, 1977);
however, thin sand layers (Komar, 1969) and Holocene sand
deposits on the suprafan and in Monterey East Valley (Hess
and Normark, 1976) indicate infrequent turbidite deposition.
The primary modes of sediment transport onto the fan
today are hemipelagic sedimentation and turbidity currents
initiated by the slumping of material from the walls of the
canyon. Greene (1970) was the first to suggest active 2
Northern California
Point Reyes
Study Area
Figure 1. Location of study area. 3 slumping of the canyon walls and later mapped slump features in detail (Greene, 1977). Chinburg (1985) estimated that
150,000,000 m3 of sediment was involved in slumping in the upper 25 km of the canyon. The slumping in the canyon may be caused by the oversteepening of the canyon walls through sedimentation and undercutting of the canyon walls by bottom currents (Chinburg, 1985). A potential recent source of sediment to the Monterey Fan is the dumping at the head of the Monterey Canyon of dredge spoil from the dredging of
Moss Landing Harbor.
No sediment accumulation rates have been established on the fan using any radioisotope, although Wilde (1965) calculated an average sedimentation rate of 1 cm/1000 yr using the total sediment volume of the fan and its estimated age (Oligocene). Hein and Griggs (1972) estimated a sedimentation rate of 27 cm/1000 yr on the northwest levee of the Monterey-Ascension Fan Valley using the depth of an isochronous biostratigraphic marker. This marker was dated at 12,500 yr B.P. and denotes a change from a dominance of planktonic foraminiferans below to a dominance of radiolarians above (Duncan et al., 1970).
Purpose of Study
The purpose of this study is to examine sediment accumulation rates using the radioisotope 210Pb at selected locations on the Monterey Fan and to determine if the 4
Monterey Fan can be considered active during this high stand of sea-level. Box, piston, and gravity cores curated at the U.S.G.S. and Scripps Institution of Oceanography were sampled for this study. These cores were taken at locations along the Monterey fan-valley axis, across the northern levee of the fan-valley, on the suprafan, and within the Sur submarine slide south of the main channel (fig. 2; Table 1).
The cores that were selected along the axis of the canyon are located downslope of slumps that have formed by failure of the canyon walls (Greene, 1970; Chinburg, 1985).
This section of the canyon contains an oxbow meander that was described by Shepard (1966) and is where the greatest sediment accumulation rates may occur. The axial gradient is relatively constant (3.6/1000) through the meander
(Komar, 1969). Sediment accumulation rates will vary as a result of the slumping of material from the canyon walls between core sites.
Two cores were selected from the suprafan to determine if turbidity currents generated by slumping were great enough to transport sediment to the area. The suprafan is a morphological unit. of modern, active, sand-rich fans and is the most active depositional lobe at the end of the fan valley that develops due to rapid deposition of sediment
(Normark, 1978; Shanmugam and Moiola, 1988). Normark et al.
(1985) recognized small, unleveed channels on the suprafan and reported that 3.5-kHz profiles show a strong, diffuse 5
Figure 2. Map showing bathymetry (Chase et al., 1975) and core locations on the Monterey Deep-Sea Fan. Squares, triangles, asterisks, and solid circles represent core locations on the levee, suprafan, fan-valley, and Sur slide, respectively. 6
TABLE 1. Location, length, type, and origin of cores used in the study.
Cruise*, original Latitude Longitude Water Core core No., and Depth Length type** (m) (em)
Supra fan
Mtc II 98 G 35" 10.5 1 N 123" 18 1 W 4072 131 Mtc II 100 G 34' 57.5'N 123" 30 1 W 4135 57
Levee
S3-78-SC 14 G 36" 21.5 1 N 123' 12.49 1 W 3369 381 S3-78-SC 15 G 36" 23.53 1 N 123' 20.52 1 W 3491 472 S3-78-SC 16 G 36" 25.25 1 N 123' 32.75'N 3631 407
Fan Valley
~ltc II 69 Bx 3 6 ' 2 2 • 5 1 N 122' 30 1 w 2551 23 Mtc I 49 Bx 3 6 ' 2 a • 8 • N 122' 44.3 1 W 3319 35 Mtc II 83 Bx 3 6' 14 . B 1 N 122' 47.2 1 W 3394 22 Mtc II 84 Bx 3 6' 13. 9 1 N 122' 51.1 1 \V 3433 30 Mtc II 87 Bx 36' 20' N 122' 53.9'1'1 3509 15 Mtc II 90 Bx 36' 17.5 1 N 123' 2.6 1 1'1 3493 11 Mtc I 43 Bx 35' 58.2 1 N 123' 11.5 •w 3639 38
Sur slide
S15-79-NC 1 PG 36' 8.5 1 N 122' 40,00 1 1'1 3362 15 S15-79-NC 2 PG 35' 59.99 1 N 122' 46.01 1 W 3508 59 S15-79-NC 4 PG 35' 47.B5 1 N 123' 00.27 1 1'1 3721 85
* Cruises
Mtci MONTCANYON I 1964 R/V "Horizon" Mtcii MONTCANYON II 1965 R/V "Argo" S3-78-SC Leg 3 1978 R/V "Seasounder" S15-79-NC Leg 15 1979 R/V 11 Seasounder 11
** Core types: G = Gravity, Bx = Box, PG = Gravity 7 surface reflection suggesting that this is a sand-rich environment. The cores collected indicate that the suprafan is presently mantled by a layer of mud less than 1 m thick
(Hess, 1974). The 210pb geochronology will be used to study the rates of sediment accumulation in the suprafan region of the fan.
The cores at selected locations across the northern levee are located at a bend in the canyon where overtopping by turbidity currents would most likely occur (Piper and
Normark, 1983). A turbidity current would need to overflow a 400-m-deep fan channel in order to be deposited on the levee. Mud waves in this area on the northern levee indicate that low-density turbidite deposition has occurred
(Normark et al., 1980). Slopes in this region are the steepest (5.5/1000) anywhere on the fan (Normark et al.,
1985). The sediment accumulation rates at each of the sites will be compared to determine if this is a zone of sediment bypass.
The three cores located on the sur submarine slide to the south of the canyon were sampled to determine if the age of the slide can be estimated. The slide and associated
debris flow are between 3 1 200 m and 3,750 m water depth and cover more than 1000 km2 of the eastern side of the
Monterey Fan. The slide crosses the main fan-valley south of the study cores from the fan-valley. Normark and
Gutmacher (1988) found that the slide is covered by 23 to 8
87 em of mud and turbidite sand and have estimated the age of the slide to be up to 1500 years B.P. Sediment accumulation rates will be used to verify this estimate of the age of the slide.
Lead-210 Geochronology
Few studies of recent sediment accumulation (<100 yr) on deep-sea fans have been made. Most studies of sediment accumulation rates use the radioisotope 14c and must average the rate over thousands of years (Drake et al., 1978; Doyle,
1979; Kindinger et al., 1983). The use of 210pb geochronology, however, makes it possible to examine sediment accumulation rates over the past 100 years. The isotope 210Pb has a half-life of 22.6 yr, belongs to the
238u radioactive decay series and is a product of the decay of 222Rn (fig. 3) in the atmosphere. Through precipitation and dry fallout, the 210Pb is removed from the atmosphere and supplied to the ocean (Bruland, 1974). The maximum age that can be measured is 135 yr (Nittrouer et al., 1979).
The activity of 210Pb is determined by measuring the activity of its grand-daughter 210Po (Fleer and Bacon,
1984). This method assumes secular equilibrium between
210Pb and 21°Po. Secular equilibrium occurs when the parents (21 0Pb) have a much longer half-life than that of the daughter (210Po) (Faure, 1977). Unless a physical chemical process has separated the twa isotopes, the 9
u2ss u2s4 4.5x1 Q9yr 2.5x1o5yr 1/' ~~:,: /' 1
Th234 Th230 24.1 days 7.5x104yr l Ra22D 1.6x1 o3yr 1 Rn222 3.8 days l p 214 p 21o p0 21s 0 0 3.1 min 1.6 x1 Q·4sec /38.4days Bi214/ 8i21o 1/ 19.7min l / 5.0days 1 Pb214 Pb21o Pb2os 26.8 min 22.6 yr stable
Figure 3. The uranium-238 decay series (after Broecker, 19 7 4) • 10 activity of the 210po is equal to the activity of the 210Pb.
Eakins and Morrison (1974) tested this assumption in part of a sediment core and found that 210Pb and 210Po were in equilibrium. Initially 210pb and 210Po might not be in equilibrium when the sediment is deposited. In order for equilibrium to exist, the deposition of sediment depleted in
210Pb must not be the chief source of 210Po and the residence time of 210pb in the sediment must be longer than the half-life of 210po (Robbins, 1978). Within a year, equilibrium between 210Pb and 210Po should be reached.
Under conditions where the accumulation rate is on the order of millimeters per year, only the upper few millimeters of sediment would be affected.
Lead-210 geochronology was first used to study a marine environment in the santa Barbara Basin where the counting of the annual varves had been used to date sediments (Koide et al., 1972). It was found that the 210Pb and varve chronologies agreed quite well. Nittrouer et al. (1979) successfully used 210pb geochronology to investigate sediment accumulation on the Washington continental shelf and produced quantitative results consistent with the qualitative results obtained with other sedimentological tools. Thornbjarnson et al. (1986) measured modern sediment accumulation rates, examined their distribution throughout the Quinault Submarine Canyon, and were able to establish a budget of modern sediment accumulation in the canyon and 11 compare this with the flux of sediment from the Columbia
River. In the Wilmington Submarine Canyon sedimentary processes were identified using 210Pb geochronology and sedimentary structures recognized in radiographs (Sanford and Kuehl, 1987).
Lead-210 geochronology also has been used to interpret recent geochemical changes that have resulted from the last one hundred years of industrial growth and which are preserved in areas of sediment accumulation. Bruland et al.
(1974) found in sample cores that Ag, Cd, Cr, Cu, Mo, u, and
Zn were accumulating at higher rates in 1970 than one hundred years earlier in the Santa Monica, Santa Barbara, and San Pedro basins off the southern California coast.
This suggests that heavy metal pollution from surface run off has increased with industrial development in the area.
Initial Assumptions
The 210Pb within the sediment consists of 226 Ra supported 210Pb and atmospherically derived excess 210Pb.
An important assumption in 21°Pb geochronology is that despite any variations that may have occurred in the sediment accumulation rates, the initial concentration of excess 210pb in the water column was constant at each stage in the accumulation of unsupported 210Pb in the sediment.
Therefore, the supply of excess 210Pb to the sediment has been constant and 210pb activities supported by the decay of 12
226Ra are also assumed constant with depth. Another important assumption is that Pb does not migrate in the sediment (Goldberg and Bruland, 1974). Any significant decreases in the concentration of 210Pb in the sediment will be taken as evidence that the dilution of the 21°pb concentration was caused by an acceleration in sediment accumulation (Appleby and Oldfield, 1978). METHODS
Samples were taken at one centimeter intervals in the cores and dried overnight at 35" c, and then pulverized using a mortar and pestle. One to three grams of the sample were spiked with a known amount of 208Po tracer to determine processing and plating efficiency. Samples were then digested in 10 ml of hot 12 M HCl and B ml of H2o 2 for four hours to dissolve organic material and sediment coatings and release Po into solution. The sediment was then separated from the solution by centrifugation. The sediment was twice washed with 2 M HCl and centrifuged to remove any remaining
Po. The wash was added to the original solution. The solution then was heated to near dryness.
The solution containing the extracted 21Dpo and the
2DBpo tracer was diluted with 2 M HCl to 100 ml and then 5 ml of 20 percent hydroxylamine hydrochloride were added to reduce chromate and ferric iron in solution. The addition of 1 ml of 10 percent bismuth nitrate was done to dilute the amount of 2 10si and an alpha decay product of 23 2Th, 212si.
To facilitate the plating of Po and complex Fe, 2 ml of 25 percent sodium citrate was added (Flynn, 1968). The pH was adjusted to 2 using NH 40H and the solution was allowed to stand for 15 minutes to verify that the chromate and ferric iron had been complexed or reduced. If the solution did not turned from a yellow color to clear, an additional 2 ml of 14
25 percent sodium citrate and 5 ml of 20 percent hydroxylamine hydrochloride were added. Both of the polonium isotopes were spontaneously plated onto a suspended
1.5-cm-diameter polished silver planchet for a period of 4 hours at 90" c.
The alpha decay was measured using a four-channel
Tennelec model 404A alpha spectrometer which was connected to a Davidson model 1056A multichannel analyzer and HP85 microcomputer. The counting error was minimized by analyzing each sample in the spectrometer until at least
1000 counts were recorded. Blank samples were treated as regular samples except that no sediment was used in the process. The results from the blanks indicated no significant 210po contamination. A method developed by
Fleer and Bacon (1984) was used to convert the disintegrations into activities.
Unsupported 210pb activities were calculated by subtracting the 210Pb activity supported by the decay of
226Ra from the total 210Pb activity. The 226Ra-supported
210Pb activity was calculated from the samples at greater depths within the core where the 210Pb activity is constant.
In cores where the depths of background levels of 210Pb were not reached due to a thick surface mixed layer or region of rapid sediment accumulation, a value of 1.02 dpm(g was assumed for the background 21 0pb activity (Carpenter, et al., 1982; Lavelle et al., 1986). This background 210Pb 15 activity is an average of the 22 6Ra-supported 210Pb activities from other study cores. All activities are expressed relative to dry weight.
The sediment accumulation rate, s, in gjcm2;yr, is given by the equationS= -pA/b, where b is the slope of the least squares fit to a graph of the logarithm of the excess 21°pb activity versus sediment depth, A is the density of the material deposited, and p is the 21Dpb radioactive decay constant. The average density of slope sediment, 2.65 gjcm3 , was used in the calculation of the sedimentation rate.
Samples containing reworked relict sediment or coarse grained sediment will have a low initial activity and must be eliminated from the accumulation rate calculation
(Nittrouer et al., 1979). Samples with low initial activities due to coarse-grained sediment or reworked relict sediment were not used in the calculation of the sediment accumulation rate. Layers consisting entirely of coarse grained sediment were noted and disregarded in the calculations. In available core radiographs and photos, sedimentary and biological structures were examined to determine the depth and degree of mixing.
An idealized profile of excess 210Pb activity from a region of moderate sediment accumulation will display three layers (fig. 4). The upper layer has a nearly constant 16
Total 210Pb Activity (dpm/g)
0 Surface Mixed Layer
------~ ------Region of Radioactive _ 1 o c Decay (j) ------~ ------~ -:r 20 -n 210 3 Background Levels of Pb - - 30
Figure 4. An idealized profile of Pb-210 activity (total) for a sediment core. The surface mixed layer (SML) is a zone mixed by physical and biologic processes. The underlying layer is the region of radioactive decay, where there is a logarithmic decrease in excess 21°pb activity. ~e lowermost region is the layer of the background level of 0Pb supported by 226Ra. 17
210Pb activity due to mixing by biological and physical processes and is referred to as the surface mixed layer
(SML). The thickness of this layer indicates the depth of reworking at the sample site. The layer underlying the SML
is the layer of radioactive decay displaying a logarithmic decrease in the excess 210Pb activity as the depth
increases. Sediment accumulation rates are calculated from the slope of this line plotted as the depth below the SML.
Below the layer of radioactive decay is the layer of the background level of 21°Pb supported by 226Ra.
The 21°Pb geochronology will be affected in regions with a thick surface mixed layer, low accumulation rates,
and the presence of reworked relict sediments or coarse grained sediments (Nittrouer et al., 1979). Low sediment
accumulation rates result in a zone of radioactive decay that is very thin and difficult to sample.
The term "sediment accumulation rate" is defined as the
amount of dry sediment deposited per unit surface area per
unit time. Robbins and Edgington (1975) found that the water content of the surface sediments may be as high as 90 percent and decrease to 75 percent within 20 em depth. Due
to a progressive decrease in water content with depth as a
result of the increasing weight of overlying sediment, the
sedimentation rate (in cmjyr) in the Santa Barbara Basin
changes by a factor of five over a 5 m depth even though the
sediment accumulation rate is constant (Bruland, 1974). The 18
sedimentation rate (A), expressed in cmjyr, is related to the accumulation rate, (S), expressed in gjcm2;yr, by the relation A=pS, where pis the in situ density, in gjcm3, of the deposited material (Bruland, 1974). care must be taken when comparing the data, because of the changes in the sedimentation rate with depth due to compaction of the sediment. Sediment accumulation rates (in gjcm2;yr) will be converted to sedimentation rates (in cmjyr) only when needed to compare results with those of other workers. RESULTS
Radiochemical Data
The sediment accumulation rate was calculated in all of the cores studied. The excess 21°pb activities are plotted versus depth in each of the cores {fig. 5a through 51). The raw data are tabulated in the appendix. Table 2 summarizes the calculated sediment accumulation and sedimentation rates for the cores. Sediment accumulation rates vary from 0.03 to 2.04 gjcm2;yr.
Erratic changes from the expected logarithmic decrease in the 210Pb profiles at certain depths beneath the SML indicate that periods of steady-state accumulation were interrupted by periods of different depositional conditions.
Unusual excess 21°Pb at discrete horizons below regions of sediment in which excess 210Pb activity decreases logarithmically indicate relict mixing by benthic fauna
(Carpenter et al., 1985). The profiles for cores 98G and
43Bx indicate that either mixing has disrupted the sediment or the accumulation rate has changed through time (Figs. 5a and 51).
Suprafan
The excess 21°pb activity profile for core 98G displays no SML (fig. 5a). Either the top of the sediment in the core was lost during the coring operation or the sediment 20
Excess 210 Pb Activity (dpm/g)
10 c (!) "'C -::r 20 -(') -3 98G 30
Sa. 40
Figure 5. Excess Pb-210 activity profiles in sediment cores versus depth. Thickness of the surface mixed layer was estimated from these profiles. See text for discussion. Sediment accumulation rates were calculated from the slope of the least squares fit to the graph. 21
Excess 210 Pb Activity (dpm/g) 1.0 5 10 50 ~----~----~~~~~----~------~-r 0
10 c (1} '"0 ::T- 20 -(') -3 30 100G
5b. 40
Excess 210 Pb Activity (dpm/g)
10 c (t) "C 14G ::T- 20 -(') -3 210 No Pb 30 5c. 40 22
Excess 210 Pb Activity (dpm/g) 1~.0----~--~~~5~~~1LQ----~--~~~-r50 O
c 10 (II "C 15G -:I" 20 -(') -3 210 No Pb 30 5d. 40
Excess 210 Pb Activity (dpm/g)
c 10 (II "C -:r 20 -(') 16G -3 30
5e. 40
I 23
210 Excess Pb Activity (dpmfg) 1.0 5
210 No Pb 10 c (D t "C 210 ..... No Pb 69Bx 20 ::T (') -3 30 - 5f. 40.
Excess 210 Pb Activity (dpm/g) 1.0 5 10 24
210 Excess Pb Activity (dpm/g) 1.LQ----~--~~~5~~~1LQ--~~--~~~-5+Q 0
c 10 (I) "C -:I" (') 20 -3 83Bx - 30
5h. 40
Excess 210 Pb Activity (dpm/g) 1.0 5 10 50 0
c 10 (I) "C -:I" No 210 Pb 20 -(') -3 84Bx 30
5i. 40 25
210 Excess Pb Activity (dpm/g) 1.0 5 10 0.35
-I-= ----~~~-=~--:;>+! 10 ~------;.- c ft) "e 20 -::r 0 -3 30 - 87Bx 5j. 40
Excess 210 Pb Activity (dpm/g) 1.0 5 10 26
Excess 210 Pb Activity (dpm/g} 1.0 5 10 50 0
0 10 (I) "C ::r ...... - (') 20 -3 43Bx 30
5!. 40
210 Excess Pb Activity (dpm/g} 1.0 5 10 50 ~------~------~~----~~~--~To 27
Excess 210 Pb Activity (dpm/g) 1.0 5 10 50 a
1a c (l) "1:1 20 -::r -(') 2PG 3 3a -
5n. 40
Excess 210 Pb Activity (dpm/g) 1.LQ----~--~~~5~~~1LQ----~--~~~-5Qr a
c 10 (II "1:1 ::r ...... - (') 4PG 20 -3 30
5o. 40 28
TABLE 2: Sediment accumulation and sedimentation rates.
Core Water Mass accumulation Sedimentation No. Depth rate rate (m) (gjcm2;yr) (cmjyr)
Suprafan
+98u 3851 0.05 0.02 l 0.03 0.01 100 4018 0.56 0.21
Levee
14 3369 0.12 0.04 15 3491 2.04 0.77 16 3631 0.58 0.22
Fan-Valley
69 2551 0.09 0.03 49 3319 0.47 0.18 83 3394 0.61 0.23 84 3433 0.17 0.06 87 3509 0.33 0.12 90 3493 0.28 0.11 +43 u 3639 0.51 0.19 l 0.20 0.08
Sur Slide
1 3362 0.48 0.18 2 3508 0.40 0.15 4 3721 1. 35 0.51
+ Accumulation rate changes down core. 29 has not been disturbed at the surface by benthic organisms.
The sediment accumulation rate from the sediment-water
interface to a depth of 4 em is 0.05 gjcm2;yr. Between the
depths of 4 em and 22 em, mixing has disrupted the sediment
and created a buried SML. From a depth of 22 em to 34 em the sediment accumulation rate is 0.03 gjcm2;yr.
In the excess 210Pb activity profile for core
100G, the SML extends from the sediment-water interface to a
depth of 18 em, suggesting intense bioturbation or that the
top of the sediment in the core was disturbed during the
coring operation (fig. 5b). The sediment accumulation rate below the depth of 18 em is 0.56 gjcm2;yr.
Levee
The sediment accumulation rate for the interval of 4 to
20 em in core 14G is 0.12 gjcm2;yr. The excess 210pb
activity profile in core 14G displays a surface layer of low
excess 210Pb. The photo and radiograph (figs. 6 and 7) of
the core reveal faint 2- to 3-mm-thick laminae below a depth
of 8 em. These laminae appear discontinuous due to
bioturbation. overlying the laminae is a massive,
unconsolidated mud that has not undergone compaction and
dewatering. The low excess 210Pb activity in the top 1 em
of this massive mud indicates that relict reworked sediment
was deposited during the latest depositional event.
Cores 15G and 16G have sediment accumulation rates of 30
Figure 6. Photograph from piston core 14G showing 2- to 3-mm-thick laminae beginning at a depth of 8 em. l Overlying the laminae is a massive hemipelagic mud. ~ 31
1(
11
12
13
14
15
16
17
18
Figure 7. Radiograph from gravity core 14G. 32
2.04 and 0.58 gjcm2;yr respectively. There is no distinct
SML in the profiles for either of these cores (fig. Sd and
5e) . This indicates that no mixing has occurred, the rate of sediment accumulation was sufficient to prevent mixing by the benthic fauna, or the top layer of sediment was lost during the coring operation. Photos of cores 15G and 16G
(figs. B, 9, 10 and 11) reveal faint 2- to 3-mm-thick laminae, below a depth of 23 and 18 em, respectively. The massive, unconsolidated mud overlying the laminae indicates a constant rate of deposition on the levee.
Fan-Valley
Cores 69Bx, 49Bx, and 83Bx are located along the fan valley, upchannel from the oxbow meander. The cores display increasing rates of sediment accumulation with increasing water depth. core 69Bx was collected the furthest upchannel and, because of the high sand content, had the least penetration of the fan-valley cores. The excess 210Pb activity profile for core 69Bx displays no SML and no excess
210Pb was found below a depth of 6 em (fig. Sf). The rate of sediment accumulation calculated in core 69Bx is
0.09 gjcm2 jyr. Core 49BX has a s-cm-thick SML and a sediment accumulation rate of 0.47 gjcm2;yr. Below a depth of 17 em, the excess 21°pb activity profile for core 49Bx indicates that mixing has occurred (fig. 5g). The sediment accumulation rate for core 83Bx is 0.61 gjcm2/yr. No SML is 33
Figure a. Photograph from gravity core l5G showing 2- to 3-mm-thick laminae below a depth of 23 em. Overlying the laminae is a massive hemipelagic mud. 34
Figure 9. Radiograph from the trip core of gravity core 16G. 35
to 3 laminae is a massive hemipelagic mud. 36
1(
11
12
14
15
16
17
18
Figure 11. Radiograph from trip core of gravity core 16G. 37 displayed in the excess 210Pb activity profile for core 83Bx
(fig. Sh). The decrease in excess 210Pb is erratic to a depth of 21 em, indicating that mixing has occurred.
Core B4Bx was collected at the apex of the oxbow meander. A sediment accumulation rate of 0.17 g/cm2jyr was calculated for core 84Bx. The excess 21°Pb activity profile has a 3-cm-thick SML, and at a depth of 12 em no excess
210Pb was found (fig. si). The absence of excess 210pb represents reworked relict sediment that has been transported by a turbidity current formed by slumping.
Below a depth of 12 em the profile indicates that mixing has occurred.
Cores 87Bx and 90Bx were collected down the fan-valley from the oxbow meander. The sediment accumulation rate for
87Bx is 0.33 gjcm2;yr. The top 3 em in the profile for core
87Bx consists of sediment with a low excess 210Pb activity
(< 1.5 dpmjg), indicating that reworked relict sediment has been deposited (fig. Sj). The excess 210Pb profile for core
90Bx has abrupt changes in the amount of excess 21°pb at the depths of 2 em and 10 em that would prevent the calculation of a sediment accumulation rate (fig. Sk). If these data are not included in the calculation, the sediment accumulation rate is 0.28 gjcm2;yr.
Core 43Bx is the located the farthest down channel in the fan-valley. The sediment accumulation rate in the upper
14 em of the profile is 0.51 g/cm2Jyr (fig. 51). Below the 38 depth of 14 em the sediment accumulation rate is
0.20 gjcm2;yr. This change in the rate of sediment accumulation could be the result of a change in the sedimentation rate, an artifact created through bioturbation, or penetration by the core of a slump deposit from the walls of the fan-valley. A major slump from the wall found by Komar (1969) blocks a part of the fan-valley upchannel of core site 43Bx and may be the cause of the rate change.
Sur Slide
Cores 1PG, 2PG, and 4PG are located on the Sur slide south of the fan-valley. core 1PG was collected in a hummocky area overlain by mud-clast debris flows. cores 2PG and 4PG were collected in a transition zone between the slide and debris flow deposits (Normark and Gutmacher,
19 8 8) .
The excess 210Pb activity profile for core 1PG displays a relatively constant decrease in excess 210Pb with depth to the bottom of the core (fig. 5m). The sediment accumulation rate for core 1PG is 0.48 gjcm2/yr. Minor erratic values between the depths of 2 em and 4 em and at a depth of 6 em may have been caused by mixing or the deposition of reworlced relict sediment by low-density turbidity currents.
The sediment accumulation rate for core 2PG is
0.40 g/cm2jyr. No SML is displayed in the excess 210Pb 39
activity profile. At depths greater than 20 em, the profile
indicates that mixing has taken place (fig. 5n).
Core 4PG has a sediment accumulation rate of 1.35 gjcm2;yr. The excess 21Dpb activity profile for core
4PG shows erratic changes at the depths of 10 and 25 em
(fig. 5o). These changes are inferred to represent reworked
sediment deposited during low-density turbidity flows or bioturbation. Core 4PG is the only one of the Sur slide cores that penetrates to the layer of the background level of 210Pb supported by 226Ra. The radiograph of core 1PG (fig. 12) displays a massive unconsolidated mud. The radiograph for core 2PG (fig. 13) reveals faint 1-mm-thick
laminae below a depth of 16 em. The laminae indicate that this area on the slide has been a region of turbidite deposition. These laminae are not visible in the photograph
of core 2PG (fig. 14). 40
Figure 12. Radiograph from the trip core of gravity core lPG. 41
1
2E
27
28
29
30
31
32
33
34
35
36
Figure 13. Radiograph from the trip core of gravity core 2PG. 42
Figure 14. Photograph from the trip core of gravity core 2PG. DISCUSSION
The distribution of sediment accumulation rates for the
Monterey Fan is presented in figure 15. The range in the rates of sediment accumulation on the Monterey Fan indicates that several different depositional processes are active on the fan.
Supra fan
The sediment accumulation rate calculated for core lOOG
(0.56 gjcm2jyr) and the upper part of core 98G (0.05 gjcm2;yr) indicate that different depositional processes are active at these two core locations. The higher sediment accumulation rate in core 100G, which is downslope of core
98G, suggests that it may be located within a distributary channel. The lower rate calculated for core 98G indicates that it may be located on a levee or within an interchannel region of the suprafan.
The sedimentation rate calculated using 210pb geochronology greater than the 0.01 cmjyr calculated by
Hess (1974) in core 98G using the depth of a biostratigraphic marker. A problem with the rate calculated by Hess is that core 98G could have been influenced by carbonate dissolution. The biostratigraphic marker could have been produced by the dissolution of planktonic foraminiferans as they descend through the water column. Figure 15. Distribution of sediment accumulation rates on the Monterey Fan. Complex depositional conditions are indicated by varying rates of sediment accumulation on the suprafan, Sur slide, and within the fan-valley. 45
The sedimentation rate calculated by Hess (1974) therefore must be used with caution.
Levee
Cores 14G, 15G, and 16G form an east-west transect on
the northern levee. These cores trace a path that a low
density turbidity current would travel if flow stripping were to occur at the bend in the fan-valley. This muddy
flow would deposit material on the flank of the levee as it
moves towards the abyssal plain. Flow stripping involves
the overflow of the muddy upper layer of the turbidity
current (Piper and Normark, 1983). Elongate sediment mounds
on the northern Monterey Fan levee, observed by Normark et
al. (1980) 1 are thought to be formed by the flow stripping of low concentration turbidity currents approximately 100-
to 800-m-thick.
The range of sediment accumulation rates indicates that
the depositional conditions are not the same across the
levee. Core 14G has sediment accumulation rate of 0.12
gjcm2;yr. This rate is the lowest rate of the levee cores
suggesting that the core is located in a region of sediment
bypass or erosion during the flow stripping of turbidity
currents. core 15G has a sediment accumulation rate of 2.04
gjcm2;yr. This high accumulation rate on the levee may be
the result of low-density turbidity currents depositing
sediment in this region on the levee. A sediment 46 accumulation rate of 0.58 cmjyr at core 16G indicates that few of the low-density turbidity currents deposit sediment in this area.
Hein and Griggs (1972) calculated a sedimentation rate of 0.03 cmjyr on the levee to the northeast of the study cores using the depth of biostratigraphic marker. A sedimentation rate of approximately 0.04 cmjyr at the crest
(core 14G) is approximately equivalent to that calculated by
Hein and Griggs (1972).
Reasons for the slight differences in the sedimentation rates calculated by Hein and Griggs (1972) and those calculated using 21°Pb geochronology are the location of the cores sampled on the levee and the time span from which the rates are calculated. The location of the core used by Hein and Griggs (1972) is further upslope and closer to the crest of the levee than core 14G. This indicates that little deposition, or even erosion, may have occurred at the crest of the levee. The sedimentation rate found by Hein and
Griggs (1972) supports this hypothesis. The sedimentation rate calculated using 210pb geochronology covers a considerably shorter time span than the 12,500 yr used by
Hein and Griggs (1972). Periods of erosion or nondeposition in a 100 yr time span can be identified using 210pb geochronology, whereas these events are more likely to occur within a 12,500 yr time span and can not be identified. By identifying events of nondeposition or erosion using 210pb 47 geochronology, more accurate rates of sedimentation can be calculated and extrapolated over time. On this levee, these techniques produce the same results because despite any changes in the depositional conditions through time, the average sedimentation rates over the past 12,500 yr are equal to rates over the past 100 yr.
Fan-Valley
The sediment accumulation rates may vary along the fan-valley due to changes in the slope, the size and frequency of the turbidity currents initiated in the canyon, and the pending of sediments, which is caused by the slumping of material from the channel walls. Up channel from the oxbow meander the sediment accumulation rates increase (0.09 to 0.61 gjcm2;yr) with increasing water depth. Turbidity currents generated by the slumping of sediment from the canyon walls can create this type of sedimentation pattern if this region of the fan-valley is a zone of sediment bypass.
As a turbidity current travels through a bend in the channel, material may be eroded from the channel walls and become incorporated into the flow. This eroded relict material reduces the excess 210Pb activity of the sediment when deposited and results in erratic excess 21°pb activity profiles. In the top 3 em of core 87Bx (fig. 5j), and in core 90Bx, between the depths of 2 and 3 em (fig. 5k), a low 48
excess 210pb activity suggests the deposition of relict
sediment by a turbidity current.
Down channel from the oxbow meander the sediment
accumulation rate abruptly increases in core 43Bx. This
change indicates a change in the depositional conditions and may be due to turbidity currents overtopping an obstacle blocking the canyon. core 43Bx is located downslope of the
Sur slide. Turbidity currents formed by slumping in the Sur
slide may have deposited sediment in the fan-valley and
created a barrier in the channel. The sedimentation rate
for the upper layer of core 43Bx is nearly the same as the
rate (0.19 cmjyr) on the suprafan at the location of core
lOOG.
Sur Slide
Normark and Gutmacher (1988) used core 4PG and a hemipelagic sedimentation rate of 15 cm/1000 yr to estimate
an age of several hundred to 1500 years B.P. for the Sur
slide. The hemipelagic sedimentation rate was based on a paleoenvironmental study by McGann (1986) of Monterey-fan
levee sediments from a core located 50 km northwest of the
slide. Only the hemipelagic intervals since the time of movement were used in this calculation because turbidites
are theoretically deposited "instantaneously" relative to
hemipelagic mud.
Sediment accumulation rates calculated using 210pb 49 geochronology in cores 1PG, 2PG, and 4PG are 0.48, 0.40, and
1.35 cmjyr respectively. The sediment accumulation rate at core location 4PG is greater than the rates at core locations 1PG and 2PG. This may be caused by low-density turbidity currents initiated from either the region downslope of core 2PG or from another direction such as a seamount (fig. 2).
The thickness of the sediment overlying the slide at core locations 1PG, 2PG, and 4PG is 38, 87, and 73 em respectively. The age of the slide v1as calculated from each of the cores using the sedimentation rates and the thickness of the overlying sediment. Cores 1PG, 2PG, and 4PG gave the slide ages of 211, 580, and 143 years, respectively.
Although depositional conditions may vary between core locations, the ages for cores 1PG and 4PG are less than the age range estimated by Normark and Gutmacher (1988). The age of the slide is here estimated to be between 140 and 600 years B.P. The age calculated using core 2PG agrees with the youngest age estimate of "several hundred years" made by
Normark and Gutmacher (1988).
Effect of Moss Landing Harbor Dredge Spoil
The excess 210Pb activity profiles for the cores up the channel from the oxbovl meander do not imply a recent change in the sediment accumulation rate. The dumping of dredge spoil from Moss Landing Harbor into the head of Monterey 50
Canyon does not appear to have changed the sediment accumulation rates within the study area. The dredge spoil is either being trapped in the upper section of the fan-valley or is too widely dispersed to be detected. Only the profile for core 43Bx exhibits an increase in sediment accumulation. If this increased sedimentation rate were caused by the dumping of dredge spoil, then other cores upslope would also reveal an increase in the sedimentation rate. comparison with other Deep-Sea Canyons The Quinault submarine canyon is one of the conduits between the Washington continental shelf and the Cascadia Basin for sediment originating from the Columbia River (Thornbjarnson et al., 1986). The primary sedimentary regime in the Quinault canyon is hemipelagic deposition (Nelson, 1976). Depending upon the location in Quinault canyon, the sedimentation rates calculated using 21°pb geochronology range from 0.08 to 0.36 cmjyr. The sediment accumulation rates reported by Thornbjarnson et al. (1986) in the Quinault Canyon are comparable to rates in the Monterey fan-valley. sanford and Kuehl (1987) used 210Pb geochronology to determine the sedimentation rates in the Wilmington Canyon area of the North Atlantic Continental Margin. Rates of sedimentation were found to be 0.1 to 0.2 cmjyr on the open 51 slope and greater than 0.6 cmjyr within the meander bends in the canyon. The rates within the oxbow meander in the
Monterey fan-valley are significantly lower than those in the Wilmington Canyon meander bend. The meander bends in the Wilmington Canyon may be located closer to the shelf than the oxbow meander in the Monterey fan-valley. The sedimentation rates on the open rise near the Wilmington canyon are similar to those on the levee of the Monterey
Fan.
Sediment Accumulation and Depositional Processes
Hemipelagic deposition can produce similar rates of sediment accumulation over a wide area. Core 98G on suprafan has the lowest rate of sediment accumulation (0.03 to 0.05 gjcm2jyr) on the fan, suggesting that hemipelagic sedimentation is not the primary depositional process active on the fan.
Decreasing rates of sediment accumulation down the Sur slide indicate that the depositional conditions are not the same. Sediment accumulation rates at core locations lPG and
2PG parallel the rates calculated on the suprafan. The higher sediment accumulation rate at core location 4PG at the base of the slide indicates that other depositional processes have been active in the area. The higher rate of sediment accumulation could be caused by deposition from turbidity currents. 52
Within the fan-valley, sediment accumulation rates increase with increasing water depth, abruptly drop, and then slowly increase again with increasing water depth. Chinburg (1985) recognized slumps that blocked the canyon and caused the pending of sediment upslope of the slump. Within the channel meander, undercutting of the fan-valley walls may have caused slumping. Sediment accumulation rates are observed be highest within these traps created by slumping. Due to low-density turbidity currents, hemipelagic sedimentation is reduced in the fan-valley.
Implications Regarding Sea-Level Stow et al. (1985) reported that the sediment type, tectonic setting and activity, and sea-level variations are the primary controls on fan development and deep-sea sedimentation. Rates of sedimentation on submarine fans are thought to be influenced primarily by low stands of sea level (Shanmugam and Moiola, 1988). The Astoria Fan is situated off the continental rise of Oregon. changes in sea-level are thought to control the rates of sedimentation on the Astoria Fan (Shanmugam et al., 1985). During the low stand of sea-level in the Late Pleistocene, sedimentation rates within the interchannel areas were estimated to be greater than 0.05 cm;yr (Nelson, 1976). The sedimentation rates within the interchannel areas decreased to less than 0.01 cm;yr and 0.02 cm;yr in the main channels during the 53
Holocene rise of sea-level.
Sedimentation rates calculated for the Astoria Fan during both high and low stands of sea-level are less than the rates on the Monterey Fan. This indicates that eustatic sea-level changes may not be the primary control of sedimentation on the Monterey Fan. Both the Astoria and
Monterey Fans are sand-rich fan systems and therefore have a similar sediment type; however, the tectonic setting of each is different (Nelson, 1985). Unlike the Astoria Canyon, the head of Monterey Canyon is not isolated from the shoreline by a continental shelf. A constant supply of sediment to the head of Monterey canyon is therefore guaranteed by long-shore drift and erosion by tidal currents in Elkhorn
Slough (Yancey, 1968; Schwartz, 1983). This would indicate that the tectonic setting is the primary control of sedimentation on the Monterey Fan. CONCLUSION
Sediment accumulation rates on the Monterey Fan display trends at different locations. Large differences in the rates of sediment accumulation (0.05 to 0.56 gjcm2;yr)
indicate that the depositional conditions are not the same across the suprafan. Hemipelagic sedimentation is not the dominant depositional process on the suprafan during the present high stand of sea-level.
Within the fan-valley, varying rates of sediment accumulation (0.09 to 0.61 gjcm2jyr) indicate that
complex depositional conditions are present. Relatively steep gradients have created zones of sediment bypass, and the slumping from the channel walls has resulted in the pending of sediments along the axis of fan-valley.
The sediment accumulation rates on the sur slide are
0.40 to 1.35 gjcm2;yr. The age of the slide is inferred to be between 140 and 600 yr B.P. This older age estimate is equivalent to the youngest age estimate made by Normark and
Gutmacher (1988) in a previous study.
The dumping of dredge spoil from Moss Landing Harbor is
not reflected by an increase in sediment accumulation rates
on the fan. The sediment is not being transported out onto
the fan or it is not sufficient to cause a measurable
increase in sedimentation.
The sedimentation rates on the Monterey Fan and Astoria 55
Fan were compared. It was found that the tectonic setting, rather than variation in sea-level, is the primary control of sedimentation on the Monterey Fan. REFERENCES CITED
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APPENDIX 1: Total Pb-210 activity data for study cores. [The 226Ra-supported 210Pb for the core.]
SUB-BOTTOM DEPTH (em) 14G 15G 16G
0-1 3.03 +j-0.28 9.06 +j-0.39 20.40 +j-0.81 4-5 * 13.90 +j-0.60 8.32 +j-0.23 6-7 9.26 +j-0.43 8-9 10.16 +j-0.32 9-10 7.00 +j-0.45 8.03 +j-0.27 9.25 +j-0.41 11-12 5.43 +j-0.22 12-13 6.31 +/-0.28 16-17 2.95 +j-0.20 19-20 4.53 +j-0.47 4.67 +j-0.17 26-27 0.84 +j-0.15 0.82 +j-0.19
[ 1. 02 dpmjg] [1. 02 dpmjg] [1.02 dpmjg]
1PG 2PG 4PG
0-1 29.53 +/-1.09 15.35 +/-1. 05 7.18 +j-0.35 1-2 21.76 +j-0.73 16.58 +j-0.52 2-3 24.68 +j-0.66 9.35 +j-0.43 4-5 22.30 +j-0.73 6-7 10.60 +/-1.21 7.97 +j-0.34 7-8 11.94 +/-0.69 8-9 5.51 +j-0.34 9-10 6.92 +j-0.27 5.39 +/-0.34 2.31 +j-0.27 12-13 14-15 3.83 +/-0 .14 19-20 1.96 +j-0.21 3.27 +j-0. 29 20-21 * 3.31 +j-0.49 21-22 3.64 +/-0.14 24-25 * 3.95 +j-0. 28 25-26 * 1. 98 +j-0 .11 29-30 * 2.52 +/-0.41 34-35 2.16 +j-0.36
[1.02 dpmjg] [1. 02 dpmjg] [1. 02 dpmjg] * Not used in the calculation of the sediment accumulation rate. 62
SUB-BOTTOM DEPTH (em) 43Bx 49Bx 69Bx
0-1 39.70 +j-1. 23 48.72 +j-1.59 42.57 +j-1. 08 2-3 12.57 +j-0.29 34.45 +/-1.14 12.45 +j-0.57 4-5 7.53 +j-0. 34 41.25 +/-1.11 1. 76 +j-0.26 6-7 7.79 +j-0.41 9.85 +j-0.58 1.12 +j-0 .11 8-9 5.32 +j-0.30 5.99 +/-0.26 0.98 +j-0.09 10-11 5.16 +j-0.30 6.55 +j-0.28 0.92 +j-0.12 12-13 4.08 +j-0.24 3.28 +j-0.29 1. 09 +j-0.06 14-15 11.35 +j-0. 76 0.77 +j-0.18 0.96 +j-0 .11 16-17 6.13 +j-0.33 2.43 +j-0.21 0.83 +j-0.10 18-19 4.18 +j-0.35 * 3.79 +j-0.24 0.87 +j-0 .11 20-21 0.88 +j-0.17 * 4.33 +j-0.19 0.91 +j-0.21 22-23 2.16 +j-0. 20 0.96 +j-0.09
(1. 02 dpmjg] (1. 02 dpmjg] (0.96 dpmjg]
83Bx 84Bx 87Bx 0-1 15.00 +j-0.39 45.41 +/-1. 00 * 2.49 +j-0.28 2-3 9.55 +j-0.43 49.41 +/-1. 31 * 1. 37 +j-0. 21 4-5 8.11 +j-0.33 27.74 +/-0.73 10.24 +j-0.53 6-7 5.70 +j-0.43 7.66 +j-0.39 8.27 +j-0.38 8-9 7.35 +/-0.28 2.05 +j-0.21 5.12 +j-0.44 10-11 3.23 +j-0.24 1. 75 +j-0.16 2.50 +j-0.20 12-13 5.89 +j-0.20 0.93 +/-0.10 2.71 +j-0.11 14-15 5.45 +j-0.28 * 1.11 +j-0.22 * 7.25 +j-0.31 16-17 2.00 +/-0.15 * 3.08 +j-0.23 18-19 1. 96 +j-0.24 1. 00 +j-0.14 19-20 2.69 +j-0.17 20-21 6.23 +j-0.19 * 1. 78 +j-0.20 22-23 * 3.70 +j-0.19 [ 1. 02 dpmjg] [ 1. 02 dpmjg] [ 1. 02 dpmjg]
90Bx
0-1 39.58 +j-0.92 * Not used in the calculation 2-3 * 5.97 +/-0.26 of the sediment accumulation 4-5 13.90 +j-0.46 rate. 6-7 7.82 +j-0239 8-9 5.68 +j-0.21 9-10 3.68 +j-0.09 10-11 * 7.43 +j-0.26
(1. 02 dpmjg] 63
SUB-BOTTOM DEPTH (em) 98G 100G
0-2 17.66 +j-0.44 * 7.57 +j-0.17 2-3 14.74 +/-0.51 * 6.83 +j-0.26 4-5 6.85 +j-0.40 * 18.18 +/-0.66 6-7 7.66 +/-0.48 * 10.94 +j-0.40 8-9 6.68 +j-0.27 * 14.38 +j-0.28 10-12 4.17 +j-0.11 * 15.41 +j-0.32 12-13 * 6.76 +j-0.31 * 11.55 +j-0.35 14-15 * 7.04 +j-0.31 16-17 * 5.05 +/-0.20 * 9.98 +j-0.22 18-19 6.87 +/-0.30 15.66 +j-0.31 20-22 7.98 +j-0.44 12.72 +j-0.44 22-23 4.40 +/-0.14 9.91 +j-0.47 30-31 2.40 +j-0.28 3.58 +j-0.09
[1.02 dpmfg] [1.02 dpm/g]
* Not used in the calculation of the sediment accumulation rate.