Environ. Sci. Technol. 1989, 23, 294-303
Scientist for the Alligator Rivers Region. We wish to thank Ranger Uranium Mines for property access and unpublished water analyses, Nicholas Currey for technical assistance, and Judith Dean and Glen Riley for numerous comments which clarified the manuscript. Registry No. 'OSPb, 13966-28-4; 204Pb,13966-26-2; 20BPb,
Ritchie, J. A. DSIR New Zealand, Chemistry Division, Report No. C.D. 2164, 1973; 24 p. Florence, T. M.; Batley, G. E. Talanta 1975,22, 201-204. Florence, T. M.; Batley, G. E. Talanta 1976,23, 179-186. Florence, T. M.; Batley, G. E. Talanta 1977,24, 152-158. Batley, G. E.; Gardner, D. Estuarine Coastal Mar. Sci. 1978, 7, 59-70.
13966-27-3.
Bio-Rad Laboratories Chemical Division Technical Bulletin
Literature Cited (1) Gulson, B. L. Lead Isotopes in Mineral Exploration; Elsevier: Amsterdam, 1986. (2) Dickson, B. L.; Gulson, B. L.; Snelling, A. A. J. Geochem. Exwlor. 1987.27. 63-75. (3) MEClennan, S . hi.;Nance, W. B.; Taylor, S. R. Geochim. Cosmochim. Acta 1980,44, 1833-1839. (4) Supervising Scientist for the Alligator Rivers Region, Annual Report 1986-1987; Australian Government Publishing Service: Canberra, 1987. (5) Schmuckler, G. Talanta 1965, 12, 281-290.
Kingston, H. M.; Barnes, I. L.; Brady, T. J.; Rains, T. C.; Champ, M. A. Anal. Chem. 1978,50, 2064-2070. Flegal, A. R.; Patterson, C. C. Earth Planet. Sci. Lett. 1983,
114, 1973.
64, 19-32.
Flegal, A. R.; Schaule, B. K.; Patterson, C. C. Mar. Chem. 1984,14, 281-287.
(15) Ludwig, K. R.; Grauch, R. I.; Nutt, C. J.; Nash, J. T.; Frishman, D.; Simmons, K. R. Econ. Geol. 1987,82,857-874. (16) Florence, T. M.; Batley, G. E. CRC Crit.Rev. Anal. Chem. 1980, 9, 219-296.
Received for review March 15,1988. Accepted September 9,1988.
History of Metal Pollution in the Southern California Bight: An Update Bruce P. Finney" and Chih-An Huh College of Oceanography, Oregon State University, Corvallis, Oregon 9733 1
Box cores collected in 1985 and 1986 along an offshore transect in the Santa Monica Basin, CA, were analyzed for organic carbon, calcium carbonate, U-series radionuclides, and a suite of major and minor elements. Downcore profiles of metals and organic carbon reflect anthropogenic influence and diagenetic processes. The deep basin cores show pronounced subsurface maxima in Pb, Zn, Cr, and organic carbon during the time interval 1960-1970. Increases from the base of the core to this time horizon are consistent with increasing anthropogenic inputs to the Santa Monica Basin. The near-surface decreases in heavy-metal accumulation reflect recent improvements in waste water treatment. Sediment compositions cannot be fully explained by simple mixtures of sewage particles and natural sediment because of factors such as diagenesis of sewage during transport and sedimentation, additional metal sources, and variations in surface water productivity. The yellow-brown surface layer in deep basin cores is relatively enriched in Fe, Co, Cu, and P. This layer forms by reduction of Fe followed by upward diffusion and precipitation as amorphous oxyhydroxides; P, Cu, and CO, released during early diagenesis, are associated with this phase. Introduction Numerous studies have documented that man may perturb the geochemical cycles of trace elements in the coastal marine environment (e.g., ref 1-3). Sediment cores recovered near regions of high human population and high industrial activity contain, in part, a record of the history of pollution ( I , 4 , 5 ) . The sedimentary record may be distorted, however, by bioturbation and other sediment mixing processes, anomalous sedimentation events such as slumps and turbidites, and diagenesis within the sediments. Sediments in the anoxic basins of the southern California borderlands are well suited for studies of the influence of human activities on sedimentation. The sediments accumulate rapidly enough so that they may be
* Present address: Department of Geology, Duke University Marine Laboratory, Beaufort, NC 28516. 294
Environ. Sci. Technol., Vol. 23, No. 3, 1989
sampled with high resolution over the past several hundred years. In addition, bioturbation is not a significant factor in disturbing the sedimentary record in portions of the nearshore basins. As part of the DOE CaBS (California Basin Study) program, we have studied cores from the Santa Monica Basin collected during cruises in 1985 and 1986. The last reports of heavy metals and other elements in cores from this basin are those by Bruland et al. ( I ) and Ng and Patterson ( 4 ) on cores recovered in 1971 and 1977, respectively. Because of the improvements in waste treatment and pollution control during the intervening years, a fresh assessment of the sedimentary record in this region seems necessary. Our goal is to add to the understanding of how sediments monitor the marine environment by determining the important transport pathways and chemical processes that affect metals during transit to and burial in sediments. To add to the earlier studies in this basin (e.g., ref 1, 4, 6, and 7), our strategy is to increase the number of elements and sedimentary components studied in an offshore transect of newly collected cores which are sampled at a high resolution. Site Characteristics The Santa Monica Basin (Figure 1) is one of the inner basins of the California Continental Borderland, lying directly offshore of Los Angeles. The center of the basin is -30 km from land and the maximum water depth is 910 m. The bottom water in the basin is nearly anoxic below the sill depth of 737 m. The basin is landward of the continental slope and the California current; the currents are associated with the Southern California Countercurrent and Davidson Current (part of the California Current System) and flow predominantly to the northwest. Currents are highly variable over seasonal and annual time scales. Upwelling events are sporadic and of relatively short duration compared to major upwelling regions. Winds are predominantly westerly, but offshore winds occur during periods of Santa Ana conditions. Within 40 km of the deep basin are the outfalls of the two largest waste water treatment plants in the Southern California
0013-936X/89/0923-0294$01.50/0
0 1989 American Chemical Society
Figure 1. Core locations and bathymetry of the Santa Monica Basin. The core studied by Bruland et al. ( 1 ) is indicated by an X. Locations of Hyperion and JWPCP outfalls are also shown.
Bight, the Joint Water Pollution Control Project (JWPCP) plant of the Los Angeles County Sanitation District and the Hyperion plant of the City of Los Angeles. Studies of U-series radionuclides in basin sediments (8) showed that sedimentation and bioturbation rates decrease with water depth or distance offshore. Sediment accumulation rates ranged from about 80 to less than 20 mg cm-2 year-'. Sedimentation is more dynamic in the basin slope region with slumps, turbidites, and abrupt changes in sedimentation rate (8, 9). In the deep basin the sediments contain a yellow-brown surface layer overlaying green sediments. Basin slope sediments are green throughout the core and have a coarser texture than the deep basin.
Methods Cores were collected during the Cross-I (October 1985) and Basin-I (May 1986) cruises, onboard the R. V. New Horizon, by using a Soutar box corer or the MANOP bottom lander. These coring devices have been demonstrated to capture undisturbed samples; examination of the cores and laboratory analysis of 234Thand other constituents indicated undisturbed recovery of the sediment-water interface. Cores were immediately subsampled aboard ship and kept frozen until analysis in the laboratory. The samples were weighed before and after drying to determine their water contents and then ground to a fine powder in a mortar and pestle. The concentrations of major and minor elements were determined with a Phillips PW 1600 X-ray fluorescence (XRF) spectrometer. A set of more than 60 USGS, NBS, international,
and in-house standards was used to calibrate the machine. The precision for most determinations is within 5% and the accuracy within 10%. Organic carbon and calcium carbonate were measured by the LECO wet oxidation technique of Weliky et al. (IO). 210Pbwas measured in the cores by the techniques described in Huh et al. (8). All data were corrected for salt content based on XRF-determined C1 content. Results and Discussion Sediment Chronologies and Mixing Rates. Sediment accumulation rates determined from profiles of excess 2'oPb were used to assign dates to the downcore samples. For purposes of simplifying the following discussion, we will focus on profiles from two cores, Basin-I LBCl (910 m) and Cross-I BC12 (750 m), which are representative of the deep basin and basin slope, respectively (Figure 1). The 210Pb-basedaccumulation rate for LBCl is 13.6 mg cm-2 year-l (Figure 2). Sediment mixing in the anoxic deep basin, which is relatively low and decreases by 3 orders of magnitude in the top 3 cm, should not significantly alter the sedimentary record (8). The excess 210Pb profile in BC12 indicates an accumulation rate of 78 mg cm-2 year-' (Figure 2). Sediment mixing in this core, evidenced by burrows and the 210Pbprofile, is extensive to depths of 4 cm or greater. Lead, Chromium, Zinc, and Organic Carbon. The heavy metals Pb, Zn, and Cr display subsurface maxima in cores from both the basin and slope regions (Figures 3 and 4). In cores recovered from the deep basin the maxima are pronounced and found at depths ranging from about 1 t o 2 cm. These peaks correspond to the time Environ. Sci. Technol., Vol. 23, No. 3, 1989
295
Pb-210 (ex)/Al
Pb-210(ex)/Al 100
S=l3.6mg/cm2/yr/ S=13.6 mg/cm2/yr
.I
1 1
Table I. JWPCP Fluxes (Metric Tons per Year)O 10
S=78rngicm2/yr S=78 rnglcni2/yr
"J1
Zn Cr Pb cu suspended solids Pb/ Zn Pb/Cr Zn/Cr
1971
year 1981
1985
1400 460 140 270 167000 0.10 0.30 3.0
250 110 45 80 84000 0.18 0.41 2.3
95 40 25 30 43000 0.26 0.62 2.4
'From Stull et al. (13). Table 11. Excess Inventories, Peak Excess Fluxes, Present Day Excess Fluxes, and Background Fluxes for the Santa Monica Basin Cores
2.0'
'
Basin4 LBCl
I
/
interval of 1960-1970 (Figure 3). The onset of rapid increases in heavy-metal contents occurred around 1930. Cores from the basin slope have more diffuse maxima deeper in the core (Figure 4). The apparent ages of the heavy-metal increases in this core are earlier, and the apparent time intervals represented by the metal enrichments are longer, compared to LBC1, probably because this core is subject to significant mixing of material by bioturbation. Data on these metals from the core studied by Bruland et al. (I),recovered near LBCl in 1971 (Figure l),showed increases from the base of the core to the sediment-water interface. Several factors may account for the differences we observe. If the coring operation failed to sample the sediment-water interface of the core studied by Bruland et al. (I),then the offsets could be due to core top loss. However, water content data ( I ) and shipboard observations (11) indicate this was not the case. Another reason for an offset could be that the Bruland et al. (1)core was sampled at a much coarser scale than LBCl (1cm vs 0.25 cm). However, even if our data were averaged at l-cm intervals, we would still observe the downcore peak. Yet another possibility is that the location of the downcore peak is controlled by diagenetic processes, even if the ultimate source for most of these metals is anthropogenic (12). Diagenetic zones in LBCl may be offset from those in the core studied by Bruland et al. (I) due to either spatial redox intensity variations or redox changes since 1971. Because all of our cores from the deep basin show downcore maxima in these metals, we can rule out variable redox conditions across the deep basin as the main factor controlling the differences. The most likely explanation for the differences between the 1971 and 1986 profiles is that the downcore distributions of these elements are controlled largely by anthropogenic inputs to the basin. This conclusion was reached by previous workers studying the California Borderland basins by comparing the fluxes of various natural and anthropogenic sources (1,2,4), and from changes in the isotopic composition of Pb in the sediments (4, 6). If sediments in the basin record changes in human activities 296
Envlron. Sci. Technoi., Vol. 23, No. 3, 1989
peak excess
background
present exce8s
Pb Zn Cr Corg
62 105 243 24000
Cross-I BC12 1.1-1.2 (1945-1975) 1.5-1.9 (1945-1971) 3.7-4.4 (1956-1971) 505-580 (1939-1971)
1.2 10 8.4 2000
0.17 0.59 2.0 28
Pb Zn Cr
22 27 52 10000
Basin-I LBCl 0.52 (1968) 0.77 (1959) 1.5 (1963) 290 (1968)
0.18 1.9 1.5 500
0.21 0.00 0.48 116
Cross.1 BClZ
8 Figure 2. The ratio of excess *"Pb to AI plotted against cumulative sediment dry mass (on a salt-free basis) and calculated sediment accumulation rates, for the Basin-I LBCl (33'44.56' N, 118°50.79' W) and Cross-I BC12 (33'50.10' N, 118°41.30' W) cores. Excess *"Pb has been normalized to AI to avoid bias caused by variable amounts of authigenic Fe oxyhydroxldes and calcium carbonate.
flux, fig ern+ year-'
excess inventory, pg cm-2
Cor%
'Core top sample (except for organic carbon, where the second sample is used).
over time scales of less than a decade or so, then the major improvements in waste water treatment since the early 1970s (Table I), and reductions in other sources of metal pollutants such as the phasing out of leaded gasoline around 1970 ( I 4 ) , may be reflected in the near-surface decreases in metal concentrations we observe. Recent decreases in accumulation of these metals, attributed to decreased environmental loadings, have also been noted in cores from the Santa Barbara Basin ( I 5 ) ,Puget Sound (5),Lake Michigan (16),and in corals from near Bermuda (14). Both LBCl and BC12 have downcore maxima in organic carbon that occur a t the same depth as the heavy-metal peaks (Figures 3 and 4). The enrichment in organic carbon is 58 and 29%, respectively, of the mean background content found at depth in the two cores. The surface organic carbon maxima found in the core studied by Bruland et al. (1) correlates with the time of this peak. The factors most likely to produce changes in sedimentary organic carbon content with time are variations in productivity or preservation of organic carbon produced by plankton in surface waters, or variations in the input of terrestrial organic carbon. To help evaluate the sources and transport pathways of metals and organic carbon to the Santa Monica Basin, we have partitioned the chemical composition of the sediments into two fractions, which we term background and excess. The background component is defined for each core by the composition a t depth where the metal or organic carbon to AI ratio reaches a constant level. The excess fraction is calculated by subtracting the background fraction from the measured value. The excess inventories of Pb, Zn, Cr, and organic carbon in each core are shown in Table I1 along with background and excess fluxes. The
Ni (ppm)/Al (%) 0.6
0
1.0
1.4
. " " ' . '
10.0
8.0
1.8
12.0
.1980
2
i
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-1970
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.19m
.1930
.1930
u .
zr : h
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Cr (ppm)/Al (%) 25 3s .
'
. '
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Organic Carbon (%)/AI (%) "
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Cu (ppm)/Al (%)
P (%)/AI (%) 4
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.
5
'
6
7
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'
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.1930
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.1900
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Calcium Carbonate (%) 12.0
0
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78
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82
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'
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0
2U
Co @pm)/Al (%)
Mn (ppm)/Al (%)
20.0
.1930
.1930
4-
.1900
.1900
6-
6-
8'
8'
,1930 h
4-
Y
.1900
,1820
.1520
S (%)/AI (%) 0.04
0.08
0.12
0.16 .1980 ,1970
,1930 h
v
.1900
6-
,1820
8J
Figure 3. Downcore distributions of Pb/AI, Zn/Al, Cr/AI, organic carbon/Al, calcium carbonate, Ba/Al, S/AI, Fe/AI, Co/AI, P/AI, Cu/AI, Mn/AI, and Ni/AI since about 1800 in core Basin-I LBC1. Ages of the sediment horizons, from the *"Pb-based chronology, are Indicated. The data in this and the following figure have been normalized to AI to avoid blas caused by variable amounts of authigenic Fe oxyhydroxides and calcium carbonate. Environ. Sci. Technol.. Vol. 23, No. 3, 1989
297
Pb (ppm)/Al (%) 1
2
3
Fe (%)/AI (9%)
4
5
0.60
0
0.65
0.70
0.75
6.0
8.0
7.0
0 5 5
10
-!
10
E15
15
0
v
20 20
25
25
,1820
30
,1820
1
30'
Cr (ppm)/AI (%) 14
16
18
20
P (%)/AI (%)
22
Cu (ppm)/Al ( % )
24
0
O
5
5-
"
"
'
.
3.5
'
10
.1930
6
$15
.1900
h
0
15:
4.5
5
.1960
10:
.lo
4.0
0
-1900
15
,1900
v
20
20-
25
20
25.l820
30
25 .I820
,1820
30-
30
Calcium Carbonate ( % ) 6.0
6.2
6.4
6.6
6.8
Mn (ppm)/AI (70) 7.0
Co (ppm)/Al(%) 8
.5
0
0
0
5
5
5
U1°
10
10
5
-
E15
=15
D
15
3
v
20
20
20
25
25
25
30
30
30 S (%)/AI (70)
0
.1900
Figure 4. Downcore distributions of PbIAI, Zn/Al, Cr/AI, organic carbonlAl, calcium carbonate, Ba/Ai, S/AI, FelAi, ColAI, PIAI, Cu/AI, Mn/AI, and NVAI since about 1800 in core Cross-I BC12 (note different depth and concentration scales from Figure 3). Ages of the sediment horizons, from the "'Pb-based chronology, are indicated. 298
Environ. Sci. Technol., Voi. 23, No. 3, 1989
80
y = 2.14 + 1 . 4 0 ~ R
3
0.76
I
y
= -.M + 0 . 0 5 4 ~R = 0.86
h
8
0
20 30 40 Pb (excess, ppm)
10
0
50
10
30
20
40
50
Pb (excess, ppm)
h
k
loo
a2
0
0
20 30 40 Pb (excess, ppm)
10
k 0
50
40
20
60
80
Zn (excess, ppm)
L M 0
u
I 0
0
20
40
60
80
Zn (excess, ppm)
Flgure 5. Plots of excess Zn vs Pb, Cr vs Pb, and Cr vs Zn in the two cores studied. Here, and in the following figures, core Cross-I BC12 is Indicated by the open symbol. The line through the origin is the mixing line expected if the compositlon of the sediment resulted from the addition of JWPCP sewage particulates (composition given in Table 111). The other line is the least-squares fit to the data.
bulk sediment accumulation rate is a factor of 6 higher in the shallower core and the excess inventories listed in Table I1 are a factor of 3-4 greater. This is reflected in the relative enrichments of the excess component (Le., more distinct maxima in LBC1). A hypothesis that explains both the organic carbon and heavy-metal variations is that the sediments receive a direct input of sewage particles. The similar downcore profiles of Pb, Zn, Cr, and organic carbon in both cores (Figures 3 and 4) suggest that the chemical composition of sediments may be influenced by the deposition of sewage particles, which are relatively enriched in organic carbon and metals (2). Sediment studies in areas adjacent to outfalls indicate that a significant proportion of sewage particles escape deposition in the immediate shelf area (2, 17),and thus some of this material may be deposited in the nearshore basins. The distributions of Pb, Zn, Cr, and organic matter in Santa Monica Bay and Palos Verdes Shelf sediments (13, 18) suggest that sewage material is transported from the JWPCP outfall northwest along the Palos Verdes Shelf and down Redondo Canyon into the deep basin. Therefore, the decreases in anthropogenic flux
50
100
150
Cr (excess, ppm) Flgure 6. Plots of excess organic carbon vs excess Pb, Zn, and Cr in the two cores studied. The line through the origin is the mixing line expected if the composttion of the sediment resulted from the addition of JWPCP sewage particles (composition given in Table 111). The other line is the least-squares fit to the data.
from BC12 to LBCl (Table 11) are consistent with the hypothesis of particulate sewage input. Sediment and sediment trap studies in the California borderland basins (8,19-22) indicate that near-bottom transport of particles is in fact an important mechanism in supplying materials to the deep basins. Thus it seems possible that the inner basins of the Southern California Bight receive an input of sewage particles. The relationships between excess heavy metals (Figure 5) and excess heavy metals and organic carbon (Figure 6) demonstrate that the organic carbon and metal contents of these sediments may be described by mixing natural sediment with a component enriched in organic carbon and heavy metals. Table I11 and Figures 5 and 6 summarize the results of a mixing model that assumed that the excess component is derived from addition of sewage particles. JWPCP sewage composition data are used because it is the source most likely to affect the study area, as discussed, due to the prevailing currents. In addition, JWPCP inputs dominated the flux of anthropogenic partkulate matter to the Southern California Bight from 1937 until the early 1980s (2, 13). The composition of the sediments is not fully explained by the sewage mixing model. For example, the organic Environ. Sci. Technol., Vol. 23, No. 3, 1989
299
Table 111. Composition of the Excess or Anthrononenic Comoonent of Sediment Cnrrpannndinfl
Basin-I LBCl (1.0-1.5 cm, 1968-1972) excessb predictedC 2.3 2.3 44.5 53 116
Cross-I BC12 (3.0-4.0 cm, 1971) excessb predict ed
Sewage" (JWPCP, 1971)
0.45
32
13.5 21 51
8 58 24 16
570 4100 1700 1120 220 150 1.2 3 100
21.5 0
11
1 0 0
0.50 2.8
0.09
0.06
0.2
0
7.2
Tinle Period of About Raa111te1-l from
0.45
41 295 122 81 16
11
tn
3
2
0.02 0.04 1.4
"From Bruland et al. (I),Myers (171, and Sweeney et al. (22). *Concentration above the base-line level (constant element/Al ratio) at depth in the core. Calculated by assuming that all the excess organic carbon is derived from sewage particles and that no degradation of oreanic carbon or loss of metals occurs.
carbon concentration found around 1971 in LBCl can be explained by a 7% contribution of sewage particles (Table 111). While the amount of excess P b and Cr predicted by the mixing model is similar to that observed, the predicted excess Zn is 240 ppm greater than observed. Processes that may result in interelement relationships different from those predicted include inhomogeneous distributions of metals between different size fractions of sewage particles, supply of metals and organic carbon from other sources, and mobilization of metals and organic carbon from sewage particles. The limited available data suggest that particle size fractionation cannot account for the observed Zn deficiency (23-25). Of the three metals, Cr does not have significant nonsewage sources ( I ) , whereas nonsewage sources (primarily atmospheric) are most likely to be important for P b (I,26,27). Roughly two-thirds of the P b deposited in the basin is estimated to be sewage-derived, however (4). Thus, the discrepancies between the predicted and observed excess Zn/Cr and Zn/Pb ratios would not be expected from inputs from other sources. Studies of mobilization of metals from Southern California sewage indicate that Zn is preferentially mobilized relative to Cr and probably also to P b (23,25,27,28).Our data are in accord with these trends and suggest significant mobilization of Zn in the far field. Previous studies indicate that at least 25-40571 of the sewage-derived organic carbon will degrade in the sediments due to bacterial degradation (17,22).It is surprising that the mixing model accounts for the organic carbon, Cr, and P b contents in the deep basin core, because other work has shown that the organic matter will decompose in sediments a t greater rates than trace elements are mobilized (13,22,29). If this is the case, an additional source is required to account for some of the excess organic carbon. As previously mentioned, an alternative source that may supply some excess organic carbon to the sediments is surface water production. Enhanced accumulation of marine organic carbon may result either from increases in surface water productivity or from changes in factors controlling organic carbon preservation in the sediments (30-32). The chief factor thought to enhance carbon preservation in sediments is a decrease in bottom water oxygen (30). However, because bottom water in the basin has been virtually anoxic throughout recent times, it seems unlikely that this mechanism alone could account for the organic carbon changes. Surface water productivity in the 300
Environ. Sci. Technol., Vol. 23, No. 3, 1989
Southern California Bight varies significantly on seasonal and annual time scales. Smith and Eppley (33)estimated primary production in this region for the period 1920-1979. Their time series of estimated productivity averaged at 5-year intervals bears some resemblance to the sedimentary organic carbon record with relatively low values in the 1920s and 1930s and highest values in the 1960s. Thus some of the recent organic carbon maxima may be due to increased productivity. Supply rates of anthropogenic metals may also be influenced by productivity variations through process such as scavenging and rapid particle transit (34). Calcium Carbonate. The downcore calcium carbonate distributions are different in the two cores (Figures 3 and 4). In the slope core, the profile shows little downcore variation, with an average calcium carbonate content of 6.5%. In the deep basin core, the calcium carbonate contents are much higher and show marked downcore variations ranging from 12 to 20%. X-ray diffraction analysis and other studies (19) indicate that the carbonate is predominantly of biogenic origin. If plankton community compositions in surface water and calcium carbonate saturation levels in bottom water have not significantly varied over these time scales, then the calcium carbonate profiles may also be a time series record of surface water productivity (31, 35). The calcium carbonate record in BC12 may not be indicative of surface water processes, because a significant proportion of biogenic and detrital material is supplied to the slope region by near-bottom transportation pathways (36). In the deep basin, downcore records of calcium carbonate and organic carbon are positively correlated, though the most recent calcium carbonate maxima are accompanied by significantlygreater organic carbon levels than older calcium carbonate maximum of equal magnitude (Figure 7). Because sewage has low calcium carbonate contents, mixtures of small amounts of sewage particles with natural sediment would not be expected to result in marked calcium carbonate variations (Table 111). Thus, the downcore patterns of organic carbon and calcium carbonate in LBCl are compatible with the explanation that the recent organic carbon maxima resulted from both increased productivity and sewage input. Barium. The subsurface peak in the Ba profile in LBCl corresponds to about 1959, slightly earlier than the time horizon observed for peaks of Pb, Cr, and organic carbon (Figure 3 and Table 11). The age of this peak is similar to that determined by Ng and Patterson (4) for Ba en-
1900:
1850: 1800 1 1750:
1700 1650 1600
3
4
5
Organic Carbon, %
10
12
14
16
16
20
CaCO3, Vi
Flgure 7. Records of organic carbon and calcium carbonate over the last -400 years in Basin4 LBC1. The ages were estimated from the *’OPb-based sediment accumulation rate. Note that the organic carbon maximum centered at 1970 is significantly greater than maxima further downcore, while amplltude of the downcore calcium carbonate variations is more constant.
richments in cores from the Santa Monica and San Pedro Basins. They attributed this peak to the possible “waxing and waning” of a primary industrial source. Other work suggests that the Ba enrichment may be due to barite from oil-drilling operations and dumping in the San Pedro Basin (37). Subsequent transport of this material into the Santa Monica Basin may occur over the deep sill between the two basins (21). The Ba enrichment predicted from the addition of sewage particles i s much less than observed in LBC1, but compatible with the Ba profile observed in BC12 (Figure 4). The natural Ba flux is apparently much greater than the flux of anthropogenic Ba transported to the shallower site. Sulfur. Excess S correlates well with organic carbon (Figure 8) and the heavy metals in LBC1, but not in BC12, The positive correlations of heavy metals and organic carbon with S cannot be accounted for by mixing sewage particles with normal background sediment (Table I11 and Figure 8). Nor can the excess S be balanced by sufficient excess cations to be predominantly in a sulfide phase, as evidenced by the inverse relation of Fe to S in the S-rich zone in LBCl (Figure 3). In addition, X-ray diffraction analysis on S-rich samples did not detect significant quantities of sulfides. These observations suggest that a substantial portion of the excess S may be in an organic form, and that the source of this S is probably seawater sulfate. Recent work by Francois (38) indicates that 30-5070 or more of the S in organic-rich sediments may be organic sulfur. The good correlation of excess organic carbon and S in the deep basin core suggests that organic sulfur is added to the sediments in proportion to organic carbon content; additional processes appear to influence the near-surface S distribution in the shallower core. Iron, Cobalt, and Phosphorus. It appears that diagenetic processes within the surface sediments of the deep basin strongly influence the downcore solid-phase profiles of Fe. The Fe content of the yellow-brown surface layer in these sediments ranges from 8 to 11% , in contrast to the green sediment below with Fe contents of 5-6.570 (Figure 3). Mineralogical and major element analyses indicate that the color change does not result from a major change in sediment type. It seems likely that the Fe enrichment results from reduction of Fe in the sediments during the oxidation of organic matter (391, followed by upward diffusion to the sediment-water interface where it precipitates as amorphous oxyhydroxides. This is consistent with pore water studies at the site, which show dissolved Fe concentrations increasing from seawater values at the sediment-water interface to a maximum at
0.0
0.2
0.4
0.6
S (excess, 9%) Flgure 8. The relationship between excess organic carbon and excess S in the two cores studied. The line through the origin is the mixing line expected if the composition of the sediment resulted from the addition of JWPCP sewage particles (composition given In Table 111). The other line is the least-squares fit to the data.
1-2-cm depth (40),and the color change observed in the sediments (41). The slope core does not show any pronounced Fe enrichments (Figure 4). The slope region is less reducing than the deep basin (40),and diagenetic signals are less evident due to the 6-fold faster sedimentation rates, extensive bioturbation, and a much broader Fe oxidation zone. Interestingly, the surface Fe enrichment was not found in the core studied by Bruland et al. (I). If this difference is not due to core top loss, then it may indicate that redox conditions in deep basin surface sediments have changed since 1971. This idea is consistent with higher contents of arganic carbon in the sediments during the 1960s and early 1970s (Figure 3), Periods of greater organic carbon flux should result in more reducing conditions in sediments (32) due to greater depletion of oxidants. More reducing conditions may have resulted in diffusion of dissolved Fe from the sediments, and thus, the Fe-rich surface layer would not form. Fluxes calculated from Fe pore water data (40) indicate that the Fe-rich layer could form in 15 years, in support of this hypothesis. The Fe-rioh surface layer of the deep basin core shows marked surface enrichments in P, Co, and Cu (Figure 3). The affinity of P and Co with Fe oxyhydroxides has been noted in ferromanganese nodules and crusts (42). During early diagenesis in the sediments, P, Cu, and Co are released near the sediment-water interface (40,43-45) where they are likely adsorbed to the Fe oxyhydroxides (43). Copper. The Cu profiles in deep basin and slope cores display different characteristics (Figures 3 and 4). The profile in LBCl shows both a subsurface peak associated with the Pb, Zn, and Cr maxima, and a surface enrichment. In BC12 there is a very slight surface enrichment. Small contributions of sewage particles should result in easily detectable Cu enrichments in sediments because sewage particles are strongly enriched in Cu (1120 ppm; ref 1). The amount of excess Cu observed in both cores is far less than that predicted by the sewage mixing model (Table 111). While much of the excess Cu in LBCl may be of anthropogenic origin, some Cu is associated with the Ferich phase. These observations suggest that the Cu profiles are strongly influenced by diagenetic processes and not solely related to anthropogenic supply; it appears that extensive Cu cycling occurs before burial. Manganese and Nickel. Downcore Mn abundances vary little across the basin (Figures 3 and 4). The Mn/A1 ratio is similar in both cores, and downcore variations are Environ. Sci. Technol., Vol. 23, No. 3, 1989
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within 5% of the mean value. This is consistent with the large natural flux of Mn relative to anthropogenic sources in the Southern California Bight ( I ) . The concentration of Mn in sewage particles is 150 ppm ( I ) , much less than that found in the sediments. Thus inputs of waste water particles are not likely to noticeably influence sedimentary Mn abundances (Table 111). In addition, Mn is easily solubilized in anoxic environments and may be mobilized from sewage particles (25). In contrast, downcore profiles of Ni differ between the cores (Figures 3 and 4). Ni has little downcore variation in BC12, but is enriched by -15% during the past 50 years in LBC1. This Ni enrichment is much less distinct than the enrichment in Pb, Zn, and Cr. Sewage particles are enriched in Ni (220 ppm; ref 1)relative to the background concentration in the sediments, but less so than Pb, Zn, and Cr. Because of the faster rate of sedimentation of natural to anthropogenic particles in BC12, Ni enrichments are not likely to be detectable (Table 111). The amount of excess Ni observed in LBCl is generally compatible with the simple mixing model (Table 111). However, Ni may be solubilized from the sewage particles (25) or take part in diagenetic reactions in the sediment (43).
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Conclusions Profiles of Zn, Pb, and Cr in cores recovered from the Santa Monica Basin indicate that pollution levels in sediments have decreased dramatically since the time of peak contamination around 1970 (Table 11). The concentration of Zn currently observed in near-surface sediments from the deep basin is similar to base-line levels deposited before about 1930. The present day fluxes of anthropogenic P b and Cr deposited in these sediments are 40 and 35%, respectively, of their maximum flux. These changes are in accord with improvements in waste water treatment at the JWPCP plant, which have led to drastic reductions in inputs of these materials to the Southern California Bight. The sedimentary record reveals that even at sites relatively far (25-40 km) from the outfalls, sediments respond quickly (on the order of a few years or less) to changes in pollution control. The well-documented association of sewage-derived metals with particulate matter (2, 3, I7), and the strong relationship between these metals and organic carbon in the sediment cores, strongly suggest that changes in the composition of Santa Monica Basin sediments are due in part to the addition of sewage particles. The chemical composition of the polluted component of the sediments does not directly reflect the composition of the waste water particulates, however. Factors such as degradation and diagenesis of sewage particles during transport and sedimentation and supply of metals from atmospheric and other sources, are superimposed upon this input. Biological processes in the water overlying the site influence metal cycling through the scavenging and transportation of metals, fine particulate material, and organic carbon to the sea floor. In addition, diagenetic processes associated with the breakdown of organic carbon also influence sedimentary metal profiles. In the deep basin, reduction of Fe in the sediments results in a near-surface enrichment of authigenic Fe oxyhydroxides. Downcore profiles of elements such as Co, P, and Cu are strongly influenced by this process. Acknowledgments We would like to thank R. Jahnke, A. Soutar, J. Singleton, and the Captain and Crew of the R. V. New Ho302 Environ. Sci. Technol., Vol. 23, No. 3, 1989
rizon for help during coring operations and S. Niemnil, A. Ungerer, and G. Campi for laboratory assistance. Registry No. C, 7440-44-0; Pb, 7439-92-1; Zn, 7440-66-6; Cr, 7440-47-3; Cu, 7440-50-8; Ni, 7440-02-0; Mn, 7439-96-5; S, 770434-9; Ba, 7440-39-3; Fe, 7439-89-6; Al, 7429-90-5; Co, 7440-48-4; P, 7723-14-0; CaC03, 471-34-1. Literature Cited (1) Bruland, K. W.; Bertine, K.; Koide, M.; Goldberg, E. D. Enuiron. Sci. Technol. 1974, 8, 425-432. (2) Galloway, J. N. Geochim. Cosmochim. Acta 1979, 43, 207-218. (3) Katz, A.; Kaplan, I. R. Mar. Chem. 1981, 10, 261-299. (4) Ng, A,; Patterson, C. C. Geochim. Cosmochim. Acta 1982, 46, 2307-2321. (5) Bloom, N. S.; Crecelius, E. A. Mar. Chem. 1987,21,377-390. (6) Chow, T. J.; Bruland, K. W.; Bertine, K.; Soutar, A.; Koide, M.: Goldbere. E. D. Science 1973. 181. 551-552. (7) Bekne, K. Goldberg, E. D. Enujron. Sci. Technol. 1977, 11, 297-299. (8) Huh, C. A.; Zahnle, D. L.; Small, L. F.; Noshkin, V. E. Geochim. Cosmochim. Acta 1987,51, 1743-1754. (9) Haner, B. E.; Gorsline, D. S. Mar. Geol. 1978, 28, 77-87. (10) Weliky, K.; Suess,E.; Ungerer, C. A,; Muller, P. J.; Fischer, K. Limnol. Oceanogr. 1983,28, 1252-1259. (11) Soutar, A. University of California, San Diego, personal communication, 1987. (12) Ridgway, I. M.; Price, N. B. Mar. Chem. 1987,21,229-248. (13) Stull, J. K.; Baird, R. B.; Heesen, T. C. J. Water Pollut. Control Fed. 1986, 58, 985-991. (14) Shen, G. T.; Boyle, E. A. Earth Planet. Sci. Lett. 1987,82, 289-304.
