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238U disequilibrium
Estuarine and Coastal l]'Iarine Science (r979) 9, 739-747
Particle Reworking in Sediments from the New York Bight Apex: Evidence from 234Th/2sU Disequilibrium
J. Kirk Cochran and Robert C. Aller" Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 , U.S.A. Received 25 September x978 and in revised form 25 ~fay 1979
Keywords: sediments; radionuclides; bioturbation; New York Bight Two diver-collected cores of mud-rich sediment from the New York Bight apex have been analyzed for s=sU and " : T h decay series nuclides with emphasis on " ' T h / " s U disequilibrium. Excess 2a~Th is present in both cores and shows exponential decrease in the top 4 cm. Biogenie reworking by a deposit-feeding community characterized by a Nucula proxima-Nephtys incisa assemblage apparently controls the form of the "~Th profiles. Mixing coefficients of 0.3--o.6 • zo- 6 cm z s- ~ are calculated. No decrease of tt~ with depth (o-r I cm) is found nor is there any vertical structure in profiles of '~ 8Th. Episodic deposition followed by periods of stability cause the observed homogeneity of the longer lived nuclides as well as laminated horizons in the sediment. 23'U/=38U isotope ratios reflect addition of either sea water or sewage source uranium to the collection area.
Introduction Disequilibrium in the marine environment between *'3~Th (t~ = 24 day) and its parent, z3su, has been well documented (Bhat et al., I969; Matsumoto, x975; Aller & Cochran, i976 ). Removal of 2atTh from nearshore waters to underlying sediments makes this nuclide a useful chronometer for studying near-interface sedimentary processes such as rates of particle mLxing by infauna, rates of diagenetie remobilizatlon of trace metals, and the short term stability of tile sediment pile. In a previous study (Aller & Cochran, x976 ) these applications of 231Th/23su disequilibrium were discussed with regard to muddy sediment from a site in central Long Island Sound. The samples were from an area undisturbed by large scale human activity such as dumping. This paper extends our study to the relatively physically unsheltered and human-impacted continental shelf sediments of the New York Bight.
Study area and sample collection The New York Bight apex, shown in Figure x, has been directly affected by man's activities; dredge spoil, sewage sludge, acid waste and cellar dirt are all dumped there (Gross, x976 ). Most of the floor of the Bight is covered with fine to medium grained sand and isolated gravel patches. Muddy sand and mud are abundant only in the Hudson Shelf Valley and the Christiaensen Basin (Freeland et aL, x976; Stubblefield et aL, x977). Temporal and spatial ~ address: Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, U.S.A. 739
740
ft. 1ft.. Cochrans R. C. Aller
studies of New York Bight sediments suggest that some of the dumped material, and sewage sludge in particular, is mobile and is transported away from the dumpsite (Stubblefield et al., x977)40*35 '
40030
'
40~
'
40*20 , 74~
,
73o55 '
73o50 '
73~
,
73o40 ,
Figure z. Map of the New York Bight apex showing coring locations N, C and D. DS = dredge spoil dumpsite, CD ----cellar dirt dumpsite, SS = sewage sludge dumpsite, AW -----approximate position of the acid waste dumpsite. The inset map shows the location of the New York Bight apex in the northeastern United States (after Freeland et aL, z976). Samples analyzed in this study were obtained during a cruise of the NOAA ship George B. Kelez from xz-i 4 May z975. During this cruise, plexiglas box corers operated by divers were used to recover sediment cores z2-x 3 cm in length from three stations (N: C and D) in muddy sand sediment northwest of the dredge spoil dumpsite (Figure x). As described in Aller and Cochran (z976), two types of corers were used: a 73 em2 corer which can be X-rayed to examine sediment structure and a larger 2z2 cm ~ corer which is useful for chemical studies. The sampling strategy was to take one core of each type at each station. Because the large area core from station C (core C-zb) had a partially disturbed interface, it was used only for examination of the contained macrofauna. Instead, the X-ray core (core C-za) was sampled for radiochemistry after X-radiography. Coring attempts at stations D and N recovered only an X-ray core from station N (core N-x) and a large area core from station D (core D - I ) . The latter was sampled for radiochemistry.
Methods Cores were stored refrigerated after collection. Core D - I was sampled within a few hours of collection, but core C--2a was first transported to the laboratory for X-radiography. This resulted in a lapse of N z days from collection to sampling for this core. Sediment was removed from the cores in x-z cm inerement~ and placed into containers of known volume. Water content, ignition loss and dry bulk densities were determined after drying at xxo ~ and ashing for 24-48 h at 55 ~ ~ Radiochemical analyses for U and T h isotopes and ul~
Particle reworking in sediments
74 x
were made on samples leached in hot 6 N-HCI and followed the procedures of Aller & Cochran (t976) for *'3~Th and Thomson et al. (x975) for 21~ Macrofauna were separated from core C-zb by sieving through a x m m mesh sieve. The sample was stored in a Rose Bengal ethanol solution for later separation and identification.
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60
70
80 0
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8
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CORE D-I
I I0
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Figure z . Water content (%) and ignition loss (%) profiles for cores C-za and D-I. Results
Physical properties Water contertt and weight loss ort ignitiort (mainly reflecting the organic content) data are plotted in Figure z . The per cent HzO and ignition loss of core D-z vary irregularly down the length of the core. Core C-za displays a regular decrease in water content (75-64%) to " 9 cm and a more rapid decrease below. Organic content as indicated by ignition loss is constant or increases slightly to " 9 cm and then rapidly decreases below that depth. I a the cores studied, as well as for others examined from tile New York Bight by Beaninger et al. (x976), minima irt water content are correlated with minima ia organic content and both appear to reflect grain size, with sandier sediment having lower water and organic content. The higher overall water and organic content of core C--za relative to core D-x suggests that the former core has a higher percentage of firte material. The regular % water decrease over the top 9 em of core C-za may be due to compaction and suggests that this zone is rather homogeneous.
Macrofauna The retained maerofauna present in core C--zb consisted of the bivalves Nueula proxlma (z 3 individuals) and MuRnia lateraEs (3); the polychaetes Nephtys incisa (7) and Pherusa aflinls
742
J . K . Cochran ~ R. C. Jtller
(I); and the burrowing anemone Ceriantheopffs antericanus (3). Except for Mulinia, these species arc listed by Peatce et aL (I976) as typical inhabitants of high organic carbon areas in the New York Bight apex. The abundances found in the present study (for a o-o2z m 2 core area) are well within the ranges reported by Pearce et aL (x976). Nucula in our sample is represented by an age class at least one year old. The existence of such a relatively old age class has significance in the interpretation of 234Th profiles. A comparable bottom fauna, with the additional deposit-feeding bivalve Yoldia limatula, is present at station N W C irt Long Island Sound, where previous 231Th studies were made (Aller & Cochran, z976; Rhoads et al., z977). X-radiographic results Plates I and 2 show X-radiographs taken at Stations N and C. Station N is nearest the dredge spoil (,,,2 km) dump site and shows no well defined biogenic structures. In contrast, the X-radiograph from Station C reveals abundant infauna. Core C-za displays evidence of intensive biological reworking in the upper 5 cm. Biogenic structures formed by Nucula, Nephtys and Ceriantheopsis are readily apparent. Numerous burrows, some extending to ,,~IO cm, truncate the fine laminations which are present throughout most of the core. Laminations are absent in the top N2 cm where the intensive reworking by Nucula has destroyed them. Visually, core C--za was characterized by a reddish brown oxidized layer z - 2 cm thick overlying black sediment.
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Depth(crn) Figure 3. Excess ttel'h activity vs. depth for cores C-2a and D-I. I~lixing coefflclents, Dn, are calculated from the initial 23tTh decrease. See text for discussion.
Radiochemical results The results of radiochemical analyses ate given in Table z and Figure 3.2S*Th is present in excess (xs) of 23sU in both cores. In core C--za, the excess drops off to near zero by 5 cm with higher values observed from 6- 9 cm. The 23*Thz, inventory of o--6 cm in this core is 5.i dpm cm -z compared to the theoretical value of 5"z dpm cm -2 supportable at steady state from the 238U dissolved in the immediately overlying water column. Core D - I has 23~Th, specific activities comparable to core C-za despite the increase in coarse sediment which appears to
Particle rezcorklng in sediments
743
TABLE 1. Radiochemical data for New York Bight cores Depth Density incore (gash/ (cm) cmt) tStTh* Core C-2a (4o~
73~
o-z
0"300
x-2
0-362
2-3
0.362
3-4
0 " 3 8 3 2.26
z'63 4-0.06 z'84 4-0"07 2.o1 +0"09
4-0-06 4-5
o'416
2"30 4-0"09
5-6
o'414
z'95 4-0"06 z.95
6-7
0"436
8-9
4-0-08 0 " 4 4 8 2"39 4.0"10
Core D-z (4o~ o-z 0"640
o'9xo
73~ %.%6 4-0'05 o.82 4-0"04 0'89 +0"04 0"89 4-0"04 z"3x 4-0.08 z'39
z.xo
4-o.xo 1"39
x-2
z'z3
2-3
x'x I
3-4
z'o6
4-5 5-6 9-zz
0"897
~~
t*'Th~,* tssU*
~t~
tSOTh/ t2~Th/ " ' U / tStTh** aSaTh** ttsU** U/Th***
22 m water depth) z.3 o +0"05 z'45 4.0"05
8"30 4-o.z8 5"45 4.0.20
z't5 4-0"03 z'33 4-0-04
x'43 4-0'07 1"57 4-0"05 z-72 4-0"07 z.49
0"92 4.0'08 0"53
4-0"05 x "5~
4-0'07
4-0"03
4-0.06 z.76
o-6o 4.o.zo o'73
z'38 4-0.02 z'4z
4-0-08
4-o.12
+0"03
x'44 4-0"03 x'42 4-o-xo 4-0"04 o'x3 z'39 4-0"07 4.0"03 o.21 1.31
2z m water depth) o'79 7"27 o.6z 4-0"03 +o'z3 ~o'o2 o-56 2.IO 0"43 +0"03 4.0.08 4.o.ol 0.57 z.2o 0"48 4-0"03 4-0.06 4-0.02
5"93 4-0"05 ~
0.80 4-0'03 o-79 4-0"03
0"99 4-0.02 --
6"28 4-0-07 ~
o'72 4.0.02 o'73
0"97 x'o9 4.0"03 4-0.02 -x-zt
+o.ox
4-0"03
7"04 :ko.o6 6"05 4-0.06 6"25
0"75 4-0.02 0'77 4.0"02 0"77 4-0.02 0"73
0"95 z'o8 4-0"03 4-0.02 o'96 z'H
4.0.06
4-0'02
4-0"03
2.47 4-0'03 --
0.68 +0.02 0-67 4.0"02 0-64 4-0.02
0"93 z'o8 4-0"03 4-0'03 -1"o4 4.0"03 0.88 z'o7 4-0"02 4-0"04
-
-
2"It +0"03
0"65
0.86
o.6o
4.0'03 o'89
4-0-06 I'X3
-':o.o2 0"72
+0"05 0"95
4-0"09 0'96
4-0-03
o'7I 4-0.02 0"75 4-0.02
~
4.0"07 4-0'07 0-89 0"45 4-O"IO 4-0.06 4-0.06
x.zz 4-0"03 z.zz 4.0"03
--
0-73
3"4~
4-o.o2 0-68
~o'o3
+0.02
0.68 4-0.02 2"62 0"64 4-0"04 4-0-02
0"98 1"12 4-0"03 4-0.02 ~ z'z5
0"24 o'25
0"24 0.22 o.2t
0":'3
4-0"03
--
0"24 0.20
4.0"02
o.z8 o.18 o'I8
z'oo
0"23
0"95
+0"03 1.o6
o'29
4.0-03
4-0-04
--
I'o4 4-0-03 z'o5 4-0"03
0"96 4-0'03
o'27
o.z8
*Activities in dpm/g. *StThexcess activity is corrected to time of core collection. **Activity ratio. ***Weight ratio. dilute the concentrations of the other radionuclides (e.g. 2aZTh, ~asU, 21~ I n addition, the excess IS~Th is mixed to a greater depth; significant excess is present at zo cm. T h i s leads to a higher inventory of 2StTh,s in the core (x 1.2 d p m cm -2 to 6 cm depth). Figure 3 shows that excess 2a4Th in core D-x drops off quasi-exponentially, but there is a break i n slope at 4 cm. T h e constant and anomalously high activity from 4 to 6 coincides with a zone of high water and organic content. I n both cores, the 12SThp3:Th activity ratios are dose to z.o and are fairly constant with depth. Lacking 2~Ra data it ~annot be determined if there is excess 22STh in these sediments as noted for other marine deposits (Koide et aL, 1973), but the patterns ate distinctly unlike those of Long Island Sound sediments, which show a m i n i m u m ~28Th/~SZTh ratio of ,-, o-7o at depths in the sediment column of 7-25 cm ( T h o m s o n et al., z975; Aller, I977; Cochran, x979)T h o m s o n et al. (I975) and Aller & Cochran (i976) both found a 23sU trend in Long Island Sound sediments of increasing concentrations with depth. T h o m s o n et al. (z975) attributed this pattern exclusively to uranium loss from the oxTgenated zone of a slump deposit, whereas Aller & Cochran (I976) recognized the generality of the profile in the Sound. T h e y
744
J. K. Cochran ~- 1~. C. .411er
ascribed the obsen'ed distribution to a steady state loss of uranium from the oxygenated zone and supply by irrigation and uptake in the reduced sediment at depth. In the New York Bight sediments analyzed in the present study, however, there is no consistent U variation with dept h tha t Cannot b.e explained by dilution with a low activity sediment fraction, such as sand. This is illustrated by the nearly constant U/Th weight ratios with depth in both cores, although the U/Th ratios of core D - I (average o.i8) are somewhat less than those of core C-an (average o.22). The ~3*Ur-3gU activity ratios in both cores are significantly greater than I.o, reaching as high as x.x5. 21~ activities are in excess of measured ~176 activities (Table I) or ~-~ activities measured by total dissolution of sediment from an adjacent site (1-4-t.6 dpm 2aURa/g; L.K. Benninger, personal communication and in preparation). There is no decrease of -"l~ with depth in either core. Discussion
(i) Sediment accumulation and 2312'hx~ profiles In general, the distribution of any short lived radionuclide in nearshore surficial sediments is governed by the competing processes of biotmbation, physical disturbance and sediment accumulation. If sediment accumulation is negligible on the time scale of 4-5 half lives of the nuclide considered, the depth profile of the nuclide reasonably cart be ascribed to either biogcnic or physical mixing of particles. Interpretation of ~3~Th profiles in the New York Bight is confounded by inadequate knowledge of the rate of sediment accumulation. The general area from which the cores were collected is the repository for much of the fine sediment present in the Bight. h loreover the dredge spoil dumpsitc less than 5 km to the southi~ast has shbaled xo m in 37 }'ears (Freeland et al., i976 ). This is equivalent to a sediment accumulation rate of 27 cm year-~. If we assume no mixing and use the exponential decrease of 23~'1'hwith depth to calculate a sedimentation rate, values of io-2o cm year -~ are obtained. Willie' these values may be plausible in terms of the rate calculated above, this explanation ignores the fact that Stations C and D are removed from the dredge spoil dumpslte and that bioturbation is active at these sites. Indeed, if sediment accumulation or physical reworking werea s rapid as io-2o cm year -~, it is doubtful that a bottom community such as Plate 2 shows could have established itself at Station C (Moore& Scruton, I957; .McCall, i976 ). This, in fact, may be the reason for the absence of fauna in core N-I (Plate I), taken nearest the dredge spoil dumpsite. An alternate explanation consistent with previous studies (Swift eta!., i976; Harris, x976 ) is that accumulation of mud in the Bight apex is episodic and probably rapid when it occurs. A rapid influx of mud at Site C could give rise to the observed internal laminations either due to interlayering of silt or sediment with a slightly different density or to differing content of colloidal organics (e.g. Coleman, x966). Episodic deposition is then followed by macrofaunal colonization with subsequent partial or total destruction of the laminae through bioturbation. This exPlanation implies that the ~ is brought to the sediment by constant fesuspension and resettling of surface sediment and that the 2atTh profiles are generated mainly by biogenie reworking. The second explanation is also consistent with the presence of a Nttcrtla age class> x year old. Because the x-radiograph of core C-aa (Plate 2) shows no obvious escape structures from depth, it seems unlikely that the Nucula were transported to the site in an episode of rapid sediment deposition. Thus their presence implies that the bottom has remained relatively stable physically for at least the age of the animals. The other short lived nuclides which may shed light on the sedimentation processes at Stations C and D are 2x~ (t~ = z2 year) and 2-~ (t~ = I. 9 year). These nuclides have
"G
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,~
Particle reworMng in sediments
745
been used successfully to determine chronologies for sediments accumulating under an anoxic water column (Koide et aL, I973), but in other nearshore sediments, their profiles may be dominantly controlled by physical or biological reworking (Benninger et aL, x976, x979). Both nuclides display essentially constant values with depth in the cores studied (Table x). The failure of excess 2t~ to decrease with depth is consistent with either art explanation of rapid contintious sedimentation or episodic sedimentation followed by bioturbation. Indeed 2:~ decreases with depth in longer cores (up to 4 ~ cm) taken elsewhere in the Bight are due mainly to physical mixing processes rather than net sediment accumulation (Bennlnger et aL, r976 and in preparation). Similarly the 22STh data give no clue as to which explanation is correct. In estuarine sediments, the 2-~STh/23ZTh profile is controlled by the addition of excess 22STh from the water column and the loss of its parent, 2-~ from the sediment (Thomson et al., I975). Only at depth in the sediment column are equilibrium values of 2"~ reached. Although 2"~ is being lost from New York Bight sediments (Kaufman et al., I973), the constancy of ~2STh ratios does not reflect this loss. If episodic rapid sedimentation is occurring then the ~ profiles imply that a relatively short period of time ( < xo year) has elapsed since the most recent addition of sediment. (2) Particle reworking rates If the excess 2StTh profiles are due to bioturbation, it is possible to obtain a measure of the reworking rates from them. A common assumption in treating such data is that particle mixing is analogous to eddy diffusion (e.g. see Goldberg & Koide, x962). Accepting this analogy and ignoring compaction, then the one-dimensional distribution in sediment of an irreversibly adsorbed nuclide is given by:
9OA
0 (DsaA~
a-7 = Ox
where A DB CO
2 X
t
OA
T2x ] -
O)
----- 23~Th activity in the sediment = particle mixing coefficient = sediment accumulation rate = decay constant for 2a~Th depth in sediment, origin fixed at sediment-water interface, positive axis into sediment time
In some cases it is not possible to ignore the effects Of compaction on 23:Th profiles, but in the present instance, most compaction occms in the top o-x era, where only one 23a'rh value is available. It seems reasonable to ignore this effect. If sedimentation is episodic, then the contribution of net accumulation to the form of the 23~Th profile is negligible relative to mixing, and w2<<4LOn holds during the development of the profile. Assuming DB is constant over the depth interval of exponential decrease of excess Z3:Th and that sufficient time has elapsed to attain steady state (OA/Ot = o), then the solution to equation (x) for the boundary conditions x = o, A = A0 and x-o-oo,A-~-o is A = A0 exp (-- (2/DB)~'x)
(z)
Application of this model to the data is hindered by the fact that in neither ease does the 2a~Th,s decrease exponentially down the entire length of the core. In core C-2a, it increases below 4 cm, and in core D-z there is a break in slope at 4 em. The high values at depth in core
746
~. K . Cochran & R. C. Aller
C--2a may be related directly to singular events such as non-diffusive subduction of surficlal sediment by the Ceriantheopsis (Plate 2). The break in slope in core D - I occurs at the same place as a maximum in the water and organic content and may be related either to a deposition event or a horizon of preferred burrow construction. (The high values remain even when 2*tTh~, is normalized to 23sU.) Despite these difficulties, a comparison of the initial ~3~Th~, gradients in the two cores can be made by applying equation (2) to the upper 4 cm. After accounting for the finite sampling intervals to produce a best-fit, this results in a D B of 0. 3 • i o - 6 cm 2 s-X for core C-za and 0.6 • 1o-e cm e s-x for core D-x. If diffusion-like mixing operates only to a finite depth ( ~ 4 cm) on a 2Z~Th time scale, D 8 values can be calculated using the finite mixing depth model of Benninger et aL (1979). T h e resultant DB's, 0"3 • xo -6 cm ~ s -x for core C-2a and 0-4• Io -6 cm 2 s -a for core D - I , are not significantly different from those determined using equation (2). The mixing coefficients determined for New York Bight sediments are comparable to that obtained at a site with similar fauna in Long Island Sound (Dr = 0.2 • IO-6 cm e s-1 in a core taken April, I975; Aller & Cochran, 1976 as modified in Aller, 1977). (3) Uranium concentrations and activity ratios A striking feature of the uranium data is that the ea~U/23sU activity ratios are genezally greater than 1.o and in core C-2a are as high as the value for sea water: i.x 5. In Long Island Sound sediments, increase in the 23~U/23sU activity ratio was correlated with an increase in 2ssU concentration with depth (Aller & Cochran, 1976 ), suggesting U loss from oxygenated sediment and gain of oceanic U in reducing sediment. High 23~/23su activity ratios found in the present study cart be explained by dredging and dumping or local resuspension and deposition of reducing sediment to which oceanic uranium has been added. A complicating factor in interpreting New York Bight sediment uranium activity ratios is the possible contribution of uranium from sewage sludge, dredged from New York Harbor and adjacent areas. Samples of raw and digested sludge from Massachusetts and Rhode Island analyzed at Yale (John Thomson, unpublished data) have U / T h weight ratios of No" 9 and 231U/z~su ratios as high as 1.4. Contamination of lithogenous mud by a small amount of sewage sludge thus also could explain the U data. Independent criteria are required to assess the importance of each of these alternatives.
Acknowledgements This work has been supported by NOAA/ERL Grant No. 04-4-o22-35 from the Marine EcoSystems Analysis (MESA) Program's New York Bight Project. We thank D. J. P. Swift and NOAA-AOML personnel for helping us obtain samples and Drs K. K. Turekian, IS. K. Benninger and D. J. P. Swift, for reviewing the manuscript. Both authors were supported by NSF Doctoral Fellowships.
References Aller, R. C. & Cochran, J. K. x976 23~Th/'~sUdisequilibrium in near-shore sediment: particle reworking and diagenetie time scales. Earth andPlantetary Science Letters 29, 37-5o. Aller, R. C. 1977 The influence of maerobenthos on chemical diagenesis of marine sediments, PhD. thesis, Yale Universit3", New Haven, Conn., 599 ppBenninger, L. K., Aller, R. C., Cochran, J. K. & Turekian, K. K. 1976 Lead-21o geochronology of contemporary near-shore sediment: status and problems. Transactionr, American Geophysical Union 57, 93 x.
Particle rezcorklng in sediments
747
Benninger, L. K., Aller, R. C., Cochran, J. K. & Turekian, K. K. x979 Effects of biologlcal sediment mixing on the 2~~ chronology and trace metal distribution in a Long Island Sound sediment core. Earth and Planetary Science Letters 43, 241-z59 9 Bhat, S. G., Krishnaswami, S., Lal, D., Rama & ~'Ioore, W. S. 1969 t3~Th/l~sU ratios in the ocean. Earth and Planetary Science Letters 5, 483-49 x. Cochran, J. K. 1979 The geochemistry of t2eRa and Z2+Rain marine deposits, PhD. thesis, Yale Universif'y, New Haven, Conn., 26o pp. Coleman, J. M. x966 Ecological changes in a massive freshwater clay sequence. Transactions Gulf Coast Association of Geological Societies 16, 159-174. Freeland, G. L., Swift, D. J. P., Stubblefield, W. L. & Cok, A. E. x976 Surficial sediments of the NOAA-MESA study areas in the New York Bight. In ]~Iiddle Atlantic Contbtental Shelf and the New York Bight (M. G. Gross, ed.). American Society of Limnology and Oceanography Spec. Symp. 2, 9o-zoL Goldberg, E. D. & Koide, M. x962 Geochronological studies of deep-sea sediments by the ionium] thorium method. Geochlmica et Cosmoehimlca Acta 26, 417-45o. Gross, M. G. x976 Sources of urban wastes. In: 3Iiddle Atlantic Continental Shelf and the New York Bight (M. G. Gross, ed.) American Society of Limnology and Oceanography, Spee. Symp. 2, z5 o-I6x.
Harris, W. I-I. 1976 Spatial and temporal variation in sedimentary grain-size facies and sediment heavy metal ratios in the New York Bight Apex. In 3liddle Atlantic Continental Shelf and the New York Bight (M. G. Gross, ed.) American Society of Limnology and Oceanography, Spce. Syrup. z, I O 2 - I 2 3.
Kaufman, A., Trier, R. M., Broecker, W. S. & Feely, H. W. 1973 Distribution of tZSRain the world ocean,ffournal of GeophysicalResearch 78, 8827-8848. Koide, M., Bruland, K. W. and Goldberg, E. D. (1973) Th-zz8/Th-23= and Pb-2xo geochronologies in marine and lake sediments. Geochimica et CosrnochimlcaActa 37, r 17x-1187. Matsumoto, E. z975 S~Th-23sU radioactive disequilibrium in the surface layer of the ocean. Geochlmica et Cormochimlca Acta 39, 2o5-=I=. McCall, P. L. 1976 The influence of disturbance on conununity patterns and adaptive strategies of the infaunal benthos of central Long Island Sound, PhD. thesis, Yale University, New Haven, Conn., 198 pp. ~Toore, D. G. & Seruton, P. C. (x957) Minor internal structures of some recent unconsolidated sediments, American Association of Petroleum GeologistsBulletin 4x, 2723-=75I. Pearce, J. B., Caraeeiolo, J. V., Halsey, M. B. & Rogers, L. H. 1976 Temporal and spatial distributions of benthic macro-invertebrates in the New York Bight. In ]~liddle Atlantic Continental Shelf and the New York Bight (M. G. Gross, ed.) American Society of Limnology and Oceanography, Spee. Syrup. 2, 394-403 . Rhoads, D. C., Aller, R. C. & Goldhaber, M. B. 1977 The influence of colonizing benthos on physical properties and chemical diagenesis of the estuarine sea floor. In Ecology of 31arine Benthos (B. C. Coull, ed.) Belle SV. Baruch Library of Marine Science No. 6, Univ. of South Carolina Press, Columbia, S.C., 113-138. Stubblefield, W. L., Permenter, R. W. & Swift, D. J. P. 1977 Time and space variation of surficial sediments of the New York Bight. Estuarlne atut Coastal ~larine Science 5, 597--6079 Swift, D. J. P., Freeland, G. L., Gadd, P. E., Han, G., Lavelle, J. W. & Stubblefield, W. L. x976 Morphologic evolution and coastal sand transport, New York-New Jersey Shelf. In 3Iiddle Atlantic Continental Shelf and the New York Bight (M. G. Gross, ed.) American Society of Limraologyand Oceanography, Spec. Syrup. 2, 69-89. Thomson, J., Turekian, K. K. & l~,IcCaffrey,R. J. x975 The accumulation of metals in and release from sediments of Long Island Sound. In Estuarine Research Vol. I. Academic Press, Inc., New York, pp. 28--44.
Particle Reworking in Sediments from the New York Bight Apex: Evidence from 234Th/2sU Disequilibrium
J. Kirk Cochran and Robert C. Aller" Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 , U.S.A. Received 25 September x978 and in revised form 25 ~fay 1979
Keywords: sediments; radionuclides; bioturbation; New York Bight Two diver-collected cores of mud-rich sediment from the New York Bight apex have been analyzed for s=sU and " : T h decay series nuclides with emphasis on " ' T h / " s U disequilibrium. Excess 2a~Th is present in both cores and shows exponential decrease in the top 4 cm. Biogenie reworking by a deposit-feeding community characterized by a Nucula proxima-Nephtys incisa assemblage apparently controls the form of the "~Th profiles. Mixing coefficients of 0.3--o.6 • zo- 6 cm z s- ~ are calculated. No decrease of tt~ with depth (o-r I cm) is found nor is there any vertical structure in profiles of '~ 8Th. Episodic deposition followed by periods of stability cause the observed homogeneity of the longer lived nuclides as well as laminated horizons in the sediment. 23'U/=38U isotope ratios reflect addition of either sea water or sewage source uranium to the collection area.
Introduction Disequilibrium in the marine environment between *'3~Th (t~ = 24 day) and its parent, z3su, has been well documented (Bhat et al., I969; Matsumoto, x975; Aller & Cochran, i976 ). Removal of 2atTh from nearshore waters to underlying sediments makes this nuclide a useful chronometer for studying near-interface sedimentary processes such as rates of particle mLxing by infauna, rates of diagenetie remobilizatlon of trace metals, and the short term stability of tile sediment pile. In a previous study (Aller & Cochran, x976 ) these applications of 231Th/23su disequilibrium were discussed with regard to muddy sediment from a site in central Long Island Sound. The samples were from an area undisturbed by large scale human activity such as dumping. This paper extends our study to the relatively physically unsheltered and human-impacted continental shelf sediments of the New York Bight.
Study area and sample collection The New York Bight apex, shown in Figure x, has been directly affected by man's activities; dredge spoil, sewage sludge, acid waste and cellar dirt are all dumped there (Gross, x976 ). Most of the floor of the Bight is covered with fine to medium grained sand and isolated gravel patches. Muddy sand and mud are abundant only in the Hudson Shelf Valley and the Christiaensen Basin (Freeland et aL, x976; Stubblefield et aL, x977). Temporal and spatial ~ address: Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, U.S.A. 739
740
ft. 1ft.. Cochrans R. C. Aller
studies of New York Bight sediments suggest that some of the dumped material, and sewage sludge in particular, is mobile and is transported away from the dumpsite (Stubblefield et al., x977)40*35 '
40030
'
40~
'
40*20 , 74~
,
73o55 '
73o50 '
73~
,
73o40 ,
Figure z. Map of the New York Bight apex showing coring locations N, C and D. DS = dredge spoil dumpsite, CD ----cellar dirt dumpsite, SS = sewage sludge dumpsite, AW -----approximate position of the acid waste dumpsite. The inset map shows the location of the New York Bight apex in the northeastern United States (after Freeland et aL, z976). Samples analyzed in this study were obtained during a cruise of the NOAA ship George B. Kelez from xz-i 4 May z975. During this cruise, plexiglas box corers operated by divers were used to recover sediment cores z2-x 3 cm in length from three stations (N: C and D) in muddy sand sediment northwest of the dredge spoil dumpsite (Figure x). As described in Aller and Cochran (z976), two types of corers were used: a 73 em2 corer which can be X-rayed to examine sediment structure and a larger 2z2 cm ~ corer which is useful for chemical studies. The sampling strategy was to take one core of each type at each station. Because the large area core from station C (core C-zb) had a partially disturbed interface, it was used only for examination of the contained macrofauna. Instead, the X-ray core (core C-za) was sampled for radiochemistry after X-radiography. Coring attempts at stations D and N recovered only an X-ray core from station N (core N-x) and a large area core from station D (core D - I ) . The latter was sampled for radiochemistry.
Methods Cores were stored refrigerated after collection. Core D - I was sampled within a few hours of collection, but core C--2a was first transported to the laboratory for X-radiography. This resulted in a lapse of N z days from collection to sampling for this core. Sediment was removed from the cores in x-z cm inerement~ and placed into containers of known volume. Water content, ignition loss and dry bulk densities were determined after drying at xxo ~ and ashing for 24-48 h at 55 ~ ~ Radiochemical analyses for U and T h isotopes and ul~
Particle reworking in sediments
74 x
were made on samples leached in hot 6 N-HCI and followed the procedures of Aller & Cochran (t976) for *'3~Th and Thomson et al. (x975) for 21~ Macrofauna were separated from core C-zb by sieving through a x m m mesh sieve. The sample was stored in a Rose Bengal ethanol solution for later separation and identification.
%HzO
,,,l(
30
o_
40
~
40
50
,I
~
.
60
% IGNITION LOSS
~
, ..... '"l'[ 13~) 0
4
,
,
,
, , , , I
.50
60
70
80 0
8
J
4
12
,
8
12
!
CORE D-I
I I0
"
I
!
I
!
~
!
!
!
I
1
Figure z . Water content (%) and ignition loss (%) profiles for cores C-za and D-I. Results
Physical properties Water contertt and weight loss ort ignitiort (mainly reflecting the organic content) data are plotted in Figure z . The per cent HzO and ignition loss of core D-z vary irregularly down the length of the core. Core C-za displays a regular decrease in water content (75-64%) to " 9 cm and a more rapid decrease below. Organic content as indicated by ignition loss is constant or increases slightly to " 9 cm and then rapidly decreases below that depth. I a the cores studied, as well as for others examined from tile New York Bight by Beaninger et al. (x976), minima irt water content are correlated with minima ia organic content and both appear to reflect grain size, with sandier sediment having lower water and organic content. The higher overall water and organic content of core C--za relative to core D-x suggests that the former core has a higher percentage of firte material. The regular % water decrease over the top 9 em of core C-za may be due to compaction and suggests that this zone is rather homogeneous.
Macrofauna The retained maerofauna present in core C--zb consisted of the bivalves Nueula proxlma (z 3 individuals) and MuRnia lateraEs (3); the polychaetes Nephtys incisa (7) and Pherusa aflinls
742
J . K . Cochran ~ R. C. Jtller
(I); and the burrowing anemone Ceriantheopffs antericanus (3). Except for Mulinia, these species arc listed by Peatce et aL (I976) as typical inhabitants of high organic carbon areas in the New York Bight apex. The abundances found in the present study (for a o-o2z m 2 core area) are well within the ranges reported by Pearce et aL (x976). Nucula in our sample is represented by an age class at least one year old. The existence of such a relatively old age class has significance in the interpretation of 234Th profiles. A comparable bottom fauna, with the additional deposit-feeding bivalve Yoldia limatula, is present at station N W C irt Long Island Sound, where previous 231Th studies were made (Aller & Cochran, z976; Rhoads et al., z977). X-radiographic results Plates I and 2 show X-radiographs taken at Stations N and C. Station N is nearest the dredge spoil (,,,2 km) dump site and shows no well defined biogenic structures. In contrast, the X-radiograph from Station C reveals abundant infauna. Core C-za displays evidence of intensive biological reworking in the upper 5 cm. Biogenic structures formed by Nucula, Nephtys and Ceriantheopsis are readily apparent. Numerous burrows, some extending to ,,~IO cm, truncate the fine laminations which are present throughout most of the core. Laminations are absent in the top N2 cm where the intensive reworking by Nucula has destroyed them. Visually, core C--za was characterized by a reddish brown oxidized layer z - 2 cm thick overlying black sediment.
~
CORED-I
CORE C'2o
5 " DS9 5.8 X IO'rc= 2 $'~ 2GX~'7~2f
I
f_
o~
T
;
I
,
6
T
d
ao'o
6
8
I0
~..
Depth(crn) Figure 3. Excess ttel'h activity vs. depth for cores C-2a and D-I. I~lixing coefflclents, Dn, are calculated from the initial 23tTh decrease. See text for discussion.
Radiochemical results The results of radiochemical analyses ate given in Table z and Figure 3.2S*Th is present in excess (xs) of 23sU in both cores. In core C--za, the excess drops off to near zero by 5 cm with higher values observed from 6- 9 cm. The 23*Thz, inventory of o--6 cm in this core is 5.i dpm cm -z compared to the theoretical value of 5"z dpm cm -2 supportable at steady state from the 238U dissolved in the immediately overlying water column. Core D - I has 23~Th, specific activities comparable to core C-za despite the increase in coarse sediment which appears to
Particle rezcorklng in sediments
743
TABLE 1. Radiochemical data for New York Bight cores Depth Density incore (gash/ (cm) cmt) tStTh* Core C-2a (4o~
73~
o-z
0"300
x-2
0-362
2-3
0.362
3-4
0 " 3 8 3 2.26
z'63 4-0.06 z'84 4-0"07 2.o1 +0"09
4-0-06 4-5
o'416
2"30 4-0"09
5-6
o'414
z'95 4-0"06 z.95
6-7
0"436
8-9
4-0-08 0 " 4 4 8 2"39 4.0"10
Core D-z (4o~ o-z 0"640
o'9xo
73~ %.%6 4-0'05 o.82 4-0"04 0'89 +0"04 0"89 4-0"04 z"3x 4-0.08 z'39
z.xo
4-o.xo 1"39
x-2
z'z3
2-3
x'x I
3-4
z'o6
4-5 5-6 9-zz
0"897
~~
t*'Th~,* tssU*
~t~
tSOTh/ t2~Th/ " ' U / tStTh** aSaTh** ttsU** U/Th***
22 m water depth) z.3 o +0"05 z'45 4.0"05
8"30 4-o.z8 5"45 4.0.20
z't5 4-0"03 z'33 4-0-04
x'43 4-0'07 1"57 4-0"05 z-72 4-0"07 z.49
0"92 4.0'08 0"53
4-0"05 x "5~
4-0'07
4-0"03
4-0.06 z.76
o-6o 4.o.zo o'73
z'38 4-0.02 z'4z
4-0-08
4-o.12
+0"03
x'44 4-0"03 x'42 4-o-xo 4-0"04 o'x3 z'39 4-0"07 4.0"03 o.21 1.31
2z m water depth) o'79 7"27 o.6z 4-0"03 +o'z3 ~o'o2 o-56 2.IO 0"43 +0"03 4.0.08 4.o.ol 0.57 z.2o 0"48 4-0"03 4-0.06 4-0.02
5"93 4-0"05 ~
0.80 4-0'03 o-79 4-0"03
0"99 4-0.02 --
6"28 4-0-07 ~
o'72 4.0.02 o'73
0"97 x'o9 4.0"03 4-0.02 -x-zt
+o.ox
4-0"03
7"04 :ko.o6 6"05 4-0.06 6"25
0"75 4-0.02 0'77 4.0"02 0"77 4-0.02 0"73
0"95 z'o8 4-0"03 4-0.02 o'96 z'H
4.0.06
4-0'02
4-0"03
2.47 4-0'03 --
0.68 +0.02 0-67 4.0"02 0-64 4-0.02
0"93 z'o8 4-0"03 4-0'03 -1"o4 4.0"03 0.88 z'o7 4-0"02 4-0"04
-
-
2"It +0"03
0"65
0.86
o.6o
4.0'03 o'89
4-0-06 I'X3
-':o.o2 0"72
+0"05 0"95
4-0"09 0'96
4-0-03
o'7I 4-0.02 0"75 4-0.02
~
4.0"07 4-0'07 0-89 0"45 4-O"IO 4-0.06 4-0.06
x.zz 4-0"03 z.zz 4.0"03
--
0-73
3"4~
4-o.o2 0-68
~o'o3
+0.02
0.68 4-0.02 2"62 0"64 4-0"04 4-0-02
0"98 1"12 4-0"03 4-0.02 ~ z'z5
0"24 o'25
0"24 0.22 o.2t
0":'3
4-0"03
--
0"24 0.20
4.0"02
o.z8 o.18 o'I8
z'oo
0"23
0"95
+0"03 1.o6
o'29
4.0-03
4-0-04
--
I'o4 4-0-03 z'o5 4-0"03
0"96 4-0'03
o'27
o.z8
*Activities in dpm/g. *StThexcess activity is corrected to time of core collection. **Activity ratio. ***Weight ratio. dilute the concentrations of the other radionuclides (e.g. 2aZTh, ~asU, 21~ I n addition, the excess IS~Th is mixed to a greater depth; significant excess is present at zo cm. T h i s leads to a higher inventory of 2StTh,s in the core (x 1.2 d p m cm -2 to 6 cm depth). Figure 3 shows that excess 2a4Th in core D-x drops off quasi-exponentially, but there is a break i n slope at 4 cm. T h e constant and anomalously high activity from 4 to 6 coincides with a zone of high water and organic content. I n both cores, the 12SThp3:Th activity ratios are dose to z.o and are fairly constant with depth. Lacking 2~Ra data it ~annot be determined if there is excess 22STh in these sediments as noted for other marine deposits (Koide et aL, 1973), but the patterns ate distinctly unlike those of Long Island Sound sediments, which show a m i n i m u m ~28Th/~SZTh ratio of ,-, o-7o at depths in the sediment column of 7-25 cm ( T h o m s o n et al., z975; Aller, I977; Cochran, x979)T h o m s o n et al. (I975) and Aller & Cochran (i976) both found a 23sU trend in Long Island Sound sediments of increasing concentrations with depth. T h o m s o n et al. (z975) attributed this pattern exclusively to uranium loss from the oxTgenated zone of a slump deposit, whereas Aller & Cochran (I976) recognized the generality of the profile in the Sound. T h e y
744
J. K. Cochran ~- 1~. C. .411er
ascribed the obsen'ed distribution to a steady state loss of uranium from the oxygenated zone and supply by irrigation and uptake in the reduced sediment at depth. In the New York Bight sediments analyzed in the present study, however, there is no consistent U variation with dept h tha t Cannot b.e explained by dilution with a low activity sediment fraction, such as sand. This is illustrated by the nearly constant U/Th weight ratios with depth in both cores, although the U/Th ratios of core D - I (average o.i8) are somewhat less than those of core C-an (average o.22). The ~3*Ur-3gU activity ratios in both cores are significantly greater than I.o, reaching as high as x.x5. 21~ activities are in excess of measured ~176 activities (Table I) or ~-~ activities measured by total dissolution of sediment from an adjacent site (1-4-t.6 dpm 2aURa/g; L.K. Benninger, personal communication and in preparation). There is no decrease of -"l~ with depth in either core. Discussion
(i) Sediment accumulation and 2312'hx~ profiles In general, the distribution of any short lived radionuclide in nearshore surficial sediments is governed by the competing processes of biotmbation, physical disturbance and sediment accumulation. If sediment accumulation is negligible on the time scale of 4-5 half lives of the nuclide considered, the depth profile of the nuclide reasonably cart be ascribed to either biogcnic or physical mixing of particles. Interpretation of ~3~Th profiles in the New York Bight is confounded by inadequate knowledge of the rate of sediment accumulation. The general area from which the cores were collected is the repository for much of the fine sediment present in the Bight. h loreover the dredge spoil dumpsitc less than 5 km to the southi~ast has shbaled xo m in 37 }'ears (Freeland et al., i976 ). This is equivalent to a sediment accumulation rate of 27 cm year-~. If we assume no mixing and use the exponential decrease of 23~'1'hwith depth to calculate a sedimentation rate, values of io-2o cm year -~ are obtained. Willie' these values may be plausible in terms of the rate calculated above, this explanation ignores the fact that Stations C and D are removed from the dredge spoil dumpslte and that bioturbation is active at these sites. Indeed, if sediment accumulation or physical reworking werea s rapid as io-2o cm year -~, it is doubtful that a bottom community such as Plate 2 shows could have established itself at Station C (Moore& Scruton, I957; .McCall, i976 ). This, in fact, may be the reason for the absence of fauna in core N-I (Plate I), taken nearest the dredge spoil dumpsite. An alternate explanation consistent with previous studies (Swift eta!., i976; Harris, x976 ) is that accumulation of mud in the Bight apex is episodic and probably rapid when it occurs. A rapid influx of mud at Site C could give rise to the observed internal laminations either due to interlayering of silt or sediment with a slightly different density or to differing content of colloidal organics (e.g. Coleman, x966). Episodic deposition is then followed by macrofaunal colonization with subsequent partial or total destruction of the laminae through bioturbation. This exPlanation implies that the ~ is brought to the sediment by constant fesuspension and resettling of surface sediment and that the 2atTh profiles are generated mainly by biogenie reworking. The second explanation is also consistent with the presence of a Nttcrtla age class> x year old. Because the x-radiograph of core C-aa (Plate 2) shows no obvious escape structures from depth, it seems unlikely that the Nucula were transported to the site in an episode of rapid sediment deposition. Thus their presence implies that the bottom has remained relatively stable physically for at least the age of the animals. The other short lived nuclides which may shed light on the sedimentation processes at Stations C and D are 2x~ (t~ = z2 year) and 2-~ (t~ = I. 9 year). These nuclides have
"G
I
to
i
r
0
75 Z g Z e., o
E
0
~
,&.~
,-Cf~ P--d,
a
[Facing p. 744
"o
e- , _
c-
.:
e-~
r" E r~
I ~.'.] o ~ o~
.
,~
Particle reworMng in sediments
745
been used successfully to determine chronologies for sediments accumulating under an anoxic water column (Koide et aL, I973), but in other nearshore sediments, their profiles may be dominantly controlled by physical or biological reworking (Benninger et aL, x976, x979). Both nuclides display essentially constant values with depth in the cores studied (Table x). The failure of excess 2t~ to decrease with depth is consistent with either art explanation of rapid contintious sedimentation or episodic sedimentation followed by bioturbation. Indeed 2:~ decreases with depth in longer cores (up to 4 ~ cm) taken elsewhere in the Bight are due mainly to physical mixing processes rather than net sediment accumulation (Bennlnger et aL, r976 and in preparation). Similarly the 22STh data give no clue as to which explanation is correct. In estuarine sediments, the 2-~STh/23ZTh profile is controlled by the addition of excess 22STh from the water column and the loss of its parent, 2-~ from the sediment (Thomson et al., I975). Only at depth in the sediment column are equilibrium values of 2"~ reached. Although 2"~ is being lost from New York Bight sediments (Kaufman et al., I973), the constancy of ~2STh ratios does not reflect this loss. If episodic rapid sedimentation is occurring then the ~ profiles imply that a relatively short period of time ( < xo year) has elapsed since the most recent addition of sediment. (2) Particle reworking rates If the excess 2StTh profiles are due to bioturbation, it is possible to obtain a measure of the reworking rates from them. A common assumption in treating such data is that particle mixing is analogous to eddy diffusion (e.g. see Goldberg & Koide, x962). Accepting this analogy and ignoring compaction, then the one-dimensional distribution in sediment of an irreversibly adsorbed nuclide is given by:
9OA
0 (DsaA~
a-7 = Ox
where A DB CO
2 X
t
OA
T2x ] -
O)
----- 23~Th activity in the sediment = particle mixing coefficient = sediment accumulation rate = decay constant for 2a~Th depth in sediment, origin fixed at sediment-water interface, positive axis into sediment time
In some cases it is not possible to ignore the effects Of compaction on 23:Th profiles, but in the present instance, most compaction occms in the top o-x era, where only one 23a'rh value is available. It seems reasonable to ignore this effect. If sedimentation is episodic, then the contribution of net accumulation to the form of the 23~Th profile is negligible relative to mixing, and w2<<4LOn holds during the development of the profile. Assuming DB is constant over the depth interval of exponential decrease of excess Z3:Th and that sufficient time has elapsed to attain steady state (OA/Ot = o), then the solution to equation (x) for the boundary conditions x = o, A = A0 and x-o-oo,A-~-o is A = A0 exp (-- (2/DB)~'x)
(z)
Application of this model to the data is hindered by the fact that in neither ease does the 2a~Th,s decrease exponentially down the entire length of the core. In core C-2a, it increases below 4 cm, and in core D-z there is a break in slope at 4 em. The high values at depth in core
746
~. K . Cochran & R. C. Aller
C--2a may be related directly to singular events such as non-diffusive subduction of surficlal sediment by the Ceriantheopsis (Plate 2). The break in slope in core D - I occurs at the same place as a maximum in the water and organic content and may be related either to a deposition event or a horizon of preferred burrow construction. (The high values remain even when 2*tTh~, is normalized to 23sU.) Despite these difficulties, a comparison of the initial ~3~Th~, gradients in the two cores can be made by applying equation (2) to the upper 4 cm. After accounting for the finite sampling intervals to produce a best-fit, this results in a D B of 0. 3 • i o - 6 cm 2 s-X for core C-za and 0.6 • 1o-e cm e s-x for core D-x. If diffusion-like mixing operates only to a finite depth ( ~ 4 cm) on a 2Z~Th time scale, D 8 values can be calculated using the finite mixing depth model of Benninger et aL (1979). T h e resultant DB's, 0"3 • xo -6 cm ~ s -x for core C-2a and 0-4• Io -6 cm 2 s -a for core D - I , are not significantly different from those determined using equation (2). The mixing coefficients determined for New York Bight sediments are comparable to that obtained at a site with similar fauna in Long Island Sound (Dr = 0.2 • IO-6 cm e s-1 in a core taken April, I975; Aller & Cochran, 1976 as modified in Aller, 1977). (3) Uranium concentrations and activity ratios A striking feature of the uranium data is that the ea~U/23sU activity ratios are genezally greater than 1.o and in core C-2a are as high as the value for sea water: i.x 5. In Long Island Sound sediments, increase in the 23~U/23sU activity ratio was correlated with an increase in 2ssU concentration with depth (Aller & Cochran, 1976 ), suggesting U loss from oxygenated sediment and gain of oceanic U in reducing sediment. High 23~/23su activity ratios found in the present study cart be explained by dredging and dumping or local resuspension and deposition of reducing sediment to which oceanic uranium has been added. A complicating factor in interpreting New York Bight sediment uranium activity ratios is the possible contribution of uranium from sewage sludge, dredged from New York Harbor and adjacent areas. Samples of raw and digested sludge from Massachusetts and Rhode Island analyzed at Yale (John Thomson, unpublished data) have U / T h weight ratios of No" 9 and 231U/z~su ratios as high as 1.4. Contamination of lithogenous mud by a small amount of sewage sludge thus also could explain the U data. Independent criteria are required to assess the importance of each of these alternatives.
Acknowledgements This work has been supported by NOAA/ERL Grant No. 04-4-o22-35 from the Marine EcoSystems Analysis (MESA) Program's New York Bight Project. We thank D. J. P. Swift and NOAA-AOML personnel for helping us obtain samples and Drs K. K. Turekian, IS. K. Benninger and D. J. P. Swift, for reviewing the manuscript. Both authors were supported by NSF Doctoral Fellowships.
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