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Radiocarbon dating of a core from Long Island sound
Estuarine and Coastal ikfarine Science (1979) 9, 171-180
Radiocarbon Long Island
Dating Sound
of a Core from
Gaboury J. Benoit, Karl K. Turekian Larry K. Benninger
and
Department of Geology and Geophysics, Yale University, Connecticut 06520, U.S.A.
New Haven,
Received 30 June 1978
Keywords : Radiocarbon dating; sedimentation tion; Long Island Sound; Connecticut coast
rates ; estuarine sedimenta-
Radiocarbon measurements sequentially with depth in a diver-obtained core from Long Island Sound has been interpreted in terms of sediment accumulation rates and sources of organic carbon in the sediment. A sediment accumulation rate of 0.075 fo.013 cm year-’ is determined from the data below IO cm in the core. An age of 2320 year BP is obtained for the sediment-water interface by extrapolation. This means that the dominant carbon component preserved in the core is soil-derived. The contributions of fossil fuel carbon and bomb radiocarbon associated with plankton are also evaluated.
Introduction The delivery of organic carbon to estuarine sediments provides several types of information about the sedimentary regime of the system based on the radiocarbon content. The most obvious is the determination of rates of sediment accumulation. But the variation of the level of 14C activity with depth in a core raised from the estuarine floor also provides insights in the sources of the organic matter in the sediments as a function of time. The special input of soil carbon, fossil carbon, planktonic carbon or sewage sludge, each has a sufficiently distinctive 14C activity to provide important insights into source as well as dating problems. We report in this paper our results on a core from a well-studied region of Long Island Sound. Although the information has been obtained in only one estuarine type we believe that it provides the basis for understanding other estuarine systems as well.
Description
of sampling
site
The core analyzed for radiocarbon is one raised from the well-studied Northwest Control site and is described by Benninger et al. (1979). Sampling station Northwest Control (NWC) is located at 41’10.4’N latitude, 72’56*3’W longitude about 6 km south of New Haven Harbor. The bottom lies at a depth of 14 m. Occasionally, severe storms have been observed to disturb the sediment-water interface in this area. Aller & Cochran (1976) noted storm induced laminations in the top 3 cm of a box core sampled soon after a northwest storm in the Fall. 0302-~52~/79/080171+10
$02.00/o
0 rg7g Academic Press Inc. (London) Ltd.
172
G.J. Benoit, K. K. Turekian & L. K. Benninger
NWC sediments support a stable, low diversity, sediment feeding epifauna. Principal organisms are the protobranch bivalves Yoldia Zimatula and NucuZu annulutu. The @fauna thoroughly rework the near-interface sediments leaving it well mixed to a depth of about 4 cm. A feeding lag layer at 6 cm and abundant fecal pellets in the top few centimeters (Aller & Cochran, 1976) attest to the high rate of biological activity at this site. Below the depth reworked by Yoldiu Zimatula, burrows refilled by surface sediments and shells and characterized by high water content, metals, and excess 2loPb activity are observed (Benninger et uZ., 1979). X-radiographs of the core used in this study, NWC 102975 (Benningcr et al., xgyy), show the top IO cm to be free of structural features, presumably as a result of intense physical and biological reworking. At depth, one commonly finds burrow structures characterized by high water content and the presence of shell fragments. In particular refilled burrows were observed at 25-30 cm, 45 cm, 55 cm, 70 cm, and IIS cm. Shell debris normally found in a layer at 3-4 cm from the surface lies within I cm of the surface in core N1YC 102975. The reason for this is not known, but one possibility is that the upper 2 to 3 cm of the core was lost some time prior to sampling. Methods and results The method of coring and sampling is described in Benninger et al., (1979). In brief, a 120 cm gravity core was collected in a length of IO cm internal diameter polyvinylchloride sewer pipe. A scuba diver assisted coring operations to assure recovery of the semi-fluid sediment water interface. r4C was measured using the liquid scintillation counting method of Noakes et al. (1965) and Polach & Stipp (1967). The Y a1e radiocarbon line is described in Nozaki & Turekian (1977). A discussion of the refinements of the technique as used on relatively tow organic matter concentration in sediment and high volatile organic material such as sewage sludge and plankton follows. Approximately IOO g of sediment previously dried at IIO “C was treated with 500 ml of cold, 27” HCl to dissolve CaCO,. The sample was centrifuged and decanted, mixed with distilled water and centrifuged again. After decanting, the wet sediment was redried at IIO “C, ground in a mortar and pestle, and burned at about 900 “C in a stream of pure oxygen at a reduced pressure. The dried, ground, sewage sludge sample was burned at a greatly reduced oxygen pressure in order to prevent explosive combustion. The dried zooplankton sample burned so violently, even at reduced oxygen pressure, that a five-fold dilution of the sample with previously burned sediment was used as a moderator. The CO, generated by combustion was passed successively through solid CuO maintained at 500 “C, a series of chemical scrubbers (Ag,SO,, K,MnO,, and H,S04-K,CrO,), and a dry ice trap and collected finally as purified solid CO, in two liquid nitrogen cooled traps in series. The gas was expanded to a 5 liter bulb and its pressure recorded. Generally less than 0.1 moles was recovered. Reported values of carbon concentration are based on these pressure measurements. Next, the CO, was reacted with hot (800 “C) Li metal forming lithium carbide. The lithium carbide after cooling to room temperature was converted to acetylene by the controlled addition of an excess of distilled water. Yields of acetylene from carbon dioxide average 95 %. The acetylene, dewatered through a phosphoric acid scrubber, was trimerized to benzene on an activated chromium catalyst. Benzene released from the catalyst by heating was collected in a dry ice cooled trap. Unreacted gas was commonly detected in the catalyst tube when a vacuum was applied even after a day or more had elapsed. The final benzene
Radiocarbon dating of a core from Long Island Sound
‘73
TABLE I. Radiocarbon ages and organic content of core NWC 102975, 41’10.4’N 72”56*3’W, 14 m depth, zooplankton and sewage sludge
Depth in core (cm) Core
%H,O
% LO1
59'9 56.3 55.8 56.4 56.3 56.0 53.8 52'3 52.6 52.6 53.6 50'9
5'5 5'7 5'2 5'2 5'5 4'9 4.6 4'4 4'7 5'0 4'5 4'0
o-5 5-10 IO-15 15-20 20-2 j
25-30 35-40 60-65 80-85 85-90 roo-I05 IOj-110
Zooplankton Sewage sludge
%C I'92
I.47 1.38 1.31
I'33 1,26 I'21 I.19 I'02
% c: % LOI 0'35 0.26 0.26 0.25 0.24 0.26 0.26 0.27
Age & IO (year BP) 272O-c310
5I201k3IO 24805250 2780~220 2200,210
29103 340 2800+ 220 318oi280
0'22
(>0.89)
(>0.18)
0.27 0.26
1'20 1'03
3370$280 3770*290
3'33'5
-103oiEo
31.6
-298oi70
yield generally did not exceed 80% because of the loss of that unreacted gas and because of the loss of benzene during liquid transfer to counting vials. It was critical to minimize loss during transfer because the quantity of synthesized benzene was small (typically 0.7 g for sediment samples). The recovered benzene was made up to 3 ml total volume in 6 ml low K counting vials by adding ‘dead’ benzene. Samples were prepared for counting by addition of I ml of a scintillation solution: 0.35% PPO and 0.07% dimethyl POPOP in a 3 :I mixture of benzene and toluene. Counter stability
was monitored
by comparison
with an internal
hot standard
counts min-l). Blank values around 7.4 counts min-l were obtained using dead benzene. Measurements were made relative to 95% activity of the NBS oxalic acid standard. Ages were calculated using the true 5720 year half-life and are reported as years Before Present (1950). The reported I cr error is the counting error alone. (2000
‘9 0 I
1000 I
2000
age ( years 6.P)
3OOO4ooO 5ooo I
IO-
I +t+
6ooO I
2030-
40j
so-
;
60-
2
ivSO90 IOOI IO-
120I
I
I
I
I
Figure r. 14C age OS.depth for core NWC
I 102975
I
I74
G. J. Benoit, K. K. Turekian & L. K. Benninger
Organic carbon content and loss on ignition (LOI) data are summarized in Table I. The concentration of organic carbon decreases rapidly from a high value at the surface to a nearly constant value below 25 cm. Radiocarbon activity measurements for NWC 102975, a zooplankton sample and a homogenized sewage sludge sample are listed in the same table. The most salient features of the sediment data are a decreasein age towards the surface, an extrapolated age of about 2000 years BP for the sediment-water interface, and a singular old age of about 5000 years BP for the section of the core between 5 and IO cm from the surface (Figure I).
Discussion The determination
of sediment accumulation rate
The long term sediment accumulation rate in NWC 102975 was determined using only data for parts of the core deeper than IO cm for reasons discussed below. A least squares fit to the 8 deepest measurements gives a slope yielding an accumulation rate of o.o7j&o.o13 cm year -l for regression of age on depth. This number is within I 0 of the value of 0.063 cm year-l inferred from the measured thickness of sediments at this site and the time since inundation by the rising postglacial sea (Bokuniewicz et al., 1976). Benninger et al. (1979) predicted that a long-lived tracer like 14C would show the effect of mixing at least to the maximum depth of excess zloPb intrusion in filled burrows, although the size of the effect could not be determined. The net effect would be an increase in apparent sediment accumulation rate, as measured by 14C, over the true rate. The significance of the extrapolated
surface 14C age
The age of the sediment extrapolated from the data below IO cm to the present-day sedimentwater interface is 23203310 years. We interpret that number as the age of that fraction of deposited carbon, when freshly settled, which survives oxidation and is buried below IO cm. The measured i4C age of sediments at the surface can differ from this extrapolated age if the 14C concentration of the total carbon deposited is different from the residual carbon left after oxidation in the mixed zone. For example, present-day zooplankton from Long Island Sound have an age of -1030~80 years BP (because of the bomb effect) and this could contribute at the present time to the 14C burden of the freshly deposited organic complex. The addition of 14C-free fossil organic matter from coal and oil products as the result of industrialization could lower the 14C concentration. Surface ages greater than o years have been reported for the organic fraction of marine sediments from the Baltic Sea (Erlenkeuser et al., 1975) and southern California nearshore basins (Emery, 1960). These have been explained as the result either of inclusion of fossil carbon from land or biological fractionation and selective decomposition (Emery, 1960). It seems unlikely that selective oxidation or decomposition could cause the sediments to become so isotopically light that they would have a zero age near 2000 years BP. Simple peroxide oxidation of marine sediments by Sackett & Thompson (1963) left the residue enriched in 13C (and therefore 14C by inference) and only by a few parts per thousand. Selective oxidation, such as the decarboxylization of amino acids, can cause enrichment of 12C relative to heavier isotopes. Sackett & Thompson (1963) calculated that such a process could not account for even I per mil enrichment of 12C relative to 1sC in Gulf of Mexico, sediments. A 2000 years age corresponds to a 614C value of -220 per mil, corresponding to a 613Cchange of -110 per mil.
Radiocarbon dating of a core from Long Island Sound
Studies conducted in the Gulf of Mexico using 13Cas a tracer indicate that within several miles of the shore, carbon incorporated into marine sediments comes mostly from the land (Sackett & Thompson, 1963; Sackett, 1964; Newman et al., 1973; Hedges & Parker, 1976). Measurements of 14C activity in organic matter in soils from around the world show that soil carbon is invariably quite old (Campbell et aZ., 1967; Scharpenseel et al., 1969; Herrera 2%Tamers, 1970; Gracanin, 1970; Scharpenseel, 1970; Conry & Mitchell, 1971; Schnitzer & Kahn, 1972; Martel & Paul, 1974). Typically soil carbon ranges in age from about o near the surface to between 2000 and 5000 years BP at a depth of I m. Although we found no data for New England soils we would expect these soils to show the same antiquity as the measured ones. Since sediment delivered to the sea by streams is a product of mass wasting, soil carbon deposited in sediments should have an age like that at intermediate depth in the soil column. The extrapolated surface age is therefore consistent with a terrcstrial source of organic matter in Long Island Sound sediments. Soil carbon can be delivered to Long Island Sound directly from the land by rivers although some may first be deposited on the continental shelf and later carried to the Sound by residual drift currents (Bumpus, 1965) and the landward flow of dense salt water near the bottom (Gross & Bumpus, 1972; Gordon & Pilbeam, 1975) in normal estuarine circulation. This second possibility is raised by the work of Bokuniewicz et al. (1976) and Wakeland (1978). Bokuniewicz et aZ. argued on the basis of mass balance calculations that as much as Ko’?
The explanation
of near-interface
14C variations
The top IO cm of the core deserve special consideration. The upper half of this zone is known to be very well mixed and the 5 to IO cm interval indicates a radiocarbon age nearly 2000 years older than would be expected at that depth. Stable metal concentrations greater than background levels are, except in burrow fillings, limited to this zone (Benninger et cd., 1978).
Erlenkeuser et aZ. (1975) measuring 14C activity in 2 cores ISO cm and 200 cm long taken from the western Baltic Sea at a water depth of 28 m observed an age distribution similar to that exhibited by NWC 102975. Using a chronology based on the 14C depth profile they noted that the 1% activity began to decrease below the expected trend at a depth corresponding to an age of 140 years, reached a minimum in sediments apparently deposited 60 years ago, then increased towards the surface, exceeding the extrapolated sediment-water interface value. Except for the difference in sediment accumulation rate (0.14 cm years-‘), surface age (800 years BP), and sampling interval (1-2 cm) the curves for Long Island Sound and the Baltic Sea show a striking resemblance and may be explained in a similar way. Erlenkeuser et aZ. (1975) explained the trend as the result of increasing addition of anthropogenically mobilized fossil carbon which was later balanced by the sudden increase in atmospheric l*C activity concommitant with the start of nuclear weapons testing. The return to a nearly preindustrial age at the surface is then explained as a fortuitous balance between the effect of ‘hot’ bomb carbon and ‘dead’ fossil carbon.
176
G.J. Bend,
K. K. Turekian &’ L. K. Benninger
The radiocarbon distribution in sediments near the top of NWC 102975 can be simply modelled. The model will help to determine whether or not the explanation proposed for the Baltic Sea data is reasonable for Long Island Sound. Three independent carbon components are considered: (I) a residual, biologically refractory carbon fraction derived from soils and plankton, (2) anthropogenically mobilized fossil carbon; and (3) easily biodegradable planktonic carbon. A two-level model will be used in which the 5 cm deep upper level is treated as a well mixed box as required by the 23*Th data (Aller & Cochran, 1976). The level below this mixed layer is assumed to be too slowly mixed to affect the 14C distribution seriously. Within a few decimeters of the surface the organic carbon concentration of NWC 10297s becomes a constant 1.1% fraction of the total solids present. When freshly settled this component of the total carbon has an activity that corresponds to the 2320 sediment-water interface age calculated by extrapolation for this core. This carbon fraction will be called ‘normal’ carbon and is symbolized C,. The concentration of normal carbon over time or distance in the sedimentary column is: [C,] = k = I-I% The initial specific activity of normal carbon relative to the oxalic acid standard calculated to 1950 as ‘present’ is: A NO e-‘WJO) = 0.755 = A, The effect of industrial mobilization of fossil carbon became significant about 90 years ago (Suess, 1955). In addition to influencing the i4C specific activity of the CO, in the atmospheric reservoir, unoxidized carbon from fossil carbon sources are also expected to be found in sediments (Erlenkeuser et al., 1975). The ‘fossil’ carbon reaching Long Island Sound as particles is symbolized C, and is assumed to have a specific activity, A,=o. Its delivery rate to Long Island Sound can be approximated by a step function where rate of supply = o for t<1885 AD and rate of supply after t = 1885 is a constant. Use of such a simple function may be justified since increased burning of fossil fuels around Long Island Sound has been partly balanced by improved emission control technology. The concentration of fossil carbon in the upper mixed layer gradually approaches the input value at a rate determined by the residence time of sediment in the mixed layer. The residence time, t, is the length of time required to accumulate a thickness of sediment equal to the depth, M, defining the bottom of the mixed layer by sediment accumulation at rate S. If we let M = 5 cm and S = 0.075 cm year-r then: ~=-zz
M
5 cm
S
0.075 cm year-r
= 66.6 year
The equation relating fossil carbon concentration at time E after the beginning of fossil carbon injection in the mixed layer to the input concentration, assuming no decomposition of the fossil carbon, can be constructed as follows: At time t fossil carbon enters the box at the constant rate [C,,,] S and leaves at the constantly changing rate [C&j S. The rate of change in concentration of fossil carbon in the box is clearly:
Radiocarbon
dating
of a core from
Long Island
Sound
-.
‘7’7
and :
so:
ln L,,l-Lwl ( L,,l
)
= -- t ‘c
therefore:
[CFctJ= L,,l
(I--e-f’T)
[C Fwml =
x0.74
Setting t = 90 years gives:
F&l
Concentration at depth x> 5 cm at time t after the beginning of deposition of fossil carbon is the same as it was in the upper box at time [(M+tS)--xl/s, therefore:
The present day average concentration in depth intervals S-IO and IO-IS cm can be calculated by integrating the concentration over the given ranges and dividing by the interval length. [cl
= d
1::
[C, (x, t,] 8 X= 2’
1xs (x-M xi
exp ( - (“‘+z))-“)
)
In this problem Ax = 5 cm. [C,,,] was determined by iteration of these equations until the best fit to the data was obtained. The measured data are consistent with a value of [CFIN] = 0.70% of total deposited solids commencing in the year 1885 AD. This corresponds to a delivery rate of fossil carbon to the sediments equal to 0.59 mg cmm2 year-l for an average solids density of 2.5 g cmm3, a water content of 55%, and a sediment accumulation rate of 0.075 cm year-l. This value is in good agreement with measurements of elemental carbon in sediments from Lake Whitney (New Haven, CT), which indicate a fossil input rate averaging 0.8 mg cme2 year-r over the past 90 years (Bertine & Mendek, 1978). IJsing values of t, r, S, n/r, Ax, and [C,,,] given above yields the following results : [C Fc90yJ(o-5 cm) = 0.70 (r-e-so167) = 0.52% [C FcsOyJ(5-10 cm) = ‘$
[CFcsOyJ(IO-xg
[(ro-5e-1’75)-(5-5e-6~75)]
cm) = OF [(rr+75-5e”)-(ro-5e-1.75)]
= 0.39% = 0.038%
The third component is planktonic carbon, C,. For the past 20 years planktonic t*C activity has been artificially raised by the presence of bomb generated l*C in the atmosphere. A measurement made for this study showed Long Island Sound zooplankton to have an activity of 1.13 relative to the pre-industrial atmosphere. Riley (1956) calculated that about 72 g me2 of planktonic carbon reaches the bottom of Long Island Sound annually. That is equivalent to 8.5% of the total amount of accumulating solids. C P will be assigned a
178
G. J. Benoit,
K. K. Turekian
&’ L. K. Benninger
value such that within the precision of the measurement the 14C activity in the 5 cm agrees with the measured value. The condition that must be fulfilled is that the average i4C activity in the top 5 cm of sediments, irO-r,cm,must be equal to the weighted sum of the 14C activities of the three carbon components, C p, C,, and C,, present in that depth range. The equation is :
where:
Gh-5
=
[C,1,-,+[C~1,-,+[C,1~-5
Recalling that A, = o, substituting for [CT],,-s, and solving for [C,],,+ gives: [C do-5
=
&-wm
([C,I,-,+CCNI,-,)-AA,[C,I,-,
_
o.650/:,
A p-30-5
The residence time of C, in the mixed layer must be (0*65/&s) x365 days = 28 days. No C, passes to depth in the sediment because it is recycled to the overlying water before it can be isolated. Concentrations and activities of C,, C,, and C, can now be calculated for all depths in the core; the results are summarized in Table 2. Most measured values are in good agrecment with model results. All measured ages agree within I cr with the calculated values, and calculated total carbon estimates are generally within IO”/~ of the measurements however the model gives uniformly low results in the range IO to 30 cm. Perhaps this reflects gradual decomposition of plankton that is carried beneath the upper 5 cm. If that is true then the model must be adjusted to give a higher C, concentration in the 5-10 cm zone in order to compensate for the added C, at this depth. It is possible to increase the amount of C!, in the S-IO cm zone relative to the o-5 cm zone if the C, input function is changed from a square wave to some function which more nearly duplicates the input rate indicated by elemental carbon concentration in Lake Whitney sediments (Bertine & Mendek, 1978). Since the maximum input rate apparently occurred some time ago, the concentration of C, would then be greater at depth than near the surface in Long Island Sound. Overall these adjustments would cause the goodness of fit to decrease since the calculated ages TABLE 2. Results from model calculation
(see text)
Depth interval
[%I
[$I;1
[$Pl
xca,,
(cm)
?o
/O
/o
0' /"
o-5 S-10 IO-15 15-20 20-25 25-30 35-40 60-65 85-90 100-105
1’10
0’52
0.65
2’27
1’10
1’10
0’0
1’10
0'0
I’10
0’0
1’10
0'0 0'0 0'0
0'0 0'0 0'0 0'0 0'0 0'0 0'0 0'0 0'0
1'49 1'14
1’10
0'39 0.038 0'0
1’10
1’10 1’10
-4No = 0'749
A,
= 0.0
Ap
= I.133
1’10 1’10 1’10 1’10 1'10 1'10
1’10
xlm,
Age,,,,
iigcnw.
0' /O
(year BP)
(year BP)
1'92 1'47 1.38 1'31 1'33 1.26
3090 4990 2780 2550 2620 2690
2720
1’21
2820
I.18 (>0.89)
1’20
3150 3490 3690
s12* 2480 2780
2200 2910 2800 3180 3370 3770
Radiocarbon
dating
of a core from Long Island Sound
179
between 10-30 cm would become younger and the total carbon values of the O-IO cm range would increase. With the single exception discussed above, the model, based on reasonable assumptions, yields predictions in good agreement with the available data. The observed distribution of 14C could be produced by a steady addition of soil organic matter and planktonic carbon over at least the past 1400 years recorded by the length of the core, followed by a short period of inclusion of fossil carbon and plankton enriched with bomb laC. Summary i4C activity decreases approximately exponentially with depth in sediments between IO and IIO cm from the surface in northern central Long Island Sound. The rate of decrease suggests a sediment accumulation rate of 0~0~~~0~013 cm year-l for this site over the past 1400 year. Diffusive mixing as measured by the zloPb depth profile can account for only 0.001 cm year-r of this value. The extrapolated sediment-water interface age at site NWC is 2320 year BP. This is the age, when freshly settled, of that part of the organic carbon which is not metabolized before burial to a depth of IO cm. Soil carbon is probably the main component of this fraction, although some refractory planktonic carbon may also be included. Very low 14C activity between 5-10 cm and near normal activity between 5 cm and the surface can be modelled in terms of non-steady state addition of soil carbon, fossil carbon, and bomb enriched planktonic carbon. Sediments between 5 and IO cm appear older than expected because they contain anthropogenically mobilized fossil carbon. Sediments in the top 5 cm of the core have a more normal age because the dilution of normal soil carbon by ‘dead’ fossil carbon observed in the 5 to IO cm interval is here compensated by inclusion of high activity, nuclear age, planktonic carbon. The model suggests that the residence time of easily metabolizable planktonic carbon in the sediment is around 28 days. Acknowledgements We wish to thank Dr Y. Nozaki for expertly guiding one of us (GJB) through the techniques of radiocarbon analysis. This research was supported by the U.S. Department of Energy through grant EY-76-S-02-3573, References disequilibrium in nearshore sediment: particle reworking .4ller, R. C. & Cochran, J. K. 1975 s”‘Th/“““U and diagenetic time scales. Earth and Planetary Science Letters 29, 37-50. Benninger, L. K., Aller, R. C., Cochran, J. K. & Turekian, K. K. 1979 Effects of biological sediment in a Long Island Sound sediment mixing on the 210Pb chronology and trace metal distribution core. Earth and Planetary Science Letters (in press). Bertine, K. K. & Mendek, M. F. 1978 Industrialization of New Haven, Ct. as recorded in reservoir sediments. Environmental Science and Technology 12, 201-207. Bokuniewicz, H. J., Gebert, J. & Gordon, R. B. 1976 Sediment mass balance of a large estuary, Long Island Sound. Estuarine and Coastal Marine Science 4, 523-536. Bumous. D. F. 1964 Residual drift alone the bottom on the continental shelf in the middle Atlantic Bight area. L%nology and Oceanography ro (suppl.) (Redfield volume), Rso-R53. Campbell, C. A., Paul, E. A., Rennie, D. A. & hlccallum, K. J. 1967 Applicability of the carbondating method of analysis to soil humus studies. Soil Science 104, 217-224. Corny, M. J. & Mitchell, G. F. 1971 The age of Irish Plagen soils. In Paleopedology, (Yaalon, D. H., ed.) International Society for Soil Science, pp. Izg-138.
180
G. J, Benoit,
K. K. Turekian
& L. K. Benninger
Emery, K. 0. 1960 The Sea of Southern California. Wiley. New York, 366 pp. Erlenkeuser, H., Suess, E. & Wilkomm, H. 1975 Industrialization affects heavy metal and carbon isotope concentrations in recent Baltic Sea sediments. Geochimica et Cosmochimica Acta 38,823.-842. Gordon, R. B. & Pilbeam, C. C. 1975 Circulation in central Long Island Sound. journal of Geophysics Research 80, 414-422. Gracanin, 2. 1970 Age and development of the hummocky meadow in the Lechtaler Alps. In Paleopedology, (Yaalon, D. H., ed.) International Society for Soil Science, pp. 129-138. Gross, M. G. & Bumpus, D. F. 1972 Residual drift of near-bottom waters in Long Island Sound, 1969. Limnology and Oceanography 17, 636-638. Hedges, J. L. & Parker, P. L. 1976 Land-derived organic matter in sediments from the Gulf of Mexico. Geochimica et Cosmochimica Acta 40, ro19-rozg. Herrera, R. & Tamers, M. A. 1970 Radiocarbon dating of tropical soil associations in Venezuela. In Paleopedology, (Yaalon, D. H., ed.) International Society for Soil Science, pp. 109-115. Martel, Y. A. & Paul, E. A. 1974 The use of radiocarbon dating or organic matter in the study of soil genesis. Soil Science Society of America Proceedings 38, 501-506. Newman, J. W., Parker, P. L. & Behrens, E. W. 1973 Organic carbon isotope ratios in Quaternary cores from the Gulf of Mexico. Geochimica et Cosmochimica Acta 37, 225-238. Noakes, J. E., Kim, S. M. & Stipp, J. J. 1965 Chemical and counting advances in liquid scintillation age dating, 6th International Conf. on Radiocarbon and Tritium dating Proc., Pullman, WA. 7-11 June, 1965, 68-92. Nozaki, Y. & Turekian, K. K. 1977 Yale University geology and geophysics radiocarbon dates. I. Radiocarbon 19, 133-141. Polach, E. A. & Stipp, J. J. 1967 Improved synthesis technique for methane and benzene radiocarbon dating. InternationalJournal of Applied Radiation Isotopes 18, 359-364. Riley, G. A. 1956 Oceanography of Long Island Sound, 1952-1954: production and utilization of organic matter. Bulletin of the Bingham Oceanogruphic Collection 15, 324.-344. Sackett, W. M. & Thompson, R. R. 1963 Isotopic organic carbon composition of recent continental derived elastic sediments of eastern gulf coast, Gulf of Mexico. Bulletin of the American Association of Petrology and Geology 47, 525-53 1. Sackett, W. M. 1964 The depositional history and isotopic organic carbon composition of marine sediments. Marine Geology 2, 173-185. Scharpenseel, M. W., Petig, F. & Tamers, M. A. 1969 University of Bonn natural radiocarbon measurements II. Radiocarbon II, 3-14. Scharpenseel, M. W. rg7o Radiocarbon dating of soils-problems, troubles, hopes. In Puleopedology (Yaalon, D. H., ed.) International Society for Soil Science, pp. 77-88. Schnitzer, M. & Kahn, S. V. 1972 Humic substances in the environment. Marcel Dekker, Inc. New York, pp. 126-129. Wakeland, M. E., Jr. 1978 Sources of fine-grained sediments in Long Island Sound. Abstracts with Programs. 13th annual meeting Northeast Section of the Geological Society of America Vol. IO, No. 2, p. 90.
Radiocarbon Long Island
Dating Sound
of a Core from
Gaboury J. Benoit, Karl K. Turekian Larry K. Benninger
and
Department of Geology and Geophysics, Yale University, Connecticut 06520, U.S.A.
New Haven,
Received 30 June 1978
Keywords : Radiocarbon dating; sedimentation tion; Long Island Sound; Connecticut coast
rates ; estuarine sedimenta-
Radiocarbon measurements sequentially with depth in a diver-obtained core from Long Island Sound has been interpreted in terms of sediment accumulation rates and sources of organic carbon in the sediment. A sediment accumulation rate of 0.075 fo.013 cm year-’ is determined from the data below IO cm in the core. An age of 2320 year BP is obtained for the sediment-water interface by extrapolation. This means that the dominant carbon component preserved in the core is soil-derived. The contributions of fossil fuel carbon and bomb radiocarbon associated with plankton are also evaluated.
Introduction The delivery of organic carbon to estuarine sediments provides several types of information about the sedimentary regime of the system based on the radiocarbon content. The most obvious is the determination of rates of sediment accumulation. But the variation of the level of 14C activity with depth in a core raised from the estuarine floor also provides insights in the sources of the organic matter in the sediments as a function of time. The special input of soil carbon, fossil carbon, planktonic carbon or sewage sludge, each has a sufficiently distinctive 14C activity to provide important insights into source as well as dating problems. We report in this paper our results on a core from a well-studied region of Long Island Sound. Although the information has been obtained in only one estuarine type we believe that it provides the basis for understanding other estuarine systems as well.
Description
of sampling
site
The core analyzed for radiocarbon is one raised from the well-studied Northwest Control site and is described by Benninger et al. (1979). Sampling station Northwest Control (NWC) is located at 41’10.4’N latitude, 72’56*3’W longitude about 6 km south of New Haven Harbor. The bottom lies at a depth of 14 m. Occasionally, severe storms have been observed to disturb the sediment-water interface in this area. Aller & Cochran (1976) noted storm induced laminations in the top 3 cm of a box core sampled soon after a northwest storm in the Fall. 0302-~52~/79/080171+10
$02.00/o
0 rg7g Academic Press Inc. (London) Ltd.
172
G.J. Benoit, K. K. Turekian & L. K. Benninger
NWC sediments support a stable, low diversity, sediment feeding epifauna. Principal organisms are the protobranch bivalves Yoldia Zimatula and NucuZu annulutu. The @fauna thoroughly rework the near-interface sediments leaving it well mixed to a depth of about 4 cm. A feeding lag layer at 6 cm and abundant fecal pellets in the top few centimeters (Aller & Cochran, 1976) attest to the high rate of biological activity at this site. Below the depth reworked by Yoldiu Zimatula, burrows refilled by surface sediments and shells and characterized by high water content, metals, and excess 2loPb activity are observed (Benninger et uZ., 1979). X-radiographs of the core used in this study, NWC 102975 (Benningcr et al., xgyy), show the top IO cm to be free of structural features, presumably as a result of intense physical and biological reworking. At depth, one commonly finds burrow structures characterized by high water content and the presence of shell fragments. In particular refilled burrows were observed at 25-30 cm, 45 cm, 55 cm, 70 cm, and IIS cm. Shell debris normally found in a layer at 3-4 cm from the surface lies within I cm of the surface in core N1YC 102975. The reason for this is not known, but one possibility is that the upper 2 to 3 cm of the core was lost some time prior to sampling. Methods and results The method of coring and sampling is described in Benninger et al., (1979). In brief, a 120 cm gravity core was collected in a length of IO cm internal diameter polyvinylchloride sewer pipe. A scuba diver assisted coring operations to assure recovery of the semi-fluid sediment water interface. r4C was measured using the liquid scintillation counting method of Noakes et al. (1965) and Polach & Stipp (1967). The Y a1e radiocarbon line is described in Nozaki & Turekian (1977). A discussion of the refinements of the technique as used on relatively tow organic matter concentration in sediment and high volatile organic material such as sewage sludge and plankton follows. Approximately IOO g of sediment previously dried at IIO “C was treated with 500 ml of cold, 27” HCl to dissolve CaCO,. The sample was centrifuged and decanted, mixed with distilled water and centrifuged again. After decanting, the wet sediment was redried at IIO “C, ground in a mortar and pestle, and burned at about 900 “C in a stream of pure oxygen at a reduced pressure. The dried, ground, sewage sludge sample was burned at a greatly reduced oxygen pressure in order to prevent explosive combustion. The dried zooplankton sample burned so violently, even at reduced oxygen pressure, that a five-fold dilution of the sample with previously burned sediment was used as a moderator. The CO, generated by combustion was passed successively through solid CuO maintained at 500 “C, a series of chemical scrubbers (Ag,SO,, K,MnO,, and H,S04-K,CrO,), and a dry ice trap and collected finally as purified solid CO, in two liquid nitrogen cooled traps in series. The gas was expanded to a 5 liter bulb and its pressure recorded. Generally less than 0.1 moles was recovered. Reported values of carbon concentration are based on these pressure measurements. Next, the CO, was reacted with hot (800 “C) Li metal forming lithium carbide. The lithium carbide after cooling to room temperature was converted to acetylene by the controlled addition of an excess of distilled water. Yields of acetylene from carbon dioxide average 95 %. The acetylene, dewatered through a phosphoric acid scrubber, was trimerized to benzene on an activated chromium catalyst. Benzene released from the catalyst by heating was collected in a dry ice cooled trap. Unreacted gas was commonly detected in the catalyst tube when a vacuum was applied even after a day or more had elapsed. The final benzene
Radiocarbon dating of a core from Long Island Sound
‘73
TABLE I. Radiocarbon ages and organic content of core NWC 102975, 41’10.4’N 72”56*3’W, 14 m depth, zooplankton and sewage sludge
Depth in core (cm) Core
%H,O
% LO1
59'9 56.3 55.8 56.4 56.3 56.0 53.8 52'3 52.6 52.6 53.6 50'9
5'5 5'7 5'2 5'2 5'5 4'9 4.6 4'4 4'7 5'0 4'5 4'0
o-5 5-10 IO-15 15-20 20-2 j
25-30 35-40 60-65 80-85 85-90 roo-I05 IOj-110
Zooplankton Sewage sludge
%C I'92
I.47 1.38 1.31
I'33 1,26 I'21 I.19 I'02
% c: % LOI 0'35 0.26 0.26 0.25 0.24 0.26 0.26 0.27
Age & IO (year BP) 272O-c310
5I201k3IO 24805250 2780~220 2200,210
29103 340 2800+ 220 318oi280
0'22
(>0.89)
(>0.18)
0.27 0.26
1'20 1'03
3370$280 3770*290
3'33'5
-103oiEo
31.6
-298oi70
yield generally did not exceed 80% because of the loss of that unreacted gas and because of the loss of benzene during liquid transfer to counting vials. It was critical to minimize loss during transfer because the quantity of synthesized benzene was small (typically 0.7 g for sediment samples). The recovered benzene was made up to 3 ml total volume in 6 ml low K counting vials by adding ‘dead’ benzene. Samples were prepared for counting by addition of I ml of a scintillation solution: 0.35% PPO and 0.07% dimethyl POPOP in a 3 :I mixture of benzene and toluene. Counter stability
was monitored
by comparison
with an internal
hot standard
counts min-l). Blank values around 7.4 counts min-l were obtained using dead benzene. Measurements were made relative to 95% activity of the NBS oxalic acid standard. Ages were calculated using the true 5720 year half-life and are reported as years Before Present (1950). The reported I cr error is the counting error alone. (2000
‘9 0 I
1000 I
2000
age ( years 6.P)
3OOO4ooO 5ooo I
IO-
I +t+
6ooO I
2030-
40j
so-
;
60-
2
ivSO90 IOOI IO-
120I
I
I
I
I
Figure r. 14C age OS.depth for core NWC
I 102975
I
I74
G. J. Benoit, K. K. Turekian & L. K. Benninger
Organic carbon content and loss on ignition (LOI) data are summarized in Table I. The concentration of organic carbon decreases rapidly from a high value at the surface to a nearly constant value below 25 cm. Radiocarbon activity measurements for NWC 102975, a zooplankton sample and a homogenized sewage sludge sample are listed in the same table. The most salient features of the sediment data are a decreasein age towards the surface, an extrapolated age of about 2000 years BP for the sediment-water interface, and a singular old age of about 5000 years BP for the section of the core between 5 and IO cm from the surface (Figure I).
Discussion The determination
of sediment accumulation rate
The long term sediment accumulation rate in NWC 102975 was determined using only data for parts of the core deeper than IO cm for reasons discussed below. A least squares fit to the 8 deepest measurements gives a slope yielding an accumulation rate of o.o7j&o.o13 cm year -l for regression of age on depth. This number is within I 0 of the value of 0.063 cm year-l inferred from the measured thickness of sediments at this site and the time since inundation by the rising postglacial sea (Bokuniewicz et al., 1976). Benninger et al. (1979) predicted that a long-lived tracer like 14C would show the effect of mixing at least to the maximum depth of excess zloPb intrusion in filled burrows, although the size of the effect could not be determined. The net effect would be an increase in apparent sediment accumulation rate, as measured by 14C, over the true rate. The significance of the extrapolated
surface 14C age
The age of the sediment extrapolated from the data below IO cm to the present-day sedimentwater interface is 23203310 years. We interpret that number as the age of that fraction of deposited carbon, when freshly settled, which survives oxidation and is buried below IO cm. The measured i4C age of sediments at the surface can differ from this extrapolated age if the 14C concentration of the total carbon deposited is different from the residual carbon left after oxidation in the mixed zone. For example, present-day zooplankton from Long Island Sound have an age of -1030~80 years BP (because of the bomb effect) and this could contribute at the present time to the 14C burden of the freshly deposited organic complex. The addition of 14C-free fossil organic matter from coal and oil products as the result of industrialization could lower the 14C concentration. Surface ages greater than o years have been reported for the organic fraction of marine sediments from the Baltic Sea (Erlenkeuser et al., 1975) and southern California nearshore basins (Emery, 1960). These have been explained as the result either of inclusion of fossil carbon from land or biological fractionation and selective decomposition (Emery, 1960). It seems unlikely that selective oxidation or decomposition could cause the sediments to become so isotopically light that they would have a zero age near 2000 years BP. Simple peroxide oxidation of marine sediments by Sackett & Thompson (1963) left the residue enriched in 13C (and therefore 14C by inference) and only by a few parts per thousand. Selective oxidation, such as the decarboxylization of amino acids, can cause enrichment of 12C relative to heavier isotopes. Sackett & Thompson (1963) calculated that such a process could not account for even I per mil enrichment of 12C relative to 1sC in Gulf of Mexico, sediments. A 2000 years age corresponds to a 614C value of -220 per mil, corresponding to a 613Cchange of -110 per mil.
Radiocarbon dating of a core from Long Island Sound
Studies conducted in the Gulf of Mexico using 13Cas a tracer indicate that within several miles of the shore, carbon incorporated into marine sediments comes mostly from the land (Sackett & Thompson, 1963; Sackett, 1964; Newman et al., 1973; Hedges & Parker, 1976). Measurements of 14C activity in organic matter in soils from around the world show that soil carbon is invariably quite old (Campbell et aZ., 1967; Scharpenseel et al., 1969; Herrera 2%Tamers, 1970; Gracanin, 1970; Scharpenseel, 1970; Conry & Mitchell, 1971; Schnitzer & Kahn, 1972; Martel & Paul, 1974). Typically soil carbon ranges in age from about o near the surface to between 2000 and 5000 years BP at a depth of I m. Although we found no data for New England soils we would expect these soils to show the same antiquity as the measured ones. Since sediment delivered to the sea by streams is a product of mass wasting, soil carbon deposited in sediments should have an age like that at intermediate depth in the soil column. The extrapolated surface age is therefore consistent with a terrcstrial source of organic matter in Long Island Sound sediments. Soil carbon can be delivered to Long Island Sound directly from the land by rivers although some may first be deposited on the continental shelf and later carried to the Sound by residual drift currents (Bumpus, 1965) and the landward flow of dense salt water near the bottom (Gross & Bumpus, 1972; Gordon & Pilbeam, 1975) in normal estuarine circulation. This second possibility is raised by the work of Bokuniewicz et al. (1976) and Wakeland (1978). Bokuniewicz et aZ. argued on the basis of mass balance calculations that as much as Ko’?
The explanation
of near-interface
14C variations
The top IO cm of the core deserve special consideration. The upper half of this zone is known to be very well mixed and the 5 to IO cm interval indicates a radiocarbon age nearly 2000 years older than would be expected at that depth. Stable metal concentrations greater than background levels are, except in burrow fillings, limited to this zone (Benninger et cd., 1978).
Erlenkeuser et aZ. (1975) measuring 14C activity in 2 cores ISO cm and 200 cm long taken from the western Baltic Sea at a water depth of 28 m observed an age distribution similar to that exhibited by NWC 102975. Using a chronology based on the 14C depth profile they noted that the 1% activity began to decrease below the expected trend at a depth corresponding to an age of 140 years, reached a minimum in sediments apparently deposited 60 years ago, then increased towards the surface, exceeding the extrapolated sediment-water interface value. Except for the difference in sediment accumulation rate (0.14 cm years-‘), surface age (800 years BP), and sampling interval (1-2 cm) the curves for Long Island Sound and the Baltic Sea show a striking resemblance and may be explained in a similar way. Erlenkeuser et aZ. (1975) explained the trend as the result of increasing addition of anthropogenically mobilized fossil carbon which was later balanced by the sudden increase in atmospheric l*C activity concommitant with the start of nuclear weapons testing. The return to a nearly preindustrial age at the surface is then explained as a fortuitous balance between the effect of ‘hot’ bomb carbon and ‘dead’ fossil carbon.
176
G.J. Bend,
K. K. Turekian &’ L. K. Benninger
The radiocarbon distribution in sediments near the top of NWC 102975 can be simply modelled. The model will help to determine whether or not the explanation proposed for the Baltic Sea data is reasonable for Long Island Sound. Three independent carbon components are considered: (I) a residual, biologically refractory carbon fraction derived from soils and plankton, (2) anthropogenically mobilized fossil carbon; and (3) easily biodegradable planktonic carbon. A two-level model will be used in which the 5 cm deep upper level is treated as a well mixed box as required by the 23*Th data (Aller & Cochran, 1976). The level below this mixed layer is assumed to be too slowly mixed to affect the 14C distribution seriously. Within a few decimeters of the surface the organic carbon concentration of NWC 10297s becomes a constant 1.1% fraction of the total solids present. When freshly settled this component of the total carbon has an activity that corresponds to the 2320 sediment-water interface age calculated by extrapolation for this core. This carbon fraction will be called ‘normal’ carbon and is symbolized C,. The concentration of normal carbon over time or distance in the sedimentary column is: [C,] = k = I-I% The initial specific activity of normal carbon relative to the oxalic acid standard calculated to 1950 as ‘present’ is: A NO e-‘WJO) = 0.755 = A, The effect of industrial mobilization of fossil carbon became significant about 90 years ago (Suess, 1955). In addition to influencing the i4C specific activity of the CO, in the atmospheric reservoir, unoxidized carbon from fossil carbon sources are also expected to be found in sediments (Erlenkeuser et al., 1975). The ‘fossil’ carbon reaching Long Island Sound as particles is symbolized C, and is assumed to have a specific activity, A,=o. Its delivery rate to Long Island Sound can be approximated by a step function where rate of supply = o for t<1885 AD and rate of supply after t = 1885 is a constant. Use of such a simple function may be justified since increased burning of fossil fuels around Long Island Sound has been partly balanced by improved emission control technology. The concentration of fossil carbon in the upper mixed layer gradually approaches the input value at a rate determined by the residence time of sediment in the mixed layer. The residence time, t, is the length of time required to accumulate a thickness of sediment equal to the depth, M, defining the bottom of the mixed layer by sediment accumulation at rate S. If we let M = 5 cm and S = 0.075 cm year-r then: ~=-zz
M
5 cm
S
0.075 cm year-r
= 66.6 year
The equation relating fossil carbon concentration at time E after the beginning of fossil carbon injection in the mixed layer to the input concentration, assuming no decomposition of the fossil carbon, can be constructed as follows: At time t fossil carbon enters the box at the constant rate [C,,,] S and leaves at the constantly changing rate [C&j S. The rate of change in concentration of fossil carbon in the box is clearly:
Radiocarbon
dating
of a core from
Long Island
Sound
-.
‘7’7
and :
so:
ln L,,l-Lwl ( L,,l
)
= -- t ‘c
therefore:
[CFctJ= L,,l
(I--e-f’T)
[C Fwml =
x0.74
Setting t = 90 years gives:
F&l
Concentration at depth x> 5 cm at time t after the beginning of deposition of fossil carbon is the same as it was in the upper box at time [(M+tS)--xl/s, therefore:
The present day average concentration in depth intervals S-IO and IO-IS cm can be calculated by integrating the concentration over the given ranges and dividing by the interval length. [cl
= d
1::
[C, (x, t,] 8 X= 2’
1xs (x-M xi
exp ( - (“‘+z))-“)
)
In this problem Ax = 5 cm. [C,,,] was determined by iteration of these equations until the best fit to the data was obtained. The measured data are consistent with a value of [CFIN] = 0.70% of total deposited solids commencing in the year 1885 AD. This corresponds to a delivery rate of fossil carbon to the sediments equal to 0.59 mg cmm2 year-l for an average solids density of 2.5 g cmm3, a water content of 55%, and a sediment accumulation rate of 0.075 cm year-l. This value is in good agreement with measurements of elemental carbon in sediments from Lake Whitney (New Haven, CT), which indicate a fossil input rate averaging 0.8 mg cme2 year-r over the past 90 years (Bertine & Mendek, 1978). IJsing values of t, r, S, n/r, Ax, and [C,,,] given above yields the following results : [C Fc90yJ(o-5 cm) = 0.70 (r-e-so167) = 0.52% [C FcsOyJ(5-10 cm) = ‘$
[CFcsOyJ(IO-xg
[(ro-5e-1’75)-(5-5e-6~75)]
cm) = OF [(rr+75-5e”)-(ro-5e-1.75)]
= 0.39% = 0.038%
The third component is planktonic carbon, C,. For the past 20 years planktonic t*C activity has been artificially raised by the presence of bomb generated l*C in the atmosphere. A measurement made for this study showed Long Island Sound zooplankton to have an activity of 1.13 relative to the pre-industrial atmosphere. Riley (1956) calculated that about 72 g me2 of planktonic carbon reaches the bottom of Long Island Sound annually. That is equivalent to 8.5% of the total amount of accumulating solids. C P will be assigned a
178
G. J. Benoit,
K. K. Turekian
&’ L. K. Benninger
value such that within the precision of the measurement the 14C activity in the 5 cm agrees with the measured value. The condition that must be fulfilled is that the average i4C activity in the top 5 cm of sediments, irO-r,cm,must be equal to the weighted sum of the 14C activities of the three carbon components, C p, C,, and C,, present in that depth range. The equation is :
where:
Gh-5
=
[C,1,-,+[C~1,-,+[C,1~-5
Recalling that A, = o, substituting for [CT],,-s, and solving for [C,],,+ gives: [C do-5
=
&-wm
([C,I,-,+CCNI,-,)-AA,[C,I,-,
_
o.650/:,
A p-30-5
The residence time of C, in the mixed layer must be (0*65/&s) x365 days = 28 days. No C, passes to depth in the sediment because it is recycled to the overlying water before it can be isolated. Concentrations and activities of C,, C,, and C, can now be calculated for all depths in the core; the results are summarized in Table 2. Most measured values are in good agrecment with model results. All measured ages agree within I cr with the calculated values, and calculated total carbon estimates are generally within IO”/~ of the measurements however the model gives uniformly low results in the range IO to 30 cm. Perhaps this reflects gradual decomposition of plankton that is carried beneath the upper 5 cm. If that is true then the model must be adjusted to give a higher C, concentration in the 5-10 cm zone in order to compensate for the added C, at this depth. It is possible to increase the amount of C!, in the S-IO cm zone relative to the o-5 cm zone if the C, input function is changed from a square wave to some function which more nearly duplicates the input rate indicated by elemental carbon concentration in Lake Whitney sediments (Bertine & Mendek, 1978). Since the maximum input rate apparently occurred some time ago, the concentration of C, would then be greater at depth than near the surface in Long Island Sound. Overall these adjustments would cause the goodness of fit to decrease since the calculated ages TABLE 2. Results from model calculation
(see text)
Depth interval
[%I
[$I;1
[$Pl
xca,,
(cm)
?o
/O
/o
0' /"
o-5 S-10 IO-15 15-20 20-25 25-30 35-40 60-65 85-90 100-105
1’10
0’52
0.65
2’27
1’10
1’10
0’0
1’10
0'0
I’10
0’0
1’10
0'0 0'0 0'0
0'0 0'0 0'0 0'0 0'0 0'0 0'0 0'0 0'0
1'49 1'14
1’10
0'39 0.038 0'0
1’10
1’10 1’10
-4No = 0'749
A,
= 0.0
Ap
= I.133
1’10 1’10 1’10 1’10 1'10 1'10
1’10
xlm,
Age,,,,
iigcnw.
0' /O
(year BP)
(year BP)
1'92 1'47 1.38 1'31 1'33 1.26
3090 4990 2780 2550 2620 2690
2720
1’21
2820
I.18 (>0.89)
1’20
3150 3490 3690
s12* 2480 2780
2200 2910 2800 3180 3370 3770
Radiocarbon
dating
of a core from Long Island Sound
179
between 10-30 cm would become younger and the total carbon values of the O-IO cm range would increase. With the single exception discussed above, the model, based on reasonable assumptions, yields predictions in good agreement with the available data. The observed distribution of 14C could be produced by a steady addition of soil organic matter and planktonic carbon over at least the past 1400 years recorded by the length of the core, followed by a short period of inclusion of fossil carbon and plankton enriched with bomb laC. Summary i4C activity decreases approximately exponentially with depth in sediments between IO and IIO cm from the surface in northern central Long Island Sound. The rate of decrease suggests a sediment accumulation rate of 0~0~~~0~013 cm year-l for this site over the past 1400 year. Diffusive mixing as measured by the zloPb depth profile can account for only 0.001 cm year-r of this value. The extrapolated sediment-water interface age at site NWC is 2320 year BP. This is the age, when freshly settled, of that part of the organic carbon which is not metabolized before burial to a depth of IO cm. Soil carbon is probably the main component of this fraction, although some refractory planktonic carbon may also be included. Very low 14C activity between 5-10 cm and near normal activity between 5 cm and the surface can be modelled in terms of non-steady state addition of soil carbon, fossil carbon, and bomb enriched planktonic carbon. Sediments between 5 and IO cm appear older than expected because they contain anthropogenically mobilized fossil carbon. Sediments in the top 5 cm of the core have a more normal age because the dilution of normal soil carbon by ‘dead’ fossil carbon observed in the 5 to IO cm interval is here compensated by inclusion of high activity, nuclear age, planktonic carbon. The model suggests that the residence time of easily metabolizable planktonic carbon in the sediment is around 28 days. Acknowledgements We wish to thank Dr Y. Nozaki for expertly guiding one of us (GJB) through the techniques of radiocarbon analysis. This research was supported by the U.S. Department of Energy through grant EY-76-S-02-3573, References disequilibrium in nearshore sediment: particle reworking .4ller, R. C. & Cochran, J. K. 1975 s”‘Th/“““U and diagenetic time scales. Earth and Planetary Science Letters 29, 37-50. Benninger, L. K., Aller, R. C., Cochran, J. K. & Turekian, K. K. 1979 Effects of biological sediment in a Long Island Sound sediment mixing on the 210Pb chronology and trace metal distribution core. Earth and Planetary Science Letters (in press). Bertine, K. K. & Mendek, M. F. 1978 Industrialization of New Haven, Ct. as recorded in reservoir sediments. Environmental Science and Technology 12, 201-207. Bokuniewicz, H. J., Gebert, J. & Gordon, R. B. 1976 Sediment mass balance of a large estuary, Long Island Sound. Estuarine and Coastal Marine Science 4, 523-536. Bumous. D. F. 1964 Residual drift alone the bottom on the continental shelf in the middle Atlantic Bight area. L%nology and Oceanography ro (suppl.) (Redfield volume), Rso-R53. Campbell, C. A., Paul, E. A., Rennie, D. A. & hlccallum, K. J. 1967 Applicability of the carbondating method of analysis to soil humus studies. Soil Science 104, 217-224. Corny, M. J. & Mitchell, G. F. 1971 The age of Irish Plagen soils. In Paleopedology, (Yaalon, D. H., ed.) International Society for Soil Science, pp. Izg-138.
180
G. J, Benoit,
K. K. Turekian
& L. K. Benninger
Emery, K. 0. 1960 The Sea of Southern California. Wiley. New York, 366 pp. Erlenkeuser, H., Suess, E. & Wilkomm, H. 1975 Industrialization affects heavy metal and carbon isotope concentrations in recent Baltic Sea sediments. Geochimica et Cosmochimica Acta 38,823.-842. Gordon, R. B. & Pilbeam, C. C. 1975 Circulation in central Long Island Sound. journal of Geophysics Research 80, 414-422. Gracanin, 2. 1970 Age and development of the hummocky meadow in the Lechtaler Alps. In Paleopedology, (Yaalon, D. H., ed.) International Society for Soil Science, pp. 129-138. Gross, M. G. & Bumpus, D. F. 1972 Residual drift of near-bottom waters in Long Island Sound, 1969. Limnology and Oceanography 17, 636-638. Hedges, J. L. & Parker, P. L. 1976 Land-derived organic matter in sediments from the Gulf of Mexico. Geochimica et Cosmochimica Acta 40, ro19-rozg. Herrera, R. & Tamers, M. A. 1970 Radiocarbon dating of tropical soil associations in Venezuela. In Paleopedology, (Yaalon, D. H., ed.) International Society for Soil Science, pp. 109-115. Martel, Y. A. & Paul, E. A. 1974 The use of radiocarbon dating or organic matter in the study of soil genesis. Soil Science Society of America Proceedings 38, 501-506. Newman, J. W., Parker, P. L. & Behrens, E. W. 1973 Organic carbon isotope ratios in Quaternary cores from the Gulf of Mexico. Geochimica et Cosmochimica Acta 37, 225-238. Noakes, J. E., Kim, S. M. & Stipp, J. J. 1965 Chemical and counting advances in liquid scintillation age dating, 6th International Conf. on Radiocarbon and Tritium dating Proc., Pullman, WA. 7-11 June, 1965, 68-92. Nozaki, Y. & Turekian, K. K. 1977 Yale University geology and geophysics radiocarbon dates. I. Radiocarbon 19, 133-141. Polach, E. A. & Stipp, J. J. 1967 Improved synthesis technique for methane and benzene radiocarbon dating. InternationalJournal of Applied Radiation Isotopes 18, 359-364. Riley, G. A. 1956 Oceanography of Long Island Sound, 1952-1954: production and utilization of organic matter. Bulletin of the Bingham Oceanogruphic Collection 15, 324.-344. Sackett, W. M. & Thompson, R. R. 1963 Isotopic organic carbon composition of recent continental derived elastic sediments of eastern gulf coast, Gulf of Mexico. Bulletin of the American Association of Petrology and Geology 47, 525-53 1. Sackett, W. M. 1964 The depositional history and isotopic organic carbon composition of marine sediments. Marine Geology 2, 173-185. Scharpenseel, M. W., Petig, F. & Tamers, M. A. 1969 University of Bonn natural radiocarbon measurements II. Radiocarbon II, 3-14. Scharpenseel, M. W. rg7o Radiocarbon dating of soils-problems, troubles, hopes. In Puleopedology (Yaalon, D. H., ed.) International Society for Soil Science, pp. 77-88. Schnitzer, M. & Kahn, S. V. 1972 Humic substances in the environment. Marcel Dekker, Inc. New York, pp. 126-129. Wakeland, M. E., Jr. 1978 Sources of fine-grained sediments in Long Island Sound. Abstracts with Programs. 13th annual meeting Northeast Section of the Geological Society of America Vol. IO, No. 2, p. 90.