profiles in relation to sedimentation in some Irish lakes ... Over the past three decades, dating of lake sediments .... 0.25. *Catchment of Glendalough Upper only.
Verh. Internat. Verein. Limnol.
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Stuttgart, December 2000
Measurement and interpretation of 210Pb, 137Cs, 134Cs and 241Am profiles in relation to sedimentation in some Irish lakes N. V. Nowlan, P. I. Mitchell, D. A. Murray, L. León Vintró and E. M. Seymour
Introduction Over the past three decades, dating of lake sediments using unsupported 210Pb (210Pbex) activity profiles has become a standard technique for the determination of recent (100–150 years) sedimentary geochronologies. These chronologies have been deduced on the basis of one or other of two fundamentally different assumptions underlying 210Pbex accumulation, i.e. the constant initial concentration (CIC) hypothesis, which assumes that the initial 210Pbex concentration in the deposited sediment is always the same regardless of changes in the sediment accumulation rate, and the constant rate of supply (CRS) hypothesis, which assumes that the supply of 210Pbex to the accreting material is constant in time (APPLEBY & OLFIELD 1992). Since chronologies derived from the same set of 210Pbex data using these hypotheses often show significant differences, the question as to which is the best representation can usually be resolved by the application of an additional, independent dating tool, such as that based on anthropogenic fallout markers. The resolution of this issue can provide valuable information on the key processes driving 210 Pbex accumulation in the lake in question. Of the fallout nuclides, 137Cs, 134Cs and 241Am have most often been used as stratigraphic markers to resolve ambiguous or uncertain 210Pbex chronologies. Under favourable circumstances, the activity profile of each of these nuclides should reflect their deposition histories, providing accurate dates corresponding to specific events. In the case of 137Cs, the onset of nuclear weapons testing in the early 1950s, the main weapons fallout peak in 1963–1964 and the Chernobyl accident in 1986 provide three clear points of reference. In addition, 241Am formed by ingrowth from weapons fallout 241Pu, and 134Cs present in Chernobyl fallout but long since decayed to background in weapons fallout, provide a useful means of distinguishing the weapons fallout peak from the Chernobyl peak (APPLEBY et al. 1991, MITCHELL et al. 1992).
In this paper, we summarise the results of applying the unsupported 210Pb dating method to the determination of sediment chronologies and representative accumulation rates in a selection of Irish lakes. Particular attention is given to reconciling the 210Pbderived chronologies with those derived from the 137 Cs, 134Cs and 241Am profiles. The validity of applying the CRS hypothesis rather than the CIC hypothesis to the data sets involved is also briefly discussed. Accurate dating is a prerequisite for effective limnological studies of environmental change and the current work is aimed at providing a realistic sediment chronology at the various sites examined.
Methodology Sediment cores (diameter: 4.3 cm) were retrieved from ten different lakes in the period 1992–1997 using a piston-type corer the details of which have been described elsewhere (MURRAY 1976). The cores were carefully divided into 1- or 2-cm sections and, following drying and homogenisation, the 210Pb, 214 Pb, 214Bi, 134Cs, 137Cs and 241Am content of each section was determined by low energy photon spectrometry using a semi-planar (n-type) germanium detector fitted with a thin beryllium entrance window. 241Am was readily detectable in most of our samples with this system via its gamma emission at 59.5 keV (MITCHELL et al. 1992). Sediment chronologies were established by applying both the CRS and the CIC hypotheses to the measured 210Pbex data. These chronologies were then compared with those indicated by the 137Cs, 134Cs and 241Am markers referred to above.
Results and discussion A summary of the accumulation rates determined for the various lakes on the basis of the CRS hypothesis is given in Table 1, together with lake surface area, catchment area and estimated 210Pbex supply rates and 137Cs inventories. 0368-0770/00/0027-01 $ 1.00 ©2000 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart
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Table 1. 210Pbex supply rates, 137Cs inventories and sediment accumulation rates for some Irish lakes. Location
Surface area 2
L. Derg L. Ree L. Gill (Core 1) L. Gill (Core 2) L. Ramor L. Conn Glendalough (Lower) L. Bray (Lower) L. Mullagh L. Dan L. Lavey
km 117.5 105 14 14 7.37 50 0.20 0.29 0.34 1.1 0.10
Catchment 2
km 10,500 4540 384 384 248 414 18.7* 1.28 1.26 -
210
137
Pbex –2
Bq m year 365 621 216 241 432 176 57 182 131 148 122
Cs inventory
–1
–2
Bq m 13,030 10,910 9,660 14,790 7770 5640 1190 1310 2360 2490 6090
Accumulation rate g cm–2 year–1 cm year–1 0.15 0.85 0.15 1.30 0.10 0.48 0.15 0.49 0.18 0.74 0.04 0.38 0.03 0.19 0.02 0.31 0.03 0.57 0.03 0.35 0.03 0.25
*Catchment of Glendalough Upper only.
Accumulation rates spanned the range 0.02 to 0.18 g cm–2 year–1 (0.19 to 1.30 cm year–1), though in a number of lakes these rates showed abrupt changes at certain dates. Some of these hiatuses could be linked to disturbances induced by afforestation within the catchment zone, while in others improved drainage appeared to be responsible. We stress that the accumulation rates given in Table 1 are merely indicative, measured as they were at only one or two sampling sites in each case. It is convenient to divide the lakes studied into two groups, namely those showing relatively high accumulation rates and those with low accumulation rates. The first group, which includes Loughs Derg, Ree, Gill and Ramor, are characterised by catchment zones that are between one and two orders of magnitude greater than the corresponding lake surface areas. This is evidenced by the considerably higher 210Pbex supply rates measured for the first group, as well as by the appreciably higher 137Cs inventories. The mean 210Pbex supply rate for the second group, at 150 ± 25 Bq m–2 year–1, is consistent with the value of 140 Bq m–2 year–1 suggested by APPLEBY & OLFIELD (1992) as the mean 210Pbex atmospheric flux over northern hemisphere land masses and confirms that, for these lakes, local catchment effects are relatively
unimportant. Interestingly, studies of 210Pbex fluxes derived from deposition densities in Irish peat bogs yielded values in the range 57–107 Bq m–2 year–1 (MCGARRY 1991, MITCHELL et al. 1992). The low supply rate for Glendalough (Lower) is probably explained by the fact that much of the 210Pbex input is trapped in the upper lake, which drains into the lower lake studied here. A similar observation applies to 137Cs. We are unclear as to the reason for the high 137Cs deposition density recorded in Lough Lavey (Co. Cavan). Comparison of the chronologies determined on the basis of the CRS and CIC hypotheses with those derived from the 137Cs, 134Cs and 241 Am markers showed that, in those cases where the two 210Pbex chronologies differed, the CRS hypothesis gave dates which were in better agreement with those deduced using our anthropogenic markers. In general, CIC dates tended to be older than CRS dates. This observation is consistent with the large body of data now available which suggests that at many sites worldwide the dominant processes governing the supply of 210Pbex to lake sediments are those underlying the CRS hypothesis (APPLEBY & OLFIELD 1992). We should add that in most of the cores examined there was evidence of limited vertical mobility of 137Cs and 241Am; specifi-
N. V. Nowlan et al., Dating of Irish lake sediments using unsupported 210Pb profiles 210
Pbex (Bq kg-1)
100 1996
137
241
Cs (Bq m-2)
200
500
600
1000
3
Am (Bq m-2) 6
Accumulation rate (g cm-2 y-1) 12
9
3
0.01
0.02
0.03
0.04
0.05
0.06
0
(a) Lough Conn
1976
1965
4
D e p th (cm )
Y e a r (C R S )
1987
8
12
1954
16
1943
20
210
Pbex (Bq kg-1)
10 1993
100
137
500
200
Cs (Bq m-2)
400
600
Accumulation rate (g cm-2 y-1)
Organic content (%) 800
10
20
30
40
0.05
0.10
0.15
0.20 0.25
0.30
0
1989
4
1985
8
1980
1975
D e p th (cm )
Y e a r (C R S )
(b) Lough Derg
12
16
1970
20
1965
24
1960
28
210
Pbex (Bq kg-1)
10 1998
100
137
Cs (Bq m-2)
500
300
600
900
1200 1500
134
Cs and
4
8
241
Am (Bq m-2) 12
16
Accumulation rate (g cm-2 y-1) 0.04
0.08
0.12
0.16
0.20
0
(c) Lough Gill 134
Cs
4
1983
1975
D e p th (cm )
Y e a r (C R S )
1991
8
12
1967
16
1956
20
241
Am
Fig. 1. Vertical chronologies (CRS hypothesis), radionuclide profiles and sediment accumulation rates for: (a) Lough Conn, (b) Lough Derg and (c) Lough Gill (Core 1). The error bars represent uncertainties of ±1σ.
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cally, small traces of both nuclides were detected in sediments which had been deposited prior to the onset of weapons fallout. We attribute this to a combination of physical and biological turbation of newly accumulated sediment and diffusion of labile species in interstitial pore waters. Further, and as expected, there was clear evidence of a time lag of several years or more in the delivery of fallout nuclides to most of the lake beds studied, particularly those with larger catchments. To illustrate the nature and extent of the database summarised in Table 1, three examples are shown in Fig. 1. In the case of Lough Conn, the accumulation rate has been low and relatively constant for over 40 years. Further, the weapons fallout 137Cs peak is unresolved from the Chernobyl peak, a feature typical of a low sedimentation regime. Nevertheless, the onset of the weapons fallout peak is clearly evident in both the 137Cs and 241Am profiles at a depth of about 12 cm (which corresponds to the year 1965). Similar profiles were found for Lough Dan and Glendalough (Lower). In the case of Lough Derg, the accumulation rate is considerably higher and the two caesium peaks are well resolved, though somewhat broadened. There is a clear hiatus ca. 1986–1988 in the measured 210Pbex profile, and this is reflected in abrupt changes in both the accumulation rate and the organic content of the sediment. Similar discontinuities were observed for Loughs Bray (Lower), Lavey and Gill (Core 2). In the case of Lough Gill (Core 1), also characterised by a higher accumulation rate, the 210 Pbex chronology is supported by the 137Cs chronology, as evidenced by two well-resolved fallout peaks. That the more recent (or uppermost) is of Chernobyl origin is incontrovertible, given the presence of a synchronous 134Cs peak. Similarly, the earlier peak is mirrored almost exactly by a peak in the 241Am profile, confirm-
ing that it is of weapons fallout origin.
Conclusions Accumulation rates in the ten lakes examined were found to lie in the range 0.02–0.18 g cm–2 year–1. In some lakes significant changes in the sedimentation regime were detected which could be linked to hiatuses associated with recent afforestation and/or improved drainage. In most cases the chronologies established by 210Pb dating were confirmed by 137Cs, 134 Cs and 241Am dating, provided hold-up within the catchment zone was taken into account. In virtually all cases, the chronology established with the CRS (as opposed to the CIC) hypothesis agreed best with that determined using the anthropogenic tracers, 137 Cs and 241Am. Some limited vertical mobility of 137 Cs and 241Am was evident in most cores, though this does not appear to have had a significant impact on the positions of the Chernobyl and weapons fallout peaks.
References APPLEBY, P. G. & OLFIELD, F., 1992: Application of lead-210 to sedimentation studies. – In: IVANOVICH, M. & HARMON, R. S. (eds): Uranium-Series Disequilibrium: Application to Earth, Marine and Environmental Sciences: 731–778. – Clarendon Press, Oxford. APPLEBY, P. G., RICHARDSON, N. & NOLAN, P. J., 1991: 241Am dating of lake sediments. – Radiobiologia 214: 35–42. MCGARRY, A. T., 1991: Nuclear Fallout and Heavy Metal Deposition in Ombrogeneous Peats in Ireland. – Ph.D. dissertation, National University of Ireland, Dublin. MURRAY, D. A., 1976: A light weight corer for sampling soft subaqueous deposits. – Limnol. Oceanogr. 21(2): 341–344. MITCHELL, P. I., SCHELL, W. R., MCGARRY, A., RYAN, T. P., SANCHEZ-CABEZA, J. A. & VIDAL-QUADRAS, A., 1992: Studies of the vertical distribution of Cs-134, Cs-137, Pu-238, Pu-239,240, Am-241 and Pb-210 in ombrogeneous mires at mid-latitudes. – J. Radioanal. Nucl. Chem. (Articles) 156(2): 361–387.
Authors’ addresses: N. V. NOWLAN, P. I. MITCHELL, L. LEÓN VINTRÓ, E. M. SEYMOUR, Department of Experimental Physics, University College Dublin, Belfield, Dublin 4, Ireland. D. A. MURRAY, Department of Zoology, University College Dublin, Belfield, Dublin 4, Ireland.
