210Pb dating

The basic principle for 210Pb dating is that gaseous 222Rn is emitted to the atmosphere from the lithosphere, surface waters and airborne dust and there decays to 210Pb. After formation in the troposphere, 2l0Pb becomes attached to aerosol particles which reside in the atmosphere for only 30 days or less depending on season, latitude, frequency of rainfall, size and altitude of the aerosols, Nevissi et al., [17]1, Schell [26], and Poet et al., [19]. — —... 

Figure 11. Hustedt pH categories (26), diatom-inferred pH values (weighted averaging), calibration radiocarbon dates, 210Pb dates, and pH periods in the history of Lilia Oresjon, southwest Sweden. Each bar and point represents one of the 700 samples analyzed from each 0.5-cm interval from the 350-cm-long core. (Reproduced with permission from reference 68. Copyright 1990 Royal Society of London.)...

Armentano, T.V., and Woodwell, G.M. (1975) Sedimentation rates in a Long Island marsh determined by 210Pb dating. Limnol. Oceanogr. 20, 452-455. 

Chrzastowski, M.J., Kraft, J.C., and Stedman, S.M. (1987) Coastal Delaware sea-level rise based on marsh mud accumulation rates and 210Pb dating. Geol. Soc. Am. (Abstracts and Programs) 9, 8. 

Bollhofer A, Mangini A, Lenhard L, Wessels M, Giovanoli F, Schwarz B (1994) High Resolution 210Pb dating of Lake Constance sediments Stable lead in Lake Constance. Environ Geol 24, 267-274. 

El-Daoushy, M. F. A. F., The determination of 210Pb and 226Ra in lake sediments and dating application, Uppsala, Sweden report UUIP-979 (1976). 

In figure 6 are presented some of the available 210Pb profiles in deep sea sediments. The data available to-date yield values ranging between (20-400) cm2/103 year for particle mixing coefficients, K [69]. The data also show that the mixing coefficients do not exhibit any systematic trend either with the sediment type or with sedimentation rate [69]. 

This model requires an excess of sulfate over reducible carbon. Concentrations may be measured in solutions squeezed from sediment cores, diffusion coefficients are known from standard chemical data tables and sedimentation rates determined from 14C, 210Pb, or 230Th dating. Therefore, this model finds its best use in the recovery of the kinetics of organic matter decay. A discussion of this and similar equations and numerical applications may be found in Berner (1980). 

For Little Rock Lake, a single core from each of its two basins was analyzed and dated in stratigraphic detail. The remaining cores were analyzed for Hg content in three coarse intervals as described, but none of these profiles was actually dated. Instead the sedimentation rates were inferred from a series of five nearby cores that had been dated by 210Pb for other purposes (16). The mean sedimentation rates from dated cores collected at similar depth in the same basin were used to calculate Hg accumulation for each undated profile. 

Dating and Sediment Accumulation. Stratigraphic Patterns Lead-210 profiles from profundal cores from each study lake show conformable declines in unsupported activity to an asymptote of supported 210Pb typically below 40-60 cm deep (Figure 3). Supported activity, attained at shallower depths in Thrush (28 cm) and Kjostad (36 cm), indicates slower linear rates of sedimentation at these sites. The activity profiles for several lakes, most notably Thrush and Kjostad, are almost perfectly exponential and thus indicate nearly constant sediment accumulation rates. Others, such as Cedar and Mountain, show flat spots and kinks that probably represent shifts in sediment flux. 

Figure 7. Plots of Hg concentration versus 210Pb age for deep-water cores. Dates older than about 1800 are extrapolations based on mean dry-mass sediment accumulation rates.

It would be an oversimplification to suggest that a pair of cores could be considered duplicates or that information from a single core could be extrapolated to basin-wide processes. However, carefully chosen core sites can provide a basis for reasonable estimates and hypothesis formation. Both basins of LRL have similar morphometric and edaphic conditions (Figure 1), suggesting similar depositional regimes and histories. Core locations were chosen near sites where extensive pore-water measurements had been made (4, 17, 59) and where sediment cores had been dated by 210Pb. Mean mass accumulation rates calculated over the top 4-5 cm were comparable between basins 105 and 126 g/m2 per year for the treatment and reference basin, respectively (65, 66). 

Methods for dating sediment cores using 210Pb and 137Cs were similar to those described by Robbins and Edgington (34). 137Cs content was de-... 

Figure 3. Sediment core from Little Rock Lake (3 m) depicting dry bulk particle (A) and pore-water (A) HgT concentration. Sediments were dated by 13rCs and 210Pb. Inset Coal use in the United States according to a 1986 report to the National Academy of Sciences.

The specific activity (A) of a radionuclide represents the observed counting rate in a sample. Radionuclides are useful in evaluating processes over time scales that are four to five times their half-lives, where beyond that period there is typically only 1% or less of the nuclide remaining and cannot be measured effectively (Nittrouer et al., 1984). For example, 210Pb has a half-life of 22.3 y and a useful life of approximately 112 y thus, dating sediment or rocks older that about 140 y is beyond the range of the nuclide. 

Much of the work to date in estuaries and adjacent marsh/shelf environments using 210Pb has been to determine sediment accumulation and accretion rates (Armentano and Woodwell, 1975 Krishnaswami et al., 1980 Church et al., 1981 Kuehl et al., 1982 Olsen et al., 1985 Paez-Osuna and Mandelli, 1985 McKee et al., 1986 Lynch et al., 1989 Bricker-Urso, 1989 Moore, 1992 Smoak et al., 1996 Dellapenna et al., 1998, 2001 Benoit, 2001 Corbett et al., 2003). Lead-210 is considered to be a reliable method for dating sediments deposited over the last 100 to 110 y (Krishnaswami et al., 1971). In the absence of bioturbation/mixing the activity gradient of excess 210Pb in sediments, which is the net result of accumulation and radioactive decay, can provide information on the sedimentation rate of recent sediments. Unfortunately, in many... 

Lead-210 is produced by radioactive decay of 222Rn and can enter estuaries as a dissolved/complexed ion or in association with particles from the ocean, river, and atmosphere. Much of the work to date in estuaries and adjacent marsh/shelf environments using 210Pb has been to determine sediment accumulation and accretion rates. 

Benoit, G. (2001) 210Pb and 137 Cs dating methods in lakes a retrospective study. J. Paleolimnol. 25, 455-465. 

In natural lacustrine and slowly-accumulating reservoir sediments, core dating with the isotope °Pb has been used extensively (Schell and Earner, 1986). Appleby and Oldfield (1983) found that the constant rate of °Pb supply model (CRS) provides a reasonably accurate sedimentation chronology. The basic assumption of the CRS model is that the rate of supply of excess °Pb to the lake is constant. This model, thus, assumes that the erosive processes in the catchment are steady and give rise to a constant rate of sediment accumulation (MAR) (Appleby and Oldfield, 1983). In practice, for reservoirs, this assumption is rarely met because, for example, an increase in the MAR caused by land disturbances, such as those associated with the urban development, transports additional surficial soils and sediments to the lake. This additional erosion increases the MAR and also increases the rate of supply of °Pb to the lake. In general, because excess 210pb... 

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