Alternative 210Pb dating: results from the New Guinea Highlands and Lough Erne

Abstract

RECENT lake sediments can be dated using 210Pb and fall-out 137Cs. Pennington et al.1, and Robbins and Edgington2 have set out the assumptions used in calculating dates and estimating accumulation rates from the declining concentration of unsupported 210Pb in the near-surface sediments of Blelham Tarn and Lake Michigan respectively. In both cases, as in other papers3,4, an essential assumption is a constant initial concentration (c.i.c.) of unsupported 210Pb per unit dry weight in the sediment at each depth, whether or not any variations may have occurred in the rate of accumulation. This assumption requires that in undisturbed cores, unsupported 210Pb concentrations should always decline monotonically with depth. Figure 1 shows unsupported 210Pb concentrations in cores from Lough Erne, Northern Ireland and Lake Ipea, Papua New Guinea. The profiles are ‘kinked’ and show at one or more points, a marked increase in unsupported 210Pb concentration with depth. The levels at which this occurs in the cores range from 6 to 30cm. The increases cannot, therefore, be the result of the anomalously low surface concentrations noted elsewhere4. Associated biological, chemical and geophysical studies show that the profiles have not been significantly disturbed by physical or biological mixing. These profiles are not consistent with the c.i.c. deposition model and are regarded as evidence for the dilution of unsupported 210Pb by accelerated sediment accumulation. If such dilution has taken place without leading to a kink in 210Pb concentrations, the assumption of c.i.c. will lead to underestimation of the true age of the sediment below the onset of acceleration. In the case of the ‘kinked’ profiles, dates are not calculable using the c.i.c. deposition model alone. We have adopted an alternative approach to calculating 210Pb dates using as our main assumption a constant rate of supply (c.r.s.) of unsupported 210Pb to the sediment per unit time and deriving dates from the integrated activity of the radionuclide. Previous authors have referred to the possibility of calculating dates using this assumption5,6 but have not given a full account of the method or evaluated the results of its application. Details of the methods used here are set out elsewhere7. This paper compares and briefly evaluates the two alternative models as applied to the sediments of Lough Erne and Lakes Ipea and Egari. The c.r.s. based dates obtained have been compared with those derived either from c.i.c. based 210Pb dates or, in the case of the ‘kinked’ profiles, from a combination of 137Cs dating8 and c.i.c. based calculations. Figure 2 plots the resulting age against depth curves from Lough Erne, Fig. 3, some of the results from Lakes Ipea and Egari.