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 papers^3,4, 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 assumption^5,6 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.