²¹⁰Pb dating of annually laminated lake sediments from Finland

Abstract

USE of ²¹⁰Pb dating is increasing rapidly and applications include studies of accelerated eutrophication in major lakes¹, salt-marsh accretion², the recent history of heavy metal pollution³ and accelerating soil erosion resulting from subsistence agriculture⁴. As dating models have increased in variety and complexity, it is important to compare models against precise and unambiguous independently derived time scales. In each area of application of ²¹⁰Pb dating, the inferences drawn from the calculated age–depth curves and the estimates of changing flux rates are often highly dependent on the ²¹⁰Pb dating model used. In this report ²¹⁰Pb-derived estimates of lake sediment age and dry-mass sedimentation rates are compared with ages and rates calculated directly by counting annual laminations. The results support a model of ²¹⁰Pb dating which assumes a constant net rate of supply (c.p.s.) of unsupported ²¹⁰Pb to the sediment despite fluctuations in dry mass sedimentation rates. Our findings underline the need for empirical evaluation of alternative ²¹⁰Pb dating models in the widest possible range of contexts. They also cast doubt on some published studies in which strongly ‘kinked’ profiles of unsupported ²¹⁰Pb concentration have been interpreted within the framework of conventional constant initial concentration (c.i.c.) assumptions.