P. G. Appleby

The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment

Summary The paper describes a method of calculating sediment age from 210Pb profiles where changes in accumulation rate have affected initial 210Pb concentration. Previously published age/depth profiles derived from 210Pb measurements have assumed a constant initial concentration of unsupported 210Pb per unit dry weight at each stage of accumulation, despite variations in accumulation rates. In sediments from three sites which have experienced rapid acceleration in accumulation in recent times, the assumption is not compatible with the measured unsupported 210Pb concentrations. the alternative assumption of a constant rate of supply of unsupported 210Pb to the sediments per unit time allows calculation of age throughout the profiles irrespective of evidence for rapidly accelerating accumulation. The methods of calculation used are outlined, illustrated and discussed in the light of evidence obtained from the sediments of Lough Erne and Lough Neagh, N. Ireland and lakes Ipea and Egari in the Highlands of Papua New Guinea. They provide a means of dating variations in accumulation rate resulting from human activity in the drainage basin.

The assessment of 210Pb data from sites with varying sediment accumulation rates

The last few years have seen a dramatic growth in the use of 210Pb sediment dating. Despite this, considerable doubt still surrounds the nature of the processes by which 210Pb is deposited in lake sediments, and this has lead to a situation where there is a choice of dating models offering different interpretations of 210Pb data. In assessing 210Pb data it is therefore essential to first of all determine whether data is consistent with the assumptions of the dating model, and to then compare the 210Pb chronology with independent dating evidence. We have tested 210Pb data from a wide variety of sites, and our calculations indicate that the crs (constant rate of 210Pb supply) model provides a reasonably accurate chronology when the total 210Pb contents of cores from neighbouring locations are comparable.

210Pb dating by low background gamma counting

Lead-210 and radium-226 measurements by direct gamma assay can now provide a record of changing concentrations in lake sediments sufficiently reliable and precise to form a suitable basis for age/depth and dry-sedimentation-rate calculations. There are additional benefits in terms of non destructive sample preparation and simultaneous assay for other environmentally significant gamma-emitting radioisotopes (e.g. 137Cs and 241Am). Results from L. Fleet, S. W. Scotland illustrate the value of this approach especially in lakes with disturbed catchments where variable input of supported 210Pb has occurred.

Three decades of dating recent sediments by fallout radionuclides : a review

This paper reviews the 210Pb dating programme carried out at the University of Liverpool over a period of three decades since its inception by Frank Oldfield in the mid-1970s. The work at Liverpool has included studies of the basic processes controlling the supply of 210Pb to the various natural archives, as well as the development of a detailed and systematic methodology for dating the environmental records in these archives. The assumption that 210Pb fallout at any given location is constant when measured on timescales of a year or more has been tested using data on 210Pb concentrations in UK rainwater spanning more than 40 years, and a simple atmospheric model developed to account for observed spatial variations in the flux over large land masses. The basic assumptions of the CRS dating model have been tested in detail using mass balance studies of fallout radionuclides in catchment/lake systems, and in general using data from a large number of separate studies. The Liverpool data base is used to illustrate the wide ranging applicability of the 210Pb dating method. Results are shown from lakes varying in size from the very small to the very large, and with sedimentation rates ranging from the very fast to the very slow. The particular difficulties of dating sediment records from Desert Lakes and Polar Regions with very low atmospheric fluxes are discussed, with unexpected results in some cases.

Two thousand years of atmospheric arsenic, antimony, and lead deposition recorded in an ombrotrophic peat bog profile, Jura Mountains, Switzerland

A peat core from a Swiss bog reveals significant enrichments of As, Sb and Pb extending back to Roman times, indicating that the anthropogenic fluxes of these metals have exceeded the natural fluxes for more than 2000 years. The isotopic composition of Pb provides no evidence of vertical downward Pb migration, suggesting that the bog has faithfully preserved the historical record of atmospheric Pb deposition. Age dating using210Pb (verified independently using pollen markers) provides the chronology of changing metal concentrations during the past 150 years. Present day enrichment factors (relative to the metal/Sc ratios of typical crustal rocks) are of the order of 20 times (As), 70 (Sb), and 130 (Pb). Given the potential toxicity of As and Sb, these new findings suggest that the environmental significance of these, and perhaps other less common trace elements, deserve more attention.

Self-absorption corrections for well-type germanium detectors

Corrections for self-absorption are of vital importance to accurate determination by gamma spectroscopy of radionuclides such as 210Pb, 241Am and 234Th which emit low energy gamma radiation. A simple theoretical model for determining the necessary corrections for well-type germanium detectors is presented. In this model, self-absorption factors are expressed in terms of the mass attenuation coefficient of the sample and a parameter characterising the well geometry. Experimental measurements of self-absorption are used to evaluate the model and to determine a semi-empirical algorithm for improved estimates of the geometrical parameter.

Comparing palaeolimnological and instrumental evidence of climate change for remote mountain lakes over the last 200 years

This paper compares the palaeolimnological evidence for climate change over the last 200 years with instrumental climate data for the same period at seven European remote mountain lakes. The sites are Øvre Neådalsvatn (Norway), Saanajärvi (Finland), Gossenköllesee (Austria), Hagelseewli (Switzerland), Jezero v Ledvici (Slovenia), Estany Redó (Spain, Pyrenees), and Nižné Terianske Pleso (Slovakia). We used multiple regression analysis to transfer homogenised lowland air temperature records to each of the sites, and these reconstructions were validated using data from on-site automatic weather stations. These data showed that mean annual temperature has varied over the last 200 years at each site by between 1 and 2 °C, typical of the high frequency variability found throughout the Holocene, and appropriate, therefore, to test the sensitivity of the various proxy methods used. Sediment cores from each site were radiometrically dated using 210Pb, 137Cs and 241Am and analysed for loss-on-ignition, C, N, S, pigments, diatoms, chrysophytes, Cladocera and chironomids. Comparisons between the proxy data and the instrumental data were based on linear regression analysis with the proxy data treated as response variables and the instrumental data (after smoothing using LOESS regressions) as predictor variables. The results showed few clear or consistent patterns with generally low or very low r2 values. Highest values were found when the data were compared after smoothing using a broad span, indicating that some of the proxy data were capturing climate variability but only at a relatively coarse time resolution. Probable reasons for the weak performance of the methods used include inaccurate dating, especially for earlier time periods, the influence of confounding forcing factors at some sites e.g., air pollution, earthquakes, and the insensitivity of some methods to low amplitude climate forcing. Nevertheless, there were trends in some proxy records at a number of sites that had a relatively unambiguous correspondence with the instrumental climate records. These included organic matter and associated variables (C and N) and planktonic diatom assemblages at the majority of sites and chrysophytes and chironomids at a few sites. Overall for longer term studies of the Holocene, these results indicate the need to be cautious in the interpretation of proxy records, the importance of proxy method validation, the continuing need to use reinforcing multi-proxy approaches, and the need for careful site and method selection.

DATING OF RECENT LAKE-SEDIMENTS IN THE UNITED-KINGDOM AND IRELAND USING SPHEROIDAL CARBONACEOUS PARTICLE (SCP) CONCENTRATION PROFILES

Spheroidal carbonaceous particles (SCPs) produced from fossil-fuel combustion are found in lake sediments and provide a historical record of atmospheric pollutant deposition. The sediment record of these particles is remarkably consistent and the main features are now seen throughout Europe and beyond. Once the SCP record from a region has been established using accurately dated cores, SCP profiles can be used to determine dates at other sites in the region. This technique was developed in Sweden but until recently has had limited use in the UK and Ireland as the most useful dating feature, the subsurface peak, was not present in all cores. In recent cores this can now almost always be observed. Dating features for UK and Irish cores are found to differ slightly between regions. The potential exists for extra dates to be allocated to a profile by using characterisation of SCP surfaces to determine changes in fuel-type use. Dates could then be allocated to these changes using documentary sources to derive combustion histories. In future years, SCP dating will become increasingly valuable especially for mid- to late-nineteenth-century sediments as continual decay renders210 Pb progressively less useful for this time interval.

Were the Larsemann Hills ice-free through the Last Glacial Maximum?

Lake sediments in the Larsemann Hills contain a great diversity of biological and physical markers from which past environments can be inferred. In order to determine the timing of environmental changes it is essential to have accurate dating of sediments. We used radiometric (210Pb and 137Cs), radiocarbon (AMS 14C) and uranium series (238U) methods to date cores from eleven lakes. These were sampled on coastal to inland transects across the two main peninsulas, Broknes and Stornes, together with a single sample from the Bolingen Islands. Radiometric dating of recent sediments yielded 210Pb levels below acceptable detection limits. However, a relatively well-defined peak in 137Cs gave a date marker which corresponds to the fallout maximum from the atmospheric testing of atomic weapons in 1964/65. Radiocarbon (AMS 14C) measurements showed stratigraphical consistency in the age-depth sequences and undisturbed laminae in some cores provides evidence that the sediments have remained undisturbed by glacial action. In addition, freshwater surface sediments were found to be in near-equilibrium with modern 14CO2 and not influenced by radiocarbon contamination processes. This dating program, together with geomorphological records of ice flow directions and glacial sediments, indicates that parts of Broknes were ice-free throughout the Last Glacial Maximum and that some lakes have existed continuously since at least 44 ka bp. Attempts to date sediments older than 44 ka bp using 128U dating were inconclusive. However, supporting evidence for Broknes being ice-free is provided by an Optically Stimulated Luminescence date from a glaciofluvial deposit. In contrast, Stornes only became ice-free in the mid to late Holocene. This contrasting glacial history results from the Dålk Glacier which diverts ice around Broknes. Lakes on Broknes and some offshore islands therefore contain the oldest known lacustrine sediment records from eastern Antarctica, with the area providing an ice-free oasis and refuge for plants and animals throughout the Last Glacial Maximum. These sediments are therefore well placed to unravel a unique limnological sequence of environmental and climate changes in East Antarctica from the late Pleistocene to the present. This information may help better constrain models of current climate changes and ensure the adequate protection of these lakes and their catchments from the impacts of recent human occupation.

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

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.