[en] As sediments can accumulate unsupported lead-210 by different processes, there are uncertainties concerning the appropriateness of procedures for calculating lead-210 dates. The applicability of CRS (constant rate of Pb-210 supply) models and CIC (constant initial Pb-210 concentration) models is considered. Variants of both models can be devised to take account of postdepositional processes such as sediment mixing and redistribution. It is argued that the CRS model should be the primary dating model at a wide range of sites and that empirical testing should be built into dating strategies

[en] In this chapter the processes giving rise to the ²¹⁰Pb/²²⁶Ra disequilibrium are examined and their application to studying sedimentation time-scaled, particularly lacustrine, marine sediments and rain-fed peat bogs is discussed. (UK)

[en] In radiometric assay by gamma spectrometry using well-type detectors, sample sizes are in many cases insufficient to fill the sample holder to the full height at which the detector was calibrated. There is thus a need to determine the detection efficiency of the detector as a function of sample height. This paper considers a simple mathematical model for estimating this relation using a fairly small number of basic parameters characterising the detector. The model is tested empirically using a thin disk of UO₃ placed at different heights in the sample holder. The empirical results show that the efficiency for such a disk placed at the top of a well is just 36% of the efficiency when placed at the bottom. The variations in relative efficiency with energy (over the range 63-1001 keV) are fairly small for disks placed near the bottom or top of the well, but significantly larger for disks at mid height. Although the modelled results broadly follow the same pattern as the empirical results, there is a discrepancy that appears to be due to uncertainties in the precise location of the crystal relative to the end cap, and also an effective dead layer at the front surface of the crystal possibly caused by forward scattering of higher energy photons emitted at a shallow angle to the front face of the detector following partial interactions. A corrected model that takes these into account is in excellent agreement with the empirical results. Calculations for different sized samples show that the nominal detector efficiency (excluding self-absorption) can vary with sample height by as much as 25%

[en] This communication compares and evaluates the results of two alternative dating models as applied to recent lake sediments, and details are given of application of the methods to sediments of Lough Erne in Northern Ireland and lakes of the New Guinea highlands. The first method uses ²¹⁰Pb data and the second method employs fall-out data. (U.K.)

[en] ²⁴¹Am and ¹³⁷Cs activities in surface and core samples of saltmarsh sediment from parts of the Galloway and south Cumbria coastlines are reported. The study concentrates on areas of saltmarsh which have been colonised by Spartina anglica Hubbard., in view of its ability to trap fine grained sediments and to colonise previously bare mudflats beyond the outer edge of pre-existing marsh. At each site downprofile variations of ²⁴¹Am and ¹³⁷Cs are presented, and most appear to show a more or less complete record of the history of discharges from Sellafield. The total inventories of the radionuclides are calculated for core samples and compared with published results from previous studies of intertidal sediments in the immediate vicinity. The relationship between particle size and radionuclide activity and the use of magnetic measurements as a relatively simple way of normalizing for this effect are examined. (Author)

[en] The residence time of "2"1"0Pb created in the atmosphere by the decay of gaseous "2"2"2Rn is a key parameter controlling its distribution and fallout onto the landscape. These in turn are key parameters governing the use of this natural radionuclide for dating and interpreting environmental records stored in natural archives such as lake sediments. One of the principal methods for estimating the atmospheric residence time is through measurements of the activities of the daughter radionuclides "2"1"0Bi and "2"1"0Po, and in particular the "2"1"0Bi/"2"1"0Pb and "2"1"0Po/"2"1"0Pb activity ratios. Calculations used in early empirical studies assumed that these were governed by a simple series of equilibrium equations. This approach does however have two failings; it takes no account of the effect of global circulation on spatial variations in the activity ratios, and no allowance is made for the impact of transport processes across the tropopause. This paper presents a simple model for calculating the distributions of "2"1"0Pb, "2"1"0Bi and "2"1"0Po at northern mid-latitudes (30°–65°N), a region containing almost all the available empirical data. By comparing modelled "2"1"0Bi/"2"1"0Pb activity ratios with empirical data a best estimate for the tropospheric residence time of around 10 days is obtained. This is significantly longer than earlier estimates of between 4 and 7 days. The process whereby "2"1"0Pb is transported into the stratosphere when tropospheric concentrations are high and returned from it when they are low, significantly increases the effective residence time in the atmosphere as a whole. The effect of this is to significantly enhance the long range transport of "2"1"0Pb from its source locations. The impact is illustrated by calculations showing the distribution of "2"1"0Pb fallout versus longitude at northern mid-latitudes. - Highlights: • We model the atmospheric distribution of "2"1"0Pb and its daughter radionuclides. • We estimate the "2"1"0Pb tropospheric residence time from the modelled activity ratios. • The results suggest a mean tropospheric residence time for "2"1"0Pb of around 10 days. • Reservoir effect of the stratosphere significantly extends the actual residence time.

[en] Magnetic susceptibility, ²¹⁰Pb, ¹³⁷Cs, ¹⁴C, and paleomagnetic secular variation are used to establish the chronology of sedimentation in a set of cores from Loch Frisa in western Scotland. The ¹⁴C dates obtained are not compatible with the chronology derived from all the other techniques and this is ascribed to inwash of old particulate carbon from the watershed. Central cores show little evidence of changes in sedimentation rate over the last 150 years whereas marginal cores contain evidence of major increases arising from ploughing and from drainage associated with catchment afforestation since 1935. The results illustrate the value of a multiple core approach to sedimentation and erosion studies even where complex lake morphometry precludes calculation of sediment budgets

No abstract available

[en] Diatoms are sensitive indicators of various limnological conditions, and the distribution of many taxa is strongly related to lakewater pH or factors correlated with pH. Indices and predictive equations based on this relationship allow the quantitative reconstruction of past lakewater pH from diatom remains in lake sediments. The magnitude, timing, and causes of recent lake acidification in both moorland and afforested catchments in the Welsh uplands were evaluated. Diatom-based reconstruction techniques and Pb-210 dating were used to evaluate evidence for recent acidification of lakes and carbonaceous particles, magnetic and geochemical analyses were used to trace the depositional history of atmospheric pollutants at each site. Documentary records of catchment history and pollen and geochemical analyses of the lake sediments were used to evaluate whether any observed acidification can be correlated with changes in grazing pressure or burning in moorland catchments or with ploughing and forest growth in afforested lake catchments. analyses have been completed from one afforested and six non-afforested lakes in mid and north Wales. Case studies are given of a moorland catchment and an afforested catchment. Results and similar paleoecological analyses from other sites in north and mid Wales suggest extensive acidification of upland lakes has occurred since 1850. The onset of acidification has varied between sites from as early as 1850 to the mid-1940's. The pattern of acidification also varies greatly, with a rapid decline in pH at some sites and a more gradual decline at others. The magnitude of pH decline ranges from 0.5 to 1.5 units

[en] Accurate dating by ²¹⁰Pb is of crucial importance to a wide range of programs studying environmental records stored in natural archives such as lake sediments or peat bog accumulations. There are two simple models, commonly referred to as the CRS and CIC models. Of these, the CRS (constant rate of ²¹⁰Pb supply) model is perhaps the most widely used. The main principles of this model are exemplified by cores from three Finnish lakes with annually laminated sediments, all of which contained layers recording dilution of the atmospheric ²¹⁰Pb flux by increased sedimentation. ²¹⁰Pb dates calculated using the CRS model were in good agreements with those determined by laminae counting. There are however circumstances where the CIC (constant initial concentration) model is appropriate, e.g. in a core from Devoke Water, where the CRS model was invalidated by an abrupt discontinuity in the sediment record. In a very real sense these models should in the first instance be regarded as tools whose purpose is to determine, as far as practicable, the processes that have generated the data contained in the sediment record. In each case the ²¹⁰Pb data must be individually assessed in light of any independent chrono-stratigraphic evidence such as that provided by ¹³⁷Cs or ²⁴¹Am. The object of this presentation is to highlight the conceptual framework that forms the basis of this assessment and to show how it can be used to solve problems that have arisen in a number of practical cases. These include the use of hybrid models where there has been a variable 2¹⁰Pb supply, corrections for inaccuracies in the calculation of radiometric inventories, and the impact of large variations in ²²⁶Ra activity