N. Richardson

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.

Radiocarbon dating of a recent high latitude peat profile: Stor Åmyrân, northern Sweden

The value of both high-precision (HP) and AMS 14C determinations in the fine-resolution dating of recent peat at Stor Åmyrân, near Umeå, northern Sweden has been explored. A detailed chronology of accumu lation, based on 14C, an occurrence of the Askja AD 1875 tephra and 241Am measurements has been derived for the last 700 years. This shows significant changes in the mass balance of the peat profile, with minimum rates of net carbon sequestration overlapping with the period of minimum spring and summer temperature documented by published tree-ring evidence. The paper presents a methodology for calculating empirically past changes in the rate of net carbon sequestration in boreal peatlands and for exploring the effects of climatic variations (and possible future global warming) on these rates.

Radiometric dating (210Pb, 137Cs, 241Am) of recent ombrotrophic peat accumulation and evidence for changes in mass balance

The paper summarizes and evaluates the results of 210Pb, 137Cs and 241Am analysis on 37 recent ombrotrophic peat profiles. From these and previously published data, it is concluded that radiometric measurements alone cannot be relied on to give an accurate chronology of peat accumulation. Where constrained by profiles of 241Am activity in the upper part and one or more pollen-dated horizons below this, 210Pb can provide good chronologies of peat accumulation provided the constraint does not displace the calculated 210Pb dates by more than c. 10%. Profiles of 137Cs activity, in most cases, fail to contribute significantly to dating recent peat. At somewhat degraded sites, the residual dry mass of peat representing the last c. 150 years is reduced. Further analysis of well-dated profiles suggests that this may reflect loss of catolelm peat newly exposed to aerobic decomposition processes after a fall in water table. Several mechanisms are proposed for the displacement and net loss of lead (including 210Pb) from peat profiles.

210Pb dating of lake sediments and ombrotrophic peats by gamma essay

210Pb levels in lake sediments and ombrotrophic peats have been measured by gamma assay, using low background well-type and planar Germanium detectors. The associated 226Ra concentrations have been measured by gamma emissions from 214Pb at 352 keV. A comparison of 137Cs emissions at 32 keV and 662 keV, and 210Pb and 214Pb emissions for samples in radioactive equilibrium, indicates that self-absorption in lake sediment samples does appear to be significant at lower energy levels. Attenuation factors to correct for this have been determined. Results from a number of sites are presented. The analysis of peat samples has been facilitated by ashing material at 450°C prior to measurement.

Inventories and fluxes of 210Pb, 137Cs and 241Am determined from the soils of three small catchments in Cumbria, UK

Abstract Inventories and vertical profiles of fall-out 210 Pb, 137 Cs (both weapons test and Chernobyl) and 241 Am (a decay product of 241 Pu) have been measured in the soils of three catchments in Cumbria, UK. Soil types varied from mineral soils to highly organic peat bog. The 210 Pb inventories in different cores from the same catchment were relatively uniform, the standard deviation of measurements being around 30%. Mean annual fluxes of 210 Pb calculated from the soil inventories were 63 Bq m −2 year −1 (Brotherswater catchment), 63 Bq m −2 year −1 (Blelham Tarn catchment), and 97 Bq m −2 year −1 (Devoke Water catchment) per metre of rainfall. From these and earlier data published in the literature, the mean UK 210 Pb flux is estimated to be 77 ± 14 Bq m −2 year −1 per metre of rainfall. Inventories of 137 Cs (weapons test and Chernobyl) were more variable, but mean values for each of the catchments were in agreement with independent studies. The mean weapons test deposition (decay corrected to 1986) was 2790 Bq m −2 per metre of rainfall, compared to a UK average of 3160 Bq m −2 derived from extensive national surveys. Although 241 Am levels in Brotherswater soils were comparable with those expected from weapons test fall-out, significantly higher values were recorded at Blelham Tarn and Devoke Water. Well-resolved peaks in 241 Am activity some distance below the present surface suggest that this radionuclide is relatively immobile within the soil column. The depth of penetration of 75% of the inventory into the soil column was similar for 210 Pb, 137 Cs (weapons) and 241 Am, all being significantly greater than Chernobyl 137 Cs. The vertical profiles of 137 Cs (weapons) suggest that there has been little mobility during the 15 years since a previous study (Cawse 1983).

241Am and 137Cs activity in fine grained saltmarsh sediments from parts of the N.E. Irish Sea Shoreline

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.

The use of ultra-sound in the preparation of carbonate and clay sediments for chironomid analysis

Initial investigations of Holocene carbonate sediment from Hawes Water, Northwest England, yielded lower numbers of chironomid head capsules than anticipated. Standard techniques used to prepare sediment for chironomid analysis were ineffective in breaking up the coarse crystalline sediment structure sufficiently. This led to large amounts of sediment being retained and increased sample processing times. The low yield of head capsules also meant that more sediment was needed to produce adequate numbers of head capsules for analysis. The use of ultra-sound as part of the sediment processing was investigated. This technique reduced the amount of sediment left for sorting and yielded significantly more head capsules which were of equivalent structural condition and cleaner than those produced by conventional methods. The technique was extended to clay samples where similar results were obtained although shorter treatment times are recommended. The proportion of Tanytarsini and Tanypodinae heads increased significantly in carbonate and clay samples, respectively; both sediment types showed a significant decline in the proportion of Chironomini. The results indicate that ultra-sonic preparation of samples will yield a more accurate representation of chironomid assemblages in sediments leading to greater sensitivity and reliability in analysing past environmental conditions.

Early-Holocene temperature variability inferred from chironomid assemblages at Hawes Water, northwest England

This paper presents the first high-resolution chironomid-inferred mean July air temperature (C-IT) reconstruction for the early Holocene from England. The reconstruction is based on a core recovered from a terrestrialised carbonate bench at Hawes Water, a small hard-water lake in northwest England. The record shows that temperatures rose rapidly after the Younger Dryas with temperatures reaching 14.0°C at the beginning of the Holocene. Over millennial timescales, the temperature record points to a slight rise in temperatures towards the top of the sequence at around 8000 years before the year 2000 (b2k). The trend is punctuated by a series of cool oscillations (at 11 200, 11 400, 10 700, 10 400, 9300 and 8300 b2k) and by a short period (11 300 to 10 250 b2k) when temperatures were considerably warmer (up to 14.9°C). The five cool oscillations coincide with temperature reversals found elsewhere in the North Atlantic region and in the Greenland ice core records. These cool events correlate well with both meltwater fluxes from the Laurentide and Scandinavian ice sheets and periods of low solar activity. Two of these oscillations (at 9300 and 8300 b2k) vary significantly from the early-Holocene mean. C-IT shift rapidly during both these events and temperatures fall ~1.6°C below the early-Holocene mean trend for ~ 50—60 years. The results presented here provide an insight into the instability of the early-Holocene climate in the British Isles and demonstrate the sensitivity of chironomids to rapid climatic events during the early Holocene.

The mineral magnetic record in recent ombrotrophic peat synchronised by fine resolution pollen analysis

Mineral magnetic measurements of recent ombrotrophic peat have been used to reconstruct particulate pollution history. This requires that the magnetic record is not seriously distorted by post-depositional dissolution, authigenic growth, diagenetic change, or downwash of the magnetic minerals. Fine-resolution pollen analysis supports the view that at each site magnetic changes between profiles are synchronous. It thus strengthens the chronological and palaeoenvironmental value of the magnetic record.

The magnetic record of inorganic fly ash deposition in lake sediments and ombrotrophic peats

Interest in identifying a geological marker signifying the starting point for the Anthropocene has prompted an exploration of the stratigraphic record of inorganic particulates generated by industrial activities. Magnetic measurements of recent lake sediments and ombrotrophic peats are here used to reconstruct the history of deposition of inorganic fly-ash spheres resulting mainly from solid fuel combustion and metal smelting. The chronologies used have been based on moss-increment counting, radioisotope dating and pollen analysis. The sites come from the United Kingdom, Scandinavia and North America. In several cases where detailed chronologies of both fly-ash deposition and local industry can be compared, the sequence of concentration-linked magnetic measurements appears to capture accurately the record of industrial development despite incontrovertible evidence from other peat-based records for some selective dissolution of magnetic minerals. The dates at which magnetic concentration increases begin range from the 16th century in the peat profiles around the head of Morecambe Bay, South Cumbria, in North-West England where early iron manufacture using charcoal-fuelled bloomery hearths is well documented, to the mid-20th century at the remotest sites in Arctic Scandinavia. The lake sediment profiles used here come mainly from the United Kingdom and, in most cases, they date increases to the late 19th century or the first decades of the 20th century. Any attempt to use the magnetic record of fly-ash deposition in lake sediments and/or peats to mark the date chosen as the onset of the Anthropocene would require careful choice of site location and archive, bearing in mind the issue of selective magnetic mineral dissolution.