Delayed inputs of hot ¹³⁷Cs and ²⁴¹Am particles from Chernobyl to sediments from three Finnish lakes: implications for sediment dating

Abstract

Anomalous peaks in the ¹³⁷Cs and ²⁴¹Am records from three lake sediment cores from southern Finland appear to be due to the presence of micron-size hot radioactive particles, almost certainly originating in fallout from the 1986 Chernobyl accident. Since the imbedding sediments all post-date 1986 by several decades, it appears that they were initially deposited on the catchments of these lakes and transported to the lakes some years later. The activities of the particles were determined using a sequential splitting process. Two of the particles were found to contain ¹³⁷Cs with activities of 64 ± 4 mBq and 266 ± 15 mBq respectively. The third contained ²⁴¹Am with an activity of 17 ± 2 mBq, but no evidence of significant amounts of ¹³⁷Cs. The delayed input of such particles into the sedimentary records highlights the need for care in using ¹³⁷Cs or ²⁴¹Am as chronostratigraphic markers in areas subject to significant levels of contamination from Chernobyl fallout.

Introduction

Artificial radionuclides such as ¹³⁷Cs and ²⁴¹Am play an important role in dating lake sediments, and the wide range of environmental records they contain. Matches between well dated features in the ¹³⁷Cs (or ²⁴¹Am) fallout record and clearly identifiable features in the activity versus depth record are used to date specific sediment layers (Foucher et al. 2021). Although of limited value on their own, these chronostratigraphic dates play an important role in validating the more detailed chronologies obtained from ²¹⁰Pb records, or correcting ²¹⁰Pb dates in the event of any discrepancies (Appleby 2001). Since the early 1970s, concentration peaks identified as recording the period of maximum fallout from the atmospheric testing of thermonuclear weapons have been used to identify the 1963 sediment depth (Pennington 1981; Appleby et al. 1991). In many parts of north-west Europe, including Finland (Arvela et al. 1990), high levels of fallout from the Chernobyl reactor fire have resulted in the presence of a second peak that can be used to identify the 1986 depth (Erlinger et al. 2008). Fallout from this source took place during a relatively short period of time, from 26th April 1986 through to the middle of May (Persson et al. 1987). Radioactive debris deposited directly onto the surface of a lake will in most cases have a relatively short residence time in the water column. Although some will exit the lake via its outflow, a significant fraction will be delivered to the bed of the lake and incorporated into the sediment record for that year. The record in subsequent years will contain further inputs from fallout deposited on the catchment and delivered to the lake by various catchment/lake transport processes (Appleby et al. 2019). Inputs via this indirect pathway will generally be much lower than direct inputs at the time of the accident, and sediments with highest concentrations of Chernobyl radionuclides can for the most part confidently be dated to 1986. Although concentrations in post-1986 sediments frequently vary in response to factors such as different levels of catchment runoff, or post-depositional migration coupled with preferential absorption onto fine-grained clays, small features in this part of the record are for the most part easily distinguished from the 1986 peak itself. In a recent exception to this, the ¹³⁷Cs record in a core from a Norwegian fjord was found to contain a very distinct peak in a sediment sample deposited several decades after 1986 (Appleby et al. 2022). Detailed analyses showed that this feature was due to the presence of a single micron-size hot ¹³⁷Cs particle. Although its activity was just 15 mBq, the ¹³⁷Cs concentration in this near surface sample (15.6 Bq kg⁻¹, 61% of which was due to the particle) was twice as high as in the slice containing the concentration peak attributed to direct fallout from the 1986 event (7.4 Bq kg⁻¹). The most likely origin of the particle was fallout onto the landscape during the passage of the Chernobyl cloud and transport to the fjord some decades later, though in this case a second potential source was discharge into the marine environment from nuclear installations such as those at Sellafield or Dounreay in the UK.

The study by Arvela et al. (1990) showed that although there were significant amounts of Chernobyl fallout in many parts of Finland, highest levels were mainly recorded in the south of the country (Fig. 1). In a recent palaeolimnogical study of lakes in southern Finland, three sites were found to have anomalous features in their Chernobyl fallout records. At two sites, Koirusjärvi and Nuasjärvi, the ¹³⁷Cs record had two distinct peaks. At a third site, Kiantajärvi, a single sample had an unusually high ²⁴¹Am activity.

Fig. 1

Distribution of Chernobyl fallout in Finland, modified after Arvela et al. (1990). Also shown are the locations of the study sites, and a comparison lake, Karipääjärvi, from a region with a low level of Chernobyl fallout. Results from Karipääjärvi have been reported earlier in Ruppel et al. (2013, 2015)

Much of the Chernobyl fallout over Finland was carried on particles in the sub-micron range (Kauppinen et al. 1986), though there were reports that it did include “hot particles” of up to 10 µm in size (Anttila et al. 1987; Raunemaa et al. 1987; Saari et al. 1989). The term “hot particle” is not well defined (Sandalls et al. 1993), but in this context is normally used to describe highly radioactive particles up to around 10 µm in diameter released into the environment from a nuclear accident or device. Although activity levels in particles released by the Chernobyl accident were initially dominated by short-lived radionuclides such as ¹⁴¹Ce, ¹⁴⁴Ce, ⁹⁵Zr, ⁹⁵Nb, ¹⁰³Ru and ¹⁰⁶Ru, in the longer term they are mainly due to ¹³⁷Cs, and to a lesser extent, ²⁴¹Am. The objectives of the present paper were to investigate the causes of the observed anomalies in the sediment records, the possibility that they might be due to delayed inputs of hot particles from the catchment, and any possible implications for the use of ¹³⁷Cs and ²⁴¹Am as chronological markers in dating lake sediments.

Site description

The study lakes (Nuasjärvi, Kiantärvi and Koirusjärvi) are all located in the southern boreal to middle boreal vegetation zones in central Finland (Fig. 1). Mean annual precipitation (1991–2020) for the area ranges from 585 to 678 mm. Average temperatures (1991–2020) lie between − 8.2 and − 9.5 °C during January, and 16.2–17.1 °C during July (Jokinen et al. 2021). The lakes have ice cover from November to May. All have relatively large catchment areas, though the exceptionally large value for Koirusjärvi reflects the fact that it is hydrologically connected to a much larger lake, Kallavesi. For more detailed physiogeographical and limnological data, see Table 1.

Table 1 Physiogeographical and limnological data of the study lakes

Methods

Sample collection and preparation

Sediment cores were retrieved from the sedimentation basins of the lakes in October 2020 using a HTH-kayak gravity corer (Renberg and Hansson 2008). The cores were subsampled on site at 0.5-cm intervals between 0 and 5 cm depth, and at 1-cm intervals between 5 and 20 cm depth. All samples were stored in plastic ziplock bags in a dark cold room at +4 °C within eight hours of retrieval. The samples were freeze-dried prior to radiometric analysis. Dry bulk densities were determined from the water content data.

Radiometric analysis

Subsamples from each core were analysed by direct gamma assay using Ortec HPGe GWL series well-type coaxial low background intrinsic germanium detectors (Appleby et al. 1986) in the Liverpool University Environmental Radioactivity Laboratory, preparatory to dating by ²¹⁰Pb and ¹³⁷Cs. ²¹⁰Pb was determined via its gamma emissions at 46.5 keV, and ²²⁶Ra by the 295 keV and 352 keV γ-rays emitted by its daughter radionuclide ²¹⁴Pb following 25 days storage in sealed containers to allow radioactive equilibration. ¹³⁷Cs and ²⁴¹Am were measured by their emissions at 662 keV and 59.5 keV respectively. Detection limits were ~ 8 mBq for ²¹⁰Pb, ~ 5 mBq for ²²⁶Ra and ~ 2 mBq for ¹³⁷Cs and ²⁴¹Am. The absolute efficiencies of the detectors were determined using calibrated sources and sediment samples of known activity. Corrections were made for the effect of self-absorption of low energy γ-rays within the sample (Appleby et al. 1992). Supported ²¹⁰Pb in each sample was assumed equal to the ²²⁶Ra activity, and unsupported (fallout) ²¹⁰Pb calculated by subtracting this from the total ²¹⁰Pb activity.

Results

Results of the radiometric analyses carried out on each core are shown in Figs. 2, 3, and 4. Table 2 lists some of the basic radiometric parameters for each site, including the maximum unsupported ²¹⁰Pb activity, unsupported ²¹⁰Pb inventory, mean ²¹⁰Pb supply rate, and ¹³⁷Cs inventory. The mean annual atmospheric ²¹⁰Pb flux is estimated to be significantly less than 100 Bq m⁻² y⁻¹ (Paatero et al. 2015). Significantly higher ²¹⁰Pb supply rates at all three core sites can be attributed to factors such as sediment focussing, and allochthonous inputs of fallout deposited on the catchment. All three sites have relatively large catchment areas compared to the size of the lake.

Fig. 2

Fallout radionuclides in the Koirusjärvi sediment core showing a ¹³⁷Cs and b unsupported ²¹⁰Pb concentrations versus depth in the core. ²⁴¹Am concentrations were below detection limits in this core. The ¹³⁷Cs record (a) has two distinct peaks, in the 6–7 cm and 11–12 cm sections. High concentrations show that both features almost certainly originate in fallout from the 1986 Chernobyl accident. The dashed line shows the estimated ¹³⁷Cs concentration in the anomalous sample attributable to the sediment matrix. The markers show the ¹³⁷Cs concentrations in the first three splits containing the hot particle

Fig. 3

Fallout radionuclides in the Nuasjärvi sediment core showing a ¹³⁷Cs and ²⁴¹Am and b unsupported ²¹⁰Pb concentrations versus depth in the core. The ¹³⁷Cs record (a) has two distinct peaks, in the 4–4.5 cm section, and more broadly between 7 and 10 cm. High concentrations again show that both features almost certainly originate in fallout from the 1986 Chernobyl accident. The dashed line shows the estimated ¹³⁷Cs concentration in the anomalous sample attributable to the sediment matrix. The markers show the ¹³⁷Cs concentrations in the first three splits containing the hot particle

Fig. 4

Fallout radionuclides in the Kiantajärvi sediment core showing a ¹³⁷Cs and ²⁴¹Am and b unsupported ²¹⁰Pb concentrations versus depth. The ¹³⁷Cs and ²¹⁰Pb records both suggest a hiatus in the sediment record at 12 cm. An unusually high ²⁴¹Am concentration was recorded in the 6–7 cm sample. The markers show the ²⁴¹Am concentrations in the first three splits containing the hot particle

Table 2 Radiometric parameters for the three Finnish lake sediment cores

The unsupported ²¹⁰Pb activity versus depth records all deviated significantly from a simple exponential decline, indicating systematic changes in the sedimentation rates. ²¹⁰Pb dates for Koirusjärvi and Nuasjärvi were calculated using the constant rate supply (CRS) model (Appleby and Oldfield 1978). This has generally proved to be the most reliable means for dating non-exponential records. Because of an apparent hiatus in the Kiantajärvi record, for this core the model was applied in a piecewise way using the methods outlined in Appleby (2001). A table of dates for each site, including their uncertainties, is given in Viksted et al. (2022, p. 68).

The ¹³⁷Cs inventories have been corrected for decay since 1986, the year of fallout from the Chernobyl nuclear accident. The high ¹³⁷Cs inventories, most notably at Koirusjärvi, confirm that all three sites were significantly impacted by fallout from the 1986 Chernobyl accident. This is further supported by the fact that the values of the inventories are consistent with the fallout data shown in Fig. 1. Data from Finnish sites less affected by Chernobyl fallout suggest that contributions from nuclear weapons test fallout are unlikely to be more than around 1000 Bq m⁻².

The ¹³⁷Cs record in the Koirusjärvi core (Fig. 2a), the most heavily impacted site, has two well-defined peaks, in the 6–7 cm and 11–12 cm sections. Although these features would normally be assumed to record two distinct events, fallout from the 1986 Chernobyl accident and the 1963 fallout maximum from the atmospheric testing of nuclear weapons, the high concentrations (> 1500 Bq kg⁻¹) suggest that both peaks contain ¹³⁷Cs originating in fallout from the Chernobyl accident, with the deeper peak in all probability dating from that time. This is confirmed by the ²¹⁰Pb dates, which place 1986 within the 11–12 cm section. Sediments in the 6–7 cm section containing the more recent ¹³⁷Cs peak were dated to 2009 ± 2 years, more than two decades after the Chernobyl fallout event. The year of maximum fallout from the atmospheric testing of nuclear weapons, 1963, is placed within the 14–15 cm section. The absence of a ¹³⁷Cs peak at this depth can be attributed to downwards migration of the very high levels of Chernobyl ¹³⁷Cs (Klaminder et al. 2012).

To determine whether the original 6–7 cm subsample was representative of the slice as a whole, a second subsample from the same slice was analysed and found to have a similar ²¹⁰Pb activity but an 11% lower ¹³⁷Cs activity. The possibility that the discrepancy was due to the presence of a hot particle was examined using a sequential splitting method (e.g. Falk et al. 1988; Pöllänen et al. 1999). The original higher activity subsample was divided into two equal parts that were then reanalysed. They again had similar ²¹⁰Pb activities, but this time there was a 40% discrepancy in the ¹³⁷Cs concentrations, 2049 ± 20 Bq kg⁻¹, compared to 1477 ± 20 Bq kg⁻¹. Repeated splitting of the high activity subsamples followed a similar pattern. By the fifth split the ¹³⁷Cs concentration in the high activity half had increased to 5994 ± 109 Bq kg⁻¹, compared to 1334 ± 64 Bq kg⁻¹ in the low activity half. Since the low activity halves all had similar ¹³⁷Cs concentrations, the most likely explanation of these results is that the ¹³⁷Cs peak in the 6–7 cm sample was mainly due to the presence of a single hot ¹³⁷Cs particle. Excluding this particle, ¹³⁷Cs activity appeared to be uniformly distributed throughout the sediment matrix. The mean concentration in the matrix based on a weighted average of measured activities in the low activity splits was calculated to be 1441 ± 81 Bq kg⁻¹. This is very similar to the mean of the ¹³⁷Cs activities in the adjacent 5–6 cm and 7–8 cm samples, 1270 ± 14 Bq kg⁻¹ and 1442 ± 16 Bq kg⁻¹, respectively.

For each high activity split the total ¹³⁷Cs activity can be divided into two components, one due to the sediment matrix and the other due to the presumed hot particle. The activity due to the matrix is calculated by multiplying the mass of the split by the ¹³⁷Cs concentration in the matrix. Excluding the value from the first split where the uncertainty was necessarily very large, the results, shown in Table 3, yield a consistent value for the activity of the hot particle of between 256 and 281 mBq with a weighted mean of 266 ± 15 mBq. Higher weights were given to smaller splits where the uncertainties were necessarily smaller. A similar analysis of concentrations in the 11–12 cm section showed that in this case the ¹³⁷Cs activity was uniformly distributed throughout the sample and that the high concentration was due to elevated levels of fallout. Successive splits of this sample had relatively similar ²¹⁰Pb and ¹³⁷Cs concentrations.

Table 3 ²¹⁰Pb and ¹³⁷Cs concentrations in the matrix and successive high activity splits of the 6–7 cm slice from the Koirusjärvi core, and total ¹³⁷Cs activities in these splits attributable to the matrix and the presumed hot particle

The ¹³⁷Cs record in the Nuasjärvi core (Fig. 3a) also has two distinct peaks, a broad feature between 7 and 10 cm, and more narrowly defined feature within the 4–4.5 cm sample. There is also a small peak in ²⁴¹Am concentrations slightly deeper in the core, between 9 and 12 cm. High concentrations again suggest that both ¹³⁷Cs peaks originate in fallout from the 1986 Chernobyl accident, with the deeper and more substantial feature being a direct record of that event. This is confirmed by the ²¹⁰Pb results, which place 1986 at a depth of around 8 cm. Since the ²¹⁰Pb dates also place 1963 at a depth of around 10 cm, the ²⁴¹Am peak most probably records the early 1960s peak in fallout of its parent radionuclide ²⁴¹Pu from the atmospheric testing of nuclear weapons (Appleby et al. 1991). The relatively small gap between the 1963 and 1986 dates is attributed to a much lower sedimentation rate at that time. The CRS model calculations suggest a value of around 0.1 cm y⁻¹, compared to 0.24 cm y⁻¹ for the post-1986 period. The absence of a 1963 ¹³⁷Cs peak can again be attributed to downwards migration of Chernobyl ¹³⁷Cs, as can the presence of significant amounts of ¹³⁷Cs at depths predating the 1953 onset of global fallout from the atmospheric testing of nuclear weapons.

Sediments in the 4–4.5 cm section containing the more recent ¹³⁷Cs peak were dated to 2009 ± 2 years. An analysis carried out on a second subsample from this section suggested that the anomaly was again due to the presence of a single hot particle, and this was confirmed by carrying out a sequential splitting analysis as described above. During this process ¹³⁷Cs concentrations in the hot splits increased from an initial value of 593 ± 8 Bq kg⁻¹ to 1952 ± 58 Bq kg⁻¹ in the 5th split (Table 4). The results again suggested that, excluding the hot particle, ¹³⁷Cs was relatively uniformly distributed throughout the section with a mean concentration of 487 ± 19 Bq kg⁻¹. Using this value, calculations of the ¹³⁷Cs activity in each hot split due to the presumed hot particle yielded a consistent value of between 57 and 64 mBq, with a weighted mean of 64 ± 4 mBq. A similar analysis of sediments in the 8–9 cm section showed that in this case the ¹³⁷Cs activity was uniformly distributed throughout the sample and that the high concentrations were due to elevated levels of fallout. Successive splits had relatively similar ²¹⁰Pb and ¹³⁷Cs concentrations.

Table 4 ²¹⁰Pb and ¹³⁷Cs concentrations in the matrix and successive high activity splits of the 4–4.5 cm slice from the Nuasjärvi core, and total ¹³⁷Cs activities in these splits attributable to the matrix and the presumed hot particle

Results from the Kiantajärvi core suggest that there is a hiatus in the sediment record at a depth of around 12 cm. At this depth there is an abrupt jump in unsupported ²¹⁰Pb concentrations from 57 ± 17 Bq kg⁻¹ in the 12–13 cm sample to 337 ± 22 Bq kg⁻¹ in the 11–12 cm sample, and a similar jump in ¹³⁷Cs concentrations from 76 ± 4 Bq kg⁻¹ below to 337 ± 8 Bq kg⁻¹ above. Although there is no clear feature identifying the 1986 Chernobyl event, high ¹³⁷Cs concentrations in all samples above 12 cm suggest that they all post-date 1986. Near constant values in the unsupported ²¹⁰Pb concentrations suggest that sediments above the hiatus span no more than a few years. One of the samples within this part of the core, at 6–7 cm, had an unusually high ²⁴¹Am activity of 25 ± 1 Bq kg⁻¹. At all other depths ²⁴¹Am concentrations were below detection limits. Since fallout from the Chernobyl accident was known to include hot ²⁴¹Am particles (Falk et al. 1988), a sequential splitting analysis was carried out to determine whether that could account for the high activity in this sample. The results, shown in Table 5, were consistent with the presence of a single hot particle. At each stage of the analysis one of the splits was shown to contain all of the ²⁴¹Am activity, and the other negligible ²⁴¹Am activity. Calculated values of the activity in the hot split had a consistent value of between 15 and 21 mBq. The weighted mean was 17 ± 2 mBq. There was no evidence of significant ¹³⁷Cs activity on the hot particle, possibly indicating that it was a Group 1 particle as defined by Falk et al. (1988). All of the splits had essentially the same ¹³⁷Cs concentration. Although there are large uncertainties over the dating of this core, it is almost certain that sediments containing the hot particle were deposited at some time during the past decade.

Table 5 ²¹⁰Pb and ¹³⁷Cs concentrations in successive splits of the 6–7 cm slice from the Kiantajärvi core, and ²⁴¹Am concentrations in successive hot subsamples. Total ²⁴¹Am activities in the hot splits are presumed to be that of the hot particle

Discussion and conclusions

Although their precise origin is to some extent immaterial, the hot particles reported here almost certainly originated in fallout from the Chernobyl cloud during its passage over Finland in late April and early May 1986. All three cores are from areas known to have been heavily impacted by fallout from Chernobyl. Further, measurements carried out in the immediate aftermath of the accident showed that hot particles from the cloud were deposited in Sweden (Persson et al. 1987) and Finland (Raunemaa et al. 1987). Releases from the Ignalina nuclear power plant in Lithuania (Marčiulionienè et al. 2015) appear to have been relatively localised. Atmospheric releases from other nuclear installations in the Former Soviet Union (Suokko and Reicher 1993) were almost certainly either too localised or too remote. Although the Kyshtym accident in 1957 did release a large amount radioactive material into the atmosphere, the prevailing winds carried it mainly in a north–north east direction from that site (Norwegian Radiation Protection Agency 2007).

A study of hot particles collected one year after the Chernobyl accident (Falk et al. 1988) showed that they included fragments from the damaged reactor as well as highly radioactive inclusions formed within the fuel elements. Although initially the radioactivity would have been mainly associated with short-lived fission products, only longer-lived products such as ¹³⁷Cs and some transuranic elements including ²⁴¹Am are presently above detection limits. One such particle from Sweden (Falk et al. 1988) had a ¹³⁷Cs activity of 400 mBq (179 mBq decay corrected to 2021) but negligible ²⁴¹Am. Another from the Gotland Deep in the Baltic Sea (Pöllänen et al. 1999) contained both ¹³⁷Cs (59 mBq decay corrected to 2021) and ²⁴¹Am (100 mBq). Particles deposited in Finland appear to have been of a similar size, up to around 10-µm in diameter (Raunemaa et al. 1987). Assuming a similar composition to those collected in Sweden, the Nuasjärvi and Koirusjärvi ¹³⁷Cs particles are estimated to have diameters of between 6–11 µm and 10–18 µm respectively, and the Kiantajärvi ²⁴¹Am particle a diameter of around 6 µm.

Hot particles from the study sites account for only a small fraction of the ¹³⁷Cs inventory in the sediments. The sediment matrix accounted for 97% of the total ¹³⁷Cs activity in the core slices from Nuasjärvi checked for homogeneity. In the Koirusjärvi core the corresponding figure was 96%. There was no evidence of the presence of hot particles in the remaining slices, in all probability they are likely to account for less than 1% of the total ¹³⁷Cs inventory. Since particles deposited directly onto the surface of the lake would have been either incorporated in the sediment record for 1986 or lost from the lake via the outflow, those detected in the present study were almost certainly deposited on the catchment and only transported to the lake some years later. Studies of catchment/lake transport (Appleby et al. 2019) have shown that significant quantities of ¹³⁷Cs deposited on the catchment may continue to be transported to the lake decades after the last fallout event. Regardless of contribution or pathway, these findings do, however, show that at sites significantly impacted by fallout from the Chernobyl accident, delayed inputs of hot particles originating in fallout from the 1986 Chernobyl accident can have a disproportionate impact on ¹³⁷Cs and ²⁴¹Am records and hence on their reliability as chronostratigraphic markers. This has relevance not just for the ¹³⁷Cs/²⁴¹Am dates themselves but perhaps most importantly for their use in validating ²¹⁰Pb chronologies. It is widely recognized that neither of the standard ²¹⁰Pb models can be relied on absolutely and independent validation is essential, even where the ²¹⁰Pb dates appear to be unequivocal. The importance of reliable chronostratigraphic dates for validating ²¹⁰Pb dates is highlighted by data from Karipääjärvi, a small lake around 500 km further north in Finnish Lapland (Fig. 1). Figure 5 shows radiometric records from a core collected from this site in 2010. The results have been previously reported in Ruppel et al. (2013, 2015). The ¹³⁷Cs record has two distinct peaks, within the 1–1.25 cm and 3–3.25 cm sections, respectively. The presence of a small ²⁴¹Am peak within the 3–3.25 cm section showed that the earlier peak in this case recorded the 1963 weapons test fallout maximum. Confidence in the reliability of the ¹³⁷Cs date allowed it to be used as a reference point (Appleby 2001) to correct a small but significant discrepancy with the raw ²¹⁰Pb dates. The superficial similarity of the Karipääjärvi records to those from Koirusjärvi shows that such confidence is not always justified and that ¹³⁷Cs dates can be misleading. Where there is any doubt, samples containing potentially anomalous features should be investigated more thoroughly for possible causes other than direct fallout.

Fig. 5

Fallout radionuclides in the Karipääjärvi sediment core showing a ¹³⁷Cs and ²⁴¹Am and b unsupported ²¹⁰Pb concentrations versus depth. Substantially lower levels of Chernobyl fallout are reflected in the greatly reduced ¹³⁷Cs concentrations compared to the study sites. Although the ¹³⁷Cs record is superficially similar to that at Koirusjärvi (Fig. 2a), association of the deeper peak at 3–3.25 cm with a similar but smaller ²⁴¹Am peak suggests that this feature most probably records the 1963 fallout maximum from the atmospheric testing of nuclear weapons. The ²¹⁰Pb calculations, which take account of dilution due e.g. to episodes of more rapid sedimentation, suggest that the later ¹³⁷Cs peak almost certainly records fallout from the 1986 Chernobyl accident