1. 210 Pb dating of environmental records stored in natural archives Peter G. Appleby Department of Mathematical Sciences University of Liverpool, Liverpool, L69 3BX, UK. Third International Conference on Po and Radioactive Pb Isotopes (INCO-PoPb-2015) October 11-14, 2015, Kusadasi, Turkey

2. Origins of 210 Pb dating The basic methodology 210 Pb dating of was established in a ground-breaking paper by Goldberg (1963).

3. The method was first applied by Goldberg to dating Greenland glacier cores. He suggested two possible assumptions for interpreting the 210 Pb records: (1) A constant rate of accumulation of 210 Pb, leading to the equation relating the cumulative activity A above a layer to the age t of that layer. (2) A constant rate of accumulation of both 210 Pb and water, leading to the equation relating the activity B x in a layer of depth x and age t to the activity B 0 in the surface layer.

4. Other applications quickly followed, initially to the measurement of accumulation rates in ice sheets (Crozaz et al. 1964, Crozaz & Langway 1966) and glaciers (Piciotto et al. 1967). Further applications were made during the next few years to the dating of lake sediments (Krishnaswami et al. 1971), marine sediments (Koide et al. 1972), salt marshes (Armentano & Woodwell, 1975) and peat bog sequences (Aaby et al. 1979). Most of these early applications were concerned with using the technique to determine the mean accumulation rate, essentially using Goldberg’s second assumption.

5. In the case of a glacier, assuming a constant (water equivalent) accumulation rate v, ice of depth x will have age t = x/v. Goldberg’s second equation then becomes: The mean accumulation rate can then be calculated by measuring the gradient  of a best exponential fit to the data. The plot shows data from a Greenland ice core (Crozaz & Langway 1966).

6. Coupled with a growing confidence in the technique and the fidelity of environmental records stored in natural archives this led to the development and testing of methods for dating cores where accumulation rates (of ice, sediment or peat) may have varied through time. This had been essentially foreshadowed in Goldberg’s original paper. An increasing number of cases arose where the 210 Pb activity versus depth relationship was clearly non- exponential.

7. the well-defined 137 Cs peak suggests that the sediments have preserved a good record of fallout radionuclides. Example of a non-exponential 210 Pb record Although the 210 Pb record in this lake sediment core (from Pirunkuru, Finland) is clearly non-exponential

8. The potential reliability of environmental records stored in lake sediments is illustrated by these records of pre- and post- Chernobyl 137 Cs in cores from Nylandssjon (Sweden). The cores were dated by counting annual laminae. Twenty years after the events the weapons test fallout peak was retained in the 1963/4 varve and the Chernobyl fallout peak in the 1986 varve.

9. Basic equations Accumulating sediments, ice or peat samples acquire an initial 210 Pb activity through direct or indirect exposure to the natural atmospheric fallout, and via in situ decay from 226 Ra.

10. In most circumstances it can be assumed that the intermediate short-lived decay products of 226 Ra are in equilibrium with 226 Ra and that this initial activity decays with time in accordance with the radioactive decay law: By measuring the present day 210 Pb and 226 Ra concentrations C Pb (t) and C Ra these equations can be used to determine the sediment age t provided reliable estimates can be made of the initial 210 Pb activity C Pb (0). Writing C uns = C Pb – C Ra for the unsupported activity this equation can be rewritten

11. Writing P(t) for the 210 Pb supply rate delivered to the sediments the initial unsupported 210 Pb activity they acquire can be written where r(t) is the mass accumulation rate (dry mass in the case of sediments or peat, water equivalent in the case of ice) at that time. The principal source of unsupported 210 Pb activity is normally assumed to be atmospheric fallout P. This can reasonably be assumed constant on time scales of a year or more. P(t) will be driven by, but not necessarily equal to, the atmospheric flux P.

12. Simple Models There are two standard simple models used in 210 Pb dating: CRS model – assumes a constant rate of supply of 210 Pb to the core site regardless of variations in the mass accumulation rate. Dates are calculated using the equation where A(z) is the residual 210 Pb inventory beneath the layer of depth z and age t(z). Foreseen in Goldberg’s paper it was developed and tested more extensively in the late 1970s by Appleby & Oldfield (1978) and Robbins (1978). CIC model – assumes a constant initial 210 Pb concentration. Dates are calculated using the equation where C(z) is the present 210 Pb concentration in the layer of depth z.

13. Theoretical Justification The CRS model would appear to be relatively well justified in the case of peat cores where there is limited scope for spatial redistribution of the direct atmospheric flux P. (c.f. Appleby 2001) where  Pb is a catchment/lake transport parameter,  the catchment lake area ratio, F Pb the fraction of 210 Pb entering the water column transferred to the sediment record, and f a sediment redistribution factor. In lakes transport processes governing the supply of fallout to the core site are more complicated and can be represented by the equation The supply rate P may be reasonably constant if the various transport parameters are stable, or their impact small.

14. Distribution of the 210 Pb supply rate over the bed of Blelham Tarn, Cumbria (Appleby et al. 2003) The atmospheric flux was estimated to be 147 Bq m -2 y -1. High supply rates at the SW end of the lake adjacent to a stream entering the lake are largely due to inputs from the catchment. Other parts are dominated by direct fallout with some focussing into the NE basin. Fallout 210 Pb entering a lake can be distributed quite unevenly.

15. The CIC model is most likely to be valid for ice cores where initial 210 Pb concentrations will be mainly governed by the constant mean annual 210 Pb concentration in precipitation. Variations in the accumulation rate caused by snow drift may lead to non-exponential concentration versus depth records. This model may also be valid for lake sediment cores where the production of sediment has been relative stable but sedimentation rates at particular sites have varied due to changes in the pattern of sediment accumulation. It is unlikely to be valid for peat cores due to the effect of organic decay.

16. CFCS model Sites where 210 Pb supply rates and mass accumulation rates are both stable will be characterised by exponential concentration versus depth records. At such sites the CRS and CIC models will yield similar results. The mean accumulation rate is calculated from the gradient of a best exponential fit to the data. 210 Pb records from Øvre Neadalsvatn (Norway) and Braya Sø (Greenland)

17. A priori application of either of the simple models to estuarine or marine cores with non-exponential concentration versus depth records is highly problematic without independent validation of the results. As we will see below, even simple exponential records cannot always be trusted.

18. Model Validation In most of our work on dating lake sediment cores although the CRS model has proved to be generally the more reliable, our experience has shown that neither of the two simple models is universally valid. Model validation is an essential part of the dating process This is most commonly achieved using chronostratigraphic dates e.g. from 137 Cs records 137 Cs dating is becoming increasingly important as it covers an increasing part of the 210 Pb dating time-span

19. In the above example from Pirunkuru, Finland, the irregular 210 Pb record precluded use of the CIC or CFCS models. The 1963 depth was however independently determined by a well-defined 137 Cs peak.

20. The validity of the CRS model dates was supported by an excellent agreement between the 210 Pb and 137 Cs dates

21. The 1963 and 1986 depths were however independently determined from the 137 Cs and 241 Am records. In this example (from Karipaajarvi, Finland), the 210 Pb dates were apparently unequivocal, the CRS and CIC models giving similar results.

22. They showed that in this case neither 210 Pb model was correct, demonstrating that even the simplest records can’t always be trusted.

23. Resolution of Dating Discrepancies Although the CRS model has proved to be generally reliable, corrections do need to be made in those cases where there are significant discrepancies between the 210 Pb and 137 Cs dates. Because of the complicated nature of the transport processes governing the supply of 210 Pb to the sediment record, it is unlikely that any more general and widely applicable process based model can be found. Discrepancies between 210 Pb dates (determined by either of the simple models), and independently determined chronostratigraphic dates, necessarily imply a departure from the assumptions of the simple models.

24. Correction Procedures Any correction procedure must be simple, practicable, and evidence based. The approach we have taken is to apply the simple models in a piecewise way to different sections of the core, using 137 Cs or other chronostratigraphic dates as reference points.

25. Given two reference depths z 1,z 2 in the core with independently determined ages t 1,t 2 the mean 210 Pb supply rate during the period of time spanned by this section of the core can be calculated using the formula where  A is the unsupported 210 Pb inventory within that section of the core.

26. If we assume that the calculated 210 Pb supply rate P is constant during that time, the age t(z) of sediments of depth z (z 1 ≤ z ≤ z 2 ) is given by the equation where and  A(z,z 2 ) is the 210 Pb inventory in sediments between depths z and z 2. This equation dates sediments at depths z 1, z 2 to times t 1, t 2.

27. Using this approach the concept of a constant 210 Pb supply rate for the entire period is in effect replaced by that of a variable 210 Pb supply rate, approximated by a series of constant steps.

28. In this example (from Ulmener Maar Lake, Germany), although the 210 Pb record is highly irregular the 137 Cs record has two peaks clearly identifying the 1963 and 1986 depths.

29. Discrepancies between the raw CRS model 210 Pb dates and the 1963 and 1986 137 Cs dates indicated small but significant variations in the 210 Pb supply rate. The corrected chronology was calculated by applying the CRS model in a piecewise way.

30. In another example of an irregular 210 Pb record, in this case from Blelham Tarn, Cumbria, UK, the 137 Cs, 241 Am, and 239+40 Pu records clearly identify the 1986 and 1963 depths.

31. A small but significant discrepancy between the raw CRS model 210 Pb dates and the 137 Cs/ 241 Am/ 239+40 Pu dates was due to an increase in the 210 Pb supply rate at this core site associated with a recent increase in sedimentation rates (though not in the same proportion). Corrected 210 Pb dates were calculated by applying the CRS model piecewise to the pre- and post-1963 sediments.

32. Quality and Reliability of Sediment Records The potential reliability of sediment records can be tested by repeat coring at selected sites over a period of years In view of their well-defined origin, radionuclide records can also be regarded as indicators of the quality of sediment records. Good quality 210 Pb and 137 Cs records suggest that records of other environmental indicators should also be trusted. as demonstrated by the 137 Cs records in cores from Nylandssjon (Sweden) collected during the period 1986 to 2007 (Klaminder et al. 2012).

34. Concluding Remarks 210 Pb dating used in conjunction with 137 Cs records has proved to be a highly flexible and very reliable means for dating environmental records stored in a range of different natural archives. Using just the above simple models applied as a whole or in part our centre has over the past 30 years successfully dating several hundred cores

35. with sediment accumulation rates ranging from the extremely slow (0.033 cm -1 ) to the extremely fast (4.4 cm y -1 ).

36. Radiometric records in a core from Lake B (Greenland). 210 Pb/ 226 Ra equilibrium is reached at a depth of just 3 cm. In this core from Freshwater Lake (Dominican Republic) equilibrium is reached at a depth of more than 200 cm. Slow Cores Fast Cores

37. Sites have ranged from the Arctic to the Antarctic

38. 210 Pb, 226 Ra and 137 Cs records in a lake sediment core from Tenndammen (Svalbard). Fallout 210 Pb and 137 Cs records in a sediment core from Heywood Lake, Signy Island (South Orkney Islands) Arctic cores Antarctic cores

39. from lakes ranging from the extremely small to the extremely large

40. Area 41,471 km 2 Area 0.012 km 2 Large and Small Lakes

41. and from desert to high-rainfall environments.

42. 210 Pb, 226 Ra and 137 Cs records in a sediment core from Lake Qarun, Egypt (mean annual rainfall ~10 mm y -1 ). 210 Pb, 226 Ra and 137 Cs records in a sediment core from Lac du Speke, Uganda(mean annual rainfall ~2500 mm y -1). Desert Regions High-Rainfall Regions

43. 210 Pb dates that have not been validated e.g. by 137 Cs must always be regarded with some caution. 137 Cs dates are becoming increasingly important now that they span up to two or more 210 Pb half-lives. Records of the 1963 fallout maximum from the atmospheric testing of nuclear weapons can be used to validate the recent chronology and improve the reliability of the early part of the record. Records of fallout from the 1986 Chernobyl accident (where they exist) can provide further checks on any recent changes.

44. 210 Pb and 137 Cs records can also provide significant information about the process of sediment accumulation. Comparing mean 210 Pb supply rates at specific sites in the lake with estimates of the atmospheric flux can provide information on the extent and nature of sediment focussing, and the importance of indirect inputs from the catchment. Irregularities in the 210 Pb record may be linked to specific events such as a sediment slump, or a major disturbance in the catchment.

45. 210 Pb is an ideal tracer for studying transport processes within catchment lake systems. Transport models validated by 210 Pb can be used to reconstruct quantitative histories of atmospheric pollution (trace metals, POPs) from their sediment records. And finally:

46. Thank you for your attention