Core chronologies

The top 8 – 12cm of sediment from cores PONE, PTHE, KHAR and VORK5 were dated by measurement of Lead-210 (210Pb) as detailed in section 3.3.4.1. Due to the half-life of 210Pb this technique is only applicable to samples less than 150 years old. Therefore samples deeper than 20cm below the sediment- water interface in the cores KHAR, NERU and VORK5 were dated by radiocarbon analysis as described in section 3.3.4.2.

3.3.4.1 Lead-210 dating

Lead-210 (210Pb) is a naturally occurring radionuclide produced by the decay of radon gas (222Rn) in the atmosphere, as part of the uranium-series decay chain. Deposited in sediments, it decays to stable 206Pb over an interval of approximately 150 years (half-life 22.3 yrs). By determining the ratio of 210Pb to 206

Pb in the sediment in relation to its depth, the time elapsed since the sediment was deposited can be estimated (Olsson 1986). The method has proved reliable in stable environments, with uniform and continuous sedimentation rates, where the dating calculations are unambiguous (Appleby 2001). In these circumstance dates are calculated using the CRS (constant rate of 210Pb supply) model. However in areas with non-uniform accumulation, such as sites where environmental conditions have varied over the last 100-150

years (Appleby 2004), it may be necessary to select an alternative dating model. The most widely used alternative is the CIC (constant initial concentration) model which assumes that sediments have a constant initial 210

Pb concentration regardless of accumulation rates (Appleby 2001). Caesium-137 (137Cs, half-life 30 years) and Americium-241 (241Am, half-life 432.2 years) are artificially produced radionuclides introduced by atmospheric fallout from nuclear weapons testing and the Chernobyl reactor accidents. These radionuclides produce well-defined peaks of known age and are used as an independent dating technique to verify the 210Pb chronology.

Sediments from the cores PONE, PTHE, KHAR and VORK5 were sub-sampled at 1 – 2 cm intervals for the top 12 – 19 cm of each core and the sub-samples freeze-dried to give approximately 1g dry weight of sediment. Although the Putorana cores had been sectioned at 0.25cm intervals it was necessary to amalgamate samples to provide sufficient material for analysis due to the high water content of surface samples. Sediment accumulation rates are generally low in arctic lakes, therefore amalgamation will increase the age range of the sample and so decrease precision but could not be avoided. Freeze-dried samples were analysed for 210Pb, 226Ra, 137Cs and 241Am by Handong Yang in the Bloomsbury Environmental Isotope Facility (BEIF) at University College London. The concentrations were determined by direct gamma assay, using an ORTEC HPGe GWL series well-type coaxial low background intrinsic germanium detector. Lead-210 was determined via its gamma emissions at 46.5keV, and 226Ra by the 295keV and 352keV gamma rays emitted by its daughter isotope 214Pb following 3 weeks storage in sealed containers to allow radioactive equilibration. 137Cs and 241Am were measured by their emissions at 662kev and 59.5kev respectively. The absolute efficiencies of the detector were determined using calibrated sources and sediment samples of known activity. Corrections were made for the effect of self absorption of low energy gamma rays within the sample.

Radiometric dates were calculated using the CRS (constant rate of 210Pb supply) and CIC (constant initial 210Pb concentration) dating models and compared to the 1963 depths determined from the 137Cs/241Am stratigraphic

records to determine which model was most appropriate (Appleby 2001; Appleby 2004). Mean sedimentation rates were calculated in cm/y and 210Pb dates calculated using the appropriate model (Appleby and Oldfield 1978; Appleby 2001). The sediment accumulation rate was extrapolated down core, for the Putorana cores PONE and PTHE, to provide an age estimate of sediment age beyond the 210Pb record. Older sediments, below 20cm depth, from the Vorkuta cores were radiocarbon dated as described below, section 3.3.4.2. Dates from both procedures were combined to form an age-depth model for these lakes. Details of the models used and the age-depth chronologies are described in chapter 7 for the Putorana samples and chapter 8 for the North-east European Russia sites.

3.3.4.2. Radiocarbon dating

Radiocarbon dating was one of earliest radiometric methods to be developed and is still one of the most widely used dating techniques for material less than 50 kyrs old. 14C is formed when free neutrons, formed by cosmic radiation, interact with atmospheric nitrogen in the upper atmosphere. The isotope is fixed through photosynthesis in equilibrium with the atmospheric concentration. On death the organism no longer incorporates 14C and decays with a half life of 5735 yr. The samples were converted to graphite at the NERC Radiocarbon Laboratory, East Kilbride by digesting approximately 2 – 3 g of wet sediment in 2M hydrochloric acid for 8 hours at 80°C. The residual was washed free from acid with deionised water then dried on filter paper in a vacuum oven and homogenised. The total carbon in a known weight (approximately 10 mg) of pre-treated sample was recovered as CO2 by heating with CuO in a sealed quartz tube and the gas converted to graphite by Fe/Zn reduction (Bryant, pers. comm., 2008). 14C was analysed at the SUERC AMS laboratory (under radiocarbon dating allocation numbers 1243.1007 and 1289.0408) and expressed as conventional radiocarbon years BP (relative to AD 1950) and % modern 14C, both expressed at + 1σ confidence level. Radiocarbon determinations were converted to calibrated dates using the OxCal v4 program (Bronk Ramsey 1995, 2001, 2008).