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3.3.2.1. Wet density, dry weight, loss-on-ignition and carbonate content of sediments

The wet density was calculated from the weight of a known volume of sediment. Dry weight, loss-on-ignition and carbonate content were determined after combustion at 105, 550 and 925°C respectively as described by Heiri et al. (2001).

3.3.2.2. Carbon and nitrogen isotopic composition

The isotopes 13C and 15N were analysed in bulk sediment samples from the PONE, PTHE and KHAR cores at the UC Davis Stable Isotope Facility, California, USA. C/N ratios were calculated to examine the relative importance of autochthonous and allochthonous sources of organic material in the lake sediments, whilst 13C/12C analyses is used to determine the dominant source of carbon in the lake (Talbot and Laerdal 2000).

Samples for bulk organic matter (δ13

C) analysis were prepared by adding 5ml of 10% hydrochloric acid to 0.5g of wet sediment to remove carbonates. Once the reaction had ceased, approximately 2 hours, the samples were centrifuged at 1500 rpm for 4 minutes and the supernatant decanted. Samples were washed, three times, with deionised water and air dried at 40°C. Additional 0.5g samples of wet sediment were air dried, untreated, for δ15N analysis. Dried samples were ball milled and 8-19mg subsamples encapsulated in tin capsules. Samples were analysed from Lake Kharinei (KHAR) at 1cm intervals for the first

10cm depth then at 2-4cm intervals to the base of the core. PONE and PTHE were analysed at 0.5cm intervals to 17cm depth, then 1cm intervals.

The samples were analyzed using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). Samples were combusted at 1020°C in a reactor packed with chromium oxide and silvered colbatous/cobaltic oxide. Following combustion, oxides were removed in a reduction reactor (reduced copper at 650°C). Nitrogen and CO2 were separated on a Carbosieve GC column (65°C, 65 mL/min) before entering the IRMS. During analysis, samples are interspersed with laboratory standards, which have been previously calibrated against NIST Standard Reference Materials (IAEA-N1, IAEA-N2, IAEA-N3, IAEA-CH7, and NBS-22). The results were expressed relative to the international standards PDB (PeeDee Belemnite) and Air for carbon and nitrogen, respectively (Sharp 2005).

3.3.2.3. Preparation of larval head capsules

The modern chironomid distribution and abundance in the study sites was investigated by analysing the uppermost 0 – 1 cm of the sediment core. For downcore samples between 0.5 and 2 g sediment were analysed. Wet sediments were deflocculated in 10% potassium hydroxide (KOH) at 70°C for 5 minutes then left to stand in hot water for 20 minutes (Brooks and Birks 2000a; Brooks et al. 2005) . Samples were washed through 90 and 212 µm sieves and the re-suspended sediment sorted in a grooved Bogorov tray using a dissecting microscope at 25 – 50 times magnification. A minimum of 50 head capsules per sample is considered sufficient for reliable temperature reconstructions (Heiri and Lotter 2001; Larocque 2001; Quinlan and Smol 2001). Therefore if less than 50 head capsules were isolated additional sediment was prepared until between 50 and 300 head capsules were separated from each sample. Isolated head capsules were stored in 80% ethanol. Prior to mounting, the head capsules were progressively dehydrated by transferring to 100% ethanol, then Euparal essence, for approximately 5 minutes in each. The head capsules were mounted, 2 per 6mm coverslip, with the ventral side up, in Euparal.

Identifications were primarily based on mentum and, where present, mandible characteristics with reference to Wiederholm (1983), Schmid (1993), Makarchenko and Makarchenko (1999), Brooks et al. (2007) and the national Chironomidae collection at the Natural History Museum, London, UK. Tanypodinae head capsules were identified using the morphology of the paraligula for Procladius or ventral and dorsal pore arrangement as described in Rieradevall and Brooks (2001). Chironomid head capsules that retained the entire mentum or greater than half were counted as one head capsule, and those with half a mentum as half a head capsule. Head capsules comprising less than half the mentum were not counted.

Not all the chironomid larval assemblages included in this study were prepared or identified, solely, by the author. The researchers and organisations involved in the collection of the surface sediments are detailed in sections 2.2.1 to 2.2.4. Samples from northeast European Russia (Lake Vanuk-ty, Lake Mitrofanovskoe and lakes prefixed F- or TDR) were collected between 1995 and 2001 as part of the EU-funded SPICE (ICA2-CT2000-10018) and TUNDRA (ENV4-CT97-0522) projects. Although chironomids had been prepared and identified from these lakes by L. Nazarova, Alfred Wegener Institute, Potsdam, additional material was prepared and identified by the author to ensure over 100 head capsules were identified for each lake. This higher number should ensure the sub- sampled assemblage more accurately reflects the population (Heiri and Lotter 2001). From Nazarova’s original material, taxa which were identified at higher resolution in Brooks et al. (2007) (i.e. Tanytarsini, Tanypodinae, Cricotopus/Orthocladius spp. and Chironomus morphotypes) were re-examined

to ensure consistent taxanomy. These taxa were also re-examined in material from Central Yakutia (prefixed Y-) prepared and identified by Larisa Nazarova. For the remaining taxa, head capsules were selected at random to check for taxonomic consistency between Larisa Nazarova and the author. As identifications appeared consistent Nazarova’s identications and counts were used for taxa not specified above. Material from the Lower Lena River (prefixed LS-) was collected by John Smol of Queen’s University, Kingston and prepared by David Porinchu of Ohio State University. Due to differences in taxonomic resolution and the method of preparation; with multiple head capsules (5-150)

under each coverslip, all head capsules on these slides were re-identified and counted by the author.