Sediment coring

All eleven UK UWMN lake sites were cored between 1989 and 1992 in order to undertake a full palaeolimnological assessment at each site. The results were published by Juggins et al. (1996), and further work combining the sediment record with the annual sediment trap samples was reported by Battarbee et al. (2008).

Sediment cores were collected from the deepest section of each lake using either a Glew or a Mackereth mini corer, operated from an inflatable boat.

Core Preparation and Lithostratigraphy

Cores were extruded in the laboratory and sectioned at 0.5 or 1 cm intervals. Basic lithostratigraphic analyses, notably dry weight, loss on ignition (LOI) and wet density, were performed on sub-sections of the core material. These analyses provide evidence of changing sediment inputs from the catchment and are important in the calculation of sediment accumulation rates.

The percentage dry weight was obtained by weighing approximately 2 g of wet sediment from each slice in a pre-weighed crucible, drying the sediment at 105°C for 24 hours and re-weighing the crucible. Percentage LOI was determined by placing the crucible containing the dried sediment in a muffle furnace at 550°C for two hours. Wet density was resolved in one of two ways: a pre-weighed 2 cm³ container was filled with wet sediment and then re-weighed; or, the volumetric displacement of water by a wet sub-sample was recorded.

Core dating

For each sediment core radiometric methods were used to construct an empirically tested and carefully evaluated chronology of sedimentation for at least the last 150 years. Dating was carried out by the Depatment of Mathematics and Theorectical Physics at the University of Liverpool.

210Pb occurs naturally in lake sediments as one of the radioisotopes in the 238U decay series. Its half-life of 22.26 years makes it well suited to dating sediments laid down over the past 100-150 years. 210Pb dates were calculated using both the constant rate of 210Pb supply (CAS) model and the constant initial 210Pb concentration (CIC) model (Appleby and Oldfield 1978). Factors governing choice of model and interpretation of results are discussed in Appleby and Oldfield (1983) and Oldfield and Appleby (1984). Additional (corroborative) dating towards the top of sediment profiles was provided by analysis of 137CS and 241Am isotopes which relate to atmospheric nuclear weapons testing.

Sediments were measured for 210Pb, 226Ra, 137CS and 241Am by direct gamma assay using a well-type coaxial low background intrinsic germanium detector (Appleby et al. 1986). Background suppression was achieved by means of a 10 cm thick lead castle, a 30.05 cm diameter x 30.05 cm long, sodium iodide escape suppression shield and a 0.3 cm thick copper cylinder fitting over the end of the detector and sample. Samples with masses ranging from 0.5-3.0 g were placed in specially fabricated plastic sample holders designed to fit inside the well recess of the detector. Data were acquired using an analogue to digital converter interfaced to a microcomputer and then transferred to an IBM 3083 computer for analysis. The system achieved a resolution of 1.5 KeV at 100 KeV and 2 KeV at 1 MeV. 210Pb was measured by its gamma emissions at 46.5 KeV and 226Ra by the 352 KeV gamma rays emitted by its daughter isotope 214Pb. 137Cs and 241Am were measured by their emissions at 662 KeV and 59.5 KeV respectively.

Diatoms

Between 0.05-0.10 g of dried sediment was weighed into a beaker and oxidised with approximately 20 ml of 30% hydrogen peroxide on an electric hotplate at 90°C. Samples were then washed by concentration in a centrifuge set at 1200 rev. min-1 for four minutes followed by resuspension in distilled water, this process was repeated at least three times. Afterwashing the sample was diluted with sufficient distilled water to produce a suitable working concentration. Approximately 0.5 ml of suspension was placed on a round 19 mm diameter cover slip and allowed to dry overnight. The cover slip was mounted on a glass slide using 'Naphrax' or 'Mikrops 163' mounting media.

The diatoms were identified and counted using a research microscope with phase contrast and a x100 oil immersion objective, providing a magnification of >x1000. At least 500 valves were counted from each sample. Counts for individual taxa were expressed as percentages of the total and plotted for each sample level down the sediment core.

Diatom concentration data facilitate the assessment of changes in diatom productivity independent of changes in sediment accumulation rate. Similarly, absolute diatom counts are valuable in the interpretation of percentage data since they enable real changes in the abundance of a particular taxon to be distinguished from the apparent changes caused by fluctuations in other taxa. Consequently diatom concentrations were determined according to the microsphere method of Battarbee and Kneen (1982).

pH was reconstructed from the sediment core diatom data using the SWAP training set Munro et al. (1990), Stevenson et al. (1991 ) and the statistical methods described by Birks et al. (1990).

Sediment Chemistry

Sediment chemistry analysis was performed at the Limnology Laboratory, University of Ulster. The methods summarised below yielded good recoveries and high levels of precision, a full description is given in Rippey et al. (1982).

Extruded sediment core material was sub-sampled for trace metal (indicators of atmospheric contamination) and major cation (indicators of catchment disturbance) analysis. The samples were lightly ground and dried overnight at 105°C. Between 0.3-0.5 g was weighed to 0.0001 g accuracy into teflon beakers. In turn, 10 ml of hydrofluoric acid, 10 ml of nitric acid and 5 ml of perchloric acid were added and evaporated to dryness. When all the silicates and organic matter had been decomposed 1 ml of hydrochloric acid and some distilled water were added and the salts dissolved. This was transferred with washings to a 25 ml volumetric flask and made up to volume. Samples were stored in clean polypropylene bottles.

All metals were determined by flame atomic absorption spectrophotometry (Perkin-Elmer 2380). Deuterium lamp background correction was used and the samples were diluted fifty- fold with 0.1% lanthanum chloride for the major cations. Curve correction was used for the major cations and zinc. Five blanks were included with each set of sediment samples to check for contamination.

Spheroidal Carbonaceous Particles

Analysis of carbonaceous particles from lake sediment cores provides a stratigraphic record of the history of the deposition of pollutants from fossil-fuel combustion. Laboratory techniques followed the method of Rose (1994).

Sediment sub-samples of 0.2 g were treated with 30 ml 6M potassium hydroxide and 4 ml 30% hydrogen peroxide to remove the organic and humic fractions and to break up the sediment for further reaction. Hydrochloric acid soluble salts were removed by heating the residue with 30 ml 6M hydrochloric acid at 80°C for two hours. Similarly, siliceous materials were broken down and the silicon removed as SiF4, by the addition of 20 m1 40% hydrofluoric acid and heating at 150°C for three hours. Remaining fluorides and residual organic and humic matter were removed by further treatments with hydrochloric acid and potassium hydroxide and hydrogen peroxide respectively.

Following the digestion a known fraction of the residual suspension was evaporated on to coverslips which were mounted using 'Naphrax' diatom mountant, and the whole of each coverslip counted at x400 using a light microscope.

Results were expressed in terms of concentration per gramme of dry sediment and presented as sediment depth (time)/ concentration curves.

References

Appleby, P. G. & Oldfield, F. (1978) The calculation of Pb-210 dates assuming a constant rate of supply of unsupported Pb-210 to the sediment. Catena, 5, 1-8.
Appleby, P. G. & Oldfield, F. (1983) The Assessment of Pb-210 Data from Sites with Varying Sediment Accumulation Rates. Hydrobiologia, 103, 29-35.
Appleby, P. G., Nolan, P. J., Gifford, D. W., Godfrey, M. J., Oldfield, F., Anderson, N. J. & Battarbee, R. W. (1986) Pb-210 Dating by Low Background Gamma-Counting. Hydrobiologia, 143, 21-27.
Battarbee, R. W. & Kneen, M. J. (1982) The Use of Electronically Counted Microspheres in Absolute Diatom Analysis. Limnology and Oceanography, 27, 184-188.
Battarbee, R. W., Monteith, D. T., Juggins, S., Simpson, G. L., Shilland, E. M., Flower, R. J. & Kreiser, A. M. (2008) Assessing the accuracy of diatom-based transfer functions in defining reference pH conditions for acidified lakes in the United Kingdom. The Holocene, 18, 57-67.
Birks, H. J. B., Line, J. M., Juggins, S., Stevenson, A. C. & Braak, C. J. F. T. (1990) Diatoms and pH Reconstruction. Philosophical Transactions of the Royal Society of London.Series B, Biological Sciences Palaeolimnology and Lake Acidification, 327, 263-278.
Juggins, S., Flower, R. J. & Battarbee, R. W. (1996) Palaeolimnological evidence for recent chemical and biological changes in UK acid waters monitoring network sites. Freshwater Biology, 36, 203-219.
Munro, M. A. R., Kreiser, A. M., Battarbee, R. W., Juggins, S., Stevenson, A. C., Anderson, D. S., Anderson, N. J., Berge, F., Birks, H. J. B., Davis, R. B., Flower, R. J., Fritz, S. C., Haworth, E. Y., Jones, V. J., Kingston, J. C. & Renberg, I. (1990) Diatom Quality-Control and Data Handling. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences, 327, 257-261.
Oldfield, F. & Appleby, P. G. (1984) Empirical testing of Pb-210-dating models for lake-sediments. In: Lake sediments and environmental history, 93-124, Leicester University Press, Leicester.
Rippey, B., Murphy, R. J. & Kyle, S. W. (1982) Anthropogenically Derived Changes in the Sedimentary Flux of Mg, Cr, Ni, Cu, Zn, Hg, Pb, and P in Lough-Neagh, Northern-Ireland. Environmental Science & Technology, 16, 23-30.
Rose, N. L. (1994) A note on further refinements to a procedure for the extraction of carbonaceous fly-ash particles from sediments. Journal of Paleolimnology, 11, 201-204.
Stevenson, A. C., Juggins, S., Birks, H. J. B., Anderson, N. J., Battarbee, R. W., Berge, F., Davis, R. B., Flower, R. J., Haworth, E. Y., Jones, V. J., Kingston, J. C., Kreiser, A. M., Line, J. M., Munro, M. A. R. & Renberg, I. (1991) The surface waters acidification project palaeolimnology programme: Modern diatom/lake-water chemistry data-set. -86. ENSIS Ltd, London.