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2.4 Geochemical analyses

We used the geochemical analysis as a power instrument for the reconstruction of limnic palaeoenvironment (Boyle 2001). The samples were taken from core #63, located in the deep portion of the palaeolake, with a total thickness of organic deposits of 7.5m. The data obtained using X-ray fluorescence spectrometry (XRF) and the loss-on-ignition (LOI) method were processed with the use of correlation and factor analyses. Two groups were identified based on the correlation analysis:

  1. Terrigenous elements, the alumosilicate constituents: Al203, TiO, SiO2, Fe2O3, Cr
  2. Bio- and chemogenic elements: CaO, MnO, S, C (LOI)

The factor analysis has singled out those factors which affected the sedimentation environment:

  1. Terrigenous elements accumulated in shallow lakes, as opposed to bio- and chemogenic elements formed in deeper basins. This opposition is viewed as an indication of the lake-level changes (Dean 1997)
  2. Na2O as opposed to K2O and SiO2 are viewed as reflecting the basin's temperature, as the plagioclase which includes Na2O is more easily decomposed at a higher temperature as compared to the potassium-rich feldspar and silica. Hence, the increased concentrations of the plagioclase and Na2O are seen as indicative of a cooler climate (Dean 1997).
  3. P2O5 and MnO as opposed to all other elements are viewed as an indication of the total biogenic and anthropogenic impact. The anthropogenic signal is based on the concentration of P2O5.
  4. The SiO2/Al203 ratio in deep-water lake sediments reflects the presence of biogenic silica and hence may be viewed as an indicator of productivity (Gavshin and Khlystov 2000).
  5. The Fe2O3/CaO ratio is viewed as indicating the humidity of climate.

As a result, several geochemical units have been identified in the studied sequence (Fig. 17):

Figure 17: Geochemical analysis: X ray fluorescence spectrometry (XRF)

  1. Depth: 7.50-6.10m. Light-olive lake mud. 14C age: 9300 BP. Low concentration of terrigenous elements: Al203 6.3-6.2%, TiO2 0.33-0.27%, MgO 1.9-1.6%; high proportion of bio- and chemogenic elements: CaO 30.2-24.7%, MnO 0.39-0.32, C (LOI) 19.6-0.32. SiO2 total, 30.6-40.0%. Interpretation: Accumulation in a deep lake. At 7.00-6.10m SiO2 biogenic: 6.5%, a signal of higher productivity. MnO 0.22%, indicates a biogenic and anthropogenic impact.
  2. Depth: 6.10-5.50m. Olive lake mud with shells. 14C age: 9000 BP. An increased concentration of terrigenous elements: Al203 12.1%, TiO2 0.63%, MgO 1.9%. Reduced proportion of bio- and chemogenic elements: CaO 11.8%, MnO 0.069%, C (LOI) 5.8. SiO2 total 54.5%. Interpretation: lower lake-level. Reduced concentration of SiO2 biogenic: 4.5%, a signal of lower productivity. MnO concentration is slightly increased, 0.27%.
  3. Depth: 5.50-4.90m. Olive lake mud with numerous shells. 14C age: 8000-7800 BP. A reduced concentration of terrigenous elements: Al203 7.9%, TiO2 0.4%, MgO 1.6%. Increased proportion of bio- and chemogenic elements: CaO 21.0%, MnO 0.13%, C (LOI) 14.2%. SiO2 total, 45.1%. Interpretation: increased volume of water. Increased concentration of SiO2 biogenic: 6.5%, a signal of higher productivity. MnO concentration is reduced, 0.23%.
  4. Depth: 4.90-4.40m. Dark-olive lake mud. 14C age: 7500-7300; 6200 BP. Increased concentration of terrigenous elements: Al203 9.4%, TiO2 0.45%, MgO 2.0%. Reduced proportion of bio- and chemogenic elements: CaO 20.2%, MnO 0.087%, C (LOI) 12.8%. SiO2 total, 42.0%. Interpretation: a fall of lake-level. Reduced concentration of SiO2 biogenic: 4.5%, a signal of lower productivity. A slight increase of P2O5, 0.30%.
  5. Depth: 4.40-3.90m. Olive lake mud. 14C age: 5600 BP. Reduced concentration of terrigenous elements: Al203 7.0%, TiO2 0.33%, MgO 1.6%. Increased proportion of bio- and chemogenic elements: CaO 20.5%, MnO 0.22%, C (LOI) 11.2%. SiO2 total, 49.2%. Interpretation: a rise of lake-level. A rapid concentration of SiO2 biogenic: 7.0%, a signal of high productivity. A decrease of P2O5, 0.23%.
  6. Depth: 3.90-3.10m. Greyish-greenish lake mud. 14C age: 4500-3900 BP. A rapid increase in the concentration of terrigenous elements: Al203 11.2%, TiO2 0.57%, MgO 1.7%. Low proportion of bio- and chemogenic elements, CaO 3.7%, MnO 0.046%, C (LOI) 6.3%. SiO2 total, 63.8%. Interpretation: an abrupt fall of lake-level. Low concentration of SiO2 biogenic: 5.5%, a signal of lower productivity. Reduced concentrations of P2O5 (0.24%) and MnO (0.05%), signal reduced anthropogenic and biogenic impact.
  7. Depth: 3.10-3.00m. Brownish lake mud with plant remains. 14C age: 3700-3500 BP. The concentration of terrigenous elements basically unchanged: Al203 12.6%, TiO2 0.59%, MgO 1.7%. A slightly increased proportion of bio- and chemogenic elements: CaO 3.7%, MnO 0.078%, C (LOI) 7.3%. SiO2 total 60.6%. Interpretation: a rise of lake-level. Reduced concentration of SiO2 biogenic: 5.0%, a signal of still lower productivity. Increased concentration of P2O5, 0.41% shows a stronger anthropogenic signal.
  8. Depth: 3.00-2.70m. Brownish lake mud with numerous plant remains. 14C age: 3200-3000 BP. Increased concentration of terrigenous elements: Al203 14.7%, TiO2 0.66%, MgO 2.1%. Increased proportion of bio- and chemogenic elements: CaO 5.7%, MnO 0.11%, C (LOI) 5.8%. SiO2- 56.5%. Interpretation: a further fall of lake-level. Reduced concentration of SiO2 biogenic: 3.7%, a signal of still lower productivity. Increased concentration of P2O5, 0.61% shows a further increased anthropogenic signal.
  9. Depth: 2.70-2.00m. Brownish lake mud with plant remains. 14C age: 2500 BP. Reduced concentration of terrigenous elements: Al203 13.4%, TiO2 0.61%, MgO 1.8%. Proportion of bio- and chemogenic elements: CaO 6.1%, MnO 0.19%, C (LOI) 6.0%. SiO2 total, 56.5%. Interpretation: a slight rise of lake-level. Increased concentration of SiO2 biogenic: 4.5%, a signal a low productivity. Increased concentration of P2O5, 0.72% shows a further increased anthropogenic signal.
  10. Depth: 2.00-1.70m. Dark brownish lake mud with plant remains.
  11. Depth: 1.70-1.20m. Dark brownish lake mud with numerous plant remains. C14 age: 1000 BP
  12. Depth: 1.20-0.70m. Dark-brownish lake mud with plant remains.
  13. Depth: 0.70-0.50m. Brownish lake mud with plant remains. Reduced concentration of terrigenous elements unchanged: Al203 13.2-12.4%, TiO2 0.62-0.53%, MgO 1.8-1.7%. Proportion of bio- and chemogenic elements: CaO 6.4-3.8%, MnO 0.24-0.12%, C (LOI) 7.8-4.8%. SiO2 total 57.2-63.7%. Interpretation: a paludination of lake which gradually turns into mire. Increased concentration of SiO2 biogenic: 4.5%, a signal of still lower productivity. Increased concentration of P2O5, 0.51-0.91% shows a further increased anthropogenic signal.
  14. Depth: 0.50-0.15m. Peat.
  15. Depth: 0.15-0.00m. Peat. A slight increase in the concentration of terrigenous elements: Al203 14.3-15.1%, TiO2 0.69-0.80%, MgO 1.7-1.9%. Proportion of bio- and chemogenic elements is reduced: CaO 4.8-4.9%, MnO 0.12%, C (LOI) 3.9-2.6%. SiO2 total 58.3-60.6%. P2O5, 1.0-0.96%.

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