Sirvi Kuupäev , alustades "2010-05-20T05:40:52Z" järgi

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Chemical and isotope evolution of groundwater in the Cambrian-Vendian aquifer system in Estonia
(2010-05-20T05:40:52Z) Raidla, Valle
The thesis studies the formation of chemical and isotope composition of groundwater in the Cambrian-Vendian aquifer system over the last 27,000 years. In North-Estonia, the Cambrian-Vendian aquifer system grounwaters are characterised by low 14C activities (1.4 – 6.8 pmc) and considerably more negative δ18OH2O values (-22 to -18‰ VSMOW) compared to local precipitation (-10 to -12‰VSMOW). The phenomenon has been explained by the origin of these waters from glacial meltwater covering Estonian territory from 27,000 to 10,000 radiocarbon years ago. At the same time, 14C activities in the groundwater suggest that the isolation age from the atmosphere is 35,000 to 15,000 radiocarbon years. This discrepancy can be explained by the fact that through geochemical processes 14C datings seem to be older than they actually are. In order to find the actual residence time of the groundwater, profound knowledge of geochemical processes influencing chemical and isotope composition of the groundwater is needed. Chemical composition of the Cambrian-Vendian groundwater is the result of a long-term interaction between the water and its surrounding rocks. The mixing model used in this thesis confirms mixing of fresh glacial water with relict mineral water. Organic material found in Cambrian clays has released carbon, which has participated in both sedimentary and dissolution processes, thus influencing the carbon cycle in Cambrian-Vendian aquifer system. The carbon, through bacterial oxidation and dolomite dissolution processes, has affected the 14C considerably. During the research, it also became clear that both crystalline basement rocks and cation exchange processes have had an important geochemical effect on 14C age calculations. The correction calculations made to 14C age, based on the given geochemical model of carbon, yield the age of 25,000 to 12,000 radiocarbon years. This corresponds to the period when the Estonian territory was covered by a continental glacier. Western waters proved to have a younger age, which most likely suggests that, during the existence of the glacier, there was a more intense water exchange in the western part of the aquifer system.