GEOCHEMISTRY AND STABLE ISOTOPIC SIGNATURES OF DEEP GROUNDWATERS AND BRINE FROM THE PERMAFROST ZONE OF THE SIBERIAN PLATFORM, RUSSIA

An archival data set of groundwater samples taken in five areas across central Siberia from Irkutsk (Lake Biakal) in the south to Udachny in the north was examined for geochemistry and stable isotopes (²H, ¹⁸O, ³⁷Cl). Sample depths ranged from surface springs to deep boreholes, down to 3395m and total dissolved solids ranged from several thousands mg/l to oven 350 g/l. Chemistry varied widely but was dominated by Na-HCO₃-SO₄ waters in the near surface and generally Na-Cl-SO₄ brines at depths below 1000 meters. The research goals focused on first examining the chemistry and isotopic signatures of deep groundwaters in this area of Russia and secondly to attempt discern the influence of chemical processes associated with permafrost on the evolution of groundwaters. Over much of the area, permanently frozen ground exists to depths of 1000 meters and may have been in place for several million years. As might be predicted d¹⁸O signatures of shallow permafrost impacted groundwaters are depleted (< -15 ‰) with isotopic signatures showing enrichment with depth and increasing concentration. d³⁷Cl signatures range between –1.0 ‰ and +1.5 ‰ with slightly more enriched fluids at shallower depths (< 1000m). By far the most interesting chemical anomaly in the data set are the large number of samples particularly at depth with excessive Br to Cl ratios compared to deep brines formed in shield and sedimentary rocks would wide. Ratios of Br/Cl in excess of 0.02 are common (Cl/Br=50). Such ratios are probably due to dissolution/recrystallization of salt (halite), which is commonly associated with all rock types in the region. Similar ratios in Gulf Coast brines of the USA have been attributed to similar processes. Research is ongoing to recognize the extent of the impact of permafrost chemical processes on determining the isotopic and chemical signature of these groundwaters.