Deep Life and Gases in the Outokumpu Deep Borehole: Base line Information for Nuclear Waste Disposal in Crystalline Rock
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Deep Life and Gases in the Outokumpu Deep Borehole: Base line Information for Nuclear Waste Disposal in Crystalline Rock Lasse Ahonen1, Ilmo Kukkonen1, Taru Toppi1, Mari Nyyssönen2, Malin Bomberg2, Aura Nousiainen2 and Merja Itävaara2 1 Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland 2 VTT Technical Research Centre of Finland, P.O. Box 1000, FIN-02044 VTT, Finland
ABSTRACT Results of microbiological and geochemical sampling in the Outokumpu deep (2.5 km) borehole are presented. The results indicate that the discharging fractures control the observed variations in the microbial populations at different depths, which evidently reflect true variations in the microbial populations in fractures of the crystalline bedrock.
INTRODUCTION Safe geological disposal of nuclear waste requires that biogeochemical processes possibly affecting the repository environment – near field and far field – are properly understood and accounted for in the disposal concept. Several studies have confirmed the existence of deep subterranean biosphere extending far deeper than the planned nuclear waste repositories [1,2]. However, the origin, diversity, metabolism and energetic constraints of these microorganisms are not yet fully understood. The Outokumpu deep borehole located in the classical Outokumpu ore province in eastern Finland was drilled within the Outokumpu Deep Drilling Project of the Geological Survey of Finland in 2004 – 2005. The deep drilling project was carried out in an international framework, partly supported by the International Continental Scientific Drilling Program (ICDP). Geologically the Outokumpu deep drill hole (2516 m) is situated in the Palaeoproterozoic sequence of metasedimentary, igneous and ophiolite-derived rocks in the central part of the Fennoscandian Shield. The mica gneiss – granite dominated rock association of the Outokumpu drill hole is typical to the Fennoscandian bedrock and thus comparable with the disposal sites considered within the Nordic nuclear waste management programs. In addition, the drill hole intersects a less typical rock type environment, namely an ophiolitic rock assemblage derived from 1.96 Ga age oceanic lithosphere. This rock type assemblage hosts massive Cu-Zn-Co sulphide deposits in the Outokumpu area. Graphitic black schist layers in the mica gneiss and ophiolite sequence represent original organic material, dating back to the period of increasing oxygen concentration in the Earth’s geological history. The Outokumpu deep borehole provides an excellent platform for studying many different themes in geology, geophysics, petrophysics, fluid and gas chemistry and geothermics in the deep crystalline bedrock. Existence of deep saline gas-rich fluids in the Outokumpu region has been known since the 1980’s [3]. Isotopic composition of the most important dissolved gas component, methane, indicates that both abiotic and biological processes contribute to its presence in crystalline rocks [4,5]. Studies of microbial distribution and diversity in the Fennos
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