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Ancient microbial activity in deep hydraulically conductive fracture zones within the Forsmark target area for deep geological nuclear waste disposal, Sweden
Linnæus University, Department of Biology and Environmental Science, 39182 Kalmar, Sweden.
Swedish Museum of Natural History, Department of Paleobiology. University of Southern Denmark, Department of Biology and Nordic Center for Earth Evolution, Campusvej 55, Odense M, DK-5230, Denmark.
Department of Biology and Environmental Science, Linnaeus University, 392 31 Kalmar, Sweden.
Swedish Museum of Natural History, Department of Geology.
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2018 (English)In: Geosciences, Vol. 8, article id 211Article in journal (Refereed) Published
Abstract [en]

Recent studies reveal that organisms from all three domains of life—Archaea, Bacteria, and even Eukarya—can thrive under energy-poor, dark, and anoxic conditions at large depths in the fractured crystalline continental crust. There is a need for an increased understanding of the processes and lifeforms in this vast realm, for example, regarding the spatiotemporal extent and variability of the different processes in the crust. Here, we present a study that set out to detect signs of ancient microbial life in the Forsmark area—the target area for deep geological nuclear waste disposal in Sweden. Stable isotope compositions were determined with high spatial resolution analyses within mineral coatings, and mineralized remains of putative microorganisms were studied in several deep water-conducting fracture zones (down to 663 m depth), from which hydrochemical and gas data exist. Large isotopic variabilities of 13Ccalcite (􀀀36.2 to +20.2‰V-PDB) and 34Spyrite (􀀀11.7 to +37.8‰V-CDT) disclose discrete periods of methanogenesis, and potentially, anaerobic oxidation of methane and related microbial sulfate reduction at several depth intervals. Dominant calcite–water disequilibrium of 18O and 87Sr/86Sr precludes abundant recent precipitation. Instead, the mineral coatings largely reflect an ancient archive of episodic microbial processes in the fracture system, which, according to our microscale Rb–Sr dating of co-genetic adularia and calcite, date back to the mid-Paleozoic. Potential Quaternary precipitation exists mainly at ~400 m depth in one of the boreholes, where mineral–water compositions corresponded.

Place, publisher, year, edition, pages
2018. Vol. 8, article id 211
Keywords [en]
deep biosphere, crystalline crust, stable isotopes, methanogenesis, sulfate reduction, calcite, pyrite, bedrock fractures, nuclear waste disposal
National Category
Other Earth and Related Environmental Sciences
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-2875DOI: 10.3390/geosciences8060211OAI: oai:DiVA.org:nrm-2875DiVA, id: diva2:1256192
Funder
Swedish Research Council, 2017-04129Available from: 2018-10-16 Created: 2018-10-16 Last updated: 2018-10-23Bibliographically approved

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Publisher's full texthttp://www.mdpi.com/2076-3263/8/6/211/pdf

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