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Geosciences 2018, 8(6), 211; https://doi.org/10.3390/geosciences8060211

Ancient Microbial Activity in Deep Hydraulically Conductive Fracture Zones within the Forsmark Target Area for Geological Nuclear Waste Disposal, Sweden

1
Department of Biology and Environmental Science, Linnaeus University, 392 31 Kalmar, Sweden
2
Department of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark
3
Department of Geosciences, Swedish Museum of Natural History, Box 50 007, 104 05 Stockholm, Sweden
*
Author to whom correspondence should be addressed.
Received: 24 April 2018 / Revised: 4 June 2018 / Accepted: 7 June 2018 / Published: 11 June 2018
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Abstract

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 δ13Ccalcite (−36.2 to +20.2‰ V-PDB) and δ34Spyrite (−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 δ18O 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. View Full-Text
Keywords: deep biosphere; crystalline crust; stable isotopes; methanogenesis; sulfate reduction; calcite; pyrite; bedrock fractures; nuclear waste disposal deep biosphere; crystalline crust; stable isotopes; methanogenesis; sulfate reduction; calcite; pyrite; bedrock fractures; nuclear waste disposal
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Drake, H.; Ivarsson, M.; Tillberg, M.; Whitehouse, M.J.; Kooijman, E. Ancient Microbial Activity in Deep Hydraulically Conductive Fracture Zones within the Forsmark Target Area for Geological Nuclear Waste Disposal, Sweden. Geosciences 2018, 8, 211.

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