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Biogenic processes in crystalline bedrock fractures indicated by carbon isotope signatures of secondary calcite.
Swedish Museum of Natural History, Department of Geology. (Nordsim)ORCID iD: 0000-0003-2227-577X
2016 (English)In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 67, 30-41 p.Article in journal (Refereed) Published
Abstract [en]

Variation in 13C/12C-isotope ratios of fracture filling calcite was analyzed in situ to investigate carbon sources and cycling in fractured bedrock. The study was conducted by separating sections of fracture fillings, and analyzing the 13C/12C-ratios with secondary ion mass spectrometry (SIMS). Specifically, the study was aimed at fillings where previously published sulfur isotope data indicated the occurrence of bacterial sulfate reduction. The results showed that the δ13C values of calcite were highly variable, ranging from −53.8‰ to +31.6‰ (VPDB). The analysis also showed high variations within single fillings of up to 39‰. The analyzed calcite fillings were mostly associated with two calcite groups, of which Group 3 represents possible Paleozoic fluid circulation, based on comparison with similar dated coatings within the Baltic Shield and the succeeding Group 1–2 fillings represent late-stage, low temperature mineralization and are possibly late Paleozoic to Quaternary in age. Both generations were associated with pyrite with δ34S values indicative of bacterial sulfate reduction. The δ13C values of calcite, however, were indicative of geochemical environments which were distinct for these generations. The δ13C values of Group 3 calcite varied from −22.1‰ to +11‰, with a distinct peak at −16‰ to −12‰. Furthermore, there were no observable depth dependent trends in the δ13C values of Group 3 calcite. The δ13C values of Group 3 calcite were indicative of organic matter degradation and methanogenesis. In contrast to the Group 3 fillings, the δ13C values of Group 1–2 calcite were highly variable, ranging from −53.8‰ to +31.6‰ and they showed systematic variation with depth. The near surface environment of <30 m (bsl) was characterized by δ13C values indicative of degradation of surface derived organic matter, with δ13C values ranging from −30.3‰ to −5.5‰. The intermediate depth of 34–54 m showed evidence of localized methanotrophic activity seen as anomalously 13C depleted calcite, having δ13C values as low as −53.8‰. At depths of ∼60–400 m, positive δ13C values of up to +31.6‰ in late-stage calcite of Group 1–2 indicated methanogenesis. In comparison, high CH4 concentrations in present day groundwaters are found at depths of >300 m. One sample at a depth of 111 m showed a transition from methanogenetic conditions (calcite bearing methanogenetic signature) to sulfate reducing (precipitation of pyrite on calcite surface), however, the timing of this transition is so far unclear. The results from this study gives indications of the complex nature of sulfur and carbon cycling in fractured crystalline environments and highlights the usefulness of in situ stable isotope analysis.

Place, publisher, year, edition, pages
2016. Vol. 67, 30-41 p.
National Category
Other Earth and Related Environmental Sciences
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-1965DOI: 10.1016/j.apgeochem.2016.01.010OAI: oai:DiVA.org:nrm-1965DiVA: diva2:1051025
Available from: 2016-11-30 Created: 2016-11-30 Last updated: 2016-12-01Bibliographically approved

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Publisher's full texthttp://www.sciencedirect.com/science/article/pii/S0883292716300105

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CiteExportLink to record
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