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Spheroidal dolomites in a Visean karst system - bacterial induced origin?

Nielsen, P., Swennen, R., Dickson, J.A.D., Fallick, A.E., and Keppens, E.M. (1997) Spheroidal dolomites in a Visean karst system - bacterial induced origin? Sedimentology, 44 (1). pp. 177-195. ISSN 0037-0746 (doi:10.1111/j.1365-3091.1997.tb00431.x)

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Publisher's URL: http://dx.doi.org/10.1111/j.1365-3091.1997.tb00431.x

Abstract

Spheroidal dolomite crystals occur in the karstified top of a Dinantian dolomite sequence in eastern Belgium. The spheroidal dolomite crystals are best developed at the base of the karst system. The dolomite crystals are characterized by a spherulitic or dumb-bell inclusion pattern, and are overgrown by dolomite cements with a rhombohedral outline. They are considered to be bacterially related precipitates based on, (1) textural similarities with documented bacteriogenic precipitates, (2) the presence of 'bacterial'microspheres and framboidal pyrite embedded within the dolomite, and (3) their general geological setting. The geochemical characteristics of the dolomites and associated minerals support a bacterial origin. The ubiquity of framboidal pyrite, depleted in 34S (δ34S=— 22.4 to — 25.5%oCDT), testifies to a period of bacterial sulphate reduction. The isotopic composition of the spheroidal dolomites (δ13C=— 2.4 to - 3.2%oPDB and δ18O=— 3.8 to - 3.4%oPDB) suggest a contribution from oxidized organic carbon produced during bacterial sulphate reduction. Sulphate reduction may also result in a concomitant 18O depletion if the system is nearly closed. It is however, evident from the sulphur isotopic composition of associated framboidal pyrite that the system was fairly open. The 18O depletion of the spheroidal dolomite crystals (δ18O=— 3.8 to — 3.4%oPDB) and their occurrence adjacent to, and within karst cavities suggests a mixing zone origin, with a significant proportion of freshwater in it. The rhombohedral cement-overgrowths have calculated δ18O values in the range of 0 to +5.3%oPDB, which reflect precipitation from normal to slightly evaporated contemporaneous seawater.

Item Type:Article
Status:Published
Refereed:Yes
Glasgow Author(s):Fallick, Prof Anthony
Authors: Nielsen, P., Swennen, R., Dickson, J.A.D., Fallick, A.E., and Keppens, E.M.
Subjects:Q Science > QE Geology
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Sedimentology
Journal Abbr.:Sedimentology
ISSN:0037-0746
ISSN (Online):1365-3091
Published Online:14 June 2006

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