Groundwater table fluctuations recorded in zonation of microbial siderites from end-Triassic strata

Weibel, R., Lindström, S., Pedersen, G.K., Johansson, L., Dybkjær, K., Whitehouse, M.J., Boyce, A.J. and Leng, M.J. (2016) Groundwater table fluctuations recorded in zonation of microbial siderites from end-Triassic strata. Sedimentary Geology, 342, pp. 47-65. (doi: 10.1016/j.sedgeo.2016.06.009)

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Abstract

In a terrestrial Triassic–Jurassic boundary succession of southern Sweden, perfectly zoned sphaerosiderites are restricted to a specific sandy interval deposited during the end-Triassic event. Underlying and overlying this sand interval there are several other types of siderite micromorphologies, i.e. poorly zoned sphaerosiderite, spheroidal (ellipsoid) siderite, spherical siderite and rhombohedral siderite. Siderite overgrowths occur mainly as rhombohedral crystals on perfectly zoned sphaerosiderite and as radiating fibrous crystals on spheroidal siderite. Concretionary sparry, microspar and/or micritic siderite cement postdate all of these micromorphologies. The carbon isotope composition of the siderite measured by conventional mass spectrometry shows the characteristic broad span of data, probably as a result of multiple stages of microbial activity. SIMS (secondary ion mass spectrometry) revealed generally higher δ13C values for the concretionary cement than the perfectly zoned sphaerosiderite, spheroidal siderite and their overgrowths, which marks a change in the carbon source during burial. All the various siderite morphologies have almost identical oxygen isotope values reflecting the palaeo-groundwater composition. A pedogenic/freshwater origin is supported by the trace element compositions of varying Fe:Mn ratios and low Mg contents. Fluctuating groundwater is the most likely explanation for uniform repeated siderite zones of varying Fe:Mn ratios reflecting alternating physiochemical conditions and hostility to microbial life/activity. Bacterially mediated siderite precipitation likely incorporated Mn and other metal ions during conditions that are not favourable for the bacteria and continued with Fe-rich siderite precipitation as the physico-chemical conditions changed into optimal conditions again, reflecting the response to groundwater fluctuations.

Item Type:Articles
Additional Information:This study was funded by Geocenter Denmark (grants 3-2010 and 6- 2013). This paper is published with the permission from the Geological Survey of Denmark and Greenland. Jette Halskov is thanked for drafting the figures. Höganäs Bjuf AB is greatfully acknowledged for permissiion to perform field work and core drilling in the quarry at Norra Albert. A.J. Boyce is supported by NERC funding of ICSF (R8/H10/76) and SUERC.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian
Authors: Weibel, R., Lindström, S., Pedersen, G.K., Johansson, L., Dybkjær, K., Whitehouse, M.J., Boyce, A.J., and Leng, M.J.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Sedimentary Geology
Publisher:Elsevier
ISSN:0037-0738
ISSN (Online):1879-0968
Published Online:22 June 2016

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