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

Weibel, R.; Lindström, S. LU ; Pedersen, G. K.; Johansson, Leif LU ; Dybkjær, K.; Whitehouse, M. J.; Boyce, A. J. and Leng, M. J. (2016) In Sedimentary Geology 342. p.45-65
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... (More)

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.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Groundwater fluctuations, Microbial activity, Oxygen and carbon isotopic composition, Siderite concretions, Sphaerosiderite, Spheroidal siderite
in
Sedimentary Geology
volume
342
pages
21 pages
publisher
Elsevier
external identifiers
  • scopus:84978136498
  • wos:000382596200004
ISSN
0037-0738
DOI
10.1016/j.sedgeo.2016.06.009
language
English
LU publication?
yes
id
4e3215a9-92b1-4378-b6fc-30015b52d1b3
date added to LUP
2016-12-15 13:47:02
date last changed
2017-09-24 05:04:46
@article{4e3215a9-92b1-4378-b6fc-30015b52d1b3,
  abstract     = {<p>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 δ<sup>13</sup>C 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.</p>},
  author       = {Weibel, R. and Lindström, S. and Pedersen, G. K. and Johansson, Leif and Dybkjær, K. and Whitehouse, M. J. and Boyce, A. J. and Leng, M. J.},
  issn         = {0037-0738},
  keyword      = {Groundwater fluctuations,Microbial activity,Oxygen and carbon isotopic composition,Siderite concretions,Sphaerosiderite,Spheroidal siderite},
  language     = {eng},
  month        = {08},
  pages        = {45--65},
  publisher    = {Elsevier},
  series       = {Sedimentary Geology},
  title        = {Groundwater table fluctuations recorded in zonation of microbial siderites from end-Triassic strata},
  url          = {http://dx.doi.org/10.1016/j.sedgeo.2016.06.009},
  volume       = {342},
  year         = {2016},
}