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Using SIMS to decode noisy stratigraphic d13C variations in Ediacaran carbonates
Xiao, S.; Cui, H.; Kang, J.; McFadden, K.A.; Kaufman, A.J.; Kitajima, K.; Fournelle, J.H.; Schwid, M.; Nolan, M.; Baele, J.-M.; Valley, J.W. (2020). Using SIMS to decode noisy stratigraphic d13C variations in Ediacaran carbonates. Precambr. Res. 343: 105686. https://hdl.handle.net/10.1016/j.precamres.2020.105686
In: Precambrian research. ELSEVIER SCIENCE BV: Amsterdam; London; New York; Oxford; Tokyo. ISSN 0301-9268; e-ISSN 1872-7433
Peer reviewed article  
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Author keywords
    SIMS; Stable carbon isotopes; Ediacaran; Chemostratigraphy; South China; Namibia
Auteurs  Top 
  • Xiao, S.
  • Cui, H.
  • Kang, J.
  • McFadden, K.A.
  • Kaufman, A.J.
  • Kitajima, K.
  • Fournelle, J.H.
  • Schwid, M.
  • Nolan, M.
  • Baele, J.-M.
  • Valley, J.W.
Abstract
    Carbonate carbon isotope (d13Ccarb) chemostratigraphy is a valuable tool in Precambrian stratigraphic correlation. The effectiveness of this tool rests on the assumption that d13Ccarb data record global seawater signals. However, in some cases d13Ccarb data exhibit rapid and noisy stratigraphic variations that may have been influenced by authigenic or diagenetic carbonate minerals. To assess the contribution of non-primary minerals towards bulk carbonate carbon isotope values, we acquired SIMS (secondary ion mass spectrometry) d13Ccarb data, electron microscopic data, and CL (cathodoluminescence) microscopic data from two Ediacaran successions—the lower Member II of the Doushantuo Formation in South China and the Mooifontein Member of the Zaris Formation in southern Namibia. The Doushantuo samples came from a stratigraphic interval with noisy meter-scale d13Ccarb variations of up to 10‰, whereas the Mooifontein Member is characterized by consistent d13Ccarb values with limited meter-scale variations of less than ~4‰. Our data show that the meter-scale stratigraphic variations in d13Ccarb are also mirrored in the SIMS data at µm-mm scales in both Doushantuo and Mooifontein samples. In the Doushantuo samples, SIMS d13Ccarb values of authigenic calcite vary by up to 10‰ over µm-mm scales and can be either higher or lower than those of the co-existing dolomite matrix, which also appears to be affected by authigenic carbonate on the basis of petrographic observation. Bulk-sample d13Ccarb values measured on powders microdrilled from the same SIMS specimens are within the range of SIMS d13Ccarb values. Thus, we infer that bulk-sample d13Ccarb values of Doushantuo samples represent mixtures of different carbonate components. The Doushantuo and Mooifontein SIMS d13Ccarb data validate the traditional method of using chemostratigraphic consistency to evaluate authigenic/diagenetic alteration, but given that secondary phases may be 13C-enriched relative to matrix carbonate, caution is warranted when taking maximum values to approximate primary chemostratigraphic trends.
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STARDUST II is a project funded by the INTERREG III A programme (France/Walloon Region/Flanders French-Flemish subprogramme) of the European Community's Regional Development Fund.
Hosted by the Flanders Marine Institute (VLIZ)