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Comprehensive geobiological characterization of a bituminous carbonate facies with Ediacara-type fossils

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Type of Publication:
Duda, Jan-Peter; Blumenberg, Martin; Thiel, Volker; Zhu, Maoyan; JoachimReitner,
Page 1 Geophysical Research Abstracts
The Shibantan Member (Dengying Formation, Ediacaran Period) is one of only few carbonate settings withEdiacara-type fossils worldwide (e.g. Ding & Chen, 1981; Sun, 1986; Xiao et al., 2005; Shen et al., 2009; Chenet al., 2014). However, only little is known about the sedimentology and biogeochemistry of the environments inwhich these organisms throve. Here we provide a comprehensive geobiological characterization of the ShibantanMember, addressing the interplay between sedimentary and (bio-) geochemical processes.Sedimentary analysis revealed that black laminated limestones of the lower Shibantan Member were depositedafter a sudden local deepening in a subtidal lower- to middle ramp environment close to the storm wave base, whilethe dark wavy dolomites of the upper Shibantan Member were deposited in a subtidal middle ramp environmentbetween storm- and fair weather wave base. Sedimentation in the Shibantan basin was generally highly dynamicas evidenced by a distinct slumping horizon and mass-flow deposits that were possibly due to synsedimentarytectonic processes.The microbial-mat associated biota including Ediacara-type fossils is restricted to the lower Shibantan Member.Sedimentary analysis of this part reveals a close relationship between autochthonous mat growth and allochthonousand/or para-autochthonous event deposition. During deposition of the lower Shibantan Member the water columnwas probably temporarily stratified, with a sub- to anoxic water layer (evidenced by Ni/Co-, V/(V+Ni) and V/Scratios) overlain by a oxygenated upper layer (evidenced by negative Ce anomalies and low V/Cr ratios). However,such stratification was not permanent, as mixing by storm events is evidenced by hummocky cross stratifica-tion structures. 13C-enrichments in carbonates of the Lower Shibantan Member (δ13C = +3.3 to +4.0 VPDB)together with 13C-depletions of syngenetic n-alkanes cleaved from the respective extraction residue using cat-alytic hydropyrolysis (HyPy; δ13C = -31.7 to -36.3 VPDB) indicate a significant withdrawal of 12C by primaryproducers that thrived within the microbial mats. At the same time, sulphurised biomarkers in the bitumen andHyPy-treated extraction residue hint at organic matter decomposition and concomitant sulphide production bysulphate-reducing bacteria. Given the sedimentological evidence for periodical ventilation of the water column bystorms, sulphide oxidising bacteria were possibly favoured whenever oxygen became available at the sediment-water interface.Taken together, the environments in which the microbial-mat-associated biota including Ediacara-type fossilsthrove were highly dynamic due to a complex interplay of geological and biological processes.
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