Microbial activity records in Marinoan Snowball Earth postglacial transition layers connecting diamictite with cap carbonate (Otavi Group, NW-Namibia)

Gyollai, Ildikó and Polgári, Márta Piroska and Fintor, Krisztián and Pál-Molnár, Elemér and Popp, F and Koeberl, C (2017) Microbial activity records in Marinoan Snowball Earth postglacial transition layers connecting diamictite with cap carbonate (Otavi Group, NW-Namibia). AUSTRIAN JOURNAL OF EARTH SCIENCES, 110 (1). pp. 4-20. ISSN 0251-7493

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Abstract

This study concerns the microbial mat formation in postglacial layers of the Marinoan Snowball Earth glaciation. A high resolution investigation was carried out on the transition layers of Ghaub to Maieberg Formation (start of Keilberg Member) of the Otavi Group in NW-Namibia (Neoproterozoic). Macroscopic characterization, optical rock microscopy, XRD mineralogy, Raman spectroscopy investigations were done and geochemical (major and trace element content) as well as carbonate carbon isotopic data were collected. Three types of microbial contributions were determined: (1) primary, synsedimentary Fe-rich biomats; (2) secondary biomats from biodegradation of Fe-bearing minerals (pyrite, chlorite); and (3) pseudo-secondary structures coating on clasts. These microbial mats of iron-oxidizing bacteria consumed iron from different sources, such as hydrothermal solutions or ironbearing minerals. On the basis of the morphology of Fe-biomats, and the mineralogical and geochemical signatures, we suggest that the Marinoan postglacial transition layers (Ghaub-Maieberg boundary) formed under neutral, suboxic conditions in brackish water at the studied locality.

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