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

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.____________________________________________________________________________________ Austrian Journal of Earth Sciences Volume 110/1 Vienna 2017 Microbial activity records in Marinoan Snowball Earth postglacial transition layers connecting diamictite with cap carbonate (Otavi Group, NW-Namibia)__________________________________________ 1)2) 2)3)*) 4) 4) 5) 1)6) Ildikó GYOLLAI , Márta POLGÁRI , Krisztián FINTOR , Elemér PÁL-MOLNÁR , Friedrich POPP & Christian KOEBERL 1) Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; 2) Research Center for Astronomy and Geosciences, Geobiomineralization and Astrobiological Research Group, Institute for Geology and 2) Geochemistry, Hungarian Academy of Sciences, 1112 Budapest, Budaörsi str. 45, Hungary; 3) Eszterházy Károly University, Dept. of Physical Geography and Geoinformatics, Leányka str. 6, 3300 Eger, Hungary; 4) Szeged University, Department of Mineralogy, Geochemistry and Petrology, Egyetem str. 2-6, 6702 Szeged, Hungary; 5) Department of Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; 6) Natural History Museum, Vienna, A-1010 Burgring 7, Vienna, Austria; *) Corresponding author, rodokrozit@gmail.com Snowball Earth; deglaciation; microbial mats; iron-oxidizing bacteria; biodegradation; cap carbonate; Otavi Group; NW-Namibia