Investigating the ecology of post-Sturtian (~716 Ma) microfossil assembleges in a cap carbonate, the Rasthof formation, northern Namibia


During the Neoproterozoic Era, there is extensive geochemical, sedimentological and paleomagnetic evidence that indicate the entire Earth was locked in an icehouse environment at least two times. Carbonates from the aftermath of the first of these global glaciations, the Sturtian glaciation, comprise the Rasthof Formation (~716 Ma), northern Namibia. Previous work has shown that microfossils are abundant in the basal microbialaminites of two previously described localities from the Rasthof: Okaaru and Ongongo. This work characterizes the microfossils at two new localities: South Ombepera and Ombepera, as a way to constrain the ecological and/or environmental factors that may have influenced microfossil distribution during Rasthof time. Abundant, mineral-rich, spheroidal microfossils were extracted from the thinlylaminated microbialaminites of South Ombepera (N\u3e200) but were consistently absent from thickly-laminated microbialaminite facies. These microfossils dominantly consist of a round, hollow test with a minority of oval, compressed forms and more rarely, vase shaped tests. Few microfossils were extracted from microbialaminites of Ongongo (N\u3c10). Rare, agglutinated tube structures are also present in residues from both microbialaminite facies (N=6). Previous work demonstrated that these forms are consistent with testate amoebae and Foraminifera, respectively. The presence of different morphotypes in similar facies across localities suggests that small-scale ecological differences characterize these assemblages, much like modern settings. Additionally, the abundance of microfossils in thinly-laminated microbialaminite as opposed to the lack of tests in thickly-laminated microbialaminite indicates that taphonomic and preservational processes vary between facies. This important discovery will guide future sampling of Neoproterozoic carbonates. A better understanding of the ecology and taphonomy of these assemblages is valuable in understanding the development and fossilization of microbial ecosystems in the aftermath of the Sturtian glaciation

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