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Carbonates, Snowball Earth (Geology), Fossils-Namibia, Fossils-Zambia, Fossils-Mongolia, Fossils-Alaska, Cap carbonates, Snowball Earth, Post-Sturtian, Microfossils, Zambia, Namibia, Mongolia, Arctic Alaska
Cap carbonates of Cryogenian age may contain insights into the previously scarce fossil record of one of Earth's most dynamic intervals. Recent analyses of several cap carbonates from different continents have resulted in the identification of fossil groups present immediately after the Sturtian Glaciation (~716 Ma), the first of two glacial intervals comprising the Snowball Earth Event. Samples collected from a drill core in Zambia (by F. Macdonald, Harvard University) include limestones of the Kakontwe Formation, which caps the Grand Conglomerate, a glacial diamictite of Sturtian age. These samples contain agglutinated testate microfossils which resemble modern agglutinated testate amoebae. More recent analysis on the Taishir Formation—a Mongolian cap carbonate of equivalent age (also collected by F. Macdonald, Harvard University)—show consistency in morphology and mineralogy with the microfossils of the Kakontwe Formation. The forms commonly found at these locations include spherical and ovoid tests, sometimes having flat edges or containing slits. While fossils found in the Kakontwe Formation tend to be more varied (ranging from spherical to ovoid with size ranges of ~ 90 Ф_m to ~200 Ф_m), structures found in the Taishir Formation tend to be in the smaller range (~50 Ф_m to ~ 140 Ф_m) and are mostly spherical with distinct features such as blunt edges or slits. EDS analysis demonstrate more consistency among the samples showing predominantly alumno-silicate and clay minerals on the surfaces of the tests. Additional distinct structures have been identified in Unit K1 of the Katakturuk Dolomite as well as an unnamed limestone of the Third and Fourth Range in Arctic Alaska, both age equivalents to the cap cabonates of Zmabia and Mongolia. These include spherical structures resembling those found in the Kaknotwe and Taishir Formation and a new coiled structure unlike fossils previously identified for this time interval. These microfossils provide insight into the ecosystem of this warming period, and contribute to our knowledge of the fossil record of the post-Sturtian interval.
Moore, Kelsey Reed, "Microfossil assemblages in Cryogenian (~716 Ma) cap carbonates of Namibia, Zambia, Mongolia and Arctic Alaska" (2015). Honors Project, Smith College, Northampton, MA.
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