MICROFOSSIL ASSEMBLAGES IN THE CRYOGENIAN NONGLACIAL INTERLUDE (~660-640 MA) OF NAMIBIA, ZAMBIA, MONGOLIA AND ARCTIC ALASKA

Strata deposited during the Cryogenian nonglacial interlude preserve a fossil record of life during one of Earth’s most dynamic intervals. We investigated this record by paired acid dissolution and petrographic analysis of limestone samples from the Kakontwe Formation (a post-Sturtian cap carbonate of Zambia), the Taishir Formation (a Mongolian cap carbonate of equivalent age), and the Black Limestone of the Fourth Range in Arctic Alaska (deposited later during the Cryogenian nonglacial interlude). Limestones of the Kakontwe Formation contain abundant agglutinated testate microfossils resembling structures previously described from the post-Sturtian Rasthof Formation of Namibia. Morphologically and mineralogically similar microfossils were also found in the Taishir Formation; the most common microfossils included spherical and ovoid tests with ~10 mm thick, organic-rich walls, sometimes having flat edges or containing slits. While fossils found in the Kakontwe Formation exhibit more varied shapes and sizes (ranging from spherical to ovoid with size ranges of ~ 90 μm to ~200 μm), structures found in the Taishir Formation are generally smaller (~50 μm to ~ 140 μm) and are mostly spherical with distinct features such as blunt edges or slits. The surfaces of all test walls contain predominantly alumino-silicate and clay minerals. Uniquely preserved microfossils were identified in the deep-water limestone of the Fourth Range. These are coiled structures composed of pyrite, and we interpret these as internal molds of Obruchevella, a common Neoproterozoic microfossil. Together, the global occurrence of similar microfossil assemblages in Cryogenian limestones reveals that diverse and widespread microbial ecosystems flourished between global glaciations.