CARBONACEOUS BIOFILM PRESERVATION ON CAST MOLD VASE-SHAPED MICROFOSSILS FROM THE CRYOGENIAN RIO PARDO BASIN, NORTHEASTERN BRAZIL.

Vase-shaped microfossils (VSMs) are preserved in many Neoproterozoic strata worldwide, with several assemblages found in post-Tonian units. Here, a new microfossil assemblage is reported from the Cryogenian glacial interlude of the Serra do Paraíso Formation, Rio Pardo Basin, composed of carbonaceous cast mold vase-shaped microfossils. Samples collected from dolostone-pelite rhythmite and silicified microbialite facies yielded a total of 26 possible specimens of VSMs identified in thin sections. Samples subjected to partial dissolution in HCl and HF also showed a palynological recovery of palynomorphs attributed to VSMs. Certain specimens were measured and analyzed using MEV-EDS and Raman spectroscopy to determine their mode of preservation. With a total length ranging from 58.16 μm to 247.15 μm (x̄=122.65 μm), three species and one genus are possibly identified: Cycliocyrillium simplex, Cycliocyrillium torquata, Limeta lageniformis?, and Bonniea sp. Carbonaceous biofilm is preserved on most specimens, as shown by Raman spectroscopy, which indicates the presence of kerogen on the test walls in association with silica, contributing to their opaque appearance. Raman mapping shows kerogen to be sparsealy distributed on the microfossil walls, while silica is restricted to their interior forming a type of cast mold. Some palynological specimens exhibit a dolomite extrusion at the aperture, which is also visible in thin section specimens. A possible explanation for these extrusions is that the early diagenetic cementation of the VSM interior, coupled with the silicification of the dolomite matrix helped to preserve a cast mold of the VSM tests, with some specimens even preserving the neck aperture as a cast. The preservation of carbonaceous material on their wall may be associated with the original composition of the organism’s vesicle or may represent an organic coating linked to taphonomic byproduct, which could explain the mineral protuberance emerging from the aperture, interpreted as a cast formed from the neck. Early diagenesis also preserved carbonaceous material in similar VSM assemblages from the Cryogenian Urucum Formation, Jacadigo Group, although cementation in those specimens occurred from the outside of the tests. Our findings support the claim that VSM-rich assemblages persisted throughout the Cryogenian glacial interlude.