ORGANIC-WALLED MICROFOSSILS PRESERVATION IN SUBSURFACE AND OUTCROP SAMPLES FROM THE NEOPROTEROZOIC EDIACARAN OF BRAZIL AND NAMIBIA

Organic-walled microfossils, such as acritarchs, are important biostratigraphic and paleoenvironmental indicators in Neoproterozoic strata, especially for late Ediacaran - early Cambrian. Their global distribution provides valuable insights into eukaryotic evolution and the environmental conditions preceding the Cambrian explosion. Despite their significance, finding well-preserved acritarchs in outcrop samples remains challenging, primarily due to surface weathering, oxidation, and other post-depositional processes that degrade organic matter. This study aims to assess the differences in acritarch preservation between subsurface and surface samples by comparing drill core material from the ICDP-GRIND project from Namibia (Nama Group) and Brazil (Corumbá Group) with outcrop samples collected from Neoproterozoic units in Brazil. Taphonomic processes, common in organic-walled microfossils, were investigated, including fragmentation, pyritization, aluminosilicification, and carbonization. Data was collected with a combination of Raman spectroscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS) methods. Raman spectroscopy was used for the diagenetic classification of kerogen and to evaluate thermal maturity. SEM-EDS analysis enabled detailed mapping of minerals, helping to identify zones of pyritization and aluminosilicification associated with acritarch structures. The results show that drill core samples exhibit significantly better microfossil preservation than outcrop samples. In the core material, acritarchs are less fragmented, retain clearer morphological features, and in some cases, show the presence of pylome/excystment structures potentially related to biological processes. In contrast, the outcrop samples often show severe degradation, likely linked to prolonged exposure to surface weathering. This study underscores the importance of subsurface sampling for the recovery of high-quality microfossil assemblages and highlights the role of integrated analytical techniques in understanding fossil preservation. These findings contribute to refining biostratigraphic frameworks and enhance the reconstruction of Neoproterozoic ecosystems and environmental conditions [Processos Fapesp 16/06114-6; 23/07995-0; 24/08023-4].