Paleoenvironmental insights into the Quaternary evolution of the southern Brazilian coast based on fossil and modern diatom assemblages

Diatom assemblages provide a strong basis for detailed interpretations of paleoceanography and diatom paleoecology of the southern Brazilian coast. Nine cores obtained in the coastal plain, shelf and continental slope and thirteen surface sediment samples of the Patos Lagoon, provide an excellent opportunity to use a paleoecological approach to study detailed Quaternary environmental changes in the southern coasts. In the interval studied, the basin sedimentation occurred in a marine-dominated environment related to humid periods prior to 43,500 year BP. Abundant diatom taxa are highlighted as excellent indicators of environmental changes, including salinity, sediment composition and transport by incised-valleys, providing additional insights into coastal evolution. Paralia sulcata is dominant in all analyzed cores, occurring continuously from Late Pleistocene to Holocene, but is rare in the modern sediments. Non-marine diatom assemblages occurred continuously, but in very low densities in periods before and after the Last Glacial Maximum. Abrupt changes in sedimentation have been recorded related to shifts from marine-dominated to shallow-estuarine deposition. During the Holocene, sea-level rose rapidly which is responsible for the progressive coastal flooding prior to 8420-7930 year BP. The Holocene barrier system became more continuous and Patos Lagoon developed. Consequently, the secondary former inlets from the Jacuí and Camaquã rivers are closed and only one single inlet (Rio Grande channel) remains active. The spatial distribution of modern assemblages is controlled by salinity gradient, wind action, and climate forcing. The bottom sediments of Patos Lagoon are dominated by freshwater diatom Aulacoseira veraluciae, Aulacoseira sp. 2 and marine-estuarine species of Cyclotella litoralis.