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For the first time in the extreme southern Brazilian coastal plain, we analyzed palynomorphs in a continuous sequence, using an outcrop profile of relict muds in Hermenegildo Beach (33°42′12″S, 53°18′57″W), Santa Vitória do Palmar, Rio Grande do Sul state. Five radiocarbon dates were obtained. The results revealed the plant succession in an ancient water reservoir near the coastal line. 7500–4000 BP: Marine organisms and freshwater taxa, with herbaceous marsh at the margins. Marine taxa were markedly present from 6000 to 5000 BP, the time of the maximum transgression. Regionally, the vegetation was dominated by grasslands and forest groupings. 4000–2000 BP: Regressive marine phase, with an increased record of freshwater organisms due to increased rainfall. A large increase in herbaceous marsh components resulted in a gradual filling-in of the water reservoir and the development of regional vegetation. 2000–1760 BP: With the end of the marine regression, progressive desalinization promoted the local herbaceous marsh development. Regionally, grassland and forest also developed, indicative of climate amelioration. Local plant succession ceased during this phase of herbaceous marsh ca. 1760 BP, when beach sands covered the sedimentary sequence. This study reveals the phases of a hydrosere in a coastal water reservoir, influenced by marine oscillations, with gradual desalinization and filling-in that culminated in an herbaceous marsh deposit. These results provide a basis for understanding many other similar relict muds spread throughout the coastal plain in the extreme south of Brazil. [ABSTRACT FROM AUTHOR]

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In the northernmost of Rio Grande do Sul coastal plain, southern Brazil, the first comparative palynological study of the sedimentary profiles of two present-day Atlantic rainforests was conducted in Torres municipality: Pirataba forest (29°15′ S, 49°51′ W), 17 km from the coastline, and Faxinal forest (29°21′ S, 49°45′ W), 2 km from the coastline. Ten radiocarbon dates and additional grain influx diagrams allowed for a more accurate analysis. The results suggested that regional and local events affected the northern coastal plain vegetation. The inner coastal plain of the Pirataba site was characterized by a cold, dry regional climate and a small number of pioneer forest species at the end of the last glacial stage (24,000–10,000 bp); a regionally variable temperature increase with slightly humid and dry phases in the early/mid-Holocene (10,000–6700 bp); and regional high temperature and humidity, and expanding Atlantic rainforests, in the mid-Holocene (6700 bp onwards). Near the coastline in the Faxinal site, the following were detected: marine transgression and damage to herbaceous marsh plants, and elevated temperature in the mid-Holocene (7000–6700 bp); regional marine regression, gradual soil desalination and scarce herbaceous marsh in the mid/late Holocene (6700–3500 bp); and regional high temperature and humidity, with forest expansion over the desalinated herbaceous marsh in the late Holocene (3500 bp onwards). The Pirataba and Faxinal forests expanded because of the high temperatures, an even greater increase in regional rainfall, and improved soil conditions at the end of the late Holocene (1500 bp onwards). The hydrosere occurring due to a water reservoir in Faxinal highlights the importance of preserving the lacustrine bodies of the region as generators of coastal forests. [ABSTRACT FROM AUTHOR]