Dry and humid phases in the highlands of southern Brazil during the last 34,000 years, and their influence on the paleoenvironments of the region

The palynology of the last 34,000 years was determined for a sedimentary profile from a bog in São Francisco de Paula, Rio Grande do Sul Eastern Plateau, using radiocarbon dates (profile: 29°19′05″S, 50°08′18″W). This is a rare older palynological sequence studied in the Quaternary of southern Brazil. Between 34,300 and 28,000 BP, the records suggest a cold and semi-arid climate, with rarefied vegetation that was dominated by sparse grassland, the Araucaria forest probably in refuges. Between 28,000 and 23,500 BP, a cold and humid climate was clearly detected at the last glacial stage. The vegetation generally developed with a grassland dominion, and a water reservoir was in the study site. Between 23,500 and 12,000 BP, cold and semi-arid climate promoted the reduction of grassland, and the local water reservoir began to be filled by sediment. Between 12,000 and 10,000 BP, the rise in temperature and humidity resulted in the development of regional vegetation, and the Araucaria forest probably expanded a little from the refuges. Herbaceous marsh dominated the study site. Between 10,000 and 7500 BP, an elevated temperature and dry conditions promoted a general decrease in vegetation cover. Between 7500 and 3000 BP, a progressive increase in humidity permitted the development of local and regional vegetation and the arborous pioneers migrated from the refuges, with the expansion of forest taxa, such as Araucaria angustifolia (Bertol.) Kuntze. There was the beginning and expansion of the bogs from 6000 BP onward. The grassland decreased after 4000 BP probably due to the forest cover. The records indicate a high regional humidity after 3000 BP, which is favorable to Araucaria forest. However, the temperature elevation during this phase probably reduced the reproductive capacity of the Araucaria forest taxa, resulting in a lower rate of expansion on the grassland. The same factor probably reduced the regional bogs from 2000 BP onward.