Phytolith signature on the Araucarias Plateau - Vegetation change evidence in Late Quaternary (South Brasil)

The Araucarias Plateau is a geomorphological unit that occupies approximately three-quarters of the terrain in the southern area of South Brazil, between the Iguazu and Uruguay Rivers. The plateau displays different altitudinal levels (600 to <1400 m a.s.l.) that are locally recognized as remnants of incomplete planation surfaces. These surfaces are supported by basic and acidic volcanic rocks (Neocretaceous Period) in the Paraná Basin. Highly developed soils are very common on these surfaces, while on the other hand colluvial, colluvial-alluvial, alluvial sediments and paleosols are also found. We employed these soils and paleosols (surface formations) as stratigraphic level markers of the pedogenesis of the regional Upper Quaternary and propose their importance as records of the paleoenvironmental conditions of the Araucarias Plateau in certain areas. These surface formations were dated by 14C and show ages between 41.16 ± 0.48 ka BP and 0.83 ka BP. We attempt to use the phytolith signature to help understand the paleovegetation changes and infer the role of climate change on the three surfaces of the studied area (Surface 2 and 5). On Surface 2, in analysing the vertical distribution of the phytolith assemblage it is perceived that there are trends of concentrations of phytoliths at depths, which mostly coincide with the limit of pedostratigraphic units. The results suggest that the predominant vegetable formation was dominated by monocotyledons during the Late Quaternary, with alternating dry and humid climatic conditions in Upper Holocene, and a continuous cold climate regime during MIS 2. The soil on Surface 2 (1201-1300 m a.s.l.) and Surface 5 (901-1000 m a.s.l.) produced phytolith analysis results that suggest the change in the paleovegetation is associated with climatic variations on theses surfaces during the Holocene. The phytolith assemblage indicates a vegetation with a predominance of monocotyledons (bilobates, saddle, rondel), and dicotyledons (irregular and globular psilate), associated with a warmer climate in the Lower Holocene. At middle Holocene the vegetation had an increase in abundance of Poaceae (bilobates and saddles) highlighting an opening in the vegetation associated with a drier climate than the previous period. The forest expanded into an Araucaria Forest in the modern period, which would have developed from the Upper Holocene (∼2000 years), under colder more humid climatic conditions, similar to the present. The phytolith analysis was shown to be effective in identifying variations in the vegetation and inferring climatic changes in the Late Quaternary on Araucarias Plateau, in the subtropical zone of Brazil.