Landscape Evolution And Unusual Geomorphological-Pedological-Chronological Relations In An Alluvial Plain Associated With Early Amerindian Settlement In Southeastern Brazil

This article deals with geomorphological, sedimentary, paleopedological, chronological (OSL and 14C) and archaeological records within a segment of the Jacare-Guasu River alluvial plain in southeast Brazil, aiming to evaluate climatic, hydrological and landscape transformations over the past 115 ka. The results show alternations between humid and drier climates that contribute to different drainage conditions, accounting for seven paleosurfaces and five paleosols amid sandy and gravel deposits along Late Pleistocene and Holocene. Micromorphological and OSL data characterize a humid and warm phase in the Eemian associated to the formation of a paleosol under good drainage conditions, followed between 111 ka and the end of Pleistocene by sandy deposition intercalated with the genesis of two new soils and by a sedimentary discontinuity. The lithics artifacts records of Amerindian settlements since the Pleistocene-Holocene transition contributed to the demarcation of paleosurfaces that formed concurrently with landscape and river level changes, and the density of human activities was found to vary simultaneously with environmental transformations in the area. A phase of ephemeral torrential channels under a drier climate, and predominance of grassland vegetation in the Pleistocene end (similar to 12.4 ka) was followed by the set up of perennial meandering channels of dimensions four times greater than that of the current Jacare-Guacu river, formed under increase of forestal associations and rise in water table, with aggradation of up 7 m, sandy deposition and formation of two new soils in the Early Holocene; from Middle Holocene, the fluvial system experienced incision (<8.5 m), avulsion (<600 m), decrease of channel size and uneven erosion of the deposits and paleosols previous formed in the area. Such changes in the landscape contributed to the establishment of lower terraces with older (115-111 ka) deposits and paleosols linked to good drainage conditions and higher terraces with newer (12.4-8.0 ka) deposits and paleosols linked to poor drainage conditions. This pattern of connections between terrace levels, soil types and ages of deposits is different from that previously registered in the alluvial plains of southeast Brazil.