Phytoliths indicate environmental changes correlated with facies analysis in a paleo island-lake, Upper Paraná River, Brazil

Abstract

Phytolith analysis has become a useful tool for paleoenvironmental studies. Phytoliths are produced by plants and are composed of biogenic silica, giving them higher resistance to diagenetic and oxidation processes that commonly occur in tropical sediments. Here, we analyzed the preserved phytolith assemblages in a sedimentary core recovered from an island in the Upper Paraná River. Deposition and preservation of distinctive phytoliths of different vegetation types in the sedimentary facies have provided evidence of the formation of the fluvial island and the floral succession since the Late Pleistocene. Three distinctive phases were described in the island formation. The sandy base (Sr facies) of the core (240 cm) was dated to 14,620 cal yr BP. Most phytoliths in this phase were characteristic of the Podostemaceae family -- aquatic plants adapted to intense water flow. At 97 cm depth, organic sedimentary facies (Fm facies) were dated to 7382 cal yr BP. At this phase, the phytolith assemblage is composed mainly of parallelepipedal bulliform, cuneiform bulliform, and elongate psilate; such morphotypes are primarily produced by grasses. Towards the core top is a layer of massive fine sediments (Vm facies) with greater abundance of plant fragments. The assemblage is composed of short cells, predominantly the bilobate morphotype, which is characteristic of the Panicoideae subfamily (Poaceae) and indicative of soil humidity.