Doline pond sediments reveal Late Holocene hydro-geomorphological changes in the highlands of the Pantanal, western Brazil

In the southern highlands of the Upper Paraguay River watershed, hundreds of small and shallow ponds exist, but little is known about their origins or potential to record hydroclimate signals in their sediments. Here, we present a multi-proxy evidence that reveals details about the paleolimnological evolution of these shallow tropical lakes. The available data support the conclusion that pond formation resulted from Late Holocene environmental changes that affected the regional hydro-geomorphology. Biological proxies suggest that two distinct environmental phases shaped the regional landscape. In the older phase, sponge spicules and phytoliths are absent, and the sediment geochemistry suggests that karstification processes within the subsurface helped to facilitate topographic closure and doline formation. Sediments of the younger depositional phase are characterized by abundant grass and palm phytoliths. Sponge spicules are also present in these deposits, including the freshwater species Dosilia pydanieli Volkmer-Ribeiro1992, Metania spinata (Carter, 1881), Radiospongilla amazonensis Volkmer-Ribeiro and Becker Maciel, 1983, and Tubella variabilis Bonetto and Ezcurra de Drago1973. These microfossils, especially D. pydanieli, are suggestive of perennial lentic ecosystems. The elemental geochemistry of the youngest sediments is dominated by Si, P, and S; a positive correlation between microfossil content and wt. % S may reflect a linkage between nearshore plant biomass and offshore organic sedimentation. Our results suggest an aquatic transition favoring the formation of stable doline ponds with abundant supra-littoral vegetation occurred over the last millennium, most likely the result of a wetter climate in this region of midwestern Brazil.