Source and distribution of pollen and spores in surface sediments of a plateau lake in southeastern Amazonia

Abstract

In the past two decades, several paleobotanical studies on lakes have indicated a drier climatic condition in Amazonia during the Last Glacial Maximum (LGM) and early-mid Holocene, which likely resulted in the contraction of the tropical forest and expansion of savanna. However, these investigations in the eastern Amazon are scarce, and the reliability and precision of the inference remains largely limited. In this study, palynological analyses were carried out in surficial sediments of Violão Lake, southeastern Amazonia, to assess the distribution and sources of pollen and spores from montane savanna and forest, based on surrounding vegetation types, catchment basin and wind patterns, and also to identify modern pollen rain characteristics of the ecosystems developed under edaphic conditions. Considering the spatial distribution of pollen and spores in the lake basin, the montane savanna pollen are predominant in the eastern and western borders and subordinate in the central portion of the lake, whereas those of forest formations are mainly present in its western margin. Pteridophytes and algae spores show a distinct surface distribution compared to montane savanna pollen, while the algae spores have a good correlation with forest formation pollen. The correlation of these data along with vegetation types of the catchment basin indicates that the absence of pollen grains from species, that present large occurrence along the catchment basin, may be partially related to phenological characteristics that avoided pollen losses. However, in general, montane savanna pollen in the bottom sediments tends to represent a signal of the regional vegetation developed over ferruginous duricrusts in the Serra do Carajás, while forest formation pollen may be used to identify changes in forest structure of the catchment basin of Violão Lake. As montane savanna is developed under edaphic conditions in hydric stress substrate, a systematic description of pollen grains clearly demonstrates that only changes in forest vegetation can be used as a reliable indicator of paleoclimate changes.