Influence of the local environment on lacustrine sedimentary phytolith records

Fossil phytolith assemblages from soils and lake sediments are widely used to reconstruct past vegetation patterns and dynamics, especially in tropical environments where a phytolith index has been quantitatively calibrated to estimate tree cover. Most calibration studies have focused on comparing phytolith assemblages in modern soil with the present aboveground vegetation. However, little is known about phytolith taphonomy and the representativeness of phytolith assemblages and indices in lake sediments. The aim of this study is to improve the understanding of the composition of phytolith assemblages and their influx into recent lake sediments by comparing them with local land cover assessments. This study focused on three lakes in the Central African Republic located in savanna, forest-savanna mosaic, and forest. It appears that the local environment surrounding the lake was critical for interpreting current and past assemblages, and the ratio of dicotyledon to Poaceae diagnostic phytoliths (the D/P index). The presence of a riparian forest in an open environment had no impact on phytolith assemblages or the D/P index, which instead reflected both the local and regional landscapes. Conversely, contributions from a marsh, if present in a forested environment, completely dominated the phytolith records, which consequently reflected only the local environment of the lake. A high correlation between the Poaceae phytoliths influxes and the charcoal accumulation rates for the two lakes surrounded by a riparian forest has also been found. This finding, which was associated with a high proportion of burned Poaceae phytoliths, provides evidence that Poaceae phytoliths are mainly transported by wind in ash clouds produced by fires.