Understanding Holocene variations in the vegetation of Sao Joao River basin, southeastern coast of Brazil, using phytolith and carbon isotopic analyses

To understand variations in the vegetation of the southeastern coast of Brazil during the Holocene and to identify roughly when they took place, we make use of phytolith morphology and carbon stable isotope analyses coupled with 14C dating of soil profiles. The soil profile studied is located in the Sao Joao River Basin close to Cabo Frio, Rio de Janeiro State. We evaluated the soil characteristics, which showed great variations in granulometry, with larger texture in upper horizons that enabled the movement of particles to lower layers. The accumulation soil horizons revealed several palaeoclimatic aspects: higher phytolith stocks, a higher density of trees, and a lower water stress at the bottom of the profile, and a predominance of C3 grasses throughout the Holocene. Carbon stable isotopic ratios of the soil profile confirm the predominance of C3 plants, except for E/Bt horizon which was enriched in 13C. The soil organic matter mean residence time ranges from post-bomb (topsoil) to 10,245 years cal BP at the base of the profile. The results of this study are compatible with other evidence of a more humid environment in the first half of the Holocene.