Regresar

Quaternary evolution, particle distribution in soils and sedimentary environment in mangroves in São Paulo State, Brazil

Abstract:

Particle distribution in soil or sediments of coastal plains contributes to the understanding of the sedimentation processes in estuaries. It is an important attribute for applications in studies of palaeoenvironmental reconstruction, geochemical cycles and environmental pollution such as heavy metal and oil spill contamination, which are relatively common in these sites due to human impact. With the objective of characterizing the sedimentary environments according to the particle size and the process of quaternary evolu tion, 14 mangroves along the São Pa ulo State coast were studied. Particle size analyses were carried out with samples of the layers 0-20 and 60-80 cm. The clay, silt, total sand size and five sand fractions were determined. Radiocarbon dating was carried out by liquid scintillation counting and accelerator mass spectrometry in humin traction of soil organic matter and by thermoluminescence of quartz grains, of samples of different layers. The results of particle size were treated according to the Folk and Ward statistical parameters. The mangrove soils of São Paulo State are holocenic with ages from 410 to 3,700 yr B.P. down to 80 cm depth. In some cases, this holocenic substratum overlies a pleistocenic sandy layer, as identified in SG1 (65-77cm = 11,000years and 90-95cm = 24,700yr), PM. (72-79 cm = 60,000yr), and in RF where the 40-50 cm layer was estimated at 12,200 years. The mangrove soil textures varyied from sandy to clayey. Sandy soils were identified on Cardoso Island, on the Guaratuba river plain and along the northern coast. These mangroves had developed on restructured sediments of old sand ridges and along the banks of the rivers that drain the coastal plains (SG1, SG2, GUA, RE). These soils also occur close to the river mouths where waves have a strong influence (RF). The mangrove soils that consist mainly of fine particles (silt and clay) are located in the protected parts of the coastal line such as the Cananéia Channel (PM, BAG), Mar Pequeno (IGUA) and in the Santos estuary of the Baixada Santista (ITA, IRI, COS, CRU). Periods of steady tidal that are caused by the alternating high and low tide cycles would allow silt settling, explaining the higher silt content in PM and BAG and across the studied layers in IGUA and ITA.