Estabelecimento e expansão dos manguezais de Laguna-SC: efeito do aquecimento global ou resultado de processos sedimentares?

Abstract

The integration of pollen, isotopes (δ13C, δ15N, C/N), sedimentary features, 14C and 210Pb dating from four sediment cores (LAG-3, LAG-4,LAG-5 e LAG-6). Sampled from a sandbar at Santo Antônio Lagoon, Laguna City, Santa Catarina, allowed the paleoenviromental reconstruction of the last 900 years BP. The dates reveal two facies associations along these cores: (A) Sandbar, represented by massive sand (facies Sm), and (B) tidal flat, represented by lenticular heterolithic bedding (Hl) and wavy heterolithic bedding (Hw). The deposits of sand bar was accumulated between >940 e ~431 cal years BP, probably under influence of a stable relative sea level (RSL) or rise of RSL during the last 1000 years. The pollen data preserved along the sandbar phase indicate a predominance of trees, shrubs herbs and some palms originating from vegetation units surrounding the lagoon. The relation δ13C (-24‰ - 15‰) and C/N (6-30) of this facies association reveals a strong contribution of organic matter from marine algae and terrestrial plants C3 and C4. During the accumulation of tidal flat deposits, occurred during the last 60 years, there was the establishment mainly of Spartina with some spaced Laguncularias shurbs. The relation δ13C (-24‰ - 16‰) and C/N (7-22) reveals an origin of sedimentary organic matter similar to the period of sandbar deposits. Based on this data and distribution gradient of Spartina and on mangrove’s trees genus along the Santa Catarina coast, is reasonable to propose that the recent colonization of Laguncularia at region of Laguna has been caused by gradual increase in minimum winter temperatures observed during last 50 years. Whether the climatic trend is the main force driving the expansion of Laguncularia trees to southern Brazil, the surface of the sandybars and tidal flats of the lagoon margin of southern Brazil, now largely occupied by Spartina, will be gradually colonized and/or replaced not only by Laguncularia, but also by Avicennia, followed during the next years, by Rhizophora.