Foraminíferos como indicadores paleoecológicos do Holoceno no Manguezal de Guaratiba, Baía de Sepetiba, Rio de Janeiro

Abstract

Mangroves are estuarine ecosystems, representing the transition between continental and marine environments, and having their training related to fluctuations in sea level during the Quaternary. In Guaratiba Mangrove, several studies about changes in sea level, more precisely in the Holocene, have been conducted under sedimentological, geochemical, palynological and micropaleontological approaches. Among the micropaleontological studies, need to be highlight those using benthic foraminifera, micro-organisms widely used as paleoenvironmental and paleoecological indicators of Holocene. In the present study was collected through a Russian Peat Borer, a core (T1) in Guaratiba Mangrove, which was analyzed for abiotic parameters such as particle size distribution, organic matter (OM), carbonate, total organic carbon (TOC) and sulfur (S), and biotic ones, such as fauna of benthic foraminifera. Ecological indices and cluster analysis were also used, allowed to establish four faunal associations (I, II, III and IV), as well as the environmental factors that most influenced the distribution of fauna. The correlation with foraminiferal assemblages of other cores dated by Carbon 14 (C14), as well as other works that deal with the of Sepetiba Bay evolution, allowed the establishment of three cycles of emersion-submersion, divided on five phases: 1) Transgressive Phase: level of mollusks shells concentration in lagoon deposits, formed by fine sediments without foraminifera, probably occurred latter than a regression; 2) Transgressive Phase: formation of a bay, with the exclusive presence of species of calcareous foraminifera (Association III) with higher richness and decrease in TOC values; occurred about 3,800 years BP; 3) Transgressive Phase: period of submersion, presence of typically estuarine foraminifera species (Association IV), between 3,500 years BP and 2,700 years BP; 4)Transgressive Phase: characterized by alternating between the formation of shallow bays and marine lagoons (higher levels of richness in the faunal associations), lower values of OM and TOC and increase in the proportion of fine sediments; event started about 2,700 years BP; and 5) Regressive Phase: agglutinate foraminifera fauna, resistant to conditions of salinity and acidity characteristics of confined environments such as mangroves, besides the increase in sand content, showing the final stages of Sepetiba Bay by the Marambaia Restinga; event started around 2,400 years BP, extending to the present. The results show the importance of the correlation between lateral cores for the paleoenvironmental interpretation in Sepetiba Bay, beyond the identification of transgression and regression stages that became closer to the curve of sea level variation proposed by Suguio et al. (1985) for the State of Rio de Janeiro coast