A influência marinha nas águas do lago Arari (ilha de Marajó-Pa) durante o Holoceno com base em indicadores biológicos e isotópicos.

Abstract

This work aimed the paleoenvironmental reconstruct of the drainage basin area of the Lake Arari (Marajó Island-PA) during the last 7.250 yr B.P. There was performed the characterization of environment study, determining the current physico-chemical data of lake water and their botanical composition (phytoplankton and macrophytes). From the nine sediment cores sampled from the Lake Arari, herbaceous plain, and on the east coast of the Marajó Island, were analyzed the grains size, sedimentary structures, pollen, isotopic (_15N and _ 13C) and elemental (C/Nmolar) content that allowed the lito, bio and chemostratigraphy correlation between the sampling sites. The ages of events were determined by radiocarbon. The integrated analysis of these data allowed the identification of three phases: Phase 1, from 7.328-7.168 and 2.306-2.234 cal. yr B.P., the sediments are classified as clayey silt and sandy silt, showing predominantly lenticular and wavy structures with mangrove pollen and values of C/N molar ratio and _ 13C _ 15N compatible with a lagoonal depositional environment. In Phase 2, from 2.306-2.234 to ~500 cal. yr B.P., occurs a decrease in the energy flow, the sedimentary deposits present parallel lamination structure or massive deposits. Although the pollen data indicate the absence of mangroves in this phase, the isotope and elemental values continue to indicate preferential contribution of aquatic marine organic matter. In phase 3, from ~500 cal. yr B.P. to modern, is established the lake system, because the sediment deposition reflects relatively low energy flow and marine influence becomes progressively smaller with larger contribution of freshwater algae. Besides, occurs a slight increase in organic matter derived from terrestrial plants due to the expansion of the herbaceous plain, which it colonizes the Arari Lake's drainage. Currently the mangrove forests are restricted to east littoral area of the Marajó Island. According to the model proposed in this work, the decrease in marine influence, with consequences for the hydrodynamics, organic matter and vegetation of the study area, was produced by the interaction between the sea-level rise and river water discharge.