Paleoenvironmental evolution during the Holocene of the coastal plain of Maricá, Rio de Janeiro, Brazil, through silica biomineralizations

Abstract

Climate change and mean sea level fluctuations were determining factors in the Quaternary evolution of the Maricá coastal plain (Rio de Janeiro, Brazil). In this context, this study aims to contribute to the knowledge on the paleoenvironmental conditions of the Maricá coast in the Holocene, with the aim of identifying the main environmental changes that occurred to understand the evolution of this coastal plain. Three geological boreholes were carried out, reaching depths of up to 12 m, in which silica biomineralizations (sponge spicules, diatom frustules, and phytoliths) were analyzed along with the granulometry, organic matter content, and age (14C-AMS dating) of the sediments. The analysis of the bioindicators present in the sediments enabled the identification of three phases with distinct paleoenvironmental conditions related to the evolution of the Maricá coastal plain: (1) between 8500 and 6500 cal years BP, there was a wetter period with a predominance of very dense shrub/tree vegetation, occupying the slopes and the coastal plain; marine bioindicators (sponge spicules, diatoms frustules, and bivalves) indicate the existence of an ancient shallow-water lagoon, sporadically connected to the sea; (2) between 6500 and 3000 cal years BP, a less humid period predominated, with herbaceous vegetation dominating and increasing hydrodynamic energy, possibly caused by events of wave transposition over the barrier; concomitant with the process of colmation; and (3) the third phase, starting at 3000 cal years BP, is characterized as a new period of higher humidity with the return of shrub/tree vegetation, albeit less dense than in the first phase, again corresponding to an environment with low hydrodynamic energy. The decrease in the amount of sponge spicules evidences the reduction of the paleolagoon water depth and its colmation in this period.