Paleoecological history of a Holocene coastal paleolagoon from southern Brazil based on palynomorph and diatom data

Abstract

This study presents new data on the paleoenvironmental evolution of the southernmost portion of the Coastal Plain of Rio Grande do Sul (CPRS) to understand how regional and local paleovegetation responded to climatic and environmental changes during the Holocene. Palynomorphs and diatoms from a 44 cm-thick peat profile collected at Hermenegildo Beach (Santa Vitória do Palmar, RS) were analyzed. The top and base of the peat profile were radiocarbon-dated, enabling the construction of an age-depth model. The data indicate the presence of a paleolagoon between 4321 and 3903 calibrated years before present (cal yrs BP) (Late Holocene), with three phases: from 4321 to 4206 cal yrs BP, warm and humid conditions with high precipitation favored the development of forest vegetation and a local herbaceous marsh. The paleolagoon was characterized by brackish-to-freshwater conditions. From 4206 to 4018 cal yrs BP, regional and local vegetation declined, indicating drier climatic conditions. Changes in the paleolagoon’s salinity were evidenced by the presence of marine-brackish diatoms. From 4018 to 3903 cal yrs BP, the regional vegetation, composed of forest and grassland taxa, expanded again. However, indicators of the local herbaceous marsh decreased in expression. The paleolagoon returned to brackish-to-freshwater conditions. After 3903 cal yrs BP, the paleolagoon and its associated local vegetation were buried by eolian deposits, interrupting the vegetation succession process. The results highlight the importance of integrating palynological and diatom microfossil analyses to better understand the dynamics of coastal paleoenvironments and the impacts of climatic events during the Holocene.