Oceanographic and sedimentological influences on carbonate geochemistry and mineralogy in hypersaline coastal lagoons, Rio de Janeiro state, Brazil

Abstract

The Região dos Lagos situated along the coast east of Rio de Janeiro, Brazil, is dominated by a semiarid microclimate attributed to the occurrence of oceanographic upwelling at nearby Cabo Frio. The upwelling is strongly associated with the dominance of NE winds during austral spring/summer and the directional change of the shoreline orientation. Some coastal hypersaline lagoons from this region have been studied intensively over the last 25 yr because they represent relatively rare sites of modern primary dolomite precipitation. Comparison of environmental signals in three lagoons indicates that during the last ~ 3.0 kyr, changes in oceanographic parameters may have influenced biogeochemical processes associated with the production of carbonate‐bearing sediments. The timing of a decrease in sea level may have influenced a period of more intense upwelling, which coincides with precipitation of stoichiometric dolomite at ~ 2.3 kyr BP in the lagoons located along the coast directly west of Cabo Frio. The dolomite found in the lower sections of cores from Lagoa Vermelha and Brejo do Espinho contains more positive δ 18 O values, indicating greater evaporation with a period of increased semiarid conditions corresponding to greater terrestrial input. The lower δ 13 C values indicate re‐equilibration with the input of new carbonate ions derived from the decomposition of organic matter during dolomite formation. In contrast, the sediment core from Lagoa Salgada, located northeast of Cabo Frio, contains no dolomite, and very positive δ 13 C values recorded in carbonate sediments are attributed to microbially mediated methanogenesis, whereas δ 18 O values remain relatively constant throughout the core at around zero.