Occurrence of calcareous foraminifera and calcite-carbonate equilibrium conditions: a case study in Minho/Coura estuary (Northern Portugal)

Abstract

The Minho/Coura estuary, northern Portugal, has been studied for the influence of hydrochemical and geochemical parameters on living (Rose Bengal stained) foraminiferal assemblages. Our previous works showed a prevalence of agglutinated species in the inner estuary and tidal marsh and significant abun- dance of calcareous species only in the estuary mouth. The new results clarify syndepositional aspects of the calcareous species related to calcite dissolution, such as thin tests that frequently lack the carbonate layer and show typical destruction features. Sites from the estuary mouth, tidal marsh and river were sampled, under Spring and Autumn conditions, in order to describe physical-chemical features and to allow the geochemical modelling of the solution in which calcification occurs. In partic- ular, the chemical equilibrium regarding calcite was evaluated from the saturation index (SI). Such modelling suggests strongly undersaturated conditions at the majority of the sampling sites, in both sampling periods. The water quality data revealed strong spatial and temporal var- iability, mainly in the tidal marsh environment, and also the existence of a stressed medium regarding calcite saturation state. The observed geochemical trends provide a plausible explana- tion for (1) the scarcity and distribution of living calcareous foraminifera in the Minho/ Coura estuary and (2) the destruction features observed in the carbonate tests.