A comparative study of the influence of geochemical parameters on the distribution of foraminiferal assemblages in two distinctive tidal marshes

Abstract

The influence of typical geochemical parameters on living foraminiferal assemblages was examined and compared in two estuaries from northern Portugal: Minho and Lima. Seawater, river waters and interstitial waters from tidal marshes were analyzed for the following parameters: pH, electric conductivity, Redox potential, salinity, alkalinity (field measurements), sulphate, chloride, fluoride, bromide, sodium, potassium, calcium and magnesium. Field and laboratory data enable d the estimation of solubility equilibrium conditions, specifically the saturation index (SI) for calcite. Although both rivers drain the same region, the results indicate distinctive conditions concerning geochemically relevant parameters, such as SI for carbonates and Mg/Ca ratios. In Minho tidal marsh, interstitial waters are strongly undersaturated with respect to carbonate phases (SIcalcite between -3,98 and -1,14). Accordingly, in this environment there is an incidence of agglutinated species. Calcareous species are only present in the estuary mouth and showing clear evidences of destruction in its calcium carbonate tests. On the other hand, in Lima estuary calcareous species are the dominant ones exhibiting well preserved tests without evidences of dissolution, as predicted by the geochemical conditions, namely SIcalcite>0,01 and Mg/Ca ratio<2,28. In combining both abiotic data and information on foraminiferal assemblages, this comparative study demonstrates a methodology for identifying the main geochemical controls on foraminiferal distribution trends.