Hydrochemistry of peritidal stromatolite pools and associated freshwater inlets along the Eastern Cape coast, South Africa

Modern stromatolites are rare due to several factors that restrict their occurrence, although the conditions promoting their formation are not always well understood. This study performed a comparative analysis of the hydrochemistry of a well-developed peritidal stromatolite pool fed by a freshwater inlet, versus a site with no stromatolite growth, along the Eastern Cape coast of South Africa. Hydrochemical data were modelled with USGS PHREEQC 3.4.0 software to determine the saturation state of water samples at a range of temperature and salinity conditions. The well-developed stromatolite pool always favoured carbonate mineral precipitation, whereas this was not true for the site without stromatolite growth. The chemical potential of the systems indicated that any calcite precipitated was biomediated. Furthermore, as opposed to the site without growth the stromatolite-bearing site was saturated with calcite precursor mineral phases. It was found that nutrients, flow rates, and other physicochemical parameters also influence stromatolite development. The carbonate chemistry of additional freshwater inlets feeding stromatolite-bearing pools and the substrate the inlets emanate from was also investigated. The carbonate content of freshwater inlets along the coast is likely derived from a calcareous source inland from where seeps occur. This study importantly showed that a combination of parameters, viz. physicochemical, hydrochemical, thermodynamical and nutritional, influences the precipitation of carbonate minerals in these peritidal stromatolite-systems.