Porewater chemistry and inferred metastability of coastal wetlands in the Townsville region, North Queensland

Abstract

The geochemistry of coastal wetlands in the Townsville region, north Queensland, is described from a geological perspective, using the mineral stability and diagenesis of mangrove and salt flat sediments. A total of 186 porewater samples were collected from Cape Bowling Green, Gordon Creek, the Townsville Town Common, and the adjacent area of Mt. Low, dissolved concentrations of Fe², Mn², Ca², Na+, K+, Sr², Rb+ and Ba² were measured. The mangrove swamp sediments have a neutral pH (ca.pH 6.9) and reducing (ca. Eh – 250mV) and the salt flats are very slightly basic (ca. pH 7.3) and oxidizing (ca. Eh +100mV), reflecting a general increase of redox potential with increasing elevation. Similarly, increasing salinity landward across the intertidal region is reflected in the porewater sale (Ca², Mg²+, Na+, K+) concentrations and associated presence of evaporate minerals (halite , gypsum, jarosite). However, early depletion of salts indicate that evaporation trends do not simply reflect seawater concentration, but also clay mineral absorption and metabolic uptake by mangrove and samphire vegetation. Overall, geochemical trends across the intertidal zone reflect a continuum of physical (e.g. transport and evaporation) and superimposed chemical (diagenetic) processes between the marine and terrestrial environments. The unstable mineralogy and non-linear evaporation trends, indicate that the intertidal zone is more complex than expected from simple thermodynamic modeling.