HYDROGEOCHEMICAL CYCLING IN A TROPICAL ENVIRONMENT; ST. LUCIA, LESSER ANTILLES

The Caribbean island of St. Lucia provides an ideal site to examine the chemical weathering of volcanic minerals in terrestrial soils, rivers, and modern marine sediments. St. Lucia is part of the Lesser Antilles, an 850-km island arc located on the eastern edge of the Caribbean Plate where it overrides the Atlantic Plate. St. Lucia’s tropical temperature, high humidity and steep relief contribute to rapid mineral weathering rates that create soils that are depleted in primary minerals relative to the underlying bedrock. Typical of tropical fluvial systems, these extreme weathering and precipitation rates result in high fluxes of riverine dissolved and suspended matter, ultimately delivering significant solute concentrations to the ocean. The weathering processes on St. Lucia highlight the significance of tropical weathering environments in global near-surface elemental cycles.

Water samples were collected from precipitation, rivers, and estuary and ocean environments. The waters were analyzed for pH, cations, anions, total alkalinity, and stable isotopes (d¹⁸O_H2O, dD_H2O). Soil, riverine, and marine sediment cores were collected and analyzed for total inorganic carbon (TIC) and total carbon. Mineralogy was determined by XRD and SEM/EDS on bulk and clay sediment fractions.

Preliminary results indicate that the weathering of predominantly andesitic rocks produces terrestrial runoff and secondary minerals that are rich in Fe and Fe-oxides. Optical and SEM analyses reveal that the terrestrial sediments contain a relatively high proportion of Fe-oxides. XRD analyses show that primary igneous minerals weather to smectite, kaolinite, and an opaline silica phase. Pore water geochemical profiles show significant sulfate reduction, generating H₂S that reacts with terrestrial-derived Fe to form pyrite. Finally, the near-shore and pore water samples are less saline than standard seawater, suggesting dilution of the near-shore waters due to freshwater discharge via submarine and/or overland flow.