SUBMARINE GROUNDWATER DISCHARGE OF PHOSPHORUS AND IRON FROM CARBONATE COASTLINES UNDER RISING SEA LEVELS, YUCATAN PENINSULA, MEXICO

Submarine groundwater discharge (SGD) is now recognized to be globally significant, with a flux of fresh and re-circulated marine water reaching 80-160% of the river discharge. Carbonate coastlines are of particular interest, with some karst coastal springs in the Bahamas and the Yucatan each discharging 10⁸-10¹⁰ m³/year, in addition to distributed diffuse discharge. The adsorption and desorption of phosphate within carbonate substrate is a chemical equilibrium between dissolved orthophosphate (PO₄^3-) and iron hydroxide or calcium carbonate bound phosphate, mediated by pH and oxygen availability. Laboratory experiments using Key Largo limestone from the Everglades have shown that phosphate is adsorbed onto the carbonate sediment when bathed in fresh water, and desorbed and dissolved out of the carbonate matrix when flushed with marine water (Price et al., 2010). Rising sea levels raise the elevation of the typically oxic fresh-water lens in density stratified coastal aquifers, while also flushing anoxic evolved marine-water through increasing volumes of carbonate rock. We hypothesize the potential for significant discharge of P and Fe from carbonate coastlines such as the Yucatan Peninsula, due to desorption of calcium carbonate bound phosphorus under shifting redox conditions, as well as the release of the iron oxide bound phosphorus by elevated dissolution rates along the fresh-saline density interface. Initial results of bulk rock composition from vertical sample profiles obtained while cave diving will be used to constrain the magnitude of potential P and Fe discharge. The release of geologically sequestered P and Fe from carbonate substrate may represent a previously unappreciated source of limiting nutrients along low latitude carbonate coastlines flanked by coral reef systems sensitive to eutrophication, and may prove valuable in understanding the growing list of eutrophic sites with no apparent anthropogenic nutrient source (e.g. Belize atolls).