Carbon and phosphorus processing in a carbonate karst aquifer and delivery to the coastal ocean
In siliciclastic systems, submarine groundwater discharge (SGD) provides a fraction of freshwater and nutrients delivered to coastal waters, but in many carbonate karst terrains SGD represents the predominant source of terrestrial water and solutes. Solute concentrations may be strongly modified by reactions in subterranean estuaries, altering chemical fluxes via SGD. In carbonate settings, feedbacks between organic carbon remineralization and calcium carbonate mineral (CaCO3) dissolution and precipitation may alter carbon dioxide (CO2) and phosphorus (P) concentrations and fluxes associated with SGD. To assess these effects, we sampled water from multiple submarine springs along the east coast of the Yucatan peninsula, as well as inland cenotes, and a coastal groundwater well. We measured ammonium (NH4), phosphate (PO4), major element, dissolved inorganic and organic carbon concentrations (DIC and DOC), fluorescent characteristics of colored dissolved organic matter (CDOM) and modeled calcite saturation indices (SIcal) and the partial pressure of dissolved CO2 (PCO2). These data indicate that reactions along a hypothetical flow path to the coast control the composition of terrestrial fresh water entering the subterranean estuary. Non-conservative mixing between brackish groundwater and lagoon water show that CaCO3 dissolution and CO2 production from organic matter remineralization occur within the subterranean estuary. Although both organic matter remineralization and carbonate dissolution should liberate P, molar N:P ratios in spring discharge are higher than the Redfield Ratio of 16:1, suggesting net sequestration of remineralized P through sorption to carbonate minerals within the STE. This result indicates that SGD, the primary source of water and nutrient to this coastal zone, results in P limitation in coastal water and is a source of CO2 despite the buffering effects CaCO3 dissolution. This result also emphasizes the importance of biogeochemical reactions within subterranean estuaries for estimates of local and global impacts of SGD composition on solute delivery to the oceans.