COASTAL CARBONATE CRITICAL ZONES: AN EXAMPLE FROM THE YUCATAN PENINSULA (Invited Presentation)

The critical zone is the region of Earth’s terrestrial surface that extends from the top of the canopy to the base of groundwater. It provides important ecosystem and human life sustaining resources, and yet carbonate terrains have few dedicated study locations (critical zone observatories) and coastal systems have not previously been considered from a critical zone perspective. Coastal carbonate critical zones are impacted by hydrologic connections through conduit systems that drain to the coast. With accelerating sea level rise, these high permeability connections could enhance salinization of inland water supplies and alter nutrient delivery to coastal waters. The eastern Yucatan Peninsula, where precipitation is efficiently drained through conduits to submarine springs in coastal lagoons, provides an ideal region to study these processes. High resolution logging data (15 minute frequency) over a 12 day period showed that the springs reverse flow direction when lagoon water levels increased to >0.08 m above the average lagoon elevation during the sampling period. During reversals, oxygenated saline lagoon water mixes with discharging anoxic brackish water. Spectroscopic analyses of water samples collected from the springs reveal that marine water introduces a reactive component of organic matter into the mixing zone. Remineralization of the organic matter elevates pCO₂, which drives carbonate mineral dissolution, as well as increases NH₄ and PO₄ concentrations over values expected from conservative mixing of inland fresh water and lagoon water. Because P adsorbs to carbonate mineral surfaces, carbonate mineral dissolution should contribute P in addition to that derived from organic matter remineralization. However, N/P ratios of the mixed water are greater than the Redfield ratio, with no clear mechanism for N loss, which suggests carbonate mineral adsorption causes net P sequestration, even during carbonate mineral dissolution. The coupled reactions of organic mineral remineralization, carbonate dissolution, and P adsorption could drive P-limitation in the lagoon. These results reflect just some important critical zone processes, and their links to environmental change, that occur in coastal carbonate systems.