HYDROGEOLOGIC CONTROLS ON ORGANIC AND INORGANIC CARBON PROCESSES IN KARST SUBTERRANEAN ESTUARIES

Carbonate mineral dissolution in carbonate karst landscapes is largely driven by the remineralization of organic carbon (OC) to CO₂, and both processes impact landscape-scale carbon budgets. In coastal karst terrains, conduits and fractures transmit saltwater tens of kilometers from the coast inland under freshwater lenses and form a biogeochemically active freshwater-saltwater mixing zone known as a karst subterranean estuary (KSEs). While organic carbon (OC) remineralization drives KSE biogeochemical processes and carbonate mineral dissolution, little is known of how variations in quantity and quality of OC sources affect dissolution in KSEs and more broadly within the carbonate critical zone. To assess OC inputs and carbon processing in KSEs, we evaluate carbonate chemistry, dissolved OC (DOC) quantity, and its quality through spectroscopic characterization of colored dissolved organic matter (CDOM) at two types of KSEs representing hydrogeologic end members: an open, shallow (<5 m below water table) coastal (<1 km from the coast) KSE in the Bahamas with relatively high contributions of surface water runoff, and two semi-enclosed, deep (>20 m below water table) 20-km inland KSEs on the Yucatan peninsula with relatively low contributions of surface water runoff. Relationships between salinity and DOC concentrations differ between KSE type. DOC concentrations decrease with salinity at the open shallow KSE but increase with salinity at the semi-enclosed deep KSEs. At both KSE types, higher DOC concentrations correspond with more recalcitrant and allochthonous CDOM and DOC remineralization drives CO₂ production. However, more carbonate mineral dissolution occurs largely in freshwater in deep semi-enclosed KSEs while less carbonate dissolution occurs predominately in brackish water of the shallow open KSE. These differences are likely due to elevated OC lability, limited atmospheric exchange, and longer water residence times of the deep KSEs and suggest that freshwater lens thickness and contributions from surface water runoff are critical to C processes in KSEs and perhaps within other portions of the carbonate critical zone. Because karst terrains comprise approximately 15% of continental land surface, controls of heterogeneity in KSEs may provide insights into landscape-scale C budgets.