The origins and fate of dissolved organic carbon in a density-stratified carbonate aquifer on a tropical coastal landscape

Flooded caves within carbonate coastlines serve as important conduits for carbon transport and transformation prior to groundwater expulsion into the sea. To investigate the sources, magnitude and biogeochemical reactions regulating carbon sources within an unconfined coastal aquifer, we analyzed the concentrations and δ¹³C values of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) at four sites along a flow path, a 6-km shoreline-perpendicular transect of flooded coastal caves in the northeastern Yucatan Peninsula, Mexico. The study revealed significantly higher DOC concentrations after regional rainfall compared to a mid-summer drought, suggesting precipitation as a key driver of the downward DOC flux from the surface. The decomposition of organic matter in the saturated soils of mangroves and tropical forest is the source of high DOC (on average, 678 µM; δ¹³C-DOC = −28‰) in the shallow fresh groundwater. The regional seaward tendency of DOC in the upper aquifer to become more ¹³C-enriched is primarily driven by increased mixing near the coast with saline groundwater, which is lower in concentration (70 µM, on average) and more ¹³C-depleted (δ¹³C-DOC = −26‰) than the seawater DOC (158 µM; −19‰). Evidence for net DOC consumption, along with positive and negative changes in δ¹³C-DOC values, is consistent with microbe-mediated transformation of organic matter primarily occurring in the upper aquifer’s low salinity waters. Diminishing DOC, coinciding with 4- to 5-fold increase in DIC concentrations while δ¹³C-DIC becomes more positive, implies that organic matter diagenesis also enhances carbonate dissolution. These landscape-level observations reveal hydrologic and biogeochemical factors that regulate the internal functioning of a coastal aquifer ecosystem and influence the quality and quantity of carbon exported to the coastal sea, where the Mesoamerican Barrier Reef resides.