ORGANIC CARBON QUALITY IN SUBMARINE GROUNDWATER DISCHARGE AND IMPLICATIONS FOR MINERAL DISSOLUTION IN A COASTAL CARBONATE AQUIFER

Terrestrial organic carbon (OC) plays a critical role in coastal carbon budgets. While it is often assumed to be recalcitrant, terrestrial OC may become bioavailable following mixing with marine OC in a process referred to as priming. The bioavailability of OC is important in oxidation of OC to CO₂, which contributes to dissolution of carbonate minerals. To evaluate linkages between coastal carbonate dissolution and bioavailability of OC, we sampled water discharging from four springs offshore of the Yucatan Peninsula and measured chemical parameters and dissolved OC quantity and quality. All meteoric water recharging the Yucatan aquifer flows to the ocean as submarine groundwater discharge, and exhibits variable compositions and salinity. We characterized the main constituent of terrestrial OC, colored dissolved organic matter (CDOM), using fluorescence techniques and compared the results between spring discharge and adjacent seawater. Salinity at the vents averaged 20.3±4.7 ppt and was negatively correlated with CDOM abundance. Statistical analyses of the fluorescence data using PARAFAC identified five-components, including two terrestrial humic-like, one microbial humic-like, and two protein-like components. CDOM of spring discharge was mostly terrestrial (59.2±0.8%), with lower abundances of microbial (30.1±0.4%) and protein (10.7±0.6%) components. Four components mixed conservatively between spring discharge and seawater (R²>0.95), suggesting mixing controlled spring CDOM distributions rather than OC processing. Only one protein-like component was non-conservative, suggesting preferential remineralization since proteins are typically bioavailable. This component constituted a small portion of spring discharge CDOM (0.3±0.4%) but was 14% of the seawater CDOM. These results suggest that only a small portion of groundwater OC is bioavailable within the vent system, and thus carbonate mineral dissolution due to groundwater OC processing may be limited. Conservative mixing of CDOM within the vent system implies that groundwater retains most OC prior to discharge, thus vents could represent important point sources of terrestrial OC to the coastal ocean and should be considered in coastal carbon budgets.