Assessing hydrologic and biogeochemical controls on pore-water dissolved inorganic carbon cycling in a subterranean estuary: A 14C and 13C mass balance approach

Abstract

Submarine groundwater discharge (SGD) studies have focused on nutrient fluxes to coastal zones in many regions of the world. However the role of SGD in carbon cycling and fluxes is less well studied. We traced SGD-derived dissolved inorganic carbon (DIC) through a subterranean estuary using Δ 14C and δ 13C to evaluate how water residence times and carbon sources affected cycling and potential DIC fluxes to coastal waters. Samples were collected from the Indian River Lagoon subterranean and surface estuaries for analysis of DIC concentrations and natural ∆ 14C-DIC and δ 13C-DIC abundances. Pore-water samples were collected from multiple depths at three piezometer stations along a 250-m long transect perpendicular to shore and extending across the seepage face (about 20–22 m from shore). This approach captured the gradient from rapid, fresh groundwater discharge at the shoreline, across the groundwater–seawater subterranean mixing zone, and into purely marine offshore sediments beyond the seepage face. Freshwater and marine end-member DIC samples were also collected from a beach well and overlying lagoon surface waters beyond the seepage face, respectively. Older ∆ 14C-DIC and more enriched δ 13C-DIC values were observed at the shoreline than at the beach well (∆ 14C: − 431‰ ± 31, δ 13C: − 9.1‰ ± 1.0 versus ∆ 14C: − 108‰, δ 13C: − 16.9‰, respectively), indicating the influence of carbonate dissolution along the groundwater flow path. Pore-water DIC isotopic values indicate the groundwater plume extends from the base of the shoreline profile (115cmbsf) to the distal end of the freshwater seepage face 20–22 m offshore. The application of dual isotope, two- and three-end-member mixing models shows DIC sources change with increasing distance from shore, transitioning from a groundwater dominated system nearshore, to a mixing zone, and then into the seawater recirculation system beyond the mixing zone seepage face. Contributions from organic carbon remineralization increased with distance from shore and had varying effects on carbonate dissolution, depending on flow paths, residence times, and redox conditions. Our study demonstrates the connectivity of unconfined coastal aquifers to surface water estuaries and illustrates how hydrologic and biogeochemical complexities within the subterranean estuary can significantly contribute to the variations in sources of DIC. Furthermore, these findings highlight the importance of qualitative tracing of DIC sources together with an in-depth consideration of in situ biogeochemistry and quantitative SGD estimates. Understanding how spatial changes in hydrology and biogeochemical conditions contribute to the composition and flux of SGD-derived older DIC to coastal waters elucidates the role subterranean estuaries may ultimately play in the global carbon budget.