GEOCHEMICAL EVOLUTION OF GROUNDWATER ALKALINITY ON MOOREA AND TETIAROA (FRENCH POLYNESIA) USING δ13C AND MAJOR IONS

Coral reefs are sensitive to local and global stressors such as nutrient loading, ocean acidification (OA), and rising seawater temperatures. The alteration of seawater due to climate change has continued to degrade reef health globally as a result of increased ocean water acidification. Submarine groundwater discharge (SGD), defined as meteoric water, connate water, recirculated seawater, or a combination of thereof, discharged from coastal sediments to the open ocean, can function as a critical nutrient pathway to coral reefs. Past studies have indicated that SGD-derived alkalinity may counteract OA-related reef degradation, but SGD alkalinity sources have not yet been specifically evaluated. High-level silicate-rock aquifers tend to produce little alkalinity on volcanic islands due to low mineral solubility and short groundwater residence times. Towards the coast, dissolution of secondary calcites and paleo-reef detritus coupled with processes such as dedolomitization may significantly elevate alkalinity yields. To better understand the alkalinity supply from groundwater to coastal reefs, specific controls of (1) aquifer composition and (2) pH/redox gradients on aquifer mineral solubilities were assessed. On Moorea, a volcanic high-island, increased SGD alkalinity yields are expected to be concentrated only where dissolution features through extensive carbonate caprocks exist; initial evidence suggests silicate weathering is the most prominent weathering type occurring. On Tetiaroa, a nearby carbonate atoll, the carbonate aquifers should generate high alkalinity concentrations, but SGD alkalinity yields should be lower than on Moorea, considering the low hydraulic gradient. An alkalinity budget model based on δ¹³C isotopes of dissolved inorganic carbon and concentrations of base anions produced by the dissociation of dominant acids (carbonic and sulfuric) will be produced to determine redox gradients, acid sources, and aquifer material. Examining SGD pathways and terrestrial alkalinity sources and documenting the spatial extent of SGD related pH buffering in the coastal zone provides valuable insights on reef resilience against global stressors.