CHARACTERIZING HYDROSTRATIGRAPHY OF A TROPICAL FRINGING REEF, MO'OREA, FRENCH POLYNESIA

Submarine groundwater discharge, often abbreviated SGD, is thought to be an important geochemical and hydrologic force in tropical coral reefs that influences terrestrial fluxes into the system. Sub-reef stratigraphy serves an important role in this process, as different layers may accelerate or inhibit groundwater flow based on factors like conductivity. Previous research has suggested SGD travels through a submarine aquifer below the reef, and is inhibited from moving vertically due to a thick carbonate confining layer known as the reef flat plate (often more than 1 m thick). The reef flat plate serves as an important regulator in the hydrogeological system of tropical coral reefs, as it is thought to have a major control on groundwater flow and channeling. The suspected origin of this feature has been previously contested in research, with hypotheses ranging from consequences of sea level fluctuations, to super saturation of calcium carbonate in the water column, which is the most accepted model. This research aims not only to classify sub-reef stratigraphy, but also to quantify the makeup and speculate on the origins of the reef flat plate using petrographic methods. Samples were collected of the reef flat in Mo’orea, French Polynesia in order to provide direct insight into formation and composition by implementing the use of thin sections and XRD. Thin sections provide evidence on cementation, diagenetic environmental effects, and potential biologic activity, all which may contribute to reef flat plate formation and development. Future research will include in-depth classification of the samples and how they fit into reef stratigraphy, as well as potential XRD analysis. Classifying samples is not only important to understanding the petrology and formation of the reef flat plate, but also to characterizing the general hydrologic dynamics of the entire reef system. SGD and its movement plays an important role in overall reef sustainability, as it is hypothesized transport of terrestrial alkalinity may safeguard against increasing ocean acidification.