Dissolved carbon in pore waters from the carbonate barrier reef of Tahiti (French Polynesia) and its basalt basement

Abstract

This paper deals with dissolved inorganic carbon (DIC) and organic carbon (DOC) in pore waters from a 150 m deep hole drilled through the carbonate barrier reef of Tahiti and its underlying basalt basement. Alkalinity-pH measurements were used to calculate the DIC species concentration, and DOC was analysed according to the high temperature catalytic oxidation technique. Salinity was used as a conservative tracer to help identify water origin and mixing within the hole. Water mixing, calcium carbonate dissolution and mineralization of organic carbon combined to form three distinct groups of pore water. In the deeper basalt layers, pore water with alkalinity of 1.4 meq kg−1 pH of 7.6 and p(CO2) of 1.2 mAtm was undersaturated with respect to both aragonite and calcite. In the intermediate carbonate layer, pore water with alkalinity of more than 2.0 meq kg−1, pH of 7.70 and p(CO2) of 1.4 mAtm was supersaturated with respect to both aragonite and calcite. The transition zone between those two groups extended between 80 and 100 m depth. The shift from aragonite undersaturation to supersaturation was mainly attributed to the mixing of undersaturated pore waters from the basalt basement with supersaturated pore waters from the overlaying limestone. In the top of the reef, inputs from a brackish water lens further increased p(CO2) up to 5.6 times the atmospheric P(CO2).