Flux of Suspended Calcium Carbonate (CaCO3), Fanning Island Lagoon

Abstract

A plume of turbid, CaCO3-laden water (0.24 mg/liter) is expelled from English Harbor, Fanning Atoll, on outgoing tides. On incoming tides, the concentration is 0.36 mg CaCO3/liter. At the two other passes of the atoll, incoming CaCO3 concentrations also are higher than outgoing concentrations. Lagoon waters contain 1 mg CaCO3/liter in the clear central portion of the lagoon and 4 mg CaCO3/liter elsewhere. Offshore concentrations out of the plume area are 0.03 mg CaCO3/liter. The lagoon and plume CaCO3 material is reef-derived detritus (aragonite and high-Mg calcite). Offshore CaCO3 particles are primarily coccoliths (low-Mg calcite). During a 24-hour survey 10 tons of suspended CaCO3 were transported into the lagoon. It is likely that the English Harbor plume represents little or no sediment loss from the lagoon. The plume debris is interpreted to be material produced on the outside fringing reefs, sucked into the lagoon on incoming tides, and subsequently expelled. Production of CaCO3 in the lagoon may be filling the lagoon faster than sea level is rising.