A GROUNDWATER CHEMICAL DRILL EXPLAINS CAVES FORMATION ON NIUE ISLAND, SOUTH PACIFIC

The Carribbean karst model indicates that mixing of freshwater with seawater in carbonate coastal areas leads to karst development and cave formations because dissolution of carbonates is significantly enhanced in the mixing zone. Here we extend the Caribbean karst model to the Pacific carbonate islands in order to explain the occurrence of coastal caves there. Our study focuses on the large carbonate island of Niue in the tropical South Pacific that possesses a single, unconfined, freshwater lens with an average thickness of approximately 100 m that floats on top of seawater. Carbonate speciation data based on 18 groundwater wells indicate that phreatic groundwaters are undersaturated for calcite, aragonite and dolomite. The degree of carbonate undersaturation diminishes from the recharge sites in the center of the island to the margins as the groundwaters are buffered by carbonate dissolution in their outflow path. Theoretical mixing lines between the phreatic groundwaters and South Pacific seawater end members show that undersaturation is greatly enhanced by mixing with seawater in the initial phases of mixing (up to 30% seawater). Thereafter the mixtures become saturated to carbonate and loose their corrosive attribute. Hence coastal caves on Niue are likely restricted to sites where initial mixing occurs. The formation of inland caves on Niue is primarily due to the corrosive nature of phreatic groundwater that acts as a chemical drill in the carbonate bedrock as the freshwater lens responds to sea level fluctuations.