Hydrodynamic behavior of caves formed in the fresh-water lens of carbonate islands

Abstract

Flank margin caves are developed in carbonate islands at the edge of the fresh-water lens during sea-level highstands. The development of these caves in the Bahamas was constrained by the short period of glacioeustatic sea-level highstand available during the last interglacial (10,000–15,000 years) and by the small size of the fresh-water lens at the time of the sea-level highstand. The mixing of fresh and saline water, and bacterial oxidation and reduction of organic matter are the chemical processes that drive the origin of the flank margin caves. In the fresh-water lens, the average specific discharge (discharge per unit volume of aquifer) drastically increases at distances less than 100 m from the shoreline, as the lens thins. This area is named theactive edge of the fresh-water lens. The higher specific discharge results in enhanced dissolution, which, coupled with the chemistry of the distal margin of the lens, increases porosity and hydraulic conductivity. As a consequence the thickness of the fresh-water lens decreases with time and the average specific discharge continues to increase at the active edge of the lens. Mapped flank margin caves in the Bahamas show short penetration distances into the eolian ridges containing them (averging 29 m), perpendicular to the paleoshoreline, indicating development within the active edge.