BOCA CHICA: A GYPSUM-PRECIPITATING ANCHIALINE POOL IN MARGARITA ISLAND, VENEZUELA

Boca Chica is a landlocked, km-wide, permanent water body that receives infiltrating seawater, is consistently saturated with respect to gypsum in its bottom waters, and occasionally reaches halite saturation. Boca Chica probably is an extension of the adjacent Cariaco Trough because subsidence keeps up with both clastic and chemical sedimentation. In dry 1987, the surface of Boca Chica lay at -4.2 m, whereas in wetter 2000 it lay at -2.3 m. Gypsum plus organic matter underlay the central portion of Boca Chica in both years, with a few crystals of halite apparent in 1987. Besides infiltrated seawater (distance of 186 m), Boca Chica receives storm runoff from adjacent hills (700 m high), rain water, and groundwater. In 2000, Boca Chica consisted of two water masses, each roughly 2 m thick, separated by a half-meter-thick mixing zone. The upper mass was about three times seawater salinity (100 to 118 ppt, down to 1.7 m depth). This mass extended to the bottom of Boca Chica around its shallow perimeter. From 1.7 to 2.2 m, the salinity increased from 118 ppt to 226 ppt whereas from 3 to 4 m depth, it ranged from about 230 to 240 ppt. Deeper than 4 m, salinity reached 244 ppt. The temperature of the upper water mass was that of ambient air, indicating wave mixing down to 1.7 m, whereas the lower mass graded downward to more than 15 ^o C hotter (reaching 46.5 ^o C). Overturning of this hot deep water occasionally kills swimmers. Boca Chica is an ideal laboratory to study salt deposition but is threatened by dredging to construct a tourist resort.