Oceanographic Setting
Cariaco Basin is a depression on the northern continental shelf of Venezuela with a total area of approximately 103 km2 (160 km long and 40 km wide, and as such is the world's largest anoxic basin of truly oceanic character (Richards, 1975). The basin is oriented east-west and consists of two main sub-basins, each of about 1400 m water depth, separated by a saddle at about 900 m depth. The basin is bounded to the north by a sill that connects Margarita Island to Cabo Codera, with the maximum sill depth being 140 m. This shallow sill exerts a strong influence on the hydrographic and chemical properties of the Cariaco water column, since only waters above the sill depth are freely exchanged with Caribbean surface waters. Due to this limited exchange, waters below the sill depth are poorly ventilated and our recent sampling indicates that anoxic conditions exist below approximately 250-275 m.  This depth for the oxic/anoxic interface is shallower than previously reported (300-350 m) and suggests that this boundary has shoaled in recent years.