A BIOGEOCHEMICAL SURVEY OF THE ANOXIC BASIN GOLFO-DULCE, COSTA-RICA

The waters and surface sediments of a 200 m deep tropical fjord were i nvestigated with respect to chemical zonation, rates of respiratory pr ocesses, and benthic fluxes. Oxygen was found to ca. 100 m water depth , but only the upper part of the oxycline was associated with the pycn ocline, which was situated above the sill located at 60 m. Nitrate-ric h water entered the bay at sill depth, and denitrification was indicat ed at anoxic depths. Hydrogen sulfide was only found in low concentrat ions near the bottom, and nitrate was a possible oxidant for the hydro gen sulfide. The observations reproduced those of a survey in 1969 to a large extent. This suggests that water exchange is frequent enough t o prevent the development of strongly reducing conditions in the botto m water. High rates of dark oxygen uptake indicated intense carbon cyc ling within the euphotic zone. The rates decreased rapidly with depth, and at anoxic depths, rates of denitrification and sulfate reduction were more than 100-fold lower than the surface oxygen uptake. The chem ical zonation as well as x-radiographs indicated that sediments underl ying oxic bottom water were strongly irrigated and bioturbated while a noxic sediments were laminated and composed of turbidites. The rates o f carbon oxidation were 5 - 10 times higher in the sediments underlyin g oxic bottom water than in those at the bottom of the basin. Sulfate reduction was a dominating process, accounting for about 50% and 100% of the carbon oxidation at the oxic and anoxic sites respectively. Eve n at the anoxic sites, hydrogen sulfide did not accumulate in the pore water and a high sedimentation of reactive iron phases is suggested a s a contributing cause.