CALCIUM-CARBONATE DISSOLUTION RATES IN HYDROTHERMAL VENT FIELDS OF THE GUAYMAS BASIN

Analysis of bivalve shell fragments that were embedded in epoxy blocks , mounted on titanium stakes, and deployed by DSRV Alvin at 5 sites in the Southern Trough of the Guaymas Basin (27 degrees 00'N, 111 degree s 24.55'W; depth 2012 m) indicates significant variation of calcium ca rbonate dissolution in in situ exposures of more than 900 days. Arrays of shell fragments of six bivalve species (i.e., Bathymodiolus thermo philus, Calyptogena magnifica, Calyptogena sp., Corbicula fluminea, cr assostrea virginica and Mytilus edulis) were positioned -17 cm, -7 cm and -2.5 cm below the sediment-water interface and 2.5 cm, 7 cm and 17 cm above the sediment-water interface in hydrothermal vent fields of the basin. Maximum dissolution rates for both calcite (mean = 86 mu m/ yr) and aragonite (mean = 312 mu m/yr) were found in epoxy blocks loca ted at the deepest point sampled in the sediment column (depth = 17 cm ). Minimum dissolution rates of calcite and aragonite were found 7 cm (mean = 26 mu m/yr) and 2.5 cm (mean = 96 mu m/yr) above the sediment- water interface, respectively. Intermediate rates of dissolution were recorded 17 cm above the sediment-water interface (mean = 40 mu m/yr f or calcite and 126 mu m/yr for aragonite). Mean rates of aragonite dis solution ranged from 59 mu m/yr (site 5; clam area) to 227 mu m/yr (si te 3; clam area), and those of calcite dissolution ranged from 13 mu m /yr (site 3; clam area) to 94 mu m/yr (site 4; bacterial mat area). Di ssolution rates were consistently highest in the bacterial mat area (s ite 4; mean = 94 mu m/yr for calcite and 223 mu m/yr for aragonite). R ates of calcium carbonate dissolution reported here for hydrothermal v ent fields of the Guaymas Basin compare favorably with those of Rose G arden (Galapagos Rift) and 21N (East Pacific Rise) hydrothermal vent s ites. These results have important implications for assessing biologic al rate processes in deep-sea hydrothermal vent environments.