Calcium carbonate dissolution rates in deep-sea bivalve shells on the East Pacific Rise at 21 degree N: results of an 8-year in-situ experiment

Analysis of shell fragments of two common deep-sea hydrothermal vent bivalves, the vesicomyid clam Calyptogena magnifica and the mytilid Bathymodiolus thermophilus, deployed more than 100 m from any active hydrothermal vent location at 20 degree 50'N, 109 degree 06'W on the East Pacific Rise indicates significant variation of calcium carbonate dissolution in in-situ exposures of more than 8 years. Shell fragments embedded in epoxy blocks, mounted on a buoyed and anchored polypropylene line, and deployed by DSV Alvin (depth = 2615 m) were continuously exposed to ambient seawater ( similar to 2 degree C) conditions immediately above the seafloor. The mean dissolution rate for the aragonitic shell of C. magnifica amounted to 27.7 mu m/yr. Dissolution rates varied significantly among different shell layers of the clam, with the middle fine to irregular complex crossed lamellar shell layer exhibiting the lowest rates (mean = 22.2 mu m/yr) and the irregular prismatic shell layer the highest rates (mean = 36.9 mu m/yr). In the mytilid B. thermophilus, the dissolution rate of the aragonitic shell layer averaged 41.6 mu m /yr, while that of the calcitic shell layer was immeasurable. The rates of calcium carbonate dissolution reported here for a ridge-crest site remote from any active hydrothermal vent are much lower than those previously documented for active vent sites at 21 degree N, the Guaymas Basin (Southern Trough), and Galapagos Rift (Rose Garden). Assuming a constant rate of dissolution, we estimate that empty adult shell valves of C. magnifica at the experimental site would dissolve completely in a period of similar to 300 years, which has important implications for determining the longevity of hydrothermal vent activity along the rise axis.