Calcium carbonate dissolution rates in deep-sea bivalve shells on the East Pacific Rise at 21°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°50′N, 109°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 (∼2°C) conditions immediately above the seafloor. The mean dissolution rate for the aragonitic shell of C. magnifica amounted to 27.7 μm/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 μm/yr) and the irregular prismatic shell layer the highest rates (mean = 36.9 μm/yr). In the mytilid B. thermophilus, the dissolution rate of the aragonitic shell layer averaged 41.6 μm/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°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 ∼300 years, which has important implications for determining the longevity of hydrothermal vent activity along the rise axis.