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

Analysis of shell fragments of two common deep-sea hydrothermal vent bivalv es, the vesicomyid clam Calyptogena magnifica and the mytilid Bathymodiolus thermophilus, deployed more than 100 m from any active hydrothermal vent l ocation at 20 degrees 50'N, 109 degrees 06'W on the East Pacific Rise indic ates significant variation of calcium carbonate dissolution in in-situ expo sures of more than 8 years. Shell fragments embedded in epoxy blocks, mount ed on a buoyed and anchored polypropylene line, and deployed by DSV Alvin ( depth = 2615 m) were continuously exposed to ambient seawater (similar to 2 degrees C) conditions immediately above the seafloor. The mean dissolution rate for the aragonitic shell of C. magnifica amounted to 27.7 mu m/yr. Di ssolution rates varied significantly among different shell layers of the da m, with the middle fine to irregular complex crossed lamellar shell layer e xhibiting the lowest fates (mean = 22.2 mu m/yr) and the irregular prismati c shell layer the highest rates (mean = 36.9 mu m/yr). In the mytilid B. th ermophilus, the dissolution rate of the aragonitic shell layer averaged 41. 6 mu m/yr, while that of the calcitic shell layer was immeasurable. The rat es of calcium carbonate dissolution reported here for a ridge-crest site re mote from any active hydrothermal vent are much lower than those previously documented for active vent sites at 21 degrees N, the Guaymas Basin (South ern Trough), and Galapagos Rift (Rose Garden). Assuming a constant rate of dissolution, we estimate that empty adult shell valves of C. magnifica at t he experimental site would dissolve completely in a period of similar to 30 0 years, which has important implications for determining the longevity of hydrothermal vent activity along the rise axis. (C) 1999 Elsevier Science B .V. All rights reserved.