ELSEVIER Palaeogeography, Palaeoclimatology, Palaeoecology 154 (1999) 293–299 Calcium carbonate dissolution rates in deep-sea bivalve shells on the East Pacific Rise at 21ºN: results of an 8-year in-situ experiment Michael J. Kennish L , Richard A. Lutz Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08903, USA Received 3 September 1997; revised version received 4 April 1999; accepted 16 April 1999 Abstract 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 20º50 0 N, 109º06 0 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 D 2615 m) were continuously exposed to ambient seawater (¾2ºC) 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 D 22.2 μm=yr) and the irregular prismatic shell layer the highest rates (mean D 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 21ºN, 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 ¾300 years, which has important implications for determining the longevity of hydrothermal vent activity along the rise axis.  1999 Elsevier Science B.V. All rights reserved. Keywords: deep-sea bivalves; calcium carbonate; shell dissolution rates; death assemblages 1. Introduction Although the general distribution of calcium car- bonate in deep-sea sediments has been known for more than a century (Murray and Renard, 1891; Murray and Hjort, 1912; Revelle, 1944; Bramlette, 1961; Berger, 1976; Arrhenius, 1988; Morse and Mackenzie, 1990; Kennish and Lutz, 1992; Millero, L Corresponding author. Tel.: C1-908-932-6555; Fax: C1-908- 932-8578; E-mail: kennish@ahab.rutgers.edu 1996), many questions remain unanswered with re- gard to the formation, deposition, and fate of min- eralized carbonates in the world’s oceans. The life history and ecology of most of the carbonate-secret- ing pelagic organisms inhabiting the surface waters of the open oceans are largely unknown. In ad- dition, the dissolution kinetics of biogenic calcite and aragonite, which greatly influence carbonate de- position in the deep ocean, are not unequivocally established (Morse and Berner, 1979; Morse and Mackenzie, 1990). Clearly, more empirical data are 0031-0182/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII:S0031-0182(99)00102-9