MINERALOGY AND SEASONAL GROWTH OF SOUTH PACIFIC MUSSEL VALVES

Two species of South Pacific mussels, Aulacomya ater and Choromytilus chorus, were collected off the coasts of Chile and Peru in July of 2006. Little sclerochronological analysis has been done regarding A. ater and C. chorus, even though they are among the most common mussels in the region. A major concern in employing these species as a paleoclimate and paleoupwelling proxy for the South Pacific is to define the mineralogy, assess seasonality of shell growth, and define the periodicity of the shell micro-structure. This fundamental research will facilitate later paleoclimate research projects in the South Pacific, and most directly, a project already underway involving the use of radiocarbon sampling to study upwelling on the coast of Peru from the Pleistocene to modern.

A. ater and C. chorus valves were analyzed via x-ray diffraction (XRD) and Raman spectroscopy to determine the mineralogy of the mussels’ valves. C. chorus is mostly calcite with aragonite in some zones, while A. ater is mostly aragonite with less calcite. An abrupt and continuous boundary between the calcite layer and the aragonite layer of C. chorus can be seen in cross sections along the longest growth axis, while the boundary between the calcite layer and the aragonite layer of A. ater is not as distinct, and thus may render the species problematic for oxygen isotope analysis.

Four specimens of C. chorus valves were sampled at high resolution along ontogeny for oxygen isotopes. By comparing the oxygen isotope profile to regional temperature records and local water δ¹⁸O, an approximate lifespan of between one and two years of age has been obtained for each of the four specimens of C. chorus analyzed. Analyses of the oxygen isotope profiles of C. chorus suggest nearly continuous shell growth throughout the year for the smaller specimens, with a marked decrease or cessation in shell growth for two larger specimens analyzed.