Date of Award
Master of Science in Oceanography
Marine Geology and Geophysics
Michael A. Arthur
Mollusc shells contain a potentially detailed record of both the life history and the environmental changes experienced by the mollusc during growth, and have been used to interpret past environmental conditions based on chemical analyses of the shell. Most of these previous studies, however, did not include detailed sampling of local environmental conditions during the time of shell growth to provide an important environmental database for direct comparison to the shell record. Thus, most geochemical interpretations of shell records presume reflection of environmental conditions, without rigorous testing of the accuracy of the relationship. The comparison of environmental conditions to the shell record is especially important if mollusc shells are to be used as reliable paleoenvironmental monitors.
This master's thesis examines the stable carbon and oxygen isotopic signals from the shells of Mytilus edulis, Pitar morrhuana, and Nucula annulata grown in the Marine Ecosystems Research Laboratory mesocosm tanks at the Graduate School of Oceanography and M. mercenaria collected from Narragansett Bay, Rhode Island. The shells of Pitar morrhuana, Mytilus edulis and M. mercenaria were incrementally sampled in order to obtain detailed isotopic profiles of the shell records. In this thesis I compare the isotope results from the shells to the observed shell growth patterns and to carbon and oxygen isotopic measurements of the water in Narragansett Bay. In addition, to develop a better understanding of ecological influences on growth rate, we set out to assess the suitability of Pitar morrhuana, Mytilus edulis and M. mercenaria as monitors of environmental conditions in the bay.
The isotopic analyses, in conjunction with the detailed data available from the MERL mesocosms, have enabled us to accurately interpret the �18O and �13C signals in the shells of these bivalves in terms of the seasonal timing of growth for Pitar and Mytilus and the known variations in the chemical and physical properties of the water in which they grew. The data indicate that the shells of both Pitar (aragonite) and Mytilus (calcite-outer layer only) appear to be forming at oxygen isotopic equilibrium with the ambient water and that the 018O profiles from these specimens are primarily controlled by variations in water temperature. The oxygen isotopes also indicate that in Narragansett Bay Mytilus has a longer growing season than Pitar by approximately one month at either end of the growing season.
The � 18O analyses of shell samples of M. mercenaria, in conjunction with the water isotopic measurements, indicate that the shell of this species does not form in isotopic equilibrium with the surrounding sea-water. The measured o18O of the shell aragonite is typically 1.5% depleted relative to the values predicted with the aragonite-water isotopic fractionation equation of Grossman and Ku (1986). Although the isotopic fractionation is a problem for estimating absolute temperatures using the observed shell values, the oxygen isotopic profiles can be interpreted in terms of relative (high-low) seasonal temperature variations. The amplitude and cycles of �18O variations in the outer shell layer enable one to determine the seasonal timing of growth of M. mercenaria living in Narragansett Bay. These observations are verified by the analyses of samples taken from the outer shell edge of specimens collected during different times of the year. The growth history of the specimens, as revealed by the oxygen isotopes, indicates that the longest growing seasons occur during the years of fastest shell growth and it appears that with increasing age the growing season is restricted to warmer temperatures.
The influence of productivity on the � 13C compositions of the Pitar and the Mytilus shell carbonate and possible associations with the carbon cycling in the MERL tanks and Narragansett Bay were also noted. The different eutrophication levels of the mesocosms apparently had a profound affect upon the � 13C composition of the Nucula shells. Nucula specimens from tanks with higher nutrient loadings (8x, 16x, and 32x) had much heavier shell � 13C values than specimens from the normal nutrient level (control) mesocosms. Apparently much of the lighter carbon (C-12) was drawn out of the water column (and stored in the organic matter) due to the higher primary productivity in the tanks with increased nutrient loading.
The � 13C measurements of the both the shell edge and the incremental samples from the M. mercenaria specimens do not exhibit any obvious seasonal trends. The � 13C values obtained are much lighter than predicted, indicating that a portion of the carbon isotopic signal is probably derived from the uptake of isotopically light metabolic carbon during shell formation. This isotopically light metabolic carbon is most likely from the normal respiration occurring within the organism.
Allard, David J., "Stable Isotopic Analyses of Selected Narragansett Bay Molecules" (1988). Open Access Master's Theses. Paper 1038.