Oxygen and carbon isotope records of cultured freshwater pearl mussel <Emphasis Type="Italic">Hyriopsis</Emphasis> sp. shell from Lake Kasumigaura,Japan |
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Authors: | Toshihiro Yoshimura Rei Nakashima Atsushi Suzuki Noriko Tomioka Hodaka Kawahata |
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Institution: | (1) Graduate School of Frontier Sciences, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan;(2) Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan;(3) Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan;(4) National Institute for Environmental Studies (NIES), 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan |
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Abstract: | We present oxygen and carbon isotope ratios and the morphological structure of the cultured freshwater pearl mussel (Hyriopsis sp., Unionidae) shell and pearl. The number of first-order fluctuations of δ18O of the outer shell layer along the maximum growth axis was consistent with the number of cultured years. The dominant factor
controlling annual δ18O fluctuations was water temperature with a minor contribution from the variation in δ18O of ambient water, especially during the rainy season. The δ13C values were approximately constant throughout the life of the mussel, suggesting that the contributions of body size to
δ13C of the shell were minor. We observed nine distinct disturbance rings on the outer surface of the shell. Five rings coincided
with the five winter peaks of the δ18O profile, indicating winter growth cessation below approximately 10°C, probably because of either inactive growth at low
water temperatures or reproduction. Summer disturbance rings were not observed in all years. Moreover, some summer rings showed
discontinuity in the inner structure. These findings suggest that summer growth cessation may be caused by occasional events
such as heavy rains, as the decrease of dissolved oxygen concentration. The δ18O profile and shell structures indicated that shell aragonite was precipitated at close to equilibrium conditions with respect
to the oxygen isotope composition of the ambient water. Hyriopsis sp. shells can potentially be used for reconstruction of past hydrologic conditions. The δ18O of a pearl indicated that calcification occurred over a temperature range of at least 13–23°C. The optimal temperature for
pearl calcification in this species is lower than that for marine pearl calcification. |
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