High resolution reconstruction of water temperature estimated from oxygen isotopic ratios of a modern Tridacna shell based on freezing microtome sampling technique

A modern Tridacna (Hippopus hippopus) was collected on September 5, 1993 from a coral reef at Ishigaki Island in Ryukyu Archipelago, Japan, in order to investigate the environmental changes recorded in the shell. We adopted freezing microtome technique for high resolution analysis of the oxygen isotope of the shell. A portion of the inner layer, where daily growth lines are observed under a microscope, was cut out vertically from the inner surface. A stick measuring 20 x 1 x 2 mm was then placed in water and frozen. Using a microtome, 50 μm interval samples were pealed off the stick in a room maintained at -20degC. We determined the date of the formation of each sample by measuring the distance from the inner surface of the shell and by counting the daily growth lines. The water temperatures estimated from the oxygen isotopic values at 1-2 day intervals agreed very well (correlation coefficient: 0.90) with the observed water temperatures which have a seasonal temperature range between about 20degC and 32degC. This implies that the Tridacna precipitates its shell in isotopic equilibrium with the surrounding sea water and that the oxygen isotope values are mainly determined by the water temperature. (author)