Recorders of reef environment histories: stable isotopes in corals, giant clams, and calcareous algae

A 60-year isotopic record from a mid-Holocene fossil giant clam (Tridacna gigas) in the Ryukyu Islands: physiological and paleoclimatic implications

“…The seasonal amplitudes of δ 18 O in T. gigas in the present study are similar to those of a modern Hippopus shell described by Watanabe andOba (1995, 1999), although the measured amplitude depends on the sampling resolution relative to growth rate (1 mm sampling gave a range of 2.0 ‰, in Watanabe and Oba, 1995; 50δm sampling of the same shell gave a range of 2.75‰, in Watanabe and Oba, 1999). The δ 18 O variation reflects variations in both sea-surface temperature (SST) and the δ 18 O of ambient seawater, because Tridacna calcifies essentially in isotopic equilibrium with surrounding seawater (Aharon, 1983;Aharon and Chappell, 1986;Aharon, 1991;Watanabe and Oba, 1999). Aharon (1983) described the relationship between temperature and the oxygen isotopic ratios of T. gigas and water using the following linear equation: Similar relationships have been observed for other aragonite molluscs (Grossman and Ku, 1986).…”

“…We cannot say whether the same applies to our 6000-year-old Tridacna. Koike and Oba, personal comm., Tridacna squamosa; in Watanabe unpublished data), the Great Barrier Reef (Tridacna gigas; in Aharon, 1991) and in a fossil Tridacna shell from a late Quaternary emerged coral reef in New Guinea (Aharon et al, 1980) (Fig. 5) .…”

“…Earlier workers mainly used fossil Tridacna shells and examined the evidence for late Quaternary sea-level and temperature changes, although they generally did not have enough sampling resolution for reconstructing past environmental variations on a seasonal scale (Aharon et al, 1980;Aharon, 1983;Aharon and Chappell, 1986). However, some recent studies have examined the isotopic values of modern specimens in order to calibrate Tridacna shells against prevailing environmental and physiological variables (Jones et al, 1986;Romanek et al, 1987;Romanek and Grossman, 1989;Pätzold et al, 1991;Aharon, 1991;Watanabe and Oba, 1999). In the present study, we have reconstructed a high-resolution stable isotope record for a 14 C-dated fossil shell of the giant clam Tridacna gigas.…”

Unusual Crystal Formation in Organisms - Exceptions that Confirm Biomineralization Rules

“…The corralinacean algae make an important contribution to sediment production and reef limestone formation (Lewin, 1962;Cabioch & Giraud 1986;James et al, 1988), producing over 50% of the calcareous material in coral reefs (Aharon, 1991). In shallow tropical waters, carbonate deposition of calcareous green and red algae can reach levels of 61% of bulk sediments (Wefer, 1980).…”

“…The fact that magnesium calcite is the dominant component is in agreement with statements of numerous literature data (Linck, 1930;Babička, 1936;Vinogradov, 1953;Chave, 1954;Chave & Wheeler, 1965;Walker & Moss, 1984;James at al., 1988;Borowitzka, 1989;Aharon, 1991;Pueschel et al, 1992). Most of the authors found that the magnesium content in Corallinacea ranged from 2 to 30 molar % (M %).…”