The tropical giant clam Hippopus hippopus shell, a new archive of environmental conditions as revealed by sclerochronological and δ¹⁸O profiles

The use of the sclerochronology and geochemistry of a New Caledonian (South West Pacific) giant clam Hippopus hippopus shell as markers of environmental changes has been investigated. Growth increment thickness and δ¹⁸O ratios were measured on 4 years of shell growth of a modern specimen. During the last year, this giant clam was placed in a tank equipped for high-frequency environmental monitoring. Because shell is secreted in isotopic equilibrium with the seawater, the palaeo-sea surface temperature (SST) equation obtained faithfully reproduces the seasonal SST amplitudes. Growth increment thickness changes are seasonal and, for more than 50%, governed by the SST changes. The transplantation from the in situ site to the tank and reproduction events reduces the strength of growth and SST relationships. Nevertheless, growth increment thickness measurements can give information on average, minimal and maximal past SST in diagenetically altered shells. A peculiar growth event characterized by a short, drastic and significant decrease has been identified and attributed to an intense upwelling event. This study further highlights the use of giant clam shell δ¹⁸O as a SST proxy but also demonstrates that H. hippopus growth increment thickness changes provide useful information on past environmental settings and on exceptional events, for example, intense upwellings.