H2O2 treatment of recent coral aragonite: oxygen and carbon isotopic implications

Coral skeletons contain relatively little organic material and inorganically precipitated carbonate cement, although these quantities are enough to bias geochemical measurements. Preliminary treatment is therefore often necessary to remove these materials. Hydrogen peroxide seems to be the most efficient reagent. This strong oxidant destroys organic matter and its acid pH promotes dissolution of carbonate cements that precipitate after the coral skeleton has formed. We tested the isotopic effects (both on 18O/16O and 13C/12C ratios) of such a chemical treatment on coral aragonite. We demonstrate that only one treatment is necessary to eliminate different contents without alteration of coral aragonite and we verified that there is no isotopic exchange between the oxygen atoms from CaC03 and H2O2. We measured carbon and oxygen isotopic compositions from samples that cover 2.5 years of coral growth and that were split in two ways: untreated and H2O2-treated. The H2O2-treated subsamples reveal a clear seasonal oscillation that is not evident in the untreated isotopic record. Therefore, hydrogen peroxide treatment should be systematically carried out before isotopic analyses in order to perform palaeoclimatic reconstructions.