Pre-treatment effects on coral skeletal δ13C and δ18O

Pre-treatments are often used to remove organic “contaminant” material prior to isotopic analyses of coral skeletal samples. Here we conducted three experiments to test the pre-treatment effect of water, 30% hydrogen peroxide (H2O2), and household bleach [5.25% sodium hypochlorite (NaClO3) and 0.15% sodium hydroxide (NaOH)], on the stable isotopic composition of coral skeletal samples. First, using a mass balance approach we calculated the expected change in skeletal δ13C due to the removal of all organic carbon. The model showed that (1) the removal of organic carbon (which has a low δ13C value relative to skeletal δ13C) from the skeletal sample should theoretically result in a higher δ13C value of the remaining organic-carbon-free carbonate, and that (2) only at the highest concentrations of skeletal organic carbon within the tissue layer of corals is the contribution of the organic carbon to the overall δ13C skeletal value potentially large enough to be detectable by mass spectrometry. We then conducted two sets of experiments to test the model where we pre-treated a large number of skeletal samples from five species of corals with water, H2O2, bleach, or no pre-treatment for 24 h. Skeletal δ13C generally decreased significantly with water, bleach, and H2O2 pre-treatments which is contrary to the model-predicted increase in δ13C following such pre-treatments. Thus, organic carbon within the skeleton is not a net source of contamination to δ13C analyses. Skeletal δ18O decreased the most with water and bleach pre-treatments. In addition, the effect of H2O2 or bleach pre-treatments on either δ13C or δ18O was not consistent among species or locations. The direction of change in δ13C and δ18O with pre-treatments was no different for skeletal samples taken within or below the tissue layer. Based on our results, we suggest that pre-treatment is not necessary and recommend that pre-treatment not be performed on coral skeletal samples prior to stable isotope analysis to avoid any pre-treatment-induced variability that could significantly compromise inter-colony and inter-species comparisons.