12:00 PM - 12:15 PM
[BPT07-12] Effects of carbonate chemistry and metabolism induced by ocean acidification on stable isotope fractionation in molluscan shell carbonate
Keywords:ocean acidification, stable isotopes, mollusca, biomineralization, proxy, pH
Stable oxygen isotope compositions of S. broughtonii had significant negative correlations with pH (-0.48‰ /pH, at 17°C; -0.61‰ / pH, at 25 °C). These of P. sachalinense, H. discus discus, and H. gigantea showed non-significant relationships with pH and small variations (within 1 per mil). The oxygen isotope fractionation in four species of our study are smaller than that of synthetic calcite (-1.42‰ / pH, Zeebe et al., 1999).
The significant negative correlations between stable carbon isotope compositions and pH appeared in S. broughtonii, H. discus discus, and H. gigantea which had non-significant pH effects on calcification, and the slopes of these relationships of shell carbonate were lower than these of dissolved inorganic carbon (DIC) of seawater. We estimated the equilibrium values of carbon isotope compositions at each pCO2 treatment, and the difference between the carbon isotope compositions of shell carbonate and equilibrium values showed gradual increases of carbon isotope values with decreasing pH in S. broughtonii, H. discus discus, and H. gigantea. Thus, the pCO2-induced change in metabolism might appear in carbon isotopes of shells of these species as the metabolic effect. On the other hand, in P. sachalinense which showed a decrease in calcification in our culture experiment of ocean acidification, the difference between the carbon isotope compositions of shell carbonate and equilibrium values did not indicate a significant pH dependency. This result might be attributable to differences in metabolic responses to acidified seawater.
The findings of our study will contribute to the correction of isotopic paleotemperature of biogenic carbonate and the understanding of acidification effects on metabolism of marine calcifiers.