Effect of seawater pH, Mg2+ and carbonic anhydrase on marine biogenic carbonates and their δ18O values : future and paleo applications

Uchikawa, Joji
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[Honolulu] : [University of Hawaii at Manoa], [August 2012]
Ocean pH affects marine CaCO3 cycling and the oxygen isotope (δ18O) values of biogenic CaCO3. Based on a carbon--cycle model and laboratory experiments, this study explores future perspectives of ocean acidification and characterizes the mechanism and paleoclimatic implications of the pH effect on δ18O values of biogenic CaCO3. For a release of 5,000 Pg of carbon in 500 years, the surface ocean pH and atmospheric CO2 will rise to 7.4 and 1,900 ppmv. Eventually anthropogenic CO2 will be sequestered via continental weathering followed by CaCO3 burial in sediments. But these processes will not effectively mitigate the predicted carbon--cycle perturbations on centennial timescales. Studying the impact of CO2--forcing on global temperatures from past climate events is crucial for predicting future global warming. But paleotemperature reconstructions using δ18O values of biogenic CaCO3 can be hampered by past changes in seawater pH. For example, planktonic foraminiferal δ18O values could have been biased by +0.4‰ at maximum (up to 2 °C of underestimated temperature) due to pH decline during the Paleocene--Eocene Thermal Maximum (~56 Ma). A proposed explanation for the pH effect on δ18O values of biogenic CaCO3 [Zeebe, 1999, 2007] assumes 18O equilibrium in the CO2--H2O system in the calcification microenvironments and that equilibrium fractionation between dissolved CO2 species and H2O by Beck et al. [2005] holds in seawater. These assumptions were evaluated by quantitative BaCO3 precipitation experiments focusing on the effect of carbonic anhydrase (CA) and Mg2+ on the kinetics and equilibrium of 18O partitioning in the CO2--H2O system. CA accelerates 18O equilibration in the CO2--H2O system by catalyzing CO2 hydration. Calculations suggest that 18O equilibration within the timescales of calcification is possible with 10--8 to 10--7 M of CA. Mg2+ is the most important cation for complex formation with CO32--in seawater. Although the MgCO30 abundance in the total dissolved CO2 was varied up to 30% in the experiments, the results revealed no discernible influence on equilibrium 18O fractionation in the CO2--H2O system. These outcomes contribute to fundamental understanding of vital effects on δ18O values of biogenic CaCO3.
Ph.D. University of Hawaii at Manoa 2012.
Includes bibliographical references.
ocean pH, Mg2+, carbonic anhydrase
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