Deciphering Climate Archives: Establishing the Inorganic Basis of an Isotope Fractionation Phenomenon in Biogenic CaCO3

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Under this award the PIs will investigate the inorganic basis for understanding the link between the ocean's CO2 chemistry and 13C/18O fractionation in biogenic carbonates. This may improve paleoclimate reconstructions and understanding of long-term climate changes. A large body of climate reconstructions is based on fossil remains of once living organisms. In the marine environment, one of the most important organisms employed for these reconstructions on time scales ranging from decades to hundreds of million of years are foraminifera and corals. In the CaCO3 secreted by several of these organisms, a seemingly universal co-fractionation of stable carbon (a13C) and oxygen (a18O) isotopes has been discovered. In foraminifera, the a18O-effect appears to be driven by the seawater pH which plays a central role in a current, intense debate on CO2 and Phanerozoic climate change. The universality of the fractionation effect strongly suggests a common mechanism, likely of physico-chemical nature, which is hitherto unknown. Inorganic precipitation experiments on calcite seed crystals will be conducted, first in simple CaCl2 solutions and subsequently in seawater. The carbonate chemistry of the solutions will be varied to cover a wide range of pH values. The isotopic composition of the calcite crystals (d13C, d18O, and d48Ca) are then measured mass-spectrometrically. Applying a technique of seeded crystallization combined with 13C, 18O, and 48Ca isotopes spiking will allow very precise mass balance calculations and hence a very precise determination of the C- and O-isotope fractionation between water and calcite overgrowth. The proposed work will establish whether the co-fractionation is a thermodynamic phenomenon solely controlled by the solution's carbonate chemistry or whether it is a biological effect. Development of a new model for the calculation of the carbonate chemistry and stable isotope fractionation will accompany the experimental work in order to obtain an original and complete picture of the fractionation process. The Broader Impacts include education of undergraduate students and graduate students in areas such as laboratory techniques, theoretical modeling, and climate/paleoclimate research.