Digital SAT Reading and Writing Practice Question #471

In paleoclimatology, reconstructing ancient ocean pH often relies on measuring the boron isotope ratio (\(^{11}\text{B}/^{10}\text{B}\)) found in the fossilized calcium carbonate shells of marine foraminifera. In a purely thermodynamic model, the fractionation of these isotopes from seawater into the shell is strictly dependent on the pH of the water, as the baseline isotopic composition of the ocean remains remarkably constant over millions of years. However, recent analyses indicate that a simple thermodynamic model is insufficient; researchers must also account for species-specific "vital effects"—biological processes intrinsic to the organism that actively alter the internal chemistry of the calcifying fluid, thereby modifying the isotopic fractionation independently of external pH.