Testing how boron-based proxies work using a general geochemical model of coral calcification

During calcification, corals choreograph the flow of ions, manipulate aqueous speciation, and modulate surface chemistry. Due to differences in the balance between these processes, different species of coral exhibit distinct paleoproxy calibrations and divergent sensitivities to environmental change. Despite this complexity, internally-consistent, reproducible geochemical patterns are found across coral species, and several trace element and isotope systems act as useful environmental proxies. A mechanistic understanding of biomineralization could resolve this apparent contradiction, indicate what ultimately governs paleoproxies, and better predict the sensitivity of coral to environmental change. Specifically, we need a framework that can integrate disparate geochemical data with physiological measurements into a unified geochemical model of coral calcification.

Using a modeling approach, we show that a reduced set of dynamical parameters describes the geochemistry of coral calcification. These parameters represent the balance between specific ion pumping, seawater transport, and cross-membrane diffusion. This framework allows us to invert observed patterns in skeletal geochemistry into a small set of parameters that completely describe biomineralization, and the enigmatic 'vital effect'. Because the modeling framework is general, we can explore the wide range of biomineralization strategies represented in the literature in an unbiased way. Using this framework we address fundamental questions in coral geochemistry:

- What is the balance between environmental and physiological control for the δ11B paleo pH proxy?

- Under what conditions do coupled geochemical measurements, like δ11B and B/Ca, constrain calcifying fluid pH and chemistry? What are the theoretical limits on the accuracy of these constraints?

- To what extent does calcifying fluid chemistry change with changes to the surrounding seawater? What does this relationship mean for the adaptive capacity of coral?