Biomineralisation and boron proxies: how do shells record archives of seawater carbon chemistry?

Carbonate biominerals play a key role in the ocean carbon cycle, and preserve vital archives of past climate change in their geochemistry. The abundance and isotopic content of boron in carbonate biominerals provide our best records of ocean carbon chemistry and pH, which have proved instrumental in studying past episodes of CO2-induced climate change [1]. The boron proxies are based on the theory that carbonates solely incorporate B(OH)4- in proportion to seawater B(OH)4-/HCO3- or B(OH)4-/CO32-, capturing both the state of the ocean C system and the pH-dependent isotopic composition of B(OH)4-. However, significant modification of internal carbon chemistry is required to facilitate calcification, and substantial proton export has been observed during carbonate formation [2]. The pH, carbon and boron chemistry at the site of calcification cannot be the same as that of external seawater. How, then, do biominerals appear to record seawater B(OH)4-? While unanswered, this question raises serious problems for our use and interpretation of the B proxies.I will explore this question using a quantitative model of B transport processes during biomineralisation. Three key fluxes dominate biomineral formation: CaCO3 precipitation, the exchange of seawater with the external environment, and ion transport across membranes by diffusion or active pumping [3]. By reducing the problem to the balance between these three key fluxes, it is possible to explore a wide range of biomineralisation scenarios with minimally restrictive assumptions. Including both the transport of B(OH)4–, and the transport and passive diffusion of membrane-permeable B(OH)3 within this framework captures the full range of potential biomineralisation scenarios and B transport processes. Sets of B geochemical data from biominerals grown in known conditions then provide crucial constraints that reveal: (1) A mechanism that allows biomineral boron to be sensitive to seawater pH and carbon chemistry, despite significant differences in chemistry and pH at the site of calcification, and (2) the ion transport dynamics of the calcification environment (e.g. ‘closed’ vs. ‘open’ or Rayleigh- vs. transport-dominated system). Together, this adds confidence to the use of the B palaeo-proxies in all carbonate biominerals, and provides a new lens through which B geochemistry can be used to understand biomineralisation mechanics, which is particularly relevant to the resilience of coral calcification to changing ocean carbon chemistry [4].