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Boron proxies and biomineralisation: the possible, the impossible and the likely.

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 CO 2 -induced climate change. The boron proxies are based on the theory that carbonates solely incorporate B(OH) 4 in proportion to seawater B(OH) 4 /HCO 3 or B(OH) 4 /CO 3 2, capturing both the state of the ocean C system and the pH-dependent isotopic composition of B(OH)4-. However, models of biomineralisation invoke significant modification of internal carbon chemistry to facilitate calcification, and substantial proton export has been observed during carbonate formation. 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 interpretation and use of the B proxies.