Carbonate minerals formed by marine organisms play a central role in the global carbon cycle and record an invaluable archive of ancient climate in their trace element and isotopic composition. These shells are formed from dissolved chemicals in seawater via a set of biological processes collectively known as ‘biomineralisation mechanisms’. There are considerable uncertainties surrounding the precise nature of these mechanisms, which have two main consequences: (1) it is difficult to predict the response of calcifying organisms to future climate change and ocean acidification, which is a source of uncertainty in future climate predictions, and; (2) biomineralisation modulates trace element and isotope incorporation into the mineral phase, and introduces substantial uncertainties into the climate archives derived from them.

Most studies of biomineralisation consider the biological and mineralogical processes that transfer ions from seawater to the site of mineral growth, and reduce the problem to consider idealised organism growing in seawater with an average ‘bulk’ composition. This neglects the modification of the surrounding environment by the act of biomineralisation. Growing calcium carbonate from seawater creates distinct boundary layers around calcifying organisms, distancing the environment of calcification from bulk seawater chemistry. The dynamics of this environment, and how it will change in response to changes in the bulk physical and chemical environment is a major area of uncertainty in our understanding of biomineralisation in future oceans, and in the recording of climate archives.