We describe a procedure for measuring the thickness and mass of calcite particles that works for most calcite particles<4.5-μm thick. The calcite particles are observed in cross-polarized light, which enables the light transmitted through the calcite particles to be correlated with their thickness. Three polarizing planes are used to minimize the darkening of crystals at some orientations (black cross). This allows direct measurement of the thickness without recourse to a transfer function. This procedure has been used recently to determine the degree of calcification of coccoliths, which provides an indicator of ocean acidification. It takes only a few minutes per sample, and it is an improvement over the former protocol, which did not allow measurement of the thickness and mass of particles thicker than 1.5 μm.

Coccolithophores are single-celled photosynthesizing marine algae, responsible for half of the calcification in the surface ocean, and exert a strong influence on the distribution of carbon among global reservoirs, and thus Earth’s climate. Calcification in the surface ocean decreases the buffering capacity of seawater for CO2, whilst photosynthetic carbon fixation has the opposite effect. Experiments in culture have suggested that coccolithophore calcification decreases under high CO2 concentrations ([CO2(aq)]) constituting a negative feedback. However, the extent to which these results are representative of natural populations, and of the response over more than a few hundred generations is unclear. Here we describe and apply a novel rationale for size-normalizing the mass of the calcite plates produced by the most abundant family of coccolithophores, the Noëlaerhabdaceae. On average, ancient populations subjected to coupled gradual increases in [CO2(aq)] and temperature over a few million generations in a natural environment become relatively more highly calcified, implying a positive climatic feedback. We hypothesize that this is the result of selection manifest in natural populations over millennial timescales, so has necessarily eluded laboratory experiments.