Background
The authors write - and correctly so - that "current atmospheric CO2 concentrations are unprecedented for at least the past 800,000 years," citing Siegenthaler et al. (2005); and they therefore say "it is crucial to understand the effects of ocean acidification on entire biological communities," and especially so, we would add, in the case of marine calcifiers.

What was done
Cubillos et al., as they describe it, "adapted an existing method to estimate coccolith calcite weight using birefringence (Beaufort, 2005)," which they felt was needed "to suit the large coccoliths of Coccolithus pelagicus," focusing "only on the central area, which is the thickest and most robust part of the coccolith," and applying the result "to fossil and sediment trap material from the South Tasman Rise area of the Southern Ocean, based on three sediment samples each from the Last Glacial Maximum, the Holocene and the Transition between the two periods, as well as modern-day trap samples.

What was learned
"Most strikingly," as the five researchers report, "it appears that modern Coccolithus populations in the Southern Ocean are, on average, more heavily calcified than their fossil counterparts."

What it means
Cubillos et al. remark that their work reveals "substantial non-linearity and independency of variations (plasticity) in coccolith size, shape and volumetric weight between the investigated time intervals," which phenomena they say they hope to study "in more detail" in order to "unravel what environmental conditions are related to intra-specific phenotypic variability in ancient and modern coccolithophores."

References
Beaufort, L. 2005. Weight estimates of coccoliths using the optical properties (birefringence) of calcite. Micropaleontology 51: 289-297.

Siegenthaler, U., Monnin, E., Kawamura, K., Spahni, R., Schwander, J., Stauffer, B., Stocker, T.F., Barnola, J.M. and Fischer, H. 2005. Supporting evidence from the EPICA Dronning Maud Land ice core for atmospheric CO2 changes during the past millennium. Tellus Series B: Chemical and Physical Meteorology 57: 51-57.