Fig. 1: Calibration.

In all panels, closed black squares refer to N. pachyderma collected from plankton tow samples, open dark grey squares represent N. pachyderma from core tops²⁷, and open turquoise squares represent N. incompta from core tops³⁹. Inverted red triangles refer to N. pachyderma core top data from the South Atlantic and the Antarctic continental shelf⁵⁹, while yellow triangles refer to N. pachyderma sediment trap data from the Antarctic Peninsular²⁴. a shows measured Mg/Ca values. b Shows normalised Mg/Ca values against water-column [\({{{\rm{CO}}}}_{3}^{2-}\)] for tows and δ¹⁸O_c derived [\({{{\rm{CO}}}}_{3}^{2-}\)] values for core tops. The grey shading connotes the 95% confidence intervals associated with the regression. c shows Mg/Ca values corrected for [\({{{\rm{CO}}}}_{3}^{2-}\)] seawater carbonate-ion concentration. The blue shading connotes the 95% confident intervals of fully propagated uncertainties for the slope of the calibration relationship, while the grey shading connotes the fully predicted uncertainties which would apply to the full uncertainty on a single data point. d shows δ¹⁸O_c values for N. pachyderma vs water column [\({{{\rm{CO}}}}_{3}^{2-}\)]. N. pachyderma from plankton tows are corrected for a + 2‰ offset to account for the difference between ontogenetic and crust calcite. The blue shading connotes the 95% confidence intervals for the regression combining both tow and core top data, while the yellow and red shading connotes the 95% confidence intervals for the tows and core top regressions, respectively. In addition, function plots are shown for other unicellular planktonic calcifying organisms^53,54. Source data for all panels are provided as a Source Data file.