Grain size analysis. Grain size measurements were performed using image analysis with the freely available software Fiji on cross-section images of vaterite specimens at different densities. A detailed procedure and examples are given in the Supplementary Information.Strain rate modelling. The calculation of the theoretical strain rate of the vaterite samples during compaction was performed using the established pressure solution creep model outlined by Zhang et al.15 A detailed procedure and Matlab scripts are given in the Supplementary Information.