Extended Data Fig. 5: Boron isotope calibration for modern D. dianthus.

Data are from open ocean sites^44,56,60, with two additional recent (<1,650 years ago) samples from the Southern Ocean from this study. Water column δ¹¹B of borate (B(OH)₄⁻) values are as previously published or are calculated from carbonate chemistry data from nearby GLODAPv2 sites for the new samples, as described^47,54. Note that the sensitivity of δ¹¹B in carbonates to pH is based on the pH sensitivity of δ¹¹B of borate. pH itself is not easily shown on a plot like this, as the relationship between δ¹¹B of borate and pH is also somewhat influenced by water temperature, salinity, and depth⁵⁴. A power law function was fitted to the data using Matlab’s curve fitting toolbox (solid line: δ¹¹B_Coral = −1.82¹⁴ × δ¹¹B_B(OH)4−^−12.22 + 27.03; R² = 0.57). Dashed lines show the 95% confidence intervals and give a measure of calibration uncertainty as shown in the error bar in Extended Data Fig. 1, although data from a given site may be able to record relative changes in pH more sensitively, as seen in many paleo-proxies.