Figure 3.

Relative changes in ocean pH derived from planktonic foraminiferal boron isotope (δ¹¹B) measurements (a). Surface dweller M. velascoensis was used to generate the 1209 (black circles), 1263 (red crosses), 865 (green triangles) [10] and M. subbotinae 401 (orange squares) [12] δ¹¹B records (a). Deep dweller Subbotina spp. were used in δ¹¹B reconstructions at Bass River (blue diamonds) and Millville (purple diamonds) (a). All records were normalized to a pre-event ocean pH = 7.75 by adjusting the intercept of the assumed δ¹¹B_foram to calibration following a similar approach to previous studies [10,12] (b,c). Ocean pH anomalies (ΔpH) were estimated by computing a relative δ¹¹B change compared with an average of pre-CIE values (c). A constant seawater δ¹¹B value of 38.9 ± 0.4‰ for the duration of the PETM was assumed [12] and no vital effect was considered to estimate ΔpH (c). Age is denoted in kiloyears relative to the CIE. Error envelopes on ocean pH estimates include the analytical uncertainty of 2 s.e. of repeat sample analyses (n = 2–3), and conservative uncertainty estimates for temperature (±2°C) and salinity (±2 units) (b). (Online version in colour.)