Exploring the Potential for Time‐Variable Vital Effects in Planktic Foraminifera, Globigerinoides Ruber

The boron isotopic composition (δ 11 B) of planktic foraminifera is a powerful proxy for reconstructing past seawater pH and thus atmospheric CO 2 . Globigerinoides ruber has been used extensively to generate high resolution δ 11 B‐CO 2 records for the last 3.5 million years. However, these records rely on the assumption of stability in the empirical δ 11 B ruber ‐to‐δ 11 B borate calibration, which was established using culture and core‐top data obtained from modern specimens. It remains unclear whether this modern calibration is applicable on geological timescales. Here, we investigate this in a number of ways. Firstly, we examine the depth distribution of G. ruber during the Plio‐Pleistocene Transition. We then report a suite of geochemical proxies including δ 11 B for G. ruber across a range of test sizes and morphotypes and finally compare G. ruber δ 11 B reconstructions of seawater borate ion, pH and CO 2 to those from Trilobatus trilobus , another surface dwelling planktic foraminifera species. Taking these data together, we find no evidence for a change in G. ruber vital effects or depth habitat during the Plio‐Pleistocene Transition with respect to those of the Holocene. We conclude that the utilization of the present δ 11 B ruber ‐to‐δ 11 B borate calibration is suitable for at least the last 3 million years.