Exploring the potential for time-variable vital effects in planktic foraminifera, Globigerinoides ruber

The boron isotopic composition (δ11B) of planktic foraminifera Globingeroides ruber has been used extensivly to reconstruct past seawater pH and thus atmospheric CO2 for the last 3.3 million years. However, it is uncertain if calibrations based on modern individuals are applicable further back in time given the potential for evolving ‘vital effects’. Here we present stable carbon (δ13C), oxygen (δ18O) and δ11B data measured from G. ruber across a range of size fractions from 2.3–3.0 million years ago (Ma) to investigate any potential change in vital effects over the last 3 million years. Additionally, we present paired δ11B G. ruber-Trilobatus trilobus from the 2.3–3.0 Ma interval to investigate if there is any meaningful difference in CO2 estimates between species. We also investigate potential changes in depth habitat through comparison of δ18O and δ13C between G. ruber morphotypes sensu stricto and sensu lato. 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 utilisation of the present δ11Bruber-to-δ11Bborate calibration is suitable for at least the last 3 million years.