Atmospheric CO₂ Estimates for the Late Oligocene and Early Miocene Using Multi-Species Cross-Calibrations of Boron Isotopes

The boron isotope (delta B-11) proxy for seawater pH is a tried and tested means to reconstruct atmospheric CO2 in the geologic past, but uncertainty remains over how to treat species-specific calibrations that link foraminiferal delta B-11 to pH estimates prior to 22 My. In addition, no delta B-11-based reconstructions of atmospheric CO2 exist for wide swaths of the Oligocene (33-23 Ma), and large variability in CO2 reconstructions during this epoch based on other proxy evidence leaves climate evolution during this period relatively unconstrained. To add to our understanding of Oligocene and early Miocene climate, we generated new atmospheric CO2 estimates from new delta B-11 data from fossil shells of surface-dwelling planktic foraminifera from the mid-Oligocene to early Miocene (similar to 28-18 Ma). We estimate atmospheric CO2 of similar to 680 ppm for the mid-Oligocene, which then evolves to fluctuate between similar to 500-570 ppm during the late Oligocene and between similar to 420-700 ppm in the early Miocene. These estimates tend to trend higher than Oligo-Miocene CO2 estimates from other proxies, although we observe good proxy agreement in the late Oligocene. Reconstructions of CO2 fall lower than estimates from paleoclimate model simulations in the early Miocene and mid Oligocene, which indicates that more proxy and/or model refinement is needed for these periods. Our species cross-calibrations, assessing delta B-11, Mg/Ca, delta O-18, and delta C-13, are able to pinpoint and evaluate small differences in the geochemistry of surface-dwelling planktic foraminifera, lending confidence to paleoceanographers applying this approach even further back in time.