Orbital CO₂ reconstruction using boron isotopes during the late Pleistocene, an assessment of accuracy

Abstract. Boron isotopes in planktonic foraminifera are a widely used proxy to determine ancient surface seawater pH, and by extension atmospheric CO₂ concentration and climate forcing on geological time scales. Yet, to reconstruct absolute values for pH and CO₂, we require a δ¹¹B_foram-borate to pH calibration and independent determinations of ocean temperature, salinity, a second carbonate parameter, and the boron isotope composition of seawater. Although δ¹¹B-derived records of atmospheric CO₂ have been shown to perform well against ice core-based CO₂ reconstructions, these tests have been performed at only a few locations and with limited temporal resolution. Here we present two highly resolved CO₂ records for the late Pleistocene from ODP Sites 999 and 871. Our δ¹¹B-derived CO₂ record shows a very good agreement with the ice core CO₂ record with an average offset of 4.6 ± 49 (2σ) ppm, and a RMSE of 25 ppm, with minor short-lived overestimations of CO₂ (of up to ~50 ppm) occurring during some glacial onsets. We explore potential drivers of this disagreement and conclude that partial dissolution of foraminifera has a minimal effect on the CO₂ offset. We also observe that the general agreement between δ¹¹B -derived and ice core CO₂ is improved by optimising the δ¹¹B_foram-borate calibration. Despite these minor issues a strong linear relationship between relative change in climate forcing from CO₂ (from ice core data) and pH change (from δ¹¹B) exists over the late Pleistocene, confirming that pH change is a robust proxy of climate forcing over relatively short (<1 million year) intervals. Overall, these findings demonstrate that the boron isotope proxy is a reliable indicator of CO₂ beyond the reach of the ice cores and can help improve determinations of climate sensitivity for ancient time intervals.