Orbital CO 2 reconstruction using boron isotopes during the late Pleistocene, an assessment of accuracy

• Published in: Climate of the Past
• Main Authors: E. de la Vega, T. B. Chalk, M. P. Hain, M. R. Wilding, D. Casey, R. Gledhill, C. Luo, P. A. Wilson, G. L. Foster
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: Environmental pollution; TD172-193.5; Environmental protection; TD169-171.8; Environmental sciences; GE1-350; ice core; Planktonic foraminifera
• Online Access: https://doi.org/10.5194/cp-19-2493-2023; https://doaj.org/article/9df58c4735b445218d9903a69a5d21d3

Boron isotopes in planktonic foraminifera are a widely used proxy to determine ancient surface seawater pH and by extension atmospheric CO 2 concentration and climate forcing on geological timescales. Yet, to reconstruct absolute values for pH and CO 2 , we require a δ 11 B foram-borate to pH calibration and independent determinations of ocean temperature, salinity, a second carbonate parameter, and the boron isotope composition of seawater. Although δ 11 B-derived records of atmospheric CO 2 have been shown to perform well against ice-core-based CO 2 reconstructions, these tests have been performed at only a few locations and with limited temporal resolution. Here we present two highly resolved CO 2 records for the late Pleistocene from Ocean Drilling Program (ODP) Sites 999 and 871. Our δ 11 B-derived CO 2 record shows a very good agreement with the ice core CO 2 record with an average offset of 13±46 ( 2 σ ) and an RMSE of 26 ppm, with minor short-lived overestimations of CO 2 (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 CO 2 offset. We also observe that the general agreement between δ 11 B-derived and ice core CO 2 is improved by optimising the δ 11 B foram-borate calibration. Despite these minor issues, a strong linear relationship between relative change in climate forcing from CO 2 (from ice core data) and pH change (from δ 11 B) 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 CO 2 beyond the reach of the ice cores and can help improve determinations of climate sensitivity for ancient time intervals.