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

• Main Authors: Vega, Elwyn, Chalk, Thomas B., Hain, Mathis P., Wilding, Megan R., Casey, Daniel, Gledhill, Robin, Luo, Chongguang, Wilson, Paul A., Foster, Gavin L.
• Format: Text
• Language: English
• Published: 2023
• Subjects: ice core; Planktonic foraminifera
• Online Access: https://doi.org/10.5194/cp-2022-93; https://cp.copernicus.org/preprints/cp-2022-93/

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 time scales. 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 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 4.6 ± 49 (2σ) ppm, and a RMSE of 25 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.