05 Jan 2023
 | 05 Jan 2023
Status: this preprint is currently under review for the journal CP.

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

Elwyn de la Vega, Thomas B. Chalk, Mathis P. Hain, Megan R. Wilding, Daniel Casey, Robin Gledhill, Chongguang Luo, Paul A. Wilson, and Gavin L. Foster

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 δ11Bforam-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 δ11Bforam-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.

Elwyn de la Vega et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2022-93', Anonymous Referee #1, 13 Feb 2023
  • RC2: 'Comment on cp-2022-93', William Gray, 22 Mar 2023

Elwyn de la Vega et al.

Elwyn de la Vega et al.


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Short summary
We evaluate how faithfully the boron isotope values of foraminifera record atmospheric CO2 and climate forcing by comparing it to the high-fidelity CO2 record from the Antarctic ice cores. We evaluate potential factors and find that, partial dissolution of foraminifera shells, assumptions of seawater chemistry, and the biology of foraminifera all have a negligible effect on reconstructed CO2. This gives confidence in the use of boron isotopes beyond the interval when ice core CO2 is available.