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

Boron isotopes in planktonic foraminifera are a widely used proxy to determine ancient surface seawater pH and by extension atmospheric CO.sub.2 concentration and climate forcing on geological timescales. Yet, to reconstruct absolute values for pH and CO.sub.2, we require a [delta].sup.11 B.sub.foram-borate to pH calibration and independent determinations of ocean temperature, salinity, a second carbonate parameter, and the boron isotope composition of seawater. Although [delta].sup.11 B-derived records of atmospheric CO.sub.2 have been shown to perform well against ice-core-based CO.sub.2 reconstructions, these tests have been performed at only a few locations and with limited temporal resolution. Here we present two highly resolved CO.sub.2 records for the late Pleistocene from Ocean Drilling Program (ODP) Sites 999 and 871. Our [delta].sup.11 B-derived CO.sub.2 record shows a very good agreement with the ice core CO.sub.2 record with an average offset of 13±46 (2Ï) and an RMSE of 26 ppm, with minor short-lived overestimations of CO.sub.2 (of up to â¼50 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.sub.2 offset. We also observe that the general agreement between [delta].sup.11 B-derived and ice core CO.sub.2 is improved by optimising the [delta].sup.11 B.sub.foram-borate calibration. Despite these minor issues, a strong linear relationship between relative change in climate forcing from CO.sub.2 (from ice core data) and pH change (from [delta].sup.11 B) exists over the late Pleistocene, confirming that pH change is a robust proxy of climate forcing over relatively short (