An evaluation of benthic foraminiferal B/Ca and δ 11B for deep ocean carbonate ion and pH reconstructions

Abstract

Reliable reconstructions of deep ocean carbonate ion concentration, [CO 3 2−], and pH are crucial to understand mechanisms responsible for the past atmospheric CO 2 variations observed in ice cores. However, it is challenging to reconstruct past deep water [CO 3 2−] and pH and literature results from different proxies conflict, warranting careful investigations on possible reasons for the existing inconsistencies. Here, we present the first down core B/Ca and δ 11B records measured in an epifaunal benthic foraminifer Cibicidoides wuellerstorfi from the Caribbean Sea during the last 160 kyr. The two proxies yield quantitatively comparable deep water [CO 3 2−] and pH results, showing high values during glacials relative to inter-glacials (differences in [CO 3 2−] and pH are ∼ 35 μmol/kg and ∼ 0.15, respectively), consistent with past ocean circulation changes in the Caribbean Sea. Our data provide convincing evidence that both proxies serve as faithful proxies to estimate deep ocean [CO 3 2−] and pH, despite our incomplete understanding of boron incorporation into foraminiferal carbonates.