Shifting ocean carbonate chemistry during the Eocene‐Oligocene climate transition: Implications for deep‐ocean Mg/Ca paleothermometry

To date, no conclusive evidence has been identified for intermediate or deep water cooling associated with the >1‰ benthic δ 18 O increase at the Eocene‐Oligocene transition (EOT) when large permanent ice sheets first appeared on Antarctica. Interpretation of this isotopic shift as purely ice volume change necessitates bipolar glaciation in the early Oligocene approaching that of the Last Glacial Maximum. To test this hypothesis, it is necessary to have knowledge about deep water temperature, which previous studies have attempted to reconstruct using benthic foraminiferal Mg/Ca ratios. However, it appears likely that contemporaneous changes in ocean carbonate chemistry compromised the Mg/Ca temperature sensitivity of benthic foraminifera at deep sites. New geochemical proxy records from a relatively shallow core, ODP Site 1263 (estimated paleodepth of 2100 m on the Walvis Ridge), reveal that carbonate chemistry change across the EOT was not limited to deep sites but extended well above the lysocline, critically limiting our ability to obtain reliable estimates of deep‐ocean cooling during that time. Benthic Li/Ca measurements, used as a proxy for [CO 3 2− ], suggest that [CO 3 2− ] increased by ∼29 μ mol/kg at Site 1263 across the EOT and likely impacted benthic foraminiferal Mg/Ca. A [CO 3 2− ]–benthic Mg/Ca relationship is most apparent during the early EOT when the overall increase in [CO 3 2− ] is interrupted by an apparent dissolution event. Planktonic δ 18 O and Mg/Ca records suggest no change in thermocline temperature and a δ 18 O seawater increase of up to 0.6‰ at this site across the EOT, consistent with previous estimates and supporting the absence of extensive bipolar glaciation in the early Oligocene.