Benthic foraminiferal Cibicidoides species from oxygen-depleted Pacific waters: morphology, I/Ca and implications for understanding the glacial ocean

In studies of paleoclimate and paleoceanography, Cibicidoides spp. are arguably the most widely used benthic foraminiferal species for reconstructing deep-sea temperature, chemistry and circulation. Most Cibicidoides species are generally considered to live epifaunally, and - when abundant - have been suggested to indicate well oxygenated bottom water conditions. We report the presence of Cibicidoides wuellerstorfi (Rose Bengal stained) living in hypoxic to suboxic waters. I/Ca is low in specimens collected in low oxygen waters, consistent with the current understanding of this proxy as an indicator of redox conditions. Areal percentage of pores in the foraminiferal tests increases with the decrease in I/Ca, most likely related to oxygen depletion, as previously concluded for some infaunal species. We suggest that Cibicidoides spp. I/Ca may be used, at least, as a semi-quantitative proxy for bottom water [O2]. The exact threshold values for Cibicidoides I/Ca and corresponding [O2] may require further constraints from live-collected specimens. We demonstrate the potential application of the I/Ca proxy by documenting a down-core record from equatorial Pacific ODP Site 1242. Our data indicate that bottom water [O2] was lower during the Last Glacial Maximum, supporting results from an independent quantitative [O2] proxy, Δδ13C (Hoogakker et al., 2015).