Boron isotopes and B/Ca ratios in benthic foraminifera: proxies for the deep ocean carbonate system

Accurate records of the state of the ocean carbonate system are critical for understanding past changes in pCO2, ocean acidification and climate. The chemical principles underlying the proxy of oceanic pH provided by the boron isotope ratio of foraminiferal carbonate are relatively well understood, but the proxy's reliability has been questioned. We present 76 new Multi-Collector Inductively-Coupled Plasma Mass Spectrometry (MC-ICPMS) δ11B measurements on a range of benthic foraminifera from 23 late-Holocene samples from the Atlantic that reaffirm the utility of the δ11B-pH proxy. Our boron isotope measurements on ~ 10 benthic foraminifera tests typically yield a precision of ~ ± 0.25‰ at 2 s.d. (equivalent to ~ ± 0.03 pH units). δ11B values of epifaunal species are within analytical uncertainty of those predicted from a simple model assuming sole incorporation of B(OH)4− from seawater and no vital effects, using the independently determined fractionation factor of 1.0272 between 11B/10B of aqueous boron species. Infaunal foraminifera are consistent with this model, but record the combined effects of lower pore-water δ11B and pH. No influence of partial dissolution or shell size on δ11B is observed. We have also measured the B/Ca ratios of the same samples. For individual Cibicidoides species, B/Ca shows a good correlation with Δ[CO32−], but the B/Ca of different co-occurring species morphotypes varies considerably. These effects are not seen in δ11B, which may therefore provide a more robust proxy of the ocean carbonate system. Whilst in theory δ11B and B/Ca can be combined to provide a quantitative reconstruction of alkalinity and dissolved inorganic carbonate (DIC), in practice this is precluded by propagated uncertainties. δ11B data give significant constraints on foraminifera calcification mechanisms, and seem most simply explained by incorporation of B(OH)4− into a HCO3− pool, which is then completely incorporated in foraminiferal CaCO3. Our demonstration of the predictable variation of δ11B with pH, across a wide range of species and locations, provides confidence in the application of MC-ICPMS measurements of foraminiferal δ11B to reconstruct past changes in the ocean carbonate system.