Boron isotopes and B/Ca 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 pCO(2), 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) delta B-11 measurements on a range of benthic foraminifera from 23 late-Holocene samples from the Atlantic that reaffirm the utility of the delta B-11-pH proxy. Our boron isotope measurements on similar to 10 benthic foraminifera tests typically yield a precision of similar to +/- 0.25 parts per thousand at 2 s.d. (equivalent to similar to +/- 0.03 pH units). delta B-11 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 B-11/B-10 of aqueous boron species. Infaunal foraminifera are consistent with this model, but record the combined effects of lower pore-water delta B-11 and pH. No influence of partial dissolution or shell size on delta B-11 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 [CO32-], but the B/Ca of different co-occurring species morphotypes varies considerably. These effects are not seen in delta B-11, which may therefore provide a more robust proxy of the ocean carbonate system. Whilst in theory delta B-11 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. delta B-11 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 delta B-11 with pH, across a wide range of species and locations, provides confidence in the application of MC-ICPMS measurements of foraminiferal delta B-11 to reconstruct past changes in the ocean carbonate system. (c) 2010 Elsevier B.V. All rights reserved.