Size and Species Effects; a Core-top Investigation into the Boron Isotope Proxy for Paleo-pH

A promising proxy of pCO2 is the boron isotopic composition of foraminiferal carbonate, yet recently there has been significant debate as to its validity. We have addressed this debate by both establishing a novel analytical procedure for the analysis of B isotope ratios in foraminifera and using this technique to provide a new calibration of the proxy. We have developed a total evaporation NTIMS technique that tackles the problem of instrumental fractionation in a novel way. Strikingly we have been able to reproduce the certified value of the NIST 951 standard using two different loading matrices. Total evaporation also minimises sample requirements and we are able to measure as little as 400pg B with a reproducibility of 0.7 permil (2sd). We have added to previous calibrations of the proxy by analysing the boron isotopic composition of 5 species of planktic foraminifera from core-tops from three sites located in the equatorial Atlantic, Indian and Pacific Oceans. By analysing symbiont-bearing and symbiont-barren or facultative foraminifera from several size fractions from these three locations, we are able to test our understanding relative to known variations in water column conditions and foraminiferal micro-environments. We find that surface dwelling, symbiont-bearing species have higher δ11B than predicted from inorganic equilibria, pinned to reproduce the deeper symbiont barren species, but this is consistent with models of the influence of symbionts on the pH and boron speciation within the foraminiferal microenvironment. This study also shows a sample size dependence of δ11B in G. sacculifer from Pacific and Indian Ocean cores. In contrast recent G. sacculifer from the Atlantic show no sample size variations and we interpret this contrast to reflect partial dissolution in Pacific and Indian samples not present in our Atlanitc core top. This work emphasises some previously noted complexity in the B isotope proxy but that such complications are compatible with current understanding of ocean chemistry and foraminiferal habits and does not prevent careful application of the B isotopic proxy to constrain a record of changing carbonate equilibrium in the oceans.