The precise and accurate isotopic measurement of sub-nanogram samples of foraminiferal hosted boron by Total Evaporation N-TIMS; applications to paleoceanography

Several recent experiments have confirmed early hypotheses that seawater pH is the dominant factor in determining the boron isotopic composition of foraminiferal calcite. Such a proxy for the paleocarbonate chemistry of the oceans is extremely useful as it can potentially provide information concerning the CO₂ content of the atmosphere in the past. However, despite several important studies, boron isotope measurements of foraminifera has yet to become a routine tool in paleoceanography. Previous foraminiferal studies have exclusively utilised negative ion thermal ionisation mass spectrometry (N-TIMS), due to the superior sensitivity of analysing boron as BO2- ions. However, because B has only two isotopes, mass bias effects have been minimised by analysing relatively large samples (5-100 ng; 30-300 foram tests) coupled with rejection of up to 30% of analyses. This relatively large sample size, the requirement of running in triplicate to achieve acceptable precision, and the necessary rejection of 1 in 3 analyses, has contributed to the limited application of this technique to paleoceanographic problems. In this contribution we present details of a novel Total Evaporation N-TIMS approach that allows the accurate analysis of sub-ng amounts of boron (equating to less than 10 foraminiferal tests) to a precision of c. 0.5 per mil (2. s.d.). Automatic analysis and a relatively short acquisition period (c.1-2 hours) enable a large sample through put (12-20 analyses per day). Application of the approach to a number of fundamental technical problems (e.g. inter- and intra-species isotope variation and the effect of test size), as well as an investigation into the isotopic composition of plankton tow and monospecific benthic foraminifera will also be presented.