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Abstract: In this study we make the first attempt to inter-calibrate boron isotope (δ11B) measurements on marine biogenic carbonates measured by four different laboratories, each using a different analytical technique. The importance of such calibrations lies in the major implications of relatively small changes in δ11B (<2‰), which are typical of palaeoclimate applications of the δ11B-pH proxy. Despite the variety of mass spectrometric techniques used by the different laboratories in this study (two variants of negative ion thermal ionisation mass spectrometry and two variants of multicollector inductively coupled plasma mass spectrometry), for samples without a complex sample matrix, such as boric acid, there are no significant interlaboratory biases: the pooled 2sd of these samples is 0.39‰, which is within the measurement uncertainties reported by each laboratory (which range between 0.18 and 0.80‰). For seawater, a common in-house reference material, we find a similarly good agreement (δ11B=39.65±0.41‰, 2sd) despite four different sample preparation procedures. Reported δ11B for calcium carbonate samples have a pooled 2sd of 1.46‰, which is larger than the measurement uncertainty reported by each laboratory. We have attempted to identify the source of this interlaboratory variability, and find that overall sample size (in terms of available B) and B/Ca ratio (i.e. amount of boron relative to matrix) may play a role, but the exact mechanisms remain uncertain. Observed variations in reported δ11B for the CaCO3 samples are, however, relatively systematic for each laboratory for a given sample matrix (i.e. similar B/Ca and amount of boron for analysis). This implies that relative differences in δ11B in a sample set of a given matrix can be reconstructed by the four laboratories involved in this study more accurately than the absolute boron isotope ratios. [Copyright &y& Elsevier]

Abstract: Values of δ 11B reported in the literature for Holocene samples of the same foraminiferal species show large variability (6‰) relative to cited precision (<1‰). This is indicative of significant inter-laboratory biases and raises concerns about the accuracy of foraminiferal proxy pH records. To investigate this problem we have measured the δ 11B of modern ocean carbonates using several different analytical procedures. We have used total evaporation negative thermal ionisation mass spectrometry (TE-NTIMS), with various sample preparation and loading procedures and multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). The δ 11B of pure boric acid solutions measured by TE-NTIMS and MC-ICPMS agree well, demonstrating no fundamental biases between the techniques. Yet the δ 11B values measured by TE-NTIMS for non-foraminiferal carbonates are about 2‰ lighter than those by MC-ICPMS whereas foraminifers measured by the same standard TE-NTIMS procedure are 2 to 6‰ heavier than those by MC-ICPMS. Significantly, we found that foraminiferal samples kept in acidic solution over several months yielded lower δ 11B values (up to 5‰ lower) and better reproducibility when re-measured by TE-NTIMS. We infer that organic material, released on foraminiferal dissolution, causes biases in NTIMS measurements. No residual organics should be present in the MC-ICPMS measurements as matrix is separated from sample before analysis. Moreover we demonstrate by standard addition the absence of any matrix influence in the MC-ICPMS procedure beyond the inherent uncertainties in the standard addition approach (∼±0.35‰ 2sigma). Degradation of the inferred organic residue, either by long term storage in acidic solution or by loading samples in 30% H2O2, reduces but does not totally remove the (variable) bias between TE-NTIMS and MC-ICPMS δ 11B measurements. Despite these problems, careful analysis of similar samples by NTIMS may permit data to be obtained with consistent relative differences that still yield valuable proxy records, but there is clearly considerable potential for inaccuracies to result in such an approach from hard to detect changes in matrix. [Copyright &y& Elsevier]