Technical note: Single-shell [delta].sup.11B analysis of Cibicidoides wuellerstorfi using femtosecond laser ablation MC-ICPMS and secondary ion mass spectrometry

The boron isotopic composition ([delta].sup.11 B) of benthic foraminifera provides a valuable tool to reconstruct past deep-water pH. As the abundance of monospecific species might be limited in sediments, microanalytical techniques can help to overcome this problem, but such studies on benthic foraminiferal [delta].sup.11 B are sparse. In addition, microanalytics provide information on the distribution of [delta].sup.11 B at high spatial resolution to increase the knowledge of biomineralization processes, for example. For this study, we investigated the intra- and inter-shell [delta].sup.11 B variability of the epibenthic species Cibicidoides wuellerstorfi, which is widely used in paleoceanography, by secondary ion mass spectrometry (SIMS) and femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). While the average [delta].sup.11 B values obtained from these different techniques agree remarkably well with bulk solution values to within ±0.1 0/00, a relatively large intra-shell variability was observed. Based on multiple measurements within single shells, the SIMS and LA data suggest median variations of 4.8 0/00 and 1.3 0/00 (2Ï), respectively, while the larger spread for SIMS is attributed to the smaller volume of calcite being analyzed in each run. When analytical uncertainties and volume-dependent differences in [delta].sup.11 B variations are taken into account for these methods, the intra-shell variability is estimated to be on the order of â¼3 0/00 and â¼0.4 0/00 (2Ï) on a â¼20 and 100 µm scale, respectively. In comparison, the [delta].sup.11 B variability between shells exhibits a total range of â¼3 0/00 for both techniques, suggesting that several shells need to be analyzed for accurate mean [delta].sup.11 B values. Based on a simple resampling method, we conclude that â¼12 shells of C. wuellerstorfi must be analyzed using LA-MC-ICPMS to obtain an accurate average value within ±0.5 0/00 (2Ï) to resolve pH variations of â¼0.1. Based on our findings, we suggest preferring the conventional bulk solution MC-ICPMS over the in situ methods for paleo-pH studies, for example. However, SIMS and LA provide powerful tools for high-resolution paleoreconstructions, or for investigating ontogenetic trends in [delta].sup.11 B.