Technical note: Single-shell δ 11 B analysis of Cibicidoides wuellerstorfi using femtosecond laser ablation MC-ICPMS and secondary ion mass spectrometry

[摘要] The boron isotopic composition ( δ 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 δ 11 B are sparse. In addition, microanalytics provide information on the distribution of δ 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 δ 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 δ 11 B values obtained from these different techniques agree remarkably well with bulk solution values to within ±0.1  ‰, 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 ‰ and 1.3 ‰ ( 2 σ ), 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 δ 11 B variations are taken into account for these methods, the intra-shell variability is estimated to be on the order of ∼3  ‰ and ∼0.4 ‰ ( 2 σ ) on a ∼20 and 100  µm scale, respectively. In comparison, the δ 11 B variability between shells exhibits a total range of ∼3  ‰ for both techniques, suggesting that several shells need to be analyzed for accurate mean δ 11 B values. Based on a simple resampling method, we conclude that ∼12 shells of C. wuellerstorfi must be analyzed using LA-MC-ICPMS to obtain an accurate average value within ±0.5  ‰ ( 2 σ ) to resolve pH variations of ∼0.1 . 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 δ 11 B .

[学科分类] 大气科学