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

• Main Authors: M. Raitzsch, C. Rollion-Bard, I. Horn, G. Steinhoefel, A. Benthien, K.-U. Richter, M. Buisson, P. Louvat, J. Bijma
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-11-01
• Series: Biogeosciences
• Online Access: https://bg.copernicus.org/articles/17/5365/2020/bg-17-5365-2020.pdf

<p>The boron isotopic composition (<span class="inline-formula"><i>δ</i>¹¹B</span>) 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 <span class="inline-formula"><i>δ</i>¹¹B</span> are sparse. In addition, microanalytics provide information on the distribution of <span class="inline-formula"><i>δ</i>¹¹B</span> at high spatial resolution to increase the knowledge of biomineralization processes, for example. For this study, we investigated the intra- and inter-shell <span class="inline-formula"><i>δ</i>¹¹B</span> variability of the epibenthic species <i>Cibicidoides wuellerstorfi</i>, 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 <span class="inline-formula"><i>δ</i>¹¹B</span> values obtained from these different techniques agree remarkably well with bulk solution values to within <span class="inline-formula">±0.1</span>&thinsp;‰, 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&thinsp;‰ and 1.3&thinsp;‰ (<span class="inline-formula">2<i>σ</i></span>), 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 <span class="inline-formula"><i>δ</i>¹¹B</span> variations are taken into account for these methods, the intra-shell variability is estimated to be on the order of <span class="inline-formula">∼3</span>&thinsp;‰ and <span class="inline-formula">∼0.4 ‰</span> (<span class="inline-formula">2<i>σ</i></span>) on a <span class="inline-formula">∼20</span> and 100&thinsp;<span class="inline-formula">µm</span> scale, respectively. In comparison, the <span class="inline-formula"><i>δ</i>¹¹B</span> variability between shells exhibits a total range of <span class="inline-formula">∼3</span>&thinsp;‰ for both techniques, suggesting that several shells need to be analyzed for accurate mean <span class="inline-formula"><i>δ</i>¹¹B</span> values. Based on a simple resampling method, we conclude that <span class="inline-formula">∼12</span> shells of <i>C. wuellerstorfi</i> must be analyzed using LA-MC-ICPMS to obtain an accurate average value within <span class="inline-formula">±0.5</span>&thinsp;‰ (<span class="inline-formula">2<i>σ</i></span>) to resolve pH variations of <span class="inline-formula">∼0.1</span>. 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 <span class="inline-formula"><i>δ</i>¹¹B</span>.</p>