The boron isotope palaeo-pH/CO2 proxy is one of the key quantitative tools available to reconstruct past changes in the concentration of CO2 in the atmosphere. In particular, marine calcifying organisms have been shown to be useful archives of this proxy, enabling quantitative variations in pH/CO2 to be reconstructed throughout the Cenozoic. In order to provide an alternative proxy archive to the widely used planktonic foraminifera, we investigated the symbiont-bearing, high-Mg, shallow-dwelling, tropical large benthic foraminifera (LBF) species Operculina ammonoides and present a calibration of the relationship between the shell boron isotopic composition and seawater pH. We investigated specimens collected from both several reefs as well as grown in laboratory culture experiments in which pH and DIC were decoupled from each other, measuring newly-formed chambers using laser-ablation as a sample introduction technique. Based on our laboratory culture samples, the resulting linear relationship between the in situ boron isotopic composition of aqueous borate ion (B(OH)4−) and the shells of O. ammonoides is characterised by a gradient of 0.38-0.10+0.12. In contrast, the boron isotopic composition of the field collected samples displays a near 1:1 relationship with B(OH)4−. We suggest that the shallow slope of the laboratory culture regression is the result of the difference between their micro-environment carbonate chemistry and that of the surrounding seawater driven by a pH dependence of the relative rates of calcification and photosynthesis. Based on a model of the effect of these processes on the diffusive boundary layer, we show that this effect is expected in laboratory culture experiments free from micro-turbulence, but not in the foraminifer’s natural environment. As such, we demonstrate the utility of these organisms as proxy archive, while also highlighting how laboratory experimental design has the potential to drive important changes in the micro-environment and resulting shell chemistry of organisms of this size. Given that the genus Operculina originated in the late Palaeocene, this work paves the way towards deep-time palaeo-pH/CO2 reconstructions using foraminifer species which have a very closely related modern representative.

Recent developments in spatially-resolved boron isotopic analysis using laser ablation as a means of sample introduction to MC-ICPMS instruments (LA-MC-ICPMS) increasingly allow researchers to explore the spatial heterogeneity of the boron isotopic composition of a range of geochemical applications, for example in palaeoclimatology and mantle petrology. However, previous work has shown that a diffuse interference centred near 10B, when measuring samples with a calcium-rich matrix, can significantly bias especially the measurement on 10B, affecting the accuracy of boron isotope measurements. Although several correction approaches have yielded sufficiently accurate analyses of delta 11B in calcium carbonate, the root cause of this interference is still not fully resolved. Here, we explore the various potential sources of inaccuracy in boron isotope measurements made using (LA-)MC-ICPMS by experimenting with dry and wet plasma conditions, in both solution and laser ablation mode (in the former case, our solution (Ca-Mg)/B ratios broadly mimic those found in natural samples). In solution mode, we find that irrespective of wet or dry plasma conditions, the introduction of a Ca-containing matrix yields a baseline up to similar to 4 and similar to 14 times higher around m/z approximate to 10 for wet and dry plasma conditions, respectively, compared to both a Mg-only matrix and lack of matrix. In order to explore this further, we performed mass scans around m/z approximate to 10 during laser ablation of different carbonates with varying matrix [Ca]. These show that the m/z approximate to 10 interference scales linearly with a mixture of the calcium content of the analyte matrix and 40Ar4+ ion beam intensity, as previously hypothesised. Moreover, by experimenting with different plasma loading scenarios during the ablation of CaCO3, i.e. varying laser spot sizes, we find that permil-level inaccuracies in delta 11B may occur when the analyte ablated mass is significantly different than that of the standard used to calibrate instrumental mass bias. This is important given that we also show that different commonly-used reference materials ablate at very different rates, which illustrates the need for a careful standardisation approach irrespective of broader matrix effects when sub-permil level accuracy and precision are desirable when utilising LA-MC-ICPMS. New empirical evidence for the different sources of inaccuracy during boron isotope measurement of carbonates using (LA-)MC-ICPMS.

El/Ca data and high resolution images of 3 laser-ablation tracks on a fossil giant calm. The following Isotopes were monitored 11B, 23Na, 24Mg, 27Al, 43Ca, 88Sr, 89Y and 138Ba. The data was measured with laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) using a 3 x 33 µm laser slit. El/Ca ratios were calibrated using NIST SRM 612 as bracketing external standard (Jochum et al., 2011) with updated Mg values from Evans & Müller (2018) and 43Ca as the internal standard; data quantification follows Longerich et al. (1996) and was performed using the software iolite 4 (Paton et al., 2011). For details see main text. References: Evans, D., & Müller, W. (2018). Automated Extraction of a Five-Year LA-ICP-MS Trace Element Data Set of Ten Common Glass and Carbonate Reference Materials: Long-Term Data Quality, Optimisation and Laser Cell Homogeneity. Geostandards and Geoanalytical Research, 42(2), 159–188. https://doi.org/10.1111/ggr.12204 Jochum, K. P., Weis, U., Stoll, B., Kuzmin, D., Yang, Q., Raczek, I., Jacob, D. E., Stracke, A., Birbaum, K., Frick, D. A., Günther, D., & Enzweiler, J. (2011). Determination of Reference Values for NIST SRM 610–617 Glasses Following ISO Guidelines. Geostandards and Geoanalytical Research, 35(4), 397–429. https://doi.org/10.1111/j.1751-908X.2011.00120.x Longerich, H. P., Jackson, S. E., & Günther, D. (1996). Inter-laboratory note. Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. Journal of Analytical Atomic Spectrometry, 11(9), 899–904. https://doi.org/10.1039/JA9961100899 Paton, C., Hellstrom, J., Paul, B., Woodhead, J., & Hergt, J. (2011). Iolite: Freeware for the visualisation and processing of mass spectrometric data. Journal of Analytical Atomic Spectrometry, 26(12), 2508–2518. https://doi.org/10.1039/C1JA10172B

Abstract Shells of the giant clam Tridacna can provide decade‐long records of past environmental conditions via their geochemical composition and structurally through growth banding. Counting the daily bands can give an accurate internal age model with high temporal resolution, but daily banding is not always visually retrievable, especially in fossil specimens. We show that daily geochemical cycles (e.g., Mg/Ca) are resolvable via highly spatially resolved laser‐ablation inductively coupled plasma mass spectrometry (LA‐ICPMS; 3 × 33 μm laser slit) in our Miocene (∼10 Ma) specimen, even in areas where daily banding is not visually discernible. By applying wavelet transformation on the measured daily geochemical cycles, we quantify varying daily growth rates throughout the shell. These growth rates are thus used to build an internal age model independent of optical daily band countability. Such an age model can be used to convert the measured elemental ratios from a function of distance to a function of time, which helps evaluate paleoenvironmental proxy data, for example, regarding the timing of sub‐seasonal events. Furthermore, the quantification of daily growth rates across the shell facilitates the evaluation of (co)dependencies between growth rates and corresponding elemental compositions.

Refinement of the measurement of the boron isotopic composition of carbonate and silicate materials via LA-ICPMS pushes samples size requirements into the range of SIMS (figure redrawn after Foster et al., 2018).