Determination of δ 11 B in Planktonic Foraminifera at the ng Level: Application to the Ontogenetic Variability in Globigerina bulloides
Paulhac Buisson, M.; Louvat, P.; Karancz, S.; Tian, R.; Raitzsch, M.; Bijma, J.; Rollion-Bard, C. (2025). Determination of δ 11 B in Planktonic Foraminifera at the ng Level: Application to the Ontogenetic Variability in Globigerina bulloides. Geochem. Geophys. Geosyst. 26(10): e2024GC011845. https://dx.doi.org/10.1029/2024gc011845
In: Geochemistry, Geophysics, Geosystems. American Geophysical Union: Washington, DC. ISSN 1525-2027; e-ISSN 1525-2027, more
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| Authors | | Top |
- Paulhac Buisson, M.
- Louvat, P.
- Karancz, S., more
- Tian, R.
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- Raitzsch, M.
- Bijma, J.
- Rollion-Bard, C.
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| Abstract |
The boron isotope ratio (δ11B) of planktonic foraminifera is a robust proxy for determining oceanic pH and inferring atmospheric pCO2 during the Cenozoic. However, measuring δ11B in these small and low [B] samples (3–16 ppm) is challenging, as a precision below 0.8‰ (2SD) is required, equivalent to ∼0.1 pH units, which reflects the scale of anthropogenic acidification or glacial/interglacial alternations. Moreover, uncertainties in paleoclimate reconstructions from biocarbonates are due to biomineralization processes, requiring species-specific calibrations of the pH-δ11B relationship to account for the so-called “vital effect.” While such calibrations exist for many species, the influence of foraminiferal size on δ11B remains poorly studied. We measured δ11B of Globigerina bulloides (80–190 tests, 3–5 ppm [B]), for three different size fractions (250–315, 315–400, and >400 μm), collected from the upper part of a core from the Chilean Margin. We validated a novel analytical method combining microdistillation for B purification and micro-direct injection (μ-dDIHEN) to the Multi-Collector Inductively Coupled Plasma Mass Spectrometer for accurate and precise (0.1–0.5‰, 2SD) δ11B analysis of the smallest samples (only 1–2 ng B in solution). This approach injects only 10 μL of solution, producing short injection peaks that minimize blank effects through transient signal processing, thereby improving paleo-pH reconstructions from small-sized foraminifera. Our results show no size-dependent variations in δ11B of symbiont-barren G. bulloides, confirming that its microenvironment is primarily influenced by respiration and calcification. This contrasts with symbiont-bearing species, where δ11B varies with symbiont density due to photosynthetic activity. |
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