Determination of δ ¹¹ B in Planktonic Foraminifera at the ng Level: Application to the Ontogenetic Variability in <i>Globigerina bulloides</i>

Abstract The boron isotope ratio (δ 11 B) of planktonic foraminifera is a robust proxy for determining oceanic pH and inferring atmospheric p CO 2 during the Cenozoic. However, measuring δ 11 B 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‐δ 11 B relationship to account for the so‐called “vital effect.” While such calibrations exist for many species, the influence of foraminiferal size on δ 11 B remains poorly studied. We measured δ 11 B of Globigerina bulloides (80–190 tests, 3–5 ppm [B]), for three different size fractions (250–315, 315–400, and &gt;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) δ 11 B 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 δ 11 B of symbiont‐barren G. bulloides , confirming that its microenvironment is primarily influenced by respiration and calcification. This contrasts with symbiont‐bearing species, where δ 11 B varies with symbiont density due to photosynthetic activity.