Species- and size-dependent δ¹¹B signatures of abrupt climate events in the Equatorial Atlantic

Boron isotopes (δ¹¹B) measured in planktonic foraminifera are widely used to reconstruct past surface ocean pH and atmospheric pCO₂, yet their application in tropical regions relied on understanding of species-specific ecology and size-dependent vital effects. Here we present new data from Globigerinoides ruber (sensu stricto) and Trilobatus sacculifer spanning the last ~60 kyr from Tropical Atlantic Ocean. Our dataset comprises paired analyses of multiple size fractions (200–250 µm, 250–300 µm, 300–355 µm, and ≥355–400 µm), which enabled an assessment of species’ vital effects and how they vary with size. During the Younger Dryas, Heinrich Stadial 1 and Heinrich Stadial 4, G. ruber varied from ~19.0–19.5‰ (200–250 µm), increased to ~19.0–20.0‰ (250–300 µm) and reached values up to ~20–21‰ in the largest studied size fraction (300-350 µm). This positive relationship between δ¹¹B and test size demonstrates a pronounced size-dependent enrichment, consistent with strong biological control and near-surface calcification. In contrast, T. sacculifer exhibits lower δ¹¹B values during the same intervals, ranging from ~18.5–19‰ (200–250 µm), 18.25-17.80 ‰ (250-300 µm), 18.2-19‰ (300-355 µm). Paired species analyses from identical depth horizons reveal persistent interspecific offsets, with G. ruber recording higher δ¹¹B values than T. sacculifer across all size fractions. These offsets are maintained throughout YD, HS1, and HS4. The magnitude of size-related offsets within each species (up to ~1‰) is comparable to the expected glacial–interglacial δ¹¹B signal, underscoring the first-order importance of size fraction. We conclude that robust δ¹¹B-based reconstructions in the Equatorial Atlantic require strict size-fraction control and species-specific ecological interpretation. These findings highlight that different planktonic foraminifera record distinct levels of the upper ocean carbonate system during periods of rapid climate change, providing new constraints on tropical ocean buffering during abrupt climate events.

Keywords: Boron Isotopes, Globigerinoides ruber, Trilobatus sacculifer, Tropical Atlantic Ocean