Palaeoceanographic proxies are essential in reconstructing past climate change and accurate estimates of past conditions require reliable calibrations. The chemical signature of benthic foraminiferal calcite reflects bottom water conditions, but the effects of their microhabitat, biomineralization, metabolism, ontogeny can challenge a straightforward interpretation of past seawater conditions. In this study we compared stable isotopes of oxygen (δ18OBF) and carbon (δ13CBF) values of age-constrained benthic foraminifera from surface sediments of the Southeast Pacific (SEP) with contemporary in-situ water column values for δ18O equilibrium calcite (δ18Oeqcal) and δ13C of dissolved inorganic carbon (δ13CDIC). The ages of benthic foraminifera, based on radiocarbon and oxygen isotope stratigraphy, show a cluster of older surface sediments sites at intermediate water depths off north Chile, likely associated with sediment erosion. Epifaunal benthic foraminifera δ18OBF values reveal a 1:1 relationship with δ18Oeqcal of ambient bottom waters across the region, while infaunal foraminifera show positive δ18OBF-δ18Oeqcal offsets. Epifaunal benthic foraminifera δ13CBF values closely match ambient bottom waters δ13CDIC in most cases. In contrast, infaunal benthic foraminifera show negative δ13CBF-δ13CDIC offset. At sites shallower than 1000 m, both epifaunal and infaunal benthic foraminifera display significant positive δ13CBF-δ13CDIC offsets. This offset is likely attributed to a combination of microbiological processes occurring within the bottom and pore waters under low to intermediate oxygen conditions, influencing the carbonate pool and the inorganic carbon speciation in the SEP. This is not likely a regionally restricted phenomenon and may have broader implications for improving the reliability of δ13C- and δ18O-based proxies in palaeoceanographic studies.

Calcareous benthic foraminifera can develop pores in their test wall to facilitate gas exchanges (e.g., O2, CO2) with the surrounding seawater. The patterns of these pores, i.e., porosity, pore density, and pore size, can vary based on environmental factors, including bottom water dissolved oxygen concentration (BWDO). Specifically, some species react with increased test porosity to lower BWDO levels, highlighting their proxy potential for reconstructing past BWDO concentrations. To validate this proxy in the Southeast Pacific (SEP), the pore patterns of six benthic foraminifera species were compared with BWDO. The specimens, dated Holocene to modern, were collected from surface sediment samples along the SEP (12°–44°S) from 24 to 3,252 m water depth. Porosity, pore density, and pore size were measured on the umbilical and spiral sides, including all visible chambers, and the penultimate and antepenultimate chambers (PAC) on both sides. Our study reveals species-specific pore pattern responses to BWDO, with most species increasing their test porosity to adapt to lower BWDO. Notably, Cibicidoides species show increased porosity on the umbilical side, whereas Planulina species do so primarily on the spiral side. This indicates that using the pore patterns of several species together is not recommended in the SEP. Instead, focusing on individual species provides stronger inverse correlations between BWDO, porosity, and pore density, especially for C. wuellerstorfi. These findings support the use of benthic foraminifera pore patterns as reliable indicators for reconstructing quantitively past BWDO, with an error range down to ±5–20 µmol/kg.