Oxygen and Carbon Isotope Variations in Modern Southeast Pacific Benthic Foraminifera: Palaeoceanographic Implications

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Abstract
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.