Revisiting Oxygen-18 and Clumped Isotopes in Planktic and Benthic Foraminifera

Abstract

Foraminiferal isotopes are widely used to study past oceans, with different species recording conditions at different depths. Their δ18O values record both seawater oxygen-18 and temperature according to species-specific fractionation factors, while their Δ47 signatures likely depend only on temperature. We describe an open-source framework to collect/combine data relevant to foraminiferal isotopes, by constraining species-specific oxygen-18 fractionation factors (18α) based on culture experiments, stratified plankton tows or core-top sediments; compiling stratified plankton tow constraints on living depths for planktic species; extracting seawater temperature, δ18O, and chemistry from existing databases for any latitude, longitude, and depth-range; inferring calcification temperatures based on the above data. We find that although 18α differs between species, its temperature sensitivity remains indistinguishable from inorganic calcite. Based on > 2,600 observations we show that, although most planktic δ18O values are consistent with seawater temperature and δ18O over their expected living depths, a sizable minority (12%–24%) have heavier-than-predicted δ18O, best explained by calcification in deeper waters. We use this framework to revisit three recent Δ47 calibration studies of planktic/benthic foraminifera, confirming that planktic Δ47 varies systematically with oxygen-18-derived temperature estimates, even for samples whose δ18O disagrees with assumed climatological conditions, and demonstrating excellent agreement between planktic foraminifera and modern, largely inorganic Δ47 calibrations. Benthic foraminifera remain ambiguous: modern benthic Δ47 values appear offset from planktic ones, yet applying equilibrium Δ47 calibration to the Cenozoic benthic foraminifer record of Meckler et al. (2022, https://doi.org/10.1126/science.abk0604) largely reconciles it with δ18O-derived temperatures, with discrete Δ47/δ18O discrepancies persisting in the Late Paleocene/Eocene/Plio-Pleistocene.