Revisiting oxygen‐18 and clumped isotopes in planktic and benthic foraminifera

Abstract

Abstract Foraminiferal isotopes are widely used to study past oceans, with different species recording conditions at different depths. Their δ 18 O values record both seawater oxygen‐18 and temperature according to species‐specific fractionation factors, while their Δ 47 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 ( 18 α ) 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, δ 18 O, and chemistry from existing databases for any latitude, longitude, and depth‐range; inferring calcification temperatures based on the above data. We find that although 18 α differs between species, its temperature sensitivity remains indistinguishable from inorganic calcite. Based on > 2,600 observations we show that, although most planktic δ 18 O values are consistent with seawater temperature and δ 18 O over their expected living depths, a sizable minority (12%–24%) have heavier‐than‐predicted δ 18 O, best explained by calcification in deeper waters. We use this framework to revisit three recent Δ 47 calibration studies of planktic/benthic foraminifera, confirming that planktic Δ 47 varies systematically with oxygen‐18‐derived temperature estimates, even for samples whose δ 18 O disagrees with assumed climatological conditions, and demonstrating excellent agreement between planktic foraminifera and modern, largely inorganic Δ 47 calibrations. Benthic foraminifera remain ambiguous: modern benthic Δ 47 values appear offset from planktic ones, yet applying equilibrium Δ 47 calibration to the Cenozoic benthic foraminifer record of Meckler et al. (2022, https://doi.org/10.1126/science.abk0604 ) largely reconciles it with δ 18 O‐derived temperatures, with discrete Δ 47 /δ 18 O discrepancies persisting in the Late Paleocene/Eocene/Plio‐Pleistocene.