W. R. Gray

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.

The Southern Hemisphere westerly winds influence deep ocean circulation and carbon storage. While the westerlies are hypothesized to play a key role in regulating atmospheric CO 2 over glacial‐interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal δ 18 O from across the Southern Ocean and emergent relationships within an ensemble of climate models to reconstruct changes in the Southern Hemisphere surface westerlies over the last deglaciation. We infer a 4.8° (2.9–7.1°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (20 ka) relative to the mid‐Holocene (6.5 ka). Climate models from the Palaeoclimate Modeling Intercomparison Project substantially underestimate this inferred equatorward wind shift. According to our reconstruction, the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO 2 ( R 2 = 0.98). Experiments with a 0.25° resolution ocean‐sea‐ice‐carbon model suggest that shifting the westerlies equatorward reduces the overturning rate of the ocean below 2 km depth, leading to a suppression of CO 2 outgassing from the polar Southern Ocean. Our results support a role for the westerly winds in driving the deglacial CO 2 rise, and suggest outgassing of natural CO 2 from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming.