Abrupt hydrographic changes in the equatorial Pacific and subtropical Atlantic from foraminiferal Mg/Ca indicate greenhouse origin for the thermal maximum at the Paleocene‐Eocene Boundary

The Paleocene‐Eocene Boundary (PEB) was marked by an extraordinary climatic event, hypothesized to originate from a large perturbation to the carbon cycle which fueled global warming, the rapid dissociation of oceanic methane hydrates. The pattern of surface warming interpreted from existing sea surface temperature records is not consistent with a greenhouse origin for this event, which would have fueled sea surface warming globally. Although oxygen isotope (δ 18 O)‐based reconstructions indicate polar warming, results for the tropics and subtropics are ambiguous because of uncertainties associated with interpreting planktonic foraminiferal δ 18 O. To remedy this, we have constructed high‐resolution temperature records based on Mg/Ca of multiple species of both surface and thermocline‐dwelling planktonic foraminifera across the PEB in the equatorial Pacific and subtropical Atlantic. During the carbon isotope excursion (CIE), surface temperatures increased by 3.5°–4°C and thermocline temperatures warmed by 3°C. Estimates of surface water and thermocline salinity based on paired Mg/Ca and δ 18 O data indicate a pattern of hydrographic changes in the equatorial and subtropical oceans that is different from previously proposed, with a more vigorous hydrologic cycle during warming. The pattern of warming and salinity changes are consistent with this being a greenhouse‐induced global warming event, and the timing of hydrographic changes relative to the CIE supports the hypothesis that gradual warming of intermediate/deep waters triggered methane hydrate dissociation.