Extreme warmth and heat-stressed plankton in the tropics during the Paleocene-Eocene Thermal Maximum

Global ocean temperatures rapidly warmed by similar to 5 degrees C during the Paleocene-Eocene Thermal Maximum (PETM; similar to 56 million years ago). Extratropical sea surface temperatures (SSTs) met or exceeded modern subtropical values. With these warm extratropical temperatures, climate models predict tropical SSTs > 35 degrees C-near upper physiological temperature limits for many organisms. However, few data are available to test these projected extreme tropical temperatures or their potential lethality. We identify the PETM in a shallow marine sedimentary section deposited in Nigeria. On the basis of planktonic foraminiferal Mg/Ca and oxygen isotope ratios and the molecular proxy TEX86H, latest Paleocene equatorial SSTs were similar to 33 degrees C, and TEX86H indicates that SSTs rose to > 36 degrees C during the PETM. This confirms model predictions on the magnitude of polar amplification and refutes the tropical thermostat theory. We attribute a massive drop in dinoflagellate abundance and diversity at peak warmth to thermal stress, showing that the base of tropical food webs is vulnerable to rapid warming.