Isotopic evidence for temperate oceans during the Cambrian Explosion

The Cambrian Explosion was a key event in the evolution of life on Earth. This event took place at a time when sea surface temperatures have been proposed to reach about 60 °C. Such high temperatures are clearly above the upper thermal limit of 38 °C for modern marine invertebrates and preclude a major biological revolution. To address this dichotomy, we performed in situ δ(18)O analyses of Cambrian phosphatic brachiopods via secondary ion mass spectrometry (SIMS). The δ(18)O(phosphate) data, which are considered to represent the most primary δ(18)O(seawater) signature, were identified by evaluating the diagenetic alteration of the analyzed shells. Assuming ice-free conditions for the Cambrian ocean and no change in δ(18)O(seawater) (-1.4‰ to -1‰; V-SMOW) through time, our temperatures vary between 35 °C ± 12 °C and 41 °C ± 12 °C. They are thus clearly above (1) recent subequatorial sea surface temperatures of 27 °C–35 °C and (2) the upper lethal limit of 38 °C of marine organisms. Our new data can therefore be used to infer a minimal depletion in early Cambrian δ(18)O(seawater) relative to today of about -3‰. With this presumption, our most pristine δ(18)O(phosphate) values translate into sea surface temperatures of about 30 °C indicating habitable temperatures for subequatorial oceans during the Cambrian Explosion.