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 degrees C. Such high temperatures are clearly above the upper thermal limit of 38 degrees C for modern marine invertebrates and preclude a major biological revolution. To address this dichotomy, we performed in situ delta O-18 analyses of Cambrian phosphatic brachiopods via secondary ion mass spectrometry (SIMS). The delta O-18(phosphate) data, which are considered to represent the most primary delta O-18(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 delta O-18(seawater) (-1.4 parts per thousand to -1 parts per thousand; V-SMOW) through time, our temperatures vary between 35 degrees C +/- 12 degrees C and 41 degrees C +/- 12 degrees C. They are thus clearly above (1) recent subequatorial sea surface temperatures of 27 degrees C-35 degrees C and (2) the upper lethal limit of 38 degrees C of marine organisms. Our new data can therefore be used to infer a minimal depletion in early Cambrian delta O-18(seawater) relative to today of about -3 parts per thousand. With this presumption, our most pristine delta O-18(phosphate) values translate into sea surface temperatures of about 30 degrees C indicating habitable temperatures for subequatorial oceans during the Cambrian Explosion.