Horizon 1 1 17 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES Nature Communications art. 1453 [7 p.] MAROC 2018 fdi:010072752 ENG Nature Communications 2041-1723 ISI:000429921000010 10.1038/s41467-018-03890-w https://www.documentation.ird.fr/hor/fdi:010072752 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers18-05/010072752.pdf 9 Horizon (IRD) The evolution of the seawater oxygen isotopic composition (delta O-18) through geological time remains controversial. Yet, the past delta O-18(seawater) is key to assess past seawater temperatures, providing insights into past climate change and life evolution. Here we provide a new and unprecedentedly precise delta O-18 value of -1.33 +/- 0.98% for the Neoproterozoic bottom seawater supporting a constant oxygen isotope composition through time. We demonstrate that the Ait Ahmane ultramafic unit of the ca. 760 Ma Bou Azzer ophiolite (Morocco) host a fossil black smoker-type hydrothermal system. In this system we analyzed an untapped archive for the ocean oxygen isotopic composition consisting in pure magnetite veins directly precipitated from a Neoproterozoic seawater-derived fluid. Our results suggest that, while delta O-18(seawater) and submarine hydrothermal processes were likely similar to present day, Neoproterozoic oceans were 15-30 degrees C warmer on the eve of the Sturtian glaciation and the major life diversification that followed. 064 ; 032 UR234 Brésil / Maroc