Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

The evolution of the seawater oxygen isotopic composition (δ18O) through geological time remains controversial. Yet, the past δ18Oseawater is key to assess past seawater temperatures, providing insights into past climate change and life evolution. Here we provide a new and unprecedentedly precise δ18O value of −1.33 ± 0.98‰ for the Neoproterozoic bottom seawater supporting a constant oxygen isotope composition through time. We demonstrate that the Aït 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 δ18Oseawater and submarine hydrothermal processes were likely similar to present day, Neoproterozoic oceans were 15–30 °C warmer on the eve of the Sturtian glaciation and the major life diversification that followed.
Uncertainty regarding the evolution of the oxygen isotopic composition of seawater casts doubt on past temperature reconstructions. Here, the authors present a new, precise δ18O value for the Neoproterozoic, and propose that ocean temperatures on the eve of the Sturtian glaciation were 15–30 °C warmer than present.