Using europium (Eu) redox state to trace the evolution of seawater chemistry over the past 4 billion years

The aim of this study is to determine the redox state of Eu hosted in banded iron formations (BIFs) deposited on seafloor at different times of the Earth’s history using XANES. We recently discovered that Eu isotopic compositions were fractionated in hydrothermal cells and reported the first d153Eu/151Eu values different from zero that we measured in several BIF samples worldwide and modern hydrothermal precipitates. All these samples also display significant Eu enrichments. It has been proposed that the processes responsible for Eu enrichments and isotopic fractionation are related to the reduction of Eu3+ to Eu2+ at high temperature in hydrothermal cells. However, no correlation was found between Eu enrichment and isotopic anomalies. Determining the valence state of Eu in modern hydrothermal deposits and Archean BIF with variable Eu enrichments and isotopic anomalies will be key to understand the physicochemical conditions in which sediments were deposited in ancient oceans.