Black shale Mo isotope record reveals dynamic ocean redox during the Mesoproterozoic Era

Eukaryotes reached ecological importance in the late Neoproterozoic Era, some one billion years after their emergence. Their slow rise to prominence has been ascribed to prolonged environmental stagnation, but testing this idea requires an appraisal of the evolution of atmospheric and ocean chemistry. Establishing a nuanced geochemical history is, however, challenging due to the paucity of well preserved sedimentary rocks. Here, we present new Mo isotope ratios from black shale units spanning similar to 1560 to similar to 1170 Ma. These results, combined with literature data, reveal potential episodic expansions of oxygenated and/or mildly reducing conditions during the Mesoproterozoic Era, suggesting fluctuating oxygen availability that could have exerted a crucial control on the evolution of eukaryotes.