Mercury isotopes record development and attenuation of photic zone euxinia during the earliest Cambrian

The Ediacaran-Cambrian transition period (ca. 550-521 Ma) witnessed the decline of Ediacaran Biota and the rapid diversification of Cambrian biota. Numerous studies have linked the biological evolution to redox conditions in the early Cambrian ocean, which are still strongly debated. Photic zone euxinia (PZE), characterized by the development of anoxic conditions and build-up of free hydrogen sulfide (H2S) in the photic zone, would have seriously challenged the survival of early organisms and have been proposed to cause several extinctions. However, this potentially deleterious environmental condition has not been recognized at the Ediacaran - Cambrian boundary. Here, we analyzed mercury (Hg) contents and stable isotopes, carbon isotopes (813Ccarb and 813Corg), and organic carbon contents (TOC) from drill core recovered from the Tarim Basin, to constrain the potential impact of PZE in the earliest Cambrian ocean. A significant negative shift in Hg mass-independent isotope fractionation (MIF) is recorded in the basal Cambrian negative carbon isotope excursion (BACE) of the Yurtus Formation, combined with similar Hg isotope compositions in the Yangtze area (Tongren section), suggesting that the photoreduction of Hg(II) associated with the regional development of the PZE during the earliest Cambrian. Integrated with biotic records, our results suggest that the development of PZE in the earliest Cambrian ocean may have significantly limited habitable space. Instead, the attenuation of PZE could restore aerobic photosynthesis by removing H2S, creating a suitable habitat for the diversification of early animals.