Sub-millimeter molybdenum and uranium isotopes track millennial redox events in the Cambrian ocean

Submillimeter-scale redox archives in organic-rich mudrocks elucidate dynamic environmental stress shaping early marine animal ecosystems. During the late Cambrian, benthic animals, including trilobites, thrived in intermittently oxygen-deficient (predominantly euxinic) outer shelf environment where the astronomically-tuned Scandinavian Alum Shale was deposited. We present ultrahigh-resolution (0.25 mm) molybdenum and uranium isotope (δ98Mo-δ238U) and concentration profiles, revealing millennial-scale redox fluctuations that intermittently disrupted bottom-water euxinia. Varying ocean circulation, redox buffer supplies, and/or reduced productivity likely created short-lived aerobic niches for opportunistic benthic animals. Distinct δ98Mo-δ238U patterns indicate recurrent chemocline shoaling with toxic hydrogen sulfide incursions into surface waters, resembling metal drawdown in modern restricted basins. These fluctuations punctuated a million-year-long global oceanic anoxic event linked to the Steptoean Positive Carbon Isotope Excursion. Following this excursion, local redox conditions oscillated around a globally more oxygenated baseline, coinciding with animal diversifications in Baltica and worldwide. These millennial-scale redox changes underscore the need for higher-resolution geochemical approaches in paleoenvironmental reconstructions. Millennial-scale bottom water oxygenation events occurred in the Cambrian Ocean in regions inhabited by early animals, according to analyses of sub-millimetre stratigraphic profiles of molybdenum and uranium isotopes.