Thallium isotope ratios in shales from South China and northwestern Canada suggest widespread O₂ accumulation in marine bottom waters was an uncommon occurrence during the Ediacaran Period

The tempo and magnitude of ocean oxygenation during the rise of animals in the Ediacaran Period (635 to 539 million years ago, or Ma) is debated. Reconciling this debate is key to understanding what role, if any, oxygen played in the rise of large morphologically complex life on Earth. We reconstruct a portion of Ediacaran ocean oxygenation history using thallium isotope records (reported as ε²⁰⁵Tl) captured in shales from South China (Wuhe section) and northwestern Canada (Goz A and Sekwi Brook sections). At first blush, our ε²⁰⁵Tl data from these two locations are very different. South China shales reveal two pronounced and extremely negative Tl isotope excursions, in mid-members II and III of the Doushantuo Formation (where ε²⁰⁵Tl_authigenic reach nadirs of −10.4 ± 0.2; 2SD and − 9.1 ± 0.2; 2SD, respectively). In contrast, northwestern Canada shales reveal fairly invariant values (average ε²⁰⁵Tl_authigenic = −2.1 ± 1.6; 2SD, n = 75). These disparate records are in much better agreement, however, if we cull from our dataset shales that were inferred in previous work to have been deposited in restricted settings. By comparison to analogous modern settings (e.g., the Black Sea), ε²⁰⁵Tl data in these shales most likely fingerprint local-scale processes. On the other hand, and also by comparison to modern settings (e.g., Cariaco and Santa Barbara basins), shales deposited in an open ocean setting probably captured a globally homogenous seawater ε²⁰⁵Tl (ε²⁰⁵Tl_SW). Moving forward with this assumption, we find that Ediacaran open ocean ε²⁰⁵Tl_SW values were predominantly comparable to bulk upper continental crust (ε²⁰⁵Tl = ~−2). These relatively heavy open ocean ε²⁰⁵Tl_SW imply limited Mn oxide burial in global marine sediments for large periods of time during the Ediacaran, and by inference also limited accumulation of O₂ in global marine bottom waters. Only for what appear to be very brief episodes of time (≪5–10 million years) at ~635 Ma and ~580 Ma did open ocean ε²⁰⁵Tl_SW reach values considerably lighter than bulk upper continental crust (for example, ε²⁰⁵Tl_SW as low as −4.4 ± 0.2; 2SD at ~635 Ma and − 5.3 ± 0.3: 2SD at ~580 Ma). These much rarer negative ε²⁰⁵Tl_SW values require short-lived episodes of globally enhanced Mn oxide burial in marine sediments, and by implication also brief episodes of enhanced accumulation of O₂ in global marine bottom waters. When interpreting geochemical data, it is important to make global-scale inferences only after identifying and accounting for local-scale overprints. Applying this strategy ourselves, we find compelling evidence for an Ediacaran global ocean that only very briefly experienced widespread bottom water oxygenation during the rise of animals.