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

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Chemical Geology



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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 ε205Tlauthigenic 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 ε205Tlauthigenic = −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 (ε205TlSW). Moving forward with this assumption, we find that Ediacaran open ocean ε205TlSW values were predominantly comparable to bulk upper continental crust (ε205Tl = ~−2). These relatively heavy open ocean ε205TlSW 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 ε205TlSW reach values considerably lighter than bulk upper continental crust (for example, ε205TlSW as low as −4.4 ± 0.2; 2SD at ~635 Ma and − 5.3 ± 0.3: 2SD at ~580 Ma). These much rarer negative ε205TlSW 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.


Thallium; Isotopes; Ediacaran; Oxygen; Redox; Ediacaran Biota; Mn oxides


Earth Sciences | Geochemistry | Physical Sciences and Mathematics



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