Abstract 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 e205Tl) captured in shales from South China (Wuhe section) and northwestern Canada (Goz A and Sekwi Brook sections). At first blush, our e205Tl 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 e205Tlauthigenic 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 e205Tlauthigenic = −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), e205Tl 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 e205Tl (e205TlSW). Moving forward with this assumption, we find that Ediacaran open ocean e205TlSW values were predominantly comparable to bulk upper continental crust (e205Tl = ~−2). These relatively heavy open ocean e205TlSW 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 e205TlSW reach values considerably lighter than bulk upper continental crust (for example, e205TlSW as low as −4.4 ± 0.2; 2SD at ~635 Ma and − 5.3 ± 0.3: 2SD at ~580 Ma). These much rarer negative e205TlSW 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.

中文翻译：

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华南和加拿大西北部页岩中的铊同位素比率表明，在埃迪卡拉纪期间，海洋底层水域中广泛的 O2 积累并不常见

摘要 关于埃迪卡拉纪（635 至 5.39 亿年前，或 Ma）动物兴起期间海洋氧化的速度和幅度存在争议。协调这场争论是理解氧气在地球上形态复杂的大型生命的兴起中扮演什么角色（如果有的话）的关键。我们使用从华南（五河段）和加拿大西北部（Goz A 和 Sekwi Brook 段）的页岩中捕获的铊同位素记录（报告为 e205Tl）重建了埃迪卡拉纪海洋氧化历史的一部分。乍一看，我们这两个位置的 e205Tl 数据非常不同。华南页岩在豆山沱组中段Ⅱ和Ⅲ（其中e205Tlauthigenic达到-10.4±0.2；2SD和-9.1±0.2；2SD的最低点）有两次明显且极负的Tl同位素偏移。相比之下，加拿大西北部页岩显示出相当不变的值（平均 e205Tlauthigenic = -2.1 ± 1.6；2SD，n = 75）。然而，如果我们从之前的工作中推断出沉积在受限环境中的页岩数据中剔除，这些不同的记录会更加一致。与类似的现代环境（例如黑海）相比，这些页岩中的 e205Tl 数据最有可能是局部尺度过程的指纹。另一方面，与现代环境（例如 Cariaco 和 Santa Barbara 盆地）相比，在开阔海洋环境中沉积的页岩可能捕获了全球均质的海水 e205Tl (e205TlSW)。继续这一假设，我们发现埃迪卡拉纪公海 e205TlSW 值主要与大块上陆地壳相当（e205Tl = ~-2）。这些相对较重的公海 e205TlSW 意味着在埃迪卡拉纪期间的很长一段时间内，全球海洋沉积物中的锰氧化物埋藏有限，并且推断也限制了 O2 在全球海洋底水中的积累。仅在~635 Ma 和~580 Ma 看起来非常短暂的时间段（≪5-1000 万年）中，公海 e205TlSW 的值比大块上陆地壳轻得多（例如，e205TlSW 低至 -4.4 ± 0.2；~635 Ma 时的 2SD 和 − 5.3 ± 0.3：~580 Ma 时的 2SD）。这些非常罕见的负 e205TlSW 值需要全球增强的 Mn 氧化物埋藏在海洋沉积物中的短期事件，并且意味着也需要全球海洋底层水中 O2 增强积累的短暂事件。在解释地球化学数据时，重要的是只有在识别和解释局部尺度的叠印之后才能进行全球尺度的推论。自己应用这一策略，我们发现了埃迪卡拉纪全球海洋的令人信服的证据，该海洋在动物崛起期间仅非常短暂地经历了广泛的底水氧化。