Abstract The c. 2.0 Ga Zaonega Formation of the Onega Basin (NW Russia) has been central in efforts to understand what led to the initial rise (Great Oxidation Event, GOE) and postulated fall in free atmospheric oxygen and associated high-amplitude carbon cycle excursions, the Lomagundi-Jatuli Event (LJE) and subsequent Shunga Event during Paleoproterozoic time. The Formation accumulated shortly after the LJE and encompasses both the recovery in the carbon cycle and hypothesised contraction of the oceanic oxidant pool. However, interpreting the correct environmental context recorded by geochemical signatures in the Zaonega rocks is difficult due to a complex depositional and diagenetic history. In order to robustly constrain that history, we undertook a multiproxy study (mineralogy, petrography, carbon isotope and rare earth element composition) of carbonate beds in the upper part of the Zaonega Formation recovered in the 102-m composite section of the OnZap drill-cores. Our findings differentiate primary environmental signatures from secondary overprinting and show that: (i) the best-preserved carbonate beds define an upwards increasing δ13Ccarb trend from c. −5.4‰ to near 0‰; and that (ii) large intra-bed δ13Ccarb variations reflect varying contributions of methanotrophic dissolved inorganic carbon (DIC) to the basinal DIC pool. Rare earth element and yttrium (REYSN) patterns confirm a marine origin of the carbonate beds whereas a consistent positive EuSN anomaly suggests a strong high temperature hydrothermal input during accumulation of the Zaonega Formation. Importantly, the presence of a negative CeSN anomaly in the REYSN pattern indicates an oxygenated atmosphere–ocean system shortly after the LJE and indicates that models invoking a fall in oxygen at that time require reassessment.

中文翻译：

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Lomagundi-Jatuli 事件后的含氧条件：俄罗斯古元古代 Zaonega 组的碳同位素和稀土元素特征

摘要 C. 2.0 奥涅加盆地（俄罗斯西北部）的 Ga Zaonega 地层在努力了解导致初始上升（大氧化事件，GOE）和假设的大气游离氧下降和相关的高振幅碳循环偏移，Lomagundi - Jatuli 事件 (LJE) 和随后的古元古代时期的 Shunga 事件。该地层在 LJE 后不久积累，包括碳循环中的恢复和海洋氧化剂库的假设收缩。然而，由于复杂的沉积和成岩历史，很难解释 Zaonega 岩石中地球化学特征记录的正确环境背景。为了有力地限制这段历史，我们进行了一项多代理研究（矿物学、岩石学、在 OnZap 钻芯的 102 米复合剖面中回收的 Zaonega 组上部碳酸盐岩层的碳同位素和稀土元素组成）。我们的研究结果将主要环境特征与二次叠印区分开来，并表明：（i）保存最完好的碳酸盐岩层定义了从 c 开始向上增加的 δ13Ccarb 趋势。−5.4‰至接近0‰；(ii) 大的床内 δ13Ccarb 变化反映了甲烷营养溶解无机碳 (DIC) 对盆地 DIC 库的不同贡献。稀土元素和钇 (REYSN) 模式证实了碳酸盐岩床的海洋起源，而一致的正 EuSN 异常表明 Zaonega 地层堆积过程中存在强烈的高温热液输入。重要的，