Redox-stratified seawater during the GOE: Evidences from rare earth elemental and C-O isotopic compositions of Paleoproterozoic BIF and carbonate rocks from the Taihua Group, North China Craton
Redox-stratified seawater during the GOE: Evidences from rare earth elemental and C-O isotopic compositions of Paleoproterozoic BIF and carbonate rocks from the Taihua Group, North China Craton
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DOI：
10.1016/j.oregeorev.2023.105424
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发表时间：
2023-03
影响因子：
3.3
通讯作者：
Caiyun Lan;X. Long
中科院分区：
文献类型：
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作者：
Caiyun Lan;X. Long
古元古代大氧化事件（GOE）以大气成分的剧烈变化而闻名，导致氧气成为全球表面过程的持久成分。剧烈的环境变化极大地影响了海洋和大气化学。为了研究古大洋运动过程中海洋的氧化还原状态，揭示古大洋运动过程中海洋的氧化还原结构，本文对古大洋运动过程中的条带状铁建造、绿柱石大理岩、碳酸盐岩华北陆缘庐山地区早古元古代上太华群含石墨大理岩。BIF样品表现出典型的主元素和微量元素特征，类似于非常纯的海洋化学沉积物，如页岩归一化的正La，Ga和Y异常，指示海洋沉积环境。在这些BIF样品中，负的Ce异常和Ce异常的缺乏反映了海洋中的氧和亚氧/缺氧水的存在。橄榄质大理岩样品的特征是地壳物质（如Al、Ti、Zr、Hf）含量极低，Y/Ho比值高（46-66），也意味着纯海洋化学成因。除部分受污染样品外，其余含石墨大理岩样品均表现出类似于橄榄质大理岩的特征。中朝北东辽河群关门山组2.2- 2.1Ga碳酸盐岩的特征是：中朝北东辽河群关门山组的碳酸盐岩具有明显的Ce正异常和较高的轻、重稀土元素（Pr/Yb（SN））比值。结合BIF样品中的Ce异常，我们认为上太华群沉积时的海水是由浅部的好氧水向深部的缺氧水氧化还原分层的。此外，含石墨大理岩记录了柱体中富含营养物质、有利于微生物生长和高生产力的同时代地表水。华北克拉通早古元古代松山群和吕梁群中的IFs也表现出明显的正、负Ce异常和氧化还原敏感元素的富集。因此，这些证据表明，氧化还原分层海水广泛发生在GOE。
The Paleoproterozoic Great Oxidation Event (GOE) is known for the dramatic change in atmospheric composition that led to oxygen becoming a persistent component of global surface processes. Drastic environmental changes greatly affected ocean and atmosphere chemistry. However, the distribution of dissolved oxygen in the ocean during the GOE is still unclear as rare contemporary sediment-records have been found. In order to identify the redox state of both shallow and deep water, and reveal the ocean redox structure during this time, we present new major and trace elemental and C-O isotopic data of bulk samples of the banded iron formation (BIF), dolomitic marble, and graphite-bearing marble of the early Paleoproterozoic upper Taihua Group in the Lushan area, southern margin of the North China Craton (NCC). The BIF samples exhibit typical major and trace element characteristics akin to very pure marine chemical sediments, such as the shale-normalized positive La, Ga, and Y anomalies, indicative of a marine depositional setting. In these BIF samples, both negative Ce anomalies and the lack of Ce anomalies reflect the presence of both oxic and suboxic/anoxic water in the ocean. Samples of the dolomitic marble are characterized by extremely low amounts of crustal material (such as Al, Ti, Zr, Hf) and high Y/Ho ratios (46–66), also implying a pure marine chemical origin. Excluding some contaminated samples, the rest of the graphite-bearing marble samples display similar features of the dolomitic marble. Significant positive Ce anomalies and high light to heavy REE (Pr/Yb(SN)) ratios in both dolomitic marble and graphite-bearing marble are essentially consistent with the 2.2–2.1 Ga carbonates from the Guanmenshan Formation of the Liaohe Group in the NE Sino-Korean Craton. Combined with the Ce anomalies in the BIF samples, we consider that the seawater when the upper Taihua Group deposited was redox-stratified from oxic shallow water to deeper anoxic water. Besides, the graphite-bearing marble record coeval surface water in the column that was enriched in nutritive materials and beneficial to microbiology and high productivity. Moreover, the early Paleoproterozoic IFs in the Songshan Group and the Lüliang Group in the NCC also exhibit significant both positive and negative Ce anomalies and enrichment of redox sensitive elements. These evidences, therefore, suggest that redox-stratified seawater occurred extensively during the GOE.