Limited oxygen production in the Mesoarchean ocean
Limited oxygen production in the Mesoarchean ocean
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DOI：
10.1073/pnas.1818762116
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发表时间：
2019-04-02
影响因子：
11.1
通讯作者：
Bekker, Andrey
中科院分区：
文献类型：
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作者：
Ossa, Frantz Ossa;Hofmann, Axel;Bekker, Andrey
太古代是地球表面主要缺氧的时期，厌氧过程控制着生物必需元素循环。与有据可查的新太古代[2.8至25亿年前(Ga)]的“氧绿洲”相比，中太古代(3.2至2.8 Ga)表面海洋氧化的幅度、空间范围和潜在原因仍然存在争议。在这里，我们报告了 Delta N-15 和 Delta C-13 值以及 Mozaan 群（南非 Pongola 超群）中太古代页岩的当地海水氧化还原数据，这些页岩是在大约 2.95 至 2.85 Ga 之间的锰（羟基）氧化物沉淀增强期间沉积的。铁和 Mn 氧化还原系统与中太古代缺氧海洋中的氧绿洲一致，但 Delta N-15 数据表明存在一个基于钼的固氮生物圈，没有令人信服的证据表明存在显着的需氧氮循环。我们认为，与新太古代相比，溶解的 O-2 水平要么太低，要么范围太有限，无法在中太古代海洋中形成大型且稳定的硝酸盐库。由于生物 N-2 固定在这种环境中明显活跃，产生 O-2 的生物体的生长和增殖可能受到氮以外的营养物质（例如磷）的抑制，这可能限制了中太古代时期含氧条件的扩张。
The Archean Eon was a time of predominantly anoxic Earth surface conditions, where anaerobic processes controlled bioessential element cycles. In contrast to "oxygen oases" well documented for the Neoarchean [2.8 to 2.5 billion years ago (Ga)], the magnitude, spatial extent, and underlying causes of possible Mesoarchean (3.2 to 2.8 Ga) surface-ocean oxygenation remain controversial. Here, we report delta N-15 and delta C-13 values coupled with local seawater redox data for Mesoarchean shales of the Mozaan Group (Pongola Supergroup, South Africa) that were deposited during an episode of enhanced Mn (oxyhydr)oxide precipitation between similar to 2.95 and 2.85 Ga. Iron and Mn redox systematics are consistent with an oxygen oasis in the Mesoarchean anoxic ocean, but delta N-15 data indicate a Mo-based diazotrophic biosphere with no compelling evidence for a significant aerobic nitrogen cycle. We propose that in contrast to the Neoarchean, dissolved O-2 levels were either too low or too limited in extent to develop a large and stable nitrate reservoir in the Mesoarchean ocean. Since biological N-2 fixation was evidently active in this environment, the growth and proliferation of O-2-producing organisms were likely suppressed by nutrients other than nitrogen (e.g., phosphorus), which would have limited the expansion of oxygenated conditions during the Mesoarchean.