The evolution of the oceanic redox state before the Great Oxidation Event traced by stable tungsten isotope analyses in iron formations

通过铁地层中的稳定钨同位素分析追踪大氧化事件之前海洋氧化还原态的演变

• 批准号: 404681299
• 负责人: Dr. Florian Kurzweil, Ph.D.
• 金额: --
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/404681299?language=en
• 关键词: evolution; oceanic; redox; state; before

Free oxygen represents an essential basis for the evolution of complex life forms on the modern habitable Earth. However, in earliest Earth’s history oxygen concentrations were more than 100.000 times lower compared to present levels. The essential increase in environmental oxygen concentrations occurred during several major oxygenation steps and was tightly linked to the evolution of simple life forms and primary production by photosynthetic bacteria. We plan to trace earliest increases in oxygen concentrations by stable tungsten isotope measurements in iron formations deposited between 3.2 to 2.4 billion years ago. The deposition of these iron formations requires the oxidation of dissolved Fe2+ and the subsequent formation of poorly soluble Fe3+ hydroxides. Thereby, isotopically light tungsten is preferentially adsorbed onto such Fe3+ hydroxides leaving behind isotopically heavy seawater. In times of globally increased iron formation deposition the global tungsten isotopic composition of seawater and sediments will shift towards heavier values due to the homogeneous distribution of tungsten in the early Earth’s oceans. Thus, tungsten stable isotopes might be the first qualitative tracer for the global extend of iron formation deposition at a specific time. In comparison to other geochemical proxies such as Mo isotopes, W isotope variation is already expected as a result of slight increases in the oceanic redox state. Therefore, W isotope measurements allow for a more distinct investigation of earliest and only marginal changes in the marine redox state. Moreover, the deposition of iron formations in oxygen-rich shallow marine water habitats is closely related to biologic activity (i.e. photosynthetic cyanobacteria). Accordingly, tungsten isotopes represent a novel geochemical approach, which can also potentially trace the earliest increases in oxygen production and the very beginning of photosynthetic activity, an essential process for the evolution of complex life forms.

游离氧是现代宜居地球上复杂生命形式进化的重要基础。然而，在地球最早的历史上，氧气浓度比现在的水平低100.000多倍。环境中氧浓度的基本增加发生在几个主要的氧化步骤中，并与简单生命形式的进化和光合细菌的初级生产密切相关。我们计划通过稳定的钨同位素测量，追踪32亿至24亿年前沉积的铁建造中氧浓度最早的增加。这些铁形成物的沉积需要溶解的Fe2+的氧化，随后形成难溶的Fe3+氢氧化物。因此，同位素较轻的钨优先吸附在这些Fe3+氢氧化物上，留下同位素较重的海水。在全球铁建造沉积增加的时期，由于早期地球海洋中钨的均匀分布，海水和沉积物的全球钨同位素组成将向更重的值转变。因此，钨稳定同位素可能是特定时间全球范围内铁建造沉积的第一个定性示踪剂。与其他地球化学指标，如Mo同位素相比，由于大洋氧化还原状态的轻微增加，W同位素的变化已经被预期到。因此，W同位素测量可以更清楚地研究海洋氧化还原状态的最早和最小的变化。此外，富氧浅海生境中铁的沉积与生物活动(即光合作用的蓝藻)密切相关。因此，钨同位素代表了一种新的地球化学方法，它还可能追踪到氧气产生的最早增加和光合作用活动的开始，这是复杂生命形式进化的基本过程。