Cr(III)-mobility and isotopic effects by adsorption on Mn- and Fe-(hydr-)oxides in the Critical Zone and in marine sediments: Doubts that Mesoarchean Cr isotopic variations directly record free atmospheric oxygen

临界区和海洋沉积物中锰和铁（氢）氧化物吸附引起的 Cr(III) 迁移率和同位素效应：对中太古代 Cr 同位素变化直接记录大气中游离氧的怀疑

• 批准号: 277010024
• 负责人: Professor Dr. Ronny Schönberg, Ph.D.
• 金额: --
• 依托单位: Arbeitsgruppe Isotopengeochemie
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2019-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/277010024?language=en
• 关键词: Cr; III; mobility; isotopic; effects

Stable chromium isotopic variations recently found in up to 3 billion year old banded iron formations (BIFs; 53Cr enriched) and paleosols (53Cr-depleted) were interpreted to provide evidence for free atmospheric oxygen at that time. This, however, is some 600 million years earlier than the 2.4 Gyr Great Oxidation Event (GOE) defined by the disappearance of mass-independent fractionation of sulfur isotopes (MIF-S) and macroscopic manifestations of oxidative weathering in the rock record. The interpretation of these Cr isotope data is based on the presumption that Cr in soils underwent oxidative weathering forming a soluble 53Cr-enriched Cr(VI)-pool that was transported into the oceans and deposited in BIFs via reductive interaction and adsorption with iron. Meanwhile, oxidative weathering left behind a 53Cr-depleted soil reservoir. New evidence has been put forward, such as heavy stable Cr isotopic compositions of serpentinized ultramafic rocks, the lack of any Cr isotopic variations in Proterozoic near-surface chemical sediments deposited under (low-) O2 conditions or our own 53Cr-enriched values of a 1.85 Gyr old post-GOE paleosol with clear oxidative loss of Cr and retention of Fe, that argue against such an uniformitarian interpretation of stable Cr isotope data in ancient chemical sediments and paleosols. In this study we aim to investigate Cr mobility in reduced aquifers and oceans and stable Cr isotope fractionation associated with adsorption of Cr(III) onto different mineral surfaces. Results from Cr isotope fractionation experiments and Cr speciation and isotope composition of a groundwater profile will be compared to Cr isotopes from a 2.45 Gyr pre-GOE paleosol profile to investigate alternative models for Cr isotope variations in ancient sediments.

最近在长达 30 亿年的带状铁地层（BIF；富含 53Cr）和古土壤（贫 53Cr）中发现的稳定铬同位素变化被解释为当时大气中游离氧的存在提供了证据。然而，这比 2.4 Gyr 大氧化事件 (GOE) 早约 6 亿年，GOE 是由硫同位素质量无关分馏 (MIF-S) 的消失和岩石记录中氧化风化的宏观表现所定义的。对这些 Cr 同位素数据的解释基于以下假设：土壤中的 Cr 经历了氧化风化，形成可溶性富含 53Cr 的 Cr(VI) 池，该池被输送到海洋中，并通过还原相互作用和与铁的吸附沉积在 BIF 中。与此同时，氧化风化留下了贫 53Cr 的土壤库。已经提出了新的证据，例如蛇纹石化超镁铁岩的重稳定 Cr 同位素组成、在（低）O2 条件下沉积的元古代近地表化学沉积物中缺乏任何 Cr 同位素变化或我们自己的 53Cr 富集值1.85 Gyr 的古土壤，具有明显的 Cr 氧化损失和 Fe 保留，反对对古代化学沉积物中稳定 Cr 同位素数据的均变论解释和古土壤。在这项研究中，我们的目的是研究减少含水层和海洋中的铬迁移率，以及与不同矿物表面吸附 Cr(III) 相关的稳定铬同位素分馏。 Cr 同位素分馏实验以及地下水剖面的 Cr 形态和同位素组成的结果将与 2.45 Gyr 前 GOE 古土壤剖面的 Cr 同位素进行比较，以研究古代沉积物中 Cr 同位素变化的替代模型。