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.

最近在多达30亿年历史的带铁地层（BIF； 53CR富集）和古溶（53cr耗尽）中发现的稳定的同位素变异被解释为当时提供自由大气氧的证据。然而，这比2.4 GYR大氧化事件（GOE）早6亿年，该事件是由硫同位素（MIF-S）的质量独立分馏（MIF-S）和岩石记录中氧化风化的宏观表现而定义的。这些CR同位素数据的解释是基于这样的假设：种族中的CR经历了氧化性风化，形成了可溶性53CR的CR（VI）池，该池被运输到海洋中，并通过还原性相互作用沉积在BIFS中，并与铁一起吸收。同时，氧化风化留在了53 cr耗尽的土壤储层后面。 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古代化学沉积物和古溶质中稳定CR同位素数据的统一解释。在这项研究中，我们旨在研究含水层和海洋减少的CR迁移率以及与CR（III）在不同矿物表面上吸附相关的稳定的CR同位素分馏。 CR同位素分馏实验以及CR形成和地下水谱的同位素组成的结果将与来自2.45 Gyr Pre-Goe古质溶液的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。