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

稳定的铬同位素变化最近发现在长达30亿年的带状铁形成（BIFs; 53铬富集）和古土壤（53铬贫化）解释提供了证据，当时的大气中的自由氧。然而，这比2.4 Gyr大氧化事件（GOE）早了约6亿年，该事件由硫同位素的质量无关分馏（MIF-S）的消失和岩石记录中氧化风化的宏观表现定义。这些铬同位素数据的解释是基于这样的假设，即土壤中的铬进行氧化风化，形成可溶性的53铬富集的铬（VI）-池，被运送到海洋中，并通过还原相互作用和铁的吸附沉积在BIF。与此同时，氧化风化留下了一个贫53铬的土壤水库。提出了新的证据，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1.85 Gyr年龄的后GOE古土壤的53 Cr富集值具有明显的Cr氧化损失和Fe保留，这与古化学沉积物和古土壤中稳定铬同位素数据的均变论解释不同。在这项研究中，我们的目的是调查减少含水层和海洋和稳定的铬同位素分馏与铬（III）吸附到不同的矿物表面铬的流动性。从铬同位素分馏实验和铬形态和地下水剖面的同位素组成的结果将进行比较，铬同位素从2.45 Gyr前GOE古土壤剖面，探讨古沉积物中的铬同位素变化的替代模型。

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.