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

最近在长达30亿年的条状铁建造(BIF；富53Cr)和古土壤(53Cr亏损)中发现的稳定铬同位素变化为当时大气中的游离氧提供了证据。然而，这比2.4Gyr大氧化事件(GOE)早了约6亿年，该事件是由硫同位素质量无关分馏的消失(MIF-S)和岩石记录中氧化风化的宏观表现所定义的。对这些铬同位素数据的解释是基于这样的假设：土壤中的铬经历了氧化风化，形成了一个可溶的富~(53)Cr(VI)池，该池被输送到海洋中，并通过与铁的还原作用和吸附作用沉积在BIF中。与此同时，氧化风化留下了一个贫化了53Cr的土壤水库。新的证据被提出，如蛇纹石化超镁铁质岩石的重稳定铬同位素组成，在(低)氧气条件下沉积的元古界近地表化学沉积物中没有任何铬同位素变化，或者我们自己的1.85Gyr古老的戈尔后古土壤的~(53)Cr富集值，具有明显的铬氧化损失和铁的保留，这些都反对这种对古代化学沉积物和古土壤中稳定铬同位素数据的统一解释。在这项研究中，我们旨在研究铬在减少的含水层和海洋中的移动性，以及与不同矿物表面吸附铬(III)相关的稳定的铬同位素分馏。地下水剖面的铬同位素分馏实验结果和铬的形态和同位素组成将与2.45Gyr前GOE古土壤剖面的铬同位素进行比较，以研究古沉积物中铬同位素变化的替代模式。

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.