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30亿年前的带状铁地层（富53Cr）和古土壤（贫53Cr）中发现了稳定的铬同位素变化，为当时大气中游离氧的存在提供了证据。然而，这比2.4 Gyr的大氧化事件（GOE）早了大约6亿年，这是由硫同位素的质量无关分异（MIF-S）的消失和岩石记录中氧化风化的宏观表现所定义的。这些Cr同位素数据的解释是基于这样的假设，即土壤中的Cr经历了氧化风化，形成了一个可溶性的富含53cr的Cr（VI）池，通过与铁的还原相互作用和吸附被输送到海洋中并沉积在bif中。同时，氧化风化留下了一个53cr枯竭的土壤水库。新的证据，如蛇纹岩化超基性岩石的重稳定Cr同位素组成，（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goe后古土壤中Cr的53cr富集值明显氧化损失和铁的保留，这些证据都反驳了对古代化学沉积物和古土壤中Cr稳定同位素数据的均变解释。在这项研究中，我们的目的是研究Cr在含水层和海洋中的迁移率，以及与Cr（III）在不同矿物表面吸附有关的稳定Cr同位素分馏。Cr同位素分馏实验结果和地下水剖面的Cr形态和同位素组成将与2.45 Gyr前goe古土壤剖面的Cr同位素进行比较，以探讨古沉积物中Cr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.