Stable chromium (Cr) isotopes have emerged as a new tool for tracking broad-scale changes in Earth's surface oxygen levels. Carbonates are one proposed sedimentary Cr isotope archive. In order to contribute to the development of a robust framework for interpreting carbonate Cr isotopic compositions and evaluating their ability to record the global redox state, we explored Cr isotope systematics of modern and Archean carbonate successions—representing end member oxic and anoxic Earth system states. We generated new data from carbonate platform sediments from the Great Bahama Bank with variable post-depositional histories to better understand the effects of diagenetic alteration on carbonate-bound Cr and its isotopic composition in a modern well-oxygenated ocean-atmosphere system. More specifically, we investigated the effects of dolomitization and aragonite-to-calcite neomorphism in marine and meteoric fluids. We also present δ⁵³Cr values from three carbonate successions, the ∼3.0 Ga Chobeni Formation (South Africa), ∼2.8 Ga Mosher Carbonate Formation (Canada), and ∼2.65 Ga Cheshire Formation (Zimbabwe), that were deposited under an anoxic atmosphere. We find that modern Bahamian carbonates have a large range of almost exclusively positive δ⁵³Cr values (from −0.04 to 2.88‰). The δ⁵³Cr values appear to be altered during both meteoric diagenesis and dolomitization but there may also be instances of rock-buffered conditions that may best preserve depositional δ⁵³Cr values. These observations provide a baseline for carbonate Cr isotope behavior in a well-oxygenated ocean-atmosphere system. Our Archean carbonate successions contain both positively fractionated and crustal δ⁵³Cr values, ranging between −0.37 and 0.89‰. The Cr isotope fractionation observed in Archean strata is most plausibly linked to either non-redox dependent Cr isotope fractionation, local Cr redox cycling, late-stage diagenetic alteration, or some combination thereof. Given the significant effects of post-depositional alteration on the isotopic composition of carbonate-bound Cr, we suggest that Cr isotope values from ancient carbonate sediments should be generally interpreted with caution and cannot be straightforwardly linked to changes in atmospheric oxygen abundance.

稳定的铬（Cr）同位素已经成为一种追踪地球表面氧含量大范围变化的新工具。碳酸盐岩是一种拟议的沉积铬同位素档案。为了有助于开发一个强大的框架来解释碳酸盐Cr同位素组成并评估其记录全球氧化还原状态的能力，我们探索了现代碳酸盐岩和太古代碳酸盐演替的Cr同位素系统-表示末端成员有氧和无氧地球系统的状态。我们从大巴哈马河岸的碳酸盐台地沉积物中产生了新的数据，并得出了不同的沉积后历史，以更好地了解成岩作用对现代氧合良好的海洋-大气系统中碳酸盐结合的Cr及其同位素组成的影响。进一步来说，我们研究了白云石化作用和文石到方解石新晶在海洋和大气中的作用。我们还介绍了δ ^53个从三个碳酸盐继承Cr值，则〜3.0嘎Chobeni形成（南非），〜2.8嘎莫舍碳酸盐岩地层（加拿大）和~2.65嘎柴郡形成（津巴布韦），缺氧气氛下沉积在。我们发现，现代巴哈马碳酸盐有一个大的范围几乎完全积极的δ ^53个Cr值（从-0.04到2.88‰）。该δ ⁵³ Cr值出现在两台大气成岩作用和白云岩中改变，但也有可能是岩石缓冲条件情况下，可能最好地保持沉积δ ⁵³Cr值。这些观察结果为氧合良好的海洋-大气系统中碳酸盐Cr同位素行为提供了基线。我们的宙碳酸盐继承同时含有正和分馏地壳δ ⁵³ Cr值，-0.37和0.89之间的范围内‰。在太古宙地层中观察到的Cr同位素分馏与非氧化还原依赖性Cr同位素分馏，局部Cr氧化还原循环，后期成岩作用变化或它们的某些组合最合理地相关。考虑到沉积后蚀变对碳酸盐结合的Cr同位素组成的重大影响，我们建议应谨慎地解释古代碳酸盐沉积物中的Cr同位素值，不能直接将其与大气氧含量的变化联系起来。