The chromium (Cr) isotope system is an important redox proxy for understanding Earth's oxygenation history. However, quantitative application of this proxy is hindered by a lack of clear understanding of the global Cr isotope mass balance. Estuarine environments potentially play important roles in influencing the global oceanic Cr mass balance because riverine Cr derived from continental weathering must first travel through estuaries before entering the open ocean. We collected water, suspended particulate matter, and sediment samples across a salinity gradient in the Mobile Bay Estuary located in the Northern Gulf of Mexico, with a purpose of evaluating whether Cr behaves conservatively in estuarine environments. A simple conservative mixing model between the fresh Mobile River water and Gulf of Mexico seawater alone was insufficient to explain the Cr concentration and isotope data, suggesting that Cr behaves non-conservatively in this estuary system. A model incorporating conservative mixing, partial Cr(VI) reduction and partial scavenging of the produced Cr(III) is developed to reproduce the observed data. Our data and modeling exercise suggest that up to 60% of Cr(VI) is reduced to Cr(III) in the Mobile Bay Estuary with a Cr isotope fractionation factor of −1.2‰, and 40% of the produced Cr(III) is removed from solution. Although these parameters may not be the sole solution to the system of equations involved, they are consistent with previous estimates. Future redox-specific Cr isotope data in estuarine environments are needed to place finer constraints on these parameters in estuaries, which is critical for estimating global marine Cr isotope mass balance using riverine Cr concentration and isotope data.

铬 (Cr) 同位素系统是了解地球氧化历史的重要氧化还原代理。然而，由于缺乏对全球 Cr 同位素质量平衡的清楚了解，该代理的定量应用受到阻碍。河口环境可能在影响全球海洋 Cr 质量平衡方面发挥重要作用，因为来自大陆风化的河流 Cr 在进入公海之前必须首先穿过河口。我们在位于墨西哥湾北部的莫比尔湾河口的盐度梯度上收集了水、悬浮颗粒物和沉积物样品，目的是评估 Cr 在河口环境中的行为是否保守。仅在莫比尔河新鲜水和墨西哥湾海水之间的简单保守混合模型不足以解释 Cr 浓度和同位素数据，表明 Cr 在该河口系统中表现出非保守性。开发了一个包含保守混合、部分 Cr(VI) 还原和部分清除产生的 Cr(III) 的模型来重现观察到的数据。我们的数据和建模练习表明，在莫比尔湾河口，多达 60% 的 Cr(VI) 被还原为 Cr(III)，Cr 同位素分馏因子为 -1.2‰，并且 40% 的产生的 Cr(III) 被还原为 Cr(III)从溶液中去除。尽管这些参数可能不是所涉及方程组的唯一解，但它们与先前的估计一致。