Chromium is a redox sensitive element that exhibits a large range of isotopic compositions in Earth’s surface environments because of Cr(VI)-Cr(III) transformations. This property of Cr has been exploited as a tracer of Earth’s oxygenation history using marine sediments. However, paleoredox applications using Cr are difficult to implement due to its complicated cycling, which creates spatial variability in seawater δ⁵³Cr values. Applications are further hindered by the potential for variability in the major inputs of Cr, such as submarine volcanism, to mask redox processes. Two previous reports of negative excursions in sedimentary δ⁵³Cr values during the middle Cretaceous Ocean Anoxic Event 2 (OAE 2) demonstrate these complications. Observed negative shifts in marine sediments conflict with the positive shifts expected in response to the increased drawdown of isotopically light Cr(III) prompted by the expansion of anoxic depositional sinks. In this study, a marine carbonate succession cored from the Eagle Ford Formation in Texas, USA, in the southern part of the Western Interior Seaway, depicts the negative 1.5‰ δ⁵³Cr excursion occurring in two steps, with the second step reaching peak minimum values indistinguishable from isotopically unfractionated igneous sources. In contrast to published δ⁵³Cr records, each step stratigraphically matches proxy evidence for increased eruption frequency and/or intensity of volcanic activity using combined ¹⁸⁷Os/¹⁸⁸Os, ⁸⁷Sr/⁸⁶Sr and Os concentration proxies previously measure from the same core, supporting higher inputs of volcanically sourced Cr to the oceans as the driver for the negative Cr isotope excursion.

铬是一种氧化还原敏感元素，由于 Cr(VI)-Cr(III) 的转变，在地球表面环境中表现出大范围的同位素组成。Cr的这一特性已被利用作为地球氧化历史的示踪剂，使用海洋沉积物。然而，使用 Cr 的古氧化还原应用由于其复杂的循环而难以实施，这会导致海水 δ ⁵³ Cr 值的空间变化。Cr 的主要输入（例如海底火山活动）可能会掩盖氧化还原过程，从而进一步阻碍了应用。先前关于沉积 δ ^{53负偏移的两份报告}中白垩纪海洋缺氧事件 2 (OAE 2) 期间的 Cr 值证明了这些并发症。观察到的海洋沉积物负变化与预期的正变化相冲突，这些正变化是由于缺氧沉积池的扩大而导致同位素轻铬 (III) 的下降增加。在这项研究中，位于美国德克萨斯州西部内陆航道南部的 Eagle Ford 组取芯的海相碳酸盐岩层序描述了 1.5‰ δ ⁵³ Cr 负偏移分两步发生，第二步达到峰值最小值与同位素未分馏的火成源无法区分的值。与公布的 δ ^53相比Cr 记录，使用先前从同一核心测量的¹⁸⁷ Os/ ¹⁸⁸ Os、⁸⁷ Sr/ ⁸⁶ Sr 和 Os组合浓度代理，每个步骤在地层上与增加喷发频率和/或火山活动强度的代理证据相匹配，支持更高的火山来源输入Cr 到海洋是负 Cr 同位素偏移的驱动力。