Terrestrial sources as the primary delivery mechanism of mercury to the oceans across the Toarcian Oceanic Anoxic Event (Early Jurassic) Earth and Planetary Science Letters

This study evaluates the utility of sedimentary mercury (Hg) contents as a proxy for fingerprinting ancient massive volcanism, which is often associated with biogeochemical perturbations. Herein we present new Hg geochemical data from anoxic marine basins across the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) as a test of the complex Hg cycle. The T-OAE was likely initiated by the main eruptive phase of the Karoo–Ferrar large igneous province, which caused a subsequent cascade of environmental perturbations and resulting mass extinction. At present the leading interpretation of sedimentary Hg anomalies has been volcanogenic outgassing as the primary source. Our study and compilation results suggest, however, that Hg/TOC anomalies were restricted to shallow-water, and/or proximal environments, while deep-water, more distal depositional settings document no significant Hg-related anomalies. Furthermore, asynchronous stratigraphic deviations in Hg enrichments favor terrestrially sourced materials and local redox variability, rather than direct volcanogenic emissions, as a primary control mechanism. Additionally, Hg isotope signatures from our only study site documenting an Hg anomaly are also consistent with a terrestrial Hg origin during the T-OAE. Therefore, our results suggest that Hg anomalies in the geological record need to be re-evaluated as a “smoking gun” proxy that only infers volcanogenic inputs. Grant no. NA11SEC4810001

本研究评估了沉积汞（Hg）含量作为示踪古大规模火山活动的代用指标的实用性，而古大规模火山活动通常与生物地球化学扰动相关。在此，我们报道了来自托阿尔阶海洋缺氧事件（T-OAE；约1.83亿年）期间各缺氧海盆的新汞地球化学数据，以检验复杂的汞循环过程。托阿尔阶海洋缺氧事件可能由卡鲁-费拉尔大火成岩省的主喷发阶段触发，进而引发一系列环境扰动及随后的生物大灭绝。目前学界对沉积汞异常的主流解释认为，火山脱气作用是汞的主要来源。然而，我们的研究及汇编结果显示，汞/总有机碳（TOC）异常仅局限于浅水环境和/或近源沉积环境，而深水更远源沉积环境则未出现显著的汞相关异常。此外，汞富集的不同步地层偏移表明，陆源物质及局部氧化还原条件变化是汞富集的主要控制机制，而非直接的火山脱气排放。另外，我们仅有的一处记录了汞异常的研究位点的汞同位素组成，同样与托阿尔阶海洋缺氧事件期间陆源汞来源的结论相符。因此，我们的研究结果表明，地质记录中的汞异常作为仅能指示火山输入的“确凿证据”代用指标，需要被重新评估。项目编号：NA11SEC4810001