Fraction-specific V/Cr diagnoses seasonal hypoxia off the Changjiang Estuary

To establish more precise redox proxy for diagnosing seasonal hypoxic events, different fractions of V and Cr are analyzed through a modified Tessier method in surface sediment and a sediment core collected within frequent hyxpoxic regions off the Changjiang Estuary, targeting exchangeable, carbonate-bound, reducible, oxidizable, and residual fractions. These different fractions of V and Cr and contents of general geochemical parameters (e.g. Al, Fe), as well as some calculated parameter (VOXID/CrOXID) in our study, are presented in the uploaded file "Geochemical parameters". Different from permanently hypoxic systems, the hypoxia in the Changjiang Estuary is seasonal/periodic. Therefore, the dissolved oxygen concentrations (DO) in bottom water are key parameter in quantitatively reflecting its redox conditions. Dataset of DO concentrations in bottom water off the Changjiang Estuary in the past decade (2014 - 2023) are compiled from previous studies and showed in the uploaded file "DO dataset off the Changjiang Estuary". Comprehensive details regarding the sources of these data are provided in the main article. Different fractions of trace metals in marine sediments is rarely investigated. To prove the potential of fraction-specific RSEs as geochemical proxies in discriminating different redox sedimentary environment, on a global scale, the dataset of oxidizable fraction of various RSEs in limited studies is compiled, which is exhibited in the upload file "RSE data compilation". Comprehensive details regarding the sources of these data are provided in the main article.为构建用于诊断季节性低氧事件的更精准氧化还原代用指标（redox proxy），本研究针对长江口外（Changjiang Estuary）常年频发低氧区域采集的表层沉积物与沉积物岩芯，采用改进的Tessier分级提取法（modified Tessier method）对钒（V）、铬（Cr）的不同赋存形态进行分析，目标形态涵盖可交换态、碳酸盐结合态、铁锰氧化物结合态、有机结合态与残渣态。本研究中获得的V、Cr各赋存形态含量、常规地球化学参数（如铝Al、铁Fe）含量，以及部分计算得到的参数（如VOXID/CrOXID）均整理于上传文件"Geochemical parameters"中。 与永久性低氧系统不同，长江口低氧具有季节性/周期性特征。因此，底层水溶解氧浓度（Dissolved Oxygen, DO）是定量反映其氧化还原环境的关键参数。本研究整合了2014-2023年近十年长江口外底层水DO浓度数据集，该数据集整理自既往研究，收录于上传文件"DO dataset off the Changjiang Estuary"中。关于这些数据的来源细节已在正文中详细说明。 海洋沉积物中痕量金属的不同赋存形态鲜有研究。为验证分形态氧化还原敏感元素（Redox-Sensitive Elements, RSEs）作为地球化学代用指标、在全球尺度下区分不同氧化还原沉积环境的潜力，本研究整合了已有有限研究中各类RSEs的可氧化态（有机结合态）数据集，该数据集展示于上传文件"RSE data compilation"中。关于这些数据的来源细节已在正文中详细说明。