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.