Characteristics and evolution of iron speciation during early diagenesis in the sand-mud transition zone of the East China Sea Shelf

Abstract

The early diagenesis of iron is a critical component of the biogeochemical cycling within shelf sediments; however, its variability across different sedimentary environments has been less known. This study analyzed two sediment cores (EH2-23 A and EH2-23B) from the sand-mud transition zone of the East China Sea (ECS) shelf, with a focus on sediment grain size, total organic carbon (TOC) content, ²¹⁰Pb and ¹³⁷Cs dating, iron speciation and porewater chemistry. The results reveal that the upper section of EH2-23 A and the lower section of EH2-23B comprise palimpsest sandy sediments, while the middle section of EH2-23 A consists of relict sandy sediments, all of which are characterized by low TOC%, older ages, and limited contents of highly reactive iron (Fe_HR), indicating minimal diagenetic activity. The lower section of EH2-23 A features interbedded sandy and muddy sediments, shaped by fluctuating hydrodynamic conditions and influenced by submarine groundwater discharge. In contrast, the upper section of EH2-23B comprises younger, TOC-rich muddy sediments, where early diagenesis is predominantly driven by sulfate reduction. Sandy sediments consistently exhibit higher degrees of pyritization (DOP) compared to muddy sediments, reflecting prolonged diagenetic processes. Depth-dependent patterns of the ratios of Fe_HR to total iron (Fe_HR/Fe_T) and pyrite to Fe_HR (Fe_py/Fe_HR) are classified into four distinct types. These findings contribute to a deeper understanding of early diagenesis and the C-S-Fe cycle in river-dominated shelf environments.