Geochemical insights into authigenic pyrite formation in central Yellow Sea sediments: influence of sedimentary environment and microbial sulfate reduction

Abstract

Authigenic pyrite is a key archive of paleo-redox conditions and sulfur cycling in marine sediments, yet its formation mechanisms under variable marginal sea environments remain incompletely understood. This study investigates the formation of authigenic pyrite in Holocene sediments from the central Yellow Sea, focusing on its morphology, sulfur isotopes, and environmental controls. Pyrite formation is primarily constrained by the availability of reactive organic carbon, as revealed by total organic carbon–total sulfur relationships (C/S ratios) and reactive iron content. High C/S ratios suggest that the supply of organic matter, rather than the availability of sulfate or iron, plays the dominant role. Framboidal pyrite is the most common morphology. These framboids consist of tightly packed microcrystals, primarily octahedral and truncated octahedral in shape, indicating rapid precipitation under early diagenetic, low-oxygen, and low-energy conditions. The sulfur isotopes of pyrite (δ³⁴S_pyr, Vienna Canyon Diablo Troilite; V-CDT) range from −47.4 ‰ to −7.8 ‰, reflecting large fractionations associated with microbial sulfate reduction (MSR). The vertical δ³⁴S_pyr profile shows transitions from open- to closed-system MSR regimes, modulated by changes in sedimentation rate, freshwater input, and stratification linked to the Yellow Sea Cold Water Mass. These findings reveal the interplay between sedimentary environment, pyrite morphology, and sulfur isotope composition, offering insights into sulfur cycling in modern marginal marine settings and ancient analogues.