Molecular insights into divergent dissolved organic sulfur evolution along two plume pathways under hypoxia in the Yangtze River Estuary

Dissolved organic sulfur (DOS) is a crucial yet poorly constrained component linking carbon and sulfur cycles in coastal oceans. To understand how hydrographic dynamics influence its fate, we investigated the chemical evolution of DOS along two distinct plume pathways from the Yangtze River Estuary to the East China Sea: the northward-extending offshore branch (Yangtze River Estuary Northward, YEN) and the southward-confined coastal branch (Yangtze River Estuary Southward, YES). We combined concentration measurements with ultrahigh-resolution mass spectrometry to characterize solid-phase extracted DOS. Results revealed that DOS concentrations decreased seaward but were consistently higher in YEN than in YES. At the molecular level, DOS in the high-salinity zone of YEN exhibited significantly lower aromaticity and higher oxygen-to-carbon ratios compared to YES, indicating divergent photochemical and microbial processing between the pathways. Crucially, in the subsurface hypoxic zone of YEN, we observed a marked increase in both the molecular diversity of dissolved organic matter (DOM) and the relative abundance of sulfonated molecular formulas, providing direct molecular evidence that hypoxia actively facilitates DOM sulfurization. Our results establish that plume trajectory and oxygen availability are key regulators of DOS molecular composition and transformation pathways, offering a mechanistic framework for sulfur-carbon coupling in large river estuaries and marginal seas.