Shelf-slope variability and permafrost influence shape refractory porewater dissolved organic matter across the East...

Dissolved organic matter (DOM) plays a central role in Arctic carbon cycling. However, its molecular composition in sediment porewaters, which links carbon burial and benthic-pelagic exchange, remains poorly constrained. Under ongoing Arctic amplification, increasing terrestrial inputs and subsea permafrost thaw are expected to modify early diagenetic carbon processing across continental shelves. Here, we investigated sediment porewater DOM along an East Siberian Sea-Chukchi shelf-slope transect using fluorescence spectroscopy and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to resolve molecular composition, sources, and transformation pathways. Porewater DOM was dominated by lignin- and carboxyl-rich alicyclic molecule (CRAM)-like compounds (33.6-66.5%), indicating persistent terrestrial influence coupled with extensive microbial reworking. Nearshore sediments exhibited elevated dissolved organic carbon (DOC) concentrations, enhanced nutrient regeneration, and stronger humic-like fluorescence signatures, together with higher proportions of carbohydrate-, protein-, and aliphatic-like formulas, consistent with fluvial and permafrost-derived inputs and active early diagenesis. In contrast, slope porewaters were enriched in lipid-like, unsaturated hydrocarbon-like, and high-m/z compounds (∼712), reflecting advanced in situ microbial transformation. Chukchi shelf sediments displayed enhanced condensed aromatic signatures but limited accumulation of structurally stabilized refractory DOM. Nitrogen-containing formulas comprised a substantial fraction of porewater DOM (44.8 ± 8.3%, up to 57.1% on the slope), highlighting pronounced heteroatom enrichment during microbial reprocessing. Refractory components, including CRAM (5.0-45.2%) and island-of-stability (IOS) compounds (0-8.3%), exhibited marked spatial variability, with the outer shelf acting as a preferential zone for refractory DOM accumulation. These results demonstrate that carbon stabilization along the Arctic margin is governed by the interplay among terrestrial supply, permafrost influence, microbial transformation, and hydrodynamic regime. Continued Arctic warming may therefore shift the balance between rapid carbon turnover and long-term stabilization in shelf sediments.