Abstract Sediment–water exchange of dissolved organic matter (DOM) represents a critical yet poorly constrained component of the marine carbon cycle. Here, we combine quantitative, molecular, and carbon isotope (δ 13 C and F 14 C) analyses to investigate the production, composition, and export of porewater DOM in the German Bight, North Sea, and evaluate its broader implications using a global data set. In the German Bight, transport‐regulated diagenetic pathways govern porewater DOM signatures. Diffusion‐limited fine‐grained sediments act as “selective reservoirs”, accumulating younger, terrestrially derived, and compositionally less refractory DOM via preferential remineralization of labile marine carbon. Conversely, permeable sands function as “biocatalytic filters” where advection‐driven oxygen infiltration fosters rapid mineralization of fresh intermediates, leaving a transformed, refractory residual pool. Globally, sediment organic carbon (OC) exerts a first‐order control on porewater dissolved organic carbon (DOC) concentration and F 14 C signatures, while its δ 13 C is influenced by external inputs such as methane‐derived carbon at seeps. Benthic fluxes export younger, reactive compounds that facilitate the molecular renewal of bottom‐water DOM. By utilizing negative‐pressure techniques to minimize sampling artifacts, we obtain diffusive DOC flux estimates (0.48 and 0.12 mol C m −2 yr −1 for shelf and slope/deep‐sea sediments, respectively) that avoid the systematic overestimation inherent in post‐freeze‐thaw centrifugation. Collectively, these findings emphasize that porewater DOM is a dynamic intermediary, regulated by sediment transport regimes, that actively shapes the quantity, composition, and long‐term cycling of the marine DOM reservoir.
Wei et al. (2026) studied this question.