Rapid Down-Slope Transport of Fresh Dissolved Organic Matter to the Deep Ocean in the Eastern North Atlantic

Intense convective mixing in the central North Atlantic is a major gateway for dissolved organic matter (DOM) into the deep ocean, sustaining elevated dissolved organic carbon (DOC) concentrations. Rapid down-slope transport on adjacent Irish and Hebrides Margins represents another, less-explored mechanism contributing to the deep-sea DOM reservoir. Our analyses of solid-phase extractable DOM (SPE-DOM) in bottom waters in this region showed 7–11 μM higher DOC concentration and 190–330 years youngerSPE-DOM radiocarbon ages compared to similar depths in the open eastern North Atlantic. We estimated a down-slope DOC flux of 43 Tg C yr⁻¹ from the Irish and Hebrides shelves. During transport, conservative mixing, dominated by physical rather than biological/chemical processes, determined the molecular DOM composition, while minor particulate organic matter degradation introduced less-refractory DOM with terrigenous characteristics. Thus, rapid down-slope transport emerges as an efficient conduit for delivering fresh DOM into the deep ocean.