Lateral transport as a major control for old organic carbon ages in the SW Iberian margin

On continental margins, the vertical flux of particulate organic carbon (POC) attenuates rapidly. However, the role of lateral transport in redistributing and preserving older carbon is a major unresolved question. To quantify these carbon pathways, we integrate data from three complementary sources in the SW Iberian margin, a key mid-latitude region for (paleo)climate studies: a year-long (December 2023-November 2024) sediment trap time-series (four traps intercepting subsurface and deep layers on the mid- and lower slope), discrete-depth in-situ pump sampling (>1000 L per depth) at six stations, and surface sediment samples.

Complementary CTD and hydrographic data revealed distinct water masses: the Eastern North Atlantic Central Water (ENACW; ~50–500 m), underlain by the warm, saline Mediterranean Outflow Water (MOW; 500–1600 m), characterized by elevated turbidity, and the Northeast Atlantic Deep Water (NEADW; >1700 m).

The annual sediment trap record reveals subsurface Δ¹⁴C-POC ranges between -20 and -75‰ (i.e., 100-550 ¹⁴C yr BP), while deep-water POC shows highly variable, older signatures, varying between -50 and -130‰ (i.e., 350-1070 ¹⁴C yr BP). A pronounced Δ¹⁴C depletion in May at both moorings, coincident with MOW intensification onshore, signals a major lateral injection of aged carbon.

Along the water column during the oligotrophic season, discrete-depth samples show that POC concentrations peak at the fluorescence maximum (above 100 m depth) before declining sharply. Δ¹⁴C values above 100 m indicate POC that has incorporated bomb-¹⁴C. Below ~100 m, Δ¹⁴C decreases markedly, especially within local turbidity maxima across all water masses. Notably, Δ¹⁴C depletion within the MOW-intermediate nepheloid layer (INL) was not distinct from other INLs, suggesting that lateral transport operates broadly along the margin. Preliminary data indicate higher aluminum (Al) at depth at all stations, suggesting the lateral supply of resuspended sediments. Ongoing Al and δ¹³C-POC analyses will clarify the origin of this and sediment trap material.

In surface sediments, Δ¹⁴C and δ¹³C of sedimentary OC indicate the increase of more recalcitrant (older, potentially terrestrial) OC offshore. Critically, sedimentary OC (¹⁴C age: 875-4800 ¹⁴C yr BP) is consistently older than coeval, bomb-¹⁴C–bearing planktic foraminifera. This decoupling demonstrates that laterally advected, mineral-protected organic matter is preferentially sequestered, while vertically exported labile carbon is degraded.

Our findings establish lateral transport as a major control on the age and redistribution of OC in this dynamic margin. We conclude that accurate carbon cycling models must explicitly account for lateral supply (particularly via nepheloid layers) as a key mechanism for delivering and preserving aged carbon in deep-sea sediments, challenging the traditional paradigm of vertical export as the principal sequestration pathway.