Towards a sedimentary organic carbon inventory of the Western Mediterranean Sea

Regional studies on the origin and fate of organic carbon (OC) in marine sediments are scarce due to limited spatial data coverage and the complex interplay among biological, physicochemical, and geological processes that can influence OC content and geochemical signatures on different spatial scales. Yet, such studies are vital to constrain global carbon inventories for ocean sediments.

To shed light on the controls on the origin, distribution, and fate of sedimentary OC in continental margins and adjacent deep-sea basins, we investigate the geochemical and sedimentological characteristics of organic matter (OM) in the semi-enclosed Western Mediterranean Sea. Here, we analyze 149 core-top samples from the Western Mediterranean Sea and the adjacent Atlantic Ocean sector and explore the spatial distribution of OC content, OC-ẟ¹³C, OC-Δ¹⁴C, C/N, grain size, and mineral surface area, among others.

Most geochemical parameters depict a clear SW-NE gradient between the westernmost and the easternmost basins. This gradient reverses in the Gulf of Lions (NW Mediterranean). Thus, OC is younger and of primarily marine origin in samples from the Atlantic sector and the Alboran Sea (SW Mediterranean). In the Algerian Basin, the Balearic Sea, and the Algero-Provencal Basin the influence of terrestrial OC input increases towards the NE characterized by the presence of highly ¹³C- and ¹⁴C-depleted (aged) sedimentary OC. Finally, samples from the Gulf of Lions show a larger influence of fresh and young OC compared to other northeastern basins.

The interplay between marine primary productivity and delivery of terrestrial OC is the main factor that determines the observed gradient. Primary productivity decreases from the southwestern basins towards the NE and increases again northeasternmost basin, the Gulf of Lions. By contrast, the terrestrial OC carbon delivered by rivers and channeled to the deeper basin by canyons has an increasing influence on sedimentary OC toward the NE.

When explored from a sedimentological context, our results reveal that lateral transport of OC and OM protection by mineral surfaces potentially act as secondary controls on the OC fate in surface sediments of the Western Mediterranean Sea.

This integrated study contributes to a better knowledge of the interplay of biological, chemical, and hydrological factors that influence the amount and geochemical characteristics of sedimentary OC in the land-sea continuum and the deeper ocean, a fundamental consideration to constraining global carbon inventories.