Sources and Fate of Sedimentary Organic Matter in the Western Mediterranean Sea

Marine sediments comprise the primary long-term sink of organic matter (OM) in marine systems. Disentangling the diverse origins of OM and the influence of the main processes that determine organic carbon (OC) fate at a global scale has proven difficult due to limited spatial data coverage. Thus, comprehensive studies of the spatial distribution of the content and geochemical characteristics of sedimentary OM at basin scales provide fundamental knowledge on the role of marine sediments in the global carbon cycle. Here, we shed light on the origin of OM and the underlying mechanisms that determine its fate in a semi-enclosed basin by examining the spatial patterns in the isotopic and elemental composition of OM in 149 core-top samples from the Western Mediterranean Sea and the adjacent Atlantic Ocean sector. Our results reveal an apparent SW-NE gradient that reverses in the Gulf of Lions in most geochemical and sedimentological features. Changes in the OC content and ẟ¹³C and Δ¹⁴C signatures are ascribed to spatial variations in marine primary productivity and the influence of varying discharge of rivers and well-developed canyons that favor the cross-shelf transport of terrestrial (and petrogenic) OC. Our results also suggest the potential influence of two other mechanisms on the geochemical signatures of OM: (a) lateral transport of allochthonous OC and selective degradation of labile OM, which potentially occurs across the studied area having a greater impact toward the north-eastern region, and (b) OM protection via association with mineral surfaces, potentially having a greater influence toward the south-western basins.