Shelf-to-deep sea transfer of organic matter in the south-west Atlantic constrained by lipid biomarkers and radiocarbon

The source-to-sink dynamics of organic matter (OM) in surface sediments (0–2 cm) of the Santos Basin were investigated using a multi-proxy approach combining lipid biomarkers (sterols, alcohols, and fatty acids) and bulk radiocarbon analysis ( 14 C OC ). Samples were collected along 11 isobaths (25–2,400 m depth) from eight cross-margin transects, with additional stations from the São Paulo Plateau. Statistical analysis revealed four biogeochemical provinces. Organic-poor inner-shelf sediments (25–50 m) exhibited lipid assemblages dominated by labile compounds, including polyunsaturated fatty acids (PUFAs) and planktonic sterols, indicating fresh marine inputs. The organic-rich mid-shelf mud belt (75–100 m) showed the highest lipid concentrations yet strong microbial alteration, reflected by elevated stanol/stenol ratios, depleted PUFA signatures, and centennial-scale 14 C OC ages, indicating prolonged OM residence, active nepheloid-layer reworking, and contributions from pre-aged terrestrial material. Outer shelf and slope sediments (150–1,900 m) displayed enhanced terrestrial lipid markers and centennial- to millennial-scale 14 C OC ages, pointing to lateral input from shelf depocenters and riverine sources, with additional OM aging near the Cabo Frio region driven by upwelling-related hydrodynamics. Localized near-modern ages indicate episodic rapid vertical export of marine OM to deep-sea sediments. The São Paulo Plateau and adjacent lower slope (1,400–2,500 m) showed low lipid concentrations but molecular signatures of fresh marine OM, consistent with efficient transfer of surface carbon to the seafloor under nutrient-rich conditions. These findings highlight the importance of hydrodynamic processes, biological activity, and sediment remobilization in shaping sedimentary OM composition and preservation in subtropical margins.