Molecular and compound-specific δ13C signatures of terrigenous organic matter along the Southwestern Atlantic Margin

Continental margins are efficient sinks for organic carbon, integrating marine production with terrigenous organic matter (OM) supplied by riverine systems. Identifying and distinguishing terrestrial OM sources is therefore essential for understanding land-ocean carbon transfer. Here, molecular distributions and compound-specific δ 13 C values of long-chain n- alkanes were analyzed in surface sediments along the Southwestern Atlantic Margin (SAM; 23°–37°S) to characterize terrigenous OM inputs under modern environmental conditions. The combined use of n- alkane indices (ACL 25–33 , Norm 31 ) and homolog-specific δ 13 C values reveals three main continental contributions: (i) grassland-dominated OM exported by the Río de la Plata (RdlP), (ii) tropical rainforest-derived OM from rivers draining the São Paulo Bight, and (iii) arid vegetation-derived OM from the Paraíba do Sul River basin. OM associated with the RdlP and Paraíba do Sul rivers shows higher ACL 25–33 and Norm 31 values and relatively enriched δ 13 C signatures, whereas rainforest-derived OM is characterized by lower molecular indices and more depleted δ 13 C values. Spatial patterns indicate that RdlP-derived OM extends farther northeastward and toward the continental slope than previously recognized, while local riverine inputs and shelf circulation promote strong mixing within the São Paulo Bight. Systematic offsets between δ 13 C values of n- C 29 and n- C 31 further suggest that homolog-specific isotopic signatures reflect hydroclimatic controls and grass inputs rather than simple C 3 -C 4 mixing alone. By defining representative molecular and isotopic end-members for major terrigenous OM sources to the SAM, this study provides a robust geochemical framework for tracing continental inputs in marine sediments and establishes a reference for paleoenvironmental reconstructions of vegetation, hydroclimate, and land-ocean carbon exchange.