THE ELEMENTARY AND ISOTOPIC CHARACTERIZATION AND DYNAMICS FROM SEDIMENTARY ORGANIC MATTER ALONG THE SOUTHERN BRAZILIAN CONTINENTAL SHELF

Continental shelves are key compartments in the global carbon cycle. However, studies on organic matter (OM) in sediments remain scarce along the Southern Brazilian Continental Shelf (SBCS). As part of the Brazilian continental margin, the SBCS is still understudied regarding OM dynamics and its role in carbon export to the South Atlantic Ocean. To address this gap, the elemental and stable isotopic composition of organic carbon and nitrogen were used to characterize and differentiate surface sediments across the SBCS. Sediment samples were collected along three transects (28°-33°S) (Santa Marta-SC, Torres-RS, and Albardão-RS), extending offshore from 13.5 m to 1500 m depth Analyses of total organic carbon (TOC) and total nitrogen (TN), and stable isotope ratios (δ¹³C and δ¹⁵N) were performed. Samples were lyophilized to constant weight and TOC, TN, δ¹³C and δ¹⁵N were measured using an Organic Elemental Analyzer (OEA) coupled to a Thermo Scientific Delta V Isotope Ratio Mass Spectrometer (IRMS). δ¹³C values ranged from -21.7 to -20.0‰ in Albardão, -22.2 to -20.0‰ in Torres, and -20.7 to -20.0‰ in Santa Marta.. δ¹⁵N values ranged from 2.7 to 6.4‰ in Albardão, 2.6 to 4.8‰ in Torres, and 4.3 to 5.3‰ in Santa Marta. Higher isotopic ratios of and δ¹⁵N (i.e., less negative δ¹³C and higher δ¹⁵N) were associated with finer sediments, which were more abundant in Albardão and Santa Marta, the same areas that showed the highest TOC and TN concentrations. All δ¹³C values were within the marine range (-22.2 to -20.0‰), suggesting negligible continental OM input, further supported by C/N ratios aligned with marine sources. A strong linear correlation between TOC and TN confirmed the predominance of recent, marine-derived OM in the shelf sediments. Similar TOC-TN relationships have been observed in surface sediments from areas with minimal terrestrial input, such as the Adriatic Sea and the Arctic Ocean, contrasting with weaker correlations found off Bohai, China, were terrestrial inputs are substantial. OM content in sediments is directly influenced by grain size, which reflects the geomorphological settings of the shelf that control sediment deposition. The elemental and isotopic signatures of carbon and nitrogen proved to be effective tracers of OM sources and dynamics across the SBCS, offering valuable insights into sedimentation processes and carbon cycling in the region.