Spatial Distribution of Sedimentary Organic Matter on the Southern Continental Shelf of Rio Grande do Sul: Elemental and Isotopic Evidence

The continental shelf of Rio Grande do Sul is a region characterized by intense interactions between continental and oceanic processes, influencing the distribution and composition of sedimentary organic matter. In this context, elemental and isotopic parameters are important tools for characterizing organic matter and understanding the biogeochemical dynamics of the environment. Thus, this study aimed to analyze the spatial distribution of total organic carbon (TOC), total nitrogen (TN), TOC/TN ratio, δ¹³C, and δ¹⁵N in surface sediments from the southern continental shelf of Rio Grande do Sul. A total of 24 surface sediment samples were collected at depths ranging from 20 to 100 m during the INCT Biodiversidade da Amazônia Azul cruise in September 2025, using a Van Veen grab sampler. Samples were freeze-dried, decarbonated by exposure to HCl vapor, oven-dried, and analyzed at the Integrated Analysis Center of the Federal University of Rio Grande using an elemental analyzer coupled to an isotope ratio mass spectrometer (EA-IRMS). The stations closest to the coast, characterized by higher hydrodynamic energy and coarser sediments, presented concentrations below the quantification limit, preventing the acquisition of elemental and isotopic data. TOC and TN showed similar spatial patterns, with a bathymetric gradient characterized by increasing concentrations toward the outer shelf, particularly at 100 m depth, as well as a tendency toward lower values in the southern portion of the study area. TOC concentrations ranged from 0.181 to 1.366%, with a mean of 0.7 ± 0.4%, while TN ranged from 0.023 to 0.172%, with a mean of 0.08 ± 0.05%. The TOC/TN ratio ranged from 7.70 to 9.58, with a mean of 8.4 ± 0.6, indicating a predominantly marine-derived source of organic matter. The spatial patterns of TOC and TN were also associated with sediment grain size and hydrodynamic conditions, as the stations closest to the coast, characterized by coarser sediments and higher hydrodynamic energy, presented lower concentrations. The δ¹³C values ranged from −21.28 to −19.23‰, with a mean of −20.5 ± 0.5‰, showing a narrow range of variation and reinforcing the predominance of marine-derived organic matter indicated by the TOC/TN ratio. In contrast, δ¹⁵N values ranged from 5.10 to 8.96‰, with a mean of 7 ± 1‰, exhibiting higher spatial variability and suggesting an influence of regional nutrient distribution and biogeochemical processes operating across the continental shelf. Overall, the results indicate a predominance of marine-derived organic matter on the southern continental shelf of Rio Grande do Sul, while the spatial patterns of TOC, TN, and δ¹⁵N suggest that the distribution of sedimentary organic matter is influenced by bathymetric, latitudinal, and grain-size gradients, as well as regional oceanographic dynamics.