Organic matter in the deltaic clinoform of the São Francisco River (Eastern Brazil)

The São Francisco River is one of the most important rivers in Brazil. The wave-dominated delta at its mouth includes a well-developed muddy clinoform on the adjacent shelf. This study aimed to understand sedimentary organic matter (OM) distribution and the relative contribution of terrestrial/fluvial and marine sources. A dense and evenly distributed sampling grid was used to evaluate the contribution of different OM sources and the influence of local factors. The following parameters were evaluated: grain size, total organic carbon (TOC), total nitrogen (TN), carbonates, and carbon isotopes (δ 13 C). The spatial distribution of OM properties shows similarities with the spatial arrangement of the various sedimentary provinces of the clinoform (topset, foreset, and bottomset) and the lateral contributions of sediments from neighboring reef bottoms, advected by coastal flows. TOC and TN varied respectively between 0.11 and 1.56% and between 0.02 and 0.20%. The distribution of these parameters in combination with %CaCO 3 shows that the major contributors of TOC to the study area are the São Francisco River, coastal reefs located northwards, and shelf reef bottoms located northeastwards and southwestwards from the deltaic clinoform. The best indicator of river influence was provided by δ 13 C, with values ranging between − 23 and − 19‰. The spatial distribution of δ 13 C shows a good agreement with the various sedimentary elements of the clinoform, reflecting the pattern of river plume expansion observed during the last decades, which is characterized by low discharges, compared to historical values. Our dense and evenly spaced sampling grid and its integration with local geological, geomorphological, and geophysical data allowed to better understand the sources and origin of organic matter and its spatial distribution in this deltaic setting.