δ13C, δ15N and TOC/TN as indicators of the origin of organic matter in sediment samples from the estuary of a tropical river

Highlights: • Grain size does not influence the distribution of TOC in the sediments. • The organic matter on the estuary is predominantly from C3 plants. • The highest TOC and TN values occur close to the city of Ituberá. • The TOC/TN, δ¹³C and δ¹⁵N ratios do not indicate anthropogenic contribution. The present study aimed to determine the total organic carbon (TOC), total nitrogen (TN), the carbon‑nitrogen ratio (TOC/TN), carbon isotope (δ¹³C), and nitrogen isotope (δ¹⁵N) in five sediment cores collected from upstream to downstream of the Rio Serinhaem estuary, State of Bahia, Northeast Brazil, in order to investigate the origin of the deposited organic matter (OM).Significant positive correlation was found between TOC and NT (r_s = 0.75); TOC/TN and TOC (r_s = 0.64); δ¹⁵N and TOC (r_s = 0.72); and δ¹⁵N and TOC/TN (r_s = 0.63). The values of δ¹³C and δ¹⁵N found are characteristic of terrestrial sources. The TOC/TN ratio confirmed the data found for δ¹³C and δ¹⁵N, which recorded the origin of organic matter from terrestrial C3 type plants. Upstream of the estuary, the highest means of TOC/TN were found (T1 = 36.9 and T2 = 24.4), as reflected by the increase in TOC content or reduction in TN. The OM along the estuary is predominantly from plants with a C3 photosynthetic pattern, indicating that the Serinhaem River estuary is considered a relatively well-preserved environment.