Unraveling sedimentary organic matter in the Oiapoque estuarine-marine ecosystem: Insights from elemental composition and isotopic fingerprints on the Amazon coast

The Oiapoque River estuary, located on the border between Brazil and French Guiana, is an understudied Amazon region characterized by intense discharge of fluvial sediment and terrestrial organic matter (OM). Influenced by environmental factors such as temperature, salinity, tides, and currents, the sedimentary OM composition reflects both autochthonous production and adjacent land biomass. This study aimed to identify and differentiate primary sources of sedimentary OM in the Amazon coastal zone using geochemical parameters such as total organic carbon (TOC), total nitrogen (TN), their relative atomic ratio (expressed as C/N), and carbon and nitrogen isotopic ratios (δ¹³C and δ¹⁵N). Two surface sediment sampling campaigns were conducted in 2018 during the wet and dry seasons. The sediments showed a strong correlation between TOC and TN across both campaigns (R² = 0.98 and 0.94); however, results suggested the contribution of inorganic nitrogen forms (e.g., NH₄⁺). The C/N ratio indicated predominant OM from terrigenous input (∼24) at upstream sites and marine input (∼5 and ∼ 9) at downstream ones. The δ¹³C analysis revealed low variability along the estuary, with values around −27 ‰ upstream and near the mouth (typical of land or C3 plant contributions), intermediate values around −24 ‰ in the central portion (mixed sources), and higher values around −22 ‰ near the mouth (marine inputs). The δ¹⁵N values (∼3 ‰) were primarily found in the inner region, at the center of the estuary, and near the mouth where river discharges occur, associated with terrestrial plants. The elemental and isotopic composition of the sedimentary OM indicates a predominance of terrigenous OM, particularly near areas influenced by the Uáça and Ouanary rivers, reflecting land use and human occupation. Estuarine dynamics, influenced by rainfall and tidal currents, affect the distribution of marine inputs and the mixture of allochthonous and autochthonous sources throughout the estuary.