Geochemical mapping of modern sedimentary organic matter deposited in a World Heritage subtropical estuary affected by human activities: stable isotopes and molecular approaches.

The Paranaguá Estuarine System (PES) plays a crucial ecological and economic role in the coastal zone of Brazil, containing significant remaining portions of the Atlantic rainforest, which is considered a World Heritage Site (WHS) by UNESCO. At the same time, the PES accommodates the largest bulk grain port terminal and is the leader in container handling in Latin America. In this study, we combined elemental, isotopic, and molecular markers (n-alkanes and n-alkanols) with a comprehensive sample set of surface sediments to understand and distinguish the primary sources of sedimentary organic matter (OM) and evaluate the environmental quality of a WHS under human pressure. The C/N ratio and δC ranged from 4.98 to 17.9 and - 27.15 to - 22.88‰, respectively. These results suggest that the primary bulk OM origin is the terrigenous contribution from river inputs. The total long-chain n-alkanes and n-alkanols results ranged from 0.29 to 12.3 µg g and 0.19 and 998.1 µg g, respectively, and the n-alkanes diagnostic indices (e.g. CPI and TAR) agree with elementary and isotopic proxies. The river contribution, together with tidal currents, dominates the local hydrodynamics. However, the estuarine mouth region and northern sector are strongly influenced by marine-sourced OM. It was also observed that the low-energy estuarine hydrodynamics controls the OM deposition in Antonina Bay, which favours the deposition of fine sediments. In Paranaguá Bay, due to a mixing zone with the intrusion of fresh and marine waters, there are sites of preferential accumulation of OM caused by fine particles' flocculation. This geochemical mapping of sedimentary OM composition may provide important insights into future scenarios of environmental changes and may be applied to track the deposition and accumulation of anthropogenic residues from the continental basin and port activities.