Tracing Carbon Sequestration Potential and Chronology of Nitrogen Pollution in a Large Tropical Estuary

Despite evident carbon–nitrogen pollution in tropical estuaries, the chronology and origin of accelerated carbon–nitrogen pollution are still largely unexplored. Here, we report that the Cochin Estuary, India, showed more than five decades of carbon and nitrogen accumulation at Site 6. The estuary serves as a major carbon reservoir, sequestering approximately 38.4 Gg C yr–1 and as an important nitrogen sink. The δ13C signatures indicated a mix of marine and terrestrial carbon sources, whereas the δ15N signatures revealed significant pollution from anthropogenic nitrogen, with enrichment (5.9 to 9.6‰) observed in the surface sediment layer (0–2 cm) and extending several centimeters deeper into the core. It was also evident that nitrogen pollution, through the inputs from the adjacent fertilizer industry, existed since 1972. Physicochemical variables such as pH, oxidation–reduction potential, and dry bulk density showed negative associations with carbon, whereas water content and dry bulk density were positively associated with nitrogen. The findings highlight the dual role of estuaries as sites of pollutant accumulation and significant carbon reservoirs, emphasizing both their importance in regulating carbon and nitrogen cycles and their vulnerability to long-term contaminant buildup, thereby underscoring the need for their protection and careful management within global carbon and nitrogen strategies.