Estuaries are boundary zones connecting terrestrial and marine environments, where the transport, production, transformation, and decomposition of organic matter actively occur. The source and behavior of carbon, nitrogen, and phosphorus in particulate organic matter (POM) are important components of biogeochemical cycling in estuarine ecosystems. The Geum and Seomjin estuaries represent contrasting estuarine systems in Korea. The Geum Estuary is a barrage-controlled estuary connected to the Yellow Sea, whereas the Seomjin Estuary is a naturally open estuary connected to the South Sea. This study compares the elemental composition and radiocarbon of particulate organic matter (POM) in the two estuaries. Surface water samples were collected at five to six stations along the salinity gradient from the inner estuary to offshore during November 2025 and February and May 2026. Particulate organic carbon (POC), particulate nitrogen (PN), and particulate phosphorus (PP) were analyzed. Preliminary results showed that POC concentrations in the Geum Estuary were nearly three times higher than those in the Seomjin Estuary during November and February. C:N ratios were close to the Redfield ratio (approximately 7) in both estuaries, suggesting that POM was mainly derived from aquatic sources. Analyses of PP and radiocarbon are ongoing, and additional samples to be collected in August 2026 will complete the one-year dataset.