As　one　of　the　main　interfaces　of　the　Earth　system,　estuaries　show　the　strongest　land–sea　interaction　in　the　carbon　cycle,　which　links　terrestrial　ecosystems　to　the　marginal　sea.　Furthermore,　estuaries　are　considered　as　one　of　the　most　active　intermediate　reservoirs　for　both　terrestrial　and　marine　matter　due　to　complex　hydrodynamic　processes　regulated　by　the　river　runoff,　wave　and　tide.　Processing　of　organic　matter　(OM)　in　tidal　estuaries　modifies　its　transfer　and　transformation　from　the　river　to　the　sea,　so　studies　of　on　the　source　and　distributions　of　estuarine　OM　can　help　us　understand　the　behavior　of　production,　exchange,　transport　and　burial　of　diverse　OM　within　this　transition　zone　before　entering　the　marginal　sea.　In　this　paper,　we　took　the　Minjiang　River　Estuary　(MRE)　as　a　typical　system　in　which　there　is　strong　influence　of　the　tide.　The　source,　composition　and　spatial　distribution　of　OM　in　surface　sediments　of　MRE　were　deciphered　based　on　multiple　organic　geochemical　properties　for　source-specific　biomarkers　(n-alkanes,　n-alkanols,　sterols)　and　bulk　OM.　Results　show　that　sedimentary　organic　components　were　negatively　correlated　with　sediment　grain　size,　which　indicates　fine　particles　such　as　silt　and　clay　are　the　major　carriers　of　the　OM　signals　in　tidal　estuaries.　Source-specific　biomarker　proxies　indicate　that　in　terms　of　source　diversity　the　sedimentary　OM　in　the　MRE　shows　mixed　signals　of　terrestrial　and　marine　sources,　and　the　proportion　of　terrestrial　OM　decreases　with　the　increase　in　distance　from　the　land.　The　fractional　contributions　of　OM　from　the　riverine　(i.e.,　terrestrial),　marine　and　deltaic　sources　were　quantitatively　estimated　using　a　Monte　Carlo　(MC)　three-end-member　mixing　model　based　on　C/N　and　δ13C　values,　and　the　average　contributions　of　the　three　sources　are　40　±　10%,　48　±　10%　and　12　±　4%,　respectively,　with　little　contribution　from　deltaic　sources.　The　dispersion　of　sedimentary　OM　from　different　sources　in　the　MRE　is　primarily　controlled　by　the　depositional　environment　determined　by　dynamic　conditions　and　tidal　processes　play　a　significant　role　in　the　redistribution　of　sedimentary　OM　dispersion　patterns.　Compared　with　other　large　estuaries　in　southeast　China,　the　OM　accumulation　contribution　in　the　tide　dominated　small　and　medium-sized　estuaries　such　as　the　MRE　which　is　largely　dependent　on　riverine　and　marine　deliveries.　The　MRE　has　a　high　potential　for　both　terrestrial　and　marine　organic　carbon　(OC)　burial,　with　an　accumulation　rate　of　3.39　±　1.83　mg　cm−2　yr−1　for　terrestrial　OC,　and　an　accumulation　rate　of　3.18　±　0.68　mg　cm−2　yr−1　for　marine　OC　in　muddy　sediment,　making　it　an　important　contributor　to　the　sedimentary　carbon　sink　of　the　marginal　sea.　©　2023　by　the　authors.