VEGETATION HISTORY OF THE COASTAL PLAINS FROM SAMBOROMBÓN BAY (∼35.5° S), ARGENTINA, SINCE 7800 ¹⁴C YEARS. The palynological analysis from a sedimentary sequence, outcropping on the right bank of the Salado River (35° 55′ S - 57° 53′ W), 30 km from the present coastline of the Samborombón Bay, allowed to reconstruct the vegetation history for the central bay area since c. 7800 ¹⁴C years BP. Results indicate the development of different plant communities: middle-marsh halophytic vegetation between 7800 and 7000 ¹⁴C years BP, replaced by low-marsh brackish grasses between 7000 and 5400 ¹⁴C years BP in relation to sea-level highstand; a transitional high salt-marsh halophytic community, developed between 5400 and 4300 ¹⁴C years BP; and, brackish marsh vegetation and grasslands between 4300 and 1700 ¹⁴C years BP, with progressive decrease of estuarine conditions due to sea-level fall and littoral ridge progradation. These results along with those from the south of the Samborombón Bay and Mar Chiquita coastal plain (southeastern Buenos Aires province) showed a similar vegetation in the region, characterized by low-marsh and coastal lagoon halophytic grasses, developed between 5400 and 4300 ¹⁴C years BP; and a local differentiation after 4300 ¹⁴C years BP related to the geomorphology of each area: a brackish marsh vegetation in the Samborombón Bay, psammophytic communities at the south of the bay and middle salt-marsh halophytic communities in Mar Chiquita coastal plain. These changes reveal the sensitive response of vegetation from coastal marshes to the sea-level fluctuations and geomorphologic evolution.