SALINITY FLUCTUATIONS DURING THE HOLOCENE IN THE MAR CHIQUITA COASTAL LAGOON (BUENOS AIRES PROVINCE, ARGENTINA): A DIATOM-BASED QUANTITATIVE APPROXIMATION. Diatom assemblages recovered from a sediment core obtained from the north of Mar Chiquita coastal lagoon (Buenos Aires Province) were studied, in order to characterize the paleosalinity fluctuations during the Holocene. Data were analyzed quantitatively by means of the application of a paleosalinity transfer function available for estuaries of the southeastern Buenos Aires Province. Two main stages during the evolution of the lagoon were inferred. Between c. 8670 and 2500 yr ¹⁴C BP, marine planktonic and tychoplanktonic diatoms, such as Paralia sulcata (Ehr.) Cleve and Cymatosira belgica Grunow, indicate the presence of an environment subject to tidal action, with salinities ranging between 20 and 30‰. Towards c. 2500 years ¹⁴C BP the assemblages changed abruptly, being dominated by the tychoplankters Staurosira construens var. venter (Ehr.) Hamilton, Staurosirella pinnata (Ehr.) Williams & Round and Pseudostaurosira brevistriata (Grun.) Williams and Round, that also dominate in modern sediments of the sampling site. A significant decrease of the marine influence that culminated with the establishment of modern environmental conditions, with salinities lower than 7‰, was interpreted.