ESTUARINE PALEOENVIRONMENTAL RECONSTRUCTIONS USING DIATOMS, NORTHERN PATAGONIA, ARGENTINA

FAYÓ ROCÍO; ESPINOSA MARCELA A.

Barcelona

Simposio; VII International Symposium on Marine Sciences; 2020

Universitat de Barcelona

Estuaries and deltas are common along modern coasts and are well represented in the geologic record because of the high preservation potential of their deposits. Globally, estuaries are more abundant than deltas because of the relatively short time available for coastal sedimentation since the onset of the Holocene sea level highstand. Deltaic systems evolution links both estuarine and progradational stages. The study area includes three estuarine mesotidal environments from Patagonia region: Río Colorado delta (39°40´S; 62°08´W), Río Negro (40° 57´ S; 62° 49? W) and Río Chubut estuaries (43° 22´ S; 65º 03´W). They are dominated by littoral drift essentially and the plains were flooded by the sea since 7,000 years BP ago. Five sediment cores were analyzed. Diatom content of fossil samples was studied and compared with modern assemblages using specific multivariate analysis (DCA, NMDS) and diatom zones were characterized by cluster analysis (single linkage, Euclidean distance) in the fossil sequences. The study of diatom assemblages from Holocene sequences of Patagonia coast allowed reconstructing the environmental evolution in relation to salinity, tidal effects and sea level changes. Colorado delta evolved from tidal channels flooded when the sea transgression occupied this area to tidal flats and marshes during the late Holocene regression. The gradually increase of the fluvial influence is associated with the eastward progradation of the deltaic lobe during the last ca. 7,000 years BP. At Río Negro estuary, shallow vegetated brackish/freshwater environment evolved into marine conditions, tidal channels and marshes during the last 2,000 years BP. A marine coastal environment (estuary) at 4,376±69 14C years BP that evolved into a salt marsh about 672±39 14C years BP was inferred at the inlet of the Chubut River. Salinity-preference data for diatoms showed how connections between the wetland and the sea varied over time with sea level changes and gradual infilling.