Diatomeas holocenas del delta del Río Colorado, Argentina. Una aproximación paleoambiental a partir del estudio de Fragilarioides pequeñas

FAYO, ROCIO; ESPINOSA, M. A; ISLA, FEDERICO IGNACIO

REVISTA BRASILEIRA DE PALEONTOLOGIA

SOC BRASILEIRA PALEONTOLOGIA

Lugar: Porto Alegre; Año: 2020 p. 1 - 14

1519-7530

Holocene diatoms of the Colorado River Delta, Argentina. A paleoenvironmental approach from the study of ?small fragilarioids?. Diatom assemblages of a sedimentary succession in the deltaic plain of the Colorado River (Argentina) were studied in order to reconstruct the paleoenvironmental conditions of the delta in response to Holocene sea-level changes. The core was extracted by vibracoring from Las Isletas (LI), located at 22 km from the coast (39 ° 43´5.7 "S, 62 ° 20´49.7" W). It has 191 cm of depth. Diatom analysis of 35 subsamples defined three diatom zones (DZ) by cluster analysis -CONISS-. The sedimentary succession was dominated by small fragilarioids, being Punctastriata glubokoensis the most abundant species; the accompanying flora was fresh- brackish and allochthonous marine-coastal taxa. At the base, subzone DZIA (191-163 cm, 7,315±64 yr 14C BP) included sandy lithological units and presented abundant marine-coastal species. The overlying subzone DZIB (163-96 cm) had low abundance of allochthonous marine species. The uppermost zone, DZII (96 cm from the top) integrated the silt-sandy lithological units and it was formed ca. 2,218±29 yr 14C BP. DZII lacked marine species. The LI sequence represents the infilling of a river-estuarine channel with tidal influence ca. 7,300 yr 14C BP, that gradually evolved into a fresh-brackish river channel, similar to the current river, ca. 2,200 yr 14C BP. During the middle Holocene transgression, the greatest marine influence would have transported coastal marine species upstream to the LI site; the increase of fluvial influence associated with the progradation of the delta lobe would have favored the ubiquity and persistence of the small fragilarioids in the fluvial-delta assemblages of the Colorado River during the last ca. 7,000 yr 14C BP.