Holocene diatoms of the colorado river delta, argentina. A paleoenvironmental approach from the study of “small fragilarioids”.

Abstract

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. A 191 cm depth core was extracted by vibracoring from Las Isletas (LI), located 22 km from the coast (39°43’5.7”S, 62°20’49.7”W). A cluster analysis – CONISS of 35 subsamples defined three diatom zones (DZ). The sedimentary succession is dominated by small fragilarioids, being Punctastriata glubokoensis Williams, Chudaev & Gololobova the most abundant species; the accompanying flora included fresh-brackish and allochthonous marine-coastal taxa. At the base, subzone DZIA (191–163 cm, 7,315±6414C yr BP) included sandy lithological units and presented abundant marine-coastal species. The overlying subzone DZIB (163–96 cm) has low abundance of allochthonous marine species. The uppermost zone, DZII (96 cm from the top) integrate the silt-sandy lithological units, and it was deposited ca. 2,218±2914C yr BP. DZII lacked marine species. The LI sequence represents the infilling of a river-estuarine channel with tidal influence ca. 7,30014C yr BP that gradually evolved into a fresh-brackish river channel, similar to the current river, ca. 2,20014C yr 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. 7,00014C yr BP.