Diatomeas holocenas del delta del río Colorado, Argentina. Una aproximación paleoambiental a partir del estudio de “fragilarioides pequeñas”

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. 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 allochthonousmarine-coastal taxa. At the base, subzone DZIA (191–163 cm, 7,315±64 14C 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±29 14C yr BP. DZII lacked marine species. The LI sequence represents the infilling of a river-estuarine channel with tidal influence ca. 7,300 14C yr BP that gradually evolved into a fresh-brackish river channel, similar to the current river, ca. 2,200 14C 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. 7,000 14C yr BP. Keywords: Fragilariaceae, delta, sea level, Holocene, Patagonia. As associacoes de diatomaceas, de uma sucessao sedimentar da planicie deltaica do Rio Colorado (Argentina), foram estudadas com o objetivo de reconstruir as condicoes paleoambientais do delta, em resposta as flutuacoes do nivel do mar no Holoceno. Um testemunho de 191 cm de profundidade foi extraido usando um vibracoring no sitio Las Isletas (LI), localizado a 22 km da costa atual (39°43’5,7”S, 62°20’49,7”O). A analise diatomologica de 35 sub-amostras permitiu definir tres zonas de diatomaceas (DZ) por meio de uma analise de agrupamento – CONISS. O testemunho foi dominado por pequenos fragilarioides, sendo Punctastriata glubokoensis Williams, Chudaev & Gololobova a especie mais abundante. A flora acompanhante apresentou especies de agua salobra-duciaquicolas e taxons marinho-costeiros aloctones. A subzona basal DZIA (191–163 cm, 7.315 ± 64 anos 14C AP) incluiu unidades litologicas arenosas e apresentou abundantes diatomaceas marinhas costeiras aloctone. A subzona sobrejacente (DZIB: 163–96 cm) exibiu poucos taxons marinhos. A zona DZII (96 cm no topo) integrou unidades litologicas de areia-siltosa, formadas ca. 2.218 ± 29 anos 14C AP. DZII e nao apresentou especies marinhas costeiras. A sequencia LI representa o assoreamento de um canal fluvial-estuarino com influencia de mares ca. 7.300 anos 14C AP que evoluiu gradualmente para um canal fluvial de aguas salobra-dulciaquicolas, semelhante ao rio atual ca. 2.200 anos 14C AP. Durante a transgressao do Meso-holoceno, o aumento da influencia marinha teria transportado especies costeiras-marinhas a montante do sitio LI. O aumento da influencia fluvial associada a progradacao do lobo deltaico teria favorecido a onipresenca e a persistencia de pequenos fragilarioides nas associacoes fluvio-deltaica do Rio Colorado durante os ultimos ca. 7.000 anos 14C AP. Palavras-chave: Fragilariaceae, delta, nivel do mar, Holoceno, Patagonia.