Non siliceous algae and acritarchs from the Holocene coastal deposits at the La Ballenera Creek, Buenos Aires, Argentina

NON SILICEOUS ALGAE AND ACRITARCHS FROM THE HOLOCENE COASTAL DEPOSITS AT THE LA BALLENERA CREEK, BUENOS AIRES, ARGENTINA. The palynological analysis of the Holocene sequence that crops out at arroyo La Ballenera (38 degrees 19' S; 57 degrees 57' W), southeastern littoral of Buenos Aires province, allows the establishment of three zones (LB-1, LB-2, LB-3) and two subzones (LB-3a and b). This analysis is based on non-pollen aquatic palynomorphs: algae (Chlorophyta, Cyanophyta and Dinoflagellata), invertebrate remains and acritarchs. The chlorophycean algae consist of colonies of Botryococcus braunii Kutzing, zygospores of Zygnemataceae (Mougeotia sp., Spirogyra sp. I and Spirogyra sp. 2) and phycomas of Prasinophyceae. Cyanophycean sheets of Gloeotrichia sp. are rare. The assemblages also contain non-marine dinoflagellate cysts of affinity with a peridinioid genus. The acritarchs are represented by Cobricosphaeridium spp. (probably copepod or anostracod eggs), Micrhystridium sp. and other species defined under an open nomenclature. Between 6,790 - 6,300 C-14 yr. B.P., these palynomorphs show the beginning of marine influence in the site and reveal a marsh enviromnent in close relation with shallow freshwater habitats. Between 6,300 C-14 - ca. 5,600 C-14 yr. B.P. the presence of dinocysts of Peridinioideae, Gloeotrichia and Botryococcus indicate the development of a marginal brackish water body, oligotrophic to mesotrophic, of low energy, with a considerable freshwater influence. From 5,600 C-14 yr. B.P. to 4,700 C-14 yr. B.P. the algae and acritarchs reflect an increase of salinity, related with a major marine influence. After ca. 4,700 C-14 yr. B.P. Cobricosphaeridium is present along with Botryococcus and other acritarchs, indicating a coastal, unstable water body of lower salinity than the aforementioned zone.