Abstract

BOREL, C. Marcela. Non siliceous algae and acritarchs from the Holocene coastal deposits at the La Ballenera Creek, Buenos Aires, Argentina. Ameghiniana [online]. 2007, vol.44, n.2, pp.359-366. ISSN 1851-8044.

The palynological analysis of the Holocene sequence that crops out at arroyo La Ballenera (38º 19' S; 57º 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 Kützing, zygospores of Zygnemataceae (Mougeotia sp., Spirogyra sp. 1 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 14C yr. B.P., these palynomorphs show the beginning of marine influence in the site and reveal a marsh environment in close relation with shallow freshwater habitats. Between 6,300 14C - ca. 5,600 14C 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 14C yr. B.P. to 4,700 14C yr. B.P. the algae and acritarchs reflect an increase of salinity, related with a major marine influence. After ca. 4,700 14C 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.

Keywords : Palynology; Algae; Acritarchs; Holocene; Buenos Aires; Argentina.