Late Pleistocene Diatoms of the Lower Basin from the Quequén Salado River, Argentina

Diatom assemblages from a site called Cueva del Tigre were studied. It is located in the lower basin of the Quequen Salado River, Buenos Aires province (38° 50′ 2.2″ S–60° 32′ 7.1″ W). The results are part of a project that includes sedimentologic, stratigraphic and palaeontologic aspects with the aim of reconstructing the environmental history of south-eastern Buenos Aires during the Late Cenozoic. The studied sedimentary succession is 1.90 m thick and has a tabular geometry with alternating levels of coarse sandstone and clayey siltstones. Diatom analysis of the 17 samples corresponding to four levels (N1, N2, N3 and N4) was performed using conventional techniques of taxonomic identification and treatment. A total of 74 diatom taxa were recognized and grouped according to salt tolerance and life form. Cluster analysis allowed dividing the sedimentary sequence into two diatom zones. The deposit begins with a freshwater/shallow lakes with associated vegetation (N1: 7 samples) dominated by Cyclotella meneghiniana Kutzing 1844 (plankton) accompanied by the epiphytes Cocconeis placentula Ehrenberg 1838 and Cymbella cistula (Ehrenberg) Kirchner 1878. The overlapping levels: N2 (4 samples), N3 (3 samples) dated on 29,360 ± 670 years 14C BP (33,128 cal. years BP) and N4 (3 samples) represent brackish conditions in a shallow pond, where benthic epipelic diatoms dominate: Caloneis westii (W.Smith) Hendey 1964, Campylodiscus clypeus Ehrenberg 1840 and Surirella striatula Turpin 1828. Today, these taxa live in temperate waters with salinities of 2–10‰. The comparison between fossil and modern samples through NMDS analysis showed that modern diatom assemblages from the Quequen Salado River are distinctly dissimilar to the Pleistocene assemblages under study, but they strongly correlate with diatom assemblages from Buenos Aires shallow lakes in agreement with the autoecological interpretation.