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

Abstract

Diatom assemblages from a site called Cueva del Tigre were studied. It is located in the lower basin of the Quequén Salado River, Buenos Aires province (38° 50´2.2"S - 60°32´7.1" W). The results are part of a project that includes sedimentological, stratigraphical and paleontological aspects with the aim to reconstructing the environmental history of southeastern 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 4 levels (N1, N2, N3 and N4) was performed using conventional techniques of taxonomic identification and treatment. 74 diatoms 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/brackish lagoon with associated vegetation (N1: 7 samples) dominated by Cyclotella meneghiniana Kützing 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 to 10 ?. The comparison between fossil and modern samples through NMDS analysis showed that modern diatom assemblages from the Quequén 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.