Diatoms: Microalgae from Pilauco

Abstract

Fossil diatoms from the site of Pilauco were studied to reconstruct the hydrologic changes in the lentic system between ∼16,000 and 14,200 cal yr BP. We studied 13 samples from the sedimentary layers PB-7 and PB-8 and counted a minimum of 400 diatom valves per sample. The taxonomic analysis identified 33 taxa at the species level and 11 taxa at the genus level. The results using the CONISS analysis revealed two notoriously different periods in the diatom succession. The PIL-1 zone was dated between 16,000 and 14,800 cal yr BP and marked by the dominant presence of benthic diatom species with an affinity for cold waters such as Fragilaria construens and Fragilaria capucina. These results indicate a cold environment assigned to the end of the last glaciation and is characterized by shallow waters and reduced humidity. The subsequent period (PIL-2) corresponds to the time frame from 14,800 to 14,200 cal yr BP and is marked by more warmer conditions favouring the growth of macrophytes serving as substrate for epiphytic diatoms such as Lemnicola hungarica and Nitzschia sp. Furthermore, the presence of Aulacoseira granulata has been considered indicative for concurrent repeated flooding of a wetland. In turn, the occurrence of prolonged dry periods longer than a season during the studied time span can be rather discarded. The diatom record at Pilauco emerged as a clear indicator for environmental conditions reflected by relative variations in temperature and humidity. The sediment layers PB-7 and PB-8 display high capacity to preserve diatom frustules and provide valuable evidence for the late Pleistocene palaeo-environmental conditions at the Pilauco site.