Are diatoms diversification and dissolution indicative of climatically-triggered hydrological changes in the Late Pleistocene sediments of Laguna Potrok Aike, Argentina?

RECASENS, C.; ARIZTEGUI, D.; MAIDANA, N. I.; PASADO SCIENCE TEAM

Berna

Congreso; XVIII INQUA-Congress; 2011

International Union for Quaternary Research

Laguna Potrok Aike is a maar lake located in southernmost Argentinean Patagonia and, as one of the few permanent lakes in the area, provides an exceptional continuous sedimentary record. This sediment record was obtained within the framework of the ICDP-sponsored project PASADO (Potrok Aike Maar Lake Sediment Archive Drilling Program), and sampled for diatom analysis in order to reconstruct a continuous history of hydrological and climatic changes since the Late Pleistocene. Diatoms are widely used to characterize and often quantify the impact of past environmental changes in aquatic systems. We use variations in diatom concentration and in their taxonomical assemblages, blended with other proxies, to track these changes. Diatom assemblages were analyzed at high resolution (sampling resolution ranging from 16 to 32 cm) on the composite core 5022-2CP. More than 216 species, varieties and forms have been identified so far in the sediment record (including several endemic species and some new to science). The quantitative analysis reveals diatom abundances ranging from nearly none to 460 million valves per gram of dry sediment, with substantial fluctuations along the record. During Glacial times, floristic assemblages are relatively stable with plankton being dominated by Discostella stelligera. Remarkable diatom concentration peaks at different depths in the core most probably reflect variations in the nutrient availability in the system. The causes for these changes in nutrients supply will be inferred from the analyses of other proxies and a more detailed age model. The Late Glacial to Holocene transition is defined by a diversification of the plankton, with the occurrence of Cyclostephanos patagonicus at approximately 15’550 cal yrs BP and its decline and stepwise replacement by indicators of more brackish conditions, Thalassiosira patagonica and Cyclotella agassizensis, from approximately 11’500 cal yrs BP to present day. The floristic results of the top of the sequence are consistent with previous results on a shorter core from the former SALSA project covering the last 16’000 cal yrs BP (Wille et al. 2007). Additionally, a high degree of valve dissolution can be noticed in the top 10 m of the record, corresponding to very low values of diatom valves concentration in the sediment. Likewise, the analysis of the superficial sediments, at the sediment-water interface, presents equally dissolved specimens of Cyclotella agassizensis. The mechanisms behind this dissolution, together with variations in the diatom abundance and species distribution will shed some new light on the behavior of this lake, its internal chemistry and nutrient supply as well as lake level variations and periods of ice-cover. Nevertheless, a correlation with other proxies is necessary to further develop these hypotheses.