Potrok Aike: diversity, ecology and paleoenvironmental changes revealed from the PASADO sediment record

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

Bremen

Workshop; 4th International PASADO Workshop; 2012

University of Bremen-Geopolar Institut

Laguna Potrok Aike is a maar lake located in the southernmost Argentinean Patagonia, in the province of Santa Cruz. As one of the few permanent lakes in the area, it provides an exceptional and continuous sedimentary record. The sediment cores from Laguna Potrok Aike obtained in the framework of the ICDP-sponsored project PASADO (Potrok Aike Maar Lake Sediment Archive Drilling Program), were 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, combined with other proxies, to track these changes. Diatom assemblages were analyzed on the composite core 5022-2CP, located ca. 700m south of the center of the lake (figure 1). The total composite profile length of 106.09 mcd (meters composite depth) was reduced to 45.80 m cd-ec (event-corrected composite profile) of pelagic deposits once gaps, reworked sections and tephra deposits were removed and this continuous deposit spans the last 51.2 cal. ka BP (Kliem et al., in review). The sampling protocol for sediment subsampling for multiproxy analyses was especially developed for the PASADO project (Ohlendorf et al., 2011). We here present the results from the diatom analysis of the composite core 5022-2CP with a resolution ranging from 16 to 32 cm, corresponding to a multi-centennial time resolution. Previous diatomological analysis from the core catcher samples of core 5022-1D (Recasens et al., 2012), allowed us to determine the dominant diatom assemblage and select the sections where higher temporal resolution was needed. Over 200 species, varieties and forms have been identified in the sediment record, including several endemic species and several species which are new to science. For example, Thalassiosira patagonica (Maidana, 1999) and Corbellia contorta (Maidana and Round, 1999) were found and described for this lake. In the framework of PASADO, a new species of the genus Cymbella has been observed for the first time in the sediments of Laguna Potrok Aike and has been described as Cymbella gravida sp. nov. Recasens and Maidana (Recasens & Maidana, in prep.) which is shown in figure 2. The combination of our study with the modern training set developped for Patagonia within the framework of PIPA (Proyecto Interdisciplinario Patagonia Austral, PIPA) will provide unique information on diatom diversity and (paleo)ecology for the Southern Hemisphere. The quantitative analysis of the sediment record reveals diatom abundances ranging from nearly none to 460 million valves per gram of dry sediment, with substantial fluctuations through time. The new results on diatom diversity and distribution in the glacial to late glacial part of the record give new information on the previously poorly known paleolimnology of this lake for that time. The bottom 5 m of the record, corresponding approximately to the last 51 to 49 cal. ka BP are dominated by Discostella stelligera morph 1 (Recasens et al. 2012) and several benthic and epiphytic species such as Cocconeis placentula and species of Pinnularia, Amphora and Nitzschia. At ca. 49 cal. ka BP, this dominating assemblage rapidly decreases and a peak of Cyclostephanos patagonicus, a big planktonic diatom, is observed. The former assemblage could be indicative of a lower lake level, or increased in-wash from the littoral area (explaining the increase in benthic diatoms, living prefereably in the shallower areas). The occurrence of C. patagonicus would indicate a freshwater input in the system and increase of the lake level, favouring the hypothesis of a previous lower level. Nevertheless, C. patagonicus abruptly disappears at ca. 48 cal. ka BP, only reappearing in the sediment record at 16.5 cal. ka BP. The gap in C. patagonicus could be explained by remarkable changes in the lake level, although no other proxies point to that fact in this time period. It is most likely that there was a change in the nutrient balance in the water column, causing the diatom flora to switch dramatically. Indeed, variations in diatom abundance and species distribution could either point toward lake level variations, changes in nutrient input or even periods of ice-cover in the lake. The diatom assemblages between 48 and 16.5 cal. ka BP reveal a stable and quite cosmopolitan flora, making it difficult to establish a fine paleoecological interpretation based only on this proxy. A correlation with other proxies is thus necessary to further develop these hypotheses. The diatom analysis of the top few meters on both profiles (5022-1D and 5022-2CP) are consistent with previous results from a shorter core retrieved during the former SALSA project, covering the last ca. 16 cal. ka BP (Massaferro et al., accepted; Wille et al. 2007). This part of the record is characterized by the presence of C. patagonicus which is slowly replaced by more brackish species, Cyclotella agassizensis and Thalassiosira patagonica at ca. 9.4 cal. ka BP which dominate up to the present. This shift in the phytoplakton composition corresponds to the previously documented salinization of the water and the lake level drop giving a signal of warming temperatures and lower moisture availability during the early and middle Holocene.