Modern and sub-recent processes of sedimentation monitored in the maar lake Laguna Potrok Aike (Santa Cruz, Argentina)

KASTNER, S.; OHLENDORF, C.; HABERZETTL, T.; LÜCKE, A.; MAIDANA, N. I.; MAYR, C.; SCHÄBITZ, F.; ZOLITSCHKA, B.

Malargë, Argentina

Conferencia; 3rd Maar Conference; 2009

Sociedad Geológica Argentina

Various interdisciplinary multi-proxy studies as well as a climate modelling approach document a unique lacustrine record of paleoclimatic and paleoecological variability at the maar lake Laguna Potrok Aike (51°58’S, 70°23’W), Santa Cruz, Argentina. Situated in the dry steppe environment of south-eastern Patagonia the 100 m deep and ca. 770 ka old maar has a high potential as a paleolimnological key site for the reconstruction of terrestrial paleoclimate conditions (Zolitschka et al., 2006). For this reason the lake was chosen as an ICDP drilling site in 2008 within the “Potrok Aike maar lake sediment archive drilling project” (PASADO). As the area is sensitive to variations in southern hemispheric wind and pressure system changes the lacustrine sediment record as well as subaerial and subaquatic lake level terraces (Anselmetti et al, in press) provide detailed information of the lake´s hydrological budget related to the variability of the Southern Hemispheric Westerlies (Mayr et al., 2007). Geochemical, palynological, diatomological and isotopic investigations were carried out with high temporal resolution on a 16 m long sediment record covering the last 16,000 years (Haberzettl et al., 2007; Mayr et al., subm.; Wille et al., 2007). To develop an understanding of the dynamics of modern processes that control the spatial distribution and characteristics of sediments in Laguna Potrok Aike it is vital to analyze spatial distribution patterns of the sediment infill. Furthermore, this information improves the potential for interpretation of the long sediment record recovered within the project PASADO. A survey of the aeral sediment distribution was carried out in 2005 by using 46 gravity cores of up to 49 cm in length. This dense grid of cores covers a range of water depth from 9 to 100 m. Additional cores were taken during the PASADO fieldwork in autumn 2008 to complement the already existing grid. As laboratory analyses are ongoing and results of the additional cores have to be integrated to the existing set of data the poster will provide new information and interpretations going beyond what is known when this abstract was written. Using the Ca and Ti data obtained by XRF- and magnetic susceptibility core scanning with 1 and 4 mm spatial resolution, respectively, the existing 46 cores were correlated and linked to an established depth/age model (Haberzettl et al., 2005). This model is transferred from a sediment short core of the deep central lake basin and based on four AMS radiocarbon ages. Core correlation prior to the 2005 sediment surface was achieved for cores from water depths exceeding 45 m. Thus surface samples were taken from all 46 cores while subsampling of selected past time windows - AD 1960, 1800, 1610, 1500, 1380 - was only possible for 25 well correlated cores from the deep central basin area. These time slices were chosen to cover paleoenvironmentally distinctive intervals with known hydrological, i.e., lake level variations. Results of XRF and magnetic susceptibility scanning as well as element concentrations (C, N, S), pollen, stable isotopes (d13Ccarb, d18Ocarb, d15N; d13Corg,) and diatom analyses are presented by distribution maps for each of the selected time slices. First results reproduce modern dynamics at the sediment surface, i.e. at 0-1 cm depth representing approx. 20 years of the youngest record (Kastner et al., in prep.). Modern sedimentation patterns reveal pronounced differences between the littoral cores down to 45 m water depth and the lake´s profundal cores separated from each other by steep slopes. Selected distribution patterns of grain size, benthic diatoms, total inorganic carbon, titanium and calcium point to frequent erosion, resuspension and redistribution of sediment at the eastern shore followed by transportation from the littoral to a profundal accumulation area. Hence, sedimentation within this terminal lake is not only influenced by lake level changes, but also by dominant westerly winds, resulting in strongest wave action at the eastern margin of the lake, internal currents, episodic inflows and the surrounding geology Spatial variations on a temporal scale are evaluated and interpreted in the context of the modern processes. Sediment distribution patterns of the analyzed latest Holocene time sections (Little Ice Age, Medieval Climate Anomaly and transitional periods) point to modifications of depositional dynamics in the deep central basin influenced by changing wind patterns (Meyer and Wagner, 2008) and varying lake levels (Haberzettl et al., 2005). Isotopic analyses, grain size distribution and XRF-scanning data reveal effects of the proximity of the analyzed sediment cores to the fluctuating paleo-shoreline. Lake level high stand and less intense westerly winds during the Little Ice Age are documented by a more homogeneous sediment distribution in the deep lake basin. In contrast, deep basin distribution patterns reveal intensified sediment redistribution during lake level low stand and intensified winds during the Medieval Climate Anomaly.