PASADO science team: Potrok Aike maar lake sediment archive drilling project (PASADO): First results of the ICDP deep drilling in the Pali Aike Volcanic Field (Santa Cruz, Argentina)

ZOLITSCHKA, B., H. CORBELLA, C. GEBHARDT, C. OHLENDORF

Malargue, Mendoza. Argentina

Workshop; IAVCEI 3rd International Maar Conference; 2009

IAVCEI 3rd International Maar Conference

“Potrok Aike maar lake sediment archive drilling project” (PASADO) – First results of the ICDP deep drilling project in the Pali Aike Volcanic Field (Santa Cruz, Argentina)   Bernd Zolitschka1, H. Corbella2, Christian Ohlendorf1 and the PASADO science team   1 Institute of Geography (Geopolar), University of Bremen, Germany – zoli@uni-bremen.de 2 Argentine Museum of Natural History, Buenos Aires, Argentina   Keywords: ICDP, PASADO, maar lake, volcaniclastic sediments, lacustrine sediments, hydrological variability.       Within the framework of the ICDP deep drilling project “Potrok Aike maar lake sediment archive drilling project” (PASADO) an international team of scientists has been attracted to carry out interdisciplinary research at the unique mid-Pleistocene maar lake of Laguna Potrok Aike in southern Patagonia. Drilling activities concluded in  November 2008 and recovered sediments will be used to test hypotheses related to the following two broad themes: o      Detailed analyses of volcanic rocks and sediments to provide insights into processes related to phreatomagmatic explosions and early sedimentation in a mid-Pleistocene maar lake. Never before the entire lacustrine sediment record has been recovered and underlying volcaniclastic sediments have been drilled and investigated in a young maar-diatreme structure. o      High-resolution quantitative climate and environmental reconstructions from orbital and suborbital (multimillennial) down to decadal timescales supported by multiple dating and stratigraphic correlation with emphasis on marine–icecore–terrestrial linkages and incorpo-ration of results from global climate modelling simulations. As drilling operations were not completed before the deadline of this abstract, we highlight here the general setting and report about achieved results based on piston coring of Late Pleistocene and Holocene lacustrine sediments, monitoring and modeling. The oral presentation, however, will focus on the activities and achievements of the ICDP deep drilling including presentation of first data. For mid to high southern latitudes climate reconstructions extending well beyond the Holocene and the Late-Glacial are mostly restricted to either marine sediments or to Antarctic ice cores. Until now, records from the continental realm are rare or not existent. Here we start to close this gap for southern South America by investigating sediment records from Laguna Potrok Aike, a ca. 770 ka old maar lake (52°S, 70°W) in the dry steppe of southern Argentina (Zolitschka et al., 2006). This terminal lake is highly sensitive to hydrological changes and its lacustrine record provides unique continental data of variations in climate, hydrology and related dust deposition (Haberzettl et al., 2007; Mayr et al., 2007a). Furthermore, it acts as a cornerstone of paleodata – climate model intercom-parison for the Southern Hemisphere (Meyer & Wagner, 2008; Wagner et al., 2007). Within the ICDP-funded project PASADO it is intended to investigate more than 400 m of sediment in an international and interdisciplinary approach possibly extending this terrestrial record to the Matuyama/Brunhes geomagnetic polarity reversal. This would be the transition from the lower to the middle Pleistocene coinciding with the marine oxygen isotope stage boundary 20/19. Therefore, we expect a high potential for paleomagnetic dating in addition to tephrochronology. This will not only increase the comparability to Antarctic ice cores considerably but also improve the correlation with marine sediment records. Lake level high and low stands are documented by detailed leveling of subaerial terraces in the catchment area and by surveying of subaquatic terraces in the lake basin with a 3.5 kHz seismic system (Anselmetti et al., in press). One low stand (ca. 8600 to 7300 cal. yrs BP) and one certain high stand (ca. AD 1480 to 1930) as well as an assumed pre-Holocene lake level high stand before ca. 13,200 cal. yrs BP have been confirmed. Process studies demonstrate that these changes in water volume influence the formation of endogenic calcite which is preserved in the sedimentary record. An understanding of the underlying climatic forcing is achieved by a comparison of modeled lake level variations with instrumental meteorological data indicating that the lake level is mainly driven by precipitation, related wind strength and wind direction. Lake levels and precipitation decrease during periods of persistently high winds from westerly directions, whereas lake levels and precipitation increase during periods of enhanced easterly winds. Such a relation is explained by a strengthening of the Southern Hemispheric Westerlies bringing dry and moisture undersaturated air masses to the Patagonian steppe while easterly winds cause more frequent occurrences of rain bringing cyclones from the South Atlantic (Mayr et al., 2007a). Since lake volume controls the autochthonous lacustrine carbonate precipitation, the amount of sedimentary calcite as well as its isotopic composition (Mayr et al., 2007b) archives these recurrence patterns of weather conditions. Reconstruction for the last 1500 years shows a lake level high-stand documented by virtually carbonate-free sediment preceded by pronounced cyclicities of calcite precipitation which are also mirrored in the oxygen isotope (δ18O) record. The high stand of 8.8 m above the present day lake level, occurred between AD 1480 and 1930 – a timing that coincides with the northern hemispheric “Little Ice Age” (Haberzettl et al., 2005; Meyer & Wagner, 2008). The distinct Holocene drought between ca. 8600 and 7300 cal. yrs BP is highlighted by a seismically and lithologically detected unconformity on a terrace at around 33 m below the present lake level (Haberzettl et al., 2008). Chronologically, this terrace coincides with increased values for inorganic carbon, higher sedimentation rates and a distinctly different isotopic composition of organic matter at the deep central basin of the lake (Haberzettl et al., 2007). These findings point to a lower lake level with increased inwash of soil material from the former lake shore which had fallen dry during this period. Before 13,200 cal. yrs BP carbonates disappear completely and we assume that this is the time of highest lake levels which is furthermore related to the formation of an outflow at ca. 21 m above present day lake level.       Acknowledgements   This paper is dedicated to the late Cristobal Kennard (Río Gallegos) who made possible our first scientific investigations of Laguna Potrok Aike in the year 2002. We are grateful to Capitan Jorge D. Moreteau and his crew with excellent technical skills. Without them PASADO would not have been possible. We also thank the members of INTA Santa Cruz, in particular Guillermo Clifton, José Larrosa and Gabriel Oliva for providing the operation's headquarters at their experimental field station Potrok Aike. Preparatory studies were supported by the German Federal Ministry of Education and Research in the framework of the German Climate Research Program (DEKLIM; grant 01 LD 0034 and 0035: project SALSA) and through the projects POTROK (DFG: ZO 102/5-1, 2, 3) and ASADO (DFG: ZO 102/8-1) within the Priority Program “ICDP-Germany” of the German Science Foundation (DFG). This research is supported by the International Continental Scientific Drilling Program (ICDP) in the framework of the “Potrok Aike Maar Lake Sediment Archive Drilling Project” (PASADO). Funding was provided by the ICDP, the German Science Foundation (DFG), the Swiss National Funds (SNF), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Swedish Vetenskapsradet (VR) and the University of Bremen.     References   Anselmetti, F., Ariztegui, D., De Batist, M., Gebhardt, C., Haberzettl, T., Niessen, F., Ohlendorf, C., Zolitschka, B. (in press). 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