Hydrological variations and Southern Hemispheric Westerlies (SHW) in Argentina ? the record of the ICDP project PASADO

ZOLITSCHKA, B; IRURZUN, M A

Génova

Conferencia; ICDP Science Conference; 2014

Semiarid conditions are prevailing at the site of Laguna Potrok Aike (52°S, 70°W; 116 m asl; diameter: 3.5 km, water-depth: 100 m), a currently terminal maar lake in the extra-Andean steppe of Patagonia. Depositional processes of autochthonous sedimentary compounds are controlled by the evaporation/precipitation ratio (E/P), which is a direct function of climate. E/P is governed by precipitation-controlled runoff and temperature- and wind-controlled evaporation. Stratigraphic and chronological correlation of the 106 m composite profile from the central deep basin with sediment cores from littoral zones and outcrops in the catchment area allows to reconstruct hydrological fluctuations during the last 51 ka providing evidence for lake-level variations in the range of >50 m. During the Late Pleistocene, the sediment facies was influenced by comparatively high inflow and little evaporation with a responding high lake level. During deglaciation and in particular during the Late Glacial (ca. 17-12 kcal BP) a lake-level lowering (higher E/P) indicates warming and/or migration of the SHW to more southern latitudes, whereas in the early Holocene a higher lake level re-established. Since 9.3 kcal BP intense carbonate precipitation together with salinity-indicating diatoms document that subsaline lacustrine conditions prevailed until today with one interruption during the neoglacial ?Little Ice Age? (LIA). For long-term hydrological variability several factors are important: 1) changes in runoff due to permafrost sealing of the ground during the glacial period, 2) variations in precipitation and wind-induced evaporation linked to changes in intensity and position of the SHW and 3) the Glacial-to-Holocene temperature increase. Based on multiproxy evidence a lake-level record for the last 51 ka was reconstructed and interpreted as the result of the SHW being in a more northerly position during the Glacial period followed by a southward movement. At 9.3 kcal BP the SHW intensified in the study area and caused a pronounced negative water balance with a lake-level lowering of more than 50 m compared to glacial conditions. Two millennia later either SHW intensity or its position changed and the lake level started to rise in a step-like manner to its Holocene maximum during the LIA. Since the 20th century strengthening of SHW increased the evaporative stress resulting again in a more negative water balance. Comparison of our data with other Patagonian records indicates that the core of the SHW shifted southward and established at 52°S around 9.3 kcal. BP. As the late Holocene lake-level high-stand during the LIA is documented for entire Patagonia, it appears to have occurred synchronously over all latitudes and thus seems to be related to a SHW weakening instead of a latitudinal shift of the SHW belt. To improve our understanding of past hydrological variability, new isotopic (87Sr/86Sr) and rock-magnetic data (ARM/SIRM ratio) will be presented and compared to geochemical and paleobiological evidences. Both parameters are not event-related and thus provide an independent and continuous record about eolian dust and runoff conditions. Sediment samples were provided by the ICDP in the framework of the Potrok Aike Maar Lake Sediment Archive Drilling Project (PASADO). We acknowledge funding by CONISET, DFG, ICDP, NSERC, SNF, VR and the University of Bremen.