High-Resolution Record of Hydrological Variability From Semiarid Southern Patagonia

A multiple-dated lacustrine record from the 100 m deep maar Laguna Potrok Aike in Santa Cruz, Argentina (52^oS, 70^o23`W) provides a continuous high-resolution (sedimentation rate >1 mm yr^-1) sediment record since >16,000 cal. BP subsampled with decadal resolution. In the semiarid desert (annual precipitation <200 mm) of southern Patagonia this is one of the few permanently water-filled lakes, which, due to its water depth, provides a continuous sedimentary record that has never dried out completely during periods of desiccation, a process frequently occurring in most of the numerous shallow lakes. Modern interannual lake level variations in the order of up to 7 m as well as fossil subaerial and subaquatic lake level terraces document a closed lake system exceptionally sensitive to changes in the hydrological regime. The total range of late Quaternary lake level fluctuations amounts to more than 50 m. Modern limnological observations show that precipitation of calcite in this hard water lake is a phenomenon coinciding with lake level fluctuations: falling lake levels increase the ionic concentration in the water column to an amount that calcite precipitates in larger quantities whereas higher lake levels do not support the production of autochthonous calcite. Thus, periods with calcite concentration of up to 38% alternate with periods in the sedimentary record with no calcite at all. As the concentration of total inorganic carbon in the sedimentary record serves as a proxy for the intensity of calcite precipitation, it is applied for the reconstruction of past lake level fluctuations at Laguna Potrok Aike. In the framework of the project South Argentinean Lake Sediment Archives and modeling (SALSA) it was thus possible to document rapid climatic changes as reflected in hydrological fluctuations. During the late Middle Ages (ca. AD 1230-1410) a rather low lake level is representative for the "Medieval Climate Anomaly". This period was followed by a prolonged period of considerably higher lake level related to the "Little Ice Age" (ca. AD 1480-1940). During the 20^th century warming the lake level dropped again, however, not reaching medieval dimensions. Large changes in water balance, like during the last millennium, are documented for the record from Laguna Potrok Aike throughout the mid-Holocene, although extended periods of high lake levels can only be reported for the ending glacial period, the Late-Glacial and the early Holocene. A possible explanation for these hydrological variations are latitudinal shifts of the southern westerlies related to an increase or a decrease of the pressure gradient between the tropics and Antarctica during the late Quaternary leading to increased or reduced precipitation in the area of investigation.