SEDIMENTOLOGY AND PHYSICOCHEMICAL LIMNOLOGY OF CRATER LAKES IN THE PALI AIKE VOLCANIC FIELD, SOUTHERN PATAGONIA (ARGENTINA)

Southern South America is the only continental land mass between 38°S and the Antarctic Circle. As such it represents a unique opportunity to reconstruct terrestrial paleoclimatic conditions in an area that is subject to shifts in polar and mid-latitude wind and pressure fields. Here we present data from crater lakes in the Pali Aike Volcanic Field (PAVF) located between 50°-52°S and 69°-71°W. The PAVF is a tectono-volcanic belt 50km wide and >150km long located 80km west of the city of Río Gallegos (Patagonia / Argentina). Several maars formed in this area between 3.78 and 0.17Ma ago. As the last glaciation, the Llanquihue, did not reach this area there is potential for long continuous deposition in these crater lakes. Though more than one hundred lakes are visible on satellite images during a field campaign in the austral summer of 2002 only two lagunas turned out to be very promising for further sedimentological and physicochemical limnological investigations due to occasionally desiccation of the other lakes. Altogether 16 gravity cores up to a length of 129cm were taken from these permanent lakes - Laguna Potrok Aike and Laguna Azul. The cores were analyzed by various chemical and physical methods. To get a general view of the lakes of this area also chemical and physical data of several episodical lagunas was collected. Laguna Potrok (max. diameter: 3470m) showed no stratification in water chemistry at all which might be due to the size and the exposure to strong winds in this area. The age of 2.153+/-150 yr cal BP of the basement of a core points to an average sedimentation rate of 0.5mm a-1. With an age of 1.073+/-92 yr cal BP the sedimentation rate of the conspicuously smaller, well stratified Laguna Azul (max. diameter: 560m), with a thermocline at 20m and sub-oxic conditions at the sediment / water interface, is almost twice as high. In both sediment records distinct shifts in the sedimentological parameters occur during the last millennium. Furthermore Laguna Potrok Aike has three major distinguishable lake terraces representing different high stands of the water body. During a field campaign in 2003 long cores will be taken from Laguna Azul and Laguna Potrok Aike. Additionally an existing 59m long record dating back to >31.600 yr BP from Magallanes Maar, a dry maar in the PAVF, will be sampled in order to get information of older epochs.