Environmental history of southern Patagonia unravelled by the seismic stratigraphy of Laguna Potrok Aike

Laguna Potrok Aike, located in southernmost Patagonia (Argentina, 52°S) is a 100 m deep hydrologically closed lake that probably provides the only continental southern Patagonian archive covering a long and continuous interval of several glacial to interglacial cycles. In the context of the planned ‘International Continental Scientific Drilling Program’ initiative ‘Potrok Aike Maar Lake Sediment Archive Drilling Project’, several seismic site surveys that characterize in detail the sedimentary subsurface of the lake have been undertaken. Long sediment cores recovered the material to date and calibrate these seismic data. Laguna Potrok Aike is rimmed steeply, circular in shape with a diameter of ~3.5 km and is surrounded by a series of subaerial palaeoshorelines, reflecting varying lake-level highstands and lowstands.Seismic data indicate a basinwide erosional unconformity that occurs consistently on the shoulder of the lake down to a depth of -33 m (below 2003 AD lake level), marking the lowest lake level during Late Glacial to Holocene times. Cores that penetrate this unconformity comprise Marine Isotope Stage 3-dated sediments (45 kyr bp) ~3.5 m below, and post-6800 cal yr bp transgressional sediments above the unconformity. This Middle Holocene transgression following an unprecedented lake-level lowstand marks the onset of a stepwise change in moisture, as shown by a series of up to 11 buried palaeoshorelines that were formed during lake-level stillstands at depths between -30 and -12 m. Two series of regressive shorelines between ~5800 to 5400 and ~4700 to 4000 cal yr bp interrupt the overall transgressional trend. In the basin, mound-like drift sediments occur after ~6000 cal yr bp, documenting the onset of lake currents triggered by a latitudinal shift or an increase in wind intensity of the Southern Hemispheric Westerlies over Laguna Potrok Aike at that time. Furthermore, several well-defined lateral slides can be recognized. The majority of these slides occurred during the mid-Holocene lake-level lowering when the slopes became rapidly sediment-charged because of erosion from the exposed shoulder sediments. Around 7800 and 4900 cal yr bp, several slides went down simultaneously, probably triggered by seismic shaking