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Environmental history of southern Patagonia unraveled by seismic stratigraphy and sediment cores of Laguna Potrok Aike Anselmetti, F. S. ; Ariztegui, D.; De Batist, M.; Gebhardt, C.; Haberzettl, T.; Niessen, F.; Ohlendorf, C.; Zolitschka, B. (2006). Environmental history of southern Patagonia unraveled by seismic stratigraphy and sediment cores of Laguna Potrok Aike. Eos, Trans. (Wash. D.C.) 87(48) In: Eos, Transactions, American Geophysical Union. American Geophysical Union: Washington. ISSN 0096-3941; e-ISSN 2324-9250

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Beschikbaar in	Auteurs
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Documenttype: Samenvatting

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Auteurs		Top
Anselmetti, F. S. Ariztegui, D. De Batist, M. Gebhardt, C.	Haberzettl, T. Niessen, F. Ohlendorf, C. Zolitschka, B.

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Abstract

Laguna Potrok Aike, located in Southernmost Patagonia (Argentina, 52°S) is a maar lake and likely provides the only continental Southern Patagonian archive that covers a long and continuous interval of several glacial- to-interglacial cycles. In the context of a proposed ICDP-drilling initiative (see Zolitschka et al., this session), several site surveys have been undertaken that characterize in detail the shallow subsurface of the lake. A variety of environmental changes are identified based on seismic stratigraphic analyses of a dense grid of high- resolution data. Long sediment cores recovered the material to date and calibrate these changes providing a chronology of events. The paleoenvironmental history reflects the complex interplay between variations in the hydrologic cycle, wind regime, and volcanic/tectonic activity. Laguna Potrok Aike is the largest and deepest lake (maximum depth 100 m) in the Pali Aike Volcanic Field. It is steeply-rimmed, circular in shape with a diameter of 3 km and surrounded subaerially by a series of paleoshorelines, reflecting its sensitivity to changes in the evaporation/precipitation ratio. Seismic subsurface data indicate a basinwide erosional unconformity, that consistently occurs on the shoulder of the lake down to a depth of -35 m (below modern lake level) marking the lowest lake level during the Late Glacial-to-Holocene times. Cores that penetrate this unconformity comprise MIS3-dated sediments (45 ka) 3.5 m below the unconformity that were deposited during high lake levels. These cores also provide an age of 6800 cal BP for the transgressional sediments burying the unconformity. This middle Holocene transgression following the maximum lake level lowstand marks the onset of a stepwise increase in moisture, as shown by a series of buried paleoshorelines that were formed during at least five lake level stillstands between the depths of -32 and -12 m. No indications point towards an intermittent lake level lowering, so that the lake level curve rises overall continuously, documenting an increased but stepwise wettening of the climate after 6800 cal BP. The seismic facies of the basinal sediments is characterized by a succession of continuous reflections. Seismic-to-core correlations indicate that most high-amplitude reflections are caused by tephra layers originating from various volcanoes of the Austral Andean Volcanic Zone and the Southern Volcanic Zone. Mound-like sediment geometries occur after ~5000 cal BP and contrast to the rather draping geometries of the older sequences. This change in sedimentation pattern documents the onset of a wind-triggered lake circulation yielding drift deposits, caused likely by a strengthening of the Southern Hemispheric Westerlies over Laguna Potrok Aike at that time. Furthermore, several well-defined lateral slides can be recognized by their transparent seismic facies. They are associated with six event horizons traceable throughout the basin. Two of these horizons are marked by simultaneous occurrence of almost ten individual slide bodies, marking either paleoearthquakes or lake level-related destabilizations of slope sediments.

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