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Contrasting Holocene vs. Late Pleistocene dynamics of sediment deposition in Laguna Potrok Aike, Argentina

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Ohlendorf, C. , Gebhardt, C. , Hahn, A. , Kliem, P. , Zolitschka, B. and the PASADO Science Team, (2010): Contrasting Holocene vs. Late Pleistocene dynamics of sediment deposition in Laguna Potrok Aike, Argentina , AGU 2010to 17 December .
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Abstract:

In the maar lake Laguna Potrok Aike (52°S, 70°W; 116 m asl.; diameter: 3.5 km, water depth: 100 m) in southern Patagonia, Argentina, in total 510 m of lacustrine sediments were recovered in the framework of the ICDP-project PASADO (Potrok Aike Maar Lake Sediment Archive Drilling Project). Quadruplicate and triplicate cores down to a maximum depth of 101.5 m below lake floor were taken with a total core recovery of 94.4 % from two drillsites located 700 m apart in the central profundal plain of the lake.Seismic refraction data reveal a funnel-shaped structure originating from phreatomagmatic maar explosions embedded in the sandstone rocks of the surrounding Santa Cruz Formation. The funnel is filled by lacustrine sediments of up to 370 m in thickness with seismic velocities (sv) of 1500-2350 m s-1 which are underlain by a unit of probably volcanoclastic origin (sv >2400 m s-1). Seismic reflection data of the uppermost 100 m of the sediments reveal stratified lacustrine sediments and a rather dynamic development of the lake: on top of pelagic sediments, a desiccation horizon is found, with sand dunes in the eastern part of the lake basin. These are overlain by a series of paleo shorelines documenting the history of lake level rise in this early new lake. While this new lake formed in the central and eastern part of the maar depression, the western part was filled by stacked coarse-grained, delta-type sediments probably deriving from the only inlet that at present is sporadically active. After this early filling of the new lake, a stage of rapid lake level rise is observed in the seismic reflection data.Seismic findings are currently verified by the analyses of a 106.08 m long composite profile created by splicing of the three drilled cores of Site 2. According to a first age model, the sedimentary record from Laguna Potrok Aike reaches back to approx. 56,000 cal. BP and exhibits contrasting lithologies downcore especially in the Pleistocene part of the record. First estimates indicate that approx. 54 % of the record consist of redeposited sediments, which in the lower part seem to represent the delta-type sediments detected in the seismic profiles. The most dramatic shift of the entire record, however, occurs around 15,600 cal. BP. At this date, a more than 1 m thick layer with a fundamentally different geochemical composition follows immediately above a ca. 2 mm thick tephra layer that originated from an eruption of the Reclùs volcano. This layer separates carbonate-free sediments in the lowermost 80 m of the record that are dominated by clastic input of fluvial and eolian origin and a high proportion of redeposition from a lake system characterised by authigenic carbonate precipitation in the uppermost 18 m of the record with calcite contents of up to 35 %, much lower percentages of redeposition and lake level variations between +21 m and -35 m with respect to present day lake level.

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