Saturday, 15 July 2006

Nature and Rates of Pedogenic Processes in Semi-arid Patagonia (Argentina).

Daniela Sauer, Institute of Soil Science and Land Evaluation, Univ of Hohenheim, Emil-Wolff-Straße 27, 70599 Stuttgart, Germany, Gerhard Schellmann, Institute of Geography, Univ of Bamberg, Am Kranen 1, D-96045 Bamberg, Germany, and Karl Stahr, Institute of Soil Science and Land Evaluation, Univ of Hohenheim, Emil-Wolff-Straße 27, 70599 Stuttgart, Germany.

Nature and rates of pedogenic processes were investigated at the eastern coast of central Patagonia by establishing a soil chronosequence on beach ridges with ages ranging from about 1380 to 6240 14C-years BP. The beach ridges are composed of reworked Patagonian Gravel ("Rodados Patagonicos"), which is a widespread fluvial or glaciofluvial sediment in Patagonia. The gravel of the beach ridges consists mainly of medium to coarse gravel. The climate in the study area is semi-arid. The mean precipitation is 287 mm a-1 with a maximum from May to July, the mean annual temperature is 12.6°C. The potential evapotranspiration and hence the water deficit in summer is significantly increased by strong westerly winds, which are characteristic for Patagonia. Soil formation is strongly influenced by eolian sediments, which have been washed into the interstitial spaces between the gravel. Different amounts of eolian material in the soils lead to differentiation into Leptosols (containing °Ü 10% fine earth in the upper 75cm) and Regosols (containing > 10% fine earth). Soil depth functions and chronofunctions of various soil properties were established. The depth functions of organic carbon and calcium carbonate contents reflect the processes of humus accumulation in the upper part of the soils and calcium carbonate leaching in the upper decimetres and accumulation immediately below. The depth function of pH indicates slight acidification in the upper centimetres. Soil chronofunctions were established by use of mean values of the horizon data of 0 - 10 cm below the desert pavement, which were weighted according to the horizon thicknesses. The depth of 10 cm was chosen, because the pedogenic changes under the given climatic conditions are most pronounced relatively close to the soil surface. Humus accumulation showed no time dependency on the time scale of this study, since the humus contents of the top soil reach equilibrium within shorter time periods. Calcium carbonate accumulation generally gets more intensive with time. This was observed in the field, but could not be quantified by laboratory analyses, because the parent material was not homogeneous with regard to calcium carbonate content, since the sediment layers of the beach ridges contained different amounts of shell fragments. Chronofunctions of pH show that within 6000 radiocarbon years of soil development pH drops from 7.0 to 6.6 in the Leptosols and from 8.1 to 7.5 in the Regosols. Both, Leptosols and Regosols show close relationships to time for Fe:Zr, Mn:Zr, and Si:Al, indicating relative enrichment of Fe and Mn and leaching of Si. Leaching of mobile elements takes place on a higher level in the Regosols than in the Leptosols, while relative enrichment of Fe and Mn take place on a lower level.

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