314 PALEOENVIRONMENTS IN THE SUBTROPICAL NORTHEASTERN ARGENTINA AS DEDUCED FROM STABLE CARBON ISOTOPES COMPOSITION OF FERRALLITIC SOILS Morrás, H. 1 , Moretti, L. 1 , Glaser,B. 3 , Hatté, C. 2 , Zech,W. 3 1. INTA-CIRN, Instituto de Suelos, 1712 Castelar, Buenos Aires, Argentina, hmorras@cirn.inta.gov.ar 2. Laboratoire des Sciences du Climat et de l´Environnement, CEA-CNRS, Gif sur Yvette, France 3. Institute of Soil Science, University of Bayreuth, Germany Key words: δ 13 C, ferrallitic soils, paleoenvironments INTRODUCTION Different biological, geological and geochemical proxy indicators may contribute to the reconstruction of paleoenvironmental changes, e.g. pollen, phytolites, fossils, paleosols, etc. More recently the use of stable carbon isotopes has gained interest to infer past climatic fluctuations. The stable carbon isotope composition of plants differs according to their photosynthetic pathway (C3, C4, and CAM). δ 13 C values of C3 plants like trees and almost all the plants in temperate and cold regions, range from approximately –32‰ to –20‰ PDB, with a mean of –27‰. In contrast δ 13 C values of C4 plants, adapted to conditions of higher hydric stress, range from –17‰ to – 9‰, with a mean of –13‰ (Mariotti, 1991; Hatté and Schwartz, 2003; Pessenda et al., 2004). Isotopic fractionation depends not only on the type of vegetation but also on the plant environment, the CO2 atmospheric concentration, the temperature and on the humidity level. During litter decomposition and humification the isotopic signal of plants is transferred into the soil organic matter (SOM). Thus, the analysis of carbon isotopic composition of SOM may allow to deduce information about the environments and climatic conditions under which plants have grown in former times. The palaeoenvironmental changes and landscape evolution of the subtropical Province of Misiones, in northeastern Argentina, has began to be analyzed only recently and some opposite interpretations have been proposed by different authors (Morrás et al., 2005 a-b). Up to now, there are no studies on the environmental evolution of this region by means of carbon isotope analyses. In contrast, there are several studies on the isotopic composition of the OM of soils in neighbouring regions of southern Brazil (vg. Desjardins et al. 1991; Pessenda et al., 2004), which provide a framework of comparison and interpretation for similar studies in subtropical Argentina. MATERIAL AND METHODS The Province of Misiones is located in the notheastern extreme of Argentina, around 27º S and 54º W. The climate is subtropical humid without dry season; mean annual temperature is around 20°C and mean annual rainfall increases from 1750 mm in the south to 1950 mm in the north. The present vegetation is a subtropical forest, except a narrow strip along the southern border with a savannah type vegetation. The typical soils are mainly Ultisols and, in lower proportion, Oxisols. This ferrallitic solum usually has a depth of 3 to 7 m above the weathered basalt, and a “stoneline” frequently appears in the lower part of the profiles, close to the saprolite. Stonelines are frequent in tropical and subtropical environments. Their genesis and that of the overlying material has experienced numerous interpretations, mainly under the headline “authochtony or allochtony”. Refering specifically to the case of Misiones and neighbouring areas in Brazil and Paraguay, Iriondo and Kröhling (1997) postulated that the material that covers the basaltic rock and in which the red soils have developed is an eolian sediment of Upper Pleistocene age, deflated from the alluvial plains of Paraná and Uruguay rivers; the authors consider this material a “tropical loess” and named it as “Oberá Formation”. On the other side, and in accordance with the traditional interpretations (Sanesi, 1965), our results suggest that the surface material and the “stonelines” have developed in situ through the weathering of the basaltic rock (Morrás et al., 2005 a-b). Fig. 1. Location of the profiles under study (triangles) and mean actual precipitation