Significado de la composición isotópica (ò 13 C y ò 18 0) en paleosuelos carbonatados. Mioceno de la Cuenca de Madrid

Carbon and oxygen isotope analyses from pedogenic carbonates of the N and NE areas of the Madrid Basin show ranges of ô 13C from —10 to —5.5 and 8 80 from —9.2 to —3.5 in the N area and ô13C from —8.2 to —5.5 and ô180 from —8 to —4 in the NE one. Differences between calcretes and shaIlow lake carbonates in the N area are related to differences in the evaporation rate, possible vegetation type and ground-water influence of the adjacent lake system. In the NE area a pedogenetic trend is observed and lighter values are indicative of a higher degree of pedogenetic alteration. Differences between the studied areas may indicate distinct vegetation types or a slight contamination by marine carbonates. In this paper we try to show how the interpretation of the isotopic composition of calcretes is particularly difficult in view of the large number of factors which may such values