Registro de isótopos estables -813C y 8180- en la sucesión miocena de la región suroriental de la Cuenca del Duero

Abstract

The stable isotopic record for the carbonate rocks, mostly palustrine facies, making up the chemical facies of the Miocene (middle -Aragonian- and upper -Vallesian- Miocene) section in the southeastern region of the Duero Basin makes evident: i) S'JCPD8 and 8,sOP0B are respectively-6.41-7.9 and -5.11-7.1 for the Lower Unit, -6.61-7.9 and-6.M-6.7 for the Middle U., -5.71-8.1 and-5.21-6.7 for the Upper U.; ii) the cross-plot 8UC versus 8iaO indicates a different representation for the Upper and Lower Units and the Middle Unit plots at an intermediate position; Hi) only the U. Unit shows a moderate correlación (r =

0.69, n= 27); and iv) some carbonate sequences show an increasing 81sO trend at the same time as 813C values decrease. The range of 8wO and 8'3C values recorded is consistent with the pedogenetic effects observed on the limestones -palustrine facies-, with the 813C compositions (-5.7 to -8.1, somewhat heavier than an extreme C3 vegetation signature), suggesting the contribution of organic C. The recorded trends for 8,sO and 8UC values in some carbonate sequences point to an increasing exposure intensity at their tops. The slight increase in the 8,sO values for the Upper Unit could indicate that climatic conditions were somewhat warmer during the Vallesian