Variaciones en la composición isotópica (13 C) de la materia orgánica en una secuencia cuaternaria en la parte central de México

Organic matter δ13 from a sedimentary sequence in the Cuenca de México is reported and used in paleoenvironmental interpretation. We studied 280 cm of a silty-clay. sedilliciltary sequence with some sand and ash interbeded at El Tepalcate. The section is located at the SE end of Lake Texcoco and represents a time span of approximately 17.000 years. δ13. Values ranging from -20.9 to -23.8%c were obtained along the sedimentary sequence. The isotopic values are interpreted as variations in percentage of organic matter source from C-3 and C-4 plants. Limiletic coliditiolis rnayhave oceurred in the lower part of the section, however, more evidence is still needed to corroborate this idea. Dry and probably saline conditions prevailed until 24,000 years B.P. followed by a fresh-water environment for 4,500 years. A stage of high salinity prevailed from 19,500 to 16,800 years B.P. During this period of time a saltier environment and high percentage of C-4 plants (>50%) prevailed. Saline and dry conditions, but not as extreme as in the previous stages, continued until 14,000 years B.P. An increase in salinity is observed for the following 1,500 years. Finally, a fresher environment existed in the upper part of the section related to an increase in the size of glacials around the mountains surrounding the Cuenca de México. The δ13 of the organic matter in addition to other micropaleontological and geochemical analyses such as polien, diatoins, and trace metals, is a very useful tool for paleoenvironmental interpretations