STABLE CARBON ISOTOPE SIGNATURES FROM PALEOSOL HUMUS FOR RECONSTRUCTING LATE PLEISTOCENE ENVIRONMENTS OF AXAMILPA, PUEBLA

Many questions exist about the landscape history during late Pleistocene in México and what was the response of the fauna to the prevailed climatic conditions. At this moment little is known about this topic in the south of Mexico, especially for the marine isotope stage 3 (MIS 3) which is less studied than MIS 2. In the specific case of MIS 2, which includes the Last Glacial Maximum, more arid conditions has been suggested for the center of Mexico while for other interior basins of the center-west of Mexico increase of humidity has been registered.

In this work, we used δ¹³C of paleosol humus and fossils for reconstructing the past environments in Axamilpa, south of Puebla. The stable carbon isotope composition has been widely used in the reconstruction of past environments because its accuracy in determining the type of the prevailed vegetation during the paleosols development.

The study area is located in the town of Tepexi de Rodríguez, Puebla. The section, described close to Axamilpa River from which takes the name, has a total length of around 22 meters. We identified 19 paleosols grouping them in four pedoestratigraphic units on the base of their macromorphological characteristics: Stagnogleyic, Vertic, Calcic and Humic units. From selected horizons of different units, we took samples for determining δ¹³C and phytoliths. In order to know the time of pedogenesis, some humus samples were dated, indicating that the pedostratigraphic sequence was developed during the end of MIS 3 and MIS 2.

Our results show a big contrast between the carbon isotope signature of modern soil and paleosols. The δ¹³C of modern soil correspond to C4 and CAM vegetation as it is expected, in a xeric shrubland area with no phytolith record. By the contrary all paleosols demonstrates a C3 signature. The phytolith record support the same interpretation and give us a big picture of the landscape evolution, where in the base are found a predominance of conifers, a kind of C3 vegetation, and few forms of grasses. Since then, it is seen a clear trend to the reduction of conifer forms and an increase of the C3 and C4 forms of grasses, being during the calcic pedogenesis the time of minor humidity in the area. We interpret the area as a forest during MIS 3 to become a heterogeneous landscape with increasing proportion of grassland during MIS 2.