The Tlaxcala basin paleosol sequence: a multiscale proxy of middle to late Quaternary environmental change in central Mexico

Resumen

The Central Mexican Highlands still lack a consistent scenario of Quaternary environmental evolution, especially for the period before the last glacial/interglacial cycle. We studied an extensive tephra-paleosol sequence near the city of Tlaxcala (11 paleosols grouped in 3 units: Grey, Brown and Red) in two sections (Tlalpan and Mamut) to obtain a paleoclimate proxy for the middle to late Pleistocene and the Holocene. A general paleoclimatic trend for the last 900,000 yr. is interpreted from the Tlalpan section (the base dated by K/Ar), and a more detailed record for the period starting with Marine Isotope Stage 3 (MIS3) from the Mamut section (provided with a set of 14C dates). Morphological, physicochemical and mineralogical characteristics of all buried paleosols point to pedogenic processes typical for humid ecosystems, namely: weathering and neoformation of kaolinitic-halloysitic clay, gleization, and clay illuviation, whereas the surface late Holocene soil is characterized by precipitation of carbonates, indicative of a drier climate. In Tlalpan section, the lowest Red Unit demonstrates the strongest development of weathering features, together with maximum accumulation of clay and crystallized iron oxides. The overlying paleosols have lower weathering status; the intermediate Brown Unit shows prominent features of clay illuviation whereas the upper Grey Unit is marked by surface redoximorphic properties. We hypothesize that the Red Unit paleosols correspond to the period of the Mid Pleistocene Climate Transition, when less pronounced glacial/interglacial climate cyclicity permitted more advanced soil development through long periods of landscape stability. In the Mamut section vertic features are present in the lower paleosol, weathering and clay illuviation are more pronounced in the middle one, and the incipient upper soil is dominated by gleization features. This trend indicates the change from drier climate with strong seasonality in the second half of MIS3 to uniform cool humid conditions during major part of MIS2 and then to unstable climate with uneven, occasionally excessive precipitation in the late Glacial, which promoted local synsedimentary soil formation in a wetland environment.