Mineral and organic components of the buried paleosols of the Nevado de Toluca, Central Mexico as indicators of paleoenvironments and soil evolution

Abstract

Results of earlier studies of Quaternary tephra-paleosol sequences in Central Mexico revealed contradictions between paleopedological and lacustrine records. To settle the contradictions, selected quantitative characteristics of mineral and organic components of the paleosols PT1-PT7 from the Nevado de Toluca sequence were studied as independent paleoclimate proxies. Mineralogical composition of sand and clay fractions allows assessment of weathering and secondary mineral formation, both being climate-dependent processes. Stable carbon isotope ratios of humus and phytolith morphology indicate paleovegetation. In all paleosols, the sand fractions consist mainly of unstable volcanogenic minerals, although the grains have weathering features. Short durations of pedogenesis are responsible for limited weathering status. The clay fraction is dominated by halloysite and dehydrated halloysite in units PT1-PT4, which suggests drier periods during paleosol formation, whereas major amounts of kaolinite in the units PT5-PT7 indicate extended humid pedogenesis. delta(13)C values of PT2-PT4 humus are intermediate between those characteristic of C3 and C4 plants. Phytolith assemblages which include both forms of C3 (dominant) and C4 (minor quantities) grasses agree with carbon isotope data. These data indicate plant cover changes driven by wet/dry paleoclimate oscillations. The new results, showing drier stages during the formation of Late Pleistocene paleosols PT2 and PT3, correct the primary interpretation of these units as indicating only humid pedogenesis, and settles the contradiction between paleopedological and lacustrine records. (C) 2002 Elsevier Science Ltd and INQUA. All rights reserved.