The diversity of ecosystems in southeastern Brazil can not always be related to edaphic, geomorphologic, or hydrologic factors. Mountain summits, where soils are characterized by common parent material, offer a special environment for studies of soil genesis and dating of cyclic events related to regional climate dynamics. After a detailed investigation of soils from the São José Hills (Prados - Minas Gerais State, Brazil), two soil profiles (P1 and P2) originated from arenite of the Tiradentes Formation were studied. They are characterized by successive depositions of sandy layers alternated with layers of sand enriched with organic matter. The study site lies 1,350 m above sea level and 350 m above the dominant topographical level of the region. Thirty-three layers with organic matter, alternated with sand layers, were identified in P1. Three layers in P1 (20-30, 70-80, and 100-110 cm depth) with an organic C content of 0,5, 7, and 1 dag kg-1, respectively, present radiocarbon (14C) ages < 40, 180 ± 60, and 350 ± 80 years BP, respectively, and deposition rates of 0.177 cm year-1 at depths between 110 and 170 cm and of 0.357 cm year-1 between 70 and 20 cm layer. In P2, the layers enriched with organic matter are thicker (between 10 and 130 mm), with abrupt discontinuity. They lie between 20-30, 80-90 110-120, and 170-180 cm deep, have a C content of 3, 2.5, 21, and 1.5 dag kg-1, and a 14 C age of 3580 ± 80, 3750 ± 80, 21210 ± 180, and 24060 ± 130 year-1 BP, respectively. Their deposition rates are 0.352 cm year-1 between 20 and 80 cm, 0.002 cm year-1 between 80 and 110 cm and 0.021 cm year-1 at depths between 110 and 170 cm. In both soil profiles, the C/N ratio increases with depth and age. The amount of Ti and Zr, elements of low mobility, are higher in the oldest profile layers, while Cu and Pb are more concentrated in the layers higher in organic matter. A plant fragment (diameter 5 cm, 62 cm long) found at the bottom of P2 dated back to 32220 ± 290 years BP, and is associated with the beginning of this profile formation. The soils at the summit of São José Hills had its origin in the Tiradentes Formation without contribution of other geologic material. Rain water is the main factor that adds energy to this environmental energy. Therefore, soil formation is related to climate attributes. P1 is a holocenic (Fluvic Entisol) soil, formed by episodic depositions of sand, alternated with sand enriched with organic matter. The formation of P2 (Paleosol) began in the Pleistocene and lasted until the Holocene. The morphology of its buried peat layers is related to oscillations of the water surface of a lake, reflecting drier and more humid climate phases. The assessed radiocarbonic ages are related to Pleistocene and Holocene climate alternations in P2 and holocenic in P1. Profile P2 is an appealing site for palynological studies, where ecotypes that were present in the area beginning in the late Pleistocene can be identified and related to the palaeoclimates.
Palaeossols; 14C dating; Quaternary; High altitude environment
