Paleohydrological investigation of Infernao Lake, Moji-Guaccu River watershed, Sao Paulo, Brazil

A 450 cm sediment core spanning the last 3,500 years was collected from 4.6 m of water in Lake Infernao, located in the floodplain of the Moji-Guacu R iver in SE Brazil, to reconstruct the wetland system (river and lake) paleo hydrology. A multidisciplinary approach using isotopic (C-13 and N-15), che mical (C/N ratio and lignin derived phenols) and physical analyses (% water content and density) were used in this study. A coarse sand unit in the lo wer part of the core (450-417 cm) corresponds to the paleoriver. The rest o f the sediment core (dark clay) shows a gradual change from a fluvial to a lacustrine environment. C-14 dates from the lower part of the core (417-200 cm), demonstrate an inversion and narrow span in age (3,000-3,500 yrs B.P. ) corresponding to a period of rapid fluvial sedimentation and reworking. T he lake seems to have been well established only after 3,000 yrs B.P. The c hange from a fluvial to a lacustrine environment is inferred from the isoto pic and chemical data. Sediment of the fluvial phase is characterized by hi gher delta C-13 values, lower C and N content and a lignin content represen tative of woody-tissues. Sediment of the lacustrine phase show a gradual de crease in delta C-13 values toward the top of the core reflecting the influ ence of organic carbon derived by phytoplankton and aquatic macrophytes (C- 3 plants). The trend to higher C and N content and low C/N ratios also supp ort this interpretation. The lower lignin content indicates increase contri butions of non-vascular plants (phytoplankton). Similarly, the slight incre ase of non-woody tissues (higher C/V ratios) and the predominance of angios perm plants (high S/V ratios) as organic matter source indicate an increase contribution of aquatic plants during the lacustrine phase. Our data prese nted here in addition to the geomorphological information and the available paleoclimatic scenario of pollen-inferred analyses for aquatic systems fro m the southeastern Brazil point to a climatic origin for this fluvial-lacus trine transition at about 3,000 yrs B.P. and to a climatic evolution leadin g to the present conditions and to a consequently gradual isolation of Lake Infernao from the influence of the Moji-Guacu River.