PALEOLIMNOLOGY OF QILU-HU, YUNNAN PROVINCE, CHINA

Qilu Hu is a large (A = 36.9 km2), shallow (Z(max) = 6.8 m) lake that lies at an elevation of 1797 m above msl on the Yunnan Plateau, southern China. Lake waters are hard (Mg = 3.2 meq L-1, Ca = 1.3 meq L-1), fresh (conductivity = 380-mu-S cm-1), and productive (Secchi < 40 cm). An 11-m sediment core has a basal C-14 age of 30 960 +/- 860 B.P. Sediments between 11 m and 6 m are high in % dry weight, rich in clay components Al2O3, Fe2O3, K2O, MgO, and low in organic C (less-than-or-equal-to 6.1%), carbonate-C (< 1.0%), total N (< 3.2 mg g-1), and total S (less-than-or-equal-to 1.7 mg g-1). Diatoms and pollen indicate open-water conditions between 9.0 m and 6.0 m (13420-11790 B.P.). Above 6.0 m, CaCO3 and organic matter concentrations increase relative to clastics. The transition marks a change to shallow-water conditions as inferred from diatoms and pollen, and probably reflects a shift to drier climate. Uppermost (80-0 cm) red clays were deposited rapidly, probably as a consequence of recent (decades to centuries) riparian disturbances (e.g. agriculture, lake-bottom reclamation, urban development). Dates assigned to events in the Qilu Hu profile are tentative because of potential hard-water-lake error.