Paleolimnology of Qilu Hu, Yunnan Province, China.

Qilu Hu is a large (A = 36.9 km ² ), 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 = 3.2m eq L ⁻¹ , Ca = 1.3 meq L ⁻¹ ), fresh (conductivity = 380 μS cm ⁻¹ ), and productive (Secchi < 40 cm). An 11-m sediment core has a basal ¹⁴ C age of 30960 ± 860 B.P. Sediments between 11 m and 6 m are high in % dry weight, rich in clay components Al ₂ O ₃ , Fe ₂ O ₃ , K ₂ O, MgO, and low in organic C (≤6.1%), carbonate-C (<1.0%), total N (<3.2 mg g ⁻¹ ), and total S (<-1.7 mg g ⁻¹ ). Diatoms and pollen indicate open-water conditions between 9.0 m and 6.0 m (1342011790 B.P.). Above 6.0 m, CaCO ₃ 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.