U-series chronology of sediments associated with Late Quaternary fluctuations, Balikun Lake, northwestern China

摘要

The U-series dating method was used to date the sediment core B_1 94 from Balikun Lake, Xinjiang, northwestern China. The samples were extracted with acids of different concentrations, and ~(230)Th/~(232)Th and ~(234)U/~(232)Th ratios of each sample were measured. Contamination with non-radiogenic ~(230)Th can be overcome in most instances using an isochron correction scheme for a sequential acid-leach procedure. The method can provide the most reliable dates from samples in lake of the arid-semiarid regions, where carbonate is assumed close to uranium and thorium after deposition. Where three known ages of palaeomagnetic reversals have already been obtained in the same core, the U-series ages appear to compare reasonably well with palaeomagnetic event ages. Based on the U-series chronology and stable isotope records, lake level history was reconstructed, which suggests that Balikun has been a shallow saline lake since 150 ka BP, and its level frequently changed in response to climate changes. Arid climates occurred between 129-71 ka BP and at 62-25 ka BP, corresponding to marine oxygen isotopic stages (OIS) 5 and 3, during which times Balikun Lake approached playa conditions. After 13 ka BP, the lake shrank and became a saline field and mudflat with a water table at or below the lake bed. Three major high lake stands were identified at about ca. 138-132, 69-65, and 25-18 ka BP, and the three high stand periods correspond to OIS 6, 4 and 2. Rising lake levels during the penultimate glaciation and both stades of the Last Glaciation might have resulted from the location of the lake in an arid region, where the climate was controlled mainly by the westerly winds.