Geochemical and isotopic (U, Th) variations in lake waters in the Qinghai Lake Basin, Northeast Qinghai-Tibet Plateau, China: origin and paleoenvironmental implications

A uranium (U) geochemical study of lake water samples from Qinghai Lake, Northeast Qinghai-Tibet Plateau, China, was undertaken to assess the primary controls on lake water chemistry. The 234U/238U activity ratios of lake water exhibited limited spatial and vertical variations and were relatively high compared to the values measured in rivers worldwide ((234U/238U) = 1.171), lower than in the four river discharge inside the Lake Qinghai catchment. The U concentrations also varied minimally within three vertical sampling profiles and exhibited spatial patterns in surface waters that correlated with the distribution of total dissolved solid (TDS) values and salinity concentrations. These data suggest that the variability in U, TDS, and salinity are partially controlled by evaporation or a different secondary concentration pathway. Moreover, the U concentration and 234U/238U activity ratio indicates that the length of this procedure, which is controlled by the duration of water-rock interaction, is probably an important factor to consider when accounting for the variable salinities of lakes located in similar geographical areas. The 230ThXS content is primarily derived from the decay of dissolved 234U. The various 230Thxs concentrations observed in the water column are interpreted to reflect various scavenging residence times in Qinghai Lake, ranging from 0.7 to 4 years. The lower water mass age, such as at site 123, suggests a more rapid scavenging rate. Dissolved 232Th is placed into the lake via the incomplete dissolution of lithogenic substances, such as dust aerosol, or other lithogenic sources (e.g., lake bed sediment or suspended riverine sediment). Water column 232Th concentration data suggest that the Th concentrations of the lake water are controlled by three processes: (1) the dissolution of aerosol dust, (2) the dissolution of riverine and lake bed sediment, and (3) the scavenging of Th from the water.