Using 210Pbuns and 137Cs to date recent sediment cores from the Badain Jaran Desert, Inner Mongolia, China

Dating the desert lake sediments using Pb-210(uns) and Cs-137 methods is challenging because of the low atmospheric fluxes and the unexpected irregular variations in the specific activity profiles. Due to the large numbers of permanent lakes in the Badain Jaran Desert, they provide abundant sites for investigating the desert Pb-210(uns) and Cs-137 geochronology. Research was performed to determine the reliable ages using this methodology and obtain a better understanding of the principal factors that affect the dating results. The top 20-30 cm of core SH-1 from Lake Shuanghaizi and of core YD-1 from Lake Yindeertu in Badain Jaran Desert were dated. The results support multiple conclusions. (1) Both cores have low surficial Pb-210(uns) activities (SH-1, 24.5 Bq kg(-1); YD-1, 59.7 Bq kg(-1)) compared with those of the cores from arid Northwest China, such as Lake Bosten (350 Bq kg(-1)) and Lake Wulungu (523.37 Bq kg(-1)) from Xinjiang. Although the Pb-210 record in SH-1 has multiple irregular variations with depth, the exponential decrease in the YD-1 activity profiles appears fairly regular. (2) The Pb-210(uns) activities are highly correlated with the sample salt concentration (SSC). (3) Sediment dates for core SH-1 derived from the constant rate of Pb-210 supply (CRS) model indicate that it spans a period of 65 years (1942 AD-2007 AD) with an average sedimentation rate of 0.21 cm a(-1) (0.16 g cm(-2) a(-1)). Sediment dates for core YD-1 derived from the corrected CRS model indicate that it spans a period of 121 years old (1889 AD-2010 AD) with an average sedimentation rate of 0.16 cm a(-1) (0.12 g cm(-2) a(-1)). In both cores, the Cs-137 time markers of 1963 appeared at different depths, which may result from the differing sedimentation rates at the sites of the cores. The sedimentation rates calculated from the CRS-determined age differences between the SH-1 and YD-1 cores appear to have resulted from the topographies containing variouun, us mega-dunes and regional aeolian activity patterns. We believe that the low Pb-210 flux and the unexpected fluctuations in the Pb-210 profiles are the principal factors affecting our dating results. The low Pb-210(uns) activities were caused by infrequent rainfall and mega-dune barriers. The unexpected fluctuations in the Pb-210(uns) profiles resulted from the salt crystallisation that occurred via deposition or postsedimentation. The salt crystallisation has been closely associated with abrupt regional climate changes, including rainfall, temperature and serious drought events during the 1950s-1970s. (C) 2012 Elsevier B.V. All rights reserved.