## 3 Materials and Methods  
In May 2018, a sedimentary core with 50-cm length (TLC01) (42.447775\({}^{\circ}\), 83.285435\({}^{\circ}\)) was obtained from Lake Ta-Lung-Chi at a depth of 5.5 m using a gravity corer (UWITEC, Mondsee, Austria) fitted with a 60 mm internal diameter Perspex tube. The color of the core sediment is dark brown with no bedding structure. The core log was shown in Figure 2. The core was sampled in-situ at 1.0 cm intervals, and a total of 50 samples were obtained. The samples were analyzed at the Key Laboratory of Lakes and the Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences.  
The \({}^{137}\)Cs specific activity was detected by an Ortec HPGe GWL series well-type, coaxial low background intrinsic germanium detectors (EG&G ORTEC, Oak Ridge, TN, USA). The procedure for measuring sediment grain size involved placing a small amount of sediment sample (about 0.3 g) into a 100 mL beaker, adding 20 mL of distilled water, and 10 mL of 10% hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)). The sample was then heated and brought to a boil on a hot plate throughout which a wash bottle was used to continuously clean the beaker walls from substances deposited from the reaction foam. Once the sample was fully reacted following the complete decomposition of excess hydrogen peroxide, 10 mL of 10% hydrochloric acid was added and the beaker was removed upon boiling. Then, 100 mL of distilled water was added and left overnight, after which the distilled water was removed, the excess hydrochloric acid was washed off, and the sample was neutralized. Next, 20 mL of distilled water and 10 mL of potassium hexametaphosphate with a concentration of 0.05 mol/L were added to the sample and the beaker was placed in an ultrasonic cleaner and agitated for 15 min. Agitated samples were tested with a British Malvern Mastersizer 2000 laser particle size analyzer with a relative error of less than 1%.  
The temperature (TMP) and precipitation (PRE) data were derived from Climatic Research Unit (CRU) Time-Series (TS) version 4.04 of high-resolution gridded data of month-by-month variation in climate (cru_ts_4.04) of the University of East Anglia in the United Kingdom, with a time range from 1901 to 2019 [22]. The monitoring data were obtained from the Meteorological Data Center of China  
Figure 2: The vertical distribution of \({}^{137}\)Cs specific activity in Lake Ta-Lung-Chi sediments, a mountain lake in the Tian Shan (**left**), and the established chronological record for sediment core TLC01 (**right**). Different colors represent the percentage (%) of clay (gray)/silt (blue)/sand (white) versus depth.  
Meteorological Administration. Sediment size analysis of the entire TLC01 core using an algorithm of end-member modelling analysis (EMMA) [38; 39], which was used to extract geo-meaningful end members. The HYSPLIT model [40] was used to determine the possible source of water vapor. Global Data Assimilation System (GDAS) meteorological data (GDAS1) were downloaded from the National Centers for Environmental Prediction [41] for use in the HYSPLIT model, and the daily backward parcel trajectory for 48 h from April 2019 to September 2019 was calculated using MeteoInfo (TrajStat package) [42].