An assessment and comparison of 230Th and AMS 14C ages for lacustrine sediments from Qarhan Salt Lake area in arid western China

The age framework of Qarhan Salt Lake in arid western China is still controversial due in part to (1) age discrepancy between conventional C-14 and Th-230 dating results, and (2) no AMS C-14 ages of organic carbon from drilling cores in Qarhan Salt Lake were reported until now. In order to discuss these chronological problems, upper 54.50 m lacustrine sediments from a drilling core (ISL1A) recovered from Qarhan Salt Lake were dated based on Th-230 and AMS C-14 dating techniques. Results show that (1) AMS C-14 ages of total organic carbon (TOC) from 4.65 to 30.29 m are almost in stratigraphic order and consistent with Th-230 ages of halite in the corresponding layers; (2) AMS C-14 ages of TOC from 30.29 to 54.50 m are younger with increasing depth. This phenomenon was also found in Shell Bar in the study area, suggesting that AMS C-14 ages from upper 30.29 m are more reliable while those from lower 24.21 m in ISL1A may be underestimated; (3) Th-230 ages of halite from lower 24 m lacustrine sediments are obviously older than AMS C-14 ages of TOC in the corresponding layers, which results into different age framework of salt lake sediments in Qarhan Salt Lake; (4) if extrapolating these reliable AMS C-14 ages in ISL1A, similar age framework with Th-230 ages in this core confirms that Th-230 ages are much close to the true ages of these sediments, which suggests that the forming timing of the bottom salt layer is similar to 50 ka.