HaiCheng Wei

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 14C and 230Th dating results, and (2) no AMS 14C ages of organic carbon from drilling cores in Qarhan Salt Lake were reported until now. In order to discuss these chronological problems, upper 54.50xa0m lacustrine sediments from a drilling core (ISL1A) recovered from Qarhan Salt Lake were dated based on 230Th and AMS 14C dating techniques. Results show that (1) AMS 14C ages of total organic carbon (TOC) from 4.65 to 30.29xa0m are almost in stratigraphic order and consistent with 230Th ages of halite in the corresponding layers; (2) AMS 14C ages of TOC from 30.29 to 54.50xa0m are younger with increasing depth. This phenomenon was also found in Shell Bar in the study area, suggesting that AMS 14C ages from upper 30.29xa0m are more reliable while those from lower 24.21xa0m in ISL1A may be underestimated; (3) 230Th ages of halite from lower 24xa0m lacustrine sediments are obviously older than AMS 14C 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 14C ages in ISL1A, similar age framework with 230Th ages in this core confirms that 230Th ages are much close to the true ages of these sediments, which suggests that the forming timing of the bottom salt layer is ~50xa0ka.

Holocene lake-level changes of Hurleg Lake on northeastern Qinghai-Tibetan Plateau and possible forcing mechanism

The palaeoshorelines of a lake are the direct geomorphic evidences to reflect past water levels and record information on palaeoclimatic changes. Previous research indicates that the timing and forcing mechanisms of climatic change on the northeastern Qinghai-Tibetan Plateau (NE QTP) are still controversial. Here, we report quartz optically stimulated luminescence (OSL) ages for beach deposits and accelerator mass spectrometry (AMS) 14C age for plant remains, shoreline features and geomorphic exposures that contribute to a reconstruction of the lake-level history of Hurleg Lake on the NE QTP. The results imply that (1) three high lake levels are dated to 6.8–6.4, 5.0–4.7 and 2.2–1.4 kyr, corresponding to relatively wet and stable climate from pollen records of sediment core in this lake during mid–late Holocene; (2) the highest lake-level period occurred at 5.0–4.7 kyr, which is almost synchronous with the highest effective moisture phase from synthesized lake records influenced by the Westerlies and (3) no beach deposits around Hurleg Lake and lower lacustrine deposits in Toson Lake were found during early Holocene, suggesting lower lake level of Hurleg Lake in this period. These results might imply that the Westerlies and local topography rather than Asian summer monsoon dominate the moisture availability in this region during the Holocene.