Paleoenvironmental implications of new OSL dates on the formation of the "Shell Bar" in the Qaidam Basin, northeastern Qinghai-Tibetan Plateau

A geological feature in the Qaidam Basin known as the "Shell Bar" contains millions of freshwater clam shells buried in situ. Since the 1980s, this feature in the now hyper-arid basin has been interpreted to be lake deposits that provide evidence for a warmer and more humid climate than present during late marine isotope stage 3 (MIS 3). Global climate during late MIS 3 and the last glacial maximum, however, was cold and dry, with much lower sea levels. We re-investigated the feature geomorphologically and sedimentologically, and employed optically stimulated luminescence (OSL) dating to verify the chronology of the sediments. We interpret the Shell Bar to be a remnant of a river channel formed by a stream that ran across an exposed lake bed during a regressive lake phase. Deflation of the surrounding older, fine-grained lacustrine deposits has left the fluvial channel sediments topographically inverted, indicating the erosive nature of the landscape. Luminescence ages place the formation of the Shell Bar in MIS 5 (~113-99 ka), much older than previous radiocarbon ages of <40 ka BP, but place the paleoclimatic inferences more in accord with other regional and global climate proxy records. We present a brief review of the age differences derived from ¹⁴C and OSL dating of some critical sections that were thought to represent a warmer and more humid climate than present during late MIS 3. We attribute the differences to underestimation of ¹⁴C ages. We suggest that ¹⁴C ages older than ~25 ka BP may require re-investigation, especially dates on samples from arid regions.