Biomarkers challenge early Miocene loess and inferred Asian desertification

Abstract

Fine‐grained Miocene sediments from Tianshui Basin, northeastern Tibetan Plateau, have received intense attention recently because these sediments were identified as loess. The presence of early Miocene loess pushes the timing of initiation of inland Asian desertification from 8 Ma back to 22 Ma. However, mudflat/distal fan and shallow lake sediments of Miocene have also been reported in Tianshui Basin. Consequently, the origin of these fine‐grained Miocene sediments in this area remains controversial. Here we investigate then‐alkane biomarker characteristics of Neogene sediments from a north‐south transect of exposures within Tianshui Basin and compare these molecular distributions with those published Quaternary loess to help resolve the disputed origin. We found thatn‐C23 and n‐C25 alkanes, sourced from either aquatic macrophytes or palustrine plants, are ubiquitous in the Miocene sediments from Tianshui Basin but are largely absent in Quaternary loess. This striking difference between n‐alkane distributions in the Tianshui samples and the Quaternary loess casts doubt on an eolian origin for the Tianshui samples and challenges the hypothesis of an early Miocene onset of Asian interior desertification.