Abstract: Background, aim, and scope Qinghai Lake (QHL), the largest saline lake in China, is located in the northeastern Qinghai-Tibetan Plateau (QTP), predominantly controlled by the interplay of the Asian monsoon and the westerlies. The widespread loess depositions of the QHL represent important environmental archives. It has been a focus of regional paleoenvironment and paleoclimate reconstruction on sub-orbital and millennial-scales. However, the loess accumulation usually accompanied by depositional hiatuses and erosion. Therefore, it is the prerequisite to determine the continuity of loess deposition before reconstruct the regional paleoenviromental evolution. In this study, we collected 12 samples to obtain the high-density Optically Stimulated Luminescence (OSL) chronology from a 260 cm thick loess section northwest to the previously published Heima He (HMH) loess section by Lu et al (2011). Meanwhile, the environmental implication of the common proxies in loess research, such as grain size, magnetic susceptibility and chroma, is discussed. Materials and methods The medium-sized (38—63 μm) quartz OSL single aliquot regenerative (SAR) dose protocol is used to acquire equivalent dose (De). Conventional tests set 260℃ as preheat temperature and 220℃ as cut-heat temperature. Environmental dose rates were determined using the inductively coupled plasma mass spectrometry (ICP-MS). The grain size, low frequency magnetic susceptibility and chroma (lightness, redness and yellowness) are measured at 5 cm interval. ResultsQuartz OSL ages reveal that the rapid loess sedimentation occurred during the late Glacial (13—12 ka) and in the late Holocene (~2 ka). An obvious accumulational hiatus, ~7 ka, from 11 ka to 4 ka was verified at HMHW loess section. The upper of 130 cm has an inverted chronosequence, 6 successive samples have near-identical ages and are clustered together indicating a rapid loess deposition. Mechanical components of grain size at HMHW section mainly consisted of silt with 50.94%—72.92%, 9.03%—36.91% of sand and 12.15%—18.05% of clay. The values of mean-grain size and lightness are coincided with the stratum of HMHW profile. Minimum is at the layer of palaeosol, on the contrary at loess layer. Maximum of magnetic susceptibility is 74.30×10-8 m3·kg-1 indicating that a pedogenesis process mainly at (1.52±0.16) ka. Discussion The episodic and discontinuous feature of loess deposition on a millennial time scale in QHL area has been confirmed. Compared with HMH loess section (Lu et al, 2011) at southeastern of HMHW, the loess depositions accompanied with palaeosol aggradation. The loess accumulation rate is varied with local terrain or erosive process. It is noteworthy that episodic loess accumulation and depositional hiatus have also been reported from the other profiles, at the adjacent Gonghe Basin and the western part of Chinese Loess Plateau (CLP). Conclusions (1) Consistent and rapid loess deposition period was recorded in both high-density OSL samples HMHW section and previous published OSL ages, which during the late Glacial and in the late Holocene. (2) It is reliable to reveal that the characteristics of loess deposition discontinuity are universal on the Tibetan Plateau. (3) The HMHW loess is sorted as sandy silt, indicating that it is deemed to be a product of dry and cold climate. Recommendations and perspectives It is necessary to acquire the high-density samples for the OSL dating to verify loess sedimentary continuity, when reconstructing the paleoenvironmental evolution process by loess deposition.
Keywords: high-density quartz OSL ages; rapid dust accumulation; depositional hiatus; Heima He loess section (HMHW)