SCIENCE CHINA Earth Sciences • RESEARCH PAPER • doi: 10.1007/s11430-014-4993-2 Underestimated 14C-based chronology of late Pleistocene high lake-level events over the Tibetan Plateau and adjacent areas: Evidence from the Qaidam Basin and Tengger Desert LONG Hao1,2* & SHEN Ji1† 1 State Key Laboratory of Lake Sciences and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China; 2 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China Received April 23, 2014; accepted September 25, 2014 The palaeolake evolution across the Tibetan Plateau and adjacent areas has been extensively studied, but the timing of late Pleistocene lake highstands remains controversial. Robust dating of lacustrine deposits is of importance in resolving this issue. This paper presents 14C or optically stimulated luminescence (OSL) age estimates from two sets of late Quaternary lacustrine sequences in the Qaidam Basin and Tengger Desert (northeastern Tibetan Plateau). The updated dating results show: (1) the radiocarbon dating technique apparently underestimated the age of the strata of >30 ka BP in Qaidam Basin; (2) although OSL and 14C dating agreed with each other for Holocene age samples in the Tengger Desert area, there was a significant offset in dating results of sediments older than ~30 ka BP, largely resulting from radiocarbon dating underestimation; (3) both cases imply that most of the published radiocarbon ages (e.g., older than ~30 ka BP) should be treated with caution and perhaps its geological implication should be revaluated; and (4) the high lake events on the Tibetan Plateau and adjacent areas, tradition- ally assigned to MIS 3a based on 14C dating, are likely older than ~80 ka based on OSL chronology. Tibetan Plateau, lake highstand, lacustrine sediments, 14C dating, OSL dating Citation: Long H, Shen J. 2014. Underestimated 14C-based chronology of late Pleistocene high lake-level events over the Tibetan Plateau and adjacent areas: Evidence from the Qaidam Basin and Tengger Desert. Science China: Earth Sciences, doi: 10.1007/s11430-014-4993-2 Since the 1980s, the late Quaternary evolution of closed mains, nearly all studies suggested that the high lake level lake basins from the Tibetan Plateau (TP) and adjacent are- stands occurred at 40–25 ka, corresponding to the late ma- as has been extensively studied to reconstruct past environ- rine isotope stage 3 (i.e., MIS 3a; Martinson et al., 1987). mental and climatic conditions (e.g., An et al., 2000; These study sites (circled ones in Figure 1(a)) are distributed Lehmkuhl and Haselein, 2000; Shi et al., 2001; Yang et al., over the TP, as well as the foreland areas in the deserts (e.g., 2004; Herzschuh, 2006; Chen et al., 2008; Mischke et al., the Tengger Desert, Badain Jaran Desert, and Taklamakan 2008; Daut et al., 2010; Long et al., 2010; Mügler et al., Desert) (Lehmkuhl and Haselein, 2000; Shi et al., 2001; 2010; Yang and Scuderi, 2010; Wischnewski et al., 2011; Yang et al., 2004; Yang et al., 2011). For instance, in the Yang et al., 2011; Shen, 2013). Qaidam Basin from the northeastern TP, Chen and Bowler Based on 14C dating of lake shorelines and lacustrine re- (1986) found a palaeolake shell bar (Figure 1(b)), approxi- mately 29 m above the modern level of the Qarhan Salt Lake; this shell bar consists of abundant mollusk fossils and *Corresponding author (email: [email protected]) †Corresponding author (email: [email protected]) mussels, reflecting fresh to slightly saline water conditions. © Science China Press and Springer-Verlag Berlin Heidelberg 2014 earth.scichina.com link.springer.com 2 Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.? Figure 1 Location of the study region. (a) Map showing the locations of lake highstand sites on the TP and adjacent areas. At the sites denoted by filled circles, the lake highstands dated back to the MIS 3a based on 14C dating (see Figure 2 for the radiocarbon dates of lake highstand timings). At the sites de- noted by filled squares, the lake highstands dated back to MIS 5 based on OSL or U/Th ages. The dashed rectangles denote the Qaidam Basin and Tengger Desert, respectively. (b) Map showing the Qaidam Basin. The Qarhan Salt Lake is shown by the dashed line. The filled circle denotes the location of the shell bar studied by Chen and Bowler (1986), Chen et al. (1990), and Zhang et al. (2008). (c) Map showing the Tengger Desert. The Zhuyeze Lake is denot- ed by the rectangle. The filled circles denote the locations of the three lacustrine profiles sections BJ-S1, BJ-S2, and QTL (Long et al., 2011). Long H, et al. Sci China Earth Sci January (2014) Vol.57 No.? 3 Three shell samples from the upper, middle and lower parts together (Figure 2), and showing obvious differences in of a profile from this bar dated back to 28650±670, 35100± ages between the short (i.e., 14C dating) and the long (e.g., 900, and 38600±680 a BP by the conventional 14C method, luminescence dating) chronologies. Resolution of this issue which suggests that this lake had a high water level at ca. is important because a large number of global climate models 39–28 ka BP (Chen et al., 1990). Zhang et al. (2008) further use lake sequences to assess the strength of Asian monsoons dated the same shell bar using the accelerator mass spec- and hemispheric westerlies. It appears that such a resolution trometry (AMS) method, and obtained similar age ranges. will involve a reconciliation of the dating problem; as a re- The 14C-dated high lake levels during the late MIS 3 seemed sult, direct comparison of radiocarbon and luminescence to occur not only in the Qaidam Basin but also in the western age estimates for the same sediments is necessary. Here we and central part of the TP (Figure 1(a)), e.g., Tianshuihai present age estimates on the basis of 14C or OSL method for Lake (Li et al., 1991), Longmuco Lake (Li, 2000), Ban- two sets of late Quaternary lacustrine sequences from the gongco Lake (Zheng et al., 1989; Li et al., 1991), Zabuye Qaidam Basin and the Tengger Desert, respectively, and try Lake (Zheng et al., 1996), and Selinco Lake (Li, 2000). to revisit the geochronology of highstands which were as- Similarly, there is good evidence of the MIS 3a high- signed to be developed during MIS 3. stands from the adjacent areas of the TP (Figure 1(a)). Tak- ing the Tengger Desert (Figure 1(c)) for example, while there are still many lakes in the inter-dune basins in this 1 Study area and materials region, remains of lacustrine sediments and palaeoshore- lines indicate the more extensive occurrence of lakes and The Qaidam Basin (36.6°–37.2°N, 93.7°–96.3°E), situated swamps in the past. Pachur et al. (1995) and Zhang et al. in the northeastern TP (Figure 1(a)), is bounded by the (2004) investigated in detail the palaeobeaches around the Kunlun Mountains to the south and the Aerjin Mountains Zhuyeze Lake in the Tengger Desert using radiocarbon da- and Qilian Mountain to the north (Figure 1(b)). This basin is ting of bulk organic matter or mollusk shells. Their results a large playa with an area of 5850 km2 and a mean elevation showed that the highest water level formed at ~35–30 ka BP. of 2800 m a.s.l., and contains a series of concentrated salt The 14C chronologies of lacustrine beaches also suggested lakes with a total area of 460 km2, and with the Qarhan Salt high lake levels during the MIS 3a in the Juyan Lake (Fig- Lake in the depocenter of the basin (Figure 1(b)). The av- ure 1(a)) on the northern margin of the Badain Jaran desert erage annual precipitation in this region is 25–50 mm, the (Wünnemann et al., 1998). Radiocarbon dates for lacustrine annual mean temperature is 2–4°C and the annual evapora- remains from the Manas Lake (Rhodes et al., 1996), Barkol tion exceeds 3000 mm. By using a rotational drilling system Lake (Yu et al., 2001) and Aiding Lake (Li et al., 1989) with 3-m-long metal tubes with 90-mm diameters, a 100-m- showed the MIS 3a highstand as well (Figure 1(a)). long sediment core (ISL1A Core, 37°03′50″N, 94°43′41″E) However, a set of recent studies on lake shorelines from was obtained from the central part of Qarhan Salt Lake the northeastern margin of the TP found that the highstands (Figure 1(b)). The stratigraphy of ISL1A Core shows evap- apparently dated back to MIS 3a by 14C dating actually date orate halite layers (mainly crystal salt) with some lacustrine back to the period beyond ~70 ka by optically stimulated clastic layers (i.e., silt-clay or clayey silt sediment) from luminescence (OSL) dating method (Madsen et al., 2008, ~52 m in depth to top, and that lacustrine clastic clay to silt 2014; Liu et al., 2010; Rhode et al., 2010; Long et al., was deposited from the base to ~52-m (Figure 3). Consid- 2012). The timing of late Pleistocene lake highstands from ering the dating limitation of the 14C technique, we collected the TP and its adjacent areas remains undetermined. For radiocarbon samples from the upper part (0–55 m) of this instance, OSL chronology of early shorelines around the core only. Because the sediments from the core ISL1A con- Qinghai Lake (Figure 1(a)) showed that the maximum high- tain little organic carbon and are devoid of plant macrofos- stands ~20–66 m above present-day lake levels occurred sils we sampled bulk organic matter (11 samples) for 14C approximately during 100–90 ka (Madsen et al., 2008), not dating (Figure 3 for the sampling locations).