- Email: [email protected]
Optical dating of dune sands in the northeastern deserts of China
Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429 www.elsevier.com/locate/palaeo
Optical dating of dune sands in the northeastern deserts of China Sheng-Hua Li a; , Ji-Min Sun b , Hui Zhao a a
Department of Earth Sciences, The University of Hong Kong, James Lee Science Building, Pokfulam Road, Hong Kong, PR China b Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China Received 1 February 2001; accepted 23 October 2001
Abstract Optical dating has been used to obtain the ages of fossil-stabilized sand dunes from four sections in the northeastern deserts of China. Our results indicate that the optically stimulated luminescence ages of the four sections correlate well, even though the samples were collected from different deserts about 600 km apart. Our results also indicate that active dune formation in this region lasted from the Last Glacial Maximum to about 10 ka, and that the warm climate of the Holocene was interrupted by a cold/dry dune formation episode about 3.5^1.7 ka. The Holocene Optimum in this region is between 10 and 3.6 ka, and a later warm/humid dune stabilization phase lasted from at least 1.6 to 1.0 ka. The youngest age on the uppermost sand unit yielded an age of only 40 yr, supporting the previous argument that the existence of modern active eolian sands in the regions with a mean annual precipitation of up to 450 mm is not mainly due to drought, but to extensive land cultivation over historic time. From the luminescence properties of the quartz grains, it is hypothesized that the sands in most of the sections are probably derived from more than one source, with a minor source of quartz having a different thermal history before deposition. 4 2002 Elsevier Science B.V. All rights reserved. Keywords: optical dating; quartz; sand dune; desert; Holocene climate change
1. Introduction Eolian and eolian-related deposits are important for paleoclimatic and paleoenvironmental reconstructions in arid and semi-arid regions (McKee, 1979; Ahlbrandt et al., 1983; Lancaster, 1988, 1999; Goudie, 1999; O’Connor and Thomas, 1999; Tchakerian, 1999; Thomas et al., 2000). In China active sand dunes mainly occur
* Corresponding author. Fax: +852-2517-6912. E-mail address: [email protected] (S.-H. Li).
in the northwest inland basins (e.g. the Taklimakan Desert), whereas deserts dominated by semistabilized sand dunes are mainly distributed in the northeastern semi-arid and sub-humid regions. Recent studies indicate that the southern limits of deserts in northeastern China have shifted in space and time, in response to the waxing and waning of the east Asian monsoonal circulations (Sun et al., 1998a,b, 1999). Thus northeastern China is one of the key sites on which to study the history of past environmental changes. However, previous studies focused mainly on orbital scale environmental changes, whereas changes in
0031-0182 / 02 / $ ^ see front matter 4 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 4 4 3 - 6
PALAEO 2798 29-5-02
420
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
the environment during the Holocene have not been well studied. Field investigations indicate that fossil sand dunes in the northeastern deserts of China are widely distributed, and these dunes are characterized by alternating paleosols and sand units. The paleosols are poorly developed, sandy loam soils. Low organic carbon content and possible carbon contamination from the roots of modern vegetation make it di⁄cult for accurate radiocarbon dating. In spite of this, there have been several papers concerned with paleoenvironmental reconstruction in these deserts in the Holocene, using radiocarbon dating (Wang, 1992; Li and Dong, 1998); the dates are di¡erent from site to site and no other numerical dating method has been applied. Optical dating, a technique closely related to thermoluminescence (TL) dating, has undergone extensive development recently (Wintle, 1993;
Duller, 1996; Aitken, 1998), and has been proved to be a successful method especially for dating dunes and sand sheets (Stokes and Gaylord, 1993; Stokes et al., 1997). This research uses optically stimulated luminescence (OSL) on sand-sized quartz grains to date fossil sand dunes in northeastern China in order to reconstruct the history of eolian activity in the region.
2. Geological setting and sample site Four fossil dune sections from the Hunshandake and Hulun Buir Deserts have been studied (Fig. 1). The two deserts are located in the semiarid climatic zone, controlled by the east Asian monsoon circulation. In winter the prevailing wind is northwesterly, and frequent dust storms usually occur in spring. In summer the dominant
Fig. 1. Map showing the deserts and the site locations.
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
421
Fig. 2. Stratigraphy, sampling depths and OSL ages for the four sections.
wind is southeasterly, bringing humid air masses from the Paci¢c Ocean to the deserts. The present mean annual precipitation in these deserts ranges from V450 mm in the east to V250 mm in the west, with more than 70% of the rainfall occurring in July, August and September (Wang, 1992). Under present climatic conditions, there should be no active sand dunes in northeastern China. However, human impact, especially land cultivation since the time of the Han Dynasty about 2300 yr ago, has greatly destroyed the vegetation cover (Sun, 2000). Poor land-use practices, combined with the high wind energy in the cold and dry spring season, have led to increased sand mobility and the reworking of stabilized sand dunes of Holocene age. Modern landforms in these deserts are dominated by semi-stabilized sand dunes, with active dunes mainly distributed at sites where large-scale land cultivation was once concentrated. Of the four sections, WG and HLD are 40 km apart and both are from the Hulun Buir Desert, whereas SGDL and SY are 100 km apart and derive from the Hunshandake Desert (Fig. 1). All the sections are relicts of fossil-stabilized dunes, distributed in the interiors of the deserts. The sequences at the four sections can be correlated, since they consist of an uppermost modern
sand unit, two sandy loam soils (all with A/C soil pro¢les) with an intercalated sand layer, as well as a sand unit underlying the lower soil (Fig. 2). Field observations indicate that the lower soil is slightly more developed than the upper one. In general, the sand unit underlying the lower soil unit is over 100 m thick. It is the potential source of the modern sand. However, it is di⁄cult to ¢nd exposures that show this thick bed. Thus, only the upper part of the lowest sand unit was studied (Fig. 2). Three OSL samples from each section were collected and the sample depths of all 12 samples are given in Table 1.
3. Optical dating of sediments Optical dating determines the last occasion when the mineral grains were exposed to sunlight, thereby resetting the luminescence clock. The measured signal is the OSL (Aitken, 1998). After burial, the signal grows with time as the result of exposure to ionizing irradiation, emitted by naturally occurring radioisotopes U, Th, and K in the sample itself and surroundings, and by cosmic rays. Minerals, e.g. quartz and feldspar, act as dosimeters that allow the irradiation dose since sunlight resetting is determined. The total accu-
PALAEO 2798 29-5-02
422
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
Table 1 Dose rate and OSL ages for 12 samples from northeastern China Site
Sample
He Er Hong De
HLD1 (soil) HLD2 (sand) HLD3 (soil) Wan Gong WG1 (sand) WG2 (sand) WG3 (sand) Sang Gen Da Lai SGDL1 (soil) SGDL2 (sand) SGDL3 (soil) San Yi SY1 (soil) SY2 (sand) SY3 (sand) a b c
Depth Alpha K content counting ratea (m) (%)
Water contentb (%)
Cosmic rayc (Gy/ka)
De (Gy)
(Gy/ka)
(ka)
0.35 1.25 2.25 0.35 1.30 1.95 0.75 1.30 2.75 0.85 1.75 3.05
1.3 1.9 6.5 5.7 3.2 6.6 2.8 6.6 6.4 4.2 4.8 4.3
0.29 0.25 0.21 0.29 0.26 0.23 0.28 0.25 0.21 0.27 0.25 0.21
4.53 R 0.58 7.2 R 0.8 36.9 R 3.2 0.14 R 0.04 5.93 R 0.52 48.4 R 8.2 2.86 R 0.24 10.7 R 1.0 30.4 R 4.0 4.02 R 0.33 5.18 R 0.50 33.8 R 3.6
2.90 R 0.10 2.91 R 0.12 2.97 R 0.12 3.50 R 0.11 3.20 R 0.13 3.61 R 0.13 2.74 R 0.10 3.06 R 0.14 3.08 R 0.13 3.22 R 0.14 3.11 R 0.14 3.06 R 0.14
1.56 R 0.14 2.48 R 0.29 12.4 R 1.2 0.04 R 0.01 1.85 R 0.18 13.4 R 0.5 1.05 R 0.1 3.5 R 0.36 9.86 R 1.36 1.25 R 0.14 1.67 R 0.18 11.1 R 1.3
2.39 R 0.12 2.79 R 0.13 4.14 R 0.16 5.48 R 0.18 4.03 R 0.16 7.56 R 0.21 1.97 R 0.11 3.00 R 0.13 3.48 R 0.14 3.11 R 0.14 2.00 R 0.11 2.16 R 0.12
2.31 R 0.12 2.31 R 0.12 2.39 R 0.13 2.65 R 0.13 2.48 R 0.12 2.56 R 0.13 2.25 R 0.12 2.62 R 0.13 2.44 R 0.12 2.66 R 0.13 2.76 R 0.13 2.66 R 0.13
Dose rate
OSL age
The alpha counting rate is for a 42-mm-diameter ZnS screen and is given in units of counts per kilosecond. The error for the water content is estimated at R 20%. The error for the cosmic rays dose rate is estimated at R 0.02 Gy/ka.
mulated radiation dose, known as the equivalent dose (De ), can be determined in the laboratory with regeneration technique using the natural OSL signal compared with that from a known dose. The environmental dose rate is estimated from the radioactivity and chemical analysis of the sample and its surrounding materials. The OSL age is given by the ratio between De and the dose rate (Wintle, 1993). A major advantage of optical over TL dating methods is the rapid and complete resetting of the OSL signal. Instead of about 10 h of direct sunlight exposure being required to reduce the TL signals to a ¢xed residual level (which is about 30% or more of initial TL for quartz), only about 10 min is required to reduce the OSL to 1% of its initial level (Wintle, 1993). For direct sunlight exposures of longer than 30 min, the remaining OSL signals are negligible and thus the residual OSL level for eolian sand is not a concern. Another advantage is that the De determination can be achieved using the OSL signal from a single sample aliquot (Duller, 1991). This has greatly increased the precision in De determination and also allows the detection of incompletely bleached sediments (Li, 1994; Olley et al., 1998). Additional techniques, using single aliquots, can be applied to sand in order to check for the incorporation of reworked material (Li, 2001).
4. Experimental procedures All 12 OSL samples were obtained by hammering aluminum tubes into cleaned vertical sections, then sealing inside plastic bags with tape to ensure that the sediment retained its natural water content. In the laboratory the material at each end of the tube was scraped away and used for dose rate measurements. Quartz grains were prepared following procedures of sieving, heavy liquid separation and HF etching in subdued red safe-light conditions. Raw samples were treated ¢rst with 20% H2 O2 and 10% HCl to remove organic materials and carbonates. Grains between 150 and 180 Wm were selected by dry sieving. Then grains of densities between 2.62 and 2.70 g/cm3 were separated using sodium polytungstate heavy liquid. The separated grains were treated with 40% hydro£uoric acid for 1 h and 10 min to remove feldspar grains. Separated quartz grains were mounted on 10-mm-diameter aluminum discs with Silkospay silicon oil. The purity of quartz grains was tested by monitoring for the presence of feldspar by measuring the infrared-stimulated luminescence (IRSL) and the 110‡C TL peak (see below). Discs for which IRSL signals were observed were discarded before further luminescence measurements. Luminescence measurements were made using
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
an automated RisS TL/OSL reader (Markey et al., 1997). The stimulating light source is a tungsten^halogen lamp ¢ltered from 420 to 550 nm using a Schott GG-420 ¢lter in combination with interference ¢lters. The light delivered to the sample disc is about 40 mW/cm2 . The OSL signal is detected through two 3-mm-thick Hoya U-340 ¢lters. Irradiation was carried out using a 90 Sr/90 Y beta source built into the reader. It currently delivers 0.0475 Gy/s to quartz on aluminum discs. The equivalent doses (De ) were determined using the single aliquot regeneration (SAR) procedure (Murray and Wintle, 2000). Each quartz disc underwent preheating, OSL measurement and bleaching, sensitivity measurement, and irradiation cycles with a di¡erent irradiation dose in each cycle. Three irradiation doses and one repeat dose were applied to each sample in such a way that the natural OSL intensity was observed between the OSL intensities regenerated from the given doses. The preheating was at 260‡C for 10 s. OSL measurement and bleaching were achieved by measuring the OSL signal at 125‡C for 100 s. This e¡ectively bleached the OSL signal to a negligible level for all samples. The OSL signals integrated from the ¢rst 5 s while subtracting half of the averaged OSL in the last 10 s were taken as the OSL intensity used for De value determination. Sensitivity monitoring was achieved by measuring the OSL signal created by a test dose of 0.95 Gy. The test-dose OSL signal was measured at 125‡C for 100 s following heating the disc to 160‡C and was calculated in the same way as for the OSL measurement. The OSL intensities were corrected for sensitivity change using the test-dose OSL. The De values were calculated using the corrected OSL intensities (Fig. 3). Twelve or more discs were measured for each sample. When heating the sample to 160‡C before measuring the test-dose OSL, the TL was recorded as a surrogate for OSL sensitivity change from quartz if the test-dose OSL was unsuitable (Murray and Roberts, 1998; Li et al., 1999). The TL measurement was made on the same reader by heating the sample to 160‡C with a heating rate of 3‡C/s immediately after the test-dose irradia-
423
Fig. 3. Illustration of the SAR technique for De determination of a quartz aliquot from sample WG2. The OSL was corrected for sensitivity changes. (O) Repeated regeneration dose point. (a) Corrected natural OSL signal.
tion. The TL response was the integrated TL signal from 80 to 120‡C after subtracting the background. The test-dose TL can also act as an indicator of impurity in the quartz grains, since feldspars often have TL peaks around 150‡C, i.e. di¡erent from quartz grains. The environmental dose rate to which the quartz grains were exposed was measured using a variety of techniques, and the results are summarized in Table 1. Thick source alpha counting (TSAC) was used to measure contributions from the U and Th decay chains. A Littlemore 7286 TSAC system with 42-mm-diameter ZnS screens was used. The K content was measured by £ame photometry. Water content was calculated from the sample weights before and after drying at 105‡C in an oven. The cosmic ray contribution to the dose rate was calculated from the burial depth and the altitude of the samples (Prescott and Hutton, 1994).
5. Results Equivalent dose values and dose rate results are summarized in Table 1. For each sample the De values of all discs are similar (Fig. 4). This indicates that the quartz grains were uniformly
PALAEO 2798 29-5-02
424
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
Fig. 4. Data for 12 quartz aliquots from SGDL1 and 13 aliquots from HLD2. Top: histogram of De distribution. Bottom: De plotted against the ratio (In /TL) of natural OSL and 110‡C TL from test dose.
bleached prior to deposition (Olley et al., 1998; Li, 2001) and reworking was insigni¢cant. Therefore, the De value for each sample is taken as the average of the measured discs. Su⁄cient bleaching at deposition has also been supported by the results for a currently active dune sample WG1. It gives an age of 40 R 10 yr, with a De value of 0.14 R 0.04 Gy averaged from 12 discs. The error in the OSL ages was estimated from the errors of dose rate measurement and the De determination in the way described in Aitken (1985). The errors for the De value are the 1c standard deviation among the discs. Since the precision of the De measurement using the single aliquot technique is very high, the systematic error contributed from individual aliquot De measure-
ment is relatively insigni¢cant. The error for the K content was R 5% of the measured value. The error in the water content was estimated as R 20%. The error for the cosmic ray dose rate was R 20 WGy per yr.
6. Discussion 6.1. Precision and accuracy of the optical ages There is good agreement between the De values obtained among the discs for each sample suggesting that the quartz grains were completely bleached prior to deposition and that reworking is insigni¢cant for these samples (Olley et al.,
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
1998). This is also supported by the results of plotting the De values versus the ratio of natural OSL intensity (In ) and the test-dose TL (Li, 2001). Unlike poorly bleached sediments, there is no correlation between the De value and the ratio of natural OSL to test-dose TL (Fig. 4). It is thus rea⁄rmed that optical dating is suitable for dating of eolian sand dunes. The dose rates for samples in each section, and even in di¡erent sections, are very close to each other due to similarity in mineral composition. No obvious di¡erence was observed between the soil and sand samples. In the age calculations, the moisture content used for each sample was that measured at the present time. Though it may have varied through time, changes of water content will result in a small error for the ¢nal age. The di¡erence is less than 10% of the age, if water content changes from 1 to 10%. Cosmic rays make a small contribution to the dose rate. 6.2. History of sand mobilization since the last deglaciation Fig. 2 indicates that the OSL dates in the HLD and WG sections of the Hulun Buir Desert can be correlated. Additionally, the dates of these two sections can be correlated with those of the SGDL and SY sections in the Hunshandake Desert, especially when we consider the fact that the two deserts are about 600 km apart. It is suggested that the climate was similar in the two deserts, as evidenced by the formation of sands and loam soils. The dates for the lowest eolian sand unit in the WG and SY sections are 13.4 and 11.4 ka, respectively. This suggests that, although the Last Glacial Maximum peaked at 21 ka (Bard et al., 1990), the glacial cold and dry conditions lasted at least until the last deglaciation, as evidenced by eolian sand mobilization. Dates at the bottom of the lower soils are 9.9 R 1.4 and 12.4 R 1.2 ka. When we consider the errors, these two ages can be correlated. Firstly, this suggests that the soil was developed during the Holocene, and this is in good agreement with the ¢nding of Neolithic stone artifacts in the lower soils. Additionally, it suggests that the shift from dune formation to sand stabiliza-
425
tion occurred about 10 ka, being in phase with the Last Glaciation/Holocene transition in the Chinese Loess Plateau and most other records in the world (Liu, 1985; Imbrie et al., 1984). The stratigraphic units of two Holocene soils and one intercalated sand layer found in all the sections suggest that the warm and humid summer monsoon, which dominated the climate during the Holocene, was interrupted by a cold and dry phase about 3.5V1.7 ka, evidenced by a decrease in the pollen £ux and the number of wooden and shrub assemblages, which corresponds with the intercalated sand unit (Qiu et al., 1992). OSL dating applied to the top of this soil yielded an age of 3.5 R 0.36 ka, suggesting that the Holocene Optimum in these regions occurred between 10 and 3.6 ka. The dates for the sand layer between the two Holocene soils give ages of 3.5V1.7 ka. Combined with the top age of the lower soil, the cold and dry episode, evidenced by sand mobilization, occurred between 3.5 and 1.7 ka. Additionally, although the age for the top of the upper soil yielded an age of V1.2 ka, as discussed above, all these studied sections are relicts of fossil-stabilized dunes, which means that some of the uppermost soil has probably been removed. Thus the latest warm and humid episode, evidenced by the soil development, lasted at least from 1.7 to 1.2 ka. Although there are many radiocarbon dates reported from the sandy loam soils in Chinese deserts, it is likely that radiocarbon in loam soil using conventional dating methods is not a closed system (Head et al., 1989). The reported 14 C dates have considerable scatter and are unreliable. This makes it di⁄cult to compare our results with previous studies in the region due to the lacking of other reliable dates. The results from this study are tentatively compared with the paleoclimatic studies in the Sahara region (Fig. 6). Fig. 6B is compiled from 560 radiocarbon dates on surface freshwater indicators (lake and swamp sediments, freshwater shells, diatoms). The frequency of the radiocarbon dates was explained as a primary indicator of past humid climates (Guo et al., 2000). The dates of soil development in northeastern China are consistent with the Sahara records that a humid phase occurred between V9.8 and
PALAEO 2798 29-5-02
426
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
Fig. 5. 110‡C TL signals against OSL signals of samples from the WG, SY, HLD and SGDL sections. Both signals are induced by the same test doses. (O) Aliquots with high OSL sensitivity.
3.6 ka. This suggests that the humid episode is a global event (Fig. 6). The uppermost eolian sand layer yielded an age of 40 R 10 yr, indicating it to be modern. It serves as evidence for the previous argument that the sand dunes in these deserts today are not in equilibrium with the modern climate, especially when considering the fact that all these deserts are located in semi-arid regions with a mean annual precipitation of up to 450 mm. Modern sand mobilization and the reworking of relict stabilized sand dunes are mainly due to the large-scale land cultivation since the Han Dynasty, especially since the Late Qing Dynasty of V300 yr ago (Sun, 2000).
The modern eolian sands in these deserts are thought to be derived mainly from the Last Glacial sand beds, when the overlying ‘protection layer’ of the Holocene soil was destroyed due to large-scale land cultivation (Sun, 2000). As a result, there are only sparsely distributed relicts of fossil-stabilized sand dunes that enable the history of eolian mobilization to be ascertained. 6.3. The sources of quartz grains When the test-dose TL is plotted against the test-dose OSL of each natural aliquot for all 12 samples, most discs show a similar ratio between the two signals (Fig. 5). However, a small
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
427
found in areas adjacent to the deserts. Geochemical analysis of the grains may give indications to the sources of the grains (Muhs et al., 2000).
7. Conclusion
Fig. 6. Correlation between the soil development phases in the studied regions and the frequency variations of surface freshwater indicators from the Sahara (B redrawn from Guo et al., 2000).
number of discs from each section show signi¢cantly greater OSL sensitivity when compared with the 110‡C TL. Given the fact that there were a few thousand quartz grains on each disc, it can be suggested that there are a small number of grains on such discs that are much more OSL sensitive than others to irradiation dose. These OSL sensitive discs cannot be explained by the result of feldspar contamination because all of the discs have been IRSL-screened and there is no signi¢cant TL signal around 150‡C. In other studies, the 110‡C TL peak and OSL sensitivities have been shown to be strongly dependent on the source of the quartz grains (Han et al., 1997), and their thermal history (Chen et al., 2000). Although there is not a unique reason for such a high OSL sensitivity over TL, it is likely that the OSL sensitive grains might be derived from sources di¡erent from the rest of the grains, or that these grains have experienced a di¡erent thermal history. For example, they might be associated with volcanic activity in northeastern China, where Quaternary volcanic deposits have been
OSL dating can provide a reliable chronology for desert sand dunes. Active dune formation in this region of northeastern China lasted from the Last Glacial Maximum to about 10 ka. It has also been shown that the warm climate of the Holocene was interrupted by a cold/dry dune-forming episode at about 3.5^1.7 ka. The Holocene Optimum occurred between 10 and 3.6 ka, and a later warm/humid dune stabilization phase lasted from at least 1.6 to 1.2 ka. The youngest age on the uppermost sand unit yielded an age of 40 yr, supporting the previous argument that current active eolian sands can occur in regions with a mean annual precipitation of up to 450 mm ; this activity is not due to drought, but to extensive land cultivation in recent time. The sands in most of the sections are probably derived from more than one source, with a minor source of quartz having a di¡erent thermal history before deposition. This more sensitive quartz may be associated with volcanic activity in northeastern China.
Acknowledgements The authors thank N. Lancaster and an anonymous referee for providing helpful comments. The work described in this paper was substantially supported by Grants to S.-H.L. from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project no. 7105/97P, 7112/98P), and a Grant to J.-M.S. from NKBRSF (Project G1999043402).
References Ahlbrandt, T.S., Swinehart, J.B., Maroney, D.G., 1983. The dynamic Holocene dune ¢elds of the Great Plains and Rocky Mountain basins, USA. In: Brook¢eld, M.E., Ahl-
PALAEO 2798 29-5-02
428
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
brandt, T.S. (Eds.), Eolian Sediments and Processes: Developments in the Sedimentology 38. Elsevier, Rotterdam, pp. 379^406. Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Bard, E., Hamelin, B., Fairbanks, R.G., Zindler, A., 1990. Calibration of C-14 time scales over the past 30,000 yrs using mass spectrometric U^Th ages from Barbados corals. Nature 345, 405^409. Chen, G., Li, S.-H., Murray, A.S., 2000. Study of 110‡C TL peak sensitivity in optical dating of quartz. Radiat. Meas. 32, 641^645. Duller, G.A.T., 1991. Equivalent dose determination using single aliquots. Nucl. Tracks Radiat. Meas. 18, 371^378. Duller, G.A.T., 1996. Recent developments in optical dating of sediments. Prog. Phys. Geogr. 20, 133^151. Goudie, A.S., 1999. The history of desert dune studies over the last 100 years. In: Goudie, A.S., Livingstone, I., Stokes, S. (Eds.), Aeolian Environments, Sediments and Landforms. Wiley, Chichester, pp. 1^13. Guo, Z.T., Petit-Maire, N., Kro«pelin, S., 2000. Holocene nonorbital climatic events in present-day arid areas of northern Africa and China. Glob. Planet. Change 26, 97^103. Han, Z.-Y., Li, S.-H., Tso, M.Y.W., 1997. Age dependence of luminescence signals from granitic and mylonitic quartz. Quat. Sci. Rev. (Quat. Geochron.) 16, 427^430. Head, M.J., Zhou, W.J., Zhou, M.F., 1989. Evaluation of C-14 ages of organic fractions of paleosols from loesspaleosol sequences near Xian, China. Radiocarbon 31, 680^694. Imbrie, J., Hays, J.D., Martinson, D.B., McIntyre, A., Mix, A.C., Morley, J.J., Pisias, N.G., Prell, W.L., Shackleton, N.J., 1984. The orbital theory of Pleistocene climate: Support from a revised chronology of the marine delta 18O record. In: Berger, A., Imbrie, J., Hays, J., Kukla, G., Saltzman, B. (Eds.), Milankovitch and Climate (Part I). Reidel, Dordrecht, pp. 269^305. Lancaster, N., 1988. Development of linear dunes in the southwestern Kalahari, Southern Africa. J. Arid Environ. 14, 233^244. Lancaster, N., 1999. Geomorphology of desert sand seas. In: Goudie, A.S., Livingstone, I., Stokes, S. (Eds.), Aeolian Environments, Sediments and Landforms. Wiley, Chichester, pp. 49^69. Li, S.-H., 1994. Optical dating: insu⁄ciently bleached sediments. Radiat. Meas. 23, 563^567. Li, S.-H., 2001. Identi¢cation of well-bleached grains in the optical dating of quartz. Quat. Sci. Rev. (Quat. Geochron.) 20, 1365^1370. Li, S.-H., Tso, M.Y.W., Westaway, K.E., Chen, G., 1999. Choice of the most appropriate thermal treatment in optical dating of quartz. Radiat. Prot. Dosim. 84, 495^498. Li, X.Z., Dong, G.R., 1998. Preliminary studies on formative age and causes of Hunshandake Sandy Land in China (in Chinese). J. Desert Res. 18, 16^20.
Liu, T.S., 1985. Loess and the Environment. The China Ocean Press, Beijing, 251 pp. Markey, B.G., BStter-Jensen, L., Duller, G.A.T., 1997. A new £exible system for measuring thermally and optically stimulated luminescence. Radiat. Meas. 27, 83^90. McKee, E.D., 1979. A Study of Global Sand Seas. U.S. Geological Survey Professional Paper 1052. Muhs, D.R., Swinehart, J.B., Loope, D.B., Been, J., Mahan, S.A., Bush, C.A., 2000. Geochemical evidence for an eolian sand dam across the north and south Platte rivers in Nebraska. Quat. Res. 53, 214^222. Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose single-aliquot protocol. Radiat. Meas. 29, 503^515. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57^73. Olley, J.M., Caitcheon, G.G., Murray, A.S., 1998. The distribution of apparent dose as determined by optical stimulated luminescence in small aliquots of £uvial quartz: implications for dating young sediments. Quat. Sci. Rev. (Quat. Geochron.) 17, 1033^1040. O’Connor, P.W., Thomas, D.S., 1999. The timing and environmental signi¢cance of late Quaternary linear dune development in Western Zambia. Quat. Res. 52, 44^55. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term time variations. Radiat. Meas. 23, 497^ 500. Qiu, S.W., Li, Q.S., Xia, Y.M., 1992. Paleosols in the Sandylands of the Northeastern Plain of China and the inferred environmental changes during the Holocene (in Chinese). Quat. Sci. 3, 224^232. Stokes, S., Gaylord, D.R., 1993. Optical dating of Holocene dune sands in the Ferris Dune Field, Wyoming. Quat. Res. 39, 274^281. Stokes, S., Thomas, D.S.G., Washington, R., 1997. Multiple episodes of aridity in southern Africa since the last interglacial period. Nature 388, 154^158. Sun, J.M., 2000. Origin of eolian sand mobilization during the past 2300 years in the Mu Us Desert, China. Quat. Res. 53, 73^88. Sun, J.M., Ding, Z.L., Liu, T.S., 1998a. Desert distributions during the glacial maximum and climatic optimum: Example of China. Episodes 21, 28^31. Sun, J.M., Ding, Z.L., Rokosh, D., Rutter, N., 1999. 580,000 year environmental reconstruction from eolian deposits at the Mu Us Desert margin, China. Quat. Sci. Rev. 18, 1351^1364. Sun, J.M., Yin, G.M., Ding, Z.L., Liu, T.S., Chen, J., 1998b. Thermoluminescence chronology of sand pro¢les in the Mu Us Desert, China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 144, 225^233. Tchakerian, V.P., 1999. Dune Palaeoenvironments. In: Goudie, A.S., Livingstone, I., Stokes, S. (Eds.), Aeolian Environments, Sediments and Landforms. Wiley, Chichester, pp. 261^292.
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429 Thomas, D.S.G., O’Connor, P.W., Bateman, M.D., Shaw, P.A., Stokes, S., Nash, D.J., 2000. Dune activity as a record of late Quaternary aridity in the Northern Kalahari: new evidence from northern Namibia interpreted in the context of regional arid and humid chronologies. Palaeogeogr. Palaeoclimatol. Palaeoecol. 156, 243^259. Wang, P.F., 1992. Preliminary study on the environmental
429
changes of Hulun Buir Sandy Land since the Holocene (in Chinese). J. Desert Res. 12, 13^19. Wintle, A.G., 1993. Luminescence dating of aeolian sands: an overview. In: Pye, K. (Ed.), The Dynamics and Environmental Context of Aeolian Sedimentary Systems. Geological Society London Special Publication 72, pp. 49^58.
PALAEO 2798 29-5-02
Optical dating of dune sands in the northeastern deserts of China Sheng-Hua Li a; , Ji-Min Sun b , Hui Zhao a a
Department of Earth Sciences, The University of Hong Kong, James Lee Science Building, Pokfulam Road, Hong Kong, PR China b Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China Received 1 February 2001; accepted 23 October 2001
Abstract Optical dating has been used to obtain the ages of fossil-stabilized sand dunes from four sections in the northeastern deserts of China. Our results indicate that the optically stimulated luminescence ages of the four sections correlate well, even though the samples were collected from different deserts about 600 km apart. Our results also indicate that active dune formation in this region lasted from the Last Glacial Maximum to about 10 ka, and that the warm climate of the Holocene was interrupted by a cold/dry dune formation episode about 3.5^1.7 ka. The Holocene Optimum in this region is between 10 and 3.6 ka, and a later warm/humid dune stabilization phase lasted from at least 1.6 to 1.0 ka. The youngest age on the uppermost sand unit yielded an age of only 40 yr, supporting the previous argument that the existence of modern active eolian sands in the regions with a mean annual precipitation of up to 450 mm is not mainly due to drought, but to extensive land cultivation over historic time. From the luminescence properties of the quartz grains, it is hypothesized that the sands in most of the sections are probably derived from more than one source, with a minor source of quartz having a different thermal history before deposition. 4 2002 Elsevier Science B.V. All rights reserved. Keywords: optical dating; quartz; sand dune; desert; Holocene climate change
1. Introduction Eolian and eolian-related deposits are important for paleoclimatic and paleoenvironmental reconstructions in arid and semi-arid regions (McKee, 1979; Ahlbrandt et al., 1983; Lancaster, 1988, 1999; Goudie, 1999; O’Connor and Thomas, 1999; Tchakerian, 1999; Thomas et al., 2000). In China active sand dunes mainly occur
* Corresponding author. Fax: +852-2517-6912. E-mail address: [email protected] (S.-H. Li).
in the northwest inland basins (e.g. the Taklimakan Desert), whereas deserts dominated by semistabilized sand dunes are mainly distributed in the northeastern semi-arid and sub-humid regions. Recent studies indicate that the southern limits of deserts in northeastern China have shifted in space and time, in response to the waxing and waning of the east Asian monsoonal circulations (Sun et al., 1998a,b, 1999). Thus northeastern China is one of the key sites on which to study the history of past environmental changes. However, previous studies focused mainly on orbital scale environmental changes, whereas changes in
0031-0182 / 02 / $ ^ see front matter 4 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 4 4 3 - 6
PALAEO 2798 29-5-02
420
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
the environment during the Holocene have not been well studied. Field investigations indicate that fossil sand dunes in the northeastern deserts of China are widely distributed, and these dunes are characterized by alternating paleosols and sand units. The paleosols are poorly developed, sandy loam soils. Low organic carbon content and possible carbon contamination from the roots of modern vegetation make it di⁄cult for accurate radiocarbon dating. In spite of this, there have been several papers concerned with paleoenvironmental reconstruction in these deserts in the Holocene, using radiocarbon dating (Wang, 1992; Li and Dong, 1998); the dates are di¡erent from site to site and no other numerical dating method has been applied. Optical dating, a technique closely related to thermoluminescence (TL) dating, has undergone extensive development recently (Wintle, 1993;
Duller, 1996; Aitken, 1998), and has been proved to be a successful method especially for dating dunes and sand sheets (Stokes and Gaylord, 1993; Stokes et al., 1997). This research uses optically stimulated luminescence (OSL) on sand-sized quartz grains to date fossil sand dunes in northeastern China in order to reconstruct the history of eolian activity in the region.
2. Geological setting and sample site Four fossil dune sections from the Hunshandake and Hulun Buir Deserts have been studied (Fig. 1). The two deserts are located in the semiarid climatic zone, controlled by the east Asian monsoon circulation. In winter the prevailing wind is northwesterly, and frequent dust storms usually occur in spring. In summer the dominant
Fig. 1. Map showing the deserts and the site locations.
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
421
Fig. 2. Stratigraphy, sampling depths and OSL ages for the four sections.
wind is southeasterly, bringing humid air masses from the Paci¢c Ocean to the deserts. The present mean annual precipitation in these deserts ranges from V450 mm in the east to V250 mm in the west, with more than 70% of the rainfall occurring in July, August and September (Wang, 1992). Under present climatic conditions, there should be no active sand dunes in northeastern China. However, human impact, especially land cultivation since the time of the Han Dynasty about 2300 yr ago, has greatly destroyed the vegetation cover (Sun, 2000). Poor land-use practices, combined with the high wind energy in the cold and dry spring season, have led to increased sand mobility and the reworking of stabilized sand dunes of Holocene age. Modern landforms in these deserts are dominated by semi-stabilized sand dunes, with active dunes mainly distributed at sites where large-scale land cultivation was once concentrated. Of the four sections, WG and HLD are 40 km apart and both are from the Hulun Buir Desert, whereas SGDL and SY are 100 km apart and derive from the Hunshandake Desert (Fig. 1). All the sections are relicts of fossil-stabilized dunes, distributed in the interiors of the deserts. The sequences at the four sections can be correlated, since they consist of an uppermost modern
sand unit, two sandy loam soils (all with A/C soil pro¢les) with an intercalated sand layer, as well as a sand unit underlying the lower soil (Fig. 2). Field observations indicate that the lower soil is slightly more developed than the upper one. In general, the sand unit underlying the lower soil unit is over 100 m thick. It is the potential source of the modern sand. However, it is di⁄cult to ¢nd exposures that show this thick bed. Thus, only the upper part of the lowest sand unit was studied (Fig. 2). Three OSL samples from each section were collected and the sample depths of all 12 samples are given in Table 1.
3. Optical dating of sediments Optical dating determines the last occasion when the mineral grains were exposed to sunlight, thereby resetting the luminescence clock. The measured signal is the OSL (Aitken, 1998). After burial, the signal grows with time as the result of exposure to ionizing irradiation, emitted by naturally occurring radioisotopes U, Th, and K in the sample itself and surroundings, and by cosmic rays. Minerals, e.g. quartz and feldspar, act as dosimeters that allow the irradiation dose since sunlight resetting is determined. The total accu-
PALAEO 2798 29-5-02
422
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
Table 1 Dose rate and OSL ages for 12 samples from northeastern China Site
Sample
He Er Hong De
HLD1 (soil) HLD2 (sand) HLD3 (soil) Wan Gong WG1 (sand) WG2 (sand) WG3 (sand) Sang Gen Da Lai SGDL1 (soil) SGDL2 (sand) SGDL3 (soil) San Yi SY1 (soil) SY2 (sand) SY3 (sand) a b c
Depth Alpha K content counting ratea (m) (%)
Water contentb (%)
Cosmic rayc (Gy/ka)
De (Gy)
(Gy/ka)
(ka)
0.35 1.25 2.25 0.35 1.30 1.95 0.75 1.30 2.75 0.85 1.75 3.05
1.3 1.9 6.5 5.7 3.2 6.6 2.8 6.6 6.4 4.2 4.8 4.3
0.29 0.25 0.21 0.29 0.26 0.23 0.28 0.25 0.21 0.27 0.25 0.21
4.53 R 0.58 7.2 R 0.8 36.9 R 3.2 0.14 R 0.04 5.93 R 0.52 48.4 R 8.2 2.86 R 0.24 10.7 R 1.0 30.4 R 4.0 4.02 R 0.33 5.18 R 0.50 33.8 R 3.6
2.90 R 0.10 2.91 R 0.12 2.97 R 0.12 3.50 R 0.11 3.20 R 0.13 3.61 R 0.13 2.74 R 0.10 3.06 R 0.14 3.08 R 0.13 3.22 R 0.14 3.11 R 0.14 3.06 R 0.14
1.56 R 0.14 2.48 R 0.29 12.4 R 1.2 0.04 R 0.01 1.85 R 0.18 13.4 R 0.5 1.05 R 0.1 3.5 R 0.36 9.86 R 1.36 1.25 R 0.14 1.67 R 0.18 11.1 R 1.3
2.39 R 0.12 2.79 R 0.13 4.14 R 0.16 5.48 R 0.18 4.03 R 0.16 7.56 R 0.21 1.97 R 0.11 3.00 R 0.13 3.48 R 0.14 3.11 R 0.14 2.00 R 0.11 2.16 R 0.12
2.31 R 0.12 2.31 R 0.12 2.39 R 0.13 2.65 R 0.13 2.48 R 0.12 2.56 R 0.13 2.25 R 0.12 2.62 R 0.13 2.44 R 0.12 2.66 R 0.13 2.76 R 0.13 2.66 R 0.13
Dose rate
OSL age
The alpha counting rate is for a 42-mm-diameter ZnS screen and is given in units of counts per kilosecond. The error for the water content is estimated at R 20%. The error for the cosmic rays dose rate is estimated at R 0.02 Gy/ka.
mulated radiation dose, known as the equivalent dose (De ), can be determined in the laboratory with regeneration technique using the natural OSL signal compared with that from a known dose. The environmental dose rate is estimated from the radioactivity and chemical analysis of the sample and its surrounding materials. The OSL age is given by the ratio between De and the dose rate (Wintle, 1993). A major advantage of optical over TL dating methods is the rapid and complete resetting of the OSL signal. Instead of about 10 h of direct sunlight exposure being required to reduce the TL signals to a ¢xed residual level (which is about 30% or more of initial TL for quartz), only about 10 min is required to reduce the OSL to 1% of its initial level (Wintle, 1993). For direct sunlight exposures of longer than 30 min, the remaining OSL signals are negligible and thus the residual OSL level for eolian sand is not a concern. Another advantage is that the De determination can be achieved using the OSL signal from a single sample aliquot (Duller, 1991). This has greatly increased the precision in De determination and also allows the detection of incompletely bleached sediments (Li, 1994; Olley et al., 1998). Additional techniques, using single aliquots, can be applied to sand in order to check for the incorporation of reworked material (Li, 2001).
4. Experimental procedures All 12 OSL samples were obtained by hammering aluminum tubes into cleaned vertical sections, then sealing inside plastic bags with tape to ensure that the sediment retained its natural water content. In the laboratory the material at each end of the tube was scraped away and used for dose rate measurements. Quartz grains were prepared following procedures of sieving, heavy liquid separation and HF etching in subdued red safe-light conditions. Raw samples were treated ¢rst with 20% H2 O2 and 10% HCl to remove organic materials and carbonates. Grains between 150 and 180 Wm were selected by dry sieving. Then grains of densities between 2.62 and 2.70 g/cm3 were separated using sodium polytungstate heavy liquid. The separated grains were treated with 40% hydro£uoric acid for 1 h and 10 min to remove feldspar grains. Separated quartz grains were mounted on 10-mm-diameter aluminum discs with Silkospay silicon oil. The purity of quartz grains was tested by monitoring for the presence of feldspar by measuring the infrared-stimulated luminescence (IRSL) and the 110‡C TL peak (see below). Discs for which IRSL signals were observed were discarded before further luminescence measurements. Luminescence measurements were made using
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
an automated RisS TL/OSL reader (Markey et al., 1997). The stimulating light source is a tungsten^halogen lamp ¢ltered from 420 to 550 nm using a Schott GG-420 ¢lter in combination with interference ¢lters. The light delivered to the sample disc is about 40 mW/cm2 . The OSL signal is detected through two 3-mm-thick Hoya U-340 ¢lters. Irradiation was carried out using a 90 Sr/90 Y beta source built into the reader. It currently delivers 0.0475 Gy/s to quartz on aluminum discs. The equivalent doses (De ) were determined using the single aliquot regeneration (SAR) procedure (Murray and Wintle, 2000). Each quartz disc underwent preheating, OSL measurement and bleaching, sensitivity measurement, and irradiation cycles with a di¡erent irradiation dose in each cycle. Three irradiation doses and one repeat dose were applied to each sample in such a way that the natural OSL intensity was observed between the OSL intensities regenerated from the given doses. The preheating was at 260‡C for 10 s. OSL measurement and bleaching were achieved by measuring the OSL signal at 125‡C for 100 s. This e¡ectively bleached the OSL signal to a negligible level for all samples. The OSL signals integrated from the ¢rst 5 s while subtracting half of the averaged OSL in the last 10 s were taken as the OSL intensity used for De value determination. Sensitivity monitoring was achieved by measuring the OSL signal created by a test dose of 0.95 Gy. The test-dose OSL signal was measured at 125‡C for 100 s following heating the disc to 160‡C and was calculated in the same way as for the OSL measurement. The OSL intensities were corrected for sensitivity change using the test-dose OSL. The De values were calculated using the corrected OSL intensities (Fig. 3). Twelve or more discs were measured for each sample. When heating the sample to 160‡C before measuring the test-dose OSL, the TL was recorded as a surrogate for OSL sensitivity change from quartz if the test-dose OSL was unsuitable (Murray and Roberts, 1998; Li et al., 1999). The TL measurement was made on the same reader by heating the sample to 160‡C with a heating rate of 3‡C/s immediately after the test-dose irradia-
423
Fig. 3. Illustration of the SAR technique for De determination of a quartz aliquot from sample WG2. The OSL was corrected for sensitivity changes. (O) Repeated regeneration dose point. (a) Corrected natural OSL signal.
tion. The TL response was the integrated TL signal from 80 to 120‡C after subtracting the background. The test-dose TL can also act as an indicator of impurity in the quartz grains, since feldspars often have TL peaks around 150‡C, i.e. di¡erent from quartz grains. The environmental dose rate to which the quartz grains were exposed was measured using a variety of techniques, and the results are summarized in Table 1. Thick source alpha counting (TSAC) was used to measure contributions from the U and Th decay chains. A Littlemore 7286 TSAC system with 42-mm-diameter ZnS screens was used. The K content was measured by £ame photometry. Water content was calculated from the sample weights before and after drying at 105‡C in an oven. The cosmic ray contribution to the dose rate was calculated from the burial depth and the altitude of the samples (Prescott and Hutton, 1994).
5. Results Equivalent dose values and dose rate results are summarized in Table 1. For each sample the De values of all discs are similar (Fig. 4). This indicates that the quartz grains were uniformly
PALAEO 2798 29-5-02
424
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
Fig. 4. Data for 12 quartz aliquots from SGDL1 and 13 aliquots from HLD2. Top: histogram of De distribution. Bottom: De plotted against the ratio (In /TL) of natural OSL and 110‡C TL from test dose.
bleached prior to deposition (Olley et al., 1998; Li, 2001) and reworking was insigni¢cant. Therefore, the De value for each sample is taken as the average of the measured discs. Su⁄cient bleaching at deposition has also been supported by the results for a currently active dune sample WG1. It gives an age of 40 R 10 yr, with a De value of 0.14 R 0.04 Gy averaged from 12 discs. The error in the OSL ages was estimated from the errors of dose rate measurement and the De determination in the way described in Aitken (1985). The errors for the De value are the 1c standard deviation among the discs. Since the precision of the De measurement using the single aliquot technique is very high, the systematic error contributed from individual aliquot De measure-
ment is relatively insigni¢cant. The error for the K content was R 5% of the measured value. The error in the water content was estimated as R 20%. The error for the cosmic ray dose rate was R 20 WGy per yr.
6. Discussion 6.1. Precision and accuracy of the optical ages There is good agreement between the De values obtained among the discs for each sample suggesting that the quartz grains were completely bleached prior to deposition and that reworking is insigni¢cant for these samples (Olley et al.,
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
1998). This is also supported by the results of plotting the De values versus the ratio of natural OSL intensity (In ) and the test-dose TL (Li, 2001). Unlike poorly bleached sediments, there is no correlation between the De value and the ratio of natural OSL to test-dose TL (Fig. 4). It is thus rea⁄rmed that optical dating is suitable for dating of eolian sand dunes. The dose rates for samples in each section, and even in di¡erent sections, are very close to each other due to similarity in mineral composition. No obvious di¡erence was observed between the soil and sand samples. In the age calculations, the moisture content used for each sample was that measured at the present time. Though it may have varied through time, changes of water content will result in a small error for the ¢nal age. The di¡erence is less than 10% of the age, if water content changes from 1 to 10%. Cosmic rays make a small contribution to the dose rate. 6.2. History of sand mobilization since the last deglaciation Fig. 2 indicates that the OSL dates in the HLD and WG sections of the Hulun Buir Desert can be correlated. Additionally, the dates of these two sections can be correlated with those of the SGDL and SY sections in the Hunshandake Desert, especially when we consider the fact that the two deserts are about 600 km apart. It is suggested that the climate was similar in the two deserts, as evidenced by the formation of sands and loam soils. The dates for the lowest eolian sand unit in the WG and SY sections are 13.4 and 11.4 ka, respectively. This suggests that, although the Last Glacial Maximum peaked at 21 ka (Bard et al., 1990), the glacial cold and dry conditions lasted at least until the last deglaciation, as evidenced by eolian sand mobilization. Dates at the bottom of the lower soils are 9.9 R 1.4 and 12.4 R 1.2 ka. When we consider the errors, these two ages can be correlated. Firstly, this suggests that the soil was developed during the Holocene, and this is in good agreement with the ¢nding of Neolithic stone artifacts in the lower soils. Additionally, it suggests that the shift from dune formation to sand stabiliza-
425
tion occurred about 10 ka, being in phase with the Last Glaciation/Holocene transition in the Chinese Loess Plateau and most other records in the world (Liu, 1985; Imbrie et al., 1984). The stratigraphic units of two Holocene soils and one intercalated sand layer found in all the sections suggest that the warm and humid summer monsoon, which dominated the climate during the Holocene, was interrupted by a cold and dry phase about 3.5V1.7 ka, evidenced by a decrease in the pollen £ux and the number of wooden and shrub assemblages, which corresponds with the intercalated sand unit (Qiu et al., 1992). OSL dating applied to the top of this soil yielded an age of 3.5 R 0.36 ka, suggesting that the Holocene Optimum in these regions occurred between 10 and 3.6 ka. The dates for the sand layer between the two Holocene soils give ages of 3.5V1.7 ka. Combined with the top age of the lower soil, the cold and dry episode, evidenced by sand mobilization, occurred between 3.5 and 1.7 ka. Additionally, although the age for the top of the upper soil yielded an age of V1.2 ka, as discussed above, all these studied sections are relicts of fossil-stabilized dunes, which means that some of the uppermost soil has probably been removed. Thus the latest warm and humid episode, evidenced by the soil development, lasted at least from 1.7 to 1.2 ka. Although there are many radiocarbon dates reported from the sandy loam soils in Chinese deserts, it is likely that radiocarbon in loam soil using conventional dating methods is not a closed system (Head et al., 1989). The reported 14 C dates have considerable scatter and are unreliable. This makes it di⁄cult to compare our results with previous studies in the region due to the lacking of other reliable dates. The results from this study are tentatively compared with the paleoclimatic studies in the Sahara region (Fig. 6). Fig. 6B is compiled from 560 radiocarbon dates on surface freshwater indicators (lake and swamp sediments, freshwater shells, diatoms). The frequency of the radiocarbon dates was explained as a primary indicator of past humid climates (Guo et al., 2000). The dates of soil development in northeastern China are consistent with the Sahara records that a humid phase occurred between V9.8 and
PALAEO 2798 29-5-02
426
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
Fig. 5. 110‡C TL signals against OSL signals of samples from the WG, SY, HLD and SGDL sections. Both signals are induced by the same test doses. (O) Aliquots with high OSL sensitivity.
3.6 ka. This suggests that the humid episode is a global event (Fig. 6). The uppermost eolian sand layer yielded an age of 40 R 10 yr, indicating it to be modern. It serves as evidence for the previous argument that the sand dunes in these deserts today are not in equilibrium with the modern climate, especially when considering the fact that all these deserts are located in semi-arid regions with a mean annual precipitation of up to 450 mm. Modern sand mobilization and the reworking of relict stabilized sand dunes are mainly due to the large-scale land cultivation since the Han Dynasty, especially since the Late Qing Dynasty of V300 yr ago (Sun, 2000).
The modern eolian sands in these deserts are thought to be derived mainly from the Last Glacial sand beds, when the overlying ‘protection layer’ of the Holocene soil was destroyed due to large-scale land cultivation (Sun, 2000). As a result, there are only sparsely distributed relicts of fossil-stabilized sand dunes that enable the history of eolian mobilization to be ascertained. 6.3. The sources of quartz grains When the test-dose TL is plotted against the test-dose OSL of each natural aliquot for all 12 samples, most discs show a similar ratio between the two signals (Fig. 5). However, a small
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
427
found in areas adjacent to the deserts. Geochemical analysis of the grains may give indications to the sources of the grains (Muhs et al., 2000).
7. Conclusion
Fig. 6. Correlation between the soil development phases in the studied regions and the frequency variations of surface freshwater indicators from the Sahara (B redrawn from Guo et al., 2000).
number of discs from each section show signi¢cantly greater OSL sensitivity when compared with the 110‡C TL. Given the fact that there were a few thousand quartz grains on each disc, it can be suggested that there are a small number of grains on such discs that are much more OSL sensitive than others to irradiation dose. These OSL sensitive discs cannot be explained by the result of feldspar contamination because all of the discs have been IRSL-screened and there is no signi¢cant TL signal around 150‡C. In other studies, the 110‡C TL peak and OSL sensitivities have been shown to be strongly dependent on the source of the quartz grains (Han et al., 1997), and their thermal history (Chen et al., 2000). Although there is not a unique reason for such a high OSL sensitivity over TL, it is likely that the OSL sensitive grains might be derived from sources di¡erent from the rest of the grains, or that these grains have experienced a di¡erent thermal history. For example, they might be associated with volcanic activity in northeastern China, where Quaternary volcanic deposits have been
OSL dating can provide a reliable chronology for desert sand dunes. Active dune formation in this region of northeastern China lasted from the Last Glacial Maximum to about 10 ka. It has also been shown that the warm climate of the Holocene was interrupted by a cold/dry dune-forming episode at about 3.5^1.7 ka. The Holocene Optimum occurred between 10 and 3.6 ka, and a later warm/humid dune stabilization phase lasted from at least 1.6 to 1.2 ka. The youngest age on the uppermost sand unit yielded an age of 40 yr, supporting the previous argument that current active eolian sands can occur in regions with a mean annual precipitation of up to 450 mm ; this activity is not due to drought, but to extensive land cultivation in recent time. The sands in most of the sections are probably derived from more than one source, with a minor source of quartz having a di¡erent thermal history before deposition. This more sensitive quartz may be associated with volcanic activity in northeastern China.
Acknowledgements The authors thank N. Lancaster and an anonymous referee for providing helpful comments. The work described in this paper was substantially supported by Grants to S.-H.L. from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project no. 7105/97P, 7112/98P), and a Grant to J.-M.S. from NKBRSF (Project G1999043402).
References Ahlbrandt, T.S., Swinehart, J.B., Maroney, D.G., 1983. The dynamic Holocene dune ¢elds of the Great Plains and Rocky Mountain basins, USA. In: Brook¢eld, M.E., Ahl-
PALAEO 2798 29-5-02
428
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429
brandt, T.S. (Eds.), Eolian Sediments and Processes: Developments in the Sedimentology 38. Elsevier, Rotterdam, pp. 379^406. Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Bard, E., Hamelin, B., Fairbanks, R.G., Zindler, A., 1990. Calibration of C-14 time scales over the past 30,000 yrs using mass spectrometric U^Th ages from Barbados corals. Nature 345, 405^409. Chen, G., Li, S.-H., Murray, A.S., 2000. Study of 110‡C TL peak sensitivity in optical dating of quartz. Radiat. Meas. 32, 641^645. Duller, G.A.T., 1991. Equivalent dose determination using single aliquots. Nucl. Tracks Radiat. Meas. 18, 371^378. Duller, G.A.T., 1996. Recent developments in optical dating of sediments. Prog. Phys. Geogr. 20, 133^151. Goudie, A.S., 1999. The history of desert dune studies over the last 100 years. In: Goudie, A.S., Livingstone, I., Stokes, S. (Eds.), Aeolian Environments, Sediments and Landforms. Wiley, Chichester, pp. 1^13. Guo, Z.T., Petit-Maire, N., Kro«pelin, S., 2000. Holocene nonorbital climatic events in present-day arid areas of northern Africa and China. Glob. Planet. Change 26, 97^103. Han, Z.-Y., Li, S.-H., Tso, M.Y.W., 1997. Age dependence of luminescence signals from granitic and mylonitic quartz. Quat. Sci. Rev. (Quat. Geochron.) 16, 427^430. Head, M.J., Zhou, W.J., Zhou, M.F., 1989. Evaluation of C-14 ages of organic fractions of paleosols from loesspaleosol sequences near Xian, China. Radiocarbon 31, 680^694. Imbrie, J., Hays, J.D., Martinson, D.B., McIntyre, A., Mix, A.C., Morley, J.J., Pisias, N.G., Prell, W.L., Shackleton, N.J., 1984. The orbital theory of Pleistocene climate: Support from a revised chronology of the marine delta 18O record. In: Berger, A., Imbrie, J., Hays, J., Kukla, G., Saltzman, B. (Eds.), Milankovitch and Climate (Part I). Reidel, Dordrecht, pp. 269^305. Lancaster, N., 1988. Development of linear dunes in the southwestern Kalahari, Southern Africa. J. Arid Environ. 14, 233^244. Lancaster, N., 1999. Geomorphology of desert sand seas. In: Goudie, A.S., Livingstone, I., Stokes, S. (Eds.), Aeolian Environments, Sediments and Landforms. Wiley, Chichester, pp. 49^69. Li, S.-H., 1994. Optical dating: insu⁄ciently bleached sediments. Radiat. Meas. 23, 563^567. Li, S.-H., 2001. Identi¢cation of well-bleached grains in the optical dating of quartz. Quat. Sci. Rev. (Quat. Geochron.) 20, 1365^1370. Li, S.-H., Tso, M.Y.W., Westaway, K.E., Chen, G., 1999. Choice of the most appropriate thermal treatment in optical dating of quartz. Radiat. Prot. Dosim. 84, 495^498. Li, X.Z., Dong, G.R., 1998. Preliminary studies on formative age and causes of Hunshandake Sandy Land in China (in Chinese). J. Desert Res. 18, 16^20.
Liu, T.S., 1985. Loess and the Environment. The China Ocean Press, Beijing, 251 pp. Markey, B.G., BStter-Jensen, L., Duller, G.A.T., 1997. A new £exible system for measuring thermally and optically stimulated luminescence. Radiat. Meas. 27, 83^90. McKee, E.D., 1979. A Study of Global Sand Seas. U.S. Geological Survey Professional Paper 1052. Muhs, D.R., Swinehart, J.B., Loope, D.B., Been, J., Mahan, S.A., Bush, C.A., 2000. Geochemical evidence for an eolian sand dam across the north and south Platte rivers in Nebraska. Quat. Res. 53, 214^222. Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose single-aliquot protocol. Radiat. Meas. 29, 503^515. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57^73. Olley, J.M., Caitcheon, G.G., Murray, A.S., 1998. The distribution of apparent dose as determined by optical stimulated luminescence in small aliquots of £uvial quartz: implications for dating young sediments. Quat. Sci. Rev. (Quat. Geochron.) 17, 1033^1040. O’Connor, P.W., Thomas, D.S., 1999. The timing and environmental signi¢cance of late Quaternary linear dune development in Western Zambia. Quat. Res. 52, 44^55. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term time variations. Radiat. Meas. 23, 497^ 500. Qiu, S.W., Li, Q.S., Xia, Y.M., 1992. Paleosols in the Sandylands of the Northeastern Plain of China and the inferred environmental changes during the Holocene (in Chinese). Quat. Sci. 3, 224^232. Stokes, S., Gaylord, D.R., 1993. Optical dating of Holocene dune sands in the Ferris Dune Field, Wyoming. Quat. Res. 39, 274^281. Stokes, S., Thomas, D.S.G., Washington, R., 1997. Multiple episodes of aridity in southern Africa since the last interglacial period. Nature 388, 154^158. Sun, J.M., 2000. Origin of eolian sand mobilization during the past 2300 years in the Mu Us Desert, China. Quat. Res. 53, 73^88. Sun, J.M., Ding, Z.L., Liu, T.S., 1998a. Desert distributions during the glacial maximum and climatic optimum: Example of China. Episodes 21, 28^31. Sun, J.M., Ding, Z.L., Rokosh, D., Rutter, N., 1999. 580,000 year environmental reconstruction from eolian deposits at the Mu Us Desert margin, China. Quat. Sci. Rev. 18, 1351^1364. Sun, J.M., Yin, G.M., Ding, Z.L., Liu, T.S., Chen, J., 1998b. Thermoluminescence chronology of sand pro¢les in the Mu Us Desert, China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 144, 225^233. Tchakerian, V.P., 1999. Dune Palaeoenvironments. In: Goudie, A.S., Livingstone, I., Stokes, S. (Eds.), Aeolian Environments, Sediments and Landforms. Wiley, Chichester, pp. 261^292.
PALAEO 2798 29-5-02
S.-H. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 181 (2002) 419^429 Thomas, D.S.G., O’Connor, P.W., Bateman, M.D., Shaw, P.A., Stokes, S., Nash, D.J., 2000. Dune activity as a record of late Quaternary aridity in the Northern Kalahari: new evidence from northern Namibia interpreted in the context of regional arid and humid chronologies. Palaeogeogr. Palaeoclimatol. Palaeoecol. 156, 243^259. Wang, P.F., 1992. Preliminary study on the environmental
429
changes of Hulun Buir Sandy Land since the Holocene (in Chinese). J. Desert Res. 12, 13^19. Wintle, A.G., 1993. Luminescence dating of aeolian sands: an overview. In: Pye, K. (Ed.), The Dynamics and Environmental Context of Aeolian Sedimentary Systems. Geological Society London Special Publication 72, pp. 49^58.
PALAEO 2798 29-5-02