Sheng-Hua Li

Optical dating of dune sands in the northeastern deserts of China

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.

Isochron dating of sediments using luminescence of K-feldspar grains

Sponsorship: Research Grant Council of the Hong Kong Special Administrative Region, China (project 7106/02P, 7032/05, and 7035/06).

Review and assessment of the potential of post-IR IRSL dating methods to circumvent the problem of anomalous fading in feldspar luminescence

Quartz has been the main mineral used for optically stimulated luminescence (OSL) dating of sediments over the last decade. The quartz OSL signal, however, has been shown to saturate at relatively low doses of ∼200–400 Gy, making it difficult to be used for dating beyond about 200 thou-sand years (ka), unless the environmental dose rate is low. The infrared stimulated luminescence (IRSL) from feldspars has been shown to continue to grow to higher dose levels than quartz OSL. The application of IRSL dating of feldspars, however, has long been hampered by the anomalous fading effect. Recent progress in understanding anomalous fading of the infrared stimulated luminescence (IRSL) signals in potassium-feldspar has led to the development of post-IR IRSL (pIRIR) protocols and also a multiple elevated temperature (MET) stimulation (MET-pIRIR) protocol. These procedures have raised the prospect of isolating a non-fading IRSL component for dating Quaternary deposits containing feldspars. In this study, we review the recent progress made on (1) overcoming anomalous fading of feldspar, and (2) the development of pIRIR dating techniques for feldspar. The potential and problems associated with these methods are discussed.

Optical dating of Holocene dune sands from the Hulun Buir Desert, northeastern China

Aeolian deposits from the Hulun Buir Desert of northeastern China are studied with optically stimulated luminescence dating methods to establish the chronology of dune building phases and climatic changes since the last deglaciation. Our results indicate that wet climate, marked by dune stabilization and soil development in the Hulun Buir Desert, commenced at-11 ka ago, and this early episode of dune stabilization lasted until 4.4 ka ago. This optimum climate between -11 and -4.4 ka ago is mostly the response to the strengthened monsoon circulation and increased precipitation in the Northern Hemisphere. The environment generally became arid after - 4.4 ka ago, but the dry climate was interrupted by three phases of weak soil development occurring at - 1.8-1.4, - 1.2-1.0 and - 0.84-0.5 ka ago, respectively. Such short events of dune stabilization were associated with the warm and humid climate in historical time. However, the present dune mobilization in the Hulun Buir Desert is mainly the result of poor land-use practices (land cultivation and overgrazing) since about 300 years ago.

Parameters of OSL traps determined with various linear heating rates

An analysis method is introduced for traps which give rise to optically stimulated luminescence (OSL). For pulse-annealing experiments, the relationship between the temperature at which maximum OSL depletion occurs and the linear heating rate used to deplete the trapped charge is established, and this allows trap parameters to be determined. The lifetime at ambient temperature can be obtained from these parameters assuming first-order kinetics. Experimental data for a sedimentary K-feldspar separate are presented.

A record of Holocene climate change in the Guanzhong Basin, China, based on optical dating of a loess-palaeosol sequence

Optical dating was used to obtain a chronology for a loess-palaeosol sequence in the Guanzhong Basin on the Loess Plateau in China. The sedimentary sequence recorded two dry episodes (3.7~2.4 ka and 1.2~0.8 ka ago) in the Holocene as shown by the presence of two layers of loess within the Holocene deposits. These two arid events are consistent with records of global climate change in the Holocene and also coincident with major historic events in China. This finding suggests that loess-paleosol sequences in China can record global climate changes and implies that rapid climate change affected the progress of human civilization. In addition, changes in the sedimentation rate through the section suggest that agricultural expansion may have accelerated the rate of dust accumulation.

Effect of heating on the quartz dose-response curve

We have investigated the effects of annealing quartz at temperatures up to 1000°C on both the absolute sensitivity and on the growth curve shape, using quartz extracts from three sedimentary samples from widely separated locations. Growth curves up to 670 Gy were measured using the single-aliquot regenerative-dose protocol, which allows all heat treatments and OSL measurements to be performed on a single sub-sample, with a corresponding improvement in precision when compared with multiple-aliquot protocols. It is found that the degree of dose-dependent sensitivity change is dependent on the annealing temperature. Despite this, and absolute sensitivity changes of more than three orders of magnitude, the growth-curve shape is relatively independent of annealing temperature, especially at low doses.

Study of the 110°C TL peak sensitivity in optical dating of quartz

As the 110°C TL emission in quartz uses the same luminescence centers as the OSL emission, the 110°C TL signal from a test dose may be used to monitor the OSL sensitivity change. It is thus important to study the relationship between the 110°C TL peak and the OSL sensitivity in studies related to optical dating from quartz. We have conducted a series of experiments using sedimentary quartz, where the annealing temperatures were varied between 260 and 1000°C before the measurement of OSL and 110°C TL sensitivities. Another series of experiments on two sedimentary quartz samples investigated the 110°C TL peak and OSL dose-dependent sensitivity change after different annealing temperatures. In these experiments, the 110°C TL and OSL signals from the test dose are shown to have similar sensitization characteristics: the 110°C TL sensitivity change is proportional to the OSL sensitivity change if the annealing temperature is lower than 500°C. It is concluded that the 110°C TL signal can be used to correct the OSL sensitivity change in the single-aliquot additive-dose protocol.

Lifetime determination of OSL signals from potassium feldspar

Abstract Lifetime determinations for both IRSL and GLSL signals were carried out using a method of pulse annealing with different heating rates. For a sedimentary K-feldspar sample and ambient temperature of 10°C, lifetimes of 6.6 × 10 8 and 1.3 × 10 6 years were obtained for IRSL and GLSL signals respectively. The differences in thermal stability for IRSL and GLSL are supported by results from an isothermal annealing experiment. The difference in lifetime is interpreted by different groups of traps being sampled by both stimulation sources. The implications for optical dating limits are discussed.

Preliminary results of long-term slip rates of 1999 earthquake fault by luminescence and radiocarbon dating

For the purpose of deriving the long-term slip rates of different segments along the surface rupture of 1999 Chi-chi earthquake, radiocarbon and luminescence methods (TL and OSL) were applied to the tectonically influenced geomorphic features and deposits. We obtained six age estimates on pottery, driftwood and fluvial sediments. We found that the OSL signals of the samples collected from the coarse fluvial deposits had not been totally bleached due to rapid deposition. Based on the cumulative vertical offsets, the vertical slip rates were computed for each site. Our results reveal that the slip rate of the northern segment is relatively higher than that of the southern segment, which is consistent with the coseismic offsets that occurred in the 1999 earthquake.