ZhongPing Lai

Effects of Feldspar Contamination on Equivalent dose and the Shape of Growth Curve for OSL of Silt-Sized Quartz Extracted from Chinese Loess

Effects of Feldspar Contamination on Equivalent dose and the Shape of Growth Curve for OSL of Silt-Sized Quartz Extracted from Chinese Loess This study examines the effects of feldspar contamination of different extent (by fluorosilicic acid etching for different time periods and mixing pure quartz with different percentage of unetched grains) on blue-light stimulated luminescence (BLSL) of quartz (38-63 μm) for Chinese loess. In particular, the De values and the shape of growth curves will be examined. The results show that: (1) The De determined using feldspar in Chinese loess is obviously underestimated, and infra-red stimulated luminescence (IRSL) has a greater underestimation than BLSL. For aliquots with no fluorosilicic acid etching, an IRSL De underestimation of ∼30% is observed for a sample with an expected age of ∼22 ka, ∼19% underestimation for BLSL, and ∼10% for post-IR BLSL. For quartz BLSL, the contamination of feldspar, which has not dissolved by fluorosilicic acid etching, will inevitably lead to age underestimation. We suggest that the IR-checking should be performed for every single aliquot. An aliquot should be excluded from the final De calculation if obvious IRSL signal was observed. (2) For the sample under examination blue LED stimulation can reset both IRSL and BLSL signals of feldspars, while IR stimulation cannot reset the BLSL of feldspars. (3) For multiple-mineral aliquot the growth curve shape of the post-IR BLSL is different from that of the IRSL, and within 25 Gy it is identical to that of the pure quartz BLSL. The contamination of feldspars in quartz will change the shape of a growth curve for quartz BLSL.

High-resolution summer precipitation variations in the western Chinese Loess Plateau during the last glacial

We present a summer precipitation reconstruction for the last glacial (LG) on the western edge of the Chinese Loess Plateau (CLP) using a well-dated organic carbon isotopic dataset together with an independent modern process study results. Our results demonstrate that summer precipitation variations in the CLP during the LG were broadly correlated to the intensity of the Asian summer monsoon (ASM) as recorded by stalagmite oxygen isotopes from southern China. During the last deglaciation, the onset of the increase in temperatures at high latitudes in the Northern Hemisphere and decline in the intensity of the East Asia winter monsoon in mid latitudes was earlier than the increase in ASM intensity and our reconstructed summer precipitation in the western CLP. Quantitative reconstruction of a single paleoclimatic factor provides new insights and opportunities for further understanding of the paleoclimatic variations in monsoonal East Asia and their relation to the global climatic system.

Examining quartz OSL age underestimation for loess samples from Luochuan in the Chinese Loess Plateau

When using quartz OSL to date loess samples from the Chinese Loess Plateau, it has been reported that the agreement between OSL ages and the independent ages is limited to the samples younger than ∼70 ka with a corresponding De of ∼230 Gy, and a sample with an expected age of 780 ka was dated to 107 ka, corresponding to 403 Gy. The growth curves of these samples do not saturate at doses of 700 Gy, and a linear growth part was observed for doses higher than 200 Gy. However, the maximum measured age of ∼100 ka imply that the De determined using this linear part of a growth curve could be problematic, or that the quartz OSL signal is not as stable as previously thought and has a barrier age of ∼100 ka. In the current study, we examine the reasons for the age underestimation. We examined the shape of growth curves, anomalous fading, thermal stability, etc. The results show that, for the loess samples examined, quartz OSL does not fade anomalously, and the barrier age of ∼100 ka is due to the fact that the OSL signals are less thermally stable, the lifetime of 0.311 Ma at 20°C obtained is much smaller than those for quartz samples from other regions such as Australia (∼100 Ma).

Holocene evolution of Huangqihai Lake in semi-arid northern China based on sedimentology and luminescence dating

The pattern of Holocene palaeoclimatic change in arid-semiarid northern China is debated. The terminal Huangqihai Lake is located at the northern margin of the modern East Asian summer monsoon and sensitive to climate change. We present here a sedimentary section from Huangqihai lake basin which comprises lacustrine and fluvial deposits and is about 8–9 m above the present lake level. Quartz optically stimulated luminescence (OSL) dating was employed to construct the chronology of the section. Based on the OSL chronology and the sedimentological and granulometric analysis, we conclude that (1) the Huangqihai Lake experienced a humid climate during the early part of the Holocene (from c. 10.2 ± 1.0 to 6.7 ± 0.7 ka) with a persistent and stable high lake level; (2) the lake level began to decline indicating aridification between 8.7 ± 0.8 ka and 6.7 ± 0.7 ka, and the climate was drier and variable after 6.7 ± 0.7 ka compared with that of before; (3) from 2.2 ± 0.2 to 0.93 ± 0.07 ka the climatic conditions were highly variable and continuously deteriorating, and the lake was turned into a playa for most of the time. In summary, the climate showed a general drying trend from the early Holocene to the late Holocene.

Rapid and cyclic dust accumulation during MIS 2 in Central Asia inferred from loess OSL dating and grain-size analysis

Due to lack of reliable proxies from the Westerlies-dominant region, the strength change of Northern Hemisphere Westerlies remains poorly understood. The aim of this study is to provide a reliable paleoclimatic proxy about the Northern Hemisphere Westerlies change. Here we report a 30.7 m thick loess section from the Ili basin directly controlled by the Westerlies. Based on optically stimulated luminescence (OSL) and high resolution grain-size records, we reconstruct the change history of the Westerlies strength during the last glacial period (mainly Marine Isotope Stages 2, MIS2), being similar with the Westerlies index recorded in the Qinghai Lake sediments. Within error limits, all ages are in stratigraphic order. We further compare the climatic records among the Ili loess, Qinghai Lake and the NGRIP, their similarity shows a good climatic coupling relationship among the Central Asia, East Asia and the North Atlantic, and the Westerlies plays a critical influence in transporting the North Atlantic signal to Central and East Asia.

Re-assessment of the paleoclimate implications of the Shell Bar in the Qaidam Basin, China

The Shell Bar in the Qaidam Basin, China, is a prominent geological feature composed of millions of densely packed Corbicula shells. Since the mid 1980s, it has been regarded as evidence for existence of a large lake during Marine Isotope Stage (MIS) 3 in the presently hyper-arid Qaidam Basin. Early studies suggested the bivalve shells accumulated at the shore of a large lake, whereas more recent work led to the conclusion that the Shell Bar was formed within a deeper water body. Based on our re-assessment of sediments and fossils from the Shell Bar, investigation of exposed fluvio-lacustrine sections upstream of the Shell Bar and study of nearby modern streams, we infer that the Shell Bar represents a stream deposit. Corbicula is a typical stream-dweller around the world. Preservation of Corbicula shells of different sizes, as well as occurrence of many articulated shells, provide evidence against post-mortem transport and accumulation along a lake shore. Additionally, the SE-NW alignment of the Shell Bar is similar to modern intermittent stream beds in its vicinity and corresponds to the present-day slope towards the basin centre further NW, and furthermore, the predominantly sandy sediments also indicate that the Shell Bar was formed in a stream. Abundant ostracod shells in the Shell Bar sediments originated from stream-dwelling species that are abundant in modern streams in the vicinity of the Shell Bar, or in part from fluvio-lacustrine sediments exposed upstream of the Shell Bar, as a result of erosion and re-deposition. Deflation of alluvial fine-grained sediments in the Shell Bar region and protection of the stream deposits by the large and thick-walled Corbicula shells reversed the former channel relief and yielded the modern exposure, which is a prominent morphological feature. Occurrence of Corbicula shells in the Qaidam Basin indicates climate was apparently warmer than present during the formation of the Shell Bar because Corbicula does not live at similar or higher altitudes in the region today. Because the Shell Bar is no longer considered a deposit formed within a lake, its presence does not indicate paleoclimate conditions wetter than today.

Diversity of Carbon Monoxide-Oxidizing Bacteria in Five Lakes on the Qinghai-Tibet Plateau, China

Carbon monoxide-oxidizing (COX) bacteria play an important role in controlling the flux of carbon monoxide among natural reservoirs, and thus studying their diversity in natural environments is of great significance to understanding the carbon cycle. In this study, the COX bacterial diversity was investigated in five lakes (Erhai Lake, Gahai Lake1, Gahai Lake2, Xiaochaidan Lake, Lake Chaka) on the Qinghai-Tibet Plateau and its correlation with environmental variables of the lakes was explored. Phylogenetic analyses showed that the CO-oxidizers were dominated by Proteobacteria and Actinobacteria in the Qinghai-Tibet Plateau lakes, and their relative abundance varied with salinity: in the freshwater Erhai Lake, the COX bacteria in the water were dominated by the Betaproteobacteria, in contrast to the Actinobacteridae dominance in the sediment; in the saline and hypersaline lakes of Gahai Lake1, Gahai Lake2 and Xiaochaidan Lake, alphaproteobacterial COX bacteria were dominant in the water, whereas Actinobacteridae and alphaproteobacterial COX bacteria were dominant in the sediment. In the hypersaline Lake Chaka, an unknown COX bacterial clade and alphaproteobacterial COX bacteria were dominant in the water and sediment, respectively. Statistical analyses showed that salinity, pH, and major ions (e.g., K+, Na+, Ca2+, Mg2+, SO4 2-, and Cl-) were important factors affecting the COX bacterial community compositions in the investigated lakes. Overall, our results provided insights into the COX bacterial diversity in Qinghai-Tibetan lakes.

Abundance and Diversity of Sulfur-Oxidizing Bacteria along a Salinity Gradient in Four Qinghai-Tibetan Lakes, China

Sulfur-oxidizing bacteria (SOB) play important roles in the sulfur cycle and are widespread in a number of environments, but their occurrence and relationship to geochemical conditions in (hyper)saline lakes are still poorly understood. In this study, the abundance and diversity of SOB populations were investigated in four Qinghai-Tibetan lakes (Erhai Lake, Gahai Lake 1, Gahai Lake 2 and Xiaochaidan Lake) by using quantitative polymerase chain reaction (qPCR) and soxB gene- (encoding sulfate thiohydrolase) based phylogenectic analyses. qPCR analyses showed that in the studied lakes, the total bacterial 16S rRNA and soxB gene abundances in the sediments were distinctly higher than in the overlying waters. The 16S rRNA gene abundance in the waters ranged 5.27 × 106–6.09 × 108 copies per mL and 7.39 × 1010–2.9 × 1011 copies per gram sediment. The soxB gene abundance in the waters ranged from 1.88 × 104 to 5.21 × 105 per mL and 4.73 × 106–2.65 × 107 copies per gram sediment. The soxB gene in the waters of the two hypersaline lakes (Gahai Lake 2 and Xiaochaidan Lake) was more abundant (2.97 × 105 and 5.21 × 105 copies per mL) than that in the two low-salinity lakes (1.88 × 104 and 3.36 × 104 copies per mL). Phylogenetic analysis showed that Alpha- and Betaproteobacteria were dominant SOB in the investigated lakes, and the composition of proteobacterial subgroups varied with salinity: in freshwater Erhai Lake and low-salinity Gahai Lake 1, the SOB populations were dominated by the Betaproteobacteria, whereas in hypersaline Lake Gahai 2 and Xiaochaidan Lake, the SOB populations were dominated by Alphaproteobacteria. Overall, salinity played a key role in controlling the diversity and distribution of SOB populations in the investigated Qinghai-Tibetan lakes.

Construction of a quartz OSL standardised growth curve (SGC) for aeolian samples from the Horqin dunefield in northeastern China

It has been suggested that the standardised growth curve (SGC) method can be used to de-terminate De accurately and reduce the measurement time. However, different opinions regarding the applicability of the SGC method exist. In this paper, we report the construction of quartz OSL SGC for 35 aeolian samples from different parts of the Horqin dunefield in northeastern China, and then test their applicability for De determination. Our results suggest that: 1) up to a regeneration dose of 80 Gy, the SGC of the Horqin dunefield can be fitted using the exponential plus linear equation, with the r2 value of 0.97, and all the regeneration dose points closely stick to the fitting line, implying that all samples share a very similar dose-response curve; 2) for samples with Des ranging from 1 to 50 Gy, the ratios of SAR De to SGC De fall within the range of 0.9–1.1, and the average ratio of SAR De to SGC De is 1.01±0.01, close to unity. Therefore, the construction of SGC is correct, and the SGC is an effective procedure for accurate De determination for samples from the study area.

Investigations on the degree of bleaching of quartz OSL signals using modern aeolian dust from western Loess Plateau, China

Optically Stimulated Luminescence signal of quartz extracted from modern aeolian dust with known maximum age (about decades) was analyzed in terms of degree of bleaching. The results of dose recovery tests show that the modified double single-aliquot regenerative-dose protocol with the early background subtraction is robust for dating these modern dusts using small aliquots. Bleaching of these dusts is discussed based on the distribution of De values in histograms, scatter plots of De versus sensitivity corrected natural OSL signal and comparison between measured De and expected De. The results indicate that most dusts were completely bleached but some dusts were not completely bleached. For those incompletely bleached dusts in Lanzhou area, the maximum OSL age overestimation is up to ∼1 ka, which might be caused by fast deposition accompanied by heavy sand/dust storms. The research suggests that cautions should be given to OSL ages younger than 1 ka in the western China close to deserts.