Landscape evolution and palaeoclimate in the deserts of northwestern China, with a special reference to Badain Jaran and Taklamakan

A comparative study of geomorphologic landscapes in the Badain Jaran Desert and Taklamakan Desert was made on the basis of field investigations, thermolumi-niscence and radiocarbon dating as well as analysis of sand grain texture. The sand seas of the Badain Jaran Desert differ greatly from that of the Taklamakan Desert in terms of dune morphology, interdune relief, dune stratigraphy and sand sources. The thermoluminiscence ages of the dunes suggest that the basic configuration of the megadunes in the Badain Jaran Desert has not changed at least since Middle Holocene. There were periods of more humid environment during the last glaciation and Holocene in both deserts, but events of higher precipitation in the Badain Jaran Desert were more frequent than in the Taklamakan. In both desert areas, aeolian processes have been intensified during the last 2000 a.     

巴丹吉林沙漠沉积物的地球化学特征

在第四纪沉积物研究中,特别是现代沉积环境的研究中,沉积物的地球化学特征在分析物源和重建古环境等方面发挥积极的作用.通过采集了巴丹吉林沙漠不同区域、不同类型沙丘、不同地貌部位的沙样,分析了巴丹吉林沙漠沉积物的地球化学特征,得出巴丹吉林沙漠沉积物在矿物学上和沉积学上的成熟度较低,但是具有很好的均质性的结论.这一结论对于推断巴丹吉林沙漠沉积物的物源有很好的指示意义,同时对于研究巴丹吉林沙漠高大沙山的成因也有一定的线索意义.     

Hydrochemical characteristics and recharge sources of Lake Nuoertu in the Badain Jaran Desert

We present a case study of physical and chemical indicators and isotope characteristics of Lake Nuoertu,one of the largest and deepest lakes of the Badain Jaran Desert.We analyze the concentrations of eight ions,total dissolved solids,and stable isotope composition of the lake and groundwater,as well as radioactive isotope tritium concentration in groundwater.The results show that annual and seasonal variations of the physical and chemical characteristics of Lake Nuoertu water are significantly greater than those of groundwater.The lake is uniformly mixed in the horizontal and vertical directions,and the hydrochemical types of the lake at different depths are consistent for Na–Cl–CO3–(SO4).Stable isotope composition of the lake and groundwater at Nuoertu is distributed along the local evaporation line(EL)slope,which is less than the slope of the global meteoric water line.The comparatively small slope shows the characteristic strong evaporation in the study area.Lake water isotopes are mostly in the upper right corner of the EL,whereas groundwater is mostly in the lower left corner.The main recharge source of Nuoertu lake water is groundwater,in combination with lake water and groundwater level change.The age of tufa springs around Nuoertu is about75–80 a,which shows that the initial recharge source of the lake is a mix between modern and 1952 or older;however,further research is required.     

巴丹吉林沙漠湖泊面积与水位的非线性关系模拟研究

位于内蒙古阿拉善盟的巴丹吉林沙漠是中国第二大沙漠,以其存在世界上最高的沙山以及约100个湖泊而著称.现今湖面海拔一般为1 150~1 200m,前人研究认为在过去3万年以来的某些时期存在更高的湖面.假设沙丘洼地的形态基本保持不变,可以根据现今湖泊洼地的三维形态来模拟高湖面时期的湖泊群分布特征和总面积.以90m分辨率的DEM数据为基础,对若干典型洼地等高面面积随高程的变化进行了分析,发现可以用幂函数和多项式2种非线性方程模拟.多数情况下需要多项式,但一般只要达到3阶就可以精确拟合.对于湖泊洼地,由于很少有湖水最大深度的数据,湖底最低高程往往是未知的.利用湖面以上的等高面数据建立拟合程度最佳的非线性函数可以反推最低高程,但仍然存在不确定性.这一点对恢复历史上的高湖面形态并没有影响.     

Observation of mega-dune evaporation after various rain events in the hinterland of Badain Jaran Desert, China

The formation mechanisms of the mega-dunes and lakes in the hinterland of Badain Jaran Desert,China,is the focus of extensive academic research in the field of geoscience,and an often debated topic is whether atmospheric precipitation on the mega-dune can infiltrate to recharge groundwater.In the present study,the probability distribution functions and the return period analysis of extreme daily precipitation based on long-term precipitation records for the southern margin of Badain Jaran Desert and 2-year observation of hinterland precipitation were used to classify precipitation in the desert region.The data of automatic weather station and eddy covariance system in the desert hinterland were used to analyze evaporation on the mega-dune surface after various rain events.The results showed that the rain events in the desert could be divided into three categories.The first is conventional precipitation(CP),which is below 5 mm,accounting for roughly 90%of all rain events in the desert.The second and third categories are ordinary annual maximum(OAM)and extreme precipitation(EP),in which precipitation is roughly 20 mm and more than 40 mm,respectively.The atmospheric precipitation of CP and OAM evaporated from the megadune surface in 1–3 days and 3–4 weeks,respectively.Following an EP event,the mega-dune evaporation was negatively influenced by the upper dry sand layer,and a lengthy period was required for its complete removal.The accumulative evaporation and accumulative precipitation of all three types of rain events indicated that local atmospheric precipitation had no significant contribution to recharging the groundwater system in the hinterland of Badain Jaran Desert.This research will benefit comprehensive elucidation of the formation mechanism of lakes in the hinterland of Badain Jaran Desert.     

Water resources: groundwater maintains dune landscape

The Badain Jaran desert in western Inner Mongolia in China has a unique landscape that contains 72 lakes, with a total water surface area of 23 km2, and the world's highest stationary sand dunes, which are up to 500 m tall--despite the prevailing dry and windy weather conditions. Here we present evidence of a major groundwater system that underpins the factors leading to this landscape. Our finding could transform plans for the region's water resources.     

基于遥感的长时间序列下巴丹吉林沙漠湖泊演化分裂特征分析

巴丹吉林沙漠内湖泊已存在达千年尺度,其地表残留的湖泊演化分裂特征可部分反映地下水变化规律。以往研究多依赖水化学特征的解释,本次基于国产高分卫星数据,从遥感的角度对巴丹吉林沙漠内124个古湖的演化分裂特征进行提取。研究区湖泊水体分裂特征模拟结果显示,巴丹吉林沙漠内湖泊在长时间尺度下的演变分裂特征明显、分类聚合形式显著,宏观上各演化分裂特征区间由东南向西北展布,但与研究区地形变化趋势不完全相同,二者在局部存在显著差异。     

A mid－Holocene drought interval as evidenced by lake desiccation in the Alashan Plateau,Inner Mongolia,China

The mid-Holocene in China is traditionally thought to be a warm and humid period with a strong sum-mer monsoon, and is often termed the Holocene Climatic Optimum or Megathermal Period. Here we present lakegeomorphologic and lithologicai evidence from the Alashan Plateau, part of the Mongolian Plateau, that indicates stronglake desiccation during the mid-Holocene. High resolution pollen data from Zhuyeze Lake, at the present summermonsoon margin, is also presented. These data show that present lakes and wetlands in the Juyanze Lake basin west of the Badain Jaran desert, in the Zhuyeze Lake basin between the Badain Jaran and Tengger deserts, and in lakes in the eastern Tengger desert, dried or experienced low lake levelsin the mid-Holocene around 5000-7000 cal yr BP. Pollen data further indicate that the vegetation cover declined in both the local areas and in the Qilian Mountains, suggesting the climate was drier than that associated with the presentAsian summer monsoon. This mid-Holocene drought interval was present throughout a quite large region of the south In-ner Mongolian Plateau. The period was also probably colder,at least in the high Asian plateaus and mountains.     

AMT法勘查额济纳盆地与巴丹吉林沙漠接触关系

 查明额济纳盆地东部与巴丹吉林沙漠之间的接触关系,有助于分析额济纳盆地地下水水位下降、生态环境恶化的原因。通过当地地质资料和物探方法的对比分析,选择合理有效的音频大地电磁法（AMT）物探手段,在该区域进行了原始创新性的勘查工作。勘查结果认为额济纳盆地上升、巴丹吉林沙漠下降,两侧地层岩性厚度变化较大,推断额济纳盆地与巴丹吉林沙漠为张性断层接触。研究可为分析两者之间地下水转化关系和额济纳盆地地下水水位下降原因、探讨盆地与沙漠区地下水形成机制提供依据,对恢复治理额济纳盆地生态环境具有一定帮助。     

Oxygen and carbon isotope records of calcareous nannofossils for the past 1 Ma in the southern South China Sea

Oxygen and carbon isotopic analyses have been carried out on calcareous nannofossils from ODP Site 1143 in the southern South China Sea. The results demonstrate that the δ18O values of calcareous nannofossils for the past 1 Ma vary systematically with those of planktonic and benthic foraminifera from the same site. But the average δ18O value of nannofossils is 1.869‰ higher than that of planktonic foraminifera, and is 3.855‰ lower than that of benthic foarminifera. There are systematic differences between the δ13C values of nannofossils, planktonic foarminifera and benthic foraminifera. The average δ13C of nannofossils is 0.756‰ higher than that of benthic foraminifera, and is 0.460‰ lower than that of planktonic foraminifera. The δ13C values of nannofossils exhibit a significant positive shift beginning near isotope stage 14 and lasting until stage 8. This study also finds that there is a close ralationship between the δ13C of nannofossils and the percentage of Florisphaera profunda. This implies that the δ13C of nannofossils can be used as an indicator of sea water surface paleoproductivity.     

Oxygen isotope variation in the water cycle of the Yamzho lake Basin in southern Tibetan Plateau

Given the potential use of stable isotope in the paleoclimate reconstruction from lacustrine records as well as in the local hydrology cycle, it is crucial to understand the processes of stable isotope evolution in catchment in the Tibetan Plateau region. Here we present a detailed study on the water oxygen isotope based on 2 years observation including precipitation, river water and lake water in the Yamzho Lake, south of the Tibetan Plateau. Temporal variation of local precipitation 5180 shows an apparent ＂monsoon cycle＂. In monsoon season, 5180 in waters is lower. In non-monsoon season, δ^18O in precipitation and lake water is higher and higher river δ^18O exists in spring, probably reflecting the effect of land surface evaporation, together with the higher δ^18O values in spring precipitation. It is also found that the surface lake water δ^18O varies seasonally and annually. The lower lake water δ^18O in the late summer is apparently related to the summer monsoon precipitation. The mean δ^18O value of lake water in 2007 is 1.2‰ higher than that in 2004, probably due to the less monsoon precipitation in summer of 2007, as can be confirmed from the precipitation data at the Langkazi meteorological data. It is also found that an obvious shift of vertical lake water δ^18O reflects the fast mixture of lake water. δ^18O values of lake water are over 10‰ higher than those of precipitation and river water in this region due to the evaporation fractionation. The modeled results show that the evaporation process of the lake water is sensitive to relative humidity, and the present lake water δ^18O reflects a relative humidity of 51% in the Yamzho Lake. It shows that the lake will take 30.5 years to reach present lake water δ^18O given a large shift in the input water δ^18O. The modeled results also reveal that surface lake water temperature and inflow δ^18O have slight effect on the isotopic balance process of lake water in the Yamzho Lake.     

远源地下水入侵阿拉善高原沙漠:酸溶物Sr同位素证据

选取北方西－东向气候剖面上物质组成相似的四个沙漠（塔克拉玛干、巴丹吉林、腾格里和毛乌素）,测定其地表沙酸溶物87Sr/86Sr比值、Ca2+和Sr2+浓度。Ca2+和Sr2+浓度相关性极好,在降雨量低的塔克拉玛干沙漠中最高,向东逐渐降低,在降雨量多的毛乌素沙漠中最低。降雨量是控制阳离子浓度变化的主导因素。酸溶物87Sr/86Sr比值在四个沙漠中有不同于阳离子变化的区域特征。塔克拉玛干沙漠酸溶物87Sr/86Sr比值在0.71以下,巴丹吉林和腾格里沙漠的大于0.7112,而毛乌素沙漠的在0.71-0.7112之间变化。塔克拉玛干和毛乌素沙漠地表沙酸溶物87Sr/86Sr比值仅受气候变化的制约。巴丹吉林和腾格里沙漠除了受气候因素影响外还受其它因素的制约,最可能的就是远源地下水。经计算,远源地下水对巴丹吉林沙漠的影响份额为47%,对腾格里沙漠为44%,接近气候因素的影响。     

Groundwater recharge and climatic change during the last 1000 years from unsaturated zone of SE Badain Jaran Desert

The history of groundwater recharge and cli-matic changes during the last 1000 years has been estimated and reconstructed using environmental chloride from un-saturated zone profde in the southeast Badain Jaran Desert,NW China. By using a steady-state model for duplicate un-saturated zone chloride profiles, the long-term recharge at the site was estimated to be 1.3 mm yr^-1. From one profde,which reached the water table, the climatic change events of 10-20 years duration were well preserved. There were 3 wet phases and 4 dry episodes during the recent 800 years ac-cording to the peaks and troughs of recharge rate calculated via chloride concentration and moisture content. There was a dry episode before 1290 AD. At ca. 1500-1530 AD, which is an important date, there was an abrupt change from drought to wet conditions. At the beginning of the 1800s, local climate changed from wet to dry occurred and subsequently dete-riorated over the past 200 years,The unsaturated profile was compared with the Guliya ice core records.The agreement of wet and dry phases from 1200 to 1900 AD is quite good,whilst trends diverged during the last 100 years.It seems that the large-scale climate difference took place between mountain regions and the desert basin in NW China during the 20th century,which closely correspond to the water table reduction of some l metre.     

Holocene climatic and environmental changes recorded in Baahar Nuur Lake core in the Ordos Plateau, Inner Mongolia of China

A 5.3 m lake core was drilled in Baahar Nuur Lake in the Ordos Plateau, and measurements of meangrain size, organic δ 13C (δ 13Corg), organic carbon content (TOC), C/N, carbonate content, carbonate δ 13C(δ 13Ccar) and δ 18O (δ 18Ocar) were conducted for retrieving the Holocene chronosequence of climaticchanges based on 15 AMS 14C dates. The record documented four major stages of climate change inthe Ordos Plateau: (IV) a cold and dry condition before ～7.65 14C ka BP; (III) a warm and humid stagebetween ～7.65 and ～5.40 ka BP; (II) a generally drier and cooler climate since ～5.40 ka BP with twohumid events occurring from ～4.70 to ～4.60 ka BP and from ～4.20 to ～3.70 ka BP, and (I) a dry climatecharacterized by complete desiccation of the lake after 3.70 ka BP. Stage III can be further divided intothree sub-stages: (IIIa) a warm and humid episode from ～7.65 to ～6.70 ka BP, (IIIb) a warm and relativelydry episode from ～6.70 to ～6.20 ka BP, and (IIIc) the magthermal and maghumid episode of the Holo-cene from ～6.20 to ～5.40 ka BP.     

δ^13C and water-use efficiency indicated by δ^13C of different plant functional groups on Changbai Mountains,Northeast China

Leaf δ^13C of different plant functional groups （trees, shrubs and forbs; evergreen and deciduous; annual, biennial and perennial） were examined on the Changbai Mountains, China. Life form has a significant influence on plant δ^13C, suggesting that leaf δ^13C is also ideal for distinguishing functional groups species in temperate and frigid zones with high humidity. Additionally, the difference of water-use efficiency （WUE） is significant among different plant functional groups. δ^13C and WUE are in the following order of forbs 〈 shrubs 〈 trees. Within shrubs, δ^13C and WUE are higher for evergreen shrubs than deciduous shrubs. Differences in δ^13C and WUE are significant among different lifespan herbs, and δ^13C and WUE follow the order of annual herbs 〉 biennial herbs 〉 perennial herbs, not in accordance with the pattern obtained by previous studies in deserts, suggesting that the ranking of δ^13C and WUE among annual, biennial and perennial herbs may be dependent on local water availability.     

Study of altitudinal lapse rates of δ18O in precipitation/river water with seasons on the southeast Tibetan Plateau

Seasonal δ^18O variation in water on the southeast Tibetan Plateau has been studied, showing the consistent variation pattern of δ^18O with altitude indicative of relevant atmospheric circulation processes. Study shows a similar variation pattern of fixed-site river water δ^18O with that of the precipitation δ^18O in southeast Tibet. δ^18O in regional rivers in southeast Tibet demonstrates a gradual depletion with increasing altitude, though the rates vary seasonally. The most depleted river ^18O occurs during the monsoon period, with the lowest δ^18O/altitude lapse rate. The river ^18O during the westerly period is also depleted, together with low δ^18O/altitude lapse rate. The pre-monsoon rivers witness the most enriched ^18O with least significant correlation coefficient with the linear regression, whilst the postmonsoon rivers witness the largest δ^18O/altitude lapse rate. Different coherence of seasonal δ^18O variation with the altitude effect is attributed to different moisture supplies. Though sampling numbers vary with seasons, the δ^18O-H linear correlation coefficients all reach the 0.05 confidence level, thus witnessing the variation features of δ^18O in seasonal river water due to the influence of atmospheric general circulation and land surface processes revealed from the altitudinal lapse rates.     

Foliar δ^13C and δ^15N values of C3 plants in the Ethiopia Rift Valley and their environmental controls

The foliar C and N stable isotopic compositions （δ^13C and δ^15N） and the relationships between these compositions and environmental factors of C3 plants in the Ethiopia Rift Valley were investigated. There were three distribution patterns for foliar δ^13C with mean values of -26.7‰±0.4‰, -29.7‰ ±0.6‰ , and -26.9‰± 1.2‰ in cold-moist, temperate-moist, and arid-hot environments, respectively. The δ^15N values ranged from -1.4‰ ±1.7‰ to 14.3‰ ± 0.1‰, with higher values under arid-hot conditions and the lowest values in plants growing at higher altitudes under cold-moist conditions. A strong negative relationship between mean annual precipitation and δ^15N explained more than half of the observed variation in the δ^15N values （r2= 0.54, P 〈 0.001）; a modest positive relationship was also found between δ^15N and temperature （r2 = 0.32, P 〈 0.01）. A weakly positive relationship existed between δ^13C and temperature, and changes in δ^13C values with precipitation and altitude followed quadratic curves. This suggests a shift in the effects of water and heat conditions caused by altitude on carbon isotopic discrimination.     

Landscape evolution and precipitation changes in the Badain Jaran Desert during the last 30 000 years

On the basis of field expeditions into different parts of the Badain Jaran Desert, new evidence of landscape evolution and precipitation changes during the last 30 000 years has been found from the geomorphological, sedimentological and geochronological studies of the megadune. Different cemented surfaces of palaeo-dunes have been observed in many localities, which are representatives of relatively moister environment. The chronological data show that four periods of more humid environment occurred in the past 30 000 years. The stabilization of dunes in a large scale and the existence of many palaeo-lakes and lacustrine terraces confirm the periodic fluctuations of climate. It implies that the intensity of westerlies and Eastasian monsoons varied in the region of Alashan, and the periods of higher precipitation took place even in the times close to the last glacial maximum.     

The altitude effect of δ 18O in precipitation and river water in the Southern Himalayas

The lapse rate of water isotopes is used in the study of the hydrologic cycle as well as in the estimation of uplift of the Tibetan Plateau.The greater elevation contrast in the Southern Himalayas allows for a detailed discussion about this lapse rate.We analyze variations of 18δO in precipitation and river water between 1320 m and 6700 m elevations in the Southern Himalayas,and calculate the specific lapse rate of water 18δO.The results show that the multi-year average lapse rate in precipitation over this region is 0.15‰/100 m.The one-year average lapse rate is 0.17‰/100 m from three sites along the Southern Himalayas.The two results agree,but are much lower than the global average of 0.28‰/100 m.This work also shows that there is a difference in precipitation 18δO lapse rate between the monsoon and non-monsoon seasons.The calculated precipitation lapse rate is much lower than that in surface water.     

Variability of snail growing season at the Chinese Loess Plateau during the last 75 ka

Knowledge of seasonal climate change is one of the key issues facing Quaternary paleoclimatic studies and estimating seasonal climate change is difficult,especially changes such as seasonal length on glacial-interglacial timescales.The stable isotope composition from seasonal land snail shells provides the potential to reveal seasonal climatic features.Two modern land snail species,cold-aridiphilous Pupilla aeoli and thermo-humidiphilous Punctum orphana,were collected from different climatic zones in 18 localities across the Chinese Loess Plateau,spanning 11 degrees of longitude and covering a range of 1000 km2.The duration of the snail growing season(temperature ≥10℃) was shorter(202 ± 6 d) in the eastern Loess Plateau compared with in the western Loess Plateau(162 ±7 d).The δ13C of P.aeoli shells was ?9.1‰ to ?4.7‰ and ?5.0‰ to 0.3‰ for δ18O.For P.orphana,the δ13C ranged from ?9.1‰ to ?1.9‰ and ?8.9‰ to ?2.9‰ for δ18O.Both the δ13C and δ18O differences between the two snail species were reduced from the east to the western Loess Plateau(2.8‰ to 0.2 ± 1.1‰ for δ13C and 4.7‰ to 2.9 ± 1.3‰ for δ18O).These isotopic differences roughly reflect the difference in the growing season lengths between the east and west Loess Plateau indicating that the duration of the snail growing season shortens by 15 d or 19 d if the difference decreases by 1‰ in δ13C or δ18O,respectively.Thus,the difference in δ13C and δ18O between both snail species can be used to reveal the length of the snail growing season in the past.Based on our investigation,the length of the snail growing seasons from the Xifeng region during the last 75 ka was reconstructed.During the mid-Holocene(8-3 ka),the mean isotopic difference from both snail species reached maximum values of 2.6 ± 0.7‰ and 2.1 ± 1.4‰ for δ13C and δ18O,respectively.This was followed by MIS 3 that ranged from 2.5 ± 0.4‰ for δ13C and 1.6 ± 0.8‰ for δ18O.The Last Glacial Maximum changed by only 0.2‰ and 0.4‰ for δ13C and δ18O,respectively.Therefore,we estimate that the duration of the snail growing seasons to be ～200 ± 10 d during the mid-Holocene,190 ± 6 d in MIS 3 and 160 ± 3 d during the last glacial period.