Megafossils of Betulaceae from the Oligocene of Qaidam Basin and their paleoenvironmental and phytogeographic implications

Understanding the paleoenvironment and phytogeographical history of the Tibetan Plateau,China relies on discovering new plant fossils.The Qaidam Basin has long been regarded as an ideal‘field laboratory’to investigate the paleoclimate and paleobiological evolution of the northern Tibetan Plateau.However,fossil angiosperms from the Qaidam Basin are rare,and our knowledge of its paleovegetation is poor.Here,we report fossil leaves and fruits of Betulaceae found from the Oligocene Shangganchaigou Formation of northwestern Qaidam Basin(Huatugou area).Comparative morphological analysis led us to assign the fruits to the Betula subgenus Betula and the leaves to Carpinus grandis.These findings,together with other reported fossil plants from the same locality,reveal a close floristic linkage between the Qaidam Basin and Europe during the Oligocene.The northern pathway of this floristic exchange may have crossed through the Qaidam Basin during the late Paleogene.This floristic linkage may have been facilitated by the continuous narrowing of the Turgai Strait and stronger westerlies,which transported moisture and provided favorable climatic conditions.Indeed,fossil plants collected from the Qaidam Basin suggest that during the Oligocene this region had warm and humid deciduous broad-leaf forest,which differs from the region’s modern vegetation and indicates that the Qaidam Basin may have been a suitable region for these plants to flourish and spread during the Oligocene.

Application of 9-component S-wave 3D seismic data to study sedimentary facies and reservoirs in a biogasbearing area:A case study on the Pleistocene Qigequan Formation in Taidong area,Sanhu Depression,Qaidam Basin,NW China

To solve the problems in restoring sedimentary facies and predicting reservoirs in loose gas-bearing sediment,based on seismic sedimentologic analysis of the first 9-component S-wave 3D seismic dataset of China,a fourth-order isochronous stratigraphic framework was set up and then sedimentary facies and reservoirs in the Pleistocene Qigequan Formation in Taidong area of Qaidam Basin were studied by seismic geomorphology and seismic lithology.The study method and thought are as following.Firstly,techniques of phase rotation,frequency decomposition and fusion,and stratal slicing were applied to the 9-component S-wave seismic data to restore sedimentary facies of major marker beds based on sedimentary models reflected by satellite images.Then,techniques of seismic attribute extraction,principal component analysis,and random fitting were applied to calculate the reservoir thickness and physical parameters of a key sandbody,and the results are satisfactory and confirmed by blind testing wells.Study results reveal that the dominant sedimentary facies in the Qigequan Formation within the study area are delta front and shallow lake.The RGB fused slices indicate that there are two cycles with three sets of underwater distributary channel systems in one period.Among them,sandstones in the distributary channels of middle-low Qigequan Formation are thick and broad with superior physical properties,which are favorable reservoirs.The reservoir permeability is also affected by diagenesis.Distributary channel sandstone reservoirs extend further to the west of Sebei-1 gas field,which provides a basis to expand exploration to the western peripheral area.

Salt-Lake Basin Bedrock Weathered Crust Gas Reservoir in the Altun Mountains Front of the Qaidam Basin,Western China

The bedrock weathered crust in front of the Altun Mountains in the Qaidam Basin,western China,is different from others because this is a salt-lake basin,where saline water fluid infiltrates and is deposited in the overlying strata.A large amount of gypsum infills the bedrock weathered crust,and this has changed the pore structure.Using core observation,polarized light microscopy,electron probe,physical property analysis and field emission scanning electron microscopy experiments,the characteristics of the weathered bedrock have been studied.There are cracks and a small number of dissolved pores in the interior of the weathered crust.Matrix micropores are widely developed,especially the various matrix cracks formed by tectonics and weathering,as well as the stress characteristics of small dissolved pores,and physical properties such as porosity and permeability.This‘dual structure’developed in the bedrock is important for guiding the exploration of the lake basin bedrock for natural gas.

Genesis of Neogene Formation Waters in the Central Qaidam Basin:Clues from Hydrochemistry and Stable D-O-S-Sr Isotopes

Geological explorations have revealed plentiful Neogene formation waters in anticlines in the central Qaidam Basin(QB).However,the hydrochemistry and origin of these waters are obscure.In this study,the hydrochemistry and DO-S-Sr isotopes of these formation waters were determined to study their origin and evolution.The formation waters are enriched in Na-Ca-Cl,and depleted in Mg-K-SO4-HCO3ions with elevated Li-B-Br-Sr elements.The D-O isotopes prove that the formation waters originated from weak-evaporated meteoric waters,and experienced water-rock interactions.Ion comparisons and Caexcess-Nadeficitdiagrams suggest that solute sources of these waters include evaporite dissolution,waterrock interaction,and minor residual lake brines.Bacterial sulfate reduction and water-rock interactions are supported by the high S-Sr isotopes.The enriched Li-B-Br-Sr concentrations of these waters are in accord with the high geochemical background values of the QB.Regarding the genesis of the formation waters,it can be concluded that meteoric waters from the southern Kunlun Mountains were discharged into the basin,weakly evaporated,and then infiltrated into the Neogene strata through faults leaching the soluble ions and mixing with residual lake brines,and all experienced water-rock interactions and a sulphate reduction process.

Influencing mechanism and hydrogeological implications of water level fluctuation of lakes in the northern Qaidam Basin,Qinghai-Tibet Plateau

The Qaidam Basin(QB)is a concentrated distribution area and chemical industrial bases of salt lakes in China.Lakes in the QB have been expanding during the past 20 years.Rapid lake expansion resulted in some considerable scientific issues on the protection of salt lake resources and infrastructure,and monitoring of hydrological processes at the lake-basin scale.Although the spatial-temporal trends of lake changes on the Qinghai-Tibet Plateau(QTP)have been well documented,the underlying influencing mechanism and hydrogeological implications of rapid lake changes in the QB are not well understood.Three lakes in the northern QB were selected to investigate lake water level fluctuations on different time scales based on extensive in-situ monitoring and satellite observations.The influencing mechanism and hydrogeological implications of rapid changes of terminal lakes were discussed in combination with the reported increasing precipitation rate and mass balance of glaciers in the northern QTP.Results reveal the following:(1)the fluctuation pattern of Sugan Lake was asynchronous and out of phase with that of Xiao Qaidam and Toson lakes during the monitoring period;(2)Sugan Lake water rose gradually,and the rise interval was from late April to early July.In contrast,Xiao Qaidam and Toson lakes took on a rapid and steep rise,and the rise intervalwas from late July to September;(3)the influencing mechanisms for rapid lake fluctuations are controlled by different factors:glacier and snow melting with increasing temperature for Sugan Lake and increasing precipitation for Xiao Qaidam and Toson lakes;(4)in accordance with different intervals and influencing mechanisms of rapid lake expansions in the QB,hydrological risk precaution of lakes and corresponding river catchments was conducted in different parts of the basin.This study provided an important scientific basis for assessing the hydrological process and hydrological risk precaution,and protection of salt lake resources along with rapid lake expansions in the arid area.

Development,sand control mechanism and hydrocarbon accumulation of beach-bar sandstone in a saline lake basin:A case from the Neogene of southwestern Qaidam Basin,NW China

Based on the data of field outcrops,drilling cores,casting thin sections,well logging interpretation,oil/gas shows during drilling,and oil/gas testing results,and combined with modern salt-lake sediments in the Qinghai Lake,the Neogene saline lake beach-bars in southwestern Qaidam Basin are studied from the perspective of sedimentary characteristics,development patterns,sand control factors,and hydrocarbon accumulation characteristics.Beach-bar sand bodies are widely developed in the Neogene saline lake basin,and they are lithologically fine sandstone and siltstone,with wavy bedding,low-angle cross bedding,and lenticular-vein bedding.In view of spatial-temporal distribution,the beach-bar sand bodies are stacked in multiple stages vertically,migratory laterally,and extensive and continuous in NW-SE trending pattern in the plane.The stacking area of the Neogene beach-bar sandstone is predicted to be 3000 km^(2).The water salinity affects the sedimentation rate and offshore distance of beach-bar sandstone,and the debris input from the source area affects the scale and enrichment of beach-bar sandstone.The ancient landform controls the morphology and stacking style of beach-bar sandstone,and the northwest monsoon driving effect controls the long-axis extension direction of beach-bar sandstone.The beach-bars have a reservoir-forming feature of“one reservoir in one sand body”,with thick beach-bar sand bodies controlling the effective reservoir distribution and oil-source faults controlling the oil/gas migration and accumulation direction.Three favorable exploration target zones in Zhahaquan,Yingdong-eastern Wunan and Huatugou areas are proposed based on the analysis of reservoir-forming elements.

Low organic matter abundance and highly efficient hydrocarbon generation of saline source rock in the Qaidam Basin,NW China

The geochemical analysis and experimental simulation are comprehensively used to systematically study the hydrocarbon generation material,organic matter enrichment and hydrocarbon generation model of Paleogene source rock in the Western Qaidam Depression,Qaidam Basin,NW China.Three main factors result in low TOC values of saline lacustrine source rock of the Qaidam Basin:relatively poor nutrient supply inhibits the algal bloom,too fast deposition rate causes the dilution of organic matter,and high organic matter conversion efficiency causes the low residual organic carbon.For this type of hydrogen-rich organic matter,due to the reduction of organic carbon during hydrocarbon generation,TOC needs to be restored based on maturity before evaluating organic matter abundance.The hydrocarbon generation of saline lacustrine source rocks of the Qaidam Basin is from two parts:soluble organic matter and insoluble organic matter.The soluble organic matter is inherited from organisms and preserved in saline lacustrine basins.It generates hydrocarbons during low-maturity stage,and the formed hydrocarbons are rich in complex compounds such as NOS,and undergo secondary cracking to form light components in the later stage;the hydrocarbon generation model of insoluble organic matter conforms to the traditional“Tissot”model,with an oil generation peak corresponding to Ro of 1.0%.

Establishment of an astronomical time scale for the Shizigou Formation in the Qaidam Basin,Inner Asia and orbital forced evolution of lakes during The Pliocene

The Qaidam Basin,as the largest inland basin within the Tibetan Plateau,has accumulated more than 10,000 m of Cenozoic continental sediments.It serves as a crucial research area for documenting Cenozoic climate changes and plateau uplift processes in the Asian interior.Additionally,the basin holds vast reserves of oil and gas resources,making high-resolution drilling data invaluable for studying paleoclimate.In this study,the longsequence lacustrine deposits of JS1 drill core across the Shizigou Formation in the Yiliping Depression at the western center of the basin were studied,aiming to establish an astronomical timescale for the Shizigou Formation and investigate the characteristics of paleoclimatic changes during the late Miocene to the Pliocene for the Asian interior.The analysis was carried out using high-resolution natural gamma ray(GR)data sequences,employing techniques such as spectral analysis,filtering,and wavelet analysis in cyclostratigraphy.The results indicated the presence of a stable Milankovitch orbital signal was perfectly recorded in the Shizigou Formation,primarily influenced by eccentricity cycles,with weaker obliquity and precession cycles.Using the stable and continuous 405 ka eccentricity cycle in astronomical tuning,a"floating"astronomical timescale with a duration of 6.1 Ma for the Yiliping depression's Shizigou Formation has been established.With reference to previously established stratigraphic age anchor points,an absolute astronomical timescale(2.5–8.6 Ma)has been ultimately provided for the Shizigou Formation.Simultaneously,a clear 100 ka short eccentricity cycle record has been identified during the Pliocene(5.3–2.5 Ma),which corresponds in time with the aridification within the basin during this Pliocene period.In addition,a comparison of the Pliocene natural gamma ray curve of the Qaidam Basin with global ice volume variations indicated that the basin's aridification was influenced by global cooling,with eccentricity-modulated precession cycles controlling solar radiation and subsequently affecting the evolution of lakes in the arid region of Inner Asia.

Paleogene and Early Neogene Lacustrine Reefs in the Western Qaidam Basin, China

Typical reefs in the Paleogene and early Neogene strata of the Qaidam Basin, Tibetan Plateau, China, reveal their internal structures and sedimentation environments and consist mainly of algal reef, stromatolite reef and thrombolite reef with distinct reef structures, fore-reef, back-reef and reef-plateau. The fore-reef is characterized by a combination of pinnacle reef, thrombolite and algal reef. The back reef is composed of stromatolite reef and algal reef. The pinnacle reefs (micro-atoll), most of which are several tens of centimeters in diameter (whereas some exceptionally big ones are over 200 cm in diameter), and several tens of centimeter to 2 m in height, are situated on the far front-edge of the reef; the pinnacle reef is also often of recumbent form with a gravel-filled circular hole in the center. The algal reef is in the form of dome and irregular beds, and filled with algal detritus, ostracodes, spirorbis fossils, ooid and terrigenous debris, and worm traces; cavities and scour marks are often developed. The algal reef is gray commonly when fresh and weathers to a brown color. The lacustrine thrombolite in the Qaidam Basin is light gray or deep gray when fresh, white-gray or brown when weathered, dense and homogeneous with abundant pores filled by oil and bitumen. Observed under the microscope, the thrombolite consists mainly of brown or brown-black clots with a little algal debris, ooid, pellet, ostracodes, spirorbis fossils and terrigenous debris, in some cases, terrigenous debris, even gravel, is abundant. Many features of the thrombolite suggest that it is formed in a high-energy envkonment. The stromatolite reefs developed on the lacustrine algal reef in the Qaidam Basin are very complex whether in shape or in internal structure. The simplest ones form laminated layers and the most complex ones have intensely branching structures. The size is also variable.

Geochemical characterization of aromatic hydrocarbons in crude oils from the Tarim,Qaidam and Turpan Basins,NW China

Based on the systematic study of aromatic hydrocarbons in over 100 crude oil samples collected from the Tabei and Tazhong uplifts in the Tarim Basin,the western depression area in the Qaidam Basin and the Tabei depression in the Turpan Basin,the geochemical characteristics of the marine（Tarim Basin）,saline lacustrine（Qaidam Basin）,and swamp（Turpan Basin） oils were investigated.The marine oils from the Tarim basin are characterized by relatively low abundance of diaromatic hydrocarbons such as biphenyl and naphthalene,and relatively high abundance of triaromatic hydrocarbons including phenanthrene,dibenzothiophene and fluorene.In contrast,the swamp oils from the Turpan Basin are dominated by the highest relative abundance of diaromatic hydrocarbons and the lowest relative abundance of triaromatic hydrocarbons in all the oil samples in this study.The relative abundance of diaromatic and triaromatic hydrocarbons in the saline lacustrine oils from Qaidam Basin is between that in Tarim oils and Turpan oils.Aromatic parameters based on the isomer distributions of dimethylnaphthalenes（DMN）,trimethylnaphthalenes（TMN）,tetramethylnaphthalenes（TeMN） and methylphenanthrenes（MP）,i.e.,1,2,5-trimethylnaphthalene（TMN）/1,3,6-TMN ratio,1,2,7-TMN/1,3,7TMN ratio,（2,6-＋2,7-）-dimethylnaphthalenes（DMN）/1,6-DMN ratio,1,3,7-TMN/（1,2,5-＋1,3,7-）TMN,1,3,6,7-TeMN/（1,3,6,7-＋1,2,5,6-＋1,2,3,5-）-TeMN ratio and MP index,may reflect the diversity of organic source input,thermal maturity and depositional environments.In addition,the dibenzothiophenes（DBTs）/fluorenes（Fs） and dibenzofurans（DBFs）/Fs ratios were found to the very useful and effective in determining genetic types of crude oils for the marine,saline lacustrine,and swamp depositional environments,and for oil-oil correlations.

Magnetostratigraphy and Anisotropy of Magnetic Susceptibility of the Lulehe Formation in the Northeastern Qaidam Basin

The timing of onset of deposition of the Lulehe Formation is a significant factor in understanding the genesis of the Qaidam basin and the evolution of the Tibetan Plateau. Here, we describe a detailed magnetostratigraphic and magnetic fabric study of the middle and lower parts of the Lulehe Formation. A total of 234 samples were collected from 117 sites throughout a thickness of almost 460 m of fluvial and lacustrine deposits at the Xitieshan section in the northeastern Qaidam basin. Out of these sites, 94 sites yielded well-defined characteristic remanent magnetization components by stepwise thermal demagnetization and were used to establish the magnetostratigraphy of the studied section. Based on correlation with the geomagnetic polarity timescale, the studied section spans the period from 53.8 Ma to 50.7 Ma. Our results show a three-fold decrease in sedimentation rates as well as marked change in facies from braided river to delta and shore-shallow lake around 52.6 Ma, which suggests tectonic uplift of the northeastern Qaidam basin margin ridge was rapid at the onset of formation of the Qaidam basin and subsequently weakened after 52.6 Ma. The anisotropy of magnetic susceptibility results indicate that tectonic compression stress had reached the northeastern Tibetan Plateau by the early stages of Indo-Eurasian plate collision and that the direction of stress in the study area was NE-SW. Furthermore, a weakening of tectonic compression stress around 52.6 Ma is consistent with sedimentary records. The age of initial deposition of the Qaidam basin （around 53.8 Ma） was almost synchronous with that of the Qiangtang, Hoh Xil, Xining, and Lanzhou basins, which implies that stress was transferred rapidly through the Tibetan Plateau during or immediately after the onset of Indo-Eurasian collision.

Paleozoic and Mesozoic Basement Magmatisms of Eastern Qaidam Basin,Northern Qinghai-Tibet Plateau:LA-ICP-MS Zircon U-Pb Geochronology and its Geological Significance

The eastern margin of the Qaidam Basin lies in the key tectonic location connecting the Qinling, Qilian and East Kunlun orogens. The paper presents an investigation and analysis of the geologic structures of the area and LA-ICP MS zircon U-Pb dating of Paleozoic and Mesozoic magmatisms of granitoids in the basement of the eastern Qaidam Basin on the basis of 16 granitoid samples collected from the South Qilian Mountains, the Qaidam Basin basement and the East Kunlun Mountains. According to the results in this paper, the basement of the basin, from the northern margin of the Qaidam Basin to the East Kunlun Mountains, has experienced at least three periods of intrusive activities of granitoids since the Early Paleozoic, i.e. the magmatisms occurring in the Late Cambrian （493.1±4.9 Ma）, the Silurian （422.9±8.0 Ma-420.4±4.6 Ma） and the Late Permian-Middle Triassic （257.8±4.0 Ma-228.8＋1.5 Ma）, respectively. Among them, the Late Permian - Middle Triassic granitoids form the main components of the basement of the basin. The statistics of dated zircons in this paper shows the intrusive magmatic activities in the basement of the basin have three peak ages of 244 Ma （main）, 418 Ma, and 493 Ma respectively. The dating results reveal that the Early Paleozoic magmatism of granitoids mainly occurred on the northern margin of the Qaidam Basin and the southern margin of the Qilian Mountains, with only weak indications in the East Kunlun Mountains. However, the distribution of Permo-Triassic （P-T） granitoids occupied across the whole basement of the eastern Qaidam Basin from the southern margin of the Qilian Mountains to the East Kunlun Mountains. An integrated analysis of the age distribution of P-T granitoids in the Qaidam Basin and its surrounding mountains shows that the earliest P-T magmatism （293.6-270 Ma） occurred in the northwestern part of the basin and expanded eastwards and southwards, resulting in the P-T intrusive magmatism that ran through the whole basin basement. As the Cenozoic basement thrust system developed in the eastern Qaidam Basin, the nearly N-S-trending shortening and deformation in the basement of the basin tended to intensify from west to east, which went contrary to the distribution trend of N-S-trending shortening and deformation in the Cenozoic cover of the basin, reflecting that there was a transformation of shortening and thickening of Cenozoic crust between the eastern and western parts of the Qaidam Basin, i.e., the crustal shortening of eastern Qaidam was dominated by the basement deformation （triggered at the middle and lower crust）, whereas that of western Qaidam was mainly by folding and thrusting of the sedimentary cover （the upper crust）.

Cenozoic uplift of the Tibetan Plateau:Evidence from the tectonic-sedimentary evolution of the western Qaidam Basin

Geologists agree that the collision of the Indian and Asian plates caused uplift of the Tibet Plateau. However, controversy still exists regarding the modes and mechanisms of the Tibetan Plateau uplift. Geology has recorded this uplift well in the Qaidam Basin. This paper analyzes the tectonic and sedimentary evolution of the western Qaidam Basin using sub-surface seismic and drill data. The Cenozoic intensity and history of deformation in the Qaidam Basin have been reconstructed based on the tectonic developments, faults growth index, sedimentary facies variations, and the migration of the depositional depressions. The changes in the sedimentary facies show that lakes in the western Qaidam Basin had gone from inflow to still water deposition to withdrawal. Tectonic movements controlled deposition in various depressions, and the depressions gradually shifted southeastward. In addition, the morphology of the surface structures in the western Qaidam Basin shows that the Cenozoic tectonic movements controlled the evolution of the Basin and divided it into （a） the southern fault terrace zone, （b） a central Yingxiongling orogenic belt, and （c） the northern fold-thrust belt; divided by the XI fault （Youshi fault） and Youbei fault, respectively. The field data indicate that the western Qaidam Basin formed in a Cenozoic compressive tectonic environment caused by the India--Asia plate collision. Further, the Basin experienced two phases of intensive tectonic deformation. The first phase occurred during the Middle Eocene--Early Miocene （Xia Ganchaigou Fm. and Shang Ganchaigou Fro., 43.8- 22 Ma）, and peaked in the Early Oligocene （Upper Xia Ganchaigou Fro., 31.5 Ma）. The second phase occurred between the Middle Miocene and the Present （Shang Youshashan Fro. and Qigequan Fro., 14.9-0 Ma）, and was stronger than the first phase. The tectonic--sedimentary evolution and the orienta- tion of surface structures in the western Qaidam Basin resulted from the Tibetan Plateau uplift, and recorded the periodic northward growth of the Plateau. Recognizing this early tectonic--sedimentary evolution supports the previous conclusion that northern Tibet responded to the collision between India and Asia shortly after its initiation. However, the current results reveal that northern Tibet also experi- enced another phase of uplift during the late Neogene. The effects of these two stages of tectonic activity combined to produce the current Tibetan Plateau.

Cenozoic Stratigraphy Deformation History in the Central and Eastern of Qaidam Basin by the Balance Section Restoration and its Implication

The Qaidam Basin, located in the northern margin of the Qinghai-Tibet Plateau, is a large Mesozoic-Cenozoic basin, and bears huge thick Cenozoic strata. The geologic events of the Indian- Eurasian plate-plate collision since -55 Ma have been well recorded. Based on the latest progress in high-resolution stratigraphy, a technique of balanced section was applied to six pieces of northeast- southwest geologic seismic profiles in the central and eastern of the Qaidam Basin to reconstruct the crustal shortening deformation history during the Cenozoic collision. The results show that the Qaidam Basin began to shorten deformation nearly synchronous to the early collision, manifesting as a weak compression, the deformation increased significantly during the Middle and Late Eocene, and then weakened slightly and began to accelerate rapidly since the Late Miocene, especially since the Quaternary, reflecting this powerful compressional deformation and rapid uplift of the northern Tibetan Plateau around the Qaidam Basin.

The Sedimentary Record in Northern Qaidam Basin and its Response to the Uplift of the South Qilian Mountain at around 30 Ma

The thick, Eocene to Pliocene, sedimentary sequence in Qaidam Basin at the northern margin of the Tibetan Plateau records the surface uplift history of the northeastern Tibetan plateau. In this study, we present detailed geochemistry, heavy mineral, and clay mineralogy data of the well preserved sedimentary record in the Dahongou section in the northeast of the Qaidam Basin. The results suggest that the sedimentary sequence recorded a 30 Ma young uplift/unroofing event in the northern edge of the Qaidam Basin, which is characterized by high ZTR index value and chlorite content, and low CIW＇. The results are consistent with previous sedimentological studies of the Qaidam Basin, which indicated rapid increase of the accumulation rates around 30 Ma. Based on past thermochronological data from the mountains around the Qaidam Basin and the accumulation rates of the Cenozoic basins in the northeastern Tibetan Plateau, we infer a regional uplift and denudation event along the northeastern Tibetan Plateau during early Oligocene （-30 Ma）, indicating that the Tibetan Plateau had expanded north-eastward of the study area at that time.

Standardization of Qilian Juniper ring-width series in the eastern margin of the Qaidam Basin

Standardization is one of the important procedures in dendroclimatology. We used abundant Qilian juniper （Sabina przewalskii Kom.） tree-ring samples from the eastern margin of the Qaidam Basin to develop a new standardization method, i.e. total growth curve （TGC）. The samples that contained the complete pith and reached to the growing culmination around the 40th-60th year were used to fit TGC, and the generalized negative exponential function was used to fit the curve. Usually, most cores cannot reach the arboreal pith for some reasons and it is difficult to determine the arboreal cambial age. The empirical model of initial radial growth （IRG） was employed to estimate the number of rings missing from the pith by the same data and IRG model explained 90.9% of the variance. When developing the chronology, the cambial ages of cores that contained the complete pith were regarded as beginning from the first year and others were determined by the numbers of missing and included rings in the core. Standardization was accomplished by dividing each tree-ring series by corresponding TGC. The chronologies developed by TGC can retain more low-frequency variational information and TGC helps to develop more reliable tree-ring width chronology.

Chemical and Isotopic Approach to Groundwater Cycle in Western Qaidam Basin,China

Due to the extremely arid climate in the western Qaidam Basin,the groundwater almost becomes the single water source for local residents and industrial production.It is necessary to know the reliable information on the groundwater cycle in this region for reasonable and sustainable exploitation of the groundwater resources with the further execution of recycling economy policies.This study focused on the recharge,the flow rate and the discharge of groundwater in the western Qaidam Basin through investigations on water chemistry and isotopes.Hydrological,chemical and isotopic characteristics show that the groundwater in the western Qaidam Basin was recharged by meltwater from new surface snow and old bottom glaciers on the northern slope of the Kunlun Mountains.In addition,the results also prove that the source water is enough and stable,and the rates of the circulation and renewal of the groundwater are relatively quick.Therefore,it can be concluded that the groundwater resources would guarantee the regional requirement if the meltwater volume of the mountains has not a great changes in future,moreover,water exploitation should be limited to the renewable amount of the groundwater reservoir in the western Qaidam Basin.

Geochemical Characteristics and Origin of Natural Gases in the Qaidam Basin,China

Sixty-five natural gas samples were collected from 19 oil-gasfields in theQaidam basin, China. The chemical composition and carbon isotope values of the samples weremeasured, and the geochemical characteristics and origin of the natural gases were studied. Thegases can be divided into biogenic gases, sapropelic oil-type gases, mixed type oil-type gases,coal-type gases and mixed gas. The delta^(13)C_1 values of the biogenic gases are very small and theC_2^+ contents of them are very low, ranging from -68.2 per thousand to -61 .8 per thousand and0.06 percent to 0.20 percent respectively. They have heavy delta D and delta^(13)C_(CO_2), showing aCO_2 reduction pathway. They ,are distributed in the East depression region and derived from theQuaternary source rocks. The sapropelic oil-type gases have small delta^(13)C_2 values and highC_2^+ ranging from -36.6 per thousand to -28.6 per thousand and from 33.01 percent to 47.15 percentrespectively. The mixed type oil-type gases have delta^(13)C_2 values and C_2^+ contents varyingfrom -28.6 per thousand to -24.8 per thousand and from 4.81 percent to 26.06 percent respectively.Both sapropelic oil-type gases and mixed type oil-type gases are associated with oil-type oils,distributed in the West depression region and derived from the Tertiary saltwater lacustrinesapropelic source rocks and humic source rocks respectively. The delta^(13)C_2 values of thecoal-type gases are extremely high and the C_2^+ contents are very low, changing from -23.3 perthousand to -12.5 per thousand and from 0.06 percent to 18.07 percent respectively. The coal-typegases in the Nanbaxian gasfield and the Lenghu oil-gasfields in the North fault block belt arederived from the Middle Jurassic coal-measures source rocks, whereas those in the West depressionregion are derived from the Tertiary saltwater lacustrine humic source rocks. Compared with someother basins in China, the natural gases there have obviously heavier delta^(13)C due to the heavierdelta^(13)C of different types of kerogens of the Tertiary saltwater lacustrine source rocks in theWest depression region of the basin. The mixing of natural gases is common in the West depressionregion, but the mixed gases are formed by sapropelic oil-type gases, mixed type oil-type gases orcoal-type gases, respectively, of different levels of maturity. Most of the sapropelic oil-typegases and mixed type oil-type gases in the west part are thermally immature and low-mature, but thecoal-type gases in the West depression region and the North fault block belt are mature and high- toover-mature.

Cenozoic Environmental Changes in the Northern Qaidam Basin Inferred from n-alkane Records

Cenozoic climatic and environmental changes in the arid Asian interior, and their possible relations with global climatic changes and the Tibetan Plateau uplift, have been intensively investigated and debated over past decades. Here we present 40-Myr （million years）-long n-alkane records from a continuous Cenozoic sediment sequence in the Dahonggou Section, Qaidam Basin, northern Tibetan Plateau, to infer environmental changes in the northern basin. A set of n-alkane indexes, including ACL, CPI and Paq, vary substantially and consistently throughout the records, which are interpreted to reflect relative contributions from terrestrial vascular plants vs. aquatic macrophytes, and thus indicate depositional environments. ACL values vary between 21 and 30; CP1 values range from 1.0 to 8.0; and Paq values change from 〈0.1 to 0.8 over the past 40-Myr. We have roughly identified two periods, at 25.8-21.0 Ma （million years ago） and 13.0-17.5 Ma, with higher ACL and CPI and lower Paq values indicating predominant lacustrine environments. Lower ACL and CPI values, together with higher Paq values, occurred at 〉25.8 Ma, 17.5-21.0 Ma, and 〈13.0 Ma, corresponding to alluvial fan/river deltaic deposits and shallow lacustrine settings, consistent with the observed features in sedimentological facies. The inferred Cenozoic environmental changes in the northern Qaidam Basin appear to correspond to global climatic changes.

Carbon and oxygen isotopic constraints on paleoclimate and paleoelevation of the southwestern Qaidam basin, northern Tibetan Plateau

We investigate the growth of the northern Tibetan Plateau and associated climate change by applying oxygen and carbon isotopic compositions in Cenozoic strata in the southwestern Qaidam basin. The X-ray diffraction and isotopic studies reveal that the carbonate minerals are mainly authigenic and they do not preserve any evidence for detrital carbonate and diagenesis. The isotope data show large fluctuations in the δ^(18)O and δ^(13)C values in the middle-late Eocene, indicating relatively warm and seasonal dry climate.The positive correlation of the δ^(18)O and δ^(13)C values in the Oligocene and the positive shift of the δ^(13)C values from the Eocene to Oligocene suggest that the climate changed to arid in the Oligocene. However,the δ^(18) values show negative shift, which is closely related to the global cooling event. During the Miocene, the δ^(13)C values vary between-2‰ and-4‰, whereas the δ^(18)O values show continuous negative shift. The mean δ^(18) values decrease from-8.5‰, in the early Miocene to-10.0‰, in the late Miocene. The stable isotope-based paleoaltimetry results suggest that the elevation of the southwestern Qaidam basin was approximately 1500 m in the middle-late Eocene and Oligocene. Subsequently, during Miocene the crustal uplift process started and the elevation reached approximately 2000 m in the early Miocene and 2500 m in the late Miocene, which suggests large-scale growth of the northern Tibet Plateau during the Miocene.