Lanzhou Basin Research Articles

Impact of aerosol-boundary layer interactions on PM2.5 pollution during cold air pool events in a semi-arid urban basin

Global emission reductions still must address winter fine particulate matter (PM2.5) pollution in urban basins with enclosed terrains and frequent cold air pool (CAP) events when the temperatures within the basin are colder than above it. The effects of urban basin aerosol-boundary layer interactions on PM2.5 pollution during CAP events remain unclear. Intensive boundary layer observations in January 2021 and numerical models were used to investigate this issue in the semi-arid urban Lanzhou Basin of China. The results showed that CAPs formed because of the synoptic weather system that exacerbated the warming over the basin. The CAPs in this experiment were characterized by stronger temperature inversion (TI) layers in the vertical direction and lower relative humidity, lower wind speed, and weaker turbulence at the bottom of the basin compared to other conditions. The strong TI layers below the top of the basin inhibited the vertical dispersion of pollutants in the basin and concentrated the PM2.5 within a height of 0.3 km from the bottom of the basin. During CAP events, the proportion of elemental carbon in PM2.5 increased, whereas that of secondary inorganic species decreased. Aerosol absorption increased faster than scattering during CAP events. Therefore, the mean single scattering albedo decreased from 0.85 during non-CAP periods to 0.81 during CAP events. Radiosonde-sounding observations and numerical simulations indicated that aerosols accumulating in the lower basin heated the atmosphere during the daytime and facilitated boundary layer development via the “stove effect” (absorption aerosol heats lower atmosphere to promote boundary layer development). No significant “dome effect” (absorption aerosol heats the upper boundary layer to suppress boundary layer development) occurred during the two CAP events. These findings provide a theoretical basis for scientifically-guided PM2.5 pollution control in winter in isolated urban basins.

Variability of East Asian Summer Monsoon precipitation during the Early Pleistocene based on pollen analysis from the western Loess Plateau, China

Understanding Early Pleistocene variability in precipitation associated with the East Asian Summer Monsoon (EASM) may help improve understanding of the effects of future global warming on this system. In this study, a quantitative EASM precipitation record with ∼2.7 kyr resolution was reconstructed for the Early Pleistocene (∼2.2–1.7 Ma) based on a pollen record from a loess-paleosol sequence in Lanzhou Basin, on the western Chinese Loess Plateau (CLP). The results show that the average annual precipitation (Pann) value was ∼505 mm, almost ∼1.6 times as much as today (∼310 mm), implying much stronger EASM intensity from ∼2.2–1.7 Ma. The EASM experienced a period of stronger intensity from ∼2.2 to 2.07 Ma, with Pann up to ∼525 mm and vegetation dominated by forest-steppe, as well as a period of weaker intensity between ∼2.07 and 1.99 Ma with lower average Pann (∼481 mm) and steppe dominated vegetation. A gradual intensification followed from ∼1.99 to 1.7 Ma, with increasing Pann (∼503 mm) accompanied by a significant increase in arboreal abundance. Our analysis suggests that the west to east temperature gradients across the Equatorial Pacific region influenced the Early Pleistocene EASM evolution over geological timescales. This finding differs from the traditional view that high-latitude ice sheet fluctuations were the primary driving force of Quaternary EASM. On orbital timescales, the Pann exhibits a dominant ∼41 kyr obliquity periodicity between ∼2.2–1.99 Ma and a ∼18 kyr precession periodicity between ∼1.99–1.7 Ma, indicating that Northern Hemisphere ice sheet and low-latitude solar insolation were the dominant drivers of EASM precipitation cycles in these two periods.

Phylogeography and Ecological Niche Modeling of the Alashan Pit Viper (Gloydius cognatus; Reptilia, Viperidae) in Northwest China and Adjacent Areas

The joint impacts of historical geological events and Quaternary climatic oscillations in Northwest China on species evolution have been examined extensively in plant under a phylogeographic perspective. However, animal phylogeographic analyses in this region are still limited. The Alashan pit viper, Gloydius cognatus, occurs primarily in arid Northwest China and adjacent areas. Based on variation at two mtDNA genes (ND4 and Cytb) in 27 individuals representing 24 populations, the spatial genetic structure and demographic history of G. cognatus were examined across its geographic range. Phylogenetic analyses revealed two well-supported allopatric clades (each with two distinct subclades/lineages), distributed across the southern (Qaidam Basin, Lanzhou Basin, and Zoige Basin [S1]; Loess Plateau [S2]) and northern (Ily Basin [N1]; Junggar Basin and Mongolian Plateau [N2]) regions. AMOVA analysis demonstrated that over 76% of the observed genetic variation was related to these lineage splits, indicating substantial genetic differentiation among the four lineages. A strong pattern of isolation-by-distance across the sampling populations suggested that geographic distance principally shaped the genetic structure. The four lineages diverged by 0.9–2.2% for the concatenated data, which were estimated to have coalesced ~1.17 million years ago (Mya), suggesting that the expansions of the Badain Jaran, Tengger, and Mu Us deserts during the Xixiabangma glaciation likely interrupted gene flow and triggered the observed divergence in the southern and northern regions. Subsequently, the early Pleistocene integration of the Yellow River and associated deserts expansion promoted the differentiation of S1 and S2 lineages (~0.9 Mya). Both mitochondrial evidence and ecological niche modeling (ENM) reject the signature of demographic and range contractions during the LGM for G. cognatus. In addition, ENM predicts that the suitable habitat of G. cognatus will contract in the future. As such, the conservation and management of ESUs should be a priority. Our findings provide the first insights on the lineage diversification and population dynamics of the Alashan pit viper in relation to geological history and Pleistocene climatic oscillations in arid Northwest China.

New Material of Schizotheriine Chalicothere (Perissodactyla, Chalicotheriidae) from the Xianshuihe Formation (Early Miocene) of Lanzhou Basin, Northwest China

A new schizotheriine mandible from the early Miocene Xianshuihe Formation in Lanzhou Basin, Northwest China is described here. Compared with other contemporaneous taxa, the lower jaw is most similar to Borissiakia betpakdalensis from Kazakhstan based on mandibular morphology and dental features, except for a much smaller body size which is hardly explained by intraspecific variation and suggests it represents a smaller body-sized species of Borissiakia. The type specimen of Phyllotillon huangheensis, from the same strata of the Lanzhou Basin, shares characters with the new specimen in the lower cheek teeth and the tapered morphology of the anterior horizontal ramus. Differences between both are reflected in the height of the ramus, especially the level of the symphysis, which may be an expression of sexual dimorphism. Therefore, schizotheriine mandibles from the early Miocene of Lanzhou Basin are regarded as the same taxon, and both are recognized as Borissiakia huangheensis. The occurrence of a chalicothere and other large perissodactyls may suggest that a certain amount of open woodland was distributed across the basin and that the paleoclimate might have been more humid during the early Miocene.

Contrasting responses of rivers with different sizes to extrinsic changes in the northeastern Tibetan Plateau

The formation of river terraces has been related to climate changes and surface uplift in the mountain regions. Although the formation mechanism of river terraces in the northeastern Tibetan Plateau has been extensively studied, the difference between the large river (trunk stream of Yellow River) and small rivers (tributaries of Yellow River) responding to external forcing is still poorly understood. Here, we employ multiple dating methods to date the terrace sequence of the Datong River in the northeastern Tibetan Plateau and then further correlate it with tectonic activity and climate records. Our results show that the modern Datong River in the Babaochuan Basin formed between 1.5 Ma and 1.1 Ma in response to the coupled effects of Asian monsoon enhancement and intense regional uplift. To further evaluate the effect of tectonic–climatic forcing on rivers of different scales, detailed comparison of terrace formation was carried out between the Datong River and Yellow River in the Lanzhou Basin. The comparison results indicate that under the context of relatively slow surface uplift during 0.6–0.13 Ma, the Datong River created three terraces probably ascribed to the glacial–interglacial cycles with high-amplitude fluctuations, whereas no terraces formed along the Yellow River in the Lanzhou Basin during the same interval. We conclude that the Datong River, as a smaller river than the trunk Yellow River, is probably more sensitive to the climate change.

Periclinal fold systems in thick-bedded mudstones: A case study of the Early Cretaceous Hekou Group, Lanzhou Basin, NW China

A new type of structural association comprising periclinal folds is present in thick-bedded calcareous mudstones and siltstones of the Early Cretaceous Hekou Group in the Lanzhou Basin, NW China. The periclinal fold system consists of meso-scale, open anticlines and synclines with fold axial trends in two perpendicular orientations (NE-SW and NW-SE) and two sets of conjugate thrust faults that form intersections in the same two orientations. Folded and faulted layers of the periclinal fold system are not sedimentary bedding surfaces but sub-parallel curviplanar foliations formed by compaction of the original strata soon after deposition. Many folded curviplanar foliations connect with the unfolded, conjugate thrust faults, indicating that faulting was active during and/or soon after folding. The NE-SW and NW-SE-trending fold axes and fault planes indicate bilateral, subhorizontal contraction acting simultaneously during the formation of the periclinal fold system. In some cases, converging slickenlines on curviplanar foliation planes within anticline cores point to localized isotropic contraction probably caused by scaping fluids and increased hydrostatic pressure. Similarities with the better-known polygonal fault system include development in fine-grained sediments and layer boundness, whereas main differences are a dominance of contractional faults in the periclinal fold system versus extensional faults in the polygonal fault system and a higher consolidation of sediments needed for the development of the periclinal fold system. This study suggests that collision between the Lhasa and Qiangtang Blocks combined with oblique convergence and subsequent collision along the southeastern margin of the Eurasian plate following compaction-related volume loss of fine-grained sediments led to the formation of the periclinal fold system in the Hekou Group in the Late Cretaceous.

Global climate change drove terrestrial ecosystem evolution during the late Paleocene-middle Miocene in the Lanzhou Basin, northeast Tibetan Plateau

In this paper, we present organic carbon isotope records from continental sediments spanning the late Paleocene-middle Miocene (57.9–15.0 Ma) in the Lanzhou Basin, northeastern Tibetan Plateau to investigate terrestrial ecosystem-response to global climate change. The continuous sediment δ13CTOC values that we obtained range from −27.83‰ to −20.05‰. Combining these data with the sediment C/N ratios and n-alkane distribution, we infer that the major source of sediment organic matter was terrestrial C3 plants. Furthermore, we note that both sediment δ13CTOC and δ13Cn-alkanes values gradually become more positive from 57.9 to 26 Ma and then become relatively more negative from 26 to 15 Ma. These δ13C values vary closely with global temperature changes. We propose that global temperature controlled δ13C values with cooler conditions resulting in less rainfall in inland Asia, and vice versa. Therefore, mainly driven by the global temperature, the regional climate in the Lanzhou Basin gradually changed from warm-wet to cold-dry conditions during 57.9–26 Ma, and the climatic conditions became warmer and wetter again after 26 Ma. This long-sequence terrestrial ecologic environment study obtained from the northeast Tibetan Plateau powerfully demonstrates the first-order role of global climate in regulating the long-term evolution of regional environment.

New Discovery of Oligocene Gastropod Opercula from the Yehucheng Formation in Northwestern China

AbstractFourteen species of nonmarine gastropod opercula are found to be in high abundance in the Oligocene upper member of the Yehucheng Formation, Lanzhou Basin, northwest China. They are attributed to two families, namely Bithyniidae and Assimineidae, and four genera, namely Pseudemmericia, Bithynia, Mirolaminatus and Assiminea. Among them, three species are new, and they are Bithynia paramonolithic sp. nov., B. obliquus sp. nov., and B. disregularis sp. nov. Some of these species were first discovered in Paleogene strata of the vast Northwest China and can be compared with the opercula recovered from the Paleogene deposits in East and South China. It is inferred from the paleontology and sediment associations that the climate was slightly humid and the fauna probably inhabited the fresh or brackish water of a shallow lake in the Lanzhou Basin during the late early Oligocene period.

Eccentricity-paced monsoon variability on the northeastern Tibetan Plateau in the Late Oligocene high CO2 world.

Constraining monsoon variability and dynamics in the warm unipolar icehouse world of the Late Oligocene can provide important clues to future climate responses to global warming. Here, we present a ~4-thousand year (ka) resolution rubidium-to-strontium ratio and magnetic susceptibility records between 28.1 and 24.1 million years ago from a distal alluvial sedimentary sequence in the Lanzhou Basin (China) on the northeastern Tibetan Plateau margin. These Asian monsoon precipitation records exhibit prominent short (~110-ka) and long (405-ka) eccentricity cycles throughout the Late Oligocene, with a weak expression of obliquity (41-ka) and precession (19-ka and 23-ka) cycles. We conclude that a combination of eccentricity-modulated low-latitude summer insolation and glacial-interglacial Antarctic Ice Sheet fluctuations drove the eccentricity-paced precipitation variability on the northeastern Tibetan Plateau in the Late Oligocene high CO2 world by governing regional temperatures, water vapor loading in the western Pacific and Indian Oceans, and the Asian monsoon intensity and displacement.

New material of Aprotodon lanzhouensis (Perissodactyla, Rhinocerotidae) from the Early Miocene in Northwest China

Aprotodon is an extinct genus of the family Rhinocerotidae with a peculiar morphology that is mainly embodied in the wide mandibular symphysis and a pair of prominent second low incisors. The genus once lived on the vast grasslands of Asia, from Pakistan, and Kazakhstan to China, during the Late Eocene to the Early Miocene. The scarcity and incompleteness of the record, however, make the morphological characteristics of Aprotodon rather perplexing. Here, we report on a better mandible fossil with an intact second right incisor from the Xianshuihe Formation from the Early Miocene age (approximately 21–20 Ma) in the Lanzhou Basin, Gansu Province, China. Several important features, notably the elongated strongly curved tusk‐like incisors and mandibular symphysis, which widens sharply in the anterior part of the mandible, support attribution to A. lanzhouensis. Compared with previous specimens, the material described here is distinct in the lack of p1, horizontal ramus, and the ventral morphology of the mandible symphysis. Based on the new fossil specimen, we provide new material for the understanding of this extinct peculiar rhinoceros.

Astronomical forcing of lake evolution in the Lanzhou Basin during early Miocene period

Orbital-scale East Asian summer monsoon (EASM) variability has been extensively investigated in Pliocene-Quaternary loess and marine sediments. In recent years, middle-late Miocene orbital-scale EASM variability has been depicted in lacustrine deposits from NE Tibetan basins, showing a dominant cyclicity of ∼100 kyr eccentricity. However, EASM variability during the early Miocene time remains poorly understood. Here, we investigate the sedimentary record of the early Miocene Xianshuihe Formation, Lanzhou Basin, in order to further constrain the NE Tibetan hydrological evolution and link it to orbital-scale monsoonal variability. By analyzing the lithology, magnetostratigraphy and cyclostratigraphic records of a 270-m section, we show that Lanzhou lake expansion cycles were consistently dominated by ∼100 kyr eccentricity forcing in early Miocene, except for the ∼20.8-19.8 Ma interval showing dominant obliquity control. These records are in agreement with deep-sea oxygen isotopic records, and are potentially linked to Antarctic ice sheet modulations related the precession amplitude modulation by eccentricity. A weakening of weathering intensity in Lanzhou Basin after 20.8 Ma may be attributed to orogenic barriers for westerly moisture and weakened surface temperature resulted from the rapid uplift of NE Tibet and surrounding mountains in the time interval of late Oligocene-early Miocene.

Early Pleistocene pollen record from the western Chinese Loess Plateau and its implications for the evolution of the East Asian Summer Monsoon

The East Asian Summer Monsoon (EASM) is the most important and active atmospheric circulation system in Asia. However, its evolution during the Early Pleistocene, and that evolution's associated drivers, remain controversial. Here, for the first time, a high resolution pollen record was obtained from a loess-paleosol sequence located in the Lanzhou Basin, western Chinese Loess Plateau (CLP), and used to reconstruct the evolution of EASM intensity during the Early Pleistocene (2.2–1.7 Ma). The type of vegetation indicated by analysis of the pollen assemblage is comparable to the forest-steppe vegetation type currently distributed on the eastern CLP. This area's present-day mean annual temperature (MAT) and mean annual precipitation (MAP) are ~6–10 °C and ~400–550 mm, respectively, implying the existence of a stronger EASM intensity during the 2.2–1.7 Ma period. When the auxiliary function of the diversity index was applied, the Early Pleistocene EASM, as recorded by the vegetation successions identified within the loess-paleosol sequence, showed a generally stronger intensity during the 2.2–2.06 Ma period, a relatively weak intensity between 2.06 Ma and 1.97 Ma, and then a gradual intensification from 1.97 Ma until 1.7 Ma. Further comparisons demonstrated that the zonal thermal gradient in the Equatorial Pacific may have played an important role in driving the evolution of the EASM over a tectonic timescale during the Early Pleistocene. In contrast, over a sub-orbital or orbital timescale, the EASM intensity appears to have been closely correlated to global temperature variations, given that the MATs of the Early Pleistocene were higher than today's. This would imply that both low-latitude forcing and global temperature change were important drivers influencing variations in precipitation in Northern China, against a background of ongoing global warming.

Magnetostratigraphy and Palaeoclimatic Significance of the Late Pliocene Red Clay‐Quaternary Loess Sequence in the Lanzhou Basin, Western Chinese Loess Plateau

AbstractThe history of red clay‐loess accumulation in the Chinese Loess Plateau (CLP) is the key to understanding the aridification history of the Asian interior and the uplift of the Tibetan Plateau. Here we present high‐resolution magnetostratigraphy and grain‐size records of a 265.7‐m‐long drill core of the red clay‐loess sequence at Gaolanshan in the Lanzhou Basin from the western CLP. Our results, combined with the loess‐paleosol stratigraphy and regional stratigraphic correlation, indicate that the most complete and continuous red clay‐loess sequence since ~3 Ma was first found in the western CLP, which is ~0.8 Myr older than the previously investigated Quaternary loess in the Lanzhou region, and the age of the oldest Lanzhou loess‐paleosol sequence is ~2.8 Ma. Consequently, the major enhancement of the aridification at 2.8 Ma in the Asian interior was primarily controlled by the uplift of the Tibetan Plateau and global cooling.

Magnetochronology of Mid-Miocene mammalian fauna in the Lanzhou Basin, northeastern Tibetan Plateau: Implications for Asian mammal migration

The Quantougou (QTG) Fauna in central Lanzhou Basin is an important late Mid-Miocene fauna on the northeastern Tibetan Plateau margin, but its numerical age remains a matter of debate. Here, we present a new magnetostratigraphic record for a fluvio-lacustrine section to further constrain the age of the QTG Fauna. Results suggest that the studied section spans from polarity chrons C5Cn.2n to C5n.2n or C5An.1n, with ages of ca. 16.5 Ma to 10 ​Ma or 16.5 Ma to 12 ​Ma. The QTG Fauna is located at the top of polarity chron C5r.3r or C5Ar.2r, which corresponds to an age of 11.7 Ma or 12.8 ​Ma for the fauna. Accordingly, the associated Myocricetodontinae (a subfamily of Gerbillidae, Rodentia) is suggested to have appeared in the Lanzhou Basin at 11.7 ​Ma or 12.8 ​Ma, which is the oldest Myocricetodontinae in East Asia but is still much younger than the ~20 ​Ma appearance of this subfamily in West and South Asia. Our age data support the interpretation that East Asian Myocricetodontinae originated from South Asia. The QTG fauna further suggest a dry and open grassland environment, which is consistent with global cooling after the Mid-Miocene Climatic Optimum.

A New Fossil Erinaceidae from the Shajingyi Area in the Lanzhou Basin, China

Here, we describe a new primitive Erinaceidae species, Oligoechinus lanzhouensis n. gen. n. sp., based on a specimen from late Oligocene strata of the Lanzhou Basin, Gansu Province. Its characteristics are as follows: the M1 width is longer than the length, the metastyle extends far disto‐labially, the posterior margin has strong curve and a distinct metaconule that is connected to the protocone by a postprotocrista is located in the centre of the tooth. The posterior arm of the metaconule is poorly developed. In the P4, the pterion of the metastyle is less developed and the metacone is carinate. In addition, O. lanzhouensis has a premolar larger than the canine tooth (P2 > C) and P3 had no molarisation, characters also possessed by Erinaceus. Overall, O. lanzhouensis was similar to Mioechinus based on architectural tooth features, demonstrating a closed genetic relationship. Fossil Erinaceidae are relatively rare before the Miocene. The discovery of O. lanzhouensis provides an important insight into the origin of Mioechinus in China and the early evolution of the Erinaceidae. Our analysis shows that a relatively recent ancestor of Mioechinus had probably existed in the early Oligocene and that O. Lanzhouensis was likely an ancestral type of Mioechinus gobiensis, and its systematic position should be located at a transitional position between Amphechinus and Mioechinus.

Sedimentary dynamics and climatic implications of Cretaceous loess-like red beds in the Lanzhou basin, Northwest China

The Chinese loess has been widely studied in order to reconstruct Cenozoic environmental change. Cretaceous loess-like red beds with alternating red and gray layers in the Lanzhou basin, Northwest China are well preserved and can be used for understanding the Mesozoic environmental and climatic changes. However, the red beds and gray clays are less studied and their significance is not clear. Hence, Cretaceous red beds of the basin are studied. The gray layers show horizontal lamination and are deposited in a lacustrine environment, whereas the red layers show gradational change in texture and color, and are analogous to paleosols. The red and gray clays show similar immobile trace elemental compositions and identical grain size distributions, suggesting that the sediments of the two deposits are derived from the same source and carried by identical transporting forces. The dominant 2–10 μm fine grain size component indicates that the deposits are sorted through long-distance transportation. In this context, the magnetic properties of the red beds are mainly determined by post-depositional environmental development. Hematite is the dominant magnetic mineral and some of the gray layers contains small quantities of magnetite. Therefore, the red and gray layers develop on land and under water, respectively. The magnetic parameter IRM-100mT eliminates the effects of ferromagnetic minerals and is sensitive to hard magnetic minerals. Thus, it can be used as a proxy for paleoclimatic change. Its high values in the red layers correspond to a drier paleoclimatic environment.

Pedogenic components of Xijin loess from the western Chinese Loess Plateau with implications for the Quaternary climate change

Chinese loess deposits preserve abundant information on the aridification of the Asian interior and Asian monsoon evolution during the Quaternary. Although substantial progress has been made in deciphering the paleoclimatic record of the thick loess deposits in the western Chinese Loess Plateau (CLP), the environmental significance of variations in the magnetic susceptibility (χ) and the ultrafine (<2 μm) grain-size is still unclear when compared with the central CLP. Here, we present high-resolution records of χ, frequency-dependent magnetic susceptibility (χfd) and grain-size (<2 μm and >40 μm fractions) of the 416-m-long Xijin loess core from the Lanzhou Basin, northeastern Tibetan Plateau. Our results indicate that the <2 μm fraction of the core consists of both detrital dust and material produced during pedogenesis. An integrated analysis of records of χ, χfd, grain-size (<2 μm) and clay minerals reveals that the pedogenic component of the <2 μm fraction is significantly anti-correlated with the >40 μm fraction and marine oxygen isotope record. Consequently, we conclude that the pedogenic component is a more accurate and sensitive indicator of weathering and soil formation than the χ record, and it is well suited to reconstructing high-resolution Quaternary climatic change in the western CLP. Furthermore, the proportion of the pedogenic component increased systematically over the past 2.2 million years, which may have been primarily caused by long-term global cooling since the Early Pleistocene together with the episodic uplift of the Tibetan Plateau.

Magnetochronology of the Oligocene mammalian faunas in the Lanzhou Basin, Northwest China

The fluvio-lacustrine sequence in the Lanzhou Basin, located at the northeastern margin of the Tibetan Plateau, is a rich source of Oligocene–Miocene mammalian fossils, critical to our understanding of the terrestrial Asian mammal and environmental evolution. While the Miocene mammalian faunas have been dated with magnetostratigraphy, the numerical age of the Oligocene faunas remains controversial. Here, we present new high-resolution magnetostratigraphic records from two fluvio-lacustrine sections in the central Lanzhou Basin, which provide age constraints for two Oligocene mammal assemblages in the Lower Xianshuihe Formation, the Nanpoping and Xiagou Faunas. The Lower Xianshuihe Formation is suggested to span from polarity chrons C12r to C7n, ranging from ca 31 Ma to 24 Ma. The Early Oligocene Nanpoping Fauna correlates to the C12r–C11n.2n polarity chron intervals, yielding an age of ∼31–30 Ma, while the Late Oligocene Xiagou Fauna correlates to chrons C7An–C8n.2n, with an age of ∼26–25 Ma. This magnetostratigraphy provides a more accurate age than biochronology for the Oligocene giant mammal Indricotheriinae contained in the Nanpoping Fauna and sheds new light on the chronology and distribution of Indricotheriinae in Eurasia. The faunas, in particular the Nanpoping Fauna, suggest a mixed setting of woodlands and grasslands associated with a humid environment in the Lanzhou Basin during Early Oligocene, in contrast to its present arid environment with few woodlands.

An ecological response to the Eocene/Oligocene transition revealed by the δ13CTOC record, Lanzhou Basin, NE Tibetan Plateau

The Eocene/Oligocene (E/O) transition, corresponds to an abrupt global cooling, thought to have been one of the greatest temperature changes in the mid-Cenozoic Earth’s history. Sparse studies have successfully reconstructed the terrestrial ecological response to this temperature change. Here, we report results from the study of organic carbon isotopes (δ13CTOC), together with n-alkanes biomarker analysis, in the Yongdeng Section, Lanzhou Basin, Northwest China, and discuss changes in δ13CTOC and their mechanisms. The results show that between 35.3 Ma and 31.0 Ma, δ13CTOC ranged from −26.72‰ to −21.27‰. The main change occurred at 33.4 Ma, when δ13CTOC became heavier by 3‰. At this time the long-chain n-alkane members (C27, C29 and C31) were dominant, suggesting the most likely sources of organic matter were terrestrial plants. Combining these results with existing measurements of plant δ13CTOC and sporopollen data in adjacent areas, we infer that this change at 33.4 Ma might have been caused by an increase in gymnosperm content especially coniferous trees adapted to cold climates, which have a heavier δ13CTOC than that of the angiosperms, this would have been a response to the global cooling characteristic of this period.

Late Pliocene establishment of exorheic drainage in the northeastern Tibetan Plateau as evidenced by the Wuquan Formation in the Lanzhou Basin

The fluvial archives in the upper-reach Yellow River basins provide important information about drainage history of the northeastern Tibetan Plateau (TP) associated with geomorphologic evolution and climate change. However, the Pliocene fluvial strata within this region have not been studied in detail, hence limiting the understanding of the late Cenozoic development of regional fluvial systems. In this paper, we present the results of a study of the geochronology, sedimentology, and provenance of the fluvial sequence of the Wuquan Formation in the Lanzhou Basin in the northeastern TP. Magnetostratigraphic and cosmogenic nuclide burial ages indicate that the Wuquan Formation was deposited during 3.6–2.2Ma. Furthermore, sedimentary facies, gravel composition, paleocurrent data, and detrital zircon UPb age spectra reveal that the fluvial sequence resembles the terraces of the Yellow River in terms of source area, flow direction, and depositional environment. Our results indicate that a paleo-drainage system flowing out of the northeastern TP was established by ca. 3.6Ma and that the upstream parts of the Yellow River must have developed subsequently from this paleo-drainage system. The late Pliocene drainage system fits well with the dramatic uplift of the northeastern TP, an intensified Asian summer monsoon, and global increase in erosion rates, which may reflect interactions between geomorphic evolution, tectonic deformation, and climate change.