Science China Earth Sciences

The tectonic evolution of the Caroline Plate during the Oligocene to Early Miocene was mainly controlled by two fulcrums. The west fulcrum at Palau Island remained fixed during 29–25 Ma as the Ayu Trough rifted and produced the abyssal hills parallel to its boundaries, and the Pacific Plate and part of the Caroline Plate subducted to the north. At the northeast corner of the Caroline Plate, the east fulcrum is surrounded by a series of arc seamount chains. These chains resulted from the convergence between the Caroline Plate and the Pacific Plate and crustal softening of the northeastern Caroline Plate induced by the Caroline Hotspot at ∼25 Ma. This softening of the crust also led to the dysfunction of the east fulcrum, and the west fulcrum migrated south, which caused dextral motion along the Ayu Rift and the development of an en echelon structure. The Yap Trench was a part of the spreading center in the Parece-Vela Rift at ∼29 Ma. However, sinistral motion along the Parece-Vela Rift at around 20–15 Ma led to the exposure of the subducted Yap Trench and left a triangular region with distinctive geophysical characteristics in the surrounding area.

The ring-width chronology of a Juniperus przewalskii tree from the middle of the Qilian Mountain was constructed to estimate the annual precipitation (from previous August to current July) since AD 1480. The reconstruction showed four major alternations of drying and wetting over the past 521 years. The rainy 16th century was followed by persistent drought in the 17th century. Moreover, relatively wet conditions persisted from the 18th to the beginning of 20th century until the recurrence of a drought during the 1920s and 1930s. Based on the Empirical Mode Decomposition method, eight Intrinsic Mode Functions (IMFs) were extracted, each representing unique fluctuations of the reconstructed precipitation in the time-frequency domain. The high amplitudes of IMFs on different timescales were often consistent with the high amount of precipitation, and vice versa. The IMF of the lowest frequency indicated that the precipitation has undergone a slow increasing trend over the past 521 years. The 2–3 year and 5–8 year time-scales reflected the characteristics of inter-annual variability in precipitation relevant to regional atmospheric circulation and the El Niño-Southern Oscillation (ENSO), respectively. The 10–13 year scale of IMF may be associated with changing solar activity. Specifically, an amalgamation of previous and present data showed that droughts were likely to be a historically persistent feature of the Earth’s climate, whereas the probability of intensified rainfall events seemed to increase during the course of the 19th and 20th centuries. These changing characteristics in precipitation indicate an unprecedented alteration of the hydrological cycle, with unknown future amplitude. Our reconstruction complements existing information on past precipitation changes in the Qilian Mountain, and provides additional low-frequency information

Aromatic hydrocarbons are generally main distillation of crude oil and organic extract of source rocks. Bicyclic and tricyclic aromatic hydrocarbons can be purified by two-step method of chromatography on alumina. Carbon isotopic composition of individual aromatic hydrocarbons is affected not only by thermal maturity, but also by organic matter input, depositional environment, and hydrocarbon generation process based on the GC-IRMS analysis of Upper Ordovician, Lower Ordovician, and Cambrian source rocks in different areas in the Tarim Basin, western China. The subgroups of aromatic hydrocarbons as well as individual aromatic compound, such as 1-MP, 9-MP, and 2,6-DMP from Cambrian-Lower Ordovician section show more depleted 13C distribution. The δ13C value difference between Cambrian-Lower Ordovician section and Upper Ordovician source rocks is up to 16.1‰ for subgroups and 14‰ for individual compounds. It can provide strong evidence for oil sourc correlation by combing the δ13C value and biomarker distribution of different oil and source rocks from different strata in the Tarim Basin. Most oils from Tazhong area have geochemical characteristics such as more negative δ13C9-MP value, poor gammacerane, and abundant homohopanes, which indicate that Upper Ordovician source rock is the main source rock. In contrast, oils from Tadong area and some oils from Tazhong area have geochemical characteristics such as high δ13C9-MP value, abundant gammacerane, and poor homohopanes, which suggest that the major contributor is Cambrian-Lower Ordovician source rock.

Accurate information of cropland area and spatial location is critical for studies of national food security, global environmental change, terrestrial ecosystem geophysics and the geochemical cycle. In this paper, we compared five global cropland datasets in circa 2010 of China from in terms of cropland area and spatial location, including GlobalLand30, FROM-GLC, GlobCover, MODIS Collection 5, and MODIS Cropland. The results showed that the accuracies of cropland area and spatial location of GlobeLand30 were higher than the other four products. The cropland areas of the five products varied in most of the provinces. Compared with the statistical data, the best goodness of fit was obtained using GlobeLand30, followed by MODIS Collection 5 and FROM-GLC, with MODIS Cropland and GlobCover having the poorer accuracies. Regarding the spatial location of cropland, GlobeLand30 achieved the best accuracy, followed by FROM-GLC and MODIS Collection 5, with GlobCover and MODIS Cropland having the poorer accuracies. In addition, the spatial agreement of the five datasets was reduced from agricultural production area to pastoral area and significantly affected by elevation and slope factors. Although the spatial resolution of MODIS Collection 5 was the lowest, accuracies of the cropland area and spatial location were better than those of GlobCover and MODIS Cropland. Therefore, high spatial resolution remote sensing images can help to improve the accuracy of the dataset during land cover mapping, while it is also important to select a suitable classification method. Furthermore, in northwestern and southeastern China, spectral mixing pixels are universal because of the complicated landscape and fragmentized topography and result in uncertainty and poor consistency when using the five products. Therefore, these regions require additional attention in future cropland mapping studies.

Knowledge of long-term change in vegetation and climate in semi-arid/arid regions is essential for the study of current climate and development of mitigation strategies against severe drought. Here, a fossil pollen record of annually-varved core from Sugan Lake in interior Qaidam Basin was quantitatively analyzed to reconstruct changes in vegetation, floristic diversity and drought frequency. Results of biomization suggested that regional vegetation was desert vegetation with three short-term occupations of temperate steppe/xerophytic shrub during the last 2700 years. Floristic diversity generally increased/decreased with the expansion/degradation of desert vegetation. Moisture fluctuations showed three distinct stages (extremely dry between 742BC and ∼AD500, relatively wet with an increasing trend between ∼AD500 and 1200 and relatively wet with frequent fluctuations after AD1200), interrupted by 14 drought events. Spectral analysis and continuous wavelet transform of moisture variation revealed 200- and 120-year cycles. According to cross-wavelet transform analysis, major drought frequency of ∼200-year was explicitly correlated to solar activity. It’s suggested that the centennial-scale drought frequency was mainly driven by solar activity, through modulation of large-scale atmospheric circulation. Furthermore, the effect of surface temperature-evaporation and uplifting/subsiding air flow should be notable. The climatic drought in interior Qaidam Basin could be intensified under the continuous global warming.

The fluctuation of a single lake level is a comprehensive reflection of water balance within the basin, while the regional consistent fluctuations of lake level can indicate the change of regional effective moisture. Previous researches were mainly focused on reconstructing effective moisture by multiproxy analyses of lake sediments. We carried out a series of experiments, including a transient climate evolution model, a lake energy balance model and a lake water balance model to simulate continuous Holocene effective moisture change represented by variability of virtual lake level in East and Central Asia. The virtual lake level, area, water depth and salinity are not equivalent to actual values, but we estimated relative changes of the regional effective moisture. We also explored the driving mechanisms of effective moisture change in different geographical regions. Our results indicated that gradually falling effective moisture during the Holocene in northern China was due to the combined effects of high lake evaporation caused by longwave and shortwave radiation, and low precipitation caused by reductions of summer solar insolation. A decline in effective moisture through the Holocene in the Tibetan Plateau and southern Central Asia resulted from decreased precipitation because of the weakening of the Asian summer monsoon. Increased precipitation induced by the strengthening of the westerly circulation contributed to the effective moisture rise during the Holocene in northern Central Asia.

Chỉ số tinh thể thạch anh (QCI) được sử dụng để phản ánh quá trình tinh thể hóa silica trong đá phiến Wufeng (WF) từ cuối Ordovician và đá phiến Longmaxi (LM) từ đầu Silurian, cũng như silica có nguồn gốc từ tro núi lửa trong đá phiến tuffaceous của Formation Lucaogou, nhằm phân biệt hai loại silica này. Silica trong các sinh quyển graptolite khác nhau cho thấy sự tinh thể hóa khác nhau. Các sinh quyển graptolite WF2–3 và LM1–4 có giá trị QCI thấp hơn rõ rệt so với các sinh quyển graptolite LM5–9 và các mẫu của Formation Lucaogou. Các tổ chức graptolite đóng vai trò hấp phụ, cố định và kết tủa trong việc tích tụ và làm giàu silicon trong lớp đất. Nguồn gốc sinh học gây ra quá trình tinh thể hóa thạch anh kém nhất trong các mẫu sinh quyển graptolite WF2–3 và LM1–4. Silica có nguồn gốc từ tro núi lửa trong đá phiến tuffaceous của Formation Lucaogou thể hiện quá trình tinh thể hóa thạch anh tương đối kém do cường độ diagenesis yếu hơn. Nhìn chung, mặc dù các sinh quyển graptolite WF2–3 và LM1–4 trải qua quá trình diagenesis mạnh mẽ và chứa một lượng nhỏ thạch anh mảnh vụn, nhưng silica vẫn cho thấy giá trị QCI thấp hơn so với silica có nguồn gốc từ tro núi lửa trong đá phiến tuffaceous của Formation Lucaogou. Quá trình tinh thể hóa silica sinh học kém hơn nhiều so với silica có nguồn gốc từ tro núi lửa. QCI là một chỉ số định lượng hiệu quả để thể hiện silica sinh học trong các đá phiến giàu hữu cơ và giàu silica.

The Songpan-Ganze orogenic belt on the northeastern margin of the Tibetan Plateau extends westward from the Songpan-Ganze terrain in western Sichuan to the Tianshuihai region in West Kunlun, Xinjiang. It hosts numerous giant spodumene pegmatite deposits and ore fields, including Jiajika and Ke’eryin in western Sichuan Province, Zhawulong on the border between the Sichuan and Qinghai Provinces, and Dahongliutan in Xinjiang Region. These form the Songpan-Ganze-West Kunlun (SP-GZ-WK) pegmatite-type rare-metal metallogenic belt. The pegmatite type rare-metal deposits in this belt are hosted in the metamorphic thermal domes in the metamorphosed flysh of the Triassic Xikang and Bayankalashan Groups. The mineralized pegmatites are intimately related to the Li- and volatile-rich two-mica granites that are peraluminous and have high (Li+Na+K)/(Mn+Fe+Mg+Ca+Ti) ratios. Pegmatites and granites in individual ore field throughout the belt typically form a cogenetic granite-pegmatite system, in which pegmatite dikes commonly surround granites. Spodumene is the predominant ore mineral in most pegmatites with limited hydrothermal alteration. In the granite-pegmatite systems, granitic magmas were emplaced under P-T conditions of 800–850°C and ∼550 MPa, while spodumene crystallized in an alkaline environment. The granite-pegmatite systems share similar Sr-Nd-Hf-Li isotopic compositions to the metasediments of the Xikang and Bayankalashan Groups. The δ7Li values tend to increase from the granites to the Li-poor pegmatites, whereas the reverse is observed between the Li-poor and Li-rich pegmatites. These geochronological data suggest that the granite-pegmatite systems formed in the Late Triassic and tend to be progressively younger from the outer to the inner zones of the metallogenic belt. These characteristics show that the granitic-pegmatitic melts were derived from the anatexis of the Xikang and Bayankalashan Groups during the Paleo-Tethyan orogeny in the Late Triassic. The separation of pegmatitic melts from granitic magmas can be best explained using the Jiajika-style “melt-melt immiscibility” or the Ke’eryin-style “fractional crystallization+melt-melt immiscibility” model. High-maturity terrestrial sediments are of key importance for the anatexis that results in the granite-pegmatite melts. The bidirectional tectonic stresses in the Songpan-Ganze orogenic belt may have caused the mineralization difference between the Jiajika deposit and the Ke’eryin ore field. These features indicate the controls of the combination of orogenic deformation, metapelites anatexis, and magmatic differentiation on the rare-metal mineralization of pegmatites. We suggest that pegmatites, pegmatite-parental granite, and their protoliths are important indicators for rare-metal mineralization in the SP-GZ-WK pegmatite type rare-metal metallogenic belt. Based on the widespread presence of fertile metasediments and well development of metamorphic thermal dome, highly differentiated granites, and regional zonation of pegmatites, the Zhawulong ore field is the most prospective area for rare metals and thus should be the priority for future exploration.

Carbonate carbon isotope (δ 13Ccarb) has received considerable attention in the Permian-Triassic transition for its rapid negative shift coinciding with the great end-Permian mass extinction event. The mechanism has long been debated for such a δ 13Ccarb negative excursion through the end-Permian crisis and subsequent large perturbations in the entire Early Triassic. A δ 13Ccarb-depth gradient is observed at the Permian-Triassic boundary sections of different water-depths, i.e., the Yangou, Meishan, and Shangsi sections, and such a large δ 13Ccarb-depth gradient near the end-Permian mass extinction horizon is believed to result from a stratified Paleotethys Ocean with widespread anoxic/euxinic deep water. The evolution of δ 13Ccarb-depth gradient combined with paleontological and geochemical data suggests that abundant cyanobacteria and vigorous biological pump in the immediate aftermath of the end-Permian extinction would be the main cause of the large δ 13Ccarb-depth gradient, and the enhanced continental weathering with the mass extinction on land provides a mass amount of nutriment for the flourishing cyanobacteria. Photic zone anoxia/euxinia from the onset of chemocline upward excursion might be the direct cause for the mass extinction whereas the instability of chemocline in the stratified Early Triassic ocean would be the reason for the delayed and involuted biotic recovery.

Using broadband seismic records from regional networks, we determined the focal mechanisms and depths of 37 earthquakes in the 2013 M7.0 Lushan earthquake sequence (3.4⩽M w⩽5.1) by fitting the three-component waveform data. The results show that the earthquakes are predominantly thrust events, with occasional strike-slip mechanisms. Most earthquakes occurred at depths of 10–20 km. We derived the regional distribution of the average stress field in this area using the damped linear inversion method and the focal mechanisms obtained in this study. The inversion results suggest that the Lushan region and the adjacent area are mostly under compression. The orientations of the maximum principal axes trend NW-SE, with some local differences in the stress distribution at different depths. Compared with the distribution of the stress field in the Wenchuan earthquake area, the stress field in the southwest section of the Longmenshan Fault zone (LFZ) share similar characteristics, predominantly thrust faulting with a few strike-slip events and the maximum compression axes being perpendicular to the LFZ.