Yangtze Region Research Articles

Paleodepositional conditions and organic matter accumulation mechanisms in the Upper Ordovician-lower Silurian Wufeng-Longmaxi shales, Middle Yangtze region, South China

The deposition of black shales has been widely attributed to primary productivity, preservation controlled by redox conditions, and detrital dilution controlled by sedimentation rates. It is difficult to distinguish the relative importance of productivity and redox conditions in organic matter accumulation. On the Upper Ordovician-Lower Silurian Wufeng-Longmaxi black shales, we combined petrographic features, major and trace elemental proxies, and organic carbon isotopic analysis to determine the relative importance of productivity versus anoxia on organic matter accumulation and the evolution of factors controlling organic matter accumulation across the Ordovician-Silurian boundary in the Middle Yangtze region. Our research also sheds light on the temporal variations in redox conditions that caused the Late Ordovician mass extinction.Productivity, redox conditions, detrital dilution, and basin restriction changes over time as the Wufeng-Longmaxi Formations were deposited (from upper Katian to Rhuddanian stage). Based on the stratigraphic profiles of productivity (biogenic Si and P/Al), redox conditions (Mo/Al, U/Th ratios, and DOPT values), detrital fraction flux (Al2O3), and the cross-plots of biogenic Si versus Co*Mn values, our findings indicate that organic matter accumulation was a combined effect of productivity and anoxia; however, the elevated productivity caused by upwelling that introduced plenty of nutrients to the oceans was the direct factor driving organic matter accumulation. Previous research found a simultaneous variation in stratigraphic profiles of productivity and redox proxies; however, in our datasets, we found an offset between productivity geochemical proxies (biogenic Si content) and redox conditions (Mo/Al). Biogenic Si profile peaks occur stratigraphically beneath Mo/Al profile peaks. Anoxia does not coexist with increased paleoproductivity across the Ordovician-Silurian boundary in the Middle Yangtze region, and redox conditions may be disrupted by local geological events.

Early millet cultivation, subsistence diversity, and wild plant use at Neolithic Anle, Lower Yangtze, China

This study examines the archeological seed assemblage of Anle, a middle Neolithic site located in the Lower Yangtze region, China. The Lower Yangtze is thought to be the origin of domesticated rice and most studies of this region to date have focused on rice domestication and cultivation within its paleoenvironmental setting. In contrast, we highlight here diverse uses of non-rice plant resources. In addition to large quantities of rice remains (carbonized grains and spikelet bases), we identify both foxtail and broomcorn millet, both AMS radiocarbon dated earlier than 5750 cal BP, demonstrating the dispersal of millet cultivation to the Lower Yangtze in the middle Neolithic, earlier than previously securely documented. While most wild species identified in macrobotanical assemblages are traditionally categorized as weeds or incidental intrusions among food residues, many can be exploited for food and medicinal purposes. By analyzing the ecological and functional implications of identified plants, we infer ecological niches of cultivation, gathering, and possible propagation of wild plants as food and medicine. Analyses of diversity and seasonality of plant resources identified show that residents of Anle created a complex seasonal sequence of temporally compatible crops, constructing niches for two crops (rice and millet) and actively structuring opportunities to exploit available wild plant resources in their immediate environment.

Application of Chemical Sequence Stratigraphy to the Prediction of Shale Gas Sweet Spots in the Wufeng and Lower Longmaxi Formations within the Upper Yangtze Region

Effective shale gas exploration is hindered by the need for obtaining high-resolution correlations between shale strata and the need for classifying shale facies. To address these issues, chemostratigraphy, sequence stratigraphy, and shale gas geology methods were integrated to develop a new method known as “chemical sequence stratigraphy,” which was successfully applied to the Wufeng–Lower Longmaxi Formations in the upper Yangtze region. Well Huadi 1 was used as a case study, and detailed data were acquired. Multivariate statistical analyses were applied to three defined indices having different genetic significance, namely: terrigenous input intensity (TII), authigenic precipitation intensity (API), and organic matter adsorption and reduction intensity (OARI). By analyzing the trends of these three indices, the Wufeng–Lower Longmaxi Formations were divided into five fourth-order chemical sequences (from bottom to top): LCW, MCL1-1, MCL1-2, MCL1-3, and MCL1-4. The geochemical facies were named and classified using the chemical sequence stratigraphic framework. The enrichment factor (EF) transformation of elements was conducted to determine whether an element is rich or deficient. The results showed that the favorable geochemical facies in the well were EF-Al deficient, EF-Ca rich, and EF-V rich. The organic matter content and rock brittle strength were then used as chemical parameters, and it was predicted that the LCW and MCL1-1 chemical sequences most likely comprised shale gas sweet spots. This conclusion is consistent with the drilling results and indicates that our proposed method is effective and reliable. This method is further applied to the Changning Shuanghe section, the Shizhu Liutang section, and sections in the Xindi 1 well in the upper Yangtze region. The comparative study of these four sections showed that LCW and MCL1-1 are the key chemical sequences for shale gas exploration and development in the Wufeng–Lower Longmaxi Formations within the Upper Yangtze region.

Lower Cambrian Organic-Rich Shales in Southern China: A Review of Gas-Bearing Property, Pore Structure, and Their Controlling Factors

The Lower Cambrian shales are widely developed in southern China, with greater thicknesses and higher TOC contents. Although the shale gas resource potential has been suggested to be huge, the shale gas exploration and development is not satisfactory. At present, the gas-bearing property evaluation of the Lower Cambrian shale is still a hot spot of concern. According to previous works, this paper systematically summarizes the gas-bearing characteristics and controlling factors of the Lower Cambrian shales in southern China. The buried depth of Lower Cambrian shales mainly ranges from 3000 m to 6000 m, and the thickness of organic-rich shale intervals ( TOC > 2 % ) varies from 20 m to 300 m. The TOC content and EqVRo value are generally up to 2%-10% and 2.5%-6.0%, respectively. The gas content of the Lower Cambrian shales in the Weiyuan-Qianwei block of the Sichuan Basin and the western Hubei area generally exceeds 2 m3/t, and gas composition is dominated by CH4. In southeastern Chongqing, northwestern Hunan, and northern Guizhou areas, the gas content of the Lower Cambrian shales is generally <2 m3/t, and the N2 content is generally >60%. In the Lower Yangtze region, the Lower Cambrian shale reservoirs basically contain no gas. Higher maturity, lower porosity, and less-no organic pores are suggested to be responsible for low gas contents and/or the predominate of N2 in shale gas reservoirs. Strong tectonic deformation is an important factor leading to the massive gas loss from shale reservoirs, thus resulting in no gas or only a small amount of N2 in the Lower Cambrian shales. In a word, the Lower Cambrian shale gas plays with low maturity and relatively stable tectonic condition, especially deep-ultradeep zones, may be the favorable targets for shale gas exploration.

Depositional Environment and Organic Matter Enrichment of Early Cambrian Niutitang Black Shales in the Upper Yangtze Region, China

Natural gas generation is the result of organic matter degradation under the effects of biodegradation and thermal degradation. Early Cambrian black shales in the Upper Yangtze Region are rich in organic matter and have shown great shale gas potentiality in recent years. Nevertheless, the enrichment mechanism and distribution of organic matter in these black shales between different sedimentary settings, such as intra-platform basin, slope, and deep basin, are still poorly understood. In this paper, based mainly on elemental geochemistry, a comprehensive study of the marine redox conditions, primary productivity, sedimentation rate, terrigenous input, hydrothermal activity, and water mass restrictions was conducted on the Early Cambrian Niutitang black shale in the Upper Yangtze Region. Our data showed that an intra-platform basin received a higher terrigenous input and that it deposited under more restricted conditions than the slope and deep basin settings. The primary productivity in the slope and deep basin settings was higher than that in the intra-platform basin setting. In the intra-platform basin, the productivity increased from its inner part to its margin. For the slope and deep basin settings, the high paleoproductivity generated large amounts of organic matter and its preservation was synergistically affected by the redox conditions. In contrast to the slope and deep basin, the preservation of organic matter in the inner part of the intra-platform basin was mainly controlled by redox conditions because the paleoproductivity in it was much lower than in the slope and deep basin settings. The intra-platform basin margin was the most favorable area for accumulating organic matter.

Earth’s Orbital Control on Organic Matter Enrichment in the Black Shales of the Wufeng–Longmaxi Formation in the Upper Yangtze Region, South China

China’s most successful horizon for shale-gas exploration and development is the Upper Ordovician Wufeng Formation through the Lower Silurian Longmaxi Formation in its Upper Yangtze Region. In this study, the Wufeng–Longmaxi Formation black shales in the Upper Yangtze Region are analysed to determine their microstructural characteristics, total organic carbon (TOC) content, and well log characteristics and to identify information on the parameters of Earth’s orbit from its gamma-ray (GR) data series. Then, paleoenvironmental modes are established over a long time scale. On this basis, the Earth’s orbital control on organic matter enrichment in the black shales is examined. The black shales of the Wufeng Formation and the lower Longmaxi Formation are rich in biogenic siliceous fossils and framboidal pyrite. Their TOC content, GR value, and uranium (U) content increase gradually from the bottom of the Wufeng Formation to the Guanyinqiao Member at its top, peak at the Guanyinqiao Member, and then decrease gradually up to the Longmaxi Formation. Approximately six and seven long eccentricity cycles can be identified from the GR curves of the Wufeng Formation and the lower Longmaxi Formation, respectively. During the long eccentricity minima, corresponding to Earth’s cold period, the black shales have a relatively high level of enrichment of organic matter. This study can provide an important reference for investigating the mechanism by which Earth’s orbits control the climate and sedimentary environment, as well as the mechanism of organic matter enrichment.

Provenance of the siliciclastic rocks in the Lower Yangtze region: Implications for Paleozoic tectonic switching of the northeastern South China Block

• Permian zircon U-Pb age spectra are different from those of Silurian and Devonian . • The Silurian-Devonian detritus was from the proximal igneous rocks. • Increased ca. 430 Ma zircons indicates the onset of foreland stage. • The Permian detritus was from the basement of the Cathaysia Block. • Foreland basin was replaced by intraplate basin in Late Paleozoic. Integrated analysis of zircon U-Pb dating, in-situ Lu-Hf and whole-rock geochemical data was performed on Silurian-Permian siliciclastic rocks from the Lower Yangtze region (LYZ), northeastern South China Block, to understand the tectono-sedimentary evolution during the Paleozoic. These rocks have a high chemical index of alteration (CIA = 71.3–91.8) and variable index of compositional variability (ICV = 0.2–3.5), indicating intensive weathering. The high Zr/Sc ratios (12.1–75.3) indicate that their sources contain recycled old sediments. The Silurian and Devonian samples contained abundant detrital zircons with ages of 1.0–0.8 Ga and 460–420 Ma. However, the Permian samples show bimodal age spectra with peaks at ∼ 1800 Ma and 280–260 Ma. Integrated with published geochronological data, the proportion of the 460–420 Ma zircons drastically increased from the Early to Mid-Silurian and decreased in the Early Carboniferous, indicating two tectonic transformations, respectively. Based on these variations and relative paleogeographic data, we suggest three evolution stages with different provenances for the LYZ: (1) from Cambrian to Ordovician, the LYZ region inherited the Cryogenian tectonic framework of South China Block, receiving detritus from East Gondwana. (2) from Silurian to Devonian widespread uplift in South China Block led to the development of an isolated foreland basin with independent sedimentary sources and (3) from Carboniferous to Permian, subduction of the Paleo-Pacific plate exposed the Paleoproterozoic basement of the Cathaysia Block, providing detritus for the LYZ intraplate basin.

Toward a unified and refined Ordovician stratigraphy for the western Yangtze region, South China

Strata equivalent to the majority of the Ordovician shallow-water sediments in South China occur in the western Yangtze region (present-day southwestern Sichuan and northeastern Yunnan, Southwest China), but remain to be properly documented largely due to their inaccessibility. Our research in the past decade has led to the recognition of spatial and temporal patterns, and hence a substantial stratigraphic revision of these rocks, with part of the results having been published in a series of papers. Here, we outline a unified and refined Ordovician stratigraphy of the region built chiefly on a summary of these new data, presenting a robust timeframe for exploring the environmental and biotic events during the Ordovician on a basin-wide scale in South China.

Tectonic Characteristics, Evolution, and Significance of the Zhouwang Fault, Lower Yangtze Area, Eastern China

AbstractFault geometry, kinematics, geophysics, the tectonic stress field and tectonic evolution of the Zhouwang fault in the southern Jiangnan tectonic transition zone of the Lower Yangtze region, eastern China are examined. Field observations show the fault is composed of a series of nearly E–W trending, N–S dipping faults, and four stages of tectonism (sinistral strike‐slip, thrust nappe, normal fault, and dextral strike‐slip) developed in turn. Geophysical data show that the fault trends almost linearly E–W along a flat, steep gravity gradient at shallow depth, with distinct gravity anomalies to the north and south and different in the north and south. Also, the deep part is characterized by northward dip and a gradual slowing down. Tectonic stress field analysis indicates that the fault experienced four tectonic movements: NNE–SSW compression, NNW–SSE compression, NEE–SWW extension, and E–W compression. Combined with regional tectonic background and previous research results, this indicates that: (1) the Zhouwang fault experienced sinistral strike‐slip movement during the Indosinian Period (260–200 Ma); (2) thrust nappes developed during the early Yanshanian Period (163–145 Ma); (3) a normal fault occurred in the late Yanshanian Period (125–65 Ma); and (4) dextral strike‐slip movement occurred in the Himalayan Period (ca. 50–37 Ma). The results reveal the tectonic evolution of the fault during Mesozoic deformation in the area, and also reveal the geological evolution and tectonic transformation of the Lower Yangtze region, which is key to our understanding of intracontinental deformation in eastern China.

Sinian hydrocarbon accumulation conditions and exploration potential at the northwest margin of the Yangtze region, China

Based on outcrop, drilling, logging and seismic data, the reservoir forming conditions, reservoir forming model and exploration potential of the ultra-deep Sinian Dengying Formation at the northwest margin of Yangtze craton region were examined. (1) This area is in craton rifting stage from Sinian to Early Cambrian, characterized by syn-sedimentary faults and rapid subsidence, significant sedimentary differences, and development of Dengying Formation platform margins on both sides of the rift. (2) The Sinian–Cambrian in this area has two sets of high-quality source rocks, Doushantuo Formation and Maidiping–Qiongzhusi Formation; of which, the latter has a thickness of 150–600 m and hydrocarbon generation intensity of (100–200)×108 m3/km2. (3) The mounds and shoals in the platform margin of Sinian Dengying Formation controlled by faults are thick and distributed in rows and zones; they are reformed by contemporaneous–quasi-contemporaneous and supergene karstification jointly, forming pore-type reservoirs with a thickness of 200–400 m. (4) The two sets of source rocks enter oil generation windows from Permian to Early Triassic, and the oil migrates a short distance to the lithologic traps of mounds and shoals to form a huge scale paleo-oil reservoir group; from Late Triassic to Jurassic, the oil in the paleo-oil reservoirs is cracked into gas, laying the foundation of present natural gas reservoirs. (5) The mound–shoal body at the platform margin of Dengying Formation and the two sets of high-quality source rocks combine into several types of favorable source-reservoir combinations, which, with the advantage of near-source and high-efficiency reservoir formation, and can form large lithologic gas reservoirs. The Mianyang–Jiange area is a potential large gas field with trillion cubic meters of reserves. According to seismic prediction, the Laoguanmiao structure in this area has the Deng-2 Member mound-shoal reservoir of about 1300 km2 making it a ultra-deep target worthy of exploration in the near future.

Organic Petrological Characteristics of Graptolite and Its Contribution to Buried Organic Carbon of Longmaxi Formation Shales, Middle Yangtze Region

The shale gas exploration of the Longmaxi Formation in the Yangtze Region of China has made a significant breakthrough. As an important hydrocarbon generation organism, graptolite is widely distributed in the Longmaxi Formation shales, but its hydrocarbon potential is still unclear. Taking the Longmaxi Formation shales in the Middle Yangtze Region as an example, this paper discusses the organic petrological characteristics of graptolite and its contribution to buried organic carbon. The Longmaxi shales in the study area can be divided into organic-rich shales (TOC > 2.0%) and organic-bearing shales (TOC < 2.0%). The organic-rich shales have high quartz content and low clay mineral content, which is opposite in the organic-bearing shales. Organic maceral results show that graptolite is widely distributed in nearly all the samples, while solid bitumen is relatively developed in organic-rich shale. The equivalent vitrinite reflectance obtained from the conversion of graptolite reflectance ranges from 2.46% to 2.76%, indicating that the organic matter maturity of the Longmaxi Formation shale is overmature. Combining an optical microscope and a field emission scanning electron microscope, the proportion of graptolite area to organic matter area can be obtained, the average of which is 32.71%. Solid bitumen mainly contributes to buried organic carbon of the organic-rich shales in the Longmaxi Formation, while graptolites contribute little to organic carbon burial. However, solid bitumen in the organic-bearing shales is relatively undeveloped, and graptolite is an important hydrocarbon generation organism, which is the main contributor to buried organic carbon.

Characteristics and Research Direction of Triassic in the South Yellow Sea Basin, China

To advance oil and gas exploration in the South Yellow Sea Basin, characteristics of Triassic, including its correlation with regional strata, distribution, lithology characteristics, and karst development characteristics, are studied by geological mapping. Results show that there exist Zhouchongcun Formation and Qinglong Formation in the Triassic. Zhouchongcun Formation corresponds to Dongma Formation in the lower Yangtze Region, Badong Formation in the middle Yangtze Region, and Lekoupo Formation in the upper Yangtze region. The upper Qinglong Formation corresponds to the Nanlinghu Formation and Longshan Formation in Xiyanzi district, and Jialingjiang Formation in the middle and upper Yangtze region. Its lithology is limestone, intercalated with marl, mudstone, and argillaceous siltstone. The lower Qinglong Formation corresponds to the Yinkeng Formation at the lower Yangtze group. The lower Qinglong group has the complex lithologic characteristics, which can be divided into six categories from top to bottom, including limestone weathering crust layer, limestone, marl with mudstone, mudstone with limestone, marl with limestone, and mudstone interbedding limestone and mudstone. Karst phenomenon is well developed in the lower Qinglong Formation, and the thickness of weathering crust, revealed by cz35-2-1 well, reaches 60.5 m. Triassic in the South Yellow Sea Basin is mainly distributed in the central uplift and southern depression, showing the regional differences in distribution. Karstification is probably the main reason for this significant difference. Identification of karst phenomenon and stratigraphic division method towards Triassic drilling coring are two important directions of Triassic study in the South Yellow Sea Basin.

Formation mechanism of the sealing capacity of the roof and floor strata of marine organic-rich shale and shale itself, and its influence on the characteristics of shale gas and organic matter pore development

The sealing capacities of shale, roof, and floor strata are essential for the preservation of shale gas. Sealing capacity development and its influence on shale gas composition and organic matter pore development are important but have not been addressed by existing studies. In this paper, the Upper Ordovician Wufeng-the first member of the Lower Silurian Longmaxi Fm. in the Yangtze region is selected as the study object. We dissected the J1 well in the Jiaoshiba block inside the Sichuan Basin and the Y6 well in the Youyang block outside the Sichuan Basin to analyze the gas components and carbon isotopes in recovered shale gas. In this study, two microscope techniques (i.e., FIB-SEM and FIB-HIM), porosity measurement, TOC content analysis, permeability tests, isothermal adsorption measurements, are utilized. The analytical results show that both the roof and floor strata have vertical sealing effects on the organic-rich shale due to their different physical properties compared to the shale. The shale overburden pressure increases with burial depth, and the adsorbed gas increases with increasing TOC content and thickness: all of these can lead to a decrease in permeability and an increase in self-sealing capacity. This paper determined the formation mechanism of the sealing capacity for shale, roof and floor strata. Furthermore, the control patterns of “sealing capacity strength” on the shale gas source and organic matter pore development are clarified. The main findings are as follows: when the shale, roof, and floor strata have a strong sealing capacity, only a small amount of liquid hydrocarbons can be discharged from the shale, but the natural gas formed due to liquid hydrocarbon cracking is abundant. Under such circumstances, shale gas has a mixed origin of liquid hydrocarbon cracked gas and kerogen cracked gas; the carbon isotopes are inverted; abundant pyrobitumen pores with high porosity and good connectivity are developed. When the sealing capacities are weak, the amount of liquid hydrocarbons discharged from the shale are high, and the amount of natural gas is low. The shale gas is mainly derived from kerogen cracking with weak or no carbon isotope inversion, and kerogen pores are small with poor connectivity. These findings will advance the study of shale gas formation mechanisms and provide a scientific basis for selecting shale gas exploration area.

Using expert elicitation to strengthen future regional climate information for climate services

Climate change knowledge can inform regional and local adaptation decisions. However, estimates of future climate are uncertain and methods for assessing uncertainties typically rely on the results of climate model simulations, which are constrained by the quality of assumptions used in model experiments and the limitations of available models. To strengthen scientific knowledge for climate services and climate change adaptation decisions, we explore the use of structured expert elicitation to assess future regional climate change. Using the Lower Yangtze region in China as a case study, we elicit judgements from six experts on future changes in temperature and precipitation as well as uncertainty sources, and compare it with climate model outputs from the Couple Model Intercomparison Project phase 5 (CMIP5). We find high consensus amongst experts that the Lower Yangtze region will be warmer in the coming decades, albeit with differences in the magnitude of change. There is less consensus about the direction and magnitude of future precipitation change. Compared with CMIP5 climate model outputs, experts provide similar or narrower uncertainty ranges for temperature change and very different uncertainty ranges for precipitation. Experts considered additional factors (e.g. model credibility, observations, theory and paleo-climatic evidence) and uncertainties not usually represented in conventional modelling approaches. We argue that, in context of regional climate information provision, expert-elicited judgements can characterise less predictable, or less explored, elements of the climate system and expert-elicited reasoning provides additional information and knowledge that is absent from modelling approaches. We discuss the value in bringing together multiple lines of evidence, arguing that expert elicited information can complement model information to strengthen regional climate change knowledge and help in building dialogue between climate experts and regional stakeholders, as part of a more complete climate service.

Quantitative Characterization for Pore Connectivity, Pore Wettability, and Shale Oil Mobility of Terrestrial Shale With Different Lithofacies—A Case Study of the Jurassic Lianggaoshan Formation in the Southeast Sichuan Basin of the Upper Yangtze Region in Southern China

Some major hydrocarbon-bearing basins are rich in shale with terrestrial facies in China, which may provide abundant terrestrial shale oil and gas resources. This work studied the Jurassic Lianggaoshan Formation in the Southeast Sichuan Basin of the upper Yangtze Region. Core samples were chosen for the total organic carbon content and mineral composition analyses to classify shale lithofacies. Afterward, pore connectivity, pore wettability, and shale oil mobility with different lithofacies were characterized by spontaneous imbibition, nuclear resonance, and centrifugation. Conclusions are as follows: the pore connectivity of organic-rich clay shale was mostly between moderate to good with oil-prone wettability and high mobile oil saturation. The organic-rich mixed shale has moderate to good pore connectivity, water-prone wettability, and the highest mobile oil saturation. Organic matter–bearing clay shale has bad to moderate pore connectivity. Meanwhile, its pore wettability covers oil wetting, mixed wetting, oil-prone wetting, and water-prone wetting. Its mobile oil saturation was moderate. Regarding organic matter–bearing mixed shale, the pore connectivity was bad to moderate with mixed-wetting pore wettability and moderate mobile oil saturation.

Crustal Structure and Anisotropy in the Lower Yangtze Region and its Metallogenic Implications

In this study, we performed receiver function profiling and fitted harmonic functions to the arrival time variations of Pms phases to calculate the crustal seismic anisotropy with delay time and fast polarization direction, using broadband seismic data obtained from 55 temporary stations in two linear profiles and 39 stations in the Lower Yangtze and adjacent region. Moreover, we determined the crustal thickness and Poisson’s ratio using a novel H-κ-c stacking method. Our results revealed that the Middle-Lower Yangtze Metallogenic Belt and the north east section of the Qinzhou-Hangzhou Metallogenic Belt are characterized by Moho upliftment (<32 km), a relatively high Poisson’s ratio (>0.26), local lithospheric thinning (<70 km), and a pattern of deep faults that connect the crust and asthenosphere and serve as conduits for magma upwelling. The NE-SW fast polarization direction was consistent with the SKS splitting results, and the average delay time was 0.45 s. Moreover, underplating of deep magma and upwelling along the weak zone caused local Moho uplift and ductile shear of the lower crust, resulting in the directional arrangement of amphibole and other minerals, which may be the controlling mechanism for the crustal anisotropy in the study area. The variations in crustal structure and anisotropy characteristics indicated that in the context of the northeastern Paleo-Pacific plate subduction, the existence of weak lithospheric zones and the northeastern asthenospheric flow are important conditions for metal supernormal enrichment in the Lower Yangtze region.

Fluid Properties and Genesis of Dolomites in the Devonian Guanwushan Formation of Upper Yangtze Platform, SW China

The Guanwushan Formation (GWSF) of Devonian dolomite are extensively developed in the northwest of Sichuan Basin in the Upper Yangtze region, but the properties of dolomitization fluid and the geneses are still unclear. Three types of dolomites can be divided by petrological characteristics: the fine-microcrystalline dolomites (FMD), the fine crystalline dolomites (FCD) and the medium crystalline dolomites (MCD). The order degree of these three types of dolomites increased in turn, and they all showed dark cathodoluminescence (CL) luminescence. The total amount of Rare Earth Elements (∑REE) of the dolomite was low, while the dolomite enriched with light REE and lacking heavy REE presented a distribution pattern consistent with that of limestone. The weak negative anomalies of the Ce and Eu indicated that the dolomites were formed in a weak redox environment with relatively low temperature. The dolomitization fluids were inherited from the original seawater. The respective δ13CPDB values of the three types of dolomites varied a little, indicating that they were not affected by the biological effects. Specifically, the δ18OPDB values of the FMD and FCD dolomites were higher than that of the limestone, indicating that the dolomitization fluid was influenced by evaporation at the penecontemporaneous stage. The interpretations were also supported by the 87Sr/86Sr ratios, as the 87Sr/86Sr ratios of FMD comparable to the Middle Devonian seawater. The δ18OPDB value of the MCD dolomite was lower than that of the limestone. It also showed poor automorphic extent, which clarified that the dolomite experienced more intense dolomitization in greater burial depth and at higher temperatures.

Research on the Occurrence State of Methane Molecules in Postmature Marine shales—A Case Analysis of the Lower Silurian Longmaxi Formation Shales of the Upper Yangtze Region in Southern China

The Yangtze region in southern China is endowed with abundant marine shale gas. Methane molecules exist in either adsorption state or free state in postmature marine shales, depending on the components of shales. In this study, the core samples of the selected well in the Lower Silurian Longmaxi Fm. shales from the Sichuan Basin, upper Yangtze region, southern China, were taken as study objects. We carried out TOC content, organic matter maturity, mineral component, and core gas content analyses and isothermal adsorption, FIB-SEM, and FIB-HIM experiments to analyze the occurrence state of methane molecules in postmature marine shales. The conclusions are as follows: most methane molecules exist in the organic matter pores of the postmature marine shales, and only a small amount of them exist in clay mineral pores. The organic matter pores in organic-rich shales are large in number with excellent roundness and are well connected, with large pores covering small ones. Thus, abundant free gas can be stored in the organic matter pores and pore throats, making it possible to densely and continuously adsorb methane molecules with a relatively large adsorption space. The flake-shaped clay minerals have a small number of pores with low roundness. Among the three clay minerals in postmature marine shales of the Longmaxi Fm., the I/S mixed layer offers certain reservoir spaces for adsorbed and free gases and chlorite stores a little adsorbed gas and little free gas, while illite hardly stores the adsorbed gas but contains a little free gas.

Spatial–Temporal Characteristics and Driving Factors of Disaster-Induced Grain Yield Loss in China

The current article aimed to characterize the spatial–temporal characteristics and the driving factors of disaster-induced grain yield loss. Taking the world’s largest agricultural country, China, as the research object, this study evaluated China’s disaster-induced grain yield loss from 1978 to 2019 and investigated the contribution of three driving factors, using a logarithmic mean Divisia index (LMDI) approach. The results showed that drought and flood were the main disasters causing grain yield losses in China. China’s grain disaster loss peaked in 2000 and then entered a declining period. In terms of spatial distribution, disaster-induced grain yield loss mainly concentrated in the regions including the Middle-Lower Yangtze region, the Huang-Huai-Hai region, and Northeast China. The gravity center of disaster-induced grain yield loss shifted northward in China. Disaster intensity contributed the most to change in disaster-induced grain yield loss, followed by yield loss density, and the grain planting area. Thus, addressing drought and flood disasters, and enhancing disaster prevention and reduction capacity are critical to reduce disaster-induced grain yield loss, especially in the Middle-Lower Yangtze, the Huang-Huai-Hai region, and Northeast China.

Aquatic macrophyte fluctuations since the 1900s in the third largest Chinese freshwater lake (Lake Taihu): Evidences, drivers and management implications

Diverse aquatic macrophyte communities represent an essential component of healthy shallow lake ecosystems, and information on changes in macrophytes abundance and composition is crucial for restoration initiatives and management. However, the long-term dynamics of aquatic macrophyte communities are poorly understood for most subtropical freshwater lakes. This study used macrophyte surveys from 1960 to 2014 alongside plant macrofossil and pollen remains deposited in the sediment to reconstruct, macrophyte community change over the two centuries in the southeastern bay of Lake Taihu, China. Plant macrofossils efficiently captured the main historical recorded macrophyte species, including four unrecorded submerged taxa (Vallisneria spinulosa, Zannichellia palustris, Nitella sp., and Chara sp.) by the monitoring surveys. Aquatic pollen failed to reflect the main species composition in the bay, but provided additional information on species with no macrofossils and few seeds, especially the floating-leaved and free-floating taxa (e.g., Nymphoides and Azolla). We conducted a constrained incremental sum of squares cluster analysis and a multiple factor analysis (MFA) to the macrophyte community in the core and identified three major changes over the past century, with a shift from Potamogeton malaianus (pre-1960), to Vallisneria natans, P. malaianus, Potamogeton maackianus, and charophyte species (1960–1998), followed by the proliferation of pollutant-resistant species Ceratophyllum demersum and Myriophyllum spicatum, as well as the floating-leaved plant Nymphoides (post-1998). MFA also clearly elucidated the relationships between macrophyte shifts and changes in various environmental stressors (i.e., cladocerans, particle size spectra and geochemical data). The results confirmed our hypothesis, which stated that hydrological alterations and increased nutrient inputs, driven by the increasing anthropogenic pressures were likely the main drivers of the alteration in the macrophyte composition and succession in the bay. Our study provides a unique perspective on the pre-disturbance macrophyte community composition of Eastern Lake Taihu, thereby offering useful information not only for lake managers in the Yangtze region, but also for the global decision-making for the eutrophication and restoration of large shallow lakes.