Fushun Basin Research Articles

Paleoclimatic and Paleogeographic Reconstructions of Sedimentation Conditions in the Fushun Basin Using Methods of Mathematical Statistics

Paleoclimatic and Paleogeographic Reconstructions of Sedimentation Conditions in the Fushun Basin Using Methods of Mathematical Statistics

Onset of aridification in mid-latitude Asia at 41 Ma linked to the proto-Paratethys Sea retreat

The initial aridification of mid-latitude Asia is thought to be one of the most prominent Cenozoic environmental changes in Asia. However, precisely dated, fine-grained sediments documenting initial aridity beyond central Asia are extremely rare. Here, we reconstruct the history of Middle-Late Eocene (47.8–37.8 Ma) hydroclimate change in northeast China using continuous palynological records from a lacustrine sedimentary sequence in the Fushun Basin. Our results indicate that the onset of aridification occurred concurrently with increased climatic variability at 41.2 Ma, as expressed by a sharp increase in xerophytic Ephedra and mountainous coniferous forest at the expense of an abrupt decrease in broadleaved forest, along with frequent alternations between semi-desert and swamp forest vegetation after 41.2 Ma. The climate changed markedly from a stable, warm-wet climate during the Middle Eocene to a cooler, drier climate with pronounced warm-dry and cold-wet oscillations during the Late Eocene. This finding challenges the previously prevailing idea that the East Asian monsoon was significantly enhanced in northeast China after 41–40 Ma. A comprehensive comparison of records from eastern and central Asia demonstrates that the onset of aridification and enhanced climate variability after 41 Ma were widespread across mid-latitude Asia. This may be explained by reduced moisture transport by the westerlies and strengthening of high- and low-latitude forcing linked to the coeval retreat of the proto-Paratethys Sea.

Quantitative geochemical reconstruction of Eocene paleoenvironment in Fushun Basin, northeast China

Quantitative geochemical reconstruction of Eocene paleoenvironment in Fushun Basin, northeast China

Sequence stratigraphy of the Eocene deep-water organic-rich mudstones in the Fushun Basin, Northeast China

Sequence stratigraphy study in mudstone and oil shale strata is difficulty but critical for the analysis of the quality unconventional oil and gas reservoirs. This contribution investigates the sequence stratigraphy of the Eocene thick organic-rich fine-grained sediments in the Fushun basin, according to the organic geochemical data measured in laboratory and predicted by the amplitude difference the resistivity and the density curve when they are overlapping (which was called ΔlogR). The sampling interval of ΔlogR was 0.125 m. The results show that the total organic carbon, the oil yield, and the petroleum potential of the Eocene mudstones prove a good linear correlation between values measured in lab and values predicted by the ΔlogR model. A total of 21 parasequences are identified in mudstones and can be divided into two cycles, a fully developed lower cycle and an underdeveloped upper cycle. There are four organic matter accumulation stages in a fully developed cycle, including two stages in the uprising half-cycle characterized by low values and rising values of the predicted geochemical data, respectively, followed by two stages in the declining half cycle characterized by high values and decreasing values of the predicted geochemical data, respectively. This contribution provides a basis for a further analysis of the characteristics of organic matter cycles in the oil shale strata of the Fushun Basin and areas with similar situation.

Orbitally-forced meridional shifts of the westerlies modulated the hydroclimate of northeast China during the late Eocene

The mid-latitude westerlies are a fundamental component of the global climate system. There has been extensive research about how meridional shifts of the westerlies influenced the wet-dry paleoclimate regimes of mid-latitude Asia during the Quaternary. However, how the westerlies operated in much older geological times, such as the late Eocene, particularly on an orbital time scale, is not fully understood. Here, we conducted detailed cyclostratigraphic and palynological analyses on a late Eocene (41.2–37.8 Ma) mudstone-shale sedimentary sequence from the Fushun Basin in northeast China, which uniquely records orbitally-forced meridional shifts of the westerlies. Cyclostratigraphic analyses reveal that the mudstone-shale rhythms exhibit a prominent short eccentricity cycle, with a modest expression of obliquity cycles and a weak expression of precession cycles. The palynological results show that the vegetation experienced frequent alternations between semi-desert and swamp forest, and the climate exhibited periodic oscillations between warm-dry and cold-wet climates. We propose that a combination of eccentricity-modulated low-latitude summer insolation and obliquity-forced high-latitude incipient ice sheet and meridional temperature gradient variations drove cyclical vegetation and hydroclimate fluctuations in northeast China during the late Eocene by regulating the intensity and displacement of the westerlies and the subtropical high. Our results imply that continued poleward movement of the westerlies and the subtropical high will lead to increased aridification along the southern margin of the westerlies under a continuous global warming scenario.

Middle Eocene Climatic Optimum sensitivity of a continental lake basin from geochemical records of the Fushun Basin, Northeastern China

The dating of continuous Eocene sediments in the Fushun Basin, NE China, provides a good opportunity to study the evolution of the land paleoclimate during extreme high-temperature events in mid-latitude regions. Here, the lower part of the Eocene Xiloutian Formation in the Fushun Basin was sampled, and isotope geochemistry, element geochemistry, organic geochemical analysis, magnetic susceptibility, and sediment color analysis were conducted to determine the evolution of paleoclimate in the continental lake basin during the Middle Eocene Climatic Optimum (MECO) period. The oxygen (δ18O) and carbon (δ13C) isotope data indicated the presence of phases during the MECO, with 401.5–374.3 m (41.2–40.42 Ma) representing the main stage and 374.3–371.2 m (40.42–40.32 Ma) the recovery stage. In addition, evaluation of the opening and closing of lakes using δ18O and δ13C showed that the lakes in the lower part of the Xiloutian Formation in the Fushun Basin are currently in a transition period between open and closed. The reconstructed paleotemperature and precipitation in the lower part of the Xiloutian Formation were characterized by an initial high trend followed by a low trend, with the highest values of both occurring during the main MECO stage. The lower part of the Xiloutian Formation is characterized by an oxidative oxygen environment of brackish water, and MECO events did not affect the salinity and redox state of the water. However, the total organic carbon (average 5.24%) in the dark mudstone during the main MECO stage was significantly higher than that during the cooling period (average 4.67%), indicating that the MECO promoted carbon fixation and enrichment of organic matter. Climate comparison showed that the climatic types in eastern and western regions of China during the Eocene were similar. Furthermore, climatic evolution may be influenced by the uplift of the Tibetan Plateau, and the retreat of the inland proto Paratethys Sea.

Eocene Statistics of the Semi-Deep-Deep Lake-Level Evolution of the Jijuntun Formation in the Fushun Basin, Northeast China.

The fluctuation of lake levels in semi-deep and deep lake environments has long been a central topic in the study of ancient lake evolution. This phenomenon has a significant impact on the enrichment of organic matter and the overall ecosystem. The study of lake-level changes in deep lake environments is hindered by the scarcity of records in continental strata. To address this issue, we conducted a study on the Eocene Jijuntun Formation in Fushun Basin, specifically focusing on the LFD-1 well. Our study involved finely sampling the extremely thick oil shale (about 80 m), which was deposited in the semi-deep to deep lake environment of the Jijuntun Formation. The TOC was predicted by multiple methods, and the lake level study was restored by combining logging INPEFA and Dynamic noise after orbital tuning (DYNOT) techniques. The oil shale of the target layer is type I kerogen, and the source of organic matter is basically the same. The γ ray (GR), resistivity (RT), acoustic (AC), and density (DEN) logging curves are in the normal distribution, indicating that the logging data are better. The accuracy of TOC simulated by improved Δlog R, SVR, and XGBoost models is affected by the number of sample sets. The improved Δlog R model is most affected by the change of sample size, followed by the SVR model, and the XGBoost model is the most stable. In addition, compared with the prediction accuracy of TOC by improved Δlog R, SVR, and XGBoost models, it is shown that the improved Δlog R method has limitations in the prediction of TOC in oil shale. The SVR model is more suitable for the prediction of oil shale resources with small sample size, and the XGBoost model is applicable when the sample size is relatively large. According to the DYNOT analysis of logging INPEFA and TOC, the lake level changes frequently during the deposition of ultra-thick oil shale, and the lake level has experienced five stages of rising-stabilizing-frequent fluctuation-stabilizing- decreasing. The research results provide a theoretical basis for revealing the plane change of stable deep lake lakes and provide a basis for the study of lake levels in faulted lake basins in Paleogene Northeast Asia.

The sedimentary filling differences of subsags and their impacts on oil shale and coal accumulation in a rift lacustrine sag: A case study from the Lower Cretaceous in the Zhangqiang sag, southern Songliao Basin, Northeast China

In order to characterize the sedimentary filling differences of two rift lacustrine subsags and their control factors and to construct the depositional model for coal and oil shale, the Lower Cretaceous Jiufotang and Shahai formations in the Qijiazi and Zhanggutai subsags, Zhangqiang sag, Songliao Basin, one of the most petroliferous basins in Northeast China have been analyzed. Seven facies were identified in the Lower Cretaceous strata of two subsags and were grouped into three facies associations, namely alluvial fan, fan delta and lacustrine. During the deposition of Jiufotang Formation, the Qijiazi subsag is mainly characterized by alluvial fan deposits, while the Zhanggutai subsag is dominated by fan delta and littoral-sublittoral lacustrine deposits. During the deposition of Shahai Formation, the Qijiazi subsag is characterized by fan delta, littoral-sublittoral lacustrine, and deep lacustrine deposits, whereas the Zhanggutai subsag is filled with fan delta and littoral-sublittoral lacustrine deposits. Coal and oil shale deposits are only preserved in the Shahai Formation of the Qijiazi subsag. Total organic carbon content, organic matter type, vitrinite reflectance, thermal alteration index and total reduced sulfur content also show that there are differences in organic matter preservation and redox conditions between two subsags. In the Jiufotang Formation, the total organic carbon contents are relatively low, the kerogen type is dominated by type III and the source rocks entered a mature phase in the Zhanggutai subsag, and the total reduced sulfur contents show a suboxic to oxidized environment in the Zhanggutai subsag and an oxidized environment in the Qijiazi subsag. In the Shahai Formation, the total organic carbon contents are relatively high, the kerogen type is dominated by type II and the source rocks entered an immature to mature phase in the Zhanggutai subsag, whereas the kerogen type is dominated by type III and the source rocks display an overall mature phase in the Qijiazi subsag. The total reduced sulfur contents of the Shahai Formation show the reduced environment in two subsags. Tectonics, sediment supply and redox conditions control the temporal and spatial differences in sedimentary filling and distribution of organic-rich rocks in two subsags. A comparison of rift lacustrine Qijiazi subsag and Fushun Basin shows that the rift basins are both characterized by deepening-upward to shallowing-upward successions controlled by tectonic subsidence and climate changes. The relatively high subsidence along with warmer climate resulted in the enhanced input of lake plankton and the decrease in higher plant input and triggered the transition from coal to oil shale formation.

Comparison of Change in Nanopore Structure of Oil Shale after Anhydrous and Sub/Supercritical Water Pyrolysis

Oil shale is an important part of unconventional resources, which is considered as an important replacement for traditional oil resources. At present, the underground in-situ conversion method of oil shale is mainly aimed at shallow reservoirs, and it is difficult to apply to in-situ exploitation of oil shale with a depth of more than 1000m. In view of the portability, fracturing and extraction characteristics of high-pressure sub/supercritical water, it has become the research direction of in-situ mining of deep oil shale. The evolution of pore structure of oil shale is the basis for exploring the production mechanism of sub/supercritical water pyrolysis of oil and gas, which has important theoretical and scientific significance for improving the development and utilization of in-situ injection of sub/supercritical water in deep oil shale. Therefore, in this paper, the oil shale of Jijuntun Formation of Paleogene in Fushun Basin is taken as the research object. The pyrolysis experiments of oil shale in three ways, namely, water-free (electric heating), subcritical water and supercritical water, are carried out by means of high temperature and high-pressure source rock pyrolysis simulation device. The evolution characteristics of nano-pore structure under different pyrolysis methods are studied by XRD and low temperature nitrogen adsorption technology. The results show that, compared with anhydrous pyrolysis at the same temperature, the pore size of 3-30 nm of oil shale increases significantly after sub/supercritical water pyrolysis. The phenomenon of kaolinite illite fossilization increases, and the overall weak acidic environment is more conducive to the transformation of illite into Illite and montmorillonite mixed layer minerals, easier to increase the pore space in oil shale, and conducive to the migration and accumulation of produced oil and gas.

East Asian lake hydrology modulated by global sea-level variations in the Eocene warmhouse

Lake hydrological change and its driving mechanisms in the Eocene warmhouse climate are important to evaluating environmental and ecological changes induced by global warming. Our limited understanding of lake hydrological changes during the Eocene is in part due to the scarcity of well-dated and continuous terrestrial records. Here we refine the middle-late Eocene cyclostratigraphy and decipher high-resolution lake hydrology changes using sedimentary noise modeling and proxy data at four terrestrial basins (Dongying Depression, Nanxiang Basin, Jianghan Basin, and Fushun Basin) in mid-latitudes of East Asia. Our new astrochronology provides a robust timescale for the period of 48.5-38.5 Ma, and our tuned magnetostratigraphy is at least comparable with other timescales between Chrons C18n.1r and C20n. The new astrochronology reveals synchronous lake-level changes in the four basins in East Asia following 1.2 Myr obliquity and 2.4 Myr eccentricity cycles, indicating astronomical forcing on lake-level changes. The 2.4 Myr cycles of lake-levels are generally in phase with 2.4 Myr cycles of global sea-level changes, suggesting that East Asian lake hydrology was modulated by global sea-level variations in the Eocene warmhouse. Therefore, global sea-level variations may be an important driver of East Asia hydroclimate and freshwater resources.

Occurrence Patterns and Enrichment Influencing Factors of Trace Elements in Paleogene Coal in the Fushun Basin, China

The occurrence forms of trace elements in coal are of great significance for the clean utilization, abnormal enrichment, and recovery of beneficial elements in coal. The Fushun Basin in Northeast China has thick coal deposits which provide a good opportunity for studying their geochemistry. This study aims to estimate the element enrichment of Paleogene coal seams and their influencing factors during deposition based on statistical and geochemical analyses. Compared with world hard coals, coals in the Fushun Basin feature enrichment of Ga and Sb (CC > 5), slight enrichment of V, Cr, Co, Ni, As, Rb, Zr, Nb, and Cd (2 < CC < 5), and depletion of B, Tl, Bl, and U (CC < 0.5). The CC values of the remaining elements (0.5 < CC < 2) are close to the average values for world hard coals. The main carriers of Ga, Co, Rb, Mo, As, Se, Pb, V, and Li are potassium, iron, and sulfate minerals and those of Cd, Cr, Ni, Sb, Th, Sn, U, Hf, Zr, Cs, Ta, and Nb are clay minerals. The CIA, Sr/Cu, Rb/Sr, and Ga/Rb values suggest that the studied coal seam formed under humid/warm climatic conditions. The coal seam is mainly derived from intermediate source rocks and sandstone or mudstone source rocks which were exposed to intensive chemical weathering and deposited in a freshwater setting. Additionally, paleoweathering, paleoclimate, detrital input, and provenance all contributed to the enrichment of geochemical elements in the studied Paleogene coal. The results of this study are preliminary, and the authors will continue to conduct mineralogical analysis.

Eocene hyperthermal events drove episodes of vegetation turnover in the Fushun Basin, northeast China: Evidence from a palaeoclimate analysis of palynological assemblages

The response of terrestrial ecosystems to Eocene hyperthermal events remains poorly understood. In this paper, we report palynological assemblages from an astronomically-calibrated Eocene succession in the Fushun Basin of northeast China, in which previous δ13C studies have identified the Eocene Thermal Maxima 2 and 3 (ETM2, ETM3), the Early Eocene Climatic Optimum (EECO), and the Middle Eocene Climatic Optimum (MECO). In the samples that are directly associated with hyperthermal events, palynoassemblages show significant increases in the abundance of conifer pollen, especially Abietineaepollenites and Taxodiaceaepollenites, and fern spores of Polypodiaceaesporites, while angiosperm taxa reduce in frequency. Quantitative palaeoclimate parameters derived through the application of the Coexistence Approach to palynological assemblages indicate marked changes in temperature and humidity during hyperthermal events. These changes were most significant during ETM2 when mean annual temperature increased by 3–5 °C and mean annual precipitation increased by approximately 600 mm above background levels, but similar perturbations are also associated with the other hyperthermal events. An increase in organic abundance in lacustrine oil shale facies during hyperthermal events may imply that elevated humidity drove enhanced runoff, nutrient supply, and eutrophication of the hydrologically-open lake basins at a time when terrestrial carbon export from humid forests was also enhanced. Findings demonstrate that short-term bursts of Eocene warming drove major turnovers in the mid-latitude vegetation of Asia, reworking landscapes, and transforming regional hydrology.

Eocene Paleoclimate Evolution under the Background of Warmhouse-Hothouse Conditions in the Continental Fushun Basin: Implications from Magnetic Susceptibility and Color Reflectance.

Paleocene–Eocene hyperthermal events are a current research focus in the fields of sedimentology and paleoclimatology. The Fushun Basin in northeast China contains continuous continental Eocene fine-grained rocks, and a series of Eocene hyperthermal events in the Fushun Basin have been identified. Because of the high cost of high-precision isotope data testing, it is necessary to find new and alternative paleoclimate parameters. In this study, Eocene coal and oil shale-bearing layers in the Fushun Basin are used as research objects. The high-precision data of magnetic susceptibility, color reflectance, rock composition, and cluster analyses are used to conduct a vertical comparison in the same category and compare that analysis with the identified Eocene hyperthermal events in the Fushun Basin. The preliminary results show that high-frequency-dependent susceptibility, high color reflectance a* (redness)/L* (lightness) values, and high kaolinite content in the study area have good correspondence with global hyperthermal events and can be used as effective parameters for the identification of continental basin hyperthermal events. The detailed magnetic susceptibility and color reflectance data also reveal that the Eocene strata in the Fushun Basin recorded the Late Lutetian Thermal Maximum (LLTM) and 13 short-term hyperthermal events during the Early Eocene Climatic Optimum (EECO). These results indicate that the parameters of rock physical properties can be used to study the evolution of the paleoclimate in geological history, and it has universal practicability in continental and marine fine-grained sedimentary rocks.

Quantitative reconstruction of atmospheric pCO2 sources during Eocene hyperthermal events based on data from the Fushun Basin, Northeast China

Eocene hyperthermal events are thought to be closely linked to rapid increases in atmospheric CO2. It is important to better understand the atmospheric triggers of these extreme events; however, reconstruction of pCO2 from sedimentary records remains challenging. Eocene hyperthermal events (Eocene thermal maximum, ETM; Early Eocene Climatic Optimum, EECO; Mid-Eocene Climatic Optimum, MECO) are well documented in the Fushun Basin of northeast China, but the source of organic matter (OM) used as a pCO2 archive in previous studies includes land plants, algae, mixed land and aquatic organisms in the Ypresian, Lutetian and Bartonian intervals, respectively. In this paper, we calculate pCO2 concentrations using δ13CTOC and δ13C27–31 and discuss the factors influencing discrepancies based on various calculations. Results show that both δ13CTOC and δ13C27–31 from single OM sources are reliable proxies for pCO2. During the ETM2, ETM3, EECO and MECO intervals, pCO2 was 638–2277 ppm, and palaeotemperature was approximately 2.6–8.9 °C higher than present. The background and peak values of pCO2 during hyperthermal events were compared with those related to methane hydrate, OM oxidation and volcanic sources of pCO2 carbon. The comparison revealed that pCO2 values associated with the ETM2, ETM3, EECO and MECO events were more consistent with those calculated by OM oxidation and locally overlapped with volcanic sources. These results indicate that these pCO2 estimates of hyperthermal events are predominantly derived from terrestrial OM oxidation and were influenced by volcanism.

Abrupt collapse of a swamp ecosystem in northeast China during the Paleocene–Eocene Thermal Maximum

The Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) was a geologically abrupt and extreme global warming event that had a profound effect on global climate and ecosystems. Efforts to understand the response of vegetation to this hyperthermal event have hitherto primarily focused on Europe and North America. However, a limited record from China hampers the understanding of regional terrestrial ecosystem response to PETM warming. Here, we present the a detailed palynological record from the Fushun Basin, NE China, spanning the late Paleocene to early Eocene. The palynological assemblage reveals an abrupt change in vegetation composition as a response to PETM warming. Swamps dominated by taxodioid conifers (Cupressaceae) were abruptly replaced by communities of Pinaceae conifers and the megathermal genus Aquilapollenites . This marked the abrupt collapse of the swamp ecosystem during the PETM as climate became hotter and wetter and mean annual temperature (MAT) increased from 15.6 °C to 19.7 °C. Vegetation turnover was driven by rapid global warming and localised changes due to increases in precipitation, subsequent elevated water levels, and expansion of lake systems. The marked discrepancy in the characteristics and magnitudes of the respective vegetation responses shows that the effects of PETM warming were more severe for swamp ecosystems than for other terrestrial ecosystems. The PETM warming led to swamp degradation, frequent wildfires, and concomitant peat burning; these changes may have triggered further warming through positive feedback mechanisms. • A detailed PETM palynological record in China was obtained. • Transient and abrupt change in vegetation composition during the PETM. • Abrupt collapse of swamp ecosystem in NE China during the PETM. • Quantitative estimates show the MAT increased from 15.6 to 19.7 °C. • Response of swamp ecosystem of NE China to PETM warming is remarkable.

Eocene hyperthermal events in the terrestrial system: Geochronological and astrochronological constraints in the Fushun Basin, NE China

Frequent hyperthermal events during the Eocene (55-47 Ma), primarily characterised by carbon isotope excursions (CIEs), have been identified from marine records worldwide. However, only a few excursions have been recognised and described in continental deposits, thereby limiting the understanding of the effect of global warming on terrestrial systems. Thick, dark, organic-rich mudstone containing complete astronomical age information was deposited during the Palaeogene in the Fushun Basin. Based on U–Pb zircon dating and Milankovitch cycle analysis, the astronomical time scale (ATS) of the Fushun Basin was established. Based on the high-resolution chronostratigraphic framework, combined with the negative migration characteristics of organic carbon isotopes (δ13CTOC), three hyperthermal events were identified: Eocene Thermal Maximum 2 (ETM2), Eocene Thermal Maximum (ETM3) and long-term Early Eocene Climatic Optimum (EECO). In particular, several periods of short-term hyperthermal events were identified in the EECO period: L, M, N, O, P, Q, R, S, T, U, V, W, C22nH3, C22nH4, C22nH5, C21rH1, C21rH2, C21rH3, C21rH4, C21rH5, C21nH1 and C21nH2. The records of sea and land hyperthermal events show that the records corresponding to land basins are timelier and more sensitive.

Eocene Hyperthermal Events Drove Episodes of Vegetation Turnover in the Fushun Basin, Northeast China: Evidence from a Palaeoclimate Analysis of Palynological Assemblages

Eocene Hyperthermal Events Drove Episodes of Vegetation Turnover in the Fushun Basin, Northeast China: Evidence from a Palaeoclimate Analysis of Palynological Assemblages

Factors controlling the distribution of oil shale layers in the Eocene Fushun Basin, NE China

The Fushun Basin is a typical continental basin filled with organic-rich fine-grained sedimentary rocks, such as coal, oil shale and greyish green mudstone. Currently, research on this basin predominantly focuses on the enrichment mechanisms of organic matter in coal and oil shale and the controlling factors of coal seam transformation to oil shale. However, during the transformation stage, the deposition of oil shale ceased and was converted to interbeds of grey green mudstone and thin oil shale layers. The control mechanism of this type of sedimentary succession is still not clear. In this study, the thick oil shale in the upper part of the Jijuntun Formation (Unit I) and the high-frequency interbedded mudstone and thin oil shale in the Xiloutian Formation (Units II and III) were analysed as key strata. Organic and inorganic geochemical data were analysed to reconstruct the transformation of the various types and sources of organic matter and controlling factors including the palaeoclimate, palaeoweathering, bioproductivity, terrigenous clastic input, palaeosalinity, and redox conditions. Among these, Unit I was affected by humid climate conditions predominantly. The water depth and volume were large, and the input of terrigenous debris was low. Freshwater algae blooms and anoxic conditions led to a supply of organic matter and that was well preserved. This further resulted in a high abundance of organic matter in the sedimentary rocks. Unit II was affected by semihumid-semiarid and arid climate conditions. The lake became shallower while terrigenous clastic input and lake palaeosalinity increased; these changes represented the transition stage from freshwater to brackish water. The lake still exhibited suboxic/anoxic conditions, but the biological living space decreased, resulting in a limited organic matter supply and a low abundance of organic matter in the sediment. Unit III was affected by semihumid-semiarid climate conditions, and the palaeosalinity continued to increase. The lake had brackish water, with a high bioproductivity, a decreased water volume and a limited supply of organic matter. However, brackish water stratification occurred in the lake, which created a strongly anoxic environment and good preservation conditions, resulting in a secondary high abundance of organic matter. A comprehensive analysis indicates that the environmental change caused by the palaeoclimate was the main factor driving the oil shale deposition to cease and the onset to deposit grey green mudstone and thin oil shale in the Fushun Basin, Northeast Asia.

Spatial change of precipitation in response to the Paleocene-Eocene thermal Maximum warming in China

The Paleocene-Eocene Thermal Maximum (PETM), a transient greenhouse climate interval associated with a huge release of carbon to the ocean-atmosphere system ~56 million years ago, provides a geological case study for the effect of rapid global warming on regional hydroclimate. Here we reconstruct carbon isotope discrimination (△) of C3 plants across the PETM in the Fushun and Nanyang Basins of China. The △ values present remarkable increase at both sites but with greater magnitude in the Fushun Basin during the PETM, which is explained as a response to regional precipitation. Quantitative estimate of mean annual precipitation based on the △ values suggests that East Asian continent generally became wetter with relatively more precipitation occurring at higher latitudes in response to the PETM warming. Our results support the prediction that continuous anthropogenic warming will lead to a wetter climate in East Asia with relatively more precipitation in higher-latitude lands.

Major and trace elements in mid-Eocene lacustrine oil shales of the Fushun Basin, NE China: Concentration features and paleolimnological implications

Mid-Eocene tectonic subsidence of the pull-apart Fushun Basin, NE China accommodated an up to 300-m-thick oil shale succession within the lacustrine Jijuntun Formation (Lutetian). Based on sedimentary records and bulk element geochemistry, accumulation of this oil shale succession is modeled to evaluate the principal controls on enrichment patterns of organic matter (OM) and trace metals in lacustrine organic facies. The Jijuntun Formation, consisting primarily of massive oil shale and interbedded siltstone and sandstone that grade rapidly upward into well-laminated oil shale, records an overall evolution from lacustrine–deltaic facies associations to profundal facies associations. Sedimentological characteristics of the oil shale succession display a gradual change of moderately overfilled to balanced-fill lake fill mode, consistent with the up-section drop in siliciclastic inputs reflected by detrital proxies like decreased Si/Al and K/Na ratios. Variations in the sedimentary fill states are accompanied by increasing hydrographic restriction of the Jijuntun paleo-lake, in agreement with low Mo/TOC (consistently <0.54 × 10−4), Mo depletions (average ~1 ppm), and slight U deficiencies (average <2 ppm). Slightly elevated (V + Cr)/Al values in the oil shale suggest reduced oxygen contents in the sediment column relative to oxic bottom waters. The up-section decrease in both FeT/Al and Mn/Al ratios further reveals a shift in bottom-water redox state toward less oxidizing conditions throughout deposition of this oil shale succession. In response to such a change in redox, reactive Fe–Mn shuttling across the chemocline could become intense and facilitate P recycling in water masses, resulting in the coupled Fe–Mn–P enrichments. Euphotic zones of this paleo-lake likely had moderate to high primary and export productivity, supported by the magnitude of enrichments of total organic carbon (TOC) and P nutrient as well as increasing TOC to P ratios. Aerobic respiration of sinking organic particles should occur at the sediment–water interface to reduce benthic O2 levels in the paleo-lake. However, greater aerobic remineralization of labile OM was required to consume relatively more dissolved oxygen during deposition of the lower relative to upper oil shale succession. Hence, different degrees of aerobic OM remineralization are responsible for the contrast in TOC contents between the lower (avg. = 4.7 wt%) and upper (avg. = 12.0 wt%) Jijuntun Formation.