Sediments of the western part of the Valjevo-Mionica basin （Serbia） were examined both geochemically and mineralogically to explain, on the basis of their sedimentologicai characteristics, the causes of changes in t...Sediments of the western part of the Valjevo-Mionica basin （Serbia） were examined both geochemically and mineralogically to explain, on the basis of their sedimentologicai characteristics, the causes of changes in their qualitative and quantitative composition. A total of 62 samples obtained from the drillhole at depths up to 400 m was investigated. Using correlation of the obtained data, six geochemical zones were defined, two of which being specially distinguished by their mineralogical, geochemical and sedimentological characteristics. The first one, upper zone A, consists of banded marlstones interbedded with clay and oil shales and is characterized by presence of analcite and searlesite. These minerals and very high contents of Na20 indicate sedimentation in alkaline conditions with increased salinity in arid climate. That provided pronounced water stratification, as well as higher bioproductivity in the basin and sedimentary organic matter preservation. Therefore, the zone A sediments are characterized by high organic matter contents of the type which provides good potential for production of liquid hydrocarbons. Another specific zone, zone F, contains sediments with very high MgO, K2O and Li concentrations. Their geochemical correlation, as well as almost complete absence of illite in this zone, indicates the presence of interstratified clay mineral type illite-saponite （lithium-bearing Mg-smectite）.展开更多
1 Introduction The Qarhan Playa of Qaidam Basin is located at the northeastern QinghaiTibetan Plateau(QTP),in which thick lacustrine and evaporate sediments deposited since the late Pleistocene(Chen et al.,1985,1986)....1 Introduction The Qarhan Playa of Qaidam Basin is located at the northeastern QinghaiTibetan Plateau(QTP),in which thick lacustrine and evaporate sediments deposited since the late Pleistocene(Chen et al.,1985,1986).As surrounded by capacious Gobi and yardang fields and suffered strong westerly-northwesterly wind,abundant aeolian materials were trapped in these sediment sequences in the展开更多
This study identified soft-sediment deformation structures(SSDS)of seismic origin from lacustrine sediments in the late Quaternary paleo-dammed lake at Tashkorgan,northeastern Pamir.The observed deformation structures...This study identified soft-sediment deformation structures(SSDS)of seismic origin from lacustrine sediments in the late Quaternary paleo-dammed lake at Tashkorgan,northeastern Pamir.The observed deformation structures include sand dykes,liquefied diapir and convolute structures,gravity induced SSDS,and thixotropic pillar and tabular structures.We conducted a preliminary study on the morphology,formation and trigger mechanisms of pillar and tabular structures formed by liquefaction of underlying coarse sand and thixotropy of the upper silty clay.The regional tectonic setting and distribution of lacustrine strata indicate that the most probable trigger for the SSDS in lacustrine sediments was seismic activity,with an approximate earthquake magnitude of M〉6.0;the potential seismogenic fault is the southern part of the Kongur normal fault extensional system.AMS^4C dating results indicate that the SSDS were formed by seismic events occurring between 26050±100 yrBP and 22710±80 yrBP,implying intense fault activity in this region during the late Pleistocene.This study provides new evidence for understanding tectonic activity and regional geodynamics in western China.展开更多
This paper aims to investigate the present situation and transfer mechanisms of microplastics in lacustrine sediments in the Qinghai-Tibet Plateau.The study surveyed the average abundance of microplastics in sediments...This paper aims to investigate the present situation and transfer mechanisms of microplastics in lacustrine sediments in the Qinghai-Tibet Plateau.The study surveyed the average abundance of microplastics in sediments.The abundance of microplastics in sediments of lakes from the Qinghai-Tibet Plateau is 17.22-2643.65 items/kg DW and 0-60.63 items/kg DW based on the data of the Qinghai Lake and the Siying Co Basin.The microplastic abundance in sediments from small and medium lakes is very high compared to that in other areas in the world.Like microplastics in other lakes of the world,those in the lakes in the Qinghai-Tibet Plateau mainly include organic polymers PA,PET,PE,and PP and are primarily in the shape of fibers and fragments.The microplastic pollution of lacustrine sediments in the Qinghai-Tibet Plateau is affected by natural changes and by human activities,and the concentration of microplastics in lacustrine ecosystems gradually increases through food chains.Furthermore,the paper suggests the relevant administrative departments of the Qinghai-Tibet Plateau strengthen waste management while developing tourism and pay much attention to the impacts of microplastics in water environments.This study provides a reference for preventing and controlling microplastic contamination in the Qinghai-Tibet Plateau.展开更多
Chronology is the basis for using lacustrine sediments to reconstruct the history of en-vironmental change.Radioactive-nuclides such as 14C,210Pb and 137Cs dating are mainly used to establish the chronology for recent...Chronology is the basis for using lacustrine sediments to reconstruct the history of en-vironmental change.Radioactive-nuclides such as 14C,210Pb and 137Cs dating are mainly used to establish the chronology for recent several ten thousand years.Because of being mixed with“dead carbon”,the measured 14C age is always older than the actual age which is recognized as“reservoir effect”.Cs is a kind of active metal element,and easy to migrate vertically in the sediment that leads to the error of the time marker.210Pb dating should make sure to select CIC model or CRS model.On the Tibetan Plateau,most of the lakes are alkaline closed lake where Cs is more moveable in the sediment and the reservoir effect caused by“dead carbon”on 14C dating is stronger.Based on the analysis on results of 14C,210Pb and 137Cs of the lacustrine sediments from Co Ngoin,central Tibetan Plateau,we use the simple regression method to re-calculate the 14C calendar ages,and establish the time sequence for cores CE-1 and CE-2 with result of 210Pb CRS model for the upper 35 cm and re-calculated 14C calendar age for sediments bellow 35 cm.Since 1400 cal.a BP,sedimentation inter-rupted for more than 1000 years.According to this time sequence,variations of environmental proxies confidently reflect the major climatic events in Holocene.展开更多
Climate proxies,such as total organic carbon and nitrogen(TOC,TN),carbonate content(CaCO3),grain-size and pollen of the sediment core retrieved from enclosed Lake Daihai which lies in the north China environment sensi...Climate proxies,such as total organic carbon and nitrogen(TOC,TN),carbonate content(CaCO3),grain-size and pollen of the sediment core retrieved from enclosed Lake Daihai which lies in the north China environment sensitive zone are analyzed to reconstruct the environment evolution of the area based on high-resolution radiocarbon chronology.The results reveal that the TOC and TN contents of the sediments correlate well with pollen percentage and pollen flux variations during the Holocene,and both reach their peak values simultaneously at 6.7―3.5 ka BP(calendar age,7.6―3.6 ka BP).Since 6.7 ka BP,both the CaCO3 and organic matter contents of the core have simultaneous variations,and their high values also occur during 6.7―3.5 ka BP.While during 9.0―6.7 ka BP(calendar age,10―7.6 ka BP)relatively lower level of organic matter content and pollen flux corre-spond to the higher level of carbonate content.The above relations suggest that during 6.7―3.5 ka BP,the productivity and effective precipitation were greatly improved in the lake drainage area,and this would probably strengthen the hydrodynamic conditions,enhancing organic matter,pollen and carbonate inputs from terrestrial sources.Such processes would account for the enrichment of both organic matter and carbonate in the sediments.While during 9.0―6.7 ka BP,the lower level of or-ganic matter,pollen flux but high carbonate content show depressed productivity and declined vege-tation coverage.The higher carbonate content at this stage would have probably resulted from the higher evaporation ratio of the lake water under relatively drier climate conditions.Therefore,it is in-ferred that during 6.7―3.5 ka BP,the climate was more humid with abundant rainfalls and vegetation was more flourishing in the Lake Daihai area.This can be seen as Holocene Climate Optimum(HCO).As a result,this has evident discrepancies with the traditional notion that the HCO occurs at Early Holocene or early Mid-Holocene.展开更多
Lacustrine sedimentary nitrogen isotope(δ^(15)N)records offer crucial insights into the impacts of regional anthropogenic activities and environmental changes on nitrogen cycling dynamics within the context of global...Lacustrine sedimentary nitrogen isotope(δ^(15)N)records offer crucial insights into the impacts of regional anthropogenic activities and environmental changes on nitrogen cycling dynamics within the context of global climate change.This study summarized δ^(15)N data from 27 Chinese lakes since 1750 CE,categorizing them into Types Ⅰ(eastern and southern China,densely populated area)and Ⅱ(Northwestern China[NWC]and the Qinghai-Tibet Plateau[QTP],sparsely populated area).The results showed pronounced regional and temporal variability in nitrogen isotope trends.Type Ⅰ lakes exhibited a significant positive δ^(15)N deviation post-1950 CE,linked to population growth(r=0.91,p<0.01)and economic development,with mitigation after 2000 CE likely due to enhanced environmental policies.Conversely,Type Ⅱ lakes showed coherent δ^(15)N variations with natural climate change from 1750 to 1950 CE,followed by a post-1950 CE decline exceeding natural variability.This shift is attributed to a“warming-wetting trend”in NWC and the QTP,which presumably increased terrestrial productivity,altered nitrogen cycling,and enhanced ^(15)N-depleted nitrogen input via surface runoff.Rising temperatures may have enhanced the activity and abundance of nitrogen-fixing microorganisms,jointly driving a negative δ^(15)N shift.This study highlights climate conditions and anthropogenic activities as key drivers of nitrogen cycle perturbations in China,with their influences varying spatiotemporally,thereby providing crucial insights into the nitrogen cycling dynamics in Chinese lakes.展开更多
The traces left by earthquakes in lacustrine sediments are studied to determine the occurrence of ancient earthquakes by identifying seismically induced soft-sediment deformation structures(SSDS).Dating can help recon...The traces left by earthquakes in lacustrine sediments are studied to determine the occurrence of ancient earthquakes by identifying seismically induced soft-sediment deformation structures(SSDS).Dating can help reconstruct the relative frequency of earthquakes.Identifying seismically induced seismites,which carry abundant seismic information from numerous SSDS,is both critical and challenging.Studying the deformation mechanism of SSDS and learning about the common criteria of seismically induced SSDS improve the identification of earthquake triggers.With better research into SSDS,seismic events can be effectively captured,and temporal constraints can be carried out by 14C dating and optically stimulated luminescence(OSL)dating to identify and date the occurrence of ancient earthquakes.The present contribution primarily addresses the meaning and mechanism of SSDS and their relationship with earthquake magnitude as well as the common criteria of the SSDS induced by earthquakes.展开更多
Uranium(U)provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments.The proxy potential under suboxic-oxic
In the present work,15400 yr old geochemical records of a core from the subalpine Daping swamp are presented with the aim to examine the relationship between the chemical weathering and the climatic changes in the reg...In the present work,15400 yr old geochemical records of a core from the subalpine Daping swamp are presented with the aim to examine the relationship between the chemical weathering and the climatic changes in the region of the western Nanling Mountains,China.The climate of the study region was deeply controlled by the East Asian summer monsoon.The results indicate that,in the past 15400 yrs,the values of chemical index of alteration(CIA)ranged from 73.9%to 88.2%(mean:85.3%),suggested a medium and high intensity of chemical weathering.The local exogenous clastic materials,which were derived from the weathered residues,played a key role in contributing towards the sediments.Since the climate-induced chemical weathering exerted strong influences on the geochemical features of weathered residues,the geochemical characteristics of the sediments were deeply impacted by climatic conditions.Wetter and warmer conditions would favor increased chemical weathering,resulting in more leaching of soluble and mobile elements(e.g.,Ba and Sr)and leaving the resistant and immobile elements(e.g.,Al and Ti)enriched in the weathered residues.These materials were then eroded and transported into the lake,and led to the sediments characterized by the characteristic of having depleted soluble elements.In contrast,dry and cold conditions would result in an opposite trend.In this sense,the geochemical records can serve as proxies to indicate changes of chemical weathering intensity,which were closely related to the evolution of summer monsoon.展开更多
Although extensive studies have been performed on nitrogen isotopes in lake sediments, understanding the complexity of the δ15N variation related to past environmental and climatic conditions still remains unclear. S...Although extensive studies have been performed on nitrogen isotopes in lake sediments, understanding the complexity of the δ15N variation related to past environmental and climatic conditions still remains unclear. Supported by multi-proxy records including litho-units, organic carbon isotopes, ration of total organic carbon and total nitrogen, organic matter content, bulk dry density, the accumulation rate of organic matter, median grain size, magnetic susceptibility, and Rb/Sr and Ca/Mg ratios obtained from a sedimentary sequence in a small lake （i.e., Dahu Swamp） in the eastern Nanling Mountains in South China, we deduce that variation in the δ15N values results primarily from the sources of organic matter, which are significantly influenced by climatic conditions over the past -16 000 years. A low （or high） lake level resulting from dry （or wet） conditions would lead to more （or fewer） vascular plant remains in the sediments and stronger （or weaker） denitrification associated with hypolimnetic anoxia, resulting in lower （or higher） 615N values. In addition, dry （or wet） conditions would favor weakened （or strengthened） biogeochemical activity in the catchment, which could result in less （or more） external input of δ15N-enriched OM, thus leading to decreased （or increased） δ15N values. Our results indicate that the lake sediment δ15N record in the Nanling Mountains has the potential to provide valuable insight into past East Asian monsoon climate- driven environmental changes.展开更多
This paper examines the relationship among organic geochemical proxies （TOC, C/N ratio and ~13C） and pollen assemblages in Zhuye Lake sediments since the Late Glacial. Results show that the reaction extent of organi...This paper examines the relationship among organic geochemical proxies （TOC, C/N ratio and ~13C） and pollen assemblages in Zhuye Lake sediments since the Late Glacial. Results show that the reaction extent of organic geochemical proxies and pollen as- semblages to environment changes are different. Organic geochemical proxies are sensitive to overall environmental change, while pollen assemblages indicate detailed information of environmental change. For the entire sedimentary section （except the sand layer fi＇om the bottom of the section）, when values of TOC, C/N ratio and total pollen concentrations are high, 813C values are low, and vice versa. The different responses of organic geochemical proxies and pollen records in Zhuye Lake are mainly due to their different sensitivity and diverse influencing factors in different environmental conditions.展开更多
The Nenjiang Formation was continuously cored in the SK-1 borehole(China Cretaceous Continental Scientific Drilling Project-SongKe1well).Over 95.7%recovery rate,the obtained continuous core is 827.6 m in length,coveri...The Nenjiang Formation was continuously cored in the SK-1 borehole(China Cretaceous Continental Scientific Drilling Project-SongKe1well).Over 95.7%recovery rate,the obtained continuous core is 827.6 m in length,covering the geological age ranges from late Santonian to middle Campanian.This is the most complete and continuous sedimentary deposits in the Songliao basin till now.Here we systematically presented the lithological results of the observation of the cores in 2--5 cm thickness scale at the drilling spot,and the analysis on lithofacies and sedimentary sequence,cyclic stratigraphy.13 types of sedimentary rocks deposited mainly in lacustrine,delta and fluvial environments were found in the Nenjiang Fromation,and they were subdivided into 19 microfacies including dolomite,marl,argillaceous limestone,turbidite,mudstone of still water,volcanic ash,channel-lag deposit,point bar,natural levee,crevasse splay,crevasse channel,floodplain,flood lake,distributary mouth bar,subaqueous distributary channels,subaqueous natural levee,subaqueous inter-distributary bay,distal bar and adjacent shore sand bar.The whole sedimentary column is composed of 630 sixth-order depositional cycles in the meter-scale,which can be further organized in 148 fifth-order depositional cycles,38 fourth-order depositional cycles and 5 third-order depositional cycles.Meticulous depiction(centimeter level)of the whole Formation provides an approach to study its sedimentary process with high precision.The centimeter-scale description of the sequent core presented some previously unidentified dolomite and volcanic ash layers.The cyclostratigraphy of the Nenjiang Formation was controlled by Milankovitch cycled,which is helpful to discover the climatic changes of the late Santonian to the middle Campanian in Songliao Basin.In addition,the detailed description of the sequent section is significant for the upcoming oil and gas exploration in the Songliao Basin if regarding the new type of oil/gas systems.展开更多
The mid-Pleistocene climate transition （MPT） has been widely reported in worldwide geological events. As a key issue of the Quaternary geology, it has attracted much attention from the paleoclimate community. MPT re...The mid-Pleistocene climate transition （MPT） has been widely reported in worldwide geological events. As a key issue of the Quaternary geology, it has attracted much attention from the paleoclimate community. MPT refers to a period lasting for several hundreds of thousand years, during which the dominant climate periodicity gradually extended from 41 kyr to 100 kyr （Ruddiman et al., 1989）,展开更多
The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still uncl...The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still unclear whether the boundary of the monsoon region changes according to monsoon variability. Investigations into the early Holocene environment in monsoon marginal zones are crucial for understanding the monsoon boundary changes. Zhuye Lake is located at the northwest edge of the Asian summer monsoon, the northern Qilian Mountains, which are less affected by modern summer monsoon water vapor. Previous studies have reached different conclusions regarding the early Holocene climatic and environmental changes based on different dating methods（^14C and OSL（optically stimulated luminescence）） and materials（shells, carbonate, pollen concentrates and bulk organic carbon）. In this study, we synthesized 102 ^14C dates and 35 OSL dates from ten Holocene sedimentary sections and ten paleo-shorelines in the lake basin. A comparison between ages from different dating methods and materials generally shows that carbon reservoir effects are relatively slight in Zhuye Lake while the disordered chronologies are mainly related to the erosion processes and reworking effects. In addition, proxy data, including lithology, pollen, total organic carbon and carbonate, were collected from different sites of Zhuye Lake. According to the new synthesis, the early Holocene environment was relatively humid, associated with high runoff and lake water levels. The result indicates that the monsoon boundary moves to the north during the period of the intensified monsoon. A typical arid-area lake was formed during the mid-Holocene when carbonate accumulation and high organic matter contents were the main features of this period. The lake retreated strongly during the late Holocene, showing a drought trend. Overall, the lake evolution is generally consistent with the Holocene Asian summer monsoon change, showing the monsoon influence to monsoon marginal zones.展开更多
Cenozoic fine-grained sedimentary rocks in the southwestern Qaidam Basin,as typical salinized lacustrine sediments,are significantly different from those in other freshwater lakes under the impact of the high salinity...Cenozoic fine-grained sedimentary rocks in the southwestern Qaidam Basin,as typical salinized lacustrine sediments,are significantly different from those in other freshwater lakes under the impact of the high salinity in the lake.Many fine-grained sedimentary rock samples were selected from the lower member of Neogene(N_(1))in the Gaskule Oilfield,Qaidam Basin,northwest China,with the aim to analyze and test their petrology,pore structure,and organic geochemical characteristics and determine its genesis and geological significance.Based on previous results,a new genesis of mixed sedimentary rocks is proposed.The Cenozoic lacustrine fine-grained sedimentary rocks in SW Qaidam Basin belong to mixed sedimentary rocks,which can be divided into three types,including carbonate and terrigenous clastic-bearing mudstone,carbonate-bearing to terrigenous clastic mixed sedimentary rocks,and carbonate-terrigenous clastic mixed sedimentary rocks.The geneses of mixing are quite different from that of mixed marine rocks.According to the sedimentary characteristics of the salinized lakes,three new geneses of mixed deposits are proposed:syn-sedimentary mixing,varve interbedded mixing,and biogenic mixing.The reservoir spaces of the Cenozoic lacustrine fine-grained sedimentary rocks in SW Qaidam Basin include residual intergranular pores,intercrystalline pores,microfractures,and minor dissolved pores.The reservoirs have super-low porosity and ultra-low permeability,with a porosity of 1.82%-21.04%,averaging 7.71%,and permeability of(0.0028-254.86)×10^(−3)μm^(2),averaging 2.82×10^(−3)μm^(2).The test results of total organic carbon(TOC)content show that the highest content of organic carbon reached 1.41%,and those of vitrinite reflectance show that Ro ranges from 0.525%-0.824%,with an average of 0.806%.The organic geochemical characteristics show that the fine-grained mixed sedimentary rocks of SW Qaidam Basin have the potential for self-generation and self-storage.As an unconventional reservoir,it has potential industrial value.This is the first study of fine-grained sedimentary rocks in the Qaidam Basin,which offers certain theoretical and practical values.展开更多
Continental organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. Here, the Rock-Eval pyrolysis, biomarkers, organic petrology, and stable carbon isotope have...Continental organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. Here, the Rock-Eval pyrolysis, biomarkers, organic petrology, and stable carbon isotope have been carried out on the Middle Jurassic Dameigou Formation source rocks from the northwest part of Yuka Depression, Qaidam Basin in order to study their thermal maturity, source of organic matter(OM), and palaeoenvironment changes. The Rock-Eval pyrolysis data(e.g., T_(max)), vitrinite reflectance, and biomarker-derived thermal maturity parameters(e.g., carbon preference index, Ts/(Ts+Tm), C_(29)Ts/(C_(29)Ts+C_(29)αβ hopane), C_(30) αβ/(αβ+βα) hopanes, C_(29)ααα 20S/(20S+20R) steranes, and C_(29)αββ/(αββ+ααα) steranes) suggest all studied samples stay between immature and low mature stage. The maceral compositions, stable carbon isotope compositions, n-alkane distributions, and biomarker-derived source parameters(e. g., C_(27)/C_(29)ααα 20R sterane, ternary diagram of C_(27)-C28-C_(29)steranes, C24 tetracyclic terpane) indicate both aquatic organisms and higher plants are the source of OM in the shales, but land plants are dominant. Generally low gammacerane concentration and environment-related parameters(e. g., cross-plots of C_(27)/C_(29)ααα 20R sterane vs. Pr/Ph) indicate these source rocks may be derived from lacustrine and fluvial-deltaic environments with fresh water, which is also supported by the variations of stable carbon isotopes from OM in the source rocks. However, the stable carbon isotope compositions of OM in the source rocks were influenced by multiple factors(e.g., source types and depositional environment) in the Yuka Depression. Slightly brackish condition is recorded in the upper part of the ZK6-1 well favor the formation of lacustrine algae, as confirmed by high contents of C_(27) steranes and short-chain n-alkanes. The variation of reducing to oxidizing condition of study area is possibly associated with the periodical flooded river-influenced aquatic condition during the deposition of the Middle Jurassic Dameigou Formation.展开更多
31 samples of shales and mudstones developed in reducing hypersaline andfreshwater setting and 20 oil samples of Bamianhe oilfield, Bohai Bay basin were collected for adetailed study of petroleum system. Composition a...31 samples of shales and mudstones developed in reducing hypersaline andfreshwater setting and 20 oil samples of Bamianhe oilfield, Bohai Bay basin were collected for adetailed study of petroleum system. Composition and distribution, especially significance ofsteroids, are discussed as a part of those. Abundant steroidal biomarkers, including C_(27-29)regular Steranes, C_(28-30) 4-methyl steranes, di-nosteranes and aromatic steranes, were detected.Results show that the composition and distribution of the steroids in samples studied are functionsof thermal maturity, organic source, paleoenvironment and lithology of potential source rocks.Alga-rich Es_4 shales (brackish water) developed in the south slope of depocenter (Niuzhuang sag)were found particularly rich in steroidal biomarkers including C_(30) 4-methyl steranes anddinosteranes. Es_3 mudstones (fresh water) were found devoid of dinosteranes. Distribution patternsof regular steranes are completely different from methyl steranes indicating different origins ofthe specific compounds of the fractioa Diagnostic distribution of steranes in rock extracts ofdiverse intervals makes the compounds to be essential indictors of source-rock tracing. Methylsteranes prove to be much more useful in oil-source rock correlation than regular steranes. Resultsalso show that alga-rich Es_4 shales located in the south slope are not likely the primary sourcerock responsible for the oils discovered based on the composition and distribution of steroids.展开更多
We collected two lake sediment cores （MC and DM） from the East Antarctic region for analysis of biogenic silica and other biogeochemical parameters （e.g., organic matter, C, N, S, H）. Based on synthetically compar...We collected two lake sediment cores （MC and DM） from the East Antarctic region for analysis of biogenic silica and other biogeochemical parameters （e.g., organic matter, C, N, S, H）. Based on synthetically comparative research, we focused on the potential application of biogenic silica （BSi） for the reconstruction of paleo-primary productivity in the East Antarctic lakes. Analytical results showed that a large number of diatoms were well preserved in the freshwater lake sediments, and that concentrations of biogenic silica displayed notable fluctuations over different water depths. The content of biogenic silica had a consistent profile over water depth, and this pattern changed with organic matter, reflecting their potential as eco-environmental proxies. Low lev- els of BSi and organic matter indicated reduction of lake algal production, and corresponded to decreased lake primary productivity. Due to the fragile ecosystem state and limited contribution of terrestrinl organic matter in the East Antarctic lakes, the contents of biogenic silica in the lacustrine sediments can sensitively indicate the evolutionary history of paleo-primary productivity. Overall, BSi is an ideal proxy for the reconstruction of past eeo-environmental change recorded in the lacustrine sediments on East Antarctica.展开更多
In order to further understand the full vector excursional details of the geomagnetic field, a paleomagnetic and rock magnetic study of four sites has been conducted at the type locality of Pringle Falls, Oregon where...In order to further understand the full vector excursional details of the geomagnetic field, a paleomagnetic and rock magnetic study of four sites has been conducted at the type locality of Pringle Falls, Oregon where 827 samples were drilled and spaced along a distance of 5 km, for their detailed directional and relative paleointensity studies. The profiles have registered a high-reso- lution (>10 cm/kyr) paleomagnetic record of the excursion (ca. 211+/13 ka) as recorded by diatomaceous lacustrine sediments. Remanence as well as induced magnetization experiments to investigate the reproducibility of the signal throughout the profiles have been conducted. In addition, low-field susceptibility vs. temperature analysis was performed indicating that the main magnetic carrier is pure magnetite (Curie point 575 ℃). The magnetic grain size also has indicated Single Domain-Multi-Domain (SD-MD) magnetite. The demagnetization was done by alternating field (a.f.) experiments, and the mean directions were determined by principal component analyses. In addition, induced magnetic tests were done, such as magnetic susceptibility (x) analyses, saturation IRM, anhysteretic remanent magnetization (ARM70) as well as the normalization of J17.5 mT/ARM70 to attempt to obtain relative paleointensity records of these sediments in question. The results of the induced rock magnetic tests such as the normalization studies indicate a direct correlation between the decrease of the relative paleointensity variations (i.e. lows) with respect to the directional changes. The detailed behavior of the paleosignal is highly consistent, since they are rapidly deposited sediments providing a detailed representation of the paleofield. The dissected VGP paths in 3 different phases are highly internally consistent and are defined by clockwise and anticlockwise loops traveling from the high northern latitudes over eastern North America and the North Atlantic to South America and then to high southern latitudes. They then return to the high northern latitudes through the Pacific and over to Kamchatka. This VGP behavior defines the geomagnetic signature of the Pringle Falls excursion as recorded at the type locality.展开更多
基金the Central Laboratory of NIS-Naftagas,Novi Sad for Rock-Eval determinations and the Ministry of Science and Environmental Protection of the Government of the Republic of Serbia for supporting this research
文摘Sediments of the western part of the Valjevo-Mionica basin （Serbia） were examined both geochemically and mineralogically to explain, on the basis of their sedimentologicai characteristics, the causes of changes in their qualitative and quantitative composition. A total of 62 samples obtained from the drillhole at depths up to 400 m was investigated. Using correlation of the obtained data, six geochemical zones were defined, two of which being specially distinguished by their mineralogical, geochemical and sedimentological characteristics. The first one, upper zone A, consists of banded marlstones interbedded with clay and oil shales and is characterized by presence of analcite and searlesite. These minerals and very high contents of Na20 indicate sedimentation in alkaline conditions with increased salinity in arid climate. That provided pronounced water stratification, as well as higher bioproductivity in the basin and sedimentary organic matter preservation. Therefore, the zone A sediments are characterized by high organic matter contents of the type which provides good potential for production of liquid hydrocarbons. Another specific zone, zone F, contains sediments with very high MgO, K2O and Li concentrations. Their geochemical correlation, as well as almost complete absence of illite in this zone, indicates the presence of interstratified clay mineral type illite-saponite （lithium-bearing Mg-smectite）.
文摘1 Introduction The Qarhan Playa of Qaidam Basin is located at the northeastern QinghaiTibetan Plateau(QTP),in which thick lacustrine and evaporate sediments deposited since the late Pleistocene(Chen et al.,1985,1986).As surrounded by capacious Gobi and yardang fields and suffered strong westerly-northwesterly wind,abundant aeolian materials were trapped in these sediment sequences in the
基金financially supported by the National Natural Science Foundation of China(41702372)the Open Fund of State Key Laboratory of Earthquake Dynamics(LED2017B03)
文摘This study identified soft-sediment deformation structures(SSDS)of seismic origin from lacustrine sediments in the late Quaternary paleo-dammed lake at Tashkorgan,northeastern Pamir.The observed deformation structures include sand dykes,liquefied diapir and convolute structures,gravity induced SSDS,and thixotropic pillar and tabular structures.We conducted a preliminary study on the morphology,formation and trigger mechanisms of pillar and tabular structures formed by liquefaction of underlying coarse sand and thixotropy of the upper silty clay.The regional tectonic setting and distribution of lacustrine strata indicate that the most probable trigger for the SSDS in lacustrine sediments was seismic activity,with an approximate earthquake magnitude of M〉6.0;the potential seismogenic fault is the southern part of the Kongur normal fault extensional system.AMS^4C dating results indicate that the SSDS were formed by seismic events occurring between 26050±100 yrBP and 22710±80 yrBP,implying intense fault activity in this region during the late Pleistocene.This study provides new evidence for understanding tectonic activity and regional geodynamics in western China.
基金funded by the survey projects initiated by the Ministry of Natural Resources of the People’s Republic of China(DD20189220,DD20211317,DD20211398,1212011220224,and 121201011000150022)the project of the 2015 Natural Science Basic Research Plan of Shaanxi Province(2015JM4129)+1 种基金the project of 2016 Fundamental Research Funds for the Central Universities(open fund,310829161128)the project of 2021 Fundamental Research Funds for the Central Universities(open fund).
文摘This paper aims to investigate the present situation and transfer mechanisms of microplastics in lacustrine sediments in the Qinghai-Tibet Plateau.The study surveyed the average abundance of microplastics in sediments.The abundance of microplastics in sediments of lakes from the Qinghai-Tibet Plateau is 17.22-2643.65 items/kg DW and 0-60.63 items/kg DW based on the data of the Qinghai Lake and the Siying Co Basin.The microplastic abundance in sediments from small and medium lakes is very high compared to that in other areas in the world.Like microplastics in other lakes of the world,those in the lakes in the Qinghai-Tibet Plateau mainly include organic polymers PA,PET,PE,and PP and are primarily in the shape of fibers and fragments.The microplastic pollution of lacustrine sediments in the Qinghai-Tibet Plateau is affected by natural changes and by human activities,and the concentration of microplastics in lacustrine ecosystems gradually increases through food chains.Furthermore,the paper suggests the relevant administrative departments of the Qinghai-Tibet Plateau strengthen waste management while developing tourism and pay much attention to the impacts of microplastics in water environments.This study provides a reference for preventing and controlling microplastic contamination in the Qinghai-Tibet Plateau.
基金supported by the“973”Project(Grant No.2004CB720200)the Knowledge Innovation Project of the Chinese Academy of Sciences(Grant No.KZCX3-SW-339).
文摘Chronology is the basis for using lacustrine sediments to reconstruct the history of en-vironmental change.Radioactive-nuclides such as 14C,210Pb and 137Cs dating are mainly used to establish the chronology for recent several ten thousand years.Because of being mixed with“dead carbon”,the measured 14C age is always older than the actual age which is recognized as“reservoir effect”.Cs is a kind of active metal element,and easy to migrate vertically in the sediment that leads to the error of the time marker.210Pb dating should make sure to select CIC model or CRS model.On the Tibetan Plateau,most of the lakes are alkaline closed lake where Cs is more moveable in the sediment and the reservoir effect caused by“dead carbon”on 14C dating is stronger.Based on the analysis on results of 14C,210Pb and 137Cs of the lacustrine sediments from Co Ngoin,central Tibetan Plateau,we use the simple regression method to re-calculate the 14C calendar ages,and establish the time sequence for cores CE-1 and CE-2 with result of 210Pb CRS model for the upper 35 cm and re-calculated 14C calendar age for sediments bellow 35 cm.Since 1400 cal.a BP,sedimentation inter-rupted for more than 1000 years.According to this time sequence,variations of environmental proxies confidently reflect the major climatic events in Holocene.
基金This work was supported by the National Basic Research Program of China(Grant No.2004CB720200)the National Natural Science Foundation of China(Grant No.40502020)+1 种基金the Knowledge Innovation Project of the Chinese Academy of Sciences(Grant No.KZCX2-SW-118)the Foundation of State Key Lab of Loess and Quaternary Geology(Grant No.SKLLG015).
文摘Climate proxies,such as total organic carbon and nitrogen(TOC,TN),carbonate content(CaCO3),grain-size and pollen of the sediment core retrieved from enclosed Lake Daihai which lies in the north China environment sensitive zone are analyzed to reconstruct the environment evolution of the area based on high-resolution radiocarbon chronology.The results reveal that the TOC and TN contents of the sediments correlate well with pollen percentage and pollen flux variations during the Holocene,and both reach their peak values simultaneously at 6.7―3.5 ka BP(calendar age,7.6―3.6 ka BP).Since 6.7 ka BP,both the CaCO3 and organic matter contents of the core have simultaneous variations,and their high values also occur during 6.7―3.5 ka BP.While during 9.0―6.7 ka BP(calendar age,10―7.6 ka BP)relatively lower level of organic matter content and pollen flux corre-spond to the higher level of carbonate content.The above relations suggest that during 6.7―3.5 ka BP,the productivity and effective precipitation were greatly improved in the lake drainage area,and this would probably strengthen the hydrodynamic conditions,enhancing organic matter,pollen and carbonate inputs from terrestrial sources.Such processes would account for the enrichment of both organic matter and carbonate in the sediments.While during 9.0―6.7 ka BP,the lower level of or-ganic matter,pollen flux but high carbonate content show depressed productivity and declined vege-tation coverage.The higher carbonate content at this stage would have probably resulted from the higher evaporation ratio of the lake water under relatively drier climate conditions.Therefore,it is in-ferred that during 6.7―3.5 ka BP,the climate was more humid with abundant rainfalls and vegetation was more flourishing in the Lake Daihai area.This can be seen as Holocene Climate Optimum(HCO).As a result,this has evident discrepancies with the traditional notion that the HCO occurs at Early Holocene or early Mid-Holocene.
基金National Natural Science Foundation of China,No.42171160。
文摘Lacustrine sedimentary nitrogen isotope(δ^(15)N)records offer crucial insights into the impacts of regional anthropogenic activities and environmental changes on nitrogen cycling dynamics within the context of global climate change.This study summarized δ^(15)N data from 27 Chinese lakes since 1750 CE,categorizing them into Types Ⅰ(eastern and southern China,densely populated area)and Ⅱ(Northwestern China[NWC]and the Qinghai-Tibet Plateau[QTP],sparsely populated area).The results showed pronounced regional and temporal variability in nitrogen isotope trends.Type Ⅰ lakes exhibited a significant positive δ^(15)N deviation post-1950 CE,linked to population growth(r=0.91,p<0.01)and economic development,with mitigation after 2000 CE likely due to enhanced environmental policies.Conversely,Type Ⅱ lakes showed coherent δ^(15)N variations with natural climate change from 1750 to 1950 CE,followed by a post-1950 CE decline exceeding natural variability.This shift is attributed to a“warming-wetting trend”in NWC and the QTP,which presumably increased terrestrial productivity,altered nitrogen cycling,and enhanced ^(15)N-depleted nitrogen input via surface runoff.Rising temperatures may have enhanced the activity and abundance of nitrogen-fixing microorganisms,jointly driving a negative δ^(15)N shift.This study highlights climate conditions and anthropogenic activities as key drivers of nitrogen cycle perturbations in China,with their influences varying spatiotemporally,thereby providing crucial insights into the nitrogen cycling dynamics in Chinese lakes.
基金the National Institute of Natural Hazards,Ministry of Emergency Management of China(ZDJ2019-21)the National Natural Science Foundation of China(Nos.41872227 and 41602221).
文摘The traces left by earthquakes in lacustrine sediments are studied to determine the occurrence of ancient earthquakes by identifying seismically induced soft-sediment deformation structures(SSDS).Dating can help reconstruct the relative frequency of earthquakes.Identifying seismically induced seismites,which carry abundant seismic information from numerous SSDS,is both critical and challenging.Studying the deformation mechanism of SSDS and learning about the common criteria of seismically induced SSDS improve the identification of earthquake triggers.With better research into SSDS,seismic events can be effectively captured,and temporal constraints can be carried out by 14C dating and optically stimulated luminescence(OSL)dating to identify and date the occurrence of ancient earthquakes.The present contribution primarily addresses the meaning and mechanism of SSDS and their relationship with earthquake magnitude as well as the common criteria of the SSDS induced by earthquakes.
文摘Uranium(U)provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments.The proxy potential under suboxic-oxic
基金Under the auspices of the National Natural Science Foundation of China(No.41971101,41571187,41071137)。
文摘In the present work,15400 yr old geochemical records of a core from the subalpine Daping swamp are presented with the aim to examine the relationship between the chemical weathering and the climatic changes in the region of the western Nanling Mountains,China.The climate of the study region was deeply controlled by the East Asian summer monsoon.The results indicate that,in the past 15400 yrs,the values of chemical index of alteration(CIA)ranged from 73.9%to 88.2%(mean:85.3%),suggested a medium and high intensity of chemical weathering.The local exogenous clastic materials,which were derived from the weathered residues,played a key role in contributing towards the sediments.Since the climate-induced chemical weathering exerted strong influences on the geochemical features of weathered residues,the geochemical characteristics of the sediments were deeply impacted by climatic conditions.Wetter and warmer conditions would favor increased chemical weathering,resulting in more leaching of soluble and mobile elements(e.g.,Ba and Sr)and leaving the resistant and immobile elements(e.g.,Al and Ti)enriched in the weathered residues.These materials were then eroded and transported into the lake,and led to the sediments characterized by the characteristic of having depleted soluble elements.In contrast,dry and cold conditions would result in an opposite trend.In this sense,the geochemical records can serve as proxies to indicate changes of chemical weathering intensity,which were closely related to the evolution of summer monsoon.
基金Supported by the National Natural Science Foundation of China(Nos.41071137,40671189)the Research Program of Higher Education Institution in Guangdong Province for High-level Talents,and the Natural Science Foundation of Guangdong Province(Nos.S2011010003413,8151063101000044)
文摘Although extensive studies have been performed on nitrogen isotopes in lake sediments, understanding the complexity of the δ15N variation related to past environmental and climatic conditions still remains unclear. Supported by multi-proxy records including litho-units, organic carbon isotopes, ration of total organic carbon and total nitrogen, organic matter content, bulk dry density, the accumulation rate of organic matter, median grain size, magnetic susceptibility, and Rb/Sr and Ca/Mg ratios obtained from a sedimentary sequence in a small lake （i.e., Dahu Swamp） in the eastern Nanling Mountains in South China, we deduce that variation in the δ15N values results primarily from the sources of organic matter, which are significantly influenced by climatic conditions over the past -16 000 years. A low （or high） lake level resulting from dry （or wet） conditions would lead to more （or fewer） vascular plant remains in the sediments and stronger （or weaker） denitrification associated with hypolimnetic anoxia, resulting in lower （or higher） 615N values. In addition, dry （or wet） conditions would favor weakened （or strengthened） biogeochemical activity in the catchment, which could result in less （or more） external input of δ15N-enriched OM, thus leading to decreased （or increased） δ15N values. Our results indicate that the lake sediment δ15N record in the Nanling Mountains has the potential to provide valuable insight into past East Asian monsoon climate- driven environmental changes.
基金supported by the National Natural Science Foundation of China(Grant Nos.41371009 and 41001116)the Fundamental Research Fund for the Central Universities(Grant Nos.lzujbky-2013-127 and lzujbky-2013-129)
文摘This paper examines the relationship among organic geochemical proxies （TOC, C/N ratio and ~13C） and pollen assemblages in Zhuye Lake sediments since the Late Glacial. Results show that the reaction extent of organic geochemical proxies and pollen as- semblages to environment changes are different. Organic geochemical proxies are sensitive to overall environmental change, while pollen assemblages indicate detailed information of environmental change. For the entire sedimentary section （except the sand layer fi＇om the bottom of the section）, when values of TOC, C/N ratio and total pollen concentrations are high, 813C values are low, and vice versa. The different responses of organic geochemical proxies and pollen records in Zhuye Lake are mainly due to their different sensitivity and diverse influencing factors in different environmental conditions.
基金Supported by projects of the National Key R&D Program of China(No.2019YFC0605402)National Natural Science Foundation of China(No.41790453)
文摘The Nenjiang Formation was continuously cored in the SK-1 borehole(China Cretaceous Continental Scientific Drilling Project-SongKe1well).Over 95.7%recovery rate,the obtained continuous core is 827.6 m in length,covering the geological age ranges from late Santonian to middle Campanian.This is the most complete and continuous sedimentary deposits in the Songliao basin till now.Here we systematically presented the lithological results of the observation of the cores in 2--5 cm thickness scale at the drilling spot,and the analysis on lithofacies and sedimentary sequence,cyclic stratigraphy.13 types of sedimentary rocks deposited mainly in lacustrine,delta and fluvial environments were found in the Nenjiang Fromation,and they were subdivided into 19 microfacies including dolomite,marl,argillaceous limestone,turbidite,mudstone of still water,volcanic ash,channel-lag deposit,point bar,natural levee,crevasse splay,crevasse channel,floodplain,flood lake,distributary mouth bar,subaqueous distributary channels,subaqueous natural levee,subaqueous inter-distributary bay,distal bar and adjacent shore sand bar.The whole sedimentary column is composed of 630 sixth-order depositional cycles in the meter-scale,which can be further organized in 148 fifth-order depositional cycles,38 fourth-order depositional cycles and 5 third-order depositional cycles.Meticulous depiction(centimeter level)of the whole Formation provides an approach to study its sedimentary process with high precision.The centimeter-scale description of the sequent core presented some previously unidentified dolomite and volcanic ash layers.The cyclostratigraphy of the Nenjiang Formation was controlled by Milankovitch cycled,which is helpful to discover the climatic changes of the late Santonian to the middle Campanian in Songliao Basin.In addition,the detailed description of the sequent section is significant for the upcoming oil and gas exploration in the Songliao Basin if regarding the new type of oil/gas systems.
基金financially supported by the Geological Investigation Project of China Geological Survey(No.12120113006200)
文摘The mid-Pleistocene climate transition （MPT） has been widely reported in worldwide geological events. As a key issue of the Quaternary geology, it has attracted much attention from the paleoclimate community. MPT refers to a period lasting for several hundreds of thousand years, during which the dominant climate periodicity gradually extended from 41 kyr to 100 kyr （Ruddiman et al., 1989）,
基金supported by the National Natural Science Foundation of China (41371009)the Fundamental Research Fund for the Central Universities (lzujbky-2013-127)
文摘The intensified monsoon increases summer rainfall and creates wet conditions in the Asian summer monsoon region during the early Holocene. Along with millennial-scale changes of the monsoon intensity, it is still unclear whether the boundary of the monsoon region changes according to monsoon variability. Investigations into the early Holocene environment in monsoon marginal zones are crucial for understanding the monsoon boundary changes. Zhuye Lake is located at the northwest edge of the Asian summer monsoon, the northern Qilian Mountains, which are less affected by modern summer monsoon water vapor. Previous studies have reached different conclusions regarding the early Holocene climatic and environmental changes based on different dating methods（^14C and OSL（optically stimulated luminescence）） and materials（shells, carbonate, pollen concentrates and bulk organic carbon）. In this study, we synthesized 102 ^14C dates and 35 OSL dates from ten Holocene sedimentary sections and ten paleo-shorelines in the lake basin. A comparison between ages from different dating methods and materials generally shows that carbon reservoir effects are relatively slight in Zhuye Lake while the disordered chronologies are mainly related to the erosion processes and reworking effects. In addition, proxy data, including lithology, pollen, total organic carbon and carbonate, were collected from different sites of Zhuye Lake. According to the new synthesis, the early Holocene environment was relatively humid, associated with high runoff and lake water levels. The result indicates that the monsoon boundary moves to the north during the period of the intensified monsoon. A typical arid-area lake was formed during the mid-Holocene when carbonate accumulation and high organic matter contents were the main features of this period. The lake retreated strongly during the late Holocene, showing a drought trend. Overall, the lake evolution is generally consistent with the Holocene Asian summer monsoon change, showing the monsoon influence to monsoon marginal zones.
基金This work is supported by the Science and Technology Project(Grant No.2019k02)of CNPC Qinghai Oilfield Company.
文摘Cenozoic fine-grained sedimentary rocks in the southwestern Qaidam Basin,as typical salinized lacustrine sediments,are significantly different from those in other freshwater lakes under the impact of the high salinity in the lake.Many fine-grained sedimentary rock samples were selected from the lower member of Neogene(N_(1))in the Gaskule Oilfield,Qaidam Basin,northwest China,with the aim to analyze and test their petrology,pore structure,and organic geochemical characteristics and determine its genesis and geological significance.Based on previous results,a new genesis of mixed sedimentary rocks is proposed.The Cenozoic lacustrine fine-grained sedimentary rocks in SW Qaidam Basin belong to mixed sedimentary rocks,which can be divided into three types,including carbonate and terrigenous clastic-bearing mudstone,carbonate-bearing to terrigenous clastic mixed sedimentary rocks,and carbonate-terrigenous clastic mixed sedimentary rocks.The geneses of mixing are quite different from that of mixed marine rocks.According to the sedimentary characteristics of the salinized lakes,three new geneses of mixed deposits are proposed:syn-sedimentary mixing,varve interbedded mixing,and biogenic mixing.The reservoir spaces of the Cenozoic lacustrine fine-grained sedimentary rocks in SW Qaidam Basin include residual intergranular pores,intercrystalline pores,microfractures,and minor dissolved pores.The reservoirs have super-low porosity and ultra-low permeability,with a porosity of 1.82%-21.04%,averaging 7.71%,and permeability of(0.0028-254.86)×10^(−3)μm^(2),averaging 2.82×10^(−3)μm^(2).The test results of total organic carbon(TOC)content show that the highest content of organic carbon reached 1.41%,and those of vitrinite reflectance show that Ro ranges from 0.525%-0.824%,with an average of 0.806%.The organic geochemical characteristics show that the fine-grained mixed sedimentary rocks of SW Qaidam Basin have the potential for self-generation and self-storage.As an unconventional reservoir,it has potential industrial value.This is the first study of fine-grained sedimentary rocks in the Qaidam Basin,which offers certain theoretical and practical values.
基金financially supported by the National Natural Science Foundation of China (Nos.42102215,41702144)the Science and Technology Research Project for Higher Education Institutions in Hebei Province (No.QN2021033)。
文摘Continental organic-rich shales are well developed in the Dameigou Formation within the Yuka Depression of the Qaidam Basin. Here, the Rock-Eval pyrolysis, biomarkers, organic petrology, and stable carbon isotope have been carried out on the Middle Jurassic Dameigou Formation source rocks from the northwest part of Yuka Depression, Qaidam Basin in order to study their thermal maturity, source of organic matter(OM), and palaeoenvironment changes. The Rock-Eval pyrolysis data(e.g., T_(max)), vitrinite reflectance, and biomarker-derived thermal maturity parameters(e.g., carbon preference index, Ts/(Ts+Tm), C_(29)Ts/(C_(29)Ts+C_(29)αβ hopane), C_(30) αβ/(αβ+βα) hopanes, C_(29)ααα 20S/(20S+20R) steranes, and C_(29)αββ/(αββ+ααα) steranes) suggest all studied samples stay between immature and low mature stage. The maceral compositions, stable carbon isotope compositions, n-alkane distributions, and biomarker-derived source parameters(e. g., C_(27)/C_(29)ααα 20R sterane, ternary diagram of C_(27)-C28-C_(29)steranes, C24 tetracyclic terpane) indicate both aquatic organisms and higher plants are the source of OM in the shales, but land plants are dominant. Generally low gammacerane concentration and environment-related parameters(e. g., cross-plots of C_(27)/C_(29)ααα 20R sterane vs. Pr/Ph) indicate these source rocks may be derived from lacustrine and fluvial-deltaic environments with fresh water, which is also supported by the variations of stable carbon isotopes from OM in the source rocks. However, the stable carbon isotope compositions of OM in the source rocks were influenced by multiple factors(e.g., source types and depositional environment) in the Yuka Depression. Slightly brackish condition is recorded in the upper part of the ZK6-1 well favor the formation of lacustrine algae, as confirmed by high contents of C_(27) steranes and short-chain n-alkanes. The variation of reducing to oxidizing condition of study area is possibly associated with the periodical flooded river-influenced aquatic condition during the deposition of the Middle Jurassic Dameigou Formation.
基金ThispaperissupportedbytheNaturalScienceResearchCouncilofChi na (underChina 973NationalKeyResearchandDevelopmentProgram #G1 999 4331 0 ) .
文摘31 samples of shales and mudstones developed in reducing hypersaline andfreshwater setting and 20 oil samples of Bamianhe oilfield, Bohai Bay basin were collected for adetailed study of petroleum system. Composition and distribution, especially significance ofsteroids, are discussed as a part of those. Abundant steroidal biomarkers, including C_(27-29)regular Steranes, C_(28-30) 4-methyl steranes, di-nosteranes and aromatic steranes, were detected.Results show that the composition and distribution of the steroids in samples studied are functionsof thermal maturity, organic source, paleoenvironment and lithology of potential source rocks.Alga-rich Es_4 shales (brackish water) developed in the south slope of depocenter (Niuzhuang sag)were found particularly rich in steroidal biomarkers including C_(30) 4-methyl steranes anddinosteranes. Es_3 mudstones (fresh water) were found devoid of dinosteranes. Distribution patternsof regular steranes are completely different from methyl steranes indicating different origins ofthe specific compounds of the fractioa Diagnostic distribution of steranes in rock extracts ofdiverse intervals makes the compounds to be essential indictors of source-rock tracing. Methylsteranes prove to be much more useful in oil-source rock correlation than regular steranes. Resultsalso show that alga-rich Es_4 shales located in the south slope are not likely the primary sourcerock responsible for the oils discovered based on the composition and distribution of steroids.
基金supported by the National Natural Science Foundation of China (Grant nos.40876096,41076123 and 40606003)the Open Research Funds from SOA Key Laboratory for Polar Science (Grant no.KP2007002)a special fund for excellent PhD thesis of CAS
文摘We collected two lake sediment cores （MC and DM） from the East Antarctic region for analysis of biogenic silica and other biogeochemical parameters （e.g., organic matter, C, N, S, H）. Based on synthetically comparative research, we focused on the potential application of biogenic silica （BSi） for the reconstruction of paleo-primary productivity in the East Antarctic lakes. Analytical results showed that a large number of diatoms were well preserved in the freshwater lake sediments, and that concentrations of biogenic silica displayed notable fluctuations over different water depths. The content of biogenic silica had a consistent profile over water depth, and this pattern changed with organic matter, reflecting their potential as eco-environmental proxies. Low lev- els of BSi and organic matter indicated reduction of lake algal production, and corresponded to decreased lake primary productivity. Due to the fragile ecosystem state and limited contribution of terrestrinl organic matter in the East Antarctic lakes, the contents of biogenic silica in the lacustrine sediments can sensitively indicate the evolutionary history of paleo-primary productivity. Overall, BSi is an ideal proxy for the reconstruction of past eeo-environmental change recorded in the lacustrine sediments on East Antarctica.
文摘In order to further understand the full vector excursional details of the geomagnetic field, a paleomagnetic and rock magnetic study of four sites has been conducted at the type locality of Pringle Falls, Oregon where 827 samples were drilled and spaced along a distance of 5 km, for their detailed directional and relative paleointensity studies. The profiles have registered a high-reso- lution (>10 cm/kyr) paleomagnetic record of the excursion (ca. 211+/13 ka) as recorded by diatomaceous lacustrine sediments. Remanence as well as induced magnetization experiments to investigate the reproducibility of the signal throughout the profiles have been conducted. In addition, low-field susceptibility vs. temperature analysis was performed indicating that the main magnetic carrier is pure magnetite (Curie point 575 ℃). The magnetic grain size also has indicated Single Domain-Multi-Domain (SD-MD) magnetite. The demagnetization was done by alternating field (a.f.) experiments, and the mean directions were determined by principal component analyses. In addition, induced magnetic tests were done, such as magnetic susceptibility (x) analyses, saturation IRM, anhysteretic remanent magnetization (ARM70) as well as the normalization of J17.5 mT/ARM70 to attempt to obtain relative paleointensity records of these sediments in question. The results of the induced rock magnetic tests such as the normalization studies indicate a direct correlation between the decrease of the relative paleointensity variations (i.e. lows) with respect to the directional changes. The detailed behavior of the paleosignal is highly consistent, since they are rapidly deposited sediments providing a detailed representation of the paleofield. The dissected VGP paths in 3 different phases are highly internally consistent and are defined by clockwise and anticlockwise loops traveling from the high northern latitudes over eastern North America and the North Atlantic to South America and then to high southern latitudes. They then return to the high northern latitudes through the Pacific and over to Kamchatka. This VGP behavior defines the geomagnetic signature of the Pringle Falls excursion as recorded at the type locality.
