Catchment environmental change over the 20th Century recorded by sedimentary leaf wax n-alkane δ~(13)C off the Pearl River estuary

The compound-specific stable carbon isotope compositions(δ~(13)C) of leaf wax n-alkanes from two short sediment cores recovered off the Pearl River estuary(PRE) were analyzed to check for their capability of indicating decadal scale catchment environmental change. Sedimentary long-chain n-alkanes exhibited an odd-over-even predominance, with a maximum at n-C_(29) or n-C_(31), indicating their leaf wax origin was from vascular plants. The δ~(13)C values of C_(29) and C_(31) n-alkane in all the sediment samples were in the range of -28.8‰ to -31.2‰, consistent with the C_3 plant-dominated vegetation in the Pearl River catchments. The time series of δ~(13)C records from the two cores were comparable and displayed a decreasing trend from the early 20 th century to the end of the 1970s, followed by a reversal in that change leading to continued increase for ca. 15 years. After being corrected for the effect of atmospheric CO_2 rise and δ~(13)C_(atm) decline, the δ~(13)C_(29) records largely retained their raw changing pattern; the post-1980 increase being more conspicuous. The slightly decreasing trend in corrected δ~(13)C records before around 1980 may have been caused by an increase in precipitation, whereas the subsequent increase of δ~(13)C is likely associated with the observed dry climate and/or intensive anthropogenic deforestation. Our results thus demonstrate that leaf wax n-alkanes buried in the sediments off the PRE may well reflect change in the regional climate and/or human activity in the river catchments over the past century.     

Detection and indication of 1,3,4-C27–29 triol in the sediment of northern South China Sea

After the detection of the 1,20,21-C_(29) long-chain triol in some sediments and freshwater pteridophytes, in this study, a new homologous long-chain triol, 1,3,4-C_(27-29), is detected for the first time in the Site4 B core sediment in the northern South China Sea. The hydroxyl location and length of the carbon chain of this newly discovered triol differ from those of 1,20,21-C_(29) triol. The test results of its molecular distribution and individual carbon isotope reveal that 1,3,4-C_(29) triol has a good correlation with n-C_(26-30) even carbon-numbered long-chain fatty alcohols, with R~2(n=68) values of 0.905, 0.929 and 0.903, respectively, and its carbon isotope composition, at –32.3‰±1.9‰, is similar to that of n-C_(26-30), at –29.13‰±0.87‰, –32.98‰±1.28‰, and –32.98‰±1.28‰. 1,3,4-C_(29) triol from the Site4 B core sediment and terrigenous long-chain fatty alcohol(n-C_(26-34)) show highly consistent distribution trends in the entire section; thus, the former could serve as a proxy indicator of the terrigenous input. Considering that the 1,20,21-C_(29) triol in previous research belongs to Azolla, which are fresh water pteridophytes, the 1,3,4-C_(27-29) triol identified in this study might have similar biogenetic derivation. Thus, determination of its biogenic area and growing environment could provide potential organic geochemical evidence supporting the terrigenous input and source in the northern South China Sea.     

Characterization of the negative carbon isotope shift in segment C2, its global implications as a harbinger of OAE1a

Lower Cretaceous C-isotope records show intermittent negative/positive spikes, and consistent patterns of coeval chemostratigraphic curves thus document shifts that signal simultaneous responses of temporal changes in the global carbon reservoir. The standard pattern registered by the δ ¹³C_org and δ ¹³C_carb in Lower Aptian sediments includes distinct isotope segments C1 to C8 (Menegatti et al., 1998). In the El Pui section, Organyà Basin, Spain, C-isotope segment C2 is the longest interval preceding segments C3–C6 associated with oceanic anoxic event 1a (OAE 1a), and reveals a distinct negative shift of ~1.8‰ to ~2.23‰ defining the C-isotope pattern within that interval. Total inorganic carbon (TIC), total organic carbon (TOC), δ ¹³C_org, microfacies, n-alkanes show no difference before, during, or after the negative inflection. The biomarkers indicate that organic matter (OM) mainly originates from algal/microbial sources because short-chain length homologues (≤nC₁₉) dominate. nC₂₀ through nC₂₅ indicate some contribution from aquatic vegetation, but little from higher plants (>nC₂₅), as also suggested by the terrestrial/aquatic ratio of n-alkanes or (TAR) = [(nC₂₇+nC₂₉+nC₃₁)/(nC₁₅+nC₁₇+nC₁₉)] (averages 0.085). We suggest that conjoint pulses of contemporaneous LIPs (Ontong Java) and massive explosive volcanism in northeast Asia, the Songliao Basin (SB-V), best conform to plausible causes of the negative intra-C2 carbon isotopic excursion (CIE) at that time. Because of its apparent common occurrence the intra-C2 inflection could be a useful marker harbinger to the more pronounced CIE C3, the hallmark of OAE1a.     

Altitudinal effect of soil <Emphasis Type="Italic">n</Emphasis>-alkane <Emphasis Type="Italic">δ</Emphasis>D values on the eastern Tibetan Plateau and their increasing isotopic fractionation with altitude

Stable isotope paleoaltimetry has provided unprecedented insights into the topographic histories of many of the world’s highest mountain ranges. However, on the Tibetan Plateau (TP), stable isotopes from paleosols generally yield much higher paleoaltitudes than those based on fossils. It is therefore essential when attempting to interpret accurately this region’s paleoaltitudes that the empirical calibrations of local stable isotopes and the relations between them are established. Additionally, it is vital that careful estimations be made when estimate how different isotopes sourced from different areas may have been influenced by different controls. We present here 29 hydrogen isotopic values for leaf wax-derived n-alkanes (i.e., _δD_wax values, and abundance-weighted average δD values of C₂₉ and C₃₁) in surface soils, as well as the δD values of soil water (δD_sw) samples (totaling 22) from Mount Longmen (LM), on the eastern TP (altitude ~0.8–4.0 km above sea level (asl), a region climatically affected by the East Asian Monsoon (EAM). We compared our results with published data from Mount Gongga (GG). In addition, 47 river water samples, 55 spring water samples, and the daily and monthly summer precipitation records (from May to October, 2015) from two precipitation observation stations were collected along the GG transect for δD analysis. LM soil _δD_wax values showed regional differences and responded strongly to altitude, varying from?160‰ to?219‰, with an altitudinal lapse rate (ALR) of?18‰ km^?1 (R ²=0.83; p<0.0001; n=29). These _δD_wax values appeared more enriched than those from the GG transect by ~40‰. We found that both the climate and moisture sources led to the differences observed in soil _δD_wax values between the LM and GG transects. We found that, as a general rule, ε _wax/rw, ε _wax/p and ε _wax/sw values (i.e., the isotopic fractionation of _δD_wax corresponding to δD_rw, δD_p and δD_sw) increased with increasing altitude along both the LM and GG transects (up to 34‰and 50‰, respectively). Basing its research on a comparative study of _δD_wax, δD_p, δD_rw(δD_springw) and δD_sw, this paper discusses the effects of moisture recycling, glacier-fed meltwater, relative humidity (RH), evapotranspiration (ET), vegetation cover, latitude, topography and/or other factors on ε _wax/p values. Clearly, if ε _wax-p values at higher altitudes are calculated using smaller ε _wax-p values from lower altitudes, the calculated paleowaterδD_p values are going to be more depleted than the actual δD values, and any paleoaltitude would therefore be overestimated.     

Preliminary study on the origin identification of natural gas by the parameters of light hydrocarbon

The light hydrocarbon composition of 209 natural gas samples and individual light hydrocarbon carbon isotopes of 53 natural gas samples from typical humic-sourced gas and sapropelic-sourced gas in the four basins of China have been determined and analyzed. Some identification parameters for humic-sourced gas and sapropelic-sourced gas are proposed or corrected. The differences of compound-specific δ ¹³C value of individual light hydrocarbon between humic-sourced gas and sapropelic-sourced gas have been founded. The humic-sourced gas has the distribution of δ ¹³C_benzene> ?24‰, δ ¹³C_toluene >?23‰, δ ¹³C_cyclohexane > ?24‰ and δ ¹³C_{methyl cyclohexane}> ?24‰, while the sapropelic-sourced gas has the distribution of δ ¹³C_benzene <?24‰, δ ¹³C_toluene< ?24‰, δ ¹³C_cyclohexane< ?24‰ and δ ¹³C_{methyl cyclohexane}< ?24‰. Among the components of C₇ light hydrocarbon compound, such as normal heptane (nC₇), methyl cyclohexane (MCH) and dimethyl cyclopentane (ΣDMCP), etc, relative contents of nC₇ and MCH are influenced mainly by the source organic matter type of natural gas. Therefore, it is suggested that the gas with relative content of nC₇ of more than 30% and relative content of MCH of less than 70% is sapropelic-sourced gas, while gas with relative content of nC₇ of less than 35% and relative content of MCH of more than 50% is humic-sourced gas. Among components of C_5–7 aliphatics, the gas with relative content of C_5–7 normal alkane of more than 30% is sapropelic-sourced gas, while the gas with relative content of C_5–7 normal alkane of less than 30% is humic-sourced gas. These paremeters have been suggested to identify humic-sourced gas and sapropelic-sourced gas.     

Classification and origin of natural gases from Lishui Sag,the East China Sea Basin

Natural gases discovered up to now in Lishui Sag, the East China Sea Basin, differ greatly in gaseous compositions, of which hydrocarbon gases amount to 2%–94% while non-hydrocarbon gases are dominated by CO₂. Their hydrocarbon gases, without exception, contain less than 90% of methane and over 10% of C₂ ⁺ heavier hydrocarbons, indicating a wet gas. Carbon isotopic analyses on these hydrocarbon gases showed that δ ¹³C₁, δ ¹³C₂ and δ ¹³C₃ are basically lighter than ?44‰, ?29‰ and ?26‰, respectively. The difference in carbon isotopic values between methane and ethane is great, suggesting a biogenic oil-type gas produced by the mixed organic matter at peak generation. δ ¹³ \(C_{CO_2 } \) values of nonhydrocarbon gases are all heavier than ?10‰, indicating a typical abiogenic gas. The simulation experiment on hydrocarbon generation of organic matter in a closed gold-tube system showed that the proportion of methane in natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit is obviously higher than that in natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit, consequently the proportion of heavier hydrocarbons of the former is remarkably lower than that of the latter. Moreover, δ ¹³C₁ values of natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit are about 5‰ heavier than those of natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit while δ ¹³C₂ and δ ¹³C₃ values of the former are over 9‰ heavier than those of the latter. Currently the LS36-1 oil-gas pool is the only commercial oil-gas reservoir in Lishui Sag, where carbon isotopic compositions of various hydrocarbon components differ greatly from those of natural gases produced by the Lingfeng Formation organic matter but are very similar to those of natural gases derived from the Yueguifeng Formation organic matter, therefore, natural gases in the LS36-1 oil-gas pool are mainly derived from the Yueguifeng Formation lacustrine source rock rather than the Lingfeng Formation marine or Mingyuefeng Formation coal-measures source rocks.     

Evidence from isotopic geochemistry as an indicator of eutrophication of Meiliang Bay in Lake Taihu,China

In this paper, Lake Taihu, a large shallow freshwater lake in China, is chosen as an example of reconstruction of eutrophication through the comparison between stable isotopes from dissolved nutrients and plants and water column nutrient parameters and integration of multiple proxies in a sediment core from Meiliang Bay including TN, TP, TOC, C/N, δ ¹⁵N, δ ¹³C, etc. Differences in aquatic plant species and trophic status between East Taihu Bay and Meiliang Bay are indicated by their variations in δ ¹³C and δ ¹⁵N of aquatic plants and δ ¹⁵N of NH₄ ⁺. A significant influence of external nutrient inputs on Meiliang Bay is reflected in temporal changes in δ ¹⁵N of NH₄ ⁺ and hydro-environmental parameters. The synchronous change between δ ¹³C and δ ¹⁵N values of sedimented organic matter (OM) has been attributed to elevated primary production at the beginning of eutrophication between 1950 and 1990, then recent inverse correlation between them has been caused by the uptake of ¹⁵N-enriched inorganic nitrogen by phytoplankton grown under eutrophication and subsequent OM decomposition and denitrification in surface sediments, indicating that the lake has suffered from progressive eutrophication since 1990. Based on the use of a combination of stable isotopes and elemental geochemistry, the eutrophication of Meiliang Bay in Lake Taihu could be better traced. These transitions of the lake eutrophication respectively occurring in the 1950s and 1990s have been suggested as a reflection of growing impacts of human activities, which is coincident with the instrumental data.     

Light carbon isotope events of foraminifera attributed to methane release from gas hydrates on the continental slope,northeastern South China Sea

In 2013, the China Geological Survey and Guangzhou Marine Geological Survey conducted the second Chinese gas hydrate expedition in the northern South China Sea(SCS) and successfully obtained visible gas hydrate samples. Five of the thirteen drilling sites were cored for further research. In this work, Site GMGS2-08 is selected for the stable isotopic analysis of foraminifera present in the boreholes in order to reveal the carbon isotopic characteristics of the foraminifera and their response to methane release in the gas hydrate geological system. Our results show that the methane content at Site GMGS2-08 is extremely high, with headspace methane concentrations up to 39300 μmol L~(-1). The hydrocarbon δ~(13)C values, ranging from-69.4‰ to-72.3‰ PDB, distinctly indicate biogenic generation. Based on the δD analytical results(~(-1)83‰ to~(-1)85‰ SMOW), headspace methane is further discriminated to be microbial gas, derived from CO_2 reduction. By isotopic measurement, five light δ~(13)C events are found in the boreholes from Site GMGS2-08, with foraminiferal δ~(13)C values being apparently lower than the normal variation range found in the glacial-interglacial cycles of the SCS. The δ~(13)C values of benthic Uvigerina peregrina are extremely depleted(as low as~(-1)5.85‰ PDB), while those of planktonic Globigerinoides ruber reach-5.68‰ PDB. Scanning electron micrograph(SEM) studies show that foraminiferal tests have experienced post-depositional alteration, infilled with authigenic carbonate, and the diagenetic mineralization is unlikely to be related to the burial depths. The correlation calculation suggests that the anaerobic oxidation of organic matter has only weak influences on the δ~(13)C composition of benthic foraminifera. This means that the anomalous δ~(13)C depletions are predominantly attributed to the overprinting of secondary carbonates derived from the anaerobic oxidation of methane(AOM). Furthermore, the negative δ~(13)C anomalies, coupled with the positive δ18O anomalies observed at Site GMGS2-08, are most likely the critical pieces of evidence for gas hydrate dissociation in the geological history of the study area.     

Evaluation of the carbon isotopic effects of NDIR and CRDS analyzers on atmospheric CO2 measurements

Non-dispersive infrared(NDIR) and cavity ring-down spectroscopy(CRDS) CO_2 analyzers use 12CO_2 isotopologue absorption lines and are insensitive to all or part of other CO_2-related isotopologues. This may produce biases in CO_2 mole fraction measurements of a sample if its carbon isotopic composition deviates from that of the standard gases being used. To evaluate and compare the effects of carbon isotopic composition on NDIR and CRDS CO_2 analyzers, we prepared three test sample air cylinders with varying carbon isotopic abundances and calibrated them against five standard cylinders with ambient carbon isotopic composition using CRDS and NDIR systems. We found that the CO_2 mole fractions of the sample cylinders measured by G1301(CRDS) were in good agreement with those measured by Lo Flo(NDIR). The CO_2 values measured by both instruments were higher than that of a CO_2 isotope measured by G2201i(CRDS) analyzer for a test cylinder with depleted carbon isotopic composition δ~(13)C =-36.828‰, whereas no obvious difference was found for other two test cylinders with δ~(13)C=-8.630‰ and δ~(13)C=-15.380‰, respectively. According to the theoretical and experimental results, we concluded that the total CO_2 mole fractions of samples with depleted isotopic compositions can be corrected on the basis of their 12CO_2 values calibrated by standard gases using Lo Flo and G1301 if the δ~(13)C and δ18O values are known.     

Natural gas geochemistry and its origins in Kuqa depression

According to gas compositional and carbon isotopic measurement of 114 gas samples from the Kuqa depression, accumulation of the natural gases in the depression is dominated by hydrocarbon gases, with high gas dryness (C₁/C_1–4) at the middle and northern parts of the depression and low one towards east and west sides and southern part. The carbon isotopes of methane and its homologues are relatively enriched in ¹³C, and the distributive range of δ ¹³C₁, δ ¹³C₂ and δ ¹³C₃ is ?32‰–?36‰, ?22‰–?24‰ and ?20‰–?22‰, respectively. In general, the carbon isotopes of gaseous alkanes become less negative with the increase of carbon numbers. The δ ¹³ \(C_{CO_2 } \) value is less than ?10‰ in the Kuqa depression, indicating its organogenic origin. The distributive range of ³He/⁴He ratio is within n × 10^?8 and a decrease in ³He/⁴He ratio from north to south in the depression is observed. Based on the geochemical parameters of natural gas above, natural gas in the Kuqa depression is of characteristics of coal-type gas origin. The possible reasons for the partial reversal of stable carbon isotopes of gaseous alkanes involve the mixing of gases from one common source rock with different thermal maturity or from two separated source rock intervals of similar kerogen type, multistages accumulation of natural gas under high-temperature and over-pressure conditions, and sufficiency and diffusion of natural gas.     

Experimental study on the shear behavior of the interface between cushion materials and the concrete raft

Cushion is a layer of granular materials between the raft and the ground. The shear behavior of the interface between the cushion and the raft may influence the seismic performance of the superstructure. In order to quantify such influences, horizontal shear tests on the interfaces between different cushion materials and concrete raft under monotonic and cyclic loading were carried out. The vertical pressure P_v, material type and cushion thickness h_c were taken as variables. Conclusions include: 1) under monotonic loading, P_v is the most significant factor; the shear resistance P_(hmax) increases as P_v increases, but the normalized factor of resistance μ_n has an opposite tendency; 2) for the materials used in this study, μ_n varies from 0.40 to 0.70, the interface friction angle δ_s varies from 20° to 35°, while u_(max) varies from 3 mm to 15 mm; 3) under cyclic loading, the interface behavior can be abstracted as a "three-segment" back-bone curve, the main parameters include μ_n, the displacement u_1 and stiffness K_1 of the elastic stage, the displacement u_2 and stiffness K_2 of the plastic stage; 4) by observation and statistical analysis, the significance of different factors, together with values of K_1, K_2 and μ_n have been obtained.     

Hydrogen isotopes in palmitic and stearic acids in suspended particles from the Changjiang River Estuary

Hydrogen isotopes in lipid biomarkers can trace past changes in the hydrologic cycle. Recent studies have revealed the potential of hydrogen isotopes in microalgal lipids for quantitatively reconstructing water δ~2H(δD) values and salinity. In this study we collected suspended particles along a salinity gradient from the Changjiang River Estuary(CRE), and measured δD values in fatty acids in these particles. The results indicated that δD values of water were correlated highly with salinity from the CRE, in agreement with the results from other estuaries. δD values in palmitic acid and stearic acid had a positive correlation with δD values of water from the CRE. Nevertheless, in the CRE, hydrogen isotope fractionation in fatty acids relative to water increased as salinity increased, opposite the trend in hydrogen isotope fractionation with salinity found in microalgal culture and field studies. We attribute the increase in hydrogen isotope fractionation as salinity increased to light availability, which was likely lower in the particle rich mixing zone at the end of the estuary, and potentially as well to multiple sources of fatty acids in the CRE.     

Geochemical characteristics and origin of gases from the Upper,Lower Paleozoic and the Mesozoic reservoirs in the Ordos Basin,China

The Ordos Basin, the second largest sedimentary basin in China, contains the broad distribution of natural gas types. So far, several giant gas fields have been discovered in the Upper and Lower Paleozoic in this basin, each having over 1000×10⁸m³ of proven gas reserves, and several gas pools have also been discovered in the Mesozoic. This paper collected the data of natural gases and elucidated the geochemical characteristics of gases from different reservoirs, and then discussed their origin. For hydrocarbons preserved in the Upper Paleozoic, the elevated δ ¹³C values of methane, ethane and propane indicate that the gases would be mainly coal-formed gases; the singular reversal in the stable carbon isotopes of gaseous alkanes suggests the mixed gases from humic sources with different maturity. In the Lower Paleozoic, the δ ¹³C₁ values are mostly similar with those in the Upper Paleozoic, but the δ ¹³C₂ and δ ¹³C₃ values are slightly lighter, suggesting that the gases would be mixing of coal-type gases as a main member and oil-type gases. There are multiple reversals in carbon isotopes for gaseous alkanes, especially abnormal reversal for methane and ethane (i.e. δ ¹³C₁>δ ¹³C₂), inferring that gases would be mixed between high-mature coal-formed gases and oil-type gases. In the Mesozoic, the δ ¹³C values for gaseous alkanes are enriched in ¹²C, indicating that the gases are mainly derived from sapropelic sources; the carbon isotopic reversal for propane and butane in the Mesozoic is caused by microbial oxidation and mixing of gases from sapropelic sources with different maturity. In contrast to the Upper Paleozoic gases, the Mesozoic gases are characterized by heavier carbon isotopes of iso-butane than normal butane, which may be caused by gases generated from different kerogen types. Finally, according to δ ¹³C₁-R ₀ relationship and extremely low total organic carbon contents, the Low Paleozoic gases would not be generated from the Ordovician source as a main gas source, bycontrast, the Upper Paleozoic source as a main gas source is contributed to the Lower Paleozoic gases.     

Geochemical characteristics of light hydrocarbons in cracking gases from chloroform bitumen A,crude oil and its fractions

The composition characteristics of light hydrocarbons from crude oil, chloroform bitumen A, saturated hydrocarbon fraction, aromatic hydrocarbon fraction, and asphaltene fraction during cracking have been studied systematically. The results revealed that the content of n-alkanes, branched alkanes and cycloalkanes in light hydrocarbons from the samples gradually decreased as the simulation temperature increased, and finally almost depleted completely, while the abundance of methane, benzene and its homologues increased obviously and became the main products. The ratios of benzene/ n-hexane and toluene/n-heptane can be used as measures for oil cracking levels. Variation characteristics of maturity parameters of light hydrocarbons, for example, iC₄/nC₄, iC₅/nC₅, isoheptane value, 2,2-DMC₄/nC₆, and 2-MC₆+3-MC₆/nC₇ for different samples with increasing pyrolysis temperature, are consistent with those in petroleum reservoirs, indicating that these parameters may be efficient maturity index.     

Alcohol compounds in <Emphasis Type="Italic">Azolla imbricata</Emphasis> and potential source implication for marine sediments

This study investigated the composition of long-chain alkyl diols, triols, sec-alcohols, hydroxyl acids, and other hydroxylated compounds in Azolla imbricata and compared the organic alcohol components of Azolla filiculoides, Azolla microphylla, and South China Sea (SCS) sediments in order to investigate the possible indication of Azolla being the biological source of diols and triols in SCS sediment. Large amounts of diols, monohydroxy acids, and sec-alcohols with internal hydroxy groups at ω20 were detected in the three types of Azolla. Among these, 1,ω20-diol and ω20-hydroxy acid exhibited strong even-odd predominance distribution, whereas ω20-sec-alcohol exhibited strong odd-even predominance distribution. In addition, small amounts of diols, triols, and dihydroxy acids with internal hydroxy groups at 9, 10 or ω9, ω10 were detected, among which the chain length of C₂₉ was predominate. Compounds having similar structures as those in Azolla reflected a similar biosynthetic pathway: ω20-hydroxy acid exhibiting even-odd predominance distribution is decarboxylated to ω20-sec-alcohol exhibiting odd-even predominance distribution and converted to 1, ω20-diol with even-odd predominance distribution by acyl reduction; ω9, ω10-hydroxy acid is converted to 1,20,21(1, ω9, ω10)-triol by acyl reduction, and then converted to 9,10-diol by hydrogenation and dehydration. The alcohol components in A. imbricata were clearly not the biological source of 1,13/1,14/1,15-C_{28, 30, 32} diols and 1,3,4-C_27-29 triols in the SCS sediment. Certain marine diatoms might be the source of 1,14-C_{28, 30} diol in inshore sediment, but the biological source of diols and triols in the SCS sediment requires further investigation.     

Natural gas leakage of Mizhi gas reservoir in Ordos Basin,recorded by natural gas fluid inclusion

Abundant natural gas inclusions were found in calcite veins filled in fractures of Central Fault Belt across the centre of Ordos Basin. Time of the calcite veins and characteristics of natural gas fluid inclusion were investigated by means of dating of thermolum luminescence (TL) and analyzing stable isotope of fluid inclusion. Results show that natural gas inclusion formed at 130–140°C with salinity of 5.5 wt%–6.0 wt% NaCl. It indicates that natural gas inclusion is a kind of thermal hydrocarbon fluid formed within the basin. Method of opening inclusion by heating was used to analyze composition of fluid inclusion online, of which the maximal hydrocarbon gas content of fluid inclusion contained in veins is 2.4219 m³/t rock and the maximal C₁/ΣC _i ratio is 91%. Laser Raman spectroscopy (LRS) was used to analyze chemistry of individual fluid inclusion in which the maximal hydrocarbon gas content is 91.6% compared with little inorganic composition. Isotope analysis results of calcite veins show that they were deposited in fresh water, in which the δ ¹³C_PDB of calcite veins is from ?5.75‰ to 15.23‰ and δ ¹⁸O_SMOW of calcite veins is from 21.33‰ to 21.67‰. Isotope results show that δ ¹³C_{1 PDB} of natural gas fluid inclusion is from ?21.36‰ to ?29.06‰ and δD_SMOW of that is from ?70.89‰ to ?111.03‰. It indicates that the gas of fluid inclusion formed from coal source rocks and it is the same as that of natural gas of Mizhi gas reservoir. Results of TL dating show that time of calcite vein is (32.4±3.42)×10⁴ a, which is thought to be formation time of gas inclusion. It indicated that natural gas inclusion contained in calcite veins recorded natural gas leakage from Mizhi gas reservoir through the Central Fault Belt due to Himalayan tectonic movement.     

Geochemical characteristics of pyrolysis gas from epimetamorphic rocks in the northern basement of Songliao Basin,Northeast China

The Xushen gas field, located in the north of Songliao Basin, is a potential giant gas area for China in the future. Its proved reserves have exceeded 1000×10⁸ m³ by the end of 2005. But, the origin of natural gases from the deep strata is still in debating. Epimetamorphic rocks as a potential gas source are widely spreading in the northern basement of Songliao Basin. According to pyrolysis experiments for these rocks in the semi-confined system, gas production and geochemistry of alkane gases are discussed in this paper. The Carboniferous-Permian epimetamorphic rocks were heated from 300°C to 550°C, with temperature interval of 50°C. The gas production was quantified and measured for chemical and carbon isotopic compositions. Results show that δ ¹³C₁ is less than ?20‰, carbon isotope trend of alkane gas is δ ¹³C₁<δ ¹³C₂<δ ¹³C₃ or δ ¹³C₁<δ ¹³C₂>δ ¹³C₃, these features suggest that the gas would be coal-type gas at high-over maturity, not be inorganic gas with reversal trend of gaseous alkanes (δ ¹³C₁>δ ¹³C₂>δ ¹³C₃). These characteristics of carbon isotopes are similar with the natural gas from the basin basement, but disagree with gas from the Xingcheng reservoir. Thus, the mixing gases from the pyrolysis gas with coal-typed gases at high-over maturity or oil-typed gases do not cause the reversal trend of carbon isotopes. The gas generation intensity for epimetamorphic rocks is 3.0×10⁸–23.8×10⁸ m³/km², corresponding to R _o from 2.0% to 3.5% for organic matter.     

Early dolomitisation of the Lower-Middle Ordovician cyclic carbonates in northern Tarim Basin,NW China

High-frequency metre-scale cycles are present within the Lower-Middle Ordovician carbonate successions in northern Tarim Basin, NW China. These metre-scale cycles were variably dolomitised from top to bottom. Three types of replacive dolomites were recognised, including dololaminite(very finely to finely crystalline, planar-s to nonplanar-a dolomite;type-1), patterned dolomite(finely crystalline, planar-s dolomite; type-2), and mottled dolomite(finely to medium crystalline,nonplanar-a(s) dolomite; type-3). Petrographic evidence indicate these dolomites were primarily deposited in supratidal to restricted subtidal environments, and formed in near-surface to shallow burial realms. Geochemically, all types of dolomites have similar δ13C and 87Sr/86 Sr ratios comparable to calcite precipitated in equilibrium with the Early-Middle Ordovician seawater. These geochemical attributes indicate that these dolomites were genetically associated and likely formed from connate seawater-derived brines. Of these, type-1 dolomite has δ18O values(.4.97‰ to.4.04‰ VPDB) slightly higher than those of normal seawater dolomite of the Early-Middle Ordovician age. Considering the absence of associated evaporites within type-1 dolomite, its parental fluids were likely represented by slightly evaporated(i.e., mesosaline to penesaline) seawater with salinity below that of gypsum precipitation. More depleted δ18O values(.7.74‰ to.5.20‰ VPDB) of type-2 dolomite and its stratigraphic position below type-1 dolomite indicate the generation of this dolomite from mesosaline to penesaline brines at higher temperatures in near-surface to shallow burial domains. Type-3 dolomite yields the most depleted δ18O values(–9.30‰to –7.28‰ VPDB), pointing to that it was most likely formed from coeval seawater-derived brines at highest temperatures in a shallow burial setting. There is a downward decreasing trend in δ18O values from type-1 through type-2 to type-3 dolomites, and in abundance of dolomites, indicating that the dolomitising fluids probably migrated downward from above and persisted into shallow burial conditions.     

Spatiotemporal variations of actual evapotranspiration over the Lake Selin Co and surrounding small lakes (Tibetan Plateau) during 2003–2012

Actual evapotranspiration(ET_a) over the Tibetan Plateau(TP) is an important component of the water cycle,and greatly influences the water budgets of the TP lake basins.Quantitative estimation of ET_a within lake basins is fundamental to physically understanding ET_a changes,and thus will improve the understanding of the hydro logical processes and energy balance throughout the lake basins.In this study,the spatiotemporal dynamic changes of ET_α within the Lake Selin Co(the TP's largest lake) and its surrounding small lakes and land area during 2003-2012 are examined at the basin scale.This was carried out using the well-established Water and Energy Budget-based Distributed Hydrological Model(WEB-DHM) for the land area,the Penman method for the water area when unfrozen,and a simple sublimation estimation approach for the water area when frozen.The relationships between ET_a changes and controlling factors are also discussed.Results indicate that the simulated land ET_a from the WEB-DHM reasonably agrees with the estimated ET_a values from the nonlinear complementary relationship model using appropriately calibrated parameter values at a point scale.Land ET_a displayed a non-significant increase of 7.03 mm year~(-1),and largely depends on precipitation.For the water area,the combined effects of reduced wind speed and net radiation offset the effect of rising temperature and vapor pressure deficit,and contributed to a non-significant decrease in evaporation of 4.17 mm year~(-1).Sensitivity analysis shows that vapor pressure deficit and wind speed are the most sensitive variables to the changes of evaporation from the water area.     

Structure-specific selection of earthquake ground motions for the reliable design and assessment of structures

A decision support process is presented to accommodate selecting and scaling of earthquake motions as required for the time domain analysis of structures. Code-compatible suites of seismic motions are provided being, at the same time, prequalified through a multi-criterion approach to induce response parameters with reduced variability. The latter is imperative to increase the reliability of the average response values, normally required for the code-prescribed design verification of structures. Structural attributes like the dynamic characteristics as well as criteria related to variability of seismic motions and their compliance with a target spectrum are quantified through a newly introduced index, δ _sv–sc , which aims to prioritize motions suites for response history analysis. To demonstrate the applicability of the procedure presented, the structural model of a multi-story building was subjected to numerous suites of motions that were highly ranked according to both the proposed approach (δ _sv–sc ) and the conventional one (δ _conv ), that is commonly used for earthquake records selection and scaling. The findings from numerous linear response history analyses reveal the superiority of the proposed multi-criterion approach, as it extensively reduces the intra-suite structural response variability and consequently, increases the reliability of the design values. The relation between the target reliability in assessing structural response and the size of the suite of motions selected was also investigated, further demonstrating the efficiency of the proposed selection procedure to achieve higher response reliability levels with smaller samples of ground motion.