Values Of N-alkanes Research Articles

Characterizing Wall Loss Effects of Intermediate-Volatility Hydrocarbons in a Smog Chamber with a Teflon Reactor

Intermediate-volatility organic compounds (IVOCs) serve as pivotal precursors to secondary organic aerosol (SOA). They are highly susceptible to substantial wall losses both in indoor environments and within smog chambers even with Teflon walls. Accurately characterizing the wall loss effects of IVOCs is thus essential for simulation studies aiming to replicate their atmospheric behaviors in smog chambers to ensure precise modeling of their physical and chemical processes, including SOA formation, yet a comprehensive understanding of the wall loss behavior of IVOCs remains elusive. In this study, we conducted a thorough characterization of wall losses for typical intermediate-volatility hydrocarbon compounds, including eight normal alkanes (n-alkanes) and eight polycyclic aromatic hydrocarbons (PAHs), using the smog chamber with a 30 m3 Teflon reactor. Changes in the concentrations of gaseous IVOCs with the chamber were observed under dark conditions, and the experimental data were fitted to the reversible gas–wall mass transfer theory to determine the key parameters such as the wall accommodation coefficient (αw) and the equivalent organic aerosol concentration (Cw) for different species. Our results reveal that Cw values for these hydrocarbon IVOCs range from 0.02 to 5.41 mg/m3, which increase with volatility for the PAHs but are relative stable for alkanes with an average of 3.82 ± 0.92 mg/m3. αw span from 1.24 × 10−7 to 1.01 × 10−6, with the values for n-alkanes initially showing an increase followed by a decrease as carbon numbers rise and volatility decreases. The average αw for n-alkanes and PAHs are 3.34 × 10−7 and 6.53 × 10−7, respectively. Our study shows that IVOCs exhibit different loss rates onto clean chamber walls under dry and dark conditions, with increasing rate as the volatility decreases. This study demonstrates how parameters can be acquired to address wall losses when conducting smog chamber simulation on atmospheric processes of IVOCs.

Lipid biomarkers and stable isotopes uncover paleovegetation changes in extremely species-rich forest-steppe ecosystems, Central Europe

The historical development of the vegetation of semi-dry grasslands in Central Europe is not satisfactorily understood. Long-term continuity of open vegetation or, conversely, deep-past forest phases are considered possible sources of the current extreme species diversity of these ecosystems. We aimed to reveal the trajectory of paleovegetation development in these ecosystems through detailed analysis of terrestrial in-situ soil geoarchives. We measured the bulk soil carbon and nitrogen contents, lipid molecular distribution, and compound-specific stable carbon and hydrogen isotopic signatures of mid- and long-chain n-alkanes extracted from soil and modern plant material tissues (i.e., deciduous and Pinus leaves and grass/herbaceous species).The C23–C33 n-alkane homologues were identified in soils with different abundances. Normally, C27 and C29 n-alkanes were the most abundant homologues in tree-leaf samples, while grass-derived n-alkanes were mostly C31 and C33 homologues. Soils were largely dominated by C29 and C31 n-alkanes. Odd-numbered C27–C33 soil n-alkane δ13C values ranged from −36.2‰ to −23.2‰, whereas their δ2H values showed a wider range of variability that fluctuated from −224‰ to −172‰. Molecular distribution in combination with radiocarbon analysis of soil organic matter (SOM) and δ13C and δ2H values of n-alkanes revealed a large contribution of C3 trees (both deciduous and coniferous trees/pine trees) as the main source of n-alkanes between the late Pleistocene and early Holocene (ca 15,000–8200 calibrated year before present/cal year BP). A clear shift toward more grassy/herbaceous vegetation was observed from the early Holocene (ca 11,700–8200 cal year BP) onwards. Distribution patterns of lipids and soil geochemical parameters showed that plants are the main source of SOM and that biodegradation and kinetic isotope fractionation are not the main reasons for 13C enrichment in soil profiles. Past C3 vegetation shifts as well as paleoclimate changes (i.e., aridity) can have played a role in the observed 13C depth profiles.

The Toarcian Posidonia Shale at Salem (North Alpine Foreland Basin; South Germany): hydrocarbon potential and paleogeography

AbstractThe Posidonia Shale in the basement of the North Alpine Foreland Basin of southwestern Germany represents an important archive for environmental changes during the Toarcian oceanic anoxic event and the associated carbon isotope excursion (T-CIE). It is also an important hydrocarbon source rock. In the Salem borehole, the Posidonia Shale is ~ 10 m thick. The lower 7.5 m (1763.5–1756.0 m) of the Posidonian Shale and the uppermost part of the underlying Amaltheenton Formation were cored and studied using a total of 62 samples. Rock–Eval, palynological, maceral, biomarker and carbon isotope data were collected to assess variations in environmental conditions and to quantify the source rock potential. In contrast to most other Toarcian sections in southwest Germany, TOC contents are high in sediments deposited during the T-CIE, but reach a peak in post-CIE sediments. Biomarker ratios suggest that this reflects strong oxygen-depletion during the T-CIE (elegantulum to lower elegans subzones), but also during the falciferum Subzone, which is also reflected by a prolonged dinoflagellate cyst blackout. While sediments of the tenuicostatum Zone to the elegans Subzone are thinner than in neighbouring sections (e.g., Dotternhausen), sediments of the falciferum Subzone are unusually thick, suggesting that increased subsidence might have contributed to anoxia. The T-CIE interval is very thin (0.75 m). δ13C values of n-alkanes show that the maximum negative isotope shift predates the strongest basin restriction during the T-CIE and that the carbon isotope shift is recorded earlier for aquatic than for terrigenous organisms. In Salem, the Posidonia Shale is thermally mature and highly oil-prone. The residual source petroleum potential is about 0.8 tHC/m2. Graphical Abstract

Environmental factors influencing the soil-air partitioning of semi-volatile petroleum hydrocarbons: Laboratory measurements and optimization model

The soil-air partition coefficient (KSA) values are commonly utilized to examine the fate of organic contaminants in soils; however, their measurement has been lacking for semi-volatile petroleum hydrocarbons within soil contaminated by crude oil. This research utilized a solid-phase fugacity meter to determine the KSA values of n-alkanes and polycyclic aromatic hydrocarbons (PAHs) under crucial environmental conditions. The results showed a notable increase in KSA values with the extent of crude oil contamination in soil. Specifically, in the 3 % crude oil treatment, the KSA values for n-alkanes and PAHs increased by 1.16 and 0.66 times, respectively, compared to the 1 % crude oil treatment. However, the KSA values decreased with changes in temperature, water content, and particle size within the specified experimental range. Among these factors, temperature played a significant role. The KSA values for n-alkanes and PAHs decreased by 0.27–0.89 and 0.61–0.83 times, respectively, with a temperature increase from 5 °C to 35 °C. Moreover, the research identified that the molecular weight of n-alkanes and PAHs contributed to variations in KSA values under identical environmental factors. With an increase in temperature from 5 °C to 35 °C, the range of n-alkanes present in the air phase expanded from C11 to C34, and PAHs showed elevated levels of acenaphthene (ACE) and benzo (b) fluoranthene (BbFA). Furthermore, heightened water content and particle size were observed to facilitate the volatilization of low molecular weight petroleum hydrocarbons. The effect of environmental variables on soil-air partitioning was evaluated using the Box-Behnken design (BBD) model, resulting in the attainment of the lowest log KSA values. These results illustrate that soil-air partitioning is a complex process influenced by various factors. In conclusion, this study improves our comprehension and predictive capabilities concerning the behavior and fate of n-alkanes and PAHs within soil-air systems.

Aquatic plants dominate the sources of long-chain n-alkanes in floodplain lakes in the middle and lower reaches of the Yangtze River

It has been generally accepted that long-chain n-alkanes found in lacustrine sequences are mainly derived from terrestrial plants, which is the basis for tracking paleoenvironmental changes associated with terrestrial plants. However, this application may be complicated by the possible contribution of long-chain n-alkanes from non-emergent aquatic plants. To explore this issue, a sedimentary core spanning the past 150 years from Lake Longgan in the middle and lower reaches of the Yangtze River, together with terrestrial plant and soil samples collected around the lake, was analyzed for molecular distributions and δ13C values of n-alkanes. Results show that n-alkanes with different chain lengths have similar sources, as evidenced by the strong positive correlations in concentrations (r2 > 0.88, p < 0.001) and δ13C values (r2 > 0.79, p < 0.001) among n-alkane homologs. The δ13Calk values (especially n-C29 and n-C31) indicated that n-alkanes have a mixed source, with non-emergent aquatic plants having a greater influence in shallow lakes in the middle and lower reaches of the Yangtze River (MLYR). Results from the MixSIAR model further reveal that non-emergent aquatic plants contribute a large amount of mid-chain n-alkanes and almost half (34.6% ∼ 57.7%) of the long-chain n-alkanes. These findings suggest that long-chain n-alkanes in lacustrine sequences should be used with caution when attempting to reflect terrestrial vegetation succession and the associated climate changes.

Distribution of Hydrocarbons in the Snow Cover of Natural and Urbanized Landscapes in the South of the Far East, Russia.

This study analyzed total organic carbon (TOC), petroleum products (PP), suspended materials (SM), volatile aromatic hydrocarbons (toluene, o-xylene, etc.) and n-alkanes in the snow cover of Bol'shekhekhtsirsky, Zeysky state natural reserves and Khabarovsk, on 4, 5 and 9 stations in the south of the Russian Far East in March 2020. In Bol'shekhekhtsirsky reserve, the concentrations of TOC, PP, and SM in snow samples were in the range of 1.5-2.4, 0.06-0.11, and 11.4-1.9mg/L, 1.4-1.9, 0.02-0.05, and 11-23mg/L in Zeysky reserve, while in Khabarovsk were 1.7-23.7, 0.12-1.26, and 25-294mg/L, respectively. In addition, the benzene, toluene, and o-xylene concentrations of snow samples ranges from not detected (ND) to 2.4, ND-3.1, and 1.1-2.7µg/L in Khabarovsk, ND-1.3, ND-2.1, and ND-2.7µg/L, respectively in Bol'shekhekhtsirsky reserve. Carbon preference index values of n-alkanes were consistent with anthropogenic sources for stations 7, 8 and 2 in Khabarovsk (Heat Power Plants 1, 2 and city roads). The snow of the Zeysky Reserve is not contaminated with organic pollutants, and can be used as a conditional background for the south of the Russian Far East.

Contributions of the Yangtze River and Yellow River to terrestrial organic carbon deposition in the Yangtze River Estuary during the last 700 years

Terrestrial organic carbon (OC) could be transported to marginal oceans by large rivers and deposited under different hydrological conditions. A mixture of the fresh & lithospheric OC, superimposed by the OC from different rivers, makes it challenging to distinguish sources and contributions of terrestrial OC. Here, we employed multiple geochemical proxies, including the hydrogen isotopic composition of leaf wax (δDwax), the carbon isotopic composition of total organic carbon (δ13Corg), TOC/TN ratio, and major element distributions, to investigate the variability in sources of terrestrial OC in the Yangtze River Estuary (YRE) since 1270 CE. Our results revealed that the terrestrial OC deposition in the YRE was lower before 1950 CE and considerably increased after 1950 CE. The δDwax record in core A3 indicates that the OC deposition in the YRE was primarily modulated by the variability of El Niño-Southern Oscillation (ENSO) and East Asia Summer Monsoon. The strengthened El Niño-like state since 1600 CE resulted in increased precipitation in the middle and lower reaches of the Yangtze River basin and in turn, affected the terrestrial OC deposition in the YRE. Several episodes with lower carbon preference index (CPI) values of n-alkanes (CPI < 3) were identified. These episodes were concurrent with the streamflow shift of the ancient Yellow River during the transitions from wet to dry conditions, highlighting the potential contribution of the ancient Yellow River to the aged OC deposition in the YRE.

Thermal modeling for anionic surfactant using Inverse gas chromatography and image processing techniques

Anionic surfactants are one of the necessary ingredients demanded in extensive bio-and pharmaceutical industry. Their concern on human health and environmental safety get increase and is originated from its surface thermodynamic properties. Surface thermodynamic properties have severely an effect on thermodynamic and kinetic parameters in the chemical reaction. However, there is a kind of serious differences between theoretical calculation and experimental result because of lacks of understanding on complex surface thermodynamic properties. Inverse gas chromatography (IGC) is an analytic chemistry tool providing information on physicochemical and thermodynamic parameters. Herein, a thermal modeling of surface thermodynamic properties is suggested. Net retention volume, VN, of n-alkanes and polar probes upon sodium lauryl sulfate surface properties as an anionic surfactant are analyzed at diverse temperatures range from 363.15 to 433.15 K using IGC. VN of n-alkane values are employed to assess the London dispersive surface free energy, γSL by comprehensive thermodynamic models and results into the Guttmann Lewis acid-base parameters and acid-base constants. The results reveal that an excellent linear correlation coefficient value closer to 1. The current work demonstrates how to approach surface thermodynamic characteristics of anionic surfactant and suggests a new thermal model as an advanced model. Additionally, scanning electron microscopy image analysis regarding the surface morphology has been carried out using image processing techniques. Current comprehensive approach enables to analyze and apply for an effective understanding on surfactant molecules.

A new group contribution model for prediction liquid hydrocarbon viscosity based on free-volume theory

In this paper a group contribution model for predicting the viscosity values of n-alkanes and iso-alkanes in the liquid phase based on Free-Volume Theory was developed. A database with 2129 experimental data with a temperature range up to 573.15 K and a pressure range up to 4000 bar was used to develop the model. The total weighted average percentage error is 4.894%. The model was tested on the next 472 experimental data with a temperature range up to 564.15 K and a pressure range up to 2019 bar and a mean percentage error that was obtained is 5.155%.

Characterization of the organic matter produced by Atlantic Rainforest plants and its influence in the surface sediments deposited in a protected subtropical Estuarine–Lagoon system

The aim of the present study was to characterize the distribution and sources of terrigenous organic matter (OM) in surface sediments of a subtropical estuarine–lagoon​ system surrounded by the Atlantic Rainforest. n-Alkanes and their isotopic composition were evaluated in 20 surface sediment samples, as well as in leaves from nine representative plant species collected from the Cananéia–Iguape Estuarine–Lagoon​ System (CIELS; Brazil). According to the calculated n-alkanes indices, the analyzed plants could be grouped as monocotyledonous (aquatic productivity index [Paq] > 0.1; carbon preference index [CPI] < 6), dicotyledonous (Paq < 0.1; CPI > 10), mangrove (average chain length [ACL23−33] < 29; ratio of n-C31 over n-C29 concentration [Norm31] < 0.1) and riparian (ACL23−33 > 29; Norm31 > 0.2) vegetation. δ13C n-alkane values were higher (> −26‰) in the C4 aquatic macrophyte analyzed (Spartina alterniflora), with intermediate values (−30 to −27‰) for the pteridophyte (Pteridium aquilinum), and lower values for the C3 species (< −31‰). In sediment, total n-alkanes, total organic carbon, and total nitrogen concentrations ranged from 510 to 10,000 ng g−1 in dry sediment, 0.15 to 6.37% and not detected to 0.74%, respectively, being governed by the mud composition. ACL23−33 and the Norm31 values ranged from 29.0 to 30.4 and 0.39 to 0.57, respectively. Paq and CPI values ranged from 0.06 to 0.25 and 4.2 to 6.6, respectively. These indexes showed that the main terrigenous OM source to CIELS sediment was the riparian vegetation, coming from the rivers that discharge into the system. The n-alkanes δ13C composition, ranging from −36.0 ± 0.0 to −24.8 ± 0.3‰, also showed contributions of macrophytes input to the deposited OM. The influence of mangrove trees is not dominant but increases in the CIELS eastern region, where this kind of vegetation is more developed.

N-Alkanes and their carbon isotopes (δ13C) reveal seasonal foddering and long-term corralling of pastoralist livestock in eastern Mongolia

The compound-specific δ13C values of n-alkanes from pastoralist winter campsites in Mongolia have great potential to reconstruct dietary seasonality of livestock and local vegetation. We analysed leaf wax n-alkanes and their carbon isotope ratios in common fodder plants (Artemisia mongolica, Chenopodium album, Elymus dahuricus, Elymus sibiricus and Stipa sibirica) fed to domesticates during the winter months, and also in topsoils (0–8 cm) from four livestock corrals in northern and southern Mongolia. n-Alkanes identified in fresh plant parts reflect signatures with mean δ13C stable isotope values of individual n-alkanes typical for C3 plants (−34.1‰). Plants from forest- and desert-steppe exhibit similar mean n-alkane δ13C values but significantly wider isotopic variation in the desert steppe flora. The non-anthropogenic control samples exhibit δ13C mean values significantly higher from that of dung deposits and fresh plants for the same regions. Our data support previous research that n-alkanes profiles and their carbon isotopic values vary depending on the local vegetation and environment, while simultaneously demonstrating how anthropogenic activity, such as corralling of livestock, is recorded in the n-alkane of domesticates dung validating the use of biomarker studies in pastoralist archaeological contexts for distinguishing seasonal foddering, local environmental data and to an extent grazing habitats.

Effect of burning on the distribution pattern and isotopic composition of plant biomolecules: Implications for paleoecological studies

In fire-prone biomes, the interpretation of paleoecological proxies is subjected to uncertainties since the plant biomarkers/molecules are thermally modified during vegetation fires, and the extent of alterations is yet to be well-constrained, particularly for oxic conditions. Towards this, we performed a series of controlled experiments where leaf samples from C3 (tree and shrub) and C4 (grass) plants were burned under ambient oxygen at temperatures between 200 °C and 500 °C. A topsoil sample was also heated to understand the effect of thermal degradation on organic matter (OM) already present in the soil. Our results show a reduction in the total organic carbon content and leaf wax concentrations which is consistent with the previous studies. We also observed a shift from predominantly long-chain homologues (with odd-over-even in n-alkanes or even-over-odd predominance in n-alkanoic acids; FAs) to balanced distributions with increased mid and short-chain homologues. We observed that the short, mid, and long-chain n-alkanes were mainly formed at the expense of FAs (with losses up to 100%), possibly due to oxygen-rich burning conditions. Burning of plant leaves also affected their stable isotopic compositions. In burned C3 plant leaves (tree and shrub), the bulk carbon isotope values (δ13COM) increased by 1.0–1.4‰, while in C4 grasses, it was 0.4–1.6‰ lower than their unburned counterparts. The changes in the δ13COM values are suggested to be a cumulative product of the kinetic isotope effect and source-driven isotopic fractionation. The carbon isotope values in long-chain n-alkanes (δ13Cn-alk) mostly decreased (up to 4.6‰) in burned C3 and C4 plant leaves due to the isotopic modifications associated with the generation of secondary n-alkanes from FAs. In contrast, the hydrogen isotope values of n-alkanes (δ2Hn-alk) in the burned samples were up to 86‰ higher than their initial values, mainly due to the kinetic isotope effect. Therefore, in biomes susceptible to frequent canopy and litter layer fires, unusually high δ2Hn-alk values coupled with a disproportionate change between δ13Cn-alk and δ2Hn-alk values might indicate pyrogenic OM presence within the soil. We also observed that changes in the n-alkane characteristics due to heating of soil samples were substantially lower than those in plant leaves due to the protected lipid component within organomineral complexes. In summary, we recommend careful consideration and identification of vegetation fire history before using stable isotope-enabled organic proxies for generating paleoecological records.

Isotopic and molecular distribution of leaf-wax in plant-soil system of the Gangetic floodplain and its implication for paleorecords

The stable isotopic composition of bulk organic matter (OM), as well as the molecular and isotopic composition of n-alkanes and n-alkanoic acids, are compared in various plant growth-forms (trees, shrubs and grasses) and soils from the Gangetic floodplain. Abundance of higher molecular weight (HMW) n-alkanes is found to be highest in grasses and shrubs and lowest in trees, whereas n-alkanoic acids show an opposite trend. Total lipid production is highest in trees and lowest and comparable in shrubs and grasses, reflecting differences in metabolic conditions and water use efficiency during synthesis of these biomolecules in trees and overground species. In all plant growth-forms, higher (5‰) carbon isotopic composition (δ13C) are observed in bulk OM compared to 13C/12C ratio of n-alkyl compounds, while a small difference (0.2‰) is seen in between n-alkane and n-alkanoic acids. The trees and shrubs show lower average hydrogen isotopic composition (δD) in n-alkanes (δDalk) and higher values in n-alkanoic acids (δDacid) compared to the grass species. Interestingly, the grass and shrub species register higher variation (ca. 110‰) in δD values of n-alkyl compounds, reflecting the ability of groundcover species to proliferate under varying environmental conditions. The lower D/H (ca. 74‰) variation in the tree species possibly suggests consistent dependency on the perennial water sources. Although the study site is equally dominated by C3 and C4 vegetation, mean δ13C values of bulk OM and n-alkanoic acid in soil show proximity to the C4 end-member, while n-alkanes show proximity to C3 trees. Mean δDalk values in the soil reflect the cumulative hydrological condition of the overground vegetation, while average δDacid values mimic the ambient conditions specific to the C3 species. The variation in OM mixing from different plant types in soil was triggered by soil-forming processes, which possibly resulted in variation in responses between the n-alkyl compounds. Compared to the global record, the C3-C4 vegetation of the Gangetic floodplain is characterised by a lower (ca. 0.5–2‰) difference between bulk leaf tissues and n-alkyl compounds (Δ13CBulk–alk/acid). H-isotopic fractionation with respect to the ambient water (εapp) shows higher (ca. +30‰) values for n-alkanoic acids and lower (ca. −60‰) values for n-alkanes compared to the global datasets, implying a role of regional environmental conditions on the isotopic fractionation in n-alkyl compounds in plants. As molecular and isotopic compositions of plant-derived proxies are related to environmental conditions, emphasis must be given to establish the response of proxies to regional conditions before applying them in paleoenvironmental reconstructions.

δ13C and δD Values of n-Alkanes from In-Reservoir Biodegraded Oils: Implications for Understanding the Mechanisms of Biodegradation and for Petroleum Exploration

This study investigates the magnitude and direction of stable C and H isotope shifts of n-C15–30 alkanes from biodegraded oils sourced from Type II (Oil suite S) and Type II/III (Oil suite H) kerogens. Compound-specific isotope data show a 2.0‰ 13C-enrichment and no D-enrichment of n-alkanes in the most biodegraded oil from sample suite S. Similarly, there is a 1.5–2.5‰ 13C-enrichment and no D-enrichment in Oil suite H. Overall, there is a &lt;2.5‰ δ13C and &lt;20‰ δD variability among individual n-alkanes in the whole sequence of biodegradation. N-alkanes from the least biodegraded Oil H samples are 2–4‰ 13C-enriched in comparison with the least biodegraded Oil S. However, there are no differences in the δD values of n-alkanes in these samples. Our indirect isotopic evidence suggests (1) a site-specific biodegradation process, most likely at position C-2 and/or C-3 or another site-specific process, and (2) a significant D/H exchange between organic compounds in the source rock and isotopically similar marine formation waters. We conclude that, unlike δD methodology, investigation of δ13C composition of n-alkanes has strong potential as a supplementary tool for oil–oil and oil–source-rock correlation even in biodegraded oils when n-alkanes are present.

Nature of Volatile Organic Matter in Lake Sediments as a Reflection of Paleoclimate Changes Occurring at 4 ka in the Central Qaidam Basin

This study explores the paleoclimate changes around the 4 ka BP period in the central Qaidam Basin (QB), assessing the differences in spectral characteristics and organic composition of salt lake sediments under different climate change conditions. Sediment samples (10-m-depth profile) were collected from the middle of dry salt flats in East Taijinar Lake (China). Sediment organic matter (SOM) was assessed by Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS). Results showed a significant difference in the TOC content of sediments with different lithological characteristics. A lower TOC content in salt-bearing strata was attributed to the extreme sedimentary environment with minimal exogenous inputs. FTIR spectroscopy revealed that the SOM in sediments included aliphatic C, ketones and alcohols. Sediments of salt-bearing strata generally exhibited a rise in the content of ketone C=O groups and a decrease in aliphatic C, with an equal content of alcohols compared with silty sand. Therefore, exogenous OM and the content of TOC in silty sand strata are higher than in salt-bearing strata, indicating that the paleoclimate became warmer and wetter after 4 ka BP. GC-MS analysis showed a significant difference between the phenol and aldehyde content in different strata, further indicating that the paleoclimate changed from dry to relatively warm around 4 ka BP. Seven organic compound types were identified in SOM, including aldehydes, hydrocarbons, phenols, esters, ketones, alcohols, and furans. Different strata exhibited different distributions of organic compounds, with particularly high concentrations of aldehydes in salt-bearing strata and phenols in silty sand. Correlation analysis was performed between detrital minerals and OM types in all samples. Results showed a strong positive correlation between detrital minerals and phenols and a strong negative correlation between detrital minerals and aldehydes, with a negative correlation also identified between detrital minerals and ketones. Overall, the reduction in volatile organic compounds demonstrates that the paleoclimate changed from cooler and dry to wet and warm around the 4 ka BP period in the central QB, with the carbon preference index and n-alkane values further demonstrating these results. This study also confirms the importance of volatile organic compound monitoring to assess paleoclimate changes.

Comparison of molecular distributions and carbon and hydrogen isotope compositions of n-alkanes from aquatic plants in shallow freshwater lakes along the middle and lower reaches of the Yangtze River, China

Leaf wax n-alkanes in lacustrine sequences have been widely applied for reconstructions of Late Quaternary paleoclimate histories. Such applications depend on knowledge of the factors controlling the characterization of the n-alkane paleoproxies of aquatic plants. This study analyzed the molecular distributions and carbon and hydrogen isotopic compositions of n-alkanes of aquatic plant samples collected from the middle and lower reaches of the Yangtze River (MLYR). In this collection, the n-alkane distributions of submersed/floating plants and emersed plants are distinctly different. By combining their Paq and n-alkane δ13C values, the n-alkane contributions from submersed plants and emersed plants in the MLYR can be differentiated. It is also noteworthy that the hydrogen isotope fractionation (εalk/water; avg. −166‰) between the n-C23 alkane of submersed plants in the MLYR and in precipitation agrees well with previous studies, suggesting a nearly constant εalk/water for the submersed plants. Moreover, compared to C3 dicots growing in a similar climate, submersed plants in the humid MLYR have lower n-alkane δ2H values. This feature may arise from isotopic differences in the evapotranspiration of lake water versus leaf water and in the biosynthetic fractionations of leaf waxes of these two plant groups. These results aid interpretations of the hydrogen isotope offset of the δ2H values of n-alkanes between aquatic and terrestrial plants in humid climates.

Comparison of δ2H values of leaf wax n-alkanes and n-alkanoic acids in subtropical angiosperms

Hydrogen isotope compositions of leaf wax lipids are widely applied to reconstruct changes of hydroclimate in sedimentary archives. However, whether the δ2H signals from different classes of n-alkyl lipids yield consistent information is an important but poorly explored issue. In this study, we investigated seasonal variations in the molecular distributions and hydrogen isotope compositions of n-alkanes and n-alkanoic acids of three tree species (Cinnamomum camphora, Quercus chenii, Liquidambar formosana) growing in a humid subtropical monsoon climate. Seasonal differences exist in the concentrations and δ2H values of n-alkanes and n-alkanoic acids. These features may be linked to difference in the synthesis times of n-alkanes and n-alkanoic acids. The hydrogen isotope offset between n-alkanes and n-alkanoic acids (εalkane/acid) is variable across the seasons and among the three tree species. A survey of published data reveals that the εalkane/acid values are usually not significantly different among vegetation types, but the offset values can be significantly different in trees from tropical/subtropical and temperate regions. This difference may relate to different synthesis times of leaf waxes because of the climatic conditions in different climate zones. These findings encourage caution in interpreting differences in the δ2Hwax values of n-alkyl classes in paleoenvironmental reconstructions.

Organic matter provenance and accumulation of transitional facies coal and mudstone in Yangquan, China: Insights from petrology and geochemistry

The coal measure strata of the late Paleozoic Taiyuan Formation in northern China, which are mainly composed of coal, mudstone, limestone, and sandstone, with obvious cyclothems, are important with respect to the development of unconventional natural gas. However, the organic matter provenance and enrichment mechanism in transitional facies are still controversial. In this study, the organic geochemistry, elemental geochemistry, and petrology of coal and mudstone in Yangquan were investigated. The Pr/nC17 values of n-alkanes as well as the proximate analysis indicate a high level of organic matter maturity. Organic petrology and biomarker compounds show that terrigenous higher plants represent the primary source of the organic matter in coal, with mudstone having larger aquatic organism contribution in lagoon facies. The redox index represents the larger Ceanom, U/Th, and δU values in lagoon facies mudstone, indicating weakly reducibility. Large diameter regular hexagon-shaped pyrite particles were identified in coal and coaly mudstone, indicating oxygen-containing bottom water condition. The rapid deposition rate promotes the accumulation of organic matter in coal and coaly mudstone, and the preservation of organic matter in coal is related to the redox shock. When the climate became warm and dry gradually, the input of terrestrial debris increased, and coal deposits were interrupted. The sudden transgression promotes the stratification of water column, forming oxygen-deficient environment and the growth of algae. The shallow water sediments were covered with gray-black mudstone, and the enrichment of organic matter was controlled by redox conditions.

Application of Column Chromatography for Accurate Determination of the Carbon Isotopic Compositions of n-alkanes in Diverse Environmental Samples

The carbon isotopic compositions (δ13C) of n-alkanes in various environmental samples have been previously proposed as suitable fingerprints for assessing the origin of organic matter (OM) in diverse environmental systems. However, with respect to using gas chromatography–combustion–isotope ratio mass spectrometry for the carbon isotopic analysis of n-alkanes, analytical uncertainty may often be caused by the co-elution of interfering unsaturated compounds (e.g., aromatic and branched compounds). Hence, we propose a simple but reliable method for purification that uses column chromatography. The performance of two different solid stationary phases (i.e., aluminum oxide and Ag+-impregnated silica) was compared in terms of their capacity to eliminate unsaturated compounds from total hydrocarbons and thus increase the precision of δ13C measurements. Compared to the use of an activated aluminum oxide column, elution from an Ag+-impregnated silica column allows more effective isolation of individual n-alkanes, which results in more precise δ13C measurements for diverse environmental samples. Thus, Ag+-impregnated silica column separation can be effective as a routine experimental technique for increasing the accuracy of the δ13C values for n-alkanes in OM, which includes a large proportion of unsaturated compounds.

Compound-specific δ2H and δ13C values of n-alkanes as a tool to unravel complex petroleum mixtures in the South Viking Graben, Norway

In order to resolve oil mixtures and for oil-source correlation purposes, both hydrogen (δ2H) and carbon isotope compositions (δ13C) of n-alkanes and acyclic isoprenoid alkanes have been determined in petroleum mixtures and extracts from Jurassic source rocks of distinct organic matter type, depositional setting and lithology in the South Viking Graben, Norway. δ2H values vary from −80 to −140‰ for n-alkanes and from −130 to −165‰ for pristane (Pr) and phytane (Ph) in oil and condensate samples. At the same maturity level, in the main oil generative window, individual n-alkanes from most oils and condensates show a 2H-enrichment with n-alkane chain length by 15 to 50‰. With the increase in the estimated input of hydrocarbons from terrigenous source material to petroleum mixtures, the n-alkanes (>C20) exhibit a higher 2H-enrichment while n-alkanes (C10-C14) show a higher 13C-enrichment. δ2H values of n-alkanes (>C20) are useful to discriminate mixed oils that were not well differentiated by δ13C data. δ2H values of individual n-alkanes from oils suggest a possible exchange of C-bound H with other hydrogen during their generation from precursor molecules. We find that δ2H values of n-alkanes from Jurassic source rock extracts, at the onset of oil generation, were mainly controlled by the isotopic composition of biosynthetic precursors and varied from −100 to −150‰. Combination of carbon and hydrogen isotope compositions of n-alkanes proved to be a useful tool to discriminate Jurassic petroleum mixtures from distinct sources in the Norwegian South Viking Graben.