Values Of N-alkanes Research Articles

Diffusion of n-alkanes in mesoporous 5A zeolites by ZLC method

Diffusion properties of mesostructured zeolite 5A were investigated by employing n-alkanes as probe molecules using the zero length column (ZLC) method. The mesopores were found to enhance molecule diffusion. Moreover, the effective diffusion time constant (Deff/R2) increased with mesoporosity in the zeolites between 308 K and 393 K, whereas the activation energy decreased with increasing mesopore volume. The effective diffusivity values of n-alkanes in mesoporous zeolite 5A were generally higher than that the microporous zeolite 5A sample. This clearly implied the important role of the mesopore in zeolites crystals in facilitating the transport of reaction molecules due to shorter average diffusion path length and less steric hindrance.

Evaluation of the organic matter sources using the δ13C composition of individual n-alkanes in sediments from Brazilian estuarine systems by GC/C/IRMS

The δ13C composition of individual n-alkanes (from C16 to C34) was measured from surface sediments of five Brazilian estuarine systems affected by different organic matter sources, such as harbor area, industries, urban centers and sugar cane crops, in order to determine the origins of the organic matter. The aliphatic hydrocarbon fraction was analyzed by gas chromatography–combustion–isotope ratio mass spectrometry (GC/C/IRMS). The levels of n-alkanes in the studied areas ranged from 0.34 to 18.14 μg kg−1, being relatively low in comparison to high polluted environments. The Carbon Preference Index (CPI) calculated in the C23–C34 range indicates that n-alkanes are mainly inherited from cuticular waxes of higher plants. The δ13C composition of all n-alkanes detected in the sediment samples ranged from −39.6 to −18.3‰ showing different sources for the studied estuarine systems. Through Principal Component Analysis (PCA) it was possible to verify the petrogenic influence in the n-alkane sources, especially in the Paraíba do Sul sediment samples. Differences up to 15‰ of the δ13C values between n-alkanes of odd and even carbon number (C26 and C27) also indicated mixture of petrogenic and biogenic sources in Paraíba do Sul River. High (less negative) δ13C n-alkane values of odd carbon number were obtained from two sampling sites located close to an ethanol plant, indicating residues discharge of sugar cane (C4 plant). Influence of C3 plants that are the main components of dense ombrophile forest was observed in the Itajaí-Açu sediments by the decrease of δ13C (about 10‰ compared to the Paraíba do Sul River δ13C).

The characteristics of the biomarkers and δ13C of n-alkanes released from thermally altered solid bitumens at various maturities by catalytic hydropyrolysis

Solid bitumen occurs extensively in the paleo-reservoirs of marine sequences in southern China. The fluids in these paleo-reservoirs have usually experienced severe secondary alteration such as biodegradation and/or thermal maturation. The concentrations of extractable organic matter (EOM) in the resulting solid bitumens are too low to satisfy the amount required for instrumental analysis such as GC–MS and GC–IRMS. It is also difficult to get enough biomarkers and n-alkanes by dry pyrolysis or hydrous pyrolysis directly because such solid bitumens are hydrogen poor due to high maturities. Catalytic hydropyrolysis (HyPy) can release much more EOM from solid bitumen at mature to highly over-mature stages than Soxhlet extraction, dry pyrolysis and hydrous pyrolysis. However, whether the biomarkers in hydropyrolysates can be used for bitumen-source or bitumen–bitumen correlations has been questionable. In this study, a soft biodegraded solid bitumen sample of low maturity was thermally altered to various maturities in a closed system. HyPy was then employed to release bound biomarkers and n-alkanes. Our results show that the geochemical parameters for source and maturity based on biomarkers released from these thermally altered bitumen residues by HyPy are insensitive to the degree of thermal alteration. Furthermore, the maturity parameters are indicative of lower maturity than bitumen maturation products at a corresponding temperature. This suggests that biomarker source and maturity parameters, based on the products of HyPy, remain valid for bitumens which have suffered both biodegradation and severe thermal maturation. The distributions of δ13C of n-alkanes in hydropyrolysates are also insensitive to the temperature used for bitumen artificial maturation. Hence, the δ13C values of n-alkanes in hydropyrolysates may also provide useful information in bitumen–bitumen correlation for paleo-reservoir solid bitumens.

Dispersion-corrected density functional theory calculations of the molecular binding of n-alkanes on Pd(111) and PdO(101)

We investigated the molecular binding of n-alkanes on Pd(111) and PdO(101) using conventional density functional theory (DFT) and the dispersion-corrected DFT-D3 method. In agreement with experimental findings, DFT-D3 predicts that the n-alkane desorption energies scale linearly with the molecule chain length on both surfaces, and that n-alkanes bind more strongly on PdO(101) than on Pd(111). The desorption energies computed using DFT-D3 are slightly higher than the measured values for n-alkanes on Pd(111), though the agreement between computation and experiment is a significant improvement over conventional DFT. The measured desorption energies of n-alkanes on PdO(101) and the energies computed using DFT-D3 agree to within better than 2.5 kJ/mol (< 5%) for chain lengths up to n-butane. The DFT-D3 calculations predict that the molecule-surface dispersion energy for a given n-alkane is similar in magnitude on Pd(111) and PdO(101), and that dative bonding between the alkanes and coordinatively unsaturated Pd atoms is primarily responsible for the enhanced binding of n-alkanes on PdO(101). From analysis of the DFT-D3 results, we estimate that the strength of an alkane η(2)(H, H) interaction on PdO(101) is ~16 kJ/mol, while a single η(1) H-Pd dative bond is worth about 10 kJ/mol.

Distributions of n-alkanes and their hydrogen isotopic composition in plants from Lake Qinghai (China) and the surrounding area

Various aquatic plants from Lake Qinghai, the largest inland saline lake in China, and terrestrial plants from the surrounding area were investigated for the distribution of n-alkanes and their δD values. The n-alkanes in the samples range from C15 to C33 with C preference index (CPI) values of 4.0–29.7. The n-C23 or n-C25 alkane is the dominant compound in the aquatic submerged plants. The aquatic emergent and terrestrial plants have an abundance maximum at n-C27, n-C29 or n-C31. The average chain length (ACL) values, ranging from 26.0 to 29.6, are closely related to the plant species. The n-alkanes from the aquatic plants have mean δD values of −169‰ to −121‰ and those from the terrestrial plants values of −173‰ to −109‰. The H isotopic composition (δD) and fractionation differ significantly among the plants studied. Comparison shows that additional evaporative enrichment of the lake water associated with saline lakes and humidity influence the δD values of the n-alkanes in aquatic and terrestrial plants, respectively. The mean δD values of n-alkanes in the plants decrease with increasing ACL value. The n-alkanes from the different types of plants are more depleted in D relative to environmental water and those from aquatic plants (with a mean value of −143‰) have a greater isotopic fractionation than terrestrial plants (mean value −113‰).

Coupled high-resolution marine and terrestrial records of carbon and hydrologic cycles variations during the Paleocene–Eocene Thermal Maximum (PETM)

The Paleocene–Eocene Thermal Maximum is characterized by a massive perturbation of the global carbon cycle reflected in a large, negative carbon isotope excursion associated with rapid global warming and changes in the hydrologic system. The magnitude of the carbon isotope excursion from terrestrial carbonates and organic carbon is generally larger relative to marine carbonates. However, high-resolution marine and terrestrial isotopic records from the same locality for direct comparison are limited. Here we present coupled carbon isotope records from terrestrial biomarkers (δ 13C n-alkane ), marine bulk carbonates (δ 13C carbonate), and bulk organic carbon (δ 13C organic) from the continuous sedimentary record of the Forada section in northern Italy in order to evaluate the magnitude and phase relationships between terrestrial and marine environments. Consistent with previous reports, we find that the carbon isotope excursion established from δ 13C n-alkane values is more negative than those established from δ 13C carbonate and δ 13C organic values. In contrast to the majority of PETM records, all Forada δ 13C records show a sharp 13C-enrichment immediately following the onset of the carbon isotope excursion. Further, the terrestrial δ 13C n-alkane record lags δ 13C carbonate/δ 13C organic trends by ~ 4–5 kyr—offsets that reflect the long residence time of soil organic carbon. Hydrogen isotope records from higher-plant leaf waxes (δD n-alkane ) and sea-surface temperatures (TEX 86) were established to assess hydrologic and ocean temperature trends. We find δD n-alkane values trend more positive, associated with higher temperatures prior to the onset of the carbon isotope excursion, and conclude that regional changes in the hydrologic cycle likely occurred before the onset of the carbon isotope anomaly.

Characterisation of n-alkanes and their hydrogen isotopic composition in sediments from Lake Qinghai, China

The distributions of n-alkanes and their hydrogen isotopic composition (δD) in surface and core sediments from the saline Qinghai Lake were measured to assess whether or not biological source information was recorded in the δD values of n-alkanes. The results indicate that the n-alkane distributions between shallow water surface and core sediments were similar, and closer to those of terrestrial herbaceous plants from the Qinghai Lake surrounding areas, rather than the aquatic plants living in the lake. The n-alkanes in the surface and core sediments had similar mean δD values, ranging from −185‰ to −133‰ and −163‰ to −142‰, respectively. The mean δD values of n-alkanes in the sediments showed that the even n-alkanes were heavier in D compared with the odd homologues.

Effect of precipitation regime on δD values of soil n-alkanes from elevation gradients – Implications for the study of paleo-elevation

We measured dD values of long chain n-alkanes isolated from 30 surface soil samples along two elevation transects on the Tibetan Plateau differing in precipitation regime and water source. The East Asian Monsoon precipitation dominates the wetter regime on the eastern slope (from 1230 to 4300 m) of Gongga Shan on the eastern Tibetan Plateau. Precipitation from the Polar Westerlies dominates the drier region on the slope from 1900 to 5000 m in the West Kunlun Shan on the northwestern Tibetan Plateau. The decrease in dD value with elevation in the wetter region greatly exceeded that in drier region by, � 1.9 ± 0.1‰/100 m and � 1.4 ± 1.0‰/100 m respectively. The apparent fractionation between leaf wax and precipitation ewax–p values in the wetter region (ca. � 164‰) were more negative than those in drier region (ca. � 125‰ above 3200 m). We also measured dD values in leaves of six common living trees (values from � 287‰ to � 193‰) from Gongga Shan, ranging from about 2900–4200 m. The abundance-weighted average values of the nalkanes (dDwax) show a strong reverse correlation with sample source elevation (R 2 0.78 for soils from 2 0.85 for soils from West Kunlun Shan above 3200 m), suggesting that n-alkane dDwax faithfully records the precipitation dD and that the isotopic altitude effect of precipitation controls dDwax altitudinal gradients in the mountains. The data show a fairly strong monotonic dependency of n-alkane dD values on elevation for the eastern Plateau, but a complex relationship between n-alkane dD values and elevation for the northwestern Plateau. The dDwax values at sites below 3200 m from the Kunlun Shan area exhibit an unexpected positive correlation with elevation. The study confirms the potential for using sediment dDwax values to reconstruct paleo-elevation in wetter regions, but suggests caution in applying the approach to dry regions. Our results also show it is essential to consider the intricacy of the pattern of atmospheric circulation and water sources and their influence on the lapse rate of dD values with elevation.

Oil–source correlation for the paleo-reservoir in the Majiang area and remnant reservoir in the Kaili area, South China

There are different viewpoints on the oil-source correlation of the Majiang paleo-reservoir and the neighbouring Kaili remnant reservoir in the Southern Guizhou Depression of China. Three potential source rocks in this depression could be inferred: the Lower-Cambrian marine mudstone, Lower-Silurian shale and Lower-Permian mudstone. Most of the potential source rocks are of high maturity. The solid bitumens and oil seepages in the Southern Guizhou Depression suffered severe secondary alterations, such as thermal degradation and biodegradation. The solid bitumens of the Majiang paleo-reservoir are also of high maturity. The oil seepages and soft bitumen of the Kaili remnant reservoir were severely biodegraded. All these secondary alterations may obscure oil-source correlations by routine biomarkers. Thus, it is very important to select appropriate biomarker parameters for the oil-source correlation. In this work, biomarkers resistant to thermal degradation and biodegradation and the data of organic carbon isotopic compositions were used for the correlation. The δ 13C values of n-alkanes in asphaltene pyrolysates were also used to make oil–oil and oil–source correlations between severely biodegraded oils. The results indicate that the Lower-Cambrian marine mudstones are the main source for the Ordovician–Silurian (O–S) solid bitumens of the Majiang area and the Ordovician–Silurian oil seepages and soft bitumens of the Kaili area. Remnant reservoir in the eastern Kaili area might have been charged at least twice by the oil generated from the Lower-Cambrian marine source rocks.

Evaluation of n-alkanes and their carbon isotope enrichments (δ13C) as diet composition markers

Plant cuticular n-alkanes have been successfully used as markers to estimate diet composition and intake of grazing herbivores. However, additional markers may be required under grazing conditions in botanically diverse vegetation. This study was conducted to describe the n-alkane profiles and the carbon isotope enrichment of n-alkanes of common plant species from the Mid Rift Valley rangelands of Ethiopia, and evaluate their potential use as nutritional markers. A total of 23 plant species were collected and analysed for long-chain n-alkanes ranging from heptacosane to hexatriacontane (C27 to C36), as well as their carbon isotopic ratio (13C/12C). The analysis was conducted by gas chromatography/combustion isotope ratio mass spectrometry following saponification, extraction and purification. The isotopic composition of the n-alkanes is reported in the delta notation (δ13C) relative to the Vienna Pee Dee Belemnite standard. The dominant n-alkanes in the species were C31 (mean ± s.d., 283 ± 246 mg/kg dry matter) and C33 (149 ± 98 mg/kg dry matter). The carbon isotopic enrichment of the n-alkanes ranged from −19.37‰ to −37.40‰. Principal component analysis was used to examine interspecies differences based on n-alkane profiles and the carbon isotopic enrichments of individual n-alkanes. Large variability among the pasture species was observed. The first three principal components explained most of the interspecies variances. Comparison of the principal component scores using orthogonal procrustes rotation indicated that about 0.84 of the interspecies variances explained by the two types of data sets were independent of each other, suggesting that the use of a combination of the two markers can improve diet composition estimations. It was concluded that, while the n-alkane profile of the pasture species remains a useful marker for use in the study region, the δ13C values of n-alkanes can provide additional information in discriminating diet components of grazing animals.

Response of δD values of sedimentary n-alkanes to variations in source water isotope signals and climate proxies at lake Nam Co, Tibetan Plateau

Compound-specific δD values of sedimentary n-alkanes from Nam Co were compared with oxygen isotope values from three different ice cores of the Tibetan Plateau and other climate proxies including temperature and solar radiation. This enabled estimation of the importance of source water and of environmental factors at the ecosystem scale on variations of the isotopic composition of sedimentary n-alkanes. The ice core records from Dasuopu, Puruogangri and Rongbuk glacier of the past 1000 years recording the source water isotopes show a continuous deuterium enrichment indicating a warming trend or decreased precipitation. Sedimentary n-alkane δD values also represent the same general tendency to enrichment in heavier isotopes. However, the correlation to the δ 18O values derived from the ice record is very weak. The δD values of n-alkanes agree with known climatic events such as the Little Ice Age and Medieval Warm Period showing depleted and enriched δD values, respectively. The δD values of the n-alkanes were best related to environmental drivers including temperature and solar radiation. The results suggest that δD values of sedimentary n-alkanes mainly record evapotranspiration and relative humidity that strongly influence δD values at the plant level, while variability of the source signal is less important.

A new and reliable calibration method for vibrating tube densimeters over wide ranges of temperature and pressure

A new method for accurately converting vibrating tube periods of oscillation in density values is presented. This method is based on the fundamental requirement of the non-dependence on pressure of the vibration period of the cell under vacuum. An analytical method permits to correctly evaluate the evacuated vibrating tube periods of the Anton Paar cells namely the high pressure cells, 512 and 512P, as a function of temperature. It is further shown that the previously experimental method for the determination of this parameter is not suitable for obtaining reliable density values. A new simple calibration procedure is described and tested over wide ranges of temperature, T = (283.15 to 323.15) K and pressure, P = (0.1 to 60) MPa. New recommended density values for n-alkanes (C 6, C 7, C 8, and C 10) and tetrachloromethane, calculated by the proposed method, are given and compared with literature values in terms of mutual uncertainties.

Partition coefficients for nonane and its isomers in the rat

Jet Fuel 8 (JP-8) is a major fuel source used by US and NATO military. JP-8 is a complex mixture of aliphatic and aromatic isomers of hydrocarbons. Tissue/blood partition coefficient (PC) values are chemical-specific parameters used in modeling the kinetic behavior of chemicals. The partition coefficient values for n-alkanes tend to increase with the increasing carbon number, but less is known about the trend for isomers of n-alkanes. PC values were obtained for the n-alkane nonane (C9) and five of its isomers, namely 3-methyloctane, 4-ethylheptane, 2,3-dimethylheptane, 2,2,4-trimethylhexane, 2,2,4,4-tetramethylpentane. The blood:air and tissue:air PC values correlated with the published log octanol/water (O:W) PC values for n-nonane and its isomers. Experimentally determined blood:air and tissue:air PC values for n-nonane with the largest O:W value were greatest and smallest for the isomer 2,2,4,4-tetramethylpentane with the lowest O:W value. As expected the fat tissue had the highest PC values and muscle the lowest for n-nonane and its isomers. For each tissue, a linear relationship was observed between the tissue/blood PC values for the isomers of n-nonane and n-nonane. This suggests that tissue/blood PC values for all isomers of an alkane could be estimated using data collected from only a sub-set of alkanes of equal carbon number. These reported tissue/blood PC values will support the development of a jet fuel physiologically-based pharmacokinetic (PBPK) model.

Leaf wax n-alkane δD values of field-grown barley reflect leaf water δD values at the time of leaf formation

Leaf wax n-alkanes from barley ( Hordeum vulgare) from a field in Switzerland exhibited changes in δD values on the order of 20‰ over a growing season, while source water (soil water) and leaf water varied by 40‰. Additionally the seasonal variability in δD values of leaf wax n-alkanes of different barley leaves can only be found across different leaf generations (i.e. leaves that were produced at different times during the growing season) while n-alkane δD values did not vary significantly within a leaf generation. Interestingly, δD values of n-alkanes correlated best with the δD values of leaf water at midday of the sampling day but showed no significant correlation with soil water (e.g. precipitation) δD values. These results provide empirical evidence that leaf wax δD values record leaf water enrichment, and therefore integrate the isotopic effects of precipitation and evapotranspiration. Our results show that leaf wax n-alkane δD values from grasses are ‘locked in’ early during leaf development and hence record the environmental drivers of leaf water enrichment, such as vapor pressure deficit (VPD). Our data have important implications for the interpretation of paleorecords of leaf wax δD. We suggest that leaf wax n-alkane δD values from sedimentary records could be used to estimate changes in the degree of leaf water enrichment and hence VPD.

Geochemical and climate modeling evidence for Holocene aridification in Hawaii: dynamic response to a weakening equatorial cold tongue

A 13.5 m sequence of Holocene limnic sediments from a sinkhole on Oahu, Hawaii provides a valuable paleoclimatic record for the central subtropical Pacific. δ 13C analysis of plant leaf waxes ( n-alkanes: n-C 27, n-C 29, n-C 31 and n-C 33) is used to infer vegetative changes. Average δ 13C values of the suite of n-alkanes increase from approximately −31 ± 0.5‰ at 10 ca kyr BP (calibrated thousand years before present) to about −27 ± 0.5‰ by 6 ca kyr BP and then remain roughly constant until the Polynesian arrival (about 1.15 ca kyr BP). The increase in δ 13C values of n-alkanes is interpreted to indicate a shift in the local vegetation from C 3 to C 4-dominated flora. Based on mass-balance calculations, the observed increase in the δ 13C values translates to at least a doubling of the relative abundance of C 4 plants. We argue that the expansion of C 4 plants was a response to decreased overall water availability (aridification) due to reduced wintertime precipitation. Model simulations of an orbitally-induced increase in insolation along the equator during the Holocene provide evidence for a wintertime drying trend in the eastern subtropical North Pacific. This trend is associated with boreal fall to winter warming of the cold tongue in the eastern equatorial Pacific (EEP). These model results provide a conceptual framework to explain a dynamic link between the reconstructed Holocene drying trend in Hawaii and orbitally-forced climate change in the EEP that is analogous to the modern El Niño-Southern Oscillation teleconnection.

Variation in solvent-extractable lipids and n-alkane compound-specific carbon isotopic compositions with depth in a southern China karst area soil

Because literatures about the lipid compositions of modern soils in karst areas are scarce, we have studied the soil horizons overlying the Heshang (和尚) Cave that has provided paleoclimate records from speleothem lipid contents. Our analysis reveals a series of n-alkanes, free fatty acids, n-alkanols and n-alkan-2-ones distribution and relative abundance changing with the depth, and in which the ratios of the lower molecular weight to higher molecular weight n-alkanes, free fatty acids, n-alkanols and n-alkan-2-ones have a peak at a subsurface depth of 5 to 10 cm. An accompanying peak in 17β(H), 21β(H)-hop-22(29)-ene (diploptene) and a shift to less negative n-alkane carbon isotopic values also identify this layer in the karst soil. This pattern indicates the existence of a subsurface soil layer in which the microorganisms that produce these compounds are especially abundant. The carbon isotopic values of individual plant wax n-alkanes are about 3‰ greater at the base of the 30- to 40-cm soil profile than in the surface layer, probably as a result of selective microbial degradation of n-alkanes from different primary sources. The lipids and carbon isotopic values of individual plant wax n-alkanes study of the overlying soil show a strong microbial activity in this karst soil and help in interpreting the lipid compositions and specific carbon isotopic value of n-alkanes of the stalagmites of the Heshang Cave for paleoenvironmental reconstructions.

Hydrogen isotopic characteristics and their genetic relationships for individual n-alkanes in plants and sediments from Zoigê marsh sedimentary environment

To understand internal relations of their hydrogen isotopic compositions in typical marsh environment, we, using GC-IRMS analytical technique, measured the hydrogen isotopes of individual n-alkanes in the herbaceous plant, woody plant leaf, and sediments from Zoige marsh in China. The results show significant differences in the hydrogen isotopic compositions of n-alkanes among the different kinds of plants and the different species in the same kind. δD values of n-alkanes in the herbaceous plants (from −254‰ to −184‰) are lighter than those in woody plant leaf (from −195‰ to −142‰), and the hydrogen isotopic compositions of n-alkanes in K. tibetica P. are lighter than P. pratensis L. The mean δD values of n-alkanes in the sediments from Zoige marsh reflect that they were derived from herbaceous plants, which is consistent with the peat samples being composed mainly of herbaceous plant remnants. The significant differences in hydrogen isotopic compositions of n-alkanes among the sedimentary samples are caused possibly by environment factors and the difference in input quantity of different herbaceous plants. A certain negative correlation exists between the δ13C and δD values of n-alkanes in the samples, and plant types can be distinguished using the cross plot of δD vs. δ13C values of n-alkanes in the plants. These data and recognitions provide scientific basis for hydrogen isotopic applied research of individual n-alkanes.

δD values of n-alkanes in Tibetan lake sediments and aquatic macrophytes – A surface sediment study and application to a 16 ka record from Lake Koucha

A set of lake surface sediment samples and aquatic macrophytes from the eastern and central Tibetan Plateau and a sediment core from Koucha Lake (northeastern Tibetan Plateau) were analysed for hydrogen isotope signals of aliphatic lipid biomarkers. The δD values of n-alkanes were correlated with those of meteoric water in the summer months and showed increasing Pearson correlation coefficients with increasing n-alkane chain length ( r, 0.80 for n-C 31). While in samples from the most humid regions (southeastern Tibetan Plateau), the apparent enrichment factors between meteoric water and lipids are close to those reported for other humid sites ( ε ≈ −135‰ for mid-chain and ≈ − 125‰ for long chain n-alkanes), they become smaller ( ε ≈ −118‰ to −41‰; mean value: −86‰) at sites with a negative moisture balance (northeastern and central Tibetan Plateau and Qaidam Basin). This might be caused by enrichment in deuterium of the source water used for lipid synthesis as a result of lake water evaporation and soil and leaf water evapotranspiration. In the core from Lake Koucha, decreasing δD values of all n-alkanes between 15 and 10 cal ka BP give evidence for an intensification of the Asian summer monsoon. At ca. 9 cal ka BP, a ca. 15‰ shift towards higher δD values of mid-chain n-alkanes suggests enhanced evaporation of the lake water, which outweighs higher precipitation amounts because of a temperature increase during the early Holocene monsoonal maximum. Only a few surface sediment samples show a significant offset between δD values of mid-chain n-alkanes and n-C 31 (ΔδD mid- n -C31) and no systematic dependence between this offset and environmental parameters, such as mean annual precipitation, was found. In the core, available data for most samples also do not show a significant offset, only a slight increase (≈ + 15‰) of ΔδD mid- n -C31 at ca 10.0 cal ka BP and a decrease (≈ − 17‰) at ca 6.8 cal ka BP might indicate wetter and drier conditions, respectively. However, interpretation of ΔδD mid- n -C31 throughout the record is difficult because of a lack of reliable data for n-C 31 in key sections of the core. So, more records are necessary to further test the potential applicability of ΔδD mid- n -C31 as a proxy for palaeo-hydrological conditions.

Use of biomarker distributions and compound specific isotopes of carbon and hydrogen to delineate hydrocarbon characteristics in the East Sirte Basin (Libya)

Biomarker ratios, together with stable carbon (δ 13C) and hydrogen (δD) isotopic compositions of individual hydrocarbons have been determined in a suite of crude oils ( n = 24) from the East Sirte Basin to delineate their sources and respective thermal maturity. The crude oil samples are divided into two main families (A and B) based on differences in source inputs and thermal maturity. Using source specific parameters including pristane/phytane (Pr/Ph), hopane/sterane, dibenzothiophene/phenanthrene (DBT/P), Pr/ n-C 17 and Ph/ n-C l8 ratios and the distributions of tricyclic and tetracyclic terpanes, family B oils are ascribed a marine source rock deposited under sub-oxic conditions, while family A oils have a more terrigenous source affinity. This genetic classification is supported by the stable carbon isotopic compositions (δ 13C) of the n-alkanes. Using biomarker maturity parameters such as the abundance of Pr and Ph relative to n-alkanes and the distribution of sterane and hopane isomers, family A oils are shown to be more thermally mature than family B oils. The contrasting maturity of the two families is supported by differences between the stable hydrogen isotopic compositions (δD) of Pr and Ph and the n-alkanes, as well as the δ 13C values of n-alkanes in their respective oils.

Occurrence of diploptene in moss species from the Dajiuhu Peatland in southern China

Diploptene is present in four moss species (Sphagnum palustre, Aulacomnium palustre, Polytrichum commune and Hypnum revolutum) and some surface samples collected from the Dajiuhu Peatland in southern China. It exists in enhanced amounts in surface peat samples taken from a site dominated by H. revolutum, the specie that contains the greatest content of diploptene. By comparison of the carbon isotopic values of n-alkanes and diploptene in the four moss species and the peat, we postulate that the moss could be an important contributor of diploptene, especially in environmental settings such as peatlands.