Carbon Isotopic Compositions Of N-alkanes Research Articles

Organic Geochemical Evidence for the Formation of Condensate from Coaly Source Rocks in the Wumaying Buried Hill of the Huanghua Depression, Bohai Bay Basin.

Although oil and gas from coaly source rocks have been widely discovered worldwide, the role of oil generated from coal measures in marine-continental coaly deposits during the Carboniferous-Permian period in the Bohai Bay Basin has long been a subject of debate. The recent discovery of a condensate reservoir in the Wumaying buried hill within the Huanghua Depression of the Bohai Bay Basin offers new potential insights into this issue. In this study, we employed organic geochemical methods to explore the possibility of the Carboniferous-Permian coal deposit being a primary source of the condensate. The distribution of light hydrocarbons and the biomarker assemblage indicate that the condensate did not undergo significant secondary alterations such as thermal cracking, gas invasion fractionation, or biodegradation. The hydrocarbon generation potential of the Carboniferous-Permian coaly source rocks suggests that they could be an important contributor to the formation of condensate. High pristine/phytane ratios (1.0-7.5), an abundant presence of benzene series, and the dominance of C29 steranes (>50%) within the condensate could be indicative of coaly organic matter. These features are comparable to those found in coaly source rocks. Moreover, the stable carbon isotopic compositions of n-alkanes in the condensate, ranging from -26.0 to -30.0‰, correlate well with those from coaly mudstone (-25.4 to -30.0‰). This suggests that the condensate of the Wumaying buried hill may predominantly originate from the Carboniferous-Permian coaly mudstone. When integrated with the geological background, the results distinctly demonstrate that the Carboniferous-Permian coaly source rocks have significantly contributed to the formation of the condensate reservoir in the Wumaying buried hill. This provides an essential reference for future exploration of oil and gas resources derived from the carboniferous-Permian coaly source rocks in the Bohai Bay Basin.

Characteristics of n-alkyl lipids in cultivated Sphagnum and their preservation potential in the topsoil of Sphagnum farmlands in southwest China

Sphagnum, a crucial peat-forming plant, is essential for sustaining the ecosystem and counteracting climate change. In recent years, attempts have been made to cultivate Sphagnum commercially to reduce the environmental impacts of harvesting the natural Sphagnum. However, research on the distribution of lipids in cultivated Sphagnum and their potential for preservation potential in the underlying soil is still limited. This study examined the distribution of n-alkyl lipids and the carbon isotope composition of n-alkanes in both natural and cultivated Sphagnum, as well as the underlying topsoil in southwestern China. Results show that the distributions of n-alkyl lipids in the cultivated Sphagnum are similar to those of the natural Sphagnum in the same region. Additionally, the δ13C values of mid-chain n-alkanes (C23 and C25) in the cultivated Sphagnum are significantly negative compared to those of long-chain homologs (C29 and C31; one-way ANOVA, p < 0.05), indicating that symbiotic methanotrophs are relatively active in the cultivated Sphagnum. Furthermore, the n-fatty acids and n-alkan-2-ones in the topsoil are mainly imported from Sphagnum, while the n-alkanes in the topsoil may originate from both Sphagnum and vascular plants. These findings contribute to a better understanding of the ecological benefits of Sphagnum cultivation and the contribution and fate of Sphagnum-derived carbon in soil profiles.

Geochemical behavior of aliphatic hydrocarbons during primary migration and implication for hydrocarbon expulsion: A case study in Qikou Sag, Bohai Bay Basin, eastern China

Lipid biomarker parameters and molecular carbon isotope compositions of n-alkanes were used as tracers to determine geochemical behavior of aliphatic hydrocarbons during their migration from two sequences of source rock-type mudstones (4050.8–4062.2 and 4067.4–4073 m depth, respectively) to the interbedded reservoir sandstones (4062.6–4066.3 m depth) in the Qikou Sag, Bohai Bay Basin, eastern China. Abnormally high values of the production index (0.69–0.90) and total extract/total organic carbon (TOC) ratios (0.53–1.21) in the sandstone samples indicated that the hydrocarbons in the sandstones were not produced in situ but migrated from the adjoining mudstones. A comparison of values in lipid biomarker parameters (oleanane index, gammacerane index, C23 tricyclic terpane/C30 αβ-hopane, and C29/C27 ααα20R sterane ratios) in the sandstone with the mudstone samples indicated steroids and hopanoids in the upper section of the sandstone layer (4062.6–4065.2 m depth) were transported from the overlying mudstones, while those in the lower sandstone layer (4066.3 m depth) were transported from the underlying mudstones. Molecular carbon isotope data suggest that all n-alkanes in the upper section of the sandstone layer were transported from the overlying mudstones. However, in the lower sandstone layer, most of the C15–C17n-alkanes were transported from the overlying mudstones, while the C18–C31n-alkanes were transported mainly from the underlying mudstones. These combined results suggest the short-chain n-alkanes (<C17) were expelled preferentially relative to other compounds (probably via molecule diffusion) while the long-chain n-alkanes (>C18) and steroids/hopanoids may have been expelled in a separate oil phase.

Carbon isotopes of n-alkanes allow for estimation of the CO2 pressure in the Early Jurassic - A case study from lacustrine shale and cannel boghead in the Dachanggou Basin, Xinjiang, Northwest China

The carbon isotope composition of sedimentary organic matter (OM) records abundant climate and environmental information, and a case from the Early Jurassic Badaowan Formation (northern Xinjiang) in Northwest China based on the integration of organic petrography, biomarkers, the carbon isotope composition of n-alkanes and the total organic carbon of coal and oil shale is presented. The results show that total organic carbon isotopes (δ13CTOC) are affected by the OM source composition, while carbon isotopes of n-alkanes show different variations. The fractionation of the two types of isotopes responds differently with varying precision to climate and environmental changes. The OM source is the main factor causing the fractionation of total organic carbon isotopes, and the fractionation of δ13C of n-alkanes reflects atmospheric CO2 pressure (pCO2). According to the covariation between the δ13C of n-alkanes in C3 plants and pCO2, the pCO2 during the deposition of oil shale and coal in the Lower Jurassic Badaowan Formation is restored. In the early stage of oil shale deposition, a higher pCO2 corresponds to a higher inertinite content, pointing to an increase in the fire frequency. Some of the lacustrine cannel boghead interbedded with oil shale is also correlated with a high pCO2, promoting biological productivity in lakes (including the surrounding hinterland) and the formation of lacustrine cannel boghead. The sedimentary archive variation in pCO2 exhibits periodicity, and lacustrine shale has the potential to act as a registering fluctuation of pCO2 at a higher resolution.

Characteristics of hydrocarbons in hydrothermal products of the Clam hydrothermal field from the Okinawa trough

The hydrothermal products of the Clam hydrothermal field from the Okinawa Trough were analyzed by gas chromatography–mass spectrometry to determine abundances of hydrocarbons. The n-alkanes in the hydrothermal products conformed to a bimodal distribution and exhibited an odd-to-even predominance of high molecular weight and an even-to-odd predominance of low molecular weight n-alkanes with maxima at C16 and C18. The total concentration of n-alkanes in hydrothermal sediment was much higher than that in hydrothermal sulfide and altered rock. The carbon isotopic value of individual n-alkanes in hydrothermal sediment was slightly higher than that in pelagic sediment. The concentrations and individual carbon isotopic compositions of n-alkanes suggest that the n-alkanes in hydrothermal products may be mainly the result of the metabolic activity of submarine microorganisms. Additionally, the present results suggest that the abiogenic contribution to source of hydrocarbons in hydrothermal products of the Clam hydrothermal field from the Okinawa Trough should not be ignored.

The Stable Carbon Isotopic Compositions of n-Alkanes in Sediments of the Bohai and North Yellow Seas: Implications for Sources of Sedimentary Organic Matter

Stable carbon isotopic compositions of n-alkanes in surface sediments of the Bohai and North Yellow Seas were investigated to elucidate sources of sedimentary organic matter in these seas. The long-chain n-alkanes in surface sediments are predominantly long-chain C27, C29, and C31 types, with obvious odd carbon predominance. The δ13C values of long-chain n-C27, n-C29, and n-C31 alkanes are −30.8%±0.59‰, −31.9%±0.6‰, and −32.1%±1.09‰, respectively, within the range of n-alkanes of C3 terrestrial higher plants. This suggests that sedimentary n-alkanes are derived mainly from terrestrial higher plants. Compound-specific carbon isotopic analysis of long-chain n-alkanes indicates that C3 terrestrial higher plants predominate (64%–79%), with angiosperms being the main contributors. The n-alkane δ13C values indicate that mid-chain n-alkanes in sediments are derived mainly from aquatic emergent macrophytes, with significant petroleum pollution and bacterial degradation sources for short-chain n-alkanes.

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.

Paleoenvironmental implications of molecular and carbon isotopic distributions of n-alkanes in the hominoid-bearing upper Miocene sediments from the Zhaotong Basin, southwestern China

It is crucial to understand the relationship between hominoid primates and their living environment. Located at the southeastern margin of the Tibetan Plateau, the Shuitangba (STB) outcrop from the Zhaotong Basin is famous for bearing the remains of the youngest known ape fossil in the terminal Miocene (~6.2 Ma). In this study, the molecular and carbon isotope compositions of n-alkanes, together with the loss on ignition and the bulk carbon isotope composition of total organic matter, were investigated in the STB outcrop to infer the paleoenvironmental conditions in this uppermost Miocene sedimentary sequence, with the emphasis on the habitat of the STB hominoid. In the hominoid-bearing peaty clay layer and the surrounding lignite layers, the n-alkane molecular and carbon isotope compositions support a primary contribution from terrestrial higher plants. In contrast, n-alkanes in the upper shallow lake sediments are derived from multiple origins, mainly from aquatic plants and bacteria. Both carbon preference index (CPI) and average chain length (ACL) vary closely with the lithology, but the CPI record shows the larger amplitude of change than the ACL and varies sharply during the lithological boundary. It is notable that the changes of the molecular and carbon isotope compositions of long-chain n-alkanes do not parallel with the abrupt increase of Poaceae pollen percentage during the STB hominoid appearing stage. Such an inconsistency between herb pollen percentage and n-alkane ratios may result from the different sources. Our results clearly support that these organic matter-based ratios have the potential to record the paleoenvironmental changes in ancient lacustrine sediments.

Response of grassland ecosystem to monsoonal precipitation variability during the Mid-Late Holocene: Inferences based on molecular isotopic records from Banni grassland, western India

Banni, located in the arid western India, is one of the largest tropical grasslands of the Asian continent. The net primary production in this grassland ecosystem is currently mediated by precipitation during the Indian summer monsoon (ISM). However, timing of the grassland expansion and its link to the intensity of monsoonal precipitation remains enigmatic due to the paucity of datasets. The major objective of this study is to understand the changes in monsoonal precipitation and vegetation for the last 4600 cal yr BP using hydrogen and carbon isotopic composition of n-alkanes (δDn-alkane and δ13Cn-alkane) measured from two core sediments (Chachi and Luna) in Banni region. The δ13CC29 and δ13CC31 values for Chachi core sediments vary from −30.9 ‰ to −27.2 ‰ and −34.4 ‰ to −25 ‰ respectively. The δ13Cn-alkane values from the core sediments are converted into %C4 plants based on a binary mixing model using the end-member δ13Cn-alkane values derived from the dominant modern vegetation in the Banni region. The prominent feature of the paleovegetation curve is the marked increase in the δ13Cn-alkane values after 2500 cal yr BP, which suggests proliferation of C4 grasses in this region. Similar changes after 2500 cal yr BP have also been observed in the δDn-alkane values. The δDC29 values are used to calculate δD value of paleoprecipitation that varied from 10 ‰ to −60.2 ‰. A significant increase in the δD values of paleoprecipitation (ca. 25 ‰) indicates a weakened ISM precipitation after ca. 2500 cal yr BP. The regional aridification and frequent fire events may have helped the expansion of C4 plant dominated grassland ecosystem in Banni region. Correlation between paleoclimatic records suggests that the southward migration of intertropical convergence zone and more frequent warm phases of El-Nino Southern Oscillation have triggered the weakening of monsoonal precipitation in the tropical region.

N-Alkanes as indicators of climate and vegetation variations since the last glacial period recorded in a sediment core from the northeastern South China Sea (SCS)

Variations in n-alkanes proxies (L/H, C31/C27 and ACL), molecular abundances of n-alkanes and stable carbon isotopic compositions of n-alkanes (δ13Calk) from the core STD235 (20°21.15′N, 118°22.56′E) have been used to trace characterize the climatic changes that occurred in the northern South China Sea (SCS) during the last glacial period. The ratio of low to high molecular-weight n-alkanes (L/H) and the concentration of n-alkanes indicated that organic matters in the sediments were derived from the mixture of both marine and terrestrial sources, but dominated by marine autochthonous input over the past 20 ka. Total n-alkanes were higher during the last glacial stage relative to the interglacial stage, implying the low sea level and the strengthened East Asian winter monsoon (EAWM) during the glacial time. The δ13Calk data indicated that the terrestrial plants on the adjacent continent of the present study area were dominated by C3 plants during the Holocene and experienced the expansion of C4 plants during the last glacial period. This similar result can be proved by the chain length parameter of n-alkanes (C31/C27, ACL) and the content of the herb pollen. In addition, n-alkanes distributions and C4 vegetation proportion calculated by δ13C of long-chain n-alkanes in the core STD235 recorded a short arid duration paralleled by the Heinrich event 1. Thereby we deduced that the occurrence of the dry event and the variation of regional vegetation community at the adjacent lands of the northern SCS were likely under the influence of the climate change emanated from the boreal high-latitudes.

Wax lipids in fresh and charred anatomical parts of the Celtis australis tree: Insights on paleofire interpretation

Leaf waxes have been assumed to be the dominant source of wax delivered to sediment. However, wooden branches and twigs have not been widely considered in this context and could be a potential source of wax lipids in fire places or combustion structures. Black sedimentary layers are the main material of open-air archaeological combustion structures and represent either carbonized fuel (wood) or the charred ground beneath the fire (mainly leaves) and it is difficult to discern between the two sources. To identify different plant parts as components of combustion residues, fresh and charred leaves, branches and twigs (bark and xylem) of the Celtis australis tree were analyzed for aliphatic and aromatic hydrocarbons and fatty acid concentrations, as well as the carbon isotopic composition of n-alkanes (δ13Calkane). Charred biomass was produced under limited oxygen conditions at 150, 250 (3 and 5 h), 350 and 450 °C for 1 h. The n-alkyl profiles in different parts of C. australis are sufficiently distinct to allow their identification as components of combustion structures under low combustion temperature conditions. Average chain lengths and carbon preference index ratios decrease with increasing temperature and vary among plant parts. The δ13Calkane values remained slightly unaltered up to 350 °C and changed by 3–4‰ at 450 °C. Our results provide new information on the molecular and isotopic changes that occur upon burning different plant parts, which in turn show potential for good preservation of organic matter in archaeological black layers and for positive identification of burned leaf and wood residues in them.

Assessing environmental changes in Lake Shihwa, South Korea, based on distributions and stable carbon isotopic compositions of n-alkanes

We investigate historical environmental changes in an artificial lake, Lake Shihwa in South Korea, based on bulk (TOC, TN, C/N ratio, δ13CTOC, and δ15NTN) and molecular (concentrations and δ13C of n-alkanes) parameters, by analyzing riverbank sediments (n = 12), lake surface sediments (n = 9), and lake core sediments (n = 108). Although the bulk organic parameters showed similar characteristics for all lake surface sediment samples, the distribution pattern and δ13C of n-alkanes revealed distinct differences between 2009 samples and 2012/2016 samples. This change of sedimentary organic matter characteristics can be attributed to operation of the tidal power plant that began in 2011, which improved lake water circulation and thus changed the lake sedimentary environment from anoxic to more oxic conditions. The vertical profiles of bulk and molecular lake sediment core records collected in 2009, especially at the site closest to the dike, showed a drastic shift around 1987, indicating that stronger anoxic sedimentary conditions prevailed after 1987. This is linked to sea dike construction in 1987, which prohibited sea-lake water exchange and thus deteriorated water quality in Lake Shihwa. We conclude that Lake Shihwa has experienced severe environmental changes due to human activities.

Effects of short-term weathering on the stable carbon isotope compositions of crude oils and fuel oils

A short-term simulated weathering experiment was performed on two crude oils and two heavy fuel oils under natural conditions to evaluate the effects of natural weathering processes by using gas chromatography–mass spectrometry combined with gas chromatography–isotopic ratio mass spectrometry. The results of diagnostic ratios of n-alkanes show that only odd to even predominance (OEP1, OPE2) and carbon preference index (CPI) remain stabilized during the 28 d weathering process, but they cannot effectively distinguish the four types of oils. Statistical analyses based on paired sample t-test and principal component analysis (PCA) revealed that stable carbon isotope compositions of n-alkanes in the four studied oils have no significant changes over the weathering time, and that the carbon isotope discrimination (Δδ13C) of n-alkanes is <3‰. We have provided evidence that the stable carbon isotope compositions of n-alkanes compared to n-alkanes diagnostic ratios significantly improve the efficiency and fidelity of the oil fingerprint identification.

Microbial alteration of the hydrogen and carbon isotopic composition of n-alkanes in sediments

The hydrogen (δD) and carbon (δ13C) isotope compositions of long-chain n-alkanes provide a record of paleoenvironmental conditions. At present, the role bacteria play in altering the stable isotope composition of n-alkanes during storage of sediments is unclear. We collected four sediment samples at precise depth intervals from an archaeological site and stored these samples at room temperature for three years. Then, we collected sediment from identical locations (measured to the cm level) three years later and froze these immediately. Samples stored at room temperature showed high amounts of medium-chain n-alkanes (n-C18–n-C26), which were absent from samples that were immediately frozen, and a decrease in the abundance of long-chain n-alkanes (n-C27–n-C33). Measured δD values of n-C29 and n-C31n-alkanes increased in stored samples by up to 25‰. δ13C values in stored samples are more negative than non-stored samples by up to 4.2‰. DNA analysis of the soil microorganisms showed bacteria containing genes coding for n-alkane degrading enzymes proliferated in samples during their three year storage. In particular, Rhodococcus and Aeromicrobium, genera which contain multiple coding regions for the n-alkane degrading enzymes CYP153 and AlkB, increased by an order of magnitude (from 0.7% to 7.5% of bacteria present). These data suggest that bacteria may play a larger role than previously expected in altering measured n-alkane stable isotope values during storage. We recommend that sediment samples that are exposed to significant changes in oxygen or ambient moisture after collection are extracted as soon as possible or stored refrigerated or frozen.

Protosalvinia revisited, new evidence for a land plant affinity

The Upper Devonian Protosalvinia phytoclasts from Ohio Shale and its host rock were analyzed using GC/MS, GC-ir/MS, in situ FTIR and pyrolysis–GC/MS, in order to determine its chemical composition and to discuss the taxonomic position of this enigmatic fossil. Based on molecular ratios, the maturity of the organic matter present in the phytoclasts and rocks is comparable to a bituminous rank coal. Diagenetic products of Chlorobiaceae carotenoids were observed in the hosting rocks of Protosalvinia, indicating that anoxia prevailed during the sedimentary deposition. The soluble organic matter of phytoclasts shows land-derived biomarkers including long chain n-alkanes with a slight odd/even predominance and bicyclic sesquiterpenes with eudesmane skeletons. The carbon isotopic composition of n-alkanes is consistent with a land plant origin. The macromolecular fraction of Protosalvinia consists of a condensed aromatic network. The pyrolysis products of the phytoclasts show polycyclic aromatic hydrocarbons commonly observed in coals, including naphthalenes, phenanthrenes and dibenzofurans. Based on what is known about the thermal evolution of organic matter, these components could derive from demethylation and condensation of phenylpropanoids or polysaccharides present in the original structure of Protosalvinia. The former components are exclusively metabolized by land plants.

Crude oils and organic matter of late Devonian deposits of the Timan—Pechora basin: Comparison by molecular and isotopic data

The carbon isotope composition of individual n-alkanes in samples of crude oil from the Late Devonian play of the Timan—Pechora basin and rock bitumen that reached the main stage of oil generation has been studied. The carbon isotope composition of individual alkanes of thermal bitumen obtained by autoclaving low-maturity Domanik rocks has been also determined. The carbon isotope composition of individual alkanes of the rock bitumen has been shown to correspond to that of the oil samples, a feature that indicates the syngenetic nature of deposits of these Late Devonian oils. The correlation between the oil and bitumen compositions is also confirmed by matching the composition of acyclic hydrocarbons and polycyclic biomarkers of the test oils and bitumens. For the Domanik organic matter, the carbon isotope composition of n-alkanes has been revealed to depend on the temperature of artificial maturation of the rock organic matter, a finding that potentially increases the reliability of the correlation between the oil and source rock organic matter by the isotope data.

Chemical and carbon isotopic dynamics of grass organic matter during litter decompositions: A litterbag experiment

A litterbag method was used for studying the variability in chemical and carbon isotopic compositions of four grasses during litter decomposition. After the 300d degradation, >90% of litter mass was lost for three C4 species (Setaria viridis, Eleusine indica, Amaranthus retroflexus) and one C3 species (Erigeron speciosus). The solid state 13C NMR spectra showed that mean proportion of aromatic and alkyl carbon increased from ca. 10% to 15% and ca. 10% to 20%, respectively, whereas that of O-alkyl carbon substantially decreased from ca. 70% to 50%. The carbon preference index and average chain length of n-alkanes remained relatively constant, whereas the carbon isotopic compositions of long chain n-alkanes varied <2‰. Our results demonstrate that the degradation of litters alone does not significantly change the n-alkane chemical and carbon isotopic proxies. Compared to open plant–soil systems, our litterbag experiments present much less variability in chemical and carbon isotopic compositions of n-alkanes. Based on these facts, we recommend a combined measurement of chemical and carbon isotopic properties in evaluation of carbon sources, dynamics and paleoenvironments.

Source characterization of sedimentary organic matter using molecular and stable carbon isotopic composition of n-alkanes and fatty acids in sediment core from Lake Dianchi, China

The distribution and compound-specific carbon isotope ratios of n-alkanes and fatty acids in a sediment core (63cm) collected from Lake Dianchi were examined to investigate organic matter sources in the eutrophic lake. Fatty acids included free and bound fatty acids. The carbon isotope compositions of individual n-alkanes and fatty acids from Lake Dianchi sediments were determined using gas chromatography/isotope ratio mass spectrometry (GC–IRMS). The δ13C values of individual n-alkanes (C16–C31) varied between −24.1‰ and −35.6‰, suggesting a dominance of 13C-depleted n-alkanes that originated from C3 plants and lacustrine algae. Fatty acids from the sediment extracts were analyzed for their abundances and carbon isotopic compositions. Molecular and isotopic evidence indicates that most of the short-chain fatty acids from Lake Dianchi sediment extracts are sourced from intense microbial recycling and resynthesis of organic matter. Long-chain free fatty acids are mainly derived from terrestrial sources. However, long-chain bound fatty acids are sourced from a combination of terrestrial organic matter, bacteria and algae, with the contribution from algal sources higher in the hypereutrophic stage.

Isotopic compositions and biomarkers in crude oils from the Tarim Basin: Oil maturity and oil mixing

Hydrogen and carbon isotopic compositions of n-alkanes, as well as biomarkers in various types of oil from the Tarim Basin were analyzed. Data were used to discuss the occurrence and formation of marine oils from this old composite basin, of which we have limited understanding. First, hydrogen and carbon compositions of n-alkanes clearly classify the collected oils into two distinct groups, supporting the reported oil classifications based on biomarkers. Group I, the majority of oil samples, is characterized by relative enrichment in D and n-alkane δD values vary from −110‰ to −75‰. Group II includes two heavy oils and δD values of n-alkanes range from −145‰ to −142‰. Second, variations in the δ13C value of n-alkanes (2–3‰) show good correlations with the maturity indices, indicating that kinetic isotopic fractionation is one control on the δ13C value of n-alkanes in the maturation process. δD values of n-alkanes in most oils are also evidently influenced by the oil maturity. Finally, by combining the isotopic compositions with biomarkers of oils, two distinct oil mixing processes were recognized in the oils from the Tabei and Tazhong uplifts in this old composite basin. The former process is the mixing of oils from the same source at different maturity stages, while the latter involves the mixing of oils from distinct sources at different maturity stages. Biodegraded oils may contribute to the total oils, but their proportions are minor.

Source characterization using molecular distribution and stable carbon isotopic composition of n-alkanes in sediment cores from the tropical Mundaú–Manguaba estuarine–lagoon system, Brazil

The Mundaú–Manguaba estuarine–lagoon system (MMELS) constitutes one of the most representative ecosystems in the state of Alagoas, NE Brazil. Approximately 30% or 60,000ha of the lower surrounding drainage basin of the MMELS are covered by sugar cane and a total of 250,000 inhabitants contribute untreated effluent to the system. Short sediment cores from MMELS were collected in 2007 at three sites: Manguaba Lagoon (C03), Mundaú Lagoon (C07) and Mundaú River (C08) in order to characterize the delivery and deposition of n-alkanes over the past 40yr. The n-alkanes ranged from C15 to C35, with total aliphatic hydrocarbon (TAH) concentration in the range 27.8–139.5μgg−1. An unresolved complex mixture (UCM) was observed in all sediments. The terrigenous/aquatic ratio (TAR), carbon preference index (CPI) and average chain length (ACL) showed that the terrigenous input dominated. The (δ13C) values of individual n-alkanes (C16–C33) varied between −22.6‰ and −34.2‰, suggesting a dominance of 12C-enriched n-alkanes that originated from C3 plants and lacustrine algae. The data reflect how anthropogenic input (via sewage, industrial pollution and agriculture) has influenced the organic content of the system through time.