N-alkane Ratios Research Articles

Distribution, sources and ecological risks of PAHs and n-alkanes in water and sediments of typically polluted estuaries: Insights from the Xiaoqing River

Seasonal water and sediment samples were collected from the Xiaoqing River estuary and the neighboring sea to study the spatial and temporal distributions, sources and ecological risks of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes. The results showed significant spatial and temporal differences in the concentrations of PAHs and n-alkanes under the influence of precipitation, temperature, and human activities. The concentrations of PAHs in water were lower in the wet season than in the dry season, and those in sediments were higher in the wet season than in the dry season. The concentrations of n-alkanes were higher in the rainy season than in the dry season for both water and sediments. The spatial distributions of PAHs and n-alkanes were estuarine > offshore. The concentration ranges of ∑PAHs in water and sediments were 230.66–599.86 ng/L and 84.51–5548.62 ng/g, respectively, in the wet season and 192.46–8649.55 ng/L and 23.39–1208.92 ng/g, respectively, in the dry season. The proportion of three-ring PAHs in water (57.03% and 78.27% in the wet and dry seasons, respectively) was high, followed by two-ring PAHs (27.31% and 13.59% in the wet and dry seasons, respectively). The proportion of four-ring PAHs was higher in sediments (24.79% and 32.20% in the wet and dry seasons, respectively). The ecological risk of PAHs assessed using the toxicity equivalent quotient and risk quotient was at moderate to moderately high risk levels. The high concentration of n-alkane fraction C16 (611.65–75594.58 ng/L) in the water is indicative of petroleum or other fossil fuel inputs. The main peaks of n-alkanes in river sediments were C27, C29 and C31, indicating higher inputs of plant sources. The sediments in the estuary showed dominance of both short-chain C16 and long-chain C25–C31, indicating a combined input of higher plants and petroleum. The diagnostic ratios of PAHs and n-alkanes indicated that their sources were mainly oil/coal/biomass combustion and petroleum spills attributed to frequent vehicular, vessel and mariculture activities. Given the potential ecological risks of PAHs and n-alkanes in water and sediments, future studies should focus on their bioaccumulation and biotoxicity.

Organic matter sources and distribution along land-use gradient in a Himalayan foothills River: Insights from molecular markers

The analysis of hydrocarbon biomarkers in surface sediments along the Markanda River in the foothills of the Indian Himalayas was conducted to gain insights into the distribution and composition of organic matter (OM) within the sediments. This investigation is essential for comprehending how anthropogenic changes are influencing the OM dynamics in river systems. The study involved identification and quantification of various compound groups such as n-alkanes, hopanes, steranes, polycyclic aromatic hydrocarbons (PAHs), linear alkyl benzenes (LABs) and phthalate esters along with their respective parametric ratios. The variation in distribution of n-alkanes and associated indices (odd-even carbon number predominance (OEP), average chain length (ACL), terrigenous to aquatic ratio (TAR), carbon preference index (CPI), and natural n-alkanes ratio (NAR)) were used to distinguish the natural source of organic content from those influenced by anthropogenic contamination. The detection of petroleum contamination was indicated by the presence of prominent unresolved complex mixtures (UCM) as well as specific petroleum biomarkers such as hopanes, diasteranes, and steranes. The study revealed varying concentrations of the analyzed organic pollutants, with the average of PAHs at 24.6 ng/g dw, LABs at 18.1 ng/g dw, and phthalates at 8.3 μg/g dw. The variability in concentration of the investigated compound groups across different locations indicated spatial heterogeneity, and the land use patterns appears to modulate the sources of OM in surface sediments. The source contribution of PAHs and phthalates determined by positive matrix factorization (PMF) shows the predominant sources of the anthropogenic hydrocarbons were linked primarily to petroleum/petroleum-derived products emissions, industrial discharges, cultural practices and common household waste/sewage disposal. This analysis provides insights for developing mitigation strategies and informing relevant policy changes globally, thereby contributing to the broader understanding of anthropogenic impacts on water ecosystems.

Source and geological significance of Jurassic asphalt in the Ruoqiang Sag of Southeast Depression, Tarim Basin, Northwestern China

The Southeast Depression of the Tarim Basin has a very low petroleum exploration degree, and currently, no industrial oil and gas have been discovered. Many hydrocarbon shows exist in the Ruoqiang Sag, where hydrocarbon sources and accumulation processes are unclear. Only two potential Jurassic hydrocarbon source rocks developed in the Ruoqiang Sag, the Yangye (J2y) lacustrine mudstone of the Middle Jurassic and the Kangsu (J1k) coal-bearing mudstone of the Lower Jurassic. In this study, the Jurassic asphalt found in the QD1 well, combined with the source rocks of drilling cores and outcrops, was used to explore the possible hydrocarbon source and forming process in the Ruoqiang Sag using oil-source correlation and hydrocarbon accumulation geochronology methods. The asphalts have lower light and heavy rare earth fractionation than the J2y mudstone, and the total rare earth element (ΣREE) content, chondrite-normalized curves, and δEu and Y/Ho values are similar to those of the J1k mudstone, especially for the J1k coal. The n-alkane ratios of the carbon preference index and the odd-to-even preference (OEP) of Jurassic asphalts are greater than 1.0, indicating that its source rock has experienced low thermal evolution. All Jurassic samples have similar source-related biomarkers but different maturity-related parameters, and the thermal evolution degree of asphalt is close to the J1k mudstone and coal but slightly higher than the J2y mudstone. Meanwhile, rhenium–osmium isotopes show the asphalts are formed in 192+30/−45 Ma, which is similar to the J1k sedimentation time. The asphalts contain coal macerals and probably derived from the asphaltene precipitated by the J1k coal during the early coalification. The low abundance of 25-norbornanes with a non- or slight petroleum biodegradation degree indicates the asphalt was generated relatively late and migrated into the J2y formation along the faults, possibly accompanied by the striking activities of the Altyn Tagh Fault during the Neotectonics movement. This study provides evidence for the J1k coal-bearing strata serving as the effective source rock in the Ruoqiang Sag and increases confidence for Jurassic petroleum exploration in the Southeast Depression of the Tarim Basin. The structural and lithological traps near hydrocarbon source stoves, where Jurassic strata were deeply buried and far from the Altyn Tagh Range, are favorable oil and gas exploration targets.

History of organic pollution in montane lake Issyk-Kul, Kyrgyzstan, Central Asia, inferred from a sediment core

In arid regions, montane lakes are valuable water sources and play important ecological roles. However, recent human-induced inputs of organic pollutants are threatening lake ecology in such regions and becoming a matter of great concern. To investigate pollutant histories and sources, we measured polycyclic aromatic hydrocarbons (PAHs) and n-alkanes in a dated sediment core that spans the last ∼350 years, from montane Lake Issyk-Kul (Kyrgyzstan, Central Asia). Results showed that organic pollutants were delivered to Lake Issyk-Kul in four stages and that their concentrations increased from Stage I (∼1670–1800 CE) to Stage IV (∼2000–2010 CE). Furthermore, we tracked the sources of sedimented PAHs using their ratios combined with n-alkanes data. Ratios of PAHs Ant/(Ant + Phe), Flt/(Flt + Pyr) and Bap/BghiP indicated that inputs during Stage II (∼1800–1970 CE) and Stage III (∼1970–2000 CE) came mainly from high-temperature combustion of coal and vehicle emissions. PAHs in Stage I and Stage IV, however, were mainly derived from low-temperature combustion and petrogenic sources. Diagnostic PAH ratios, combined with the natural n-alkane ratio (NAR＜0) and unresolved complex mixtures (UCM), showed that the sources of PAHs in Stage I were mainly from erosion of bedrock and partly influenced by forest wildfires, different from the source during Stage IV, which was mainly from refined petroleum caused by accidental spills. Our assessment of the contamination history of the lake indicates that toxicity risk to the waterbody from sediment PAHs is low, but recent discharges arising from traffic deserve attention.

Occurrence, distribution and sources of petroleum contamination in reef-associated sediments of the Gulf of Mannar, India

The distribution of saturated hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) was assessed in superficial sediment samples collected from Mandapam island groups, Gulf of Mannar, India. The hydrocarbon distribution pattern and the n-alkane indices (e.g., carbon preference index (CPI) and natural n-alkanes ratio (NAR)) were deployed to differentiate between the biogenic and anthropogenic sources. Petroleum pollution was indicated by the pristane/phytane ratio close to 1. Presence of a prominent unresolved complex mixture (UCM) as well as hopane concentrations further supported this assertion. The evaluation of petrogenic sources of contamination were also comprehended by various diagnostic ratios of PAHs. The sites associated with shipping activities, tourism, and located near the mainland and accessible portions of the islands exhibited high petroleum contamination. Correlation analysis underlines the significance of combining petroleum-specific marker compounds and diagnostic ratios to improve the assessment of human influence on marine ecosystems.

Geochemical evaluation and hydrocarbon generation potential of the Upper Cretaceous–Pliocene succession, offshore Nile Delta, Egypt

The offshore Nile Delta is a prominent petroleum province; nevertheless, previous research focused mainly on evaluation of Miocene–Pliocene possible source rocks because of lack of data. The current study is the first to evaluate organic geochemical features of the Upper Cretaceous–Pliocene sequence in the western offshore Nile Delta, applying TOC, Rock-Eval pyrolysis and GC-MS procedures for 137 ditch cuttings collected from three wells. The main objectives are to infer organic richness, type of kerogen, maturation, and depositional paleoenvironment of organic matter for these possible source intervals.The analysed rock samples generally display fair to good organic enrichment. Most of these samples are enriched in Type-III and Type-II/III kerogen, suggesting a high influx of terrigenous organic matter. However, Abu Madi, Appollonia, and Khoman formations contain Type-II oil-prone kerogen, reflecting more marine organic matter, whereas Kafr El Sheikh and El Wastani cuttings display the lowest TOC contents and are more enriched in non-generating inert organic matter.Biomarker ratios indicate that Kafr El Sheikh, Abu Madi, Qawasim, and Qantara formations are rich in Type-II/III kerogen (mixed organic matter) preserved under suboxic nearshore or lacustrine depositional environments. In contrast, Sidi Salem, Tineh, Appollonia, and Khoman formations contain abundant Type-II marine-algal organic matter deposited under anoxic marine depositional settings. Vitrinite reflectance data and biomarker ratios indicate immature to peak mature rock samples. The stable carbon isotopic composition of condensate oil samples suggests nonmarine waxy oils. Ratios of n-alkanes and isoprenoids for a condensate sample from the study area reveal no similarity with the Upper Cretaceous–Pliocene rock extracts.

Influence of salinity on hydrogen isotope fractionation of n-alkanes in mangrove leaves and surface sediments: A comparison across various geomorphological settings

Increasing attention has recently been paid to hydrogen isotope ratios of mangrove lipids due to their potential for reconstructing past salinity and hydrology. This proxy is based on several observations that net 2H/1H fractionation, εn-alkane/water, between mangrove leaf waxes and surface water, increases with surface water salinity. However, the extent to which hydrogeomorphic conditions, on a regional scale, affect the hydrogen isotope fractionations of n-alkanes in mangrove trees and sediments is not yet understood. This study aimed to assess the robustness of negative εn-alkane/water – salinity for six mangrove species and adjacent surface sediments from the Zhanjiang mangrove across different hydrogeomorphic settings. The results indicated that a significant inverse relationship exists between εn-C31/water and εn-C33/water and salinity for all samples from the three fringe and basin/interior mangroves, with εn-C31/water decreasing by −2.57 ± 0.87 ‰ ppt−1 and εn-C33/water decreasing by −3.64 ± 1.14 ‰ ppt−1. These trends, as well as interspecific differences among mangrove species, could be attributed to biochemical changes in response to salt stress, such as using different proportions of amino acids and soluble carbohydrates as compatible solutes. In contrast, the relationship between sedimentary εn-alkane/water and salinity differed significantly across hydrogeomorphic conditions and land-use changes, possibly due to variations in hydrology and input of allochthonous organic matter. A comparison of fitted parameters of linear regressions among riverine, fringe, and interior mangroves suggests that the slop and intercept describing the linear relationship between sedimentary εC27–33/water of basin/interior mangroves and salinity are generally consistent with that of the corresponding mangrove trees. This suggests that the basin/interior mangroves and their sedimentary deposits may be better candidates for paleoenvironmental reconstruction using hydrogen isotope ratios of long-chain n-alkanes.

Branched-chain biofuels derived from hydroisomerization of palm olein using Ni/modified beta zeolite catalysts for biojet fuel production

To overcome mass transfer limitation and adjust zeolite's acidity, the modification of beta zeolite via etching with hydrofluoric acid/ammonium fluoride (HF/NH4F) solution was revealed and the modified beta zeolite was then applied as a support of nickel (Ni) catalysts used for hydroisomerization of palm olein. HF/NH4F solution containing 0.25 M HF concentration was appropriate to enlarge original existing intercrystalline mesopores of beta zeolite (beta-0.25), which resulted to the higher pore volume. The Ni/beta-0.25 catalyst also exhibited the better Ni dispersion and lower acidity. These features suppressed overcracking and provided 43.4 wt% bio-jet fuel yield with 22.1 wt% iso-alkanes fraction when hydroisomerization was operated under 40 bar initial hydrogen (H2) pressure at 340 °C for 5 h. At 0.75 M HF concentration, the excessive leaching of aluminum species out of the zeolite framework provided beta-0.75 with very low acidity, which was ineffective to convert molecules of palm olein to bio-jet fuel range. Initial H2 pressure at 40 bar was sufficient to activate the hydroisomerization of palm olein over Ni/beta-0.25 catalyst to produce liquid biofuel containing iso−/n-alkane ratio of 2.61. The liquid biofuel/Jet A-1 blend at 50% (v/v) had freezing point and gross heating value of −58.9 °C and 43.7 MJ/kg, respectively.

Biodegradation of dissolved organic matter and sedimentary organic matter in high arsenic groundwater system: Evidence from lipid biomarkers and compound-specific carbon isotopes

Organic matter plays important roles in arsenic mobility. Whether sedimentary or dissolved organic matter is the predominant electron donors is still on debate. To fill this gap, both sediments and groundwater were collected from the Hetao Basin, a typical arid-semiarid area hosting high arsenic groundwater, to characterize diagnostic biomarkers and compound-specific carbon isotopes of lipids in dissolved organic matter (DOM) and its depth-matched sedimentary organic matter (SOM). Results showed that SOM in both clay and sand layers were mainly sourced from terrestrial higher plants, but sand SOM had additional sources from microbial and petroleum OM. Compound-specific carbon isotope compositions of DOM and SOM showed that δ13C of low molecular weight (LMW) n-alkanes (δ13CL) were more positive than those of high molecular weight (HMW) n-alkanes (δ13CH). The δ13CH in groundwater (δ13CH-DOM) and sediments (δ13CH-SOM) fall in the range of C3 plants, indicating that DOM and SOM had similar origins. More positive δ13CL than δ13CH suggests preferential degradation of LMW compounds in both sediments and groundwater. In comparison with SOM, more microbial-derived OM was observed in groundwater DOM, especially in high‑arsenic groundwater. High arsenic groundwater was accompanied by the low ratio of n-alkanes to hopanes in depth-matched SOM, and the great difference between δ13CL and δ13CH and low terrestrial/microbial ratios (TAR) in groundwater DOM. This indicates that SOM and DOM should be utilized by indigenous bacteria to fuel the reductive dissolution of Fe oxides for arsenic release into groundwater. Furthermore, more abundant LMW compounds and weaker odd-to-even predominance in DOM than those in SOM support that DOM encountered stronger biodegradation. This study highlights that preferential biodegradation of DOM (especially LMW components and petroleum sourced OM) would provide electrons for the reductive dissolution of arsenic-bearing Fe/Mn oxides and thus induce arsenic enrichment in groundwater.

Geochemical characteristics of crude oils and hydrocarbon accumulation in the Palogue Oilfield, Melut Basin, South Sudan

The Palogue Oilfield is the largest oilfield in the Melut Basin, an important petroliferous rift basin in Africa. Geochemical analyses, including gas chromatography and gas-mass chromatography were adopted to analyze the biomarker composition and geochemical characteristics of oils from the Palogue Oilfield and the neighboring Moleeta and Jamous lows. Oil-oil correlations were performed for the Palogue Oilfield and the Moleeta/Jamous lows, and the hydrocarbon accumulations of the Palogue Oilfield were studied based on the oil maturities, thermal evolution modelling of the Moleeta/Jamous lows and the petroleum system analysis of the Melut Basin. Geochemical analysis indicates that the oils of the Moleeta/Jamous lows are derived from terrestrial higher plants, with the low ratios (<1) of light n-alkanes to heavy n-alkanes (∑nC21-/∑nC22+) and the predominance of C29 regular sterane, but there still are some geochemical differences in distribution shape of n-alkanes, paleoenvironments and maturities in these two lows. The oils of the Palogue Oilfield have mixed-source features and their geochemical characteristics are more like those of the Jamous low, and a few oils are like the oils in the Moleeta low. The Jamous low has a larger area and higher thermal maturity than the Moleeta low, and the abundant oils generated by the Al Renk source rocks (upper part of early Cretaceous) in the Jamous low are the primary contributors to the formation of the giant Palogue Oilfield. The vitrinite reflectance equivalent (Rc) converted by the methyl phenanthrene ratio indicates that the oil maturities of the Palogue Oilfield range from 0.65% to 1.07%. Thermal evolution modelling of the Moleeta/Jamous lows indicates that the Jamous low generated and expelled hydrocarbons earlier than the Moleeta low. The maturity of Al Renk source rocks reached 0.65% at 70Ma (Late Cretaceous) and 1.07% at 38Ma (Eocene) in the Jamous low, and reached 0.65% at 59Ma (Paleocene) and 1.07% at present in the Moleeta low. Thus, the high-maturity oils (Ro> 1.07%) generated by the source rocks in the Jamous low since 38 Ma have not been discovered in the primary production layers of the Paleogene Samma and Yabus Formations in the higher part of the Palogue structure, a drape-anticline developing on a paleo-high between the Jamous and Moleeta lows. The maturity parameter of Ts/(Ts + Tm) shows that the oil maturities from late Cretaceous Formation in the well PS-7 and PS-8 are close to the oils in the Jamous Low and are significantly higher than the oils from the Paleogene Formation in the higher part of the Palogue structure. So, the Cretaceous Formation under the Paleogene pay zones and the Paleogene Formation in the lower part of the Palogue structure likely accumulated high-maturity oils from the Jamous low, which will enhance the exploration potential of the Palogue Oilfield. The petroleum system study indicates that the end of Oligocene (late Paleogene) is the critical moment for the Paleogene hydrocarbon accumulations in the Melut Basin. Whereas, the high-maturity oil accumulations (Ro> 1.07%) in Cretaceous formations in the Palogue Oilfield occurred after 38Ma (late Eocene), when the Al Renk source rocks in the Jamous low started to enter the high thermal evolution stage with the Ro> 1.07%.

Late quaternary hydrological changes in the southeastern amazon basin from n-alkane molecular and isotopic records in sediments of Saci lake, Pará state (Brazil)

The molecular distributions and isotopic ratios (δ13C and δD) of n-alkanes in sediments deposited during the last 35 cal kyr BP in Saci Lake, southeastern Amazonia, have been measured and interpreted for their paleoclimate information. A terrestrial origin for the odd carbon-numbered long-chain (>C27) n-alkanes has been inferred from molecular distributions. A shift in the n-alkane δ13C values across the Pleistocene-Holocene boundary coincides with a climate-induced floral change previously deduced from palynological analyses. Dry conditions during the late Pleistocene and mid-Holocene were confirmed by several n-alkane indices used to estimate the availability of moisture and by larger δD values. High ACL values indicate relatively dry conditions from 29 to 18.2 cal kyr BP and again from 8.0 and 5.0 cal kyr BP with a decrease in precipitation and an increase of grasses. Changes in the n-alkane C-29 isotopic compositions (δD and δ13C) were observed during the Pleistocene-Holocene transition, coinciding with a floristic change induced by climatic conditions that is also observed in palynological results from the same lake. The n-alkane C-29 δ13C values remain between −33.2‰ and − 35.7‰, which is representative for n-alkanes derived from C3 vegetation. These lines of evidence suggest a continuous gallery forest cover in the Saci Lake catchment for the last ~35 cal kyr BP in contrasted with the observed palynological changes which represent the dynamics of the vegetation on the upland areas. The n-alkane C-29 δD values are less negative in the Pleistocene and more negative during the Holocene in the lake record. These characteristics suggest more humid conditions during the Holocene than in the Pleistocene, probably associated with the South American Monsoon System (SAMS). However, an increase of approximately 10‰ occurs during the late Holocene, which coincides with the forest expansion around 5 cal kyr BP that is recorded in palynological data of Saci Lake and other records of the Amazon Forest. This change suggests that the D/H ratio is also influenced by the increase of the plant biomass, which would provide an enrichment of D in the isotopic signal as a function of regional moisture recycling caused by evapotranspiration.

Holocene vegetation reconstruction in the forest–steppe of Mongolia based on leaf waxes and macro-charcoals in soils

Abstract. Vegetation and climate reconstruction in the forest–steppe of Mongolia is still challenging regarding the pattern of forest and grassland distribution during the Holocene. Different sediments containing paleosols and humic layers provide geomorphological archives for landscape development in Mongolia. n-Alkane and macro-charcoal ratios represent specific indicators to distinguish the share between grasses and trees. In a preliminary study, we investigated the applicability of these two paleo-proxies from soils for vegetation reconstruction comparing different relief positions and site conditions in the northern Khangai Mountains of Mongolia. n-Alkanes that are deposited from leaf waxes in the soil have the potential to indicate vegetation composition on a local scale. Depending on site-specific environmental conditions, n-alkanes are subjected to different degrees of microbiological decomposition, which is more intensive in soils of dry steppe than of forests. Mongolian forests are often underlain by permafrost that may reduce microbiological activity. In steppe soils, the decomposition of n-alkanes increases the quantity of mid-chain n-alkanes that adulterate the biomarker proxy signal to indicate more forest share. Macro-charcoals in soils have a site-specific component, but additional eolian input of macro-charcoals from long-distance transport can provide a distinct proportion in sediments. Thus, eolian influx of wood-derived macro-charcoal can dominate the proxy signal at sites where trees were few or had never existed. Radiometric dating of several paleosols and humic layers has shown that both proxies coincide as evidence for high grassland-to-forest ratios during the Early Holocene. By contrast, the proxy signals diverge for the Late Holocene. For this period, n-alkanes generally indicate more grassland, whereas macro-charcoals show increased wood-derived proportions. We imply that this difference is caused by increased forest fires and simultaneously spreading steppe area. A main portion of leaf waxes and charcoal particles in soils directly derive from the covering and nearby vegetation, whereas large lakes and glacier may receive these biomarkers from a larger catchment area. Thus, we conclude that soil archives provide proxies on a more local and site-specific scale than other archives do. Although the temporal resolution of soil archives is lower than for the other ones, biomarker proxies for paleosols and humic layer can be related to periods of distinct geomorphological processes. Further investigations comparing the multi-proxy data of different geomorphological archives are necessary to improve the paleo-ecological reconstruction for landscape development in Mongolia.

An optimized accelerated solvent extraction and purification method for enhanced recovery of n-alkanes (notably short-chains) from environmental samples using the conventional dichloromethane/methanol solvent

Sedimentary lipid biomarker proxies are extensively used for paleoclimatic and paleoenvironmental research. Among all the lipid compounds, the non-polar n-alkanes (chain lengths n-C14 to n-C37) are one of the most widely used lipid fractions for such studies. However, the lack of optimized automated extraction and post-extraction sample workup procedure, using conventional polar solvent, from environmental samples, has been challenging. Conversely, extraction of the n-alkanes using non-polar solvents restricts the co-extraction of other polar lipid compounds like ketones, alcohols, acids, etc. which may be of interest to many researchers. Here we report an optimized total lipid extraction method using the conventional polar dichloromethane/methanol solvents with standardized Automated Solvent Extractor (ASE) parameters and post-extraction sample work-up protocols. The robustness of the method has been tested by extracting eight samples containing algae, leaf, and sediment/rock of varying age (spanning from recent to ∼252 Ma). The results of the extraction efficiency (yield) of the method were compared with commonly used methods, those using either non-polar n-hexane or polar dichloromethane/methanol solvents. For n-alkane extraction from algal biomass, the performance of the proposed method and the conventional polar solvent method are statistically comparable, but the new method performs better than the non-polar solvent method. Likewise, for the leaf and sediment samples, the performance of the proposed method is equivalent to the non-polar solvent method but outperforms the commonly used polar solvent method. The Terrestrial Aquatic Ratio (TAR) and long-chain hydrocarbon/short-chain hydrocarbon (LHC/SHC) ratios calculated from the algae and sediment samples also show that our recommended extraction approach can be confidently used in calculating n-alkane ratio proxies, which critically depend on the extraction efficiency of the short-chain n-alkanes. Taken together, the new method will be highly useful for studying the n-alkanes from a wide spectrum of natural sample matrices and provide reliable data for paleoclimatic inference.

Molecular and stable isotope analysis (δ13C, δ2H) of sedimentary n-alkanes in the St. Lawrence Estuary and Gulf, Quebec, Canada: Importance of even numbered n-alkanes in coastal systems

Sediments comprise a multitude of inorganic and organic components, with much of the composition of the organics still not fully characterized. Our research targeted n-alkanes, to determine whether compound specific carbon and hydrogen isotope analysis allows for their source identification in coastal sediments. Here, we map the current abundances and sources of straight chain n-alkanes in sediments of the St. Lawrence Estuary and Gulf using molecular (diagnostic ratios) and isotopic fingerprinting (δ13C, δ2H). n-Alkane abundances (117.11 ± 1.61 to 418.64 ± 70.20 µg/g OC), carbon preference index (CPI; 1.95 ± 0.05 to 5.09 ± 0.10), average chain length (ACL; 28.36 ± 0.02 to 28.97 ± 0.01), proportion of aquatic submerged plants and terrestrial plant inputs (Paq; 0.295 ± 0.003 to 0.377 ± 0.002), terrigenous aquatic ratio (TAR; 3.43 ± 0.16 to 7.99 ± 0.05), and n-alkane ratio (NAR; 0.169 ± 0.011 to 0.584 ± 0.011) values varied along the terrestrial-marine continuum. Large differences in the concentration weighted average (WA) δ13C and δ2H for odd and even n-alkanes were found, with WA δ13C ranging from −30.9 ± 0.3 to −33.4 ± 0.09 ‰ and −28.8 ± 0.01 to −32.3 ± 0.2 ‰, respectively, and 165.6 ± 3.6 to −200.8 ± 2.4 ‰ and −96.0 ± 2.8 to −158.7 ± 2.1 ‰ for δ2H. The diagnostic ratios were shown to misrepresent the input sources of organic matter (OM) and were inaccurate as source indicators when more than one OM source was present. With the addition of compound specific δ13C and δ2H analysis of n-alkanes, it was determined that the n-alkanes were predominantly derived from natural, rather than anthropogenic sources, with variations being driven by geographic changes in vegetation type and differing ratios of terrestrial and marine OM inputs. Importantly, compound specific isotope analysis of the even numbered n-alkanes would permit identification and tracking of petroleum-derived contaminants. Molecular data alone are ineffective for this, owing to the similarity in CPI values for petroleum-derived contaminants and highly degraded OM which is discharged by the St. Lawrence River into the estuary.

Composition and Distribution of Aliphatic Hydrocarbon Compounds and Biomarkers in Seafloor Sediments from Offshore of the Leizhou Peninsula (South China).

The offshore of Leizhou Peninsula (LP, China), which contains unique ecosystems such as mangroves, seagrass beds, and coral reefs, is an environmentally sensitive area. For this reason, the levels of aliphatic hydrocarbon including biomarkers (hopanes, steranes) in the offshore seafloor sediments were analyzed in terms of their composition, distribution, and input sources and aimed to evaluate the extent of possible petroleum hydrocarbon contamination in the sediments of coastal areas. The total aliphatic hydrocarbons (TAH) fraction, the content of total n-alkanes (nC14–nC37) (∑n-alkanes), and content of hopane + sterane are in the range of 13.76–99.53, 1.22–8.33, and 0.02–0.23 μg/g dw, respectively. The presence of unresolved complex mixture (UCM) hydrocarbons hump and petrogenic steranes and hopanes in these seafloor sediments suggest that petrogenic sourced hydrocarbon inputs were present. The stations on the peninsula’s southwest side had the lowest values of UCM/resolved aliphatic compounds (UCM/R) and UCM/n-alkanes. These findings suggest that seafloor sediments from the southwest offshore of the peninsula were likely contaminated by recently inputted petroleum hydrocarbons. The presence of relatively high ∑n-alkanes content in seafloor sediments from southwest offshore of the LP, combined with relatively low natural n-alkane ratios (NARs), indicates an increased influence of petrogenic hydrocarbons. The elevated levels of recent petrogenic hydrocarbon contamination in the sediments from the LP’s southwestern offshore were likely related to petroleum exploitation in the Beibu Gulf’s Maichen and Wushi sags.

Biomarker characteristics of Montney source rock, British Columbia, Canada

Montney Formation (MF) source rock located in northeastern British Columbia (BC), Canada, was analyzed to determine its depositional conditions and organic matter source input other than to determine their level of thermal maturity. The high total sulfur (TS) (2.23–20.86 wt.%) and good to very good total organic carbon (TOC) content (0.3–5.87 wt.%) in the analyzed samples give good evidence that the deposition of MF source rock was in a marine environment under reducing conditions. A mixed marine-terrestrial derived organic matter (OM) for the Montney source rock that was deposited in a marine dysoxic environment is deduced from the composition and distribution of different biomarker traces. Thus, the previous result is supported by the high short-chain n-alkanes ratio, accompanied by carbon preference index (CPI) around unity, high concentration of tricyclic terpanes, high C24 tricyclic/C24 tetracyclic, hopane/sterane ratios ranging from low to moderate, as well as the relationship between regular sterane compositions. During deposition of the MF source rock, it can be noticed that more land organic materials this was deduced according to the high waxiness index. From maturity ratios of Ts/(Ts + Tm), C32 22S/(22S + 22R) homohopane, moretane/hopane and 20S/(20S + 20R) and ββ/(ββ + αα) C29 it can give a conclusion that the source rock is mature to postmature of hydrocarbon generation.

Evaluation of organic indicators derived from extractable, hydrolysable and macromolecular organic matter in sedimentary tsunami deposits

Due to the high population density and sensitive infrastructures, coastal areas are highly vulnerable to various geohazards such as tsunamis. The impact of tsunamis is traceable in sedimentary archives providing an important tool for reconstructing these events. Here, geochemical and physical evidence of the resulting inundations have been discussed over the last years with increasing interest on identifying and using highly specific tsumanite indicators.To investigate a broader range of potential organic indicators, a multi-tiered geochemical analysis has been applied on tsunami deposit from Japan and Hawaii in this study with the aim to detect and quantify a more comprehensive set of suitable marker compounds. Both areas of investigation differed especially in terms of population and anthropogenic influence. The analyzed fractions of organic matter covered not only the low molecular weight compounds of the extractable and hydrolysable fraction (a part of the non-extractable fraction), but also the macromolecular matter. Three consecutive analytical approaches including extraction, alkaline hydrolysis and pyrolysis have been applied on the sedimentary organic matter to study their potential to contrast between tsunami and non-tsunami layers.The analyses of compound ratios as well as individual concentration profiles in tsunami and non-tsunami layers revealed two parameters, the TARFA (terrigenous to aquatic ratio of fatty acids) and the C29/C27 sterol ratio, which exhibit a high potential to act as tsunamite indicators. Further parameters (TARALK - terrigenous to aquatic ratio of n-alkanes, concentration profiles of long-chain n-aldehydes) showed a minor potential but are worth to be considered in organic geochemical analyses of tsunami affected sediment archives in the future.

Evaluation of Aliphatic Hydrocarbons in Surface Sediments of Lagoa Mirim (RS, Brazil).

In this study, the hydrocarbons (HCs) levels in sediments from Lagoa Mirim, situated in the south of Brazil, were verified. The methodology brought together stages of pre-sonification, soxhlet extraction, and determination by gas chromatography coupled with mass spectrometry (GC/MS). Ten sample points were evaluated where ∑n-alkanes varied between 1.46µg kg-1 ± 4.0% and 10.10µg kg-1 ± 17.6%. Diagnostic indexes were calculated, being: Carbon Preferential Index (CPI), terrestrial/aquatic ratio (TAR), unresolved complex mixture (UCM), UCM/∑n-alkanes ratio, and n-alkane ratio with Low molecular weight hydrocarbon and High molecular weight hydrocarbons (HMW/LMW). In general, the results of this study indicate a low anthropogenic impact in the environment.

Simultaneous determination of compound-specific isotopic compositions of acyclic isoprenoids and n-alkanes in light crude oil.

Stable carbon and hydrogen isotope ratios of individual n-alkanes and acyclic isoprenoids are important tools in petroleum geochemistry. However, the analysis requires baseline separation and peak profiles using gas chromatography-isotope ratio mass spectrometry to obtain accurate compound-specific isotope data. Time-consuming isolation or purification is typically conducted to separate the compounds to avoid co-elution with other compounds or matrices in crude oils. We developed a simple gas chromatography separation method to simultaneously measure the compound-specific carbon or hydrogen isotope compositions of n-alkanes and acyclic isoprenoids. It was achieved by direct injection of the whole crude condensate and light oil or the saturated fractions of different types of crude oils using a 60m DB-17ms column. This method simplifies the pre-treatment of compound-specific isotope analysis, saves manpower and time, and reduces the use of organic solvents to be more environmentally friendly.

Analyses of depositional environments of the Marcellus formation in New York using biomarker and trace metal proxies

The purpose of this study was to observe how the composition of organic matter (OM) and the extent of anoxia during deposition within the Marcellus Formation in New York varied by distance from the sediment source in eastern New York. Lipid biomarkers (n-alkanes and fatty acids) in the extractable organic component (bitumen) of the shale samples were analyzed, and proxies such as the average chain length (ACL), aquatic to terrestrial ratio (ATR) and carbon preference index (CPI) of n-alkanes were calculated. Fatty acids were relatively non-abundant due to the age of the shale bed, but n-alkane distributions revealed that the primary component of the OM was terrigenous plants. The presence of shorter n-alkane chain lengths in the samples indicated that there was also a minor component of phytoplankton and algal (marine) sourced OM. Whole rock analyses were also conducted, and cerium anomalies were calculated as a proxy for anoxia. All samples had a negative anomaly value, indicating anoxic conditions during deposition. Two samples, however, contained values close to zero and thus were determined to have suboxic conditions. Anoxia and total organic matter (TOM) did not show any spatial trends across the basin, which may be caused by varying depths within the basin during deposition. A correlation between nickel concentrations and TOM was observed and indicates that algae was the primary source of the marine OM, which supports the lipid biomarker analysis. It was determined that the kerogen type of the Marcellus Formation in New York State is type III, consistent with a methane-forming shale bed.