Analysis Of N-alkanes Research Articles

Organic geochemical and compound specific stable carbon isotope analyses of n-alkanes and saturated carboxylic acids extracted from late Paleozoic coals

AbstractIsotopic studies of coal organic matter are often conducted to gain information about past ecosystems and plant communities e.g. under changing climate conditions. While many studies focused on isotope analysis of bulk coal organic matter or extractable aliphatic hydrocarbons, the polar fraction and compounds that are bound to kerogen and not freely extractable have received less attention. The aim of this study was to provide a systematic comparison and evaluation of information gained from different molecular compounds (n-alkanes, n-carboxylic acids) extracted from Paleozoic coals before and after alkaline hydrolysis. Stable carbon isotope analysis of these compounds were used to evaluate a possible application of extractable and ‘bound’ lipids for paleoenvironmental analyses. Coal samples of different stratigraphic age, low thermal maturity (VRr < 1.02%, mostly VRr < 0.8%) and from different paleogeographic locations were selected to investigate organic matter deposited under different paleoenvironmental and paleoclimatic conditions. Molecular distributions show prominent carbon preference suggesting preservation of biologic signatures in coal organic matter. Permian Gondwana coals show similar n-alkane and n-carboxylic acid distribution which may be related to Glossopteris flora. Molecular distributions of Carboniferous coal from Europe are more variable with unimodal or bimodal patterns and maxima in the short- or long-chain range, reflecting higher diversity of paleovegetation and different contribution from algae or bacteria in limnic and paralic environments. The isotope signals of free as well as bound fatty acids show closer response for different depositional environments, whereas signals of paraffins and bulk organic matter are less variable. However, averaged fatty acid isotope signals in coals from different paleovegetational realms, e.g. Euramerica and Gondwana differed only marginally. Graphical abstract

Quaternary floodings in the Zanzibar Channel (NW Indian Ocean, Tanzania) – Identifying palaeoceanographic patterns and palaeoenvironment using a multiproxy study

Multiproxy palaeoenvironmental analyses represent a promising tool for complex reconstructions of continent-sea or air-sea interactions in shallow marine environments. This is in particular illustrated by the case of strong climatic variations in the late Pleistocene to mid-Holocene. A unique combination of geochemical proxies (143Nd/144Nd, 87Sr/86Sr, δ18O and δ13C isotope analyses, n-alkane analysis and biomarker study) together with micropalaeontological data (foraminifera, calcareous nannoplankton and diatoms) allowed to precisely characterise two marine floodings (mid-Holocene and late Pleistocene) in the Zanzibar Channel (NW Indian Ocean, Tanzania). The climate of the late Pleistocene interval was arid compared to the Holocene. The fully oligotrophic environment characteristic of the late Pleistocene flooding (∼115–130 ka) was episodically interrupted by intervals of intense rainfall with episodes of increased nutrient input during the mid-Holocene (∼5–10 ka). The dominance of seaweed meadows in the Holocene and late Pleistocene contrasts with the modern environment dominated by seagrass ecosystems. Relatively non-radiogenic εNd signatures in the Zanzibar Channel during the Holocene and late Pleistocene indicate a strong influence of riverine input draining the Precambrian basement on the African mainland. The main inflow of seawater was from the south, consistent with the flow direction of the East African Coastal Current and the directions of the March to October monsoon winds. A promising tool for future applications as indicators of seagrass/seaweed meadow type ecosystems may be the presence of specific diatom taxa.

The role of water in APCI-MS online monitoring of gaseous n-alkanes

In atmospheric pressure chemical ionization mass spectrometry (APCI-MS), [M−3H+H2O]+ ions can deliver analyte-specific signals that enable direct analysis of volatile n-alkane mixtures. The underlying ionization mechanisms have been the subject of open debate, and in particular the role of water is insufficiently clarified to allow for reliable process analytics when the humidity level changes over time. This can be a problem, particularly in online monitoring, where analyte accumulation in the ion source can also occur. Here, we investigated the role of water during APCI-MS of volatile n-alkanes by changing the carrier gas for sample injection from a dry to a wetted state as well as by using 18O-labeled water. This allowed for a distinction between gaseous and surface-adsorbed water molecules. While adsorbed water seems to be responsible for the desired [M−3H+H2O]+ signals through surface reactions with the analyte molecules, gaseous water was found to promote the formation of CnH2n+1O+ of different (and analyte-independent) hydrocarbons, revealing a reaction with hydrocarbon species which accumulated in the ion source during continuous operation. At the same time, gaseous water competed with analyte molecules for ionization and thus suppressed the formation of alkyl (CnH2n+1+) and alkenyl (CnH2n−1+) ions. The results reveal a memory effect due to hydrocarbon adsorption, which may cause severe interpretation difficulties when the ionization chamber undergoes sudden humidity changes. The use of [M−3H+H2O]+ for n-alkane analysis in alkane/water mixtures can be facilitated by constantly maintaining high humidity and hence stabilizing the ionization conditions.Graphical abstract

Rapid online analysis of n-alkanes in gaseous streams via APCI mass spectrometry.

Online monitoring of dynamic chemical processes involving a wide volatility range of hydrocarbon species is challenging due to long chromatographic measurement times. Mass spectrometry (MS) overcomes chromatographic delays. However, the analysis of n-alkane mixtures by MS is difficult because many fragment ions are formed, which leads to overlapping signals of the homologous series. Atmospheric pressure chemical ionization (APCI) is suitable for the analysis of saturated hydrocarbons and is the subject of current research. Still, although APCI is a "soft ionization" technique, fragmentation is typically inevitable. Moreover, it is usually applied for liquid samples, while an application for online gas-phase monitoring is widely unexplored. Here, we present an automated APCI-MS method for an online gas-phase analysis of volatile and semi-volatile n-alkanes. Mass spectra for n-heptane and n-decane reveal [M-H]+, [M-3H]+ and [M-3H+H2O]+ as abundant ions. While [M-H]+ and [M-3H]+ show an excessive fragmentation pattern to smaller CnH2n+1+ and CnH2n-1+ cations, [M-3H+H2O]+ is the only relevant signal within the CnH2n+1O+ ion group, i.e., no chain cleavage is observed. This makes [M-3H+H2O]+ an analyte-specific ion that is suitable for the quantification of n-alkane mixtures. A calibration confirms the linearity of C7 and C10 signals up to concentrations of ~1000-1500 ppm. Moreover, validated concentration profiles are measured for a binary C7/C10 mixture and a five-alkane C7/C10/C12/C14/C20 mixture. Compared to the 40-min sampling interval of the reference gas chromatograph, MS sampling is performed within 5 min and allows dynamic changesto be monitored.

N-Alkane Biosignatures in a High Arctic Mars Analogue Gossan Deposit

If past life ever existed on Mars, what are the multiple ways it could have been preserved in the Martian geological record? This crucial question is becoming especially relevant the more we uncover about the planet’s ancient wet history. Different acidic and sulfur rich analog environments have been proposed that are comparable to the alteration environments of iron oxides and sulfate minerals on Mars. However, some authors have hypothesized that these past Martian environments might have been cold and semi-dry, similar to polar regions on Earth. As part of the T-MARS team, we studied reactive gossans in the Canadian High Arctic, on Axel Heiberg Island, as an analog environment to similar deposits on Mars. We hypothesized that n-alkane lipids could potentially be an important form of molecular fossils entombed in varying mineral assemblages of sulphates, iron oxides, and phyllosilicates in Arctic gossans, because of their excellent preservation potential relative to most other forms of organic molecules. To determine the preservation potential of lipids in mineralogically varying acidic sulfur rich gossan deposits, this study extracted and quantified n-alkane biomarkers from three different Arctic gossans with gas chromatography–mass spectrometry (GC-MS). Total organic carbon, pH, and mineralogy were also determined. Organic matter was found to be very low in all samples (<1% wt.%). N-alkane analysis also revealed preserved even-over-odd distribution patterns in short chain n-alkanes, most likely from a microbial source, along with evidence for long-chain n-alkanes with odd-over-even distribution from higher plant sources. The presence of these unique chemical biosignatures in low organic, highly acidic, and sulfur rich Mars analog gossans of varying maturity provides evidence that sulfur deposits linked to paleo hydrothermal systems on Mars can be promising targets for preserved organic biosignatures, specifically lipid n-alkanes. The significant diversity in biosignature patterns across samples of varying mineralogy, pH, and oxidation levels within each gossan suggests that n-alkane preservation varies on a small scale in these environments. These factors alone do not definitively account for the variability of n-alkane concentrations and distributions in this study, and additional investigations of these and other influencing factors are needed to determine which specific targets to choose for biosignature search on Mars in future space missions. This exploratory study provides novel insights into the lipid biosignature content in high Arctic Mars analogue gossan deposits.

Application of whole-cell bioreporters for ecological risk assessment and bioremediation potential evaluation after a benzene exceedance accident in groundwater in Lanzhou, China

Oil spill events challenge human health and ecosystem safety, which are priority concerned issues for sustainable development. There is then an increasing demand of tools for ecological risks assessment at contaminated sites. In this study, we introduced two whole-cell bioreporters, ADPWH_alk and ADPWH_recA, to measure the available n-alkanes and the genotoxicities of total petroleum hydrocarbons in soils and groundwater which were contaminated by the Benzene Exceedance Accident in Lanzhou, China. Comparing to traditional chemical analysis methods, the whole-cell bioreporter method could provide risk assessment on cell level within a shorter time and a less cost, which is economical and environment friendly. The highest contents of available alkanes in soil and groundwater were 18,737 mg/kg and 308.4 mg/L, respectively. In addition, the available n-alkanes significantly (p < 0.01) correlated to chemical analysis of total n-alkanes. The highest genotoxicity level was found in soil and groundwater samples with lower TPHs concentration (4338.0 mg/kg and 1.4 mg/L Mitomycin C equivalent), suggesting the significant impacts of geochemical variables and alkane availability on the ecological risks of petroleum contamination. Combining chemical analysis and whole-cell bioreporter results, bioremediation strategies were suggested for groundwater and soils with higher n-alkane availability and lower ecological risks, whereas chemical oxidation were suggested for other contaminated sites. For the first time, we mapped the distribution of available n-alkanes and petroleum toxicities in a large scale soil-groundwater system using whole-cell bioreporters, showing their huge potential for rapid contaminant detection and fast risk assessment.

Global warming induces the succession of photosynthetic microbial communities in a glacial lake on the Tibetan Plateau

As an important freshwater resource in the Qinghai-Tibet Plateau, glacial lakes are being immensely affected by global warming. Due to the lack of long-term monitoring data, the processes and driving mechanisms of the water ecology of these glacial lakes in a rapidly changing climate are poorly understood. This study, for the first time, reconstructed changes in water temperature and photosynthetic microbial communities over the past 200 years in Lake Basomtso, a glacial lake on the southeastern Tibetan Plateau. Temperatures were reconstructed using a paleotemperature proxy based on branched glycerol dialkyl glycerol tetraethers (brGDGTs), the cell membrane lipids of some bacteria, and photosynthetic microbial communities were determined by high-throughput DNA sequencing. The reconstructed mean annual air temperature (MAAT) at Lake Basomtso varied between 6.9 and 8.3 °C over the past 200 years, with a rapid warming rate of 0.25 °C /10 yrs after 1950s. Carbon isotope of sediment and n-alkane analyses indicate that ≥95% of the organic matter in Lake Basomtso is derived from a mixture of terrestrial C3 plants and endogenous organic matter inputs, and the proportion of endogenous organic matter in the sediments has gradually increased since the 1960s. The sedimentary DNA analyses of the sediment core reveal that Chloracea is the most dominant prokaryotic photosynthetic microbial group (84.5%) over the past 200 years. However, the relative abundance of Cyanobacteria has increased from ≤6.8% before the 1960s to 15.5% nowadays, suggesting that warmer temperatures favor the growth of Cyanobacteria in glacial lakes. Among eukaryotic photosynthetic microorganisms, the Chlorophyceae have been gradually replaced by Dinoflagellata and Diatomacae since the 1980s, although the Chlorophyceae still had the highest average relative abundance overall (30–40%). The Pb isotopic composition, together with the total phosphorous concentration, implies that human activity exerted a minimal impact on Lake Basomtso over the past 200 yrs. However, the synchronous fluctuations of total organic carbon (TOC), total nitrogen (TN), and metal elements in sediments suggest that temperature appears to have a strong influence on nutrient input to Lake Basomtso by controlling glacial erosion. Global warming and the concurrent increase in glacial meltwater are two main factors driving changes in nutrient inputs from terrestrial sources which, in turn, increases the lake productivity, and changes microbial community composition. Our findings demonstrate the sensitive response of glacial lake ecology to global warming. It is necessary to strengthen the monitoring and research of glacial lake ecology on the Tibetan plateau, so as to more scientifically and accurately understand the response process and mechanism of the glacial lake ecosystem under global warming.

EVALUATION OF TOXICITY FOR MYSIDOPSIS JUNIAE SPECIES IN CASES OF OIL SPILL IN THE BRAZILIAN POTIGUAR BASIN

This study aimed to assess the toxicity of the water soluble fraction (WSF) and correlate the effects of three oils and their compositions, through the bioassay with the Mysidopsis juniae, the analysis of n-alkanes and specific gravity. The PET01 and PET02 oils were classified as light oils ºAPI 39.8 and 32.8 and paraphynics with more abundant n-alkane ranges from n-C10 to n-C16, and n-C9 to n-C32. The PET03 oil was classified as heavy (°API 19.7), with more abundant hydrocarbons in the range of n-C29 and n-C33. It was also possible to evaluate the concentrations of metals in petroleum, Ba (3.41-4.45 mg/kg), Fe (5.42-9.01 mg/kg), Zn (0.18-0.53 mg/kg) and Ni (1.77-3.55 mg/kg) were present in the composition and may have harmful effect even at low concentrations. The acute toxicity of WSF from the oils varied by oil type, with LC50 11.4-27.6 of % WSF. Spill simulations indicated that short-term exposures in areas spilled with these oils are less toxic, but with increasing oil and seawater contact time (&gt;1 h), the toxicity and solubility of total BTEX increases. The toxicity of the samples was directly related to the composition of the oils and categorized by the ecotoxicological risk to the environment from PET01 &gt; PET02 &gt; PET03. These results provide reliable data for ecotoxicological risk assessments in cases of oil spillage in marine environments.

Compound-specific carbon isotope evidence that the initial carbon isotope excursion in the end-Triassic strata in northwest Tethys is not the product of CAMP magmatism

Compound-specific carbon isotopic analysis of biomarkers and palynological studies from sections across the end-Triassic mass extinction in the northwestern Tethys provide constraints on the origin of the bulk organic carbon ‘initial’ isotope excursion. This excursion is commonly used to correlate among geographically disparate sections that span this pivotal event associated with the rise of modern fauna. Palynological analysis is used to identify intervals of Liassic-equivalent strata in south Wales and northeast England for isotopic study. Compound-specific carbon isotope analysis of n-alkanes and regular isoprenoids from these intervals reveals a trend different than that displayed by bulk organic matter, suggesting the latter signal is not derived from large perturbations in the global carbon cycle but from localized source changes. The most pronounced changes in compound-specific isotope profiles are negative excursions in pristane and phytane, which occur within the positive phase of the bulk carbon isotope excursion and correspond to shifts in total organic carbon and increased heterotrophy. These findings indicate that a re-evaluation of temporal relationships between positive climate feedbacks and their effects is necessary to more accurately understand the likely global nature of the end-Triassic extinction, and more generally, carbon cycle perturbations associated with other mass extinction events.

Late-season biosynthesis of leaf fatty acids and n-alkanes of a mature beech (Fagus sylvatica) tree traced via 13CO2 pulse-chase labelling and compound-specific isotope analysis.

Leaf cuticular waxes play an important role in reducing evapotranspiration via diffusion. However, the ability of mature trees to regulate the biosynthesis of waxes to changing conditions (e.g., drought, light exposition) remain an open question, especially during the late growing season. This holds also true for one of the most widely distributed trees in Central Europe, the European beech tree (Fagus sylvatica L.). In order to investigate the ongoing formation of wax constituents like alkanes and fatty acids, we conducted a 13CO2 pulse-chase labelling experiment on sun-exposed and shaded branches of a mature beech tree during the late summer 2018. The 13C-label was traced via compound-specific δ13C isotope analysis of n-alkanes and fatty acids to determine the de-novo biosynthesis within these compound classes. We did not observe a significant change in lipid concentrations during the late growing season, but we found higher n-alkane concentrations in sun-exposed compared to shaded leaves in August and September. The n-alkane and fatty acid composition showed ongoing modifications during the late growing season. Together with the uptake and following subsequent decrease of the 13C-label, this suggests ongoing de-novo biosynthesis, especially of fatty acids in European beech leaves. Moreover, there is a high variability in the 13C-label among individual branches and between sun-exposed and shaded leaves. At the same time, sun-exposed leaves invest more of the assimilated C into secondary metabolites such as lipids than shaded leaves. This indicates that the investigated mature beech tree could adjust its lipid production and composition in order to acclimate to changes in microclimates within the tree crown and during the investigated period.

Microscale solid-liquid extraction: A green alternative for determination of n-alkanes in sediments

In this study, a green microscale solid-liquid extraction method using a miniaturized device combined with cleanup via dispersive micro-solid-phase extraction (MSLE-DµSPE) was developed for the determination by gas chromatography-quadrupole mass spectrometry (GC-MS) of n-alkanes in marine sediments. The main factors affecting the performance of this novel method were optimized using multivariate statistical tools. The MSLE-DµSPE method was validated considering the matrix-matched calibration, recovery, detection and quantification limits, ruggedness and accuracy. Under the optimum conditions, the method detection limits for n-alkanes ranged from 0.0051 to 0.0279 mg kg−1, and the quantification limits ranged from 0.0171 to 0.0930 mg kg−1. Correlation coefficients (R2) ≥0.99 were obtained for all compounds within the linear region (0.0025–0.200 mg L−1). The mean recovery for most n-alkanes ranged from 60.6 to 119%, with intraday and interday relative standard deviation (RSD) <20%. Evaluation of the MSLE-DµSPE method using Analytical Eco-Scale, Green Analytical Procedure (GAPI), and Analytical Greenness (AGREE) assessment metrics demonstrated the green potential of the developed method. Finally, the proposed method was successfully applied to marine sediment samples and the n-alkanes from C12 to C40 were detected with total concentrations in the range of 0.98–7.61 mg kg−1. This study represents the first application of a green microscale procedure to the analysis of n-alkanes in marine sediments.

Baseline levels and characterization of hydrocarbons in surface marine sediments along the transportation corridor in Hudson Bay: A multivariate analysis of n-alkanes, PAHs and biomarkers

Hudson Bay is a small arctic inland shelf sea which receives large amounts of freshwater from riverine discharges, with marine flow from the north and the Atlantic. A warming climate has resulted in an expanded open water season which will result in an increase in shipping of fuel oil and petroleum to communities and mines on the western shore, increasing the risk of hydrocarbon releases. To evaluate the status of hydrocarbons, surface sediments were collected at 34 locations in the transportation route and offshore and analysed for several types of hydrocarbons. Total hydrocarbons varied by over 25 times between sites, reaching a maximum of 1116 μg/g OC (organic carbon basis) in Hudson Strait due to low molecular weight n-alkanes from marine primary production. The gross mean for all sites was 344 μg/g OC (GSD = 173–682), roughly equivalent to other remote sites in the Canadian Arctic with no known local hydrocarbon source. n-alkanes accounted for >90 % of residues. Diagnostic ratios (e.g., Carbon Preference Index (CPI), Odd-Even Predominance (OEP)) indicated mixed sources of n-alkanes, likely due to the input from vascular plants and ombrotrophic peat in northern and western watersheds, and primary production within the Bay. The elevated proportion of high molecular weight n-alkanes at deep water sites is consistent with lotic particulate organic matter deposited in the nearshore environment and redeposited offshore. Ʃ36PAHs were a small fraction (1.9 %) of hydrocarbons, with a gross mean of 5.68 μg/g OC (GSD = 3.30–9.79). PCA separated deep water sediments from nearshore and community samples due to 4 alkylated naphthalenes which usually indicate a petrogenic source but probably indicates a natural source due to the lack of other petrogenic markers. Priority PAHs (i.e., Ʃ16PAH) varied from 31.5 % to 56.6 % of the Ʃ36PAH residues. The concentrations of individual PAHs were well below the Interim Sediment Quality Guidelines recommended by the Canadian Council of Ministers of the Environment.

Fast quantitative analysis of n-alkanes, PAHs and alkenones in sediments

The study of different organic biomarker classes is essential to elucidate global Earth dynamics since different biogeochemical processes play a role in regulating environmental and climatic conditions. However, multiproxy analysis generally consists of labor-intensive and time-consuming methodologies, which hamper the study of a large number of samples. Here, we develop and validate a fast analytical method to quantify different classes of organic biomarkers (PAHs, n-alkanes and alkenones) in sediments. This new method sequentally extracts and fractionates the target compounds using a pressure liquid extraction (PLE) system, which allows us to selectively obtain the analytes by reducing the time of analysis, sample handling and solvent usage. We show that our method provides reproducible results and high recoveries (>70%), and can be applied to a wide range of sedimentary environments, such as oceanic basins, continental slope and shelf, and lakes. Moreover, the method provides reproducible estimates of paleoclimatic indices, such as the carbon preference index (CPI), the average chain length (ACL) and the U37K'-derived sea-surface temperature (SST). Therefore, this new method enables fast quantitative multiproxy analysis of oceanic, coastal and lake sediments.

Origin, transport, and retention of fluvial sedimentary organic matter in South Africa's largest freshwater wetland, Mkhuze Wetland System

Abstract. Sedimentary organic matter (OM) analyses along a 130 km long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St Lucia, Africa's most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. Combining bulk OM analyses, such as Rock-Eval®, and source-specific biomarker analyses of plant-wax n-alkanes and their stable carbon (δ13C) and hydrogen (δD) isotopic composition showed that fluvial sedimentary OM originating from inland areas is mainly deposited in the floodplain and swamp area of the wetland system but not in the downstream lake area. A distinctly less degraded OM signature, i.e., a considerably lower degree of transformation of unstable components (higher I index) and lower contribution of refractory and persistent fractions (lower R index) as well as recognizably higher δD values compared to samples from upstream sub-environments, characterizes surface sediments of Lake St Lucia. The offset in δD indicates that the contributing vegetation, although similar to upstream vegetation inputs in terms of photosynthetic pathway (δ13C) and alkane distribution pattern, experienced different hydrological growth conditions. The results suggest that under current conditions hinterland sedimentary OM is deposited throughout the wetland system up to the Mkhuze Swamps, which ultimately captures the transported OM. Consequently, samples from the downstream located Lake St Lucia show locally derived signals instead of integrated signals encompassing the river catchment. This finding raises important constraints for future environmental studies as the assumption of watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore might not hold true in certain settings.

Characterizations and comparison of low sulfur fuel oils compliant with 2020 global sulfur cap regulation for international shipping

The International Marine Organization 2020 Global Sulfur Cap requires ships to burn fuels with <0.50% S and some countries require <0.10% S in certain Sulfur Emission Control Areas but little is known about these new types of fuels. Using both traditional GC–MS and more advanced chromatographic and mass spectrometry techniques, plus stable isotopic, δ13C and δ2H, analyses of pristane, phytane and n-alkanes, the organic components of a suite of three 0.50% S and three 0.10% S compliant fuels were characterized. Two oils were found to be near identical but all of the remaining oils could be forensically distinguished by comparison of their molecular biomarkers and by the profiles of the heterocyclic parent and alkylated homologues. Oils could also be differentiated by their δ13C and δ2H of n-alkanes and isoprenoids. This study provides important forensic data that may prove invaluable in the event of future oil spills.

A Multi-1D modeling and hydrocarbon expulsion assessment of a sulfur-rich Thebes-S organofacies, Abu Rudeis-Sidri oil field, Eastern Egypt

Most organic geochemical analyses focused on the source rock potentiality and ignored the contribution of numerical modeling to assess the generation potential of source rocks. The present study investigated the impact of the geologic evolution on thermal maturity, hydrocarbon generation potential of the Thebes-S organofacies to assess the hydrocarbon expulsion and exploration success of the Abu-Rudeis Sidri oil field. The Lower Eocene Limestone of the Thebes-S organofacies is considered as one of the most significant high sulfur-content source rocks within the central Gulf of Suez. The source rock intervals were depicted in five well locations using Passey et al.,1990 method (ΔlogR) and published geochemical data. Then, the results of one-dimensional petroleum system modeling performed at multiple well locations were evaluated using the petroleum system modeling approach. The gas ratio analysis and star plot of n-alkanes in the ARM-5 well of Thebes-S and the previously studied Duwi-S organofacies were also used for genetical comparisons. The uncertainty analysis of the key input parameters of the Thebes-S organofacies was applied. The results of well-calibrated 1D models show differences in maturation histories of source rocks, and hence, variations in oil generation potential. The Thebes-S organofacies generated hydrocarbon at a lower maturity, in which the oil generation windows were reached at depths from 2025 to 2404 m TVDSS, whereas the gas generation windows were reached at depths from 2652 m to 2775 m TVDSS. Tectonically the central Gulf of Suez is characterized by three major rift phases: the Oligo-Miocene (25-23 Ma), the Lower Miocene (21-19 Ma), and the Mio-Pliocene (6–5.2 Ma). The source rock approached the main oil window in Pliocene (∼4.42–1.95 Ma) and the thermogenic gas window at a later stage between 1.27 and 0.64 Ma. Besides the tectonic evolution, the burial depth was the major factor in the thermal maturation and hydrocarbon generation processes. Modeling results confirm insignificant hydrocarbon expulsion from the Thebes-S organofacies. This can be attributed to the following factors: (1) less overburden (2) low hydrocarbon saturation level; (3) late gas generation. These findings strengthen the idea that the oil was sourced from Duwi-S, rather than Thebes-S organofacies. This could suggest that the oil reservoirs from Thebes’ organofacies in the study area are self-sourced.

Lake eutrophication in northeast China induced by the recession of the East Asian summer monsoon

Lakes are one of the most important freshwater resources on Earth and they provide a wide range of ecosystem services. However, due to rapid economic development and the intensification of human activities, many lakes have become eutrophic, which may threaten their status as water resources. Human activities have played a significant role in lake eutrophication, but whether this role is independent of, or coupled with, natural climate change requires further study. We selected Dali Lake, a large lake affected by human activity within the ancient warfare borders, to clarify the ecological response of a lake to climate change and human activity. We used analyses of sedimentary n-alkanes and AMS 14C dating to reconstruct the paleolimnological evolution of Dali Lake since 15 cal kyr BP, and specifically to assess the timing and causes of eutrophication. The results show that the short-chain n-alkanes (C17–19-alkanes) in Dali Lake are mainly produced by bacteria and algae within the lake, and that the sedimentary absolute abundance of short-chain n-alkanes (A17–19-alkanes) can be used as a proxy for assessing the ecological status of the lake. The ecological status of Dali Lake was the most stable during the early to middle Holocene, when the East Asian summer monsoon was strong, but bacterial and algal outbreaks occurred during three episodes of a weakened summer monsoon—the Older Dryas, Younger Dryas, and the Common Era—when the lake experienced different degrees of eutrophication. During the recession of the East Asian summer monsoon, the weakening of precipitation recharge of the lake led to a reduction in lake area and an increase in nutrient concentrations in the lake water, while aeolian dust input was an additional nutrient source, leading to bacterial and algal outbreaks. During the Common Era, lake eutrophication occurred in the context of both summer monsoon recession and enhanced human activities, but their combined effects did not lead to more intense lake eutrophication than was caused by monsoon recession during the Younger Dryas. We conclude that, although human activities have enhanced the eutrophication of Dali Lake, the reduction in lake size due to monsoon recession and the resulting increase in the salinity and nutrient concentration of the lake water, combined with increased aeolian inputs, were a more important trigger of lake eutrophication.

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.

Semi-closed artificial thermal maturation of type I source rock with the presence of natural formation water: hydrocarbon generation and individual n-alkane stable carbon isotope partitioning pattern

Hydrous pyrolysis is a significant laboratory method to investigate oil generation with the presence of water. Generally, distilled water is utilized in hydrous pyrolysis but not natural formation water, which is ubiquitous in oilfields and has elevated salinities. To understand the influence of formation water on hydrocarbon generation and stable carbon isotopic ratios of expelled oils, two series of semi-closed hydrous pyrolysis experiments were conducted in a wide temperature range (250–550 °C) for the type I Green River Shale source rocks using distilled water and formation water with a salinity of 76 g/L. Oil generation peak occurred at a lower temperature (325 °C) in experiments using formation water compared to experiments using distilled water (375 °C), suggesting that the presence of formation water has facilitated the oil generation. The presence of formation water also promoted the hydrocarbon expulsion at lower temperatures in hydrous pyrolysis (325–350 °C). Stable carbon isotopic fractionation between unpyrolyzed kerogen and n-alkanes (n-C17 to n-C28) generated during the primary stage of oil generation (325–375 °C) is minor (<2.6‰) in the presence of formation water, but smaller relative to those in experiments using distilled water at individual temperature points. Positive correlations between pyrolysis temperatures and δ13C ratios of n-C17, n-C18 and n-C19 during oil generation were observed in the experiments using formation water, which is consistent with the knowledge that 12C–12C bonds are more readily to be cleaved than 13C–12C bonds. Outcomes of this study not only support that stable carbon isotopic analysis of n-alkanes is a powerful tool for fingerprinting oil source in sedimentary basins, but also highlight the significance of using formation water as a medium in hydrous pyrolysis to improve the understanding of chemical/isotopic variations during hydrocarbon generation.

Seasonal variation of leaf wax n-alkane δ2H values: Differences between Quercus aquifolioides (an evergreen tree) and Stipa bungeana (a perennial grass) from the southeastern Tibetan Plateau

To understand the cause of known isotopic differences between grasses and trees, their seasonal variability and environmental drivers, we present a parallel study of a perennial grass, Stipa bungeana, and an evergreen tree, Quercus aquifolioides, in the monsoon-dominated southeastern Tibetan Plateau (TP), by conducting fortnightly paired δ2H analyses of precipitation (δ2Hp), leaf water, and n-alkanes (δ2Hwax) over a period of 2 y. Similar to previous investigations, we show that δ2Hwax in both species are correlated to δ2Hp throughout two growing seasons. However, for the first time, we identified a time lag of 2 months in variation of δ2Hwax relative to δ2Hp in new Q. aquifolioides leaves, whereas nearly no lagged phenomena were observed in S. bungeana and old mature Q. aquifolioides leaves. The δ2Hwax in new Q. aquifolioides leaves correlated significantly with the total amount of n-alkanes. These findings suggest that the turnover rate of n-alkanes in new Q. aquifolioides leaves is possibly changing with their synthesis, and that is a possible mechanism of the observed delayed phenomenon. A paired t-test showed that the difference of δ2Hwax between trees and grasses becomes insignificant after time-lag correction. Moreover, the apparent fractionation in Q. aquifolioides was significantly correlated with weekly or fortnightly precipitation amount, temperature, relative humidity, and wind velocity, while in S. bungeana only weekly or fortnightly precipitation amount. Our high temporal resolution observations hence confirm that climate variables are important drivers of plant δ2Hwax and, give significant implications for δ2Hwax-base paleoclimate reconstructions in global monsoonal regions.