Distribution of n-alkanes in lacustrine sediments from subtropical lake in Brazil

The geochemical composition of lake bed sediments of a tropical reservoir in Brazil have been investigated. The C, N, P composition showed almost no variation between the different sampling points. All samples contained inorganic phosphorus (IP) ranging from 52 to 70%. The Redfield ratios show that the lake is without significant anthropogenic inputs. Most of the organic matter is composed by higher plants decomposition revealed in the total organic carbon (TOC):nitrogen (N) ratio ranging from 15.4 to 57.2. Also, TOC:organic phosphorus (OP) ranged from 265.3 to 933.6, suggesting that most of the organic matter has terrestrial origin from wood plants. The ratios suggest that most organic matter is influenced by the terrestrial characteristics of the watershed. Furthermore, the relative abundance of n-alkane homologues in the sediments was investigated. All samples have been analyzed for the n-alkanes C₈ to C₄₀. The sediments were dominated by n-alkanes C₂₅–C₃₈. It is concluded that n-alkane in sediments mainly come from terrestrial plants, however there is a contribution of submerged aquatic plants, especially in those sites in deeper areas of the lake. On the basis of P_aq index the n-alkanes in sediments comes from terrestrial plants, however there is a contribution from emerged/floating plants.The investigated lake seems to be considered as meso to eutrophic. Because of the relatively high primary productivity in the lakes, there is a substantial organic-matter flux to the sediments, which rapidly becomes anoxic. According to the pristine/phytane ratio the sediments represent an anoxic environment with values found between 0.38 and 1.72.     

Monocyclic alkanes in Ordovician organic matter

The major compounds in the C₁₅₊ branched/cyclic alkane fractions of two Ordovician oils (Pine Unit and Midland Farms oils) and an immature Ordovician rock (Guttenberg Oil Rock) are monocyclics. One series of these compounds was identified as n-alkylcyclohexanes and another tentatively identified as methyl-n-alkylcyclohexanes. The carbon number distribution of these compounds resembled those of the n-alkanes found in these samples with an odd carbon number predominance. It is suggested that the monocyclic alkanes, in these Ordovician samples, may be principally derived from the cyclisation of straight-chain algal fatty acids, by mechanisms that involve decarboxylation. However, there is evidence from the m/z 97 fragmentograms of these samples, to indicate that methyl-n-alkylcyclohexanes can also be derived from fatty acids by a less preferred mechanism that does not involve decarboxylation or from other precursors.In addition, a sample of kerogen from the Guttenberg Oil Rock was hydrously pyrolysed. The saturated hydrocarbon products of this experiment showed very similar distributions (including monocyclics) to those observed in the Ordovician oils. This suggests that although the oils and the Guttenberg Oil Rock come from widely differing geographical locations, their precursor (algal derived) organic matter was very similar.     

Origin of organic sulphur compounds and sulphur-containing high molecular weight substances in sediments and immature crude oils

The distribution patterns of Organic Sulphur Compounds (OSC), occurring in certain sediments and immature crude oils, were compared with those of the corresponding hydrocarbons. Because of the complexity of the OSC mixtures, they were desulphurized to hydrocarbons (n-alkanes, isoprenoid alkanes, steranes, triterpanes and branched alkanes). The hydrocarbons produced by desulphurization of the OSC exhibited distribution patterns different from those of the hydrocarbons originally present. Therefore reaction of elemental sulphur with these hydrocarbons at elevated temperatures must be considered as an unlikely origin for these OSC. Sulphur incorporation reactions on an intramolecular basis with suitable functionalized precursors at the early stages of diagenesis are probably the major origin for these OSc. Desulphurization of high molecular weight fractions also produced hydrocarbons, dominated by n-alkanes up to C₄₀. Therefore it is assumed that these substances contain n-alkanes, 2,5-dialkyl-thiophenes and -thiolanes linked to each other by sulphur briddges. These findings stronly suggest that sulphur-containing high molecular weight substances are formed by the same sulphur incorporation reactions as OSC, but in an intermolecular fashion.     

Carbon isotopic composition of individual n-alkanes in asphaltene pyrolysates of biodegraded crude oils from the Liaohe Basin, China

Biodegraded oils are widely distributed in the Liaohe basin, China. In order to develop effective oil-source correlation tools specifically for the biodegraded oils, carbon isotopic compositions of individual n-alkanes from crude oils and their asphaltene pyrolysates have been determined using the gas chromatography–isotope ratio mass spectrometry technique. No significant fractionation in the stable carbon isotopic ratios of n-alkanes in the pyrolysates of oil asphaltenes was found for anhydrous pyrolysis carried out at temperatures below 340°C. This suggests that the stable carbon isotopic distribution of n-alkanes (particularly in the C₁₆–C₂₉ range) in the asphaltene pyrolysates can be used as a correlation tool for severely biodegraded oils from the Liaohe Basin. Comparison of the n-alkane isotopic compositions of the oils with those of asphaltene pyrolysates shows that this is a viable method for the differentiation of organic facies variation and post-generation alterations.     

Detection of petroleum contamination in river sediments from Quebec City region using GC–IRMS

Isotopic analysis by compound specific gas chromatography–isotope ratio mass spectrometry (GC–IRMS) is used to detect and characterize petroleum pollution in surficial sediments along the St Lawrence River, near Quebec City. Unusually mature n-alkane distributions have been found in some recent intertidal sediments in the region. GC–IRMS results suggest that the n-alkanes are not derived from indigenous organic sources because they carry δ¹³C values between −30.0 and −27.0‰, as well as very small isotopic differences between odd and even numbered n-alkanes, which are both typically associated with petroleum products. Comparison of these sediments with bunker fuel, an oil used in the shipping industry, has shown a close isotopic correlation in some sites, which is further supported by biomarkers. Overall, the contamination has been dispersed along the river but is generally localized around the industrial region where hydrocarbon transfer from shore storage to ships takes place. This study illustrates how GC–IRMS can be used effectively in the detection and characterization of petroleum pollutants in sediments.     

Experimental investigation of the compositional variation of acyclic paraffins during expulsion from source rocks

Anhydrous non-isothermal heating experiments were conducted under controlled compressive stress on cylindrical plugs of six oil shales from Permian through Eocene age. The objective of this study was to compare the distribution of acyclic paraffins in initial, residual and expelled organic matter and to highlight causes of compositional differences resulting from expulsion. Pristane generation from kerogen is highest in the Eocene Messel shale and affects the pristane / phytane (pr / ph) ratio commonly used as a redox proxy. The isoprenoid to n-alkane ratios (pr / n-C₁₇, ph / n-C₁₈) decrease during generation and are lowest in the residual bitumen due to preferential generation and retention of n-alkanes. The n-alkane distribution shows that only lacustrine shales produce high wax oils. Evaporative fractionation leads to loss of n-alkanes up to n-C₂₀ with boiling points below 350 °C. This demonstrates that lacustrine and marine shales may lead to accumulation of low wax oils due to evaporative fractionation after expulsion.     

Latitudinal and downcore (0–750 ka) changes in n-alkane chain lengths in the eastern equatorial Pacific

The n-alkane C₃₁/(C₂₉ + C₃₁) ratios from surface sediments in the eastern equatorial Pacific (EEP) exhibit higher values to the north and lower values to the south across the southern edge (2–4°N) of the Intertropical Convergence Zone (ITCZ). Since plants tend to synthesize longer chain length n-alkanes in response to elevated temperature and/or aridity, the higher C₃₁/(C₂₉ + C₃₁) ratios at northern sites suggest a higher contribution of vegetation under hot and/or dry conditions. This is consistent with the observation that northern sites receive higher levels of plant waxes transported by northeasterly trade winds from northern South America, where hot and dry conditions prevail. Furthermore, from a sediment core covering the past 750 ka (core HY04; 4°N, 95°W) we found that C₃₁/(C₂₉ + C₃₁) ratios exhibit a long-term decrease from MIS (marine oxygen isotope stage) 17 to 13. During this period, the zonal SST (sea-surface temperature) gradient in the equatorial Pacific increased, suggesting an increase in Walker circulation. Such intensified Walker circulation may have enhanced moisture advection from the equatorial Atlantic warm pool to the adjacent northern South America, causing arid regions in northern South America to contract, which may explain long-term decrease in n-alkane chain lengths.     

CPI values of terrestrial higher plant-derived long-chain n-alkanes: a potential paleoclimatic proxy

Carbon Preference Index (CPI values) of higher plant-derived long-chain n-alkanes extracted from 62 surface soil samples in eastern China exhibited a specific pattern of variations, namely gradual increase with the increasing latitudes. Such regular variations existed in both forest soil and grassland soil. Our data implied that CPI values of higher plant-derived long-chain n-alkanes had a certain connection with climatic conditions, and such a connection was not influenced by vegetation types. Together with previous data from marine sediments, loess/paleosol sequences, tertiary red clay and modern plants, our observation made us conclude that CPI values of higher plant-derived long-chain n-alkanes may be used as an excellent proxy for paleoclimatic studies.     

n-alkane distribution coupled with organic carbon isotope composition in the shell bar section, Qarhan paleolake, Qaidam basin, NE Tibetan Plateau

Lipids extracted from lacustrine deposits in the paleolake Qarhan of the Qaidam basin in the northeastern Tibetan Plateau were determined by conventional gas chromatography-mass spectrometry. Several series of biomarkers were identified, mainly including n-alkanes, n-alkan-2-ones, n-alkanoic acids, branched alkanes, triterpenoids and steroids, indicative of various biogenic contributions. On the basis of cluster analysis, the n-C₁₅, n-C₁₇, n-C₁₉ alkanes were proposed to be derived from algae and/or photosynthetic bacteria, the n-C₂₁, n-C₂₃, n-C₂₅ homologues from aquatic plants, and the n-C₂₉, n-C₃₁ homologues from vascular plants. In contrast, the n-C₂₇ alkane is not categorized in the n-C₂₉ and n-C₃₁ group of alkanes, probably due to more complex origins including both aquatic and vascular plants, and/or differential biodegradation. Stratigraphically, layers-2, 4 and 5 were found to show a close relationship in n-alkane distribution, associated with a positive shift in carbon isotope composition of bulk organic matter (δ¹³C_org), inferring a cold/dry period. Layers-1 and 6 were clustered together in association with a negative δ¹³C_org, excursion, probably indicating a relatively warm/humid climate. The potential coupling between the n-alkane distributions and δ¹³C_org, suggests a consequence of vegetation change in response to climate change, with the late MIS3 being shown to be unstable, thought to be the climatic optimum in the Tibetan Plateau. Our results suggest that the cluster analysis used in this study probably provides an effective and authentic method to investigate the n-alkane distribution in paleolake sediments.     

The variations of stable carbon isotope ratio of land plant-derived n-alkanes in deep-sea sediments from the Bering Sea and the North Pacific Ocean during the last 250,000 years

Two piston cores, one located far from the continents (The North Pacific Ocean: ES core), and another located comparatively closer to the continents (The Bering Sea: BOW-8a core) were investigated to reconstruct environmental changes on source land areas. The results show significant contribution of terrestrial organic matter to sediments in both cores. The δ¹³C values of n-C₂₇, n-C₂₉, and n-C₃₁ alkanes in sediments from the North Pacific ES core show significant glacial to interglacial variation whereas those from the Bering Sea core do not. Variations of δ¹³C values of land plant n-alkanes are related to the environmental or vegetational changes in the source land areas. Environmental changes, especially, aridity, rainfall, and pCO₂ during glacial/interglacial transitional periods can affect vegetation, and therefore C₃ / C₄ plant ratios, resulting in δ¹³C changes in the preserved land plant biomarkers. Maximum values of δ¹³C as well as maximum average chain length values of long chain n-alkanes in the ES core occur mostly at the interglacial to glacial transition zones reflecting a time lag related to incorporation of living organic matter into soil and transportation into ocean basins via wind and/or ability of C₄ plants to adapt for a longer period before being replaced by C₃ plants when subjected to gradual climatic changes. Irregular variations with no clear glacial to interglacial trends in the BOW-8a core may result from complex mixture of aerosols from westerly winds and riverine organic matter from the Bering Sea catchments. In addition, terrestrial organic matter entering the Bering Sea could originate from multiple pathways including eolian, riverine, and ice rafted debris, and possibly be disturbed by turbidity and other local currents which can induce re-suspension and re-sedimentation causing an obliterated time relation in the Bering Sea biomarker records.     

Hydrocarbon biomarkers from Ordovician sediments and the fossil alga Gloeocapsomorpha prisca Zalessky 1917

Numerous Ordovician oils worldwide are known to show unusual and distinctive distributions of hydrocarbons which, it has been suggested, are derived from a “unique benthonic mat-forming non-photosynthetic prokaryotic organism”, Gloeocapsomorpha prisca Zalessky 1917, which is the major contributor of organic matter. Organic matter-rich sediments from the Canning and Amadeus Basins of Australia, known to contain G. prisca fossils and to have the characteristic predominance of odd carbon number (C₁₃–C₁₉) n-alkanes, were investigated to determine other hydrocarbon distributions. Sediments from both basins contained abundant n-alkylcyclohexanes with odd carbon number predominance (C₁₃–C₁₉) and methyl-n-alkylcyclohexanes (C₁₄–C₂₀) which, in immature sediments, showed an even carbon number predominance. The isomer distribution of these latter compounds was determined by direct comparison with synthetic standards. The sediments from both basins also contained very similar distributions of steranes and pentacyclic triterpanes and the derived kerogens had a characteristically light carbon isotope signature.Pyrolysis of a G. prisca-rich kerogen yielded a hydrocarbon mixture with a similar composition to the sediment extract, except that there was a marked increase in the relative abundance of pristane, phytane, alkylcyclohexanes, steranes and hopanes relative to n-alkanes. We argue on the basis of the geological, geochemical and palynological data that G. prisca was probably planktonic, photosynthetic and very possibly eukaryotic and that the striking character of Ordovician oils and sediments derive from bacterial and other diagenetic imprints superimposed on the primary signature of this organism.     

柴达木盆地昆特依地区晚更新世正构烷烃分布特征及其环境变迁

为探讨柴达木盆地晚更新世的环境变迁,同时为青藏高原隆升对气候变化的影响提供基础资料,对柴达木盆地昆特依地区ZK1404钻孔沉积物中的正构烷烃分布特征以及ASM~(14)C测年数据进行了分析。结果显示,昆特依地区晚更新世沉积物的正构烷烃总体上呈现以高碳数为主的分布特征,多数样品以nC_(27)、nC_(29)为主峰,部分以nC_(31)为主峰,呈现出显著的奇碳优势。少数样品中低于nC_(21)的短链正构烷烃相对丰度较高,主要以nC_(17)、nC_(20)为主峰碳数,无明显的奇偶优势。根据正构烷烃分布特征、AMS~(14)C测年数据、区域地质调查资料及沉积环境判断,柴达木盆地昆特依地区晚更新世(33600～26370a B.P.)气候环境以温凉干旱与温凉干旱偏湿交替出现为特征,与现代大陆性干旱荒漠气候截然不同。     

A compound-specific n-alkane δC and δD approach for assessing source and delivery processes of terrestrial organic matter within a forested watershed in northern Japan

We measured molecular distributions and compound-specific hydrogen (δD) and stable carbon isotopic ratios (δ¹³C) of mid- and long-chain n-alkanes in forest soils, wetland peats and lake sediments within the Dorokawa watershed, Hokkaido, Japan, to better understand sources and processes associate with delivery of terrestrial organic matter into the lake sediments. δ¹³C values of odd carbon numbered C₂₃-C₃₃n-alkanes ranged from −37.2‰ to −31.5‰, while δD values of these alkanes showed a large degree of variability that ranged from −244‰ to −180‰. Molecular distributions in combination with stable carbon isotopic compositions indicate a large contribution of C3 trees as the main source of n-alkanes in forested soils whereas n-alkanes in wetland soil are exclusively derived from marsh grass and/or moss. We found that the n-alkane δD values are much higher in forest soils than wetland peat. The higher δD values in forest samples could be explained by the enrichment of deuterium in leaf and soil waters due to increased evapotranspiration in the forest or differences in physiology of source plants between wetland and forest. A δ¹³C vs. δD diagram of n-alkanes among forest, wetland and lake samples showed that C₂₅-C₃₁n-alkanes deposited in lake sediments are mainly derived from tree leaves due to the preferential transport of the forest soil organic matter over the wetland or an increased contribution of atmospheric input of tree leaf wax in the offshore sites. This study demonstrates that compound-specific δD analysis provides a useful approach for better understanding source and transport of terrestrial biomarkers in a C3 plant-dominated catchment.     

Simulation experiments on the variation of leaf n-alkanes in aquatic environments

The leaves of six plant species and the corresponding leaf residues collected in water from the two-year simulation experiments were analyzed in nalkane distributions by gas chromatography (GC) and gas chromatography-mas spectrometry (GC/MS). The leaf nalkanes keep unchanged in the dominant homologues when soaked in tap water for two years. The most significant change was observed in carbon preference index (CPI), with enhanced values being found in leaf residues collected from water. This is contradictory with the previous reports showing the lower CPI values during sinking and burial processes in natural aquatic environments. The elevated CPI values from leaf residues might be related to the low amount of microorganisms in the water used in the simulation experiment, and the enhanced solubility of even-carbon-numbered n-alkanes via van der Waals attraction. In contrast with herbaceous plants, the woody plants appear to show relatively great variations in both the CPI and the average chain length (ACL) values of n-alkanes after submerged in water for two years. Our data clearly show the differentiated decomposition between woody and herbaceous leaves, with the woody leaves suffered from much stronger decomposition. This observation suggests that in comparison with the grassland, the forest vegetation might result in relatively low authentic signals to be preserved in the n-alkane distributions in aquatic sediments.     

Pyrolysis-GC characterization of whole rock and kerogen-concentrate samples of immature Jurassic source rocks from NW-Germany

Nine rock samples from three Jurassic stratigraphic units of a shallow core from NW Germany were analyzed by pyrolysis-gas chromatography. The units contain a mixed Type-II/III kerogen (Dogger-α), a hydrogen-rich Type-II kerogen (Lias-), and a hydrogen-poor Type-III kerogen (Lias-δ). All of the kerogen was immature (R_o = 0.5%). Two sets of kerogen concentrates (“AD”: HCl/HF followed by a density separation, and “A”: only acid treatment) prepared from the rock samples were also analyzed to make a detailed comparison of the pyrolysates of rock and corresponding kerogen-concentrates.Hydrogen-index (HI) values of the kerogen concentrates prepared from organic-carbon poor rock were nearly 200% higher than HI values of the rock samples. Changes in HI were minimal for the samples containing Type-II kerogen. The A and AD samples from the C_org-poor rock yielded pyrolysates with n-alkane series of very different molecular lengths. Pyrograms of the rock samples had n-alkane series extending to n-C₁₄; the chromatograms of the A samples reached the n-C₁₄-nC₂₀ range. The AD samples from C_org-poor rock and all three sample types from the C_org-rich rock had n-alkane series up to n-C₂₉. The benzene/hexane and toluene/heptane ratios for the C_org-poor rock and A samples were far higher than for the AD samples, which had ratios similar to those of all three sample types from the C_org-rich rocks. These results indicate that choice of kerogen preparation method is critical when C_org-poor samples are analyzed.     

Biological marker compounds as indicators of the depositions! history of the Maoming oil shale

The Eocene Maoming oil shale from Guangdong Province occurs as a laterally uniform stratigraphic section, typically 20–25 m thick, from which the aliphatic hydrocarbon constituents of six representative samples were investigated using GC and C-GC-MS. The sediments evaluated included the basal lignite, a vitrinite lens from the overlying claystone, and four intervals from the massive oil shale bed. As expected, the lignite and vitrinite differ markedly from the oil shales. The lignite is dominated by bacterial hopanoids and components of higher plant origin, including C₂₉ steroids and triterpenoids such as oleanenes. Visually, the oil shale samples show corroded and degraded phytoclasts, spores, wispy particles of fluorescent organic material attributable to dinoflagellates and, especially in the uppermost sample, colonial algal bodies. The distributions of biological markers in the oil shales show many features in common, notably a dominance of dinoflagellate-derived 4-methylsteroids, and a significant proportion of higher-plant derived n-alkanes with marked odd-over-even carbon number predominance. Overall, they exhibit several features that resemble characteristics of the Messel shale. The hydrocarbons of the lowest shale horizon suggest that there may have been a gradual transition between deposition of the original peat and the subsequent oil shales. The aliphatic hydrocarbons of the uppermost shale are dominated by a number of C₃₁ and C₃₃ botryococcane homologues and other unusual branched alkanes possibly derived from green algae. All of the samples are immature. Overall, molecular and microscopic examination of the stratigraphic succession of the Maoming oil shale suggests a shallow, lacustrine environment within which peats were deposited. This lake subsequently deepened to support abundant algal populations, especially dinoflagellates, culminating in a dominance of botryococcoid algae.     

地质样品正构烷烃组分分离纯化的部分问题探究

用实验室自配的标准正构烷烃样品分别经硅胶或氧化铝柱层析后发现,等量溶剂洗脱的情况下硅胶对长链正构烷烃无吸附而氧化铝具有一定吸附性。进一步通过用植物样品测试4种层析柱填充方法,发现不论是只用氧化铝填充还是上部硅胶下部氧化铝和上部氧化铝下部硅胶,都会对长链正构烷烃产生一定的吸附,且这种吸附效果随着碳链的增长而增强。在实验条件下,当碳链加长到C36时,用硅胶加氧化铝填充层析柱的吸附量已达到20%左右,而只用氧化铝填充层析柱的吸附量则高达50%。故建议对研究高碳数正构烷烃的地质样品组分提取时用单一的硅胶柱层析方法。同时,实验显示对于一些杂质多的正构烷烃样品经过尿素络合后比络合前"干净"得多。18个黄土-古土壤和植物样品平均回收率为50%左右,经过尿素络合后的样品正构烷烃各组分相对含量基本不会发生改变,也不会产生明显的同位素分馏效应。因此在进行非正构组分干扰较大的正构烷烃各组分相对含量或同位素分析时,可以选择尿素络合的方法来将其纯化。     

Organic geochemistry of Recent equatorial deltaic sediments

The organic matter of recent deltaic sediments cored in the Mahakam delta, East Kalimantan, has been studied before and after physical fractionation into sands >50 μm, silts 5–50 μm and clays <5 μm. Both the lipid and non-lipid components have been investigated.Weight, carbon and nitrogen fractionation budgets were used to define three types of samples, depending on coarse particle contributions to the total amount of organic matter: a = a first type with more than 50% of the O.M. in the coarse particles, high C/N ratios and O.M. content, b = an intermediary type with medium C/N ratios and O.M. content, each fraction having quite the same O.M. content, c = a third type with less than 5% of the whole O.M. in the sands and the lowest C/N ratios and O.M. content.Concerning the global organic characteristics of the fractions, a systematic increase of C/N ratios occurs when going from clays to sands; the finer the fraction is, the more nitrogenous the compounds are. This enrichment in nitrogen is related to a persistent high rate of hydrolysable material either for argillaceous organic matter-poor sediments or for the clay fractions of all types of samples. Conversely, the type (a) coarse sediments, in particular the sandy components were resistant to acid hydrolysis with burial.Concerning the geochemical markers signatures of granulometric fractions, the distribution patterns of n-alkanes and n-fatty acids are characterized by the predominance of high molecular weight compounds >C₂₂. Carbon preference index (CPI) values are higher in the sands and silts, reflecting their enrichment in continentally-derived vegetation debris. For type (c), the fractionation revealed markers of microbial activity within the clay fractions. For all types of samples, we observed an increase with burial of the n-alkane and n-fatty acid concentrations, particularly in the clay fractions, suggesting possibly a better preservation and/or affinity of lipids with the finest fractions.     

The use of high temperature gas chromatography to study the biodegradation of high molecular weight hydrocarbons

A consequence of the biodegradation of petroleum is that lower molecular weight compounds are removed preferentially to higher molecular weight (HMW) compounds greater than triacontane (n-C₃₀). The extent to which the latter compounds are biodegraded has rarely been studied. Reasons for this include the technical difficulties associated with carrying out biodegradability tests with solid, water-insoluble substances and the limits of the analytical techniques, such as gas chromatography (GC).A quantitative high temperature GC (HTGC) method was developed to monitor the biodegradation of the aliphatic fraction of a waxy Indonesian oil by Pseudomonas fluorescens. Recoveries of over 90% were obtained for n-alkanes up to hexacontane (C₆₀) using liquid-liquid continuous extraction. After only 14 days, 80% of the aliphatic hydrocarbons had been degraded. At the end of the 136-day study, 14% of the original fraction remained. This comprised mainly C₄₀₊ compounds. No decrease in the concentrations of compounds above C₄₅ was observed. However, the use of a rapid screening biodegradation method provided tentative proof that Pseudomonas fluorescens was capable of utilising n-alkanes up to C₆₀ once the bacteria had acclimated to HMW alkanes.     

Organic geochemistry across the Permian–Triassic transition at the Idrijca Valley,Western Slovenia

Bulk and molecular stable C isotopic compositions and biomarker distributions provide evidence for a diverse community of algal and bacterial organisms in the sedimentary organic matter of a carbonate section throughout the Permian–Triassic (P/Tr) transition at the Idrijca Valley, Western Slovenia. The input of algae and bacteria in all the Upper Permian and Lower Scythian samples is represented by the predominance of C₁₅–C₂₂ n-alkanes, odd C-number alkylcyclohexanes, C₂₇ steranes and substantial contents of C₂₁–C₃₀ acyclic isoprenoids. The occurrence of odd long-chain n-alkanes (C₂₂–C₃₀) and C₂₉ steranes in all the samples indicate a contribution of continental material. The decrease of C_org and C_carb contents, increase of Rock-Eval oxygen indices, and ¹³C-enrichment of the kerogen suggest a decrease in anoxia of the uppermost Permian bottom water. The predominance of odd C-number alkylcycloalkanes, C₂₇ steranes, and C₁₇ n-alkanes with δ¹³C values ∼−30‰, and ¹³C-enrichment of the kerogens in the lowermost Scythian samples are evidence of greater algal productivity. This increased productivity was probably sustained by a high nutrient availability and changes of dissolved CO₂ speciation associated to the earliest Triassic transgression. A decrease of C_org content in the uppermost Scythian samples, associated to a ¹³C-depletetion in the carbonates (up to 4‰) and individual n-alkanes (up to 3.4‰) compared to the Upper Permian samples, indicate lowering of the primary productivity (algae, cyanobacteria) and/or higher degradation of the organic matter.