Evidence for gammacerane as an indicator of water column stratification

Accumulation and Quantitative Assessment of Terrigenous Organic Matter in Upper Oligocene to Lower Miocene Marine Source Rocks of the Qiongdongnan Basin

The intense collision between marine and terrestrial agents results in the dual-source (marine and terrigenous) characteristics of marine source rocks. Our research quantitatively assessed terrestrial organic matter and revealed the crucial role of terrestrial organic materials in the organic matter enrichment of lower Miocene to upper Oligocene marine source rocks in the Qiongdognnan Basin. The quantitative assessment was achieved using partial least-squares analysis with eight biomarker parameters associated with n-alkanes, isoprenoids, bicadinanes, taraxerane, tricyclic terpanes, and gammacerane. Differential unloading of terrestrial organic materials based on sedimentary facies of the delta-marginal sea system were observed through oleanane and bicadinane contents. It should be noted that the diagnostic ratio of oleanane was excluded from the quantitative analysis due to the dual influence from differential unloading and contact with seawater of the terrestrial organic materials. Calculation results show that the terrestrial organic matter was highest in the delta front at 70%, followed by prodelta at 59% and inner shallow marine at 57%. From the late Oligocene to the early Miocene, the proportion of terrestrial organic matter in marine source rocks continuously increased, with the highest average value observed in the second member of the Sanya Formation at 69% and the lowest occurring in the third member of the Lingshui Formation at 46%. Increasing terrestrial organic material inputs and preservation driven by the East Asian summer monsoon provided first-order control of the accumulation of organic carbon in the Qiongdongnan Basin during late Oligocene to early Miocene, rather than the bioproductivity of marine algae. The redox conditions of the water column determine the enrichment extent of organic matter.

Multi-proxy record of the Austrian Upper Triassic Polzberg Konservat-Lagerstätte in light of the Carnian Pluvial Episode

We present a multi-proxy investigation of a lower Carnian basinal succession from Polzberg in the Northern Calcareous Alps (Lower Austria). A section comprising a unique Konservat-Lagerstätte was studied based on bio- and chemostratigraphy along with geophysical methods, yielding a detailed and robust stratigraphic calibration of the Polzberg succession. The Polzberg section revealed the paleoceanographic history and helped to identify a global climatic reversal, the Carnian Pluvial Episode. The age of the Upper Triassic Reingraben formation in the Northern Calcareous Alps is refined as the Austrotrachyceras austriacum Zone within the lower Carnian (Julian 2). Ammonoids and conodonts provide a detailed biostratigraphic subdivision that serves as a basis for analyses of the faunal distribution and the paleoenvironmental evolution of the Upper Triassic Reifling Basin. The succession includes lithological and facies changes similar to those of coeval units in the Tethys. The Carnian was characterized by a weak (~ 1‰) positive δ13C trend, punctuated by a negative shift during the lower Carnian corresponding to the initiation of the Carnian Pluvial Episode, a period representing the onset of early/late Carnian transitional global greenhouse conditions. Organic maturity parameters and the conodont alteration index (CAI) show that the thermal overprint of the Polzberg section is low. Biomarker proxies suggest that the organic matter of the uppermost Göstling formation is a mixture of marine and terrestrial material deposited in a dysoxic environment. Within the overlaying Reingraben formation, the amount of marine biomass decreased gradually upwards. Oxygen-depleted conditions, probably due to water-column stratification, continued during deposition of the Reingraben formation. Bacterial sulfate reduction played a major role in organic matter degradation.

Geochemical Evaluation of Paleocene Source Rocks in the Kohat Sub-Basin, Pakistan

The Kohat sub-basin is one of the main hydrocarbon-producing sedimentary basins located in the northwest extension of the Indus Basin in Pakistan. It contains numerous proven and potential petroleum from the Cambrian to the Miocene. Conventional petroleum resources have been depleting rapidly over the last couple of years. Therefore, unconventional resources should be explored using a variety of geochemical and geophysical techniques to address the energy demands. Geochemical techniques, including total organic carbon (TOC) assessment, Rock-Eval pyrolysis, organic petrography, and biomarker studies, are essential for evaluating the potential of shale gas reservoirs to delineate future prospects in a basin. The source rock potential of the Paleocene rocks, including the Patala, Lockhart, and Hangu formations of the sub-basin, is evaluated using geochemical analyses on well cuttings from the Tolanj-01 well. The analyses include estimation of total organic carbon (TOC), Rock-Eval pyrolysis, and organic petrography (vitrinite reflectance) to evaluate the organic richness, thermal maturity, kerogen type, hydrocarbon type, and environment of deposition. Other techniques for extractable organic matter (EOM) include solid-liquid chromatographic separation of fractions, gas chromatography (GC-FID)/whole oil chromatography, and gas chromatography-mass spectrometry (GC-MS). The organic matter (TOC, wt %) analysis reveals that 18 (18) samples of the Hangu formation (0.08-1.8 wt %) show poor values, 12 (12) samples of the Lockhart formation (0.05-0.5 wt %) have poor to fair content, and 26 (26) samples of the Patala formation have poor to fair (0.08-0.19 wt %) TOC content. Rock-Eval pyrolysis studies including hydrogen index, oxygen index, Tmax, quantities of free hydrocarbons (S1, mg/g), and hydrocarbons produced from pyrolysis (S2, mg/g) are determined for the well-cut samples (56) of the Paleocene rocks. The hydrogen index values for the Hangu formation are lower than 200, and those for the Lockhart and Patala formations range between 100 and 250. A maceral analysis is also conducted on these samples, which reveal that the majority of the samples of the Paleocene units present in the basin belong to kerogen types II/III. The thermal maturity of the Hangu and Lockhart formations falls in the late-stage oil window, while that of the Patala formation falls in the peak to late oil window. The genetic potential (GP) for these rock units is also determined based on S1 and S2 values, which reveals that it is generally poor except for a few samples of the Hangu and Lockhart formations, which show fair GP values. For the organic petrography (vitrinite reflectance, R0), one sample from each unit is selected, which shows that the Hangu, Lockhart, and Patala formations fall in the category of the mature oil window with their R0 (%) values being 0.95, 0.89, and 0.82, respectively. The extracts (EOM) from these rock units are also analyzed to assess the depositional settings, biological source input, biodegradation, thermal maturity, etc. The greater values of pristine to phytane (Pr/Ph > 1) ratios for Hangu (1.33), Lockhart (1.23), and Patala (1.8) indicate an intermediate condition (suboxic), while a cross-plot of Pr/n-C17-Ph/n-C18 shows that the organic matter is deposited in a transitional setting. The ratios between C19TCT/C19 TCT + C23 TCT and C24 TeCT/C24TeCT + C23TCT biological source inputs are mainly of marine origin. Similarly, the ternary diagram of regular steranes (C27-C28-C29) shows a greater marine input. Lower values of the carbon preference index (CPI1) for Hangu (0.95), Lockhart (0.91), and Patala (1.04) indicate higher thermal maturity of the Paleocene rocks. Similarly, the methylphenanthrene index (MPI-1) values, Moretane index, and Pr/n-C17 vs Ph/n-C18 plots also show higher thermal maturity for these rock extracts.

Applying biomarkers as paleoenvironmental indicators to reveal the organic matter enrichment of shale during deep energy exploration: a review

Analysis of biomarkers in geological materials such as shales is very crucial because they can provide useful information on the depositional conditions and environments, organic matter input, thermal maturity as well as the geological age of shales in some cases. The paleoenvironment, and its impact on organic matter enrichment of the shales, plays a vital role in the exploration and development of the resource. Paleoenvironmental reconstruction can be conducted using elemental, isotopic, maceral, and biomarker proxies. However, the literature on the biomarkers for paleoenvironment reconstruction to reveal the organic matter enrichment of shales in many petroleum systems throughout the world is still insufficient. Hence, this paper seeks to critically review the application of biomarkers during paleoenvironmental reconstruction in shales. The uses of biomarkers as indicators of modern and ancient marine and brackish/saline lacustrine depositional environments are considered. This review shows that biomarkers could be used to establish the sedimentary depositional environments, redox conditions, and organic matter enrichments of shales that are critical to deep energy exploitation. Nevertheless, despite the fact that biomarkers are significant indicators of depositional conditions, secondary processes such as source facies, thermal maturity, migration, and reservoir alteration can greatly influence their uses as paleoenvironmental condition indicators in source rocks and oils. Hence, for a reliable paleoenvironmental evaluation, there is a need to combine isotopic, elemental and maceral proxies with biomarkers.

Organic Geochemical Characteristics and Organic Matter Enrichment of the Upper Permian Longtan Formation Black Shale in Southern Anhui Province, South China

Although previous studies have yielded valuable insights into shale gas reservoirs, a comprehensive understanding of the organic geochemical characteristics and organic matter enrichment of marine-continental transitional shale has yet to be achieved. The Longtan Formation transitional shales were extensively deposited in Southern Anhui Province, South China, during the Late Permian. Our analysis of twenty-two rock samples from one core (Gangdi-1 well) and two outcrops (Daoshanchong outcrop and Changqiao outcrop) revealed that the Longtan Formation shale extracts exhibit a wide range of C₁₁-C₃₅n-alkanes and acyclic isoprenoids, with unimodal, bimodal, and multimodal distributions. The carbon peak ranges from nC₁₅ to nC₂₄, with high quantities of medium-chain n-alkanes (nC₂₂-nC₂₅), indicating that the organic matter in Longtan Formation shale originates from a mixed source of higher plant debris and lower aquatic organisms. Our conclusion is supported by the ternary diagram of C₂₇-C₂₈-C₂₉ regular steranes and the variations of the δ¹³C values of C₁₅-C₃₂n-alkanes, which is higher than the corresponding value (<1.6‰) of n-alkanes from a single source. Furthermore, thermal maturity proxies based on organic petrography (R_o and T_max) and biomarkers, such as the ratios of C₃₁ 22S/(22S + 22R), C₂₉ 20S/(20S + 20R), and C₂₉ ββ/(αα + ββ), suggest that organic matter is in a mature stage of hydrocarbon generation. By analyzing the Pr/Ph ratio and pyrite morphology combined with a plot of total organic carbon (TOC) versus total sulfur (TS) and the Pr/nC₁₇-Ph/nC₁₈ diagram, we speculate that the Longtan Formation shales were chiefly developed in a dysoxic-to-oxic water environment. Finally, we establish depositional models of organic matter enrichment in deltaic and tidal flat-lagoon environments, emphasizing that the abundant mixed-sourced organic matter can significantly enhance primary productivity, and a higher sedimentation rate can distinctly shorten organic matter exposure time in the oxidized water environment, thereby promoting organic matter accumulation in such a setting.

An Overview of Lipid Biomarkers in Terrestrial Extreme Environments with Relevance for Mars Exploration

Lipid molecules are organic compounds, insoluble in water, and based on carbon-carbon chains that form an integral part of biological cell membranes. As such, lipids are ubiquitous in life on Earth, which is why they are considered useful biomarkers for life detection in terrestrial environments. These molecules display effective membrane-forming properties even under geochemically hostile conditions that challenge most of microbial life, which grants lipids a universal biomarker character suitable for life detection beyond Earth, where a putative biological membrane would also be required. What discriminates lipids from nucleic acids or proteins is their capacity to retain diagnostic information about their biological source in their recalcitrant hydrocarbon skeletons for thousands of millions of years, which is indispensable in the field of astrobiology given the time span that the geological ages of planetary bodies encompass. This work gathers studies that have employed lipid biomarker approaches for paleoenvironmental surveys and life detection purposes in terrestrial environments with extreme conditions: hydrothermal, hyperarid, hypersaline, and highly acidic, among others; all of which are analogous to current or past conditions on Mars. Although some of the compounds discussed in this review may be abiotically synthesized, we focus on those with a biological origin, namely lipid biomarkers. Therefore, along with appropriate complementary techniques such as bulk and compound-specific stable carbon isotope analysis, this work recapitulates and reevaluates the potential of lipid biomarkers as an additional, powerful tool to interrogate whether there is life on Mars, or if there ever was.

Geochemical characteristics and implications of hydrocarbon in source rocks of Chagan Sag, Yin'e basin of Inner Mongolia

Chagan Sag is one of the most significant tectonic unit in the Yin'e Basin. The special component of the organic macerals and biomarkers in the Chagan sag suggests great difference of its hydrocarbon generation process. In this paper, forty samples of source rocks are subjected to carry out on the geochemical characteristics by methods of rock-eval analysis, organic petrology and gas chromatography mass spectrometry (GC-MS) to reveal the origin, depositional environment and maturity of organic matter in Chagan Sag, Yin'e Basin of Inner Mongolia. The total organic matter of the analyzed samples ranges from 0.4 wt%∼3.89 wt% with an average of 1.12 wt%, indicating fair to excellent hydrocarbon generation potential. The rock-eval results show that the S₁+S₂ and hydrocarbon index range from 0.03 mg/g∼16.34 mg/g (avg.3.6 mg/g) and 6.24-521.32 mg/g (avg. 199.63 mg/g), suggesting most of the kerogen types are type II and III, with small amount of type I. The Tmax ranges from 428 to 496 °C, suggesting low mature to mature stage. The macerals component of morphological macerals presents with certain amount of vitrinite, liptinite and some inertinite. However, the amorphous component takes the dominant part of the macerals with account of 50%-80%. The amorphous components are dominated by sapropelite in the source rock, indicating bacteriolytic amorphous promote the organic generation process. Hopanes and sterane are widely distributed in the source rocks. The biomarker results suggest a mix of planktonic-bacterial and higher plant origins, with wide range of thermal maturity and relatively reducing depositional environment. Abnormal high content of hopanes were observed in the biomarkers, and some special biomarkers such as monomethylalkanes, long-chain-alkyl naphthalenes, aromatized de A-triterpenes, 8,14-seco-triterpenes, and A, B-cyclostane are detected in Chagan Sag. The presence of these compounds suggests that bacterial and microorganisms are of great significance to the generation of hydrocarbon in the source rock in Chagan Sag.

Paleoenvironment and Organic Characterization of the Lower Cretaceous Lacustrine Source Rocks in the Erlian Basin: The Influence of Hydrothermal and Volcanic Activity on the Source Rock Quality

Lower Cretaceous lacustrine source rocks in the Erlian Basin are highly heterogeneous. It is important to assess and explain these heterogeneities for the reconstruction of paleoenvironments and the prediction of high-quality source rock distributions. In this study, well-logging, organic, and elemental geochemical data were comprehensively analyzed for the source rocks of Member 4 of the Aershan Formation (Fm) and Member 1 of the Tengger Fm in the southern Bayindulan (BNAN), southern Wulanhua (WLHs), Anan, Aer, and southern Wuliyasitai sags of the Erlian Basin. The variability in sedimentary environments, sources of organic matter of the source rocks in different sags, and the influence of hydrothermal and volcanic activity on the source rock quality in the Erlian Basin were assessed. The results reveal that the source rocks can be divided into four types of organic facies (A, B, BC, and C). Organic facies A-B present hydrogen indices (HIs) higher than 400 mg/g and are mainly composed of mudstone and thick (average thickness >50 m) dolomitic mudstone, with biomarkers characterized by a Pr/Ph ratio lower than 1.0, a gammacerane/C₃₀ hopane (Gam/C₃₀H) ratio higher than 0.2, and a C₁₉ tricyclic terpane/C₂₃ tricyclic terpane (C₁₉/C₂₃TT) ratio lower than 0.6. Organic facies BC-C are composed of mudstone with an HI < 400 mg/g, with biomarkers characterized by a Pr/Ph ratio higher than 0.8, a Gam/C₃₀H ratio lower than 0.2, a C₁₉/C₂₃TT ratio higher than 0.6, and a sterane/hopane ratio lower than 0.4. Dolomitic mudstone belonging to organic facies A-B is mainly developed in the BNAN, WLHs, and Anan sag and is characterized by a fault-controlled distribution in the sag, a right-declined rare earth element pattern, and an enrichment in the elements of Ba, Cu, Zn, Fe, and Ni. The genesis of high HI dolomitic mudstone is associated with hydrothermal and volcanic activity because the hydrothermal fluid or hydrolysis of volcanic ash result in increasing input of reducing gas and soluble nutrient ions, thus promoting the formation of anoxic and saline Cretaceous lakes with high primary productivity.

Effects of Magmatic-Hydrothermal Activities on Characteristic of Source Rocks from Beipiao Formation in the Jinyang Basin, NE China

The Jinyang Basin is a typical volcanic-sedimentary basin, located in the southern peripheral area of the Songliao Basin. Hydrothermal activity is often closely related to the intrusion or eruption of magma. It was recently suggested that dolomite in the Jurassic Beipiao Formation was formed under the influence of magmatic-hydrothermal activity and magmatic-hydrothermal activity might have impacts on the organic matter of the source rocks. No investigation has been aimed at the effect of magmatic-hydrothermal activities on the accumulation of organic matter in the Beipiao Formation source rocks and a comprehensive study is urgent, which would be indicative in unravelling the accumulation mechanism of organic matter and useful in further petroleum exploration. To provide important insights into these issues, we carried out a detailed investigation of geological and geochemical analysis for Wolong (WL) and Dongkuntouyingzi (DK) outcrop shales from the Lower Jurassic Beipiao Formation in the Jinyang Basin. The hydrothermal indicator discrimination diagram (Zn-Ni-Co triangular plot) and rare earth element anomalies (δEu and δCe) indicate that the formation of WL samples is associated with hydrothermal activity, but DK is not. The TOC values suggest that most of the WL and DK samples are good to very good and fair to good source rocks, respectively. The Ro values suggest that both WL (Ro = 1.17%) and DK (Ro = 1.01%) samples have entered the oil-generating stage, and WL samples were influenced by the magmatic activity with higher maturity. The biomarkers such as high steranes/hopanes, high 4-methyl steranes/C29 steranes, low Pr/Ph values and high gammacerane index suggest that WL samples were deposited in an anoxic-prone saline environment with significant contributions of algal sources. Contrarily, the DK samples were deposited in oxic-prone and freshwater paleolake with significant contributions of terrigenous organic matter. The magmatic-hydrothermal activities in the Wolong area brought numerous nutrients to the lake basin, which may facilitated the reproduction of aquatic organisms. At the same time, the magmatic-hydrothermal activities increased the salinity of water and promoted the formation of a water reducing environment, which provided an excellent environment for the preservation and enrichment of organic matter. Therefore, the magmatic-hydrothermal activities in the Wolong area promoted the formation of organic-rich source rocks and the hydrocarbon generation process.

Geochemical Characteristics of Three Oil Families and Their Possible Source Rocks in the Sub-Sag A of Weixinan Depression, Beibuwan Basin, Offshore South China Sea

Three oil families from the sub-sag A of the Weixinan Depression are identified by integrated analysis of physical properties, stable carbon isotopes, and gas chromatography-mass spectrometry (GC-MS). Their similarities and differences in relative thermal maturities, depositional environments, and biological sources of organic matter (OM) are investigated. A possible oil-source correlation of this area is established. Group A1 oils, defined as low-maturity oils, are characterized by high density and high viscosity. They contain more terrigenous OM deposited in a freshwater environment with unstratified water columns reflected by a relatively high terrestrial/aquatic ratio and Pr/Ph values, low abundance of C₃₀ 4-methlysteranes, and low δ¹³C values. They are derived from the upper hydrocarbon supply combination. Group A2 oils are characterized by moderate density and viscosity and medium stable carbon isotope values. This group of oils has lower terrestrial/aquatic ratios and Pr/Ph values and a medium concentration of C₃₀ 4-methlysteranes and δ¹³C values, suggesting that the oils are derived from the shales which have more contribution from the algal input and are formed in a weakly oxidizing environment. They are a mixture generated from the source rocks in the middle and lower hydrocarbon supply combination. Groups A3 oils, defined as light oils, have low density and viscosity. The geochemical data of the A3 oils, including a less-negative stable carbon isotope, high abundance of C₃₀ 4-methylsteranes, low Pr/Ph values, and highest Ts/(Ts + Tm) ratios (Ts represents C₂₇ 18α(H)-22,29,30-trisnorneohopane and Tm represents C₂₇ 17α(H)-22,29,30-trisnorhopane), suggest that they are generated from the source rocks deposited in a subanoxic environment with the large input of dinoflagellates. The A3 oils are generated from the shales from the lower hydrocarbon supply combination. The oil-source correlation results can be further supported by the distribution of faults and structural ridge as the migration channel of petroleum developed around the sub-sag A.

Investigation on Oil Physical States of Hybrid Shale Oil System: A Case Study on Cretaceous Second White Speckled Shale Formation from Highwood River Outcrop, Southern Alberta

Nine samples collected from the Upper Cretaceous Second White Speckled Shale Formation at the Highwood River outcrop in southern Alberta were geochemically characterized for their oil contents, physical states, and chemical compositions. Cold extraction was performed on 8–10 mm and 2–5 mm chips sequentially to obtain the first and second extractable organic matter (EOM-1 and EOM-2), while Soxhlet extraction was performed on powder from previously extracted chips to obtain the third extract (EOM-3). EOM-1 can be roughly regarded as free oil and EOM-2 is weakly adsorbed on mineral surfaces, while EOM-3 may represent the oil strongly adsorbed on kerogen. While both extraction yields and Rock-Eval pyrolysates differed from their original values due to the evaporative loss during outcropping, there was a generally positive correlation between the total EOM and total oil derived from Rock-Eval pyrolysis. EOM-1 was linearly correlated with Rock-Eval S1, while the extractable S2 content was well correlated with the loss of TOC, suggesting that TOC content was the main constraint for adsorbed oils. A bulk composition analysis illustrated that EOM-1 contained more saturated hydrocarbons, while EOM-3 was enriched in resins and asphaltenes. More detailed fractionation between the free and adsorbed oils was demonstrated by molecular compositions of each extract using quantitative GC-MS analysis. Lower-molecular-weight n-alkanes and smaller-ring-number aromatic compounds were preferentially concentrated in EOM-1 as compared to their higher-molecular or greater-ring-number counterparts and vice versa for EOM-3. Fractionation between isoprenoids and adjacent eluted n-alkanes, isomers of steranes, hopanes, alkylnaphthalenes, alkylphenanthrenes and alkyldibenzothiophenes was insignificant, suggesting no allogenic charge from deep strata. Strong chemical fractionation between saturated and aromatic hydrocarbon fractions was observed with EOM-1 apparently enriched in n-alkanes, while EOM-3 retained more aromatic hydrocarbons. However, the difference between free and adsorbed state oils was less dramatic than the variation from shales and siltstones. Lithological heterogeneities controlled both the amount and composition of retained fluids. Oil that resided in shales (source rock) behaved more similar to the EOM-3, with diffusive expulsion leading to the release of discrete molecules from a more adsorbed or occluded phase to a more free phase in siltstones with more connected pores and/or fractures (reservoir). Under current technical conditions, only the free oil can flow and will be the recoverable resource. Therefore, the highest potential can be expected from intervals adjacent to organic-rich beds. The compositional variations due to expulsion and primary migration from source rocks to reservoirs illustrated in the present study will contribute to a better understanding of the distribution of hydrocarbons generated and stored within the shale plays.

Biomarker and compound-specific isotope records across the Toarcian CIE at the Dormettingen section in SW Germany

The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE; ~ 183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock-Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n-alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ¹³C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the "Unterer Stein" marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n-C₂₇ (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n-alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ¹³C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).

Influence of aphotic haloclines and euxinia on organic biomarkers and microbial communities in a thalassohaline and alkaline volcanic crater lake

Studies on microbial communities, and their associated organic biomarkers, that are found thriving in the aphotic euxinic waters in modern stratified ecosystems are scarce compared to those undertaken in euxinic photic zones. The Dziani Dzaha (Mayotte, Indian Ocean) is a tropical, saline, alkaline crater lake that has recently been presented as a modern analog of Proterozoic Oceans due to its thalassohaline classification (having water of marine origin) and specific biogeochemical characteristics. Continuous intense photosynthetic production and microbial mineralization keep most of the water column permanently aphotic and anoxic preventing the development of a euxinic (sulfidic and anoxic) photic zone despite a high sulfide/sulfate ratio and the presence of permanent or seasonal haloclines. In this study, the molecular composition of the organic matter in Lake Dziani Dzaha was investigated and compared to the microbial diversity evaluated through 16S rRNA gene amplicon sequencing, over two contrasting seasons (rainy vs. dry) that influence water column stratification. Depth profiles of organic biomarker concentrations (chlorophyll-a and lipid biomarkers) and bacterial and archaeal OTU abundances appeared to be strongly dependent on the presence of aphotic haloclines and euxinia. OTU abundances revealed the importance of specific haloalkaliphilic bacterial and archaeal assemblages in phytoplanktonic biomass recycling and the biogeochemical functioning of the lake, suggesting new haloalkaline non-phototrophic anaerobic microbial precursors for some of the lipid biomarkers. Uncultured Firmicutes from the family Syntrophomonadaceae (Clostridiales), and Bacteroidetes from the ML635J-40 aquatic group, emerged as abundant chemotrophic bacterial members in the anoxic or euxinic waters and were probably responsible for the production of short-chain n-alkenes, wax esters, diplopterol, and tetrahymanol. Halocline-dependent euxinia also had a strong impact on the archaeal community which was dominated by Woesearchaeota in the sulfide-free waters. In the euxinic waters, methanogenic Euryarchaeota from the Methanomicrobia, Thermoplasmata, and WSA2 classes dominated and were likely at the origin of common hydrocarbon biomarkers of methanogens (phytane, pentamethyl-eicosenes, and partially hydrogenated squalene).

Molecular Indicators of Sources and Biodegradation of Organic Matter in Sediments of Fluid Discharge Zones of Lake Baikal

This paper performs a detailed study of a wide set of organic-geochemical proxies in 15 sediment cores collected from the main basins of Lake Baikal (the northern, the central and the southern) where processes of focused fluid discharge were detected. A variety of studied zones includes sites with gas and hydrothermal seepage, mud volcanoes with or without gas-oil fluid discharge, gas hydrates and authigenic carbonates. The composition of the dispersed organic matter and individual hydrocarbon molecular markers (n-alkanes, dimethyl alkanes, isoprenoids, steranes, terpanes and polycyclic aromatic hydrocarbons) testify to the input from predominantly allochthonous terrestrial and autochthonous microbial and algal sources. The studied sources, maturity and biodegradation parameters of organic matter vary significantly for areas with different fluid discharge. The composition of specific biomarkers including isoprenoids and immature hopanoids reflects the lateral and vertical changes of microbial activity in sediments associated with various environmental conditions. The identified types of terpanes distribution (mature, mixed and immature) correlate well with types of fluid discharge and attest to the development of various methanogenic and methanotrophic microbial communities in sediments. Moreover, the revealed specificity of microbial molecular markers distribution allowed us to suggest the fluid discharge processes in zones where they were not previously detected.

Biomarkers, stable carbon isotope, and trace element distribution of source rocks in the Orange Basin, South Africa: implications for paleoenvironmental reconstruction, provenance, and tectonic setting

Aptian to Campanian sediments from the Western offshore to Central Orange Basin were studied by integrating molecular geochemistry, inorganic and isotopic studies to recognize their geochemical characteristics via the reconstruction of the Orange basin’s paleoweathering, paleosalinity, paleovegetation, paleoclimate, and tectonic records. Molecular analyses of both aliphatic and aromatic compounds reveal an input dominantly from a marine source. The source rocks accumulated in a reduced, anoxic, saline water column. Based on various biomarker proxies and vitrinite reflectance data, some samples are thermally mature to produce petroleum, while others are not. According to the V/Ni ratio, samples from the Orange Basin in South Africa are mainly anoxic, with only a few samples ranging from suboxic to anoxic. This is congruent with biomarker and isotope analyses that further indicate the presence of marine-derived source rocks with some terrestrial remains generating hydrocarbons. The investigated sediments are made up of intermediate igneous rocks that have undergone moderate chemical weathering. Geochemical figures on tectonic setting discriminant function diagrams revealed a continental rift of passive margin settings. As a result, the extrapolated crustal processes are directly analogous to the genesis and evolution of the Orange Basin, demonstrating Gondwana’s breaking up and the opening of the Atlantic Ocean Margin.

Messinian bottom-grown selenitic gypsum: An archive of microbial life

Primary gypsum deposits, which accumulated in the Mediterranean Basin during the so-called Messinian salinity crisis (5.97-5.33 Ma), represent an excellent archive of microbial life. We investigated the molecular fossil inventory and the corresponding compound-specific δ¹³ C values of bottom-grown gypsum formed during the first stage of the crisis in four marginal basins across the Mediterranean (Nijar, Spain; Vena del Gesso, Italy; Heraklion, Crete; and Psematismenos, Cyprus). All studied gypsum samples contain intricate networks of filamentous microfossils, whose phylogenetic affiliation has been debated for a long time. Petrographic analysis, molecular fossil inventories (hydrocarbons, alcohols, and carboxylic acids), and carbon stable isotope patterns suggest that the mazes of filamentous fossils represent benthic microbial assemblages dominated by chemotrophic sulfide-oxidizing bacteria; in some of the samples, the body fossils are accompanied by lipids produced by sulfate-reducing bacteria. Abundant isoprenoid alcohols including diphytanyl glycerol diethers (DGDs) and glycerol dibiphytanyl glycerol tetraethers (GDGTs), typified by highly variable carbon stable isotope composition with δ¹³ C values spanning from -40 to -14‰, reveal the presence of planktic and benthic archaeal communities dwelling in Messinian paleoenvironments. The compound inventory of archaeal lipids indicates the existence of a stratified water column, with a normal marine to diluted upper water column and more saline deeper waters. This study documents the lipid biomarker inventory of microbial life preserved in ancient gypsum deposits, helping to reconstruct the widely debated conditions under which Messinian gypsum formed.

Geochemistry and Organic Petrology of Middle Permian Source Rocks in Taibei Sag, Turpan-Hami Basin, China: Implication for Organic Matter Enrichment

The Taodonggou group of Middle Permian is an important source rock in Taibei sag of Turpan-Hami basin. Due to its deep burial, drilling has only been revealed in recent years. Based on organic petrology and organic geochemistry experiments, this paper studies the organic petrology, organic geochemistry, sedimentary environment, and hydrocarbon generation potential of source rocks in Taibei sag, Turpan-Hami basin, and reveals the influence of the sedimentary environment on the organic matter abundance of source rocks. The results are as follows: (1) The organic matter of the Middle Permian source rocks in Taibei sag of Turpan-Hami basin is mainly sapropelite and exinite. The vitrinite is mainly vitrodetrinite, and the exinite is mainly lamalginiite. (2) The total organic carbon content value is 0.55-6.08 wt %, and the average value is 2.58 wt %. The PG value ranges from 0.78 mg HC/g to 30.86 mg HC/g, and the average value is 4.88 mg HC/g. Chloroform asphalt "A" is 0.046-0.8767 wt %, and the average value is 0.285 wt %. The types of organic matter are mainly III and II-III, and the R _o value is 0.628-1.49 wt % (average = 0.988 wt %). The T _max distribution is 329-465 °C. The average temperature is 434.7 °C, which is in the mature stage (oil window stage). The Middle Permian source rocks are mainly very good to excellent source rocks with a good hydrocarbon generation potential. (3) The source rocks are deposited in a semihumid and semiarid climate. Organic matter is input as a mixed source. The early and late stages is dominated by terrestrial higher plants. The middle stage is dominated by lower aquatic organisms, and the sedimentary environment consists of weak reduction and weak oxidation environments. (4) In the study area, the abundance of organic matter has a weak negative correlation with CPI and a positive correlation with Pr/Ph and ∑C₂₁-/∑C₂₂+. Under the coaction of paleoclimate, organic matter input, and redox environment, the enrichment model of organic matter with high productivity and weak oxidation environment characteristics can also form excellent source rocks. This study is of great significance and provides theoretical guidance for the exploration of deep oil and gas resources.

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.

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Geochemical analyses of biomarker traces, elemental analysis, and pyrolysis provide a continuous profile of source rock.

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The OM source input, preservation conditions, and thermal maturation level were provided based on biomarker analysis.

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The ​results ​of ​the ​elemental ​analysis ​show ​that ​the ​Montney ​formation ​source ​rock ​was deposited ​in ​a ​marine ​environment.

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Biomarkers are considered an effective method for detailed and insightful information that helps evaluate any source rock.

Chemometric Classification of Oil Families in the Laizhouwan Depression, Bohai Bay Basin, Eastern China

A suite of oil-sand bitumen and crude oil samples, collected from the Laizhouwan depression, Bohai Bay basin, were geochemically investigated for molecular compositions. Three oil families (A, B, and C) were classified by hierarchical cluster analysis (HCA) and principal component analysis on nine typical biomarker ratios. Typically, family A oils are characterized by relatively low values of gammacerane/C₃₁ homohopane 22R (G/H31, 0.13-0.76) and C₃₅/C₃₄ homohopane (H35/H34, 0.39-0.78), suggesting a source from freshwater depositional environments. In contrast, family C oils show a relatively high G/H31 (2.49-5.41) and H35/H34 (1.43-2.45) ratios, indicating a source from hypersaline water depositional environments. Family B oils display ratios of G/H31 and H35/H34 in-between the range of families A and C, suggesting mixed origin. In addition, family A oils can be further classified into four subfamilies (A1, A2, A3, and A4) and family B oils into two subfamilies (B1 and B2) by HCA. The A1 oils characterized by a high C₂₄ tetracyclic terpane/C₂₆ tricyclic terpane (TeT24/TT26) ratio (1.02-1.39) are mainly distributed in the northeast, B1 oils characterized by relatively low TeT24/TT26 ratio are in the west, and A2, A3, and A4 oils with an intermediate TeT24/TT26 ratio are in the center of the depression. Oils in well L16-1-2 in the southern depression, however, show vertical variations with family C oils in the deeper reservoirs, subfamily B2 oils in the shallower reservoirs, and subfamily A4 oils in the middle-depth reservoirs. Based on the biomarker compositions, at least three oil charges were indicated: family C oils are likely sourced from the Es4 rock in the southern sag, B2 oils may be a mixture of family C with family A oils, and A4 oils without biodegradation influence may be the latest charge derived from the Es3 source rock in the northern sag. The oil families and/or subfamilies with typical genetic affinities, as well as the regular occurrence in different blocks, may indicate two major petroleum systems or multiple subsidiary oil systems existing in the Laizhouwan depression.

Carotenoid biomarkers in Namibian shelf sediments: Anoxygenic photosynthesis during sulfide eruptions in the Benguela Upwelling System

Aromatic carotenoid-derived hydrocarbon biomarkers are ubiquitous in ancient sediments and oils and are typically attributed to anoxygenic phototrophic green sulfur bacteria (GSB) and purple sulfur bacteria (PSB). These biomarkers serve as proxies for the environmental growth requirements of PSB and GSB, namely euxinic waters extending into the photic zone. Until now, prevailing models for environments supporting anoxygenic phototrophs include microbial mats, restricted basins and fjords with deep chemoclines, and meromictic lakes with shallow chemoclines. However, carotenoids have been reported in ancient open marine settings for which there currently are no known modern analogs that host GSB and PSB. The Benguela Upwelling System offshore Namibia, known for exceptionally high primary productivity, is prone to recurrent toxic gas eruptions whereupon hydrogen sulfide emanates from sediments into the overlying water column. These events, visible in satellite imagery as water masses clouded with elemental sulfur, suggest that the Benguela Upwelling System may be capable of supporting GSB and PSB. Here, we compare distributions of biomarkers in the free and sulfur-bound organic matter of Namibian shelf sediments. Numerous compounds-including acyclic isoprenoids, steranes, triterpanes, and carotenoids-were released from the polar lipid fractions upon Raney nickel desulfurization. The prevalence of isorenieratane and β-isorenieratane in sampling stations along the shelf verified anoxygenic photosynthesis by low-light-adapted, brown-colored GSB in this open marine setting. Renierapurpurane was also present in the sulfur-bound carotenoids and was typically accompanied by lower abundances of renieratane and β-renierapurpurane, thereby identifying cyanobacteria as an additional aromatic carotenoid source.

Anatomy of an extinction revealed by molecular fossils spanning OAE2

The Cenomanian–Turonian mass extinction (Oceanic Anoxic Event 2-OAE2) was a period of profound ecological change that is recorded in the sedimentary record in many locations around the globe. In this study, we provide a new and detailed account of repetitive changes in water column ecology by analyzing the organic geochemical record preserved within the OAE2 section of the Greenhorn Formation, Western Interior Seaway (WIS) of North America. Results from this study provide evidence that OAE2 in the WIS was the result of the cumulative effect of reoccurring environmental stresses rather than a single massive event. During OAE2, extreme variations in biotic composition occurred erratically over periods of several thousands of years as revealed by molecular fossil (biomarker) abundances and distributions calibrated to sedimentation rates. These cycles of marine productivity decline almost certainly had follow-on effects through the ecosystem and likely contributed to the Cenomanian–Turonian mass extinction. While the causes behind organic productivity cycling are yet unproven, we postulate that they may have been linked to repeated episodes of volcanic activity. Catastrophic volcanism and related CO₂ outgassing have been interpreted as main drivers for OAE2, though this study provides new evidence that repetitive, punctuated environmental stresses were also important episodes within the anatomy of OAE2. Following OAE2, these cycles of productivity decline disappeared, and the WIS returned to conditions comparable to pre-OAE2 levels.

Distribution and Geochemical Significance of Rearranged Hopanes in Jurassic Source Rocks and Related Oils in the Center of the Sichuan Basin, China

Gas chromatography-mass spectrometry (GC-MS) analysis has revealed extremely high abundances of rearranged hopanes in Jurassic source rocks and related crude oils in the center of the Sichuan Basin. The detected rearranged hopanes include 17α(H)-diahopanes (C₂₇D and C_29-35D), early-eluting rearranged hopanes (C₂₇E and C_29-33E), and 18α(H)-neohopanes (C₂₉Ts and Ts). Both the 17α(H)-diahopanes and the early-eluting rearranged hopanes exhibit a distribution pattern similar to that of the 17α(H)-hopane series, with a predominance of the C₃₀ member and the presence of 22S and 22R epimers of hopanes in the extended series (>C₃₀). The results of this study show that the relatively high abundance of rearranged hopanes in Jurassic source rocks in the study area is associated with their depositional environments and with clay-mediated acidic catalysis rather than, as was previously thought, thermal maturity. Shallow lacustrine facies with brackish water and a suboxic to weak reducing sedimentary environment have contributed to the enrichment of rearranged hopanes, and clay-mediated acidic catalysis may also have had a positive influence on their abundance. The distribution patterns of the diahopane series indicate that the oils from Jurassic reservoirs in the Gongshanmiao Oilfield are sourced from Jurassic source rocks. Rearranged hopanes are therefore considered to be effective biomarkers for oil-source correlation in the center of the Sichuan Basin.

Variation of salinity and nitrogen concentration affects the pentacyclic triterpenoid inventory of the haloalkaliphilic aerobic methanotrophic bacterium Methylotuvimicrobium alcaliphilum

The occurrence and activity of aerobic methanotrophs are influenced by environmental conditions, including pH, temperature, salinity, methane and oxygen concentrations, and nutrient availability. Aerobic methanotrophs synthesize a variety of lipids important for cell functions. However, culture-based experiments studying the influence of environmental parameters on lipid production by aerobic methanotrophs are scarce. Such information is crucial to interpret lipid patterns of methanotrophic bacteria in the environment. In this study, the alkaliphilic strain Methylotuvimicrobium alcaliphilum was cultivated under different salinities and different nitrate concentrations to assess the effect of changing conditions on the inventory of pentacyclic triterpenoids. The results indicate that hopanoid abundance is enhanced at lower salinity and higher nitrate concentration. The production of most pentacyclic triterpenoids was favored at low salinity, especially for aminotriol. Interestingly, 3-methyl-aminotetrol and tetrahymanol were favored at higher salinity. Bacteriohopanepolyols (BHPs), particularly aminotriol and 3-methyl-aminotriol, increased considerably at higher nitrate concentrations. Four novel N-containing BHPs—aminodiol, 3-methyl-aminodiol, and isomers of aminotriol and 3-methyl-aminotriol—were identified. This study highlights the significance of environmental factors for bacterial lipid production and documents the need for cultivation studies under variable conditions to utilize the full potential of the biomarker concept.

Response of a continental fault basin to the global OAE1a during the Aptian: Hongmiaozi Basin, Northeast China

This paper presents new research on a lacustrine anoxic event (LAE). These data include stable carbon isotope (δ¹³C_org), pyrite sulfur isotope (δ³⁴S_py), trace element and biomarker ratios from the Hongmiaozi Basin (North China) and unravel the response of continental lakes under the influence of early Aptian extreme climate conditions. According to the stratigraphic chronology (122–118 Ma) and carbon isotope correlations, terrestrial sediment was influenced by the early Aptian Oceanic Anoxic Event (OAE1a). The results show that the Xiahuapidianzi Group experienced a significant warming process under negative excursions in carbon isotopes due to the influence of increased carbon dioxide partial pressure (pCO₂). The climate varied from warm and humid to hot and arid (high Sr/Cu, low Rb/Sr, calcareous mudstone), the evaporation and salinity increased (high Sr/Ba and B/Ga), and land input sources decreased (low Zr, Ti and Th). Moreover, high total organic carbon (TOC) content was source from bacteria, algae (n-alkanes), and euxinic depositional environments (Pr/Ph, Cu/Zn and U V Mo). In the stage of continuous carbon isotopes positive excursion, organic matter accumulated rapidly. A paleolake environment model has provided a better understanding of current global climate issues under global warming caused by increased carbon dioxide concentrations.

Geochemical Characteristics of the Lower Cretaceous HengTongshan Formation in the Tonghua Basin, Northeast China: Implications for Depositional Environment and Shale Oil Potential Evaluation

The Tonghua Basin in Northeast China potentially contains shale oil and gas resources, but the exploration and development of these resources has been limited. The Sankeyushu depression represents the sedimentary center of the Tonghua Basin, and a large thickness of shale, the Hengtongshan Formation, was deposited in this depression. Exploratory engineering discoveries in recent years have confirmed that the Hengtongshan Formation has the potential to produce oil and gas. A series of methods, including inorganic and organic geochemistry and organic petrology, have been used to study the source material, organic matter maturity, depositional environment and oil-generating potential of the Hengtongshan Formation. Investigation of drill core samples has revealed that the Hengtongshan Formation in the Sankeyushu depression is mainly composed of black shale, with a small amount of plant fossils and thin volcanic rocks, and the content of brittle minerals (quartz + carbonate minerals) is high. The provenance of organic matter in the source rocks in the Hengtongshan Formation is a mixture of aquatic organisms (algae and bacteria) and higher plants, and there may be some marine organic components present in some strata.The organic matter was deposited and preserved in a saline reducing environment. Volcanism may have promoted the formation of a reducing environment by stratification of the lake bottom water, and the lake may have experienced a short-term marine ingression with the increase in the salinity. The maturity of the organic matter in all the source rocks in the Hengtongshan Formation is relatively high, and hydrocarbons have been generated. Some source rocks may have been affected by volcanism, and the organic matter in these rocks is overmature. In terms of the shale oil resource potential, the second member of the Hengtongshan Formation is obviously superior to the other members, with an average total organic carbon (TOC) of 1.37% and an average hydrogen index (HI) of 560.93 mg HC/g TOC. Most of the samples can be classified as good to very good source rocks with good resource potential. The second member can be regarded as a potential production stratum. According to the results of geochemical analysis and observations of shale oil and natural gas during drilling, it is predicted that the shale oil is present in the form of a self-sourced reservoir, but the migration range of natural gas is likely relatively large.

Molecular and isotopic evidence reveals the end-Triassic carbon isotope excursion is not from massive exogenous light carbon

The negative organic carbon isotope excursion (CIE) associated with the end-Triassic mass extinction (ETE) is conventionally interpreted as the result of a massive flux of isotopically light carbon from exogenous sources into the atmosphere (e.g., thermogenic methane and/or methane clathrate dissociation linked to the Central Atlantic Magmatic Province [CAMP]). Instead, we demonstrate that at its type locality in the Bristol Channel Basin (UK), the CIE was caused by a marine to nonmarine transition resulting from an abrupt relative sea level drop. Our biomarker and compound-specific carbon isotopic data show that the emergence of microbial mats, influenced by an influx of fresh to brackish water, provided isotopically light carbon to both organic and inorganic carbon pools in centimeter-scale water depths, leading to the negative CIE. Thus, the iconic CIE and the disappearance of marine biota at the type locality are the result of local environmental change and do not mark either the global extinction event or input of exogenous light carbon into the atmosphere. Instead, the main extinction phase occurs slightly later in marine strata, where it is coeval with terrestrial extinctions and ocean acidification driven by CAMP-induced increases in Pco₂; these effects should not be conflated with the CIE. An abrupt sea-level fall observed in the Central European basins reflects the tectonic consequences of the initial CAMP emplacement, with broad implications for all extinction events related to large igneous provinces.

Biomarker stratigraphy in the Athel Trough of the South Oman Salt Basin at the Ediacaran-Cambrian Boundary

The South Oman Salt Basin (SOSB) has been studied extensively for knowledge concerning the habitat of the enigmatic Ediacaran-Cambrian oils that are produced from that region. Geological, geochemical, geophysical, and geochronological investigations have all contributed to improved understanding of the range of late Neoproterozoic depositional environments recorded there. Of particular interest has been the deep Athel depocenter within the SOSB that features a silica-rich interval known as the Al Shomou Member or Athel Silicilyte and the co-eval A4 carbonate-evaporite sequence that straddles the Ediacaran-Cambrian boundary. The deep basin has been suggested to be anoxic and euxinic based on studies of sulfur isotopes, trace metal distributions and other proxies. Organic geochemistry has provided some clues concerning aspects of the depositional environments and microbial communities prevailing during this interval. However, ambiguities remain including a paucity of convincing molecular evidence for euxinia in the photic zone of the basin. Here, we present a comprehensive study of biomarker hydrocarbons, including steroids, triterpenoids, and carotenoids. Among the compounds detected is a distinctive array of aromatic carotenoids. Relatively low abundances of monoaromatic carotenoids, such as chlorobactane, okenane, and β-isorenieratane, suggest the possibility of transient photic zone euxinia with a shallow chemocline or, perhaps, exogenous inputs from microbial mats. However, it is the dominance of renieratane and renierapurpurane over isorenieratane in diaromatic carotenoids and their association with abundant C₃₈ and C₃₉ carotenoids that identifies cyanobacteria as major contributors to the inventory of carotenoids. Our results, based on multiple lines of molecular evidence and statistical analysis, also suggest that the Athel Silicilyte was biogeochemically distinct from the other units of the Ara Group. Overall, our study has important implications for understanding other late Neoproterozoic depositional environments.

Understanding the geobiology of the terminal Ediacaran Khatyspyt Lagerstätte (Arctic Siberia, Russia)

The Khatyspyt Lagerstätte (~544 Ma, Russia) provides a valuable window into late Ediacaran Avalon-type ecosystems with rangeomorphs, arboreomorphs, and mega-algae. Here, we tackle the geobiology of this Lagerstätte by the combined analysis of paleontological features, sedimentary facies, and lipid biomarkers. The Khatyspyt Formation was deposited in carbonate ramp environments. Organic matter (0.12-2.22 wt.% TOC) displays characteristic Ediacaran biomarker features (e.g., eukaryotic steranes dominated by the C₂₉ stigmastane). Some samples contain a putative 2-methylgammacerane that was likely sourced by ciliates and/or bacteria. 24-isopropylcholestane and 26-methylstigmastane are consistently scarce (≤0.4% and ≤0.2% of ∑C_27-30 regular steranes, respectively). Thus, Avalon-type organisms occupied different niches than organisms capable of directly synthesizing C₃₀ sterane precursors among their major lipids. Relative abundances of eukaryotic steranes and bacterial hopanes (sterane/hopane ratios = 0.07-0.30) demonstrate oligotrophic and bacterially dominated marine environments, similar to findings from other successions with Ediacara-type fossils. Ediacara-type fossils occur in facies characterized by microbial mats and biomarkers indicative for a stratified marine environment with normal-moderate salinities (moderate-high gammacerane index of 2.3-5.7; low C₃₅ homohopane index of 0.1-0.2). Mega-algae, in contrast, are abundant in facies that almost entirely consist of allochthonous event layers. Biomarkers in these samples indicate a non-stratified marine environment and normal salinities (low gammacerane index of 0.6-0.8; low C₃₅ homohopane index of 0.1). Vertical burrowers occur in similar facies but with biomarker evidence for stratification in the water column or around the seafloor (high gammacerane index of 5.6). Thus, the distribution of macro-organisms and burrowers was controlled by various, dynamically changing environmental factors. It appears likely that dynamic settings like the Khatyspyt Lagerstätte provided metabolic challenges for sustenance and growth which primed eukaryotic organisms to cope with changing environmental habitats, allowing for a later diversification and expansion of complex macroscopic life in the marine realm.

Geochemically Distinct Oil Families in the Gudong Oilfield, Zhanhua Depression, Bohai Bay Basin, China

Twenty crude oil samples were obtained from the Gudong Oilfield and their organic geochemical characteristics were analyzed. The oil samples were classified into three families by hierarchical cluster analysis and principal component analysis based on 13 source-related and depositional environment-related biomarker parameters. Oils in family I have low ratios of C₁₉/C₂₃ tricyclic terpanes and C₂₄ tetracyclic terpane/C₂₆ tricyclic terpanes, and relatively high ratios of steranes/hopanes and C₃₀ 4-methylsteranes/ααα20R C₂₉ sterane, thus indicating that microalgae were the dominant organic matter input for the source rocks of family I. The gammacerane/C₃₀ hopane ratios are higher than that of family II and family III, whereas the C₃₅/C₃₄ homohopane ratios are lower, thus indicating a suboxic, brackish water environment for the source rocks. The inferred source rock is the first member of the Shahejie Formation in the Huanghekou Sag. Family II is characterized by high ratios of C₁₉/C₂₃ tricyclic terpanes and C₂₄ tetracyclic terpane/C₂₆ tricyclic terpanes but relatively low ratios of steranes/hopanes and C₂₇/C₂₉ αααR steranes. These findings suggest that the original organic matter of the source rocks had a greater contribution from terrigenous higher plants than from microalgal. The relatively low ratios of gammacerane/C₃₀ hopane and C₃₅/C₃₄ homohopane suggest that the source rocks were deposited in an oxic environment with a low salinity, thus corresponding to the Dongying Formation in the Huanghekou Sag. Family III oils have high C₂₇/C₂₉ ααααR steranes ratios and low C₃₀ 4-methylsteranes/ααα20R C₂₉ ratios, which indicate the contribution of microalgae (especially zooplankton algae) to the source rocks. The relatively high abundance of C₃₅ homohopane and low gammacerane/C₃₀ hopane ratios suggest a weakly reducing condition with low salinity, which is in accordance with the third member of the Shahejie Formation in the Gunan Sag. The C₃₁S/(S + R) homohopane ratios imply that oil samples in this study are in the mature stage, although the ratios of C₂₉20S/(20S + 20R) and C₂₉ββ/(αα + ββ) steranes suggest that the maturity of family II is higher than that of family I and family III.

Factors controlling accumulation of organic carbon in a rift-lake, Oligocene Vietnam

Understanding of the processes of petroleum source rock (SR) accumulation in lacustrine rift basins and the behavior of lake systems as long-term carbon sinks is fragmentary. Investigation of an 800 m thick (500 m core and ~ 300 m outcrop), deep-lacustrine, Oligocene section in Vietnam, provides a rare insight into the controls and deposition of organic carbon (OC) and SR formation in continental rift basins. A multidisciplinary dataset, combining elemental data, inorganic and organic geochemistry with sedimentology, shows that the richest alginite-prone, sapropelic SR developed during periods of relative tectonic quiescence characterized by moderate primary productivity in a mainly dysoxic lacustrine basin. Increased rift activity and further development of graben morphology intensified water column stratification and anoxia, which hindered nutrient recycling. Sapropelic organic matter (OM) continued to accumulate, but with increasing amorphous OM content and decreasing total OC values. Periods of increased seasonality were characterized by thermocline weakening, enhanced mixing of water columns, increased primary productivity and diatom blooming. The results suggest that a change from dysoxia towards anoxia or extreme primary productivity does not necessarily enhance OC burial and SR quality. External nutrient input from a phosphate-rich hinterland is sufficient for sapropel formation, whereas the main limiting factor is methanogenesis.

Free and kerogen-bound biomarkers from late Tonian sedimentary rocks record abundant eukaryotes in mid-Neoproterozoic marine communities

Lipid biomarker assemblages preserved within the bitumen and kerogen phases of sedimentary rocks from the ca. 780-729 Ma Chuar and Visingsö Groups facilitate paleoenvironmental reconstructions and reveal fundamental aspects of emerging mid-Neoproterozoic marine communities. The Chuar and Visingsö Groups were deposited offshore of two distinct paleocontinents (Laurentia and Baltica, respectively) during the Tonian Period, and the rock samples used had not undergone excessive metamorphism. The major polycyclic alkane biomarkers detected in the rock bitumens and kerogen hydropyrolysates consist of tricyclic terpanes, hopanes, methylhopanes, and steranes. Major features of the biomarker assemblages include detectable and significant contribution from eukaryotes, encompassing the first robust occurrences of kerogen-bound regular steranes from Tonian rocks, including 21-norcholestane, 27-norcholestane, cholestane, ergostane, and cryostane, along with a novel unidentified C₃₀ sterane series from our least thermally mature Chuar Group samples. Appreciable values for the sterane/hopane (S/H) ratio are found for both the free and kerogen-bound biomarker pools for both the Chuar Group rocks (S/H between 0.09 and 1.26) and the Visingsö Group samples (S/H between 0.03 and 0.37). The more organic-rich rock samples generally yield higher S/H ratios than for organic-lean substrates, which suggests a marine nutrient control on eukaryotic abundance relative to bacteria. A C₂₇ sterane (cholestane) predominance among total C₂₆ -C₃₀ steranes is a common feature found for all samples investigated, with lower amounts of C₂₈ steranes (ergostane and crysotane) also present. No traces of known ancient C₃₀ sterane compounds; including 24-isopropylcholestanes, 24-n-propylcholestanes, or 26-methylstigmastanes, are detectable in any of these pre-Sturtian rocks. These biomarker characteristics support the view that the Tonian Period was a key interval in the history of life on our planet since it marked the transition from a bacterially dominated marine biosphere to an ocean system which became progressively enriched with eukaryotes. The eukaryotic source organisms likely encompassed photosynthetic primary producers, marking a rise in red algae, and consumers in a revamped trophic structure predating the Sturtian glaciation.

Coincidence of photic zone euxinia and impoverishment of arthropods in the aftermath of the Frasnian-Famennian biotic crisis

The lowermost Famennian deposits of the Kowala quarry (Holy Cross Mountains, Poland) are becoming famous for their rich fossil content such as their abundant phosphatized arthropod remains (mostly thylacocephalans). Here, for the first time, palaeontological and geochemical data were integrated to document abundance and diversity patterns in the context of palaeoenvironmental changes. During deposition, the generally oxic to suboxic conditions were interrupted at least twice by the onset of photic zone euxinia (PZE). Previously, PZE was considered as essential in preserving phosphatised fossils from, e.g., the famous Gogo Formation, Australia. Here, we show, however, that during PZE, the abundance of arthropods drastically dropped. The phosphorous content during PZE was also very low in comparison to that from oxic-suboxic intervals where arthropods are the most abundant. As phosphorous is essential for phosphatisation but also tends to flux off the sediment during bottom water anoxia, we propose that the PZE in such a case does not promote the fossilisation of the arthropods but instead leads to their impoverishment and non-preservation. Thus, the PZE conditions with anoxic bottom waters cannot be presumed as universal for exceptional fossil preservation by phosphatisation, and caution must be paid when interpreting the fossil abundance on the background of redox conditions.

Bisnorgammacerane traces predatory pressure and the persistent rise of algal ecosystems after Snowball Earth

Eukaryotic algae rose to ecological relevance after the Neoproterozoic Snowball Earth glaciations, but the causes for this consequential evolutionary transition remain enigmatic. Cap carbonates were globally deposited directly after these glaciations, but they are usually organic barren or thermally overprinted. Here we show that uniquely-preserved cap dolostones of the Araras Group contain exceptional abundances of a newly identified biomarker: 25,28-bisnorgammacerane. Its secular occurrence, carbon isotope systematics and co-occurrence with other demethylated terpenoids suggest a mechanistic connection to extensive microbial degradation of ciliate-derived biomass in bacterially dominated ecosystems. Declining 25,28-bisnorgammacerane concentrations, and a parallel rise of steranes over hopanes, indicate the transition from a bacterial to eukaryotic dominated ecosystem after the Marinoan deglaciation. Nutrient levels already increased during the Cryogenian and were a prerequisite, but not the ultimate driver for the algal rise. Intense predatory pressure by bacterivorous protists may have irrevocably cleared self-sustaining cyanobacterial ecosystems, thereby creating the ecological opportunity that allowed for the persistent rise of eukaryotic algae to global importance.

Redox-dependent niche differentiation provides evidence for multiple bacterial sources of glycerol tetraether lipids in lakes

Reliable prediction of future climate conditions requires a thorough understanding of climate variability throughout Earth’s history. Microbial molecular fossils, such as bacterial membrane-spanning tetraether lipids [branched glycerol dialkyl glycerol tetraethers (brGDGTs)], have proven to be particularly useful for the assessment of past climatic conditions, because they occur ubiquitously in the environment and show compositional changes related to temperature. However, the identity and ecology of brGDGT-producing bacteria is largely unknown, and a mechanistic basis for brGDGT-based paleoclimate reconstruction is still lacking. Here, we present insights into the ecological parameters that affect brGDGT synthesis in lakes, demonstrating that eutrophic lakes with oxygen-deprived bottom waters are the preferred sites for brGDGT-based reconstructions of continental climate.

Terrestrial paleoclimate archives such as lake sediments are essential for our understanding of the continental climate system and for the modeling of future climate scenarios. However, quantitative proxies for the determination of paleotemperatures are sparse. The relative abundances of certain bacterial lipids, i.e., branched glycerol dialkyl glycerol tetraethers (brGDGTs), respond to changes in environmental temperature, and thus have great potential for climate reconstruction. Their application to lake deposits, however, is hampered by the lack of fundamental knowledge on the ecology of brGDGT-producing microbes in lakes. Here, we show that brGDGTs are synthesized by multiple groups of bacteria thriving under contrasting redox regimes in a deep meromictic Swiss lake (Lake Lugano). This niche partitioning is evidenced by highly distinct brGDGT inventories in oxic vs. anoxic water masses, and corresponding vertical patterns in bacterial 16S rRNA gene abundances, implying that sedimentary brGDGT records are affected by temperature-independent changes in the community composition of their microbial producers. Furthermore, the stable carbon isotope composition (δ¹³C) of brGDGTs in Lake Lugano and 34 other (peri-)Alpine lakes attests to the widespread heterotrophic incorporation of ¹³C-depleted, methane-derived biomass at the redox transition zone of mesotrophic to eutrophic lake systems. The brGDGTs produced under such hypoxic/methanotrophic conditions reflect near-bottom water temperatures, and are characterized by comparatively low δ¹³C values. Depending on climate zone and water depth, lake sediment archives predominated by deeper water/low-¹³C brGDGTs may provide more reliable records of climate variability than those where brGDGTs derive from terrestrial and/or aquatic sources with distinct temperature imprints.

The effect of biodegradation on gammacerane in crude oils

Gammacerane is one of the major biomarkers widely used in depositional environment diagnosis, oil family classification, and oil-source correlation. It is generally accepted that gammacerane is more resistant to biodegradation than regular hopanes. However, whether it is biodegradable as well has not been reported in literatures. In order to investigate the effect of biodegradation on gammacerane in crude oils, 69 core samples from two biodegraded petroleum accumulations were geochemically characterized by quantitative GC-MS analysis. All samples are originated from lacustrine source rocks in China and have experienced at least level 8 degree of biodegradation on the scale of Peters and Moldowan (The biomarker guide: interpreting molecular fossils in petroleum and ancient sediments, Prentice Hall, Englewood Cliffs, 1993). Both case histories showed the concentration of gammacerane decrease with increasing severity of biodegradation, indicating the destruction of gammacerane by biodegradation. A whole series of 25-norhopanes paralleling the 17α,21β-hopanes (up to C₃₄), together with C₂₈ 18-α-25,30-bisnorneohopane, C₂₉ 25-nordiahopane and C₂₉ 25-norgammacerane, is found in the Liaohe sample suite but C₃₃, C₃₄ 25-norhopane and 25-norgammacerane are almost undetectable in the Junggar case. The gammacerane in the Liaohe case study appear to be altered simultaneously with hopanes, although the rate of gammacerane alteration is slower. Its susceptibility to biodegradation is similar to 18α(H)-22,29,30-trisnorneohopane (Ts) and 17α(H)-22,29,30-trisnorhopane (Tm) but more vulnerable than 18α-30-norneohopane (C₂₉ Ts), 15α-methyl-17α(H)-27-norhopane (C₃₀ diahopane) and pregnanes. The gammacerane in the Junggar oils appear to be less biodegradable than the Liaohe case history. It was altered simultaneously with pregnanes and C₂₉ Ts but faster than C₃₀ diahopane. The present data suggest that biodegradation sequence is not universal since the relative rates of biodegradation of different compound classes depend upon specific environmental conditions. Like the case of hopane demethylation, the mechanism of gammacerane biodegradation is not straightforward. While the conversion of gammacerane to 25-norgammacerane is not quantitatively balanced in the Liaohe case history, no 25-norgammacerane has been formed from the degradation of gammacerane in the Junggar case history. The ratio of gammacerane to regular hopanes increases with biodegradation degree especially at extreme levels of degradation, gammacerane index is no longer valid for depositional environment assessment or oil-source correlation.

Effects of soil erosion and anoxic-euxinic ocean in the Permian-Triassic marine crisis

The largest mass extinction of biota in the Earth’s history occurred during the Permian–Triassic transition and included two extinctions, one each at the latest Permian (first phase) and earliest Triassic (second phase). High seawater temperature in the surface water accompanied by euxinic deep-intermediate water, intrusion of the euxinic water to the surface water, a decrease in pH, and hypercapnia have been proposed as direct causes of the marine crisis. For the first-phase extinction, we here add a causal mechanism beginning from massive soil and rock erosion and leading to algal blooms, release of toxic components, asphyxiation, and oxygen-depleted nearshore bottom water that created environmental stress for nearshore marine animals. For the second-phase extinction, we show that a soil and rock erosion/algal bloom event did not occur, but culmination of anoxia–euxinia in intermediate waters did occur, spanning the second-phase extinction. We investigated sedimentary organic molecules, and the results indicated a peak of a massive soil erosion proxy followed by peaks of marine productivity proxy. Anoxic proxies of surface sediments and water occurred in the shallow nearshore sea at the eastern and western margins of the Paleotethys at the first-phase extinction horizon, but not at the second-phase extinction horizon. Our reconstruction of ocean redox structure at low latitudes indicates that a gradual increase in temperature spanning the two extinctions could have induced a gradual change from a well-mixed oxic to a stratified euxinic ocean beginning immediately prior to the first-phase extinction, followed by culmination of anoxia in nearshore surface waters and of anoxia and euxinia in the shallow-intermediate waters at the second-phase extinction over a period of approximately one million years or more. Enhanced global warming, ocean acidification, and hypercapnia could have caused the second-phase extinction approximately 60 kyr after the first-phase extinction. The causes of the first-phase extinction were not only those environmental stresses but also environmental stresses caused by the soil and rock erosion/algal bloom event.

The Ability of Microbial Community of Lake Baikal Bottom Sediments Associated with Gas Discharge to Carry Out the Transformation of Organic Matter under Thermobaric Conditions

The ability to compare the composition and metabolic potential of microbial communities inhabiting the subsurface sediment in geographically distinct locations is one of the keys to understanding the evolution and function of the subsurface biosphere. Prospective areas for study of the subsurface biosphere are the sites of hydrocarbon discharges on the bottom of the Lake Baikal rift, where ascending fluxes of gas-saturated fluids and oil from deep layers of bottom sediments seep into near-surface sediment. The samples of surface sediments collected in the area of the Posolskaya Bank methane seep were cultured for 17 months under thermobaric conditions (80°C, 5 MPa) with the addition of complementary organic substrate, and a different composition for the gas phase. After incubation, the presence of intact cells of microorganisms, organic matter transformation and the formation of oil biomarkers was confirmed in the samples, with the addition of Baikal diatom alga Synedra acus detritus, and gas mixture CH₄:H₂:CO₂. Taxonomic assignment of the 16S rRNA sequence data indicates that the predominant sequences in the enrichment were Sphingomonas (55.3%), Solirubrobacter (27.5%) and Arthrobacter (16.6%). At the same time, in heat-killed sediment and in sediment without any additional substrates, which were cultivated in a CH₄ atmosphere, no geochemical changes were detected, nor the presence of intact cells and 16S rRNA sequences of Bacteria and Archaea. This data may suggest that the decomposition of organic matter under culturing conditions could be performed by microorganisms from low-temperature sediment layers. One possible explanation of this phenomenon is migration of the representatives of the deep thermophilic community through fault zones in the near surface sediment layers, together with gas-bearing fluids.

A distinct pathway for tetrahymanol synthesis in bacteria

Tetrahymanol is a polycyclic triterpenoid lipid first discovered in the ciliate Tetrahymena pyriformis whose potential diagenetic product, gammacerane, is often used as a biomarker for water column stratification in ancient ecosystems. Bacteria are also a potential source of tetrahymanol, but neither the distribution of this lipid in extant bacteria nor the significance of bacterial tetrahymanol synthesis for interpreting gammacerane biosignatures is known. Here we couple comparative genomics with genetic and lipid analyses to link a protein of unknown function to tetrahymanol synthesis in bacteria. This tetrahymanol synthase (Ths) is found in a variety of bacterial genomes, including aerobic methanotrophs, nitrite-oxidizers, and sulfate-reducers, and in a subset of aquatic and terrestrial metagenomes. Thus, the potential to produce tetrahymanol is more widespread in the bacterial domain than previously thought. However, Ths is not encoded in any eukaryotic genomes, nor is it homologous to eukaryotic squalene-tetrahymanol cyclase, which catalyzes the cyclization of squalene directly to tetrahymanol. Rather, heterologous expression studies suggest that bacteria couple the cyclization of squalene to a hopene molecule by squalene-hopene cyclase with a subsequent Ths-dependent ring expansion to form tetrahymanol. Thus, bacteria and eukaryotes have evolved distinct biochemical mechanisms for producing tetrahymanol.

Focused ultrasound solid-liquid extraction for the determination of organic biomarkers in beachrocks

Beachrocks are consolidated coastal sedimentary formations resulting mainly from the relative rapid cementation of beach sediments by different calcium carbonate polymorphs. Although previous works have already studied the elemental composition and the mineral phases composing these cements, few of them have focused their attention on the organic matter present therein. This work describes an extraction methodology based on focused ultrasound solid-liquid extraction (FUSLE), followed by analysis using large volume injection (LVI) in a programmable temperature vaporizer (PTV) combined with gas chromatography-mass spectrometry (GC-MS) in order to determine organics such as polycyclic aromatic hydrocarbons (PAHs) and biomarkers (hopanes), which can increase and confirm the information obtained so far. This goal has been achieved after the optimization of the main parameters affecting the extraction procedure, such as, extraction solvent, FUSLE variables (amplitude, extraction time and pulse time) and also variables affecting the LVI-PTV (vent time, injection speed and cryo-focusing temperature). The developed method rendered results comparable to traditional extraction methods in terms of accuracy (77-109%) and repeatability (RSD<23%). Finally, the analyses performed over real beachrock samples from the Bay of Biscay (Northern Spain) revealed the presence of the 16 EPA priority PAHs, as well as some organic biomarkers which could increase the knowledge about such beachrock formation.

Unravelling ancient microbial history with community proteogenomics and lipid geochemistry

Our window into the Earth's ancient microbial past is narrow and obscured by missing data. However, we can glean information about ancient microbial ecosystems using fossil lipids (biomarkers) that are extracted from billion-year-old sedimentary rocks. In this Opinion article, we describe how environmental genomics and related methodologies will give molecular fossil research a boost, by increasing our knowledge about how evolutionary innovations in microorganisms have changed the surface of planet Earth.

Biomarker evidence for photosynthesis during neoproterozoic glaciation

Laterally extensive black shales were deposited on the São Francisco craton in southeastern Brazil during low-latitude Neoproterozoic glaciation approximately 740 to 700 million years ago. These rocks contain up to 3.0 weight % organic carbon, which we interpret as representing the preserved record of abundant marine primary productivity from glacial times. Extractable biomarkers reflect a complex and productive microbial ecosystem, including both phototrophic bacteria and eukaryotes, living in a stratified ocean with thin or absent sea ice, oxic surface waters, and euxinic conditions within the photic zone. Such an environment provides important constraints for parts of the "Snowball Earth" hypothesis.

Source diagnostic and weathering indicators of tar balls utilizing acyclic, polycyclic and S-heterocyclic components

This study represents a forensic chemical analysis to define the liability for the coastal bitumens polluting the beaches of the Mediterranean city of Alexandria. Six tar balls collected from several locations along the coast of the city were analyzed for their acyclic and polycyclic hydrocarbons as well as sulfur heterocycles using GC/FID, GC/AED and gas chromatography/mass spectrometry techniques. The analysis of one Egyptian crude oil is also included as a possible source oil. The tar ball samples were at early stages of weathering. Based on the GC traces and biomarker signatures, the tar balls could be genetically different. One sample collected from the Eastern Harbor region appears to be a Bunker C type fuel produced from Egyptian crudes. The refining process has removed the low molecular weight components. On the other hand, the wide n-alkane distribution together with the absence of an unresolved complex mixture suggests that crude oils probably from tank washings, ballast discharges or accident spills from tankers could have contributed significantly to the other tar ball samples. The distribution of source specific hopane and sterane markers revealed that the tar samples probably originate from different oil fields.