A late Quaternary sedimentary record of steryl alkyl ethers from offshore southeastern Australia

Steryl alkyl ethers (SAEs) have been reported from marine sediments ranging from Holocene to Lower Cretaceous and their production has been linked to upwelling of cold, nutrient-rich water. However, the lack of continuous stratigraphic records for them has complicated validation of the precise connection of their production, sedimentary distribution and survival to climatic conditions. Here, we report an SAE record that spans the last ca. 135 ka from a core taken offshore southeastern Australia in the Murray Canyons Group region. The SAEs are composed predominantly of C₂₇–C₃₀ steroid moieties with one or two double bonds, ether-bound to C₁₀–C₁₁ alkyl moieties. Some of the SAEs are identical to those reported in marine sediments, whereas others have not been identified previously. They do not exhibit any systematic change in distribution over the past 110 ka but sediments older than 110 ka possess a different assemblage, with a higher amount of constituents containing a C₁₁ alkyl moiety. The SAE accumulation rate increases rapidly during glacial episodes and for intervals characterized by a sharp decline in sea-surface temperature. The results suggest that SAEs may be related to marine eukaryotes that thrive when there is an influx of cold, deep waters in the Murray Canyons Group region, likely triggered by stronger westerlies over southeastern Australia forcing deeper water to reach the surface during glacials.     

Distribution and significance of novel low molecular weight sterenes in an immature evaporitic sediment from the Jinxian Sag,North China

An unusual series of C₂₂–C₂₇ monounsaturated sterenes and C₂₄–C₃₀ tetracyclic terpanes (17,21-secohopanes) were detected in relatively high concentrations in an immature evaporitic marl sediment of the Jinxian Sag, Bohai Bay Basin, North China. The site of unsaturation in these novel sterenes is assigned tentatively to the D ring on the basis of mass spectral interpretation, which also distinguishes them from reported unsaturated sterenes. Other hydrocarbon biomarker or stable isotope characteristics are indicative of microbial (e.g. methyl hopanes), phytoplankton or higher plant (depleted δ¹³C values of isoprenoids and hopanes) inputs and an anoxic carbonate depositional environment (hexacyclic hopanes; tetracyclic terpanes). The hydrocarbon composition showed no obvious biodegradation and the relatively high concentration of unsaturated terpenoids (e.g. gammacerene) and low values of other established maturity parameters (T_s/T_m = 0.23; R_o = 0.44%; T_max = 417 °C), are consistent with sediments of low maturity. The novel, low molecular weight sterenes and the tetracyclic terpanes may be early diagenetic products of microbial sources in a carbonate environment.     

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C₁–C₄, CO₂]). Molecular ratios of LMWHC (C₁/[C₂ + C₃] > 1000) and stable isotopic compositions of methane (δ¹³C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C₁/C₂₊ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C₁–C₄, CO₂]) relative to the other gas types and the virtual lack of ¹⁴C–CH₄ indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C₁–C₄, CO₂)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of ¹⁴C–CH₄ (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.     

Stable carbon isotope geochemistry of adsorbed alkane gases in near-surface soils of the Saurashtra Basin,India

The stable carbon isotopic compositions of light hydrocarbon gases adsorbed in near-surface soil and sediments from the Saurashtra basin were characterized for their origin and maturity. Saurashtra is considered geologically prospective for oil and gas reserves; however, a major part of the basin is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. Surface geochemical prospecting, based on micro-seepage of hydrocarbons from subsurface accumulations, could be advantageous in such areas. In light of this, 150 soil samples were collected from the northwestern part of Saurashtra, around the Jamnagar area, where a thick sedimentary sequence of about 2–3 km exists under 1–1.5 km of Deccan basalt. The concentration of acid desorbed alkane gases from soil samples was found to vary (in ppb) as: methane (C₁) = 3–518; ethane (C₂) = 0–430; propane (C₃) = 0–331; i-butane (iC₄) = 0–297; n-butane (nC₄) = 2–116; i-pentane (iC₅) = 0–31 and n-pentane (nC₅) = 0–23, respectively.Fifteen samples with high concentrations of alkane gases were measured for their δ¹³C₁; δ¹³C₂ and δ¹³C₃ compositions using gas chromatography–combustion-isotope ratio mass spectrometry (GC–C-IRMS). The values for methane varied from ? 27 to ? 45.4‰, ethane from ? 20.9 to ? 27.6‰, and propane from ? 20.4 to ? 29.1‰ versus the Vienna PeeDee Belemnite (VPDB). The carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gases derived from thermogenic sources. Comparisons of carbon isotopic signatures and compositional variations with the standard carbon isotopic models suggest that hydrocarbon gases found in the shallow depths of the study area are not of bacterial origin but are formed thermally from deeply buried organic matter, likely to be mainly a terrestrial source rock with a partial contribution from a marine source. These gases may have migrated to the near-surface environment, where they represent an admixture of thermally generated hydrocarbon gases from mixed sources and maturity. The maturity scale (δ¹³C versus Log Ro %) applied to the surface sediment samples of the Jamnagar area indicated the source material to be capable of generating oil and gas. The detection of thermogenic alkane gases in near-surface sediments offers the possibility of hydrocarbons at depth in Saurashtra.     

n-Alkane evidence for the onset of wetter conditions in the Sierra Nevada,California (USA) at the mid-late Holocene transition, ~ 3.0 ka

n-Alkane biomarker distributions in sediments from Swamp Lake (SL), in the central Sierra Nevada of California (USA), provide evidence for an increase in mean lake level ~ 3000 yr ago, in conjunction with widespread climatic change inferred from marine and continental records in the eastern North Pacific region. Length distributions of n-alkane chains in modern plants growing at SL were determined and compared to sedimentary distributions in a core spanning the last 13 ka. As a group, submerged and floating aquatic plants contained high proportions of short chain lengths (< nC₂₅) compared to emergent, riparian and upland terrestrial species, for which chain lengths > nC₂₇ were dominant. Changes in the sedimentary n-alkane distribution over time were driven by variable inputs from plant sources in response to changing lake level, sedimentation and plant community composition. A shift toward shorter chain lengths (nC_21, nC₂₃) occurred between 3.1 and 2.9 ka and is best explained by an increase in the abundance of aquatic plants and the availability of shallow-water habitat in response to rising lake level. The late Holocene expansion of SL following a dry mid-Holocene is consistent with previous evidence for increased effective moisture and the onset of wetter conditions in the Sierra Nevada between 4.0 and 3.0 ka.     

First-principles phase diagram calculations for the system NaCl–KCl: The role of excess vibrational entropy

First principles phase diagram calculations were performed for the system NaCl–KCl. Plane-wave pseudopotential calculations of formation energies were used as a basis for fitting cluster expansion Hamiltonians, both with and without an approximation for the excess vibrational entropy (S_VIB). Including S_VIB dramatically improves the agreement between calculated and experimental phase diagrams: experimentally, the consolute point is {X_C = 0.348, T_C = 765 K}_Exp; without S_VIB, it is {X_C = 0.46, T_C ≈ 1630 K}_Calc; with S_VIB, it is {X_C = 0.43, T_C ≈ 930 K}_Calc.     

Diagenetic state and source characterization of marine sediments from the inner continental shelf of the Gulf of Cádiz (SW Spain), constrained by terrigenous biomarkers

Surface sediments from the Gulf of Cádiz (GoC) were analyzed by alkaline CuO oxidation, in order to estimate the contribution of terrigenous organic matter (TOM) to the inner continental shelf of the southwest Iberian Peninsula. The parallel analysis of sediment samples from the two most important rivers draining to this coastal area (i.e. Guadiana River and Tinto–Odiel fluvial system) provided fundamental information regarding local terrestrial sources. Relatively constant intensive lignin parameters (S:V = 1.0 ± 0.1 and C:V = 0.22 ± 0.04) and high values of the lignin phenol vegetation index (LPVI = 155 ± 43) indicated that non-woody angiosperm tissues constitute the dominant component of vascular plant material reaching the shelf sediments. The NW to SE decreasing isotopic (¹³C) and molecular (Λ₈) signatures found among the sediments, coinciding with the Guadiana delivery plume, suggest that this river is the main terrestrial source in the inner GoC shelf. Slightly elevated values of degradation indicative ratios ([Ad:Al]_V = 0.41 ± 0.10; [Ad:Al])_S = 0.34 ± 0.07; [3,5-Bd:V] = 0.14 ± 0.05; P:[V + S] = 0.24 ± 0.09) suggested the alteration state of the shelf sediments. The two fold higher ratios of the river sediments (Guadiana: [Ad:Al]_V = 0.82 ± 0.08; [Ad:Al]_S = 0.84 ± 0.03; Tinto–Odiel: [Ad:Al]_V = 0.86 ± 0.12; [Ad:Al]_S = 0.83 ± 0.013) and the increasing degradation trend observed outward in the shelf, lead us to consider preferential sorption processes, instead of in situ diagenesis, to affect the degradation signature of the shelf sediments. Preferentially solubilized degraded OM is more likely to be sorbed and stabilized prior to transport to the marine system, showing an apparently more advanced degradation state. The use of the 3,5-Bd:V ratio in conjunction with (Ad:Al)_V revealed a composition continuum of the sedimentary OM ranging from fresh plant materials to highly altered soil humic constituents. Elemental and molecular analyses show a land to sea gradient by a NW to SE decrease of the terrestrial influence, accounting for larger terrestrial inputs (TOM: 71–98%) in those sediments near the Guadiana mouth, and predominantly autochthonous composition (TOM: 42–50%) in those located offshore. This work utilizes lignin derived biomarkers to determine the contribution of terrigenous OM delivered to this poorly described coastal area from regional rivers. Within a context of increasing international efforts to better understand the global C cycling, this study illustrates the relevance of using the alkaline CuO oxidation approach to evaluate C budgets and continental influence in river dominated ocean margins.     

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

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

Precise indices based on n-alkane distribution for quantifying sources of sedimentary organic matter in coastal systems

Precise indices based on n-alkane signatures were developed in order to determine the sources and composition of sedimentary organic matter (SOM) in coastal systems. The Arcachon Bay (France), a well-studied temperate lagoon, was used as an example of a complex coastal system sheltering a wide diversity of OM sources. Three main groups of sources were well discriminated from their n-alkane signatures: seagrass (Zostera sp.) produced mainly n-C₁₇, n-C₁₉, n-C₂₁, n-C₂₃ and n-C₂₅ alkanes, algae (Rhodophyta, Chlorophyta) produced n-C₁₅ and n-C₁₇ and the terrigenous input [Quercus sp., Spartina sp. and river suspended particulate OM (SPOM)] was characterized by n-C₂₅, n-C₂₇, n-C₂₉, n-C₃₁ and n-C₃₃. From the above and literature n-alkane fingerprints, we developed a set of indices (n-alkane ratios) to quantify the contribution of these three major sources of the SOM. At the Arcachon Bay scale, they indicated that SOM was composed mainly of seagrass (ca. 53 ± 19%) and terrestrial (ca. 41 ± 17%) material, followed by algae (ca. 6 ± 9%). Moreover, the new n-alkane indices exhibited more relevant spatial patterns than classical ones – the TAR (C₂₇ + C₂₉ + C₃₁/C₁₅ + C₁₇ + C₁₉; terrestrial to aquatic ratio) and the P_aq (C₂₃ + C₂₅/C₂₃ + C₂₅ + C₂₉ + C₃₁; aquatic plant %) – with a greater contribution from marine sources in the central part of the lagoon where a high density of Zostera seagrass was observed. Therefore, the development of precise indices adapted to the local diversity of OM sources is needed when using n-alkanes for quantifying the source composition of SOM in complex coastal systems.     

Biomarker distributions as indicators for the depositional environment of lacustrine sediments in the Valjevo-Mionica basin (Serbia)

In this study, the molecular composition and biomarker distribution of lacustrine sediments from Val-1 drillhole in the central zone of the western part of the Valjevo-Mionica basin were investigated at depth interval of 0–400 m. Former investigations have shown that the core material can be separated into six depth intervals based on bulk geochemical, mineralogical and sedimentological characteristics. Concerning the quality of organic matter, presence of specific minerals, and high salinity and anoxicity, or alkalinity, three zones are of highest interest, defined at depth intervals of 15–75 m (A), 75–200 m (B) and 360–400 m (F). The first aim of the study was to identify which biomarkers characterize these specific intervals. The second aim, addressing the transitions zones of these intervals, was to extend the changes in the characteristics of the organic substance, to reflect the changes of conditions in the depositional environment as well as to define biomarker parameters which are the most sensitive sedimentological indicators.The sediments from the hypersaline anoxic and alkaline environment show high contribution of algal precursor biomass, what is in accordance with the good quality of organic substances in the sediments from these zones. High squalane content and low content of regular isoprenoid C₂₅ are typical for hypersaline anoxic environment, whereas sediments from alkaline environment have high regular isoprenoid C₂₅ content.Transition to specific sedimentation zones is characterized by change in total organic matter content, and of both free and pyrolysis-derived, and change in hydrogen index value. In the biomarker distributions, more significant changes were detected in distributions of n-alkanes and isoprenoids, compared to polycyclic alkanes. The most intensive changes in alkane distribution are reflected in changes in n-C₁₇ content compared to n-C₂₇, and phytane compared to n-C₁₈. In addition, significant sensitivity was seen in ratios between squalane and n-alkane C₂₆ (hypersaline depositional environment), or isoprenoid C₂₅ and n-alkane C₂₂ for high alkalinity environment.This study showed that Sq/n-C₂₆ ratio can be used to assess the quality of organic substance in immature lacustrine sediments.     

Positive correlation between salinity and n-alkane δ13C values in the mangrove Avicennia marina

Carbon isotope ratio (δ¹³C) values of lipid biomarkers from plants can be used to assess water use efficiency and to reconstruct environmental conditions in the past. We assessed the effect of salinity on the δ¹³C values for leaf wax n-C₃₁ and n-C₃₃ alkanes, bulk leaf matter and leaf total lipid extracts from Avicennia marina (gray mangrove) trees growing along the Brisbane River estuary in Queensland, Australia. We observed an increase in 0.19 ± 0.053‰ (R² 0.61, p 0.008) and 0.16 ± 0.052‰ (R² 0.55, p 0.01) per salinity unit for the two n-alkanes, respectively, and of 0.087 ± 0.028‰ (R² 0.41, p 0.009) for whole leaves per salinity unit, indicating that water use efficiency of A. marina increased with the salt content of water. There was no correlation between δ¹³C values of total lipid extracts and salinity, perhaps because of a decrease in lipid concentration at higher salinity or because of varying contributions of different lipid classes to the extract. The robust relationship between salinity and δ¹³C values of leaf wax lipids provides a means of quantitatively reconstructing past salinity from carbon isotope ratios of mangrove lipid biomarkers in sediments. When paired with measurements of the hydrogen isotope ratio values of the same compounds, the approach should facilitate quantitative reconstruction of the hydrogen isotope composition of environmental water. In order for the method to successfully reconstruct past salinity and water isotopes, a mangrove source for leaf wax would need to be confirmed by palynological or other evidence, or the isotopic composition of a more source specific biomarker, such as taraxerol, would need to be measured.     

Ethane- and propane-producing potential and molecular characterization of an ethanogenic enrichment in an anoxic estuarine sediment

Ethane and propane are low molecular weight hydrocarbons observed widely at trace levels in cold marine sediments where thermogenic sources are considered insignificant, but their biological sources remain poorly constrained. In this study, several C₂ and C₃ compounds including alkenes, alcohols, thiols and carboxylic acids with a C₂ or C₃ skeleton were tested for their relative alkane-producing potential in an anoxic estuary sediment. Maximum conversion efficiency of substrates to ethane or propane was observed in the sediment supplemented with ethylene (up to 38%), followed by additions with ethanethiol (0.01%) and propanethiol (0.003%). Experiments with sterilized sediment, 2-bromoethanesulfonic acid or NaNO₃ were negative for alkane production, suggesting that methanogens were involved in alkane generation. Detailed investigation on ethanogenesis from ethylene showed that this reaction required H₂ but at reasonably low concentration (< 120 nmol dissolved H₂ l⁻¹ slurry) and caused a slight stable carbon isotope effect (ε_{ethane/ethylene} = −8.6 ± 2.4‰). The high ethane-producing potential, reasonable H₂ requirement and extensive occurrence of ethylene make ethylene reduction a plausible explanation for ethane in cold marine sediment. Phylogenetic analysis was first carried out with an ethane-producing enrichment with ethylene as the substrate and showed a dominance of homoacetogenic bacteria and the methanogenic genus Methanocalculus. Although we cannot rule out the possibility that other methanogens in the gene libraries are responsible for ethanogenesis from ethylene, the dominance of Methanocalculus, with its hydrogenotrophic cultured representatives, is in accord with our biogeochemical observation that H₂ is required for this reaction.     

Carbonate recrystallisation and organic matter maturation in heat-affected sediments from the Shaban Deep,Red Sea

Parasound profiles across the Shaban Deep in the Red Sea indicate turbiditic transport of surface sediments from the topographic height (basalt ridge) into the interior of the deep. This is supported by petrographical and (isotope-) geochemical evidence in the East Basin of the Shaban Deep where the presence of variable mixtures of authochtonous and allochthonous sediment compounds had been found.The uppermost 170 cm of both sediment cores 17008-1 and 17009-3 reveal “normal” stable oxygen isotope values for the planktonic foraminifera Globigerinoides ruber near ?1‰ which is indicative for carbonate formation in Red Sea surface water around 27 °C. However, below 182 cm in core 17008-1 highly variable δ ¹⁸O values for G. ruber between 0.26 and ?10.68‰ occur which are not the result of temperature-controlled oxygen isotope fractionation between foraminiferal carbonate and Red Sea surface water. The lowest δ¹⁸O values of ?10.68‰ measured for highly-altered foraminifera shells suggests carbonate precipitation higher than 90 °C.Organic petrographical observations show a great diversity of marine-derived macerals and terrigenous organic particles. Based on petrographical investigations sediment core 17008-1 can be subdivided in intervals predominantly of authochtonous character (i.e. 1, 3, 5 corresponding to core depths 0–170 cm, 370–415 cm, 69–136 cm), and allochthonous/thermally altered character (e.g. 2 and 4 corresponding to core depths 189–353 cm and 515–671 cm). Allochthonous/thermally altered material displays a wide to an extremely wide range of maturities (0.38–1.42% R_r) and also natural coke particles were found.Similarly, the organic geochemical and pyrolysis data indicate the predominance of well-preserved, immature algal and bacterial remains with a minor contribution of land plant material. Sediments below 170 cm (core 17008-1) contain contributions of re-sedimented pre-heated material most likely from the area of the basaltic ridge. This is documented by individual coke particles reduced hydrogen indices and elevated T_max values up to 440 °C.An “oil-type” contribution (evidenced by mature biomarkers, hopene/hopane ratios, elevated background fluorescence, n-alkane distribution) is also present in the sediments which most likely originated at greater depth and impregnated the surface sediments.The heat source responsible for recrystallisation of foraminiferal carbonate and maturation of organic particles in Shaban Deep sediments most likely is attributed to modern basalt extrusions which now separate the Shaban Deep subbasins.     

Calibrating n-alkane Sphagnum proxies in sub-Arctic Scandinavia

Moss covered, high latitude wetlands hold large amounts of terrestrial organic matter (OM), which may be vulnerable to expected climate warming. Molecular analysis of fluvially transported material from these regions can distinguish between different sources of terrestrial OM. Sphagnum moss may represent one of the major sources. This study aimed to quantitatively establish a molecular proxy for identifying Sphagnum-derived OM from high latitude peatlands in the sub-Arctic coastal ocean. We collected and analyzed Sphagnum species throughout northern Sweden and Finland. Results show that the C₂₅/(C₂₅ + C₂₉) n-alkane ratio is most suitable for terrestrial OM source apportionment in these coastal regions since, compared to other n-alkane Sphagnum proxies, it shows (i) the least variation between species, (ii) the most constant values for different latitudinal regimes and (iii) the largest dynamic range to the higher plant end member in two-source mixing models. Application of the proxy to surface sediments and suspended particulate matter in the sub-Arctic northern Baltic Sea shows that 68–103% of the terrestrial OM fraction is derived from Sphagnum-rich peatland. We recommend that future studies on terrestrial OM fluxes into (sub-)Arctic regions should apply the C₂₅/(C₂₅ + C₂₉) proxy to improve insight into the contribution of Sphagnum-derived terrestrial OM from climate-vulnerable, high latitude wetlands.     

Organic geochemistry of the Vindhyan sediments: Implications for hydrocarbons

The organic geochemical methods of hydrocarbon prospecting involve the characterization of sedimentary organic matter in terms of its abundance, source and thermal maturity, which are essential prerequisites for a hydrocarbon source rock. In the present study, evaluation of organic matter in the outcrop shale samples from the Semri and Kaimur Groups of Vindhyan basin was carried out using Rock Eval pyrolysis. Also, the adsorbed low molecular weight hydrocarbons, methane, ethane, propane and butane, were investigated in the near surface soils to infer the generation of hydrocarbons in the Vindhyan basin. The Total Organic Carbon (TOC) content in shales ranges between 0.04% and 1.43%. The S₁ (thermally liberated free hydrocarbons) values range between 0.01–0.09 mgHC/gRock (milligram hydrocarbon per gram of rock sample), whereas the S₂ (hydrocarbons from cracking of kerogen) show the values between 0.01 and 0.14 mgHC/gRock. Based on the T_max (temperature at highest yield of S₂) and the hydrogen index (HI) correlations, the organic matter is characterized by Type III kerogen. The adsorbed soil gas, CH₄ (C₁), C₂H₆ (C₂), C₃H₈ (C₃) and nC₄H₁₀, (nC4), concentrations measured in the soil samples from the eastern part of Vindhyan basin (Son Valley) vary from 0 to 186 ppb, 0 to 4 ppb, 0 to 5 ppb, and 0 to 1 ppb, respectively. The stable carbon isotope values for the desorbed methane (δ¹³C₁) and ethane (δ¹³C₂) range between −45.7‰ to −25.2‰ and −35.3‰ to −20.19‰ (VPDB), respectively suggesting a thermogenic source for these hydrocarbons. High concentrations of thermogenic hydrocarbons are characteristic of areas around Sagar, Narsinghpur, Katni and Satna in the Son Valley. The light hydrocarbon concentrations (C₁–C₄) in near surface soils of the western Vindhyan basin around Chambal Valley have been reported to vary between 1–2547 ppb, 1–558 ppb, 1–181 ppb, 1–37 ppb and 1–32 ppb, respectively with high concentrations around Baran-Jhalawar-Bhanpur-Garot regions (Kumar et al., 2006). The light gaseous hydrocarbon anomalies are coincident with the wrench faults (Kota – Dholpur, Ratlam – Shivpuri, Kannod – Damoh, Son Banspur – Rewa wrench) in the Vindhyan basin, which may provide conducive pathways for the migration of the hydrocarbons towards the near surface soils.     

Elastic constants of single-crystal Pt measured up to 20 GPa based on inelastic X-ray scattering: Implication for the establishment of an equation of state

Phonon velocities and densities for Pt were measured based on inelastic X-ray scattering from ambient pressure to 20 GPa in order to independently determine its equation of state (EOS). Phonon velocities were determined with sine dispersion relations. C_ij values were obtained by fitting phonon velocities and densities to the Christoffel equation. We found that the obtained C_ijs were in good agreement with previously reported C_ijs at ambient condition. Based on the C_ij values in various conditions, experimental pressures were calculated. The EOS of Pt as a primary pressure scale was determined based on the experimental pressures. We report K’ = 5.17 with fixed K_T = 274.1 GPa and V₀ = 60.360 ų for Vinet EOS. Our scale is in good agreement with several previously published scales based on shock experiments and XRD.     

Organic geochemistry and depositional environment of the Aptian bituminous limestone in the Kale Gümüşhane area (NE-Turkey): An example of lacustrine deposits on the platform carbonate sequence

The Jurassic–Lower Cretaceous aged carbonate sequence is widely exposed in the southern zone of Eastern Pontides. Aptian black bituminous limestone is found in the upper part of this sequence in the Kale area (Gümüşhane). This limestone contains faunal remains (e.g., gastropod, ostracod, characean stems and miliolid type benthic foraminifera) that indicate a freshwater, lacustrine depositional environment.The total organic carbon (TOC) values of the bituminous limestone samples range from 0.11–1.30% with an average TOC value of 0.54%. The hydrogen index (HI) varies from 119–448 mg HC/g TOC (average HI 298 mg HC/g TOC) indicating that the limestone contains gas prone as well as oil prone organic matter. Pyrolysis data prove that the organic matter content in the bituminous limestone consists of Type II kerogen. The average T_max value for bituminous limestone samples is 438 °C (434–448 °C). Bitumen/TOC ratios for bituminous limestone are 0.05 and 0.04. The T_max values and the ratios indicate that the bituminous limestone samples contain early mature to mature organic matter.Analysis of solvent extracts from the two richest bituminous limestones show a predominance of high carbon number (C₂₆–C₃₀) n-alkanes. The Pr/Ph ratio and CPI value are 1.34 and 0.96, respectively. C₂₉ is the dominant sterane, with C₂₉ > C₂₇ > C₂₈. The bituminous limestone samples have low C₂₂/C₂₁ ratios, high C₂₄/C₂₃ tricyclic terpane ratios and very low C₃₁R/C₃₀ hopane ratios (<0.25). These data are consistent with the bituminous limestones being deposited in a lacustrine environment.     

Thermal degradation of rye and maize straw: Lipid pattern changes as a function of temperature

Future climatic conditions may coincide with an increased potential for wildfires in grassland and forest ecosystems, whereby charred biomass would be incorporated into soils. Molecular changes in biomass upon charring have been frequently analysed with a focus on black carbon. Aliphatic and aromatic hydrocarbons, known to be liberated during incomplete combustion of biomass have been preferentially analysed in soot particles, whereas determinations of these compounds in charred biomass residues are scarce. We discuss the influence of increasing charring temperature on the aliphatic and aromatic hydrocarbon composition of crop grass combustion residues. Straw from rye, representing C₃ grasses and maize, representing C₄ grasses, was charred in the presence of limited oxygen at 300, 400 and 500 °C. Typical n-alkane distribution patterns with a strong predominance of long chain odd-numbered n-alkanes maximising at C₃₁ were observed in raw straw. Upon combustion at 300 °C aliphatic hydrocarbons in char were dominated by sterenes, whereas at 400 °C sterenes disappeared and medium chain length n-alkanes, maximising around n-C₂₀, with a balanced odd/even distribution were present. At a charring temperature of 500 °C n-alkane chain length shifted to short chain homologues, maximising at C₁₈ with a pronounced predominance of even homologues. Even numbered, short chain n-alkanes in soils may thus serve as a marker for residues of charred biomass. Aromatic hydrocarbons indicate an onset of aromatization of biomass already at 300 °C, followed by severe aromatization upon incomplete combustion at 400–500 °C. The diagnostic composition of aliphatic and aromatic hydrocarbons from charred biomass affords potential for identifying residues from burned vegetation in recent and fossil soils and sediments.     

Geochemical and isotopic approach to maturity/source/mixing estimations for natural gas and associated condensates in the Thrace Basin,NW Turkey

The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW–SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH₄, thermogenic C₂₊ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH₄ is bacteriogenic (Calculated δ¹³C_C1–C = −61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH₄. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (δ¹³C_C1–C = −35.80‰; Hamitabat Field) and Group-3 (δ¹³C_1–C = −49.10‰; Değirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C₂₊ gases. The Group-2 C₂₊ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (δ¹³C = −21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C₂₊ gases, produced from one field, were generated from isotopically light marine kerogen (δ¹³C = −29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C₂₊ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (δ¹³C = −21‰; −29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (R_eq = 0.55–0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin.     

Organic matter distribution in the modern sediments of the Pearl River Estuary

We determined biomarker concentrations and distributions for surface sediments from 54 sites in the Pearl River Estuary, China. We focus on a suite of four biomarker-based indicators for relative terrestrial to marine organic matter (OM) source: the branched-isoprenoid tetraether (BIT) index, the ratio of high/low molecular weight n-alcohols [(ΣC_26–34/(ΣC₁₆₊₁₈ + ΣC_26–34)], an analogous ratio for n-fatty acids and the ΣC₂₉-steroids/(ΣC₂₉-steroids + brassicasterol) ratio. All four exhibit the same terrestrial to marine transition seen in previous bulk δ¹³C studies, but with an abrupt decrease in the relative terrestrial contribution across the delta front to pro-delta transition. Concentrations of terrestrially-derived biomarkers show no systematic decrease across the transition. Instead, the decrease in the proportion of terrestrial OM is due to a decrease in the sedimentation rate and associated terrestrial OM burial across the delta toe. This suggests that diagenetic controls on the fate of terrestrial OM, such as increased biodegradation where sedimentation rate is low, are subordinate to sedimentological processes. Biomarker-derived temperature values are cooler than expected for the lower Pearl River catchment, suggesting that the dominant component of the terrestrial OM is derived from the cooler upland regions of the catchment. The dominance of input from more distal terrain with greater topographic relief is evidence for the importance of geomorphological control on terrigenous OM transport. Collectively, the results demonstrate the importance of sedimentological processes in the supply, deposition and transport of terrestrial OM.