Hopanoic Acids Research Articles

Organic geochemistry of Palaeogene coals from Greenland and Svalbard

AbstractThe organic geochemistry and coal petrology of Palaeogene coals from northeast Greenland (Thyra Ø Island and Kronprins Christian Land) and central Spitsbergen (Longyearbyen and Grumantbyen) were studied using Rock–Eval and gas chromatography–mass spectrometry, as well as microphotometry and maceral group analyses. Bulk data and biomarker distributions of the coals demonstrate a low coal rank for both, but a lower coalification degree of coals from Greenland (0.49–0.55% VRr) compared to those from Svalbard (0.68–0.75% VRr). Maceral group analyses revealed relatively similar distributions with a strong predominance of vitrinite. The generally high abundance of hopanoids (hopanes/hopenes and hopanoic acids) implies a strong bacterial reworking of the organic matter, whereas sulphur occurrences indicate a marine influence after organic matter deposition. A great variety of higher plant biomarkers was detected in all coals. Distinctive compounds recorded in the coals are aliphatic and aromatic diterpenoids as well as partly hydrogenated picenes, suggesting strong input of conifers and angiosperms. Pristane/phytane ratios indicate that the organic matter in the ancient swamps was deposited in an oxic, fluvio-deltaic setting at both sites. This study provides a detailed geochemical investigation of understudied coals from northeast Greenland. Moreover, it enhances our understanding of probably interrelated Palaeogene depositional settings from Greenland and Spitsbergen in terms of their palaeoecology, primary input into coal swamps, and individual thermal history. Graphical abstract

Highly efficient purification of long-chain alkenones using silver dimercaptotriazine flash chromatography

Long-chain alkenones (LCAs) are important tools for paleotemperature reconstructions in lacustrine and marine environments. However, some neutral lipid compounds that co-elute with LCAs often occur in natural sediment samples, which seriously interferes with the identification and quantification of the LCAs. In this study, we report a new flash column chromatography based on silver(I)-dimercaptotriazine (Ag-DMT) functionalized silica material (≡Si-DMT-Ag) for purifying and isolating the individual LCAs from complex lake and marine sediment samples. Compared to silver(I)-mercaptopropyl (Ag-MP) functionalized (≡Si-(CH2)3-S-Ag) and silver nitrate (SiO2 + AgNO3) impregnated silica materials used by previous studies, the Ag-DMT stationary phase displays the best efficiency in removing co-eluting compounds (including wax ester, hopanoid, hopanoic acid, sterane, alkanoic acid, and some unknown compounds) from the LCAs in our study lake and marine sediment samples. The Ag-DMT material also shows its high retention capacity for the LCAs, probably due to a strong interaction of more positively charged silver sites on Ag-DMT with double bonds. Employing the Ag-DMT chromatography, we develop an optimal solvent elution scheme to efficiently isolate the individual LCAs with the same chain length but different numbers of unsaturation for their isotopic analysis in the future. Our study provides a highly effective method for eliminating co-eluting compounds and isolating individual LCAs for paleoclimate studies.

Variation in organic matter across the Cretaceous-Paleogene boundary in New Zealand supports the “Living Ocean” model of biotic recovery

The Cretaceous-Paleogene (K-Pg) boundary represents the most recent mass extinction in Earth history. Although it is widely accepted that a bolide impact in North America caused the extinction, many questions persist about how this impact affected Earth's climate and environment. Here we explore changes in organic matter inputs across the K-Pg in an impact-distal section in New Zealand (mid-Waipara River section, North Canterbury, New Zealand). The section is 21.2 m thick, including 1.2 m of uppermost Cretaceous and 20 m of lower Paleocene sediment, the latter spanning ~1 Myr, albeit interrupted by at least one major unconformity. We examine the abundance and distribution of n-alkanes and n-alkanoic acids (mixed sources), acyclic isoprenoids (pristane and phytane), steranes (eukaryotes), hopanes and hopanoic acids (bacteria) and GDGTs (glycerol dialkyl glycerol tetraether lipids) to explore changes in the relative contribution of marine vs terrestrial sources. Minimal differences in the biomarker assemblages between sediments below and above the K-Pg boundary suggest a rapid recovery of the non-fossilizing phytoplankton community, similar to that observed in other distal settings. However, the organic matter source became variable immediately after the impact event. Variations in the concentrations and distribution of the high-molecular-weight n-alkanes and n-alkanoic acids, terrestrial-aquatic ratios, and GDGT branched to isoprenoidal tetraether indices indicate an increase in the absolute and proportional abundance of terrestrial soil- and plant-derived organic matter just after the impact event. This co-occurs with an apparently transient decline in marine productivity, indicated by a decrease in concentrations of low-molecular-weight n-alkanoic acids, concentrations of pristane + phytane (of putative algal chlorophyll origin), and sterane/hopane ratios. This suggests that the bolide did affect the mid-Waipara biomarker-producing bacterial and algal assemblages, distinct from what has been observed at other distal sites. However, it remains unclear if this was a direct consequence of the impact or the wider post-impact reorganisation of environment and oceanography as has been previously observed in this region.

An alternative method for the separation and analysis of acidic biomarkers from crude oil samples

This study aimed to develop and/or optimize a method of separation and analysis of neutral and acidic constituents from oil samples from Brazilian sedimentary basins and make a broad characterization of these samples through the respective molecular fossils. Three oil samples (A, B and C) of different origins and biodegradation were analyzed in this work. The neutral fractions were subjected to classic separation processes using gradient elution and analyzed by gas chromatography–mass spectrometry (GC–MS). The acidic fraction was isolated using KOH modified silica as stationary, adapted in SPE, and analyzed by GC–MS as silylated derivatives. The molecular parameters for B oil suggest a marine evaporitic depositional environment, with less continental influence, low siliciclastic sediments input, low oxygenation, hypersaline, stratified water column, and persistent photic zone euxinic. For the A and C oils, the parameters indicate a significant similarity between the samples, differing essentially only in the absence of acyclic n-alkanes and isoprenoids because of different stages of biodegradation. The presence of TPP and β-carotane in samples A and C allows us to classify them as mixed origin, with characteristics of lacustrine freshwater and saline depositional environments. Biomarkers associated with dinoflagellates of marine origin in the saturated fraction (dinosteranes) and the aromatic fraction (triaromatic dinosteranes) were detected in all three samples and suggest dinoflagellates were important primary producers during deposition time. 25-Norhopanes were detected only at low concentrations, and biomarker-based geochemical parameters indicate the biodegradation did not change the distribution of steranes and hopanes. The 25-Norhopanoic acids detection also agree with the detection of 25-norhopanes in the A and C oils, supports the same order of biodegradation, C > A ≫ B. The low abundance of tricyclic terpanoic acids in oil B acidic fraction agrees with the low abundance of tricyclic terpanes into the corresponding neutral fraction. Unlike hopanes, hopanoic acids distribution shows the ββ-isomers, biologically and thermally less stable configuration. Especially in oil B, series of 3-carboxysteranes, 3-carboxymethylsteranes were observed. In summary, acidic fractions data from three samples indicate that acidic biomarkers provide parallel information and complementary to that obtained from the hydrocarbon biomarkers. The extraction methods, derivatization and analysis of carboxylic acids from oil samples proved to be efficient, leading to quantitative recoveries and good yields.

A novel redox indicator based on relative abundances of C31 and C32 homohopanes in the Eocene lacustrine Dongying Depression, East China

A suite of oils and bitumens from the Eocene Shahejie Formation (Es) in Dongying Depression was geochemically characterized to illustrate impact of source input and redox conditions on distributions of pentacyclic terpanes. The Es4 developed under highly reducing, sulfidic hypersaline conditions, while Es3 formed under dysoxic, brackish to freshwater conditions. Oils derived from Es4 are enriched in C32 homohopanes (C32H), while those from Es3 are prominently enriched in C31 homohopanes (C31H). The C32H/C31H ratio shows positive correlation with homohopane index (HHI), gammacerane index, and negative correlation with pristane/phytane ratio, and can be used to evaluate oxic/anoxic conditions during deposition. High C32H/C31H ratio (> 0.8) is an important characteristic of oils derived from anoxic environments, while low values (< 0.8) indicate dysoxic conditions and extremely low values (< 0.4) indicate strong oxic conditions. The C32H/C31H ratio can be applied for deposition condition diagnosis because carboxyl group of C32 hopanoic acids might be reduced to C32 homohopanes under anoxic conditions, and oxidized to C31 homohopane under oxic conditions. Advantages to use C32H/C31H ratio as redox condition proxy compared to the HHI and gammacerane indexes are wider valid maturity range, less sensitive to biodegradation influence and better differentiating reducing from oxic environments.

Characterization of Severely Biodegraded Crude Oils Using Negative-Ion ESI Orbitrap MS, GC-NCD and GC-SCD: Insights into Heteroatomic Compounds Biodegradation

A slightly and two severely biodegraded crude oils with the same origin were analysed using negative-ion electrospray ionization Orbitrap mass spectrometry (ESI Orbitrap MS), gas chromatography-nitrogen chemiluminescence detector (GC-NCD), and GC-sulfur chemiluminescence detector (GC-SCD) to investigate the composition of heteroatomic compounds and their fate during severe biodegradation and to provide insights into biodegradation pathway of hopanes, nitrogen- and sulfur-containing compounds. Twelve heteroatomic compound classes, including O1–O5, N1, N2, N1O1–N1O3, N1S1 and O3S1, were detected and assigned unambiguous molecular formulae. The O1 species are likely phenols with additional naphthenic and/or aromatic rings. Carboxylic acids (O2 species) are originated from oxidation of hydrocarbons, and the tricyclic naphthenic acids are the most resistant, followed by bicyclics. Hopanes could be biodegraded by demethylation or by unstable hopanoic acids as intermediates to yield 25-norhopanes. The N1 species are pyrrolic compounds with naphthenic and/or aromatic rings and are dominated by carbazole analogues. Carbazoles with more aromatic rings are more resistant to biodegradation. The N1 species could be converted to N1O1 and N1O2 compounds via ring-opening and hydroxylation pathways. The N1S1 species contain a pyrrolic and cyclic sulfide structure, which are highly recalcitrant to biodegradation. Benzothiophenes and dibenzothiophenes might be biodegraded via the complete pathway or the sulfur-specific pathway rather than by other pathways to yield acidic oxygenated sulfur compounds.

New paleoenvironmental proxies for the Irati black shales (Paraná Basin, Brazil) based on acidic NSO compounds revealed by ultra-high resolution mass spectrometry

Variations of the acidic NSO (Nitrogen, Sulphur, and Oxygen) compound composition of the Lower Permian Irati black shales and Serra Alta shales were assessed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) using electrospray ionization (ESI) in negative ion mode to test their significance for the regional paleoenvironmental reconstruction by comparison with known features in the northeastern and central-eastern Paraná Basin, Brazil. The high abundance of the S1Ox class for the basal Irati black shales in the northeastern basin reflects a sulfide-rich environment, whereas high O>2 classes in the Serra Alta shales indicate a high input of terrestrial organic matter deposited in oxic waters. Here, eight parameters based on O1 and O2 compounds are suggested as new paleoenvironmental proxies: phenol index (%DBE 4; O1 class); C27/C28 DBE 4 (O1 class); C27/C28 DBE 5 (O1 class); Even/OddFA; TARFA Odd (terrigenous/aquatic ratio); TARFA Even; C36 hopanoic acid index; and hopanoic/steranoic acids ratio. Higher values of the phenol index, the TARFA and Even/OddFA indicate higher land plant input during the final black shale deposition. Variations of C27/C28 DBE 4 and C27/C28 DBE 5, the first being based on the distribution of methylated isoprenoidyl phenols, can be used to reconstruct paleosalinity; here higher values indicate higher salinity. The C36 hopanoic acid index is higher for the marine hypersaline samples from the northeastern basin, while a significant bacterial biomass signal is stored as a higher hopanoic/steranoic acids ratio for samples from the central-eastern basin.

The contribution of prokaryotes and terrestrial plants to Maldives inter-atoll sapropels: Evidence from organic biomarkers

The composition of organic matter in early Miocene sediment cores obtained from International Ocean Discovery Program Expedition 359 (IODP Exp. 359) was analysed using tetramethylammonium hydroxide (TMAH) thermochemolysis, a method used to determine concentrations of lignin phenols, fatty acids, sterols and hopanoids. These early Miocene sediment cores contain thick sequences (∼100 m) of interbedded light (organic-poor) and dark (organic-rich) sedimentary layers. A total of 49 samples from cores between 704.4 mbsf to 803.95 mbsf (∼20–23 Ma) at Site U1466 (518 m water depth) was selected with particular emphasis placed on the dark layers, although the light layers were also sampled. While organic compounds were below detection limits in the lighter (white) coloured sedimentary layers, the dark layers, with concentrations of organic carbon up to 20%, contained varying concentrations of all analysed compounds. Although sterols were present, hopanoids were more abundant in the dark layers. The contribution of prokaryotic organisms, estimated from the ratio ([hopanols and hopanoic acids]/[sterols + hopanols and hopanoic acids]), indicates that it was prokaryotic organisms rather than eukaryotic organisms that contributed the majority (∼90%) of the organic matter in the dark layers. In conjunction with this record, more negative δ15N values in the darker layers suggest that the prokaryotic organisms were nitrogen-fixing microorganisms (presumably cyanobacteria). The long-to-short fatty acid ratios, together with sterol distributions and the concentration of lignin phenols, suggest the input of terrestrial plant material occurred during a high total organic carbon (TOC) interval (∼770–810 mbsf), which coincides with more negative carbon isotope ratios. Additionally, the long-to-short fatty acid ratio in this interval was significantly higher in the dark layers than the light layers, suggesting that terrestrial environments suitable for vegetation growth expanded during the interval represented by the dark layers. Furthermore, the ratio of cinnamyl lignin phenols to vanillyl lignin phenols (C/V) is relatively high during these intervals. From these biomarker tracers, we speculate that the terrestrial vegetation was dominated by herbaceous plants adapted to frequent sea-level changes.

Novel Hopanoic Acid and Depside from the Lichen Dirinaria applanata

Novel Hopanoic Acid and Depside from the Lichen Dirinaria applanata

Exceptional preservation of hopanoid and steroid biomarkers in Ediacaran sedimentary rocks of the East European Craton

Thermally unstable hopanes with a biological 17β,21β (H)- configuration (ββ hopanes), hopenes, hopanoic acids and hopanols were identified as important constituents of Ediacaran to Cambrian sedimentary rocks of the East European Craton. Relatively high abundances of ββ hopanes in relation to αβ hopanes were identified in the Petersburg area and eastern Belarus, while in Volyn samples these compounds were found in traces. In addition, polar hopanoids including hopanols and hopanoic acids were found in most of the Petersburg and some Belarus and Volyn sedimentary rocks. The estimated equivalent of vitrinite reflectance for samples of lower maturity, measured based on C31ββ/(αβS + αβR + ββ) ratio is in the range 0.28–0.49% Rr, while for those of higher thermal maturity this parameter corresponds to 0.41–0.57% Rr. The values of the C31 22S/(S + R) ratio are in agreement with above data and are in the range of 0.1–0.3 for the Belarus and Petersburg samples, of 0.3–0.4 for Volyn, and of 0.4–0.5 for Lithuania, where ββ hopanes and hopanols were not detected or are present as traces. Moreover, there is good correlation (R2 = 0.8) between C31ββ/(αβS + αβR + ββ) and C31ENE/(H + ENE) ratio values (defined as ratio of C31 hopenes to αβ hopanes), which proves that less-stable ββ hopanes and hopenes are enriched in the same immature sedimentary rocks. The remarkable occurrence of a significant portion of sedimentary hopanes retaining the biological stereochemical configuration confirms that exceptionally immature Precambrian rocks can be found in some instances when these strata have undergone only a mild burial temperature history.

Fatty acids and other biomarkers in two Early Jurassic concretions and their immediate host rocks (Lias δ, Buttenheim clay pit, Bavaria, Germany)

Two carbonate- and phosphate-rich concretions from Early Jurassic strata at Buttenheim (Bavaria, Germany) and their immediate claystone host rocks were analysed to study the mechanisms of biomarker preservation in concretions. Superimposed on a common sedimentary background signal, distinctive biomarkers reflect microbial processes involved in concretion formation. When normalized to the HCl-insoluble insoluble rock portion, concretions show ∼1.5–3 times higher TOC values than their host rocks, but identical absolute quantities of most extractable hydrocarbons. In contrast, n-fatty acids and α,ω-dicarboxylic acids are, by an order, more abundant compared to the host rocks, suggesting an enhanced preservation or selective accumulation mechanism for carboxylic acids in the concretions. Further, linear and iso-/anteiso-fatty acids with unusually short carbon chains (as low as C8) are exclusively observed in the concretions and are interpreted as biodegradation products of lipids derived from in-situ bacteria. Likewise, strongly enhanced neohop-13(18)-enes and hopanoic acids with more ‘immature’ isomer distributions indicate an in-situ contribution of microbial biomass during formation of the concretions. Pyrolysis released ample additional saturated short-chain n-fatty acids from the kerogens of the concretions, but not from the host rocks. We hypothesize that these compounds are remnants of locally accumulated OM rich in fatty acyl moieties (e.g. plant polymers or triglyceride storage lipids) whose hydrolysis products were initially precipitated as fatty acid salts (‘adipocere’). During shallow burial, these substances may have delivered a steadily flowing energy source for sedimentary microorganisms, with their anaerobic decomposition supplying both carbonate and metal ions. In concert with other factors, this may have promoted carbonate precipitation and thus, the formation of the Buttenheim concretions.

Distributions of geohopanoids in peat: Implications for the use of hopanoid-based proxies in natural archives

Hopanoids are pentacyclic triterpenoids produced by a wide range of bacteria. Within modern settings, hopanoids mostly occur in the biological 17β,21β(H) configuration. However, in some modern peatlands, the C31 hopane is present as the ‘thermally-mature’ 17α,21β(H) stereoisomer. This has traditionally been ascribed to isomerisation at the C-17 position catalysed by the acidic environment. However, recent work has argued that temperature and/or hydrology also exert a control upon hopane isomerisation. Such findings complicate the application of geohopanoids as palaeoenvironmental proxies. However, due to the small number of peats that have been studied, as well as the lack of peatland diversity sampled, the environmental controls regulating geohopanoid isomerisation remain poorly constrained. Here, we undertake a global approach to investigate the occurrence, distribution and diagenesis of geohopanoids within peat, combining previously published and newly generated data (n = 395) from peatlands with a wide temperature (−1 to 27 °C) and pH (3–8) range. Our results indicate that peats are characterised by a wide range of geohopanoids. However, the C31 hopane and C32 hopanoic acid (and occasionally the C32 hopanol) typically dominate. C32 hopanoic acids occur as αβ- and ββ-stereoisomers, with the ββ-isomer typically dominating. In contrast, C31 hopanes occur predominantly as the αβ-stereoisomer. These two observations collectively suggest that isomerisation is not inherited from an original biological precursor (i.e. biohopanoids). Using geohopanoid ββ/(αβ + ββ) indices, we demonstrate that the abundance of αβ-hopanoids is strongly influenced by the acidic environment, and we observe a significant positive correlation between C31 hopane isomerisation and pH (n = 94, r2 = 0.64, p < 0.001). Crucially, there is no correlation between C31 hopane isomerisation and temperature. We therefore conclude that within peats, αβ-hopanoids are acid-catalysed diagenetic products and their occurrence at shallow depths indicates that this isomerisation is rapid. This shows that geohopanoid ββ/(αβ + ββ) indices can be used to reconstruct pH within modern and ancient peat-forming environments. However, we only recommend using ββ/(αβ + ββ) indices to interrogate large amplitude (>1 pH unit) and longer-term (>1 kyr) variation. Overall, our findings demonstrate the potential of geohopanoids to provide unique new insights into understanding depositional environments and interpreting terrestrial organic matter sources in the geological record.

Negative electrospray Fourier transform ion cyclotron resonance mass spectrometry determination of the effects on the distribution of acids and nitrogen-containing compounds in the simulated thermal evolution of a Type-I source rock

The thermal maturity level impact on polar compounds was analyzed for a suite of hydrous pyrolysis (HP) products using a 7.2 Tesla LTQ FTICR-MS instrument. The sample suite was analyzed via electrospray ionization in the negative ion mode focusing on the polar compounds, i.e., nitrogen-, sulfur-, and oxygen-containing (NSO) compounds. The HP experiments were performed under isothermal conditions for 2 h at eleven different temperatures (300, 310, 320, 325, 330, 340, 345, 350, 355, 360 and 365 °C) to simulate the full range of thermal levels starting at early bitumen generation to maximum oil generation. The maturity suite samples consist of one immature bitumen (original sample), eleven expelled oil samples and eight residual bitumen samples. In general, Ox compounds are destroyed by decarboxylation and dehydration with increasing maturity; sulfur compounds decrease; the aromaticity and degree of condensation increase; and steranoic and hopanoic acids decrease. Additionally, we investigated fluid retention properties comparing the carbon number distribution between the expelled oil and residual bitumen samples from each experiment. The maturity related changes of the acidic O2 compounds in the expelled oil samples led us to create several regressions that we propose as new maturity parameters covering the full range of oil generation.

Long-chain alkenones in Baltic Sea surface sediments: New insights

C37 alkenones produced by certain haptophytes of the Isochrysidales are valuable sedimentary biomarkers used to estimate sea surface temperature (SST) in the open ocean. However, in coastal seas the role of salinity gradients on alkenone producing species and SST estimates is poorly known. Alkenones were analyzed in surface sediments from the marine Skagerrak region and the entire brackish Baltic Sea. Three types of alkenone distribution patterns were identified: type A distribution, which resembles the distribution in Emiliania huxleyi, type B distribution, which is similar to Ruttnera lamellosa, Isochrysis galbana and Pseudoisochrysis paradoxa distributions, although these haptophytes are absent from the Baltic Sea, and type C distribution, which is also found in worldwide lake sediments. These types of distribution are apparent in the percentage of C37:4 alkenone (%C37:4), which is significantly negatively correlated to sea surface salinity (SSS). The distribution of alkenones very likely results from distinct alkenone-producing haptophytes, whose spatial distribution is ultimately related to SSS, as supported by the hydrogen isotope fractionation (α) between alkenones and water. U37K and U37K″ correlate more significantly than U37K′ with both SST and SSS, probably due to the superimposed effect of changing alkenone-producing species. The application of U37K and U37K″ as SST proxies results in unrealistic Holocene temperature records for the southern Baltic Sea, but %C37:4 as a SSS proxy reveals reasonable salinity changes. Interestingly, a C32 hopanoic acid was found abundantly as the methyl ester in the ketone fractions and may represent a marker for (cyano)bacterial biomass in the Baltic Sea.

Origin and history of the oils in the Lawton oil field, southwestern Oklahoma

The Lawton oil field is located on the Wichita Mountain uplift in southwestern Oklahoma. This study focused on addressing three problems related to the Lawton oil field: (a) what is (are) the source(s) of the oils; (b) have the oils been subjected to alteration since emplacement; and (c) what is the filling history of the field? Eighteen oils and one seep sample were collected and characterized by a variety of geochemical techniques including bulk composition, biomarkers, and stable carbon isotopes in order to address these questions. Source rocks in this region are absent following the Pennsylvanian-age Wichita orogeny but biomarkers and stable isotopic data suggest the source of these oils is probably the Woodford Shale. Despite the fact that all of these oils were recovered from reservoirs at depths between 125 and 1,000 ft (38 and 305 m), none of them appeared to be extensively biodegraded as manifested by the abundance of the complete range of n-alkanes. However the presence of 25-norhopanes in all eighteen samples suggests that these oils are mixtures of various proportions of degraded and non-degraded (or slightly degraded) oils. The seep sample, and some oils, have a relatively high abundance of the C32 ββ 22R hopanoic acid, relative to the C31 ββ 22R hopanoic acid, with the presence of these ββ 22R hopanoic acids suggesting the oils have been degraded aerobically at some point in their history. It was also noted that the concentration of the C31 ββ 22R hopanoic acid is higher in the shallower reservoirs than in the deeper reservoirs possibly indicating slightly higher levels of degradation in the shallower reservoirs compared to the deeper reservoirs. A hydrocarbon accumulation model has been proposed for this field based on two main reservoir charging periods. The initial charge of crude oil was biodegraded, probably under anaerobic conditions. The second charging period took place after (or together with) the uplift of the Wichita Mountains and subsidence of the southern part of the Anadarko Basin leading to deeper burial of the source rock and production of higher maturity oil, which subsequently filled the reservoirs containing the degraded oil and may still be recharging these shallow reservoirs today.

Insoluble prokaryotic membrane lipids in a Sphagnum peat: Implications for organic matter preservation

Preservation of organic matter (OM) in the geosphere has a direct impact on carbon bioavailability, the carbon cycle and the formation of fossil fuels. We have examined some of the processes that lead to the preservation of OM by characterising insoluble OM in a Sphagnum peat bog. We focussed on the partitioning of prokaryotic biomarkers between solvent-extractable and insoluble OM fractions and how that partitioning changed with depth. The insoluble organic matter (IOM) was examined using stepwise chemical degradation involving base and acid hydrolysis. Iso- and anteiso-C15 and C17 fatty acids (FAs), hopanoic acid and bishomohopanol, and branched glycerol dialkyl glycerol tetraethers (GDGTs) – diagnostic for Bacteria – were targeted as well as archaeol and isoprenoidal GDGTs – diagnostic for Archaea. High percentages of these compounds – up to 65% – occur in IOM pools, indicating that archaeal- and bacterially derived OM is prone to insolubilization even in shallow sediments (<5cm depth). Differences in functionalities – likely related to intact polar (IP) head groups – seem to determine the insolubilization of prokaryotic lipids; specifically, we propose that compounds bearing rapidly degradable phosphate-based polar head groups are less likely to be incorporated into the insoluble OM pools. Collectively, these data indicate that microbial membrane lipids are rapidly incorporated into peat IOM during early diagenesis. We suggest that this is due to an inherently recalcitrant character of some prokaryotic cells which affords protection to their membrane lipids. However, this large proportion of insoluble prokaryotic lipids does not appear to be entirely stable at shallow burial depths, indicating that the peat IOM is a dynamic reservoir, at least during early diagenesis.

Formation of seep carbonates along the Makran convergent margin, northern Arabian Sea and a molecular and isotopic approach to constrain the carbon isotopic composition of parent methane

Authigenic carbonate deposits have been sampled with the remotely operated vehicle ‘MARUM-QUEST 4000m’ from five methane seeps between 731 and 1823m water depth along the convergent Makran continental margin, offshore Pakistan (northern Arabian Sea). Two seeps on the upper slope are located within the oxygen minimum zone (OMZ; ca. 100 to 1100m water depth), the other sites are situated in oxygenated water below the OMZ (below 1100m water depth). The carbonate deposits vary with regard to their spatial extent, sedimentary fabrics, and associated seep fauna: Within the OMZ, carbonates are spatially restricted and associated with microbial mats, whereas in the oxygenated zone below the OMZ extensive carbonate crusts are exposed on the seafloor with abundant metazoans (bathymodiolin mussels, tube worms, galatheid crabs). Aragonite and Mg-calcite are the dominant carbonate minerals, forming common early diagenetic microcrystalline cement and clotted to radial-fibrous cement. The δ18Ocarbonate values range from 1.3 to 4.2‰ V-PDB, indicating carbonate precipitation at ambient bottom-water temperature in shallow sediment depth. Extremely low δ13Ccarbonate values (as low −54.6‰ V-PDB) point to anaerobic oxidation of methane (AOM) as trigger for carbonate precipitation, with biogenic methane as dominant carbon source. Prevalence of biogenic methane in the seepage gas is corroborated by δ13Cmethane values ranging from −70.3 to −66.7‰ V-PDB, and also by back-calculations considering δ13Cmethane values of carbonate and incorporated lipid biomarkers. These calculations (Δδ13Cmethane–carbonate, Δδ13CANME–methane, Δδ13CMOX–methane) prove to be useful to assess the carbon stable isotope composition of seeping methane if this has not been determined in the first place; such an approach represents a useful tool to reconstruct fluid composition of ancient seeps. AOM is also revealed by lipid biomarkers of anaerobic methane oxidizing archaea such as crocetane, pentamethylicosane (PMI), and sn2-hydroxyarchaeol strongly depleted in 13C (δ13C values as low as −127‰ V-PDB). Biomarkers of sulphate-reducing bacteria are also abundant, showing slightly less negative δ13C values, but still significantly 13C-depleted (average values as low as −101‰). Other bacterial biomarkers, such as bacteriohopanepolyols (BHPs), hopanols, and hopanoic acids are detected in most carbonates, but are particularly common in seep carbonates from the non-OMZ sites. The BHP patterns of these carbonates and their low δ13C values resemble patterns of aerobic methanotrophic bacteria. In the shallower OMZ sites, BHPs revealed much lower contents and varying compositions, most likely reflecting other sources than aerobic methanotrophic bacteria. 230Th/U carbonate ages indicate that AOM-induced carbonate precipitation at the deeper non-OMZ seeps occurred mainly during the late Pleistocene–Holocene transition, i.e. between 19 and 15ka before present, when the global sea level was lower than today.

Methanogenesis produces strong 13C enrichment in stromatolites of Lagoa Salgada, Brazil: a modern analogue for Palaeo-/Neoproterozoic stromatolites?

Holocene stromatolites characterized by unusually positive inorganic δ(13) CPDB values (i.e. up to +16‰) are present in Lagoa Salgada, a seasonally brackish to hypersaline lagoon near Rio de Janeiro (Brazil). Such positive values cannot be explained by phototrophic fixation of CO2 alone, and they suggest that methanogenesis was a dominating process during the growth of the stromatolites. Indeed, up to 5mm methane was measured in the porewater. The archaeal membrane lipid archaeol showing δ(13) C values between -15 and 0‰ suggests that archaea are present and producing methane in the modern lagoon sediment. Moreover, (13) C-depleted hopanoids diplopterol and 3β-methylated C32 17β(H),21β(H)-hopanoic acid (both -40‰) are preserved in lagoon sediments and are most likely derived from aerobic methanotrophic bacteria thriving in the methane-enriched water column. Loss of isotopically light methane through the water column would explain the residual (13) C-enriched pool of dissolved inorganic carbon from where the carbonate constituting the stromatolites precipitated. The predominance of methanogenic archaea in the lagoon is most likely a result of sulphate limitation, suppressing the activity of sulphate-reducing bacteria under brackish conditions in a seasonally humid tropical environment. Indeed, sulphate-reduction activity is very low in the modern sediments. In absence of an efficient carbonate-inducing metabolic process, we propose that stromatolite formation in Lagoa Salgada was abiotically induced, while the (13) C-enriched organic and inorganic carbon pools are due to methanogenesis. Unusually, (13) C-enriched stromatolitic deposits also appear in the geological record of prolonged periods in the Palaeo- and Neoproterozoic. Lagoa Salgada represents a possible modern analogue to conditions that may have been widespread in the Proterozoic, at times when low sulphate concentrations in sea water allowed methanogens to prevail over sulphate-reducing bacteria.

Temperature and pH control on lipid composition of silica sinters from diverse hot springs in the Taupo Volcanic Zone, New Zealand.

Microbial adaptations to environmental extremes, including high temperature and low pH conditions typical of geothermal settings, are of interest in astrobiology and origin of life investigations. The lipid biomarkers preserved in silica deposits associated with six geothermal areas in the Taupo Volcanic Zone were investigated and variations in lipid composition as a function of temperature and pH were assessed. Lipid analyses reveal highly variable abundances and distributions, reflecting community composition as well as adaptations to extremes of pH and temperature. Biomarker profiles reveal three distinct microbial assemblages across the sites: the first in Champagne Pool and Loop Road, the second in Orakei Korako, Opaheke and Ngatamariki, and the third in Rotokawa. Similar lipid distributions are observed in sinters from physicochemically similar springs. Furthermore, correlation between lipid distributions and geothermal conditions is observed. The ratio of archaeol to bacterial diether abundance, bacterial diether average chain length, degree of GDGT cyclisation and C31 and C32 hopanoic acid indices typically increase with temperature. At lower pH, the ratio of archaeol to bacterial diethers, degree of GDGT cyclisation and C31 and C32 hopanoic acid indices are typically higher. No trends in fatty acid distributions with temperature or pH are evident, likely reflecting overprinting due to population influences.

Constraining the genetic relationships of 25-norhopanes, hopanoic and 25-norhopanoic acids in onshore Niger Delta oils using a temperature-dependent material balance

Analysis of oil samples from the Niger Delta (Nigeria) revealed a range of structurally related hopanes, including 25-norhopanes, and hopanoic and 25-norhopanoic acids. 25-Norhopanes were detected in all medium and heavily biodegraded oils and were most abundant in the heavily degraded oils. Hopanoic acids (C30-C33) and 25-norhopanoic acids (C30-C31) were most abundant in moderately degraded oils and occurred in reduced concentration in heavily degraded oils but were absent from, or in trace concentration in, slightly degraded oils. Consideration of the structures suggests that 25-norhopanoic acids form via carboxylation of 25-norhopanes or demethylation of hopanoic acids. Mass balance for the onshore Niger Delta oils suggests that formation of 25-norhopanes operates independently of 25-norhopanoic acid formation and that 25-norhopanoic acids are likely transient intermediates for only a small proportion of the 25-norhopanes.