Long-chain unsaturated ketones in recent lacustrine sediments

C₃₇–C₃₉ ketones containing 2–4 double bonds and similarly-unsaturated C₃₈–C₄₀ ethyl ketones have been detected for the first time in lacustrine sediments. Tetra-unsaturated compounds are relatively more abundant than in marine sediments. The similar molecular composition of methyl ketones and cooccurring ethyl ketones containing one additional carbon atom indicates that the two classes are biosynthetically related.No source of these potential biological markers in lacustrine sediments has been identified. The abundance and unsaturation pattern of alkenones in lacustrine sediments may reflect the relative input and species composition, respectively, of algae presumed to belong to the Phylum Chrysophyta, by analogy with the marine source.     

n-Alkan-2-ones in peat-forming plants from the Roñanzas ombrotrophic bog (Asturias, northern Spain)

We determined the distribution of lipids (n-alkanes and n-alkan-2-ones) in present-day peat-forming plants in the Roñanzas Bog in northern Spain. Consistent with the observation of others, most Sphagnum (moss) species alkanes maximized at C₂₃, whereas the other plants maximized at higher molecular weight (C₂₇ to C₃₁). We show for the first time that plants other than seagrass and Sphagnum moss contain n-alkan-2-ones. Almost all the species analysed showed an n-alkan-2-one distribution between C₂₁ and C₃₁ with an odd/even predominance, maximizing at C₂₇ or C₂₉, except ferns, which maximized at lower molecular weight (C₂₁-C₂₃). We also observed that microbial degradation can be a major contributor to the n-alkan-2-one distribution in sediments as opposed to a direct input of ketones from plants.     

Lipids of aquatic organisms as potential contributors to lacustrine sediments—II

The relationship between the lipid composition of organisms in the water column of an eutrophic lake and the lipid composition of underlying sediments, previously examined for n-alkanols and steroids, is now reported for hydrocarbons, ketones and carboxylic acids.The n-C₇ alkane and alkenoic acids from two primary sources are rapidly metabolized in the water column and surficial sediment. Bacterial biomarkers, including hopenes and fatty acids, were detected in the photosynthetic bacterial layer occurring just above the sediment-water interface.Within the sediment the apparent conversion of free n-alkanes, alkan-2-ones and ω-hydroxy acids to the corresponding bound form is noted; microbiological oxidation of n-alkanes to alkan-2-ones is supported by the detection of the intermediate alkan-2-ols with a distribution similar to that of the ketones. The geochemistry of sediment deposited c. 1900, prior to biological study of the site, was interpreted from stable biomarkers and the diagenetic changes recognised in the study of contemporary deposition. A qualitative difference in algal input to the older sediment is inferred from the low Δ⁷-sterol content and presence of 2,6,10-trimethyl-7-(3-methylbutyl)-dodecane. However, there was still significant dinoflagellate input, as indicated by the presence of 4α-methylsterols. A difference in higher-plant input to the older sediment, indicated from the n-alkane, alkene and triterpenoid ketone distributions, is consistent with the recent development of tree cover.     

Lipid geochemistry in a sediment core from Ruoergai Marsh deposit (Eastern Qinghai-Tibet plateau, China)

Peats in a sediment core from Ruoergai bog, which has a cold and moist plateau climate with major source input from herbaceous plants, have been studied by GC–MS in order to understand the composition and diagenetic processes of lipids in this depositional environment. Long chain components (C₂₁–C₃₅) predominate in the n-alkanes, n-alk-1-enes, n-fatty acids, n-alkan-2-ones and n-alkanols with a maximum of C₃₁, C_27, C₂₂ or C_24, C₂₃ or C₂₅ and C₂₂, respectively. A herbaceous origin for these long chain compounds is suggested, and this is supported by their stable carbon isotopic compositions. Diterpenoid hydrocarbons with abietane, pimarane and kaurane skeletons, some of which have not been reported often in modern sediments, are prominent and are derived from higher plants. Several triterpenoid ketones and alcohols with oleanane or lupane skeletons, and a series of des-A-triterpenoid hydrocarbons which have not been reported often in modern sediments are also present, and are assigned to a higher plant source. Hopanoids, including their alkanes, alkenes, ketones, alcohols and esters, are abundant and of bacterial origin. Steroid ketones and alkanols are dominated by C₂₉ homologues. C₂₈ and C₂₉ steroids are derived mainly from higher plants, whereas the C₂₇ component is assigned to a microbial source. The presence of short-chain n-alkanes with no odd-even carbon predominance, bacterially derived fatty acids (C₁₄, C₁₅, iso- and unsaturated acids), n-alkan-2-ones, des-A-triterpenoid hydrocarbons, hopanoids and some steroid ketones indicate that intense microbial reworking of the organic matter has taken place in this depositional environment. The chemical and biochemical conversions of some cyclic alkenes to alkanes, such as tricyclic diterpenoids, tetracyclic terpenoids and steroid ketones, are also evident with depth. The dominance of C₂₀ components in the diterpenoid hydrocarbons may reflect an oxidizing or reducing depositional condition.     

Unprecedented long chain 1-chloroalkenes and 1-chloroalkanes in the Holocene sediments of Lake El Junco,Galápagos Islands

The occurrence of long chain chloroalkenes and chloroalkanes has not been previously reported in natural sediments. Here we report the presence of long chain (C₃₀–C₃₆) n-chloroalkanes and chloroalkenes in the recent sediments of the only permanent freshwater lake in the Galápagos Islands, El Junco. Both chloroalkanes and chloroalkenes have a chlorine atom at the terminal position. The cis and trans alkenyl chlorides have double bonds near the middle of the hydrocarbon chain (ω15, 16, 17). Alkenes in the lake sediments are typically C_23:1 to C_27:1, substantially shorter than the chloroalkenes and chloroalkanes. The lipid chain lengths and the positions of functional groups imply a structural relationship between chloroalkenes and some alkenols, diols and (or) hydroxy acids that most likely derive from algae.     

Controls on alkenone unsaturation ratios along the salinity gradient between the open ocean and the Baltic Sea

Alkenone unsaturation ratios of sedimentary lipids are used as a geochemical proxy for sea surface temperatures, and interest is growing in their potential as indicators of different water masses and possibly of salinity. We analyzed the abundance of unsaturated C₃₇ to C₃₈ ketones in lipid extracts of 57 surface sediment (0-1 cm) samples along a salinity gradient from 8 to 33 psu in the transition from the Skagerrak to the Baltic Sea (NW Europe). In addition to surface sediments, we analyzed alkenones in suspended particulate matter at 13 stations—over a gradient in salinity from 25 to 33 psu—during a bloom of the coccolithophore Emiliania huxleyi. Alkenones were detected in all samples (suspended matter and sediment) with variable contributions of the tetra-unsaturated C₃₇ alkenone compound (%C_37:4; range from 2 to 10% of total C₃₇ alkenone content). Comparing the alkenone unsaturation index (U₃₇^K′) and %C_37:4 data to climatological sea surface temperature and sea surface salinity data sets revealed that SST estimated from U₃₇^K′ of saline end members (samples from the Skagerrak) is in the general range of modern SST during bloom periods of haptophytes. At salinities below ∼30 psu %C_37:4 increases to above 5% and the unsaturation ratios cease to be related to climatological annual or seasonal sea surface temperatures. On the other hand, the %C_37:4 appears to be inversely and significantly correlated to salinity: Highest C_37:4 proportions in the inner Baltic Sea are caused by an unidentified organism, but in the transition area at salinities down to 10 psu, the producer apparently is E. huxleyi. The suspended matter data together with those from the water column support the hypothesis of changing biosynthesis of alkenones under salt stress by the coccolithophore E. huxleyi, but constrain the maximum of %C_37:4 attributable to salt stress to 10% of all C₃₇ alkenones.     

Impact of temperature on long chain diol and mid-chain hydroxy methyl alkanoate composition in Proboscia diatoms: Results from culture and field studies

Long chain 1,14-diols and 12-hydroxy methyl alkanoates are biomarker lipids for Proboscia diatoms and occur widely in Quaternary sediments. To determine the effect of temperature on the lipid composition of these algae, a new Proboscia sp. culture grown at 8 °C and Proboscia indica cultures grown at 18, 21, 24 and 27 °C were examined. The results were combined with lipid data from a P. indica culture and a Proboscia alata culture, grown at 20 and 2 °C, respectively, from previous studies. The data showed a strong relationship between long chain diol and 12-hydroxy methyl alkanoate composition and growth temperature, i.e. the chain length increases and the degree of unsaturation of long chain 1,14-diols decreases with increasing growth temperature. To determine the effect of temperature on Proboscia lipid compositions in natural environments, we also analyzed fossil long chain 1,14-diols and 12-hydroxy methyl alkanoates in surface sediments derived from Proboscia diatoms living in the water column of the eastern South Atlantic. The results indicate a significant relationship between sea surface temperature and chain length distribution of saturated long chain diols, but also suggest that the relative abundances of unsaturated long chain diols and 12-hydroxy methyl alkanoates in sediments are predominantly determined by factors other than temperature.     

Tri- and tetraterpenoid hydrocarbons in the Messel oil shale

The high molecular weight constituents of the branched and cyclic hydrocarbon fraction of the Messel oil shale (Eocene) have been examined by high resolution gas chromatography and combined gas chromatography-mass spectrometry. The following compounds are present: perhydrolycopene (1; lycopane), together with one or more unsaturated analogues with the same skeleton; a series of 4-methylsteranes (2c) in higher abundance than their 4-desmethyl analogues; two series of pentacyclic triterpanes, one series (C₂₇-C₃₂) based on the hopane structure (3a-e), and the other (C₂₇-C₃₁) based on the 17α-H hopane structure (3a-d, 17αH); and an intact triterpene hop-17 (21)-ene [3c, Δ 17(21)]. Only two additional triterpanes were detected in minor concentrations, viz. 30-normoretane (3b, 21αH) and a C₃₁ triterpane based on the hopane/lupane-type skeleton. The presence of these compounds suggests a significant microbial contribution to the forming sediment. Comparison of the tri- and tetraterpenoid hydrocarbons with those of the Green River Shale indicates differences in the organisms contributing to the two sediments.     

The lipid chemistry of an interfacial sediment from the Peru Continental Shelf: Fatty acids,alcohols, aliphatic ketones and hydrocarbons

A sample of the sediment-water column interface which lies on the continental shelf under the Peru upwelling regime, has been examined for fatty acids, fatty alcohols, ketones and hydrocarbons. Fatty acids were the most abundant compound class, ranging from C₁₂-C₂₄, with 16:0 as the major component (765.5 μg/g dry sediment). The alcohols were dominated by 3,7,11,15-tetramethylhexadeca-2-en-ol (phytol), with even-chain n-alcohols in the range C₁₄-C₂₀. The ketones consisted of C₃₇-C₃₉ di- and tri-unsaturated alken-2-ones and alken-3-ones. Both alkanes and alkenes were present in the hydrocarbon fraction; the alkanes ranging from C₁₃ — C₂₀ and comprising both straight chain and isoprenoid compounds; the alkenes consisting of isomeric pairs of C₂₅ branched trienes and tetraenes. The data indicate that the organic content has been contributed very largely from marine sources (probably mainly from phytoplankton and bacteria), showing little terrigenous influence. The presence of labile compounds such as polyunsaturated fatty acids (with two to six double bonds), implies that the sediment has undergone very little diagenetic alteration, and the lipids are probably largely unchanged from the state in which they actually reached the sediment. They may therefore serve as a useful baseline in assessing diagenesis in older sediments, where diagenetic transformations are more advanced.     

Carbon isotope composition of long chain leaf wax n-alkanes in lake sediments: A dual indicator of paleoenvironment in the Qinghai-Tibet Plateau

The carbon isotope composition (δ¹³C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C₃ and C₄ photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting δ¹³C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C₂₇ and C₂₉) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. −26.7‰ to −15.7‰ for C₂₉) similar to that of terrestrial C₄ plants. Thus, the use of δ¹³C values of sedimentary C₂₇ and C₂₉ n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C₄ plants. For sedimentary C₃₁, however, the contribution from submerged plants was minor, so that the δ¹³C values for C₃₁ n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the δ¹³C values of sedimentary C₂₇ and C₂₉ n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C₄ vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the δ¹³C values of sedimentary long chain n-alkanes (C₂₇, C₂₉ and C₃₁) may carry different environmental signals. While the δ¹³C values of C₃₁ were a reliable proxy for C₄/C₃ terrestrial vegetation composition, the δ¹³C values of C₂₇ and C₂₉ n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.     

Phosphate-free ornithine lipid contents in Desulfovibrio spp. respond to growth temperature

Eight sulfate-reducing Desulfovibrio strains isolated from intertidal sediments of the North Sea were investigated for their intact polar lipid (IPL) composition. They contained two types of IPLs, phospholipids and aminolipids. The dominating phospholipids were phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and, in lower concentration, diphosphatidylglycerol (DPG). Aminolipids formed a significant IPL fraction in all strains and high resolution tandem mass spectrometry assigned them as phosphate-free ornithine lipids (OLs). In intertidal flat sediments microorganisms may face strong temperature change on varying timescales and it is crucial for the microbes to maintain constant membrane fluidity, e.g. by modification of their membrane lipid composition. We therefore investigated whether or not these strains employ the same strategies for adjusting their membrane composition to growth temperature and, in particular, how OLs are modified. In all strains the relative OL content was found to be higher at higher growth temperature, in most cases at the expense of PE content and less often PG content. The fatty acid (FA) side chains of the main PE and PG species were similar, i.e. both were dominated by C₁₇ or C₁₈, whereas C₁₅ FAs were additionally found as major OL side chains. The temperature-related side chain variation was similar for all IPLs: unsaturated FA content was lower at higher temperature. The corresponding FA patterns after hydrolysis revealed elevated branched FA content and anteiso/iso ratio at higher growth temperature. As the temperature-related changes in the IPL side chains were similar for all strains, we conclude that side chain modification plays a major role in the maintenance of membrane fluidity at higher temperature and that alternative roles of OLs in the membrane adaptation of Desulfovibrio spp. other than melting point adjustment are possible.     

Volatile C1C7 organic compounds in an anoxic sediment core from the Pettaquamscutt River (Rhode Island,U.S.A.)

Surface sediments from an anoxic marine environment, the Upper Basin of the Pettaquamscutt River, Rhode Island, were analyzed for volatile organic compounds in the C₁C₇ range. The compounds identified included methane, ethane, alkenes (C₂C₅), carbon disulfide, cyclopentane, 3-methylpentane, methylfuran, aldehydes and ketones. Ethane, methylfuran, and most of the aldehydes and ketones showed maxima at the sediment water interface. Methane levels were very high-10–100 times greater than observed in most other surface sediments examined in this laboratory.     

n-Alkane parameters from a deep sea sediment transect off southwest Africa reflect continental vegetation and climate conditions

An isobathic transect of marine surface sediments from 1°N to 28°S off southwest Africa was used to further evaluate the potential of the chain length distribution and carbon stable isotope composition of higher plant n-alkanes as proxies for continental vegetation and climate conditions. We found a strong increase in the n-C_29–33 weighted mean average δ¹³C values from −33‰ near the equator to around −26‰ further south. Additionally, C_25–35 n-alkanes reveal a southward trend of increasing average chain length from 30.0 to 30.5. The data reflect the changing contribution of plants employing different photosynthetic pathways (C₃ and C₄) and/or being differently influenced by the environmental conditions of their habitat. The C₄ plant proportions calculated from the data (ca. 20% for rivers draining the rainforest, to ca. 70% at higher latitude) correspond to the C₄ plant abundance in continental catchment areas postulated by considering prevailing wind systems and river outflows. Furthermore, the C₄ plant contribution to the sediments correlates with the mean annual precipitation and aridity at selected continental locations in the postulated catchment areas, suggesting that the C₄ plant fraction in marine sediments can be used to assess these environmental parameters.     

Occurrence of long chain 1,14-diols in Apedinella radians

Long chain 1,14-diols have been reported in diatoms of the genus Proboscia and applied as specific biomarker lipids for such algae. We report here the presence of saturated C₂₈, C₃₀ and C₃₂ 1,14-diols in a culture of the marine heterokont alga Apedinella radians (Class Dictyochophyceae, order Pedinellales). Apedinella species occur globally, although predominantly in estuarine waters, so the finding has potential implications for the use of long chain 1,14-diols as biomarkers of Proboscia diatoms and as an indicator of upwelling.     

Latitudinal distribution of terrestrial lipid biomarkers and n-alkane compound-specific stable carbon isotope ratios in the atmosphere over the western Pacific and Southern Ocean

We investigated the latitudinal changes in atmospheric transport of organic matter to the western Pacific and Southern Ocean (27.58°N-64.70°S). Molecular distributions of lipid compound classes (homologous series of C₁₅ to C₃₅n-alkanes, C₈ to C₃₄n-alkanoic acids, C₁₂ to C₃₀n-alkanols) and compound-specific stable isotopes (δ¹³C of C₂₉ and C₃₁n-alkanes) were measured in marine aerosol filter samples collected during a cruise by the R/V Hakuho Maru. The geographical source areas for each sample were estimated from air-mass back-trajectory computations. Concentrations of TC and lipid compound classes were several orders of magnitude lower than observations from urban sites in Asia. A stronger signature of terrestrial higher plant inputs was apparent in three samples collected under conditions of strong terrestrial winds. Unresolved complex mixtures (UCM) showed increasing values in the North Pacific, highlighting the influence of the plume of polluted air exported from East Asia. n-Alkane average chain length (ACL) distribution had two clusters, with samples showing a relation to latitude between 28°N and 47°S (highest ACL values in the tropics), whilst a subset of southern samples had anomalously high ACL values. Compound-specific carbon isotopic analysis of the C₂₉ (−25.6‰ to −34.5‰) and C₃₁n-alkanes (−28.3‰ to −37‰) revealed heavier δ¹³C values in the northern latitudes with a transition to lighter values in the Southern Ocean. By comparing the isotopic measurements with back-trajectory analysis it was generally possible to discriminate between different source areas. The terrestrial vegetation source for a subset of the southernmost Southern Ocean is enigmatic; the back-trajectories indicate eastern Antarctica as the only intercepted terrestrial source area. These samples may represent a southern hemisphere background of well mixed and very long range transported higher plant organic material.     

Hydrocarbons in 60 northeast Gulf of Mexico shelf sediments: a preliminary survey

Hydrocarbon results from gas chromatography of 60 recent sediment and 10 benthic algae samples delineate two distinct shelf environments in the northeastern Gulf of Mexico.Sediments off Florida (shell hashes and sands) have moderate amounts of lipids/total sediment (average 113ppm ± 80%) but low hydrocarbon levels (average 3.06 ppm ± 41%). Aliphatic hydrocarbons are dominated by a series of branched or cyclic, unsaturated C₂₅ isomers. The major n-alkane is n-C₁₇. The n-alkane and isoprenoid patterns are consistent with a marine hydrocarbon source.Sediments closer to the Mississippi River (silts and clays) contain large amounts of lipids (average 232 ppm ± 53%) and hydrocarbons (average 11.7 ppm ± 55%) to total sediment. Aliphatic hydrocarbons are mainly odd carbon number high molecular weight n-alkanes, indicating a terrigenous hydrocarbon source. Isoprenoids are present in greater abundance than in sediments off Florida (n-C₁₇/ pristane and n-C₁₈/phytane ratios ~2to 3). Relatively large amounts of n-C₁₆, together with an even distribution of n-alkanes in the range C₁₄–C₂₀ and a substantial unresolved envelope all point to a fossil fuel input to the Mississippi samples.Samples off the Alabama coast show intermediate characteristics.     

Carboxylic acid distribution patterns of temperate C3 and C4 crops

Agricultural grasses cover a major part of the land surface in temperate agro-ecosystems and contribute significantly to the formation of soil organic matter. Crop-derived lipids are assumed to be responsible for fast carbon turnover in soils. Differences in lipid distribution patterns between crops following C₃ and C₄ photosynthesis pathways have rarely been described, but could be useful for source apportionment of crop-derived input into soils or sediments. The distribution of long chain n-carboxylic acids (C₂₂, C₂₄, C₂₆) reveals significant differences between crop plants following either the C₃ or the C₄ photosynthetic carbon fixation pathway. The plant compartments leaves, stems and roots of C₄ plants contain relatively large proportions (> 40%) of n-C₂₄ carboxylic acid when compared to C₃ plants. These reveal larger relative proportions of n-C₂₂ and n-C₂₆ acids, whose relative abundance is subject to change between different plant compartments and during the growing season. The carboxylic acid ratio [CAR = n-C₂₄/(n-C₂₂ + n-C₂₆) carboxylic acids] provides distinct ratios for C₄ (> 0.67) and C₃ crops (< 0.67) and can thus be used as a molecular marker for the differentiation of crop plant biomass. In combination with the bulk stable carbon isotopic composition (δ¹³C) the CAR can be used as a tool for the estimation of the C₄ derived carbon proportion in soils or sediments.     

Novel 23,28-bisnorlupanes in Tertiary sediments. Widespread occurrence of nuclear demethylated triterpanes

A pair of isomeric C₂₈-bisnorlupanes was detected in high abundance in the saturated hydrocarbon fractions of Tertiary sediments from West Greenland and the Gulf of Suez. The compounds were isolated by preparative gas chromatography and the isomers separated by liquid chromatography. On the basis of mass spectrometric and nuclear magnetic resonance data the structures were assigned to be 17α(H)- and 17ß(H)-23,28-bisnorlupane, respectively. The significance of nuclear demethylated triterpanes is discussed in terms of their generally high abundance, their uneven distribution within a given facies, and their worldwide occurrence.     

Geochemical significance of lipids and lipid-derived substructures interlaced in kerogen

Young kerogens isolated from seven freshwater lakes, six river mouths and four marine surface sediments were subjected to pyrolysis in vacuo. Their pyrolysates were trapped and separated subsequently for determination of hydrocarbons, fatty acids and alcohols. The abundances, carbon number distributions of long (C₁₂) polymethylene chain lipid compounds and relative abundances of pristenes are proposed as possible indices applicable to discrimination between young kerogens from freshwater lacustrine and marine sediments. Some oil-shale kerogens of Eocene and Permian age were pyrolyzed in the same way, where the chain-length distributions of the pyrolysis products showed similar trends as those observed for the pyrolysis of young kerogens.     

The identification of organic input sources of sediments from the santa catalina basin using factor analysis

Individual neutral lipids, including hydrocarbons, ketones, alcohols, sterols, keto-ols and diols have been analysed from eight horizons in a sediment core taken from the Santa Catalina Basin, Calif. Variations in the concentration of individual lipid components between sites are studied using factor analysis to determine their inputs. Several sources of organic material are postulated from the composition of hypothetical factors, including methanogenic bacteria, higher plants and algae including coccolithophores, dinoflagellates and diatoms. Sources for long chain keto-ols and diols and a C₃₅ bicyclic alkene are suggested as methanogenic bacteria and algae respectively.