The stereochemistry of 2- and 3-hydroxy fatty acids in a Recent lacustrine sediment

Homologous series of 2- and 3-hydroxy fatty acids occur in both the free and the bound lipid fractions of an oligotrophic lacustrine sediment (Loch Clair, N.W. Scotland). The stereochemistry of each constituent was assigned by gas chromatographic separation of diastereoisomeric O-2(S)-phenylpropionyl methyl ester derivatives. Free and bound 2-hydroxyacids above C₂₀ showed the R configuration characteristic of by-products formed during fatty acid metabolism by α-oxidation. In the C₁₄–C₁₈ region, bound 2- and 3-hydroxyacids show the R configuration more dominantly than the corresponding free lipids. These bound 2(R)- and 3(R)-hydroxyacids are attributed to microbial cell wall lipids.     

Tightly bound aliphatic acids in Lake Biwa sediments: Their origin and stability

Tightly bound geolipids were separated from a 200 m sediment core of Lake Biwa by a second saponification of sediments from which unbound and bound lipids had been extracted. Tightly bound fatty acids, β-hydroxy acids, ω-hydroxy acids and α,ω-dicarboxylic acids were released; their concentrations ranged from 4.7–31.5, 5.6–60.5, 3.4–9.5 and 0.2–5.4 μg/g dry sediment, respectively. These geolipids were considered to be incorporated in humic substances.Tightly bound fatty acids showed a unimodal distriution with a peak at C₁₆, suggesting that they originate from algae and bacteria and that they are more stable than unbound and bound fatty acids in the sediments. Most of the total β-hydroxy acids, which probably originate from bacteria, were found in the tightly bound fraction. This suggests that a large portion of tightly bound geolipids are formed in the water column and in surface sediments as a result of microbial alteration dead algae. Each class of tightly bound lipids showed higher concentrations between depths of 3 and 15 m in the sediments, where primary production was thought to be enhanced in the past.     

Early diagenesis of fatty acids in lacustrine sediments—I. Identification and distribution of fatty acids in recent sediment from a freshwater lake

Fatty acids have been isolated and quantitatively determined from a 1.5 m sediment core of Lake Suwa, a eutrophic lake in the central districts of Japan.The fatty acids identified by combined gas chromatography-mass spectrometry were straight-chain saturated (C₁₂ to C₃₄), monounsaturated with even carbon number (C₁₆ to C₂₄) and branched-chain (iso, anteiso, 10-methyloctadecanoic) acids. The concentrations of the higher molecular weight (? C₂₀) saturated fatty acids remained nearly constant throughout the core, suggesting a high degree of preservation of those acids, whereas the monounsaturated and the lower molecular weight saturated fatty acids indicated a great decrease in concentration with depth to an approximately 20cm level. It is suggested that the microbial activity in sediments causes a significant reconstruction of the fatty acid distribution during early diagenesis.     

Maturation related changes in the distribution of ester bound fatty acids and alcohols in a coal series from the New Zealand Coal Band covering diagenetic to catagenetic coalification levels

A rank series of lignites and coals of low to moderate maturation levels (vitrinite reflectance (R₀): 0.27–0.8%) from the New Zealand Coal Band were investigated using alkaline ester cleavage experiments to reveal compositional changes of ester bound components (fatty acids and alcohols) during increasing maturation. Ester bound alcohols are found to be present in highest amounts in the very immature lignite samples (R₀: 0.27–0.29%), but show a rapid decrease during early diagenesis. Ester bound fatty acids also show an initial exponential decrease during diagenesis, but reveal an intermittent increase during early catagenesis before decreasing again during main catagenesis. This intermittent increase was related to the short chain fatty acids. To obtain a maturity related signal and to eliminate facies related scattering in the amounts of fatty acids in the coal samples, the carbon preference index of fatty acids (CPI_FA) parameter is introduced here. For the long chain fatty acids (C₂₀–C₃₂) originating from terrigenous plant debris, the CPI_FA decreases with increasing maturity, showing a strong maturation related signal. During diagenesis, the same trend can be observed for the short chain fatty acids, but the intermittent increase in the amounts of short chain fatty acids is also accompanied by high CPI_FA values. This indicates less altered organic biomass at this advanced maturation level and is in contrast to the mature CPI_FA signal of the long chain fatty acids of the same samples. One possible reason for this discrepancy could be extremely different amounts of short and long chain fatty acids in the original source organic matter of these samples. However, another intriguing explanation could be the incorporation of immature bacterial biomass from deep microbial communities containing C₁₆ and C₁₈ fatty acids as main cell membrane components. Deep microbial life might be stimulated at this interval by the increasing release of thermally generated potential substrates from the organic matrix during early catagenesis. In contrast to the fatty acids, the high amounts of alcohols in the immature lignite samples are also visible in the alkene distribution from the open system pyrolysis experiments of the organic matrix before and after saponification.     

Lipid Distribution in Marine Sediments from the Northern South China Sea and Association with Gas Hydrate

The distributions of lipids in surface and subsurface sediments from the northern South China Sea were determined. The n-alkanes were in bimodal distribution that is characterized by a centre at n-C16 –n-C20 with maximum at C18(or C19) and n-C27 –n-C31 as well as at C29(or C31). The short-chain alkanes suffered from significant losses due to their slow deposition in the water column, and their presence with a slight even carbon predominance in shallow seafloor sediments was ascribed mainly to the direct input from the benthos. The long-chain alkanes with odd predominance indicate transportion of terrigenous organic matter. Immature hopanoid biomarkers reflect the intense microbial activity for bacteria–derived organic matter and the gradual increase of maturity with burial depth. Abundant n-fatty acid methyl esters(n-FAMEs) that are in distributions coincident with fatty acids were detected in all samples. We proposed that the observed FAMEs originated from the methyl esterification of fatty acids; methanol production by methanotrophs and methanogenic archaea related to the anaerobic oxidation of methane, and sulfate reduction provided an O–methyl donor for methylation of fatty acids. The CH4 released from hydrate dissociation at oxygen isotope stage II of Cores ZD3 and ZS5, which had been confirmed by the occurrence of negative δ13C excursion and spherical pyrite aggregates, could have accelerated the above process and thus maximized the relative content of FAMEs at ZD3-2(400–420 cm depth) and ZS5-2(241–291 cm depth).     

Early diagenesis of fatty acids in lacustrine sediments—III. Changes in fatty acid composition in the sediments from a brackish water lake

A 3-m sediment core taken from Lake Suigetsu, in which a shift from fresh to brackish water occurred about three hundred years ago, has been examined for variation with depth of organic carbon and fatty acids. From the difference in total amounts of sulphur between sediments under fresh and brackish water environments, the surface sediments above approximately 35 cm depth were deduced to be accumulated under a brackish water environment. The total contents of organic carbon and fatty acids, and percentage composition of fatty acids gave discontinuous profiles above and below the 35–40 cm sediment layer. At a depth of 12.5 cm, the distribution in chain length of the fatty acids changed from a unimodal (the predominance of C₁₂-C₁₈ over C₂₀-C₃₄) to a bimodal pattern, which was mirrored by the composition diversity index (CDI).Although the fatty acids in the surface sediments (0–40 cm) from Lake Suigetsu seemed to suffer milder degradation through microbial activity than those in a core (0–150 cm) from Lake Suwa, a freshwater eutrophic lake, both lacustrine sediments showed similar trends in the alteration of fatty acid composition with depth.     

Lipid composition of the pelagic crab Pleuroncodes planipes, its feces, and sinking particulate organic matter in the Equatorial North Pacific Ocean

Fecal pellets from the pelagic crab Pleuroncodes planipes were a substantial portion of the particulate organic matter in a sediment trap deployed at the bottom of the mixed layer in the eastern tropical North Pacific Ocean. The lipids of fresh P. planipes feces were compared to lipids of the sediment trap material, of mixed zooplankton which may comprise part of the diet of the crab, and of the crab itself in order to elucidate the source of organic compounds found in the trap. Hydrocarbons, wax esters, triacylglycerols, steroidal alcohols, steroidal ketones, and fatty acids were determined by capillary gas chromatography and gas chromatography/mass spectrometry. Significant input of lipids via sedimentation of crab fecal material is indicated, and modification of dietary lipid within the gut of the crab is inferred. Labile dietary fatty acids are depleted and sterols enriched in the fecal pellets and trap material relative to the zooplankton and crab. Nuclear saturated and unsaturated 3-ketosteroids and unsaturated steroidal hydrocarbons were detected in the crab, its feces, and in the sediment trap particulate material.     

Qualitative fatty acid and n-alkane stratigraphy of the Lake Turkana Basin,Kenya

In order to improve understanding of the stratigraphy of the Lake Turkana Basin, one of the important sites in the evolution of early man, this study evaluates the usefulness of organic biological marker compounds, n-alkanes and fatty acids, for correlation of isolated sedimentary strata.Eighty-five paleosol samples were collected from well-defined sedimentary horizons in two regions (Area 103 and Area 130) of the Koobi Fora area of Lake Turkana. Results indicate that most of the organic matter present was derived from terrestrial plant waxes. In sediments where extensive diagenesis has occurred, microbial input of organic matter may have been substantial. Algae were either not an important source of organic matter, or their marker compounds have been removed or altered by degradative processes.The fate of the original paleosol organic matter has been governed to some extent by weathering processes, especially in Area 130. Weathering decreased the amount of extractable lipids, particularly fatty acids and the low molecular weight alkanes (C₁₇C₂₀); produced or retained relatively large amounts of alkanes greater than C₂₁ within a unimodal distribution; and lowered CPI values. Consequently, stratigraphic correlation by unique alkane and fatty acid distributions has been confined to short distances (many meters).Both n-alkanes and fatty acids have been retained better by association with clay minerals than by sand matrices. The alkane distribution of sandstones differs from that of clay organics in having a narrower carbon chain length distribution and lower CPI values. In Area 103, where weathering was less severe, compositional variations with stratigraphic position indicate that lipid material has been retained within each of the facies examined.     

Conversion of sedimentary fatty acids from extractable (unbound + bound) to tightly bound form during mild heating

Prolonged heating (1–248 days) of a recent sediment was conducted under mild conditions (65 and 83°C) to understand the thermal behavior of extractable (unbound and bound) fatty acids (C_12C32). In the course of the heating, the concentrations of saturated and monounsaturated fatty acids first decreased, and then increased. This indicates that extractable fatty acids can be converted to a tightly bound form upon heating, before fatty acids are extensively released from sediment matrices. Changes in molecular distributions of extractable fatty acids upon heating were also observed. The higher molecular weight fatty acids (?C₂₀) are more likely to become tightly bound than are lower molecular weight (<C₂₀). Monounsaturated fatty acids (C_16:1 and C_18:1) are more easily converted to the tightly bound form than polyunsaturated fatty acids (C_18:2 and C_18:3). These differences probably reflect their mode of occurrence in sediments, which are closely related to their origin.     

Biomarker lipids of the freshwater fern Azolla and its fossil counterpart from the Eocene Arctic Ocean

Eocene sediments recovered from the Lomonosov Ridge in the central Arctic Ocean during Integrated Ocean Drilling Program Expedition 302 contain high amounts of fossil remains of the free floating freshwater fern Azolla. Both extant Azolla and the sediments from the Arctic Azolla interval were found to contain relatively high quantities of compounds identified as 1,ω20 C₃₀–C₃₆ diols. Furthermore, structurally related mid-chain hydroxy fatty acids, long chain n-alkanols and keto-ols were discovered. The different series have a common feature: there is always a hydroxy group at the ω20 position. In addition, structurally related C₂₉ ω20,ω21 diols, C₂₉ 1,20,21 triols, C₂₉ dihydroxy fatty acids as well as a series of wax esters containing these mono- and dihydroxy lipids are reported. Selective extraction of Azolla surface lipids revealed that these compounds are most likely present in the leaf waxes of the fern. The suite of long-chain, mid-chain ω20 hydroxy wax constituents is described for the first time. As they are well preserved in the Eocene sediments, these lipids may serve as palaeo-environmental indicators of the Arctic Azolla interval and as markers for the past occurrence of Azolla in general.     

Distribution and diagenesis of organic compounds in JOIDES sediment from Gulf of Mexico and western Atlantic

Fifteen sediment samples were studied from five drill sites recovered by the ‘Glomar Challenger’ on Legs I and IV in the Gulf of Mexico and western Atlantic. This study concentrated on compounds derived from biogenic precursors, namely: (1) hydrocarbons, (2) fatty acids, (3) pigments and (4) amino acids.Carbon isotope (δC¹³) data (values < ? 26%, relative to PDB), long-chain n-alkyl hydrocarbons ( ?C₂₇7) with odd carbon numbered molecules dominating even carbon numbered species, and presence of perylene proved useful as possible indicators for terrigenous contributions to the organic matter in some samples. Apparently land-derived organic matter can be transported for distances over 1000 km into the ocean and their source still recognized.The study was primarily designed to investigate: (i) the sources of the organic matter present in the sediment, (ii) their stability with time of accumulation and (iii) the conditions necessary for in situ formation of new compounds.     

Organic geochemistry of Walvis Bay diatomaceous ooze—I. Occurrence and significance of the fatty acids

Diatomaceous ooze of the shelf off Walvis Bay (S.W. Africa) was analysed for lipid material. The lipids from the sediment consist of a mixture of hydrocarbons, alcohols, ketones, fatty acids, sterols, carotenoid pigments and chlorophylls. The total fatty acid mixture has been analysed by capillary gas chromatography mass spectrometry and shown to consist of straight chain, iso-, anteiso- and isoprenoid acids. The environmental setting of the S.W. African shelf (Walvis Bay) makes it possible to discuss these fatty acids as markers for the fate of organic matter. The acids in the sediment point to a generation during passage of algal lipids through levels of microbial activity either on or slightly above the burial surface.     

Origin and transport of sedimentary organic matter in the Yalujiang estuary,North China

The biogeochemistry of organic matter (OM) in a macrotidal estuary, the Yalujiang River, was studied during two cruises: the flood season in August 1994 and the dry season in April 1996. Surface sediments were collected in the riverine zone (RZ), the turbidity maximum zone (TMZ), and the marine zone (MZ). The molecular distribution of the n-alkanes and fatty acid series and bulk sediment characteristics, such as C:N and δ¹³C, were used to assess differences in OM source and transport from the river upstream to the marine end member. Higher C:N values typical for terrestrial sources were observed at the upper reach for both seasons. The δ¹³C of OM in surface sediments varied from −27.3‰ to −21.6‰ in the flood season and from −26.8‰ to −31‰ in the dry season. The concentrations of n-alkanes varied between 0.3–21.4 μg g⁻¹ and the variation of fatty acids was 4.8–32.9 μg g⁻¹. The data showed mixing of terrestrial and autochthonous OM in the middle and lower reaches. The distribution of lipids (n-alkanes and Carbon Preference Index) encountered in this study confirmed the importance of terrestrial OM in the sediment samples from degraded soil material. The distribution of fatty acids suggested important phytoplankton, zooplankton, and microbial signals (short-chain and unsaturated acids; ≤C₂₀). Branched fatty acids, such as the iso- and anteiso-C₁₅ and C₁₇ compounds, relfect bacterial contributions. All samples were characterized by a high proportion of mixture inputs in both seasons. A slight decreasing trend was observed with increasing salinity except for the highest percentage of mixed fatty acids in the TMZ of the flood season. Terrestrial fatty acids were approximately 20% in the flood season and 27–46% in the dry season. Differences in hydrological conditions and primary production between the TMZ, RZ, and MZ resulted in different OM distributions, which are reflected in the sources and degree of diagenesis of the sedimentary OM. Seasonal variation may be strongly influenced by hydrological characteristics rather than primary productivity and anthropogenic activities in the Yalujiang region.     

The influence of marine and terrestrial source material on the composition of petroleum

This paper consists of two interrelated parts. In the first part, the influence of the composition of sediment organic matter on crude oil composition is discussed. The second part deals with the origin of normal paraffins in petroleum.Source beds with abundant terrestrial plant matter generate heavy hydrocarbons rich in five-ring naphthenes. Unless such source beds are exposed to a high temperature for a prolonged time, the oils released are also rich in five-ring naphthenes. Such oils are rare; thus far the only examples found are some Eocene Wilcox oils from the Texas Gulf Coast and some Eocene Green River oils from the Uinta Basin, Utah. Normally, oil source beds are not rich in terrestrial plant matter and the five-ring naphthene content of the source bed hydrocarbons, as well as that of the produced oils, is low.The n-paraffins generated by oil source beds rich in terrestrial plant matter are characterized by abnormally low (C₂₁ + C₂₂)/(C₂₈ + C₂₉) ratios of 0.6–1.2. In oils of dominantly marine origin, this ratio is in the range 1.5–5.0. The ratio of marine to terrestrial organic matter in source beds appears to influence both the naphthene composition and the n-paraffin composition of the generated oils.Evidence is presented that petroleum n-parainns originate from slow thermal cracking of fatty acids contained in fats and waxes. Reaction equations are discussed which explain the major geochemical observations, including the difference in carbon-number distribution of the assumed parental fatty acids and of their descendant n-paraffins. In normal oils, which originate mostly from fat rich marine organic matter, the n-paraffin concentration tapers off above C₂₀. The molecular weight range of the fatty acids of plant waxes is considerably higher than that of fats. If plant waxes contribute strongly to the oil source material, the molecular weight distribution of the petroleum n-paraffins formed is abnormal and high carbon numbers in the C₂₄-C₃₂ range dominate.     

Organic geochemical characteristics of time-series settling particles in northern South China Sea and their implications

Organic carbon, total nitrogen, amino acids, sugars, and chlorophyll were determined in < 1 mm fractions of the samples collected by successive large aperture time-series sediment traps (Honjo-Mark M) in northern South China Sea during September 1987 to October 1988. The ratio of C/N and the relative abundance of amino acids and sugars show that organic matter in the settling particles from northern South China Sea is derived mainly from marine plantkon (especially phytoplankton). The organic carbon fluxes in our sediment traps are lower than those in other sediment traps. But the relative contents of C_org/total particulate matter are generally similar to those in the Panama Basin, Arabian Sea and Subarctic Pacific. It is suggested that monsoon-caused changes of physical and chemical conditions in the upper euphotic layer would control the fluxes of organic matter as well as its composition and transport in northern South China Sea. This project was financially supported by both Sino-German Scientific Cooperation Program and National Natural Science Foundation of China (No. 49070269, 49776297).     

Compositions and sources of extractable organic matter in Mesopotamian marshland surface sediments of Iraq: II. Polar compounds

The concentrations of polar organic compounds including n-alkanoic acids, n-alkanols, steroids and triterpenoids were determined in extracts of shallow sediments from the Mesopotamian marshlands of Iraq. The sediments were collected by a stainless steel sediment corer, extracted with a dichloromethane and methanol mixture (3:1 v:v) by ultrasonic agitation and then analyzed by gas chromatography–mass spectrometric (GC–MS). The analysis results showed that the n-alkanoic acids ranged from C₈ to C₂₀ with concentrations of 7.8 ± 1.2 μg/g sample, whereas the concentrations of n-alkanols, which ranged from C₁₂ to C₃₉ were from 28.6 ± 4.3 to 121.7 ± 18.3 μg/g sample. The steroids and triterpenoids included stenols, stanols, stenones, stanones, tetrahymanol, tetrahymanone and extended ββ-hopanes. The total concentrations of steroids and triterpenoids ranged from 26.8 ± 4.1 to 174.6 ± 26.2 μg/g and from 0.74 ± 0.11 to 11.2 ± 1.7 μg/g sample, respectively. The major sources of these lipids were from natural vegetation, microbial (plankton) residues and bacteria in the sediments, with some contribution from anthropogenic sources (livestock, sewage and petroleum). Further studies of these wetlands are needed to characterize the input rate, transformation and diagenesis of the organic matter and to assess its various sources.     

Organic geochemistry of suspended and settling particulate matter in Lake Michigan

Organic matter contained in particulate matter in Lake Michigan waters and sediments has been characterized by $\text{C}\text{N}$ ratios and by distributions of biomarker fatty acids, alkanols, sterols, and aliphatic hydrocarbons. Differences in organic constituents of particulate matter from various depths and distances from shore indicate a complex interaction of production, transformation, and destruction of the organic matter contained in sinking particles. Near-surface material contains important contributions of landderived organic matter, presumably of eolian input. Midwater particles have predominantly aquatic organic material of algal origin. At the sediment-water interface, selective suspension of the finer fractions of surficial sediments enriches bottom nepheloid layers with these sediment size classes. As a result, near-bottom particulate matter has an aquatic biomarker character. Organic matter associated with sinking particles undergoes substantial degradation during passage to the bottom of Lake Michigan, and aquatic components are selectively destroyed relative to terrigenous components.     

Bound carboxylic acids in the Alberta oil sands

The Alberta oil sands contain 1–2% organic solvent insoluble organic matter chemisorbed to the inorganic matrix. Analysis of the monocarboxylic fraction (1–14%) of the chemisorbed material has revealed the presence of C₁₂–C₃₂ normal, iso, anteiso alkenoic acids, mono and diunsaturated acids, cyclopropylalkanoic and cyclic terpenoid carboxylic acids. Some of the main components of the acyclic acids were similar to those which have been identified in various petroleums and Alberta oil sand bitumens (in small concentrations) by Mackenzie et al. (Advances in Organic Geochemistry 1981, pp 637–649, Wiley/Heyden, 1983), and attributed to bacterial degradation of the oils via the aerobic pathway of biosynthesis.The presence of these acids in the chemisorbed fraction and virtual absence in the bitumen lends additional support to earlier proposals that the Alberta oil sand bitumens have undergone severe microbial degradation.     

Evaluation of alkaline permanganate oxidation method for the characterization of young kerogen

Alkaline potassium permanganate oxidation of a young kerogen (lacustrine) and 34 model compounds (saturated and unsaturated fatty acids, hydroxy acid, aliphatic dicarboxylic acids, aliphatic alcohols, normal hydrocarbon, β-carotene, phenolic acids, benzenecarboxylic acids, carbohydrates, amino acids and proteins) were conducted, followed by GC and GC-MS analysis of the degradation products. The stability of the degradation products of kerogen in permanganate solution and the relationship between degradation products and kerogen building blocks were determined.The results showed that aliphatic acids C₁₂–C₁₆ monocarboxylic acids and C₆–C₁₀ α,ω-dicarboxylic acids) were rather susceptible to oxidation compared with benzenecarboxylic acids and the former were degraded into lower molecular weight decarboxylic acids. It was concluded that oxidation at milder conditions (60° C, 1 hr) is appropriate for qualitative and quantitative characterization of the aliphatic structure of young kerogen. It was noteworthy that benzoic acid was produced in a significant amount by oxidation of amino acids (phenylalanine) and proteins, C₁₈-isoprenoidal ketone from phytol, and C₈ and C₉ α,ω-dicarboxylic acids from unsaturated fatty acids, respectively; furthermore, 2,2-dimethyl succinic and 2,2-dimethyl glutaric acids were produced from β-carotene.     

The organic geochemistry of black sedimentary barite: significance and implications of trapped fatty acids

Fatty acids isolated in sedimentary black barite (BaSO₄) from Arkansas and Nevada were identified by gas chromatography-mass spectroscopy. The dominant or major fatty acids found in these beds of barite are C_16:0, C_18:0, and C_18:1. The occurrence and distribution of these acids in this type of rock may serve as “molecular fingerprints” of microbial biogeochemical processes. The organic matter and associated microorganisms are shown to be trapped within the finely crystalline barite, thus forming a closed system for microbial diagenesis.Important differences that occur in the distribution of the lesser or minor fatty acids probably result from: (1) the nature of the progenitor organic detritus in the environment of barite deposition: and (2) the subsequent degree of microbiological alteration of the parent organic debris swept into and trapped in the depositional environment.Three general models of sedimentary environments are proposed in which anoxic conditions may prevail and where barium sulfate (BaSO₄) may precipitate: (1) in a silled basin with semi-restricted circulation; (2) on an outer continental shelf where the slope is encroached upon by water of the oxygen minimum layer; (3) on a low-energy, inner shelf or semi-restricted embayment impinged by a wedge of anoxic water.The major geochemical and geological parameters which are believed to be the significant factors controlling the formation and high grade of these organic-rich, black bedded barites are: (1) a unique source of barium-rich fluid that only contains trace amounts of other elements; (2) the presence of an anoxic bottom environment within the depositional basin; (3) a reflux source of sulfate ion; (4) an adequate source of organic matter.The results of this study may serve as guidelines for future exploration in similar, untested sedimentary basins, especially those with rocks of middle Paleozoic age.