Distributions and hydrogen isotopic compositions of plant leaf wax from Orinus kokonorica along a general aridity gradient around Lake Qinghai,China

To quantitatively analyze the response of distributions and hydrogen isotopic compositions （SD） of plant leaf wax to moisture, and to better understand their implications for paleoclimatic reconstruction, we measured av- erage chain length （ACL） and 8D values of n-alkanes and n-fatty acids （n-FAs） from Orinus kokonorica, a typical and representative plant in Lake Qinghai area, along a distance transect extending from lakeshore to wetland to dry- land in the arid ecosystem. The results showed that the ACL values of n-alkanes and n-FAs were negatively corre- lated with soil water content （SWC） with R2~0.593 and R2=0.924, respectively. This is as a result of plant＇s response to water loss with more abundance in long-chain n-alkyl lipids under increasing aridity by analyzing relationships between the molecular ratios of long-chain n-alkyl lipids （n-alkanes and n-FAs） from O. kokonorica and SWC. The 8D values of C29 n-alkane and C28 n-FA were also negatively correlated with SWC with R2-0.778 and R2-0.760, respectively, which may due to enhanced D-enrichment in leaf water by evapotranspiration （soil water evaporation and leaf water transpiration） with increasing aridity. Our results demonstrated that moisture exerts a significant con- trol on the ACL and 8D values from O. kokonorica in an arid ecosystem. This preliminary study on a modern single plant （O. kokonorica） sets a foundation for comprehending these values as quantitative proxies for paleo-humidity reconstruction.     

Leaf wax n-alkane δD values of field-grown barley reflect leaf water δD values at the time of leaf formation

Leaf wax n-alkanes from barley (Hordeum vulgare) from a field in Switzerland exhibited changes in δD values on the order of 20‰ over a growing season, while source water (soil water) and leaf water varied by 40‰. Additionally the seasonal variability in δD values of leaf wax n-alkanes of different barley leaves can only be found across different leaf generations (i.e. leaves that were produced at different times during the growing season) while n-alkane δD values did not vary significantly within a leaf generation. Interestingly, δD values of n-alkanes correlated best with the δD values of leaf water at midday of the sampling day but showed no significant correlation with soil water (e.g. precipitation) δD values. These results provide empirical evidence that leaf wax δD values record leaf water enrichment, and therefore integrate the isotopic effects of precipitation and evapotranspiration. Our results show that leaf wax n-alkane δD values from grasses are ‘locked in’ early during leaf development and hence record the environmental drivers of leaf water enrichment, such as vapor pressure deficit (VPD). Our data have important implications for the interpretation of paleorecords of leaf wax δD. We suggest that leaf wax n-alkane δD values from sedimentary records could be used to estimate changes in the degree of leaf water enrichment and hence VPD.     

Spatial heterogeneity in lipid biomarker distributions in the catchment and sediments of a crater lake in central India

The basin-scale spatial variability in lipid biomarker proxies in lacustrine sediments, which are established tools for studying continental environmental change, has rarely been examined. It is often implicitly assumed that a lake sediment core provides an average integral of catchment sources. Here we evaluated the distribution of lipid biomarkers in a modern ecosystem and compared it with the sedimentary record. We analyzed lipid biomarkers in terrestrial and aquatic organisms and in lake surface sediments from 17 locations within the saline–alkaline Lonar crater lake in central India. Terrestrial vegetation and lake surface sediments were characterized by relatively high average chain length (ACL) index values (29.6–32.8) of leaf wax n-alkanes, consistent with suggestions that plants in drier and warmer climates produce longer chain alkyl lipids than plants in cooler and humid areas. A heterogeneous spatial distribution of ACL values in lake surface sediments was found: at locations away from the shore, the values were highest (31 or more), possibly indicating different sources and/or transport of terrestrial biomarkers. In floating, benthic microbial mats and surface sediment, n-heptadecane, carotenoids, diploptene, phytol and tetrahymanol occurred in large amounts. Interestingly, these biomarkers of a unique bacterial community were found in substantially higher concentrations in nearshore sediment samples. We suggest that human influence and subsequent nutrient supply resulted in increased primary productivity, leading to an unusually high concentration of tetrahymanol in the nearshore sediments.In summary, the data showed that substantial heterogeneity existed within the lake, but leaf wax n-alkanes in a core from the center of the lake represented an integral of catchment conditions. However, lake level fluctuation may potentially affect aquatic lipid biomarker distributions in lacustrine sediments, in addition to source changes.     

n-Alkane distribution and carbon stable isotope composition in leaf waxes of C3 and C4 plants from Angola

Palaeoenvironmental assessment of past C₃ and C₄ vegetation distributions relies on end member data from plant analyses. In southwestern Africa, end member data of the carbon number distribution of n-alkanes from leaf waxes and their carbon isotopic composition were available for the rainforest and the savannah. To complement this, we analysed the n-alkane parameters of 41 C₃ plants and 11 C₄ plants from the transition region, i.e., the wood- and shrubland of Angola. The combined results for the rainforest, the wood- and shrubland and the savannah show an increase in the average chain length (ACL) of C₃ and C₄ plants and an increasingly enriched carbon stable isotope composition for the C₃ plants from the equator towards southern Africa. The enlarged database was applied to the data of a north–south transect of deep-sea surface sediments already used in a previous study, which resulted in the proxies showing a good reflection of the vegetation on the adjacent southwest African continent in terms of %C₄ plant cover. Applying end member values for ACL and δ¹³C obtained from the enlarged database by two different averaging methods (arithmetic average and median) to the n-alkane data from the sediment transect yielded similar vegetation reconstructions. In addition, a correlation between ACL and growth height of the plants is discussed, indicating that the ACL may be useful as a tree abundance parameter. Thus, the enlarged end member database strengthens the n-alkane parameters as tools for palaeoenvironmental studies.     

Effect of leaf litter degradation and seasonality on D/H isotope ratios of n-alkane biomarkers

During the last decade, compound-specific hydrogen isotope analysis of plant leaf-wax and sedimentary n-alkyl lipids has become a promising tool for paleohydrological reconstructions. However, with the exception of several previous studies, there is a lack of knowledge regarding possible effects of early diagenesis on the δD values of n-alkanes. We therefore investigated the n-alkane patterns and δD values of long-chain n-alkanes from three different C3 higher plant species (Acer pseudoplatanus L., Fagus sylvatica L. and Sorbus aucuparia L.) that have been degraded in a field leaf litterbag experiment for 27 months.We found that after an initial increase of long-chain n-alkane masses (up to ∼50%), decomposition took place with mean turnover times of 11.7 months. Intermittently, the masses of mid-chain n-alkanes increased significantly during periods of highest total mass losses. Furthermore, initially high odd-over-even predominances (OEP) declined and long-chain n-alkane ratios like n-C₃₁/C₂₇ and n-C₃₁/C₂₉ started to converge to the value of 1. While bulk leaf litter became systematically D-enriched especially during summer seasons (by ∼8‰ on average over 27 months), the δD values of long-chain n-alkanes reveal no systematic overall shifts, but seasonal variations of up to 25‰ (Fagus, n-C₂₇, average ∼13‰).Although a partly contribution by leaf-wax n-alkanes by throughfall cannot be excluded, these findings suggest that a microbial n-alkane pool sensitive to seasonal variations of soil water δD rapidly builds up. We propose a conceptual model based on an isotope mass balance calculation that accounts for the decomposition of plant-derived n-alkanes and the build-up of microbial n-alkanes. Model results are in good agreement with measured n-alkane δD results. Since microbial ‘contamination’ is not necessarily discernible from n-alkane concentration patterns alone, care may have to be taken not to over-interpret δD values of sedimentary n-alkanes. Furthermore, since leaf-water is generally D-enriched compared to soil and lake waters, soil and water microbial n-alkane pools may help explain why soil and sediment n-alkanes are D-depleted compared to leaves.     

Production of n-alkyl lipids in living plants and implications for the geologic past

Leaf waxes (i.e., n-alkyl lipids or n-alkanes) are land-plant biomarkers widely used to reconstruct changes in climate and the carbon isotopic composition of the atmosphere. There is little information available, however, on how the production of leaf waxes by different kinds of plants might influence the abundance and isotopic composition of n-alkanes in sedimentary archives. This lack of information increases uncertainty in interpreting n-alkyl lipid abundance and δ¹³C signals in ancient settings. We provide here n-alkyl abundance distributions and carbon isotope fractionation data for deciduous and evergreen angiosperm and gymnosperm leaves from 46 tree species, representing 24 families. n-Alkane abundances are significantly higher in angiosperms than gymnosperms; many of the gymnosperm species investigated did not produce any n-alkanes. On average, deciduous angiosperms produce 200 times more n-alkanes than deciduous gymnosperms. Although differences between angiosperms and gymnosperms dominate the variance in n-alkane abundance, leaf life-span is also important, with higher n-alkane abundances in longer-lived leaves. n-Alkanol abundances covary with n-alkanes, but n-alkanoic acids have similar abundances across all plant groups. Isotopic fractionation between leaf tissue and individual alkanes (ε_lipid) varies by as much as 10‰ among different chain lengths. Overall, ε_lipid values are slightly lower (−4.5‰) for angiosperm than for gymnosperm (−2.5‰) n-alkanes. Angiosperms commonly express slightly higher Δ_leaf (photosynthetic discrimination) relative to gymnosperms under similar growth conditions. As a result, angiosperm n-alkanes are expected to be generally 3-5‰ more depleted in ¹³C relative to gymnosperm alkanes for the same locality. Differences in n-alkane production indicate the biomarker record will largely (but not exclusively) reflect angiosperms if both groups were present, and also that evergreen plants will likely be overrepresented compared with deciduous ones. We apply our modern lipid abundance patterns and ε_lipid results to constrain the magnitude of the carbon isotope excursion (CIE) at the onset of the Paleocene-Eocene Thermal Maximum (55.8 Ma). When Bighorn Basin (WY) sediment n-alkanes are interpreted in context of floral changes and modern n-alkane production estimates for angiosperms and gymnosperms, the CIE is greater in magnitude (−5.6‰) by ∼1‰ compared to previous estimates that do not take into account n-alkane production.     

Methyl ketones in high altitude Ecuadorian Andosols confirm excellent conservation of plant-specific n-alkane patterns

To reconstruct past shifts in the upper forest line (UFL) in the Northern Ecuadorian Andes we are studying the applicability of plant-specific patterns of lipids preserved in soils as proxies for past vegetation along an altitudinal transect. Longer chain length n-alkanes, (C₁₉–C₃₅) were previously found to occur in plant-specific patterns in the dominant vegetation in the area as well as in preliminary soil samples, and may serve as such a proxy. In the present study, we assessed the preservation of n-alkane patterns with depth in soils from five excavations along an altitudinal transect 3500–3860 m above sea level (m.a.s.l) in the area. We used the carbon preference index (CPI) as well as chain length distributions of n-alkanes and their most likely degradation products, n-methyl (Me) ketones, n-alcohols and n-fatty acids. Clear n-alkane patterns were found in all the soils and at all depths, while a clear relationship with the observed patterns of n-Me ketones identified them as the primary degradation product of the former. Very low average n-Me ketone/n-alkane ratio values were found, ranging from 0.03 to 0.15 at the top of the mineral soil, to 0.05–0.20 at the interface with an underlying palaeosol several thousand years old. The concurrent high CPI values indicate very limited degradation of n-alkanes with depth. Except for C₃₃, the shifts in n-Me ketone/n-alkane values were similar for all chain lengths investigated, signifying an absence of preferential degradation of individual n-alkanes. With one exception, all the soils showed a similar increase in n-Me ketone/n-alkane values with depth, indicating that the degradation rates were not influenced by altitude. This means that, even if the total concentration of n-alkanes decreases over time, the characteristic pattern remains intact, conserving their potential as a biomarker for past vegetation reconstruction in the area, as well as for investigation of degradation processes of soil organic carbon.     

Significant seasonal variation in the hydrogen isotopic composition of leaf-wax lipids for two deciduous tree ecosystems (Fagus sylvativa and Acerpseudoplatanus)

Compound specific hydrogen isotope ratios (δD) of long chain sedimentary n-alkanes, which mostly originate from the leaf waxes of higher terrestrial plants, are increasingly employed as paleoclimate proxies. While soil water is the ultimate hydrogen source for these lipids and the isotopic fractionation during biosynthesis of lipids is thought to remain constant, environmental parameters and plant physiological processes can alter the apparent hydrogen isotopic fractionation between leaf-wax lipids and a plant’s source water. However, the magnitude and timing of these effects and their influence on the isotopic composition of lipids from higher terrestrial plants are still not well understood. Therefore we investigated the seasonal variability of leaf-wax n-alkane δD values for two different temperate deciduous forest ecosystems that are dominated by two different tree species, Beech (Fagus sylvatica) and Maple (Acerpseudoplatanus).We found significant seasonal variations for both tree species in n-alkane δD values of up to 40‰ on timescales as short as one week. Also, the isotopic difference between different n-alkanes from the same plant species did vary significantly and reached up to 50‰ at the same time when overall n-alkane concentrations were lowest.Since δD values of soil water at 5 and 10 cm depth, which we assume represent the δD value of the major water source for the investigated beech trees, were enriched in autumn compared to the spring by 30‰, whereas n-alkane δD values increased only by 10‰, we observed variations in the apparent fractionation between beech leaf derived n-alkanes and soil water of up to 20‰ on a seasonal scale. This observed change in the apparent fractionation was likely caused by differences in leaf water isotopic enrichment. Based on mechanistic leaf water models we conclude that changes in the isotopic difference between water vapor and soil water were the most likely reason for the observed changes in the apparent fractionation between n-alkanes and soil water.The large variability of n-alkane concentrations and δD values over time implies a continuous de novo synthesis of these compounds over the growing season with turnover times possibly as short as weeks. The signal to reach the soil therefore represents an integrated record of the last weeks before leaf senescence. This holds true also for the sedimentary record of small catchment lakes in humid, temperate climates, where wind transport of leaf-wax lipids is negligible compared to transfer through soil and the massive input of leaves directly into the lake in autumn.     

Carbon isotope analyses of n-alkanes in dust from the lower atmosphere over the central eastern Atlantic

Atmospheric dust samples collected along a transect off the West African coast have been investigated for their lipid content and compound-specific stable carbon isotope compositions. The saturated hydrocarbon fractions of the organic solvent extracts consist mainly of long-chain n-alkanes derived from epicuticular wax coatings of terrestrial plants. Backward trajectories for each sampling day and location were calculated using a global atmospheric circulation model. The main atmospheric transport took place in the low-level trade-wind layer, except in the southern region, where long-range transport in the mid-troposphere occurred. Changes in the chain length distributions of the n-alkane homologous series are probably related to aridity, rather than temperature or vegetation type. The carbon preference of the leaf-wax n-alkanes shows significant variation, attributed to a variable contribution of fossil fuel- or marine-derived lipids. The effect of this nonwax contribution on the δ¹³C values of the two dominant n-alkanes in the aerosols, n-C₂₉ and n-C₃₁ alkane, is, however, insignificant. Their δ¹³C values were translated into a percentage of C₄ vs. C₃ plant type contribution, using a two-component mixing equation with isotopic end-member values from the literature. The data indicate that only regions with a predominant C₄ type vegetation, i.e. the Sahara, the Sahel, and Gabon, supply C₄ plant-derived lipids to dust organic matter. The stable carbon isotopic compositions of leaf-wax lipids in aerosols mainly reflect the modern vegetation type along their transport pathway. Wind abrasion of wax particles from leaf surfaces, enhanced by a sandblasting effect, is most probably the dominant process of terrigenous lipid contribution to aerosols.     

Simulation experiments on the variation of leaf n-alkanes in aquatic environments

The leaves of six plant species and the corresponding leaf residues collected in water from the two-year simulation experiments were analyzed in nalkane distributions by gas chromatography (GC) and gas chromatography-mas spectrometry (GC/MS). The leaf nalkanes keep unchanged in the dominant homologues when soaked in tap water for two years. The most significant change was observed in carbon preference index (CPI), with enhanced values being found in leaf residues collected from water. This is contradictory with the previous reports showing the lower CPI values during sinking and burial processes in natural aquatic environments. The elevated CPI values from leaf residues might be related to the low amount of microorganisms in the water used in the simulation experiment, and the enhanced solubility of even-carbon-numbered n-alkanes via van der Waals attraction. In contrast with herbaceous plants, the woody plants appear to show relatively great variations in both the CPI and the average chain length (ACL) values of n-alkanes after submerged in water for two years. Our data clearly show the differentiated decomposition between woody and herbaceous leaves, with the woody leaves suffered from much stronger decomposition. This observation suggests that in comparison with the grassland, the forest vegetation might result in relatively low authentic signals to be preserved in the n-alkane distributions in aquatic sediments.     

A compound-specific n-alkane δC and δD approach for assessing source and delivery processes of terrestrial organic matter within a forested watershed in northern Japan

We measured molecular distributions and compound-specific hydrogen (δD) and stable carbon isotopic ratios (δ¹³C) of mid- and long-chain n-alkanes in forest soils, wetland peats and lake sediments within the Dorokawa watershed, Hokkaido, Japan, to better understand sources and processes associate with delivery of terrestrial organic matter into the lake sediments. δ¹³C values of odd carbon numbered C₂₃-C₃₃n-alkanes ranged from −37.2‰ to −31.5‰, while δD values of these alkanes showed a large degree of variability that ranged from −244‰ to −180‰. Molecular distributions in combination with stable carbon isotopic compositions indicate a large contribution of C3 trees as the main source of n-alkanes in forested soils whereas n-alkanes in wetland soil are exclusively derived from marsh grass and/or moss. We found that the n-alkane δD values are much higher in forest soils than wetland peat. The higher δD values in forest samples could be explained by the enrichment of deuterium in leaf and soil waters due to increased evapotranspiration in the forest or differences in physiology of source plants between wetland and forest. A δ¹³C vs. δD diagram of n-alkanes among forest, wetland and lake samples showed that C₂₅-C₃₁n-alkanes deposited in lake sediments are mainly derived from tree leaves due to the preferential transport of the forest soil organic matter over the wetland or an increased contribution of atmospheric input of tree leaf wax in the offshore sites. This study demonstrates that compound-specific δD analysis provides a useful approach for better understanding source and transport of terrestrial biomarkers in a C3 plant-dominated catchment.     

N-Alkanes and polycyclic aromatic hydrocarbons (PAHs) profile of soil from some polluted sites in Niger Delta, Nigeria

Soil samples were collected at different locations from Etche and Obio Akpor local government area (LGA) of River State in Niger Delta. The n-alkane and polycyclic aromatic hydrocarbons were determined qualitatively and quantitatively using GC-FID. The concentration of PAHs in the soil samples ranged from 7.40 to 78.3 ng/g. The highest concentration of PAHs was recorded in Agbada 1 flow station, while the lowest concentration was recorded in Agbada 2 flow station. A significant level of pollution was also observed in the soil of Bomu pipeline at Obio Akpor LGA. Also, the distribution of n-alkanes in the samples was also used to assess the level of pollution in the studied area. Various n-alkanes and PAHs ratios were obtained to know the major source of pollution in the area under study. The main source of pollution was pyrolytic, which might be due to the gas flaring activities going on in the study area. Also, the results showed that n-alkanes could also be a complementary tool in assessing pollution and source apportionment.     

Salinity variations in the northern Coorong Lagoon,South Australia: Significant changes in the ecosystem following human alteration to the natural water regime

European settlement and drought have significantly impacted the hydrology of the Coorong, a shallow coastal lagoon complex in South Australia, which is part of a terminal wetland at the mouth of the River Murray. An increased salinity associated with lower water levels and progressive isolation from ocean flushes contributed to a severe decline in ecological diversity over the past decades. Here we have conducted a molecular and stable isotopic study of a sedimentary core from the northern Coorong Lagoon spanning more than 5000 years to investigate the recent palaeoenvironmental history of the ecosystem. Major alterations were evident in many biogeochemical parameters in sediments deposited after the 1950s coinciding with the beginning of intensified water regulations. The most prominent shift occurred in δ¹³C profiles of C₂₁–C₃₃ n-alkanes from average values of −23.5‰ to an average of −28.2‰. Further changes included decreases in carbon preference index (CPI) and average chain length (ACL) of the n-alkane series as well as significant increases in algal (e.g. C₂₀ HBI, long chain alkenes and C₂₉-alkadiene) and bacterial (e.g. ¹³C depleted short chain n-alkanes and hopanoids, δ¹³C: −35.9‰ to −30.1‰) derived hydrocarbons. Long chain n-alkanes with a strong odd/even predominance as observed here are typically attributed to terrigenous plants. In the Coorong however, terrigenous input to sedimentary OM is only minor. Therefore changes in the before mentioned parameters were attributed to a source transition from a major contribution of macrophytes towards predominantly microalgae and bacteria.δD values of C₂₁–C₃₃ n-alkanes showed a general trend towards more enriched values in younger sediments, indicating an overall rising salinity. However, the most pronounced positive shift in these profiles again occurred after the 1950s. Altogether this study demonstrates that the recent human induced changes of the Coorong hydrology, compounded by a severe drought led to an increase in salinity and alterations of primary production which have been much more significant than natural variations occurring throughout the Holocene over several thousands of years.     

Experimental investigation of the compositional variation of acyclic paraffins during expulsion from source rocks

Anhydrous non-isothermal heating experiments were conducted under controlled compressive stress on cylindrical plugs of six oil shales from Permian through Eocene age. The objective of this study was to compare the distribution of acyclic paraffins in initial, residual and expelled organic matter and to highlight causes of compositional differences resulting from expulsion. Pristane generation from kerogen is highest in the Eocene Messel shale and affects the pristane / phytane (pr / ph) ratio commonly used as a redox proxy. The isoprenoid to n-alkane ratios (pr / n-C₁₇, ph / n-C₁₈) decrease during generation and are lowest in the residual bitumen due to preferential generation and retention of n-alkanes. The n-alkane distribution shows that only lacustrine shales produce high wax oils. Evaporative fractionation leads to loss of n-alkanes up to n-C₂₀ with boiling points below 350 °C. This demonstrates that lacustrine and marine shales may lead to accumulation of low wax oils due to evaporative fractionation after expulsion.     

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.     

A comparison of terpenoid and leaf fossil vegetation proxies in Paleocene and Eocene Bighorn Basin sediments

Plant-derived terpenoids, long recognized as biomarkers, can help reveal the major taxonomic groups of land plants present in ancient environments, even if rocks and sediments do not preserve plant macro- or microfossils. Previous studies have used simple di- to triterpenoid ratios to reconstruct floral changes in the geologic past, but few have compared terpenoid ratios with estimates of floral composition from fossils. Further, reconstructions have not taken into account differences in biomarker production (i.e. concentration relative to leaf biomass) between different types of plants. Here, we have examined terpenoids from early Cenozoic fluvial rocks from the Bighorn Basin (Wyoming, USA), where fossil flora has been studied in detail. We analyzed the distributions of diterpenoids, triterpenoids and n-alkanes from leaf wax in a total of 43 samples from 15 stratigraphic horizons of late Paleocene (63 Ma) to early Eocene (53 Ma) in age. In nearly all samples, triterpenoids, derived from angiosperms, were significantly lower in abundance than conifer-specific diterpenoids, a finding that contrasted with plant fossil evidence for the same rocks. This suggests that di- to triterpenoid ratios severely underestimate the abundance of angiosperms in paleovegetation. Angiosperms dominated n-alkane production among modern plants, and we propose a new paleovegetation proxy based on the ratio of diterpenoids (conifers) to n-alkanes (angiosperms), corrected for lipid production estimated from extant vegetation. Using diterpenoids and alkanes, we infer the composition of paleovegetation to be similar to that inferred from plant fossils. Although the approach works well for the Bighorn Basin, we stress the new paleovegetation proxy will need to be evaluated for other time periods, communities, paleogeography and depositional environments with pollen or megafossil data available.     

Distributions of n-alkanes and their hydrogen isotopic composition in plants from Lake Qinghai (China) and the surrounding area

Various aquatic plants from Lake Qinghai, the largest inland saline lake in China, and terrestrial plants from the surrounding area were investigated for the distribution of n-alkanes and their δD values. The n-alkanes in the samples range from C₁₅ to C₃₃ with C preference index (CPI) values of 4.0–29.7. The n-C₂₃ or n-C₂₅ alkane is the dominant compound in the aquatic submerged plants. The aquatic emergent and terrestrial plants have an abundance maximum at n-C₂₇, n-C₂₉ or n-C₃₁. The average chain length (ACL) values, ranging from 26.0 to 29.6, are closely related to the plant species. The n-alkanes from the aquatic plants have mean δD values of −169‰ to −121‰ and those from the terrestrial plants values of −173‰ to −109‰. The H isotopic composition (δD) and fractionation differ significantly among the plants studied. Comparison shows that additional evaporative enrichment of the lake water associated with saline lakes and humidity influence the δD values of the n-alkanes in aquatic and terrestrial plants, respectively. The mean δD values of n-alkanes in the plants decrease with increasing ACL value. The n-alkanes from the different types of plants are more depleted in D relative to environmental water and those from aquatic plants (with a mean value of −143‰) have a greater isotopic fractionation than terrestrial plants (mean value −113‰).     

Biomarkers of novel ecosystem development in boreal forest soils

Novel ecosystem development is occurring within the western boreal forest of Canada due to land reclamation following oil sand surface mining. Sphagnum peat is the primary organic amendment used to reconstruct soil in these novel ecosystems. We hypothesised that ecosystem recovery would be indicated by an increasing similarity in the biomolecular characteristics of novel soil organic matter (SOM) derived from peat to those of natural boreal ecosystems. We evaluated the use of the homologous series of long chain (⩾ C₂₁) n-alkanes with odd/even predominance to monitor the re-establishment of boreal forest on these anthropogenic soils. The lipids were extracted from dominant vegetation inputs and SOM from a series of natural and novel ecosystem reference plots. Twice the concentration of n-alkanes was extracted from natural than from novel ecosystem SOM (p < 0.01). We observed unique n-alkane signatures for the source vegetation, e.g. peat material was dominated by C₃₁, and aspen (Populus tremuloides Michx.) leaves by C₂₅. The n-alkane distribution differed between the two systems (p < 0.001) and reflected the dominant vegetation input, i.e. peat or tree species. Our results indicate that further research is required to clarify the influence of vegetation or disturbance on the signature of n-alkanes in SOM; however, the use of n-alkanes as biomarkers of novel ecosystem development is a promising application.     

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

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

Occurrence and distribution of long-chain acyclic ketones in immature coals

Seven coal and carbonaceous mudstone samples were collected from outcropping Jurassic coal beds, on the margin of the Dingxi Basin, Northwestern China. The n-alkane distributions in all of the samples are characterised by high concentrations of the C₁₉–C₂₉ homologues, and very much lower amounts outside of this range. C₂₃ or C₂₄ are usually the most abundant n-alkanes. Straight chain n-alkanes from C₂₃ to C₂₉ show moderate odd-to-even C number predominances (CPI range: 1.26–2.70). Long-chain acyclic n-alkan-2-ones, n-alkan-3-ones and n-alkan-4-ones ranging from C₁₅ to C₃₃ with moderate odd-to-even C number predominances, were detected together with one isoprenoid methyl ketone (6,10,14-trimethylpentadecan-2-one) in all of the samples. The C number distributions of the three series of alkanones show a similar distribution to that of the n-alkanes, but the correspondence is not sufficient to substantiate a product–precursor relationship. It can be concluded that the n-alkan-2-ones are a mixture of the products of microbially-mediated β-oxidation of corresponding n-alkanes in the sediments and from the microbial oxidation of higher plant-derived n-alkanes prior to incorporation in the sediments. The n-alkan-3-ones and n-alkan-4-ones were formed from microbially mediated oxidation of the corresponding n-alkanes in the γ and δ positions, respectively. Generation of the ketones from higher plant n-fatty alcohols and n-alkanoic acids could be a possible way to form some of the ketones observed, but it can only play a minor role in the samples analysed.