Different source of n-alkanes and n-alkan-2-ones in a 6000 cal. yr BP Sphagnum-rich temperate peat bog (Roñanzas,N Spain)

The most widely accepted origin of n-alkan-2-ones in peats is the microbial oxidation of the related n-alkanes and/or oxidative decarboxylation of fatty acids derived from plant input. The distributions of n-alkanes and n-alkan-2-ones in 48 samples from the Roñanzas 6000 cal. yr BP peat bog profile (N Spain) do not justify a single source. The n-alkan-2-ones typically dominate the n-alkanes, maximizing at C₁₉ or C₂₅/C_27, whereas the n-alkanes maximized either at C₂₃ or at C₃₁/C₃₃. The averaged δ¹³C values of the n-alkanes ranged from −32.3‰ to −33.1‰, but those of the n-alkan-2-ones were consistently higher (−29.2‰ to −29.9‰), suggesting a different, probably bacterial, source for the ketones.     

n-Alkan-2-one distributions in a northeastern China peat core spanning the last 16 kyr

Most research on long chain methyl ketones has focused on their origins and distributions. Their application in paleoclimate studies is less common than that of other n-alkyl lipids. The goal of this research was to explore this potential by studying n-alkan-2-ones from the Hani peat sequence in northeastern China. They were identified using gas chromatography-mass spectrometry (GC-MS) and showed a distribution ranging from C₁₉ to C₃₁ with a strong odd/even predominance. This type of distribution is considered to derive from Sphagum and microbial oxidation of n-alkanes. Comparison with climate sensitive indicators and macrofossil analysis shows that microbial oxidation of n-alkanes derived from higher plants was enhanced during the warm early Holocene period. This led us to develop three n-alkan-2-one proxies - C₂₇/ΣC_23-31 (C₂₇/HMW-KET), carbon preference index (CPI_H-KET) and average chain length (ACL_(27-31)-KET) - as possible indicators of paleoclimate in the peat-forming environment. These proxies, in combination with C₂₇n-alkane δD values and peat cellulose δ¹⁸O records, might allow examination of paleo-ecosystem behavior during climatic evolution in northeastern China over the past 16,000 yr.     

Distribution of n-alkan-2-ones in Qionghai Lake sediments,southwest China,and its potential for late Quaternary paleoclimate reconstruction

Studies on long-chain n-alkan-2-ones from lake sediments remain sparse. In this study, we present an n-alkan-2-one record from Qionghai Lake, southwest China, to assess the paleoclimate significance of variations in their compositions. A homologous series of n-alkan-2-ones ranging from C₂₁ to C₃₅ were identified, with maximum concentrations of the C₂₉ or C₃₁ chain lengths and a strong odd-over-even predominance. This type of n-alkan-2-one is considered to derive mainly from microbial oxidation of the corresponding n-alkanes, and partial inputs from plants. The n-alkan-2-one-derived average chain length (ACL) and carbon preference index (CPI) values changed significantly over the past 28k cal a bp , consistent with the sediment grain size and n-alkane proxies from the same core. Generally, the high CPI_27-33-ket and low ACL_27-33-ket values indicated cold and dry climates such as for the Last Glacial Maximum (23.2–19.7k cal a bp ), Heinrich 1 event (17.6–15.6k cal a bp ) and Younger Dryas (12.8–11.6k cal a bp ), but low CPI_27-33-ket and high ACL_27-33-ket values denoted a warm and humid Holocene Climatic Optimum (7.0–4.3k cal a bp ). Therefore, n-alkan-2-ones have great paleoclimatic potential and can be applied together with other biomarkers to reconstruct a reliable paleoclimate record in lake 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.     

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.     

Environmental evolution recorded by lipid biomarkers from the Tawan loess–paleosol sequences on the west Chinese Loess Plateau during the late Pleistocene

This study provides a reconstruction of the environmental evolution since 128 ka recorded by the lipid biomarkers of the C₁₅–C₃₅ n-alkanes, the C₁₃–C₃₃ n-alkan-2-ones and the C₁₂–C₃₀ n-alkanols isolated from the Tawan loess section, Northwest China. Variations in paleoenvironment are reconstructed from the values of the carbon preference index (CPI), the average chain length (ACL), the L/H (ratio of lower-molecular-weight to higher-molecular-weight homologues), the n-alkane C₂₇/C₃₁ ratios, and the n-alkan-2-one C₂₇/C₃₁ ratio. These parameters indicate the dominance of grasses over the west Chinese Loess Plateau (CLP) during the late Pleistocene. Lower values of the CPI and the ACL values, respectively, indicate stronger microbial reworking of organic matter and changes in plant species, which are both indicative of a warmer-wetter environment. Furthermore, the fluctuations of environment recorded in the Tawan section exhibit ten phases that show obvious cycles between warm periods and cold intervals. This study reveals that changes in the biomarker proxies agree well with changes in the magnetic susceptibility and grain size, and it indicates a huge potential for paleoenvironmental reconstructions by using the n-alkan-2-one and n-alkanol proxies.     

Effect of climate change on delivery and degradation of lipid biomarkers in a Holocene peat sequence in the Eastern European Russian Arctic

Lipid biomarkers from a peat plateau profile from the Northeast European Russian Arctic were analyzed. The peat originated as a wet fen ca. 9 ka BP and developed into a peat bog after the onset of permafrost ca. 2.5 ka BP. The distributions and abundances of n-alkanols, n-alkanoic acids, n-alkanes, n-alkan-2-ones and sterols were determined to study the effect of degradation on their paleoclimate proxy information. Plant macrofossil analysis was also used in combination with the lipid distributions. The n-alkanol and n-alkanoic acid distributions in the upper part of the sequence generally correspond to compositions expected from plant macrofossil assemblages. Their carbon preference index (CPI) values increase with depth and age, whereas those of the n-alkanes decrease. The different CPI patterns suggest that n-alkanoic acids and n-alkanols deeper in the sequence may be produced during humification through alteration of other lipids. Excursions in the n-alkanoic acid content also suggest an important contribution of invasive roots to the lipid biomarker composition. The CPIs associated with these compounds show that under permafrost conditions organic material from Sphagnum is better preserved than material from vascular plants. Increasing stanol/stenol ratio values and decreasing n-alkane CPI values indicate progressive degradation of organic matter (OM) with depth. The n-alkan-2-one/n-alkane and n-alkan-2-one/n-alkanoic acid ratios were shown to be useful proxies that can reflect the degree of OM preservation and suggest that both microbial oxidation of n-alkanes and decarboxylation of n-alkanoic acids produce n-alkan-2-ones in this peat sequence.     

Molecular and isotopic stratigraphy in an ombrotrophic mire for paleoclimate reconstruction

A 40 cm deep Sphagnum-dominated peat monolith from Bolton Fell Moss in Northern England was systematically investigated by lipid molecular stratigraphy and compound-specific δ¹³C and δD analysis using gas chromatography (GC), GC-mass spectrometry (GC-MS), GC-combustion-isotope ratio-MS (GC-C-IRMS) and GC-thermal conversion-IRMS (GC-TC-IRMS) techniques. ²¹⁰Pb dating showed the monolith accumulated during the last ca. 220 yr, a period encompassing the second part of Little Ice Age. While the distributions of lipids, including n-alkan-1-ols, n-alkan-2-ones, wax esters, sterols, n-alkanoic acids, α,ω-alkandioic acids and ω-hydroxy acids, display relatively minor changes with depth, the cooler climate event was recorded in the concentrations of n-alkanes and organic carbon, CPI values of n-alkanes and n-alkanoic acids, and the ratio of 5-n-alkylresorcinols/sterols. Superimposed on the fossil fuel effect, the relatively cooler climate event was also recorded by δ¹³C values of individual hydrocarbons, especially the C₂₃n-alkane, a major compound in certain Sphagnum spp. The δD values of the C₂₉ and C₃₃n-alkanes correlated mainly with plant composition and were relatively insensitive to climatic change. In contrast the C₂₃n-alkane displayed variation that correlated strongly with recorded temperature for the period represented by the monolith, agreeing with previously reported deuterium records in tree ring cellulose spanning the same period in Scotland, Germany and the USA, with more negative values occurring during the second part of Little Ice Age. These biomarker characteristics, including the compound-specific δ¹³C and δD records, provide a new set of proxies of climatic change, potentially independent of preserved macrofossils which will be of value in deeper sections of the bog where the documentary records of climate are unavailable and humification is well advanced.     

Distribution patterns and stable carbon isotopic composition of alkanes and alkan-1-ols from plant waxes of African rain forest and savanna C3 species

Leaf wax components of terrestrial plants are an important source of biomass in the geological records of soils, lakes and marine sediments. Relevant to the emerging use of plant wax derived biomarkers as proxies for past vegetation composition this study provides key data for C₃ plants of tropical and subtropical Africa. We present analytical results for 45 savanna species and 24 rain forest plants sampled in their natural habitats. Contents and distribution patterns of long chain n-alkanes (n-C₂₅ to n-C₃₅) and n-alkan-1-ols (n-C₂₄ to n-C₃₄) as well as bulk and molecular carbon isotopic data are presented. The variations of the analysed parameters among different growth forms (herb, shrub, liana and tree) are small within the vegetation zones, whereas characteristic differences occur between the signatures of rain forest and savanna plants. Therefore, we provide averaged histogram representations for rain forest and savanna C₃ plants.The findings were compared to previously published data of typical C₄ grass waxes of tropical and subtropical Africa. Generally, trends to longer n-alkane chains and less negative carbon isotopic values are evident from rain forest over C₃ savanna to C₄ vegetation. For n-alkanols of rain forest plants the maximum of the averaged distribution pattern is between those of C₃ savanna plants and C₄ grasses. The averaged presentations for tropical and subtropical vegetation and their characteristics may constitute useful biomarker proxies for studies analysing the expansion and contraction of African vegetation zones.     

A 220 ka palaeoenvironmental reconstruction of the Fuentillejo maar lake record (Central Spain) using biomarker analysis

The sedimentary record of the volcanic lake known as the Fuentillejo maar (central Spain) offers the opportunity to determine the palaeoenvironmental evolution of the central-southern part of the Iberian Peninsula during the Middle and Upper Pleistocene. A total of 439 samples from the upper 88 m of a ca. 142 m long core (spanning the last ca. 355 ka) were examined. The interval corresponds to the last 220 ka of the record. High n-alkane CPI values and the presence of sulfur revealed that little diagenetic signal attenuation or modification had occurred. A number of n-alkane-based indices (e.g. predominant n-alkane, ACL, Paq and TAR_HC ratios, and the proportion of C₂₇, C₂₉ and C₃₁ n-alkanes with respect to the summed C₂₇ + C₂₉ + C₃₁) alkanes, showed changing conditions in the organic matter input to the maar lake over time, with episodes involving a major input of terrestrial vegetation, alternating with others in which algae or a mixed input of terrestrial plants, aquatic macrophytes and algae occurred. The proportions of C₂₇, C₂₉ and C₃₁ were useful for reconstructing the palaeoenvironmental evolution of the basin as they allowed assignment of several dry intervals during which grasses developed, while during humid phases trees expanded at the expense of grasses.In general, we found good correspondence between these episodes and the climatic cycles observed in δ¹⁸O records from ices cores in Greenland and Antarctica, as well as from marine cores. Thus, we conclude that global palaeoclimatological changes were recorded in the maar record. Likewise, Heinrich Events appeared to be reflected in the n-alkane indices. This was confirmed by single spectral analysis and cross spectral analysis with the insolation curve on 21 July at a latitude of 65°N. The approach revealed that the cycles interpreted in the Fuentillejo record (103, 41, 23 and 19 ka) showed evidence of orbital influence related to the astronomical cycles of the eccentricity variation in the Earth’s orbit, the Earth’s obliquity oscillation, and the precession variation (Milankovitch theory).     

Normal Alkane Distributions in Soil Samples along a Lhasa-Bharatpur Transect

We present n-alkane distributions in the soil samples along a transect from Lhasa on the Tibetan Plateau to Bharatpur in Nepal, which covers a large geographical area and a wide range of climatic conditions. These data allow us to assess the significance of n-alkane distributions in different vegetation types and their relationships to temperature and precipitation. In the tropical rainforest and broadleaved forest zones, n-alkanes exhibit a bimodal distribution pattern with dominant homologues around n-C_(31) and n-C_(23). The bimodal distribution of tropical rainforest n-alkanes may be a general pattern because of the presence of many lianas, epiphytic plants, algae and the strong microbial activity and degradation involved in the postdepositional process. In the warm-temperate mixed forest and needle-leaved forest zones, the long-chain alkanes have a pattern of n-C_(31) n-C_(33) n-C_(27). In the alpine shrub and grassland zone, although the most abundant homologue is n-C_(31) , relatively high n-C_(23) concentrations have also been observed in some samples. The statistical results show a good correlation between n-alkane proxies and climatic factors. The average chain length(ACL) values are positively correlated with precipitation and temperature across the transect. The carbon preference index(CPI) values show a negative correlation with temperature and precipitation along a Lhasa–Bharatpur transect. Although there is a positive relationship between ACL and temperature, it is difficult to separate the two climatic variables(temperature and precipitation) because they are well coupled in the monsoon region.     

n-alkane distribution coupled with organic carbon isotope composition in the shell bar section, Qarhan paleolake, Qaidam basin, NE Tibetan Plateau

Lipids extracted from lacustrine deposits in the paleolake Qarhan of the Qaidam basin in the northeastern Tibetan Plateau were determined by conventional gas chromatography-mass spectrometry. Several series of biomarkers were identified, mainly including n-alkanes, n-alkan-2-ones, n-alkanoic acids, branched alkanes, triterpenoids and steroids, indicative of various biogenic contributions. On the basis of cluster analysis, the n-C₁₅, n-C₁₇, n-C₁₉ alkanes were proposed to be derived from algae and/or photosynthetic bacteria, the n-C₂₁, n-C₂₃, n-C₂₅ homologues from aquatic plants, and the n-C₂₉, n-C₃₁ homologues from vascular plants. In contrast, the n-C₂₇ alkane is not categorized in the n-C₂₉ and n-C₃₁ group of alkanes, probably due to more complex origins including both aquatic and vascular plants, and/or differential biodegradation. Stratigraphically, layers-2, 4 and 5 were found to show a close relationship in n-alkane distribution, associated with a positive shift in carbon isotope composition of bulk organic matter (δ¹³C_org), inferring a cold/dry period. Layers-1 and 6 were clustered together in association with a negative δ¹³C_org, excursion, probably indicating a relatively warm/humid climate. The potential coupling between the n-alkane distributions and δ¹³C_org, suggests a consequence of vegetation change in response to climate change, with the late MIS3 being shown to be unstable, thought to be the climatic optimum in the Tibetan Plateau. Our results suggest that the cluster analysis used in this study probably provides an effective and authentic method to investigate the n-alkane distribution in paleolake sediments.     

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).     

Mathematical modeling of the aquatic macrophyte inputs of mid-chain n-alkyl lipids to lake sediments: Implications for interpreting compound specific hydrogen isotopic records

We present a systematic study of chain-length distributions and D/H ratios of n-alkyl lipids (both n-alkanes and n-alkanoic acids) in a wide range of terrestrial and aquatic plants around and in Blood Pond, Massachusetts, USA. The primary goal is to establish a model to quantitatively assess the aquatic plant inputs of the mid-chain length n-alkyl lipids to lake sediments and to determine the average hydrogen isotopic ratios of these lipids in different plants. Our results show that middle-chain n-alkyl lipids (C₂₁-C₂₃n-alkanes and C₂₀-C₂₄n-alkanoic acids) are exceptionally abundant in floating and submerged aquatic plants, in contrast to the dominance of long-chain n-alkyl lipids (C₂₇-C₃₁n-alkanes and C₂₆-C₃₂n-alkanoic acids) in other plant types, which are consistent with previously published data from Mountain Kenya and the Tibetan Plateau. Combining available data in different environmental settings allows us to establish statistically robust model distributions of n-alkyl lipids in floating/submerged macrophytes relative to other plant types. Based on the model distributions, we established a multi-source mixing model using a linear algebra approach, in order to quantify the aquatic inputs of mid-chain n-alkyl lipids in lake sediments. The results show that ∼97% of the mid-chain n-alkyl lipids (C₂₃n-alkane and C₂₂n-acid (behenic acid)) in Blood Pond sediments are derived from floating and submerged macrophytes. In addition, D/H ratios of C₂₂n-acid and C₂₃n-alkane in the floating and submerged plants from Blood Pond display relatively narrow ranges of variation (−161 ± 16‰ and −183 ± 18‰, respectively). Our study demonstrates that mid-chain n-alkyl lipids such as C₂₃n-alkane and C₂₂n-acid could be excellent recorders of past lake water isotopic ratios in lakes with abundant floating and submerged macrophyte inputs.     

Compound specific δD values of long chain n-alkanes derived from terrestrial higher plants are indicative of the δD of meteoric waters: Evidence from surface soils in eastern China

The stable carbon and hydrogen isotope composition of higher plant-derived long chain n-alkanes (δ¹³C_n-alkanes and δD_n-alkanes) from 45 surface soil samples (within well characterized vegetation zones) from eastern China (18°N–50°N) are reported. The weighted average δD_n-alkanes value for n-C₂₇, n-C₂₉ and n-C₃₁ in the samples and the annual average δD of meteoric water recorded at 12 weather stations proximal to the sampling sites show similar spatial variations. The δD of n-alkanes shows a gradual depletion in value with increasing latitude. The results demonstrate that, on a large spatial scale, the δD values of long chain n-alkanes derived from higher plants have the potential to record the δD of meteoric water, although many other factors can also influence the isotope values. There appears to be no apparent relationship between the δD of the n-alkanes extracted from the surface soil and the overlying vegetation type (i.e. forest/grassland or C₃/C₄ composition). Therefore, palaeoenvironmental studies utilizing δD_n-alkanes from higher plant-derived material in geological samples have the potential to provide additional information with regard to the past hydrological cycle.     

Calibrating n-alkane Sphagnum proxies in sub-Arctic Scandinavia

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

Effect of different wetness conditions on Sphagnum lipid composition in the Erxianyan peatland,central China

Samples of Sphagnum palustre and peat from the Erxianyan peatland, central China, were analyzed for lipids and their carbon isotopes to investigate how lipid distributions respond to hydrological change and to evaluate the importance of the contribution of microbial lipids to the peat moss. The lipids in samples collected from different hydrological settings in and around a pond and in the central part of the bog show clear variation along the hydrological gradient, with higher n-C₂₃/n-C₂₅ alkane ratio values and lower ACL (average chain length) values of long chain n-alkanes, n-fatty alcohols and n-fatty acids at the wetter sites. Although the relationship between the S. palustre lipids and the hydrological conditions can be partially overprinted in peat by an input from vascular plants, lipid ratios such as P_aq and ACL can provide useful qualitative information about Sphagnum contributions. In addition, lipid composition and carbon isotope values provide information about microbial activity associated with S. palustre. The occurrence of a high abundance of 7-methylheptadecane in submerged S. palustre is an indication of cyanobacteria in the living peat moss. The relatively ¹³C-depleted carbon isotope values of the n-C₂₃ alkane could result from the influence of symbiotic methanotrophs on the carbon available for assimilation by S. palustre.     

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

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

Differential hydrogen isotopic ratios of Sphagnum and vascular plant biomarkers in ombrotrophic peatlands as a quantitative proxy for precipitation—evaporation balance

We have developed a new approach to quantitatively reconstruct past changes in evaporation based on compound-specific hydrogen isotope ratios of vascular plant and Sphagnum biomarkers in ombrotrophic peatland sediments. We show that the contrast in H isotopic ratios of water available to living Sphagnum (top 20 cm) and in the rooting zone of peatland vascular plants can be used to estimate “?”—the fraction of water remaining after evaporation. Vascular plant leaf waxes record H isotopic ratios of acrotelm water, which carries the D/H ratio signature of precipitation and is little affected by evaporation, whereas the Sphagnum biomarker, C₂₃n-alkane, records H isotopic ratios of the water inside its cells and between its leaves, which is strongly affected by evaporation at the bog surface. Evaporation changes can then be deduced by comparing H isotopic ratios of the two types of biomarkers. We calibrated D/H ratios of C₂₃n-alkane to source water with lab-grown Sphagnum. We also tested our isotopic model using modern surface samples from 18 ombrotrophic peatlands in the Midwestern United States. Finally, we generated a 3000-year downcore reconstruction from Minden Bog, Michigan, USA. Our new record is consistent with records of other parameters from the same peatland derived from different proxies and allows us to differentiate precipitation supply and evaporative loss.     

干旱-半干旱区兰州兴隆山现代森林典型植被和土壤中类脂物分子特征及其意义

利用GC－MS对西北干旱区-半干旱区兰州兴隆山森林区典型植被与现代土壤样品中可抽提的类脂物进行了系统分析,检测出包括正构烷烃和α正构脂肪酮等一系列类脂物分子化石。在未进行族分离的情况下,检出兴隆山现代森林木本植物的正构烷烃以C₂₇或C₂₉为主峰,草本植物则以C₃₁为主峰;  土壤样品中均保留有很强的、几乎均势的C₂₇,C₂₉和C₃₁信号; α正构脂肪酮具有明显的奇碳优势,植物中以C₂₃,C₂₅或C₂₉为主峰,土壤样品以C₂₇或C₂₉为主峰;  其反映的信息是植被与微生物共同作用的结果。本研究为理解现代土壤和古土壤中类脂物所指示的植被信息、进一步探讨类脂物分子在植被(有机质)-微生物-土壤-湖泊沉积体系中存在与相互转换关系这一基础理论问题提供了科学资料。同时,为分子化石分析方法的改进和不同处理过程所得数据之间的对比和科学解释提供资料。