Composition Of N-alkanes Research Articles

Organic geochemical characteristics of Eocene crude oils from Zhanhua Depression, Bohai Bay Basin, China

Geochemical studies of crude oil and source rock play an important role in future exploration in Zhanhua Depression. In this study, thirty-one oil samples collected from Shahejie Formation in Zhanhua Depression, Bohai Bay Basin, NE China have been geochemically analyzed and their organic geochemical characteristics have been applied to differentiate groups of oils. These oil samples can be classified into two families based on multiple biomarker proxies and stable carbon isotopic values. Family I is characterized by a low ratio of pristane over phytane (Pr/Ph < 0.7), a relatively high ratio of phytane over n-C18 (Ph/n-C18), varying ratios of gammacerane over C30 hopane (Ga/C30H) and C22/C21 tricyclic terpane, and a low ratio of C19/C23 tricyclic terpane. Family II is marked by a relatively high Pr/Ph ratio (0.7–1.6), relative low ratios of Ph/n-C18 and C22/C21 tricyclic terpane, and avarying ratio of C19/C23 tricyclic terpane. Both families I and II within these crude oils can be subdivided into two families based on different values of stable carbon isotopic composition of individual n-alkanes. Moreover, the potential source rocks of oil samples in Family I and Family II were likely derived from the upper Es4 member and Es3 member, respectively, based on the correlation of organic geochemical characteristics of the oils and source rocks. The results of oil–source rock correlation provide insight into the process from oil generation to migration and to final accumulation, providing a better understanding of factors controlling oil–gas distribution for prediction of sweet spots.

Sources of Organic Matter in the Bottom Sediments of Small Rivers Estuaries in Basin of the Tatar Strait

The distribution of organic matter (OM), hydrocarbons, phytopigments and ecological–trophic groups of bacteriobenthos has been investigated in the bottom sediments of the internal estuaries of the small Muchke and Toki rivers. The morphological and hydrological characteristics of estuaries influence the quantitative and qualitative composition of the OM in bottom sediments. The investigation into the molecular and group composition of n-alkanes in bottom sediments has identified the main genetic types of OM closely associated with the specific hydrodynamics of waters and sedimentation in small tidal estuaries.

Experimental simulation study on the influence of diagenetic water medium on sedimentary n-alkanes and their respective hydrogen isotopes

In order to understand the influence of the diagenetic water medium on the hydrogen isotope of sedimentary n-alkanes, thermal simulation experiments were carried out involving the initial peat under the addition of alkaline saltwater, acidic freshwater and peat alone condition. The generated n-alkanes were analyzed in terms of molecule composition and hydrogen isotopic composition of individual n-alkanes. The results show that the addition of water had a significant effect on n-alkane nC21−/nC22+ ratio, ACL value and Pr/Ph ratio. The n-alkanes display significant heavier hydrogen isotopic composition from the alkaline saltwater added experiments, but obvious lighter hydrogen isotopic composition from the acidic freshwater added experiments. The difference of the average δD value of n-alkanes between the saltwater and freshwater experiments ranged from 6‰ to 73‰, and maximum difference occurred from the experiments of 350 °C. The exchange of inorganic hydrogen in water and organic hydrogen was more intensive in the acidic freshwater experiments than that in the alkaline saltwater experiments. The average δD value of n-alkanes generated under the participation of saltwater and freshwater media appear to be higher heavier along with the increase in experimental temperature. Also, the average δD value of n-alkanes generated from the saltwater added experiments was always higher than that from the freshwater added experiments. The correlations among the δD composition of n-alkanes generated under different condition were established. These results provide information to understand of the genesis of sedimentary n-alkanes formed under the participation of diverse diagenetic water media at various stages of maturity.

Geochemical insights into contribution of petroleum hydrocarbons to the formation of hydrates in the Taixinan Basin, the South China Sea

Methane hydrate in the South China Sea (SCS) has extensively been considered to be biogenic on the basis of its δ13C and δD values. Although previous efforts have greatly been made, the contribution of thermogenic oil/gas has still been underestimated. In this study, biomarkers and porewater geochemical parameters in hydrate-free and hydrate-bearing sediments in the Taixinan Basin, the SCS have been measured for evaluating the contribution of petroleum hydrocarbons to the formation of hydrate deposits via a comparative study of their source inputs of organic matters, environmental conditions, and microbial activities. The results reveal the occurrence of C14–C16 branched saturated fatty acids (bSFAs) with relatively high concentrations from sulfate-reducing bacteria (SRBs) in hydrate-bearing sediments in comparison with hydrate-free sediments, which is in accord with the positive δ13C values of dissolved inorganic carbon (DIC), increasing methane concentrations, decreasing alkalinity, and concentration fluctuation of ions (Cl−, Br−, SO42−, Ca2+, and Mg2+). These data indicate the relatively active microbial activities in hydrate-bearing sediments and coincident variations of environmental conditions. Carbon isotope compositions of bSFAs (−34.0‰ to −21.2‰), n-alkanes (−34.5‰ to −29.3‰), and methane (−70.7‰ to −69.9‰) jointly demonstrate that SRBs might thrive on a different type of organic carbon rather than methane. Combining with numerous gas/oil reservoirs and hydrocarbon migration channels in the SCS, the occurrence of unresolved complex mixtures (UCMs), odd-even predominance (OEP) values (about 1.0), and biomarker patterns suggest that petroleum hydrocarbons from deep oil/gas reservoirs are the most probable carbon source. Our new results provide significant evidence that the deep oil/gas reservoirs may make a contribution to the formation of methane hydrate deposits in the SCS.

Surface soil n-alkane molecular and δD distributions along a precipitation transect in northeastern China

Hydrogen isotope compositions of leaf wax long-chain n-alkanes (δDalk) buried in soils have the potential to provide records of continental paleoclimatic and paleohydrological changes. However, different factors and processes influence the δDalk signature, making interpretation of the soil isotope record complicated. The relative humidity/aridity and vegetation types (C3 vs C4 plants) are two major factors responsible for the soil δDalk variations. In most cases, the interaction between relative humidity/aridity and temperature cannot be separated. Here, we analyzed the δDalk compositions in surface soils along the Northeast China Transect (NECT), where the mean annual precipitation (MAP) varies from ca. 120 to 815 mm, yet the mean annual temperature changes little, providing a setting to test the role of relative humidity/aridity in soil δDalk variations, independent of temperature changes. At the same time, the western part of the NECT, which is mainly a grassland environment, provides a test of the effects of C3 and C4 grasses on soil δDalk changes. In the semi-humid eastern NECT region, the δDalk values and their apparent fractionation relative to precipitation exhibit a strong negative correlation with the aridity index and a close relationship with soil water content. This finding supports the idea that the soil δDalk values are mainly influenced by relative aridity and that the impact of evapotranspiration, including soil evaporation and transpiration-induced 2H enrichment of woody plant leaf water, plays a vital role in shaping soil δDalk. In the semi-arid and arid western NECT region, the δDalk values and their apparent fractionations show no relationship with any environmental factors. However, in the westernmost area, soil δDalk values tend to show a negative relationship with the increasing proportion of C4 plants. This pattern probably indicates that in the arid area the soil δDalk values vary more closely with plant types than with the environmental variables. This study provides new insights into deciphering the relationship between soil δDalk values and relative humidity/aridity, and it affirms the applicability of δDalk in paleo-hydrologic reconstructions.

Carbon and hydrogen isotopes of n-alkanes in soils reconstructed after mining disturbance.

Ecosystem reconstruction after mining disturbance is a challenge considering the multitude of factors that affect soil formation and revegetation. In the boreal forest of western Canada, peat material is often used as the organic amendment for land reclamation to upland forest. Carbon and water dynamics of peat-dominated ecosystems differ from natural upland forest soils. The objective of this work was to evaluate the evolution of soils reconstructed after mining disturbance using 13 C and 2 H analyses of n-alkane tracers. Ten soils from natural ecosystems were sampled (0-10cm) and compared with 11 soils from novel ecosystems ranging in age from 0 to 30 yr, as well as a fresh peat sample. Soils supported different vegetation, including pine (Pinus spp.), aspen (Populus spp.), and white spruce [Picea glauca (Moench) Voss]. Despite overlaps for some individual n-alkanes, we found a dominance of n-C25 in reconstructed soils, also dominant in the peat material, and a dominance of n-C27 in natural soils, one of the dominant n-alkanes in natural forest vegetation. In addition, there was a significant difference in odd n-alkane δ2 H and δ13 C values between natural and reconstructed soils (p<.05). Differences in δ2 H values, more negative for reconstructed soils than for natural soils, were attributed to changes in soil moisture, from wetter peat-dominated soils to drier upland forests; among forest types, δ2 H values were most negative under pine vegetation. The δ13 C composition of odd n-alkanes, in particular n-C27 , was significantly related to tree age (p<.05). Overall, both 2 H and 13 C isotopic signatures of odd n-alkanes exhibited differences between natural and reconstructed soils. However, within the reconstructed soils, neither isotopic signature showed a clear evolution with age since reclamation.

A rapid hydroclimate change and its influence on the Songliao Basin during the Santonian-Campanian transition as recorded by compound-specific isotopes

Reconstruction of the paleoclimate and paleoenvironment of the Songliao Basin has important theoretical and practical significance to reveal the regional continental climate characteristics and continental organic matter accumulation under the Cretaceous greenhouse climate conditions. Here, we report the composition of biomarkers and the stable carbon and hydrogen isotope compositions of n-alkanes from Nenjiang Formation sediments in the Songliao Basin, which provide information on the source of organisms, paleoenvironmental conditions, and paleoclimate changes during the Late Cretaceous. Our hydrogen isotope records of n-alkanes displayed a significantly negative shift, indicating that a dramatically wet climate occurred during the Santonian-Campanian transition. A wet climate, such as that during a prolonged rainfall event, had a significant impact on the lake and resulted in the lake level increasing and the salinity decreasing in the surface water during this period. However, the freshwater input increased the salinity difference between the upper and lower water layers, which caused stable water stratification to occur in the lake. In addition, the high total organic carbon (TOC) content indicates high primary productivity because of increased precipitation carrying more terrigenous nutrients into the lake, which resulted in eutrophication during this period. Notably, the increased aryl isoprenoid ratios and the occurrence of isorenieratane indicates that photic zone euxinia (PZE) was enhanced during the Santonian-Campanian transition, which was possibly due to the dramatic change in the hydroclimate. We speculate that the large freshwater input promoted the formation of PZE in the lake. Then, PZE restricted the habitat available for aerobic organisms, which had fatal effects on the lake ecosystem.

Petroleum sources in the Xihu Depression, East China Sea: Evidence from stable carbon isotopic compositions of individual n-alkanes and isoprenoids

In order to better understand the relative contributions of various source rocks for petroleum in the Xihu Depression, East China Sea Basin, gas chromatography-mass spectrometry analyses of the saturated and aromatic fractions and carbon isotopic analyses of the purified n-alkanes and isoprenoids were performed. The molecular composition and individual n-alkane and isoprenoid carbon isotope compositions were determined for oils and extracts of Paleogene mudstones (TOC: 0.45–2.3%), carbonaceous mudstones (12.7–19.6%), and coals (48–60%) in the Xihu Depression, so as to discriminate the hydrocarbon sources of coals and associated mudstones to the petroleum generated in the basin. Abundant diterpanes (e.g., isopimarane, phyllocladane) are present in all the samples. The differences in the δ13C values of pristane and phytane indicate separate origins for the isoprenoids. A general negative sloping profile with increasing n-alkane carbon chain length was observed in coals, mudstones and oils from the Huangyan oilfield, with individual n-alkane δ13C values between −25.6‰ and −34.4‰. These values are characteristic of the isotopic compositions of C3 plants, and are also typical of fluvial/lacustrine depositional settings. The isotope compositions of individual n-alkanes from the extracts of coals and mudstones correlate with the relative concentration of terrigenous higher plant-derived diterpenoids, which suggests that plant phyla/classes (for example gymnosperm-derived diterpanes versus angiosperm-derived diterpanes) may be a control on the carbon isotope profiles of n-alkanes. For samples dominated by terrigenous organic matter, long chain n-alkanes with a large proportion of gymnosperm input are approximately 2–3‰ enriched in 13C relative to those with angiosperm input, regardless of lithology. The connections between the oils, coals, and mudstones allow determination of the patterns and variations for different hydrocarbon contributions, and provide a way of differentiating specific source contributions to petroleum in the Xihu Depression. Oils generated from the coals have heavier δ13C values with increasing chain length within a narrow range (approx. 2–3‰) and have more heterotrophic origins, whereas oils from the mudstones are more depleted in 13C with increasing carbon number (n-C20-31) and have a greater derivation from land plants. The 825B oils in the Huangyan oilfield have a greater angiosperm-sourced hydrocarbon contribution than those of the 825AD oils, which have a greater gymnosperm-sourced hydrocarbon contribution. The tricyclic and tetracyclic diterpanes and the isotopic compositions of individual n-alkanes and isoprenoids may be useful in differentiating contributions of coals and associated mudstones to petroleum.

Wetland expansion on the continental shelf of the northern South China Sea during deglacial sea level rise

To identify environmental causes for past changes in vegetation in subtropical East Asia, we present carbon isotope compositions of plant-wax n-alkanes and provide estimates of the C4-plant contribution across the past four glacial terminations and interglacials, based on cores recovered from the northern South China Sea. Our results show a comparable C4-plant contribution between the Last Glacial Maximum (LGM) and the Holocene. An increase of the C4-plant contribution by 15–20% is found for Terminations IV, II and I relative to subsequent interglacial peaks, coeval with an expansion of Cyperaceae and Poaceae. In contrast, Termination V reveals a lower C4-plant contribution than Marine Isotope Stage (MIS) 11c. The data exhibit a long-term trend, with a stepwise increase of the C4-plant contribution across interglacials MIS 11c, 9e, 7e and 1. We suggest that no substantial changes in humidity levels over glacial-interglacial cycles occurred facilitating a similar C3/C4-plant ratio for the LGM and the Holocene. Instead, deglacial sea-level rises caused an extensive development of floodplains and wetlands on the exposed continental shelf, providing habitats for the spread of C4 sedges and grasses. The progressive subsidence of Chinese coastal areas and the broadening of the continental shelf over the late Quaternary explains the nearly absence of C4 plant occurrence during Termination V and a gradual increase of the C4-plant contribution across interglacial peaks. Taken together, changes in coastal environments should be considered when interpreting marine-based vegetation reconstructions from subtropical Asia.

The potential of δ2Hn-alkanes and δ18Osugar for paleoclimate reconstruction - A regional calibration study for South Africa.

The hydrogen isotopic composition of leaf wax-derived n-alkanes (δ2Hn-alkanes) is a widely applied proxy for (paleo)climatic changes. It has been suggested that the coupling with the oxygen isotopic composition of hemicellulose-derived sugars (δ18Osugar) - an approach dubbed 'paleohygrometer' - might allow more robust and quantitative (paleo)hydrological reconstructions. However, the paleohygrometer remains to be evaluated and tested regionally. In this study, topsoil samples from South Africa, covering extensive environmental gradients, are analysed. δ2Hn-alkanes correlates significantly with the isotopic composition of precipitation (δ2Hp), whereas no significant correlation exists between δ18Osugar and δ18Op. The apparent fractionation (εapp) is the difference between δ2Hn-alkanes and δ2Hp (εapp 2H) and δ18Osugar and δ18Op (εapp 18O), respectively, and integrates i) isotopic enrichment due to soil water evaporation, ii) leaf (and xylem) water transpiration and iii) biosynthetic fractionation. We find no correlation of εapp 18O nor for εapp 2H with temperature, and no correlation of εapp 2H with potential evapotranspiration and an aridity index. By contrast, εapp 18O correlates significantly with both potential evapotranspiration and the aridity index. This highlights the strong effect of evapotranspirative enrichment on δ18Osugar. In study areas without plant predominance using Crassulacean Acid Metabolism (CAM), coupling δ18Osugar and δ2Hn-alkanes enables to reconstruct δ2Hp and δ18Op with an offset of Δδ2H = 6 ± 27‰ and Δδ18O = 0.8 ± 3.7‰, respectively, as well as relative humidity (RH) with an offset of ΔRH = 6 ± 17%. The paleohygrometer does, however, not work well for our study areas where CAM plants prevail (reconstructed δ18Op, δ2Hp and RH are off by 3.1‰, 27.2‰ and 31.7%). This probably reflects plant-specific (phenological) adaptations and/or post-photosynthetic exchange reactions related to CAM metabolism. Overall, our findings corroborate that δ2Hn-alkanes and δ18Osugar are valuable proxies, and the paleohygrometer is a promising approach for paleoclimate reconstructions in southern Africa.

Paleoenvironmental implications of molecular and carbon isotopic distributions of n-alkanes in the hominoid-bearing upper Miocene sediments from the Zhaotong Basin, southwestern China

It is crucial to understand the relationship between hominoid primates and their living environment. Located at the southeastern margin of the Tibetan Plateau, the Shuitangba (STB) outcrop from the Zhaotong Basin is famous for bearing the remains of the youngest known ape fossil in the terminal Miocene (~6.2 Ma). In this study, the molecular and carbon isotope compositions of n-alkanes, together with the loss on ignition and the bulk carbon isotope composition of total organic matter, were investigated in the STB outcrop to infer the paleoenvironmental conditions in this uppermost Miocene sedimentary sequence, with the emphasis on the habitat of the STB hominoid. In the hominoid-bearing peaty clay layer and the surrounding lignite layers, the n-alkane molecular and carbon isotope compositions support a primary contribution from terrestrial higher plants. In contrast, n-alkanes in the upper shallow lake sediments are derived from multiple origins, mainly from aquatic plants and bacteria. Both carbon preference index (CPI) and average chain length (ACL) vary closely with the lithology, but the CPI record shows the larger amplitude of change than the ACL and varies sharply during the lithological boundary. It is notable that the changes of the molecular and carbon isotope compositions of long-chain n-alkanes do not parallel with the abrupt increase of Poaceae pollen percentage during the STB hominoid appearing stage. Such an inconsistency between herb pollen percentage and n-alkane ratios may result from the different sources. Our results clearly support that these organic matter-based ratios have the potential to record the paleoenvironmental changes in ancient lacustrine sediments.

Pericarp features of wild perennial Helianthus L. species as a potential source for improvement of technical and technological properties of cultivated sunflower

Although the anatomy of the sunflower achene has been extensively studied, not enough attention is given to the research of the pericarp characteristics as potentially important parameters in the breeding programme. Therefore, we considered that it was important to make very detail comparative analysis of morpho-anatomical characteristics of achene as well as miromorphological and n-alkane characteristics of epicuticular wax of the 18 wild perennial Helianthus species. For anatomical analyses cross-sections were obtained from the middle part of pericarp using cryotechnique procedure. Micromorphological observation, morphological and anatomical analysis of pericarp were performed using Light and Scanning Electron Microscopy. The composition of the epicuticular n-alkanes was characterized by GC-FID and GC/MS analyses. Results showed differences in the distribution of phytomelanin pigment among the species. Denser phytomelanin network in the species H. salicifolius, H. resinous, H. tuberosus, and H. hirsutus may be connected with better resistance to insects and mechanical damages. Species H. resinosus, H. giganteus, and H. hirsutus are characterized by the highest percentage of cuticle and sclerenchyma thickness in the pericarp which can be a predictor of mechanical resistance to biotic and abiotic factors. Species H. strumosus, H. maximiliani, and H. grosseserratus are characterized with thin pericarp, poorly developed cuticle and epicuticle wax, as well as thin sclerenchyma tissue. In the pericarp waxes, C29 was the dominant alkane but several samples had n-alkanes longer than C33.These long-chain n-alkanes may be related to drought tolerance in some species. Obtained results of pericarp characteristics can be useful for improving technical and technological properties of the achene as well as for preservation of collection of wild sunflower species.

Hydrogen Isotope Composition of n-Alkanes Generated during Anhydrous Pyrolysis of Peats from Different Environments

The variation and its level of hydrogen isotopic composition of individual n-alkanes in sediments from different latitudes and climatic environments from immature to post-mature are still unclear. ...

Sources and historical sedimentary record: Temporal variability of n-alkane and PAHs from the Yellow River Estuary, China

The composition and distribution of n-alkanes and polycyclic aromatic hydrocarbons (PAHs) in suspended particulate matter (SPM) from the Yellow River and a sedimentary core from the Yellow River Estuary, China, were measured in order to determine environmental changes in the Yellow River Estuary on a regional scale. The concentration of n-alkane in SPM increased along the upper-middle-lower reaches of the Yellow River. The total concentrations of n-alkanes and PAHs in the core ranged from 0.04 to 0.86 μg g−1 (avg. 0.21 μg g−1), and from 0.04 to 0.29 μg g−1 (avg. 0.15 μg g−1) on a dry wt. basis, respectively. Understanding the temporal evolution of n-alkanes provide information on terrigenous versus aquatic productivity, oil exploration at the Shengli Oilfield, and channel diversion in the Yellow River. n-Alkanes in SPM were mainly derived from mixed sources, with terrigenous inputs dominating. PAHs in the sediment core were predominantly derived from coal and biomass combustion. The variation in PAHs levels throughout the core determines changes in energy use and socio-economic development. The temporal variability in n-alkane and PAHs and their molecular diagnostic ratios revealed a trend of regional environmental change and the role of anthropogenic activity in that environmental change.

Crop breeding has increased the productivity and leaf wax n-alkane concentration in a series of five winter wheat cultivars developed over the last 60 years

Plant wax n-alkanes are a major constituent of the leaf and grain surface. In this study, we explored what can be learned from the abundance and carbon isotopic composition (δ13C) of n-alkanes in historical winter wheat cultivars. We investigated leaf and grain wax n-alkane concentration (ΣalkLand ΣalkG) and carbon isotopes (δ13CalkL and δ13CalkG) on C29 as well as bulk leaf and grain carbon isotopes (δ13CbulkL and δ13CbulkG) to assess if these wax components changed across five wheat cultivars released from the 1950s to the early 2010s. Results showed that ΣalkL and grain yield increased, while δ13CalkL and δ13CbulkL decreased across the historical wheat cultivars. We found a significant correlation between ΣalkL and shoot biomass at the early growth stage, and a strong correlation between ΣalkL at the grain-filling stage and grain yield. Grain measures, including ΣalkG, δ13CalkG, and δ13CbulkG did not correlate with crop production. Although δ13CalkL and grain yield were not correlated at the flowering stage, they were correlated at the grain-filling stage under dry conditions. Our results indicate that increased ΣalkL has been indirectly selected in breeding efforts to improve crop production in winter wheat, suggesting that greater leaf waxiness confers advantages for crop growth.

Comparison of n-alkane concentrations and δD values between leaves and roots in modern plants on the Chinese Loess Plateau

Sedimentary δDn-alkane values have been widely used as a valuable proxy for paleoenvironmental reconstruction. A number of studies have focused on δDn-alkane values that derived exclusively from leaves, while less attention has been paid to the root-derived n-alkanes and their impact on sedimentary δDn-alkane values. In this study, we sampled modern plant leaf and root materials from different growth contexts (slopes and seasons) on the Chinese Loess Plateau to compare leaf-derived n-alkanes with root-derived n-alkanes. Our results demonstrated that total n-alkane (C27–C33) concentrations varied substantially between leaf and root materials, with average values of 209 and 29.5 μg/g observed in leaves and roots respectively. The results suggest that ca. 12% of the n-alkane concentrations in sediments may be derived from plant roots. Furthermore, leaf-derived δDn-alkane values for Stipa bungeana (grass), Artemisisa vestita (shrub) and Bothriochloa ischaemum (grass) averaged −184‰, −152‰ and −198‰, compared with their root-derived δDn-alkane values of −199‰, −179‰ and −163‰, respectively. These statistically significant differences in concentrations and δD values between leaf-derived and root-derived n-alkanes suggest that the contribution of n-alkanes derived from plant roots is important for evaluating the resultant n-alkane compositions of sediments for paleoenvironmental reconstruction. Our findings indicates that the effects of root-derived n-alkanes on total sedimentary δDn-alkane values should be considered carefully in future paleoenvironmental reconstruction efforts.

Hydrogen isotope fractionation variations of n-alkanes and fatty acids in algae and submerged plants from Tibetan Plateau lakes: Implications for palaeoclimatic reconstruction

The hydrogen isotope compositions (δD) of n-alkanes and fatty acids (FAs) are widely applied in palaeoclimatic reconstructions, and the determinations of their hydrogen isotope fractionation factor values (ε) are vital for quantitatively reconstructing past precipitation variations. Currently, studies on n-alkane and FA ε values focus on terrestrial plants, which, however, show large uncertainties because of the influence of evapotranspiration. Therefore, in this study, we analysed the ε values of algae and submerged plants immersed in lakes, which are not affected by evapotranspiration, to understand the hydrogen isotope fractionation of plant lipid synthesis. By investigating the δD values of lipids (n-alkanes and FAs) in algae and submerged plants and the δD values of co-existing water (including lake bottom water, surface sediment water, and leaf water of algae and submerged plants) from five Tibetan Plateau lakes, we find that the n-alkane ε values of algae and submerged plants show narrow changes, ranging from −176 to −159‰ and −167 to −142‰, respectively. The FA ε values of algae and submerged plants also show small variations, ranging from −160 to −121‰ (except Chara) and −161 to −138‰, respectively. Therefore, the average biosynthetic hydrogen isotope fractionation of these plants is −162‰ for n-alkanes and −145‰ for FAs, and the small ε differences between FAs and n-alkanes can be related to the different magnitudes of FA utilization in n-alkane synthesis. Finally, we find that the biosynthetic hydrogen isotope fractionation factors of aquatic plants are close to those of terrestrial grasses but slightly more negative than those of terrestrial woody plants. Thus, our results are helpful for understanding the hydrogen isotope fractionation variations in terrestrial plant lipids, which is beneficial for palaeohydrological reconstructions.

Chemical composition of n-alkanes and microbially mediated n-alkane degradation potential differ in the sediments of Qinghai-Tibetan lakes with different salinity

N-alkane-based proxies are widely employed to reconstruct paleoclimate and paleoenvironment in lacustrine environments. However, little is known about the influence of microbially mediated alkane-degradation on n-alkane-derived proxies. In this study, the chemical composition of n-alkanes and microbially mediated n-alkane degradation potential were investigated in the surface sediment samples collected from seven lakes with a range of salinity from freshwater to salt saturation on the northern Qinghai-Tibetan Plateau (QTP). The results showed that the chemical composition of n-alkanes differed among the studied QTP lakes. Significant correlations were observed between salinity and some n-alkane-based paleoclimate and paleoenvironment proxies, such as ratio of C21−/C22+, average chain length (ACL) and carbon preference index (CPI). This suggested that salinity may affect the validity of some n-alkane-based paleoclimate and paleoenvironment proxies. Alkane-degrading bacteria were abundant and widespread in the studied freshwater and saline/hypersaline lakes but were minor or absent in salt-saturation lakes. The obtained alkane-degrading bacterial strains showed active ability to degrade n-hexadecane. This suggested that the salinity influence on the n-alkane distribution may be partially related to microbial degradation, which awaits further in-situ investigation. So salinity variation should be taken into account when using n-alkane-based proxies for reconstructing paleoclimate and paleoenvironment in lakes.

N-Alkane distribution in ombrotrophic peatlands from the northeastern Alberta, Canada, and its paleoclimatic implications

In this paper, the stratigraphic occurrence of n-alkanes (Cn) is investigated in three peat cores (Mildred, JPH4 and McMurray) from the ombrotrophic peatlands in northeastern Alberta, Canada, to determine their origin, biomass input, and paleoclimatic significance. The molecular composition of n-alkanes is dominated by the >C21 medium- to long-chain homologues, with a strong odd over even predominance. A number of n-alkane-based indices (e.g. predominant n-alkane, C23/C29, C23/(C27 + C31), Paq, ACL, and CPI), show changing conditions in the organic matter (OM) input from Sphagnum species or terrestrial vascular plants, which increasingly contributed upwards in the peat profiles, and through time. Paq and C23/(C27 + C31) proxies are similar in the three cores, and allow us to infer vegetation variations that coincide with the climate alternations of the Medieval Warm Period (MWP), Little Ice Age (LIA), and modern Recent Warming (RW). The utilization of ACL-ket and (C23 + C25)/(C27 + C29 + C31)-ket proxies, performed in comparison with n-alkane proxies, further support the biomarker-based reconstructions of paleoclimate variations over recent centuries. This study suggests that n-alkane homologues are sensitive to vegetation and climate changes and thus are reliable biomarkers for tracking past shifts in vegetation and climate variations in peat archives.

Savanna in equatorial Borneo during the late Pleistocene

Equatorial Southeast Asia is a key region for global climate change. Here, the Indo-Pacific Warm Pool (IPWP) is a critical driver of atmospheric convection that plays a dominant role in global atmospheric circulation. However, fluctuating sea-levels during the Pleistocene produced the most drastic land-sea area changes on Earth, with the now-drowned continent of Sundaland being exposed as a contiguous landmass for most of the past 2 million years. How vegetation responded to changes in rainfall that resulted from changing shelf exposure and glacial boundary conditions in Sundaland remains poorly understood. Here we use the stable carbon isotope composition (δ13C) of bat guano and High Molecular Weight n-alkanes, from Saleh Cave in southern Borneo to demonstrate that open vegetation existed during much the past 40,000 yrs BP. This location is at the southern equatorial end of a hypothesized ‘savanna corridor’ and the results provide the strongest evidence yet for its existence. The corridor would have operated as a barrier to east-west dispersal of rainforest species, and a conduit for north-south dispersal of savanna species at times of lowered sea level, explaining many modern biogeographic patterns. The Saleh Cave record also exhibits a strong correspondence with insolation and sea surface temperatures of the IPWP, suggesting a strong sensitivity of vegetation to tropical climate change on glacial/interglacial timeframes.