Mid-chain N-alkanes Research Articles

Paleovegetation and environment during deposition of the Late Oligocene sub-bituminous coal in the Bobov Dol Basin (SW Bulgaria) as deduced from petrographic and geochemical characteristics

Coal and carbonaceous shale samples were taken from the Late Oligocene Bobov Dol Basin, SW Bulgaria. Seams Ia, I, IIa+b, III, IV and V from the coal-bearing Bobov Dol Fm. were sampled to provide insights into the peat forming vegetation and depositional environment based on detailed organic petrological and geochemical study.The petrographic composition indicates that both coals and carbonaceous shales contain predominantly terrestrial organic matter with abundant huminite macerals and locally enriched liptinite macerals. Inertinite is rare or absent. Good tissue preservation in seams I, IV and V indicates deposition under wet sedimentary environment, whereas predominance of detrohuminite and lower Tissue Preservation Index (TPI) in seams Ia, IIa+b and III are linked to enhanced humification of the plant remains due to changes in the hydrological regime. The low percent of inertinite and the overall high Gelification Index (GI), however, argue for peat formation under wetter climatic settings, whereas low to moderate ash yields of coal samples indicate organic matter deposition in oligo- to mesotrophic topogenous mires. Reconstructed evolution of the environmental settings during peat formation denotes differences which might be caused by changes in the hydrological regime and tectonic activity.The biomarker composition shows prevalence of gymnosperm-derived sesqui- and diterpenoids, followed by lower concentration of n-alkanes (apart from the carbonaceous shale samples from seam Ia), whereas angiosperm-derived non-hopanoid triterpenoids, acyclic isoprenoids, steroids and hopanoids, are generally present in minor amounts. Diterpenoid composition indicates dominant contribution from Cupressaceae and Pinaceae conifers. Minor amounts of predominantly lupane-type non-hopanoid triterpenoids denote limited development of angiosperm vegetation, presumably mostly related to Betulaceae family. Relative abundance of mid-chain (n-C21–25) n-alkanes, associated with elevated proxy ratio (Paq) furthermore, suggest possible contribution from aquatic macrophytes to peat formation. Very low hopane concentrations and low/moderate ββ/(ββ + αβ) hopane ratios imply minor aerobic bacterial activity under mild acidic conditions. The presence of inorganic carbon suggests that environmental acidity might be controlled by the occasional input of neutral to slightly alkaline ground waters. Moderate sulfur contents and the occurrence of hopenes denote anaerobic bacterial activity after organic matter burial.Low maturity of the organic matter and limited hydrocarbon generation potential is indicated by the low huminite reflectance, hopane (C31 22S/(22S + 22R)) and sterane (αααC29 20S/(20S + 20R)) isomerization ratios, as well as Rock-Eval parameters, Tmax and Production Index (PI).

Aquatic plants dominate the sources of long-chain n-alkanes in floodplain lakes in the middle and lower reaches of the Yangtze River

It has been generally accepted that long-chain n-alkanes found in lacustrine sequences are mainly derived from terrestrial plants, which is the basis for tracking paleoenvironmental changes associated with terrestrial plants. However, this application may be complicated by the possible contribution of long-chain n-alkanes from non-emergent aquatic plants. To explore this issue, a sedimentary core spanning the past 150 years from Lake Longgan in the middle and lower reaches of the Yangtze River, together with terrestrial plant and soil samples collected around the lake, was analyzed for molecular distributions and δ13C values of n-alkanes. Results show that n-alkanes with different chain lengths have similar sources, as evidenced by the strong positive correlations in concentrations (r2 > 0.88, p < 0.001) and δ13C values (r2 > 0.79, p < 0.001) among n-alkane homologs. The δ13Calk values (especially n-C29 and n-C31) indicated that n-alkanes have a mixed source, with non-emergent aquatic plants having a greater influence in shallow lakes in the middle and lower reaches of the Yangtze River (MLYR). Results from the MixSIAR model further reveal that non-emergent aquatic plants contribute a large amount of mid-chain n-alkanes and almost half (34.6% ∼ 57.7%) of the long-chain n-alkanes. These findings suggest that long-chain n-alkanes in lacustrine sequences should be used with caution when attempting to reflect terrestrial vegetation succession and the associated climate changes.

Characteristics of n-alkyl lipids in cultivated Sphagnum and their preservation potential in the topsoil of Sphagnum farmlands in southwest China

Sphagnum, a crucial peat-forming plant, is essential for sustaining the ecosystem and counteracting climate change. In recent years, attempts have been made to cultivate Sphagnum commercially to reduce the environmental impacts of harvesting the natural Sphagnum. However, research on the distribution of lipids in cultivated Sphagnum and their potential for preservation potential in the underlying soil is still limited. This study examined the distribution of n-alkyl lipids and the carbon isotope composition of n-alkanes in both natural and cultivated Sphagnum, as well as the underlying topsoil in southwestern China. Results show that the distributions of n-alkyl lipids in the cultivated Sphagnum are similar to those of the natural Sphagnum in the same region. Additionally, the δ13C values of mid-chain n-alkanes (C23 and C25) in the cultivated Sphagnum are significantly negative compared to those of long-chain homologs (C29 and C31; one-way ANOVA, p < 0.05), indicating that symbiotic methanotrophs are relatively active in the cultivated Sphagnum. Furthermore, the n-fatty acids and n-alkan-2-ones in the topsoil are mainly imported from Sphagnum, while the n-alkanes in the topsoil may originate from both Sphagnum and vascular plants. These findings contribute to a better understanding of the ecological benefits of Sphagnum cultivation and the contribution and fate of Sphagnum-derived carbon in soil profiles.

Terrestrial versus aquatic source identification of sedimentary n-alkane and sugar biomarkers: a case study from the Bale Mountains, Ethiopia

Organic matter in sedimentary archives is abundantly used to reconstruct paleoenvironmental and climate histories. Thereby, distinguishing between the terrestrial and aquatic origin of sedimentary organic matter is often a prerequisite for robust interpretations. In this case study, we use published data for modern plants and topsoils to identify the terrestrial versus aquatic source of n-alkane and sugar biomarkers in two afro-alpine sediment archives (Lake Garba Guracha and Depression B4) in the Bale Mountains, Ethiopia. The results of our comparative approach show that the long-chain n-alkanes C29, C31, and C33 in the sedimentary archives yielded patterns similar to those typical for the potential terrestrial input. By contrast, the relative abundances of the sedimentary mid-chain n-alkanes C23 and C25, and at least partly C27, are significantly increased compared to the plants and topsoils. This suggests that they are primarily produced by aquatic macrophytes and micro-organisms. The Paq ratio (C23 + C25)/(C23 + C25 + C29 + C31) is validated as a suitable source identification proxy in our study area. The sugar biomarkers xylose (xyl) and arabinose (ara) are abundant in the plant and topsoil samples. By comparison, high relative abundances of fucose (fuc) and rhamnose (rham) are generally only observed in sediments. This indicates that these sugar biomarkers are primarily produced by aquatic macrophytes or micro-organisms. Therefore, the ratio (fuc + rham)/(ara + xyl) is a suitable sugar biomarker proxy for organic matter source identification. The relative abundances of galactose and mannose are systematically decreasing and increasing, respectively, from leaves over O-layers to Ah-horizons. Furthermore, they are not significantly different from the abundances found in the sediments. This hinders terrestrial versus aquatic source identification using galactose and mannose.

Integrated organic, inorganic geochemical and palynofacies analyses to characterize the paleoenvironment of the Tortonian (Late Miocene) Wakar Formation, offshore Nile Delta, Egypt

The subsurface Wakar Formation (Fm) has been identified in all wells of the offshore concessions in the eastern Mediterranean in Egypt. This formation is important because it represents the end member of the Late Miocene (Tortonian) siliciclastic sedimentary rock of a vertical thickness between 500 and 900 m in these fields and is a promising formation for gas production in Egypt. There are no comprehensive studies on the geochemistry and palynofacies of this Wakar Fm in literature. Therefore, sixteen collected ditch samples of the entire thickness of the Wakar Fm from a PFM-SE -1 well were investigated. The main objective is to reveal the history of depositional events that prevailed in North Africa at this time. TOC content (average = 0.81 wt%) and generative potential of the Wakar Fm are of poor and fair quantity. The organic matter (OM) belongs to types IIIC and III of coastal and delta settings as revealed by the palynofacies and geochemical data. The Tmax of the samples does not exceed 435 °C (419–432 °C) of the immature stage, which corresponds to the carbon preference index (CPI >1) (1.15–1.4). The thermal alteration index range between 2 and 2+, indicating immature to marginally mature OM. GC of the two extracted samples shows a predominance of short-chain and mid-chain n-alkanes derived mainly from algae and seagrass; the Pr/Ph ratio (3.24–3.02; suboxic) and low terrigenous/aquatic ratio (TAR; 0.23–0.47) indicate that the OM bearing sediments were accumulated in a transitional setting of the suboxic aqueous medium. The dominance of terrestrial OM and low amorphous organic matter also suggests deposition in a marginal basin under dysoxic-anoxic to suboxic-anoxic conditions. Pr/n-C17 versus Ph/n-C18 diagrams confirm that preservation occurred in weakly oxidized environments from a source of kerogen type III gathered from a swamp and transitional marine zones. The inorganic geochemical analyses (XRD and XRF) and the calculated paleoenvironmental indices (i.e; log (Fe2O3/K2O) versus log (SiO2/Al2O3)) indicate that the main composition of the Wakar Fm is a combination of iron shale and sandstone facies. The Fe-sandstone facies occupies the middle part of the formation with a thickness of 170 m, which is the pay zone in the present concession. The parent protolith of Wakar Fm is a combination of intermediate and mafic igneous rocks. The prevailing paleoclimate is arid and semi-arid conditions synchronize the base, and top of the Wakar Fm, which is responsible for weak to moderate chemical weathering as revealed from chemical index of alteration values (CIA average = 67.99). Discriminant functions exhibit a depositional setting belonging to an active continental margin and oceanic island arc between marine and terrestrial zones under oxic-dysoxic environments.

Unique distribution pattern and δ13C signature of des-A-triterpenoids from Lake Wuliangsu: Source and paleoenvironmental implications

des-A-triterpenoids have been widely recognized as indicators of terrestrial vascular angiosperms and microbial activities. However, recent studies also suggested complex sources for des-A-triterpenoids in lakes and peatlands, thus complicating their applications in paleoenvironmental reconstructions. Here we present the occurrences of aliphatic and aromatic des-A-triterpenoids from a ∼160-year sedimentary core in Lake Wuliangsu in the upper reaches of the Yellow River, China. In these sediments, des-A-triterpane was not detected, whereas di-unsaturated des-A-triterpenes are more abundant than mono-unsaturated des-A-triterpenes. The δ13C values of di-unsaturated des-A-triterpenes range from –23.0‰ to –17.6‰, with an average value of –21.0‰. Accordingly, aliphatic des-A-triterpenoids from Lake Wuliangsu show a unique distribution pattern and more positive δ13C signatures compared with those sourced from terrestrial vascular angiosperms. By comparing the concentrations and δ13C values of des-A-triterpenoids with n-alkanes and triterpenoids, we suggest that des-A-triterpenoids from Lake Wuliangsu sediments might be degraded from oleanenone, which was produced by submerged macrophytes. In this sense, aliphatic des-A-triterpenes in Lake Wuliangsu could represent a novel proxy for submerged macrophytes, which would be more precise than those of mid-chain n-alkanes, considering that mid-chain n-alkanes are usually from multiple sources. Our results also demonstrate the importance of strong aerobic bacteria activities and photodegradation for the formation of di-unsaturated des-A-triterpenes. The aromatization process of aliphatic des-A-triterpenes show contrasting variation characteristic with the formation of aliphatic des-A-triterpenes over the past ∼160 years, probably due to different rates (production vs consumption of di-unsaturated des-A-triterpenes) and prerequisites (aerobic vs anaerobic conditions) for these two synthesis pathways.

Impact of climatic and environmental factors on δD of n-alkanes in the lake surface sediments of arid Tibet

The hydrogen isotope composition (δD) of paleoclimatic archives is increasingly being used as a proxy for the reconstruction of Quaternary hydroclimatic variability, but the factors controlling δD in arid/semi-arid regions remain poorly understood. We compared the hydrogen isotope composition of individual n-alkanes in the surface sediments of a series of lakes and puddles in Tibet with local meteorological and hydrological data. The results show that the δD of short-chain and long-chain n-alkanes is sensitive to hydroclimatic conditions, with δDp being the predominant control. δD becomes less depleted where precipitation amount and humidity are lower or evaporation is stronger. In addition, temperature is a significant factor affecting δD of mid-chain n-alkanes, with a significant negative correlation, and the temperature in spring, autumn and winter is an important influence on it. We believe that the δD of long-chain n-alkanes is still a good paleohydrological index in the arid/semi-arid region; δD indices of mid-chain and short-chain n-alkanes are also useful, and more mechanism research is needed.

Organic matter and sedimentary accumulation rates in a transect of cores in the Bay of Bengal offshore Bangladesh and Andaman Sea offshore Myanmar

Eight sediment multi-cores samples collected from the Bay of Bengal and the Andaman Sea were examined to evaluate organic matter (OM) sources to the marine setting for a large sediment and carbon sink. Total organic carbon (TOC) for sediments from the northern Bay of Bengal (NBB), eastern Bay of Bengal (EBB) and the northern Andaman Sea (NAS) range from 0.34 to 0.79 wt%, 0.84 to 1.06 wt%, and 0.20 to 1.18 wt%, respectively, with higher concentrations in fine silts than in the sandy fractions suggesting hydrodynamic sorting and surface area control TOC. The TOC to total nitrogen (TN) ratio (C/N) (NBB: 5.31–8.52; EBB: 5.51–6.97; NAS: 6.44–11.92) indicates OM is a mixture of dominant contributions from marine plankton and lesser contributions from land plants. The organic carbon (δ13Corg) isotopic values display a similar range of values in each of the regions of the NBB (−12.0 to −19.9‰), EBB (−10.1 to −20.2‰) and NAS (−8.3 to −20.3‰). δ13Corg values may derive from marine phytoplankton, terrestrial C4 plants and/or aquatic macrophytes. The total nitrogen (δ15N) isotopic compositions (NBB: 3.7–4.1; EBB: 4.1–5.9; NAS: 4.2–6.5‰) diagnose input from planktonic sources or aquatic macrophytes. Hydrocarbons from the sediments have a bimodal distribution with high relative abundance of mid-chain n-alkanes (C23 and C25 peaks) and long-chain n-alkanes (C29 and C31). Average chain length (ACL25-35), carbon preference index (CPI25-35), and grassland (Pgrass) ratios suggest contributions from terrestrial vegetation including grasses. High TOC to total sulphur (TS) ratios (C/S) observed suggest well-oxygenated interval associated with vertical mixing of seawater about 1000 years ago. The average sedimentation rates of Bay of Bengal and Andaman Sea are about 0.4 mm yr−1 and 0.07 mm yr−1, respectively. The organic carbon accumulation rate of the Bay of Bengal is found to be several times higher than that of Andaman Sea. Rapid deposition of both marine and terrestrial OM in the Bay of Bengal could be a favourable sedimentary system for methane fermentation to form methane hydrate in this regime.

Holocene vegetation reconstruction in the forest–steppe of Mongolia based on leaf waxes and macro-charcoals in soils

Abstract. Vegetation and climate reconstruction in the forest–steppe of Mongolia is still challenging regarding the pattern of forest and grassland distribution during the Holocene. Different sediments containing paleosols and humic layers provide geomorphological archives for landscape development in Mongolia. n-Alkane and macro-charcoal ratios represent specific indicators to distinguish the share between grasses and trees. In a preliminary study, we investigated the applicability of these two paleo-proxies from soils for vegetation reconstruction comparing different relief positions and site conditions in the northern Khangai Mountains of Mongolia. n-Alkanes that are deposited from leaf waxes in the soil have the potential to indicate vegetation composition on a local scale. Depending on site-specific environmental conditions, n-alkanes are subjected to different degrees of microbiological decomposition, which is more intensive in soils of dry steppe than of forests. Mongolian forests are often underlain by permafrost that may reduce microbiological activity. In steppe soils, the decomposition of n-alkanes increases the quantity of mid-chain n-alkanes that adulterate the biomarker proxy signal to indicate more forest share. Macro-charcoals in soils have a site-specific component, but additional eolian input of macro-charcoals from long-distance transport can provide a distinct proportion in sediments. Thus, eolian influx of wood-derived macro-charcoal can dominate the proxy signal at sites where trees were few or had never existed. Radiometric dating of several paleosols and humic layers has shown that both proxies coincide as evidence for high grassland-to-forest ratios during the Early Holocene. By contrast, the proxy signals diverge for the Late Holocene. For this period, n-alkanes generally indicate more grassland, whereas macro-charcoals show increased wood-derived proportions. We imply that this difference is caused by increased forest fires and simultaneously spreading steppe area. A main portion of leaf waxes and charcoal particles in soils directly derive from the covering and nearby vegetation, whereas large lakes and glacier may receive these biomarkers from a larger catchment area. Thus, we conclude that soil archives provide proxies on a more local and site-specific scale than other archives do. Although the temporal resolution of soil archives is lower than for the other ones, biomarker proxies for paleosols and humic layer can be related to periods of distinct geomorphological processes. Further investigations comparing the multi-proxy data of different geomorphological archives are necessary to improve the paleo-ecological reconstruction for landscape development in Mongolia.

Seasonal variability in temperature trends and atmospheric circulation systems during the Eemian (Last Interglacial) based on n-alkanes hydrogen isotopes from Northern Finland

The Last Interglacial warm period, the Eemian (ca. 130–116 thousand years ago), serves as a reference for projected future climate in a warmer world. However, there is a limited understanding of the seasonal characteristics of interglacial climate dynamics, especially in high latitude regions. In this study, we aim to provide new insights into seasonal trends in temperature and moisture source location, linked to shifts in atmospheric circulation patterns, for northern Fennoscandia during the Eemian. Our study is based on the distribution and stable hydrogen isotope composition (δD) of n-alkanes in a lake sediment sequence from the Sokli paleolake in NE Finland, placed in a multi-proxy framework. The δD values of predominantly macrophyte-derived mid-chain n-alkanes are interpreted to reflect lake water δD variability influenced by winter precipitation δD (δDprec), ice cover duration and deuterium (D)-depleted meltwater. The δD values of terrestrial plant-derived long-chain n-alkanes primarily reflect soil water δD variability modulated by summer δDprec and by the evaporative enrichment of soil and leaf water. The δDprec variability in our study area is mostly attributed to the temperature effect and the moisture source location linked to the relative dominance between D-depleted continental and polar air masses and D-enriched North Atlantic air masses. The biomarker signal further corroborates earlier diatom-based studies and pollen-inferred January and July temperature reconstructions from the same sediment sequence. Three phases of climatic changes can be identified that generally follow the secular variations in seasonal insolation: (i) an early warming trend succeeded by a period of strong seasonality (ii) a mid-optimum phase with gradually decreased seasonality and cooler summers, and (iii) a late climatic instability with a cooling trend. Superimposed on this trend, two abrupt cooling events occur in the early and late Eemian. The Sokli δD variability is generally in good agreement with other North Atlantic and Siberian records, reflecting major changes in the atmospheric circulation patterns during the Eemian as a response to orbital and oceanic forcings.

A potential suite of climate markers of long-chain n-alkanes and alkenones preserved in the top sediments from the Pacific sector of the Southern Ocean

Investigating organic compounds in marine sediments can potentially unlock a wealth of new information in these climate archives. Here, we present pilot study results of organic geochemical features of long-chain n-alkanes and alkenones and individual carbon isotope ratios of long-chain n-alkanes from a newly collected, approximately 8 m long, located in the far reaches of the Pacific sector of the Southern Ocean. We analyzed a suite of organic compounds in the core. The results show abundant long-chain n-alkanes (C29–C35) with predominant odd-over-even carbon preference, suggesting an origin of terrestrial higher plant waxes via long-range transport of dust, possibly from Australia and New Zealand. The δ13C values of the C31n-alkane range from −29.4 to −24.8‰, in which the higher δ13C values suggest more contributions from C4 plant waxes. In the analysis, we found that the mid-chain n-alkanes (C23–C25) have a small odd-over-even carbon preference, indicating that they were derived from marine non-diatom pelagic phytoplankton and microalgae and terrestrial sources. Furthermore, the C26 and C28 with lower δ13C values (~−34‰) indicate an origin from marine chemoautotrophic bacteria. We found that the abundances of tetra-unsaturated alkenones (C37:4) in this Southern Ocean sediment core ranges from 11 to 37%, perhaps a marker of low sea surface temperature (SST). The results of this study strongly indicate that the δ13C values of long-chain n-alkanes and {U}_{37}^{mathrm{k}} index are potentially useful to reconstruct the detailed history of C3/C4 plants and SST change in the higher latitudes of the Southern Ocean.

The Stable Carbon Isotopic Compositions of n-Alkanes in Sediments of the Bohai and North Yellow Seas: Implications for Sources of Sedimentary Organic Matter

Stable carbon isotopic compositions of n-alkanes in surface sediments of the Bohai and North Yellow Seas were investigated to elucidate sources of sedimentary organic matter in these seas. The long-chain n-alkanes in surface sediments are predominantly long-chain C27, C29, and C31 types, with obvious odd carbon predominance. The δ13C values of long-chain n-C27, n-C29, and n-C31 alkanes are −30.8%±0.59‰, −31.9%±0.6‰, and −32.1%±1.09‰, respectively, within the range of n-alkanes of C3 terrestrial higher plants. This suggests that sedimentary n-alkanes are derived mainly from terrestrial higher plants. Compound-specific carbon isotopic analysis of long-chain n-alkanes indicates that C3 terrestrial higher plants predominate (64%–79%), with angiosperms being the main contributors. The n-alkane δ13C values indicate that mid-chain n-alkanes in sediments are derived mainly from aquatic emergent macrophytes, with significant petroleum pollution and bacterial degradation sources for short-chain n-alkanes.

Lipid biomarker estimates of seasonal variations of aerosol organic carbon sources in coastal Qingdao, China

Aerosols represent a significant source of organic carbon (OC) to China’s marginal seas, but their provenance and chemical composition remain understudied. At the boundary of the East Asian continent and China’s marginal seas, the coastal city of Qingdao is well-situated to study the sources and seasonal variations of organic aerosols and their contribution to marginal seas. Twenty-nine aerosol samples collected in 2015–2016 were analyzed for total OC (TOC), n-alkanes (C17–C33), n-alkanols (C20–C32) and n-fatty acids (n-FAs, C14–C32). Concentrations of TOC (5.7–18 μg/m3), n-alkanes (17.5–143 ng/m3), n-alkanols (18–41 ng/m3) and n-FAs (99.3–160 ng/m3) were highest in winter when total aerosol concentrations were highest. Organic carbon-normalized (OC-normalized) biomarker concentrations were used to apportion different sources of aerosol OC: short- and mid-chain n-alkanes (<C26) for fossil fuel residues; long-chain n-alkanes (≥C26), mid- and long-chain n-alkanols (≥C20) and mid- and long-chain n-FAs (≥C20) for terrestrial plant OC; short-chain n-FAs (<C20) for marine OC. From these we inferred higher fossil fuel OC contributions during cold seasons when aerosols were mainly transported to Qingdao by continental air masses, higher terrestrial plant OC contributions during warm seasons when terrestrial plant productivity was at its peak, and higher marine OC contributions in warm seasons when marine air masses were transported to Qingdao. Comparing these results with those from other continental and marine locations demonstrates the utility of this biomarker approach in providing semi-quantitative estimates of temporal and spatial variations in the sources of organic aerosols at the land-sea interface of marginal seas, where carbon burial rates tend to be high.

Effects of redox conditions and temperature on the degradation of Sphagnumn-alkanes

Leaf wax n-alkanes in peat deposits are important proxies for paleoenvironmental reconstructions. The n-alkane compositions of Sphagnum species, which are important peat forming plants, are commonly dominated by the C23 and C25 homologs. However, questions remain about the stability of Sphagnum-derived n-alkanes to early diagenetic alterations. In this study, a 13-month litterbag decomposition study using Sphagnum palustre collected from the Dajiuhu peatland, central China, was conducted under controlled laboratory conditions to assess the influences of redox (aerobic vs anaerobic) conditions and environmental temperatures (15 °C vs 25 °C) on the chain-length distributions and carbon isotopic compositions of Sphagnum n-alkanes. Results clearly reveal the preferential degradation under aerobic conditions of mid-chain n-alkanes relative to their long-chain homologs (C29 and C31), resulting in the increase of the relative proportions of long-chain n-alkanes. The change in n-alkane distribution is accompanied by an increase in the δ13C values of the mid-chain n-alkanes, whereas those of the long-chain n-alkanes remain nearly constant. In contrast, neither chain-length distributions nor n-alkane δ13C values were much affected under anaerobic degradation conditions. Statistical analysis reveals that redox conditions play significant roles in the degradation of Sphagnum n-alkanes, whereas the influence of air temperature is not significant. These findings will improve applications of n-alkane paleoenvironmental proxies to reconstruct the types of plants and the depositional conditions that contributed to peat sequences, particularly under settings with fluctuating water levels that affect redox conditions.

Geochemical characterization of sediments from the archaeological site Vinča – Belo Brdo, Serbia

In this study, a multidisciplinary approach was used for a detailed characterization of sediments from the archaeological site Vinča – Belo Brdo, Serbia, one of the most important Neolithic settlements in Europe. This research aimed to determine the paleoenvironmental conditions during Pleistocene and Early Holocene sedimentation prior to Early Neolithic settlement (~5800 cal BC) and provide novel insight into the interaction between humans and the environment during the Middle and Late Neolithic (5300–4550 cal BC). For the first time, organic geochemical characterization, combined with organic petrography, grain size, mineralogy, and inorganic analysis were done on sediment samples from geological and archaeological sections of the Vinča – Belo Brdo site. In the archaeological section, the layer with remains of the burned house and the overlaying leveling layer were particularly interesting.The samples from the oldest geological layers were characterized by the relatively high content of carbonates, the largest amount of soluble organic matter (OM), predominance of liptinite macerals, along with the prevalence of short and mid-chain n-alkanes indicating a marsh-lake depositional environment. The sedimentation of overlaying geological layers continued in an oxbow lake, followed by a shallow depositional environment with stronger input of vascular plants. This was indicated by an increase of the terrigenous component, a high amount of huminite maceral, especially textinite, the predominance of odd long-chain n-alkanes, as well as the presence of diterpanes, pimarane, and 16α(H)-phyllocladane.Quartz was the most abundant mineral in paleosol, while the OM had a mixed origin with a major input of microorganism in the precursor biomass. Estimated average paleosol temperature around 12–16 °C and mean annual precipitation of 938 mm/yr were favorable for ancient civilization settlement.In archaeological samples, the OM was predominantly formed by microorganisms, with a certain contribution of terrestrial plants. Their occurrence was substantiated by the presence of telohuminite, detrohuminite, resinite, and sporinite. The prevalence of n-alkane C18, along with the occurrence of inertinite macerals, semifusinite and fusinite, in the layer with the burned house remains confirmed the incomplete combustion of woody biomass at temperatures <500 °C. Organic and inorganic geochemical parameters for the leveling layer covering the destroyed buildings showed analogous composition as the lowest geological layers. This proved that Vinča inhabitants excavated material for leveling at a site in their vicinity, and used it for covering the burnt debris, clearing the areas for a new settlement. In this context, the decades long archaeological dilemma of the origin of the leveling material was resolved.

How to Deal With Multi-Proxy Data for Paleoenvironmental Reconstructions: Applications to a Holocene Lake Sediment Record From the Tian Shan, Central Asia

Multi-proxy investigations on geological archives provide valuable information about environmental variations in the past. As opposed to single-proxy studies, the combination of several proxies can reveal more detailed information and strengthen subsequent paleoenvironmental reconstructions. However, there is still no consensus about how to deal with resulting highly dimensional datasets in a statistical manner. In many cases, the interpretations of multi-proxy datasets rely on visually matching several proxy records, which can lead to incorrect or insufficient interpretations. Here we report an innovative approach that combines the novel dimension reduction technique Uniform Manifold Approximation and Projection (UMAP) and the time series analysis R package asdetect to identify and characterize Holocene environmental phases and phase boundaries in a sediment core from Lake Chatyr Kol, southern Kyrgyzstan. Despite the fact that the Holocene climate evolution of Central Asia has been intensively studied during the last decades, knowledge about regional climate development during the Holocene and the underlying mechanisms is still relatively scarce. We particularly focus on phase transitions and differentiate between event-based shifts as opposed to gradual phase transitions. For this study, long-chain alkenones were used as a paleotemperature proxy and variations in long-chain alkyl diol distributions were ascribed to relative changes of algal input. The compound-specific stable hydrogen isotope compositions (δD) of individual n-alkanes were utilized as paleohydrological proxies, with the δD of mid-chain n-alkanes reflecting changes in the δD of the lake water and the δD of long-chain n-alkanes recording the δD of the meteoric water. We show the potential of modern analysis tools for data-driven paleoenvironmental reconstructions and advocate for their more frequent implementation in multi-proxy studies.

Spectroscopic and chromatographic investigation of soil organic matter composition for different agrosystems from arid saline soils from Southeastern Tunisia

In Tunisia, several studies estimated the soil organic matter (SOM) stocks. However, little is known about SOM composition and dynamics especially in arid regions. Globally, arid soils of Tunisia are saline. This study aims to investigate the composition and stability of SOM in salt-affected soils under different vegetation covers in Southeastern Tunisia. Two depths are sampled in ten profiles from different localities. SOM composition was assessed by FTIR spectroscopy and gas chromatography-mass spectrometry (GC/MS). The results showed that, despite the differences of studied soils, SOM contents are low (< 20 g/kg) and decrease with soil depth. The lowest content of SOM is attributed to native soils which suggest that SOM in arid saline soils is mainly controlled by available litter input. FTIR spectroscopy revealed that SOM is predominated by polysaccharides, aliphatic C, and aromatic C. Polysaccharides and aliphatic C increased with soil depth for agricultural and native soils. However, aromatic C increased with soil depth for agricultural soils, while it decreased for native soils. Agricultural soils are enriched in aliphatic C and polysaccharides compared with native soils that had higher content of aromatic C. Thus, the chemical composition of SOM in arid saline soils is strongly influenced by litter input quality and decomposition rate of SOM. GC/MS analyses which showed the predominance of mid-chain n-alkanes confirm those finding. Because of their relative recalcitrance and/or their protection by association with soil minerals, polysaccharides, mid-chain n-alkanes, aliphatic C, and aromatic C may have crucial effect on SOM stabilization in arid saline soils.

Relationships between Source Inputs and Lipid Geochemistry of Lake Sediments on the Northern Tibetan Plateau, China

Surface sediment samples were collected along the margins of Lakes Dachaidan, Xiaochaidan, Gahai (Delinha), Chaka, Gahai, and Qinghai, and from the Jinyingtan peat deposit, to study the lipid geochemistry of sediments on the Tibetan Plateau, China. TOC contents of the lake sediments varied in different saline lakes, with higher values in Lakes Qinghai and Gahai, which may be related to low salinity and abundant aquatic organisms. Fatty acids and n-alkanes were the two most abundant lipid constituents. Fatty acids were dominated by short-chain, indicating they were mainly from aquatic organisms, algae and bacteria organic matter. n-Alkanes were dominated by mid-chain and long-chain n-alkanes, and were derived mainly from aquatic macrophytes and terrestrial plants. The carbon isotopic compositions of short-chain, branched, unsaturated and cyclic fatty acids, and short-chain n-alkanes show an increasing trend in lake sediment samples from less saline lakes to more saline lakes. In less saline lakes, δ13C values of algae-sourced compounds show relatively enriched 12C, because of higher primary production and CO2 assimilation. The δ13C values of long-chain fatty acids and mid-chian/long-chain n-alkanes are positively correlated and show no obvious change from west to east, indicating similar macrophytes and terrestrial higher plants input in the studied samples.

Distribution of n-alkanes and their δ2H and δ13C values in typical plants along a terrestrial-coastal-oceanic gradient

Reconstructing past responses of coastal wetlands to climate change contextualizes ongoing and future developments in these globally important ecosystems. The molecular distributions and stable isotope ratios (δ2H and δ13C) of sedimentary plant wax n-alkanes are frequently used to infer past vegetation and hydroclimate changes in wetland systems. However, there is limited modern information available about these compounds in subtropical wetlands. Here we analyzed mature leaves from 30 typical plant species and roots from 6 plant species collected in the Florida Everglades, including tree island plants, freshwater wetland plants, mangroves, and seagrass. The n-alkane abundance (2 to 884 µg/g dry weight), percent of aquatic plants ratio (Paq, 0 to 1), average chain length (ACL23-33, 24.0–30.7), concentration weighted average (CWA) δ2H (−231 to −78‰) and δ13C values (−38.9 to −14.4‰) spanned wide ranges with plant growth habit. Significant differences in n-alkane abundances, Paq, ACL23-33, CWA δ2H and δ13C values were found to exist between the leaves and roots of some emergent aquatic plants. Simple mass balance calculations of wetland aquatic plants suggest that long chain n-alkanes (e.g., C29n-alkanes) are predominantly derived from leaves rather than roots in wetland surface sediments/soils. However, the contribution from mid-chain n-alkanes (e.g., C23n-alkane) from roots may be equal to or greater than those from leaves. This implies that the differences in the isotopic compositions between root and leaf derived material need to be taken into account when interpreting down core changes in mid-chain n-alkane δ2H and δ13C values, which may be derived from variable contributions from leaves and roots rather than a change in hydroclimate or vegetation. Considering the large variation in both n-alkane distribution proxies and isotopic composition, no single molecular index or stable isotope ratio can capture multivariate changes of wetland ecosystems in the past. Nevertheless, principal component analysis shows promising potential to resolve different plant functional types. Paleo-reconstruction of subtropical aquatic ecosystems using n-alkanes will be most useful if the full molecular and isotopic distribution information of plant waxes are used.

Leaf wax &amp;lt;i&amp;gt;n&amp;lt;/i&amp;gt;-alkane patterns and compound-specific &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C of plants and topsoils from semi-arid and arid Mongolia

Abstract. Leaf wax n-alkane patterns and their compound-specific δ13C signatures are valuable proxies for paleoenvironmental reconstructions. So far, their potential has not been investigated in semi-arid to arid Mongolia. We have therefore analysed the leaf wax n-alkanes and their compound-specific δ13C signature of five plant species (Poaceae, Cyperaceae, Artemisia spp., Caragana spp. and Larix sp.) and topsoils (0–5 cm) along two transects in central and southern Mongolia. Grasses show a distinct dominance of the n-C31 homologue, whereas the shrubs Caragana spp. and Artemisia spp. are dominated by n-C29. Larix sp. is characterised by the mid-chain n-alkanes n-C23 and n-C25. From plant to topsoil, n-alkane patterns show the potential to differentiate between grass-covered sites from those covered by Caragana spp. n-Alkane concentrations and odd-over-even predominance (OEP) of the topsoils are distinctly influenced by mean annual temperature, mean annual precipitation and aridity, likely reflecting the degree of n-alkane degradation and biomass production. In contrast, the average chain length (ACL) and the n-alkane ratio (n-C31∕n-C29+n-C31) are not affected by climatic parameters and, thus, are not biased by climate. The compound-specific δ13C signatures are strongly correlated to climate, showing a significant enrichment with increasing aridity, indicating the effect of water use efficiency. Our calibration results suggest that long-chain n-alkanes and their compound-specific δ13C signatures have great potential to reconstruct paleoenvironmental and paleoclimatic conditions when used in sediment archives from Mongolia.