•Simultaneous analyses of long chain alkenones and alkenoates are achieved.•Group II/III haptophytes make C36 Δ7,14,21 and Δ14,21,28 ethyl alkenoate isomers.•All peaks including double bond positional isomers are baseline resolved.•GC optimization enhances alkenone/alkenoate-based paleoclimatic reconstructions.We report here our latest attempt to optimize gas chromatographic (GC) separation of long chain alkenones and alkenoates from haptophyte cultures and sediments. Building upon the previously reported separation using VF-200 ms column (60 m × 250 μm × 0.1 μm) (Longo et al., 2013), we tested multiple GC columns with different column length, film thickness and variable loadings of poly(trifluoropropylmethylsiloxane) stationary phase from two separate manufacturers (Agilent and Restek). We show that Rtx-200 (105 m × 250 μm × 0.25 μm) GC column completely eliminates the co-elution between long chain alkenones and alkenoates, fully resolves the previously partially co-eluting alkenones (e.g., C38:2 ethyl and C38:4 methyl alkenones) and improves the resolution of alkenone and alkenoate double bond positional isomers. Since the debut of the alkenone paleothermometer three decades ago, long chain alkenoates have always been considered an annoyance for paleotemperature reconstructions and are routinely removed by saponification. Our new separation method simplifies the sample preparation for alkenone analysis and improves the application of alkenone/alkenoate proxies, which opens the door for paleoclimate application of alkenoate-based paleothermometers alongside alkenones. We also report the structure determination of new alkenoate isomers using dimethyl disulfide derivatization.

•The alkenone-based paleotemperature reconstructions for the past 21 kyr in central Asia.•2 thousand years delay of temperature and moisture optimum relative to local summer insolation maximum.•Model simulations display a much less pronounced delay in the initiation of Holocene warm conditions.•Abrupt cold-dry event aroud 4 kyr.Numerous studies have demonstrated that there are major differences in the timing of maximum Holocene precipitation between the monsoonal East Asia and westerly dominated Central Asia, but it is unclear if the moisture differences are also associated with corresponding temperature contrasts. Here we present the first alkenone-based paleotemperature reconstructions for the past 21 kyr from Lake Balikun, central Asia. We show, unlike the initiation of Holocene warm conditions at ∼11 kyr BP in the monsoon regions, the arid central Asia remained in a glacial-like cold condition prior to 8 kyr BP and experienced abrupt warming of ∼9 °C after the collapse of the Laurentide ice sheet. Comparison with pollen and other geochemical data indicates the abrupt warming is closely associated with major increase in the moisture supply to the region. Together, our multiproxy data indicate ∼2 thousand years delay of temperature and moisture optimum relative to local summer insolation maximum, suggesting major influence of the Laurentide ice sheet and other high latitude ice sheet forcings on the regional atmospheric circulation. In addition, our data reveal a temperature drop by ∼4 °C around 4 kyr BP lasting multiple centuries, coinciding with severe increases in aridity previously reported based on multiproxy data. In contrast, model simulations display a much less pronounced delay in the initiation of Holocene warm conditions, raising unresolved questions about the relative importance of local radiative forcing and high-latitude ice on temperature in this region.

D/H ratios of leaf lipids from lacustrine aquatic macrophytes have been shown to record precipitation δD values, which can reflect precipitation-weighted mean annual temperature (MAT) in the northeastern United States. Here we report a high-resolution hydrogen isotopic record from Little Pond, Massachusetts, USA, which we compare with other paleoclimate data from the region, including a similar δD record from Blood Pond, Massachusetts. Together the two datasets provide a >16 ka record of δD variability in the region, affording new insights into Holocene climate history. First, the long-term trends in δD correlate significantly with regional temperatures inferred from alkenone records from nearby areas of the North Atlantic and lake-level inferred changes in precipitation and evaporation. The long-term δD trends reflect a period of maximum regional warmth at ca. 8–6 ka after the collapse of the Laurentide Ice Sheet. Second, unlike the positive relationship between temperature and δD observed over the long-term and during early events like the Younger Dryas, we find that a series of warm and dry events at 4.9–4.6, 4.2–3.9, 2.9–2.1, and 1.3–1.2 cal kya BP coincide with negative δD excursions from the long trends. These events were likely driven by summer drought dynamics.

•Dimethyl disulfide (DMDS) alkenone adduction is a kinetically controlled reaction.•Mono-DMDS adducts (DMDS-1) of alkenones are transient reaction products.•DMDS-1 adducts are ideal for determining alkenone double bond positions.•The yield of alkenone DMDS-1 adducts can be maximized at low temperatures.•Reaction duration and iodine catalyst were optimized for alkenone DMDS-1 yields.Long chain alkenones (LCAs) are among the most successful biomarkers for paleotemperature reconstructions. However, fundamental questions regarding the biosynthesis and cellular functions of alkenones in haptophyte algae remain poorly understood. Recent discoveries of LCAs with double bond positions and chain lengths that differ from common structures further highlight the importance of continued research into structural variations of this important class of lipid biomarkers to improve LCA applications as temperature proxies. Double bond positions on alkyl chains can be effectively determined by preparing mono-double bond adducts with dimethyl disulfide (DMDS-1), and subsequent gas chromatography–mass spectrometry (GC–MS) analysis. However, previously published procedures for adduct preparation were originally designed for mono-unsaturated fatty acids, and generally produce low product yields when applied to alkenones. Here we demonstrate that the problem originates mainly from DMDS and alkenone overreaction at high temperatures for long time periods, and, secondarily, insufficient amount of iodine catalyst. The overreaction results in DMDS reacting with multiple double bonds, and the possible formation of intermolecular linkages, creating non-volatile products. These products are of little use for elucidating alkenone structures. We demonstrate that by reducing the reaction temperature and time, and by using an optimal amount of iodine, we can maximize the yield of transient DMDS-1 adducts for alkenone structure determination.Download high-res image (91KB)Download full-size image

•δD of source water for Artic plants reflects a mixture of seasonal precipitation dominated by summer rainfall.•Net apparent fractionation between precipitation and leaf waxes in Arctic plants is similar to that of temperate regions.•Leaf waxes from Eriophorum vaginatum, a C3 graminoid, are 23–76‰ more depleted than Betula nana, a C3 shrub.•Lake sediment waxes are derived primarily from within watersheds, and εapp in lake sediments correlates with watershed-scale vegetation assemblages.Leaf wax hydrogen isotopes (δDwax) are increasingly utilized in terrestrial paleoclimate research. Applications of this proxy must be grounded by studies of the modern controls on δDwax, including the ecophysiological controls on isotope fractionation at both the plant and landscape scales. Several calibration studies suggest a considerably smaller apparent fractionation between source water and waxes (εapp) at high latitudes relative to temperate or tropical locations, with major implications for paleoclimatic interpretations of sedimentary δDwax. Here we investigate apparent fractionation in the Arctic by tracing the isotopic composition of leaf waxes from production in modern plants to deposition in lake sediments using isotopic observations of precipitation, soil and plant waters, living leaf waxes, and waxes in sediment traps in the Brooks Range foothills of northern Alaska. We also analyze a lake surface sediment transect to compare present-day vegetation assemblages to εapp at the watershed scale. Source water and εapp were determined for live specimens of Eriophorum vaginatum (cottongrass) and Betula nana (dwarf birch), two dominant tundra plants in the Brooks Range foothills. The δD of these plants’ xylem water closely tracks that of surface soil water, and reflects a summer-biased precipitation source. Leaf water is enriched by 23 ± 15‰ relative to xylem water for E. vaginatum and by 41 ± 19‰ for B. nana. Evapotranspiration modeling indicates that this leaf water enrichment is consistent with the evaporative enrichment expected under the climate conditions of northern Alaska, and that 24-h photosynthesis does not cause excessive leaf water isotope enrichment. The εapp determined for our study species average −89 ± 14‰ and −106 ± 16‰ for B. nana n-alkanes and n-acids, respectively, and −182 ± 10‰ and −154 ± 26‰ for E. vaginatum n-alkanes and n-acids, which are similar to the εapp of related species in temperate and tropical regions, indicating that apparent fractionation is similar in Arctic relative to other regions, and there is no reduced fractionation in the Arctic. Sediment trap data suggest that waxes are primarily transported into lakes from local (watershed-scale) sources by overland flow during the spring freshet, and so δDwax within lakes depends on watershed-scale differences in water isotope compositions and in plant ecophysiology. As such, the large difference between our study species suggests that the relative abundance of graminoids and shrubs is potentially an important control on δDwax in lake sediments. These inferences are supported by δDwax data from surface sediments of 24 lakes where εapp, relative to δDxylem, averages −128 ± 13‰ and −130 ± 8‰ for n-acids and n-alkanes, respectively, and co-varies with vegetation type across watersheds. These new determinations of plant source water seasonality and εapp for the Arctic will improve the δDwax paleoclimate proxy at high latitudes.

•User-friendly software environment for processing LC–MS data.•Automated peak integration of branched and isoprenoid GDGTs.•Chemometric based backend for complex baseline correction and peak deconvolution.Liquid chromatography–mass spectrometry (LC–MS) analysis of glycerol dialkyl glycerol tetraethers (GDGTs) from sediment and soil samples has become a widely adopted approach for reconstructing past ocean and continental climate variables such as temperature and pH. The LC–MS data used for constructing these GDGT climate proxies are taken directly from the peak area values of individual GDGT [M+H]+ ions, often determined from manual peak integration due to unreliable computer integrators in commercial software, particularly in cases of complex baselines, asymmetric peak shapes and peak coelution. Manual integration is not only time consuming, but also prone to user induced inconsistency when individuals utilize different criteria for peak/baseline definition. To overcome these problems, we have developed a user friendly, graphical user interface (GUI) programmed in the MATLAB environment, allowing users to efficiently and reproducibly perform batch processing and peak integration of LC–MS data. The program, “TEXPRESS” v1.0 (“tetraether index express”), incorporates modern chemometric based techniques for baseline definition and deconvolution of complex chromatographic peaks and we show that LC–MS data processed using the TEXPRESS toolbox are in strong agreement with results obtained from manual peak integration. We provide a general overview of the concepts and architecture of the TEXPRESS toolbox and discuss the advantages of chemometric based peak integration methods for processing branched and isoprenoid GDGT LC–MS data.

•Leaf wax abundance in 5 C3 plants generally increased from base to tip of leaf.•There was a base-to-tip 2H enrichment in all cases.•C isotopes showed a base-to-tip 13C depletion in most species.•Isotopic trends indicated continuous leaf wax regeneration over growth season.•C isotopic change may reflect changing water use efficiency or source δ13C value.Hydrogen and carbon isotope ratios of terrestrial plant leaf wax are powerful paleoclimate and paleoenvironmental proxies. However, further research on the underlying controls of the chemical and isotopic composition is needed in order to more quantitatively infer past climate and environmental conditions. There are systematic molecular and isotopic trends along leaves of a C4 grass Miscanthus, but whether such patterns also occur in other plant types is unclear. Here we determined intra-leaf variability in leaf wax abundance and isotope ratios for five terrestrial C3 plants. All species showed a general increasing base-to-tip trend in leaf wax abundance, suggesting a physiological need for more leaf wax in outer leaf sections. The hydrogen isotope values showed base-to-tip deuterium enrichment in all plants, but carbon isotope values showed base-to-tip 13C depletion in most species. Such intra-leaf isotopic distributions are similar to those in the C4 grass Miscanthus, suggesting similar underlying controls. While leaf wax hydrogen isotopic ratio values in general followed similar trends to corresponding leaf water, we found a significant reduction in hydrogen isotopic fractionation at the base of leaves (most pronounced for Dactylis glomerata). We attribute this to the utilization of isotopically enriched glucose transported from outer leaf sections for leaf wax synthesis at the base. The transported sugars were enriched in deuterium because of preferential consumption of 2H-depleted resources during metabolism in outer sections, in addition to biosynthesis using 2H-enriched leaf water.

•Distributions of long chain alkenones (LCAs) in surface sediments from 12 lakes determined.•The lakes are from arid NW China and have a wide range of salinity (0.7–146.3 g/l).•C41 and C42 LCAs may be characteristic of hypersaline environments.The chain length of long chain alkenones (LCAs) in ocean and lake sediments commonly ranges from C37 to C39. Alkenones > C40 are extremely rare: C41 and C42 alkenones have only been reported in a Cretaceous black shale (Cenomanian, ca. 95 Ma) from the Blake Bahama Basin, western North Atlantic (DSDP Leg 76, site 534). The significance of these exceptionally long chain alkenones is, however, unclear. We have conducted a systematic survey of LCAs in surface sediments from 12 lakes with a wide range of salinity (0.7–146.3 g/l) from arid northwestern China. They showed a general trend of increasing maximum chain length with increasing salinity. In particular, C41 and C42 LCAs occurred only in the surface sediments of hypersaline lakes (113.6–146.3 g/l), but were absent from lakes with lower salinity, indicating that these compounds may be produced by an unknown haptophyte species living in hypersaline environments.

•Isocratic separation of fatty acid ethyl esters (FAEEs) from fish oil.•Baseline resolution between ω-3 PUFAs (EPA/DHA) within 5–10 min.•Fundamental properties of the silver(I)-mercaptopropyl stationary phase examined.•Eicosapentaenoic acid purity (EPA, >95%) and docosahexaenoic acid (DHA, >99%).Omega-3 polyunsaturated fatty acids (ω-3 PUFAs) have become increasingly popular in dietary supplements worldwide and global demand for higher purity ω-3 PUFAs in clinical applications has been rising rapidly in the recent years. Traditional methods for isolating ω-3 PUFAs however, generally require multiple, cumbersome steps to obtain high purity (>95%) products. In this study, we report an efficient liquid chromatography method for purifying individual omega-3 fatty acid ethyl esters (FAEEs), eicosapentaenoic acid (EPA, C20:5) and docosahexaenoic acid (DHA, C22:6), at >95% purity using a silver(I)-mercaptopropyl stationary phase, otherwise known as silver-thiolate chromatographic material (AgTCM). We demonstrate the key variables controlling separation of FAEEs with a silver(I)-thiolate stationary phase and examine important similarities and differences between silver-thiolate and silver-ion chromatography in regards to the separation mechanism taking place. We demonstrate the separation behavior of FAEEs under various conditions such as mobile phase composition, flow rate and sample injection amount. Purification of EPA and DHA was carried out on an analytical and semi-preparative scale and the resulting purity was determined by GC-FID. We report high attainable purity for EPA (>95%) and DHA (>99%) from a single isocratic separation of fish oil FAEEs within 5–10 min using a mobile phase of heptane/acetone (95:5, % v/v).

•Improved separation of alkenones using a poly(trifluoropropylmethylsiloxane) stationary phase.•Baseline resolution of alkenones via ketone position.•Separation of novel tri-unsaturated isomeric alkenones.•Preliminary characterization of novel alkenones.•New GC–FID stationary phase for alkenone analysis.Long chain alkenones (LCAs) have played a central role in reconstructing paleo-sea surface temperature throughout much of the world oceans. The standard method for LCA separation and quantification utilizes gas chromatography (GC) with a non-polar column and flame ionization detection (FID). The method has remained nearly unchanged since the debut of alkenone paleothermometry over three decades ago. The conventional stationary phase used [poly(dimethylsiloxane)] is adequate for calculation of the U37K′ index for most marine samples, but performs poorly in the separation of methyl and ethyl C38 LCAs and fails to separate certain lacustrine LCAs, including structural isomers featuring differing double bond and carbonyl positions. Here, we demonstrate major improvements in the selectivity and resolution of numerous LCAs using a mid-polarity, dipole selective poly(trifluoropropylmethylsiloxane) GC stationary phase with FID. Various columns featuring this phase achieved baseline resolution of all C38 LCAs in a marine sediment and Emiliania huxleyi culture samples, improving peak resolution of C38 LCAs by > 350%. Such dipole selective stationary phases also reveal novel tri-unsaturated isomeric LCAs in lacustrine samples, for which we provide the first report and preliminary characterization. We conduct a thorough evaluation of LCA retention behavior on poly(trifluoropropylmethylsiloxane) and poly(dimethylsiloxane) columns and report an optimal range of operating conditions for both phases.

•Cultured Pseudoisochrysis paradoxa contained an abundant, but not dominant, C37:4 alkenone.•P. paradoxa  U37K calibration matched lake U37K calibrations and was distinct from marine calibrations.•The U37K calibration had a superior fit than the U37K′ calibration.•C37:4 should be incorporated for reconstructing temperature from P. paradoxa alkenone records.The alkenone unsaturation index (U37K or U37K′) serves as a critical tool for reconstructing temperature in marine environments. Lacustrine haptophyte algae are genetically distinct from their ubiquitous and well studied marine counterparts, and the unknown species-specific genetic imprints on long chain alkenone production by lacustrine species have hindered the widespread application of the U37K temperature proxy to lake sediment records. The haptophyte Pseudoisochrysis paradoxa   produces alkenones but its U37K calibration has never been determined.   It has an alkenone fingerprint abundant in tetraunsaturated alkenones, a hallmark of lacustrine environments. We present here the first calibration of the U37K index to temperature for a culture of P. paradoxa  . We found that the U37K index accurately captured the alkenone response to temperature whereas the U37K′ index failed to do so, with U37K′ values below 0.08 projecting to two different temperature values. Our results add a fifth species-specific U37K calibration and provide another line of evidence that different haptophyte species require different U37K calibrations. The findings also highlight the necessary inclusion of the C37:4 alkenone when reconstructing temperatures from P. paradoxa-derived alkenone records.

Compound specific hydrogen and carbon isotopic ratios of higher plant leaf waxes have been extensively used in paleoclimate and paleoenvironmental reconstructions. However, studies so far have focused on the comparison of leaf wax isotopic differences in bulk leaf samples between different plant species. We sampled three different varieties of tall grasses (Miscanthus sinensis) in six segments from base to tip and determined hydrogen and carbon isotopic ratios of leaf waxes, as well as hydrogen and oxygen isotopic ratios of leaf water samples. We found an increasing, base-to-tip hydrogen isotopic gradient along the grass blades that can probably be attributed to active leaf wax regeneration over the growth season. Carbon isotopic ratios, on the other hand, show opposite trends to hydrogen isotopic ratios along the grass blades, which may reflect different photosynthetic efficiencies at different blade locales.

We report the use of silver-thiolate chromatographic material (AgTCM) as a stable material for use in TLC. The AgTCM stationary phase operates under the same principles as silver-ion chromatography, separating compounds by degree of unsaturation; however, the AgTCM stationary phase shows considerable advantages over Ag-TLC in terms of light stability and shelf lifetime. We demonstrate the light stability of the AgTCM-TLC and its application for separations based on the degrees of unsaturation using fatty acid methyl esters (FAMEs) and polycyclic aromatic hydrocarbons (PAHs).Highlights► Silver(I)-mercaptopropyl modified silica gel used in thin layer chromatography (TLC). ► Enhanced light stability and shelf lifetime over conventional silver based TLC methods. ► Demonstration of retention properties of PAHs and FAMEs.

Understanding the natural mechanisms that control fire occurrence in terrigenous ecosystems requires long and continuous records of past fires. Proxies, such as sedimentary charcoal and tree-ring fire scars, have temporal or spatial limitations and do not directly detect fire intensity. We show in this study that polycyclic aromatic hydrocarbons (PAHs) produced during wildfires record local fire events and fire intensity. We demonstrate that high performance liquid chromatography with fluorescence detector (HPLC-FLD) is superior to gas chromatography–mass spectrometry (GC–MS) for detecting the low concentrations of sedimentary PAHs derived from natural fires. The HPLC-FLD is at least twice as sensitive as the GC–MS in selective ion monitoring (SIM) mode for parent PAHs and five times as sensitive for retene. The annual samples extracted from varved sediments from Swamp Lake in Yosemite National Park, California are compared with the observational fire history record and show that PAH fluxes record fires within 0.5 km of the lake. The low molecular weight (LMW) PAHs (e.g., fluoranthene, pyrene and benz[a]anthracene) are the best recorders of fire, whereas the high molecular weight (HMW) PAHs likely record fire intensity. PAHs appear to resolve some of the issues inherent to other fire proxies, such as secondary deposition of charcoal. This study advances our understanding of how PAHs can be used as markers for fire events and poses new questions regarding the distribution of these compounds in the environment.Highlights► Polycyclic aromatic hydrocarbons (PAHs) as a signature for fire in paleorecord. ► HPLC-fluorescence detection is superior to GC–MS for detecting PAHs. ► PAHs record fires within 0.5 km of the lake in Yosemite National Park, CA. ► Low molecular weight PAHs (e.g. fluoranthene, pyrene) are the best recorders of fire. ► PAHs appear to resolve some issues inherent to other fire proxies.

Long chain alkenone (LCA) lipids produced by haptophyte algae are ubiquitous in marine environments and are increasingly reported from enhanced conductivity, solute rich lakes worldwide. Following our survey of lakes in the United States (Toney et al., 10.1016/j.gca.2009.11.021), we examined surface sediments from 19 lakes in central Canada for alkenones and compared our data with lake characteristics. We found that 68% of surveyed lakes contained sedimentary LCAs, all have C37:4me dominated profiles. Lakes containing LCAs had salinities >1.5 g/l and SO4/CO3 ratios >3.5. LCA concentrations were highly elevated (∑C37 > 500 μg/g sediment) in sediments of meromictic lakes with permanently anoxic deep waters. Overall, the distribution of LCAs was consistent with a previous survey of lakes in the interior of the United States demonstrating that tetra-unsaturated lipids were most common in sulfate rich, saline lakes. Alkenone unsaturation indices (U37K) from the upper 10 cm of the sediment column are weakly correlated with point sampled average spring water temperature (r2 = 0.30, p = 0.067) and the range of inferred temperatures is in agreement with the average, long term potential evapotranspiration weighted temperature.Highlights► Alkenones are common in prairie lakes of interior Canada. ► High concentrations of C37 alkenones are associated with anoxic deep meromictic lakes. ► Alkenone unsaturation is marginally correlated with spring temperature.

West Greenland has had multiple episodes of human colonization and cultural transitions over the past 4,500 y. However, the explanations for these large-scale human migrations are varied, including climatic factors, resistance to adaptation, economic marginalization, mercantile exploration, and hostile neighborhood interactions. Evaluating the potential role of climate change is complicated by the lack of quantitative paleoclimate reconstructions near settlement areas and by the relative stability of Holocene temperature derived from ice cores atop the Greenland ice sheet. Here we present high-resolution records of temperature over the past 5,600 y based on alkenone unsaturation in sediments of two lakes in West Greenland. We find that major temperature changes in the past 4,500 y occurred abruptly (within decades), and were coeval in timing with the archaeological records of settlement and abandonment of the Saqqaq, Dorset, and Norse cultures, which suggests that abrupt temperature changes profoundly impacted human civilization in the region. Temperature variations in West Greenland display an antiphased relationship to temperature changes in Ireland over centennial to millennial timescales, resembling the interannual to multidecadal temperature seesaw associated with the North Atlantic Oscillation.

The reliability of chronology is a prerequisite for meaningful paleoclimate reconstructions from sedimentary archives. The conventional approach of radiocarbon dating bulk organic carbon in lake sediments is often hampered by the old carbon effect, i.e., the assimilation of ancient dissolved inorganic carbon (DIC) derived from carbonate bedrocks or other sources. Therefore, radiocarbon dating is ideally performed on organic compounds derived from land plants that use atmospheric CO2 and rapidly delivered to sediments. We demonstrate that lignin phenols isolated from lake sediments using reversed phase high performance liquid chromatography (HPLC) can serve as effective 14C dating materials for establishing chronology during the late Quaternary. We developed a procedure to purify lignin phenols, building upon a published method. By isolating lignin from standard wood reference substances, we show that our method yields pure lignin phenols and consistent ages as the consensus ages and that our procedure does not introduce radiocarbon contamination. We further demonstrate that lignin phenol ages are compatible with varve counted and macrofossil dated sediment horizons in Steel Lake and Fayetteville Green Lake. Applying the new method to lake sediment cores from Lake Qinghai demonstrates that lignin phenol ages in Lake Qinghai are consistently younger than bulk total organic carbon (TOC) ages which are contaminated by old carbon effect. We also show that the age offset between lignin and bulk organic carbon differs at different Lake Qinghai sedimentary horizons, suggesting a variable hard water effect at different times and that a uniform age correction throughout the core is inappropriate.

D/H ratios of sedimentary leaf waxes can provide useful information about past climate change. However, factors controlling δD values of higher plant leaf waxes (δDwax) are poorly understood. Here we show that δDwax values are negatively correlated with δ13C values of leaf waxes (δ13Cwax), based on a study of 35 leaf samples of 11 tree species around Blood Pond, Massachusetts (USA). Our data suggest that plant water use efficiency exerts an important control on the δDwax variation among tree species.

Long chain, odd carbon-numbered n-alkan-2-ones are abundant in peatland sediments. We show that they are present in all species of Sphagnum we have examined but are absent from ten other plant species most common to ombrotrophic bogs. Combined with literature data, we conclude that n-alkan-2-ones are viable biomarkers for Sphagnum in ombrotrophic bogs.

Paleoclimatic interpretation of the hydrogen isotope ratios of plant leaf waxes extracted from sediments requires a thorough understanding of the factors controlling the isotopic ratios. Existing studies have found relatively small variability in hydrogen isotope fractionation among plants of different photosynthetic pathways (C3, C4 and CAM) and between gymnosperms and angiosperms. However, there has been no systematic study at a single site to determine how leaf wax hydrogen isotope (D/H) ratios differ in different plant types under the same precipitation and environmental regime. Such data are nevertheless crucial for understanding the impact of past vegetation changes on the sedimentary hydrogen isotope records of leaf waxes. Here, we present a study of D/H ratios of leaf waxes from 48 species in seven types of terrestrial and aquatic C3 plants around Blood Pond, Dudley, Massachusetts, USA. The δD values of leaf waxes differ by as much as 70‰ for different plant types, with those from trees and ferns having the highest values and those from grasses having the lowest values. The large isotopic variation indicates that the apparent hydrogen isotopic fractionation between leaf waxes and precipitation is not constant for different plant types. Our results indicate that inferring precipitation D/H ratios on the basis of sedimentary leaf waxes is only viable when significant vegetation change is absent or can be accounted for isotopically.

Proxies such as plant macrofossils, humification indices and testate amoebae have been developed to reconstruct past hydrological variability in ombrotrophic peatlands.Reconstructions based on these approaches have resulted in records of decadal- to centennial-scale moisture variability in regions of North America and elsewhere. Although the various peatland moisture proxies generally show significant temporal covariance, multi-proxy approaches can refine knowledge of the multivariate nature of climate change and increase confidence in interpretations. Here, we demonstrate that ratios of the abundances of n-alkanes provide a new and efficient way of reconstructing past peatland surface moisture change. We found strong correlations among n-alkane indices, humification indices and testate amoebae-inferred water table depths from a single core. Biogeochemical proxies can be used in paleohydrological reconstructions for ombrotrophic bogs to provide a new and complementary source of data from these under-utilized paleoclimate archives.

We measured D/H ratios of individual long chain leaf wax n-alkanes isolated from a loess profile spanning the last 130 ka from Xifeng, China. The δD values show large variation (−140 to −200‰) for the past 130 ka and are highly correlated with changes in magnetic susceptibility (MS), an accepted proxy for past monsoon circulation in the Chinese Loess Plateau. Peaks in high δD values (−140 to −160‰) coincide with low MS values for loess sections formed under relatively dry conditions (marine isotope stages, MISs, 2 and 4), whereas peaks in low δD values (−190 to −200‰) coincide with high MS values for paleosol sections formed under relatively wet conditions (MISs 1, 3 and 5). We attribute the isotopic variations to the combined effects of varying aridity, which controls the soil and leaf water evaporation, and temperature and monsoon intensity, which alter the D/H ratios of precipitation. We also discovered a strong negative correlation between the molecular distributions (ratios of C31/C29 and C31/C27n-alkanes) and the δD values of n-alkanes, based on studies of modern plants and the Xifeng loess sequence. The complex relationship between MS and monsoon circulation has been difficult to understand mechanistically. However, the climatic controls on the D/H ratios of individual long chain n-alkanes and C31/C29n-alkane ratios can be readily demonstrated and calibrated from modern plants and surface soils. Therefore, these new molecular and isotopic proxies have potential as quantitative proxies for paleoenvironmental reconstruction in the Chinese Loess Plateau.

We determined the stable hydrogen isotope fractionation during progressive vaporization of three n-alkanes (heptane or C7, octane or C8, nonane or C9) at 24±1 °C, in an effort to understand the environmental isotopic fractionation of low molecular weight petroleum hydrocarbons. The measurements were carried out using continuous flow gas chromatography- high temperature conversion–isotope ratio mass spectrometry (GC/TC/IRMS). During the course of vaporization, the remaining liquid compounds show progressive deuterium (D)-depletion, indicating a preferential vaporization of D-enriched species. Up to a 14‰ change in δD values was observed during the course of vaporization, with vapor–liquid fractionation factors (α) of 1.0044±0.0002 for heptane, 1.0043±0.0001 for octane, and 1.0040±0.0002 for nonane. The observed Δ is significantly greater than the Δ13C reported previously for different compounds. Our data are fundamental for evaluating the isotopic fractionation of organic contaminants during natural evaporation processes. The results have potential applications for the study of petroleum hydrocarbons at contaminated sites. The large isotopic change during the experimental vaporization also highlights the importance of avoiding evaporation losses during the collection and preparation of low molecular weight compounds for hydrogen isotope analyses.

Seasonality of precipitation is an important yet elusive climate parameter in paleoclimatological reconstructions. This parameter can be inferred qualitatively from pollen and other paleoecological methods, but is difficult to assess quantitatively. Here, we have assessed seasonality of precipitation and summer surface wetness using compound specific hydrogen and carbon isotope ratios of vascular plant leaf waxes and Sphagnum biomarkers extracted from the sediments of an ombrotrophic peatland, Bøstad Bog, Nordland, Norway. Our reconstructed precipitation seasonality and surface wetness are consistent with regional vegetation reconstructions. During the early Holocene, 11.5–7.5 ka, Fennoscandia experienced a cool, moist climate. The middle Holocene, 7.5–5.5 ka, was warm and dry, transitioning towards cooler and wetter conditions from the mid-Holocene to the present. Changes in seasonality of precipitation during the Holocene show significant coherence with changes in sea surface temperature in the Norwegian Sea, with higher SST corresponding to greater percentage of winter precipitation. Both high SST in the Norwegian Sea and increased moisture delivery to northern Europe during winter are correlated with a strong gradient between the subpolar low and subtropical high over the North Atlantic (positive North Atlantic Oscillation).

The Tigray Plateau of Ethiopia and Eritrea is vulnerable to environmental change, yet environmental influences on the rise and fall of the civilizations that once existed there are almost unexplored. We sampled sections of gully walls for palaeoenvironmental proxies from two sites: 1) Adi Kolen on the southern outskirts of the Plateau's most developed former empire, the Aksumite, and 2) Adigrat near polities dating to at least ca. 3000 cal yr BP. A multi-proxy approach for examining local variation in palaeoenvironments was evaluated that included stable isotopic and elemental analyses (δ13CSOM, δ15N, %TOC, and %TN) of soil, and charcoal identification. An increase in δ15N values from older soils in Adi Kolen (4400 cal yr BP) and Adigrat (2900 cal yr BP) until 1200 cal yr BP is not explained by changes in δ15N that occur with time in an unchanging environment. It may instead indicate an overall decrease in rainfall from the earlier times until 1200 cal yr BP. In one Adigrat section, the decreases in organic δ13C and increases in C/N molar ratios from older to younger soil could have resulted from changes that occur over time, per se. In the remaining sections, however, δ13CSOM trends more likely reflect changes in the biomass of C4 relative to C3 plants (% C4 biomass). Changes in% C4 biomass may reflect climate and/or land use. Deciphering which may be aided by analyses of the other proxies. Identified charcoal suggests that both sites supported some juniper forest types until very recently but that forests may have been a more important and dynamic component of Adigrat's vegetation history than Adi Kolen's. If environment affected the trajectories of the kingdoms of the Tigray Plateau, these results suggest that the exact nature of the changes in climate differed among kingdoms. The kingdoms prior to 1200 cal yr BP may have been exposed to increasing aridity punctuated with relatively wetter intervals. Thereafter, general changes in climate are not apparent. Land clearing dynamics are likely to have had a more consistent effect on the trajectories of kingdoms than climate changes.

D/H ratios of leaf waxes (δDwax) derived from terrestrial plants and preserved in lake sediments can provide important information on past continental hydrology. Ideally, δDwax can be used to reconstruct precipitation D/H ratios (δDP) which is a well-established paleoclimate proxy. However, many other factors, such as vegetation and relative humidity (RH), also affect δDwax variation. How the combination of these factors affects sedimentary δDwax is unclear. Here, we use a transect of 32 lake surface sediments across large gradients of precipitation, relative humidity, and vegetation composition in the southwestern United States to study the natural factors affecting sedimentary δDwax. δD values of C28 n-alkanoic acids show significant correlation with δDP values (R2 = 0.76) with an apparent isotopic enrichment of ∼99 ± 8‰, indicating that sedimentary δDwax values track overall δDP variation along the entire transect. Leaf waxes produced by plants grown under controlled conditions (RH = 80%, 60%, 40%) show a small increase in D/H ratios as RH decreases, consistent with prediction from the Craig-Gordon model. However, the isotopic effect of RH on δDwax along the natural transect is partially countered by the opposing influence of vegetation changes. The correlation between δDwax and δDP values is significantly higher (R2 = 0.84) in the drier portions of the transect than in the wetter regions (R2 = 0.64). This study suggests that D/H ratios of sedimentary leaf waxes can be used as a proxy for precipitation δD variations, with particularly high fidelity in dry regions, although more studies in other regions will be important to further test this proxy.

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 “ƒ”—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, C23 n-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 C23 n-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.

Monocarboxylic acids (MCAs) are important astrobiologically because they are often the most abundant soluble compounds in carbonaceous chondrites (CCs) and are potential synthetic end products for many biologically important compounds. However, there has been no systematic study on the effect of parent body alteration on molecular and isotopic variability of MCAs. Since MCAs in meteorites are dominated by low molecular weight (C1–C8), highly volatile compounds, their distributions are likely to be particularly sensitive to secondary alteration processes. In contrast, the aliphatic side chains of insoluble organic matter (IOM) in CCs, whose composition has been shown to be closely related to the MCAs, may be far more resistant to secondary alteration. In the present study, we determined the distributions and isotopic ratios of free and IOM-derived MCAs in six carbonaceous chondrites with a range of classifications: Murchison (CM2), EET 87770 (CR2), ALH 83034 (CM1), ALH 83033 (CM2), MET 00430 (CV3) and WIS 91600 (C2). We compare mineralogical and petrological characteristics to the MCAs distributions to better define the processes leading to the synthesis and alteration of meteoritic MCAs. Our results show that aqueous and especially thermal alteration in the parent bodies led to major loss of free MCAs and depletion of straight relative to branched chain compounds. However, the MCAs derived from aliphatic side chains of IOM are well preserved despite of secondary alterations. The molecular and isotopic similarities of IOM-derived MCAs in different chondrite samples indicate very similar synthetic histories for organic matter in different meteorites.

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–C23 n-alkanes and C20–C24 n-alkanoic acids) are exceptionally abundant in floating and submerged aquatic plants, in contrast to the dominance of long-chain n-alkyl lipids (C27–C31 n-alkanes and C26–C32 n-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 (C23 n-alkane and C22 n-acid (behenic acid)) in Blood Pond sediments are derived from floating and submerged macrophytes. In addition, D/H ratios of C22 n-acid and C23 n-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 C23 n-alkane and C22 n-acid could be excellent recorders of past lake water isotopic ratios in lakes with abundant floating and submerged macrophyte inputs.

Long chain alkenones (LCAs) are potential biomarkers for quantitative paleotemperature reconstructions from lacustrine environments. However, progress in this area has been hindered, because the conditions necessary for the growth of haptophytes responsible for alkenone distributions in lake sediments: the predominance of C37:4 LCA are not known. Here we report the first enrichment culturing of a novel haptophyte phylotype (Hap-A) from Lake George, ND that produces predominantly C37:4-LCA. Hap-A was enriched from its resting phase collected from deep sediments rather than from water column samples. In contrast, enrichments from near surface water yielded a different haptophyte phylotype (Hap-B), closely related to Chrysotila lamellosa and Pseudoisochrysis paradoxa, which does not display C37:4-LCA predominance (similar enrichments have been reported previously). The LCA profile in sediments resembles that of enrichments containing Hap-A, suggesting that Hap-A is the dominant alkenone producer of the sedimentary LCAs. In enrichments, increased lighting appeared to be crucial for triggering alkenone production. Both U37K and U38K indices show a promising, positive relationship with temperature for Hap-A in enrichments, but the offset from the environmental calibration suggests that other factors (e.g., the growth stage or nutrients) may influence the absolute U37K value. Based on 18S rRNA gene analyses, several lakes from the Northern Great Plains, as well as Pyramid Lake, NV and Tso Ur, Tibetan Plateau, China contain the same two haptophyte phylotypes. Analysis of surface sediment from the Great Plains lakes show the Hap-A-type LCA distribution, whereas Pyramid and Tso Ur show the Hap-B type distribution. Waters of the Great Plain lakes are dominated by sulfate ions, whereas those Pyramid and Tso Ur are dominated by carbonate ions, suggesting that the sulfate to carbonate ratio may be a determining factor for the dominance of the Hap-A and Hap-B phylotypes in natural settings.

The late Quaternary temperature history on the Chinese Loess Plateau (CLP) is little known due to the absence of suitable paleothermometers. Here, we present a quantitative temperature record for the last 110 kyr from Lantian county, southern CLP, based on the distributions of bacterial tetraethers. Lantian loess temperature variations display significant correlation with the Northern Hemisphere insolation (35°N) as well as the cave monsoon records at the precession band. We attribute such correlation to both the latent heat release from the East Asian summer monsoon and the direct influence of Sun's sensible heat over the semi-arid landscape. The long term cooling during the Marine Isotope Stage (MIS) 3 and the modest temperature drop of ~ 5 °C from 79 to 65 ka are best explained by interactions among multiple climate forcings including insolation, atmospheric CO2 concentration, global ice volume, and regional glacier activities. The cold and modestly wet climate during MIS 3 may have promoted maximum regional glacial advances, whereas the minimal temperature during local LGM reduced moisture transfer and led to glacial recession. The unusually early deglacial warming at ~ 22.5 ka highlights the importance of regional climate records to decipher the complex continental climate dynamics.Highlights► A quantitative temperature record for the last 110 kyr from Lantian, southern CLP. ► Lantian temperature significantly correlates with the 35°N summer insolation. ► Some T variations are best explained by interactions of multiple climate forcings. ► Temperature, monsoonal rainfall and regional mountain glaciers interact. ► We also observe the unusually early deglacial warming at ~ 22.5 ka.