Leaf Wax N-alkanes Research Articles

Seasonal variations of leaf wax n-alkane distributions and δ2H values in peat-forming vascular plants from the Dajiuhu peatland, central China

Seasonal variations of leaf wax n-alkane distributions and δ2H values in peat-forming vascular plants from the Dajiuhu peatland, central China

Seasonal variation of leaf wax n-alkane δ2H values: Differences between Quercus aquifolioides (an evergreen tree) and Stipa bungeana (a perennial grass) from the southeastern Tibetan Plateau

To understand the cause of known isotopic differences between grasses and trees, their seasonal variability and environmental drivers, we present a parallel study of a perennial grass, Stipa bungeana, and an evergreen tree, Quercus aquifolioides, in the monsoon-dominated southeastern Tibetan Plateau (TP), by conducting fortnightly paired δ2H analyses of precipitation (δ2Hp), leaf water, and n-alkanes (δ2Hwax) over a period of 2 y. Similar to previous investigations, we show that δ2Hwax in both species are correlated to δ2Hp throughout two growing seasons. However, for the first time, we identified a time lag of 2 months in variation of δ2Hwax relative to δ2Hp in new Q. aquifolioides leaves, whereas nearly no lagged phenomena were observed in S. bungeana and old mature Q. aquifolioides leaves. The δ2Hwax in new Q. aquifolioides leaves correlated significantly with the total amount of n-alkanes. These findings suggest that the turnover rate of n-alkanes in new Q. aquifolioides leaves is possibly changing with their synthesis, and that is a possible mechanism of the observed delayed phenomenon. A paired t-test showed that the difference of δ2Hwax between trees and grasses becomes insignificant after time-lag correction. Moreover, the apparent fractionation in Q. aquifolioides was significantly correlated with weekly or fortnightly precipitation amount, temperature, relative humidity, and wind velocity, while in S. bungeana only weekly or fortnightly precipitation amount. Our high temporal resolution observations hence confirm that climate variables are important drivers of plant δ2Hwax and, give significant implications for δ2Hwax-base paleoclimate reconstructions in global monsoonal regions.

Carbon isotopic composition of leaf wax n-alkanes in mangrove plants along a latitudinal gradient in Brazil

The carbon isotopic composition (δ13C) of selected specific biomarkers provides important information on the relationship between plant species and the environment in which they developed. Here we present baseline data of δ13C values of individual n-alkanes (δ13Cn-alkanes) from leaf waxes of mangrove species – Avicennia spp., Rhizophora mangle and Laguncularia racemosa – typical of mangrove forests collected from equatorial to sub-tropical regions in Brazil. The objective was to evaluate the influence of tree physiology, local hydrology and climatic factors upon the molecular and isotopic signature of the leaf wax n-alkanes. Total n-alkanes concentration, average chain length (ACL25-35) and carbon preference index (CPI24-36) are higher for Avicennia spp. contrasting with the lower values found for R. mangle and L. racemosa. This seems to derive from the physiological characteristics of each species with respect to the intrinsic water use efficiency under the specific tidal flooding regime of mangrove forests. For Avicennia spp. inverse correlations between δ13Cn-alkanes values (δ13C27, δ13C29, δ13C31, δ13C33 and δ13CWA) and mean annual precipitation suggests plant response to local environmental conditions. For R. mangle species, relationships between ACL25-35 and mean annual potential evapotranspiration, and between CPI24-36 and mean annual temperature were observed. These results indicate that δ13Cn-alkanes values in Avicennia spp. as well as ACL25-35 and CPI24-36 in R. mangle may be useful in tracing environmental changes and, as long as preservation of reliable record in sediments is verified, may also be applied in the reconstruction of past environmental conditions. The different molecular and isotopic footprints of the mangrove species presented here have potential in future interpretations of n-alkanes as biomarkers in biogeochemical studies in mangrove-dominated coastal regions.

Comparison of molecular distributions and carbon and hydrogen isotope compositions of n-alkanes from aquatic plants in shallow freshwater lakes along the middle and lower reaches of the Yangtze River, China

Leaf wax n-alkanes in lacustrine sequences have been widely applied for reconstructions of Late Quaternary paleoclimate histories. Such applications depend on knowledge of the factors controlling the characterization of the n-alkane paleoproxies of aquatic plants. This study analyzed the molecular distributions and carbon and hydrogen isotopic compositions of n-alkanes of aquatic plant samples collected from the middle and lower reaches of the Yangtze River (MLYR). In this collection, the n-alkane distributions of submersed/floating plants and emersed plants are distinctly different. By combining their Paq and n-alkane δ13C values, the n-alkane contributions from submersed plants and emersed plants in the MLYR can be differentiated. It is also noteworthy that the hydrogen isotope fractionation (εalk/water; avg. −166‰) between the n-C23 alkane of submersed plants in the MLYR and in precipitation agrees well with previous studies, suggesting a nearly constant εalk/water for the submersed plants. Moreover, compared to C3 dicots growing in a similar climate, submersed plants in the humid MLYR have lower n-alkane δ2H values. This feature may arise from isotopic differences in the evapotranspiration of lake water versus leaf water and in the biosynthetic fractionations of leaf waxes of these two plant groups. These results aid interpretations of the hydrogen isotope offset of the δ2H values of n-alkanes between aquatic and terrestrial plants in humid climates.

Comparison of δ2H values of leaf wax n-alkanes and n-alkanoic acids in subtropical angiosperms

Hydrogen isotope compositions of leaf wax lipids are widely applied to reconstruct changes of hydroclimate in sedimentary archives. However, whether the δ2H signals from different classes of n-alkyl lipids yield consistent information is an important but poorly explored issue. In this study, we investigated seasonal variations in the molecular distributions and hydrogen isotope compositions of n-alkanes and n-alkanoic acids of three tree species (Cinnamomum camphora, Quercus chenii, Liquidambar formosana) growing in a humid subtropical monsoon climate. Seasonal differences exist in the concentrations and δ2H values of n-alkanes and n-alkanoic acids. These features may be linked to difference in the synthesis times of n-alkanes and n-alkanoic acids. The hydrogen isotope offset between n-alkanes and n-alkanoic acids (εalkane/acid) is variable across the seasons and among the three tree species. A survey of published data reveals that the εalkane/acid values are usually not significantly different among vegetation types, but the offset values can be significantly different in trees from tropical/subtropical and temperate regions. This difference may relate to different synthesis times of leaf waxes because of the climatic conditions in different climate zones. These findings encourage caution in interpreting differences in the δ2Hwax values of n-alkyl classes in paleoenvironmental reconstructions.

Temperature, precipitation, and vegetation changes in the Eastern Mediterranean over the last deglaciation and Dansgaard-Oeschger events

We here estimate past temperature and hydroclimate variations in eastern Anatolia for marine isotope stages 4 to 1 (MIS4 to MIS1) respectively using branched glycerol dialkyl glycerol tetraethers (brGDGTs) and the hydrogen isotopic composition of n-C29 leaf-wax n-alkanes (δ2Hwax) stored in the sedimentary record of Lake Van (Turkey). Our millennial-scale lipid biomarker records reflect warm and wet conditions during interstadials/interglacials and colder and dryer periods with increased evapotranspiration and aridity during stadials/glacials. The degree of methylation of the 5-methyl brGDGTs (MBT´5me) indicates increasing mean annual air temperatures (MAT) during stadial/interstadial transitions, that characterize Dansgaard-Oeschger events, and during the last glacial termination.Based on the effects of changes in precipitation amount on apparent enrichment factors between the δ2H of the C29n-alkanes and δ2H of precipitation, a total increase in annual precipitation of about 200 mm during transitions from stadials to interstadials, and of 300–350 mm during glacial-interglacial transitions can be determined, in line with previous paleoclimate reconstructions for the Eastern Mediterranean.High sterol concentrations in sediments deposited during warm and humid interstadials reflect increases in lake level, vegetation density and algal blooms, whereas lower values of the branched versus isoprenoid tetraethers ratio (BIT) likely reflects the increased niche of Thaumarchaeota resulting from enhanced windiness and mixing of the water column during colder periods. This quantitative hydroclimate reconstruction from Lake Van (Turkey), as it is optimally situated to respond to North Atlantic climate change via changes in the large-scale wind fields shed light into millennial-scale global climate variability.

Trend reversal of soil n-alkane Carbon Preference Index (CPI) along the precipitation gradient and its paleoclimatic implication

Homologue distribution proxies of leaf wax n-alkanes (normal chain alkanes) have been widely used for chemotaxonomy and paleoclimate reconstruction, while modern calibrations of them reported contrasting conclusions in different climate settings, especially between the cold arid and other climate regions. To further explore n-alkane-abiotic factor correlation in cold arid regions, this study surveyed n-alkane patterns of soil and leaf wax of three plant species along confined temperature and precipitation gradients in the Junggar Basin of northwest China. Our results show that salinity has a possible strong constraint on the Average Chain Length (ACL) of soil n-alkanes in high salinity settings, while precipitation or water stress is the leading constraint on the Carbon Preference Index (CPI) of those in low salinity settings. Precipitation also shows strong control over CPI or ACL of leaf wax n-alkanes of different plant species. Our results confirmed that n-alkane patterns are sensitive to, and constrained by abiotic factors independently on levels of plant species and averaged biome. A compilation of soil CPI from transect studies across the hot-humid to cold-arid regions of China further revealed precipitation being the first-order constraint of soil CPI, and a CPI trend reversal pattern along the precipitation gradient, where CPI increases from arid to semiarid climate and then decreases toward humid climate. Further analysis of existing CPI records showed that this CPI trend reversal may help differentiate three climate states qualitatively, i.e. arid, semiarid-semihumid and humid climates, which could be a valuable complement to existing proxies for paleoclimate reconstruction.

Duelling narratives of chironomids and pollen explain climate enigmas during The Last Glacial-Interglacial transition in North Island New Zealand

The Last Glacial-Interglacial Transition (LGIT) was a dynamic stage of Earth's history, and the difficulty of reconstructing this complex interval may be compounded by divergent proxy records, often collected from the same archive. To overcome this obstacle, we exploit the contrasting biological preferences of chironomids to both summer temperature and degree days (positive) and pollen to both the length of the growing season (positive) and winter duration (negative), recorded by a small lake in the central North Island of New Zealand. The climate proxy records are anchored to shifting zonal boundaries (e.g., southern westerly wind belt) via the hydrogen isotope ratios of leaf wax n-alkanes (δDwax). These results enable us to interrogate the structure of the LGIT and address two fundamental questions: 1) Is there evidence for Holocene-like temperatures during the early deglacial? and 2) Were early Holocene summers cool?Lake sediment δDwax values indicate a poleward retreat of the westerlies around 18,000 calibrated years before present (cal kyr BP), signalling the onset of climate amelioration for the region. Remarkably, the independently derived summer and mean annual temperature reconstructions are anti-phased. Chironomid-inferred summer temperatures surpass modern values by 17.5 cal kyr BP, whereas pollen-inferred mean annual temperatures remain supressed. Summers cool from 17.5 to 11 cal kyr BP, when they reach a minimum for the record, while winter and mean annual temperatures simultaneously warm to a maximum. The chironomid record generally traces regional insolation, although an Antarctic template imprints on this trend with declining summer temperatures during the Antarctic Cold Reversal (ACR) and warming during the Younger Dryas (YD). Forest development during the early LGIT, on the other hand, is supressed by cool, dry winters, punctuated by severe frosts, despite being set against a backdrop of overall warming; the successional pattern is best explained by a latitudinal retreat of the westerlies. Our findings underscore the complementarity of multiple bioproxy responses and reveal the importance of seasonal heat and energy distribution during the LGIT.

A long-term investigation of the variation in leaf wax n-alkanes responding to climate on Dongling Mountain, north China

Long chain n-alkanes of leaf waxes are widely used in paleoclimatic reconstructions. The application of these proxies depends on profound understandings gained from studies of the relationship between climatic factors and long chain n-alkanes in living plants. However, a lot of uncertainties still exist in the relationship due to the interactions among climatic factors, geographical and topographical factors, and phylogeny. In addition, unrealistic or inaccurate climatic data also introduce errors. To evaluate the effect of climate with minimized interfering factors, we conducted long-term measurements of leaf wax n-alkanes of the plants growing on Dongling Mountain, north China from 2004 to 2019, where a meterological observatory is located. On the whole, total n-alkane concentration (Σalk) was positively correlated mean annual precipitation (MAP); average chain length (ACL) was negatively related to MAP; carbon preference index (CPI) had poor correlations with all climate factors. Compared with Σalk, ACL is a better indicator that reflects annual precipitation. The coefficient of ACL vs. MAP is 0.0015/mm. However, MAP accounted for only 40% of the variability in the year-averaged ACL of all plants together due to the significant interspecific differences in the response of leaf wax n-alkane distribution to climatic parameters; thus, ACL should be employed in parallel with other proxies to quantitatively reconstruct paleoprecipitation. We conducted a sample reconstruction of paleoprecipitation using ACL records preserved in the annually laminated sediments of Maar Lake Twintaung, Myanmar. The results of reconstruction using ACL are completely consistent with the results of reconstruction using carbon isotope ratios of n-alkanes, and are highly consistent with historical documents.

Expanded lacustrine sedimentation in the Qaidam Basin on the northern Tibetan Plateau: Manifestation of climatic wetting during the Oligocene icehouse

The Qaidam Basin in the core area of arid Inner Asia has been considered undergoing continuous aridification over the Cenozoic. However, the Qaidam Basin is marked with expanded lacustrine sedimentation during the Oligocene, which contrasts with the fluvial or deltaic facies stratigraphically below (Eocene) and above (Miocene-present). The Oligocene lacustrine expansion challenges the idea of persistent aridification. To solve the conundrum, we reconstruct a long-term compound-specific hydrogen isotope (δ2H) record from sedimentary leaf wax n-alkanes to evaluate the paleoclimatic context before, during, and after the Oligocene lacustrine expansion. The δ2H results reveal three shifts at ca. 40 Ma, 34 Ma, and 24 Ma. The leaf wax δ2H values range from −176.8‰ to −166.7‰ from 51 to 40 Ma, followed by an abrupt increase of 23.9‰ at 40 Ma. We interpret this rapid increase as enhanced aridification due to the coeval retreat of the Paratethys Sea from the region. At 34 Ma, the δ2H plunges across the Eocene-Oligocene transition (EOT). Post-EOT δ2H values are the lowest, vary with high amplitude from −187.1‰ to −153.2‰, and are associated with the lacustrine facies expansion, indicating a wetter climate. By compiling the regional isotopic proxy studies, we observe the contrasting patterns in paleohydrology conditions since the EOT: the relaxation of aridity in the westerlies region versus the enhanced aridification in the East Asian summer monsoon region. We interpret that the west-east contrasting patterns represent the different climatic responses to global cooling: wetting in the west as a result of the enhanced moisture transport via westerlies replacing the subtropical high, and drying in the east due to the reduction in moisture content associated with weakening East Asian summer monsoon. Wetting in Inner Asia is synchronous with cooling in the ocean (North Atlantic) and on land (Xining Basin). Since 24 Ma, δ2H increases in response to warming during the latest Oligocene to the early Miocene when the subtropical high re-occupied Inner Asia, causing the aridity. This study reveals a dynamic climate in Inner Asia with different mechanisms responding to global change.

Global-scale altitude effect on leaf wax n-alkane δD values in terrestrial higher plants

Numerous field studies have shown that leaf wax n-alkane δD values (δDn-alkane) can be used as a proxy for paleoelevation reconstruction. However, the lack of systematic global analysis of δDn-alkane values with respect to altitude limits the reliability of δDn-alkane values for paleoaltimetry at the global scale. In this study, we analyzed the variation of δDn-alkane values with altitude in terrestrial higher plants based upon our compiled global δDn-alkane database. The general decrease in δDn-alkane values with altitude corresponds to the variation in precipitation δD (δDprec) with altitude, suggesting that the δDn-alkane values effectively reflect δDprec values. When eliminating the effect of latitude at the global scale, the global δDn-alkane-altitude lapse rate for all species was −1.01±0.10‰ (100 m)−1, while these lapse rates were −1.03±0.13‰ and −0.47±0.13‰ (100 m)−1 for dicots and monocots, respectively. Our study reveals that it is reliable to use δDn-alkane values to gain information about paleoelevation at the global scale, and the global δDn-alkane-altitude lapse rates provide an alternative to reconstruct paleoelevation at some δDn-alkane-uninvestigated areas.

Seasonality of the altitude effect on leaf wax n-alkane distributions, hydrogen and carbon isotopes along an arid transect in the Qinling Mountains.

Reconstructing paleoelevation allows the temporal evolution of biogeochemical processes and hydroclimate regimes to be understood and quantified. A dual-isotope biomarker of clumped hydrogen and carbon isotopes of leaf wax n-alkanes was recently proposed in humid tropical forests, and it was proven to be superior to a single-isotope proxy that was previously reported. However, it remains unknown whether the dual-isotope biomarker is suitable in arid conditions. The present study investigated leaf wax n-alkane distribution, hydrogen (δ2Hwax) and carbon (δ13Cwax) isotopes in terrestrial plants along an arid mountainous transect. We found that the effects of seasonality on n-alkane distribution, δ2Hwax and δ13Cwax were minimal for all species (p>0.05), and that species-specific δ2Hwax values remained almost unchanged for most species, in contrast to δ13Cwax values. Significant correlations between altitude and δ2Hwax values (R2=0.54, 0.58, and 0.75 for spring, summer, and autumn, respectively), instead of δ13Cwax values (R2=0.08, 0.43, and 0.12 with p=0.24, 0.01, and 0.19 for spring, summer, and autumn, respectively), were observed, suggesting that δ2Hwax values, but not δ13Cwax values, can be reliably used as a proxy for reconstructing paleoelevation in arid conditions. Therefore, it will be necessary to identify other proxies to supplement δ2Hwax values under a dual-isotope approach in future research.

Intra-leaf heterogeneities of hydrogen isotope compositions in leaf water and leaf wax of monocots and dicots

Several recent studies showed that leaf wax n-alkane δ2H values (δ2Hwax) within a leaf were heterogeneous in a small number of species. It still remains unclear whether the heterogeneity of intra-leaf δ2Hwax values is general for various species, how δ2Hwax values vary spatially and temporally, and whether there is a common explanation for the intra-leaf δ2Hwax heterogeneity in higher plants. Here we compared the hydrogen isotope compositions of leaf wax and corresponding leaf water (δ2Hlw) across leaf sections among a variety of monocot and dicot plant species. There is significant and consistent heterogeneity for both δ2Hwax and δ2Hlw, i.e., base-to-tip 2H-enrichment for monocots (except Hemerocallis citrina, and Dactylis glomerata) whereas base-to-tip and center-to-edge increases in δ2Hwax and δ2Hlw for dicots. The consistent occurrence of variations of δ2Hlw and δ2Hwax values within a leaf imply that δ2Hwax values probably inherit point-to-pint from in-situ δ2Hlw values, and thus the intra-leaf δ2Hwax heterogeneity mainly results from the spatial pattern of intra-leaf δ2Hlw values associated with veinal structures between dicots and monocots. The general heterogeneity of intra-leaf δ2Hwax values further intensifies that it is necessarily needed for in-depth understanding leaf wax biomarker.

Impacts of Norse settlement on terrestrial and aquatic ecosystems in Southwest Iceland

Norse colonization of North Atlantic islands in the 1st millennium of the Common Era led to drastic prehistoric environmental changes in these previously “pristine” landscapes. In Iceland, Norse settlement is associated with a rapid decline in birch trees and heightened soil erosion, yet the timing of Norse exploration in the North Atlantic coincided with large climate changes that also influenced Icelandic environments. To date, there are few records that disentangle climatic and human impacts on terrestrial ecosystems, and there has been very little work on the impacts of Norse arrival on Iceland’s aquatic ecosystems. Here we use a high-resolution lake-sediment record from Vestra Gislholtsvatn (VGHV), southwest Iceland to investigate these processes during the last 2,000 years. Norse arrival (c. 870 CE) in Iceland is followed by a rapid increase in sedimentation rate and a transition in leaf wax n-alkanes indicating a decrease in trees and expansion of grasses. This transition coincides with limnological changes, including increased primary productivity (i.e. C17 n-alkane and biogenic opal fluxes) and shifts in the haptophyte algal community. Many of these changes are still apparent today. Comparisons with a new winter-spring alkenone temperature reconstruction from VGHV and marine sea surface temperature records show little to no correlation between terrestrial and aquatic ecological changes and climate at this time. Similarly, volcanic eruptions recorded in VGHV are not associated with any long-term environmental changes. Rather, the VGHV record suggests that human settlement had a lasting impact on the catchment area of VGHV and changes within the lake ecosystem.

Water isotopic constraints on the enhancement of the mid-Holocene West African monsoon

During the early to mid-Holocene, changes in orbital precession led to considerable increases in West African monsoon (WAM) rainfall compared to today and shifted its reach further north. However, climate proxies and paleoclimate model simulations disagree over fundamental aspects of the mid-Holocene (MH; 6 ka BP) enhancement of the WAM. Here, we use a water isotope-enabled Earth system model (iCESM1) to, for the first time, directly compare simulated northern African hydroclimatic change between the mid-Holocene and pre-industrial era (PI) with the hydroclimate signal inferred from leaf wax n-alkanes in order to study the WAM's past spatial change. iCESM1 simulates a northernmost WAM extent of ∼24°N, which broadly agrees with the extent inferred from pollen and dust records (23–28°N) but falls short of that from leaf wax n-alkanes (27–31°N). While the isotopic composition of rainfall (δDP) inferred from leaf wax n-alkanes is lower during the MH than the PI, simulated MH δDP is higher in northwestern Africa, especially in boreal fall. This discrepancy can be reconciled by interpreting the inferred signal of leaf wax n-alkanes as being reflective of the isotopic composition of soil water (δDS) and its subsequent influence by soil evaporation. We postulate that leaf wax n-alkanes may overestimate inferred mean annual precipitation rates in the MH by not incorporating the enrichment of precipitation shown by iCESM1. Our results have broad implications for reconstruction of past hydrologic change in northern Africa and lend further support to the northernmost WAM extents inferred from pollen and dust records.

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.

Carbon isotope systematics of leaf wax n-alkanes in a temperate lacustrine depositional environment

The carbon isotope ratio (δ13C) of plant-derived organic carbon preserved in geological archives can be a valuable proxy for the relative abundance of terrestrial plants using C3 and C4 photosynthesis. In certain sedimentary archives, however, mixing of terrestrial C3- and aquatic macrophyte-sourced carbon will result in sedimentary organic matter (OM) δ13C signatures that could be misinterpreted as shifts in the abundance of C3 and C4 vegetation. There is potential for this problem to be mitigated using leaf wax n-alkane compound-specific δ13C measurements because n-alkane production differs between terrestrial vegetation and aquatic macrophytes. This approach requires an increased understanding of how mixing of terrestrial plant and aquatic macrophyte n-alkane inputs to lacustrine sedimentary archives manifests in the δ13C values of different n-alkane homologues in diverse environmental settings. This study examines a Pleistocene lacustrine sequence in southeastern Australia in which the inputs from terrestrial and aquatic macrophytes vary naturally through time, enabling the characterization of the mixing dynamics for different n-alkane homologues. Relative contributions of terrestrial vegetation and aquatic macrophytes were estimated using the relative abundance of mid-chain to long-chain n-alkanes and compared to the δ13C values of discrete n-alkane homologues. We find that δ13C values of mid- and some long-chain n-alkanes (C23–C29) are strongly impacted by mixing between C3 terrestrial- and non-emergent aquatic macrophyte-derived n-alkanes. In contrast, δ13C values of very long chain (C31–C35) n-alkanes integrated in sediments are the least affected by isotopic mixing. These results indicate that aquatic macrophyte inputs can significantly influence C29 isotopic signatures and thus the δ13C values of the very long chain n-alkanes (≥ C31) will provide the most robust quantification of n-alkane inputs from terrestrial plants and will be most useful for reconstructing the abundance C3 and C4 vegetation from temperate lake sediments.

Habitat Influence on the Molecular, Carbon and Hydrogen Isotope Compositions of Leaf Wax n-Alkanes in a Subalpine Basin, Central China

Epidermal leaf waxes of terrestrial higher plants have been widely utilized for the reconstructions of paleoenvironment and paleoclimate in peat deposits. In this study, specimens of four plant species growing in both peatland and non-peatland habitats were retrieved to compare their molecular, carbon (δ13C) and hydrogen (δ2H) isotopic compositions of leaf wax n-alkanes from a closed subalpine basin in Central China. Three of the four species show quite higher total concentrations of n-alkanes in the relatively dry non-peatland setting than in the peatland. In addition, the δ2H values of long-chain n-alkanes are generally less depleted in the peatland and are comparable among different plant species, which is interpreted as the influence of inundation condition and the possible limited supply of photosynthetic products. This study reveals different patterns of plant wax molecular and isotopic compositions between peatland and the surrounding non-peatland conditions, and confirms the paleoenvironmental potential of leaf wax ratios on the peat sequences.

Leaf wax n-alkanes in leaves, litter, and surface soil in a low diversity, temperate deciduous angiosperm forest, Central Missouri, USA

ABSTRACT The leaf wax n-alkanes in modern ecosystems serve as proxies for assessing the ecologic and climatic conditions in soils, sediments and other geologic archives, and potentially as baselines for future ecological changes. In this study, the concentrations and distributions of long-chain (C25-C35) n-alkanes in fresh mature leaves, leaf litter fall, and surface soil (0-5 cm) from 0.4 ha, low diversity (oak-hickory-maple), deciduous angiosperm forest are quantified. Owing to degradation and dilution, leaf litter (350 ± 227 μg/g dw) and soil (13 ± 13 μg/g dw) each contain significantly lower concentrations of long-chain n-alkanes than fresh mature leaves (574 ± 291 μg/g dw). The average chain length and odd-even predominance (OEP) of fresh mature leaves increased and decreased, respectively, in leaf litter and soil indicating preferential and progressive degradation of the more abundant C27 and C29 homologues relative to less abundant C31 and C33 homologues. A poor relationship between OEP and a normalised n-alkane ratio(s) demonstrates the large potential effects of inter- and inner-species variability and/or varying effects of degradation on leaf wax n-alkanes, and highlights the potential limitations of the endmember modelling in paleo reconstructions.

Lower-altitude of the Himalayas before the mid-Pliocene as constrained by hydrological and thermal conditions

Contradictory paleoaltitudinal histories of the Himalayas arrived at using isotopic paleoaltimeters and ecological evidences have constrained our further exploration of the geodynamic mechanisms driving the India-Eurasia collision and the resultant interaction between tectonic uplift and climate change. In this study, based on evidence derived from glycerol dialkyl glycerol tetraethers (GDGTs), the hydrogen isotopes of leaf wax n-alkanes (δ2Halk), we estimated the paleoaltitude of the Gyirong Basin (GB), central Himalayas, for the 7.0∼3.2 Ma period. The paleoaltitudinal reconstruction based on δ2Halk values (∼6.3 km above sea level (asl)) was consistent with that of the revised δ18Oc values (∼5.6 km asl), and higher than the present-day altitude (∼4 km asl). The reconstructed paleotemperatures based on brGDGTs (14.2 ± 4.5°C for the lake surface; 7.5 ± 3.3°C for terrestrial organic matter source areas) were consistent with the ecological evidences, indicating a low-altitude paleoenvironment. This proves that a systemic bias exists between the isotopic paleoaltimeters and the ecological evidence, rather than a problem with specific indicators. The reconstructed paleoaltitudinal differences between the δ2Halk and δ18Oc values are similar to those of brGDGT reconstructions, seemingly representing the paleoaltitudinal differences between the source areas of terrestrial organic matter inputs and the lake surface, and suggesting that the isotopic palaeoaltimeters do not specifically record the higher-altitude parts of the basinal topography. Further paleoenvironmental analysis indicated that the warmer and wetter environment present during the Late Miocene to Mid-Pliocene would, on the face of it, be better suited to the application of brGDGT paleothermometers, and this in turn would potentially lead to a systemic overestimation by isotopic paleoaltimeters. We therefore adopted the paleoaltitudinal reconstructions which used brGDGTs and concluded that the lake surface of the GB during the Late Miocene to Mid-Pliocene was 2.5 ± 0.8 km asl, and that the surrounding mountains exceeded 3.6 km asl in height, implying that the central Himalayas underwent a rapid uplift of ∼1.5 km after the Mid-Pliocene.