δDwax Values Research Articles

Abrupt change in tropical Pacific climate mean state during the Little Ice Age

The mean state of the tropical Pacific ocean-atmosphere climate, in particular its east-west asymmetry, has profound consequences for regional climates and for the El Niño/Southern Oscillation variability. Here we present a new high-resolution paleohydrological record using the stable-hydrogen-isotopic composition of terrestrial-lipid biomarkers (δDwax) from a 1,400-year-old lake sedimentary sequence from northern Philippines. Results show a dramatic and abrupt increase in δDwax values around 1630 AD with sustained high values until around 1900 AD. We interpret this change as a shift to significantly drier conditions in the western tropical Pacific during the second half of the Little Ice Age as a result of a change in tropical Pacific mean state tied to zonal sea surface temperature (SST) gradients. Our findings highlight the prominent role of abrupt shifts in zonal SST gradients on multidecadal to multicentennial timescales in shaping the tropical Pacific hydrology of the last millennium, and demonstrate that a marked transition in the tropical Pacific mean state can occur within a period of a few decades.

A >200 ka U‐Th Based Chronology From Lacustrine Evaporites, Searles Lake, CA

AbstractWell‐dated lacustrine records are essential to establish the timing and drivers of regional hydroclimate change. Searles Basin, California, records the depositional history of a fluctuating saline‐alkaline lake in the terminal basin of the Owens River system draining the eastern Sierra Nevada. Here, we establish a U‐Th chronology for the ∼76‐m‐long SLAPP‐SLRS17 core collected in 2017 based on dating of evaporite minerals. Ninety‐eight dated samples comprising nine different minerals were evaluated based on stratigraphic, mineralogic, textural, chemical, and reproducibility criteria. After the application of these criteria, a total of 37 dated samples remained as constraints for the age model. A lack of dateable minerals between 145 and 110 ka left the age model unconstrained over the penultimate glacial termination (Termination II). We thus established a tie point between plant wax δD values in the core and a nearby speleothem δ18O record at the beginning of the Last Interglacial. We construct a Bayesian age model allowing stratigraphy to inform sedimentation rate inflections. We find that the >210 ka SLAPP‐SRLS17 record contains five major units that correspond with prior work. The new dating is broadly consistent with previous efforts but provides more precise age estimates and enables a detailed evaluation of evaporite depositional history. We also offer a substantial revision of the age of the Bottom Mud‐Mixed Layer contact, shifting it from ∼130 ka to 178 ± 3 ka. The new U‐Th chronology documents the timing of mud and salt layers and lays the foundation for climate reconstructions.

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.

Temperature and moisture variability in the eastern Mediterranean region during Marine Isotope Stages 11–10 based on biomarker analysis of the Tenaghi Philippon peat deposit

The Mediterranean region is highly sensitive to climate change, particularly with regard to warming and increasing aridity. Understanding its past climate history during periods similar to the Holocene is key to understand the long-term dynamics that accompany anthropogenic climate change. Marine Isotope Stage (MIS) 11 (ca. 424–367 ka BP) is considered one of the best Holocene analogues. Despite detailed insight from Atlantic marine records and European continental records, MIS 11 temperature and rainfall evolution in the Mediterranean remains poorly understood.We present a detailed record of MIS 11–10 climate change at Tenaghi Philippon, a telmatic peatland in NE Greece. We use microbial membrane lipids (brGDGTs), the δD of n-C29 (δDwax) and distribution of n-alkanes derived from plant leaf waxes, and levoglucosan concentrations to reconstruct changes in temperature, rainfall sources and vegetation burning. Glacial-interglacial temperature patterns indicate strong Atlantic influence in the Eastern Mediterranean region.Low δDwax values and high temperatures indicate a predominance of Atlantic-sourced winter precipitation during MIS 11, and vice versa during MIS 10. The latter is attributed to a suppression of the Mediterranean storm track, probably due to a persistent high-pressure cell over most of the European continent, mainly in response to an extended ice cover during the glacial. The levoglucosan record is consistent with rapid change to drier conditions and increased vegetation burning from MIS 11 to 10. Millennial-scale oscillations allow to characterise cooling episodes previously recorded at other sites, with conditions of decreased winter precipitation, while suggesting increased seasonality during the interglacial optimum.

Evaluation of leaf wax δD and soil brGDGTs as tools for paleoaltimetry on the southeastern Tibetan Plateau

The orogenic history of the Tibetan Plateau (TP) and surrounding mountain ranges continues to be a major source of disagreement among geologists, particularly concerning the uplift models for the Cenozoic evolution of the TP and estimates for when the highest and largest plateau on Earth reached its current elevation. Quantitative reconstructions of past elevation from geologic samples are necessary to document the uplift history of TP and examine the interactions between tectonic-relief and climate over geological time-scales. Several studies establishing lipid biomarker-based paleoaltimetry based on leaf wax δD values and brGDGTs have been reported in recent years for the TP and surrounding regions, but have yet to be synthesized into a regional framework for paleoelevation determination and uncertainty analysis. Here we report new leaf wax δD and brGDGTs data developed from surface soil samples along an elevation transect spanning ~1250–3900 m.a.s.l in the Hengduan Mountains on the southeastern edge of the TP. We find that the abundance-weighted mean leaf wax δD (n-C27, n-C29 and n-C31) values (δDwax) lapse rates determined for the Hengduan Mountains and for five other nearby study locations are statistically indistinguishable, and can be combined to provide a regional δDwax lapse rate of −1.97 ± 0.04‰ (1σ)/100 m for use in regional paleoelevation studies across the southeastern TP. We also find a strong correlation (R2 = 0.71) between brGDGTs and elevation-dependent mean annual air temperature, which contributes to a number of studies in the region that support the use of fossil brGDGTs as a paleoelevation proxy. Our results reveal that δDwax and brGDGTs for the Hengduan Mountains provide similar empirical uncertainty in paleoelevation reconstruction, with standard errors of elevation estimation (SE) of ±483 m (±1σ) and ± 394 m (±1σ), respectively. We propose a paleoaltimetric approach that combines δDwax and brGDGT data, in order to derive paleoelevation estimates with lower uncertainties. In the Hengduan Mountains data set, the approach yields a SE (±286 m; ±1σ) that is 27–40% lower than when δDwax values and brGDGTs are applied separately.

Effect of continuous light on leaf wax isotope ratios in Betula nana and Eriophorum vaginatum: implications for Arctic paleoclimate reconstructions

Reconstructions of climate using leaf wax D/H ratios (δDwax) require accounting for the apparent isotopic fractionation (εapp) between plant source water and waxes. There have been conflicting publications on whether plants in the Arctic growing under 24-hour continuous light, fractionate less than temperate and tropical plants. In this study, we examine the effect of diurnal light (DL) versus 24-hour continuous light (CL) on the isotopic composition of leaf n-alkanes and n-acids in greenhouse experiments using two common Arctic plants (Eriophorum vaginatum, or tussock cottongrass and Betula nana, or dwarf birch). For E. vaginatum, the δDwax values of various wax homologues were 5–11‰ more positive for CL plants relative to their DL counterparts, whereas for B. nana, CL waxes were 3–24‰ more negative, suggesting that daylight length is not a unifying control on leaf wax D/H ratios of Arctic plants. The δ13Cwax of B. nana was more negative for plants grown in continuous light compared to diurnal light, reflecting lower water-use efficiency, associated with prolonged stomatal opening in the CL treatment. We modeled the impact of increasing stomatal conductance and effective flow path lengths (mimicking variable leaf morphologies) on the isotopic composition of leaf waters (δDlw) and find that variations in leaf-water enrichment may explain the variable δDwax responses seen between E. vaginatum and B. nana. We suggest that between-species differences in the δDlw response to light, and differences in the utilization of stored carbohydrates, were important for governing δDwax. Our greenhouse results suggest that Arctic plant leaf waxes do not consistently display reduced εapp values as a result of 24-hour day light, providing additional support for field observations.

Variable apparent hydrogen isotopic fractionation between sedimentary n-alkanes and precipitation on the Tibetan Plateau

The hydrogen isotope ratio values of sedimentary leaf wax n-alkanes (δDwax) have been considered a reliable proxy for precipitation isotope ratios (δDP) across various transects on a global basis. Records of δDwax have been used to infer past changes in hydrology, climate and altimetry, assuming nearly constant apparent hydrogen isotopic fractionation (εwax-P) between sedimentary leaf wax components and precipitation. Here, we present the results of an analysis of δDwax values from the surface sediments of 22 lakes across a precipitation and vegetation gradient on the Tibetan Plateau. The δDwax values correlated significantly with mean annual precipitation δD (r 0.90) and summer precipitation δD (r 0.91), suggesting that δDwax can record δDP in very dry regions. However, εwax-P varies with annual precipitation as well as with relative humidity. Calculation using Craig-Gordon isotope models showed that the variable εwax-P values likely resulted from intensive soil evaporation and plant evapotranspiration in this relatively arid region. This implies that variable isotopic fractionation should be taken into consideration when reconstructing paleohydrology, paleoclimate and paleoaltimetry.

Choosing a suitable εw-p by distinguishing the dominant plant sources in sediment records using a new Pta index and estimating the paleo-δDp spatial distribution in China

The hydrogen isotopic composition of plant wax (δDwax) inherits the isotopic values of the source water and isotopic fractionation that take place in lipid biosynthesis. The δDwax values of n-alkanes or n-acids in sediment records are considered to be effective paleo-δD for precipitation proxies for different timescales. However, the vegetation effect also contributes substantially to the apparent fractionation values (εw-p) between the precipitation and plant. Therefore, a method for evaluating the dominant plant source in the sediment records is key for reconstructing a reliable paleo-δDp record. In this study, based on field data from the extremely arid Qaidam Basin and the semi-arid central Chinese Loess Plateau (CLP), a plant type index (Pta), based on the traditional Paq and n-alkane C31/C29 index, was used to evaluate the dominant plant type that contributed to the sediments. In the arid and semi-arid region, the Pta values of the aquatic plants were <90, while the shrub Pta ranged from 90 to 180 and the grass Pta was >180. A test on contemporary soils and surface lake sediments also showed that the Pta index could estimate the regional vegetation distribution in modern environments. We also evaluated the dominant n-alkane source for four sediment records in China during the last millennium and reconstructed a regional paleo-δDp by choosing a different εw-p. The estimated paleo-δDp values showed a good correlation with the modern regional summer δDp. We suggest that a large number of field studies are needed to improve the Pta index for quantitatively calculating the precise ratios for various plant sources. Such data would play a key role in improving hydrogen isotopic hydroclimatic research.

Climatic and geomorphic drivers of plant organic matter transport in the Arun River, E Nepal

Fixation of atmospheric CO2 in terrestrial vegetation, and subsequent export and deposition of terrestrial plant organic matter in marine sediments is an important component of the global carbon cycle, yet it is difficult to quantify. This is partly due to the lack of understanding of relevant processes and mechanisms responsible for organic-matter transport throughout a landscape. Here we present a new approach to identify terrestrial plant organic matter source areas, quantify contributions and ascertain the role of ecologic, climatic, and geomorphic controls on plant wax export in the Arun River catchment spanning the world's largest elevation gradient from 205 to 8848 m asl, in eastern Nepal. Our approach takes advantage of the distinct stable hydrogen isotopic composition (expressed as δD values) of plant wax n-alkanes produced along this gradient, transported in river waters and deposited in flood deposits alongside the Arun River and its tributaries. In mainstem-flood deposits, we found that plant wax n-alkanes were mostly derived from the lower elevations constituting only a small fraction (15%) of the catchment. Informed by remote sensing data, we tested four differently weighted isotopic mixing models that quantify sourcing of tributary plant-derived organic matter along the Arun and compare it to our field observations. The weighting parameters included catchment area, net primary productivity (NPP) and annual rainfall amount as well as catchment relief as erosion proxy. When weighted by catchment area the isotopic mixing model could not explain field observations on plant wax δD values along the Arun, which is not surprising because the large arid Tibetan Plateau is not expected to be a major source. Weighting areal contributions by annual rainfall and NPP captured field observations within model prediction errors suggesting that plant productivity may influence source strength. However weighting by a combination of rainfall and catchment relief also captured the observed δD value pattern suggesting dominantly erosive control. We conclude that tributaries at the southern Himalayan front with high rainfall, high productivity, high relief and high erosion rates dominate plant wax exports from the catchment.

Plant leaf wax biomarkers capture gradients in hydrogen isotopes of precipitation from the Andes and Amazon

Plant leaf waxes have been found to record the hydrogen isotopic composition of precipitation and are thus used to reconstruct past climate. To assess how faithfully they record hydrological signals, we characterize leaf wax hydrogen isotopic compositions in forest canopy trees across a highly biodiverse, 3km elevation range on the eastern flank of the Andes. We sampled the dominant tree species and assessed their relative abundance in the tree community. For each tree we collected xylem and leaf samples for analysis of plant water and plant leaf wax hydrogen isotopic compositions. In total, 176 individuals were sampled across 32 species and 5 forest plots that span the gradient. We find both xylem water and leaf wax δD values of individuals correlate (R2=0.8 and R2=0.3 respectively) with the isotopic composition of precipitation (with an elevation gradient of −21‰km−1). Minimal leaf water enrichment means that leaf waxes are straightforward recorders of the isotopic composition of precipitation in wet climates. For these tropical forests we find the average fractionation between source water and leaf wax for C29n-alkanes, −129±2‰ (s.e.m., n=136), to be indistinguishable from that of temperate moist forests. For C28n-alkanoic acids the average fractionation is −121±3‰ (s.e.m., n=102). Sampling guided by community assembly within forest plots shows that integrated plant leaf wax hydrogen isotopic compositions faithfully record the gradient of isotopes in precipitation with elevation (R2=0.97 for n-alkanes and 0.60 for n-alkanoic acids). This calibration study supports the use of leaf waxes as recorders of the isotopic composition of precipitation in lowland tropical rainforest, tropical montane cloud forests and their sedimentary archives.

Plant wax δD values record changing Eastern Mediterranean atmospheric circulation patterns during the 8.2 kyr B.P. climatic event

Throughout the Holocene, the climate of the Mediterranean region has been strongly influenced by variability in the atmospheric circulation of the high and low latitudes. A prominent example for such Holocene climate perturbations is the ‘8.2 kyr B.P. climatic event’. Reorganization of Northern Hemisphere atmospheric circulation patterns resulted in variations of temperature and precipitation distribution across the Mediterranean. The effects of changing high- and low-latitude atmospheric circulation on Mediterranean climate in relation to the 8.2 kyr B.P. climatic event are, however, not well understood. Here we present a high-resolution record of stable hydrogen isotope composition of plant-wax n-alkanes (δDwax) across the 8.2 kyr B.P. climatic event from the Tenaghi Philippon peat deposit (NE Greece) in order to characterize patterns of precipitation and changes in atmospheric circulation in the Eastern Mediterranean region. Our record reveals pronounced changes in δDwax that correlate closely with previously published palynological data. A long-term decline in δDwax values characterizes the lower part of the section. The 8.2 kyr B.P. climatic event itself is connected to two distinct positive δDwax excursions: a minor shift in δDwax around 8.2 kyr B.P. and a major shift in δDwax between ca. 8.1 and 8.0 kyr B.P.. The upper part of the section shows a progressive trend towards higher δDwax values. We link shifts in δDwax to changes in Mediterranean air mass trajectories supplying precipitation to NE Greece caused by variations in the relative contributions of northerly-derived, D-depleted moisture and southerly-derived, D-enriched moisture. Possible control mechanisms for alternating air mass trajectories include changes in the influence of the Siberian High and differences in the influence of the African and Asian monsoon circulation on anticyclonic conditions in the Mediterranean region as well as regional inflow of moist air masses from the Aegean Sea.

Different altitude effect of leaf wax n-alkane δD values in surface soils along two vapor transport pathways, southeastern Tibetan Plateau

Here we present 76 hydrogen isotope values for leaf wax-derived n-alkanes (δDwax, being the abundance-weighted average δD values of C29 and C31n-alkanes) in surface soils along two vapor transport pathway transects running across the southeastern Tibetan Plateau (TP). The first is the South Himalayan (SH) Transect (with an altitude range of 1660–5050m∗ above sea level, and representing the Southern Himalayan air mass), and the second a combination of the Zayu–Bomi (ZB; altitude 1468–4800m) and Bayi-Lhasa (BL; altitude 3050–4970m) transects, together representing the Brahmaputra–Yarlung Zangbo river valley air mass. δDwax values and their relation to altitude did exhibit some variability. Nonetheless, altitude was the predominant factor in determining soil δDwax values, with a lapse rate of −2.2‰/100m (R2=0.78; n=76) based on overall soil δDwax values, despite changes in precipitation amount, relative humidity (RH) and vegetation type. Furthermore, our εwax/rw values (the isotopic fractionation between the δDwax and δD values of river water (δDrw)) remained approximately constant at −104.5‰ (ranging between −99‰ and −110‰), confirming that the relation between altitude and isotopic hydrology (river water and precipitation) controls the n-alkane δDwax altitudinal gradients for all of the sample localities in the Southern Himalaya (SH) and the southern TP. This endorses the efficacy of using δDwax to reconstruct paleoelevation. Along the SH Transect, soil δDwax values varied from −151‰ to −264‰, with an altitudinal lapse rate of −2.4‰/100m (R2=0.84, n=32). This was due to the significant change in altitude over short distances. Along the ZB Transect, soil δDwax values varied from −174‰ to −263‰, with a lower lapse rate of −1.9‰/100m (R2=0.68, n=29). The altitudinal lapse rate was −3.3‰/100m above 3000m (R2=0.63, n=19), and −1.35‰/100m below 3000m (R2=0.64, n=10); in the lower sub-transect, the latitudinal effect significantly counteracted the altitude effect. Along the BL Transect, soil δDwax values varied from −205‰ to −244‰, with an altitudinal lapse rate of −1.4‰/100m (R2=0.74, n=15). The lower lapse rate for soil δDwax reflects a combination of local continental and altitudinal effects. The differences we observed in soil δDwax altitudinal lapse rates along the two vapor transport pathways imply that they are affected by subtle local environmental conditions, moisture sources, latitude and topography. This should be considered fully during any reconstruction of paleoelevation.

Monsoon source shifts during the drying mid-Holocene: Biomarker isotope based evidence from the core ‘monsoon zone’ (CMZ) of India

A better understanding of past variations of the Indian Summer Monsoon (ISM), that plays a vital role for the still largely agro-based economy in India, can lead to a better assessment of its potential impact under global climate change scenarios. However, our knowledge of spatiotemporal patterns of ISM strength is limited due to the lack of high-resolution, continental paleohydrological records. Here, we reconstruct centennial-scale hydrological variability during the Holocene associated to changes in the intensity of the ISM based on a record of lipid biomarker abundances and compound-specific stable isotopic composition of a 10 m long sediment core from saline–alkaline Lonar Lake, situated in the core ‘monsoon zone’ of central India.We identified three main periods of distinct hydrology during the Holocene in central India. The period between 10.1 and 6 cal ka BP was likely the wettest during the Holocene. Lower average chain length (ACL) index values (29.4–28.6) and negative δ13Cwax values (−34.8‰ to −27.8‰) of leaf wax n-alkanes indicate the dominance of woody C3 vegetation in the catchment, and negative δDwax values (concentration weighted average) (−171‰ to −147‰) suggest a wet period due to an intensified monsoon. After 6 cal ka BP, a gradual shift to less negative δ13Cwax values (particularly for the grass derived n-C31) and appearance of the triterpene lipid tetrahymanol, generally considered as a marker for salinity and water column stratification, mark the onset of drier conditions. At 5.1 cal ka BP an increasing flux of leaf wax n-alkanes along with the highest flux of tetrahymanol indicate a major lowering of the lake level. Between 4.8 and 4 cal ka BP, we find evidence for a transition to arid conditions, indicated by high and strongly variable tetrahymanol flux. In addition, a pronounced shift to less negative δ13Cwax values, in particular for n-C31 (−25.2‰ to −22.8‰), during this period indicates a change of dominant vegetation to C4 grasses. In agreement with other proxy data, such as deposition of evaporite minerals, we interpret this period to reflect the driest conditions in the region during the last 10.1 ka. This transition led to protracted late Holocene arid conditions after 4 ka with the presence of a permanent saline lake, supported by the sustained presence of tetrahymanol and more positive average δDwax values (−122‰ to −141‰). A late Holocene peak of cyanobacterial biomarker input at 1.3 cal ka BP might represent an event of lake eutrophication, possibly due to human impact and the onset of cattle/livestock farming in the catchment.A unique feature of our record is the presence of a distinct transitional period between 4.8 and 4 cal ka BP, which was characterized by some of the most negative δDwax values during the Holocene (up to −180‰), when all other proxy data indicate the driest conditions during the Holocene. These negative δDwax values can as such most reasonably be explained by a shift in moisture source area and/or pathways or rainfall seasonality during this transitional period. We hypothesize that orbital induced weakening of the summer solar insolation and associated reorganization of the general atmospheric circulation, as a possible southward displacement of the tropical rainbelt, led to an unstable hydroclimate in central India between 4.8 and 4 ka.Our findings shed light onto the sequence of changes during mean state changes of the monsoonal system, once an insolation driven threshold has been passed, and show that small changes in solar insolation can be associated with major hydroclimate changes on the continents, a scenario that may be relevant with respect to future changes in the ISM system.

Major evolutionary trends in hydrogen isotope fractionation of vascular plant leaf waxes.

Hydrogen isotopic ratios of terrestrial plant leaf waxes (δD) have been widely used for paleoclimate reconstructions. However, underlying controls for the observed large variations in leaf wax δD values in different terrestrial vascular plants are still poorly understood, hampering quantitative paleoclimate interpretation. Here we report plant leaf wax and source water δD values from 102 plant species grown in a common environment (New York Botanic Garden), chosen to represent all the major lineages of terrestrial vascular plants and multiple origins of common plant growth forms. We found that leaf wax hydrogen isotope fractionation relative to plant source water is best explained by membership in particular lineages, rather than by growth forms as previously suggested. Monocots, and in particular one clade of grasses, display consistently greater hydrogen isotopic fractionation than all other vascular plants, whereas lycopods, representing the earlier-diverging vascular plant lineage, display the smallest fractionation. Data from greenhouse experiments and field samples suggest that the changing leaf wax hydrogen isotopic fractionation in different terrestrial vascular plants may be related to different strategies in allocating photosynthetic substrates for metabolic and biosynthetic functions, and potential leaf water isotopic differences.

Snapshot of cooling and drying before onset of Antarctic Glaciation

The late Eocene to Oligocene transition (EOT) witnessed a major ice advance on Antarctica. Because of the paucity of accessible outcrops and difficult drilling logistics, little is known about hydrological conditions in the Antarctic Peninsula during the late Eocene prior to the major ice advance. Here we study hydrological conditions with proxy evidence from marine sediment core NBP0602A-3C, adjacent to the tip of the Antarctic Peninsula, with sediments dated to 35.9±1.1 Ma providing a snapshot of conditions prior to the EOT. We reconstruct conditions during the latest Eocene based on plant leaf wax hydrogen isotopic evidence (δDwax) paired with previously-published evidence from pollen assemblages, and compared to climate model experiments simulating conditions before, during and after the transition. The pollen from late Eocene sediments of NBP0602A-3C indicate a Nothofagidites (southern beech) dominated landscape. In the same sediments, δDwax values average −202±7‰ (1σ, n=22) for the C28n-alkanoic acid. Uncertainty around the appropriate net fractionation between water and wax (ϵwax/w) provides the largest degree of uncertainty in paleoprecipitation δD reconstructions and we therefore use two reasonable fractionations −100 and −60‰ to constrain the likely range of average precipitation δD values to between −113 and −151‰. Model experiments confirm that isotopic compositions show low sensitivity to temperature changes at the tip of the Antarctic Peninsula, in comparison to much larger changes in the continental interior. Across the interval sampled, a decline in pollen abundance from 105 to 103 gdw−1 represents cooling and drying towards more tundra-like conditions. The 30‰ trend in δDwax values towards more positive values can best be explained by smaller fractionations as the vegetation shifted from forest to tundra, with greater water stress. Model results are broadly consistent with the data and quantify the cooling and drying across the full EOT from ca. 7 to 2 °C and 700 to 600 mma−1 with an isotopic shift in δD of only about −15‰ at the tip of the Antarctic Peninsula. Pollen and leaf wax proxies reconstruct vegetation and hydrological conditions prior to the transition, and model experiments through the EOT provide proxy-groundtruthed quantification of terrestrial transformations.

The “inverse altitude effect” of leaf wax-derived n-alkane δD on the northeastern Tibetan Plateau

The response of soil wax derived n-alkane hydrogen isotope values (δDwax, the abundance weighted average δD values of the C29 and C31n-alkanes) and isotopic lapse rates (the average change in δDwax with elevation) to factors other than elevation change should be assessed and taken into account for quantitative elevation reconstructions. To shed more light on how plant wax derived n-alkanes may be used for this purpose, soil and Gramineae samples analyzed for δDwax values. Those were derived from the foreland basin, the mountain vegetation zone and glacier catchment areas along the Beida River (a tributary in the Heihe River Basin; up to the headwaters of the Qiyi glacier), and the Xiying River catchment (up to the headwaters and glaciated areas of the Lenglong Range), located on the northeastern Tibetan Plateau. For comparison, this study analyzed surface soils from the Maxian Mountains for δDwax. As a whole, the trend in δDwax in the study regions seems to primarily reflect δDrw (δD values of river water) and/or δDp (δD values of precipitation). These data show good inverse linear relationships between Gramineae and soil δDwax values and elevation (altitude effect) in the mountain vegetation zone of the Xiying River catchment (2200–3500m) and Maxian Mountains (2800–3700m), where the elevation effect becomes the main control factor even in the dry Qilian Mountains. However, at the low elevation sites, soil and Gramineae δDwax values from the Xiying River (1750–2200m) and Maxian Mountains (2060–2800m) show the “inverse altitude effect”. The recycled fraction and trajectory of water vapor may result in these positive δDwax elevation gradients in dry continental settings. The “inverse altitude effect” has also been observed in periglacial regions below the Qiyi glacier and from the Lenglong Range. We interpret this effect as evidence of stronger fractionation processes occurring in the freeze–thaw mass exchange between the solid and liquid phases at high elevations. Thus, the “inverse altitude effect” can be attributed to the decrease of precipitation amount in the upper valley and to marked fractionation in soil water and the residual or melting snow in periglacial regions, which may complicate the isotope thermometry. When interpreting changes of long term δD and δ18O records for determining elevation change, one must consider all possible influences on the lapse rate of δDwax values with elevation, including the patterns of atmospheric circulation in the past, topography, recycling of moisture in dry continental settings, and strong fractionation processes effecting soil and glacier melt water in the periglacial regions.

Hydrogen isotopic composition of plant leaf wax in response to soil moisture in an arid ecosystem of the northeast Qinghai-Tibetan Plateau, China

The hydrogen isotopic composition of plant leaf wax (δDwax) is used as an important tool for paleohydrologic reconstruction. However, the understanding of the relative importance of environmental and biological factors in determining δDwax values still remains incomplete. To identify the effects of soil moisture and plant physiology on δDwax values in an arid ecosystem, and to explore the implication of these values for paleoclimatic reconstruction, we measured δD values of soil water (δDwater) and δDwax values in surface soils along two distance transects extending from the lakeshore to wetland to dryland around Lake Qinghai and Lake Gahai on the northeast Qinghai-Tibetan Plateau. The results showed that the δDwater values were negatively correlated with soil water content (SWC) (R 2=0.9166), and ranged from −67‰ to −46‰ with changes in SWC from 6.2% to 42.1% in the arid areas of the Gangcha (GCh) and Gahai (GH) transects. This indicated that evaporative D-enrichment in soil water was sensitive to soil moisture in an arid ecosystem. Although the shift from grasses to shrubs with increasing aridity occurred in the arid area of the GH transect, the δDwax values in surface soils from the arid areas of the two transects still showed a negative correlation with SWC (R 2=0.6835), which may be due to the controls of primary evaporative D-enrichment in the soil water and additional transpirational D-enrichment in the leaf water on the δDwax values. Our preliminary research suggested that δDwax values can potentially be applied as a paleo-humidity indicator on the northeast Qinghai-Tibetan Plateau.

Fluctuation in leaf wax D/H ratio from a southern California lake records significant variability in isotopes in precipitation during the late Holocene

The hydrogen isotopic composition of plant molecular markers in modern vegetation and marine sediments in southern California have been intensively studied. Here we report a late Holocene reconstruction from Zaca Lake in coastal southern California, together with modern hydrological and vegetation studies in the catchment. The small catchment and decadal sampling resolution throughout the 9m, 3000yr sedimentary record provided a high resolution terrigenous counterpart of nearby marine records from the Santa Barbara Basin. Today, δD values of precipitation average −51.5‰±18 (1σ, n=7). Modern plant leaf wax δD values for the C28n-alkanoic acid averaged −141‰±11 (1σ, n=10) for Quercus agrifolia, with a calculated fractionation relative to precipitation (εwax/precip) of −94‰±22 (1σ, n=10); in contrast, there was negligible production of the C28 acid by co-dominant Pinus coulteri. Downcore, the C28 acid δD values ranged between −101‰ and −177‰ (mean −150‰±8, 1σ, n=490). Abundance distributions suggested that the sedimentary C28 acid was dominated by Quercus, implying that paleoprecipitation varied between extremes of −8‰ and −92‰ (mean −63‰±14, compound 1σ, n=490). The 3000yr leaf wax D/H record from Zaca Lake revealed substantial temporal variability, greater than observed in a speleothem reconstruction of similar resolution. We suggest that the plant-based proxy may magnify the variability by sampling spring precipitation preferentially. Centennial-duration positive isotopic excursions were associated with more sub-tropical moisture sources and drier conditions including during the Medieval Climatic Anomaly, and negative excursions were associated with N Pacific sources and wetter conditions including during the Little Ice Age.

Influence of terrestrial vegetation on leaf wax δD of Holocene lake sediments

The hydrogen isotope composition of terrestrial plant leaf wax in sediments is increasingly used as a paleoclimatic indicator. Modern calibration studies suggest that paleoclimatic interpretation of leaf wax δD values requires consideration of the differences in the apparent fractionation of hydrogen isotopes among different groups of plants. However, it is not common that paleoecological data are used to help interpret leaf wax δD profiles. Here we assess the relative importance of factors influencing millennial-scale shifts in δD values of n-alkanoic acids at Steel Lake (Minnesota, USA), an extensively studied site with independent records of vegetation composition, δD of input water to the lake, and evaporation. The δD values of the n-C28 alkanoic acid (δDC28) vary between −190 and −168‰, and do not correlate with δD of input water or the extent of evaporation. However, δDC28 is negatively correlated with the δ13C values of the n-C28 alkanoic acid (δ13CC28). The correlation, along with pollen assemblage and carbonate δ13C records, suggests that Holocene shifts between forest and grassland and/or in the water use efficiency of C3 plants influenced the stratigraphic variation in leaf wax δD. Thus, paleoecological information, such as that inferred from pollen assemblages and carbon isotopes of plant-derived compounds, may aid paleoclimatic interpretation of leaf wax δD profiles from lake sediments.

Variation in n-alkane δD values from terrestrial plants at high latitude: Implications for paleoclimate reconstruction

The molecular hydrogen isotope composition ( δD) of leaf waxes from terrestrial plants is increasingly used to infer hydrological characteristics of ancient high latitude climates. Analysis of the hydrogen isotope composition of n-alkanes ( δD n -alkane) from a global dataset of individual plants growing at low and middle latitudes indicates that plant ecological life form is an important factor in determining the hydrogen isotope fractionation. However, environmental and biological controls of high latitudinal leaf wax δD values are poorly understood because of a lack of δD records from modern flora in these regions. We previously noticed smaller apparent hydrogen isotope fractionations between n-alkanes and environmental water ( ε alk - water ) in deciduous trees growing at high latitudes (>59°N; Liu, W.-G., Yang, H., 2008. Multiple controls for the variability of hydrogen isotopic compositions in higher plant n-alkanes from modern ecosystems. Global Change Biology 14, 2166–2177.) To further examine these issues, we measured δD n -alkane from a variety of plants that inhabit high latitude environments and added critically needed leaf wax δD data from grass and herbs to the existing global δD n -alkane database. Inclusion of these new data with the existing global dataset ( n = 408) confirms plant ecological life form as an important control for leaf wax δD variation for terrestrial plants living at high latitudes. Our results suggest that, while precipitation δD is captured in these high latitude plants, physiological characters such as leaf area, venation pattern and hydraulic system, that enhance transpiration rate during summer growth, may impart δD n -alkane differences among plants with different ecological life forms.