Different hydrogen isotope fractionations during lipid formation in higher plants: Implications for paleohydrology reconstruction at a global scale

Orbital modulation of subtropical versus subantarctic moisture sources in the southeast Pacific mid-latitudes

Reconstructing rainfall variability and moisture sources is a critical aspect to understand past and future hydroclimate dynamics. Here, we use changes in the deuterium content of land-plant leaf waxes from two marine sediment cores located off Chile to reconstruct changes in rainfall amount and variation in moisture sources over the last ~50 ka. The records indicate increased moisture in central Chile during precession maxima, but an obliquity modulation is evident in southern Chile. While the southern westerly winds are the dominant factor of precipitation in southern Chile by bringing moisture and perturbations from the extratropics, the subtropics represent an additional moisture source during precession maxima due to a stronger subtropical jet increasing moisture transport from the tropics to the mid-latitudes. These findings imply that a combination of orbital modulation of moisture sources and rainfall amount explains the last glacial moisture maximum and early Holocene moisture minimum in south-central Chile.

Apparent fractionation of hydrogen isotope from precipitation to leaf wax n-alkanes from natural environments and manipulation experiments

Knowledge of hydrogen isotopic fractionation (ε) of plant leaf waxes is the foundation for applying hydrogen isotope values (δ²H) in environmental reconstructions. In this work, we systematically investigated plant ε values (ε_{alk/precipitation}, ε_{alk/soil water}, ε_{alk/leaf water} and ε_{alk/lake water}, representing the isotopic fractionation between plant n-alkane δ²H and precipitation δ²H, soil water δ²H, leaf water δ²H and lake water δ²H) from the natural environments and manipulation experiments. The results show that the ε_{alk/precipitation} values of terrestrial plants have large variations (from -190 ‰ to -20 ‰) and become more negative with increasing aridity index. This phenomenon is possibly caused by the δ²H changes in source water (from precipitation to soil water and then to leaf water) during plant leaf wax synthesis under various evapotranspiration conditions in different climatic zones. The rainfall manipulation experiments show that leaf water δ²H values are generally higher than soil water δ²H values, and the latter are higher than precipitation δ²H values. This finding further demonstrates that the evapotranspiration effect on source water δ²H affects the quantification of the leaf wax apparent ε values (ε_{alk/leaf water} < ε_{alk/soil water} < ε_{alk/precipitation}). The ε_{alk/lake water} values of submerged plants display a smaller range (-153 ± 5 ‰) than the ε_{alk/precipitation} values of terrestrial plants, which is close to the terrestrial ε_{alk/precipitation} values in humid areas. Therefore, the biosynthetic ε value of terrestrial plant leaf waxes is relatively constant (ca. -153 ± 5 ‰), and the observed variable apparent ε_{alk/precipitation} values are possibly caused by the varied degree of evapotranspiration effect on the water that plants used in different climatic conditions. This effect should be considered when applying δ²H values of leaf waxes to trace environmental changes.

Holocene climatic controls on flooding regime along the Ussuri River in Northeast Asia

Knowledge of the long-term flooding response to climatic changes is critical for probing the flooding future in an oncoming warmer world. In this paper, three well-dated wetland sedimentary cores with high-resolution grain-size records were employed to reconstruct the historical flooding regime along the Ussuri River during the past 7000 years. The results show that five flooding-prone intervals marked by increased mean rates of sand-fraction accumulation occurred at 6.4-5.9 ka BP, 5.5-5.1 ka BP, 4.6-3.1 ka BP, 2.3-1.8 ka BP, and 0.5-0 ka BP, respectively. These intervals are generally consistent with the higher mean annual precipitation controlled by the strengthened East Asian summer monsoon which has been widely documented in geological records across the monsoonal regions of East Asia. Considering the prevalent monsoonal climate along the modern Ussuri River, we suggest that the regional flooding evolution during the Holocene Epoch should be generally controlled by the East Asian summer monsoon circulation which was initially linked to the ENSO activities in the tropical Pacific Ocean. While for the last interval spanning 0.5-0 ka BP, human influence, compared with the long-serving climatic controls, has played a more critical role in driving the regional flooding regime.

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 (δ²H_wax) and carbon (δ¹³C_wax) isotopes in terrestrial plants along an arid mountainous transect. We found that the effects of seasonality on n-alkane distribution, δ²H_wax and δ¹³C_wax were minimal for all species (p > 0.05), and that species-specific δ²H_wax values remained almost unchanged for most species, in contrast to δ¹³C_wax values. Significant correlations between altitude and δ²H_wax values (R² = 0.54, 0.58, and 0.75 for spring, summer, and autumn, respectively), instead of δ¹³C_wax 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 δ²H_wax values, but not δ¹³C_wax 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 δ²H_wax 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 (δ²H_wax) within a leaf were heterogeneous in a small number of species. It still remains unclear whether the heterogeneity of intra-leaf δ²H_wax values is general for various species, how δ²H_wax values vary spatially and temporally, and whether there is a common explanation for the intra-leaf δ²H_wax heterogeneity in higher plants. Here we compared the hydrogen isotope compositions of leaf wax and corresponding leaf water (δ²H_lw) across leaf sections among a variety of monocot and dicot plant species. There is significant and consistent heterogeneity for both δ²H_wax and δ²H_lw, 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 δ²H_wax and δ²H_lw for dicots. The consistent occurrence of variations of δ²H_lw and δ²H_wax values within a leaf imply that δ²H_wax values probably inherit point-to-pint from in-situ δ²H_lw values, and thus the intra-leaf δ²H_wax heterogeneity mainly results from the spatial pattern of intra-leaf δ²H_lw values associated with veinal structures between dicots and monocots. The general heterogeneity of intra-leaf δ²H_wax values further intensifies that it is necessarily needed for in-depth understanding leaf wax biomarker.

Stable isotopes of fatty acids: current and future perspectives for advancing trophic ecology

To understand consumer dietary requirements and resource use across ecosystems, researchers have employed a variety of methods, including bulk stable isotope and fatty acid composition analyses. Compound-specific stable isotope analysis (CSIA) of fatty acids combines both of these tools into an even more powerful method with the capacity to broaden our understanding of food web ecology and nutritional dynamics. Here, we provide an overview of the potential that CSIA studies hold and their constraints. We first review the use of fatty acid CSIA in ecology at the natural abundance level as well as enriched physiological tracers, and highlight the unique insights that CSIA of fatty acids can provide. Next, we evaluate methodological best practices when generating and interpreting CSIA data. We then introduce three cutting-edge methods: hydrogen CSIA of fatty acids, and fatty acid isotopomer and isotopologue analyses, which are not yet widely used in ecological studies, but hold the potential to address some of the limitations of current techniques. Finally, we address future priorities in the field of CSIA including: generating more data across a wider range of taxa; lowering costs and increasing laboratory availability; working across disciplinary and methodological boundaries; and combining approaches to answer macroevolutionary questions. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.

Using δDn-alkane as a proxy for paleo-environmental reconstruction: A good choice to sample at the site dominated by woods

Some studies have demonstrated that leaf wax δD_n-alkane values for a single species varied significantly with seasons. However, it is still not clear that the seasonality patterns of leaf wax δD_n-alkane values in higher plants. Meanwhile, few efforts have been pursued to assess the effect of the light slopes (sunny vs. cloudy) on leaf wax δD_n-alkane values. In this study, we systematically investigated plant wax δD_n-alkane values and soil n-alkane δD values along different light slopes in different seasons (spring vs. autumn), as well as the relationship of n-alkane δD values between plant leaves and soil. We found that plant wax δD_n-alkane values were D-enriched by ca. 20‰ in spring relative to autumn, and ca. 10‰ in the sunny slope than in the cloudy slope. Moreover, surface soil n-alkane δD values varied consistently with plant wax δD_n-alkane values for different seasons and light slopes. More importantly, plant wax δD_n-alkane values showed clear seasonal variations, but varied slightly with light slopes. The variations of plant wax δD_n-alkane values can be recorded in soil n-alkane δD_n-alkane values. In addition, we found that leaf wax δD_n-alkane values in a majority of species differed significantly among woods, non-woods and grasses at a site. Therefore, we suggested a good choice to sample at the site dominated by woods when leaf wax δD_n-alkane values are utilized as a proxy for the reconstruction of the paleoenvironment.

Stable hydrogen isotope values of lignin methoxyl groups of four tree species across Germany and their implication for temperature reconstruction

Stable hydrogen isotope ratios of lignin methoxyl groups (δ²H_LM values) in wood have been shown to mirror the δ²H signatures of precipitation (δ²H_precip values). Thus, δ²H_LM values were suggested to serve as a potential paleotemperature proxy since δ²H_precip values are dominantly controlled by air temperature in the mid-latitudes. A recent study where a significant δ²H_LM-temperature relationship was found for a European transect with mean annual temperatures ranging from -4 to 17°C strengthened this assumption. However, using δ²H_LM values as a paleotemperature proxy requires quantification of noise from site-, species- and biosynthetic-specific influences to determine the significance of recording smaller temperature changes. Here, we measured δ²H_LM values of tree-ring sections covering 1981-1990 and 1991-2011 of four different tree species (European beech, English oak, Scots pine, Norway spruce) at 15 sampling sites across Germany. The maximum difference in mean annual temperature between sample sites was 5°C and all sites showed small temperature increases from 1981 to 1990 to 1991-2011 (mean Δ=0.7°C). For all species investigated, the maximum difference of δ²H_LM within the tree was <10mUr or ‰ (median values) and between trees at a single site was ≤28mUr (median values). The general pattern of the spatial δ²H_LM-temperature relationship found for the European transect was confirmed here although a significant correlation was lacking. This can be explained by the lower spatial δ²H_precip-temperature correlation (R²=0.39) found for sampling sites in this study and the δ²H_LM differences between trees. Nevertheless, the temporal changes in δ²H_LM values of European beech trees correctly reflected within ±2°C the temperature change at every sampling site. Therefore, we suggest that δ²H_LM values of European beech trees have considerable potential for reconstructing temperature changes when applied on tree-ring chronologies and consider this approach particularly suited for Late Holocene climate studies.

Experimental Determination of Silicon Isotope Fractionation in Rice

Analyzing variations in silicon (Si) isotopes can help elucidate the biogeochemical Si cycle and Si accumulation processes of higher plants. Importantly, the composition of Si isotopes in higher plants has yet to be studied comprehensively and our knowledge of the distribution of Si isotopes in higher plants lags behind that of Si isotopes in marine organisms, such as diatoms. In the present study, we investigated the isotope fractionation that occurs during the uptake and transport of Si in rice, using a series of hydroponic experiments with different external concentrations of Si. We found that an active mechanism was responsible for the majority of Si uptake and transport at lower Si levels and that the uptake of Si by rice roots was significantly suppressed by both low temperature and metabolic inhibitors. In addition, light Si isotopes (28Si) entered roots more readily than heavy Si isotopes (30Si) when the active mechanism was inhibited. Therefore, we conclude that biologically mediated isotope fractionation occurs during the uptake of Si by rice roots. In addition, both active and passive Si uptake components co-exist in rice, and the fractionation effect is enhanced when more Si is absorbed by plants.

Hydrogen isotopic differences between C3 and C4 land plant lipids: consequences of compartmentation in C4 photosynthetic chemistry and C3 photorespiration

The ² H/¹ H ratio of carbon-bound H in biolipids holds potential for probing plant lipid biosynthesis and metabolism. The biochemical mechanism underlying the isotopic differences between lipids from C₃ and C₄ plants is still poorly understood. GC-pyrolysis-IRMS (gas chromatography-pyrolysis-isotope ratio mass spectrometry) measurement of the ² H/¹ H ratio of leaf lipids from controlled and field grown plants indicates that the biochemical isotopic fractionation (ε² H_{lipid_biochem} ) differed between C₃ and C₄ plants in a pathway-dependent manner: ε² H_C4  > ε² H_C3 for the acetogenic pathway, ε² H_C4  < ε² H_C3 for the mevalonic acid pathway and the 1-deoxy-D-xylulose 5-phosphate pathway across all species examined. It is proposed that compartmentation of photosynthetic CO₂ fixation into C₄ mesophyll (M) and bundle sheath (BS) cells and suppression of photorespiration in C₄ M and BS cells both result in C₄ M chloroplastic pyruvate - the precursor for acetogenic pathway - being more depleted in ² H relative to pyruvate in C₃ cells. In addition, compartmentation in C₄ plants also results in (i) the transferable H of NADPH being enriched in ² H in C₄ M chloroplasts compared with that in C₃ chloroplasts for the 1-deoxy-D-xylulose 5-phosphate pathway pathway and (ii) pyruvate relatively ² H-enriched being used for the mevalonic acid pathway in the cytosol of BS cells in comparison with that in C₃ cells.