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Reconstructing lake evaporation history and the isotopic composition of precipitation by a coupled δ18O–δ2H biomarker approach
... Shorter-chain n-alkanes (e.g., n-C 21 and n-C 23 ), on the other hand, are produced by aquatic organisms and incorporate the δ 2 H signal of the lake water, i.e., depending on the hydrological setting, evaporation may lead to isotopic enrichment 35-39 . Like δ 2 H from n-alkanes, δ 18 O from hemicellulose sugars provides valuable paleohydrological information 34,35,40,41 . Hemicellulose sugars are also produced by terrestrial (i.e., arabinose) and aquatic (i.e., fucose) sources 42 . ...
... Their δ 18 O signal mainly reflects the isotopic composition of the local precipitation (arabinose) and the lake water (fucose) modulated by evapo(transpi)rative enrichment and biosynthetic fractionation 32,33,41,43 . The "coupled isotope approach", also dubbed "paleohygrometer approach", combines δ 2 H n-alkane and δ 18 O sugar analyses to reconstruct deuterium excess (d-excess), which can be used as a proxy for evapo(transpi)rative enrichment 30,35,43,44 . This approach has been tested and applied successfully in several studies to quantify the evapo(transpi)rative enrichment of leaf water 30,35,40,[43][44][45] and lake water 35 . ...
... The "coupled isotope approach", also dubbed "paleohygrometer approach", combines δ 2 H n-alkane and δ 18 O sugar analyses to reconstruct deuterium excess (d-excess), which can be used as a proxy for evapo(transpi)rative enrichment 30,35,43,44 . This approach has been tested and applied successfully in several studies to quantify the evapo(transpi)rative enrichment of leaf water 30,35,40,[43][44][45] and lake water 35 . ...
Isotope-based records provide valuable information on past climate changes. However, it is not always trivial to disentangle past changes in the isotopic composition of precipitation from possible changes in evaporative enrichment, and seasonality may need to be considered. Here, we analyzed δ2H on n-alkanes and δ18O on hemicellulose sugars in sediments from Bichlersee, Bavaria, covering the Late Glacial and Early Holocene. Our δ2Hn-C31 record documents past changes in the isotopic composition of summer precipitation and roughly shows the isotope pattern known from Greenland. Both records show lower values during the Younger Dryas, but at Bichlersee the signal is less pronounced, corroborating earlier suggestions that the Younger Dryas was mainly a winter phenomenon and less extreme during summer. δ18Ofucose records the isotopic composition of the lake water during summer and is sensitive to evaporative enrichment. Coupling δ2Hn-C31 and δ18Ofucose allows calculating lake water deuterium-excess and thus disentangling changes in the isotopic composition of precipitation and evaporative enrichment. Our deuterium-excess record reveals that the warm Bølling-Allerød and Early Holocene were characterized by more evaporative enrichment compared to the colder Younger Dryas. Site-specific hydrological conditions, seasonality, and coupling δ2H and δ18O are thus important when interpreting isotope records.
... The compound-specific hydrogen isotope composition of leaf wax-derived n-alkanes (δ 2 Hn-alkane) and oxygen isotope composition of hemicellulose-derived sugars (δ 18 Osugar) serve as proxies in paleoclimate and -environmental studies (Hepp et al., 2017;Tuthorn et al., 2015;Zech et al., 2014aZech et al., , 2013b. The above-mentioned biomarkers are preserved for a long period of time Zech et al., 2014a) and their application was tested in different paleoclimate archives; for instance lake sediments (Hepp et al., 2015;Sachse et al., 2004;Zech et al., 2014b), loess-paleosols (Buggle and Zech, 2015;Zech et al., 2013b), and tree rings (Gessler et al., 2009). The isotope composition of δ 2 Hn-alkane and δ 18 Osugar from terrestrial vascular plants primarily reflects the isotope composition of precipitation. ...
... Additionally, our approach does not consider evaporative enrichment of soil water (Hepp et al., 2015;Tuthorn et al., 2015;Zech et al., 2013b). Moreover, most leaf-derived biomarkers are biosynthesized at an early stage of leaf ontogeny (Gershenzon et al., 2000;Tipple et al., 2013) and may not reflect the mean annual isotope composition of precipitation, but rather the isotope composition of precipitation during the growing season. ...
... Grass blades as tree rings: environmentally induced changes in the oxygen isotope ratio of cellulose along the length of grass blades. New Phytologist 155, 417-424.Hepp, J.,Tuthorn, M., Zech, R., Mügler, I., Schlütz, F., Zech, W., Zech, M., 2015. ...
Over the past decades, biomarkers (e.g. lignin, lipids) and stable isotopes (e.g. 2H and 18O) were increasingly used to reconstruct vegetation and climate changes. Hitherto, factors affecting the abundance and distribution of biomarkers and their stable isotope composition are not similar worldwide and regional calibration studies are, therefore, highly recommended. The main aim of this Ph.D. dissertation is to evaluate the potential and limitations of biomarkers and their stable isotope composition for chemotaxonomic characterization of the keystone plant species in the Bale Mountains, Ethiopia, and its implication for paleovegetation and paleoclimate reconstruction. More specifically, I focused on lignin-derived phenols, leaf wax-derived n- alkanes, stable isotopes in precipitation (δ2Hprec and δ18Oprec), and compound-specific stable isotopes (δ2Hn-alkane and δ18Osugar). Mild alkaline cupric oxide (CuO) oxidation method was used to extract phenolic compounds. Leaf wax-derived n-alkanes have been Soxhlet-extracted and purified using aminopropyl columns from leaves and topsoils (O-layers as well as Ah-horizons). Individual phenols and n-alkanes were separated by gas chromatography (GC) and detected by mass spectrometry (MS) and flame ionization detector (FID), respectively. The δ2Hprec and δ18Oprec values were measured using a Thermo Conversion Unit coupled with a ConFlo IV interface to an isotope ratio mass spectrometry (IRMS). Compound-specific δ2Hn-alkane and δ18Osugar values were measured using gas chromatography - thermal conversion - isotope ratio mass spectrometry (GC-TC-IRMS).
While conventional phenol ratios such as syringyl vs. vanillyl (S/V) and cinnamyl vs. vanillyl (C/V) phenols failed for unambiguous Erica identification, using machine-learning approaches, we found that the relative abundance of coumaryl phenols (> 0.2) and benzoic acids (0.05–0.12) can be used as a proxy to distinguish Erica from other keystone plant species. Erica in particular is characterized by relatively high cinnamyl contributions of > 40%. However, litter degradation strongly decreases the lignin phenol content and changes the lignin phenol pattern. The relative cinnamyl contribution in the soils under Erica were < 40%, while soils that developed under Festuca abyssinica exhibited relative cinnamyl contributions of > 40%.
Similarly, long-chain n-alkanes extracted from the leaf waxes allowed a differentiation between Erica versus Festuca abyssinica and Alchemilla haumannii, based on a lower C31/C29 ratio (x͂ = 1.7) in Erica. Nevertheless, this characteristic plant pattern was also lost due to degradation in the respective O-layers and Ah-horizons.
At the first hand, the isotope composition of precipitation in the Bale Mountains shows values for δ2Hprec and δ18Oprec in the range of −38 to +29‰ and −8.7 to +3.7‰, respectively. The local meteoric water line (LMWL, δ2H = 5.3 ∗ δ18O + 14.9) is characterized by a lower slope and higher intercept compared to the global meteoric water line (GMWL, δ2H = 8 ∗ δ18O + 10). Furthermore, our isotope data correlate significantly with altitude and amount of precipitation. At the same time, δ18Oprec and δ2Hprec values exhibited seasonal pattern reflecting rainy versus dry season. Thus, shortly after the end of the dry season, isotope values were enriched, while more depleted isotope values coincided with high precipitation amounts recorded in May, August, and September. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model revealed that during the dry season water vapour originates primarily from the Arabian Sea, whereas during the wet season it originates primarily from the Southern Indian Ocean.
On the other hand, the weighted mean δ2Hn-alkane and δ18Osugar values ranged from −186 to −89‰ and from +27 to +46‰, respectively and exhibited a much wider range than δ18Oprec and δ2Hprec. The δ2Hn-alkane and δ18Osugar values of leaves, O-layers, and Ah-horizons are statistically not significantly different from each other. Hence, in contrast to the biomarker contents and patterns, our dataset provides no evidence for degradation and/or root input affecting the isotope composition of the biomarkers of topsoils. The keystone plant species in the Bale Mountains are characterized by variable values of δ2Hn-alkane as well as apparent isotope fractionation (ɛapp); δ18Osugar yielded the same species-dependent trends. When considering a systematic biosynthetic fractionation offset of −160‰ and +27‰ for δ2Hn-alkane and δ18Osugar, respectively, leaf water of Erica arborea and Erica trimera is 2H- and 18O-enriched by +55 ± 5 and +9 ± 1‰, respectively, compared to precipitation. By contrast, Festuca abyssinica reveals the most depleted δ2Hn-alkane and δ18Osugar values. This can be attributed to “signal-dampening” caused by basal grass leaf growth. The intermediate values for Alchemilla haumannii and Helichrysum splendidum can be explained by plant physiological or microclimatic conditions affecting relative humidity (RH) and thus leaf water enrichment. Once the isotope composition of leaf water calculated, it is possible to reconstruct the isotope composition of source water, deuterium excess and RH using coupled δ2Hn-alkane-δ18Osugar approach. While the actual RH values range from 69 to 82% (x̄ = 80 ± 3.4%), the reconstructed relative humidity, based on a recently suggested coupled δ2Hn-alkane-δ18Osugar (paleo-) hygrometer approach yielded a mean of 78±21%.
In conclusion, chemotaxonomic differentiation of modern-day plants (leaves) is possible using phenols and n-alkanes but soil degradation processes and root input seem to render the proxies unusable for the reconstruction of the past extent of Erica in the Bale Mountains. Similarly, vegetation changes, particularly in terms of grass versus non-grassy vegetation, need to be considered in paleoclimate studies based on δ2Hn-alkane and δ18Osugar records. Furthermore, our δ2Hprec and δ18Oprec results challenge the traditional amount effect interpretation of paleoclimate isotope records from Eastern Africa and rather pinpointed to a previously underestimated source effect. And, the coupled δ2Hn-alkane-δ18Osugar (paleo-) hygrometer approach holds great potential for deriving additional paleoclimatic information compared to single isotope approaches.
... The coupled approach used in this study and its potential limitations was previously described in detail by Prochnow, et al. 29 and is originally based on a concept published by Hepp, et al. 64 . ...
... δ 18 O-δ 2 H cross plot showing the concept of the coupled isotope approach29,64 . Datapoints resemble isotope values of biomarkers and the calculated isotopic composition of lake water and precipitation. ...
It is generally accepted that a weakening of the North Atlantic thermohaline circulation caused the Younger Dryas cooling. Although the role of seasonality was emphasized previously, this aspect is rarely considered yet, and it remains elusive how this impacted hydroclimate during winters and summers across Central Europe. Here, we coupled biomarker-based δ¹⁸O and δ²H from Bergsee in southern Germany to reconstruct deuterium excess as a proxy for evaporation history from the Bølling-Allerød to the Preboreal. We compared this dataset with other biomarker isotope records in Central Europe. They are all lacking a strong isotopic depletion during the Younger Dryas, which is best explained by the summer sensitivity of the biomarker proxies: As Younger Dryas summers were relatively warm, there is an absence of the strong winter cooling signals recorded in annual water isotope records like Greenland or Lake Steißlingen. Lake evaporation at Bergsee together with other paleohydrological reconstructions draw a coherent picture of the Late Glacial hydroclimate, with strong evidence for warm and dry Younger Dryas summers. Rather than a southward shift of the Westerlies during winter, we suggest that a recently proposed feedback mechanism between North Atlantic sea ice extend, strong winter cooling and summer atmospheric blocking serves as a suitable explanation for summer dryness. Additional confidence to the robustness of these biomarker records is provided by the overall agreement of paleohydrological fluctuations during the Preboreal.
... However, this interpretation is challenged by heavy-isotope enrichment during evapotranspiration (e.g., Zech et al., 2015). Apart from studies of sedimentary cellulose (Heyng et al., 2014;Wissel et al., 2008), the oxygen stable isotope composition of sugar biomarkers (δ 18 O sugar ) emerged as a complementary paleoclimate proxy during the last decade (Hepp et al., 2015(Hepp et al., , 2017Zech et al., 2013aZech et al., , 2014aZech and Glaser, 2009). The interpretation of the δ 18 O sugar values is comparable to those of δ 2 H n-alkane . ...
... The second advantage of the proposed coupled δ 2 H n-alkaneδ 18 O sugar approach is a more robust reconstruction of the isotope composition of the source water, which can often be directly linked to the local precipitation signal (Hepp et al., 2015(Hepp et al., , 2017Tuthorn et al., 2015;Zech et al., 2013a). Therefore, we calculated the source water isotope compositions via the slopes of the LELs and the GMWL. ...
... However, this interpretation is challenged by heavy-isotope enrichment during evapotranspiration (e.g., Zech et al., 2015). Apart from studies of sedimentary cellulose (Heyng et al., 2014;Wissel et al., 2008), the oxygen stable isotope composition of sugar biomarkers (δ 18 O sugar ) emerged as a complementary paleoclimate proxy during the last decade (Hepp et al., 2015(Hepp et al., , 2017Zech et al., 2013aZech et al., , 2014aZech and Glaser, 2009). The interpretation of the δ 18 O sugar values is comparable to those of δ 2 H n-alkane . ...
... The second advantage of the proposed coupled δ 2 H n-alkaneδ 18 O sugar approach is a more robust reconstruction of the isotope composition of the source water, which can often be directly linked to the local precipitation signal (Hepp et al., 2015(Hepp et al., , 2017Tuthorn et al., 2015;Zech et al., 2013a). Therefore, we calculated the source water isotope compositions via the slopes of the LELs and the GMWL. ...
The hydrogen isotope composition of leaf-wax-derived biomarkers, e.g., long-chain n-alkanes (δ2Hn-alkane), is widely applied in paleoclimate. However, a direct reconstruction of the isotope composition of source water based on δ2Hn-alkane alone is challenging due to the enrichment of heavy isotopes during evaporation. The coupling of δ2Hn-alkane with δ18O of hemicellulose-derived sugars (δ18Osugar) has the potential to disentangle this limitation and additionally to allow relative humidity reconstructions. Here, we present δ2Hn-alkane as well as δ18Osugar results obtained from leaves of Eucalyptus globulus, Vicia faba, and Brassica oleracea, which grew under controlled conditions. We addressed the questions of (i) whether δ2Hn-alkane and δ18Osugar values allow reconstructions of leaf water isotope composition, (ii) how accurately the reconstructed leaf water isotope composition enables relative humidity (RH) reconstruction, and (iii) whether the coupling of δ2Hn-alkane and δ18Osugar enables a robust source water calculation.
For all investigated species, the n-alkane n-C29 was most abundant and therefore used for compound-specific δ2H measurements. For Vicia faba, additionally the δ2H values of n-C31 could be evaluated robustly. Regarding hemicellulose-derived monosaccharides, arabinose and xylose were most abundant, and their δ18O values were therefore used to calculate weighted mean leaf δ18Osugar values. Both δ2Hn-alkane and δ18Osugar yielded significant correlations with δ2Hleaf water and δ18Oleaf water, respectively (r2=0.45 and 0.85, respectively; p<0.001, n=24). Mean fractionation factors between biomarkers and leaf water were found to be -156 ‰ (ranging from -133 ‰ to -192 ‰) for εn-alkane/leaf water and +27.3 ‰ (ranging from +23.0 ‰ to 32.3 ‰) for εsugar/leaf water, respectively. Modeled RHair values from a Craig–Gordon model using measured Tair, δ2Hleaf water and δ18Oleaf water as input correlate highly significantly with modeled RHair values (R2=0.84, p<0.001, RMSE = 6 %). When coupling δ2Hn-alkane and δ18Osugar values, the correlation of modeled RHair values with measured RHair values is weaker but still highly significant, with R2=0.54 (p<0.001, RMSE = 10 %). Finally, the reconstructed source water isotope composition (δ2Hs and δ18Os) as calculated from our coupled approach matches the source water in the climate chamber experiment (δ2Htank water and δ18Otank water). This highlights the great potential of the coupled δ2Hn-alkane–δ18Osugar paleohygrometer approach for paleoclimate and relative humidity reconstructions.
... Zech et al. 2013bZech et al. , 2014aTuthorn et al. 2015;Hepp et al. 2017). The above mentioned biomarkers are preserved for a long period of time (Eglinton and Hamilton 1967;Glaser and Zech 2005;Eglinton and Eglinton 2008) and their application was tested in different paleoclimate archives, such as for instance in lake sediments (Zech et al. 2014b;Hepp et al. 2015), loess-paleosols (Zech et al. 2013b;Buggle and Zech 2015), and tree rings (Gessler et al. 2009). The isotopic composition of d 2 H n-alkane and d 18 O sugar from terrestrial vascular plants primarily reflects the isotopic composition of precipitation. ...
... Thus, the difference between actual and reconstructed RH might be associated with leaf-air temperature differences (Hepp et al. 2017;Zech et al. 2013b). Additionally, our approach does not consider evaporative enrichment of soil water (Zech et al. 2013b;Hepp et al. 2015;Tuthorn et al. 2015). Moreover, most leaf-derived biomarkers are biosynthesized at an early stage of leaf ontogeny (Gershenzon et al. 2000;Tipple et al. 2013) and may not reflect the mean annual isotopic composition of precipitation, but rather the isotopic composition of precipitation during the growing season. ...
The hydrogen isotopic composition of leaf wax–derived n -alkane (δ ² H n -alkane ) and oxygen isotopic composition of hemicellulose–derived sugar (δ ¹⁸ O sugar ) biomarkers are valuable proxies for paleoclimate reconstructions. Here, we present a calibration study along the Bale Mountains in Ethiopia to evaluate how accurately and precisely the isotopic composition of precipitation is imprinted in these biomarkers. n -Alkanes and sugars were extracted from the leaf and topsoil samples and compound–specific δ ² H n -alkane and δ ¹⁸ O sugar values were measured using a gas chromatograph–thermal conversion–isotope ratio mass spectrometer (GC–TC–IRMS). The weighted mean δ ² H n -alkane and δ ¹⁸ O sugar values range from − 186 to − 89‰ and from + 27 to + 46‰, respectively. Degradation and root inputs did not appear to alter the isotopic composition of the biomarkers in the soil samples analyzed. Yet, the δ ² H n -alkane values show a statistically significant species dependence and δ ¹⁸ O sugar yielded the same species–dependent trends. The reconstructed leaf water of Erica arborea and Erica trimera is ² H– and ¹⁸ O–enriched by + 55 ± 5 and + 9 ± 1‰, respectively, compared to precipitation. By contrast, Festuca abyssinica reveals the most negative δ ² H n -alkane and least positive δ ¹⁸ O sugar values. This can be attributed to “signal–dampening” caused by basal grass leaf growth. The intermediate values for Alchemilla haumannii and Helichrysum splendidum can be likely explained with plant physiological differences or microclimatic conditions affecting relative humidity (RH) and thus RH–dependent leaf water isotope enrichment. While the actual RH values range from 69 to 82% (x̄ = 80 ± 3.4%), the reconstructed RH values based on a recently suggested coupled δ ² H n -alkane –δ ¹⁸ O sugar (paleo–) hygrometer approach yielded a mean of 78 ± 21%. Our findings corroborate (i) that vegetation changes, particularly in terms of grass versus non–grassy vegetation, need to be considered in paleoclimate studies based on δ ² H n -alkane and δ ¹⁸ O sugar records and (ii) that the coupled δ ² H n -alkane –δ ¹⁸ O sugar (paleo–) hygrometer approach holds great potential for deriving additional paleoclimatic information compared to single isotope approaches.
... Zech et al., 2015), especially in respect of single proxy (δ 2 Hn-alkane)-based climate records. Apart from studies of sedimentary cellulose (Heyng et al., 2014;Wissel et al., 2008), the oxygen stable isotope composition of sugar biomarkers 70 (δ 18 Osugar) emerged as complementary paleoclimate proxy during the last decade (Hepp et al., 2015, Zech et al., 2013a, 2014a. The interpretation of the δ 18 Osugar values is comparable to those of δ 2 Hn-alkane. ...
... One of the advantages of the proposed coupled δ 2 Hn-alkane-δ 18 Osugar approach is a more robust reconstruction of the isotope composition of the source water, which can often be directly linked to the local precipitation signal (Hepp et al., 2015(Hepp et al., , 2017Tuthorn et al., 2015;Zech et al., 2013a). 505 ...
The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain n-alkanes (δ2Hn-alkane), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of source water based on δ2Hn-alkane alone can be challenging due to the alteration of the soil water isotopic signal by leaf-water heavy-isotope enrichment. The coupling of δ2Hn-alkane with δ18O of hemicellulose-derived sugars (δ18Osugar) has the potential to disentangle this effect and additionally to allow relative humidity reconstructions. Here, we present δ2Hn-alkane as well as δ18Osugar results obtained from leaves of the plant species Eucalyptus globulus, Vicia faba var. minor and Brassica oleracea var. medullosa, which grew under controlled conditions. We addressed the questions (i) do δ2Hn-alkane and δ18Osugar values allow precise reconstructions of leaf water isotope composition, (ii) how accurately does the reconstructed leaf-water-isotope composition enables relative humidity (RH) reconstruction in which the plants grew, and (iii) does the coupling of δ2Hn-alkane and δ18Osugar enable a robust source water calculation? For all investigated species, the alkane n-C29 was most abundant and therefore used for compound-specific δ2H measurements. For Vicia faba, additionally the δ2H values of n-C31 could be evaluated robustly. With regard to hemicellulose-derived monosaccharides, arabinose and xylose were most abundant and their δ18O values were therefore used to calculate weighted mean leaf δ18Osugar values. Both δ2Hn-alkane and δ18Osugar yielded significant correlations with δ2Hleaf-water and δ18Oleaf-water, respectively (r2 = 0.45 and 0.85, respectively; p
... Deuterium excess, a measure of the enrichment of heavier deuterium isotope in water compared to seawater, acts as a fingerprint to retrace the water's source (e.g., precipitation, evaporation) and identify the mechanisms that led to its mineralization (Hassen et al., 2016). For instance, high deuterium excess indicate low evaporation, suggesting that the water originated directly from precipitation (Hepp et al., 2015). Conversely, low values are indicative of significant evaporation, potentially concentrating minerals in the remaining water. ...
Groundwater is a crucial resource for various purposes, especially in the arid land of Southern Tunisia. The assessment of its properties needs a deeper understanding of the main mineralization processes. Therefore, a comprehensive characterization of shallow aquifers in Chott Djerid area has been carried out using isotope methods based on oxygen-18, deuterium, tritium, carbon-14 and carbon-13 on 46 water samples. The obtained results indicated that both oxygen-18 and deuterium pled to meteoric precipitations in the case of the Saharan aquifers. They also highlighted the mixing effect from deeper groundwaters of the underlying Senonian aquifer. This was not the case of the oasis shallow groundwaters, which resulted from the irrigation return flow. Deuterium excess showed that the dissolution of evaporites was considered the main process of salinization (contributing up to 90%), compared to evaporation for oasis groundwaters. Tritium data indicated the presence of a local recharge in the Saharan region; it was estimated by the chloride mass balance to be 13.2 mm/year. The combination of carbon-14 and carbon-13 showed two main groups of water: (1) waters of the Saharan aquifers showing a slight evolution from an old to a recent pole and (2) old waters represented by both the Senonian and the oasis aquifers. A third intermediate group highlighted the mixing effect in the case of several samples from the Saharan neighboring aquifers. In conclusion, isotope-based investigation of shallow aquifers may help decision-makers to set up a sustainable approach for groundwater management in the Djerid harsh environment.
... This suggests that δ 2 H variations in leaf water solely can explain those of nalkanes. This agrees with previous observations showing that nalkanes can be used for the reconstruction of environmental δ 2 H values of water (1,57), while plant carbohydrate δ 2 H values appear driven by plant metabolic changes (9,26). Given that δ 2 H values of n-alkanes show little sensitivity to our treatments (Table 1 Figs. 1 and 7) and that the H of n-alkanes is relatively more sourced by NADPH than H of sugars, this indicates that the δ 2 H values of the total NADPH pool (i.e., including photosynthetic and other sources of NADPH) are relatively constant along the N fertilization gradient. ...
The hydrogen isotopic composition (δ2H) of plant compounds is increasingly used as a hydroclimatic proxy; however, the interpretation of δ2H values is hampered by potential coeffecting biochemical and biophysical processes.
Here, we studied δ2H values of water and carbohydrates in leaves and roots, and of leaf n-alkanes, in two distinct tobacco (Nicotiana sylvestris) experiments. Large differences in plant performance and biochemistry resulted from (a) soil fertilization with varying nitrogen (N) species ratios and (b) knockout-induced starch deficiency.
We observed a strong 2H-enrichment in sugars and starch with a decreasing performance induced by increasing NO3−/NH4+ ratios and starch deficiency, as well as from leaves to roots. However, δ2H values of cellulose and n-alkanes were less affected. We show that relative concentrations of sugars and starch, interlinked with leaf gas exchange, shape δ2H values of carbohydrates.
We thus provide insights into drivers of hydrogen isotopic composition of plant compounds and into the mechanistic modeling of plant cellulose δ2H values.
... The coupled δ 2 H n-alkane -δ 18 O sugar approach (paleohygrometer) has previously been described in detail by e.g. Hepp et al. (2015Hepp et al. ( , 2017, Hepp et al. (2019Hepp et al. ( , 2020, Hepp et al. (2021), Lemma et al. (2021) and Tuthorn et al. (2015). The fundamental assumption of the approach is that the isotopic composition of leaf water can be reconstructed by applying biosynthetic fractionation factors (ε bio ) on the measured δ 2 H n-alkane and δ 18 O sugar values (Cernusak et al., 2022;Liu et al., 2023). ...
Fire is a natural phenomenon along South Africa's southern Cape coast, but identifying its climatic drivers has been a subject of considerable debate. This study investigates the hydroclimatic and fire dynamics from a 9.6 m sediment core from Vankervelsvlei covering the past 7.2 ka. The fen is located near the southern Cape coast within the year-round rainfall zone of South Africa. A reconstruction of hydroclimatic variability through time applies oxygen isotopes from hemicellulose-derived sugars and hydrogen isotopes from leaf wax-derived n-al-kanes. Coupling both isotopes enables a reconstruction of the atmospheric source and seasonality of precipitation as well as estimating local relative humidity. Past trends in fire activity are inferred from macro-charcoal and polycyclic aromatic hydrocarbon (PAH) analyses, the latter serving as fire biomarkers. Results indicate high fire activity at Vankervelsvlei accompanied by generally moist conditions and a year-round rainfall regime linked to both Westerly-derived winter precipitation and Easterly-and locally-derived summer precipitation from 7.2 +0.2 / − 0.2 to 4.5 +0.3 / − 0.3 cal ka BP. From 4.5 +0.3 / − 0.3 to 1.5 +0.4 / − 0.2 cal ka BP, a shift to a Westerly-derived winter rainfall regime is identified. This variation features alongside reduced fire activity and persistent drought conditions as Easterly-and locally-derived summer precipitation decreased. From 1.5 +0.4 / − 0.2 cal ka BP until present day, macro-charcoal and PAH accumulation rates show high fire activity. Paleoclimate evidence from the last two millennia suggests a variable climate with an overall increase in total moisture availability as contributions from both Westerly-derived winter precipitation and Easterly-and locally-derived summer precipitation support the year-round rainfall regime present today. Results from Vankervelsvlei support previous evidence from regional paleo-reconstructions, refining our understanding of the interplay between hydroclimatic variability and fire activity along South Africa's southern Cape coast. Our study discusses the role of large-scale climate modes, specifically the intensity of El Niño, as a potential driver of short-term hydroclimatic variability, which in turn drives fuel availability and fire activity at Vankervelsvlei during the Holocene.
... Kjellman et al., 2020;Katrantsiotis et al., 2021) have been determined. Another approach to disentangling the impact of lake water evaporation from precipitation changes is coupling δ 2 H and δ 18 O reconstructions, as demonstrated using δ 2 H of n-alkanes and δ 18 O of sugar biomarkers to develop a Lateglacial-Holocene palaeohydrological reconstruction from Himalayan Nepal (Hepp et al., 2015). Reconstructed palaeohydrology from δ 2 H lipid has also provided climatic contexts for human evolution (as reviewed by Patalano et al., 2021) and human settlements (e.g. ...
To reconstruct past environmental changes, a range of indirect or proxy approaches can be applied to Quaternary archives. Here, we review the complementary and novel insights that have been provided by the analysis of chemical fossils (biomarkers). Biomarkers have a biological source that can be highly specific (e.g. produced by a small group of organisms) or more general. We show that biomarkers are able to quantify key climate variables (particularly water and air temperature) and can provide qualitative evidence for changes in hydrology, vegetation, human–environment interactions and biogeochemical cycling. In many settings, biomarker proxies provide the opportunity to simultaneously reconstruct multiple climate or environmental variables, alongside complementary and long‐established approaches to palaeoenvironmental reconstruction. Multi‐proxy studies have provided rich sets of data to explore both the drivers and impacts of palaeoenvironmental change. As new biomarker proxies continue to be developed and refined, there is further potential to answer emerging questions for Quaternary science and environmental change.
... However, recent studies show the great potential of δ 2 H values of plant compounds to retrospectively determine hydrological and climatic conditions (Anhäuser, Hook, Halfar, Greule, & Keppler, 2018;Gamarra & Kahmen, 2015;Hepp et al., 2015Hepp et al., , 2019Sachse et al., 2012), as well as to disentangle metabolic and physiological processes (Cormier et al., 2018;Estep & Hoering, 1981;Sanchez-Bragado, Serret, Marimon, Bort, & Araus, 2019;Tipple & Ehleringer, 2018) such as the proportional use of carbon sources (i.e., fresh assimilates vs. storage compounds) for plant growth (Lehmann, Vitali, Schuler, Leuenberger, & Saurer, 2021;Zhu et al., 2020). Enabling the analysis of δ 2 H ne of NSC, especially sugar at the leaf level, will make it possible to study processes and environmental conditions which are shaping the 2 H-fractionation of carbohydrates at a much higher time resolution compared to the analysis of δ 2 H ne of cellulose. ...
The analysis of the non-exchangeable hydrogen isotope ratio (δ²Hne) in carbohydrates is mostly limited to the structural component cellulose, while simple high-throughput methods for δ²Hne values of non-structural carbohydrates (NSC) such as sugar and starch do not yet exist. Here we tested if the hot vapor equilibration method originally developed for cellulose is applicable for NSC, verified by comparison with the traditional nitration method. We set up a detailed analytical protocol and applied the method to plant extracts of leaves from species with different photosynthetic pathways (i.e., C3, C4, CAM). δ²Hne of commercial sugars and starch from different classes and sources, ranging from −157.8 to +6.4‰, were reproducibly analysed with a precision between 0.2 and 7.7‰. Mean δ²Hne values of sugar are lowest in C3 (−92.0‰), intermediate in C4 (−32.5‰), and highest in CAM plants (6.0‰), with NSC being ²H-depleted compared to cellulose and sugar being generally more ²H-enriched than starch. Our results suggest that our method can be used in future studies to disentangle ²H-fractionation processes, for improving mechanistic δ²Hne models for leaf and tree-ring cellulose, and for further development of δ²Hne in plant carbohydrates as a potential proxy for climate, hydrology, plant metabolism and physiology.
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... Organic molecular biomarkers are produced by plants and microorganisms over nearly the entire range of global climatic conditions (with the exception of extreme cold environments) and can be preserved in nearly all sedimentary sinks associated with orogenesis. As a result, organic molecular biomarkers are increasingly utilized for paleoclimate/paleohydrology and paleotopography reconstruction (e.g., Polissar et al., 2009;Hren et al., 2010;Aichner et al., 2015;Hepp et al., 2015;Tuthorn et al., 2015;Rohrmann et al., 2016;Schafer et al., 2018;Bliedtner et al., 2020). These molecules can record two specific parameters that are commonly linked to elevation, present and past: 1) the stable isotopic composition of environmental water, and 2) the temperature at which molecules are synthesized. ...
Stable isotope paleoaltimetry is one of the most commonly used approaches for quantifying the paleoelevation history of an orogen yet this methodology is often limited to arid to semi-arid climates, mountain systems with a clear orographic rainshadow and terrestrial basins. We present a new approach to reconstructing past topography and relief that uses the catchment-integrated signature of organic molecular biomarkers to quantify the hypsometry of fluvially-exported biomass. Because terrestrially-produced biomolecules are synthesized over the full range of global climate conditions and can be preserved in both terrestrial and marine sediments, the geochemistry of fluvially-transported sedimentary biomarkers can provide a means of interrogating the evolution of topography for a range of environments and depositional settings, including those not well suited for a traditional isotope paleoaltimetry approach. We show an example from Taiwan, a rapidly eroding tropical mountain system that is characterized by high rates of biomass production and short organic residence time and discuss key factors that can influence molecular isotope signal production, transport and integration. Data show that in high relief catchments of Taiwan, river sediments can record integration of biomass produced throughout the catchment. Sedimentary biomarker δ²H nC29 in low elevation river deposition sites is generally offset from the δ²H nC29 value observed in local soils and consistent with an isotope composition of organics produced at the catchment mean elevation. We test the effect of distinct molecular production and erosion functions on the expected δ²H nC29 in river sediments and show that elevation-dependent differences in the production and erosion of biomarkers/sediment may yield only modest differences in the catchment-integrated isotopic signal. Relating fluvial biomarker isotope records to quantitative estimates of organic source elevations in other global orogens will likely pose numerous challenges, with a number of variables that influence molecular production and integration in a river system. We provide a discussion of important parameters that influence molecular biomarker isotope signatures in a mountain system and a framework for employing a molecular paleohypsometry approach to quantifying the evolution of other orogenic systems.
... Lake water samples however, are different from both meteoric water lines and show distinct enrichment. Furthermore, as they are in line with the beginning evaporation signal in the aforementioned tributaries (Fig. 5), they reveal the strong evaporation driven character of these endorheic lakes on the TP (Aichner et al., 2010;Günther et al., 2015;Hepp et al., 2015;Kasper et al., 2015). ...
Two of the largest and deepest endorheic lakes on the Tibetan Plateau, Nam Co and Tangra Yumco, were targeted to infer Late Quaternary (24 ka) responses of their hydrological balance to environmental dynamics. For this purpose, high-resolution multi-proxy records were used to obtain information on lake level variations and how these are related to direct precipitation proxies. Allochthonous minerogenic input as reflected by potassium (K), is directly linked to precipitation driven by variations in Indian Summer Monsoon. Negatively correlated elements such as Ca, Sr and Mg and associated carbonate mineral phases represent autochthonous sediment production, which is coupled to evaporation in these closed systems. δ¹³C and δ¹⁸O of bulk carbonates represent lake water conditions, i.e., salinity and lake water volume, interpreted as precipitation-evaporation (P/E) balance. In combination with geomorphological features, i.e., lake level terraces and paleo-shorelines, the multy-proxy data set from Nam Co is used to reconstruct lake level dynamics. Low lake levels of >90 m below the recent one between 20 and 16 ka cal BP are associated with most enriched δ¹⁸Ocarb values and the precipitation of aragonite and high-Mg calcite in a shallow lake with highly dynamic allochthonous minerogenic input. Similar conditions are observed between 13.1 and 11.4 ka cal BP matching the Younger Dryas chronozone. Between 15.5 and 13.1 ka cal BP as well as during the Early Holocene (11.4 to 8.0 ka cal BP) distinctly depleted δ¹⁸Ocarb and δ¹³Ccarb coupled with high rates of allochthonous minerogenic input suggest substantial strengthening in precipitation. After 9.4 ka cal BP, the terrigenous input and thus precipitation is significantly reduced, whereas δ¹⁸Ocarb reveals only a slight enrichment trend and thus a slowly but steady decrease in lake level througout the Mid- to Late Holocene. The decoupling of this relation between precipitation and lake level suggest that lake level terraces of large endorheic lakes on the TP and also δ¹⁸Ocarb records are not a direct indicator of (monsoonal) precipitiation, whereas the allochtonous minerogenic input can be directly linked to precipitation dynamics. This is supported by the comparison to regional archives from the Arabian Sea and the northern Bay of Bengal, showing the very same pattern in precipitation variability. The presented monsoon precipitation record from Nam Co additionally reveals a link to supra-regional atmospheric circulation components as it is oscillating in- and anti-phase to El-Niño strength and frequency. A distinct precipitation change at 2 ka cal BP seems to have affected all archives, however leaving space for future studies to find the cause of this Late-Holocene climatic shift.
... Oxygen isotope composition has been determined for several Quaternary records in eastern Africa: for example, δ 18 O ice cores (Thompson et al., 2002), δ 18 O carbonate (Lamb et al., 2005), δ 18 O diatom (Barker et al., 2001(Barker et al., , 2007(Barker et al., , 2011Lamb et al., 2005), and δ 18 O sugar (Hepp et al., 2017). δ 18 O sugar has been successfully applied to several other paleolimnological archives (Zech et al., 2014b;Hepp et al., 2015Hepp et al., , 2019. In contrast to this relatively new proxy (Zech and Glaser, 2009), numerous δ 18 O diatom records have been published, proving the potential for reconstructing past climate changes from biogenic silica (δ 18 O diatom ) Barker et al., 2004Barker et al., , 2007Barker et al., , 2011Lamb et al., 2005;Wilson et al., 2014;Narancic et al., 2016;Cartier et al., 2019;Kostrova et al., 2019). ...
In eastern Africa, there are few long, high-quality records of environmental change at high altitudes, inhibiting a broader understanding of regional climate change. We investigated a Holocene lacustrine sediment archive from Lake Garba Guracha, Bale Mountains, Ethiopia, (3,950 m asl), and reconstructed high-altitude lake evaporation history using δ18O records derived from the analysis of compound-specific sugar biomarkers and diatoms. The δ18O-diatom and δ18O-fuc records are clearly correlated and reveal similar ranges (7.9‰ and 7.1‰, respectively). The lowest δ18O values occurred between 10–7 cal ka BP and were followed by a continuous shift towards more positive δ18O values. Due to the aquatic origin of the sugar biomarker and similar trends of δ18O-diatom, we suggest that our lacustrine δ18O-fuc record reflects δ18O-lake water. Therefore, without completely excluding the influence of the ‘amount-effect’ and the ‘source-effect’, we interpret our record to reflect primarily the precipitation-to-evaporation ratio (P/E). We conclude that precipitation increased at the beginning of the Holocene, leading to an overflowing lake between ca. 10 and ca. 8 cal ka BP, indicated by low δ18O-lake water values, which are interpreted as reduced evaporative enrichment. This is followed by a continuous trend towards drier conditions, indicating at least a seasonally closed lake system.
... The δ 2 H of plant waxes, also preserved in lake 60 sediment archives, tracks plant source water δ 2 H and is used to understand precipitation seasonality, 61 precipitation amount, effective precipitation, and relative humidity through time ( 63 Thomas et al., 2020). Recent studies, however, illustrate that some lake sediment-derived plant wax 64 compounds represent a mix of aquatic and terrestrial material (Hepp et al., 2015;Freimuth et al., 2020). 5 been in marine settings with the use of IP25 as a proxy for sea ice (Nichols et al., 1988;Belt et al., 2001; 101 Belt et al., 2007;Belt et al., 2017). ...
The hydrogen isotopic composition of lake water (δ²Hlw) reflects hydrological processes, which can yield information about evaporation and precipitation changes through time when preserved in lake sediment archives. Unfortunately, few proxies exist that record only δ²Hlw. Instead, most δ²Hlw records represent a mix of aquatic and terrestrial material. Highly branched isoprenoids (HBIs), known to be produced by diatoms in marine and lacustrine settings, may be used as a lake water proxy to directly reconstruct hydroclimate, if the hydrogen isotopic composition of HBIs (δ²HHBI) reflects the δ²Hlw. We test this hypothesis by analyzing 78 sediment samples from 12 lakes in the Adirondack Mountains in New York, for HBI concentrations and δ²H. δ²HHBI was compared to δ²Hlw, which showed an average fractionation (εHBI/lw) of −127.3 ± 15.0‰ (1σ) for all samples in all lakes. Consistency in εHBI/lw between samples implies that δ²HHBI may be used to reconstruct δ²Hlw through time, to help assess how lake systems have changed in the past. Sediment samples collected from deeper (>4 m) zones within the lake had smaller variability in εHBI/lw (±11.9‰, 1σ) than samples from shallower zones, suggesting that εHBI/lw may be sensitive to other factors, such as light availability, which may be related to differences in diatom growth habit (e.g., benthic, planktonic). Similarly, the carbon isotopes of HBIs (δ¹³CHBI) were higher for sediment samples collected in deeper zones in the lake, suggesting that δ¹³CHBI can be used to further understand differences in HBI synthesis in diatom communities living in different growth habitats.
... These compounds and their compound-specific stable hydrogen (δ 2 H n-alkane ) and oxygen (δ 18 O sugar ) isotopic composition get incorporated into soils through above-ground and root litter, abrasion, as well as grazing (dung), and they have a mean residence time of ∼40 years (leaf wax n-alkanes), while pentoses (including the hemicellulose-derived sugar arabinose) average over ∼20 years (Schmidt et al., 2011). Since δ 2 H n-alkane and δ 18 O sugar are not strongly affected by degradation effects (Zech et al., 2011(Zech et al., , 2012, they are increasingly used for paleohydrological reconstructions (Aichner et al., 2015;Hepp et al., 2015Hepp et al., , 2019Thomas et al., 2016;Rach et al., 2017;Schäfer et al., 2018;Bliedtner et al., 2020). Usually, they are interpreted to record the isotopic composition of precipitation (Sachse et al., 2006(Sachse et al., , 2012Tuthorn et al., 2015;Hou et al., 2018;Hepp et al., 2020), which in turn is long acknowledged as a valuable proxy for paleoclimate reconstructions and controlled by e.g., the temperature and amount effect, continentality, and altitude (Dansgaard, 1964). ...
Compound-specific hydrogen and oxygen isotope analyzes on leaf wax-derived n-alkanes (δ²Hn–alkane) and the hemicellulose-derived sugar arabinose (δ¹⁸Oara) are valuable, innovative tools for paleohydrological reconstructions. Previous calibration studies have revealed that δ²Hn–alkane and δ¹⁸Oara reflect the isotopic composition of precipitation, but – depending on the region – may be strongly modulated by evapotranspirative enrichment. Since no calibration studies exist for semi-arid and arid Mongolia so far, we have analyzed δ²Hn–alkane and δ¹⁸Oara in topsoils collected along a transect through Mongolia, and we compared these values with the isotopic composition of precipitation (δ²Hp–WM and δ¹⁸Op–WM, modeled data) and various climate parameters. δ²Hn–alkane and δ¹⁸Oara are more positive in the arid south-eastern part of our transect, which reflects the fact that also the precipitation is more enriched in ²H and ¹⁸O along this part of the transect. The apparent fractionation εapp, i.e., the isotopic difference between precipitation and the investigated compounds, shows no strong correlation with climate along the transect (ε2H n–C29/p = −129 ± 14‰, ε2H n–C31/p = −146 ± 14‰, and ε18O ara/p = +44 ± 2‰). Our results suggest that δ²Hn–alkane and δ¹⁸Oara in topsoils from Mongolia reflect the isotopic composition of precipitation and are not strongly modulated by climate. Correlation with the isotopic composition of precipitation has root-mean-square errors of 13.4‰ for δ²Hn–C29, 12.6 for δ²Hn–C31, and 2.2‰ for δ¹⁸Oara, so our findings corroborate the great potential of compound-specific δ²Hn–alkane and δ¹⁸Oara analyzes for paleohydrological research in Mongolia.
... The measurement of the oxygen isotopic composition (here given as the δ 18 O value) of plant-derived compounds is widely used for the reconstruction of climatic, hydrological, and physiological changes. [1][2][3][4][5][6][7][8] Variations in the δ 18 O values of plant compounds are mainly caused by corresponding variations in the δ 18 O values of their source and leaf water. 5,9 The oxygen isotopic composition of leaf water is imprinted on primary assimilates in the leaves via a gem-diol isotope exchange reaction of carbonyl groups of intermediates with the surrounding water. ...
RATIONALE
The oxygen isotopic composition (here shown as the δ¹⁸O value) of soluble sugars in leaves and phloem tissue holds valuable information about plant functions in response to climatic changes. However, δ¹⁸O analysis of sugars is prone to error and thoroughly tested methods are lacking.
METHODS
We performed three experiments to test if sample preparation modifies the δ¹⁸O values of sugars. In experiment 1, we tested effects of oven‐ vs freeze‐drying, while in experiment 2 we focused on the extraction and purification of leaf sugars. In experiment 3, we investigated the exudation and purification of twig phloem sugars as a function of exudation time and different ethylenediaminetetraacetic acid (EDTA) exudation media.
RESULTS
Freeze‐drying produced more consistent δ¹⁸O values than oven‐drying for sucrose, but not for phloem sugars. Extraction and purification of leaf sugars can be performed without a significant modification of its δ¹⁸O values. Yet, purified leaf and phloem sugars had higher δ¹⁸O values than the fraction of water‐soluble compounds, highlighting the necessity for purification of extracted sugars. Moreover, the exudation time significantly modulated the δ¹⁸O values of phloem sugars, which is probably related to changes in sugar composition. EDTA addition did not improve the determination of phloem sugars’ δ¹⁸O values.
CONCLUSIONS
We show that sample preparation of plant sugars for reliable determination of δ¹⁸O values requires a strict protocol that we provide in this manuscript. We recommend for phloem sugar a maximum exudation time of one hour to reduce the degradation of sucrose and minimise oxygen isotope exchange reactions between the resulting hexoses and water.
... Another approach to reconstruct air humidity is based on coupling δ 2 H n-alkanes and δ 18 O sugar results ('paleohygrometer') and was validated by Tuthorn et al. (2015) on topsoils from South America and recently by Hepp et al. (2019a) in a climate chamber experiment. Both approaches have already been applied successfully for paleoclimate reconstructions, for example in loess-paleosol sequences (Hepp et al., 2017;Zech et al., 2013), on subfossil wood (Voelker et al., 2015) and in lacustrine sediments (Hepp et al., 2015(Hepp et al., , 2019b. ...
The hydrogen isotopic composition of leaf wax-derived n-alkanes (δ2Hn-alkanes) is a widely applied proxy for (paleo)climatic changes. It has been suggested that the coupling with the oxygen isotopic composition of hemicellulose-derived sugars (δ18Osugar) - an approach dubbed 'paleohygrometer' - might allow more robust and quantitative (paleo)hydrological reconstructions. However, the paleohygrometer remains to be evaluated and tested regionally. In this study, topsoil samples from South Africa, covering extensive environmental gradients, are analysed. δ2Hn-alkanes correlates significantly with the isotopic composition of precipitation (δ2Hp), whereas no significant correlation exists between δ18Osugar and δ18Op. The apparent fractionation (εapp) is the difference between δ2Hn-alkanes and δ2Hp (εapp 2H) and δ18Osugar and δ18Op (εapp 18O), respectively, and integrates i) isotopic enrichment due to soil water evaporation, ii) leaf (and xylem) water transpiration and iii) biosynthetic fractionation. We find no correlation of εapp 18O nor for εapp 2H with temperature, and no correlation of εapp 2H with potential evapotranspiration and an aridity index. By contrast, εapp 18O correlates significantly with both potential evapotranspiration and the aridity index. This highlights the strong effect of evapotranspirative enrichment on δ18Osugar. In study areas without plant predominance using Crassulacean Acid Metabolism (CAM), coupling δ18Osugar and δ2Hn-alkanes enables to reconstruct δ2Hp and δ18Op with an offset of Δδ2H = 6 ± 27‰ and Δδ18O = 0.8 ± 3.7‰, respectively, as well as relative humidity (RH) with an offset of ΔRH = 6 ± 17%. The paleohygrometer does, however, not work well for our study areas where CAM plants prevail (reconstructed δ18Op, δ2Hp and RH are off by 3.1‰, 27.2‰ and 31.7%). This probably reflects plant-specific (phenological) adaptations and/or post-photosynthetic exchange reactions related to CAM metabolism. Overall, our findings corroborate that δ2Hn-alkanes and δ18Osugar are valuable proxies, and the paleohygrometer is a promising approach for paleoclimate reconstructions in southern Africa.
... Thus, our results suggest δ 18 O analysis of sucrose as the most sensitive compound that can be traced throughout the plant for reconstruction of climatic and hydrological conditions (Gessler et al., 2013;Treydte et al., 2014). Hepp et al., 2015;Zech et al., 2013), measuring the δ 18 O of sucrose and its incorporation into tree-ring cellulose along vertical gradients within individual trees might be a good starting point to trace the isotopic signal of water vapour and its environmental-hydrological information (e.g., weather and climatic conditions and atmospheric circulations patterns). Given the close relationship between oxygen and hydrogen isotopes after fog exposure ( Figure S1), water vapour-induced changes in δ 2 H values of leaf water might be also imprinted on δ 2 H biomarker such as fatty acids or n-alkanes (Cormier et al., 2018;Gamarra, Sachse, & Kahmen, 2016;Sachse et al., 2012), providing a new avenue for the reconstruction of hydrological information. ...
The 18O‐signature of atmospheric water vapour (δ18OV) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the 18O‐signal transfer differs among plant species and growth forms. We performed a 9 h greenhouse fog experiment (relative humidity ≥ 98%) with 18O‐depleted water vapour (‐106.7‰) on 140 plant species of 8 different growth forms during daytime. We quantified the 18O‐signal transfer by calculating the mean residence time of O in leaf water (MRTLW) and sugars (MRTSugars) and related it to leaf traits and physiological drivers. MRTLW increased with leaf succulence and thickness, varying between 1.4 and 10.8 h. MRTSugars was shorter in C3 and C4 plants than in CAM plants and highly variable among species and growth forms; MRTSugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs and herbs, and longest for conifers, epiphytes and succulents. Sucrose was more sensitive to δ18OV variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ18OV variations, which is important for the interpretation of δ18O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.
... Lately, more attention has been paid to d-excess change in a wide variety of paleoclimatic archives, such as ice cores, speleothems, and lake sediments in order to re-construct source precipitation conditions, coupled with subsequent analysis of potential air mass back trajectories [61][62][63][64][65][66] . Deuterium excess values may deviate from +10 (‰) due to the combination of three factors: a) a relative humidity (RH) increase in the precipitation source, b) a decrease in SST, and c) greater wind speeds (>7 ms −1 ) affecting evaporation and subsequent kinetic fractionation 44 . ...
The Mesoamerican and Caribbean (MAC) region is characterized by tropical cyclones (TCs), strong El Niño-Southern Oscillation events, and climate variability that bring unique hazards to socio-ecological systems. Here we report the first characterization of the isotopic evolution of a TC (Hurricane Otto, 2016) in the MAC region. We use long-term daily rainfall isotopes from Costa Rica and event-based sampling of Hurricanes Irma and Maria (2017), to underpin the dynamical drivers of TC isotope ratios. During Hurricane Otto, rainfall exhibited a large isotopic range, comparable to the annual isotopic cycle. As Hurricane Otto organized into a Category 3, rapid isotopic depletion coupled with a decrease in d-excess indicates efficient isotopic fractionation within ~200 km SW of the warm core. Our results shed light on key processes governing rainfall isotope ratios in the MAC region during continental and maritime TC tracks, with applications to the interpretation of paleo-hydroclimate across the tropics.
... Lately, more attention has been paid to d-excess change in a wide variety of paleoclimatic archives, such as ice cores, speleothems, and lake sediments in order to re-construct source precipitation conditions, coupled with subsequent analysis of potential air mass back trajectories [61][62][63][64][65][66] . Deuterium excess values may deviate from +10 (‰) due to the combination of three factors: a) a relative humidity (RH) increase in the precipitation source, b) a decrease in SST, and c) greater wind speeds (>7 ms −1 ) affecting evaporation and subsequent kinetic fractionation 44 . ...
The Mesoamerican and Caribbean (MAC) region is characterized by tropical cyclones (TCs), strong El Niño-Southern Oscillation events, and climate variability that bring unique hazards to socio-ecological systems. Here we report the first characterization of the isotopic evolution of a TC (Hurricane Otto, 2016) in the MAC region. We use long-term daily rainfall isotopes from Costa Rica and event-based sampling of Hurricanes Irma and Maria (2017), to underpin the dynamical drivers of TC isotope ratios. During Hurricane Otto, rainfall exhibited a large isotopic range, comparable to the annual isotopic cycle. As Hurricane Otto organized into a Category 3, rapid isotopic depletion coupled with a decrease in d-excess indicates efficient isotopic fractionation within ~200 km SW of the warm core. Our results shed light on key processes governing rainfall isotope ratios in the MAC region during continental and maritime TC tracks, with applications to the interpretation of paleo-hydroclimate across the tropics.
... Lately, more attention has been paid to d-excess change in a wide variety of paleoclimatic archives, such as ice cores, speleothems, and lake sediments in order to re-construct source precipitation conditions, coupled with subsequent analysis of potential air mass back trajectories [61][62][63][64][65][66] . Deuterium excess values may deviate from +10 (‰) due to the combination of three factors: a) a relative humidity (RH) increase in the precipitation source, b) a decrease in SST, and c) greater wind speeds (>7 ms −1 ) affecting evaporation and subsequent kinetic fractionation 44 . ...
... Deuterium excess was calculated as d-excess = δ 2 H -8·δ 18 O 26 . Lately, more attention has been paid to d-excess change in a wide variety of paleoclimatic archives, such as ice cores, speleothems, and lake sediments in order to re-construct source precipitation conditions, coupled with subsequent analysis of potential air mass back trajectories [61][62][63][64][65][66] . Deuterium excess values may deviate from +10 (‰) due to the combination of three factors: a) a relative humidity (RH) increase in the precipitation source, b) a decrease in SST, and c) greater wind speeds (>7 ms −1 ) affecting evaporation and subsequent kinetic fractionation 44 . ...
The Mesoamerican and Caribbean (MAC) region is characterized by tropical cyclones (TCs), strong El Niño-Southern Oscillation events, and climate variability that bring unique hazards to socio-ecological systems. Here we report the first characterization of the isotopic evolution of a TC (Hurricane Otto, 2016) in the MAC region. We use long-term daily rainfall isotopes from Costa Rica and event-based sampling of Hurricanes Irma and Maria (2017), to underpin the dynamical drivers of TC isotope ratios. During Hurricane Otto, rainfall exhibited a large isotopic range, comparable to the annual isotopic cycle. As Hurricane Otto organized into a Category 3, rapid isotopic depletion coupled with a decrease in d-excess indicates efficient isotopic fractionation within ~200 km SW of the warm core. Our results shed light on key processes governing rainfall isotope ratios in the MAC region during continental and maritime TC tracks, with applications to the interpretation of paleo-hydroclimate across the tropics.
... Analogous to δ 2 H n-alkane , δ 18 O sugar is affected by the isotope composition of source water, which is closely related to the local precipi-tation (δ 18 O prec ) as well as by evapotranspirative 18 O enrichment (Tuthorn et al., 2014;Zech et al., 2013bZech et al., , 2014b. Moreover, it was suggested that the coupling of δ 2 H and δ 18 O results can help to disentangle δ 2 H/δ 18 O prec changes and variable 2 H/ 18 O leaf/lake-water enrichment (Henderson et al., 2010;Hepp et al., 2015Hepp et al., , 2017Tuthorn et al., 2015;Voelker et al., 2014Voelker et al., , 2015Zech et al., 2013a). For instance, Voelker et al. (2014) presented a framework for using δ 2 H and δ 18 O of tree-ring cellulose in order to infer relative air humidity (RH). ...
Causes of the Late Glacial to Early Holocene transition phase and particularly the Younger Dryas period, i.e. the major last cold spell in central Europe during the Late Glacial, are considered to be keys for understanding rapid natural climate change in the past. The sediments from maar lakes in the Eifel, Germany, have turned out to be valuable archives for recording such paleoenvironmental changes.
For this study, we investigated a Late Glacial to Early Holocene sediment core that was retrieved from the Gemündener Maar in the Western Eifel, Germany. We analysed the hydrogen (δ2H) and oxygen (δ18O) stable isotope composition of leaf-wax-derived lipid biomarkers (n-alkanes C27 and C29) and a hemicellulose-derived sugar biomarker (arabinose), respectively. Both δ2Hn-alkane and δ18Osugar are suggested to reflect mainly leaf water of vegetation growing in the catchment of the Gemündener Maar. Leaf water reflects δ2H and δ18O of precipitation (primarily temperature-dependent) modified by evapotranspirative enrichment of leaf water due to transpiration. Based on the notion that the evapotranspirative enrichment depends primarily on relative humidity (RH), we apply a previously introduced “coupled δ2Hn-alkane–δ18Osugar paleohygrometer approach” to reconstruct the deuterium excess of leaf water and in turn Late Glacial–Early Holocene RH changes from our Gemündener Maar record.
Our results do not provide evidence for overall markedly dry climatic conditions having prevailed during the Younger Dryas. Rather, a two-phasing of the Younger Dryas is supported, with moderate wet conditions at the Allerød level during the first half and drier conditions during the second half of the Younger Dryas. Moreover, our results suggest that the amplitude of RH changes during the Early Holocene was more pronounced than during the Younger Dryas. This included the occurrence of a “Preboreal Humid Phase”. One possible explanation for this unexpected finding could be that solar activity is a hitherto underestimated driver of central European RH changes in the past.
... The occurrence of a temperature effect on oxygen-isotope fractionation during cellulose formation is still discussed (Sternberg and Ellsworth, 2011;Mayr et al., 2013). A potential methodological extension is the recent development of an analytical procedure for d 18 O analyses on hemicellulose-derived sugar biomarkers (Zech et al., 2014;Hepp et al., 2015). ...
The stable isotope composition of key chemical elements for life on Earth (e.g., carbon, hydrogen, nitrogen, oxygen, sulfur) tracks changes in fluxes and turnover of these elements in the biogeosphere. Over the past 15-20 years, the potential to measure these isotopic compositions for individual, source-specific organic molecules (biomarkers) and to link them to a range of environmental conditions and processes has been unlocked and amplified by increasingly sensitive, affordable and wide-spread analytical technology. Paleoenvironmental research has seen enormous step-changes in our understanding of past ecosystem dynamics. Vital to these paradigm shifts is the need for well-constrained modern and recent analogues. Through increased understanding of these environments and their biological pathways we can successfully unravel past climatic changes and associated ecosystem adaption.
With this review, we aim to introduce scientists working in the field of Quaternary paleolimnology to the tools that compound-specific isotope analysis (CSIA) provides for the gain of information on biogeochemical conditions in ancient environments. We provide information on fundamental principles and applications of novel and established CSIA applications based on the carbon, hydrogen, nitrogen, oxygen and sulfur isotopic composition of biomarkers. While biosynthesis, sources and associated isotope fractionation patterns of compounds such as n-alkanes are relatively well-constrained, new applications emerge from the increasing use of functionalized alkyl lipids, steroids, hopanoids, isoprenoids, GDGTs, pigments or cellulose. Biosynthesis and fractionation are not always fully understood.
... Finally, our findings have important implications for ecophysiological and hydrological sciences using oxygen (and potentially also hydrogen) isotopes of various plant biomarkers such as cellulose, hemicellulose and n-alkanes to decipher temporally and spatially integrated information about past environmental conditions and associated plant functional responses (Hepp et al., 2015;Tuthorn et al., 2015). This is especially important for organic molecules derived from plants, sediments, or soils in regions and seasons that experience high humidity conditions (e.g. ...
Our understanding of how temporal variations of atmospheric water vapour and its isotopic composition (δ ¹⁸ O V ) influence water and assimilates in plants remains limited, restricting our ability to use δ ¹⁸ O as a tracer of ecophysiological processes.
We exposed oak ( Quercus robur ) saplings under wet and dry soil moisture conditions to ¹⁸ O‐depleted water vapour ( c . − 200‰) at high relative humidity ( c . 93%) for 5 h, simulating a fog event. We then traced the step change in δ ¹⁸ O V into water and assimilates (e.g. sucrose, hexoses, quercitol and starch) in the leaf lamina, main veins and twigs over 24 h.
The immediate δ ¹⁸ O V effect was highest for δ ¹⁸ O of leaf lamina water, but 40% lower on δ ¹⁸ O of main vein water. To a smaller extent, we also observed changes in δ ¹⁸ O of twig xylem water. Depending on the individual assimilation rate of each plant, the ¹⁸ O‐label was partitioned among different assimilates, with highest changes in δ ¹⁸ O of starch/sucrose and lowest in δ ¹⁸ O of quercitol. Additionally, ¹⁸ O‐label partitioning and allocation towards leaf starch and twig phloem sugars was influenced by the plant water status.
Our results have important implications for water isotope heterogeneity in plants and for our understanding of how the δ ¹⁸ O signal is incorporated into biomarkers.
... Lately, more attention has been paid to d-excess changes in a wide variety of paleoclimatic archives, such as ice cores (Klein et al., 2016), speleothems (Demény et al., 2013), and lake sediments (Hepp et al., 2015) in order to reconstruct source precipitation conditions coupled with subsequent analysis of potential air mass back trajectories (Pfahl & Wernli, 2008;Risi, Landais, Winkler, & Vimeux, 2013). Jouzel et al. (2013) explained that d-excess values may deviate from +10 (‰) due to the combination of three factors: a) a relative humidity (RH) increase in the precipitation source, b) a decrease in sea surface temperature (SST), and c) greater wind speeds (>7 ms -1 ) affecting the evaporation regime and subsequent kinetic fractionation. ...
... Lately, more attention has been paid to d-excess changes in a wide variety of paleoclimatic archives, such as ice cores (Klein et al., 2016), speleothems (Demény et al., 2013), and lake sediments (Hepp et al., 2015) in order to reconstruct source precipitation conditions coupled with subsequent analysis of potential air mass back trajectories (Pfahl and Wernli, 2008;Risi et al., 2013). Jouzel et al. (2013) explained that d-excess values may deviate from +10 (‰) due to the combination of three factors: a) a relative humidity (RH) increase in the precipitation source, b) a decrease in sea surface temperature (SST), and c) greater wind speeds (>7 ms -1 ) affecting the evaporation regime and subsequent kinetic fractionation. ...
The last 2014-16 El Niño event was among the three strongest episodes on record. El Niño considerably changes annual and seasonal precipitation across the tropics. Here, we present a unique stable isotope data set of daily precipitation collected in Costa Rica prior to, during, and after El Niño 2014-16, in combination with Lagrangian moisture source and precipitation anomaly diagnostics. δ2H composition ranged from -129.4 to +18.1 (‰) while δ18O ranged from -17.3 to +1.0 (‰). No significant difference was observed among δ18O (P = 0.0186) and δ2H (P = 0.664) mean annual compositions. However, mean annual d-excess showed a significant decreasing trend (from +13.3 to +8.7 ‰) (P < 0.001) with values ranging from +26.6 to -13.9 ‰ prior to and during the El Niño evolution. The latter decrease in d-excess can be partly explained by an enhanced moisture flux convergence across the southeastern Caribbean Sea coupled with moisture transport from northern South America by means of an increased Caribbean Low Level Jet regime. During 2014-15, precipitation deficit across the Pacific domain averaged 46% resulting in a very severe drought; while a 94% precipitation surplus was observed in the Caribbean domain. Understanding these regional moisture transport mechanisms during a strong El Niño event may contribute to a) better understanding of precipitation anomalies in the tropics and b) re-evaluate past stable isotope interpretations of ENSO events in paleoclimatic archives within the Central America region. This article is protected by copyright. All rights reserved.
... In previous studies we found that the hemicellulose-derived neutral sugars arabinose (ara) and xylose (xyl) strongly predominate over fucose (fuc) in terrestrial soils and sediments (e.g. . By contrast, we found fuc to occur in similar abundance to ara and xyl in lacustrine sediments (Zech et al., 2014b;Hepp et al., 2015). We have therefore investigated and reviewed here neutral sugar patterns of various terrestrial and aquatic plants, including mosses as well as algal species. ...
One of the most important and at the same time most challenging issues in paleolimnological research is the differentiation between terrestrial and aquatic sedimentary organic matter (OM). We therefore investigated the relative abundance of the sugars fucose (fuc), arabinose (ara) and xylose (xyl) from various terrestrial and aquatic plants, as well as from algal samples. Algae were characterized by a higher abundance of fucose than vascular plants. Our results and a compilation of data from the literature suggest that fuc/(ara+xyl) and (fuc+xyl)/ara ratios may serve as complementary proxies in paleolimnological studies for distinguishing between terrestrial and aquatic sedimentary OM.
... Also climatically driven changes of deuterium content in local precipitation as well as changes of temperature with altitude need to be taken into account. A way out of this dilemma could be a coupled δ 2 H-δ 18 O approach in terrestrial and lacustrine paleoclimate studies as suggested by Zech et al. [67], Hepp et al. [82] and Tuthorn et al. [83]. ...
Hydrogen isotope compositions (δD) of lipids are widely used in hydrological research, yet there have been limited investigations into the relationships between salinity and δD values of aquatic plants in natural lakes, which hinders the broader application of δD compositions as indicators of paleosalinity reconstructions. In this study, we examined δD compositions of n-alkanes in aquatic plants and lake water from 58 lakes (containing 18 brackish/saline lakes) across diverse regions of China (including the Tibetan Plateau, Inner Mongolia Plateau, Chinese Loess Plateau, Tarim Basin, and Yangtze Plain). Subsequently, we examined the correlations between salinity and n-alkane δD values in aquatic plants. Our findings indicate that δD values of n-alkanes from aquatic plants record variations in lake water δD (R2 = 0.53, p < 0.01). Furthermore, δD values of lake water in brackish/saline lakes exhibit a positive correlation with salinity (R2 = 0.74, p < 0.01). Thus, a positive correlation (R2 = 0.68, p < 0.01) is observed between δD values of n-alkanes from aquatic plants and lake salinity, whose changes both co-vary with lake water δD variations. Therefore, n-alkane δD values of aquatic plants can serve as indirect indicators of salinity for reconstructing paleosalinity in brackish/saline lakes. It is important to note that paleosalinity reconstruction in lakes using n-alkane δD values is generally more effective when lake salinity experiences large variations, whereas caution should be exercised when lake salinity changes are more subtle.
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 (εalk/precipitation, εalk/soil water, εalk/leaf water and εalk/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 εalk/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 (εalk/leaf water < εalk/soil water < εalk/precipitation). The εalk/lake water values of submerged plants display a smaller range (−153 ± 5 ‰) than the εalk/precipitation values of terrestrial plants, which is close to the terrestrial εalk/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 εalk/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.
The Late Quaternary climate history of South Africa and, in particular, potential changes in atmospheric circulation have been subject to considerable debate. To some extent, this is due to a scarcity of natural archives, and on the other hand the available indirect hydrological proxies are not suited to distinguishing between precipitation originating from temperate Westerlies and tropical Easterlies. This study presents a paleoenvironmental record from Vankervelsvlei, a wetland located on the southern Cape coast in the year-round rainfall zone of South Africa. A 15 m long sediment record was retrieved from this site and analysed using a multi-proxy approach. This includes, for the first time in this region, analysis of both compound-specific δ²H and δ¹⁸O from leaf wax-derived n-alkanes and hemicellulose-derived sugars, respectively, to investigate hydrological changes during the Late Quaternary. Our data suggest the driest conditions of the past ∼250 ka likely occurred from MIS 6 to MIS 5e, which still caused sediment deposition at Vankervelsvlei, and MIS 2, during which time there was an absence of sedimentation. Moist conditions occurred from MIS 5e to 5a and during parts of MIS 3, while drier conditions prevailed between MIS 5a and early MIS 3 and at the transition from MIS 3 to MIS 2. Besides changes in the amount and proportional contribution of precipitation contributions from Westerlies during glacial and Easterlies during interglacial periods, relative sea-level change affected the continentality of Vankervelsvlei, with a distinct impact on the sites hydrological balance. High-resolution analyses of the Middle- and Late-Holocene parts of the record show moist conditions and increased Easterly/locally-derived summer precipitation contributions from 7230 ⁺¹⁶⁰/−210 to 4890 ⁺²⁸⁰/−180 cal BP and after 2840 ⁺³⁵⁰/−330 cal BP. Dry conditions, accompanied by the reduction of Easterly/locally-derived summer rainfall and increased seasonality occurred from 4890 ⁺²⁸⁰/−180 to 2840 ⁺³⁵⁰/−330 cal BP. Our findings highlight that source and seasonality of precipitation play a major role in the hydrological balance of the southern Cape coastal region. By comparing the Vankervelsvlei record to other regional studies, we infer a coherent trend in the overall moisture evolution along South Africa's southern Cape coast during the Late Quaternary.
The hydrogen isotopic composition of leaf wax-derived biomarkers, e.g. long chain n-alkanes (δ2Hn-alkane), is widely applied in paleoclimatology research. However, a direct reconstruction of the isotopic composition of paleoprecipitation based on δ2Hn-alkane alone can be challenging due to the overprint of the source water isotopic signal by leaf-water enrichment. The coupling of δ2Hn-alkane with δ18O of hemicellulose-derived sugars (δ18Osugar) has the potential to disentangle this effect and additionally allow relative humidity reconstructions. Here, we present δ2Hn-alkane as well as δ18Osugar results obtained from leaves of the plant species Eucalyptus globulus, Vicia faba var. minor and Brassica oleracea var. medullosa, which were grown under controlled conditions. We addressed the questions (i) do δ2Hn-alkane and δ18Osugar values allow precise reconstructions of leaf water isotope composition, (ii) how accurately does the reconstructed leaf-water-isotope composition enables relative humidity (RH) reconstruction in which the plants grew, and (iii) does the coupling of δ2Hn-alkane and δ18Osugar enable a robust source water calculation? For all investigated species, the alkane n-C29 was most abundant and therefore used for compound-specific δ2H measurements. For Vicia faba, additionally the δ2H values of n-C31 could be evaluated robustly. With regard to hemicellulose-derived monosaccharides, arabinose and xylose were most abundant and their δ18O values were therefore used to calculate weighted mean leaf δ18Osugar values. Both δ2Hn-alkane and δ18Osugar yielded significant correlations with δ2Hleaf-water and δ18Oleaf-water, respectively (r2 = 0.45 and 0.85, respectively; p
Sedimentary plant wax distributions and isotopic compositions are powerful, widely applied paleoenvironmental proxies. However, there is conflicting evidence on the behavior of these proxies at high-latitude sites, where extreme climate and light conditions may uniquely influence plant physiology and growth. Here, we present modern sedimentary n-alkane and n-alkanoic acid abundances and compound-specific (δ²H and δ¹³C) isotope values from a 22-lake transect extending from northwest to southernmost Greenland, covering a large latitudinal and climatic gradient. Sedimentary plant waxes are similar in abundance and carbon isotopic composition across the transect, suggesting no major differences in biologic sources. There are strong correlations (r = 0.8–0.9) between δ²H values of many long-chain sedimentary waxes and those of modelled precipitation, with n-alkanes more tightly correlated to precipitation than n-alkanoic acids. Data presented here also demonstrate that δ²H values of mid-chain sedimentary waxes do not strongly correlate to the δ²H values of lake water when it decouples isotopically from precipitation (i.e. in glacier-fed and evaporatively-enriched lakes). This calls into question the common interpretation that mid-chain sedimentary waxes can be ascribed to aquatic plants. We contextualize our Greenland data with an updated global dataset of δ²H values of modern sedimentary waxes and precipitation. This update adds 100 + lakes from recently published literature to the seminal review presented by Sachse et al. (2012). This large new compilation suggests a global average apparent fractionation including Arctic data between n-C28 alkanoic acids and annual precipitation (εC28/ANN) of −99‰, and between n-C29 alkanes and annual precipitation (εC29/ANN) of −121‰. The latter value is remarkably consistent with the value first reported by Sachse et al. (2012).
As one proxy of water evolution, hydrologic cycle and water balance in lakes, the stable isotopes of hydrogen and oxygen (δD and δ ¹⁸ O) had been wildly utilized during lake researching. However, one problem is that a few works based on stable isotopes of lake water had been done in Cold-Arid regions of China. In this paper, to analyze the relationships of δD and δ ¹⁸ O and their feedbacks on precipitation process and vaporization process, totally 144 samples of lake water, river water and precipitation water had been collected in summer Dali-nor Lake, which locates in Cold-Arid area of Inner Mongolia, North China. Meanwhile, the typical physical and chemical proxies of lake water had been measured, such as temperature, salinity, pH value, and so on. By comparing the values of δD and δ ¹⁸ O among different types of sample, we concluded that: The values of classical physical and chemical proxies of lake water offset a certain degree following depth change, and are also a little different between different regions at the same water depth. But there are no clearly fluctuations appeared, such as thermocline and halocline, in summer Dali-nor Lake. As a whole, the characteristics of water mass are essentially stable in summer Dali-nor Lake. But, the values of δD and δ ¹⁸ O both become lighter and lighter following water depth changing. For example, the value of δD and δ ¹⁸ O are about -31.723‰ and -1.785‰ in surface water, and about -31.776‰ and -1.838‰ in bottom water, respectively. The meteoric water line is δD=8.22 δ ¹⁸ O+6.82 (R ² =0.936, n=42), which shows that the change of δD and δ ¹⁸ O in precipitation had influenced both by rainfall process and vaporization process in Dali-nor Lake. The precipitation may have stronger influence on fractional distillation of δD and δ ¹⁸ O than evaporation process in July and August. Correspondingly, the evaporation process may have stronger influence on fractional distillation of δD and δ ¹⁸ O than precipitation process in June and September. As a result, the relationships of δD and δ ¹⁸ O in surface water and bottom water are different in summer Dali-nor Lake. The relationship in surface water is nearly with those in precipitation and river water, but the relationship of bottom water is close to those in groundwater. Resumptively, the value changes of δD and δ ¹⁸ O in surface water might be influenced mainly by evaporation process and precipitation process, and those might be influenced mainly by groundwater input in deeper water.
The Late Glacial to Early Holocene transition phase and particularly the Younger Dryas period, i.e. the major last cold spell in Central Europe during the Late Glacial, are considered crucial for understanding rapid natural climate change in the past. The sediments from Maar lakes in the Eifel, Germany, have turned out to be valuable archives for recording such paleoenvironmental changes.
For this study, we investigated a Late Glacial to Early Holocene sediment core that was retrieved from Lake Gemündener Maar in the Western Eifel, Germany. We analysed the hydrogen (δ²H) and oxygen (δ¹⁸O) stable isotope composition of leaf wax-derived lipid biomarkers (n-alkanes C27 and C29) and hemicellulose-derived sugar biomarkers (arabinose), respectively. Both δ²H and δ¹⁸O are suggested to reflect mainly leaf water of vegetation growing in the catchment of the Gemündener Maar. This enables the coupling of the results via a δ²H-δ¹⁸O biomarker paleohygrometer approach and allows calculating past relative air humidity values, which is the major advantage of the applied approach. Fundamental was the finding that the isotopic enrichment of leaf water due to evapotranspiration depends mainly on relative humidity. We hence use the coupled δ²H-δ¹⁸O biomarker approach to reconstruct the deuterium-excess of leaf water and in turn relative air humidity values corresponding to the vegetation period and daytime (RHdv).
Most importantly, the results of the coupled δ²H-δ¹⁸O biomarker paleohygrometer approach (i) support a two-phasing of the Younger Dryas, i.e. a relative wet phase (on Allerød level) followed by a drier Younger Dryas ending, (ii) do not corroborate overall drier climatic conditions characterising the Younger Dryas or a two-phasing with regard to a first dry and cold Younger Dryas phase followed by a warmer period along with increasing precipitation amounts, and (iii) suggest that the amplitude of RHdv changes during the Early Holocene was more pronounced compared to the Younger Dryas. One possible driver for the unexpected Lake Gemündener Maar RHdv variations could be the solar activity.
We present the results of leaf wax analyses (long-chain n-alkanes) from the 6.8 m deep loess sequence
of Möhlin, Switzerland, spanning the last ∼ 70 kyr. Leaf waxes are well preserved and occur in sufficient amounts only down to 0.4 m and below 1.8 m depth, so no paleoenvironmental reconstructions
can be done for marine isotope stage (MIS) 2.
Compound-specific δ
2Hwax analyses yielded similar values for late MIS 3 compared to the uppermost
samples, indicating that various effects (e.g., more negative values due to lower temperatures,
more positive values due to an enriched moisture source) cancel each other out. A pronounced ∼ 30 ‰
shift towards more negative values probably reflects more humid conditions before ∼ 32 ka. Radiocarbon
dating of the n-alkanes corroborates the stratigraphic integrity of leaf waxes and their potential
for dating loess–paleosol sequences (LPS) back to ∼ 30 ka.
During the last decade compound-specific D/H analysis of plant
leafwax-derived n-alkanes has become a promising and popular tool in
paleoclimate research. This is based amongst other factors mainly on the
finding that sedimentary n-alkanes primarily reflect the D/H isotopic
composition of precipitation or lake water. Recently, several authors
also suggested that D/H of n-alkanes can be used as proxy in
paleoaltimetry studies, because they found soil n-alkanes reflecting the
'altitude effect' of D/H in precipitation. Here we present results from
a D/H transect study (1700 to 4000 m a.s.l.) carried out on the humid
south-western slopes of Mt. Kilimanjaro. While D/H of precipitation
shows the expected altitude effect, i.e. more negative deltaD values
with increasing altitude, deltaD of nC29 and nC31 do not confirm this
trend or even become more positive both in the O-layers (organic layers)
and the Ah-horizons (mineral topsoils). Furthermore, deltaD of nC29 and
nC31 are similar, but not identical (R2 = 0.22 and 0.34 for the O-layers
and the Ah-horizons, respectively). Although overall our D/H n-alkane
results are in agreement with the results of Peterse et al., (2009,
Biogeosciences) reported for the south-eastern slopes of Mt.
Kilimanjaro, we suggest a re-interpretation. We explain the lacking
correlation of D/H(alkanes) with D/H(precipitation) with other factors
influencing D/H(alkanes) of plants and soils. First, large interspecies
offsets in net fractionation are reported. Indeed, plant communities
along the slopes of Mt. Kilimanjaro as well as along most altitude
transects change dramatically. Second, D-enrichment by
evapotranspiration (soil water and plant leaf-water), depending mainly
on relative air humidity (RH) is neither constant along altitude
transects nor over time. And third, soil n-alkanes do not only derive
from plant litter but also from soil microorganisms (Zech et al., 2011,
GCA), potentially adulterating the original plant D/H signal. Given that
our results highlight that n-alkanes in soils do not necessarily reflect
the D/H isotopic composition of precipitation, we conclude that care has
to be taken not to over-interpret D/H records from soils and sediments
when reconstruction D/H of paleoprecipitation. Concerning paleoaltimetry
studies, they additionally would require that D/H of paleoprecipitation
did not change significantly. Recalling the temperature-, the amount-
and the source-effect, we doubt this is case.
In 2003 sediment core Lz1024 was drilled at Lake El'gygytgyn, far east Russian Arctic, in an area of the Northern Hemisphere which has not been glaciated for the last 3.6 Ma. Biogenic silica was used for analysing the oxygen isotope composition (δ18Odiatom) in the upper 13 m long section dating back about 250 ka with samples dominated by one taxa in the <10 μm fraction ( Cyclotella ocellata ). Downcore variations in δ18O values show that glacial-interglacial cycles are present throughout the core and δ18Odiatom values are mainly controlled by δ18Oprecipitation. Changes reflect the Holocene Thermal Maximum, the Last Glacial Maximum and the interglacial periods corresponding to MIS 5e and MIS 7 with a peak-to-peak amplitude of δ18O = 5.3 ‰. Our record is the first continuous δ18Odiatom record from an Arctic lake sediment core directly responding to precipitation and dating back more than 250 ka and correlates well with the stacked marine δ18O LR04 ( r = 0.58) and δD EPICA Dome-C record ( r = 0.69). With δ18O results indicating strong links to both marine and ice-core records, records from Lake El'gygytgyn can be used to further investigate the sensitivity of the Arctic climate to both past and future global climatic changes.
Because of the scarcity of meteorological observations, the precipitation climate on the Tibetan Plateau and surrounding regions (TP) has been insufficiently documented so far. In this study, the characteristics and basic features of precipitation on the TP during an 11-yr period (2001-11) are described on monthly-to-annual time scales. For this purpose, a new high-resolution atmospheric dataset is analyzed, the High Asia Reanalysis (HAR), generated by dynamical downscaling of global analysis data using the Weather Research and Forecasting (WRF) model. The HAR precipitation data at 30- and 10-km resolutions are compared with both rain gauge observations and satellite-based precipitation estimates from the Tropical Rainfall Measurement Mission (TRMM). It is found that the HAR reproduces previously reported spatial patterns and seasonality of precipitation and that the high-resolution data add value regarding snowfall retrieval, precipitation frequency, and orographic precipitation. It is demonstrated that this process-based approach, despite some unavoidable shortcomings, can improve the understanding of the processes that lead to precipitation on the TP. Analysis focuses on precipitation amounts, type, seasonality, and interannual variability. Special attention is given to the links between the observed patterns and regional atmospheric circulation. As an example of an application of the HAR, a new classification of glaciers on the TP according to their accumulation regimes is proposed, which illustrates the strong spatial variability of precipitation seasonality. Finally, directions for future research are identified based on the HAR, which has the potential to be a useful dataset for climate, glaciological, and hydrological impact studies.
We investigated a late glacial–Holocene lacustrine sediment archive located at 4,050 m a.s.l. in the small carbonate-free catchment of Lake Panch Pokhari, Helambu Himal, Nepal. A δ18O sugar biomarker record was established by applying novel compound-specific δ18O analysis of plant sugar biomarkers (Zech and Glaser in Rapid Commun Mass Spectrom 23:3522–3532, 2009). This method overcomes analytical challenges such as extraction and purification faced by previous methods aimed at using δ18O of aquatic cellulose as a paleoclimate proxy. The δ18O results for sugar biomarkers arabinose, xylose and fucose agree well and reveal a pronounced trend towards lower δ18O values during the deglaciation and the onset of the Bølling/Allerød interstadial. By contrast, the period of the Younger Dryas is characterized by higher δ18O values. The early Holocene again reveals lower δ18O values. We suggest that our lacustrine δ18O record reflects coupled hydrological and thermal control. It is strongly related to changes in the oxygen isotopic composition of paleo-precipitation and resembles the δ18O records of Asian speleothems. With respect to the ‘amount effect,’ the record is interpreted as reflecting the Indian Summer Monsoon intensity. The precipitation signal is, however, amplified in our record by evaporative 18O enrichment that is controlled by the ratio of precipitation to evaporation. We suggest that our δ18O record reflects the variability of the Indian Summer Monsoon, which was strong during the Bølling/Allerød interstadial and early Holocene, but weak during the Younger Dryas stadial. This interpretation is corroborated by a pollen-based index for Lake Panch Pokhari that estimated the strength of the Indian Summer Monsoon versus the strength of the Westerlies. Millennial-scale synchronicity with the Greenland δ18O temperature records highlights the previously suggested strong teleconnections between the Asian Monsoon system and North Atlantic climate variability.
Paleoclimate research based on the stable isotopic composition of lake sediments is often hampered by the lack of preservation of suitable material for isotopic analysis. We examined organic material as a proxy for past water isotopic composition in a series of experiments. First, we cultured aquatic moss under constant illumination, temperature, and water 18O, and show that new cellulose records source water 18O precisely (r2 = 0.9997). Second, we analyzed paired lakewater and vegetation samples collected from sites spanning strong climatic gradients. In field conditions, the relationship between organic 18O and water 18O is more variable, though it is still controlled by environmental water isotopic composition. However, terrestrial mosses in the arctic are often significantly enriched in 18O relative to aquatic mosses in nearby lakes due to their use of different water sources. Third, we measured 18O of cellulose extracted from disseminated sedimentary organic material. In the majority of the middle- to high-arctic lakes in this study, the 18O of disseminated sediment cellulose is greatly enriched relative to the expected values based on lakewater 18O, suggesting a significant component of terrestrial cellulose. This interpretation is supported by radiocarbon dates from a Holocene sediment core in which 14C ages of sediment cellulose are 700-5000 yrs older than the enclosing sediments. We conclude that aquatic cellulose can be used as a reliable tracer of lakewater isotope ratios, but terrestrial cellulose often dominates the sedimentary cellulose pool in places such as Baffin Island where sedimentation rates are low enough to allow the degradation of aquatic cellulose. Care must be taken when interpreting sediment cellulose 18O records where diagenesis has played a role, because terrestrial cellulose is more resistant to degradation, and therefore can predominate in environments with low organic carbon burial.
This study presents changes in diatom flora assemblage composition, TOC, TOC/N and biogenic opal in a 450 cm core of Lake Panch Pokhari, Central Nepal (4,050 m asl), indicating Late Quaternary environmental fluctuations. Four Diatom Zones (DZ) were detected, with two major changes. The first one was found in ~430 cm depth (~14.8 cal. kyr BP), where the original flora characterized by Navicula digitulus Hustedt, Pinnularia rhombarea Krammer, P. aff. viridiformis var. minor Krammer, Encyonema silesiacum (Bleisch) D. G. Mann, Cymbopleura naviculiformis (Auerswald) Krammer and Nitzschia sp. was fully replaced by an assemblage consisting of Aulacoseira alpigena (Grunow) Krammer, Diatoma hyemalis (Roth) Heib., Tabellaria flocculosa (Ehrenberg) Kützing, Brachysira brebissonii Ross and Pinnularia subgibba Krammer, creating a stable diatom assemblage for ~8 kyr (DZ3). The second change was found at ~70 cm (~2.1 cal. kyr BP) when increased nutrient inputs lead to emergence of new taxa such as Fragilaria construens var. subsalina Hustedt, F. tenera (W. Smith) Lange-Bertalot, Eunotia cf. pseudopapilio Lange-Bertalot and M. Nörpel-Schempp and Gomphonema subclavatum Grunow. In order to evaluate the past environmental conditions in the Lake Panch Pokhari, the detected diatom taxa were subjected to analyses of their autecological preferences and dominance within the specific assemblage. We also assumed that TOC/N ratios >10 indicate accelerated erosion due to the strengthening of the Summer Monsoon starting at ~14.8 and between 13.7 and 12.8 cal. kyr BP. Monsoon intensity was most pronounced during the Early Holocene and at the beginning of the Late Holocene. The fluctuations of TOC and TOC/N in the Late Glacial sediments seem to correlate temporally and climatically with oscillations in the Northern Atlantic region.
Isotopic compositions of carbon-bound hydrogen in individual compounds from eight different organisms were measured using isotope-ratio-monitoring gas chromatography–mass spectrometry. This technique is capable of measuring D/H ratios at natural abundance in individual lipids yielding as little as 20 nmol of H2, and is applicable to a wide range of compounds including hydrocarbons, sterols, and fatty acids. The hydrogen isotopic compositions of lipids are controlled by three factors: isotopic compositions of biosynthetic precursors, fractionation and exchange accompanying biosynthesis, and hydrogenation during biosynthesis. δD values of lipids from the eight organisms examined here suggest that all three processes are important for controlling natural variations in isotopic abundance. n-Alkyl lipids are depleted in D relative to growth water by 113–262‰, while polyisoprenoid lipids are depleted in D relative to growth water by 142–376‰. Isotopic variations within compound classes (e.g., n-alkanes) are usually less than ∼50‰, but variations as large as 150‰ are observed among isoprenoid lipids from a single organism. Phytol is consistently depleted in D by up to 50‰ relative to other isoprenoid lipids. Inferred isotopic fractionations between cellular water and lipids are greater than those indicated by previous studies.
Global trends in the δ2H-δ18O enrichment slope of continental lakes and shallow soil water undergoing natural evaporation are predicted on the basis of a steady state isotope balance model using basic monthly climate data (i.e., temperature and humidity), isotopes in precipitation data, and a simple equilibrium liquid-vapor model to estimate isotopes in atmospheric moisture. The approach, which demonstrates the extension of well-known conceptual models in stable isotope hydrology to the global scale, is intended to serve as a baseline reference for evaluating field-based isotope measurements of vapor, surface water, and soil water and as a diagnostic tool for more complex ecosystem models, including isotope-equipped climate models. Our simulations reproduce the observed local evaporation line slopes (4–5 range for lakes and 2–3 range for soil water) for South America, Africa, Australia, and Europe. A systematic increase in slopes (5–8 range for lakes) toward the high latitudes is also predicted for lakes and soil water in northern North America, Asia, and Antarctica illustrating a latitudinal (mainly seasonality-related) control on the evaporation signals that has not been widely reported. The over-riding control on the poleward steepening of the local evaporation lines is found to be the isotopic separation between evaporation-flux-weighted atmospheric moisture and annual precipitation, and to lesser extents temperature and humidity, all of which are influenced by enhanced seasonality in cold regions.
The local meteoric water line (MWL) has been established from north to south of the Tibetan Plateau based on the measured
results of δD and δ18O in precipitation and river water, and the relationship between MWL and moisture origins discussed. The spatial and seasonal
variations ofd in precipitation and river water on the Tibetan Plateau have been studied. Results show that the spatial and seasonal variations
ofd between north and south of the Tanggula Mountains are related to different moisture origins and water recycling.
Successful petroleum exploration relies on detailed analysis of the petroleum system in a given area. Identification of potential source rocks, their maturity and kinetic parameters, and their regional distribution are best accomplished by rapid screening of rock samples (cores and/or cuttings) using the Rock-Eval apparatus. The technique has been routinely used for about fifteen years and has become a standard tool for hydrocarbon exploration. This paper describes how the new functions of the latest version of Rock-Eval (Rock-Eval 6) have expanded applications of the method in petroleum geoscience. Examples of new applications are illustrated for source rock characterization, reservoir geochemistry, and environmental studies, including quantification and typing of hydrocarbons in contaminated soils.
Over the last 28 years ice core records have been systematically recovered from ten high-elevation ice fields, nine of which are located in the low latitudes. Each core has provided new information about the regional climate and environmental change, and together their records challenge existing paradigms about the Earth's climate system. When viewed collectively, these ice core histories provide compelling evidence that the growth (glaciation) and decay (deglaciation) of large ice fields in the lower latitudes are often asynchronous, both between the hemispheres and with high latitude glaciation that occurs on Milankovitch timescales. Although stable isotopic records suggest that global-scale cooling occurred during the Last Glacial Stage (LGS), we contend that precipitation is the primary driver of glaciation in the low latitudes. This is consistent with the time-transgressive nature of precession-driven changes in insolation (and hence precipitation) such that glaciers advance/retreat in the tropics north of the equator while glaciers retreat/advance in the tropics south of the equator. Thus, the coeval inter-hemispheric retreat of glaciers in the 20th century and the beginning of the 21st century is atypical. Copyright 2005 John Wiley & Sons, Ltd.
We measured the oxygen isotope composition of both the water and dry matter components of phloem sap exported from photosynthesising Ricinus communis L. leaves. The ¹⁸O / ¹⁶O composition of exported dry matter matched almost exactly that expected for equilibrium with average lamina leaf water (leaf water exclusive of water associated with primary veins) with an isotope effect of αo=1.027, where αo=Ro / Rw , and Ro and Rw are ¹⁸O / ¹⁶O of organic molecules and water, respectively. Average lamina leaf water was enriched by 14–22‰ compared with source water under our experimental conditions, and depleted by 4–7‰, compared with evaporative site water. This showed that it is the average lamina leaf water ¹⁸O / ¹⁶O signal that is exported from photosynthesising leaves rather than a signal more closely related to that of evaporative site water or source water. Additionally, we found that water exported in phloem sap from photosynthesising leaves was enriched compared with source water; the mean phloem water enrichment observed for leaf petioles was 4.0 ± 1.5‰ (mean ± 1 s.d., n = 27). Phloem water collected from stem bases was also enriched compared with source water. However, the enrichment was approximately 0.8 times that observed for leaf petioles, suggesting some mixing between enriched phloem water and unenriched xylem water occurred during translocation. Results validated the assumption that organic molecules exported from photosynthesising leaves are enriched by 27‰ compared with average lamina leaf water. Furthermore, results suggest that the potential influence of enriched phloem water should be considered when interpreting the ¹⁸O / ¹⁶O signatures of plant organic material and plant cellulose.
During a survey of the diatom flora from a sediment core from Lake Panch Pokhari in the Nepalese Himalaya, four new Eunotia species were recorded and described: E. igorii Krstić, Levkov & Pavlov, E. zechii Krstić, Levkov & Pavlov, E. panchpokhariensis Krstić, Levkov & Pavlov and E. paramuscicola Krstić, Levkov & Pavlov. Detailed morphological descriptions of the taxa are given based on both light and scanning electron microscopy. The morphological features of each taxon were compared with similar taxa. Eunotia igorii is similar to South American, neotropical species which are larger in size and have more coarsely striated valves. Furthermore, an additional distinctive feature of E. igorii is the presence of prominent spines throughout the length of the valve margins. Eunotia zechii has three weak undulations on the dorsal valve margin, in contrast to the similar taxa E. fennica (Hustedt) Lange-Bertalot and E. neoborealis Lange-Bertalot. Eunotia panchpokhariensis has quite narrow and strongly curved valves with a large number of undulations (six or seven) on the dorsal margin. Taxa with similar morphological features are rarely found, and those with five or more dorsal undulations have larger valves. Eunotia paramuscicola also resembles E. panchpokhariensis in valve outline, but it has fever dorsal undulations and they are more uneven. The uneven expression and position of the dorsal undulations and the variable protraction of the valve ends, clearly set apart E. paramuscicola from its allied taxa. With the exception of E. paramuscicola, the three other new Eunotia species have so far not been found during extensive surveys in the Nepalese Himalaya. © 2013
In 2003 sediment core Lz1024 was drilled at Lake El'gygytgyn, far east Russian Arctic, in an area of the Northern Hemisphere which has not been glaciated for the last 3.6 Ma. Biogenic silica was used for analysing the oxygen isotope composition (δ18Odiatom) in the upper 13 m long section dating back about 250 ka with samples dominated by one taxa in the <10 μm fraction (Cyclotella ocellata). Downcore variations in δ18O values show that glacial-interglacial cycles are present throughout the core and δ18Odiatom-values are mainly controlled by δ18Oprecipitation. Changes reflect the Holocene Thermal Maximum, the Last Glacial Maximum (LGM) and the interglacial periods corresponding to MIS 5.5 and MIS 7 with a peak-to-peak amplitude between LGM and MIS 5.5 of Δ18O = 5.3‰. This corresponds to a mean annual air temperature difference of about 9 °C. Our record is the first continuous δ18Odiatom record from an Arctic lake sediment core directly responding to precipitation and dating back more than 250 ka and correlates well with the stacked marine δ18O LR04 (r = 0.58) and δD EPICA Dome-C record (r = 0.69). With δ18O results indicating strong links to both marine and ice-core records, records from Lake El'gygytgyn can be used to further investigate the sensitivity of the Arctic climate to both past and future global climatic changes.
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A detailed stable-isotope record is presented for the full length of the Greenland Ice-core Project Summit ice core covering the last 250,000 years according to a graduated timescale. It appears that the climatic stability of the Holocene is the exception rather than the rule; the last interglacial is also noted to have lasted longer than is implied by the deep-sea SPECMAP record. This discrepancy may be accounted for if the climate instability at the outset of the last interglacial delayed the melting of the Saalean ice sheets in America and Eurasia.
1] An accurate representation of the spatial distribution of stable hydrogen and oxygen isotopes in modern precipitation is required for many hydrological, paleoclimate, and ecological applications. No standardized method for achieving such a representation exists, and potential errors associated with previously employed methods are not understood. Using resampling, we test the accuracy of interpolated dD and d 18 O estimates made using four methods. Prediction error for all methods is strongly related to number of data and will likely decline with the addition of new data. The best method lowers estimation error by 10–15% relative to others tested and gives an average error, using all available data, %2.5% of the global range. We present and interpret global maps of interpolated dD, d 18 O, and deuterium excess in precipitation and the 95% confidence intervals for these values created using the optimal method. These depict global and regional patterns, make evident the robustness of interpolated isotopic patterns, and highlight target areas for future precipitation sampling. Citation: Bowen, G. J., and J. Revenaugh, Interpolating the isotopic composition of modern meteoric precipitation, Water Resour. Res., 39(10), 1299, doi:10.1029/2003WR002086, 2003.
The seasonal and spatial variations of stable isotope ratios in present day European precipitation are simulated with a simple multibox model of the mean west-east horizontal transport of the atmospheric water vapour across the European continent. Isotope fractionation during the formation of precipitation leads to an increasing depletion of heavy isotopes in the residual air moisture as it moves towards the centre of the continent. This isotopic depletion is partly compensated, particularly in summer, by evapotranspiration, which is assumed to transfer soil water into the atmosphere without isotope fractionation. The model estimates are based on horizontal water vapour flux data, varying seasonally between 88 and 130 kg m -1 s -1 for the Atlantic coast region, and on the monthly precipitation, evapotranspiration and surface air temperature data available for various locations in Europe. Both continental and seasonal temperature effects observed in the stable isotope composition of European precipitation are fairly well reproduced by the model. The calculations show that the isotopic composition of local precipitation is primarily controlled by regional scale processes, i.e. by the water vapour transport patterns into the continent, and by the average precipitation-evapotranspiration history of the air masses precipitating at a given place. Local parameters such as the surface and/or cloud base temperature or the amount of precipitation modify the isotope ratios only slightly. Implications of the model predictions for the interpretation of stable isotope ratios in earlier periods as they are preserved in ice cores and in groundwater are also discussed. DOI: 10.1111/j.2153-3490.1982.tb01801.x
Oxygen isotope records of five stalagmites from Hulu Cave near Nanjing bear a remarkable resemblance to oxygen isotope records from Greenland ice cores, suggesting that East Asian Monsoon intensity changed in concert with Greenland temperature between 11,000 and 75,000 years before the present (yr. B.P.). Between 11,000 and 30,000 yr. B.P., the timing of changes in the monsoon, as established with 230Th dates, generally agrees with the timing of temperature changes from the Greenland Ice Sheet Project Two (GISP2) core, which supports GISP2's chronology in this interval. Our record links North Atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer East Asian Monsoon originates.
In order to understand the processes controlling organic carbon deposition (i.e., primary productivity vs. terrigenous supply) and their paleoceanographic significance, three sediment cores (PS2471, PS2474, and PS2476) from the Laptev Sea continental margin were investigated for their content and composition of organic carbon. The characterization of organic matter includes the determination of bulk parameters (hydrogen index values and C/N ratios) and the analysis of specific biomarkers (n-alkanes, fatty acids, alkenones, and pigments). Total organic carbon (TOC) values vary between 0.3 and 2%. In general, the organic matter from the Laptev Sea continental margin is dominated by terrigenous matter throughout. However, significant amounts of marine organic carbon occur. The turbidites, according to a still preliminary stratigraphy probably deposited during glacial Oxygen Isotope Stages 2 and 4, are characterized by maximum amounts of organic carbon of terrigenous origin. Marine organic carbon appears to show enhanced relative abundances in the Termination I (?) and early Holocene time intervals, as indicated by maximum amounts of short chain n-alkanes, short-chain fatty acids, and alkenones. The increased amounts of fatty acids, however, may also have a freshwater origin due to increased river discharge at that time. The occurrence of alkenones is suggested to indicate an intensification of Atlantic water inflow along the Eurasian continental margin starting at that time. Oxygen Isotope Stage 1 accumulation rates of total organic carbon are 0.3, 0.17, and 0.02 g C/cm2/ky in cores PS2476, PS2474, and PS2471, respectively.
The enrichment of the heavy isotopic species (H2 18O and HDO) in surface waters and lakes was already observed during the early surveys of stable isotope variations in the hydrological cycle (Rankama 1954). Craig (1961a) noted that the isotope compositions of waters from African lakes were displaced in δ-D vs δ-18O space relative to other freshwaters whose composition follows the meteoric water line (MWL).
Many different additives are used in the textile industry and each of them has its specific function: protection from ultraviolet light, oil, water repellency, etc. Clothing and textile products are produced, consumed, and incinerated or disposed in landfills over the world. This means that the fate of their additives can be widely spread, and the potential impact of these chemicals on the environment and human health can, therefore, be present on a global scale. Perfluorocarbon compounds, triclosan, and brominated compounds have been selected as representative additives of this sector. A review of their main environmental consequences and human health damage has been undertaken. The information gathered shows that these compounds can be toxic, very persistent in the environment, and bioaccumulable.
The Rock-Eval pyrolysis method was designed to meet the needs of petroleum prospection and has now gained widespread use. It quickly provides different data on the organic content of rocks, such as the petroleum potential of series encountered, the nature of kerogens and their state of maturity. The vertical representation of results in the form of geochemical logs leads to an interpretation that is both effective and practical. Reference charts and diagrams are used to characterize source rocks (petroleum potentials, types of organic matter, degree of evolution, weathering, etc.) as well as migration phenomena. The compiling of geochemical maps on a basin-wide scale then becomes possible. Likewise, the method has left the field of petroleum prospection per se and found applications in the analyzing of coals, bituminous rocks, recent sediments and even techniques of the refining and secondary recovery of crude oils.-from English summary
A mechanistic model is presented to quantify both the physical and
biochemical fractionation events associated with hydrogen and oxygen
isotope ratios in tree-ring cellulose. The model predicts the isotope
ratios of tree-rings, incorporating both humidity and source water
environmental information. Components of the model include (1) hydrogen
and oxygen isotope effects associated with leaf water enrichment; (2)
incorporation of leaf water isotope ratio values into photosynthetic
carbohydrates along with the biochemical fractionation associated with
autotrophic synthesis; (3) transport of exported carbohydrates (such as
sucrose) from leaves to developing xylem in shoots and stems where
cellulose is formed; (4) a partial exchange of oxygen and hydrogen
isotopes in carbohydrates with xylem sap water during conversion into
cellulose; and (5) a biochemical fractionation associated with cellulose
synthesis. A modified version of the Craig-Gordon model for evaporative
enrichment adequately described leaf water δD and δ
18O values. The leaf water model was robust over a wide range
of leaf waters for both controlled experiments and field studies, far
exceeding the range of values to be expected under natural conditions.
The isotopic composition of cellulose was modeled using heterotrophic
and autotrophic fractionation factors from the literature as well as the
experimentally derived proportions of H and O that undergo exchange with
xylem water during cellulose synthesis in xylem cells of tree-rings. The
fraction of H and O from carbohydrates that exchange with xylem sap
water was estimated to be 0.36 and 0.42, respectively. The proportions
were based on controlled, long-term greenhouse experiments and field
studies where the variations in the δD and δ 18O
of tree-ring cellulose were measured under different source water
isotopic compositions. The model prediction that tree-ring cellulose
contains information on environmental water source and atmospheric vapor
pressure deficit (related to relative humidity) was tested under both
field and greenhouse conditions. This model was compared to existing
models to explain cellulose isotope ratios under a wide range of source
water and humidity conditions. Predictions from our model were
consistent with observations, whereas other models showed large
discrepancies as soon as the isotope ratios of source water and
atmospheric water deviated from each other. Our model resolves the
apparently conflicting and disparate interpretations of several previous
cellulose stable isotope ratio studies.
The oxygen isotopic composition of precipitation (δ18Oprec) is well known to be a valuable (paleo-)climate proxy. Paleosols and sediments and hemicelluloses therein have the potential to serve as archives recording the isotopic composition of paleoprecipitation. In a companion paper (Zech et al., 2014) we investigated δ18Ohemicellulose values of plants grown under different climatic conditions in a climate chamber experiment. Here we present results of compound-specific δ18O analyses of arabinose, fucose and xylose extracted from modern topsoils (n = 56) along a large humid-arid climate transect in Argentina in order to answer the question whether hemicellulose biomarkers in soils reflect δ18Oprec.
A 12.4 m core collected from the eastern part of Lake Bangong is analyzed for mineralogy, radiometric chronology and stable isotope contents of authigenic, inorganic carbonate.Isotopic compositions of the modern lake waters and of its major tribularies are first presented. Radiocarbon activity of these waters show that equilibrium with the atmospheric CO2 is not reached, because of admixture of dead carbon in solution in the system. A model is thus developed to correct the radiocarbon ages of fossil material of aquatic origin for this ageing.The core provides a continuous Holocene record. Changes in mineralogy of bulk sediments, and in 13C and 18O contents of authigenic carbonates reveal extremely large changes in environmental conditions, induced by climate changes and/or local hydrological factors (e.g. closure/opening of the lake system controlled by the altitude of the outflow sill).The eastern basin of Lake Bangong was closed before ?9.6 and after 1.2 ka B.P., in response to aridity on the catchment. Wet/warm conditions, attributed to the monsoon influence, were established suddenly at ?9.6 ka B.P. and maintained until ?6.2 ka B.P., although minor reversal events occurred between ?8.6 and 7.7 ka B.P. The return towards aridity is non linear. Dry spells at ?6.2, ?3.9-3.2, and ?1.3 ka B.P. are alternated with short-term periods with positive precipitation-evaporation balance.
Hydrogen isotope ratios of sedimentary biomarkers are known to record the climatic variability in terrestrial and marine environments. However, there is still a lack of calibration studies that can quantitatively retrace the driving forces, especially at the Tibetan Plateau. Here, we elaborate the actual influence of environmental parameters such as temperature, evapotranspiration, salinity and biosynthetic fractionation on delta D values of n-alkanes. We measured hydrogen isotope values (delta D) of n-alkanes in recent sediment and plant samples as well as delta D values of different water sources (lake and inflow water, precipitation as well as leaf, root and soil water) from six Tibetan lakes along a 10 degrees-spanning longitudinal transect covering an aridity gradient. As expected, the deuterium record can be used to distinguish two water pools at the Plateau: (I) precipitation water, which supplies water for the lake inflow and terrestrial plants, and (II) enriched lake water having a clear evaporative signal. Based on significant correlations of the source water and delta D values of n-alkanes, delta D of alkane n-C-23 record the lake water isotope composition and track the evaporative enrichment of the lake system. In contrast, delta D of alkane n-C-29 can be used to retrace the isotope composition of the inflow displaying the integrated rainfall signal in the vegetation period modified by soil and leaf water evaporation. While temperature changes are less pronounced across the Tibetan transect, the isotopic difference between C-23 and C-29 could potentially be used as a proxy to reconstruct effective moisture (precipitation minus evaporation). Areas with lower precipitation amounts and higher evaporation rates are characterized by higher Delta delta DC23-C29 values. This relationship is more pronounced in regions with mean annual precipitation below 350 mm. Even in contrasting environments, the application of the delta D proxies is very promising. Combining other Tibetan studies, it is possible to present a comprehensive picture of the usage of compound-specific hydrogen isotopes of n-alkanes on the Tibetan Plateau.
The current isotope tree ring model assumes that 42% of the sucrose oxygen exchanges with stem water during cellulose synthesis and that the oxygen isotope biochemical fractionation is c . 27‰. However, previous studies have indicated that this model can overestimate the cellulose oxygen isotope ratio of plants under salinity or water stress. Saline stress increases soluble carbohydrates and osmolytes, which can alter exchange and biochemical fractionation during cellulose synthesis.
To test the effect of salinity as well as the synthesis of osmolytes on exchange and biochemical fractionation, we grew wild‐type and a transgenic mannitol synthesizer Arabidopsis thaliana hydroponically with fresh and saline water. We then measured the oxygen isotope ratios of leaf water, stem water and stem cellulose to determine the effects on exchange and biochemical fractionation.
Biochemical fractionation did not change, but oxygen isotope exchange was twice as high for plants grown in saline water relative to freshwater‐treated plants (0.64 and 0.3, respectively). Mannitol (osmolyte) synthesis did not affect exchange or biochemical fractionation regardless of salinity.
Increases in salinity increased oxygen isotope exchange during cellulose synthesis, which may explain the overestimation of cellulose δ ¹⁸ O values under saline conditions.
The oxygen isotopic composition of cellulose is a valuable proxy in paleoclimate research. However, its application to sedimentary archives is challenging due to extraction and purification of cellulose. Here we present compound-specific δ¹⁸O results of hemicellulose-derived sugar biomarkers determined using gas chromatography–pyrolysis–isotope ratio mass spectrometry, which is a method that overcomes the above-mentioned analytical challenges. The biomarkers were extracted from stem material of different plants (Eucalyptus globulus, Vicia faba and Brassica oleracea) grown in climate chamber experiments under different climatic conditions.
The Gunt River catchment in the Central Pamirs is a representative of the headwater catchments of the Aral Sea Basin. It covers 14,000 km(2), spanning altitudes between 2000 and 6700 m a.s.l. In a monitoring network, water samples were taken at 30 sampling points every month and analysed for the stable water isotopes ((18)O and (2)H). Our first results show δ(2)H values in the range from-131.2 to-94.9 ‰ and δ(18)O values from-18.0 to-14.0 ‰. The stable isotope patterns in the catchment seem to follow a systematic way, dominated by an altitude effect with a mean Δ δ(2)H=-3.6 ‰/100 m. The observed seasonal variations can be explained by geographical aspects such as the influence of different wind systems as well as melting processes.
Leaf-wax hydrogen isotope composition (δDwax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling δDwax in the tropics remain poorly understood. We measured δDwax and the stable carbon isotope composition of leaf-waxes (δ13Cwax), including both n-alkanes and n-alkanoic acids, from modern lake sediments and soils across a marked aridity gradient in southeastern Mexico and northern Central America to investigate the importance of aridity and vegetation composition on δDwax. In this region the estimated hydrogen isotope composition of meteoric water (δDw) varies by only 25‰, and variability in δDw does not explain the relatively large variance in δDwax (60‰). Instead, the aridity index, defined as the ratio of mean annual precipitation to mean annual potential evapotranspiration (MAP/PET), explains much of the variability in the hydrogen isotope fractionation between leaf-waxes and meteoric water (εwax/w). Aridity effects are more evident in lake sediments than in soils, possibly because integration of leaf-waxes across a broad catchment masks small-scale variability in εwax/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from δ13Cwax, provides a secondary control on εwax/w for n-alkanoic acids (εn-acid/w). Low δ13Cn-acid values are associated with high εn-acid/w values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C4 grasses and C3 trees and shrubs. A similar relationship between δ13Cn-alkane and εn-alkane/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in δDwax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies.
Natural variations of oxygen 18 and deuterium abundances in groundwaters, as compared to the local precipitation, are discussed as a hydrologic tool. It is shown that the residual scatter in time and space of the isotope content of groundwater sources is a measure of the homogeneity of these systems rather than of their age or size and that the concept of a regional slope of evaporation lines on the δD–δ18 O diagram has only limited validity. Isotope criteria are suggested for distinguishing rainfall from past periods of different climate from present rainfall. It is concluded that as a result of the unsteady nature of the mechanisms affecting the isotope composition of groundwaters, their most reliable use, under the limitations of our present quantitative knowledge on the causes of this scatter, is as classifying parameters in regional investigations.
During a long-continued study of the lacustrine beds of the Eocene Green River Formation, an attempt was made to interpret past events from observation of present-day processes. After a search of some 40 years, four lakes have been found that are producing a kind of organic ooze judged to be a modern analogue of the precursors of rich oil shale. Two of the lakes are in central Africa and two are in Florida. All four are shallow. The ooze in all four is predominantly algal, entirely in the form of minute fecal pellets, and does not decay in warm, wet, oxidizing environments. Several of the most unusual, mummified microorganisms found in the oil shale of the Green River are illustrated as testament to the inference that the organic oozes also were resistant to decay. Studies to determine why these algal oozes do not decay are in progress, but as yet no satisfactory explanation is available.
This study characterizes the petrological changes occurring in an organic matter assemblage of Recent lacustrine sediments from Alberta, Canada, during the various steps of Rock-Eval 6® pyrolysis. The results indicate that the organic compounds released during pyrolysis at low temperatures of up to 300°C (mainly S1-compounds) are most likely derived from a ‘stain-like’ amorphous organic matter, including pigments and oils/lipid products with strong, multicolored VIS fluorescence. The fluid characteristics of the thermally labile S1-compounds possibly account for the significant grain surface coating by the organic matter. Thermal alteration of sediments at 300°C results in the transformation of some alginite and other macerals into a secondary product of blue–green fluorescing bitumen, which migrates into available free spaces.The release of S2-compounds during high temperature pyrolysis (from 300 to 650°C) causes severe alterations in the morphology of the liptinitic organic matter. A bright yellow fluorescence that appeared at this stage is likely due to thermal transformation of bitumen produced in S1 pyrolysis and other liptinitic macerals into an “oily-material”, which fills cell lumens and becomes incorporated into the mineral matter. Neither the rapid rate of heating nor the sample residence time during Rock-Eval pyrolysis are sufficient for complete thermal destruction of the secondary hydrocarbon and bitumen products produced in the sample.
There has been much recent debate about Holocene climate variation in the monsoon region of China, especially the temporal pattern of variations in precipitation, the time-transgressive nature of the Holocene precipitation maximum, and the extent to which variations in regions influenced by the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM) have been synchronous. We summarize and compare carbonate oxygen-isotope records (δ18Ocarb) from ten lakes within the present-day ISM region. We discuss their paleoclimate significance considering the present-day moisture source, isotopic composition of precipitation and the hydrological setting. The δ18Ocarb records are controlled mainly by the isotopic composition of lake water, which in turn is a function of regional Precipitation/Evaporation (P/E) balance and the proportion of precipitation that is monsoon-derived. We normalized the δ18Ocarb data and used these records to generate an integrated moisture index. This index, along with oxygen-isotope records from speleothems and carbon-isotope records (δ13Corg) from peats within the monsoon region, suggests that Holocene climate was broadly synchronous across the monsoon region and, within the limits of accuracy of the existing age models, provides no strong evidence for previously-proposed anti-phasing of the ISM and the EASM. Stable-isotope records from lake sediments and peat bogs have excellent potential for providing high-quality paleoclimate data for monsoon Asia, and complement high-resolution speleothem sequences, which are only found in certain localities.
Optical and geochemical techniques were applied to organic matter of Late Quaternary sediments from Lakes Victoria and Rukwa, East Africa. Variations in total organic carbon (TOC) and hydrogen content (expressed as Hydrogen Index, HI) can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences. TOC and HI both decline as an exposure surface is approached, mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation. Inert organic material of very low HI is typically concentrated at the overlying transgression surface. The carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces. Although weathering may be partially responsible, isotopic variations are mainly due to changes in the relative importance of material from plants using C4 photosynthesis, such as grasses and sedges, growing around the shore and on the exposed lake muds, and of material from planktonic algae and terrestrial C3 plants.
Isotopic effects on the molecular diffusion coefficients of H2 16O,HD16O, and H2 18O have been determined by two independent methods. In the first one, the evaporation of liquid water under controlled laminar flow conditions was investigated. In the second one, the transport of water vapor through a diffusion tube was measured. The molecular diffusion coefficient of natural water vapor in nitrogen was redetermined. At 21 °C and P=760 Hg mm,D=0.244±0.004 cm2/sec. For the isotopic molecules we have DHD16O/DH216O=0.9757±0.0009 and DH218O/DH216O=0.9727±0.0007. Agreement between the data obtained in the pure gas phase experiment and the two-phase experiment shows that the condensation coefficients of the isotopically substituted molecules are identical. Interpretation of the isotopic effects on molecule diffusivities indicates that the relative mass differences between H2 16O,HD16O, and H2 18O alone cannot account for the experimental results. This is more important for the D–H substitution in which the asymetric molecule HD16O is formed. Qualitatively, the displacement of the center of gravity of the molecules H2 16O and HD16O in this substitution can explain the observed effect. Interpretation of these isotopic effects in terms of simple kinetic theory of gases indicates that for rigid spherical molecule models with diameters Γ we have the relation ΓH218O
Isotopic effects on the molecular diffusion coefficients of H 216O, HD16O, and H218O have been determined by two independent methods. In the first one, the evaporation of liquid water under controlled laminar flow conditions was investigated. In the second one, the transport of water vapor through a diffusion tube was measured. The molecular diffusion coefficient of natural water vapor in nitrogen was redetermined. At 21°C and P=760 Hg mm, D=0.244±0.004 cm2sec. For the isotopic molecules we have DHD16O/DH216O=0.9757±0.0009 and D H216O/DH216O=0.9727±0.0007. Agreement between the data obtained in the pure gas phase experiment and the two-phase experiment shows that the condensation coefficients of the isotopically substituted molecules are identical. Interpretation of the isotopic effects on molecule diffusivities indicates that the relative mass differences between H 216O, HD16O, and H218O alone cannot account for the experimental results. This is more important for the D-H substitution in which the asymetric molecule HD16O is formed. Qualitatively, the displacement of the center of gravity of the molecules H216O and HD16O in this substitution can explain the observed effect. Interpretation of these isotopic effects in terms of simple kinetic theory of gases indicates that for rigid spherical molecule models with diameters Γ we have the realtion ΓH218O < ΓH216O < ΓHD 16O. We calculate from the experimental data ratio of the coefficients of diffusion of the three isotopic molecules in air D HD16O/DH216O=0.9755±0.0009 and D H216O/DH216O=0.9723±0.0007.
The stable isotopes of oxygen and hydrogen incorporated in the water molecule (18O and 2H) have become an important tool not only in Isotope Hydrology, routinely applied to study the origin and dynamics of surface and groundwaters, but also in studies related to atmospheric circulation and palaeoclimatic investigations. A proper understanding of the behaviour of these tracers in the water cycle is required for a meaningful use of these tools in any of these disciplines. Our knowledge of the vertical distribution and the factors controlling the stable isotope ratios of oxygen and hydrogen in atmospheric moisture derives from a limited number of observations and vertical profiles in the atmosphere. An international programme jointly operated by the International Atomic Energy Agency (IAEA) and the World Meteorological Organization (WMO), and operational since 1961, has resulted in the development of a dedicated database to monitor isotope ratios in precipitation in more than 500 meteorological stations world-wide. The main features of the spatial and temporal variations of stable isotope ratios of oxygen and hydrogen in precipitation and atmospheric moisture at the global scale are presented based on the analysis of limited data on water vapour, data obtained by the Global Network for Isotopes in Precipitation (GNIP) and the few observations at high latitudes. Copyright © 2000 John Wiley & Sons, Ltd.
Ostracod assemblage changes and stable isotopes of ostracod shells were used to reconstruct the salinity history of Lake Donggi Cona on the northeastern Tibetan Plateau in response to the late glacial and Holocene climate history. Brackish and relatively unstable conditions were inferred for the earlier part of the late glacial between 18.9 and 13.4 cal ka BP and for the early to mid Holocene (11.9–6.8 cal ka BP). A saline lake existed in the intervening period probably representing colder and drier conditions during the Greenland Stadial 1 (= Younger Dryas in North Atlantic region). Freshwater conditions similar to the present stage of Lake Donggi Cona were established for the first time at 6.8 cal ka BP. This inference and the implied increase in moisture availability in the latter half of the Holocene is in contrast to most climate records from the Tibetan Plateau which suggest drier and colder conditions. However, three lake records from the eastern Tibetan Plateau including our new record show that the Holocene insolation-driven monsoonal weakening and temperature decrease caused a catchment-specific, opposite response of the moisture availability in comparison to those at the majority of sites. Catchments with relatively low precipitation at present experienced apparently a moisture availability increase during the colder second half of the Holocene in contrast to the inferred drier conditions at predominantly generally wetter sites. The increase in moisture availability at relatively dry sites mainly results from the lowered temperature and evaporation but is probably further controlled by a complex of factors including the amount and timing of precipitation, the altitudes of the lake basins and upper reaches, and the existence or development of glaciers, snow fields and frozen ground in a given catchment. Our record from Lake Donggi Cona suggests that the recently observed reduction in Yellow River discharge in response to present global warming will likely continue or even increase under warmer climatic conditions in the future.
Aiming at developing a novel tool for palaeoclimate research, we recently proposed a new method for determining the oxygen isotope composition of monosaccharides (Zech, M., Glaser, B., 2009. Compound-specific delta O-18 analyses of neutral sugars in soils using GC-Py-IRMS: problems, possible solutions and a first application. Rapid Communications in Mass Spectrometry 23, 3522-3532). In order to answer the question whether isotope fractionation and oxygen exchange reactions during litter decomposition affect the climatically controlled delta O-18 values of plant derived sugars, such as for instance xylose and arabinose from hemicelluloses,we studied the compound specific delta O-18 values of five different litter species having been decomposed in a field litterbag experiment for 27 months. While delta O-18 values of bulk organic matter yielded a systematic O-18 depletion of 3.3% (pine) to 4.6% (spruce) accompanying total cellulose decomposition of 51% (beech) to 86% (ash), delta O-18 values of individual sugars show no significant trend with time. In addition to the observed absence of isotope fractionation during decomposition, we also rule out O-18 exchange reactions affecting the delta O-18 signature of sugar molecules during diagenetic processes as well as during analytical procedure based on experimental findings and on theoretical biochemical mechanistic considerations. We conclude that our new method may become an analytical tool that elegantly overcomes extraction, purity and hygroscopicity problems of previous O-18 cellulose methods. It furthermore has the potential to be applied to a wide range of climate archives such as tree rings, lacustrine sediments and loess palaeosol sequences.
Oxygen atoms in plant products originate from CO2 , H2O and O2 , precursors with quite different d18O values. Furthermore their incorporation by different reactions implies isotope effects. On this base the resulting non-statistical 18O distributions in natural compounds are discussed. The d18O value of cellulose is correlated to that of the leaf water, and the observed 18O enrichment (+19o/oo above water), depending upon origin and bio-syntheses. Alcoholic groups introduced by monooxygenase reactions, e.g. in sterols and phenols, show d18O values near +5o/oo, in agreement with an assumed isotope fractionation factor of $1.02 on the reaction with atmospheric oxygen (d18O = +23.5o/oo). Correspondingly, "thermodynamically ordered isotope distribution'' is only observed for oxygen in some functional groups correlated to an origin from CO2 and H2O, not from O2 . The individual isotopic increments of functional groups permit the prediction of global d18O values of natural compounds on the basis of their biosynthesis. # 2001 Published by Elsevier Science Ltd.
Editor: J. Fein Keywords: Tree-ring cellulose δ 18 O Hydroclimate δ 18 O of precipitation The Asian monsoon Northern China Oxygen isotopic ratios (δ 18 O) of tree-ring cellulose, grown from 1954 to 2003 in semi-arid Northern China demonstrate a common inter-annual variability, despite coming from three different species in two distinct growth environments (an alpine meadow and a rocky ridge). The variability was significantly negative correlated with precipitation and relative humidity during the growing season. This suggests that the past summer hydroclimate can be inferred from tree-ring cellulose δ 18 O from various kinds of trees growing in semi-arid Northern China. In addition, we evaluated past changes in δ 18 O of precipitation from the tree-ring cellulose δ 18 O and relative humidity using the mechanistic model for tree-ring cellulose δ 18 O in Roden et al. (2000). By fixing the species-dependent exchange rate of oxygen between carbohydrate and xylem water for Larix principis-rupprechtii and Picea koraiensis, we could also reconstruct the variations in precipitation δ 18 O from the different tree species, which are similar to the observed δ 18 O of precipitation during 1985–2002. Although the reconstructed δ 18 O of precipitation does not have any significant relation to local temperature or precipitation during 1954–2003, its long-term variation pattern is similar to that of the Asian summer monsoon indices and δ 18 O of stalagmite in the Heshang cave (30°27′N, 110°25′E; Fig. 1), suggesting that δ 18 O of precipitation is not controlled by local meteorology but is influenced by large-scale atmospheric circulation.
a b s t r a c t New high-resolution deuterium excess (d) data from the two EPICA ice cores drilled in Dronning Maud Land (EDML) and Dome C (EDC) are presented here. The main moisture sources for precipitation at EDC and EDML are located in the Indian Ocean and Atlantic Ocean, respectively. The more southward moisture origin for EDML is reflected in a lower present-day d value, compared to EDC. The EDML and EDC isotopic records (d 18 O and d) show the main climate features common to the East Antarctic plateau and similar millennial scale climate variability during the last glacial period. However, quite large d 18 O and d differences are observed during MIS5.5 and the glacial inception with a long-term behaviour. A possibility for this long-term difference could be related to uncertainties in past accumulation rate which are used in the glaciological models. Regional climate anomalies between the two sites during MIS5.5 could also be consistent with the observed EDML-EDC d 18 O and d gradient anomalies. Simulations performed with the General Circulation Model ECHAM4 for different time slices provide a temporal temperature/isotope slope for the EDML region in fair agreement to the modern spatial slope. T site and T source records are extracted from both ice cores, using a modelling approach, after corrections for past d 18 O seawater and elevation changes. A limited impact of d on Antarctic temperature reconstruction at both EDML and EDC has been found with a higher impact only at glacial inception. The AIM (Antarctic Isotope Maximum) events in both ice cores are visible also after the source correction, suggesting that these are real climate features of the glacial period. The different shape of the AIM events between EDC and EDML, as well as some climate features in the early Holocene, points to a slightly different climate evolution at regional scale. A comparison of our temperature reconstruction profiles with the aerosol fluxes show a strong coupling of the nssCa fluxes with Antarctic temperatures during glacial period and a tighter coupling of d 18 O and T site with ssNa flux at EDML compared to EDC during the glacial period and MIS5.5.