The dominant environmental driver of leaf water stable isotope enrichment differs for H-2 compared to O-18

Author(s):  
Matthias Cuntz ◽  
Lucas A Cernusak ◽  

<p>Several important isotopic biomarkers derive at least part of their signal from the stable isotope composition of leaf water (e.g., leaf wax δ<sup>2</sup>H, cellulose δ<sup>2</sup>H and δ<sup>18</sup>O, lignin δ<sup>18</sup>O). In order to interpret these isotopic proxies, it is therefore helpful to know which environmental variable most strongly controls a given leaf water stable isotope signal. We collated observations of the stable isotope compositions of leaf water, xylem water, and atmospheric vapour, along with air temperature and relative humidity, to test whether the dominant driver of leaf water <sup>2</sup>H concentration could differ from that of <sup>18</sup>O concentration. Our dataset comprises 690 observations from 35 sites with broad geographical coverage. We limited our analysis to daytime observations, when the photosynthetic processes that incorporate the leaf water isotopic signal primarily take place. The Craig-Gordon equation was generally a good predictor for daytime bulk leaf water stable isotope composition for both δ<sup>2</sup>H (R<sup>2</sup>=0.86, p<0.001) and δ<sup>18</sup>O (R<sup>2</sup>=0.63, p<0.001). It showed about 10% admixture of source water was caused by unenriched water pools such as leaf veins or the Péclet effect. Solving the Craig-Gordon equation requires knowledge of relative humidity, air temperature, and the stable isotope compositions of source water and atmospheric vapour. However, it is not possible to invert the Craig-Gordon equation to solve for one of these parameters unless the others are known. Here we show that the two isotopic signals of δ<sup>2</sup>H and δ<sup>18</sup>O are predominantly driven by different environmental variables: leaf water δ<sup>2</sup>H correlated most strongly with the δ<sup>2</sup>H of source water (R<sup>2</sup>=0.68, p<0.001) and atmospheric vapour (R<sup>2</sup>=0.63, p<0.001), whereas leaf water δ<sup>18</sup>O correlated most strongly with air relative humidity (R<sup>2</sup>=0.46, p<0.001). We conclude that these two isotopic signals of leaf water are not simply mirror images of the same environmental information, but carry distinct signals of different climate factors, with crucial implications for the interpretation of downstream isotopic biomarkers.</p>

2012 ◽  
Vol 8 (5) ◽  
pp. 1457-1471 ◽  
Author(s):  
T. J. Daley ◽  
D. Mauquoy ◽  
F. M. Chambers ◽  
F. A. Street-Perrott ◽  
P. D. M. Hughes ◽  
...  

Abstract. Ombrotrophic raised peatlands provide an ideal archive for integrating late Holocene records of variations in hydroclimate and the estimated stable isotope composition of precipitation with recent instrumental measurements. Modern measurements of mean monthly surface air temperature, precipitation, and δD and δ18O-values in precipitation from the late twentieth and early twenty-first centuries provide a short but invaluable record with which to investigate modern relationships between these variables, thereby enabling improved interpretation of the peatland palaeodata. Stable isotope data from two stations in the Global Network for Isotopes in Precipitation (GNIP) from southern South America (Punta Arenas, Chile and Ushuaia, Argentina) were analysed for the period 1982 to 2008 and compared with longer-term meteorological data from the same locations (1890 to present and 1931 to present, respectively). δD and δ18O-values in precipitation have exhibited quite different trends in response to local surface air temperature and precipitation amount. At Punta Arenas, there has been a marked increase in the seasonal difference between summer and winter δ18O-values. A decline in the deuterium excess of summer precipitation at this station was associated with a general increase in relative humidity at 1000 mb over the surface of the Southeast Pacific Ocean, believed to be the major vapour source for the local precipitation. At Ushuaia, a fall in δ18O-values was associated with an increase in the mean annual amount of precipitation. Both records are consistent with a southward retraction and increase in zonal wind speed of the austral westerly wind belt. These regional differences, observed in response to a known driver, should be detectable in peatland sites close to the GNIP stations. Currently, insufficient data with suitable temporal resolution are available to test for these regional differences over the last 3000 yr. Existing peatland palaeoclimate data from two sites near Ushuaia, however, provide evidence for changes in the late Holocene that are consistent with the pattern observed in modern observations.


2021 ◽  
Vol 18 (19) ◽  
pp. 5363-5380
Author(s):  
Johannes Hepp ◽  
Christoph Mayr ◽  
Kazimierz Rozanski ◽  
Imke Kathrin Schäfer ◽  
Mario Tuthorn ◽  
...  

Abstract. 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.


2005 ◽  
Vol 63 (1) ◽  
pp. 31-44 ◽  
Author(s):  
Meena Balakrishnan ◽  
Crayton J. Yapp ◽  
David J. Meltzer ◽  
James L. Theler

Well-preserved aragonitic land snail shells (Vallonia) from late Pleistocene Eolian sediment in the Folsom archaeological site in New Mexico exhibit an overall decrease of δ18OPDB from maximum values of +2.7‰ (more positive than modern) to younger samples with lower average values of about −3.6‰ (within the modern range). The age of the samples (approximately 10,500 14C yr B.P.) suggests that the decrease in δ18O may manifest climatic changes associated with the Younger Dryas. Some combination of increased relative humidity and cooler temperatures with decreased δ18O of precipitation during the times of snail activity can explain the decrease in shell δ18O. A well-known Paleoindian bison kill occurred at the Folsom site during this inferred environmental transition.Average δ13C values of the aragonite shells of the fossil Vallonia range from −7.3 to −6.0‰ among different archaeological levels and are not as negative as modern values. This suggests that the proportion of C4 vegetation at the Folsom site approximately 10,500 14C yr B.P. was greater than at present; a result which is consistent with other evidence for higher proportions of C4 plants in the region at that time.


2012 ◽  
Vol 8 (1) ◽  
pp. 595-620 ◽  
Author(s):  
T. J. Daley ◽  
D. Mauquoy ◽  
F. M. Chambers

Abstract. Ombrotrophic raised peatlands provide an ideal archive for integrating late Holocene records of variations in hydroclimate and the estimated stable isotope composition of precipitation with recent instrumental measurements. Modern measurements of mean monthly surface air temperature, precipitation and δD and δ18O values in precipitation from the late twentieth and early twenty-first centuries provide a short but invaluable record with which to investigate modern relationships between these variables, thereby enabling improved interpretation of the peatland palaeodata. Data from two stations in the Global Network for Isotopes in Precipitation (GNIP) from Tierra del Fuego (Punta Arenas, Chile and Ushuaia, Argentina) were analysed for the period 1982 to 2008. In both locations, δD and δ18O values have decreased in response to quite different trends in local surface air temperature and total precipitation amount. At Ushuaia, the fall in δ18O values is associated with an increase in the mean annual amount of precipitation. At Punta Arenas, the fall in δ18O values is weakly associated with decrease in the precipitation amount and an increase in local temperatures. The pattern in both records is consistent with an increase in the zonal intensity of the southern westerly wind belt. These regional differences, observed in response to a known driver, should be detectable in peatland sites close to the GNIP stations. There is currently insufficient availability of suitably temporally resolved data to test for these regional differences over the last 3000 yr. Existing peatland palaeoclimate data from two sites near Ushuaia, however, provide evidence for changes in the late Holocene that are consistent with the pattern observed in modern observations. Furthermore, the records suggest synchroneity in millennial-scale oscillations between the Northern and Southern Hemispheres.


2019 ◽  
Author(s):  
Johannes Hepp ◽  
Bruno Glaser ◽  
Dieter Juchelka ◽  
Christoph Mayr ◽  
Kazimierz Rozanski ◽  
...  

Abstract. 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 


2013 ◽  
Vol 33 (4) ◽  
pp. 1031-1041 ◽  
Author(s):  
罗伦 LUO Lun ◽  
余武生 YU Wusheng ◽  
万诗敏 WAN Shimin ◽  
周平 ZHOU Ping

2020 ◽  
Author(s):  
Adnan Moussa ◽  
Ginevra Fabiani ◽  
Julian Klaus

&lt;p&gt;The vadose zone is a key component of the critical zone (CZ) and the interface of the atmosphere and the subsurface. A better understanding of critical zone hydrological processes is key for improving hydrological models and sustainable resource management. Isotopes of Hydrogen (&lt;sup&gt;2&lt;/sup&gt;H) and Oxygen (&lt;sup&gt;18&lt;/sup&gt;O) are a common tool to decipher hydrological processes in the CZ. However, there is still lack in understanding the spatiotemporal distribution of the soil water stable isotope composition (&lt;sup&gt;2&lt;/sup&gt;H and &lt;sup&gt;18&lt;/sup&gt;O) at catchment scale. Until today, only a few studies evaluated long-term variability and spatial patterns. Here we present results of bi-weekly measurements of the soil water stable isotope over nine months. SWI composition were measured using direct vapour equilibration and accounted for different landscape elements (eight locations per campaign) in the forested Weierbach (~0.42 km&lt;sup&gt;2&lt;/sup&gt;) experimental catchment in Luxembourg.&lt;/p&gt;&lt;p&gt;Preliminary results show that a strong similarity of &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O depth profiles between different landscape elements at the same sampling date. However, after a snowmelt event we observed a much higher variability throughout the catchment likely from different melt, fractionation, and infiltration processes. The &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O profiles throughout the landscape change consistently with time driven by a combination of rainfall and evaporation. Lc-excess data showed that soil water was experiencing kinetic evaporative fractionation in the top 30 cm of the soil throughout the year. The presented high frequent data on isotopic composition of soil pore water are useful to analyse spatial difference in vadose zone processes for better understanding soil-atmosphere interaction and flow processes. Eventually such data can be used for constraining spatially distributed hydrological models.&lt;/p&gt;


2020 ◽  
Author(s):  
Johannes Hepp ◽  
Christoph Mayr ◽  
Kazimierz Rozanski ◽  
Imke Kathrin Schäfer ◽  
Mario Tuthorn ◽  
...  

Abstract. 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 


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