Isotopic Composition Of Source Water Research Articles

Land-use-based freshwater sediment source fingerprinting using hydrogen isotope compositions of long-chain fatty acids

Rapidly changing land use patterns and frequent extreme weather events have resulted in an increased sediment flux to freshwater systems globally, highlighting the need for land-use-based sediment source fingerprinting. Application of variability in hydrogen isotope compositions (δ2H values) of vegetation-specific biomarkers from soils and sediments is relatively underexplored for land-use-based freshwater suspended sediment (SS) source fingerprinting, but has the potential to complement the information from routinely applied carbon isotope analysis and provide new insights. We analysed δ2H values of long-chain fatty acids (LCFAs) as vegetation-specific biomarkers in source soils and SS collected from the mixed land use Tarland catchment (74 km2) in NE Scotland, to identify stream SS sources and quantify their contributions to SS. Plant growth form was the primary control on source soils LCFAs (n-C26:0, n-C28:0, n-C30:0) δ2H variability, while the isotopic composition of source water had no significant control. Forest and heather moorland soils covered with dicotyledonous and gymnosperm species were differentiated from arable land and grasslands soils covered with monocotyledonous species. SS samples collected for fourteen months from the Tarland catchment with a nested sampling approach showed monocot-based land use (cereal crops, grassland) to be the major source of SS with 71 ± 11% contribution on catchment-wide scale averaged throughout the sampling period. Storm events after a dry summer period and sustained high flow conditions in the streams during autumn and early winter suggested enhanced connectivity of more distant forest and heather moorland land uses covering relatively steep topography. This was shown by an increased contribution (44 ± 8%) on catchment-wide scale from dicot and gymnosperm-based land uses during the corresponding period. Our study demonstrated successful application of vegetation-specificity in δ2H values of LCFAs for land-use-based freshwater SS source fingerprinting in a mesoscale catchment where δ2H values of LCFAs were primarily controlled by plant growth forms.

Isotopic Equilibrium Between Precipitation and Water Vapor in Northern Patagonia and Its Consequences on δ18Ocellulose Estimate

AbstractModeling work of the isotopic composition of tree ring cellulose (δ18Ocell) relies on the isotopic equilibrium assumption between atmospheric water vapor and tree source water, frequently assimilated to integrated precipitation. Here, we explore the veracity of this assumption based on observations collected during a field campaign in Río Negro province (Argentina) in February–March 2017. We examine how the observed isotopic composition of water vapor deviates from equilibrium with precipitation. This deviation, named isotopic disequilibrium (Δ18Ovap_eq), is low (between −2.0‰ and 4.1‰) and a significant relationship is observed between the isotopic composition of water vapor and its expected value at equilibrium. Negative Δ18Ovap_eq can be explained by evaporation of small raindrops (from 1% to 5% of initial droplet mass). Positive Δ18Ovap_eq can result from vegetation transpiration with transpired water accounting for 14% to 29% to ambient water vapor. The low Δ18Ovap_eq at the study site may be due to the high level of relative humidity (from 70% to 96%) favoring isotopic diffusive exchanges between the two water phases and thus promoting the isotopic equilibrium. We examine the impact of the isotopic equilibrium assumption on the calculation of δ18Ocell. A perfect agreement is shown between observed and calculated δ18Ocell provided that the isotopic composition of source water is significantly higher than the expected averaged isotopic composition of precipitation over the tree growing period.

Evaluation of bacterial glycerol dialkyl glycerol tetraether and <sup>2</sup>H–<sup>18</sup>O biomarker proxies along a central European topsoil transect

Abstract. Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as δ2H of leaf wax-derived n-alkanes (δ2Hn-alkane) or δ18O of hemicellulose-derived sugars (δ18Osugar), are increasingly used for the reconstruction of past climate and environmental conditions. Plant-derived δ2Hn-alkane and δ18Osugar values record the isotopic composition of plant source water (δ2Hsource-water and δ18Osource-water), which usually reflects mean annual precipitation (δ2Hprecipiation and δ18Oprecipiation), modulated by evapotranspirative leaf water enrichment and biosynthetic fractionation (εbio). Accuracy and precision of respective proxies should be ideally evaluated at a regional scale. For this study, we analysed topsoils below coniferous and deciduous forests as well as grassland soils along a central European transect in order to investigate the variability and robustness of various proxies and to identify effects related to vegetation. Soil pH values derived from brGDGTs correlate reasonably well with measured soil pH values but are systematically overestimated (ΔpH = 0.6±0.6). The branched vs. isoprenoid tetraether index (BIT) can give some indication whether the pH reconstruction is reliable. Temperatures derived from brGDGTs overestimate mean annual air temperatures slightly (ΔTMA=0.5 ∘C ± 2.4). Apparent isotopic fractionation (εn-alkane/precipitation and εsugar∕precipitation) is lower for grassland sites than for forest sites due to signal damping; i.e. grass biomarkers do not record the full evapotranspirative leaf water enrichment. Coupling δ2Hn-alkane with δ18Osugar allows us to reconstruct the stable isotopic composition of the source water more accurately than without the coupled approach (Δδ2H = ∼-21 ‰ ± 22 ‰ and Δδ18O = ∼-2.9 ‰ ± 2.8 ‰). Similarly, relative humidity during daytime and the vegetation period (RHMDV) can be reconstructed using the coupled isotope approach (ΔRHMDV=∼-17±12). Especially for coniferous sites, reconstructed RHMDV values as well as source water isotope composition underestimate the measured values. This can likely be explained by understorey grass vegetation at the coniferous sites contributing significantly to the n-alkane pool but only marginally to the sugar pool in the topsoils. Vegetation-dependent variable signal damping and εbio (regarding 2H between n-alkanes and leaf water) along our European transect are difficult to quantify but likely contribute to the observed underestimation in the source water isotope composition and RH reconstructions. Microclimate variability could cause the rather large uncertainties. Vegetation-related effects do, by contrast, not affect the brGDGT-derived reconstructions. Overall, GDGTs and the coupled δ2Hn-alkane–δ18Osugar approach have great potential for more quantitative paleoclimate reconstructions.

Relationship of leaf oxygen and carbon isotopic composition with transpiration efficiency in the C4 grasses Setaria viridis and Setaria italica.

Leaf carbon and oxygen isotope ratios can potentially provide a time-integrated proxy for stomatal conductance (gs) and transpiration rate (E), and can be used to estimate transpiration efficiency (TE). In this study, we found significant relationships of bulk leaf carbon isotopic signature (δ13CBL) and bulk leaf oxygen enrichment above source water (Δ18OBL) with gas exchange and TE in the model C4 grasses Setaria viridis and S. italica. Leaf δ13C had strong relationships with E, gs, water use, biomass, and TE. Additionally, the consistent difference in δ13CBL between well-watered and water-limited plants suggests that δ13CBL is effective in separating C4 plants with different availability of water. Alternatively, the use of Δ18OBL as a proxy for E and TE in S. viridis and S. italica was problematic. First, the oxygen isotopic composition of source water, used to calculate leaf water enrichment (Δ18OLW), was variable with time and differed across water treatments. Second, water limitations changed leaf size and masked the relationship of Δ18OLW and Δ18OBL with E. Therefore, the data collected here suggest that δ13CBL but not Δ18OBL may be an effective proxy for TE in C4 grasses.

δD values of n-alkanes in sediments from Gahai Lake, Gannan, China: implications for sources of organic matter

Gahai Lake in the Gannan region, located in the northeast of the Tibetan Plateau, is the largest pristine freshwater lake on the plateau. Surface sediments in the Gahai Lake were systematically sampled, and n-alkane distribution and hydrogen isotope were analyzed by a gas chromatograph-mass spectrometer and a gas chromatograph-interface-isotope ratio mass spectrometer. The δD values of n-alkanes in the sediments ranged from −247 to −161 ‰ and the mean values varied between −222 and −182 ‰. The studied sediments were divided into types I and II based on the average δD values of n-alkanes. The average δD values of n-alkanes in type I sediments were significantly higher than those in type II sediments and there were good correlations among the δD values of the n-alkanes from the two sediment types. The δD values of type I sediments indicated that the C21–C33 odd-numbered n-alkanes were derived mainly from aquatic plants in the Gahai Lake, while the δD values of type II sediments showed that C27–C33 odd-number n-alkanes were derived from terrestrial herbaceous plants. This assessment of n-alkane sources was also confirmed by their distributions. It was inferred that the medium-chain length n-alkanes of relatively high abundance, low δD values, and low carbon preference index values were derived principally from bacteria. The distribution patterns of the sedimentary n-alkane δD values for the plateau humid-climate freshwater lake and dry-climate saline lake systems were compared. The results further confirmed that sedimentary n-alkanes record the hydrogen isotopic composition of source water used by organisms. Therefore, δD values of sedimentary n-alkanes can be used as geochemical indicators for sources of sedimentary organic matter and in paleoclimate studies. Our results also showed that the δD values of n-alkanes from sediments within the lake can differ significantly for a given time period owing to their different biological sources. This should be taken into account when reconstructing paleoclimatic and hydrologic conditions using sedimentary n-alkane δD values.

The 8.2‐ka BP event in north‐eastern North America: first combined oxygen and hydrogen isotopic data from peat in Newfoundland

ABSTRACTFinding direct evidence for atmospheric circulation change in terrestrial records of Holocene climate variability remains a fundamental challenge. Here we present the first combined stable oxygen and hydrogen isotopic palaeorecord from a peatland core in Newfoundland, Canada. Sphagnum cellulose samples were isolated from a core from Nordan's Pond Bog, Newfoundland, and analysed for δD values. Combined with existing δ18O data, the resulting δD/δ18O bi‐plot correlates directly with existing measurements of the modern (late 20th century) isotopic composition of precipitation from GNIP stations in Nova Scotia and Labrador, implying a close relationship between the estimated isotopic composition of source water used by the mosses and that of the source precipitation. We use the relative variations between the two isotope records to test the hypothesis that atmospheric circulation changed in the millennium following the 8.2‐ka BP climate event. The data reveal a secondary complex isotopic response ∼200 years (8250–8050 a BP) after a primary oxygen isotopic event that is widespread in the north Atlantic region. This secondary event is characterized by a divergence in oxygen and hydrogen isotope records that can most plausibly be explained by the augmentation of precipitation moisture from a more distant and more continental vapour source.

Physicochemical analyses of surface sediments from the Lonar Lake, central India – implications for palaeoenvironmental reconstruction

We report the results of our investigations on the catchment area, surface sediments, and hydrology of the monsoonal Lonar Lake, central India. Our results indicate that the lake is currently stratified with an anoxic bottom layer, and there is a spatial heterogeneity in the sensitivity of sediment parameters to different environmental processes. In the shallow (0-5 m) near shore oxic-suboxic environments the lithogenic and terrestrial organic content is high and spatially variable, and the organics show degradation in the oxic part. Due to aerial exposure resulting from lake level changes of at least 3m, the evaporitic carbonates are not completely preserved. In the deep water (>5 m) anoxic environment the lithogenics are uniformly distributed and the delta C-13 is an indicator not only for aquatic vs. terrestrial plants but also of lake pH and salinity. The isotopic composition of the evaporites is dependent not only on the isotopic composition of source water (monsoon rainfall and stream inflow) and evaporation, but is also influenced by proximity to the isotopically depleted stream inflow. We conclude that in the deep water environment lithogenic content, and isotopic composition of organic matter can be used for palaeoenvironmental reconstruction.

Inverse relationship between salinity and n-alkane δD values in the mangrove Avicennia marina

Hydrogen isotope ratios in lipids derived from mangroves have the potential to be used for paleohydrologic reconstructions and could serve as a much needed tool for establishing past climate variability in the tropics. We assessed the effect of salinity on the apparent fractionation factor, αa, between mangrove derived n-alkanes and their source water for Avicennia marina (gray mangrove) specimens collected along a 28PSU salinity gradient in the Brisbane River Estuary. Our results indicate that there is an inverse relationship between the apparent fractionation factor and salinity. This salinity effect is large enough to override variability in the isotopic composition of source water, which plays a dominant role in determining the hydrogen isotope ratio of leaf waxes in other vascular plants. We suggest that this relationship may be due to (i) increased discrimination against deuterium during water uptake at high salinity, (ii) increased production of compatible solutes from D enriched pyruvate at high salinity, resulting in more hydrogen from D depleted NADPH being incorporated in leaf waxes, and/or (iii) increased secretion of salty brine by leaves at high salinity, resulting in (iii.a) higher relative humidity at the leaf surface, and (iii.b) introducing the possibility that D depleted water of hydration is absorbed by the leaf. Our results indicate that hydrogen isotope ratios of mangrove lipid biomarkers can be developed as a paleosalinity indicator. They also imply that care must be taken when interpreting hydrogen isotopic variations in non-source specific higher plant lipids in sediments where both mangrove and non-mangrove plants contribute organic material.

Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability

Hydrogen isotope ratios were measured on n-alkanes (n-C 12 to n-C 31) extracted from recent lake surface sediments along a N-S European transect to test if modern climate variability is recorded in these biomarkers. δD values of the n-alkanes are compared to δD values of meteoric water from the IAEA-GNIP database spanning a range from −119‰ in northern Sweden to −41‰ in southern Italy, to lake water δD values, and to mean annual temperatures, varying between −2.0°C in the north and 13.7°C in the south. δD values of the short-chained n-alkanes n-C 12 to n-C 20, excluding algal derived n-C 17 and n-C 19, are higher in the north and lower in the south. The isotopic fractionation ε for hydrogen between meteoric water and the short-chained n-alkanes is increasing from N to S by more than 100‰ and is significantly correlated to mean annual temperature for n-C 16 and n-C 18. This suggests that these n-alkanes may originate from a different source in the northern lakes, possibly due to petroleum contamination, or are synthesized using a different biochemical pathway. The n-C 17 and n-C 19 alkanes of algal origin, the n-C 21 and n-C 23 alkanes originating from water plants, and the long-chain n-alkanes n-C 25, n-C 27, n-C 29, and n-C 31 of terrestrial origin, clearly correlate with δD values of meteoric water, lake water, and mean annual temperature, indicating that they excellently record the δD value of meteoric water. The mean hydrogen isotope fractionation ε C17/w of −157‰ (SD = 13) between n-C 17 and meteoric water is fairly constant over the wide range of different climates and lake environments, suggesting only minor influence of environmental factors on this biochemical fractionation. This suggests that δD values of n-C 17 are suitable to reconstruct the isotopic composition of source water. The mean fractionation between the long-chain n-alkanes and water is −128‰ (SD = 12). The mean difference of 31‰ between both ε values is likely due to evaporative enrichment of deuterium in the leaf water. If this is the only influence on the enrichment, the difference between the δD values of terrestrial and aquatic compounds might be suitable to reconstruct terrestrial evapotranspiration of the lake environment.

There is no Temperature Dependence of Net Biochemical Fractionation of Hydrogen and Oxygen Isotopes in Tree-Ring Cellulose

The isotopic composition of tree-ring cellulose was obtained over a two-year period from small diameter, riparian zone trees along an elevational transect in Big Cottonwood Canyon, Utah, USA to test for a possible temperature dependence of net biological fractionation during cellulose synthesis. The isotope ratios of stream water varied by only 3.6‰ and 0.2‰ in δD and δ18O, respectively, over an elevation change of 810m. The similarity in stream water and macroenvironment over the short (13km) transect produced nearly constant stem and leaf water δD and δ18O values. In addition, what few seasonal variations observed in the isotopic composition of source water and atmospheric water vapor or in leaf water evaporative enrichment were experienced equally by all sites along the elevational transect. The temperature at each site along the transect spanned a range of ≥ 5°C as calculated using the adiabatic lapse rate. Since the δD and δ18O values of stem and leaf water varied little for these trees over this elevation/ temperature transect, any differences in tree-ring cellulose δD and δ18O values should have been associated with temperature effects on net biological fractionation. However, the slopes of the regressions of elevation versus the δD and δ18O values of tree-ring cellulose were not significantly different from zero indicating little or no temperature dependence of net biological fractionation. Therefore, cross-site climatic reconstruction studies using the isotope ratios of cellulose need not be concerned that temperatures during the growing season have influenced results.

Hydrogen and oxygen isotope ratios of tree-ring cellulose for riparian trees grown long-term under hydroponically controlled environments.

Saplings of three riparian tree species (alder, birch and cottonwood) were grown for over 5 months in a hydroponics system that maintained the isotopic composition of source water in six treatments, ranging from -120 to +180‰δD and -15 to +10‰δ18O. The trees were grown in two greenhouses maintained at 25°C and at either 40 or 75% relative humidity, creating differences in transpiration rates and leaf water isotopic evaporative enrichment. The cellulose produced in the annual growth ring was linearly related to source water with differences in both slope and offset associated with greenhouse humidity. The slope of the isotopic composition of source water versus tree-ring cellulose was less than 1 for both δD and δ18O indicating incomplete isotopic exchange of carbohydrate substrate with xylem water during cellulose synthesis. Tests using the outer portion of the tree-ring and new roots were similar and showed that the tree-ring values were representative of the cellulose laid down under the imposed environmental conditions. The fraction of H and O in carbohydrate substrate that isotopically exchange with medium water was calculated to be 0.36 and 0.42 respectively, and biochemical mechanisms for these observed fractions are discussed. A mechanistic model of the biochemical fractionation events for both δD and δ18O leading to cellulose synthesis was robust over the wide range of cellulose stable isotope ratios. The experimental results indicate that both water source and humidity information are indeed recorded in tree-ring cellulose. These results help to resolve some of the disparate observations regarding the interpretation of stable isotope ratios in tree-rings found in the literature.

Isotopic characteristics of ice cover in a large northern river basin

Isotope stratigraphy of river-ice covers can be applied to reconstruct time-series of the stable isotopic composition of winter streamflow, although careful attention to ice type, morphology and location of sampling is required to obtain useful results. Ice surveys conducted in the Liard River Basin, northwestern Canada, suggest that the isotopic composition of winter streamflow is relatively invariant along the main river stem. However, systematic isotopic enrichment is observed over the course of the winter for small tributaries draining discontinuous-permafrost wetlands, and is attributed to shifts in the isotopic composition of source water. Recommendations for further study required to verify the underlying physical processes associated with this response are outlined. Copyright © 1999 John Wiley & Sons, Ltd.

Isotopic characteristics of ice cover in a large northern river basin

Isotope stratigraphy of river-ice covers can be applied to reconstruct time-series of the stable isotopic composition of winter streamflow, although careful attention to ice type, morphology and location of sampling is required to obtain useful results. Ice surveys conducted in the Liard River Basin, northwestern Canada, suggest that the isotopic composition of winter streamflow is relatively invariant along the main river stem. However, systematic isotopic enrichment is observed over the course of the winter for small tributaries draining discontinuous-permafrost wetlands, and is attributed to shifts in the isotopic composition of source water. Recommendations for further study required to verify the underlying physical processes associated with this response are outlined.