Average δD Values Research Articles

On the origin and evolution of deuterium enrichment in type 1 and 2 chondritic organic solids

Rotationally resonant Deuterium Nuclear Magnetic Resonance spectroscopy (D MAS NMR) was applied to IOM isolated from a CR1 chondrite Grosvenor Mountains (GRO) 95577 and a CM2 chondrite (Murchison). It is shown that in IOM D strongly prefers the aliphatic hydrogen reservoir over the aromatic hydrogen reservoir. For GRO 95577, that has a bulk δD of 3303 ‰ (Alexander et al., 2010), the average δD value of the aromatic reservoir is 1740 ± 128 ‰ and the aliphatic reservoir is 4477 ± 105 ‰, i.e., D/H enrichments of 1.27 and 0.64, respectively, relative to the bulk. For Murchison IOM, that has a bulk δD of 811 ‰ (Alexander et al., 2010), the average δD of the aromatic reservoir is 512 ± 88 ‰ and the aliphatic reservoir is 1033 ± 64 ‰ i.e., D/H enrichments of 1.12 and 0.82, respectively, relative to the bulk. D-H exchange between D-enriched water and a type III kerogen reveals nearly equivalent D up take by both aromatics and aliphatics. Laboratory synthesis of IOM-like material in the presence of D2O reveals a high degree of deuteration with a strong preferential deuteration of the aliphatic hydrogen reservoir indicating that the δD of the water during IOM synthesis is the primary determinant of syn-IOM’s δD. The IOM in GRO 95577 and Murchison (FA and H/C × 100) lie on the molecular evolution line as defined by the IOM of the Tagish Lake clasts and Murchison IOM has experienced more molecular evolution relative to that exhibited by GRO 95577 IOM. A forward prediction derived from the D/H ratios for the aliphatic and aromatic hydrogen reservoirs in Murchison and GRO 95577, relative to their bulk D/H ratios, derived from D MAS NMR, is applied to explain the origin of the Tagish Lake trend of δD vs molecular evolution (H/C × 100). The results of this forward prediction suggest that the Tagish Lake isotopic trend results from a combination of molecular evolution (loss of predominantly aliphatic H and D) and partial D-H exchange with D depleted chondritic water during a short-term hydrothermal alteration event. Such events may be faithfully identified in chondritic organic solids and be a common occurrence, but not necessarily revealed in the mineralogy of type 1 and 2 carbonaceous chondrites.

A Comparison Study on the δD Value of Polycyclic Aromatic Hydrocarbons in Hydrous Pyrolysate of Herbaceous Peat with Different Water Medium

AbstractTo investigate the influence of diagenetic water media on the hydrogen isotopes of individual sedimentary aromatic compounds, a series of hydrous pyrolyses were conducted on herbaceous peat. Polycyclic aromatic hydrocarbons (PAHs) in hydrous pyrolysed samples and their hydrogen isotopic composition characteristics were studied. The aqueous medium demonstrated a significant influence on the hydrogen isotopic composition of the individual PAHs generated during pyrolysis. The results showed that the PAHs formed after pyrolysis in the presence of a saltwater medium with high δD value from a salt lake had a heavy hydrogen isotopic composition. The PAHs formed after pyrolysis in the presence of a fresh water medium with low δD value from a swamp had a light hydrogen isotopic composition. The difference in the average PAH δD value between the two hydrous experiments varied from –174‰ to –109‰, suggesting that the hydrogen isotopic composition of individual sedimentary PAHs can reflect the source of the diagenetic water medium. In addition, a comparative study found that the hydrogen isotopes of PAHs were superior to those of n‐alkanes in the same sample for diagenetic water indications. The results indicated that the exchange of water‐derived inorganic hydrogen and organic hydrogen was more intensive in freshwater experiments than in saltwater experiments. With an increase in the simulation temperature, the average δD value of PAHs generated in the hydrous simulation experiments showed an increasing trend, reflecting that the δD value of sedimentary PAHs formed with the participation of diagenetic water media was still closely related to the thermal maturity of organic matter. Comparative studies showed that the δD values of different types of organic compounds produced by hydrous pyrolysis of peat were in the order, PAHs > n‐alkanes > methane.

Exploring the Deuterium Excess of Cretaceous Arctic Paleoprecipitation Using Stable Isotope Composition of Clay Minerals from the Prince Creek Formation (Maastrichtian) in Northern Alaska

We report estimated stable isotope compositions of Artic paleoprecipitation using phyllosilicates sampled from three paleosols and two bentonites in the Prince Creek Formation (Maastrichtian) in northern Alaska. Previous studies reported a deuterium excess in estimates of Arctic paleoprecipitation from the Late Cretaceous by combining hydrogen and oxygen proxy sources, including pedogenic minerals, dinosaurian tooth enamel phosphates, pedogenic siderites, and n-alkane biomarkers. The new dataset produced in this study removes uncertainty on possible explanations (photosynthetic and transpiration) of the deuterium excess by producing stable hydrogen and oxygen isotopic signatures from the same source material. The δD of the phyllosilicates range from −171‰ to −72‰ VSMOW and δ18O ranges from 5.0 to 11.8‰ VSMOW. By assuming a MAT of 6.3 °C and calculating uniquely derived fractionation equations for each phyllosilicate, we report estimated isotopic composition of Late Cretaceous paleoprecipitation with an average δD value of −133‰ VSMOW, corresponding to an average δ18O value of −20.3‰ VSMOW. The estimates of Late Cretaceous paleoprecipitation do not intersect the Global Meteoric Water Line and reveal a reported deuterium excess ranging from 7 to 46 per mil. These results confirm the presence of a deuterium excess in Late Cretaceous Arctic paleoprecipitation and provide new insight to assessing possible explanations for this phenomenon.

Pore characteristics and preservation mechanism of over-6000-m ultra-deep shale reservoir in the Sichuan Basin

Investigation of pore characteristics and their preservation mechanism of over 6000 m ultra-deep shale reservoirs is of significance for shale gas exploration and development in the Sichuan Basin. In this study, the pores structure and multifractal characteristics of pores for the Longmaxi shale and Qiongzhusi shale in the Sichuan Basin are well studied by using field emission scanning electron microscopy (FE-SEM), gas adsorption and multifractal analysis. The results show that: (1) Extremely strong mechanical compaction resulting from ultra-deep burial depth can lead to the homogenization of pore structure, which is characterized by the change of bubble-like OM pores to silt or fracture shape pores and the higher average pore diameter (APD) value. (2) The Longmaxi shale and Qiongzhusi shale reservoirs have the obvious multifractal nature for different pore sizes. Samples from the Longmaxi shale and Qiongzhusi shale in well CS#1 demonstrate the higher average H values and smaller average ΔD values compared with those samples in well MS#1, indicating that shale reservoir in over-pressure condition has higher connectivity and less heterogeneity. The pore preservation in over 6000 m ultra-deep shale reservoirs are influenced by several geological factors, including 1) quartz is beneficial for the preservation of pores especially for OM pores due to its supporting effect; 2) the shale reservoirs in over-pressure show many more OM pores and higher surface porosity than those in normal pressure; 3) the direct floor of Qiongzhusi shale is likely the critical geological factor affecting the pores preservation. Therefore, the Longmaxi shale with both over-pressure condition and high quartz content is likely the best target zone for deep shale gas exploration in the Sichuan Basin.

Isotopic compositions (δD, δ18O) and end-member mixing for the control interface in a complex tidal region

Identifying the mixing processes of waters and currents in tidal reach is an important aspect of environmental management to protect freshwater resources and prevent water pollution. In this study, three field investigations conducted in a typical tidal reach in August, November and the following April focused on two isotopes (δD and δ18O) and salinity. A salinity-isotope conservative mixing model was established to differentiate water flows of the important control interface (CI) from freshwater, transition zone and saltwater end-members. Results suggested that the average δD and δ18O values during the ebb and flood tides depleted from August to November, then enriched significantly in the following April and were even higher than those in August. The δD and δ18O values in the saltwater zone enriched markedly compared with those in freshwater zone and transition zone due to the stronger evaporation occurring in the saltwater zone. Based on the revised model, the average contributions of freshwater end-member, transition zone end-member and saltwater end-member in three months were, respectively, 51.50 %, 36.93 % and 11.57 %. However, the contributions of freshwater and transition zones in April end-member were equivalent (47.45 % vs 44.31 %). Meanwhile the largest contribution of saltwater end-member was 20.56 % and occurred in August. The proportions of three end-members that contributed to CI changed with different evaporation scenarios and moisture sources of precipitation. Our research provides important information that furthers our understanding of the isotopes and their applications to environmental management in estuarine regions.

Stable Isotope Tracers of Cretaceous Arctic Paleoprecipitation

We report estimated stable isotope compositions of depositional waters and paleoprecipitation from the Cretaceous Arctic to further elucidate the role of the global hydrologic cycle in sustaining polar warmth during that period. Estimates are based on new hydrogen isotopic analyses of n-alkane biomarkers extracted from Late Cretaceous and mid-Cretaceous terrestrial deposits in northern Alaska and the Canadian High Arctic. We integrate these new results with earlier published work on oxygen isotopic analyses of pedogenic siderites, dinosaurian tooth enamel phosphates, and pedogenic clay minerals from the same field areas. Average Late Cretaceous δD values of −143‰ VSMOW corresponded with average δ18O values of −24.1‰ VSMOW, and average mid-Cretaceous δD values of −106‰ VSMOW corresponded with average δ18O values of −22.1‰ VSMOW. The distributions of water isotope δD and δ18O values from Cretaceous Arctic deposits do not intersect with the Global Meteoric Water Line, suggesting an apparent deuterium excess ranging from about 40 to 60 per mil. We considered several possible explanations for these Cretaceous results including (1) mass-balance changes in zonal patterns of evaporation and precipitation at lower latitudes, (2) concentration of 2H in leaf tissue waters from continuous transpiration by coniferous paleofloras during the Arctic growing season, and (3) concentration of 2H in the groundwaters of methane-emitting Arctic soils.

Characterization of spatiotemporal patterns of soil water stable isotopes at an agricultural field

Spatiotemporal variations in soil water content (SWC) and soil water stable isotopic compositions (SWSIC; 2H/1H (δD) and 18O/16O (δ18O)) provide critical information on elucidating land surface processes across scales. Meanwhile, little is known about the spatiotemporal characteristics of SWSIC and its driving factors. Therefore, it's necessary to improve tracer techniques of SWSIC by interpreting their spatiotemporal variability patterns as well as the correlations with other factors such as texture, soil depth and vegetation. To this end, the spatiotemporal variations in SWC and SWSIC along with their controlling factors were jointly investigated based on seven field campaigns over roughly a two-year period at an agricultural field in North China Plain. Two transects, vegetated and bared, were considered. The results of vegetated transect showed that both SWC and SWSIC exhibited considerable spatiotemporal variabilities at the field scale of ~100 m, with SWSIC displaying more complex patterns. Overall, the spatial variations in SWSIC were larger in wet seasons than in dry seasons, which decreased with increasing soil depth, largely due to less impacts of precipitation inputs and soil evaporation on SWSIC dynamics at deeper depths. The temporal stability analysis (TSA) showed that there existed temporal persistence of the spatial structure of SWSIC, particularly at deeper soil depths. Moreover, the SWSIC data in our study showed that the effect of vegetation on the SWSIC dynamics was noticeable with shading effects, root distribution and water uptake, which caused much lesser degrees of soil evaporation at the vegetated transect. What's more, the representative sites for monitoring spatial average δD values were identified, demonstrating the viability of using the TSA method to estimate the spatial average SWSIC values at field scales. These findings can improve the interpretation of SWSIC data for practical applications.

A deuterium-poor water reservoir in the asteroid 4 Vesta and the inner solar system

Recent investigations of meteorites thought to originate from the asteroid 4 Vesta have suggested an early accretion of water on rocky bodies in the inner Solar System from a carbonaceous chondrite-like source. However, these studies have been based on the hydrogen isotope compositions (δD) of late-crystallizing apatite grains in eucrites that likely do not record the primary magmatic composition. We have determined the δD and H2O concentrations in some of the earliest-formed silicates (clinopyroxenes) from several eucrites with the goal of constraining the hydrogen isotope composition of their source reservoir on their parent body. The H2O concentrations in clinopyroxenes from eucrites Juvinas, Stannern and Tirhert range from 5 to 18 μg/g, with a weighted average δD of –263 ± 70‰. Their apatites and whitlockites exhibit a higher weighted average δD of –165 ± 73‰, possibly as a result of H2 degassing during or after phosphate crystallization. Thermal metamorphism of these eucrites has most probably resulted in the loss of H, and an increase in their original δD values. While the weighted average δD value for the eucrite clinopyroxenes reported here is inferred to reflect an upper limit for the isotopic composition of the silicate mantle reservoir on their parent asteroid 4 Vesta, the average δD value of Stannern clinopyroxenes is considered to be closest to the initial δD of the source mantle (i.e., –373 ± 127‰), which is lighter than that of Earth’s depleted upper mantle and most carbonaceous chondrites. We suggest that at least some of the water in 4 Vesta (and possibly other rocky bodies in the inner Solar System) was derived from a relatively deuterium-poor reservoir in the protosolar nebula, which was incorporated into planetesimals formed early in Solar System history.

Preservation of primordial signatures of water in highly-shocked ancient lunar rocks

Spurred by the discovery of water in lunar volcanic glasses about a decade ago, the accessory mineral apatite became the primary target to investigate the abundance and source of lunar water. This is due to its ability to contain significant amounts of OH in its structure, along with the widespread presence of apatite in lunar rocks. There is a general understanding that crustal cumulate rocks of the lunar magnesian (Mg) suite are better candidates for recording the original isotopic compositions of volatile elements in their parental melts compared to eruptive rocks, such as mare basalts. Consequently, water-bearing minerals in Mg-suite rocks are thought to be ideal candidates for discerning the primary hydrogen isotopic composition of water in the lunar interior. Mg-suite rocks and most other Apollo samples that were collected at the lunar surface display variable degrees of shock-deformation. In this study, we have investigated seven Apollo 17 Mg-suite samples that include troctolite, gabbro and norite lithologies, in order to understand if shock processes affected the water abundances and/or H isotopic composition of apatite. The measured water contents in apatite grains range from 31 to 964 ppm, with associated δD values varying between −535 ± 134‰ and +147 ± 194‰ (2σ). Considering the full dataset, there appears to be no correlation between H2O and δD of apatite and the level of shock each apatite grain has experienced. However, the lowest δD was recorded by individual, water-poor (<∼100 ppm H2O) apatite grains that are either directly in contact with an impact melt or in its proximity. Therefore, the low-δD signature of apatite could be a result of interactions with D-poor regolith (solar wind derived H), facilitated by shock-induced nanostructures that could have provided pathways for migration of volatiles. In contrast, in relatively water-rich apatites (>∼100 ppm H2O), regardless of the complexity of the shock-induced nanostructures, there appears to be no evidence of water-loss or alteration in their δD. The weighted average δD value of 24 such water-rich apatites is −192 ± 71‰, and, of all 36 analyzed spots is −209 ± 47‰, indistinguishable from that of other KREEPy lunar lithologies or the Earth's deep mantle. Despite experiencing variable degrees of shock-deformation at a later stage in lunar history, water-rich apatite in some of the earliest-formed lunar crustal material appears to retain the original isotopic signature of H in the Moon.

Experimental simulation study on the influence of diagenetic water medium on sedimentary n-alkanes and their respective hydrogen isotopes

In order to understand the influence of the diagenetic water medium on the hydrogen isotope of sedimentary n-alkanes, thermal simulation experiments were carried out involving the initial peat under the addition of alkaline saltwater, acidic freshwater and peat alone condition. The generated n-alkanes were analyzed in terms of molecule composition and hydrogen isotopic composition of individual n-alkanes. The results show that the addition of water had a significant effect on n-alkane nC21−/nC22+ ratio, ACL value and Pr/Ph ratio. The n-alkanes display significant heavier hydrogen isotopic composition from the alkaline saltwater added experiments, but obvious lighter hydrogen isotopic composition from the acidic freshwater added experiments. The difference of the average δD value of n-alkanes between the saltwater and freshwater experiments ranged from 6‰ to 73‰, and maximum difference occurred from the experiments of 350 °C. The exchange of inorganic hydrogen in water and organic hydrogen was more intensive in the acidic freshwater experiments than that in the alkaline saltwater experiments. The average δD value of n-alkanes generated under the participation of saltwater and freshwater media appear to be higher heavier along with the increase in experimental temperature. Also, the average δD value of n-alkanes generated from the saltwater added experiments was always higher than that from the freshwater added experiments. The correlations among the δD composition of n-alkanes generated under different condition were established. These results provide information to understand of the genesis of sedimentary n-alkanes formed under the participation of diverse diagenetic water media at various stages of maturity.

Comparative Study of Hydrogen and Carbon Isotopic Composition of Gases Generated from the Pyrolysis of a Peat under Saltwater and Freshwater Conditions

AbstractTo understand the influence of the diagenetic water medium on the isotopic compositions of thermogenic coalbed gas, both hydrous and anhydrous closed‐system pyrolyses were performed at temperatures of 250°C to 650°C on an herbaceous marsh peat. Compared to the results of anhydrous pyrolysis, the hydrocarbon gases generated from hydrous pyrolyses have very different hydrogen isotopic compositions. However, the carbon isotopic compositions of the hydrocarbon gases became only slightly heavier in hydrous pyrolysis, compared to that from anhydrous pyrolysis. With the progress of thermal evolution from peat to a more advanced thermal maturity of vitrinite reflectance values (Ro) of 5.5% during the pyrolysis, the difference in the average δD value increased from 52‰ to 64‰ between the hydrous pyrolysis with saltwater and anhydrous pyrolysis and increased from 18‰ to 29‰ between the hydrous pyrolysis with freshwater and anhydrous pyrolysis, respectively. The difference in the average δ13C value was only 1‰–2‰ between the hydrous and anhydrous pyrolysis. The relationships between the δD values of the generated hydrocarbon gases and Ro values as well as among δD values of the hydrocarbon gas species are established. The close relationships among these parameters suggest that the water medium had a significant effect on the hydrogen isotopic composition and a minimal effect on the carbon isotopic composition of the hydrocarbon gases. The results of these pyrolyses may provide information for the understanding of the genesis of coalbed gas from herbaceous marsh material with the participation of different diagenetic water media.

Hydrogen isotopic characteristic of hydrocarbon gas pyrolyzed by herbaceous swamp peat in hydrous and anhydrous thermal simulation experiments

In order to understand the influence of diagenetic water medium on hydrogen isotope of thermogenic coalbed gas, the hydrous and anhydrous pyrolytic simulation experiments have been carried out on herbaceous peat. We studied hydrogen isotope composition and its difference and evolution pattern of the pyrolysis hydrocarbon gases It was observed that diagenetic water medium exerts a significant influence on hydrogen isotopic composition of the pyrolysis hydrocarbon gases. The experiment added water with heavier hydrogen isotopic composition than peat-formed environmental water generated hydrocarbon gases with heavier hydrogen isotopic compositions. When peat was subject to continuous pyrolysis until 5.5%Ro, average δD values of the generated methane, ethane and propane were increased by 74‰, 42‰ and 66‰, respectively. It was considered that the reason for hydrous simulation experiment to increase hydrogen isotopic value of pyrolytic hydrocarbon gases is due to isotopic exchange between water-derived hydrogen and organic hydrogen. Mathematical models between the δD values of the hydrocarbon gases generated under the participation of freshwater with higher δD values and Ro values as well as between δD values of the generated hydrocarbon gases were established. These research results provide a scientific basis for the genetic study of thermogenic coalbed methane pyrolyzated by coal-forming materials formed in herbaceous marsh under the participation of diagenetic water media.

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 abundance and isotopic composition of water in eucrites

AbstractVolatile elements play a key role in the dynamics of planetary evolution. Extensive work has been carried out to determine the abundance, distribution, and source(s) of volatiles in planetary bodies such as the Earth, Moon, and Mars. A recent study showed that the water in apatite from eucrites has similar hydrogen isotopic compositions compared to water in terrestrial rocks and carbonaceous chondrites, suggesting that water accreted very early in the inner solar system given the ancient crystallization ages (~4.5 Ga) of eucrites. Here, the measurements of water (reported as equivalent H2O abundances) and the hydrogen isotopic composition (δD) of apatite from five basaltic eucrites and one cumulate eucrite are reported. Apatite H2O abundances range from ~30 to ~3500 ppm and are associated with a weighted average δD value of −34 ± 67‰. No systematic variations or correlations are observed in H2O abundance or δD value with eucrite geochemical trend or metamorphic grade. These results extend the range of previously published hydrogen isotope data for eucrites and confirm the striking homogeneity in the H‐isotopic composition of water in eucrites, which is consistent with a common source for water in the inner solar system.

Thermal simulation study on the influence of coal-forming material on the isotopic composition of thermogenic coalbed gas

In order to understand the influence of coal-forming materials and slight differences in coal maceral composition on the isotopic composition of thermogenic coalbed gas, closed-system isothermal pyrolysis experiments were performed on forest and herbaceous swamp peats and on coals with slight differences in maceral compositions. The carbon and hydrogen isotopic compositions of the hydrocarbon gases (methane, ethane and propane) generated during the pyrolysis of the samples were determined. The results showed that the carbon and hydrogen isotopic compositions of the hydrocarbon gases generated from the herbaceous swamp peat were lighter than those generated from the forest swamp peat. At pyrolysis intervals from peat to vitrinite reflectance values (Ro) of 2.5%, 3.5% and 5.5%, this difference in the average δD value of the generated methane was 18‰, 11‰ and 9‰, and in the δ13C value of methane was 3.4‰, 3.8‰ and 1.8‰, respectively. The reason for the difference of trend can be explained as follows. The methane generated from the coal containing slightly higher exinite content had noticeably lighter carbon isotopic composition. However, the hydrogen isotopic composition of the generated methane was primarily related to coal-forming environmental factors, such as latitude, δD value of rainwater, and diagenetic fluid of freshwater or seawater. These results provide a basis for studying the isotopic geochemistry of coalbed gas.

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

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

Hydrogen isotope ratios of individual n-alkanes in plants from Gannan Gahai Lake (China) and surrounding area

Gannan Gahai Lake, in the northeast of the Tibetan Plateau, is the largest freshwater lake in the Gannan plateau, and is rarely influenced by human activity. Aquatic plants in the lake and terrestrial plants in the surrounding area were systematically collected, and the n-alkane distributions and hydrogen isotopic composition investigated for the first time. The distributions exhibited a unimodal/bimodal distribution with maxima at C17 or C21 and at C23 to C31, and were dominated by odd numbered components with carbon preference index (CPI) values of 2.3–14.7. Average chain length (ACL) values, ranging from 25.7 to 29.6, correlated with the dominant n-alkane carbon number. The average δD values of the n-alkanes ranged from –246‰ to –130‰. In general, the δD values were higher for woody plants, lower for herbaceous plants and in between for aquatic plants. The data show that the hydrogen isotopic composition of the plant n-alkanes was influenced by species, leaf surface size, plant water source and environment. The δD values tended to be lower with increasing ACL values, which results from the reduction in leaf water evaporation. The isotopic fractionation during n-alkane synthesis was larger for herbaceous plants, smaller for woody plants and in between for aquatic plants.

Compound-specific hydrogen isotope composition of n-alkanes in combustion residuals of fossil fuels

The hydrogen isotope compositions (δD) of n-alkanes present in the combustion residuals of fossil fuels (coal, gasoline, and diesel) were measured using GC-IRMS to distinguish between coal soot and vehicle exhaust. The n-alkane δD values of industrial and domestic coal soot ranged from −95.3‰ to −219.6‰ and −128.1‰ to −188.6‰, respectively, exhibiting similar tendencies. The δD values of the C15–C18n-alkanes in both types of coal soot were nearly consistent, and the δD values of the C19–C24n-alkanes exhibited a zigzag profile. The δD values of C16–C22n-alkanes in gasoline exhaust exhibited a saw-tooth distribution, decreased with the carbon number, and were more positive than the δD values of C16–C22n-alkanes in diesel exhaust, which increased with the carbon number. However, the δD values of the C23–C29n-alkanes in gasoline and diesel vehicle exhaust were mostly consistent. The weighted average δD values of the C16–C19n-alkanes in industrial and domestic coal soot were similar to the average δD values in gasoline and diesel vehicle exhausts; however, the average δD values of the C21–C29n-alkanes in vehicle exhausts were richer in D than those in coal soot.

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

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

ORIGIN OF TANZANITE AND ASSOCIATED GEMSTONE MINERALIZATION AT MERELANI, TANZANIA

Tanzanite is the blue vanadium-bearing variety of zoisite. The only known source of gem-quality tanzanite, to date, is the Merelani area of northeastern Tanzania, a structurally complex region consisting of granulite facies metacarbonate and metasilicate rocks overprinted by amphibolite to greenschist facies retrogression. Gem-quality green grossular garnets, known as tsavorite, are also found in the area, as well as a variety of other rare minerals and gemstones (such as axinite, dravite, fluorapatite, prehnite and tremolite). The δ18O values of tanzanite, tremolite, tsavorite and kyanite from the Merelani area are all high (average of 19.1, 19.5, 18.5 and 18.7‰, respectively) and are consistent with high-T O-isotope equilibrium, and the minimal involvement of externally derived fluids. The whole-rock δ18O values of graphite-bearing gneisses that host the tanzanite range from 16.8 to 21.6‰, whereas the marbles have somewhat higher values (24.0 and 25.0‰). Graphite-free garnet-bearing gneisses from the Telele Hills, ~33 km south of the mining area, have much lower δ18O values (~7 to 10‰). The high loss on ignition (LOI) values, ~8 weight % and ~10 weight %, of the graphite-bearing gneisses from the mining area do not correlate with CaO or MgO content, which is consistent with the presence of up to 10% graphite with minimal carbonate being present. The average δ13C value of the graphite from the garnet-kyanite gneiss in the mining area is −23.8‰, whereas most of the associated carbonate rocks have δ13C values close to zero. This lack of C-isotope equilibration between the adjacent carbonate and silicate rocks, together with the lack of change in bulk-rock δ18O values at the contact between carbonate and silicate units suggests that the high δ18O value of the graphite-bearing gneiss reflects that of its protolith. The average δD value of the tanzanite and tremolite is −77 and −27‰, respectively, which is consistent with their formation at in equilibrium with fluids having δD values of −36 and −11‰, respectively, assuming a temperature of formation of ~630°C. High δ18O values calculated for the fluids in equilibrium with tanzanite are consistent with a strongly rock-buffered system and hence a ‘metamorphic origin’. Calculated fluid δD values are close to zero, and suggest that the fluid is formation water, ultimately of marine or meteoric origin. The negative δ13C of the graphite and the high δ18O values of the rocks and minerals are consistent with previous suggestions that the protolith of the graphite-bearing gneisses was black shale. The major and trace element compositions of the rocks and minerals are consistent with this hypothesis