Heavy Isotope Ratios Research Articles

Aircraft measurements of water vapor heavy isotope ratios in the marine boundary layer and lower troposphere during ORACLES

Abstract. This paper presents aircraft in situ measurements of water concentration and heavy water isotope ratios D/H and 18O/16O during the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) project. The aircraft measurement system is also presented. The dataset is unique in that (1) it contains both total water and cloud condensed water isotope ratios; (2) it spans sufficient space and time to enable construction of spatially resolved climatology of isotope ratios in the lower troposphere; and (3) it is paired with a wealth of complementary measurements on atmospheric thermodynamic, chemical, aerosol, and radiative properties. Aircraft sampling took place in the southeast Atlantic marine boundary layer and lower troposphere (Equator to 22∘ S) over the months of September 2016, August 2017, and October 2018. Isotope measurements were made using cavity ring-down spectroscopic analyzers integrated into the Water Isotope System for Precipitation and Entrainment Research (WISPER). From an isotope perspective, the 300+ h of 1 Hz in situ data at levels in the atmosphere ranging from 70 m to 7 km represents a remarkably large and vertically resolved dataset. This paper provides a brief overview of the ORACLES mission and describes how water vapor heavy isotope ratios fit within the experimental design. Overviews of the sampling region and sampling strategy are presented, followed by the WISPER system setup and calibration details. The three data formats available to the users are each covered (latitude–altitude curtains, individual vertical profiles, and time series), with illustrative examples to highlight some features of the dataset and provide a plausibility check. The curtains and profiles demonstrate the dataset's potential to provide a comprehensive perspective on moisture transport and isotopic content in this region. Finally, measurement uncertainties are provided. Curtain and vertical profile data for all sampling periods can be accessed at https://doi.org/10.5281/zenodo.5748368 (see Henze et al., 2022). Time series data for the September 2016, August 2017, and October 2018 sampling periods can be accessed at https://doi.org/10.5067/Suborbital/ORACLES/P3/2016_V3, https://doi.org/10.5067/Suborbital/ORACLES/P3/2017_V3, and https://doi.org/10.5067/Suborbital/ORACLES/ P3/2018_V3, respectively (see references for ORACLES Science Team, 2020a–c, 2016 P3 data, 2017 P3 data, and 2018 P3 data).

Stable isotopes of deep soil water retain long-term evaporation loss on China's Loess Plateau

Evaporation from the land surface enriches heavy isotope ratios (2H/1H and 18O/16 O) in shallow soils, and downward water movement will carry the fractionation signal to deep soils. However, how to acquire the evaporation from water stable isotopes in deep soils remains untested. Here, we measured water stable isotope composition in the deep soils (2–10 m) across 20 sites on China's Loess Plateau. Our results show that the line-conditioned excess (lc-excess) in deep soils of these sites was invariable with depth at each site, but ranged between −14.0‰ and − 4.1‰ among these sites, indicating differing degree of enrichment in heavy water isotopes between sites. Moreover, the mean lc-excess in deep soils water was significantly correlated to mean annual precipitation (R2 = 0.57), potential evapotranspiration (R2 = 0.25), and the Budyko dryness (R2 = 0.68), indicating that deep soil water lc-excess reflects land surface climate conditions. Furthermore, the deep soils correspond to a timescale of approximately 100 years at one site and more than 27 years at the remaining sites. These results together indicate that stable isotopes of deep soil water retained long-term land surface evaporation effects. Further, by implementing the steady-state isotope mass balance model into the lc-excess framework, we derived a new method to estimate evaporation loss fraction (f). Our f estimates at these sites varied between 5% and 15%, which may represent the lower bound of the actual evaporation to precipitation ratio. Nevertheless, our work suggests that in these and the other similar regions, deep soil is a novel archive for long-term soil evaporation loss, and f may be estimated through a snapshot field campaign of stable isotope measurements.

Isotopic dynamics of precipitation and its regional and local drivers in a plateau inland lake basin, Southwest China

Shrinkage of plateau lakes under climate strength has drawn growing attention. Because of its intricate implication to hydro-meteorological condition and climate system, stable isotopes in precipitation (e.g. δ2Hp and δ18Op) provide us a powerful tool to understand the climate-hydrologic dynamics in shrinking lakes. However, how the regional atmospheric circulation, moisture sources and local fractionation processes drive isotopic variability from temporal to spatial scale has rarely been reported for remote plateau lakes. Hence, we collected a total of 98 rainfall samples at the south and the north shores of Chenghai lake, Yunnan-Guizhou Plateau to study the potential driving forces of precipitation isotope variability during the wet season of 2019. Based on backward trajectories of air masses obtained from HYSPLIT model, 68% of moisture came from δ18O depleted ocean (Indian Ocean, Bay of Bengal, South China Sea and Pacific Ocean), and the rainout process promoted the isotopic depletion when moisture arrived at the study basin. Evapotranspiration increased the heavy isotope ratios in precipitation originated from continents (northern China inland and western continents). The temporal dynamics of δ18Op and δ2Hp were in phase with the convection activities intensity underlined the influence from large-scale atmospheric circulation. Local meteorological factors played a secondary role in isotope variability. Precipitation amount-effect strongly affected isotope ratios while mild anti-temperature effect was observed at daily scale. Interestingly, the rainfall isotope ratios showed different mechanisms in govern at lake south shore and north shore, with a distance of 19 km in between. This south-to-north difference can be explained by either lower 1.03% sub-evaporation in the south shore or 7% of recycled moisture contributing to precipitation in the north shore. Our findings discover the driving forces for δ18Op variation and provide solid interpretations for hydro-climate change in Southwest China.

A detailed life history of a pleistocene steppe bison (Bison priscus) skeleton unearthed in Arctic Alaska

Detailed paleoecological evidence from Arctic Alaska’s past megafauna can help reconstruct paleoenvironmental conditions and can illustrate ecological adaptation to varying environments. We examined a rare, largely articulated and almost complete skeleton of a steppe bison (Bison priscus) recently unearthed in Northern Alaska. We used a multi-proxy paleoecological approach to reconstruct the past ecology of an individual representing a key ancient taxon. Radiocarbon dating of horn keratin revealed that the specimen has a finite radiocarbon age ∼46,000 ± 1000 cal yr BP, very close to the limit of radiocarbon dating. We also employed Bayesian age modeling of the mitochondrial genome, which estimated an age of ∼33,000–87,000 cal yr BP. Our taphonomic investigations show that the bison was scavenged post-mortem and infested by blowflies before burial. Stable carbon and oxygen isotope (δ13C and δ15N) analyses of sequentially sampled horn keratin reveal a seasonal cycle; furthermore, high δ15N values during its first few years of life are consistent with patterns observed in modern bison that undertook dispersal. We compared sequential analyses of tooth enamel for strontium isotope ratios (87Sr/86Sr) to a spatial model of 87Sr/86Sr values providing evidence for dispersal across the landscape. Synthesis of the paleoecological findings indicates the specimen lived during interstadial conditions. Our multi-proxy, paleoecological approach, combining light and heavy isotope ratios along with genetic information, adds to the broader understanding of ancient bison ecology during the Late Pleistocene, indicating that ancient bison adopted different degrees of paleo-mobility according to the prevailing paleoecological conditions and climate.

Isotopic interaction and source moisture control on the isotopic composition of rainfall over the Bay of Bengal

Abstract The isotopic interaction between ambient vapor and raindrops has been investigated for the first time at Port Blair, Andaman Islands, an environment having the minimal contribution of the continental moisture. Rainwater and ambient vapor samples were collected on a daily timescale during the Indian summer monsoon season of 2015. Oxygen (Hydrogen) heavy isotopic ratios of rainwater and ambient vapor are positively correlated and a difference in the seasonal average values was found to be 9.5‰ (65.8‰) at ~28 °C. On a daily scale, the oxygen isotope ratios of ambient vapor and rainwater are significantly correlated over a wide range of rainfall amount, but correlation weakens for rainfall exceeding ~36 mm/day. The isotopic variability of rainfall appears to be modulated by the interaction with the ambient vapor, which in turn is determined by the source moisture. In a case study, it is estimated that the vapor-rainwater isotopic exchange could lead to 18O enrichment (depletion) in raindrops (vapor) about 0.41‰ (approx. 30% of the total change), which is far above (>4σ) of the experimental uncertainty.

Identification of Waters Incorporated in Laguna Lake, Republic of the Philippines, Based on Oxygen and Hydrogen Isotopic Ratios

We examined the oxygen and hydrogen isotopic ratios of surface waters and groundwaters in the catchment of Laguna Lake, Republic of the Philippines, to identify the types of water that combine and control these ratios in the lake water. The oxygen and hydrogen isotopic ratios of water samples collected from rivers, lakes, springs, and irrigation canals were determined using cavity ring-down spectroscopy. The lake water data deviated from the meteoric line of the Philippines by between −13.5‰ and −10‰, and between −11.5‰ and −1.5‰ in the dry and wet seasons, respectively. The values for the groundwaters and surface waters were mainly between −8‰ and 3‰ throughout the year. In addition to rainwater, evaporative concentration, which may have an almost constant effect throughout the year, was the main control on the oxygen and hydrogen isotopic properties of Laguna Lake. The contributions of the surface waters and groundwaters to the oxygen and hydrogen isotopic ratios of the lake were relatively constant. Based on their isotopic properties, the waters within the water catchment area of Laguna Lake can be roughly divided into lake water with heavier isotopic ratios, and groundwater and surface water with lighter isotopic ratios.

Gas geochemistry of the Mobile Bay Jurassic Norphlet Formation: Thermal controls and implications for reservoir connectivity

The Mobile Bay gas field is located offshore Alabama in the northern Gulf of Mexico. Production is from eolian dunes of the Jurassic Norphlet sandstone at depths exceeding 6100 m (20,000 ft) and temperatures greater than 200C. Reservoir connectivity and compositional variation, including the distribution of nonhydrocarbon gases (H2S and CO2), are critical factors in production strategy. To evaluate the controls on compositional variation and connectivity, detailed molecular and isotopic analyses were conducted for 29 wells. Analysis of volatiles in fluid inclusions suggests that the field was originally filled with oil that subsequently cracked to gas. In addition to the thermal destruction (cracking) of oil, the process of thermochemical sulfate reduction (TSR) continues to destroy the remaining hydrocarbons through oxidation of gas and reduction of sulfate to form H2S and CO2. The variable extent of the TSR process at Mobile Bay results in a wide range of hydrocarbon and H2S compositions. Condensates are almost exclusively composed of diamondoids whose composition appears controlled by H2S concentrations. In contrast to hydrocarbon and H2S contents, CO2 concentrations are relatively constant throughout the field. Carbon isotopic ratios for CO2 correlate positively with those for wet-gas hydrocarbons but are heavier than expected for CO2 originating from hydrocarbon oxidation via TSR. The narrow range of CO2 contents and heavy isotope ratios suggests that CO2 is regulated by water-rock equilibration and carbonate precipitation. The destruction of the hydrocarbon gas and mineralization of the carbon dioxide product create a volume reduction and an associated drop in reservoir pressure. This process creates several internal sinks (or exits) that may control the spill direction for gas in the field.

Temperature dependence of isotopic fractionation in CO 2 photolysis

The effect of temperature on the heavy isotope ratios of the products (CO and O 2) of photolysis of CO 2 by ∼185 nm photons was studied. The isotope ratios 18O/ 16O and 17O/ 16O in O 2 and CO and 13C/ 12C in CO increase with temperature by ∼1‰ per K. The increase in 17O/ 16O in O 2 is related linearly to the increase in 18O/ 16O with a slope 0.8 contrary to the mass dependent value of 0.5. Variations in factors related to the transition dipole moment and Boltzmann distribution of the vibrational state populations are probably responsible for the observed change. Temperature dependence of the gas production rate agrees with the reported cross-section variations.

Isotope ratios of precipitation and water vapor observed in Typhoon Shanshan

Isotope ratios of precipitation and water vapor were observed during the passage of Typhoon Shanshan at Ishigaki Island, southwestern Japan, on 15–16 September 2006. Such high‐resolution isotopic observations allow for qualitative understanding of atmospheric moisture cycling; they revealed that isotope ratios of both the precipitation and water vapor decreased radially inward in the cyclone's outer region; anomalously high isotope ratios appeared in the cyclone's inner region; and d‐excess tended to decrease in the cyclone's inner region. In the cyclone's outer region, the water vapor was isotopically depleted due to the rainout effect which involves both condensation efficiency as reflected in inwardly increasing cloud thickness and isotopic exchange between falling droplets and the ambient water vapor. In contrast, water vapor in the cyclone's inner region was isotopically enriched due to weak rainout effect in conjunction with intensive isotopic recharge from the sea spray and sea surface with heavy isotope ratios. Since water vapor mainly acts as a source of precipitation, the isotope ratios of precipitation also had systematic variation. A unique circumstance is the intensity of isotopic exchange with almost saturated surface air and high winds, causing anomalously high isotope ratios and low d‐excess values in the cyclone's inner region.

Reconstructing Neolithic groups in Sarawak, Malaysia through lead and strontium isotope analysis

Heavy stable isotope analysis of mid-late Holocene Neolithic burials from Sarawak (Malaysia) identifies groups not apparent in mortuary treatment or inferred subsistence. Isotope ratios of strontium ( 87Sr/ 86Sr) and lead ( 208Pb/ 204Pb, 207Pb/ 204Pb, 206Pb/ 204Pb) from adult tooth enamel show distinct groups at Niah Cave's West Mouth organized by dietary catchment. Two roughly contemporary sites, Gua Sireh and Lobang Angin, are included to assess regional heterogeneity in northern Borneo. Heavy stable isotope ratios for humans differ from sampled modern plants and archaeological fauna from West Mouth, and suggest a non-local origin for some of the West Mouth human remains. Distinct heavy isotope ratios characterize all three sites and show regional differentiation broadly consistent with geological expectations. Results supplement paleodiet and morphological analyses and have implications for clarifying Niah Cave mortuary use during the mid-late Holocene. Further and more precise geographic placement will follow through additional landscape sampling for strontium and lead isotope values.

Relationship between the variation of isotopic ratios and the source of summer precipitation in eastern Siberia

The response of the isotopic ratio of summer precipitation in eastern Siberia to precipitation sources is estimated by examination of stable isotope ratios in the summer precipitation and by atmospheric data analysis, based on water balance considerations. The isotopic ratio (δ18O) was analyzed in samples of precipitation that were collected daily or weekly, from May 2000 to August 2000, in the Yakutsk region of eastern Siberia. The results of the isotopic data show that during spring, the variation of isotopic ratio is controlled mainly by the Rayleigh distillation process. During summer, on the other hand, the variation of isotopic ratio bears no relation to the Rayleigh process, and isotopic variation at this time is mainly caused by changes in the sources of precipitation. This is reinforced by comparison with the analysis of atmospheric data and observed isotopic data. The comparison shows that only in the summer samples, there was a good correlation (R2 = 0.88) between heavier isotopic ratios and increases in the proportion of the moisture supply that originates from the west, which is the dominant source of precipitation (about one half of the moisture). This linear relationship indicates the summer precipitation in eastern Siberia is a composite of source water from the west with heavy isotopic ratio and other sources with light isotopic ratio. The origin of this light isotopic ratio in summer precipitation might be recycled water from the land surface. Recycled water with light isotopic ratio results from the contribution of thaw water.

A methane fuse for the Cambrian explosion: carbon cycles and true polar wander

The dramatic diversification of animal groups known as the Cambrian Explosion (evolution’s ‘Big Bang’) remains an unsolved puzzle in Earth Science. The Vendian–Cambrian interval is characterized by anomalously high rates of apparent plate motion, interpreted as True Polar Wander (TPW), and by more than a dozen large, high-frequency perturbations in carbon isotopes that dwarf all others observed through the past 65 million years. We suggest that these biological, tectonic, and geochemical events are intimately related in the following fashion. First, tropical continental margins and shelf-slopes which formed during fragmentation of the supercontinent Rodinia accumulated massive quantities of isotopically-light organic carbon during Late Neoproterozoic time, as indicated by strikingly heavy isotope ratios in inorganic carbon during interglacial intervals. Second, an initial phase of Vendian TPW moved these organic-rich deposits to high latitude, where conditions favored trapping biogenic methane in layers of gas hydrate and perhaps permafrost. Continued sedimentation during Late Vendian time added additional hydrate/gas storage volume and stabilized underlying units until the geothermal gradient moved them out of the clathrate stability field, building up deep reservoirs of highly pressurized methane. Finally, a burst of TPW brought these deposits back to the Tropics, where they gradually warmed and were subjected to regional-scale thermohaline eddy variation and related sedimentation regime changes. Responding to the stochastic character of such changes, each reservoir reached a critical failure point independently at times throughout the Cambrian. By analogy with the Late Paleocene Thermal Maximum event, these methane deposits yield transient, greenhouse-induced pulses of global warming when they erupt. Temperature correlates powerfully with biodiversity; the biochemical kinetics of metabolism at higher temperature decrease generation time and maintain relatively rich and dense invertebrate populations. Repeated thermal pulses along with progressive disruption and alteration of global ocean circulation patterns by TPW could cause the increase in diversity that accompanied the radiation of metazoans. We suggest that a methane ‘fuse’ ignited the Cambrian Evolutionary Explosion.

Geochemistry of the magmatic–hydrothermal system of Kawah Ijen volcano, East Java, Indonesia

Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemical and isotopic analysis. An extremely low pH (<0.3) lake contains SO4–Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce “immature” solutions. The strong D and 18O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ratios also modifies the lake D and 18O content. The large ΔSO4–S0 (23.8–26.4‰) measured in the lake suggest that dissolved SO4 forms during disproportionation of magmatic SO2 in the hydrothermal conduit at temperatures of 250∼280°C. The lake δ18OSO4 and δ18OH2O values may reflect equilibration during subsurface circulation of the water at temperatures near 150°C. Significant variations in the lake's bulk composition from 1990 to 1996 were not detected. However, we interpret a change in the distribution and concentration of polythionate species in 1996 as a result of increased SO2-rich gas input to the lake system.Thermal springs at Kawah Ijen consist of acidic SO4–Cl waters on the lakeshore and neutral pH HCO3–SO4–Cl–Na waters in Blawan village, 17km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO4/Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO4–Cl water and ground water.The lakeshore fumarole discharges (T=170∼245°C) have both a magmatic and a hydrothermal component and are supersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is ∼260°C. The proportions of the oxidized, SO2-dominated magmatic vapor and of the reduced, H2S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interaction of SO2-bearing magmatic vapors with the summit acidic hydrothermal reservoir. This idea is supported by the lower H2S/SO2 ratio deduced for the gas producing the SO4–Cl reservoir feeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid–vapor zone at about 350°C.Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H2S in the lake water. This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar to the present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H2S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quenched liquid sulfur products show δ34S values similar to those found in the banded deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lake bottom.

D/H and 18O/16O ratios of water in the crater lake at Kusatsu-Shirane volcano, Japan

The D/H and 18O/16O ratios of water in the active crater lake situated on the Kusatsu-Shirane volcano, Japan are about 20 and 6‰, respectively, higher than local meteoric water. The ratios show seasonal variations superimposed on a gradual change over nine years. The isotopic ratios started to increase in early 1990 and decrease in the spring of 1995. The seasonal variation which is high in winter and low in summer correlates with the temperature difference between lake water and ambient air. The large temperature difference in winter enhances the evaporation of lake water and produces the enriched isotopic ratios relative to the ratios in summer. The accumulation of snow and the decrease in the flux of meteoric water into the lake strengthens the winter-time isotopic enrichment. The enriched isotopic ratios of the lake water over a long time result from the addition of an end member with heavy isotopic ratios contained in a thermal fluid supplied to the lake. Considering the water balance in the lake, the isotopic ratios of the thermal fluid were found to be close to the lake water itself, suggesting the circulation of the lake water seeping through lake floor. Based on the correlation between Cl−concentration and the isotopic ratios, the contribution by the heavy end member was estimated to be 25–36% relative to the enrichment by evaporation. The heavy end member could be a liquid phase evolved from a parental fluid, which is a mixture of local meteoric water and a magmatic fluid as found in high-temperature volcanic gases.

Laboratory-simulated thermal maturation of different types of sediments from the Williston Basin, North America—effects on the production rates, the isotopic and organo-geochemical composition of various pyrolysis products

Ten sediments of different type, age and maturity have been subjected to open-system pyrolysis, three of them additionally to closed-system pyrolysis for comparison. Various parameters have been investigated with respect to the thermally-induced maturation process. Production rates of various pyrolysis products basically depend on the type and maturity of the organic matter. Low temperatures (200–300°C, sometimes even 350°C) are mainly characterized by the preponderance of carbon dioxide, while at higher temperatures either methane or the pyrolysate fraction are the predominant pyrolysis products. Some rank parameters have been analysed to follow the stage of evolution of the organic matter. These measurements include the H/C ratio and the 0.5 (Pr/C 17 + Ph/C 18) ratio both decreasing with increasing maturity as well as the conversion factor ( C-factor) of the organic matter and the Pr/Ph ratio both increasing with temperature. All these parameters display maximum variations between 350° and 400°C, i.e. that temperature interval where most pronounced changes with the n-alkane distribution patterns also occur. Production rates and these rank parameters correlate with the process of hydrocarbon generation, which naturally operates during the catagenetic stage of evolution. During this artificial maturation, systematic variations along with the carbon isotope composition have been observed. Kerogens exhibit a small displacement (in the size of 0.4-1.0‰) towards heavier values with increasing pyrolysis temperature due to the preferential release of isotopically light decomposition products. Pyrolysates display a marked 13C enrichment of 1.9-3.0‰ for immature sediments with increasing temperature. This isotope effect basically seems to depend on the maturity, and not on the type of the organic matter. Gaseous pyrolysis products generally obey the theory of the intramolecular 13C heterogeneity. Displacements towards heavier isotope ratios at higher temperatures (500° and 600°C) seem to be due to a kinetic isotope effect reflecting the preferential cleavage of 12C− 12'C over 12C– 13C bonds. At closed-system pyrolytical conditions, much more pronounced differences along with the generation and distribution of various pyrolysis products and their carbon isotope composition do occur. It can be demonstrated that the pyrolysate fraction as well as the C 2−4 gaseous hydrocarbons serve as additional sources for the formation of methane at pyrolysis temperatures as high as 500° and 600°C, respectively.

C 13 and O 18 compositions in some fresh-water carbonates associated with ultramafic rocks and serpentinites: western United States

All carbonates associated with the ultramafic rocks and serpentinites of the western United States are shown by their stable isotope ratios to be of near-surface, low-temperature origin. These include vein materials that have been previously classified as hydrothermal. New laboratory and natural data were obtained on the equilibrium isotope relations between hydromagnesite and water. The origins of travertines in the ultramafic area are easily distinguished on the basis of their stable isotope ratios. The extremely heavy isotope ratios of nesquehonites suggest an intricate evaporating-film mechanism of formation.