Oxygen Isotope Analysis of Nanomole Phosphate Using PO3- Fragment in ESI-Orbitrap-MS.

The oxygen isotope composition of phosphate is a useful tool for studying biogeochemical phosphorus cycling. However, the current Ag3PO4 method is not only tedious in PO43- extraction and purification but also requires a large-sized sample at the micromole level, thereby limiting its application. Here, we present an approach to measuring the oxygen isotope composition, δ18O, of dissolved phosphate at the nanomole level using electrospray ionization Orbitrap mass spectrometry (ESI-Orbitrap-MS). We compared the reproducibility of δ18O measurements using the H2PO4- ions (m/z = 97 and 99 for H2P16O4- and H2P18O16O3-, respectively) and using the PO3- fragment ions (m/z = 79 and 81 for P16O3- and P18O16O2-, respectively) generated by source fragmentation and by higher-energy collisional dissociation, respectively. The results demonstrate that phosphate δ18O can be more reliably measured by the PO3- ions than by the H2PO4- ions. PO3- generated by source fragmentation at 40 V achieved the highest reproducibility for δ18O based on precision tests. Furthermore, the mass spectrum for a 50:50 µM mixed solution of phosphate and sulfate revealed that PO3- ions resulting from source fragmentation at 40 V are the predominant species in the Orbitrap analyzer. Notably, P16O3- ions (m/z: 79) are not interfered with by 32S16O3- (m/z: 80) ions. This is in contrast to the case for 1H2P16O4- ions, which share the same m/z value with 1H32S16O4- ions and exhibit much lower signal intensity than HSO4- ions. Using the PO3- fragment method and six phosphate standards with a wide range of δ18O values, we obtained a calibration line with a slope of 0.94 (R2 = 0.98). The overall uncertainty for ESI-Orbitrap-MS phosphate δ18O measurement was 0.8‰ (n = 30; 1 SD). With much room for improvement, the PO3- fragment method presents a better approach to measuring the phosphate oxygen isotope composition, applicable to nanomole sample sizes in a liquid phase.

A new phosphate purification method for igneous weathering profiles.

RATIONALE: The oxygen isotope composition of phosphate (δ18 OPO4 ) is widely employed for reconstructing paleotemperature and tracing biogeochemical phosphorus cycling. However, existing phosphate purification protocols do not work well for igneous rocks and igneous weathering profiles (IWPs). A reliable purification method is needed for measuring δ18 OPO4 in these materials. METHODS: Our phosphate purification method includes two steps of cation exchange resin treatment separated by a step of calcium phosphate precipitation (R-Ca-R method). In addition, the silver phosphate precipitation in our procedure is featured by slow evaporation to crystallization until the appearance of ammonium nitrate or silver nitrate crystals. We evaluated our methods on weathered and pristine igneous rocks, phosphorite rocks, KH2 PO4 , and (NH4 )2 HPO4 solutions. RESULTS: Our purification method converted over 99.9% phosphate in solution to calcium phosphate, which can be easily decalcified by cation resin. The improved silver phosphate precipitation method produced high phosphate yields (97.1%-99.5%) and retained original δ18 OPO4 within analytical uncertainty (2σ = 0.6‰). We applied the purification and precipitation method on five igneous rocks and IWPs, and obtained δ18 OPO4 values ranging from 6.4‰ to 8.0‰. Duplicate phosphate extractions yielded δ18 OPO4 values differing by less than 0.3‰. CONCLUSIONS: We developed a new phosphate purification method that is applicable to phosphate extraction in igneous rocks and IWPs. We also proposed a reliable indicator for the termination of silver phosphate precipitation. Our method can achieve high phosphate yield and reproducible δ18 OPO4 value.

Sulfate triple-oxygen-isotope evidence confirming oceanic oxygenation 570 million years ago.

The largest negative inorganic carbon isotope excursion in Earth's history, namely the Ediacaran Shuram Excursion (SE), closely followed by early animal radiation, has been widely interpreted as a consequence of oceanic oxidation. However, the primary nature of the signature, source of oxidants, and tempo of the event remain contested. Here, we show that carbonate-associated sulfate (CAS) from three different paleocontinents all have conspicuous negative 17O anomalies (Δ'17OCAS values down to -0.53‰) during the SE. Furthermore, the Δ'17OCAS varies in correlation with its corresponding δ34SCAS and δ18OCAS as well as the carbonate δ13Ccarb, decreasing initially followed by a recovery over the ~7-Myr SE duration. In a box-model examination, we argue for a period of sustained water-column ventilation and consequently enhanced sulfur oxidation in the SE ocean. Our findings reveal a direct involvement of mass-anomalously 17O-depleted atmospheric O2 in marine sulfate formation and thus a primary global oceanic oxygenation event during the SE.

A transient peak in marine sulfate after the 635-Ma snowball Earth.

SignificanceEarth system's response to major perturbations is of paramount interest. On the basis of multiple isotope compositions for pyrite, carbonate-associated sulfate, carbonates, and organics within, we inferred that the much-debated, enigmatic, extremely 13C-depleted calcite cements in the ∼635-Ma cap carbonates in South China preserve geochemical evidence for marine microbial sulfate reduction coupled to anaerobic oxidation of methane. This interpretation implies the existence of a brief interval of modern-level marine sulfate. We determined that this interval coincides with the earliest Ediacaran 17O-depletion episode, and both likely occurred within ∼50 ky since the onset of the 635-Ma meltdown, revealing an astonishing pace of transformation of the Earth system in the aftermath of Earth's latest snowball glaciation.

Isotope fractionation during capillary leaking in an isotope ratio mass spectrometer.

RATIONALE: When isotope composition is measured in dual-inlet mode with an isotope ratio mass spectrometer (IRMS), reference gas may be gradually enriched in heavy isotopes due to preferential loss of light ones from the bellows over time. Quantifying the degree of isotopic enrichment of the reference gas is imperative for high-precision isotopic analysis (i.e. at per meg level). METHODS: O2 and CO2 leaking experiments were performed with the dual-inlet system of an IRMS (Thermo Fisher® MAT 253 Plus). During each experiment, the drop of gas pressure in the bellows with time was recorded and isotope ratios of residual gas at various time intervals were analyzed. RESULTS: Isotopic enrichment of residual O2 gas could be as large as 1‰ for δ18 O when a large fraction (>75%) of initial gas was lost. The evolution of isotope compositions of the remaining gas can be well described by a pressure-dependent Rayleigh fractionation equation. When the pressure in the bellows is within 10-50 mBar, the isotope fractionation factor (α18 O) for O2 leaking ranges from 0.99911 to 0.99982 and the characteristic relationship of α17 O and α18 O is from 0.5123 to 0.5124. CONCLUSIONS: Isotope fractionation associated with capillary leaking from bellows is pressure-dependent. We recommend that the reference gas should be reloaded frequently, especially after a measurement with a low analyzing pressure for the analysis of small amounts of sample.

Vapor isotopic evidence for the worsening of winter air quality by anthropogenic combustion-derived water.

Anthropogenic combustion-derived water (CDW) may accumulate in an airshed due to stagnant air, which may further enhance the formation of secondary aerosols and worsen air quality. Here we collected three-winter-season, hourly resolution, water-vapor stable H and O isotope compositions together with atmospheric physical and chemical data from the city of Xi'an, located in the Guanzhong Basin (GZB) in northwestern China, to elucidate the role of CDW in particulate pollution. Based on our experimentally determined water vapor isotope composition of the CDW for individual and weighted fuels in the basin, we found that CDW constitutes 6.2% of the atmospheric moisture on average and its fraction is positively correlated with [PM2.5] (concentration of particulate matter with an aerodynamic diameter less than 2.5 µm) as well as relative humidity during the periods of rising [PM2.5]. Our modeling results showed that CDW added additional average 4.6 µg m-3 PM2.5 during severely polluted conditions in the GZB, which corresponded to an average 5.1% of local anthropogenic [PM2.5] (average at ∼91.0 µg m-3). Our result is consistent with the proposed positive feedback between the relative humidity and a moisture sensitive air-pollution condition, alerting to the nontrivial role of CDW when considering change of energy structure such as a massive coal-to-gas switch in household heating in winter.

Large equatorial seasonal cycle during Marinoan snowball Earth.

In the equatorial regions on Earth today, the seasonal cycle of the monthly mean surface air temperature is <10°C. However, deep (>1 m) sand wedges were found near the paleoequator in the Marinoan glaciogenic deposits at ~635 million years ago, indicating a large seasonal cycle (probably >30°C). Through numerical simulations, we show that the equatorial seasonal cycle could reach >30°C at various continental locations if the oceans are completely frozen over, as would have been the case for a snowball Earth, or could reach ~20°C if the oceans are not completely frozen over, as would have been the case for a waterbelt Earth. These values are obtained at the maximum eccentricity of the Earth orbit, i.e., 0.0679, and will be approximately 10°C smaller if the present-day eccentricity is used. For these seasonal cycles, theoretical calculations show that the deep sand wedges form readily in a snowball Earth while hardly form in a waterbelt Earth.

Evidence of metasomatism in the interior of Vesta.

Diogenites are a group of meteorites that are derived from the interior of the largest protoplanet Vesta. They provide a unique opportunity to understanding together the internal structure and dynamic evolution of this protoplanet. Northwest Africa (NWA) 8321 was suggested to be an unbrecciated noritic diogenite meteorite, which is confirmed by our oxygen and chromium isotopic data. Here, we find that olivine in this sample has been partly replaced by orthopyroxene, troilite, and minor metal. The replacement texture of olivine is unambiguous evidence of sulfur-involved metasomatism in the interior of Vesta. The presence of such replacement texture suggests that in NWA 8321, the olivine should be of xenolith origin while the noritic diogenite was derived from partial melting of pre-existing rocks and had crystallized in the interior of Vesta. The post-Rheasilvia craters in the north-polar region on Vesta could be the potential source for NWA 8321.

Unusual Improvement of Pseudocapacitance of Nanocomposite Electrodes: Three-Dimensional Amorphous Carbon Frameworks Triggered by TiO2 Nanocrystals.

Both nanocrystals and carbon materials have attracted considerable attention in lithium-ion batteries (LIBs) because of their fast kinetics for lithium storage or long-life cycles. However, the easy aggregation of nanocrystals and high-temperature doping process of carbon materials seriously hindered their application in LIBs. Here, we report the development of unprecedented TiO2-x@C nanocomposite electrodes through a unique "melting low-temperature pyrolysis" strategy. It is found that the continuous and interconnected three-dimensional amorphous carbon frameworks (3DCFs) in the composites are closely connected with TiO2 nanocrystals by Ti-O-C covalent bonding, forming robust 3D framework architectures. Interestingly, we find that TiO2 nanocrystals can greatly improve the pseudocapacitance of TiO2-x@C nanocomposite electrodes with increasing cycles, which significantly exceeds previously reported TiO2-based anodes and carbon materials. Furthermore, for the first time, the unusual improvement of pseudocapacitance of TiO2-x@C electrodes is carefully investigated by means of dQ/dV curves and electrochemical kinetic analysis to reveal the extra contribution of lithium storage. 3DCF, a "lithium-ion reservoir", possesses an unexpected capacity enhancement behavior that is triggered by TiO2 nanocrystals and exhibits bicontinuous pathways for both rapid ion and electron transport. In this case, TiO2 nanocrystals stabilizing the 3DCF acted as a conductive agent during charge and discharge. Our findings confirm that the 3DCF triggered by TiO2 nanocrystals boosted the electrochemical performance of TiO2-x@C nanocomposite electrodes, especially the pseudocapacitance enhancement. The unique characteristics of ingenious combination of TiO2 nanocrystals and amorphous carbon materials make them attain superior electrochemical properties in all known TiO2- and carbon-based anodes (289 mA h g-1 at 5 A g-1 after 4000 cycles). Above all, our findings reveal previously ignored fundamental aspects of pseudocapacitance improvement of nanocomposite electrodes and offer new hope for structural design and carbon coating process of high-performance anode materials.

Transient marine euxinia at the end of the terminal Cryogenian glaciation.

Termination of the terminal Cryogenian Marinoan snowball Earth glaciation (~650-635 Ma) is associated with the worldwide deposition of a cap carbonate. Modeling studies suggest that, during and immediately following deglaciation, the ocean may have experienced a rapid rise in pH and physical stratification followed by oceanic overturn. Testing these predictions requires the establishment of a high-resolution sequence of events within sedimentary records. Here we report the conspicuous occurrence of pyrite concretions in the topmost Nantuo Formation (South China) that was deposited in the Marinoan glacial deposits. Sedimentary facies and sulfur isotope data indicate pyrite precipitation in the sediments with H2S diffusing from the overlying sulfidic/euxinic seawater and Fe (II) from diamictite sediments. These observations suggest a transient but widespread presence of marine euxinia in an ocean characterized by redox stratification, high bioproductivity, and high-fluxes of sulfate from chemical weathering before the deposition of the cap carbonate.

Triple oxygen isotope evidence for limited mid-Proterozoic primary productivity.

The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative ∆17O values (down to -0.88‰) observed in sulfates, except for those from the terminal Cryogenian period2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO2 and O2 and the photosynthetic O2 flux at this time3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O2 flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO2 (2-30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O2 was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean4-6 and Phanerozoic primary production7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O2 over geologic time8.

Assessing Pyrite-Derived Sulfate in the Mississippi River with Four Years of Sulfur and Triple-Oxygen Isotope Data.

Riverine dissolved sulfate (SO42-) sulfur and oxygen isotope variations reflect their controls such as SO42- reduction and reoxidation, and source mixing. However, unconstrained temporal variability of riverine SO42- isotope compositions due to short sampling durations may lead to mischaracterization of SO42- sources, particularly for the pyrite-derived sulfate load. We measured the sulfur and triple-oxygen isotopes (δ34S, δ18O, and Δ'17O) of Mississippi River SO42- with biweekly sampling between 2009 and 2013 to test isotopic variability and constrain sources. Sulfate δ34S and δ18O ranged from -6.3‰ to -0.2‰ and -3.6‰ to +8.8‰, respectively. Our sampling period captured the most severe flooding and drought in the Mississippi River basin since 1927 and 1956, respectively, and a first year of sampling that was unrepresentative of long-term average SO42-. The δ34SSO4 data indicate pyrite-derived SO42- sources are 74 ± 10% of the Mississippi River sulfate budget. Furthermore, pyrite oxidation is implicated as the dominant process supplying SO42- to the Mississippi River, whereas the Δ'17OSO4 data shows 18 ± 9% of oxygen in this sulfate is sourced from air O2.

Identifying apparent local stable isotope equilibrium in a complex non-equilibrium system.

RATIONALE: Although being out of equilibrium, biomolecules in organisms have the potential to approach isotope equilibrium locally because enzymatic reactions are intrinsically reversible. A rigorous approach that can describe isotope distribution among biomolecules and their apparent deviation from equilibrium state is lacking, however. METHODS: Applying the concept of distance matrix in graph theory, we propose that apparent local isotope equilibrium among a subset of biomolecules can be assessed using an apparent fractionation difference (|Δα|) matrix, in which the differences between the observed isotope composition (δ') and the calculated equilibrium fractionation factor (1000lnß) can be more rigorously evaluated than by using a previous approach for multiple biomolecules. We tested our |Δα| matrix approach by re-analyzing published data of different amino acids (AAs) in potato and in green alga. RESULTS: Our re-analysis shows that biosynthesis pathways could be the reason for an apparently close-to-equilibrium relationship inside AA families in potato leaves. Different biosynthesis/degradation pathways in tubers may have led to the observed isotope distribution difference between potato leaves and tubers. The analysis of data from green algae does not support the conclusion that AAs are further from equilibrium in glucose-cultured green algae than in the autotrophic ones. CONCLUSIONS: Application of the |Δα| matrix can help us to locate potential reversible reactions or reaction networks in a complex system such as a metabolic system. The same approach can be broadly applied to all complex systems that have multiple components, e.g. geochemical or atmospheric systems of early Earth or other planets.

Enantiomer signature and carbon isotope evidence for the migration and transformation of DDTs in arable soils across China.

Due to the adverse impact of DDTs on ecosystems and humans, a full fate assessment deems a comprehensive study on their occurrence in soils over a large region. Through a sampling campaign across China, we measured the concentrations, enantiomeric fractions (EFs), compound-specific carbon isotope composition of DDT and its metabolites, and the microbial community in related arable soils. The geographically total DDT concentrations are higher in eastern than western China. The EFs and δ13C of o,p'-DDT in soils from western China show smaller deviations from those of racemic/standard compound, indicating the DDT residues there mainly result from atmospheric transport. However, the sources of DDT in eastern China are mainly from historic application of technical DDTs and dicofol. The inverse dependence of o,p'-DDT and p,p'-DDE on temperature evidences the transformation of parent DDT to its metabolites. Initial usage, abiotic parameters and microbial communities are found to be the main factors influencing the migration and transformation of DDT isomers and their metabolites in soils. In addition, a prediction equation of DDT concentrations in soils based on stepwise multiple regression analysis is developed. Results from this study offer insights into the migration and transformation pathways of DDTs in Chinese arable soils, which will allow data-based risk assessment on their use.

Non-traditional stable isotope behaviors in immiscible silica-melts in a mafic magma chamber.

Non-traditional stable isotopes have increasingly been applied to studies of igneous processes including planetary differentiation. Equilibrium isotope fractionation of these elements in silicates is expected to be negligible at magmatic temperatures (δ(57)Fe difference often less than 0.2 per mil). However, an increasing number of data has revealed a puzzling observation, e.g., the δ(57)Fe for silicic magmas ranges from 0‰ up to 0.6‰, with the most positive δ(57)Fe almost exclusively found in A-type granitoids. Several interpretations have been proposed by different research groups, but these have so far failed to explain some aspects of the observations. Here we propose a dynamic, diffusion-induced isotope fractionation model that assumes Si-melts are growing and ascending immiscibly in a Fe-rich bulk magma chamber. Our model offers predictions on the behavior of non-traditional stable isotope such as Fe, Mg, Si, and Li that are consistent with observations from many A-type granitoids, especially those associated with layered intrusions. Diffusion-induced isotope fractionation may be more commonly preserved in magmatic rocks than was originally predicted.

Lost cold Antarctic deserts inferred from unusual sulfate formation and isotope signatures.

The Antarctic ice cap significantly affects global ocean circulation and climate. Continental glaciogenic sedimentary deposits provide direct physical evidence of the glacial history of the Antarctic interior, but these data are sparse. Here we investigate a new indicator of ice sheet evolution: sulfates within the glaciogenic deposits from the Lewis Cliff Ice Tongue of the central Transantarctic Mountains. The sulfates exhibit unique isotope signatures, including δ(34)S up to +50‰ for mirabilite evaporites, Δ(17)O up to +2.3‰ for dissolved sulfate within contemporary melt-water ponds, and extremely negative δ(18)O as low as -22.2‰. The isotopic data imply that the sulfates formed under environmental conditions similar to today's McMurdo Dry Valleys, suggesting that ice-free cold deserts may have existed between the South Pole and the Transantarctic Mountains since the Miocene during periods when the ice sheet size was smaller than today, but with an overall similar to modern global hydrological cycle.

Significant human impact on the flux and δ(34)S of sulfate from the largest river in North America.

Riverine dissolved sulfate (SO4(2-)) flux and sulfur stable isotope composition (δ(34)S) yield information on the sources and processes affecting sulfur cycling on different spatial and temporal scales. However, because pristine preindustrial natural baselines of riverine SO4(2-) flux and δ(34)S cannot be directly measured, anthropogenic impact remains largely unconstrained. Here we quantify natural and anthropogenic SO4(2-) flux and δ(34)S for North America's largest river, the Mississippi, by means of an exhaustive source compilation and multiyear monitoring. Our data and analysis show that, since before industrialization to the present, Mississippi River SO4(2-) has increased in flux from 7.0 to 27.8 Tg SO4(2-) yr(-1), and in mean δ(34)S from -5.0‰, within 95% confidence limits of -14.8‰ to 4.1‰ (assuming normal distribution for mixing model input parameters), to -2.7 ± 1.6‰, reflecting an impressive footprint of bedrocks particular to this river basin and human activities. Our first-order modern Mississippi River sulfate partition is 25 ± 6% natural and 75% ± 6% anthropogenic sources. Furthermore, anthropogenic coal usage is implicated as the dominant source of modern Mississippi River sulfate, with an estimated 47 ± 5% and 13% of total Mississippi River sulfate due to coal mining and burning, respectively.

Dynamic model constraints on oxygen-17 depletion in atmospheric O2 after a snowball Earth.

A large perturbation in atmospheric CO2 and O2 or bioproductivity will result in a drastic pulse of (17)O change in atmospheric O2, as seen in the Marinoan Oxygen-17 Depletion (MOSD) event in the immediate aftermath of a global deglaciation 635 Mya. The exact nature of the perturbation, however, is debated. Here we constructed a coupled, four-box, and quick-response biosphere-atmosphere model to examine both the steady state and dynamics of the MOSD event. Our model shows that the ultra-high CO2 concentrations proposed by the "snowball' Earth hypothesis produce a typical MOSD duration of less than 10(6) y and a magnitude of (17)O depletion reaching approximately -35‰. Both numbers are in remarkable agreement with geological constraints from South China and Svalbard. Moderate CO2 and low O2 concentration (e.g., 3,200 parts per million by volume and 0.01 bar, respectively) could produce distinct sulfate (17)O depletion only if postglacial marine bioproductivity was impossibly low. Our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean-atmosphere exchange only if partial pressure of atmospheric O2 is larger than 0.02 bar during the snowball period and records of weathering-derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown. In any case, a snowball Earth is a precondition for the observed MOSD event.