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The isotope composition of seawater sulfate is an important tracer of sulfur, carbon, and oxygen cycles in Earth's deep past. Carbonate- associated sulfate (CAS) extracted by acid digestion is widely used as a proxy for sulfate in paleo-seawater from which the carbonate minerals precipitated. Early and late diagenesis, weathering, and laboratory processing can in some cases compromise original seawater sulfate signals. Here, we report that extracted CAS can also be severely contaminated by recent atmospheric sulfate, especially when the sampled carbonates are from outcrops in arid to semi-arid climates or in heavily polluted regions. Our evidence comes from triple oxygen isotope compositions of sequentially extracted water-leachable sulfate and acid-leachable sulfate from carbonates of diverse ages from northwestern and north-central China and southwestern North America. Independent of the age of the rocks, almost all the waterleachable sulfates and half of the acid-leachable sulfates bear positive 17O anomalies, clearly distinguishable from those of typical seawater sulfate. Because secondary atmospheric sulfate (SAS) is the only source of sulfate known to bear positive 17O anomalies, we conclude that sulfate extracted from carbonate outcrops in these regions has a significant component of SAS. Because SAS generally has a much lower δ34S value than paleo-seawater sulfate, it could shift the δ34S of the extracted CAS to lower values and in some cases even lower than that of the co-occurring pyrite, i.e., the "super-heavy pyrite" enigma reported in geological records. Our findings call for a re-evaluation of many published, outcrop-based CAS data and conclusions. [ABSTRACT FROM AUTHOR]

Reconstruction of the pre-Pleistocene climate of continental interiors has been hampered by a lack of climatic proxies and limited temporal resolution. We developed a new approach based on the oxygen isotope composition ([Delta]18O) of hematite, which occurs as a coating on fossil vertebrates, and used this method to study climatic change across the Paleocene-Eocene boundary in the Bighorn basin, Wyoming. Hematite coatings form in soils or shallow sediments, and therefore may be used to monitor near-surface conditions. Hematite [Delta]18O values are measured by combining selective leaching, laser fluorination, and mass-balance calculation. These values, in conjunction with [Delta]18O values for co-occurring carbonates, are used to assess the plausibility of published hematite-water oxygen isotope fractionation relations. Though uncertainty remains as to the appropriate fractionation, all are relatively insensitive to temperature variations. Thus, differences in hematite [Delta]18O values most likely reflect shifts in surface-water [Delta]18O values. Analysis of hematites spanning the Paleocene-Eocene transition reveals a roughly 4[per thousand] decrease in the [Delta]18O of surface water in very early Eocene time. This episode probably reflects a cooling event (perhaps as great as 6 [degrees] C) that started 0.7 m.y. after the terminal Paleocene [Delta]13C excursion, which corresponds to a pulse of extreme marine warming. Following the cooling, which lasted -0.6 m.y., temperatures rebounded to values close to those for late Paleocene time. The cooling episode coincides remarkably well with other indicators of environmental and climatic change from the basin, such as mammalian turnover events and mean annual temperatures estimated from leaf physiognomy.

Abstract: We have recently reported non-mass-dependent 17O depletion in sulfate deposited after the Marinoan glacial meltdown at ca. 635 million years ago. Further investigation linking the Δ17O of barite to its sedimentological–geological context in Marinoan South China reveals that the 17O depletion in sulfate is most pronounced at sites near paleo-continents, supporting the hypothesis that atmospheric O2 was the source of the depletion which was transferred to sulfate via oxidative weathering of sulfides. Host minerals or rocks for the Marinoan 17O depletion have been limited to barite (southern China and western Africa) and carbonate-associated sulfate (CAS) in limestone lenses within a diamictite (Svalbard). If the Marinoan 17O depletion event is indeed related to an extraordinary atmospheric condition, the signal should be global in its distribution. The Kimberley region of Western Australia was close to a continent in the late Neoproterozoic and will serve as a test of our hypothesis that the anomalous 17O depletion may be widely recorded in the rock records of this time period. We report here that the CAS in the Moonlight Valley (MV) cap dolostones, Texas/Mabel Downs (TMD), the eastern Kimberley region, Western Australia is anomalously depleted in 17O (Δ17O value as low as −0.68‰). The magnitude of the anomaly decreases gradually toward the overlying Ranford mud-silt-sandstones. This finding not only expands the geographic distribution of the depleted 17O signal, but also the type of host rocks or minerals that the anomalous sulfate resides. The CAS in the MV cap dolostones in Palm Spring (PS) section, ∼150km south of TMD, however, does not bear a 17O depletion. Neither does the CAS in the Egan cap dolostones. The presence and absence of pronounced 17O anomalies in the two time-equivalent yet spatially different MV cap dolostones are consistent with the paleogeography that indicates TMD was close to the continent while PS was at an open-ocean environment. While sharing some of the same sedimentological features with that of the MV cap dolostones at TMD, the Egan cap has distinct δ13C and δ18O values for dolostones and distinct Δ17O, δ34S and δ18O values for CAS, supporting an earlier assignment of the Egan glaciation to be younger than Marinoan in the late Neoproterozoic. [Copyright &y& Elsevier]

Abstract: Distinct, non-mass-dependent 17O depletion was reported in barite from Marinoan cap dolostones (~635Ma) in South China, and has been interpreted as an indication of an extremely high-pCO2 atmosphere (Bao et al., 2008). There is a significant variation in the magnitude of the 17O anomaly even among samples from the same outcrops. Understanding the origins of this barite and particularly the source of sulfate is critical to interpreting the anomalous 17O signature and its implication on sulfur and oxygen cycles at the aftermath of Marinoan glacial meltdown. In this study, we analyzed 160 micro-sampled barite samples from 20 hand specimens collected from two cap dolostone sections in South China. Together with field, petrographic, and Sr isotope data, the expanded dataset 1) confirms the large variability in ∆17O (from −0.09‰ to −0.87‰, VSMOW), δ18O (+13.3‰ to +23.5‰, VSMOW), and δ34S (+20.0‰ to +45.5‰, VCDT) of barite; 2) demonstrates a hyperbolic relationship between the ∆17O and δ34S of various barite samples with more negative Δ17O values generally corresponding to lower δ34S values; 3) reveals that individual barite crystal fans and fans of the same layer possess a well-clustered set of δ18O, Δ17O, and δ34S values; and 4) shows, correspondingly, that barite crystal fans of different layers bear different sets of Δ17O, δ18O, and δ34S values. The study suggests that 17O-depleted barite crystals were formed under supersaturation when Ba2+ from sulfate-free deepwater came to mix with sulfate-bearing shallow water. There were multiple upwelling episodes which resulted in multiple barite layers. The large variability in sulfur and triple-oxygen isotope composition and the high 87Sr/86Sr ratios indicate that the two sites from South China were sufficiently close to the continent so that the isotopic composition of sulfate was easily influenced by changes in riverine flux when seawater sulfate concentration at that time was exceedingly low. Our depositional model for the barite is consistent with the proposed sequence of events after the meltdown of Marinoan glaciation in South China by Zhou et al. (2010), and is also consistent with the proposed atmospheric origin of sulfate 17O depletion by Bao et al. (2008). [Copyright &y& Elsevier]

Volcanic eruptions release a large amount of sulphur dioxide (SO2) into the atmosphere. SO2 is oxidized to sulphate and can subsequently form sulphate aerosol, which can affect the Earth's radiation balance, biologic productivity and high-altitude ozone concentrations, as is evident from recent volcanic eruptions. SO2 oxidation can occur via several different pathways that depend on its flux and the atmospheric conditions. An investigation into how SO2 is oxidized to sulphate—the oxidation product preserved in the rock record—can therefore shed light on past volcanic eruptions and atmospheric conditions. Here we use sulphur and triple oxygen isotope measurements of atmospheric sulphate extracted from tuffaceous deposits to investigate the specific oxidation pathways from which the sulphate was formed. We find that seven eruption-related sulphate aerosol deposition events have occurred during the mid-Cenozoic era (34 to 7 million years ago) in the northern High Plains, North America. Two extensively sampled ash beds display a similar sulphate mixing pattern that has two distinct atmospheric secondary sulphates. A three-dimensional atmospheric sulphur chemistry and transport model study reveals that the observed, isotopically discrete sulphates in sediments can be produced only in initially alkaline cloudwater that favours an ozone-dominated SO2 oxidation pathway in the troposphere. Our finding suggests that, in contrast to the weakly acidic conditions today, cloudwater in the northern High Plains may frequently have been alkaline during the mid-Cenozoic era. We propose that atmospheric secondary sulphate preserved in continental deposits represents an unexploited geological archive for atmospheric SO2 oxidation chemistry linked to volcanism and atmospheric conditions in the past. [ABSTRACT FROM AUTHOR]

Abstract: A large amount of calcite pseudomorphs after gypsum were discovered in a volcanic ash bed and its underlying sand/siltstone in the Gering Formation (late Oligocene), northwestern NE, USA. The highly positive 17O anomalies in the residual sulfate indicate that the sulfate could only have come from the oxidation of sulfur gases by ozone in the atmosphere. We previously proposed that it was most likely the result of an extreme tropospheric sulfate haze (dry fog) event that was associated with volcanic eruptions in the west. To test the dry fog hypothesis, we examine here two alternative scenarios that also involve atmospheric sulfate deposition but do not imply a dramatic volcanic and dry fog source: (1) is it possible that some playas in arid or semiarid regions accumulate the type of atmospheric sulfate deposits as we see in the mid-Gering bed by the inflow of surface water or groundwater? (2) Could a period of hyperaridity and stability during the late Oligocene have caused a significant accumulation of atmospheric background sulfate onto the mid-Gering ash bed surface? Sulfates from a variety of playa settings were collected and measured for their 17O anomalies. The sulfate isotope (both O and S) and concentration profiles of two volcanic ash beds that are currently exposed in the central Atacama Desert (Chile) and in Taylor Valley of the Antarctica Dry Valleys were studied along with their sedimentary features. These two ash beds were chosen for comparison because of their significant levels of atmospheric sulfate accumulated during prolonged and hyperarid climatic conditions. Results showed no or minimal 17O anomalies in any of the playa sulfate samples from around the world, and none of the parameters in the mid-Gering ash profile matches those in either of the two volcanic ash profiles in modern hyperarid settings. Conclusion: The observed 17O-anomalous gypsum deposit in the mid-Gering ash bed is most likely the result of a series of extreme dry fogs that resulted in an intense pulse of atmospheric sulfate deposition to a semiarid playa setting. [Copyright &y& Elsevier]

Reports measurements of delta [sup18]O and delta [sup17]O values of sulphates from a range of dry-valley soils. Suggestion that Antarctic sulphate comes not just from sea salt but also from the atmospheric oxidation of reduced gaseous sulphur compounds; Possibility that the distributions of sulphate are largely explained by differences in particle size and transport mode which exist between sea-salt aerosols and aerosols formed from biogenic sulphur emission.

Presents a study of the oxygen-isotope values of two massive sulphate mineral deposits which show large isotopic anomalies despite being formed in surface environments. Origination of deposits in the central Namib desert and the sulphate-bearing Miocene volcanic ash-beds in North America; Questions regarding the source of the oxygen isotope; Possibility of the chemical origin of variable isotope anomalies on planets other than Earth.

Frequently occurred severe haze events in heavily populated China in recent years have beenattributed to extensive industrial and residential emissions, together with enhancedsecondary aerosol formation under stagnant meteorological condition, especially inwinter. Sulfate, as the major inorganic component of fine particles (PM2.5), has beenfound to form rapidly under high relative humidity conditions. Currently, a largegap exists in model-estimated sulfate concentration and field observation, withthe latter often being magnitudes higher. While model simulation is improving,multiple oxygen and sulfur isotopic compositions of atmospheric sulfate offer anindependent, often specific constraint on atmospheric secondary sulfate formationpathways. Here we report inorganic components, together with sulfur (δ34S) and triple-oxygenisotopes (δ18O, Δ17O) of sulfate in PM2.5 samples collected every 12 hours in the city ofBeijing, northern China in winter 2015. During the sampling period, PM2.5 concentrationwent up to as high as 178.9 ± 154.9 μg m−3. Sulfate had an average concentration of 20.9 ±20.9 μg m−3, with a mean δ34S values of 5.5±2.4‰ ranging from 1.3‰ to 9.4‰Ṫhe δ34Sis higher during heavily polluted days (average at 6.8‰) than that during relatively cleandays (average at 2.0‰). Meanwhile, the average sulfate Δ17O (Δ17O = δ18O -0.5305×δ17O) during heavy haze episodes was 0.62 ± 0.33‰ suggesting that aqueousoxidation pathway of S(IV)–H2O2 played a significant role on the enhanced sulfateformation during the evolution of haze episodes associated with fog processing. [ABSTRACT FROM AUTHOR]

Triple oxygen isotope (Δ′17O) analysis of CO 2 or carbonate is increasingly applied in diverse fields in earth and environmental sciences. One of the widely-used analytical methods for Δ′17O of CO 2 is through Pt-catalyzed oxygen isotope exchange between CO 2 and O 2. However, large inter-laboratory inconsistencies in oxygen isotope fractionation between the two gases have been reported, potentially hindering data comparison across different laboratories. By examining the oxygen isotope fractionations in thermal diffusion experiments for pure O 2 and CO 2 gases as well as CO 2 -O 2 mixtures, we demonstrate that thermal-gradient-induced (TG-induced) isotope fractionation is the main reason for the inconsistencies. The degree of TG-induced oxygen isotope fractionation is affected by temperature contrasts and volume ratios between the hot and cold parts of the extraction line as well as gas composition. Larger thermal contrasts generated larger oxygen isotope fractionations. CO 2 fractionated less than O 2 in the same thermal gradient conditions. For CO 2 -O 2 mixtures, TG-induced oxygen isotope fractionation of O 2 was reduced while that of CO 2 was enhanced, compared with the pure gases. Based on the results of thermal diffusion experiments, we found that the anomalous mass fractionation exponents (0.2 ∼ 0.9) observed in the Pt-catalyzed CO 2 -O 2 isotope exchange experiments can be rationalized quantitatively by a combination of isotopic exchange equilibrium and TG-induced oxygen isotope fractionation. Our model suggests that by weakening TG-induced oxygen isotope fractionation and by increasing the volume fraction of hot zone of a reactor, we can achieve a high precision (<10 per meg) for Δ′17O CO2 through CO 2 -O 2 exchange. Considering the widely existing thermal-gradient conditions in nature, the findings in this study may shed light on the origin of non-mass-dependent isotope fractionation. [ABSTRACT FROM AUTHOR]

The estimated contribution of "primary" combustion sulfate to the total atmospheric sulfate flux is inconsistent, hindering our interpretation of the atmospheric "missing sulfate". We expect combustion sulfate to inherit the characteristic triple oxygen isotope composition of air O 2 to variable degrees. To test the hypothesis, we extracted sulfate from soot samples collected from 48 residential chimneys throughout China. The mean sulfate's Δ′17O 0.5305 and δ18O values are −0.27 ± 0.06‰ and 12.8 ± 3.6‰, respectively, and the Δ′17O 0.5305 and δ18O values correlate negatively and extend to air O 2 's triple oxygen isotope composition as an endmember, suggesting that combustion sulfate is generated dominantly by S(IV) multiphase oxidation via O 2 in the plume, with possible additional pathways through •OH in aqueous films or droplets and NO 2 at soot surface. Oxygen sources (i.e., air O 2 , fuel, ambient moisture) and oxidation kinetics determine the triple oxygen isotope composition of combustion sulfate. The characterization of combustion sulfate's triple isotope composition helps to improve global and regional atmospheric sulfur chemistry models. • Triple oxygen isotope composition of combustion sulfate has an average value of −0.27‰ for the Δ′17O and 12.8‰ for the δ18O. • Oxygen sources (O 2 , fuel, water) and oxidation kinetics determine combustion sulfate's triple oxygen isotope composition. • The "primary" combustion sulfate is in fact "secondary" sulfate formed in chimney conduits and hot plumes. [ABSTRACT FROM AUTHOR]

Abstract We report a petrography, mineral chemistry, oxygen and chromium isotopic study of Grove Mountains (GRV) 020043 together with a subset of other acapulcoites and lodranites. GRV 020043 is a petrologic type 4 chondrite, with chondrules of diverse types and sizes, and is composed of low-Ca pyroxene (40 vol.%), olivine (24 vol.%), diopside (8 vol.%), plagioclase (10 vol.%), Fe-Ni metal (kamacite and taenite), troilite and some accessory minerals (chromite and apatite). The olivine in GRV 020043 has an average fayalite content (Fa) of 10.7 mol.% with the low-Ca pyroxene having an average ferrosilite content (Fs) of 10.8 mol.%. The whole rock oxygen isotopic composition of GRV 020043 is +3.226 ± 0.267‰, +0.797 ± 0.131‰, and −0.927 ± 0.017‰ for δ18O, δ17O, and Δ17O, respectively, with a bulk chromium isotopic compositions of ε54Cr = −0.48 ± 0.10. These characteristics of GRV 020043 are different from all established or ungrouped chondrites but agree with those of the acapulcoite-lodranite clan. We therefore suggest that GRV 020043 represents the chondritic precursor of acapulcoite-lodranite parent body. The similarity of bulk oxygen and chromium isotopic compositions among GRV 020043, Acapulco, Northwest Africa (NWA) 468 (metal-rich lodranite), NWA 8118 (lodranite), NWA 8287 (acapulcoite), and NWA 8422 (lodranite) indicates that they originated from a common oxygen and chromium reservoir in the protoplanetary disk or may have derived from a parent body with a differentiated multilayer structure. [ABSTRACT FROM AUTHOR]

Rationale: The oxygen isotope composition of phosphate (δ18OPO4) 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 δ18OPO4 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, KH2PO4, and (NH4)2HPO4 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 δ18OPO4 within analytical uncertainty (2σ = 0.6‰). We applied the purification and precipitation method on five igneous rocks and IWPs, and obtained δ18OPO4 values ranging from 6.4‰ to 8.0‰. Duplicate phosphate extractions yielded δ18OPO4 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 δ18OPO4 value. [ABSTRACT FROM AUTHOR]

The field of isotope geochemistry began with the study of oxygen isotope geothermometry, most notably for carbonates. For traditional oxygen isotope geothermometry only the relationship between one rare isotope, oxygen-18, and the common isotope, oxygen-16, is used because for most terrestrial processes the 17 O- 16 O relationship scales with the 18 O- 16 O relationship and is thought to not grant any new information. However, theoretical analysis predicts a small temperature-dependence of the equilibrium triple oxygen isotope relationship and instrumentation and techniques now allow for high-precision determination of the oxygen isotope composition for all three oxygen isotopes for a variety of sample types. To set the groundwork for triple oxygen isotope geothermometry, here we present new calibrations based on statistical thermodynamics and density functional theory for both the traditional two isotope and the recently introduced triple isotope thermometer for pairs of quartz, calcite, dolomite, fluorapatite, hematite, magnetite and liquid water. The results compare well with previous studies on 18 O/ 16 O fractionation where theoretical and experimental data are available. Of the models given here, pairs of quartz, calcite, dolomite and fluorapatite with water, hematite or magnetite show promising temperature sensitivities as triple isotope thermometers with acceptable uncertainties for surface and low-T hydrothermal environments. [ABSTRACT FROM AUTHOR]

The Marinoan panglaciation ( ca 650 to 635 Ma) is represented in north-east Svalbard by the 130 to 175 m thick Wilsonbreen Formation which contains syn-glacial carbonates in its upper 100 m. These sediments are now known to have been deposited under a CO2-rich atmosphere, late in the glaciation, and global climate models facilitate testing of proposed analogues. Precipitated carbonates occur in four of the seven facies associations identified: Fluvial Channel (including stromatolitic and intraclastic limestones in ephemeral stream deposits); Dolomitic Floodplain (dolomite-cemented sand and siltstones, and microbial dolomites); Calcareous Lake Margin (intraclastic dolomite and wave-rippled or aeolian siliciclastic facies); and Calcareous Lake (slump-folded and locally re-sedimented rhythmic/stromatolitic limestones and dolomites associated with ice-rafted sediment). There is no strong cyclicity, and modern analogues suggest that sudden changes in lake level may exert a strong control on facies geometry. Both calcite and dolomite in stromatolites and rhythmites display either primary or early diagenetic replacive growth. Oxygen isotope values (−12 to +15‰VPDB) broadly covary with δ13C. High δ13C values of +3·5 to +4·5‰ correspond to equilibration with an atmosphere dominated by volcanically degassed CO2 with δ13C of −6 to −7‰. Limestones have consistently negative δ18O values, while rhythmic and playa dolomites preserve intermediate compositions, and dolocretes possess slightly negative to strongly positive δ18O signatures, reflecting significant evaporation under hyperarid conditions. Inferred meltwater compositions (−8 to −15·5‰) could reflect smaller Rayleigh fractionation related to more limited cooling than in modern polar regions. A common pseudomorph morphology is interpreted as a replacement of ikaite (CaCO3·H2O), which may also have been the precursor for widespread replacive calcite mosaics. Local dolomitization of lacustrine facies is interpreted to reflect microenvironments with fluctuating redox conditions. Although differing in (palaeo)latitude and carbonate abundance, the Wilsonbreen carbonates provide strong parallels with the McMurdo Dry Valleys of Antarctica. [ABSTRACT FROM AUTHOR]

• Massively bedded barite (MBB) deposits occur mostly in Paleozoic sedimentary basins. • Sulfate-limited euxinic water is prone to be sulfate-free which accumulates hydrothermal Ba2+. • MBB deposits form when Ba2+-rich water encountering sulfate-bearing water. • Petrology, hyalophane, and δ 34 S and Sr isotope data support the new model. Existing models for massively bedded barite (MBB) deposits (e.g., sedimentary exhalative and diagenetic/ cold-seep) satisfy some geological and geochemical observations, but none explain the Paleozoic clustering of MBB deposits in Earth history. Here we bring seawater redox history into the picture and propose a sulfate-limited euxinic seawater (SLES) model where hydrothermally sourced Ba2+ accumulates as dissolved ion while other metal ions precipitate as insoluble metal sulfides. A subsequent encounter of this Ba2+-rich water mass with a sulfate-bearing one results in the deposition of MBB, thus physically separated from the accompanied metal sulfide deposits. We tested the SLES model using early Cambrian MBB deposits in South China through petrographic and isotope analyses. Syngenetic sphalerite and barite layers show a clear separation. A wide range of 87Sr/86Sr values (0.7082 to 0.7120) of the MBB supports a mixing of seawater and hydrothermal sources. A large range of δ 34 S values (32.2 to 61.1‰) of the MBB and the occurrence of mineral hyalophane supports an overall sulfate-limited but sulfate-concentration temporally and spatially heterogeneous ocean. Our model established the genetic link between MBB deposits and the accompanied metal sulfide deposits in the Paleozoic. The dearth of MBB deposits before and after the Paleozoic is due to widespread ferruginous oceans with little sulfate and complete oxygenated oceans with too much sulfate, respectively. The Paleozoic clustering of the MBB deposits is a consequence of a critical redox transition in Earth history. [ABSTRACT FROM AUTHOR]

• Thin beds of barite crystal fans occur in the basal Ediacaran, East Finnmark, Norway; • Barite formed in a shallow marine to subaerial, freshwater-dominated setting; • Barite's Δ17O and δ18O values are among the lowest in all palaeocontinents; • Barite in cracks and fissures is post-depositional. Basal Ediacaran barite, which has mass-anomalous depleted 17O, supporting Snowball Earth models, likely grew in shallow-marine settings, but sedimentological constraints lack details. Environmental conditions in the Varanger Palaeovalley during the basal Ediacaran are well preserved in the Nyborg Formation, where barite forms < 15 mm high syn syn-sedimentary crystal fans on Archean basement or thin intervening sediments. Sedimentary evidence suggests crystal fans formed in low-energy, very shallow-marine to subaerial facies. Rare earth element and yttrium signatures in associated carbonates suggest non-saline growth environments. Post-depositional fluid-flow replaced crystal fan barite with silica (now quartz) and redeposited barite as: <1 × 0.2 mm "dispersed" grains in sediments below fans; <5 mm grains replacing calcite filling desiccation cracks; <1 mm grains in basement fissures; <3 µm grains replacing matrix calcite; irregular grains in detrital sheet-silicate + calcite grains. Caledonian dissolution-reprecipitation reworked barite into brittle fractures and stylolites. Barite Δ17O values are amongst the most negative (-0.9 – -1.25 ‰) and δ18O values the lowest recorded (9.9 – 16.9 ‰) anywhere, consistent with a high latitude Baltica at ∼ 635 Ma. δ34S values (17.7 – 24.5 ‰) lie within the known range for basal Ediacaran barite. [ABSTRACT FROM AUTHOR]

Abstract: In order to understand spatial variations of stable isotope geochemistry in the Quruqtagh basin (northwestern China) in the aftermath of an Ediacaran glaciation, we analyzed carbonate carbon isotopes (δ 13 C carb), carbonate oxygen isotopes (δ 18 O carb), carbonate associated sulfate sulfur (δ 34 S CAS) and oxygen isotopes (δ 18 O CAS), and pyrite sulfur isotopes (δ 34 S py) of a cap dolostone atop the Ediacaran Hankalchough glacial diamictite at four sections. The four studied sections (YKG, MK, H and ZBS) represent an onshore–offshore transect in the Quruqtagh basin. Our data show a strong paleobathymetry-dependent isotopic gradient. From the onshore to offshore sections, δ 13 C carb values decrease from −2‰ to −16‰ (VPDB), whereas δ 18 O carb values increase from −4‰ to −1‰ (VPDB). Both δ 34 S CAS and δ 34 S py show stratigraphic variations in the two onshore sections (MK and YKG), but are more stable in the two offshore sections (H and ZBS). δ 18 O CAS values of onshore samples are consistent with terrestrial oxidative weathering of pyrite. We propose that following the Hankalchough glaciation seawater in the Quruqtagh basin was characterized by a strong isotopic gradient. The isotopic data may be interpreted using a three-component mixing model that involves three reservoirs: deep-basin water, surface water, and terrestrial weathering input. In this model, the negative δ 13 C carb values in the offshore sections are related to the upwelling of deep-basin water (where anaerobic oxidation of dissolved organic carbon resulted in 13 C-depleted DIC), whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite. The new data provide evidence for the oceanic oxidation following the Hankalchough glaciation. [Copyright &y& Elsevier]

• Calcites precipitated from the solutions with identical chemical and isotopic compositions have different δ 13 C values. • The DBL thickness affects the pH and the δ 13 C value of HCO − 3 at the mineral-solution interface. • Our finding offers a mechanistic interpretation on the variability of the ε calcite / DIC in riverine or cave environments. It is often assumed that the carbon isotope composition (δ 13 C) of carbonate minerals records that of the dissolved inorganic carbon (DIC) species with insignificant disequilibrium effect. However, results from field observations and laboratory experiments have shown that the δ 13 C difference between calcite and solution can vary up to 3‰ even under a similar set of solution composition and temperature, raising uncertainties on the δ 13 C's paleoclimate and paleoecology implications. One likely cause is the variable calcite precipitation rates and pH values induced by different thicknesses of the stagnant liquid layer between solid and well mixed bulk solution (i.e., diffusion boundary layer, DBL). To test this hypothesis, we selected a well-studied natural travertine deposit (Baishuitai, SW China) which consists of meter-scale travertine terraces. Calcite crystals at both the pool bottom and the rim of the terraces are inorganically precipitated from solutions identical in chemical, isotopic composition and temperature, but different only in hydrodynamics. This difference results in thicker DBL for water at the pool-bottom than for water flowing across the rim. We found that the δ 13 C and Mg/Ca ratios are higher while Sr/Ca ratios are lower for the pool-bottom calcites than for the rim calcites. By applying a mass transfer model, we quantitatively link the differences in carbon isotope and elemental compositions of the abiogenic calcites to the different hydrodynamic conditions. The inverse variation in Mg/Ca and Sr/Ca ratios in calcites arises from the different precipitation rates between at the pool-bottom and at the rim, while the consistently lower δ 13 C for calcites at the rim is due to their higher pH at the solid-solution interface than for calcites precipitated at the pool-bottom. In contrast, calcite precipitation rate has little influence on carbon isotope fractionation between calcite and HCO − 3 . Our results demonstrate the role of DBL thickness in governing the δ 13 C of HCO − 3 at mineral surface, which can assist to interpret the variable δ 13 C values of calcites in riverine or cave environments. [ABSTRACT FROM AUTHOR]

Several hundred meteorites with a total mass of over 100 kg were collected as the Alatage Mountain (AM) strewn field located in the Kumtag desert, Xinjiang Province, China. Twelve AM meteorites were studied in this work. Petrography, mineralogy, bulk chemistry, bulk oxygen isotopic compositions, and light noble gas concentrations and isotopic compositions were determined for all or for a selection of the meteorites. The studied meteorites are L‐chondrites that suffered a very strong impact; impact melt veins and melt pockets are widely distributed. More than 50% of the troilite exists in the form of blebs and veins in olivine and pyroxene. Some of these meteorites are impact melt recrystallized rocks (e.g., AM 037). The strong impact caused the decomposition of troilite, which led in AM 034 to the sulfidization reaction of olivine. The metal in most meteorites is almost completely altered, and the troilite has been significantly oxidized. Weathering resulted in the depletion of Mg, Fe, Co, and Ni, and the enrichment of Sr, Ba, Pb, and U in these meteorites. The cosmic ray exposure (CRE) ages measured for these specimens range between 6.2 ± 1.9 Ma and 9.0 ± 2.7 Ma, depending on the cosmogenic nuclide used. The average CRE age is 7.6 ± 1.3 Ma. Both 4He and 40Ar gas retention ages indicate that the strong impact which caused the shock effects occurred about 320 Ma ago. [ABSTRACT FROM AUTHOR]

Diffusional isotope fractionation has been widely used to explain lithium (Li) isotope variations in minerals and rocks. Isotopic mass dependence of Li diffusion can be empirically expressed as D 7 Li D 6 Li = 6 7 β , where D is the diffusivity of a Li isotope. The knowledge about temperature and compositional dependence of the β factor which is essential for understanding diffusion profiles and mechanisms remains unclear. Based on the potential energy and interatomic forces generated by deep neural networks trained with ab initio data, we performed deep potential molecular dynamics (DPMD) simulations of several Li pseudo-isotopes (with mass = 2, 7, 21, 42 g/mol) in albite, hydrous albite, and model basalt melts to evaluate the β factor. Our calculated diffusivities for 7Li in albite and model basalt melts at 1800 K compare well with experimental results. We found that β in albite melt decreases from 0.267 ± 0.006 at 4000 K to 0.225 ± 0.004 at 1800 K. The presence of water appears to slightly weaken the temperature dependence of β , with β decreasing from 0.250 ± 0.012 to 0.228 ± 0.031 in hydrous albite melt. The calculated β in model basalt melt takes much smaller values, decreasing from 0.215 ± 0.006 at 4000 K to 0.132 ± 0.015 at 1800 K. Our prediction of β in albite and hydrous albite melts is in good agreement with experimental data. More importantly, our results suggest that Li isotope diffusion in silicate melts is strongly dependent on melt composition. The temperature and compositional effects on β can be qualitatively explained in terms of ionic porosity and the coupled relationship between Li diffusion and the mobility of the silicate melt network. Two types of diffusion experiments are suggested to test our predicted temperature and compositional dependence of β. This study shows that DPMD is a promising tool to simulate the diffusion of elements and isotopes in silicate melts. [ABSTRACT FROM AUTHOR]

Diffusional isotope fractionation occurs in geochemical processes (such as magma mixing, bubble growth, and crystal growth), even at magmatic temperatures. Isotopic mass dependence of diffusion is commonly expressed as D i D j = m j m i β , where D i and D j are diffusion coefficients of two isotopes whose masses are m i and m j. How the dimensionless empirical parameter β depends on temperature, pressure, and composition remains poorly constrained. Here, we conducted a series of first-principles molecular dynamics simulations to evaluate the β factor of Mg isotopes in MgSiO 3 and Mg 2 SiO 4 melts using pseudo-isotope method. In particular, we considered interactions between Mg isotopes by simultaneously putting pseudo-mass and normal-mass Mg atoms in a simulation supercell. The calculated β for Mg isotopes decreases linearly with decreasing temperature at zero pressure, from 0.158 ± 0.004 at 4000 K to 0.121 ± 0.017 at 2200 K for MgSiO 3 melt and from 0.150 ± 0.004 at 4000 K to 0.101 ± 0.012 at 2200 K for Mg 2 SiO 4 melt. Moreover, our simulations of compressed Mg 2 SiO 4 melt along the 3000 K isotherm show that the β value decreases linearly from 0.130 ± 0.006 at 0 GPa to 0.060 ± 0.011 at 17 GPa. Based on our diffusivity results, the empirically established positive correlation between β and solvent-normalized diffusivity (D i / D Si) seems to be applicable only at constant temperatures or in narrow temperature ranges. Analysis of atomistic mechanisms suggests that the calculated β values are inversely correlated with force constants of Mg at a given temperature or pressure. Good agreement between our first principles results with available experimental data suggests that interactions between isotopes of major elements must be considered in calculating β for major elements in silicate melts. Also, we discuss diffusion-controlled crystal growth by considering our calculated β values. [ABSTRACT FROM AUTHOR]

The Proterozoic Eon spans Earth's middle age during which many important transitions occurred. These transitions include the oxygenation of the atmosphere, emergence of eukaryotic organisms and growth of continents. Since the sulfur and oxygen cycles are intricately linked to most surface biogeochemical processes, these transitions should be recorded in changes to the isotopic composition of marine and terrestrial sulfate minerals. Here we present oxygen (∆17O, δ18O) and sulfur (∆33S, δ34S) isotope records of Proterozoic sulfate from currently available data together with new measurements of 313 samples from 33 different formations bearing Earth's earliest unambiguous evaporites at 2.4 Ga through to Ediacaran aged deposits. This record depicts distinct intervals with respect to the expression of sulfate isotopes that are not completely captured by established intervals in the geologic timescale. The most salient pattern is the muted ∆17O signatures across the GOE , late Proterozoic and Ediacaran with values that are only slightly more negative than modern marine sulfate, contrasting with highly negative values across the mid-Proterozoic and Cryogenian. We combine these results with estimates of atmospheric composition to produce a gross primary production (GPP) curve for the Proterozoic. Through these results we argue that changes in GPP across Earth history likely help account for many of the changes in the Proterozoic Earth surface environment such as rising atmospheric oxygen, large fluctuations in the size of the marine sulfate reservoir and variations in the isotopic composition of sedimentary sulfate. [ABSTRACT FROM AUTHOR]

Abstract: This study investigates the sulfur and oxygen isotope fractionations of dissimilatory sulfate reduction and works to reconcile the relationships between the oxygen and sulfur isotopic and elemental systems. We report results of experiments with natural populations of sulfate-reducing bacteria using sediment and seawater from a marine lagoon at Fællestrand on the northern shore of the island of Fyn, Denmark. The experiments yielded relatively large magnitude sulfur isotope fractionations for dissimilatory sulfate reduction (up to approximately 45‰ for 34S/32S) with higher δ18O accompanying higher δ34S, similar to that observed in previous studies. The seawater used in the experiments was spiked by addition of 17O-labeled water and the 17O content of residual sulfate was found to depend on the fraction of sulfate reduced in the experiments. The 17O data provides evidence for recycling of sulfur from metabolic intermediates and for an 18O/16O fractionation of ∼25–30‰ for dissimilatory sulfate reduction. The close correlation between the 17O data and the sulfur isotope data suggests that isotopic exchange between cell water and external water (reactor water) was rapid under experimental conditions. The molar ratio of oxygen exchange to sulfate reduction was found to be about 2.5. This value is slightly lower than observed in studies of natural ecosystems [e.g., Wortmann U. G., Chernyavsky B., Bernasconi S. M., Brunner B., Böttcher M. E. and Swart P. K. (2007) Oxygen isotope biogeochemistry of pore water sulfate in the deep biosphere: dominance of isotope exchange reactions with ambient water during microbial sulfate reduction (ODP Site 1130). Geochim. Cosmochim. Acta 71, 4221–4232]. Using recent models of sulfur isotope fractionations we find that our combined sulfur and oxygen isotopic data places constraints on the proportion of sulfate recycled to the medium (78–96%), the proportion of sulfur intermediate sulfite that was recycled by way of APS to sulfate and released back to the external sulfate pool (∼70%), and also that a fraction of the sulfur intermediates between sulfite and sulfide were recycled to sulfate. These parameters can be constrained because of the independent information provided by δ18O, δ34S, δ17O labels, and Δ33S. [Copyright &y& Elsevier]

Rationale: The Δ 36 S standard deviation measured in a conventional isotope ratio mass spectrometer such as MAT 253 is ca 0.1‰ to 0.3‰. At this precision, it is difficult to resolve the origin of non-mass-dependent sulfur isotope fractionation in tropospheric sulfate aerosol and in Martian meteorites or small deviations from the canonical mass-dependent fractionation laws. Interfering ions with m/z at 131 of 36 SF 5 + are suggested by the community as the cause of the poor precision, but the exact ion species has not been identified or confirmed., Methods: Here we examined the potential interfering ions by using a Thermo Scientific ultrahigh-resolution isotope ratio mass spectrometer to measure SF 6 working gas and SF 6 gases converted from IAEA-S1/2/3 Ag 2 S reference materials., Results: We found that there are two resolvable peaks to the right of the 36 SF 5 + peak when a new filament was installed, which are 186 WF 4 2+ followed by 12 C 3 F 5 + . However, only the 12 C 3 F 5 + interference peak was observed after more than three days of filament use. 12 C 3 F 5 + is generated inside the instrument during the ionization process. Avoiding the interfering signals, we were able to achieve a Δ 36 S standard deviation of 0.046‰ (n = 8) for SF 6 zero-enrichment and 0.069‰ (n = 8) for overall measurement start from silver sulfide IAEA-S1., Conclusions: Aging the filament with SF 6 gas can avoid the interference of 186 WF 4 2+ . Minimizing the presence of carbon-bearing compounds and avoiding the interfering signals of 12 C 3 F 5 + from 36 SF 5 + , we can improve Δ 36 S measurement accuracy and precision, which helps to open new territories for research using quadruple sulfur isotope composition., (© 2024 John Wiley & Sons Ltd.)

The oxygen isotope composition of phosphate is a useful tool for studying biogeochemical phosphorus cycling. However, the current Ag 3 PO 4 method is not only tedious in PO 4 3- 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, δ 18 O, of dissolved phosphate at the nanomole level using electrospray ionization Orbitrap mass spectrometry (ESI-Orbitrap-MS). We compared the reproducibility of δ 18 O measurements using the H 2 PO 4 - ions ( m / z = 97 and 99 for H 2 P 16 O 4 - and H 2 P 18 O 16 O 3 - , respectively) and using the PO 3 - fragment ions ( m / z = 79 and 81 for P 16 O 3 - and P 18 O 16 O 2 - , respectively) generated by source fragmentation and by higher-energy collisional dissociation, respectively. The results demonstrate that phosphate δ 18 O can be more reliably measured by the PO 3 - ions than by the H 2 PO 4 - ions. PO 3 - generated by source fragmentation at 40 V achieved the highest reproducibility for δ 18 O based on precision tests. Furthermore, the mass spectrum for a 50:50 μM mixed solution of phosphate and sulfate revealed that PO 3 - ions resulting from source fragmentation at 40 V are the predominant species in the Orbitrap analyzer. Notably, P 16 O 3 - ions ( m / z : 79) are not interfered with by 32 S 16 O 3 - ( m / z : 80) ions. This is in contrast to the case for 1 H 2 P 16 O 4 - ions, which share the same m / z value with 1 H 32 S 16 O 4 - ions and exhibit much lower signal intensity than HSO 4 - ions. Using the PO 3 - fragment method and six phosphate standards with a wide range of δ 18 O values, we obtained a calibration line with a slope of 0.94 ( R 2 = 0.98). The overall uncertainty for ESI-Orbitrap-MS phosphate δ 18 O measurement was 0.8‰ ( n = 30; 1 SD). With much room for improvement, the PO 3 - fragment method presents a better approach to measuring the phosphate oxygen isotope composition, applicable to nanomole sample sizes in a liquid phase.

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 17 O anomalies (Δ' 17 O CAS values down to -0.53‰) during the SE. Furthermore, the Δ' 17 O CAS varies in correlation with its corresponding δ 34 S CAS and δ 18 O CAS as well as the carbonate δ 13 C carb , 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 17 O-depleted atmospheric O 2 in marine sulfate formation and thus a primary global oceanic oxygenation event during the SE., (© 2023. The Author(s).)

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: O 2 and CO 2 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 O 2 gas could be as large as 1‰ for δ 18 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 (α 18 O) for O 2 leaking ranges from 0.99911 to 0.99982 and the characteristic relationship of α 17 O and α 18 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., (© 2022 John Wiley & Sons Ltd.)

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 13 C-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 17 O-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.

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 [PM 2.5 ] (concentration of particulate matter with an aerodynamic diameter less than 2.5 μm) as well as relative humidity during the periods of rising [PM 2.5 ]. Our modeling results showed that CDW added additional average 4.6 μg m -3 PM 2.5 during severely polluted conditions in the GZB, which corresponded to an average 5.1% of local anthropogenic [PM 2.5 ] (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.

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.

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 TiO 2- 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 TiO 2 nanocrystals by Ti-O-C covalent bonding, forming robust 3D framework architectures. Interestingly, we find that TiO 2 nanocrystals can greatly improve the pseudocapacitance of TiO 2- x @C nanocomposite electrodes with increasing cycles, which significantly exceeds previously reported TiO 2 -based anodes and carbon materials. Furthermore, for the first time, the unusual improvement of pseudocapacitance of TiO 2- x @C electrodes is carefully investigated by means of d Q /d V 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 TiO 2 nanocrystals and exhibits bicontinuous pathways for both rapid ion and electron transport. In this case, TiO 2 nanocrystals stabilizing the 3DCF acted as a conductive agent during charge and discharge. Our findings confirm that the 3DCF triggered by TiO 2 nanocrystals boosted the electrochemical performance of TiO 2- x @C nanocomposite electrodes, especially the pseudocapacitance enhancement. The unique characteristics of ingenious combination of TiO 2 nanocrystals and amorphous carbon materials make them attain superior electrochemical properties in all known TiO 2 - 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.

The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today 1 . However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆ 17 O) 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 ∆ 17 O values (down to -0.88‰) observed in sulfates, except for those from the terminal Cryogenian period 2 . 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 CO 2 and O 2 and the photosynthetic O 2 flux at this time 3 . Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O 2 flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO 2 (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 O 2 was between 0.1-1% or 1-10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean 4-6 and Phanerozoic primary production 7 , these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O 2 over geologic time 8 .

The ~635 Ma Marinoan glaciation is marked by dramatic Earth system perturbations. Deposition of nonmass-dependently (17)O-depleted sulfate (SO4(2-)) in worldwide postglacial sediments is, thus far, unique to this glaciation. It is proposed that an extremely high-pCO2 atmosphere can result in highly (17)O-depleted atmospheric O2, or the Marinoan Oxygen-17 Depletion (MOSD) event. This anomalous (17)O signal was imparted to sulfate of oxidative weathering origin. However, (17)O-depleted sulfate occurs in limited sedimentary intervals, suggesting that Earth surface conditions conducive to the MOSD had a finite duration. An MOSD duration can, therefore, provide much needed constraint on modeling Earth system responses at that time. Unfortunately, the sulfate (17)O record is often sparse or lacks radiometric dates. Here, we report 11 barite layers from a post-Marinoan dolostone sequence at Wushanhu in the South China Block. The (17)O depletion fluctuates in magnitude in lower layers but is persistently absent up section, providing the most confident first and last sedimentary appearance of the anomaly. δ(13)C chemostratigraphy is used to correlate the Wushanhu section to two proximal sections on the same shallow platform that lack barite layers but have published U-Pb dates that occur in dolostone and shale. Assuming a similar pattern and rate for carbonate and shale deposition among the different sections, we estimate the MOSD duration at 0-0.99 My. This number can be further constrained by new radiometric dates from equivalent sequences worldwide, thus underpinning models on the nonsteady-state Earth system response in the immediate aftermath of the Marinoan meltdown.

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.

Cryogenic or heating methods have been widely used in experiments involving gas purification or isolation and in studying phase changes among solids, liquids, or gases for more than a century. Thermal gradients are often present in these routine processes. While stable isotopes of an element are known to fractionate under a thermal gradient, the largely diffusion-driven fractionation is assumed to be entirely mass-dependent. We report here, however, that distinct non-mass-dependent oxygen isotope fractionation can be generated when subjecting rarefied O(2) gas in a closed system to a simple thermal gradient. The Δ(17) O value, a measure of the (17) O anomaly, can be up to -0.51‰ (standard deviation (s.d.) 1σ = 0.03) in one of the temperature compartments. The magnitude of the (17) O anomalies decreased with increasing initial gas pressures. The authenticity of this phenomenon is substantiated by a series of blank tests and isotope mass-balance calculations. The observed anomalies are not the result of H(2) O contamination in samples or in isotope ratio mass spectrometry. Our finding calls attention to the importance of thermal gradient-induced isotope fractionation and to its implications in laboratory procedures, stable isotope geochemistry, and the physical chemistry of rarefied gases., (Copyright © 2010 John Wiley & Sons, Ltd.)

The oxygen isotope composition of terrestrial sulfate is affected measurably by many Earth-surface processes. During the Neoproterozoic, severe "snowball" glaciations would have had an extreme impact on the biosphere and the atmosphere. Here, we report that sulfate extracted from carbonate lenses within a Neoproterozoic glacial diamictite suite from Svalbard, with an age of approximately 635 million years ago, falls well outside the currently known natural range of triple oxygen isotope compositions and indicates that the atmosphere had either an exceptionally high atmospheric carbon dioxide concentration or an utterly unfamiliar oxygen cycle during deposition of the diamictites.