Huiming Bao

Triple oxygen isotope evidence for elevated CO2 levels after a Neoproterozoic glaciation

Understanding the composition of the atmosphere over geological time is critical to understanding the history of the Earth system, as the atmosphere is closely linked to the lithosphere, hydrosphere and biosphere. Although much of the history of the lithosphere and hydrosphere is contained in rock and mineral records, corresponding information about the atmosphere is scarce and elusive owing to the lack of direct records. Geologists have used sedimentary minerals, fossils and geochemical models to place constraints on the concentrations of carbon dioxide, oxygen or methane in the past. Here we show that the triple oxygen isotope composition of sulphate from ancient evaporites and barites shows variable negative oxygen-17 isotope anomalies over the past 750 million years. We propose that these anomalies track those of atmospheric oxygen and in turn reflect the partial pressure of carbon dioxide () in the past through a photochemical reaction network linking stratospheric ozone to carbon dioxide and to oxygen. Our results suggest that was much higher in the early Cambrian than in younger eras, agreeing with previous modelling results. We also find that the 17O isotope anomalies of barites from Marinoan (∼635 million years ago) cap carbonates display a distinct negative spike (around -0.70‰), suggesting that by the time barite was precipitating in the immediate aftermath of a Neoproterozoic global glaciation, the was at its highest level in the past 750 million years. Our finding is consistent with the ‘snowball Earth’ hypothesis and/or a massive methane release after the Marinoan glaciation.

Origins of sulphate in Antarctic dry-valley soils as deduced from anomalous 17O compositions.

The dry valleys of Antarctica are some of the oldest terrestrial surfaces on the Earth. Despite much study of soil weathering and development, ecosystem dynamics and the occurrence of life in these extreme environments, the reasons behind the exceptionally high salt content of the dry-valley soils have remained uncertain. In particular, the origins of sulphate are still controversial; proposed sources include wind-blown sea salt, chemical weathering, marine incursion, hydrothermal processes and oxidation of biogenic sulphur in the atmosphere. Here we report measurements of δ 18O and δ17O values of sulphates from a range of dry-valley soils. These sulphates all have a large positive anomaly of 17O, of up to 3.4‰. This suggests that Antarctic sulphate comes not just from sea salt (which has no anomaly of 17O) but also from the atmospheric oxidation of reduced gaseous sulphur compounds, the only known process that can generate the observed 17 O anomaly. This source is more prominent in high inland soils, suggesting 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.

Oxygen isotope fractionation in ferric oxide-water systems: low temperature synthesis

The magnitude and temperature-sensitivity of oxygen isotope fractionation in ferric oxide-water systems remain uncertain. In this study, three different synthetic methods are used to investigate the temperature dependence of the fractionation between water and hematite, akaganeite, and goethite at near-surface temperatures. Our results reveal two similarities among these ferric oxide-water systems. First, the fractionation of oxygen isotopes between water and ferric oxide is small (i.e., ferric oxide-water fractionation factors [α] are very close to 1.000). Second, these α values are relatively insensitive to change in temperature (T). Hematite-water has a slightly higher α value and a greater temperature sensitivity than goethite-water at surface temperatures. While the issue requires further study, we speculate that differences in the washing and drying protocols applied to final precipitates may be one of the factors that have contributed to the discrepancies among published α-T curves. Owing to the rapid exchange of oxygen among the various hydrolytic Fe(III) species and ambient water, oxygen isotope equilibrium is probably attained between water and the ferric oxide gels and poorly-ordered ferrihydrite that are the initial precipitates in nearly all natural settings. Aging experiments suggest that isotopic compositions carried by ferric oxide gels and ferrihydrite are almost entirely erased by later exchange with ambient water during the maturation processes leading to formation of either hematite or goethite. These results suggest that dissolution and reprecipitation occur in the supposedly “solid-state transformation” from ferrihydrite to hematite. Thus the δ18O value of natural crystalline ferric oxides may provide a record of the long-term average δ18O value of local surface water, rather than that of the water from which the solid ferric oxide first formed.

Stretching the Envelope of Past Surface Environments: Neoproterozoic Glacial Lakes from Svalbard

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 ∼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.

Sulfate oxygen-17 anomalies in desert varnishes

Rock varnishes are ubiquitous in arid regions on Earth, and are believed to be commonly present on Mars. Here we report high water-soluble sulfate and nitrate contents in desert varnishes from the Death Valley region of southwestern U.S.A., and that sulfate in varnishes possess δ17O/δ18O ratios that do not fall on terrestrial mass-dependent fractionation line. Sulfate from wet and dry atmospheric deposition is probably the source of the δ17O anomalies. The anomalies are only moderately lower than that of aerosol sulfates collected from the greater Los Angeles area, indicating probably more than half of the sulfate in desert varnish is supplied by atmospheric deposition. This finding suggests that Earth surface environments are constantly accumulating δ17O-anomalous sulfate from the atmosphere; arid and stable conditions facilitate the preservation of these atmospheric signatures. This finding also indicates that different δ17O/δ18O ratios found in different components in the Martian meteorites may result from atmospheric chemical processes.

Paleocene–Eocene climatic variation in western North America: Evidence from the δ18O of pedogenic hematite

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 (δ 18 O) 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 δ 18 O values are measured by combining selective leaching, laser fluorination, and mass-balance calculation. These values, in conjunction with δ 18 O 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 δ 18 O values most likely reflect shifts in surface-water δ 18 O values. Analysis of hematites spanning the Paleocene-Eocene transition reveals a roughly 4‰ decrease in the δ 18 O of surface water in very early Eocene time. This episode probably reflects a cooling event (perhaps as great as 6 °C) that started 0.7 m.y. after the terminal Paleocene δ 13 C 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.

Reinstate regional transport of PM2.5 as a major cause of severe haze in Beijing

Building upon fine particulate matter (PM2.5) data and the accompanying meteorological conditions in the fall of 2013, Guo et al. (1) conclude that local aerosol nucleation and growth dominantly contributed to severe haze in Beijing, whereas regional transport of PM2.5 played an “insignificant” role. Guo et al.’s conclusion is surprising because it not only requires an uncharacteristic aerosol formation mode for Beijing, but also goes against a broadly held view that “the entire eastern China region, or at least the North China Plain” should be looked at “as a single air basin” if Beijing’s air quality is to be improved (2). Instead of curtailing the emission of aerosol precursor gases in a broader region, Guo et al.’s (1) conclusion would advocate a campaign on limiting traffic and emission within and around the immediate vicinity of Beijing. Already, Beijing’s municipal government has increased urban traffic controls as its number one tactic in an effort to reduce PM2.5.* However, we believe that an in-depth scrutiny of the data makes science more accurate and … [↵][1]2To whom correspondence may be addressed. Email: shaocaiyu{at}zju.edu.cn or bao{at}lsu.edu. [1]: #xref-corresp-1-1

Timing the deposition of 17O-depleted barite at the aftermath of Nantuo glacial meltdown in South China

Barite in basal Ediacaran cap carbonates in South China shows distinct, non-mass-dependent depletion in 17O, a signal most likely reflecting an extremely high p CO2 atmosphere during Marinoan glacial meltdown. The precise geological context of the barite within the cap carbonate was not defined, however, and such information is crucial to an accurate interpretation of the anomalous 17O signal. Based on an extensive field survey of Marinoan cap carbonates in South China and detailed sedimentological, petrographic, and isotope data, we propose here a unified sequence of events that followed the deposition of the Nantuo diamictite: the cap dolostone was first deposited and later uplifted due to isostatic rebound, undergoing karstic dissolution in both shallow platform and transitional facies of the Yangtze Block; subsequent transgression initiated a consistent sequence of mineral deposition on karstic surfaces or within cavities. The 17O-depleted barite, either visible on outcrops or identifiable in thin sections, is among the first minerals precipitated when the transgression flooded the karstified cap dolostone. The calcite with extremely negative δ13C values occurs at the last stage of the cavity filling, well after the deposition of cap dolostone and the 17O-depleted barite, suggesting that the signal of methane hydrate was registered much later in the cap dolostone. A similar recognition of karstic dissolution and subsequent barite deposits in cap dolostones in northwest Africa and northwest Canada suggest that deposition of the 17O-depleted barites may be a global event, recording an atmospheric-biological-hydrological condition shortly (within 0–1.6 m.y.) after the initial meltdown of the Marinoan snowball Earth.

Oxygen-17 excesses of the Central Namib gypcretes: spatial distribution

Abstract We present here sulfate oxygen isotopic data (72 samples with both δ18O and δ17O) systematically collected from the Central Namib Desert. Surface soils from two shore-inland (west–east) transects exhibit a gradual increase in the sulfate oxygen-17 excess (Δ17O=δ17O−0.52 δ18O) until at ca. 70 km inland, where no continuous gypcrete deposit is observed further east (inland). The oxygen isotopic compositions for water-soluble sulfates extracted from soils and gypcretes range from 8.3 to 13.3‰ and 0.06 to 1.11‰ for δ18O and Δ17O, respectively. The lateral pattern is similar to what has been seen in the cold deserts of the Antarctic dry valleys. However, unlike the dry valleys, no discernible correlation is found between δ18O and Δ17O, or between the depth of soil horizon and Δ17O in the Namib. Possible explanations include a relatively smaller component of dimethylsulfide (DMS)-derived sulfate in the total gypsum deposits and/or more active surface processes (e.g., flooding and leaching) in the Central Namib Desert than in the Antarctic cold deserts. Although current state of knowledge is insufficient to delineate quantitatively the sulfate contributions from different sources and reactions, the measurement of sulfate Δ17O does identify an unmistakable atmospheric sulfate component and provides additional independent information regarding sources and reactions.

Oxygen isotopic composition of ferric oxides from recent soil, hydrologic, and marine environments

Low-temperature synthesis experiments on ferric oxide–water systems have resulted in disparate oxygen isotope fractionation–temperature (α-T) curves. In this study, recent ferric oxides, mostly goethites of Holocene age, were collected and analyzed from a variety of modern soil, stream, and marine environments, where formation temperature and the oxygen isotopic composition (δ18O) of the water from which ferric oxides precipitated can be independently measured or estimated. This allows comparison of experimental α-T relationships with data from natural systems. Selective dissolution methods were refined for the pretreatment of fine-grained minerals in order to obtain reliable δ18O values for pure and crystalline ferric oxides. The difference (Δδ18O) between the δ18O value of goethite and that of local mean meteoric water ranges from −1.5 to +6.3‰ for soil goethites from New Jersey, Indiana, Michigan, Iowa, South Dakota, and Taiwan. We argue that these variations are largely the result of differences between the δ18O of formation water and that of local mean meteoric water, induced probably by 18O-enrichment of soil waters by evaporation or other processes in soil horizons where ferric oxides are forming. A marine goethite sample from Scotland and a subaqueous bog iron sample from New Jersey, which can not be biased by evaporative processes, provide crucial natural evidence that the difference in δ18O between goethite and formation water is ∼−1.5‰ at ∼10°C. This result is consistent with our prior laboratory synthesis results (Bao and Koch, 1999), but in conflict with other experimental calibrations. Given the highly variable δ18O value of soil or other surface water, as well as the potential of initially formed ferric oxides for reequilibration with subsurface burial fluids during maturation to crystalline phases, an understanding of formational and diagenetic conditions is absolutely essential when attempting to use the oxygen isotope composition of ferric oxides as a paleoclimatic proxy.